《Imaging Systems For Medical Diagnostics》——12. X-ray components and systems (1) X射線元件和系統（1）

《Imaging System 醫學影像》@EnzoReventon

《Imaging Systems For Medical Diagnostics》——12.X-ray components and systems——12.1 The X-ray tube X光射線管

==================================================================================

12.1 The X-ray tube X射線管

12.2 X-ray generators X射線發生器

12.3 X-ray image detectors X射線圖像探測器

12.4 X-ray systems X射線系統

12.5 Cone-beam CT with C-arm systems 帶C臂系統的5錐束CT

12.6 Mammography 乳房X線照相術

==================================================================================

12.1 The X-ray tube X射線管

The X-ray tube constitutes the source of radiation in an X-ray system and plays a decisive role in determining the degree of image quality produced by the system. The following factors are of primary importance:

X射線管是X射線系統中的輻射源，在确定系統産生的圖像品質方面起着決定性的作用。以下因素至關重要：

• Variable radiation hardness (i.e. penetration capacity): the capacity to vary radiation hardness over a wide range by means of the magnitude of the voltage applied to the tube permits optimal adaptation to the object of examination and the method of examination.

  可變輻射硬度（即穿透能力）：通過施加在管子上的電壓大小在大範圍内改變輻射硬度的能力，允許對檢查對象和檢查方法進行最佳調整。

  PS.x射線的強度和硬度:中管電流表示強度，管電壓表示硬度。

• Variable radiation intensity (dose rate): the capacity to control radiation intensity over a wide range via the tube current offers the same adaptive advantages.可變輻射強度（劑量率）：通過管電流控制寬範圍輻射強度的能力提供了相同的自适應優勢。
• Focal spot size and energy distribution in the focal spot: these play a role in determining the radiation source’s modulation transfer function (MTF) and thus contribute to contrast and resolving power. 焦斑尺寸和焦斑中的能量分布：它們在确定輻射源的調制傳遞函數（MTF）中起作用，進而提高對比度和分辨率。

Tungsten emitter:鎢發射體

Ceramic bushing:陶瓷襯套

Source for heating current:加熱電流的來源

Cathode assembly:陰極元件

HV_generate:高壓發生器

Electron beam:電子束

Tungsten target:鎢靶

Envelope /at least partially HV_insulating:外殼/至少部分高壓絕緣

X_ray window:X射線視窗

X_rays:X光

An evacuated envelope contains devices for high voltage insulation and the cathode assembly, including the emitter and the anode situated opposite one another. The electrons that are released by the emitter at low speed are focused by the shape of the cathode assembly and accelerated to 30 to 65% of the speed of light by the applied high voltage, as can be expressed by:

真空外殼包含用于高壓絕緣的裝置和陰極元件，包括彼此相對的發射極和陽極。發射極以低速釋放的電子通過陰極元件的形狀聚焦，并通過施加的高電壓加速至光速的30%至65%，如下所示：

e：電子電荷，U：陰極和陽極之間的電壓m0：電子品質，c：光速，v：電子速度

The accelerated electrons abruptly decelerate as they strike the anode surface. As described in section 6.1, no more than 1% of the overall electron energy is converted into X-rays. A preponderance of the total energy is converted into unwanted heat, which needs to be dissipated by the anode.加速電子撞擊陽極表面時突然減速。如第6.1節所述,不超過總電子能量的1%被轉換成X射線。總能量的絕大部分被轉化為不需要的熱量，需要通過陽極進行散熱。

In the following sections, we describe the function of the emitter, the formation of the electron beam and the resulting focal spot, the different types of anodes (including specific types of bearings used in X-ray tubes outfitted with rotating anodes), the calculation of maximum anode loads and their dependency on anode design, the function and the design of tube envelopes and the function and design of tube housings.在以下章節中，我們将介紹發射器的功能、電子束的形成和産生的焦點、不同類型的陽極（包括配備旋轉陽極的X射線管中使用的特定類型的軸承）、最大陽極負載的計算及其對陽極設計的依賴性，管殼的功能和設計以及管殼的功能和設計。

12.1.1 Emitters 發射器

The vast majority of X-ray tubes used today are equipped with a cathode assembly consisting of two parts: the electron source (emitter) and the auxiliary electrode surrounding it (Wehnelt electrode). While emitters generally consist of a helically wound tungsten wire with a diameter of 0.2 to 0.3 mm, flat emitters made of tungsten sheet are also used. Both kinds of emitters are heated directly by electrical current. In what follows, we offer a few basic principles of thermionic emission. Readers may wish to refer to the relevant textbooks for a more comprehensive discussion [12.1-12.3].

目前使用的絕大多數X射線管都配備有陰極元件，陰極元件由兩部分組成：電子源（發射極）和其周圍的輔助電極（韋内爾電極）。而發射器通常由直徑為0.2-0.3mm的螺旋纏繞鎢絲組成。也使用由鎢片制成的3 mm扁平發射器。這兩種發射器都由電流直接加熱。在接下來的内容中，我們将介紹一些熱電子發射的基本原理。讀者可以參考相關教科書進行更全面的讨論[12.1-12.3]。

Thermionic emission 熱離子發射

Electrons in metal are bound by a 3D potential constituted by their positively charged atomic cores (fig. 12.2). An electron has the capacity to leave the crystal as soon as it acquires sufficient energy (e.g. by means of heating or by the absorption of a photon). The height of the barrier is referred to as the work function W. Typical work function values for metals range from 2 to 5 eV and depend on the degree of crystallographic order, surface orientation and (critically) the purity of the surface (i.e. with regard to adsorbed species or coatings). An electric field E at the surface of the metal can lower the escape barrier by an amount ΔW. The quantum mechanical probability that an electron with an energy level of < ( ε V a c ε_Vac εV​ac – ΔW) will tunnel through the wall (Schottky effect) increases as E increases. This process is referred to as field emission. Electric field strengths for field emission currents sufficient for technical purposes are obtained in the vicinity of kinks or tip-shaped metal surfaces.金屬中的電子被帶正電的原子核構成的三維勢束縛（圖12.2）。一個電子一旦獲得足夠的能量（例如通過加熱或吸收光子），就有能力離開晶體。勢壘的高度稱為功函數W。金屬的典型功函數值範圍為2至5 eV（電子伏），取決于結晶順序、表面取向和（關鍵的）表面純度（即吸附物種或塗層）。金屬表面的電場E可以将逃逸勢壘降低ΔW。能級<（ ε V a c ε_Vac εV​ac – ΔW）的電子穿過壁的量子力學機率（肖特基效應）随着E的增加而增加。此過程稱為場發射。在扭結或尖端形狀的金屬表面附近獲得足以用于技術目的的場發射電流的電場強度。

金屬固體中電子的能量圖。 ε F ε_F εF​：費米能級（ ε V a c ε_Vac εV​ac：真空能級，電子通過固體表面逃逸到真空中所需的最小能量）。固體表面的電勢随外部電場E1、2、3的強度而變化。

Assuming that all of the electrons that leave the surface of the solid are removed by outer fields, the actual rate at which they leave the solid will equal the maximum achievable emission current density, i.e. the saturation current density. This density is given by the Richardson-Dushman equation (readers may wish to refer to the texts cited under [12.4, 12.5] for a more thorough discussion):

假設所有離開固體表面的電子都被外場移除，它們離開固體的實際速率将等于可達到的最大發射電流密度，即。E飽和電流密度。該密度由理查森-杜什曼方程給出（讀者可能希望參考[12.4,12.5]中引用的文本進行更深入的讨論）：

j s j_s js​：表面電流密度，h：普朗克常數，W：功函數，k：玻爾茲曼常數，T：固體溫度

其中：

E: 外部電場強度, ε 0 ε_0 ε0​: 介電真空常數

The derivation of this equation relies on the assumption that every electron with an appropriate energy level and direction may pass the surface. Quantum-mechanical considerations indicate that this is not entirely accurate. When one takes the transmission and reflection probabilities of the metal electrons into account, the saturation current density is reduced to：

這個方程的推導依賴于一個假設，即每一個具有适當能級和方向的電子都可以通過表面。量子力學的考慮表明這并不完全準确。當考慮金屬電子的透射和反射機率時，飽和電流密度減小到:

ε F ε_F εF​：費米能量

These equations offer detailed descriptions of the influence that the experimental parameters have on the saturation emission current. Indeed, the above outlined correlations have been used to experimentally determine the work functions of pure metal surfaces. However, it is important to bear in mind that these equations do not permit straightforward quantitative assessments of emitter emission currents in X-ray tubes. The reasons for this are outlined in our discussion of technical emitters below.

這些方程較長的描述了實驗參數對飽和發射電流的影響。事實上，上述關聯式已用于實驗确定純金屬表面的功函數。然而，重要的是要記住，這些方程不允許對X射線管中的發射極發射電流進行直接的定量評估。下面我們對技術發射器的讨論概述了産生這種情況的原因。

The direct proportionality of the tube current (i.e. the total electron current between cathode and anode) to the emitter’s saturation current density holds as long as all emitted electrons are taken up by the electric field and accelerated towards the anode. However, a space charge region will form in the vicinity of the emitter surface as the number of emitted electrons increases (e.g. via an increase in the emitter temperature). This space charge modifies the electric field at the emitter surface and leads to a situation in which the tube current is no longer governed by the number of electrons escaping the cathode, but by the total voltage applied between cathode and anode (Langmuir-Child relation):

隻要所有發射的電子被電場吸收并向陽極加速，管電流（即陰極和陽極之間的總電子電流）與發射極飽和電流密度的直接比例就保持不變。然而，随着發射電子數量的增加（例如通過發射極溫度的增加），發射極表面附近将形成一個空間電荷區。這種空間電荷改變了發射極表面的電場，并導緻管電流不再由逃離陰極的電子數決定，而是由施加在陰極和陽極之間的總電壓決定（Langmuir-Child關系）：

U：陽極和陰極之間的電位差

d：陽極和陰極之間的距離

飽和電流密度j_RDS和空間電荷電流密度j_LC，繪制為發射極溫度的函數。顯示了三種不同管電壓U1、U2和U3的曲線圖。

飽和電流密度j_RDS和空間電荷電流j_LC，繪制為管電壓U的函數。顯示了三種不同發射極溫度T1、T2和T3的曲線圖。

管陽極電流IA，針對三種不同管電壓U1、U2和U3的加熱電流IH繪制.

管陽極電流IA，針對多個燈絲加熱電流IH（範圍為4-5.8A）的管電壓UA圖。

Space-charge and pure saturation current represent ideal conditions. Detailed investigations have shown that under common operating conditions some emitter areas operate in the saturation current regime while space-charge limit conditions apply to neighboring areas. Nevertheless, the characteristics of real X-ray tubes also reflect the dependencies described by the above equations, as can be seen in figs. 12.3-12.6.空間電荷和純飽和電流代表理想條件。詳細的研究表明，在常見的工作條件下，一些發射極區域在飽和電流狀态下工作，而空間電荷限制條件适用于相鄰區域。然而，真實X射線管的特性也反映了上述方程描述的相關性，如圖所示 12.3-12.6.\

Technical emitters in medical X-ray tubes 醫用X射線管中的發射器

While many geometric configurations have been deployed to help bring about the emission of electrons from a heated solid, the most widely used emitter in medical X-ray applications is the helix-shaped tungsten filament (as depicted in fig. 12.7).雖然已經部署了許多幾何結構來幫助實作加熱固體的電子發射，但醫用X射線應用中最廣泛使用的發射器還是螺旋形鎢絲（如圖12.7所示）。

鎢絲螺旋加熱絲

A second type of emitter, the flat emitter (cf. fig. 12.8), has recently gained acclaim in connection with its application in mammography X-ray tubes and, even more recently, in connection with its application in high-end, rotating envelope X-ray tubes [12.6].第二種類型的發射器，扁平發射器（見圖12.8），最近因其在乳腺X射線攝影管中的應用，以及最近在高端旋轉包絡X射線管中的應用而受到好評[12.6]。

平面發射器，采用雷射切割技術由鎢片制成

The following parameters are crucial for the maximum achievable tube current:以下參數對于可實作的最大管電流至關重要:

• Emitter area contributing to the emission 有助于排放的發射器面積
• Maximum applicable emitter temperature compatible with a sufficiently long tube service life 與足夠長的使用壽命相容的最大适用發射器溫度
• Work function W 功函數W
• Field gradient at the emitter surface 發射極表面的場梯度

While the influence of the emitter area is straightforward, a point-like electron source (or at least a source with a homogeneous spatial distribution of emission) is required, especially for high resolution imaging. The concept of a flat emitter is suited to the task of increasing the emitting area. However, care must be taken to ensure uniform temperature distribution. Temperature distribution is not only crucial for emission homogeneity, but also for achieving a sufficient emitter configuration service life. The helix-shaped emitter exhibits a rather complex temperature distribution and does not allow for even a rough calculation of the emission current via the above formulas.

雖然發射極面積的影響是直接的，但需要點狀電子源（或至少是發射具有均勻空間分布的源），特别是對于高分辨率成像。平面發射器的概念适用于增加發射面積的任務。但是，必須注意確定溫度分布均勻。溫度分布不僅對發射均勻性至關重要，而且對實作足夠的發射極配置使用壽命也至關重要。螺旋形發射器顯示出相當複雜的溫度分布，并且不允許通過上述公式對發射電流進行粗略計算。

While X-ray tube currents may range up to 1.5 to 2.0 A, the electron emission surface area should not exceed around 20 mm2 so as to ensure proper focusing across the whole current and voltage operation range. Given that this requires emitter temperatures as high as 3,000 K, tungsten is almost the only feasible material for use in the construction of emitters [12.7].而X射線管電流的範圍可能高達1.5-2.0A時，電子發射表面積不應超過20 m m 2 mm^2 mm2左右，以確定在整個電流和電壓工作範圍内正确聚焦。考慮到這需要高達3000 K的發射器溫度，鎢幾乎是建造發射器的唯一可行材料[12.7]。

An emitter’s work function W is mainly determined by the chosen material – pure tungsten shows 4.5 eV. However, the work function can be modified by the surface adsorption of other materials. The application of special coatings on the emitter (e.g. LaB6) enables one to lower the effective work function and achieve a higher electron emission rate. Unfortunately, coating service life is often limited, which leads to a considerable decrease in emission performance as the emitter’s service life increases. This problem has prevented broader use of coated emitters in medical applications。發射器的功函數W主要由所選材料決定-純鎢顯示4.5電子伏。然而，功函數可以通過其他材料的表面吸附來修改。在發射極上塗覆特殊塗層（如 L a B 6 LaB_6 LaB6​）可以降低有效功函數并獲得更高的電子發射率。不幸的是，塗層的使用壽命通常是有限的，這導緻随着發射器使用壽命的增加，發射性能大大降低。這個問題阻礙了塗層發射器在醫療應用中的廣泛應用。

The gradient of an external electrostatic potential at the emitter surface has a significant influence on the emission current. On the one hand, high field gradients amplify the saturation emission current due to the lowering of the potential barrier. On the other hand, they also lead to a depletion of the space-charge region and bring the emitter operation mode into the saturation current’s current-limited regime. The steepness of the electrical field at the emitter surface is mainly governed by the design of the cathode cup, i.e. the elements surrounding the emitter that enable the focusing of the electron beam to the anode.

發射極表面的外部靜電電勢梯度對發射電流有顯著影響。一方面，由于勢壘的降低，高場梯度放大了飽和發射電流。另一方面，它們也會導緻空間電荷區耗盡，并使發射極工作模式進入飽和電流的限流區。發射極表面電場的陡度主要取決于陰極杯的設計。E發射極周圍的元件，使電子束能夠聚焦到陽極上。

The helix-shaped tungsten wire filament represents a good compromise in that it accounts for the various – partly contradictory – emitter requirements. This filament combines, for instance, easy manufacturability, simple handling, long service life, reliable operation and the capacity to achieve high emission currents. Helix-shaped tungsten filaments provide good mechanical stability throughout the manufacturing process. The wires are usually treated with potassium to inhibit high temperature creep in the wire. The filaments are heated by an electric current flowing through the wire. Therefore, the emission temperature can be easily controlled via the applied heating current.螺旋形鎢絲代表了一個很好的折衷方案，因為它考慮了各種發射極要求（部分互相沖突）。例如，這種燈絲具有易于制造、操作簡單、使用壽命長、運作可靠和實作高發射電流的能力。螺旋形鎢絲在整個制造過程中提供良好的機械穩定性。電線通常用鉀處理，以抑制電線的高溫蠕變。燈絲由流過導線的電流加熱。是以，可通過施加的加熱電流輕松控制發射溫度。

Slow emission-current response time represents a disadvantage associated with the helix-shaped tungsten emitter. For instance, in angiographic applications it is required to vary the emission current between “on” (100 mA) and “off” (0 mA) states in the ms time regime. Moreover, modern CT applications have led to a demand for fast tube current variations. While the latter can be achieved via fast heating current variations, this approach is no longer sufficient for requirements in angiography.

慢發射電流響應時間代表了與螺旋形鎢發射極相關的缺點。例如，在血管造影應用中，需要在ms時間範圍内的“開”（100 mA）和“關”（0 mA）狀态之間改變發射電流。此外，現代CT應用已導緻對快速管電流變化的需求。雖然後者可以通過快速加熱電流變化實作，但這種方法已不足以滿足血管造影的要求。

Emitter heating circuits are usually fed by medium-frequency power sources. This proves sufficient given the fact that the time dynamics are mainly governed by the wire’s thermal inertia. The use of thinner filament wires in order to avoid the comparably high heat capacity is not feasible owing to mechanical instability and a shortening of operation time. While quick heating can be ensured by pushing the filament, the cooling process, inevitably determined by heat dissipation factors (mostly radiation), represents the speed delimiting factor. This is why all filament-operated X-ray tubes rely on fast tube current modulation, usually realized either via primary pulsing (i.e. modulation of the tube voltage) or electrode control (i.e. the cathode cup is set to a negative potential relative to the filament potential). In both cases, the filament is continuously heated during operation to the temperature required for maximum tube current.

發射極加熱電路通常由中頻電源供電。鑒于時間動力學主要由導線的熱慣性控制，這證明了這一點。由于機械不穩定和操作時間縮短，使用較薄的燈絲以避免相對較高的熱容是不可行的。雖然通過推動燈絲可以確定快速加熱，但冷卻過程不可避免地由散熱系數（主要是輻射）決定，代表了速度定界系數。這就是為什麼所有燈絲操作的X射線管都依賴于快速管電流調制，通常通過**初級脈沖（即管電壓調制）或電極控制（即陰極杯設定為相對于燈絲電位的負電位）**實作。在這兩種情況下，燈絲在運作期間持續加熱至最大管電流所需的溫度。

However, given clear filament service life constraints, it would not be feasible to maintain a permanent maximum heating current. Fig. 12.9 offers an example of the relationship between filament service life and applied filament current. The limited filament service life can be explained in terms of wire evaporation, which – like the electron emission rate – depends exponentially on wire temperature.然而，鑒于燈絲使用壽命的明确限制，維持永久最大加熱電流是不可行的。圖12.9提供了燈絲使用壽命和應用燈絲電流之間關系的示例。有限的燈絲使用壽命可以用金屬絲蒸發來解釋，金屬絲蒸發與電子發射率一樣，與金屬絲溫度呈指數關系。

Wire temperature surges cause tungsten to evaporate at an increased rate. The tungsten wire begins to thin at the highest temperature level. This thinning leads to increased electrical resistance at the same level, which, in turn, leads to a further increase in temperature – a self-accelerating process that quickly leads to the destruction of the filament. In order to ensure a sufficiently long filament service life, it is necessary to limit the application of high heating currents to short periods (e.g. whenever, as in the case of acquisition mode, high emission current is needed) and to reduce the heating current as soon as possible. This task requires active control: preventing the filament from exceeding the emission threshold when in standby mode and refraining from pushing to the desired operating temperature until immediately before the start of image acquisition.

金屬絲溫度波動導緻鎢以更快的速度蒸發。鎢絲在最高溫度下開始變薄。這種變薄會導緻相同水準的電阻增加，進而導緻溫度進一步升高——這是一個自加速過程，會迅速導緻燈絲損壞。為了確定足夠長的燈絲使用壽命，有必要将高加熱電流的應用限制在短時間内（例如，在采集模式下，每當需要高發射電流時），并盡快降低加熱電流。此任務需要主動控制：防止燈絲在待機模式下超過發射門檻值，并在圖像采集開始前避免推至所需的工作溫度。

12.1.2 Specifications for focal spot size and electron beam shape 焦點尺寸和電子束形狀規範

When the accelerated electrons hit the anode surface, their energy is mainly transformed into heat. The amount and spatial density of power deposited onto the anode surface limits the anode’s service life. Excessively high heat densities can exert a degree of thermomechanical stress that destroys (or even melts) the anode’s structure. However, a small focus size with high power density is required for sharp images. To overcome these conflicting demands, Götze introduced the line focus already in 1918 [12.8]. This concept is still in use in almost every medical X-ray tube manufactured today. Fig. 12.10 offers a sketch of the principles behind the line focus.

當加速電子撞擊陽極表面時，它們的能量主要轉化為熱量。沉積在陽極表面的功率量和空間密度限制了陽極的使用壽命。過高的熱密度會産生一定程度的熱機械應力，破壞（甚至熔化）陽極的結構。然而，對于清晰的圖像，需要具有高功率密度的小焦距。為了克服這些互相沖突的需求，格茨在1918年就引入了線聚焦[12.8]。這一概念至今仍在幾乎所有制造的醫用X射線管中使用。圖12.10提供了線焦點背後的原理草圖。

A line focus enables the visible focal spot size to vary across the image and thus to influence image sharpness (fig. 12.11). A tube’s effective focal spot size [12.9] is usually measured using pinhole or slit cameras perpendicular to the tube housing assembly’s axis at the height of the focal spot.

線焦點使可見焦點的大小在整個圖像上發生變化，進而影響圖像的清晰度（圖12.11）。管的有效焦點尺寸[12.9]通常使用針孔或狹縫相機測量，該相機垂直于管殼元件的軸，位于焦點高度處。

光學長度l和寬度w的線聚焦背後的原理。當選定的陽極角度α小于45°時，探測器可見的光學焦點長度l對應于陽極表面上電聚焦的更大長度l。這種幾何效應用于進一步提高光學X射線功率密度，使其超過給定的技術限制，進而達到最大陽極功率密度。

In addition to focal spot size, image quality is also influenced by the distribution of X-ray intensity across the entire focal spot and by the extra focal radiation that arises as electrons that have been scattered back from the anode surface in the direction of the cathode then fall back again onto the anode far from the desired focal spot area.

除了焦斑尺寸外，圖像品質還受到整個焦斑上X射線強度分布的影響，以及由于電子從陽極表面沿陰極方向散射回來，然後再次落到遠離所需焦斑區域的陽極上而産生的額外焦輻射的影響。

The relationship between the spatial X-ray intensity profile and achievable image quality is given by the modulation transfer function (MTF), as described in chapter 9.

空間X射線強度分布和可實作圖像品質之間的關系由 調制傳遞函數（MTF） 給出，如第9章所述。

觀察者從不同方向觀察旋轉陽極X射線管的同一焦斑時可見的焦斑幾何形狀。

除了焦點的絕對尺寸外，其強度分布的形狀對可實作的圖像品質也有重要影響。在相同的焦斑寬度下，高斯光強分布焦斑的調制傳遞函數優于矩形光強分布焦斑。

Fig. 12.12 offers an example of a typical intensity profile and its corresponding MTF. Given the same focal spot width, the modulation transfer function of a focal spot exhibiting a Gaussian intensity profile is superior to a focal spot exhibiting a rectangular intensity profile because the MTF of the Gaussian intensity profile offers a certain advantage at higher spatial frequencies. Moreover, the additional maxima at higher spatial frequencies may cause additional image quality problems due to aliasing. Fig. 12.13 shows the relationship between the focal spot width and the MTF.

圖12.12提供了典型強度分布及其相應MTF的示例。給定相同的焦斑寬度，顯示高斯強度分布的焦斑的調制傳遞函數優于顯示矩形強度分布的焦斑，因為高斯強度分布的MTF在較高的空間頻率下提供一定的優勢。此外，較高空間頻率下的附加最大值可能由于混疊而導緻附加圖像品質問題。圖12.13顯示了焦斑寬度和MTF之間的關系。

Focal spot size and focal spot intensity distribution are crucial factors when it comes to image quality and anode service life. A given emitter design enables one to influence both spot size and intensity distribution via the electric and magnetic fields that form the electron beam.

焦點尺寸和焦點強度分布是影響圖像品質和陽極使用壽命的關鍵因素。給定的發射極設計可以通過形成電子束的電場和磁場影響光斑大小和強度分布。

為不同寬度的焦點計算調制傳遞函數。盡管焦斑寬度的差異很小，但對MTF的影響是顯而易見的。

Forming the electron beam 形成電子束

The movement of an electron in an electric and magnetic field is described by the Lorentz equation:電子在電場和磁場中的運動由洛倫茲方程描述：

The Poisson equation describes the relationship between the gradient of an electrical potential φ and a charge density:

泊松方程描述了電勢梯度φ和電荷密度之間的關系：

Computer algorithms that have finite element methods and that apply these equations have been developed to iteratively calculate the electron paths of simplified cathode geometries. Unfortunately, a precise understanding of all of the boundary conditions has not yet been established, (e.g. the exact temperature at each point on the emitter surface, or the exact charge of insulating parts in the vicinity of the cathode). This and the required geometrical simplifications reduce the reliability of simulation results. Therefore, any computer calculation will require validation by experimental results and subsequent computer model adaptations until the results generated by both approaches converge (at least for a fixed set of parameters in a realistic scenario). Fig. 12.14 shows a computer simulation of a bifocal cathode with intertwining focal spots.

采用有限元方法并應用這些方程的計算機算法已被開發出來，用于疊代計算簡化陰極幾何結構的電子路徑。不幸的是，對所有邊界條件的精确了解尚未建立（例如，發射極表面上每個點的精确溫度，或陰極附近絕緣部件的精确電荷）。這和所需的幾何簡化降低了模拟結果的可靠性。是以，任何計算機計算都需要通過實驗結果和随後的計算機模型調整進行驗證，直到兩種方法産生的結果收斂為止（至少在現實場景中對于一組固定的參數）。圖12.14顯示了具有交織焦點的雙焦點陰極的計算機模拟。

雙焦點陰極元件電場中電子軌迹的計算機模拟。上部電子束被優化為最小焦點寬度。較低的電子束以更大的焦斑寬度為代價使管功率最大化。

Fig. 12.15a-d shows the equipotential lines and electron trajectories for different cathode configurations. While the figures shown are based on the geometry of a helically wound tungsten filament, the following explanations are also valid for other types of emitters (e.g. flat emitters made of tungsten sheet).

圖12.15a-d顯示了不同陰極配置的等電位線和電子軌迹。雖然所示圖基于螺旋纏繞鎢絲的幾何形狀，但以下解釋也适用于其他類型的發射器（例如，鎢片制成的扁平發射器）。

Given that the electrons emerge from the emitter at different angles with thermal velocity distribution, a very wide electron beam would reach the anode (fig. 12.15a). Small focal spots are achieved by using an additional electrode that has the same potential as the emitter (Wehnelt electrode) and that surrounds the emitter. The Wehnelt electrode changes the equipotential lines and presses the electron trajectories together (fig. 12.15b). This leads to a crossover of the outer trajectories, and thereby focuses the electron beam. Electrons from the back of the emitter are avoided by placing the emitter into a narrow slot. The shape of the whole assembly resembles that of a cup. Focusing is influenced by the position of the emitter in relation to the inner surface of the focal cup, the width of the slot and the elevation of the cup. The focusing parameters are selected in a way that ensures that the crossover is near the surface of the anode.

假設電子從發射極以不同的角度以熱速度分布出現，一個非常寬的電子束将到達陽極（圖12.15a）。小焦點是通過使用一個附加電極來實作的，該電極具有與發射器相同的電勢（Wehnelt電極）并包圍發射器。韋内爾電極改變等電位線，并将電子軌迹壓在一起（圖12.15b）。這導緻外部軌迹交叉，進而聚焦電子束。通過将發射器放入窄縫中，可以避免來自發射器背面的電子。整個元件的形狀類似于杯子。聚焦受發射器相對于焦杯内表面的位置、狹縫寬度和焦杯高度的影響。聚焦參數的選擇應確定交叉點5靠近陽極表面。

a) Cylindrical cathode filament opposite a plane anode (basic configuration).

a） 與平面陽極相對的圓柱形陰極燈絲（基本配置）。

b) Basic configuration as in fig. 12.15a with an additional Wehnelt electrode focusing the electron beam (Wehnelt configuration). Wehnelt electrode at the same potential as the cathode filament.

b） 基本配置如圖12.15a所示,帶有聚焦電子束的附加Wehnelt電極（Wehnelt配置）。在與陰極燈絲相同的電位下使用Wehnelt電極。

One distinct disadvantage associated with the Wehnelt electrode is the reduction in the anode potential (fig. 12.15b). The equipotential lines move away from the emitter as the depth of the focal cup increases and as its width decreases.

與Wehnelt電極相關的一個明顯缺點是陽極電位降低（圖12.15b）。等電位線随着焦杯深度的增加和焦杯寬度的減小而遠離發射器。

Additional focusing capacity can be achieved by applying a grid potential that is slightly more negative than the emitter’s potential (fig. 12.15c). This effect can be used to create very small focal spots, which are necessary in some angiographic applications.

額外的聚焦能力可以通過施加一個比發射器的電勢略負的栅極電勢來實作（圖12.15c）。這種效果可用于建立非常小的焦點，這在某些血管造影應用中是必要的。

c) Wehnelt configuration as in Fig. 12.15b. This time the Wehnelt electrode is charged negatively compared to the cathode potential. This leads to a stronger focusing of the electron beam compared that shown in Fig. 12.15b.

c） Wehnelt配置如圖12.15b所示。這一次，與陰極電位相比，韋内爾電極帶負電荷。與圖12.15b所示相比，這導緻電子束的聚焦更強.

If the difference in potential between emitter and focal cup is increased (fig. 12.15d), the electric field near the emitter changes its sign and the tube current comes to a stop. This effect can be used to switch the tube current on and off quickly (i.e. by quickly changing the gating voltage) without having to heat or cool the filament. This technology was widely used before recent developments in high voltage generation enabled primary pulsing, i.e. the direct modulation of tube voltage.

如果發射器和焦杯之間的電位差增大（圖12.15d），發射器附近的電場改變其符号，管電流停止。這種效應可用于快速開關管電流（即通過快速改變選通電壓），而無需加熱或冷卻燈絲。這項技術在最近的高壓發電一次脈沖技術發展之前得到了廣泛應用。E管電壓的直接調制.

d) Wehnelt configuration as in fig. 12.15c. If the negative potential applied to the Wehnelt electrode reaches a critical level, the electron beam loses its focus and is entirely blocked. This effect can be used to switch an X-ray tube almost instantaneously.

d） Wehnelt配置如圖12.15c所示。如果施加在韋内爾電極上的負電位達到臨界水準，電子束就會失去焦點，完全被阻擋。這種效應幾乎可以在瞬間切換X射線管。

The use of an additional focusing lens is necessary in some applications to achieve the desired focal spot geometry. When a focal cup is positioned near an insulating ceramic part, it may also be necessary to use an additional potential sheet to avoid interference from ceramic charging effects. If the anode is positioned at a large distance to the cathode, an additional focal lens of anode or ground potential is placed between anode and cathode to ensure that a sufficiently strong electrical field reaches the surface of the emitter.

在某些應用中，需要使用額外的聚焦透鏡，以獲得所需的焦點幾何形狀。當焦點杯位于絕緣陶瓷部件附近時，可能還需要使用額外的電位片，以避免陶瓷充電效應的幹擾。如果陽極與陰極的距離較大，則在陽極和陰極之間放置陽極或地電位的附加聚焦透鏡，以確定足夠強的電場到達發射器表面。

In such a configuration, the electrons move in a potential free area after passing the focal lens. An expansion of the electron beam vertical to the trajectory is observed due to electrostatic repulsion. This effect increases as the tube current increases and the tube voltage decreases. Before passing the focal lens near the cathode, the same electrostatic rejection that is exhibited by the electrons influences the focusing effect of the focal lens and moves the position of the focal crossover point. This effect also has an impact on the dimensions of the focal spot at the anode. The selection of an appropriate focusing configuration can lead to a balancing out of both effects for a wide range of tube currents. Fig. 12.16 shows the variation of focal spot width in relation to tube current and tube voltage for a typical CT tube. A noticeable increase in the focal spot width is observed only at high tube currents and small tube voltages (80 kV).

在這種配置中，電子通過聚焦透鏡後在無電勢區域中移動。由于靜電斥力，觀察到電子束垂直于軌道的擴充。這種效應随着管電流的增加和管電壓的降低而增加。在通過靠近陰極的聚焦透鏡之前，電子所表現出的相同靜電抑制影響聚焦透鏡的聚焦效果并移動聚焦交叉點的位置。這種效應也會影響陽極上焦點的尺寸。選擇合适的聚焦配置可以在廣泛的管電流範圍内平衡這兩種效應。圖12.16顯示了典型CT管的焦點寬度相對于管電流和管電壓的變化。隻有在高管電流和小管電壓（80 kV）下，才能觀察到焦點寬度的顯著增加。

Focal spot width plotted against tube current I A I_A IA​ for different tube voltages. The focal spot’s minimum width increases along with the tube current I A I_A IA​.

針對不同管電壓的管電流 I A I_A IA​繪制的焦點寬度。焦點的最小寬度随着管電流 I A I_A IA​的增加而增加。

Versatile electron beam deflection 多功能電子束偏轉

It became necessary to increase the distance between anode and cathode in certain X-ray tubes in order to enable the introduction of additional components designed to actively control the focal spot on the target. Controlled deflection of the electron beam can be achieved by deploying a horseshoe electromagnet or a capacitor plate between the tube’s anode and cathode.

在某些X射線管中，有必要增加陽極和陰極之間的距離，以便能夠引入設計用于主動控制目标焦點的附加元件。電子束的可控偏轉可以通過在電子管的陽極和陰極之間布置一個馬蹄形電磁鐵或一個電容闆來實作。

Such flying focal spot tubes enable one to quickly switch the electron beam between two different points on the same anode radius in circumferential direction. The CTsystem detector thus gathers information from two different focal spots at nearly the same time, an increase in information that enables improved image quality. Fig. 12.17 shows the technical assembly of a high-end CT tube with magnetic flying focal spot technology.

這種飛行的焦點管使人們能夠在圓周方向上相同陽極半徑上的兩個不同點之間快速切換電子束。是以，CTsystem探測器幾乎同時從兩個不同的焦點收集資訊，資訊量的增加可以提高圖像品質。圖12.17 顯示了采用磁飛焦點技術的高端CT管的技術組裝。

The electromagnetic forming and deflection of the electron beam in the latest generation of X-ray tubes is a prerequisite for the whole principle of tube operation: the rotating envelope X-ray tube has a cylindrical design that is invariant to rotation and built around a single emitter in the rotation center (fig. 12.18). The anode is an integral part of the tube envelope and is directly exposed to the surrounding cooling oil [12.10, 12.11]. The whole tube rotates around its symmetry axis, while the electron beam is continuously deflected to a fixed spot in space on the anode plate by means of an external magnetic deflection unit that is positioned around the constriction in the tube envelope (fig. 12.19) [12.12, 12.13].

最新一代X射線管中電子束的電磁形成和偏轉是整個射線管工作原理的先決條件：旋轉包絡X射線管具有圓柱形設計，旋轉不變，并圍繞旋轉中心的單個發射器建構（圖12.18）。陽極是管道外殼的一個組成部分，直接暴露在周圍的冷卻油中[12.10,12.11]。整個電子管圍繞其對稱軸旋轉，同時電子束通過外部磁偏轉裝置連續偏轉到陽極闆上的一個固定點，該裝置位于電子管外殼的收縮部分周圍（圖12.19）[12.12，12.13]。

現代旋轉陽極X射線管配置的橫截面（西門子阿克倫）。用于控制電子束偏轉的線圈內建在陰極和陽極之間。一個飛行的焦點可以通過交變磁場産生。該技術用于進一步提高圖像品質。

Two R-coils in the magnetic deflection unit are responsible for radial deflection. This enables adjustments in the focal spot path. Two additional Phi-coils (mounted vertically to the R-coils) enable deflection in the phi-direction. This is required in order to position the focal spot relative to the X-ray window of the housing and to create a flying focal spot [12.14]. Finally, the four Q-coils are quadruple coils used to influence the focal spot geometry. Their field is used to change the originally round focal spot geometry to an oval shape. This changes the important ratio of focal spot width to length. An additional electrical increase in focal spot length can be achieved by fast-wobbling deflection in the R-direction [12.15]. In this case, the wobble frequency needs to be sufficiently faster than the image sampling frequency in order to avoid image distortion.

磁偏轉裝置中的兩個R線圈負責徑向偏轉。這樣可以調整焦點路徑。另外兩個Phi線圈（垂直安裝在R線圈上）可實作Phi方向的偏轉。這是為了相對于外殼的X射線視窗定位焦點和建立飛行焦點[12.14]所必需的。最後，四個Q線圈是用于影響焦斑幾何形狀的四個線圈。它們的場用于将原來的圓形焦點幾何體更改為橢圓形。這會改變焦點寬度與長度的重要比率。通過R方向上的快速擺動偏轉，可以實作焦距的額外電增加[12.15]。在這種情況下，抖動頻率需要足夠快于圖像采樣頻率，以避免圖像失真。

旋轉包絡X射線管（西門子Straton 120）。

a） 原理：左邊陰極産生的電子通過一個被磁偏轉系統（見圖12.19）包圍的窄通道加速到右邊的陽極闆上。

b） 技術實作：信封已被切開進行示範。

The entire magnetic deflection system is driven by a microprocessor-controlled electronic deflection unit.

整個磁偏轉系統由微處理器控制的電子偏轉裝置驅動。

由Straton 120旋轉包絡X射線管部署的磁偏轉系統。電子束通過R-、Phi-和Q-線圈成形和偏轉。 R線圈負責徑向偏轉,另外兩個Phi線圈（垂直安裝在R線圈上）可實作Phi方向的偏轉.

12.1.3 The anode assembly 陽極元件

As has been mentioned in various contexts, more than 99% of the power applied to an X-ray tube is converted into heat (3.1). Efficient heat dissipation represents one of the greatest challenges faced in the development of high power X-ray tubes. Given its importance with respect to the functioning and service life of an X-ray tube as a whole, the anode is usually the prime subject of tube design [12.16, 12.17]. Medical X-ray tubes can be divided into three categories:

正如在各種情況下所提到的，X射線管99%以上的功率被轉換成熱量（3.1）。高效散熱是高功率X射線管發展中面臨的最大挑戰之一。考慮到陽極對整個X射線管的功能和使用壽命的重要性，陽極通常是射線管設計的首要主題[12.16,12.17]。醫用X射線管可分為三類：

• Tubes with stationary anodes 固定陽極管
• Tubes with rotating anodes 旋轉陽極管
• Rotating envelope tubes 旋轉包絡管

In the remaining sections of this chapter, we discuss these three tube types with respect to their specific anode designs, offer an overview of various bearing systems applied in rotating anode tubes, describe motor configurations for the driving of rotating anodes and present various schemes and methods for calculating design-specific anode load capacities.

在本章剩餘部分中，我們将讨論這三種管類型的具體陽極設計，概述旋轉陽極管中應用的各種軸承系統，描述用于驅動旋轉陽極的電機配置，并給出計算設計特定陽極負載能力的各種方案和方法。

Tubes with stationary anodes 固定陽極管

Fig. 12.20 offers an illustration of a stationary anode. A tungsten rhenium target is embedded in a copper block. The electrons are focused onto a focal spot on the target surface. The deposited thermal energy is carried away from the heated focal spot via thermal conduction through the copper block towards its stem and hence to the outside of the tube. The copper block is connected (vacuum-tight) to the tube envelope.

圖12.20 提供了固定陽極的圖示。鎢铼靶嵌入銅塊中。電子聚焦在目标表面的焦點上。沉積的熱能通過熱傳導從加熱焦點帶走，通過銅塊朝向其杆，進而到達管的外部。銅塊連接配接（真空密封）到管外殼。

固定陽極（a：鎢铼靶；b：焦點；c：銅塊；d：銅塊插座。在某些應用中，在插座中鑽有冷卻劑循環通道；e：管外殼和銅塊之間的真空緊密連接配接）。

One way of increasing the admissible tube power (although hardly ever used for diagnostic X-ray tubes) is to improve anode stem cooling by creating additional convection between the copper stem and the surrounding cooling medium by means of cooling boreholes in the anode block [12.18]. Water or oil is then pumped through these boreholes to improve cooling.

增加容許管功率的一種方法（盡管幾乎從未用于診斷X射線管）是通過在銅杆和周圍冷卻媒體之間通過冷卻陽極塊中的鑽孔創造額外的對流來改善陽極杆冷卻[12.18]。然後通過這些鑽孔泵送水或油，以改善冷卻效果。

The anode is often outfitted with an electron capture hood with a beryllium window (fig. 12.21). While X-rays can pass the beryllium window without sustaining a substantial decrease in intensity, electrons reflected at the anode surface are prevented from reaching the glass envelope and are thus prevented from reducing the disruptive strength of the tube envelope. Due to the cup-shaped geometry of the capture hood, a field-free space is created in close proximity to the target surface. This design is thus also effective at stopping reflected electrons from falling back to the target surface and thereby helping to reduce undesired extra-focal radiation.

陽極通常配有帶铍窗的電子俘獲罩（圖12.21）。雖然X射線可以通過铍視窗而不會持續強度的大幅降低**，但陽極表面反射的電子無法到達玻璃外殼**，是以無法降低管外殼的破壞強度。由于捕獲罩的杯形幾何結構，在目标表面附近建立了一個無場空間。是以，這種設計也能有效阻止反射電子落回目标表面，進而有助于減少不需要的額外聚焦輻射。

帶固定陽極的管（1：電子捕獲罩；2：铍窗；3：銅冷卻塊）

Due to their simple and robust design, stationary anode X-ray tubes distinguish themselves in terms of their high reliability and long service lives. However, their low power makes them unsuitable for many applications. In radiographic applications, the upper power limit is mainly determined by the maximum admissible temperature at the boundary between the tungsten target and the copper block. Temperature surges that exceed the melting point of copper will destroy the anode.

由于其簡單而堅固的設計，固定陽極X射線管以其高可靠性和長使用壽命而脫穎而出。然而，它們的低功耗使得它們不适合于許多應用。在射線照相應用中，功率上限主要由鎢靶和銅塊之間邊界處的最高容許溫度決定。超過銅熔點的溫度波動會破壞陽極。

Tubes with rotating anodes 旋轉陽極管

The first tubes with rotating anodes were built in 1933 [12.19]. Compared to stationary anodes, they offer the advantage of enabling one to distribute the thermal energy that is deposited onto the focal spot across the larger surface of a focal ring. This permits an increase in power for short operation times (fig. 12.22). However, as the anode is now rotating in a vacuum, the transfer of thermal energy to the outside of the tube envelope depends largely on radiation, which is not as effective as the liquid cooling used in stationary anodes. Rotating anodes are thus designed for high heat storage capacity and for good radiation exchange between anode and tube envelope. Another difficulty associated with rotating anodes is the operation of a bearing system under vacuum and the protection of this system against the destructive force of the anode’s high temperatures. Appropriate designs for minimized heat flux into the bearing system are required.

第一批帶有旋轉陽極的管子建于1933年[12.19]。與固定陽極相比，它們的優點是能夠将沉積在焦環較大表面上的熱能分布到焦斑上。這允許在短時間内增加功率（圖12.22）。然而，由于陽極現在在真空中旋轉，熱能傳遞到管殼外部在很大程度上取決于輻射，輻射不如固定陽極中使用的液體冷卻有效。是以，旋轉陽極設計用于高蓄熱能力以及陽極和管外殼之間的良好輻射交換。與旋轉陽極相關的另一個困難是軸承系統在真空下的操作，以及保護該系統免受陽極高溫破壞力的影響。需要進行适當的設計，使進入軸承系統的熱通量最小化。

In the early days of rotating anode X-ray tubes, the heat storage capacity of the anode was the main hindrance to high tube performance [12.20-12.22]. This has changed with the introduction of the following new technologies:

在旋轉陽極X射線管的早期，陽極的蓄熱能力是管高性能的主要障礙[12.20-12.22]。随着以下新技術的引入，情況發生了變化：

• Graphite blocks brazed to the anode dramatically increase heat storage capacity and heat dissipation.釺焊在陽極上的石墨塊極大地提高了儲熱能力和散熱能力。
• Liquid anode bearing systems (sliding bearings) provide heat conductivity to the surrounding cooling oil. 液體陽極軸承系統（滑動軸承）為周圍的冷卻油提供導熱性。
• Rotating envelope tubes allow direct liquid cooling for the backside of the rotating anode. 旋轉包殼管允許對旋轉陽極的背面進行直接液體冷卻。

Materials for rotating anodes 旋轉陽極材料

Tungsten has been deployed as a target material in nearly all X-ray tube anodes designed for medical applications since 1909 (6.1.4). The anode plates of rotating anode tubes usually include a 1 to 2 mm thin layer of tungsten-rhenium alloy deposited onto a plate of molybdenum [12.23]. The rhenium increases the ductility of the tungsten, reduces thermomechanical stress and increases anode service life thanks to a slower roughening of the anode surface (always accompanied by a dose reduction) [12.24]. The ideal commercial and technological alloy has been determined to be composed of 5 to 10% rhenium and 90 to 95% tungsten (fig. 12.23).

自1909年（6.1.4）以來，幾乎所有用于醫療應用的X射線管陽極都使用鎢作為靶材料。旋轉陽極管的陽極闆通常包括沉積在钼闆上的一層1至2 mm厚的鎢铼合金[12.23]。铼增加了鎢的延展性，降低了熱機械應力，并提高了陽極的使用壽命，這得益于陽極表面的緩慢粗糙化（總是伴随着劑量降低）[12.24]。理想的商業和技術合金已确定由5%至10%的铼和90%至95%的鎢組成（圖12.23）。

X射線管陽極劑量效率與管負載數量的關系圖。圖中顯示了鎢層中铼含量在0%至15%之間的不同陽極表面成分。随着铼含量的增加，耐磨損性增加。

As mentioned, the introduction of graphite blocks brazed to the backside of the molybdenum plate represents an advance in rotating anode technology. The graphite block in this design significantly increases the heat storage capacity of the anode, while requiring only a slight increase in overall anode weight. Furthermore, heat dissipation is accelerated by the larger anode surface and the superior emission coefficient of graphite compared to molybdenum [12.25]. Fig. 12.24a-c shows various anode designs offering different heat storage capacities.

如上所述，在钼闆背面釺焊石墨塊的引入代表了旋轉陽極技術的進步。這種設計中的石墨塊顯著增加了陽極的蓄熱能力，同時隻需要稍微增加陽極的總重量。此外，與钼相比，更大的陽極表面和更高的石墨發射系數加速了散熱[12.25]。圖12.24a-c顯示了提供不同蓄熱能力的各種陽極設計。

各種陽極類型。

a） ，b）帶有石墨環的陽極，石墨環釺焊在陽極背面。

c） 帶冷卻迷宮的陽極（瓦裡安專用）

molybdenum:钼

graphite:石墨

Molybdenum and graphite are brazed together with zirconium or, for higher operating temperatures, with titanium or other specially designed brazing alloys.

钼和石墨與锆釺焊在一起，或者在更高的工作溫度下，與钛或其他特殊設計的釺焊合金釺焊在一起。

Ball bearing systems 滾珠軸承系統

Ball bearing systems deployed in rotating anode X-ray tubes must be designed for operation under extreme conditions:

部署在旋轉陽極X射線管中的滾珠軸承系統必須設計為在極端條件下運作：

• They form a connection between the very hot anode plate and the cold environment. 它們在極熱的陽極闆和寒冷的環境之間形成連接配接。
• They need to operate under vacuum. 它們需要在真空下工作。
• They need to withstand high turning speeds of 100 to 150 Hz and substantial weight loads (e.g. a 10 kg anode plate in a typical CT machine equals a 100 kg effective load due to the centrifugal acceleration). 它們需要承受100至150赫茲的高轉速和大量的重量負載（例如，由于離心加速度，典型CT機器中的10千克陽極闆相當于100千克的有效負載）。

The lubricants need to fulfill certain specifications. For instance, they need to be constructed of soft deformable materials, resist alloy formation with the carrier material, remain stable at high temperatures and exhibit low vapor pressure under vacuum. These criteria limit the selection of lubricants to thin layers of lead or silver.

潤滑劑需要滿足某些規格。例如，它們需要由軟變形材料制成，抗與載體材料形成合金，在高溫下保持穩定，在真空下表現出低蒸汽壓。這些标準将潤滑劑的選擇限制在鉛或銀的薄層上。

A sufficient reduction in the transfer of heat into the bearing system can be achieved by increasing the distance between the anode plate and the bearing and including materials of low heat conductivity. The bearing temperature must be kept below 400°C to achieve good results.

通過增加陽極闆和軸承之間的距離，包括低導熱性的材料，可以充分減少進入軸承系統的熱量傳遞。軸承溫度必須保持在400°C以下，以獲得良好的結果。

In order to compensate for the wide operational temperature range and the high temperature gradients between the rotating and stationary parts of the bearing system, the bearings are designed with high degrees of axial and radial clearance, with special temperature compensation mechanisms and with a spring for preloading the system. Furthermore, only very small tolerances are accepted in the case of the individual bearing parts.

為了補償軸承系統旋轉部件和固定部件之間的寬工作溫度範圍和高溫度梯度，軸承設計有高度的軸向和徑向間隙，帶有特殊的溫度補償機構和彈簧，用于預加載系統。此外，對于單個軸承零件，僅接受非常小的公差。

Liquid (sliding) bearings 液體（滑動）軸承

The recent introduction of liquid metal bearings represents an innovation in rotating anode X-ray technology. These bearings use the aquaplaning effect of liquid metals and offer the following advantages:

最近引進的液态金屬軸承代表了旋轉陽極X射線技術的一項創新。這些軸承利用液态金屬的滑水效應，具有以下優點：

• The bearing system is free of wear and tear. 軸承系統無磨損。
• The generation of running noise is minimal. 運作噪音的産生最小。
• Additional anode cooling can be achieved by fast heat flux through the liquid metal in the bearing system. 通過軸承系統中液态金屬的快速熱流，可以實作額外的陽極冷卻。

The diagram in fig. 12.25 illustrates the principle. Two bodies are in motion relative to one another. The liquid between the bodies accumulates and forms a bow wave. This effect leads to a pressure buildup in the wedge between the two bodies. If the pressure exceeds a certain level, the liquid will enter the gap between the bodies until a film of liquid separates the bodies along their entire contact surface.

如圖12.25 所示。說明了這一原則。兩個物體相對運動。兩個物體之間的液體聚集形成一個弓形波。這種效應會導緻兩個閥體之間的楔塊内壓力積聚。如果壓力超過某一水準，液體将進入兩個物體之間的間隙，直到一層液體沿着整個接觸面将兩個物體分開。

配備液體軸承的X射線管的典型元件。由于液體軸承的旋轉部件和固定部件之間有較大的接觸面，是以沉積在陽極中的熱量可以快速傳導到冷卻油中。

Liquid bearing:液體軸承

Radial bearing:徑向軸承

Axial bearing:軸向軸承

Rotating part:旋轉部件

Rotor:轉子

Anode disk:陽極盤

Envelope:封套

Stationary part:固定部件

Fig. 12.26 offers an illustration of a liquid bearing used in X-ray technology. The bearing’s stationary shaft is designed as a hollow body. This permits the active cooling of the bearing and thus also of the anode via the injection of a cooling agent. The thin ~20 μm gap between the stationary shaft and the rotating bearing body is filled with liquid metal. An eutectic of gallium, indium and tin with a melting temperature of between –10°C and –11°C is often used. In addition to the main radial bearing (i.e. formed by the stationary shaft and the rotating bearing body) an axial bearing is formed between the disk (with a larger diameter in the middle of the shaft) and the corresponding hollow ring in the bearing body (fig. 12.26). This construction enables one to achieve both radial and axial guidance and stabilization.

圖12.26提供了X射線技術中使用的液體軸承的圖示。軸承的固定軸設計為空心體。這允許通過注入冷卻劑對軸承和陽極進行主動冷卻。固定軸和旋轉軸承體之間約20μm的薄間隙充滿液态金屬。通常使用熔點在-10°C至-11°C之間的镓、铟和錫共晶。除了主徑向軸承（即由固定軸和旋轉軸承體形成）外，在軸承（在軸的中部具有較大直徑）和軸承體中的相應空心環之間形成軸向軸承（圖12. 26）。這種結構可以實作徑向和軸向導向和穩定。

The load capacity of a liquid bearing depends essentially on the diameter and length of the design, on the rotation speed, on the viscosity of the liquid and on the size of the gap filled with liquid. It is necessary to ensure that the liquid metal does not escape from the bearing under any operating conditions and that the liquid metal does not form an alloy with other bearing parts. This can be achieved by selecting appropriate materials, using a surface coating and deploying accurate bearing parts. The anodes in commercially available X-ray tubes equipped with liquid bearings are typically run at a rotation speed of 150 Hz. The maximum heat transfer from the anode through the bearing into the cooling system is about 1 to 2 kW.

液體軸承的承載能力主要取決于設計的直徑和長度、轉速、液體粘度和充滿液體的間隙大小。必須確定液态金屬不會在任何操作條件下從軸承中逸出，并且液态金屬不會與其他軸承零件形成合金。這可以通過選擇合适的材料、使用表面塗層和部署精确的軸承零件來實作。配有液體軸承的商用X射線管中的陽極通常以150 Hz的轉速運作。從陽極通過軸承進入冷卻系統的最大熱傳遞約為1至2 kW。

Tubes with rotating envelopes 旋轉包殼管

A rotating envelope tube combines the advantages of stationary and rotating anodes:

旋轉包絡管結合了固定陽極和旋轉陽極的優點：

• The backside of the anode is directly exposed to the cooling agent, as can be inferred from the stationary anode configuration. The anode thus cools down in a matter of seconds (fig. 12.27) and no additional heat storage capacity at the anode (e.g. graphite) is required.陽極的背面直接暴露于冷卻劑中，這可以從固定陽極配置推斷出來。是以，陽極在幾秒鐘内冷卻（圖12.27），并且陽極（例如石墨）不需要額外的蓄熱能力。
• Given that the anode (together with the tube envelope) rotates in the manner of a conventional rotating anode, the thermal energy deposited at the focal spot is distributed across a focal ring (as with conventional rotating anode assemblies). The same performance thus results for brief high power X-ray generation, as is achievable with a (cold) rotating anode of the same diameter. 假設陽極（連同管外殼）以正常旋轉陽極的方式旋轉，則沉積在焦點處的熱能分布在焦環上（與正常旋轉陽極元件一樣）。是以，與相同直徑的（冷）旋轉陽極一樣，短時間高功率X射線産生也具有相同的性能。

A number of mechanisms deployed in the development of rotating envelope tubes are protected by patents. One specific mechanism has recently been deployed in the development of a product currently sold under the brand name Straton. The most challenging task in the operation of this tube is the generation and stabilization of the focal spot relative to the non-rotating environment.

在旋轉包絡管的開發中部署的許多機制受到專利保護。最近，在開發目前以Straton品牌銷售的産品時，部署了一種特定的機制。該管運作中最具挑戰性的任務是相對于非旋轉環境的焦點的産生和穩定。

不同類型X射線管的冷卻曲線。通過部署在旋轉包絡管（Straton 120）中的直接冷卻原理可實作的冷卻速度比傳統X射線管快幾個數量級。

12.1.4 Anode load capacity 陽極負載能力

The impact of the electrons heats the anode. In order to avoid damage related to thermal stress and to prevent the evaporation of material, it is important to have access to information on the temperature of the anode base, the focal ring and the focal spot.

電子的沖擊使陽極發熱。為了避免與熱應力相關的損壞并防止材料蒸發，擷取陽極基座、焦環和焦點的溫度資訊非常重要。

Anode disk temperature 陽極盤溫度

The disk temperature is derived from the equilibrium of the power supplied by the electrons P and dissipated by radiation P R a d P_{Rad} PRad​ and thermal conduction P C o n d P_{Cond} PCond​.

磁盤溫度由電子P提供的能量平衡導出，并由輻射 P R a d P_{Rad} PRad​和熱傳導 P C o n d P_{Cond} PCond​耗散.

c i c_i ci​ m i m_i mi​：單陽極元件的熱容（取決于溫度）

The subscript i in the formula is used to account for the various materials in anodes composed of several components (e.g. metallic disks, graphite rings and other materials).

公式中的下标i用于說明陽極中由多種成分組成的各種材料（例如金屬盤、石墨環和其他材料）。

The anode disk dissipates its heat power largely via thermal radiation, as described by the Stefan-Boltzmann law:

正如Stefan-Boltzmann定律所述，陽極盤主要通過熱輻射耗散其熱功率：

A i （ T ） A_i（T） Ai​（T）：組分i的陽極吸收（取決于溫度） S i S_i Si​：陽極組分i的表面積

不同情況下陽極闆溫度與時間的關系圖：在2℃下運作期間加熱。5千瓦、5千瓦和25千瓦；空載冷卻

In the case of anodes with liquid metal bearings, a noticeable part of the anode heat is also dissipated by the liquid metal via thermal conduction. The efficiency of the dissipation depends on thermal conductivity κ, bearing surface S B S_B SB​ and the temperature difference between the anode and the cooling oil.

對于帶有液态金屬軸承的陽極，液态金屬通過熱傳導也會散發陽極熱量的顯著部分。散熱效率取決于導熱系數κ、支承面 S B S_B SB​以及陽極和冷卻油之間的溫差。

κ：管道的典型導熱系數， S B S_B SB​：軸承表面

Fig. 12.28 shows anode disk heating and cooling curves for different loads P (fig. 12.28).

圖12.28顯示了不同負載P下陽極盤的加熱和冷卻曲線（圖12.28）。

Focal spot and focal ring temperature rise 焦斑和焦環溫升

The temperature of the focal spot is significantly higher than the temperature of the anode disk. Fundamental work has been done on this subject by Bouwers [12.26] and Osterkamp [12.27] for stationary anodes. The rise in temperature ϑ for short load times (< 0.05 s for standard focal spot dimensions) can be approximated by:

焦點的溫度明顯高于陽極盤的溫度。Bouwers[12.26]和Osterkamp[12.27]在固定陽極方面已經完成了這方面的基礎工作。短負載時間内（标準焦點尺寸小于0.05 s）的溫升可近似為:

P：功率輸入， A F A_F AF​=2δl焦斑面積，t：負載周期, λ：熱導率，c：比熱，ρ：品質密度

and the rise for long loading times can be approximated by:

長加載時間的上升可近似為:

δ：焦斑半寬

The maximum permissible focal spot temperature in the case of stationary anodes is limited by the tolerable temperature at the boundary between the anode tungsten and copper and (during very long loading times) by the anode’s thermal capacity and the heat dissipation to the surrounding oil.

在固定陽極的情況下，最大允許焦點溫度受陽極鎢和銅之間邊界的容許溫度以及（在很長的加載時間内）陽極的熱容量和對周圍油的散熱的限制。

While in the case of stationary anodes the time t in eq. (12.11) corresponds to the period during which the load is applied, it is necessary to replace this factor in the case of rotating anodes by an interval Δt in order to describe the duration the focal spot is hit by the electron beam at one revolution:

在固定陽極的情況下，等式（12.11）中的時間t對應于施加負載的時間，在旋轉陽極的情況下，有必要用間隔Δt替換該系數，以描述焦點在一圈内被電子束擊中的持續時間：

δ：焦斑半寬，R：焦軌半徑，f：陽極旋轉頻率

The temperature rise at the focal spot for a rotating anode is thus approximated by:

是以，旋轉陽極焦點處的溫升近似為:

The focal track on the target is formed by the multitude of all surface elements heated by the electron beam. The track is visible on used targets as a highly roughened circle (fig. 12.29). Focal track temperature ϑ T r a c k ϑ_{Track} ϑTrack​ increases as the number of revolutions n increases (fig. 12.30):

靶材上的焦點軌迹是由電子束加熱的大量表面元素形成的。 軌迹在使用過的目标上是一個高度粗糙的圓（圖12.29）。焦點軌迹溫度 ϑ T r a c k ϑ_{Track} ϑTrack​随着轉數n的增加而增加（圖12.30）：

k：陽極厚度、熱輻射、徑向熱擴散的系數n=t f：時間t内的轉數

The focal ring temperature of an anode rotating at 100 Hz or 150 Hz increases during a 1-second load by a factor of 10 or 12, respectively. The reduction in the focal spot temperature that results from the increased rotation frequency is partially compensated for by the fact that the focal ring temperature increases as the frequency increases.

在1秒負載期間，以100Hz或150Hz旋轉的陽極的焦環溫度分别增加10或12倍。由于旋轉頻率的增加而導緻的焦斑溫度的降低被焦環溫度随頻率的增加而增加的事實部分補償。

典型旋轉陽極X射線管的陽極闆幾何形狀

旋轉陽極的焦點、焦點軌迹和目标基座的溫度與X射線産生期間的轉數或經過的時間t成反比

By combining eqs. (12.14 and 12.15) one obtains the anode power necessary to achieve the total focal spot temperature rise:

通過結合等式。（12.14和12.15）獲得實作焦點總溫升所需的陽極功率：

l：焦點長度

Temperature balance and time-related temperature changes 溫度平衡和與時間相關的溫度變化

The absolute temperature in the focal spot T F o c u s T_{Focus} TFocus​ follows as：

焦點 T F o c u s T_{Focus} TFocus​ 中的絕對溫度如下所示:

The temperature rise in the focal spot takes place within milliseconds, the temperature equalization between disk and focal track takes some seconds. Each of the three quoted temperatures (focal spot, focal track and anode disk base temperature) is subject to specific limitations relating to the materials and processes used.

焦斑的溫升在毫秒内發生，磁盤和焦軌之間的溫度均衡需要幾秒鐘。三個引用溫度（焦點、焦點軌迹和陽極盤基座溫度）中的每一個都受到與所用材料和工藝相關的特定限制。

An upper temperature limit is reached when an arc discharge occurs between the anode and cathode or envelope as a consequence of the ionization of the evaporating metal. A second limitation in the maximum focal spot temperature is created by focal track wear and tear caused by thermomechanical stress and resulting in dose reduction. The maximum permissible focal spot temperature is set well below these limitations so as to safeguard the tube and secure an acceptable service life.

由于蒸發金屬的電離作用，陽極和陰極或外殼之間發生電弧放電時，溫度達到上限。最大焦點溫度的第二個限制是由熱機械應力引起的焦點軌迹磨損造成的，并導緻劑量降低。最大允許焦點溫度設定遠低于這些限制，以保護管并確定可接受的使用壽命。

Anode temperature simulation 陽極溫度模拟

Calculating anode temperatures on the basis of the above described formulas is not sufficient to determine the dynamic spatial and temporal temperature distributions in complex tube designs. In such cases, the object can be physically and geometrically modeled via FEM (finite element-method) with the help of a dedicated computer simulation program. Here, the temperature is calculated for each small element, taking into account contributions from all neighboring areas. This enables one to obtain the spatial temperatures and the thermal mechanical stress for the whole object in relation to the applicable input power and time (fig. 12.31). Furthermore, it also yields parameters such as temperature rise, temperature gradients in the focus, temperature in the focal track, temperature in the brazing layer and stress in the target surface.

根據上述公式計算陽極溫度不足以确定複雜管道設計中的動态時空溫度分布。在這種情況下，可借助專用計算機模拟程式，通過FEM（有限元法）對對象進行實體和幾何模組化。這裡，考慮到所有相鄰區域的貢獻，計算每個小元素的溫度。這使得我們能夠獲得整個物體相對于适用輸入功率和時間的空間溫度和熱機械應力（圖12.31）。此外，它還産生諸如溫升、焦點中的溫度梯度、焦點軌迹中的溫度、釺焊層中的溫度和目标表面中的應力等參數。

12.1.5 X-ray tube vacuum envelopes X射線管真空封套

Vacuum enclosure 真空外殼

Correct X-ray tube functioning depends on the capacity of the electrons to avoid collisions with gas particles as they move from the cathode to the anode. This requirement makes the establishment of mean free paths in the order of several meters necessary (corresponding to gas pressures smaller than 10-4 mbar). The key role of all tube envelopes is to make such a vacuum possible and to maintain it for the entire service life of the tube.

正确的X射線管功能取決于電子在從陰極移動到陽極時避免與氣體粒子碰撞的能力。這一要求使得需要建立幾米量級的平均自由路徑（對應于小于10-4 mbar的氣體壓力）。所有管道外殼的關鍵作用是使真空成為可能，并在管道的整個使用壽命内保持真空。

旋轉陽極闆内熱分布的理論計算；使用有限元方法進行計算。

The manufacturing process usually includes the deployment of turbomolecular pumps to evacuate the X-ray tube to an internal pressure of less than 1 0 − 6 10^{-6} 10−6 mbar. The enclosure’s maximum permissible effective leakage rate may not exceed 1 0 − 15 10^{-15} 10−15 mbar l/s [12.28]. Operation at high power levels increases the initial internal pressure owing to the desorption of bound gasses caused by the increased temperature in the tube components. This process, however, is normally reversible. Furthermore, the rise in pressure can be counterbalanced by application of so-called getter materials (e.g. porous zirconium sinter bodies) in the tube [12.29].

制造過程通常包括部署渦輪分子泵，将X射線管排空至小于 1 0 − 6 10^{-6} 10−6mbar的内部壓力。外殼的最大允許有效洩漏率不得超過 1 0 − 15 10^{-15} 10−15毫巴升/秒[12.28]。由于管元件中溫度升高導緻束縛氣體解吸，是以在高功率水準下運作會增加初始内部壓力。然而，這個過程通常是可逆的。此外，壓力上升可以通過在管道中使用所謂的吸氣劑材料（例如多孔锆燒結體）來抵消[12.29]。

In what follows, we discuss the design and the choice of materials.接下來，我們将讨論設計和材料的選擇。

X - ray tube envelopes used to be made exclusively of special glass materials (fig. 12.32a). As the technology developed, these were then replaced by so-called metal center-part tubes (fig. 12.32b) whose configurations were characterized by the use of glass only at the anode and cathode sites and the use of metals such as stainless steel, copper and Cu-Ni alloys for the center part of the tube. In the case of bipolar tubes, the center part is usually grounded in order to divert the high number of reflected electrons that originate from the anode focal spot and strike the tube envelope. Up to 15-20% of the original input power can be carried by these electrons to the tube envelope. The percentage of reflected energy is even higher (up to 30-35%) in the case of an anode-grounded tube design. Modern tube envelopes are made almost entirely of metal (fig. 12.32c). Ceramic inserts of highly purified aluminum oxide are used at the anode and cathode sites (metal ceramic tubes) for purposes of insulation.

X射線管的外殼過去隻由特殊的玻璃材料制成（圖12.32a）。随着技術的發展，這些管被所謂的金屬中心部分管（圖12.32b）所取代，其結構特點是僅在陽極和陰極位置使用玻璃，并且在管的中心部分使用不鏽鋼、銅和銅鎳合金等金屬。在雙極管的情況下，中心部分通常接地，以轉移來自陽極焦點的大量反射電子并撞擊管外殼。高達15-20%的原始輸入功率可由這些電子傳送到管殼。在陽極接地管設計的情況下，反射能量的百分比甚至更高（高達30-35%）。現代的管子外殼幾乎全部由金屬制成（圖12.32c）。在陽極和陰極位置（金屬陶瓷管）使用高純度氧化鋁陶瓷嵌件進行絕緣。

不同類型的管封套。a） 玻璃外殼，b）金屬玻璃外殼，c）帶陶瓷絕緣體的金屬外殼

Mechanical and thermal stability 機械和熱穩定性

In addition to providing a vacuum enclosure, the tube envelope also serves as a backbone for the mechanical fixing of the individual X-ray tube components. Particularly high demands are placed on the mechanical stability of the envelope in order to maintain the stability of the focusing position on the anode. The highest forces acting on the tube enclosure are caused by the centrifugal acceleration of more than 20 g (g= earth acceleration) in modern CT scanners:

除了提供真空外殼外，該管外殼還用作單個X射線管元件機械固定的主幹。為了保持陽極上聚焦位置的穩定性，對外殼的機械穩定性提出了特别高的要求。作用在管外殼上的最大力是由現代CT掃描器中超過20 g（g=地球加速度）的離心加速度引起的：

a：離心加速度，r：機架半徑f：CT機架的旋轉頻率

Typical values for the parameters are r = 0.6 m and f = 1 to 3 Hz.

參數的典型值為r=0.6米，f=1至3赫茲。

The tube cover is also required to withstand the difference between the tube vacuum pressure and the pressure within the tube housing. This pressure difference may be higher than 3 bar in specific designs.

管蓋還需要承受管真空壓力和管殼内壓力之間的差異。在特定設計中，該壓差可能高于3bar。

Metal enclosures also represent a safety feature in that they protect the patient from anode system bursting at high rotational speeds.

金屬外殼也代表了一種安全特性，因為它們可以保護患者免受陽極系統在高轉速下爆裂的影響。

Finally, tube envelopes are also required to exhibit sufficient mechanical resistance to strong local stresses that arise in connection with temperature changes during tube operation. High temperature gradients in the envelope material are especially possibile in the vicinity of the focal spot, where the above-mentioned reflected electrons hit the tube envelope.

最後，還要求管道外殼具有足夠的機械阻力，以抵抗管道運作期間因溫度變化而産生的強局部應力。包絡材料中的高溫梯度尤其可能出現在焦點附近，上述反射電子撞擊管包絡。

Electrical insulation 電絕緣

In addition to proper vacuum enclosure and mechanical stability, the tube envelope is also required to provide electrical insulation for the applied tube voltages relative to one another and to the system ground. Special glass or ceramic feed-through elements are used to connect the cathode and anode to the high voltage power supply. The high voltage potentials at the anode and cathode must remain reliably insulated throughout the tube’s service life. The glass bulbs used in the past proved deficient in this regard on account of the formation of vaporized metal films on their inner surfaces. While production is currently dominated by metal ceramic tube envelopes, the introduction of this technology was also marked by certain drawbacks. For instance, the high specific resistance of the ceramic materials led to unwanted local charging effects and associated electrical breakdowns. This problem was finally resolved by the introduction of special shielding electrodes [12.30].

除了适當的真空外殼和機械穩定性外，還需要管外殼為施加的管電壓互相之間和系統接地提供電氣絕緣。特殊的玻璃或陶瓷饋電元件用于将陰極和陽極連接配接到高壓電源。陽極和陰極的高壓電位必須在管子的整個使用壽命内保持可靠絕緣。過去使用的玻璃燈泡在這方面存在缺陷，因為在其内表面上會形成蒸發的金屬膜。雖然目前生産主要是金屬陶瓷管封套，但這種技術的引入也存在一些缺陷。例如，陶瓷材料的高比電阻導緻不必要的局部充電效應和相關的電氣故障。通過引入特殊的屏蔽電極[12.30]，這個問題最終得以解決。

Heat dissipation 散熱

Nearly all dissipated heat is required to leave the tube during operation via the tube envelope so that it can be absorbed by the cooling medium circulating around the tube envelope in the tube housing. High performance X-ray tubes rely on the capacity of all tube envelope materials to deliver high heat conductivity and a high absorption coefficient for thermal radiation (at the vacuum side).

在運作過程中，幾乎所有的散熱都需要通過管殼離開管，以便被管殼中圍繞管殼循環的冷卻媒體吸收。高性能X射線管依賴于所有管殼材料的能力，以提供高導熱性和高熱輻射吸收系數（在真空側）。

Maximum spectral intensity distribution for tubes designed for radiation cooling lies within a wavelength band of around 2 μm at a temperature of 1,500 K [12.31]. Optimal radiation transfer to the envelope depends on an optimization of the absorption coefficient for the inner surface of the tube enclosure, especially with respect to the regions of the main heat exchange in the vicinity of the anode. This can be achieved by mechanical roughening (A Ý 0.5) or the use of a coating (A Ý 0.9).

設計用于輻射冷卻的管的最大光譜強度分布位于1500 K溫度下約2μm的波長範圍内[12.31]。到外殼的最佳輻射傳遞取決于管外殼内表面吸收系數的優化，尤其是陽極附近的主熱交換區域。這可以通過機械粗化（AÝ0.5）或使用塗層（AÝ0.9）來實作。

Heat dissipation in the case of directly cooled tubes (e.g. rotating envelope tubes) is dominated by heat conduction through the solid envelope material. The thermal conduction coefficients of the applied materials thus play a dominant role.

直接冷卻管（如旋轉外殼管）的散熱主要是通過固體外殼材料進行的熱傳導。是以，應用材料的熱傳導系數起主導作用。

Magnetic properties 磁性

In order to increase image quality in CT applications, an external electromagnetic deflection system consisting of magnetic coils is sometimes used as a means of electronically controlling the focal spot position. In this case, the tube enclosure is required to be non-magnetic, at least in the relevant regions. Furthermore, the specific electrical resistance has to be large enough in order to avoid excessive eddy current losses. While glass offers ideal properties in light of the requirements, metals such as austenitic steel and Cu-Ni alloys can also be used [12.7].

為了提高CT應用中的圖像品質，有時使用由磁線圈組成的外部電磁偏轉系統作為電子控制焦點位置的手段。在這種情況下，要求管外殼至少在相關區域為非磁性。此外，比電阻必須足夠大，以避免渦流損耗過大。 雖然玻璃根據要求提供了理想的性能，但也可以使用奧氏體鋼和銅鎳合金等金屬[12.7]。

X-ray absorption properties X射線吸收特性

Only a small fraction of the X-rays emerging isotropically from the focal spot contributes to imaging. All other X-rays need to be blocked to protect the operator and the patients. For this reason, envelope materials that offer high X-ray absorption rates are desirable, especially for the half space above the anode plate. Assuming that appropriate envelope materials are selected, a good proportion of the unwanted radiation will be absorbed close to its origin. This permits a reduction in the amount of shielding material that is incorporated in the tube housing to meet requirements relating to radiation leakage.

隻有一小部分的X射線從焦點各向同性地射出，有助于成像。所有其他X射線都需要屏蔽，以保護操作員和患者。是以，需要提供高X射線吸收率的外殼材料，尤其是陽極闆上方的半空間。假設選擇了合适的包層材料，在靠近其原點的位置将吸收相當大比例的不必要輻射。這允許減少管殼中包含的屏蔽材料量，以滿足與輻射洩漏相關的要求。

In contrast, the used X-ray beam should ideally remain uninfluenced when passing the tube envelope, unless a degree of modulation is desired (e.g. to harden the radiation). This contradiction is solved by incorporating X-ray windows into the tube envelope:

相比之下，所使用的X射線束在通過管殼時應理想地保持不受影響，除非需要一定程度的調制（例如，使輻射硬化）。通過将X射線視窗合并到管封套中，解決了這一沖突：

• In glass tubes, the thickness of the tube envelope is diminished in the region where the applied X-ray beam leaves the tube. 在玻璃管中，在應用的X射線束離開管的區域内，管外殼的厚度減小。
• In metal tubes, specially designed metal windows consisting of beryllium or titanium (also for radiation hardening) are used. 在金屬管中，使用由铍或钛（也用于輻射硬化）組成的特殊設計的金屬窗。

12.1.6 Casing design 套管設計

The first X-ray tubes developed by Roentgen at the end of the 19th century had no additional casing besides the tube envelope. Today, the housing of an X-ray tube is an integral part of the system. It combines various functions necessary for the safe operation of the X-ray tube:

19世紀末，倫琴發明的第一批X射線管除了管殼外沒有額外的外殼。如今，X射線管的外殼是該系統不可分割的一部分。它結合了X射線管安全運作所需的各種功能：

• X-ray shielding to protect the patient and operator 保護患者和操作員的X射線屏蔽
• High voltage tube connection and insulation 高壓管連接配接和絕緣
• Mechanical interface between tube and system environment 管道與系統環境之間的機械接口

典型旋轉陽極管周圍的外殼和外圍部件

西門子阿克倫管位于其外殼内。X射線視窗在頂部清晰可見。

Potential materials for shielding include heavy metals such as lead, molybdenum and tantalum. Lead is often used because it is cheap and easily available in sufficient quantities. Lead sheets are typically glued to the inner wall of the tube housing. Given that raw lead would have a negative impact on the quality of the cooling and insulation oil, all lead surfaces are usually coated.

潛在的屏蔽材料包括鉛、钼和钽等重金屬。鉛之是以經常被使用，是因為它便宜且容易獲得足夠的數量。鉛闆通常粘在管殼内壁上。考慮到未加工的鉛會對冷卻油和絕緣油的品質産生負面影響，所有鉛表面通常都有塗層。

High-voltage connection and insulation 高壓連接配接和絕緣

The design of the tube housing assembly is required to guarantee a sufficient distance between the high voltage parts and the casing. Edges should be rounded off in order to avoid high electrical field strength. The risk of arcing can be reduced by spatially concentrating parts possessing high electrical potential towards the center of the tube housing and by introducing long insulation (creeping) distances (e.g. along roughened polymer or ceramic insulators). Furthermore, filling the space between the X-ray tube and the tube envelope with oil will provide both electrical insulation and cooling.

管殼元件的設計需要保證高壓部件和外殼之間有足夠的距離。邊緣應磨圓，以避免高電場強度。通過将具有高電勢的零件空間集中在管殼中心，并引入長距離絕緣（爬電）距離（例如沿粗糙的聚合物或陶瓷絕緣子），可以降低電弧風險。此外，用油填充X射線管和管外殼之間的空間将提供電氣絕緣和冷卻。

Most medical X-ray tube systems today are filled with mineral oil for purposes of cooling and insulation. However, air-cooled tube systems are also available. If oil is used for the sake of cooling, it is necessary to make sure that a sufficiently strong stream of oil is directed to the hot spots in the tube envelope.

如今，大多數醫用X射線管系統都充滿了礦物油，用于冷卻和隔熱。但是，也可以使用風冷管系統。如果為了冷卻而使用機油，則必須確定将足夠強的機油流引導至管殼中的熱點。

A large portion of the heat that is generated during the operation of classical rotary anode tubes is stored in the anode plate and then slowly transferred to the tube envelope (and to the surrounding cooling oil) during the necessary cool-down phase following tube operation. The overall cooling system is required to provide continuous cooling of the average tube power (e.g. via the deployment of a heat exchanger connected to the primary oil circuit).

傳統旋轉陽極管運作期間産生的大部分熱量儲存在陽極闆中，然後在管運作後的必要冷卻階段緩慢轉移到管殼（以及周圍的冷卻油）。整個冷卻系統需要提供平均管功率的連續冷卻（例如，通過部署連接配接到主油路的熱交換器）。

In the case of rotating envelope tubes, the generated heat is transferred almost instantaneously from the anode to the surrounding cooling oil during operation. This design relies on turbulent and significantly faster cooling oil streams and dramatically increased oil volumes per time period to fully support tube performance. A high performance cooling system with sufficiently high peak power is required. The introduction of an oil reservoir in the oil circuit represents an alternative. Here, the reservoir acts as a heat reservoir (~10 l oil have the same heat storage capacity as 1,000 cm3 of the typical anode materials used in conventional rotating anode tubes). The use of latency heat reservoirs (as commonly used in automobiles) would also represent an option.

在旋轉外殼管的情況下，産生的熱量在運作期間幾乎瞬間從陽極轉移到周圍的冷卻油。這種設計依賴于湍流和顯著更快的冷卻油流，以及每一時間段顯著增加的油量，以充分支援管道性能。需要具有足夠高峰值功率的高性能冷卻系統。在油路中引入儲油器是一種替代方案。在這裡，蓄熱器充當蓄熱器（~10升油的蓄熱能力與傳統旋轉陽極管中使用的1000 cm3典型陽極材料相同）。使用蓄熱器（通常用于汽車）也是一種選擇。

The only rotating envelope tube system that is commercially available today (Siemens Straton) uses an oil reservoir for the interim storage of thermal energy.

目前市場上唯一的旋轉包絡管系統（西門子Straton）使用儲油器臨時儲存熱能。

The cooling capacity of today’s tube cooling systems ranges (depending on the throughput of cooling air) from around 7 to 10 kW. This amounts to a cooling capacity equal to the task of dissipating the heat emission from an open fire.

今天的管式冷卻系統的冷卻能力範圍（取決于冷卻空氣的流量）約為7到10千瓦。這相當于一個冷卻能力，相當于從明火中散發熱量的任務。

Mechanical interface 機械接口

The housing is the mechanical interface between the tube and the X-ray system (e.g. a CT scanner or an angiography system). The rotation of the CT gantry in CT applications (fig. 12.36) generates centrifugal forces that act on the housing and the tube (up to 30 g). Both the tube envelope (as mentioned above) and the entire tube housing assembly with its weight of about 50 kg are required to resist these forces and the additional mechanical stress caused by temperature gradients.

外殼是管子和X射線系統（例如CT掃描器或血管造影系統）之間的機械接口。CT應用中CT機架的旋轉（圖12.36）産生離心力，作用在外殼和管道上（高達30g）。管外殼（如上所述）和整個管殼元件（重量約為50 kg）都需要抵抗這些力和溫度梯度引起的附加機械應力。

In addition to the tube envelope, the tube housing is also required to guarantee the stability and the accuracy of the focal spot position when exposed to high centrifugal forces and thermal stress. Sophisticated design methods such as 3D-CAD systems and FEM (finite element method) simulations represent essential tools of modern tube housing design when it comes to fulfilling these complex requirements.

除了管外殼外，還需要管外殼，以確定在暴露于高離心力和熱應力時焦點位置的穩定性和準确性。複雜的設計方法，如3D-CAD系統和FEM（有限元法）模拟，在滿足這些複雜要求時，代表了現代管殼設計的基本工具。

CT掃描器的開放式機架（卸下蓋子）視圖（顯示旋轉和離心力）