Photons, spectra, energy spectra, dual energies and photon counting

Photons are from the 2014 Glossary of Radiological Medicine and Protection. It is a type of elementary particle, which is not charged and has a rest mass of zero. Photons, short for light quanta, are elementary particles that transmit electromagnetic interactions, and are a canonical boson that was proposed by Einstein in 1905.

photon

X-rays are also essentially a type of photon, typically requiring high-speed electrons to bombard the anode metal face. The resulting X-rays are generally very continuous and are called radiative radiation. As the tube voltage increases, the electron energy increases, so that the inner electrons are also excited, resulting in holes in the inner layer, and the outer electrons will return to the inner layer, thus forming photons with a wavelength of 0.1 nm. Since the outer electrons generally emit quantized energy, the wavelengths are generally concentrated, which forms the X-spectral characteristic spectral line, which usually requires a tube voltage of 70kV and above to excite the characteristic spectrum.

X-ray bulb

Electromagnetic spectroscopy (EMS)

Spectrum (spectrum) is a medical imaging technology term published by the National Science and Technology Terminology Approval Committee in 2020, from the first edition of "Medical Imaging Technology Terminology". Refers to the pattern of electromagnetic waves arranged sequentially by wavelength or frequency.

X-ray spectra, tungsten target 100kV photon number vs. photon energy diagram

Energy spectrum (energy spectrum) is a medical imaging technology term published by the National Science and Technology Terminology Approval Committee in 2020, which is from the first edition of "Medical Imaging Technology Terminology". Refers to the distribution curve of X-ray photon energy and number of photons.

For X-rays, spectra and energy spectra have the same meaning.

Energy spectrum curve

In the field of CT imaging, spectral CT and energy spectrum CT usually express the same meaning, and both essentially belong to CT dual-energy imaging. In the CT dual energy we call the energy spectrum curve, which refers to the change curve of the CT value of the material under different energies (keV).

CT dual-energy CT imaging is a medical imaging technology term published by the National Science and Technology Terminology Approval Committee in 2020, from the first edition of "Medical Imaging Technology Terminology". Imaging techniques that use X-ray beams of two energies to scan the object being scanned.

At present, the main implementation methods are dual source, kV fast switching, double layer detector, homologous dual beam, kV slow switching, etc., the main purpose of which is to obtain two different energy rays.

For the history of dual-energy imaging, see: Fifty Years of CT Energy Imaging (1971-2021): An Exploration from Dual Energy to Multi-Energy; For Dual Energy Realization Methods, see: Implementation Methods of Dual Energy CT.

Different dual-energy realization methods

Dual-source dual-energy CT imaging: Two sets of X-ray tubes and detectors are set up in the frame of a computed tomography device, arranged at an angle, one X-ray tube produces X-rays of high-level voltage, and one X-ray tube produces X-rays of low tube voltage. The two systems independently collect data information, match the image data space, and perform dual-energy analysis.

For more information about dual-source CT, see: Technical Principles and Clinical Applications of Dual-Source CT.

Dual-energy technology with energy spectroscopy purification can better separate information from high and low energies for more accurate results

Photon counting is the next generation of CT imaging technology, as early as 20 years ago, Siemens scientists began the basic theoretical research of photon counting CT. With advances in high-speed processing electronics, it has become increasingly possible to image CT detectors to identify and count X-ray photons in a short period of time. Previously, although the concept of photon counting has been applied to mammography equipment and nuclear medicine PET systems, the challenge of achieving photon counting in CT devices is even greater. The X-ray photon count rate of CT scan can reach up to 10 /mm /s, while the count rate in breast machine and PET system is generally lower than 100/mm /s, so photon counting CT puts forward higher requirements for the speed of data processing.

At present, traditional CT based on dual-energy imaging can realize the simultaneous identification and quantitative analysis of up to two substances, and the simulation calculates the iodine map, calcium map, virtual flat scan, virtual unpower image, etc. for clinical diagnosis. Photon counting CT can read CT data from different energy domains at the same time by setting multiple thresholds (T0, T1, T2, T3), and this multi-energy imaging method provides the possibility of specific substance imaging and multi-material decomposition. For example, two different contrast agents are separated at the same time: iodine and gadolinium, iodine and bismuth, or other heavy elements (tungsten and nano gold, etc.), so as to achieve simultaneous imaging of multi-contrast media and targeted molecular imaging. In addition to this, by optimizing weighting different energy domains, the CNR of the image can also be further improved, especially in enhanced CT examination.

For more information on photon counting CT, see: What is Photon Counting CT?

NAEOTOM Alpha Photon Counting CT

Another concept that is more confusing is spectral counting. Spectral counting is a concept in the field of proteomics, defined as the total number of spectra of a protein, and has been widely accepted in proteomics research as a practical, label-free, semi-quantitative measurement of protein abundance.

Workflow in labelless mode

Source: Xi Zone

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