Central nervous system malformation is one of the most common congenital malformations, and the neonatal neural tube defect rate is (1~2)/1000. Long-term follow-up studies have shown that the incidence of central nervous system abnormalities may be as high as 1%.
The average gestational age of fetal neurological examination in the first trimester is 12~15 weeks, and the fetal skull is thin at this stage, which can comprehensively observe the intracranial structure from multiple angles, so as to diagnose severe cranial malformations as soon as possible.
Indications for fetal nervous system ultrasound
If ultrasound screening reveals suspected cranial or spinal abnormalities in the fetus, a special ultrasound examination of the fetal nervous system should be performed. Indications are as follows:
- Routine screening for suspected central nervous system or spinal abnormalities
- Suspicion of central nervous system or spinal abnormalities at NT screening
- Family history of hereditary central nervous system or spinal abnormalities
- History of pregnancy with fetal brain or spinal malformations
- Fetal congenital heart disease
- Monochorionic twin pregnancy
- Suspicion of congenital intrauterine infection
- Known history of exposure to neurological teratogens
- Chromosomal microarray abnormalities of undetermined significance
Advantages of 3D imaging technology
3D ultrasound is very different from 2D: 3D images are reconstructed by computers from multiple 2D images, while 4D images are dynamic forms of 3D images. 3D can not only obtain some planes that are difficult to obtain in 2D, but also can be used for fetal surface imaging and volume measurement, multi-plane tomography, thin-slice 3D, dynamic Z-plane, four-dimensional ultrasound, cardiac STIC technology, vascular stereostructure diagram, vascular distribution percentage, etc.
The advantages of 3D ultrasound are mainly reflected in two aspects: first, it can obtain perfectly aligned images on three orthosecting planes with multi-plane images, and second, it can present as much as possible a thicker brain "slice", which improves the "signal-to-noise ratio" of all planes and significantly improves image quality. These advantages support the recommendation of 3D imaging for targeted neurosonography.
Check the scope of application
1. Imaging of surface structure. This is the original application of 3D ultrasound in fetal imaging. In the first trimester, the entire fetus can be revealed, as well as the head, abdominal wall, and limbs, while in the second trimester, the fetus's head, face, fingers (toes) and other relatively small structures can be displayed. Once there are malformations on the surface of the fetus, such as cleft lip, encephalocele, spina bifida, omphalocele, gastroschis, limb or hand and foot deformities, three-dimensional imaging can make pregnant women and their families clear at a glance, so that obstetricians or pediatricians can consult more in combination with the actual situation, and facilitate the selection of the next treatment plan.
2. Examination of skeletal system. Compared with 2D ultrasound, 3D ultrasound can clearly visualize the skeletal system, including the skull, spine, ribs, and limb bones, which is helpful for observing bone development and the presence of deformities. The most commonly used is three-dimensional imaging of the spine, which shows the entire spine from the cervical spine to the sacrococcygeal vertebrae, counts the number of vertebral arches, counts the number of bilateral ribs, and finds abnormalities such as hemivertebral bodies, scoliosis, and missing or fused ribs. In addition, the examination of the cranial suture of the skull found premature closure of the cranial synostosis, which undoubtedly improved the diagnosis of microcephaly.
3. Three-dimensional reconstruction of the median sagittal plane of the brain. Due to the position of the fetus in utero, 2D ultrasound often only obtains a cross-section of the brain. Three-dimensional reconstruction of the median sagittal plane of the brain can show the midline structures such as septum pellucidulus, corpus callosum, third ventricle, fourth ventricle, cerebellar vermis and posterior fossa, measure the diameter of cerebellar vermis, and diagnose or differential diagnose common central nervous system abnormalities such as corpus callosum loss and Dandy-walker syndrome.
4. Multi-plane tomography scanning. This 3D ultrasound is similar to a CT scan, with a continuous, parallel cross-sectional view, as if the probe is slowly moving in parallel, showing the examination site from top to bottom or left to right.
5. Acquisition of the median sagittal plane of the fetal face. In the median sagittal plane, the nasal bone can be observed, the nasal bone length, nasal length, maxillofacial angle, philtrum length, nasal process length, anterior-posterior nasal distance, facial height, maxilla, nasal root, and mandible angle can be measured.
Whenever fetal brain abnormalities are suspected on ultrasound, ranging from mild ventricular dilation to severe abnormalities such as hydrocephalus and midline displacement, detailed neurosonography is required. Using TUI in all three orthogonal planes, all coronal, sagittal and axial slices required for a detailed neurosonogram can be achieved with a single fetal head scan. This includes the following:
Although the potential to assess the fetal nervous system at an early stage is significantly improved, screening for most central nervous system abnormalities is also essential with neuroultrasound follow-up after 20 weeks' gestation. Except in particularly fatal or near-fatal anomalies, such as exoencephaly-anencephaly, bulging cephaly, and anecedence, diagnosis is simple and does not require subsequent scanning.
Expert presentation
Bibliography:
Ultrasound Obstet Gynecol. 2021 Apr; 57(4):661-671. ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 2: performance of targeted neurosonography PMID: 33734522
Authors: Chen Kai and Cheng Zhijian, Department of Ultrasound Medicine, Jiangxi Provincial People's Hospital
Correspondent: Le Xiwen Luo Shan
Editor: Huang Menglin
Reviewer: Chen Shirui
Issued: Daley Red
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