Objective
The aim of this study was to evaluate a rapid 3-dimensional ultrasound-assisted technique for evaluation of the corpus callosum as an integral part of the anatomic survey.
Study Design
Transabdominal 3-dimensioal gray scale and power Doppler volumes of the fetal brain were acquired in 102 consecutive healthy fetuses at 20–23 postmenstrual weeks. Offline analysis was performed by 2 of the authors using a systematic approach of “volume manipulation.” Diagnostic-quality visualization of the corpus callosum and the pericallosal arteries on the median plane was recorded by the 2 examiners independently.
Results
The median plane was easily obtained in all cases. Diagnostic-quality images of the corpus callosum were recorded in 93.1% and 99.0% and of the pericallosal arteries in 94.4% and 95.5% of the cases, by the 2 examiners, respectively.
Conclusion
Three-dimensional ultrasound enables a rapid and easy evaluation of the corpus callosum that may facilitate its inclusion as an integral part of the routine anatomic survey.
The corpus callosum, a thick plate of dense myelinated fibers, is the main connection between the 2 cerebral hemispheres. Its formation (anterior to posterior) is initiated in the embryonic period and is completed by 18–20 postmenstrual weeks. This structure is thought to play a sentinel role in integrating motor, sensory, and cognitive functions in the human brain.
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Partial and complete agenesis of the corpus callosum (AGCC) are relatively common brain anomalies thought to occur in 0.1–0.7% of the general population. Higher prevalence was documented in selected populations, such as developmentally delayed children (2–3%), and among fetuses with dilatation of the lateral ventricles (3–10%). The poor outcome associated with both complete and partial AGCC depends mainly on the presence of associated fetal anomalies and the fetal karyotype. Nevertheless, even fetuses with isolated AGCC, which account for only one-third of all cases, have been associated with a significant risk of developmental delay.
Acceptable guidelines for the routine fetal brain examination during the second-trimester anatomic survey have been provided by the American College of Obstetricians and Gynecologists, the American Institution of Ultrasound in Medicine, and the International Society of Ultrasound in Obstetrics and Gynecology. These guidelines are based on transabdominal axial brain planes on which the falx, thalami, cavum septi pellucidi, lateral ventricles with the choroids, cerebellum, and cisterna magna should be demonstrated. These axial planes, however, are not sufficient to evaluate the corpus callosum. Therefore, the diagnosis of AGCC on these axial planes is based on indirect clues, such as colpocephaly, absence of the cavum septi pellucidi, and the widely separated parallel frontal horns. However, these indirect signs are not always present at the time of the anatomic survey and may progress to be apparent only later in the mid or third trimester. To date, direct evaluation of the corpus callosum is not officially recommended as an integral part of the basic routine anatomic survey.
To directly visualize the corpus callosum, a meticulous scanning technique, in which the fetal brain is scrutinized in the coronal and midsagittal (median) planes, has to be implemented. Figure 1 demonstrates a simplified anatomic diagram of the median plane providing evaluation of the entire corpus callosum and pericallosal artery. Malinger and Zakut have shown that dedicated transvaginal fetal neurosonography, performed by an expert examiner, provides excellent visualization of the corpus callosum. Similar findings were found by other investigators, who, in cases of vertex presentation, successfully used the transvaginal approach in the evaluation of the corpus callosum. Unfortunately, this practical technique did not gain popularity in the United States and is perceived by many as cumbersome and time consuming. This attitude might be attributed mainly to the lack of expertise or training of most diagnostic fetal ultrasound providers in obtaining the median plane using transvaginal sonography, as well as in interpreting the “nontraditional” brain planes (coronal and sagittal). In addition, some clinicians are reluctant to use the transvaginal approach even when the fetus is in vertex presentation.
Three-dimensional (3D) ultrasound has emerged in recent years, facilitating the examination of an entire volume of ultrasound information, which can be scrutinized using an infinite number of planes to obtain the desired view. The technique allows the simultaneous presentation of the image in the 3 ortogonal planes. In this study, we sought to investigate a systematic approach using the 3D multiplanar mode to obtain a diagnostic-quality imaging of the corpus callosum on the median plane. We particularly aimed for a transabdominal, rapid, and simple application, which might facilitate the demonstration of the corpus callosum as an integral part of the routine second-trimester anatomic survey.
Materials and Methods
This prospective, observational study was conducted in our ultrasound unit between January 2008–September 2008. The study population included 102 consecutively scanned sonographically normal fetuses presenting for a routine second-trimester anatomic survey at 20–23 postmenstrual weeks. Exclusion criteria included multiple gestations and known congenital or chromosomal anomalies.
Transabdominal 3D volume acquisition of the fetal brain was performed by 1 of 3 sonographers, who are experienced in 3D techniques. All scans were performed using a single General Electric Voluson E8 (Milwaukee, WI), equipped with a 5–8 MHz transabdominal 3D transducer, which was designated for that purpose. There was no selection as to which patients were assigned to this examination room. Initially, a gray scale volume of the fetal brain was acquired. Depending on the fetal position, the volume acquisition was obtained in the sagittal or coronal planes, using the anterior fontanelle, the sagittal, or metopic sutures as acoustic windows. If fetal position did not enable easy and rapid access to these structures and acquisition in these planes, we acquired the volume in the axial plane at the biparietal diameter level. Additional volume was acquired using the power Doppler mode (with the pulse repetition frequency set at 0.6 kHz) to image the brain vasculature in 90 patients. We used an acquisition sweep angle of 45° to include the entire fetal brain within the volume. In most cases, several volumes were acquired, accounting for possible motion artifacts that would have precluded our evaluation. The sonographers were instructed to acquire the volumes during the routine anatomic brain survey and not to extend the scan beyond the routine American Institution of Ultrasound in Medicine guidelines to scrutinize the brain or optimize the visualization of the corpus callosum before the acquisition. Visualization of the corpus callosum by 2-dimensionl (2D) ultrasound was not performed before the acquisition of the volume. Subsequently, all volumes were deidentified and stored on a serially numbered computer disk. Offline analysis was conducted using dedicated software (4D View; General Electric) on 1 of 2 personal laptop computers. Each volume was examined separately and independently by 2 of the authors, versed in fetal neurosonography. A systematic approach of “volume manipulation” on the multiplanar display mode was performed in which the volumes were positioned into a standard, symmetric orientation of the orthogonal planes. This resulted in display of the coronal plane in box A, the median plane in box B, and the axial plane in box C. In this manner the entire corpus callosum could be depicted on the median plane ( Video 1 ). Moreover, the axial and coronal planes were simultaneously displayed and were evaluated as well. The volume containing the power Doppler was positioned in a similar fashion to trace the anterior cerebral artery and its branch, the pericallosal artery, on the median plane ( Video 2 ). Diagnostic-quality visualization of the entire corpus callosum (on the gray scale volume) and the pericallosal artery (on the power Doppler volume) on the median plane was then recorded by the 2 examiners, who were blinded to each other’s evaluations and results.
The agreement rate between the 2 examiners was also calculated. This was derived from the number of cases in which both examiners reached a similar conclusion, either both adequately visualized the structure or both did not adequately visualize it. All images were reviewed for quality assurance by the first author (E.B), who is a Fellow in maternal-fetal medicine. Furthermore, this author also performed an independent offline evaluation of all of the acquired volumes.
The study was approved by the institutional review board committee.
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