Fig.12.The diagrams demonstrate the different angles in which the coronal views are obtained with transvaginal and with 3D neuroscans, respectively. The sections recreated from the 3D volume are parallel to each other and not radiating from a common point(the anterior fontanelle) as is the case with conventional 2D transvaginal neurosonography and neonatal neuroscan. 图12。该图分别显示经阴道超声和三维超声扫查获得颅脑冠状面的角度不同。三维超声获得的切面互相平面，而不是从前囟门扇扫的传统二维经阴道和新生儿神经系统超声的切面。
3.As discussed previously,the power angiography mode can be used in selected cases to obtain a volume imaging of various main vessels in the brain.The authors are interested mainly in the course of the pericallosal branch of the anterior cerebral artery.In cases of space-occupy-ing lesions, the anatomy of the vessels may be of use in determining the size and extent of the lesion.In cases of brain tumor,its vascularity helps to evaluate the nature of the lesion. Using 2D US to obtain the‘‘perfect’’ median (midsagittal)plane requires experience in transvaginal brain scan and at times may not be obtained because of unfavorable fetal position or presentation. Manipulating the 3D US to obtain the ‘‘perfect’’median plane can be achieved easily by aligning the axial and coronal planes in the right position(discussed later). 3.如前所讨论的，能量血管造影术模式在某些病例中使用能够获得大脑一些主要血管的容积影像。作者们主要对发自大脑前动脉的胼周动脉的分支走形感兴趣。在占位性病变，这些血管的解剖位置在确定病灶的大小和严重程度上有用。大脑肿瘤的血管分布有助于评估病灶的性质。二维经阴道超声获得大脑“完美的”正中面（正中矢状面）需要经验，有时候因为胎儿位置或体位不佳而难以获得。三维超声通过校准右位横断面和冠状面能够获得“完美的”正中面（见后诉）。 In some cases, in which the artery deviates from the midline because of pressure from a structure(such as a cyst),the course of the displaced artery can be followed using 2D US scans only with great difficulty or not at all.3D angiography may assist in tracing the deviant course of the artery with the combined use of the marker dot. 某些病例，因为结构占位压力（比如囊肿）导致血管偏离中线，二维超声扫描偏移的动脉困难很大，但是三维超声却一点也不会。三维血管造影术联合标记点技术能够帮助跟踪偏移走形的动脉。
1.In general,the limitations of 3D US are few and mainly reflect lack of adequate experience because of its long learning curve.19
2 The quality of an image displayed in the orthogonal planes(or any other render or display)can be only as good as the 2D US image in the acquisition plane. In addition,the reconstructed third plane(C plane)always will have lower resolution and an increased level of artifacts.The quality may be improved somewhat by using the thick-slice or the VCI application(discussed previously).
1.总体而言，三维超声的局限性很少，缘于其学习周期长而主要反映出缺乏足够的经验。19 2.显示在正交平面上的图像（或其他渲染图像）的质量与二维超声图像一样好。此外，重建的第三平面（C平面）可能分辨率低且伪像增加。这些可能通过使用厚层成像或VCI技术得到一定程度的改善。（如前讨论） 3.Acoustic shadowing in 2D US image in the acquired plane results in fixed acoustic shadow also limiting the 3D volume. For example, a 2D US image of the fetal brain obtained through the occipital bone may have poor quality of the intracranial structures as the thick bone creates an acoustic shadow. Similarly,a 3D volume obtained in this manner includes this fixed acoustic shadow embedded in the volume.Therefore, the prerequisite for a good 3D US image is to generate good-quality 2D US image. 3.二维超声图像某些切面的固定的声影也限制了三维容积超声的使用。例如，通过枕骨的胎儿颅脑二维超声图像，缘于厚厚的颅骨产生声影导致颅内结构显示质量欠佳。类似的，这种方式获得的三维容积超声也同样在容积内包含了固定存在的声影。因此，好的三维超声图像的首要条件是二维超声图像质量要好。
4.Fetal motion is a major limitation causing artifacts that may preclude the acquisition of a good 3D US volume making it necessary to obtain additional volumes. This usually is overcome by decreasing the acquisition time of the volume using the low-or medium-quality sweep and by obtaining the volume in a time period with no fetal or maternal movements. The authors frequently instruct patients to hold their breath at the time of volume acquisition to limit any possible movement.This is true for volumes requiring long acquisition times,such as those containing color or power Doppler information. 4.胎动引起的伪像妨碍了形成好的三维容积图像，使得我们需要扫查更多的容积数据，这也是三维超声的一个主要的局限性。这可以通过使用低-中质量扫查缩短扫查时间，和在母体和胎儿都没有活动的时间段内扫查来克服。作者常常引导母体在扫描的时候屏住呼吸来避免任何不必要的获得。这种方法是正确的，因为有些扫查需要很长时间，比如包括彩色和能量多普勒信息的检查的时候。
5.The surface-rendering mode requires a fluid-tissue interface that sometimes may be compromised by an unfavorable fetal position, a fetal hand over the fetal face,or a fetal face too close to the anterior placenta. The electronic scalpel or the electronic eraser may assist in eliminating structures that are not desired or that block the target structure. 5.表面成像模式需要组织界面，时常为不好的胎儿体位限制，胎儿手遮住面部，或是胎儿面部太靠近前壁胎盘。电子手术刀或电子橡皮擦有助于消除不需要的结构或遮挡目标的结构。 6.Additional drawbacks that apply to 3D US just as in 2D US are the difficulty of obtaining a good image in patients who are obese or present with oligohydramnios and a nonfavorable fetal position limiting the acquisition of the volume. 7.3D US requires more expensive, updated ultrasound machines and software and is regarded as experimental by most health care carriers. For now, its use is limited by availability, financial aspects, and inexperienced users. 6.三维超声的缺点就像二维超声应用里面的一样，在肥胖的病人或羊水过少，胎儿体位不好时候难以获得好的超声图像。 7.三维超声需要昂贵的，现代化的超声仪器和软件，在大部分卫生保健单位被认为是实验性的。到现在为止，三维超声的使用受限于它的实用性，经济原因和经验不足的使用者。
Specific limitations to the use of 3D US during fetal neuroscan are few. Generally,a high-frequency transvaginal ultrasound probe produces better images with higher resolution. The fetus must be in vertex presentation and the probe should be directed to perform a transvaginal transfontanellar neuroscan.In cases of breech presentation, acquisition of the volume should be attempted through the fontanelles, the sutures, or the thinner temporal bone to obtain a better view of the intracranial structures. At times, external version of the fetus into vertex presentation is warranted in fetuses with strong suspicion of a brain anomaly. Imaging the posterior fossa may be compromised by the acoustic shadow of the thick petrous ridge of the skull limiting the ability to image the brainstem. Volume acquisition through the posterior fontanelle or the posterior section of the sagittal suture may assist in overcoming this problem. Another limiting factor is the rare case of craniosynostosis in which the early fusion of the sutures limits easy scanning access to the brain that may be partially overcome by using the higher-frequency transvaginal transducer.