磁共振成像在妇产科应用中的新进展 [精华]

磁共振成像在妇产科应用中的新进展,1983年Smith等首次报道了胎儿的MRI检查。在80年代中期，产前MRI研究主要在子宫和卵巢异常方面，随后出现了胎儿异常MRI的报道。早期应用的常规自旋回波序列采集时间长，胎儿运动使影像质量下降。最近5年来，快速MR技术在产科的应用，解决了胎动伪影这一难题，正成为高分辨超声诊断胎儿异常的有力补充。,一、胎儿MR检查的安全性,目前，没有确实的证据说明短期暴露于磁场损害了发育中胚胎，也没有MR检查导致胎儿畸形的报道。Baker等报道20例经MRI检查的胎儿，随访3年未见相关异常。可是由于磁场的潜在的、尚不清楚的效应，暴露应受限制，MRI仅在诊断效益明显大于理论损害的情况下进行。磁共振协会安全委员会认为，当其它非放射性检查手段不合适或者为获得更多信息需要暴露于放射线(如X线平片、CT)时，可以使用MR检查。美国食品与药品管理局(FDA)认为，MR检查的安全性未完全证实，所以检查前应征得孕妇的同意。许多研究者认为，在妊娠最初3个月，正在发育的胎儿有可能受到各种物理因素的损害，应避免在此期使用MR检查。MRl检查对比剂(GdDTPA)有轻度延缓动物发育的作用，被认为是妊娠C类药，只能在明显利大于弊时才能使用，不作为常规检查。,二、MR技术及其进展,(一)常规SE序列 通常见于早期报道。阵列线圈用于早期妊娠。晚期因母体体积增大，推荐使用体线圈。孕妇仰卧位，膝稍弯曲，足先进，行横断、矢状、冠状面T1WI、T2WI，整个检查时间较长，孕妇难以耐受，同时胎动伪影严重，所以，早期研究多使用镇静剂来改善影像质量。,(二)快速MR序列1回波平面成像(echo planar imagimg，EPl)：EPI是当今最快速的成像方法，每层扫描时间在30100ms，每秒可获取图像达20幅。但与RARE序列相比，有以下不足之处：信噪比稍低。磁敏感性伪影明显。化学位移伪影较重。需要特殊硬件，包括特殊的梯度系统、高速的数据采集系统和图像处理系统。从而限制了EPI的发展，目前在产科应用不普及，多用来评估胎儿脏器的体积。2快速SE序列(fast spin echo，FSE；turbo spin echo，TSE)：快速SE序列最早于1984年提出，当时称为弛豫增强快速采集，即RARE(rapid acquisition with relaxation enhancement)序列。其原理是90度RF脉冲之后，用一系列180度脉冲产生多个回波信号。快速SE序列最大优点是与常规SE序列扫描相比，速度成倍提高，但对比度和分辨率稍有下降，运动伪影仍稍重。典型的序列参数为TRTE 3 000138ms，回波链长度(ETL)为29，6mm层厚，矩阵116x256，FOV285x380，每层扫描需25s。3半傅里叶采集单次激发快速自旋回波(halffourier acquisition single shot turbo SE，HASTE)：又叫半傅里叶快速采集弛豫增强(hallfourier rapid acquisition with relaxation enhancement)。实质上是半傅里叶成像方法与快速SE结合的产物，主要反映T2对比，影像具有重T1加权特征。HASTE序列的成像速度极快(仅次于EPI序列)，每层采集时间少于400ms，每次采集间隔1s，1个序列13层影像需要17s，最突出的优点是有效地减少运动伪影，可提供胎儿组织器官最好的对比分辨。典型的参数包括60ms回波时间(TE)，ETL为72，一次采集，4mm层厚，FOV24cmX24cm，矩阵192X256，130度脉冲。HASTE序列是目前胎儿MRI研究中最常用的序列。4快速GRE序列：快速小角度激发(fast low-angle shot，FLASH)是最常用的快速GRE序列。它通过稳态不相干(SSl)技术对剩余横向磁化进行散相处理，从而减少MR扫描时间。FLASH序列反映T1WI，每层采集时间小于20s，影像受胎动影响质量可有轻度下降，识别出血或脂肪很有效。典型的序列参数包括：1264(TRTE)，80度翻转角，FOV24cmX 32cm，矩阵96X 256，5mm层厚，一次采集。,(三)三维体积成像(threedimensional volume imaging) 与二维成像不同的是，三维体积成像不使用层面选择梯度，其三维上的空间定位由第二个相位编码梯度来完成。二维博里叶成像中，噪声来自整个容积，而信号取自成像层面，因而信噪比(SNR)较低；三维成像信号则来自整个容积，所以其SNR较高。三维成像的最大缺点是总的成像时间较长。Kinoa等利用稳态自由进动序列(steady state free precession，SSFP)研究胎儿标本的脑室及原始基质的体积变化。作者认为其研究结果有助于宫内胎儿的临床超声和MRI检查。,三、正常胎儿的MRI表现,许多研究指出，胎儿影像受胎龄和器官大小影响。生殖囊在6周左右可见，表现为高信号的内膜中的一个低信号卵圆形块，在9周，能显示胎儿极向。在1216周，胎儿头、肺、肝及肢体可见，部分胎儿心脏在15周左右可见。25周后，所有胎心均能在MRI中显示。到26周，整个头、腹部、肢体清晰可见，胎儿大脑和蛛网膜下腔用T2WI显示较好，面部、眼球、咽喉也可显示。在腹部，肾、肝、胆囊、胃和膀胱显示清晰。假如在定位像扫描和随后的影像采集之间发生胎儿运动，标准方位的影像难以获得，对肢体尤其如此。,Figure 1a. Ovarian edema in a 28-year-old woman with twins (initially quadruplets) at 14 weeks gestation who had undergone ovulation induction and had an enlarging solid left ovarian mass at US. Sagittal turbo spin-echo MR image (5,139/90, two acquisitions, 140 x 256 matrix, echo train length of seven, 5-mm section thickness, 26 x 35-cm field of view, acquisition time of 3 minutes 3 seconds) of the gravid uterus (U) and enlarged (14 x 9 x 8 cm) left ovary with small peripheral follicles (arrows). The follicles have low signal intensity, which is likely due to hemorrhage. The enlargement of the left ovary was histopathologically shown to be caused by massive ovarian edema. Because of the long imaging time, fetal structures could not be identified. Half-Fourier RARE MR image (single shot/60; field of view, 26 x 35 cm; matrix, 192 x 256; echo train length, 72; one signal acquired; section thickness, 5 mm) in the coronal plane demonstrates the gravid uterus (U) with the body of one fetus (arrowhead), hyperstimulated right ovary (open arrow), and edematous left ovary (O). Coronal RARE MR image obtained slightly more anteriorly shows fetuses (one of which is not alive) in three gestational sacs. 图1.女性，28岁，多胎怀孕14周，卵巢水肿，曾行促排卵处理，超声示左侧卵巢增大的实性肿块。 矢状位turbo SE序列(5,139/90, 2次采集,矩阵 140 x 256 , 回波链长度7, 层厚5mm , FOV 26 x 35cm ,采集时间 3 分3 秒) 显示妊娠子宫(U)， 左侧卵巢增大(14 x 9 x 8 cm)，见小的周围卵泡（箭头）。卵泡因出血呈低信号，病理提示左侧卵巢增大是由于卵巢块样水肿引起，由于检查时间长，胎儿机构不能鉴别； 冠状位半傅立叶 RARE 序列 (single shot/60; field of view, 26 x 35 cm; matrix, 192 x 256; echo train length, 72; one signal acquired; section thickness, 5 mm) 显示妊娠子宫(U)，胎儿体部（箭头），过度刺激的右侧卵巢（开箭），水肿的右侧卵巢(O)； 稍靠前的冠状位 RARE 序列显示3个妊娠囊内的胎儿（1个未成活）。,(一)中枢神经系统发育及MRI表现,了解正常胎儿的脑发育，对认识胎儿颅内疾病至关重要。妊娠1620周的胎儿，大脑轮廓光滑，无脑回，仅有一个浅而宽的外侧裂，在2729周部分区域形成脑沟，从30周开始整个大脑皮层脑沟均出现，到晚3个月大脑变大，蛛网膜下腔变小。在2125周由于生理性脑积水，正常脑室较大，直到25周开始缩小。T1WI、T2Wl MRI显示胎儿大脑呈多层结构，这些层样结构的厚度和可视性由胎龄大小及影像质量来决定。一些研究者描述5层结构，其它依据胎龄不同为3或4层。在2328周的胎儿大脑，Girard等描述5层(T1Wl)，3个高信号层，2个低信号层。由内向外是：脑室周围高信号代表原始基质(germinal matrix)。邻近原始基质的低信号代表未髓鞘化的间质。高信号中间层代表移行的胶质细胞。低信号的实质。最周边一个高信号的皮质带。28周以后多层结构消失。在离体研究中，Brisse等描述16周呈3层结构，1922周呈4层，更大的胎儿显示层数更少：24周3层；34周仅2层。MRI高信号在胎龄小者代表高细胞密度，在胎龄大者代表髓鞘形成。在显示脑发育方面，MRI优于US，MRI是唯一能显示胎儿在于宫内髓鞘形成过程的技术。21周基底节呈T1WI高信号，T2WI低信号。皮质下移行层(中间层)呈同样表现，开始于23周是由于细胞密度增加。23周在背侧脑干，3l周在内囊后肢，35周在视放射，36周在放射冠出现T1WI高信号，T2WI低信号，是由于髓鞘形成所致。,Figure 2. MR images in a fetus at 12 weeks gestation in a 25-year-old woman with a left ovarian chocolate cyst. Sagittal and coronal half-Fourier RARE images (/87) reveal the dilated ventricle, or so-called normal fetal hydrocephalus. The brainstem (arrow in a) and choroidal plexus (arrowhead in b) are noted. The cortex is shown to have a smooth surface. In a and b, * = ventricle. （孕12周胎儿脑发育）,Figure 3. Transverse half-Fourier RARE MR image (/87) of a fetus at 20 weeks gestation in a 32-year-old woman with deep venous thrombosis reveals the dilated ventricle and the subarachnoid space (arrow). The cortex still shows absent sulcation. The cerebral cortex has three layers. The innermost layer is of low signal intensity (arrowheads) and corresponds to the germinal matrix . （孕20周胎儿脑发育）,Figure 4. Coronal half-Fourier RARE MR image (/87) of a fetus at 22 weeks gestation in a 35-year-old woman with leiomyomas reveals the sylvian fissure (arrowheads). The ventricle was slightly dilated but is not apparent in this section. （孕22周胎儿脑发育),Figure 5. MR images of a fetus at 28 weeks of gestation in a 32-year-old woman with leiomyomas. Sagittal half-Fourier RARE image (/87 repetition time msec/effective echo time msec) reveals the formation of multiple sulci (arrowheads) in the central sulcal area but reveals no infolding of the cortex. * = lateral ventricle. Transverse half-Fourier RARE image (/87) also shows multiple sulci in the parietal lobe. The thalami (curved arrows) are shown to have low signal intensity. * = ventricle. The size of the ventricular system at the level of the frontal horn angles and the midbody of the lateral ventricles was expressed as a fraction of the diameter of the brain at the same level (ventricle-to-brain ratio). Arrowheads point to the measured widths of the anterior horn of the lateral ventricle, and straight arrows indicate the diameter of the brain at the same level. （孕28周胎儿脑发育）,Figure 6. Transverse half-Fourier RARE MR image (/87) of a fetus at 34 weeks gestation in a 39-year-old woman with deep venous thrombosis reveals the formation of multiple sulci (arrowheads), but opercular formation has not yet occurred. （孕34周胎儿脑发育）,Figure 7. Transverse half-Fourier RARE MR image (/87) of a fetus at 37 weeks gestation in a 32-year-old woman with leiomyomas shows that brain sulcation in the fetus is similar to that in the adult. The basal ganglia () show low signal intensity. Ventricular dilatation is not seen. （孕37周胎儿脑发育）,(二) 非中枢神经系统器官的正常MRI表现,胎儿肺因含水，T1WI呈低信号，T2Wl呈高信号。胎心在T1WI及T2WI皆表现为信号缺失区，在T2WI，位于高信号的两肺之间的胎心轮廓更为清晰，但其内部结构显示不清，由于流空效应，MRI同样能显示心脏周围大血管结构。肝脏T1WI呈中、高信号，T2WI呈中、低信号，占据上腹部大部分。在T2WI上膈肌位置被高信号的肺和低信号的肝勾画，结构较为清楚。胆囊、胃T2WI呈高信号(含液体)。肾、脾和胰腺的显示稍困难。膀胱清晰可见表现为T1WI低信号，T2WI高信号。皮肤、皮下脂肪、外生殖器和面部轮廓清晰。在晚3个月，骨结构于T1WI呈低信号(相对于中等信号的肌肉和高信号的皮下脂肪)，T2Wl在高信号的羊水中肢体易于辨认。,Figure 8. Axial RARE MR image (4.4/64 effective, 120 flip angle, 6-mm section thickness) through the level of the heart (large straight arrow) shows a normal chest in a fetus at 30 weeks gestational age. The spinal canal (small straight arrow) is shown posteriorly. The lungs (curved arrows) are homogeneous and high in signal intensity. 图8.轴位通过心脏平面（长直箭）RARE MR(4.4/64 effective, 120翻转角, 6-mm 层厚)显示一孕30周胎儿正常胸部，椎管（小直箭）显示于后，肺部（弯箭）呈均匀高信号。,Figure 9. Sagittal half-Fourier RARE MR images (TE, 60 msec) of a fetus at 26 weeks gestation. Slightly oblique and off-midline view shows the corpus callosum (arrows), massa intermedia , and parieto-occipital fissure (arrowhead). Slightly lateral to a, a smooth cerebral surface, along with the early appearance of the precentral (arrow 1) and central (arrow 2) sulci, is depicted. The central sulcus is a landmark of 20 weeks gestation, and the precentral sulcus is a landmark of 24 weeks gestation. （孕26周正常胎儿）,四、MRI在胎儿疾病诊断中的应用,(一)中枢神经系统病变1脑室扩大 (ventriculomegaly，VM)：US检查显示脑室扩大比较容易，但确定扩大的原因或判断预后常较困难，MRI对US有很好的补充作用。T2WI对脑室的显示较好，大脑和邻近结构的表现有助于了解脑室扩大的原因。扩大的脑室壁不规则，有分隔，邻近的脑实质异常，常提示感染和破坏性改变，从而与先天性疾病相鉴别。扩大的标准是以三角区间径10mm为正常上限，间径的测量应该在标准横断面像上获得。一旦作出脑室扩大的诊断，不仅要注意中枢神经系统异常，而且也应注意全身的其它相关异常。,Figure 10. Severe ventriculomegaly in a fetus at 30 weeks gestation. Oblique axial US scan shows a 34-week head size and massively enlarged lateral ventricles. It is difficult to assess the cortical mantle. The posterior fossa appears to be small, and there is a dilated fourth ventricle (arrow), but the cerebellum is difficult to visualize ultrasonographically. Sagittal and coronal RARE MR images (single shot/60) demonstrate a definite rim of smooth cortical tissue (arrowheads in b), malformed brain stem (arrow in b), and Dandy-Walker malformation (M). These findings make hydranencephaly unlikely and suggest a cerebrocerebellar malformation, such as Walker-Warburg syndrome, with aqueductal stenosis. This type of information is important in counseling patients prior to delivery. 图10. 孕30周胎儿，重度脑室扩张。 斜轴位超声扫描示头大如34周，侧脑室巨大，难以评估皮质结构，后颅窝较小，四脑室扩张（箭），但超声难以显示小脑。矢状位 及冠状位 RARE序列（单次激发/60)显示皮质的精确边界（图b箭头），变形的脑干（图b箭），Dandy-Walker畸形(M) 。这些发现提示并非脑积水，而是脑实质发育畸形，如并有导水管狭窄的Walker-Warburg综合征，重要的是这类畸形应在产前劝告病人。,2胼胝体畸形(anomalies of the corpusc allosum，ACC)：,胼胝体是连接两侧大脑半球的结构，大约在817周出现，MRI可在20周左右显示胎儿胼胝体。单纯的ACC不常见，ACC的发现常提示有其它大脑畸形，发生率约68。Sonigo等认为ACC的发生率比较高，主要包括完全缺如、部分缺如、胼胝体萎缩及异常增厚。产前诊断主要依靠US，MRI可以证实US诊断并发现其它相关的中枢神经系统畸形。他们研究50例胎儿，US诊断或怀疑ACC的占66，仅能显示16的相关中枢神经系统畸形；MRI更正了33病例的US诊断，能显示61的相关中枢神经系统畸形。Garel等认为胎儿胼胝体非常薄，因为部分容积效应，在矢状面不易显示，可在垂直于胼胝体的方位上显示(如冠状面显示体部，横断面显示膝部和压部)。诊断ACC要结合间接征象，如顶部脑沟、脑回呈放射状排列，放射形的中央沟，三脑室上移，侧脑室向两侧移位等。20例ACC胎儿，US诊断正确率80，MRI为95。有报道MRI最早能诊断ACC在215周(依据间接征象)。,Figure 11. Agenesis of the corpus callosum in a fetus at 34 weeks gestation. Coronal RARE MR image (single shot/60) shows the characteristic vertical orientation of the frontal horns (arrows) in agenesis of the corpus callosum. Arrowheads point to the region where the corpus callosum should be depicted. The US scan demonstrated only asymmetric mild ventriculomegaly. (Reprinted, with permission, from reference 55.) 图11. 孕34周胎儿，胼胝体缺入。冠状位RARE序列（单次激发/60)显示特征性的垂直方向的额角（箭），箭头所指应为胼胝体位置。超声仅显示轻微脑室扩张。,3神经元移行异常(neuronal migration disorder，*)包括无脑裂畸形、灰质异位、多小脑回畸形、脑裂畸形和单侧巨脑等。产前US诊断这些畸形困难且少见报道，MRI能够显示正常的脑细胞移行，对*的诊断优于US。Levine等指出，HASTE序列MRI可清楚地显示胎儿的脑沟、脑裂，能够预测胎儿皮层发育过程，正常胎儿MRI表现稍滞后于神经解剖标本0-8周。Garel等对173例大样本胎儿脑沟行MRI研究，他们认为T1WI、T2WI MRl可很好地显示胎儿脑皮层发育，可以对胎儿正常脑沟的评估提供一个参考标准，同时有利于产前MRI诊断脑回异常。研究脑回最好的时期是2834周。Sonigo等报道20例胎儿*，MRI能够诊断54的脑裂畸形，80的无脑回畸形，73的多小脑回畸形和100的脑裂畸形，而US仅有2例考虑脑回异常。,4出血性疾病 (haemorrhagic injuries)：,胎儿脑内出血通常较广泛，可以引起宫内或产后死亡、积水性无脑畸形或继发性脑积水。脑室内出血可用US诊断，MRI能很好地补充。MRI同时可以显示脑实质病变，这些病灶的显示可以解释许多脑瘫和神经损伤的病例，而在MRI检查之前往往被误认为是产伤，这些认识对医学一法律问题相当重要。,Figure 1. Coronal MR image in a fetus with subependymal hemorrhage at 18 weeks of gestational age. The patient was referred to our institution because the fetus had an enlarged cisterna magna that appeared normal at confirmatory US. Half-Fourier single-shot RARE MR image (echo spacing of 4.2 msec, echo time of 60 msec, echo train length of 72, one signal acquired, 2.8-mm section thickness, 21 x 24-cm field of view, 192 x 256 matrix, 19-second sequence duration, and section acquisition of 420 msec) shows an area of low signal intensity (arrow) in the left subependymal region. This finding was confirmed on all images. Subependymal hemorrhage in utero has unknown clinical importance. The baby had normal US findings in the head at birth and is developing normally. This is an example of a fetus with normal confirmatory US findings with an MR imaging finding that changed the diagnosis but not case management, since the patient continued the pregnancy, and peripartum care was not affected. 冠状位MRI示孕18周胎儿室管膜下出血,5结节性硬化(tuberous sclerosis，TS):,产前US诊断TS主要依靠显示多发性心脏横纹肌瘤。这种肿瘤与1580的TS相关。TS的MRI表现为T1WI脑室周围室管膜下出现结节状高信号，直径1-4mm。Sonigo等报道8例TS胎儿，产前US均诊断正常，MR发现5例有TS(625)，3例未作出诊断，提示产前MRI正常不能完全除外TS。,Figure 2. Coronal MR image in a fetus with tuberous sclerosis at 33 weeks of gestational age. US images (not shown) demonstrated multiple cardiac rhabdomyomas. Half-Fourier single-shot RARE MR image (echo spacing of 4.2 msec, echo time of 60 msec, echo train length of 72, one signal acquired, 3-mm section thickness, 30 x 40-cm field of view, 128 x 256 matrix, 20-second sequence duration, and section acquisition of 420 msec) shows multiple small subependymal tubers (arrows). These were not visualized at US (or on MR images obtained at 22 weeks not shown). This is an example of a fetus strongly suspected of having a CNS abnormality despite normal US findings with an MR finding that changed diagnosis but not case management, since the patient continued the pregnancy. 图2. 冠状位MRI示孕33周胎儿结节性硬化（tuberous sclerosis，TB）,6其它：MRI对胎儿后颅窝畸形、Galen静脉瘤、颅内肿块、脑膜脑膨出等均有较高的诊断价值.,Figure 12. Posterior encephalocele in a fetus at 20 weeks gestation. Axial US scan shows a posterior encephalocele. A small amount of tissue (arrow) is seen within the sac. On the axial RARE MR image (single shot/60), the structures of the posterior fossa appear to be normal. A portion of the encephalocele (arrowheads) is seen posteriorly. The information that the majority of the brain appeared to be normal was helpful to the patient in deciding to continue the pregnancy. Postnatally, a meningocele sac was excised, and the neural content returned to the posterior fossa. At the time this article was written, the baby was doing well at 1 year of age. 图12.孕20周脑膨出胎儿， 轴位超声显示脑膨出，囊内可见少量组织（箭）； 轴位RARE序列MRI（单次激发/60)显示后颅窝结构正常，可见脑膨出（箭头）。大脑正常有助于孕妇决定继续怀孕，胎儿出生后，可切除膨出的脑膜，脑实质归回后颅窝。,Figure 13. Sagittal half-Fourier RARE MR image (TE, 60 msec) of a fetus with a large encephalocele (solid arrow) at 22 weeks gestation. None of the normal fissures or sulci are seen. The open arrow indicates pleural effusion. 脑膨出,图14.孕34周胎儿， Dandy-Walker综合征。轴位 及矢状位 RARE序列（单次激发/60)显示Dandy-Walker畸形(M)。,图15. 孕27周胎儿，蛛网膜囊肿。 轴位超声扫描示脑外占位（extraaxial collection），但难于评估脑室的受侵。冠状位 及矢状位 RARE序列（单次激发/60)清楚显示病灶性质为巨大蛛网膜囊肿，对周围结构有占位效应.,(二)颈部病变,囊性水瘤(cystic hygromas) 又叫淋巴管瘤，可以发生胎儿的任何部位，头、颈、腋窝是最常见的部位。在胎儿囊性水瘤是喉后区肿块最常见的原因，可以压迫或包绕气管。先天性颈部畸胎瘤 是发生在颈部的少见疾病，通常起源于颈部前侧，横跨中线，肿瘤通常是良性的，大部分为实性成份，可以有囊变和钙化，肿块可引起气道阻塞。鉴别这些病变与其它可以出现在颈后部的病变(如脑膨出、脑膜膨出)相当重要。当US诊断不清时，MRI能够提供后颈项区更多的解剖细节。在快速T2WI序列，囊性成份可见，口咽部和气管树由于充满羊水呈高信号，与肿块分界清楚，肿块的急性出血、钙化也可显示，有助于鉴别畸胎瘤与淋巴管瘤。这些巨大的颈部肿块，先前认为是致命的，随着宫内手术的进步，部分胎儿生命可得到挽救。产前MRI评估大肿块的结构及它们与气管、颈部大血管的关系极为有用，有助于宫内手术的顺利进行。,(三) 胸部病变,在胎儿胸部最常见的占位病灶包括先天性膈疝(CDH)、先天性肺囊腺瘤畸形(CCAM)、支气管肺隔离症(BPS)。任何胸内占位都可以引起肺发育不良，发育不良的程度由肿块产生时的胎龄和肿块大小所决定。目前宫内治疗的开展使精确的产前诊断更为重要。,1先天性膈疝(CDH)：,是最常见的胎儿胸部占位，目前CDH诊断的主要手段是US。但US视野小，对胎儿肺与突人的腹腔脏器分辨较差，大多数情况下要通过间接征象来判断上腹部器官的位置，如需要准确评估胃的位置及判断门脉血管走行来确定肝脏位置，所以，US误、漏诊率达41%。MRI对疝入的脏器特别是肝脏显示清楚，对CDH诊断明显优于US。正常胎儿肺T2WI呈高信号；而疝入胸腔的肝脏在T1WI呈高信号，T2WI呈低信号；胃为含液结构，T1WI低信号，T2WI高信号；含有胎粪的扩张肠袢，T1WI呈明显高信号，这些脏器的信号特征有明显区别，在MRI上鉴别不难。许多学者认为心脏、纵隔移位及双肺受压在横断面观察较好，冠状面T1WI对肝脏的观察很有帮助，与膈嵴相应的位置容易识别。另外，MRI可以直接测量肺体积，准确诊断肺发育不良并进行定量分析，对CDH的胎儿外科手术很有帮助。,Figure 16. Left-sided congenital diaphragmatic hernia in a fetus of 22 weeks gestational age. Coronal T1-weighted fast low-angle shot MR image (174/4.1, 80 flip angle, 6-mm section thickness) shows the high-signal-intensity liver (straight arrows) extending into the chest above the diaphragmatic ridge (curved arrow). The meconium-filled bowel (arrowhead) has high signal intensity and extends to the apex of the chest. （左侧先天性膈疝）,2先天性肺囊腺瘤畸形(CCAM)：,CCAM是由于细支气管过度生长，引起小叶扩张。大多数CCAM为单侧，可以发生在任何叶段。Hubbard等将其MRI表现分为3型：I型为大囊型；型为小囊型(较多直径小于1cm的小囊)；型为囊实型(实性肿块内有较多小囊)。I型和型CCAM MRI表现为信号强度很高的肿块(几乎与羊水接近)，显著高于周围正常肺组织信号，型为中、高信号，信号高于肺组织，但低于羊水，且不均匀。CCAM宫内切除可以有50一60胎儿存活，产前MRI对宫内手术计划很有帮助。,Figure 17. CCAM in a fetus of 29 weeks gestational age. Sagittal US image through the fetus with the spine (small straight arrows) directed toward the transducer. There is a large hyperechoic mass (curved arrows) in the right side of the chest. A large, dominant cyst (long straight arrow) is present within the hyperechoic lesion. The lung lesion is substantially more echogenic than the liver (L). Axial RARE MR image (4.4/64 effective, 120 flip angle, 6-mm section thickness) through the chest at the level of the heart (H) shows a large high-signal-intensity heterogeneous mass (curved arrows) arising from the right lung and crossing the midline. A small amount of normal right lung (black arrow) can be seen anterior to the heart, which is shifted to the left. The left lung (open arrow) can be seen posterior to the heart. There also is marked subcutaneous edema (arrowhead), consistent with hydrops. Axial RARE MR image (4.4/64 effective, 120 flip angle, 6-mm section thickness) obtained through the chest at the level of the heart (H) at gestational age of 32 weeks, 3 weeks after in utero removal of the CCAM lesion, shows marked growth of both lungs (long arrows). A small amount of right-sided pleural fluid (short arrow) is present.,3支气管肺隔离症(BPS)：,BPS是支气管的前肠畸形，叶外型是产前最常见的类型。这些病灶与正常支气管树缺乏联系，多见于左后基底段。在MRI上，病灶比正常肺组织信号明显要高、均匀、边界清楚。US可以诊断本病，但不能显示病灶本身，MRI对中等大小的病灶也能显示，但MRI不能显示BPS的供血动脉，与US相比是为不足。,Figure 19. Bronchial stenosis of the right middle lobe in a fetus of 23 weeks gestational age. Axial RARE MR image (4.4/64 effective, 120 flip angle, 6-mm section thickness) through the chest at the level of the heart (H) shows shift of the heart to the left side of the chest. The right middle lobe (curved arrow) is enlarged. A normal vessel can be seen coursing through the right lung (arrowhead). The markedly enlarged right middle lobe has homogeneous signal intensity that is only minimally higher than that of the left lung