Ultrasonography is the first choice for screening fetal structural malformations.
Ultrasonography is non-invasive, fast, real-time, inexpensive and accurate, Compared with other complicated imaging examinations, ultrasound examination has its irreplaceable advantages. However, ultrasound examination also has its disadvantages: It is difficult to pass through the fetal skull, the visual field is small, the examination results are often not satisfactory when there are maternal obesity, maternal hysteromyoma, oligohydramnios, multiple births, etc. Moreover, ultrasound examination is affected by the fetal position and often requires a long time or multiple examinations. In addition, ultrasound diagnosis is more or less limited by the operator’s technical level.
Fetal magnetic resonance imaging (MRI) is a very important supplementary method for obstetric ultrasound examination.
In 1983, Smith et al. Reported MRI examination of fetus for the first time. Fetal MRI has developed rapidly in recent years, With the development of fetal intrauterine surgery and neonatal surgery in delivery room, Fetal MRI can make further targeted diagnosis for babies with doubts about ultrasound examination, can make timely and correct diagnosis and classification of diseases in the fetal period, and help formulate appropriate follow-up surgical plans. In addition, fetal MRI can also help mothers to choose delivery methods.
Is MRI safe during pregnancy?
Due to the existence of ionizing radiation, X-ray examination and CT examination are generally not recommended during pregnancy, but MRI examination is recommended. Because MRI is safe for the fetus and will not have adverse effects on fetal development, both from the imaging principle and from the evidence of evidence-based medicine.
Advantages of fetal MRI
There is no ionizing radiation; High soft tissue resolution; The field of vision is large, and can be scanned on any section to display the whole picture of the fetus. It is less affected by the mother’s situation. It is not affected by fetal bone and amniotic fluid volume. It has high accuracy in the diagnosis of fetal central nervous system diseases, oligohydramnios-related diseases and thoracic diseases. The image has good intuition and resolution in other parts, and can quantitatively evaluate pulmonary hypoplasia. For twins and large gestational weeks, the image display is obviously better than ultrasound examination, especially when the fetal head enters the pelvis or the fetal skull ossifies in the third trimester of pregnancy. Fetal MRI can scan any section and accurately carry out various measurements. It has a good characterization of fetal diseases and can also carry out functional imaging such as magnetic resonance spectroscopy (MRS). MR images are less affected by the technical level of examination operators and are convenient for preservation and consultation.
Recommended Indications for Fetal MRI
1. The first choice of indications and the greatest diagnostic value are fetal central nervous system abnormalities: Such as brain dysplasia, obvious widening of lateral ventricle or posterior cranial fossa, Dandy-Walker malformation, corpus callosum dysplasia, Chiari malformation, cerebral hemorrhage, brain tumor, hydrocephalus, anencephaly, anfissure of forebrain, neural tube insufficiency, meningocele, etc. It is worth mentioning that, When the fetal corpus callosum is dysplasia or absent, Ultrasound often indicates bilateral lateral ventricle widening, but it is impossible to distinguish whether corpus callosum exists. At this time, fetal MRI can help to diagnose whether lateral ventricle widening is caused by corpus callosum dysplasia, or whether simple lateral ventricle widening slightly without brain parenchyma dysplasia, which is crucial to pregnancy outcome.
2. Fetal maxillofacial deformities: such as cleft lip, cleft palate, orbital dysplasia or eyeball loss, etc., can be further confirmed and supplemented by fetal MRI, and whether other related deformities are combined can be found at the same time.
3. Fetal chest tumor lesions: Ultrasonography showed that the fetal lung echo was enhanced, The most common is fetal lung cystadenomatoid malformation (CCAM) or isolated lung. Both are benign lesions but are often difficult to distinguish. Fetal MRI can be scanned by various sections to determine the source of blood supply in abnormal lesions, thus determining the nature of lesions, because some CCAMs are self-limiting, i.e. Disappear automatically after birth without treatment. Isolated lungs are divided into intrapulmonary type and extrapulmonary type, which can be treated surgically after birth.
4. Fetal abdominal tumor lesions: This kind of situation is often difficult to determine, fetal MRI can determine the source of the lesion, thus further defining the nature of the lesion. For example, hepatic hemangioendothelioma, renal mesodermal nephroma, etc. Cystic lesions include choledochal cyst, digestive tract duplication malformation, (female) ovarian benign cyst, etc.
5. Abnormal fetal genitourinary system: Ultrasound found that the fetal renal pelvis was separated, MRI can further clarify the cause of fetal hydronephrosis, Such as ureteropelvic junction obstruction (UPJO), bladder outflow tract obstruction, ectopic ureter opening, etc. Fetal renal dysplasia such as horseshoe kidney, ectopic kidney, polycystic dysplasia kidney (MCDK), infantile polycystic kidney, MRI can be used to further judge renal function. If ultrasound does not see renal echo in the renal region, fetal MRI can judge whether fetal renal dysplasia or (pelvic) ectopic kidney through different section scanning and diffusion function imaging.
6. Fetal gastrointestinal abnormalities: MRI examination can be used to further clarify the diagnosis and confirm the further treatment plan. These abnormalities mainly include congenital esophageal atresia, duodenal atresia, small intestine and colon obstruction or atresia, anal atresia, meconium peritonitis, etc.
7. Abnormalities of fetal limbs: Fetal MRI can help further identify these abnormalities, For example, to confirm whether it is equinovarus or only temporary varus, Fetal MRI can determine whether the cystic focus near the limb is edema caused by amniotic band or soft tissue lymphangioma due to limb absence or hypoplasia, polydactyly/toe or polydactyly/toe malformation. If the fetal lower limbs are found to be unclear or fused, fetal MRI can help judge whether the baby changes posture mischievously or is a sign of human fish sequence.
8. Fetal spinal abnormalities: such as spina bifida, myelomeningocele, hemivertebra, tethered cord, sacrococcygeal teratoma, etc. Fetal MRI can be clear at a glance and is justified.
9. Fetal cardiac abnormalities: MRI diagnosis of fetal cardiac abnormalities is relatively complicated, and it is better to go to a pediatric hospital with a better cardiac specialty for examination.
10. Twin (multiple) fetuses: IVF treatment often yields twins or triplets, and problems follow. For example, Twin Transfusion Syndrome (TTTS), Twin Reverse Arterial Perfusion Sequence Syndrome, Twin/Multiple Death, Fetal MRI can help monitor the growth and development of the intrauterine fetus, observe whether the central nervous system of the surviving fetus is abnormal, and which one is lost.
11. Other circumstances requiring fetal MRI examination: such as obesity of pregnant women, polyhydramnios/oligohydramnios, and complex malformation, fetal head entering pelvis in the third trimester of pregnancy, ossification of fetal skull, placenta previa, placental abruption (hemorrhage), placenta accreta, scar pregnancy, uterine malformation of pregnant women, fetal intrauterine growth restriction, fetal gastroschisis and omphalocele, etc.
At present, fetal MRI has been applied more and more. It has become an important supplementary means of obstetric ultrasound examination. It can also provide additional information that cannot be found by ultrasound. The noise of fetal MRI examination is relatively loud, and the baby in the uterus will often have obvious fetal movement in the examination. During fetal MR examination, pregnant women generally do not use sedatives and contrast agents, and should try to avoid scanning within 3 months of pregnancy (early pregnancy). MRI images of fetus after 4 months of pregnancy are easy to understand.
Conclusion: Ultrasound is the first choice for screening fetal malformations. For some organ and system abnormalities (especially the central nervous system), fetal MRI can help to further diagnose.
Author: Duan Tao
The article was reprinted by Clove Garden authorized by the author.