subject: MRI of the Fetal Chest [print this page] MRI of the Fetal Chest MRI of the Fetal Chest
In most instances ultrasound examination is excellent to evaluate anomalies of the fetal abdomen and pelvis. There areinstances, however, when MRI can be helpful. In fetuses with oligohydramnios, ultrasound evaluation can be extremelydifficult, whereas fetal MRI imaging is unaffected by a lack of amniotic fluid. It is often difficult to distinguish proximal fromdistal small-bowel obstruction on prenatal ultrasound examination. There are different MRI signal characteristics that arehelpful in these cases, distinguishing proximal from distal small bowel.
Proximal bowel willhave high to intermediate signal intensity on T 2-weighted sequences, similar to amniotic fluid, while distal bowel hasintermediate to low signal intensity. Conversely, distal bowel has a high signal intensity on T 1-weighted sequencesbecause of meconium filling the bowel lumen. Using these sequences, dilated loops of bowel may be distinguished aseither proximal or distal.
The imaging of the fetal urinary tract with ultrasound examination is excellent and rarely improved on by MRI. Exceptionsto this include cases of obstructive uropathy complicated by oligohydramnios, polycystic kidneys, and renal tumors. Thisis also true of sacrococcygeal teratomas (SCTs), which arise from Hensen's node at the tip of the coccyx. SCTs are mostcommonly exophytic, but can also extend into the pelvis or abdomen with compression of bladder and intestines.Fetal MRI is not indicated as a primary imaging method in any fetal anomaly or condition. As discussed above, however,there are instances in which the information provided by fetal MRI complements that obtained by prenatal ultrasound examination. The role of MRI in prenatal diagnosis is still evolving. At present, MRI is best used selectively in cases inwhich prenatal sonography is unable to make a definitive diagnosis, where a larger field of view is required, or inacquiring specific information necessary for selection for fetal intervention.
Real-time interventional MRI is currentlyavailable in a select few centers, and it is anticipated that reports of its use in fetal therapy will soon appear.MRIs in CCAM vary in appearance, depending on their size and number and size of cysts. The larger the size andnumber of cysts, the higher the signal intensity of the CCAM on sequences with T 2-weighting. MRI is able to distinguishCCAM from normal compressed lung and determine the lobe of the lung from which the tumor arises. BPS has very highsignal intensity as compared with normal lung and is very homogeneous, with discrete margins.
