Chapter 46: MAGNETIC RESONANCE IMAGING IN OBSTETRICS

When first used in the mid-1980s, magnetic resonance (MR) image acquisition was slow, necessitating maternal sedation and even temporary fetal paralysis with intramuscular injection to limit motion artifact. The last few decades have brought many technological advances allowing very fast MR acquisitions. Newer vendor-specific protocols have been used to effectively image the fetus. Images are acquired in 1 second or less, eliminating the need for sedation. Motion artifact, although still present, is significantly reduced. MR hypothetically produces superior image resolution to ultrasonography, based on its underlying technology, and is not as hindered by bony interfaces, maternal obesity, oligohydramnios, or an engaged fetal head. Referral for MR evaluation is most frequently encountered in the setting of suspected fetal central nervous system (CNS), thoracic, genitourinary, or gastrointestinal disorders. Maternal indications include evaluation of suspected pelvic masses, placental invasion, pelvimetry, pelvic floor, and cervix.

A current commentary of the American College of Obstetrics and Gynecology was published summarizing the National Institutes of Health Workshop on fetal imaging technology, including present and future fetal MR applications, which are discussed below.¹

A primary concern to referring physicians and their patients is the safety of both mother and fetus. MR utilizes no ionizing radiation, but questions arise regarding fluctuating electromagnetic fields, high sound intensity levels, and fetal heart rate patterns during the procedure.

Several animal and tissue studies have been conducted to determine the biologic effects of electromagnetic fields with many conflicting reports. An early study looked at the long-term effects of repetitive exposure to a static 1.5-tesla (T) magnetic field on human lung fibroblasts.² Study and control group proliferation was similar, indicating no adverse effect from repetitive MR exposure. Human studies are rare. A large epidemiologic retrospective study compared spontaneous abortion rates, infertility rates, incidence of low birth weight, and premature delivery among nurses and technologists working with MR before and after employment.³ There were no increased incidences of adverse outcomes in the MR-exposed group. Two studies reported follow-up on children exposed to echo-planar MR in utero. The first by Baker et al completed a 3-year follow-up on 20 children with no demonstrable increase in the occurrence of disease or disability.⁴ These patients were imaged with a 0.5-T superconductive magnet from 21 weeks to term. In the second case-controlled prospective observational study of 20 infants exposed to echo-planar imaging, pediatric assessments completed on these infants at 9 months of age were normal.⁵

In reference to the sound intensity and exposure, one study assessed the sound level experienced by the fetal ear during an MR procedure by having a volunteer swallow a microphone connected to a thin lead and filling the stomach with a liter of fluid to represent an amniotic sac.⁶ There was at least a 30-dB attenuation in intensity from the body surface to within the fluid-filled stomach, which reduced the acoustic sound pressure down from the dangerous threshold of 120 dB to an acceptable level less than 90 dB. This level is lower than the 135 dB experienced when vibroacoustic stimulation is used. No evidence of hearing loss was found in one study after 450 babies were exposed to this noise intensity from vibroacoustic stimulation.⁷ Studies have also looked at fetal heart rate patterns during MR procedures and reported no change in heart rate patterns or incidence of fetal movements.⁸

Recommendations from a recent document from the American College of Radiology concerning safe practices for MR have determined that MR can be performed in pregnancy if the data are needed to affect the care of the fetus or mother during pregnancy.⁹ Furthermore, because of the potential for dissociation of the chelate molecule in the amniotic fluid, MR contrast agents should not be routinely administered. The longer the chelate molecule remains in a protected space such as the amniotic sac, the greater the potential for dissociation of the potentially toxic gadolinium ion, manifest as in nephrogenic systemic fibrosis in at-risk adults.⁹ It is also recommended that women undergoing MR imaging during pregnancy provide informed written consent documenting that they understand the potential risks and benefits of the procedure.

All fetal and maternal studies are performed on a 1.5-T magnet or less, and it is recommended that informed consent be obtained for all patients in the settings of either institutional review board (IRB)-approved protocols or as a clinically indicated procedure. In addition, all women complete a written MR safety screening questionnaire including information regarding metallic implants, pacemakers, or other metal- or iron-containing device that may impact the study.⁹ Iron supplementation may rarely cause artifact in the colon but does not usually affect the fetal resolution. Portable devices and monitors must be evaluated using U.S. Food and Drug Administration labeling criteria to prevent ferrous objects from entering unsafe zones within the MR areas.⁹

In over 1500 MR procedures performed by us during the past 8 years, maternal anxiety secondary to claustrophobia and/or fear of the MR equipment has occurred in less than 1% of our patients. To reduce maternal anxiety in this small group of women we prescribe either oral Valium (5 to 10 mg) or Ativan (1 to 2 mg) for sedation, and an obstetrician is in attendance. Otherwise, anxiolytics are not routinely employed. The recommended weight limit is 350 pounds on our current MR unit; therefore, the extremely obese woman may not be a candidate for MR imaging.

Women are placed in the supine or right lateral decubitus position. A torso coil is used in most circumstances, with the occasional use of either the body or cardiac coil depending on maternal size, fetal size, and area of interest. A series of 3-plane localizers are obtained relative to the maternal coronal, sagittal, and axial planes. In every fetal case, the gravid uterus is imaged in the maternal axial plane (7-mm slices, 0 gap) with a T2-weighted fast acquisition, typically a single-shot fast spin echo sequence (SSFSE), half-Fourier acquisition single-shot turbo spin echo (HASTE), or RARE (rapid acquisition with relaxation enhancement) depending on the brand of machine and manufacturer (Tables 46-1 and 46-2). These acquisitions are particularly good in identifying the fetal and maternal anatomy. Next, a fast T1-weighted acquisition such as spoiled gradient echo (SPGR) is performed (7-mm thickness, 0 gap). The pertinent fetal and maternal anatomic parameters identified on these sequences are noted on the MR patient report.

Orthogonal images of targeted fetal or maternal structures are then obtained. In these cases, 3- to 5-mm slice thickness, 0 gap T2-weighted acquisitions are performed in the coronal, sagittal, and axial planes (Figure 46-1). Depending on the anatomy and underlying suspected abnormality, T1-weighted images can be performed to evaluate for subacute hemorrhage, fat, or location of normal structures that appear bright on these sequences, such as liver and meconium in the colon (Figure 46-2).^10,11 Short T1 inversion recovery images (STIR) may provide differentiation in cases where the water content of the abnormality is similar to the normal structure, such as thoracic masses and normal lung. Occasionally, fluid-attenuated inversion recovery (FLAIR) images may be obtained in the case of central nervous system abnormalities to evaluate the ventricular system and other cerebral spinal fluid–containing spaces, and diffusion-weighted imaging (DWI) may be employed to evaluate for restricted diffusion and ischemia.^10,11

Our series will also include an axial brain 3- to 5-mm T2-weighted sequence to obtain head biometry for the purposes of assigning an approximate gestational age from the biparietal diameter and head circumference in all cases (see Figure 46-1A).¹² The thickness of the slice should be applied based on the size of the fetus, size of the area of interest, and considerations of the trade-off between the potential of improved conspicuity and signal-to-noise ratios. The smaller the slice thickness, the less the signal-to-noise ratio that may affect resolution.

Some nomograms of the head have been proposed, which include only the brain and none of the bony calvarium as a better indictor of actual brain volume.¹³ For the purpose of gestational age assignment, we prefer the more standard inclusion of the skull, as defined by historic sonographic nomograms.^14,15 The brain itself can be measured when there is an abnormality and suspected parenchymal loss.

The most common indication of the fetal MR evaluation is for suspected central nervous system abnormalities on sonographic evaluation and accounts for approximately 70% of our practice. MR images of the fetal CNS are hypothetically superior to that of ultrasound because of the improved resolution. Ultrasonography remains the modality of choice for diagnosing CNS abnormalities. However, MR is a very useful adjunct in the antenatal diagnosis of some suspected CNS anomalies. Fast T2-weighted images produce excellent tissue contrast, and cerebrospinal fluid–containing structures are bright. This allows exquisite detail of the posterior fossa, midline structures, and cortex. Near-field attenuation secondary to the fetal skull on ultrasonography is not encountered with MR and allows the easy determination of bilateralism of CNS lesions.

Another major advantage of MR is the ability to acquire images in the axial, coronal, and sagittal planes in reference to the fetus or the maternal pelvis. Sagittal fetal images are very helpful, for example, in evaluating the corpus callosum in cases referred for mild ventriculomegaly (Figure 46-3). T1-weighted images are also used to assess whether a CNS lesion contains fat or represents hemorrhage (Figure 46-4).

Similar to sonographic scans, CNS biometry can be measured and is routinely done in our cases. Routine measurements include biparietal diameter, occipital frontal distance, cerebellar width, cisterna magna depth, and bilateral atrial measurements.^12,16,17 Ventricle and cisterna magna measurements on 60 fetuses beyond 14 weeks to term gestation found atrial measurements to be slightly smaller on MR compared with ultrasonography, but determined the abnormal sonographic cutoff of greater than 10 mm was greater than 2 SD above the mean on MR (Figure 46-5).¹⁶ Cisterna magna measurements were gestational age dependent with cutoff values less than 4 and greater than 11, also similar to ultrasonography (Figure 46-6). Nomograms for multiple components of brain biometry, including the corpus callosum length and cerebellar vermis, have also been published.^13,18 The measurements of corpus callosum and vermis lengths are routinely done on all MR studies of the central nervous system (see Figure 46-1C).

Cortical maturation can be evaluated by MR images, and cerebral gyration and sulcation patterns reflect the embryologic development.^19,20 Fetuses with a CNS abnormality may have significant lag time in cortical development.¹⁹ Ultrasonography is very limited in evaluating very subtle migrational abnormalities at early gestational age. These migrational CNS disorders such as Walker-Warburg are problematic, but MR is becoming more reliable in assessing for cortical maturation, especially later in gestation (Figure 46-7).^{20,21, and 22}

Four large studies assessed the second-opinion MR examination in the setting of a suspected CNS abnormality demonstrated on ultrasonography in regard to its ability to confirm, change the diagnosis, and possibly alter clinical management. In the first study with 66 cases of abnormalities found on confirmatory sonogram, MR added additional information in 57.6% of cases, and changed the diagnosis in 40% of cases.²³ Clinical management was clearly changed in 9 cases. In 73 MR examinations performed for CNS anomalies, another group found that 46% of pregnancies were managed differently than the way they would have been managed if the diagnosis had been based only on a sonographic basis.²⁴ Additional information was provided in 46 of 72 pregnancies (64%), and the diagnosis was changed in 20 of those 46 cases. Clinical management was altered in 8 cases. MR was more likely to confirm sonographic diagnosis prior to 24 weeks, but beyond 24 weeks a change of diagnosis or additional information was gleaned more frequently.²⁵ A British study confirmed the findings of the other studies, where MR imaging either changed the diagnosis or gave additional information that altered management in 35 of 100 cases.²⁶

Presently, the most common reason for fetal MR referral is isolated ventriculomegaly, which may be associated as a highly variable outcome.^{27,28,29, and 30} Additional abnormalities commonly associated with ventriculomegaly may not be defined on sonographic examination. The most common change in diagnosis was from marked ventriculomegaly to a more precise diagnostic category, such as aqueductal stenosis or hydrancephaly (Figure 46-8), and from mild ventriculomegaly to more specific diagnoses, including agenesis of the corpus callosum and migrational abnormalities (Figure 46-9). A more exact diagnosis impacts patient counseling and, to a lesser degree, clinical management.^{25,26,27,28,29,30, and 31} This finding was confirmed in a recent paper, which found that MR imaging provided important information in the setting of ventriculomegaly, especially in fetuses with other findings of the central nervous system by sonographic evaluation.²⁹ At the present time there is considerable variability in the central nervous system diagnosis in the setting of ventriculomegaly secondary to modality differences (ultrasonography or MR) and observational errors. Agreement was 60% with sonography and 53% with MR in this series.³¹

MR evaluation of the fetal spine has been performed. In one series additional spinal cord abnormalities were seen on 10% of cases in the setting of sonographically detected bony anomalies of the spine.³² Two of the 3 defects were diastematomyelia, and one was in the setting of lumbosacral myelomeningocele. The third was a segmental spinal dysgenesis.

MR is used in the setting of thoracic masses and abnormalities not only to aid in diagnosis, but to delineate the extent and location of the mass and to determine the volume of remaining lung tissue. As further discussed in Chapter 13, congenital cystic adenomatoid malformations (CCAM), bronchopulmonary sequestrations (BPS), congenital diaphragmatic hernias (CDH), chylothoraces, and abnormalities leading to pulmonary hypoplasia, such as renal and skeletal dysplasias, are commonly referred for MR evaluation. MR does help identify morphologic features of chest lesions, evaluates their size, and evaluates volume of normal lung tissue (Figure 46-10). The tissue contrast between normal lung tissue and thoracic lesions is excellent in most cases. Certain acquisitions such as STIR are needed in some cases to distinguish sequestrations and adenomatoid malformations from normal lung tissue (Figure 46-11).³³

MR is an excellent adjunct to sonographic diagnosis of CDH to determine the position of the liver and which abdominal contents are contained within the chest.^{34,35,36,37,38, and 39} Similar to sonography, authors have looked at predictive indicators of lethal pulmonary hypoplasia and the need for extra membranous oxygenation (ECMO).^34,35 Calculated fetal lung volumes by MR have been found to be predictive of pulmonary hypoplasia in some series,^{36,37, and 38} but other series did not find it to be predictive of lethal pulmonary hypoplasia.^37,38 Evaluation of the liver involvement in the hernia either by qualitative or quantitative methods by MR has been found to be particularly useful (Figure 46-12).^33,39,40

Organ volumes are determined by first outlining all areas of tissue on successive MR images. Volume is then determined by summing the area of each region of interest and multiplying by the slice thickness of the acquisition. In addition, findings of CDH noted above the lung volume/gestational age ratio is significantly different between lethal and nonlethal groups in the setting of renal disorders, and may aid in patient counseling.⁴¹

Ultrasonography can be used to detect many genitourinary abnormalities and is the primary diagnostic modality of choice. However, oligohydramnios and/or maternal habitus may hinder the sonographic examination, or a complex anomaly may need further assessment. In a study involving 24 fetuses with either oligohydramnios or suspected urinary tract abnormality, MR evaluation alone made the diagnosis in 8 cases, and added information in 5 of 12 cases of oligohydramnios and 3 of 10 cases with normal amniotic fluid.⁴² Another series found MR to be an excellent technique for revealing the complex anatomy of the genitourinary system.⁴³

Determining the origin of a cystic mass within the fetal abdomen may be difficult especially in the third trimester. MR can help differentiate genitourinary from gastrointestinal abnormalities based on the different appearance of the colon and the genitourinary tract found on T1- and T2-weighted images.^{44,45,46, and 47} Very precise MR characteristics of the abdominal solid organs, gastrointestinal tract, and genitourinary system have been described.^47,48 In one series, the signal intensity of meconium in the fetal colon and urine in the fetal bladder were evaluated in 80 fetuses (Figure 46-13).⁴⁸ These characteristic signals by MR were most helpful after 24 weeks. In 17 of 29 cases referred for either suspected gastrointestinal or genitourinary system abnormalities, T1-weighted images added additional information. The lack of signal in a contracted bladder on T2-weighted sequences was significantly associated with lethal renal abnormalities (P < .001) and was found to be very helpful in the setting of renal agenesis and oligohydramnios (Figure 46-14).⁴⁹

As more fetal abnormalities have been considered candidates for antenatal surgery, MR has emerged as the modality of choice for preoperative and follow-up evaluations. The role of fetal surgery is beyond the scope of this chapter, but some of the MR contributions to diagnosis and follow-up will be discussed.

The most common indication for fetal surgery presently in the United States is ablation of vascular anastamoses in the setting of monochorionic twin-to-twin transfusion syndrome.^{50,51, and 52} MR is often employed to asses for fetal complications of this syndrome, which are most often manifest in the central nervous system.^53,54 Monochorionic twin gestations are referred to assess for the possibility of hemorrhage or periventricular leukomalacia, in the setting of twin-to-twin transfusion syndrome or in the case of demise in one twin. This is particularly important when placental ablation of the vascular anastamoses is being considered (Figure 46-15).

As noted above, MR may provide important information in the setting of congenital diaphragmatic hernias and masses of the lungs. Criteria for type and timing of fetal surgery may be affected by MR findings, such as degree of liver involvement, size of the lesion, and amount of normal lung volume.

Sacrococcygeal teratomas are also candidates for fetal surgery. MR may aid in determining the degree of intrinsic involvement and help stage these lesions (Figure 46-16).^55,56

Airway issues arise in a number of fetal abnormalities. Fetal giant neck tumors, either lymphangiomas or teratomas, can be treated with fetal surgery or undergo an EXIT (ex utero intrapartum treatment) procedure at the time of delivery. MR is very helpful in defining the extent of these lesions as well as their affect on the oral and hypopharnyx (Figure 46-17).^{57,58, and 59} Micrognathia is an abnormality that may necessitate an EXIT procedure prior to delivery (Figure 46-18).^60,61 Congenital high airway obstruction syndrome (CHAOS) is another candidate for an antenatal procedure or EXIT. MR is very helpful in defining the level of the lesion as well as the dramatic findings in the chest and abdomen, including hyperexpanded lungs and ascites (Figure 46-19).^60,61

Fetuses who are candidates for the ongoing management of myelomeningocele study (MOMS) clinical trial will undergo a fetal MR to evaluate the fetal brain. A large trial demonstrating improvement of the subarachnoid space by MR after fetal surgery was the rationale for considering this technique.^62,63 Fetal MR can consistently demonstrate the abnormal findings of the Arnold-Chiari II malformation (Figure 46-20).^{62,63,64, and 65} In one series, MR and ultrasonography were equally accurate in assignment of the lesion level of the defect, but were off at least one level at least 20% of the time.⁶⁵ This agrees with another study, which found no additional information between these 2 modalities before 24 weeks.²²

Diagnosing placenta accreta, increta, or percreta is important to prepare for the potential of significant hemorrhage at delivery and to counsel patients effectively. Ultrasonography is adept at diagnosing myometrial invasion, and MR evaluation of the placenta has been considered as an adjunct to ultrasonography when cases are indeterminate.^66,67 Many reports describe the usefulness of MR in cases of placental invasion. One report compared the accuracies of transabdominal and transvaginal gray-scale ultrasonography with color Doppler sonography, power Doppler sonography, and MR imaging. Transvaginal gray-scale ultrasonography was found to be adequate in most cases, specifically diagnosing 6 out of 7 cases of placenta accreta. The remaining case was a posterior placenta accreta diagnosed accurately by MR. MR based on this report appeared to be helpful only in cases where the placenta was out of range of both transabdominal and transvaginal sonography.⁶⁵ One large series compared MR to ultrasonography and found MR to be superior in the evaluation of placental invasion of the myometrium (sensitivity 0.88, specificity 1.00), compared to ultrasonography (sensitivity 0.77, specificity 0.96).⁶⁶ This series employed gadolinium intravenous contrast, and although it was not applied to the criteria for invasion, some have questioned its safety in pregnancy. Our experience in the evaluation of invasion by MR has been less promising, since the diagnosis with ultrasonography is excellent (Figure 46-21). With sonographic imaging, using color-flow mapping to demonstrate large intraplacental lakes in the setting of myometrial thickness less than 1 mm, the sensitivity for myometrial invasion was 100%, with a corrected specificity of 90%.⁶⁸ One could argue that sensitivity of 100% would be the desired outcome in this setting.

As mentioned previously, MR may be used to determine the volume of specific organs and potentially reconstruct a 3D image. A study of 4 women who underwent serial MR imaging showed that the volume of the fetus, and fetal liver, lungs, and brain could be determined with relative ease.⁶⁹ The first study to use fetal volume to determine fetal weight and compare it to sonographic estimated weight prior to delivery was performed in 11 women.⁷⁰ The median difference between actual and estimated weight was 3% for MR and 6.5% for ultrasonography, (P < .01; R = 0.97). The same authors found that the liver volume was smaller in growth-restricted fetuses and saw the potential for more accurate estimation of weight by MR volumetry.⁷⁰ Other small studies have confirmed the accuracy of MR to determine weight.^71,72,73 The main variables that differ between these studies are the methods of volume determination and the presumed fetal density. Equations ranged from 1 g/mL × volume, 0.12 + 1.031 g/mL × volume, and 1.07 g/mL × volume. The basic method involves an MR acquisition with a slice thickness of at most 8 to 10 mm through the entire fetus. In each image the area of the fetus or region of interest is calculated either by using specific software or manual tracings. The areas are summed and then multiplied by the slice thickness. This gives the fetal volume, which is multiplied by the presumed fetal density. A recent study tested the accuracy of MR weight estimation in comparison with sonographic weight estimation by an experienced examiner (Figure 46-22).⁷³ A total of 80 women were enrolled and underwent both MR and sonographic procedures within 3 hours of delivery. Baker's equation of 0.12 + 1.031 g/mL × volume was used. The correlations were 0.95 (0.92, 0.97) and 0.85 (0.77, 0.90) for MR and sonography respectively, P < .0001. Axial 8-mm, coronal 3-mm, and sagittal 5-mm acquisitions were part of the study protocol resulting in a total magnet time of around 15 minutes. Subsequent post-processing time to determine volume was on average 30 minutes. Using a smaller slice thickness did not improve the 3% absolute error, and for future purposes we will use an 8-mm slice thickness.⁷⁴

As previously noted, an axial image of the entire gravid uterus is routinely obtained by us prior to obtaining orthogonal fetal imaging results in a complete fetal survey, and is performed regardless of the indication for the procedure. Several reports have been published looking at normal fetal anatomy as seen on fetal MR, but no comparisons have been made to ultrasonography.^{75,76, and 77} One study applied the American College of Obstetrics and Gynecology recommendations for prenatal ultrasonography to this initial sequence and found 85% of the anatomy was adequately visualized.⁷⁸ This same group subsequently evaluated term fetuses utilizing axial, coronal, and sagittal MR acquisitions, and found that 99% of the required anatomy was visualized, if the fetal heart was excluded (Figure 46-23).⁷⁹ In both studies, it was no surprise that the nonfetal components including the placenta, qualitative assessment of amniotic fluid, and the maternal pelvis were visualized every time. MR performs better beyond 20 weeks because of increasing embryonic development and larger fetal size.

MR Pelvimetry

Pelvimetry has been used in obstetrics for the assessment of pelvic capacity prior to the delivery of a breech fetus. Several studies have evaluated its clinical usefulness and found it to be of little value in this setting. MR pelvimetry was first evaluated in 1985. A large randomized trial of MR pelvimetry in breech-presenting fetuses found that cesarean section rates were equal in the pelvimetry group versus controls.⁸⁰ Interesting studies have been performed on predicting labor dystocia and difficult vaginal delivery by using MR pelvimetric data along with fetal measurements.^81,82 One study evaluated several methods to predict cephalopelvic disproportion (CPD) using ultrasonography and MR pelvimetry in 38 women at risk for dystocia. Overall positive predictive value was between 41% and 59% for CPD. A recent study of fetal and maternal pelvic volumetry performed on 101 women found several MR-determined maternal pelvic measurements to be associated with women who were at risk to develop dystocia (Table 46-3), but this did not accurately predict the ones which require a cesarean delivery (Figure 46-24).⁸³

Maternal Complications of Pregnancy

Because it does not employ harmful radiation, MR imaging has become the procedure of choice when diagnosing many maternal complications, including central nervous system findings of pregnancy-induced hypertension, acute-onset abdominal pain, and suspected pelvic masses, to name a few.^{84,85,86, and 87} A comprehensive review is beyond the scope of this chapter, but MR has emerged as the primary imaging modality for evaluation of suspected appendicitis and when the origin of a pelvic mass in pregnancy cannot be determined with certainty by ultrasonography alone.^88,89

Pelvic Floor

The effect of pregnancy on the female pelvic floor has been the subject of extensive research in the field of gynecologic urology. MR evaluation of the pelvic floor has been performed, with the intention of defining the pathology.^{90,91,92, and 93} A comprehensive review of this literature is not intended in this chapter, but one study did look at the pelvic floor in term nulliparous women with uncomplicated pregnancies with MR.⁹⁴ This study described the levator ani muscle anatomy using three-dimensional (3D) MR post-processing technique in 84 women. They found the levator ani muscle was well preserved in 93% of women, but that the morphometry was variable in that the shape was characterized as U-shaped in 53% and V-shaped in 47% of these term nulliparas (Figure 46-25). It was concluded that pregnancy itself is not related to pelvic floor issues, but more likely the stresses of labor and delivery.

Cervix

MR has been utilized to evaluate the gravid cervix.^{95,96,97, and 98} One of the roles has been to determine the relationship of the MR appearance of the cervix to cervical ripeness (Figure 46-26).⁹⁸ A large study found no correlation to the MR T2 relaxation time of the cervix and the clinical Bishop Score or labor outcome. Based on the present T2 relaxation time applications, MR did not appear useful in the assessment of cervical ripening.⁹⁹

As MR acquisition times improve and technology allows for better resolution of structures and movement, there are 3 potential directions that fetal imaging may proceed. One is volume acquisition and 3D post-processing. This would occur similarly to spiral computed tomography (CT) with multiple channel detectors, where one large high-resolution image can be processed in any plane. The previously mentioned pelvic floor study looked at the bony and soft tissue structure of the maternal pelvis, utilizing 2-mm, 0 gap acquisitions and subsequent 3D planar postprocessing.⁸³ Applying this to the fetal anatomy of interest would require faster acquisition times, the ability to correct for fetal motion, and improved field of view options. New vendors have been developing these systems. An interesting study looked at combined orthogonal views that created a volume of the fetal brain with subsequent post-processing of improved images, which corrected for motion and limited resolution.¹⁰⁰

The second direction would be the real-time MR for evaluation of the fetal heart and other moving structures. A feasibility study reviewed MR studies of the chest to ascertain certain components of the fetal heart.¹⁰¹ They found, among other things, that T2-weighted true FISP (fast imaging with steady state precession) was helpful in defining certain atrial ventricular, conal truncal, and venous relationships. This technique would have to be coupled with an acquisition that would capture the anatomy in some predictable fashion when the fetal heart rate is 120 to 160 beats per minute.

Finally, many of the already established acquisitions that assess physiological functions may be explored in the fetus. This includes spectroscopy for issues of brain and pulmonary maturity, which has been discussed in small series.^102,103 Other possibilities include diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) for evaluation of the fetal central nervous system^{104,105, and 106} and placental hypoxemia/ischemia (Figure 46-27).¹⁰⁷ DWI and ADC mapping has the added advantage in the fetal brain of providing diffusion tensor and white matter track imaging.^{104,105, and 106} Many of these emerging technologies are still in the experimental stage and should undergo very rigorous investigation before their clinical application becomes recommended.

Highlighted References