Computer-assisted imaging of the fetus with magnetic resonance imaging

Computerized Medical Imaging and Graphics, Vol. 20, No. 6, pp. 491-496, 1996 Copyright 0 1996 ElsevierScienceLtd. All rights reserved Printed in Great Britain 089%111/96 $15.00 + .OO

Pergamon

PII: sog95-6111(!x)oou4~

COMPUTER-ASSISTED

IMAGING OF THE FETUS WITH MAGNETIC RESONANCE IMAGING* Patrick M. Colletti

USC School of Medicine,

LAC/USC Imaging Science Center, 1744 Zonal Avenue, Los Angeles, CA 90033, USA (Received

9 February

1996)

Abstract-The purposeof this paperis to reviewthe useof magMic reuomu~ imaging(MRI)oftbefetasaadto proposefatnre tedmiqws and applications.lnstitn~nal re!viewboard apprWedMB image8of the fetns were acquiradia66patieatswithsoaog+hUy saspe&dfetalabnorn&tks.Axial,eoraacll,aad short TE imageswere obtalnd In addition, 12 stadie8were@onBed wftb rapid scans

give addltionaIlnfonaathmto that of sonograpbyin fetal aaomalks,par&aMy tbo8elilvoIvblgtbeeentmlnervous system,andia the detectionof fat, blood,and mecoahm.MRI of tbe fetuscan demonstratenormaland abnormal stnxtares. Newerte&a@rs with faster hagiag will allowfor greaterpossMity of computerassisted manipulation of data. Copyright 0 1996ElsevierScienceLtd. Key

Words: MRI, Fetus; Fetus, MRI; Fetal MRI, computer processing; Fetal MRI, 3D acquisition

INTRODUCTION

TECHNIQUE

Sonography continues to represent the standard for fetal imaging. While sonographic data are processed in a digital form, lack of industrial standard and paucity of digital ports has made advanced image processing of sonographic data somewhat more difhcult when compared with other digital imaging modalities, such as computed tomography and magnetic resonance imaging (MRI), where data are generally in a format that can be digitally processed easily. Magnetic resonance imaging data are routinely manipulated in 3D reformats, surface renderings, and maximum intensity projection renderings among other possible image displays. Thus, a discussion of magnetic resonance imaging would be important in a review of computerized imaging of the fetus. Despite the fact that multiple case reports (l12) and institutional review board approved MRI fetal evaluations (13-46) have been performed, little work on image processing has been reported in these studies. This paper reviews the current state-of-theart in fetal MRI.

The basis for most magnetic resonance imaging studies of the fetus involves the acquisition of short TR, short TE Tl-weighted sequences. Tl-weighted images emphasize fetal fat (19, 41) and will give reasonable contrast in the central nervous system (20, 35, 45). These examinations generally use 2-4 signal averages (NSA) and 128 phasing encodings which yield satisfactory images of the fetus in approximately 2-5 min, provided there is relatively little fetal motion. Unfortunately, often fetal motion completely obscures the detail of fetal MR imaging and many studies give limited information. The severely abnormal fetus is often relatively immobile and good images may be acquired more easily. Subtle lesions may be easily lost due to fetal motion, however. There are two potential solutions to reduce these motion artifacts. These include: (1) pharmacological intervention for fetal sedation; and (2) very rapid imaging techniques (Fig. 1). Fetal sedation can be achieved with relative safety by using maternal agents such diazepam (43, 47, 48). The risk to a fetus for a maternal single oral dose of 10 mg of diazepam should be negligible. Motion-free fetal images have also been achieved after the fetus has been sedated for in utero transfusion with administration of pancuronium bromide by umbilical vein (12, 25, 28, 49).

*This paper was presented on June 24, 1994 at the National Institute of Child Health and Human development workshop on Computer Assisted Imaging of Embryonic and Fetal Development in Bethesda, Maryland. 491

492

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Fig. 1. Images performed using a pelvic phased array coil on a 24 yr old woman who at 28 weeks of pregnancy show considerable motion artifact on the axial TR 4OO/TE 17 views through the fetal abdomen (upper images). Axial fast spoiled GRASS TR 11.6/TE 4.2 with a 30” tip angle show much improved resolution. Flow within blood vessels appears as white in these images (a = aorta, i = inferior vena cava). Liver (L) and spleen (S) are clearly identified as is high signal meconium within the colon (C). Hydronephrotic left kidney is identified (arrows). This was not identified until 2 weeks later by sonography. Fast imaging with gradient echo (l&22) or echo planar techniques can avoid motion artifact. Because Tl-weighting is desirable, transverse magnetization spoiling (spoiled GRASS) or inversion recovery prepared (IR prepped) sequences should be advantageous over standard GRASS sequences. While gradient echo images may be performed as short of a time as 700 ms, gross fetal motion may still obliterate important structures. One way of improving the likelihood of motion free images is to obtain multiple rapid gradient echo images at the same level. These images may be viewed as a movie (18). It is likely that at least some of the images will be during relative motion free periods. A more elegant method of rapid imaging involves the use of echo planar techniques. These techniques produce images in time ranges from 64 to 128 ms, allowing for a motion-free demonstration of fetal anatomy. These may also be repeated rapidly, yielding a movie display of fetal cardiac motion (26). Special gradient systems are required for producing echo planar images. These are only now becoming commercially available and thus few studies (14, 19, 29, 42) have been performed with echo planar systems.

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Fetal anatomy by MM MR imaging of the embryo and fetus in the first and second trimesters has generally been difficult due to the small size of the structures relative to the size of the receiving coil. From about gestational week 10 until just before labor, gross fetal motion increases (46). Identification of the normal and abnormal fetal anatomy in the first half of pregnancy has been difficult to achieve reliably in vivo. It is possible that the advent of improved surface coils and echo planar imaging may allow for improved imaging of the embryo and fetus prior to 21 weeks gestational age. The fetal brain is well demonstrated with magnetic resonance imaging. Images emphasizing Tl-weighting are generally superior to those with T2weighting and the ventricles and general brain morphology can usually be discerned. In the late third trimester, relative high signal within the areas of myelination (Fig. 2) in the internal capsule can be identified with Tl-weighting. High signal in utero subdural hematoma has been reported (6). This is

Fig. 2. A 39 yr old pregnant woman at 33 weeks has an axial TR 588/TE 17 spin echo image of the pelvis which showsa sa&al image of the fetal brain. Note the relatively high signal in areas of active myelination in the region of the internal capsule (arrows).

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Fig. 4. 3D reformatted IR prepped fast spoiled GRASS data aquisition shows the fetus with prominent signal in the rectum (r).

Fig. 3. Targetedmaximum intensity projection from a 3D inversion prepared fast spoiled GRASS data acquisition performedwith a TR of 12.9,TE of 2.3 (Fr), TI 400with a 30” tip angle acquisition shows high signal within the intestine due to the short Tl of meconium(arrows). Liver = L. This in the samepatient asin Fig. 2.

likely due to the short Tl relaxation time of reduced hemoglobin products. One problem with evaluation of the brain is the di@culty in obtaining orthogonal views in a moving fetus. Again, rapid oblique imaging should help with this problem. Alternatively, three-dimensional acquisitions with anatomically orthogonal reformats may be advantageous Gradient echo imaging in the fetal brain may show increased signal within normally flowing blood vessels and thus ljow within cerebral arteries and veins can be demonstrated with gradient echo techniques.

The fetal spine may be difficult to evaluate because orthogonal views are not easily obtained. Nonetheless, large defects may be demonstrated. MRI may be useful to demonstrate fat within the skin overlying a closed neural tube defect. Sacral

defects and teratomas have also been demonstrated by in utero MRL The fetal heart is generally well demonstrated with either spin echo images (dark flowing blood) or gradient echo (bright flowing blood) images. The cardiac chambers and the aorta along with the great vessels are usually well seen (8). Fetal heart motion has been displayed with echo planar imaging at OST (26). Normal and abnormal fetal lungs (46) have been demonstrated with MRI. Fetal lung volumes have been calculated and correlated with fetal age using echo planar MRl (14). Within the abdomen, the fetal liver is generally well seen with a relatively high signal compared to the remainder of the organs. Low signal in the liver due to iron deposition from fetal hemochromatosis (7) has been reported. Fetal aorta, inferior vena cava, portal vein and hepatic veins can usually be demonstrated along with the umbilical vein. Flow causes high signal within abdominal vessels with gradient echo techniques. Fetal gall bladder may also be identified. The fetal spleen can occasionally be demonstrated. The fetal intestine is demonstrated quite well with magnetic resonance imaging due to the shortened relaxation time and bright signal from meconium (Fig. 3). This may be useful in detecting intestines within hernias and within gastroschisis. Intestinal communications in conjoint twins may be demonstrated if dilute gadolinium DTPA is administered directly into the stomach of one twin (11). Normal fetal kidney is not well seen and renal agenesis (2) is difficult to confirm. The hydronephrotic kidney may be quite prominent however. Maternal intravenous gadolinium chelate crosses the

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Fig. 7. Multiple targeted MIP images of the fetal liver from the data set used in Fig. 5.

Fig. 5. Axial (5a) and coronal (5b) of a 3D IR prepped fast spoiled GRASS fetal image of the brain at 31 weeks in a 31 yr old woman. Diffuse high signal in the brain was associated with diffuse necrosis on follow-up in a still born fetus.

Fig. 6. Multiple targeted MIP images of the fetal brain from the data set used in Fig. 5.

Fig. 8. A 32 yr old woman presents at 30 weeks of pregnancy. Targeted MIP image of the fetal circulation shows jugular veins (J), superior vena cava (S), heart (II), abdominal aorta (A), and iliac vessels (I).

Computer-assisted imaging of fetus with MRI

placenta and can be detected in the fetal kidney. Thus, in utero evaluation of renal anatomy and function may be possible. Within the fetal pelvis, fluid within the fetal bladder and meconium within fetal rectum (Fig. 4) can usually be demonstrated. Occasionally, a fetal phallus can be demonstrated in a manner similar to ultrasound. Demonstrating the fetal extremities may be difficult due to the non-orthogonal orientation of the structures. Nonetheless, many times fetal limbs are quite well shown, including hands and feet. Fetal fat may be identiied and measured (19) with MRI and intrauterine growth retardation (41) and macrosomia (19) may be evaluated. MR pelvimetry can give an estimatiou of fetal shoulder size (27) to help predict dystosia. Three-dimensional data acquisitions and reformatting would be extremely useful for demonstration of non-orthogonal structures such as the fetal brain (Fig. 5) and fetal extremities. ‘Targeted’ maximum intensity projections can be used to selectively view fetal organs (Figs 6 and 7). MR Angiographic displays of fetal vessels (Fig. 8) should also be possible if scamling times can be reduced to appropriate short intervals or with appropriate fetal sedation. SUMMARY

AND CONCLUSIONS

The advent of faster imaging techniques including fast 3D acquisitions should allow for marked enhancement of fetal magnetic resonance imaging in the future. Fetal MRI might occasionally become useful as a follow-up for selected suspected abnormalities detected by sonography. The most promising areas for magnetic resonance imaging are in the evaluation of fetal central nervous system and in the detection of fat, hemorrhage, and meconium within the fetus.

Acknowledgement-The author is grateful to American Health Services, Inc. for the sypport of this project.

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About the AU~~~-PATSUCK M. COLLETTI reaiverl his BS degree ia chemical engineering from Rutgers University, where he continued his studies to receive the MMS in 1971, aad the MD in 1975 from the Rutgers Medical School. Dr. Colletti completed a surgery internship at the Los Angeles County/University of Southern California Medical Center in 1976. This was followed by a radiology residency in diagnostic radiology, aad a fellowship in nuclear medicine. Since 1980, he has held academic appoiatmerits with the USC School of Medicine as well as staff physician status with the LAC/USC Medical Center. Presently, he holds the rank of Associate Professor of Radiology with the USC School of Medicine, and is the chief of magnetic resonance imaging for the LAC/USC Medical Center. He is also serving as the medical director for the LAC/USC Imaging Science Center.