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Medical University of Vienna uses MRI for fetal imaging

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Prayer, Daniela, M.D. Vienna, University Hospital of Vienna Austria
Brugger, Peter, R.T. Vienna, University Hospital of Vienna Austria

T2-weighted imaging has been the standard in fetal MRI*, but it isn't the only useful method by far. Dr. Daniela Prayer and Dr. Peter Brugger at Medical University of Vienna, are employing T1-weighted sequences, static and dynamic steady state (i.e., Balanced-FFE) sequences, and echoplanar and diffusion-weighted imaging methods, to reveal possible fetal pathology and anatomy and to track fetal development. As a complementary imaging modality to ultrasound, MRI visualization is virtually unaffected by maternal obesity or fetal position and brain imaging is unrestricted by the skull. MRI affords exceptional soft tissue contrast resolution and the ability to differentiate internal organs and gray and white matter. MRI also provides multi-planar imaging capability and a full-fetus FOV.

 

Dr. Daniela Prayer.

 

Clinicians optimize numerous techniques for MRI of fetus

Optimal use of MRI for fetal studies requires attention to several key technical and practical issues, among which are choice of sequence and sequence parameters, fetal motion and position, and allowable examination times. One of the obvious initial challenges in fetal MRI is adjusting the position of the coil so that the receptor elements are as close as possible to the fetal regions of interest says Daniela Prayer, M.D., radiologist at Medical University of Vienna.

 

"A multistack, multiplanar B-FFE sequence that provides three orthogonal directions helps us locate the fetus," she says. "The next challenge is to select a pulse sequence that will account for maternal breathing and often energetic fetal motion, so ultra-fast sequences and techniques such as SENSE are useful."

 

In addition to imaging speed, clinicians require high spatial resolution and contrast, small FOVs, high SNR and no artifacts. Most of the requirements are related to the small size of fetal structures. For example, the head of the fetus measures about 65 x 50 mm in the 21st gestational week (GW), which specifies an FOV of about 200 mm. Further reductions in the FOV compel decreased matrixes, resulting in degraded image quality, Dr. Prayer notes.

Planning sequences account for fetus in motion

At Medical University Vienna, fetal MRI studies begin with a T2-weighted sequence in an orthogonal plane through the organ of interest. During the actual examination, each sequence serves as a scout for the next, Dr. Prayer explains. Fetal motion is unavoidable but can be coped with.

 

"Because the fetus is moving, you don't plan your examination, the fetus plans the examination," she says. "The desired result is a series of contiguous images, because we need to see every organ in three orthogonal planes."

 

A common dilemma in planning fetal MRI is the so-called "banana problem," in which the developing baby reclines in such a way that acquiring axial slices is possible only through the thorax, at which point the abdomen curves (banana-like) out of plane.

 

"The solution is to plan axial slices radially from thorax to abdomen to accommodate this curvature," she advises. "Of course, we adjust our field-of-view to the age of the fetus, as smaller fetuses require a small FOV and vice-versa."

  Radial stack for planning axial images from thorax to abdomen
"Banana problem"
Radial stack for planning axial images from thorax to abdomen

T2-weighted sequences are workhorses

T2-weighted imaging is standard in fetal MRI as it affords superb images of fetal anatomy at all gestational ages. Optimal image quality can be achieved by adjusting the TE based on gestational age. For example, regarding the fetal brain, in the 28th GW, a long (140 ms) TE may enable best delineation of the internal capsule and basal ganglia, while in a later GW (e.g. 39th GW) the same long TE can result in motion artifacts due to a longer sequence duration. In addition, what works well for fetal brain imaging does not necessarily work well in the body. Reducing the TE in the body can help visualize more organ detail. T2-weighted sequences with long TE's (e.g., 250) work very well in the imaging of cystic lesions, Dr. Prayer adds.

 

Vienna clinicians use a fetal MRI version of an MRCP sequence used for abdominal imaging of adults. This thick-slab T2-weighted sequence uses a 25-50 mm slab to create images that are 3D-like in appearance and take just one second to acquire, so fetal motion typically doesn't affect them. This sequence is ideal for assessing overall fetal proportions, extremities and superficial lesions, but does not work with anhydramnios, Dr. Prayer notes.

 

TE 100 ms TE 120 ms TE 140 ms
TE 100 ms
TE 120 ms
TE 140 ms

Image contrast is better at long TE (140 ms) in this fetal brain in the 28th GW.

 

TE 140 ms TE 100 ms
TE 140 ms
TE 100 ms

In this older fetus (GW 36) the lower TE 100 ms yields better image

quality, because the TE 140 ms image suffers from fetal motion.

 

TE 80 ms TE 250 ms
TE 80 ms
TE 250 ms

Long TE (250 ms)T2-weighted images work well to visualize the content of cystic lesions.

 

T2-weighted imaging of a small fetus and a larger fetus.

 

A reduced TE in the body can help visualize more organ detail.

 

In fetal MRI, T1-weighted sequences critical in specialized studies

T1-weighted sequences are known for several drawbacks in fetal MRI, including long duration, rather thick (~ 5 mm) slices and a requirement to perform these sequences in breath-hold to avoid maternal respiratory artifact, Dr. Prayer observes. "Nevertheless, T1 information is needed to demonstrate, for example, hemorrhage and calcification, myelin - as seen in the brain stem - or cell density in the fetal basal ganglia," she says.

 

"In addition, T1-weighted sequences help visualize superficial fat, the pituitary gland and meconium. In fact, these sequences enable meconium-based colonography."

 A T1-weighted sequence was used to visualize meconium.
A T1-weighted sequence was used to visualize meconium.

 

The T1-weighted sequence, moreover, is the only method capable of visualizing methemoglobin associated with intraamniotic bleeding. The liver and thyroid also will appear slightly more hyperintense in T1-weighted sequences, as will vessels within the fetus or umbilical cord due to their greater flow sensitivity.

Balanced sequences offer speed for dynamic studies

Balanced, or steady state free precession (SSFP), sequences provide high T2-weighted contrast, and while they require a minimum FOV of 260 mm, they supply very thin, high SNR, over-contiguous slices for studies of fetal extremities and abdominal structures. Additional imaging applications include the fetal thorax and great vessels.

 

"We use dynamic Balanced-FFE when rapid fetal motion [e.g., cardiac, swallowing, bowel] would cause blurring using a conventional sequence," Dr. Prayer observes. "We use five to six frames per second, a large FOV of 320 mm and the slice thickness can vary between 12 mm and 50 mm depending on what we want to see. We use thicker slices for viewing the whole fetus, but if we want to only see him swallowing we use 10-12 mm slices."

 Dynamic B-FFE of a fetus (GW 28) with cardiac tumor (arrow), used to see if tumor impairs contractility of ventricle. 5-6 fps., FOV 320 mm, slices 12-50 mm.
Dynamic B-FFE of a fetus (GW 28) with cardiac tumor (arrow), used to see if tumor impairs contractility of ventricle. 5-6 fps., FOV 320 mm, slices 12-50 mm.

 

 

Fetus with large lesion of the neck.

The B-FFE movie shows that the fetus can still swallow and breathe.

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Fetal skeleton, hemorrhages visualized with EPI

Before echoplanar imaging (EPI), it was conventional knowledge that MRI was unable to visualize skeletal structure. Echoplanar sequences nicely demonstrate the developing baby's skeleton and venous sinuses, although resolution is never pristine. EPI sequences also are suitable for imaging old hemorrhages.

Morphology and function clarified with DWI

Diffusion-weighted sequences have become very important in fetal imaging, providing information about ultrastructure and tissue function.

 

"We use DWI in the brain to show the maturation of white matter, to detect acute ischemic lesions and to assess pressure on tissue," Dr. Prayer explains. "We also use DWI to visualize the kidneys' diffusionweighted presentation and determine how it may be related to function. In the placenta, it is used to depict ischemic changes.

 

"The most important factor determining DWI image quality," she continues, "is placing the region of interest in the coil center to avoid acquiring blurry images."

 

In white matter imaging of the fetus, in particular, anisotropy is a quality of the mature white matter - as the myelin sheaths restrict random, multi-directional proton motion. "To some degree, this also is true for unmyelinated tissue in the fetus," Dr. Prayer observes. "Since it is possible to perform anisotropy imaging, it is feasible also to conduct DTI fibertracking. The prerequisites are that we have a relatively older fetus, post- 28th gestational week, for example, and the head has to be fixed in the small pelvis or sedation must be administered due to the one-minute scan time."

 

Representative images can depict fibers in the corpus callosum and in the internal capsule. In addition, it is possible to calculate Apparent Diffusion Coefficient (ADC). "The anisotropic images, and to some degree the isotropic images too, show lamination of the fetal brain, particularly the germinal zone, which is bright, and changes in contrast on the ADC," she says. "All of these findings are important because their appearance changes with fetal age."

 

DWI can even serve as an alternative for T2-weighted sequences for organs outside of the CNS, Dr. Prayer notes. "Often, on T2-weighted images, the kidneys don't display much contrast against the surrounding tissue," she says. "On the diffusion-weighted images, kidneys appear very bright."

 DTI fibertracking (fetus GW 31)
DTI fibertracking (fetus GW 31)

FLAIR techniques shed light on brain

While the use of FLAIR techniques is limited in applications demanding high spatial resolution, they provide good contrast in the fetal brain, she says. "The basal ganglia appear much brighter than on T1-weighted images," she notes. "Additionally, FLAIR sequences help us characterize a certain tissue or fluid, such as fluid within an abdominal cyst."

Scan time and image interpretation are critical

Fetal MRI examination time can vary widely depending on the number of fetuses - two fetuses doubling study time and so on - and on whether there is complex pathology, which also lengthens examination time. In addition, excess fetal motion - particularly in small fetuses surrounded by more fluid - can impact on scan time.

 

"We try not to examine longer than 45 minutes, as pregnant mothers have difficulty remaining in one position for long," Dr. Prayer says. "Image interpretation is based on at least 1,000 images per case to assess morphology, pathology and volumetry. With this volume of images, interpretation is always done by a team."

Fetal MRI's future: edging into mainstream

As in many other applications, MRI shows certain distinct advantages over traditional imaging modalities and fetal MRI is no exception. "We expect that as we build more clinical experience, MRI will begin to be considered as a more conventional and clinically useful diagnostic imaging modality in cases where use of ultrasound is impractical," Dr. Prayer says.

 

*Note:  Although there have been no documented adverse fetal effects reported, international organizations advise that pregnant patients should not be scanned during the first trimester. Scanning of pregnant patients at fields higher than 1.5T is not permitted.



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