Review: Solving Technical Problems in Fetal MRI

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Fetal MRI is a rapidly evolving field that is transforming prenatal care. It allows earlier detection of complex congenital anomalies, complements ultrasound diagnoses and can provide additional information to guide prenatal counseling and improve postnatal management.

However, the diagnostic utility of fetal MRI can be limited by technical barriers that reduce image quality and introduce artifacts. In Pediatric Radiology, Fedel Machado-Rivas, MD, research fellow, John E. Kirsch, PhD, director of the Human Imaging Core at the Martinos Center for Biomedical Imaging, and Michael S. Gee, MD, PhD, chief of Pediatric Radiology and associate program director of the Radiology Residency Program, all from the Massachusetts General Hospital Department of Radiology, and a colleague present solutions to common problems.

Optimizing Image Quality

Grainy images indicate a low signal-to-noise ratio (SNR). This can be addressed by increasing the field strength or increasing the number of radiofrequency coils, which improves SNR independent of field strength. Coil elements can be combined in parallel arrays to further reduce noise.

Blurry images can result from low spatial resolution, which depends on voxel size. Decreasing the field of view (FOV) improves spatial resolution but sacrifices SNR, so it is most successful during 3-T imaging, where there is inherently higher SNR. Another approach is to increase the matrix size, but that increases acquisition time. Rapid acquisition is important in order to avoid motion artifacts from fetal movements and maternal respiration.

Excessive specific absorption rate (SAR)—The SAR is the radiofrequency power deposition generated by the excitation and refocusing pulses that create electrical currents in tissues and produce heat. At any field strength, the FDA limit for SAR for pregnant women is 4 W per kilogram of maternal weight, averaged over six minutes (this is automatically programmed into commercial scanners).

Radiofrequency power deposition increases with field strength, so the SAR limit is more easily reached at 3-T. It's also important to know that commercial software adjusts the SAR to enable more heat dissipation, which can increase acquisition time.

To maintain SAR below the FDA limit, lower the flip angles (either excitation or refocusing), increase the repetition time length, decrease the number of slices or shorten the echo-train length.

Reducing Artifacts

Motion Artifacts

Five strategies to minimize motion artifacts are:

• Maximize image acquisition speed by using single-shot T2-weighted imaging or balanced steady-state free precession (bSSFP) sequences
• Decrease the total number of slices, which decreases acquisition time in steady-state sequences
• Increase the repetition time
• If the source of motion is in the phase-encoding direction, swap the phase-encoding and frequency-encoding directions
• Ask the patient to hold her breath (fast acquisitions)

Wraparound Artifacts

In fetal MRI a small FOV is required in order to maintain spatial resolution, but maternal tissue surrounds the area of interest, typically resulting in a "wraparound artifact" of maternal structures mismapped on top of the fetus. Two strategies are:

• Increase the number of phase-encoding steps in the phase-encoding direction (typically by 50%) so that the FOV includes the aliasing structures. There are tradeoffs, however. In steady-state sequences, acquisition time is increased proportionally to the oversampling. In single-shot sequences, SAR is increased slightly, but possibly enough to be important if 3-T is used, considering the already high SAR
• Swap the phase-encoding long axis for the frequency-encoding direction to minimize surrounding structures outside the FOV. The frequency-encoding direction does not have a penalty for oversampling, so this approach changes the direction of the acquisition plane with virtually no increase in acquisition time or SAR

Banding Artifacts

bSSFP is often used in fetal MRI to provide relatively high SNR and fast acquisition time. However, off-resonance effects from B0 nonuniformity are apt to produce phase-shift and phase-accumulation errors. This results in a bandlike signal loss in areas of B0 nonuniformity, typically at the edges of the FOV or near tissue interfaces.

Decrease repetition time—Bands are spaced in intervals inversely proportional to the repetition time. Therefore, decreasing repetition time widens the band intervals and minimizes the banding effect.

Lower the field strength—Banding artifacts typically worsen at 3-T because of a proportional increase in the Larmor precession frequency. This is compounded by the restriction on repetition time that is necessary to avoid excessive SAR.

Inhomogeneities in the Radiofrequency Field

Changes in the physical propagation of permittivity can distort the radiofrequency field, resulting in a wide variation in flip angles. This manifests as areas of decreased signal intensity (shading) or loss of tissue contrast. This problem is augmented at higher field strengths because of the resulting reduction in wavelength of the radiofrequency transmission field.

Besides imaging at lower field strength, multichannel transmit systems have been applied to fetal MR imaging with some success. Commercial dual-drive transmission systems have flexible transmission settings that permit more degrees of freedom, which allows for shaping the transmission field in an elliptical polarization model that resembles the abdomen more accurately and minimizes inhomogeneities.

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