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FETAL MRI PROTOCOL |
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Sagittal Trufi Sag to Mother trufi
Coronal Trufi Coro to Mother trufi
Axial Trufi Ax to Mother trufi
Sagittal haste Sag to Baby haste
Coronal haste Coro to Baby haste
Axial haste Ax to Baby haste
Sagittal haste Sag Thick to Baby haste
Sagittal haste Sag Thin to Baby haste
Axial Ax DTI to Baby(6 directions) epi
---Don't stop scanning if baby moves. Scan the next couple of slices,
baby probably went back to the same position
-- Number of slices need to match the number of concatenations.
Because we want to scan one slice at a time. Give at least 3 seconds
-- Do not go blow 250mm FOV
--Place both body matrix coils side by side
Technique of image acquisition
Clinical utilization of FMRI sequences have 2 essential requirements: 1) fast temporal resolution to account for fetal movement; 2) high, tissue signal-to-noise ratio (SNR), given the inability to administer gadolinium-based contrast agents. Of the commercially available 3-dimensional MRI sequences, 2 meet these criteria. One is gradient echo imaging with balanced steady state preservation of residual transverse magnetization (steady state free precession, SSFP).11 The other is volume interpolated (VI) gradient echo imaging.12 Three-dimensional SSFP is applied to depict fluid-sensitive material and structures, including amniotic fluid, placenta, cord, brain, ventricles, spinal canal, airway, lungs, pleural space, heart, aorta, kidneys, ureters and bladder. When using this sequence, the relative T2/T1 hyperintensity provides high tissue contrast between the viscera, placenta and amniotic fluid. Three-dimensional SSFP is also useful for depiction of the bowel, liver, spleen, the spine and extremities. Three-dimensional VI is useful for depiction of T1-sensitive material and solid viscera including meconium, hemorrhage, placenta, bowel, liver, spleen, thyroid gland and the pituitary gland.
For all 3-dimensional FMRI acquisitions, it is imperative to prescribe the parameters and image coverage such that the scan time is <10 to 15 sec, while spatial resolution and SNR are maximized. Depending on the gestational age, potential space in the amniotic cavity and the maternal fasting status, scan times >10 to 15 sec will increase the potential opportunity for fetal motion . Exam coverage should be restricted to no more than the boundaries of the uterus. Thin-section images are ideal for high spatial resolution, but at the expense of longer scan times and reduced SNR. Increasing the matrix and repetition time will improve SNR, but will also increase the scan time.
A 1.5T magnet is used. Imaging at or after 18 weeks' gestation is preferred. Beyond this period, the fetus is large enough to image many anomalies, and motion is usually not as problematic. The mainstay of fetal MR imaging has been fast single-shot T2-weighted imaging. In general, slice thicknesses of between 4 and 7 mm are used with 0 to 1 mm interslice spacing (repetition time [TR] 2150, echo time [TE] 80Ef, matrix 256 x 256). Body coils and larger phased-array coils have been used most commonly. Smaller surface coils are sometimes useful for directed imaging of specific parts of fetal anatomy if the area of interest is not too far from the surface. Typically, 10 to 15 slices are acquired in 20 to 25 seconds. As fetal motion is unpredictable, shorter sequences are optimal. An initial fast localizing sequence is performed, and sequences are then acquired in orthogonal planes relative to each previous sequence. It is most effective to be ready to set up each sequence as rapidly as possible. One does not have the luxury of time, and the most successful examination is usually performed by personnel who are thoroughly familiar with fetal anatomy and fetal anomalies.
Fast T1-weighted imaging has also emerged into the fetal MRI arsenal. Children's Hospital Boston uses an inversion recovery single-shot fast spin-echo (SSFSE) technique (TR 2530, TE 35.5, inversion time [TI] 2000, matrix 256 x 192) . Slice thickness selection is similar to the T2-weighted sequences. The T1-weighted technique has had particular utility in the brain, looking for hemorrhage, and in the abdomen, where the T1-weighted conspicuity of the liver has been reported to be helpful in assessing liver position in fetuses with congenital diaphragmatic hernia (CDH).Newer fast imaging techniques are developing rapidly and are expected to have important fetal applications. One must always be aware of the level of energy deposition, however, when considering such applications.
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