FETAL MRI PROTOCOL
TIPS
3-Plane Scout Gre
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
Optional Scans
Axial Ax DTI to Baby(6 directions) epi
Ax/Sag/Coro T1-flash fl
TIPS
- 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 between each slice.
- 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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