T2-Weighted Fast MR Imaging with True FISP Versus HASTE
Comparative Efficacy in the Evaluation of Normal Fetal Brain Maturation
Hsiao-Wen Chung1,2,
Cheng-Yu Chen2,
Robert A. Zimmerman3,
Kwo-Wei Lee2,
Chueng-Chen Lee2 and
Shy-Chi Chin2
1
Department of Electrical Engineering, National Taiwan University, No. 1,
Section 4, Roosevelt Rd., Taipei, Taiwan 10764, Republic of China.
2
Department of Radiology, Tri-Service General Hospital and National Defense
Medical Center, No. 8, Section 3, Ting-Chou Rd., Taipei, Taiwan 100, Republic
of China.
3
Department of Radiology, The Children's Hospital of Philadelphia, 34th St. and
Civic Center Blvd., Philadelphia, PA 19104.
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Fig. 1A. —Brainstem myelination in normal fetus at 22 weeks' gestation.
Magnified axial half-Fourier acquisition single-shot turbo spin-echo MR image
at mid pons level shows that, compared with supratentorial brain where
myelination has not begun and no point-spread-function blurring is seen,
myelination of pontine tegmentum (arrowheads) is blurred because of
T2 decay in myelinated white matter.
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Fig. 1B. —Brainstem myelination in normal fetus at 22 weeks' gestation.
Magnified axial fast MR image with steady-state free precession at same level
as A shows early brainstem myelination.
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Fig. 2A. —Neuronal migration in normal fetus at 22 weeks' gestation.
Magnified coronal half-Fourier acquisition single-shot turbo spin-echo MR
image shows four-layer pattern of cerebral mantle comprising, from inner to
outer layers, germinal matrix, row of migrating neurons, intermediate zone,
and immature cortex. Arrows indicate boundaries between layers.
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Fig. 2B. —Neuronal migration in normal fetus at 22 weeks' gestation.
Magnified coronal fast MR image with steady-state free precession at same
level as A shows comparable imaging quality in supratentorial anatomy
depiction during second trimester. Arrows indicate boundaries between
layers.
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Fig. 3A. —Perirolandic myelination in normal fetus at 33 weeks'
gestation. Magnified axial half-Fourier acquisition single-shot turbo spinecho
MR image suffers from severe blurring along phase-encoding direction
(arrows). Myelination was distorted by blurring and hence was less
conspicuous.
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Fig. 3B. —Perirolandic myelination in normal fetus at 33 weeks'
gestation. Magnified axial fast MR image with steady-state free precession at
same level as A clearly shows early perirolandic myelination as
hypointense signal (arrowheads). Note that fat-water boundary was
conspicuously delineated in this image because of out-phase nature of TE (2.3
msec) in gradient-echo images.
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Fig. 4. —Graph shows theoretic signal change as function of T1/T2 for
fast imaging with steady-state precession (true FISP), plotted for three
excitation flip angles using formula in literature
[11,
13]. Scale on x-axis
corresponds to myelination process that causes increase of T1/T2 from 2500/400
msec (approximately 6.3%) to 900/80 msec (approximately 11.3%). For 70°
flip angle (solid line) used in this study, myelinated white matter
has signal intensity of 50% less than that in nonmyelinated white matter in
true FISP images. This decreasing change provides conspicuous contrast in true
FISP images, revealing myelination as low-signal-intensity areas. Dotted
line=30° flip angle, dashed line=50° flip angle.
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Fig. 5. —Graph shows effect of steady-state-free-precession (SSFP)
angle in one TR on signal of fast imaging with steady-state free precession at
70° flip angle plotted for (from top to bottom) cerebral spinal fluid,
nonmyelinated white matter, adult gray matter, and adult white matter. Note
that if 180° phase alternation is used for radiofrequency excitation,
signal is relatively uniform for all brain tissues with about ± 90°
tolerance in SSFP angle. With automatic shimming, a line width of 64 Hz at 1.5
T, measured at one tenth of maximum, was routinely achievable. Banding-free
images can thus be obtained at TR of 8.0 msec or less. Also note significant
signal difference between nonmyelinated and myelinated white matter because of
changes in T1/T2, which provides poor contrast between adult gray and white
matter.
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