MR Ventriculocisternography By Using 3D Balanced Steady-State Free Precession Imaging: Technical Note
Ming-Long Wua,
Cheng-Wen Koe,
Ting-Yi Chenb,
Ming-Ting Wuf,
Hsiao-Wen Chungc,
Teng-Yi Huangd and
Yi-Ru Lina
a Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China
b Department of Radiology, Veterans General Hospital-Taipei, Taipei, Taiwan, Republic of China
c Department of Radiology, Tri-Service General Hospital, Taipei, Taiwan, Republic of China
d Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China
e Department of Computer Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, Republic of China
f Department of Radiology, Kaohsiung Veterans General Hospital, Kaohsiung, Taiwan, Republic of China
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FIG 1. Steady-state signal intensities of CSF, gray matter (GM), and white matter (WM) as a function of flip angle. A flip angle of 70° is often chosen in MR ventriculocisternography to maximize CSF-parenchyma contrast.
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FIG 2. Steady-state signals across the imaging slab of a smoothed synch radio-frequency pulse lasting for 0.9 ms in time duration, plotted for two designated flip angles of 40° (dotted line) and 70° (thin solid line) along with the ideal flip-angle profile (thick solid line), all normalized to the value at the slab center. Note that, at larger designated flip angles, the actual slab profiles for the SSFP signals tend to become wider.
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FIG 3. Percentage section aliasing, calculated as widening of the full-width-at-half-maximum of the steady-state signal intensity profile relative to the desired slab thickness, plotted as a function of the designated flip angle (dotted line). Experimental measurements (circles) from our subjects agreed fairly well with the theoretical deductions. Note that, at a 70° flip angle, the percentage section aliasing is nearly 30%, meaning that about one-third of the sections would be subject to interference from CSF outside the image slab. By lowering the flip angle to 30° or 40°, the percentage section aliasing decreases to <20%.
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FIG 4. Increased section aliasing artifacts in 3D SSFP imaging on a 33-year male subject. Several inner sections, the 13th (A), 17th (B), and 21st (C) anterior sections in a 84-section volume acquired with a designated flip angle of 70°, are shown to exhibit prominent aliased CSF signals (A–C, arrows) from the lateral ventricles actually located at a posterior region (D). The posterior section also shows evidence of section aliasing from anterior region (D, arrow).
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FIG 5. Contrast between CSF and white matter calculated as a function of the designated flip angle (dotted line), superimposed by experimental values (circles) measured from the images obtained from the healthy subjects recruited in our study. The measurements showed that the CSF-parenchyma contrast reduces as flip angle decreases, in good agreement with theoretical derivations.
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FIG 6. Images acquired from a 23-year-old man healthy subject showing comparison of section aliasing artifacts in 3D SSFP imaging, as a function of the designated flip angle. The 8th anterior section in a 36-section volume acquired with a designated flip angle of 70° (A), exhibits prominent aliased signals from CSF (arrows). By lowering the flip angle to 50° (B) and 30° (C), the section aliasing decreases (arrows) and becomes invisible, respectively. Also note the continuous reduction in the contrast between CSF and brain parenchyma from A to C.
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