Biomedical Engineering Reference
In-Depth Information
(PSF) and corresponding experiments. The optimal number of TE segments
for the least artifact was determined to be one-half of the number of slices. Two
types of artifacts caused by VTE were predicted and subsequently observed. The
signal distribution near the bifurcation and the siphon was much more uniform
with VTE, and the flow-related signal loss was greatly reduced (see Fig. 3.30).
The resultant MR angiograms provided improved vessel detail. The results show
that VTE improved the quality of flow-compensated 3D TOF MRA.
3.4.17 Cardiac-Triggered Free-Breathing 3D Balanced
Fast Field-Echo Projection MRA
A two-dimensional pencil-beam aortic labeling pulse was developed for the renal
arteries [17]. For data acquisition during free breathing in eight healthy adults
and seven consecutive patients with renal artery disease, real- time navigator
technology was implemented. This technique allows high spatial resolution and
high contrast renal MR angiography and visualization of renal artery stenosis
without exogenous contrast agent or breath hold (see Fig. 3.31). Initial promising
results warrant larger clinical studies.
3.4.18 Cervical MRA
Initial experience with intracranial and cervical MRA at 3.0 T was re-
ported. Phantom measurement s (corrected for relaxation effects) show S/N
(3
.
0T)
=
2
.
14
+
/
−
0
.
08
×
S/N (1.5 T) in identical-geometry head coils [18]. A
3.0 T TOF intracranial imaging protocol with higher-order autoshimming was
developed and compared to 1.5 T 3D TOF in 12 patients with aneurysms. A com-
parison by two radiologists showed the 3.0 T to be significantly better (P
<
0.001)
for visualization of the aneurysms (see Fig. 3.29). The feasibility of cervical and
intracranial contrast enhanced MR angiography (CEMRA) at 3.0 T was also
examined. The relaxivity of the gadolinium contrast agent decreased by only
about 4-7% when the field strength was increased from 1.5 T to 3.0 T. Cervi-
cal 3.0 T CEMRA was obtained in eight patients available for direct compari-
son. Image comparison suggested 3.0 T to be favorable field strength for cer-
vical CEMRA. Voxel volumes of 0.62-0.73 mm
3
were readily achieved at 3.0 T
with the use of single-channel transmit-receive head or cervical coil, a 25 mL
