Biomedical Engineering Reference
In-Depth Information
The dependence of RF power deposition on TR for CEMRA was calculated and
described.
3.4.19 Magnetization Transfer MRA with
RF Labeling Technique
A method for MT angiography using an RF labeling technique was suggested.
The method utilized a slice-selective spin-lock pulse sequence for tagging the
spins of inflowing blood [19]. The pulse sequence begins with a spatially selective
90
◦
(x) RF pulses, followed by a nonselective composite locking pulse of 135
◦
(y) - n[360
◦
(y)] - 135
◦
(y) and by a 90
◦
(
−
x) pulse. A spoiler gradient was then
applied. A rapid imaging stage, which yielded a T1 rho-weighted signal from
the tagged spins, completed the sequence. Untagged spins were thoroughly de-
phased and consequently suppressed in the image. Thus, contrast was obtained
without an injection of a contrast material or image subtraction. Furthermore,
the flow of the tagged bolus could be visualized. The sequence was implemented
on phantoms and on human volunteers using a 1.5 T scanner. The results indi-
cated the feasibility of the suggested sequence.
3.4.20 Oscillating Dual-Equilibrium Steady-State
Angiography (ODESSA)
A novel technique of generating non-contrast angiograms was proposed [20].
This method utilized a modified steady-state free precession (SSFP) pulse se-
quence (see Fig. 3.32). The SSFP sequence was modified such that flowing
material reaches a steady state that oscillates between two equilibrium values,
while stationary material attains a single, non-oscillatory steady state. Subtrac-
tion of adjacent echoes results in large, uniform signal from all flowing spins
and zero signal from stationary spins. Venous signal can be suppressed based
on its reduced T2. ODESSA arterial signal was more than three times larger
than that of traditional phase-contrast angiography (PCA) in the same scan time,
and also compares favorably with other techniques of MR angiography (MRA).
Pulse sequences are implemented in 2D, 3D, and volumetric-projection modes.
Angiograms of the lower leg, generated in as few as 5 seconds, showed high
arterial signal-to-noise ratio (SNR) and full suppression of other tissues.
