Biomedical subjects

William R Overall

Publications and source records attributed to William R Overall.

3 recordsLinked to original sources

PubMed2003

Steady-state sequence synthesis and its application to efficient fat-suppressed imaging.

A new synthesis algorithm, based on the Shinnar-Le Roux (SLR) transform, can be used to generate fully refocused steady-state pulse sequences with arbitrary magnetization profiles as a function of off-resonant precession. This is accomplished by appropriate periodic oscillation of the RF excitation magnitude and phase from echo to echo. The technique is applied to the design of refocused steady-state free precession (SSFP) sequences with flat profiles, providing the opportunity for banding-artifact-free imaging with steady-state contrast. The algorithm is also used to generate refocused-SSFP sequences with an arbitrarily broad region of attenuated signal. These sequences are implemented and applied to the problem of steady-state fat suppression. Preliminary results show signal levels that agree well with theory, and a broad region of suppressed signal at each echo. Total imaging time is kept identical to that of a standard refocused-SSFP experiment through echo equalization and interleaving. 3D images from the leg of a normal volunteer acquired in 44 s demonstrate the applicability of the technique to fat-suppressed imaging.

Adipose Tissue↗

PubMed2002

Oscillating dual-equilibrium steady-state angiography.

A novel technique of generating noncontrast angiograms is presented. This method, called oscillating dual-equilibrium steady-state angiography (ODESSA), utilizes a modified steady-state free precession (SSFP) pulse sequence. The SSFP sequence is modified such that flowing material reaches a steady state which oscillates between two equilibrium values, while stationary material attains a single, nonoscillatory steady state. Subtraction 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 is 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 s, show high arterial signal-to-noise ratio (SNR) and full suppression of other tissues.

Artifacts↗

PubMed2002

Fast phase-contrast velocity measurement in the steady state.

A new method of encoding flow velocity as image phase in a refocused steady-state free precession (SSFP) sequence, called steady-state phase contrast (SSPC), can be used to generate velocity images rapidly while retaining high signal. Magnitude images with refocused-SSFP contrast are simultaneously acquired. This technique is compared with the standard method of RF-spoiled phase contrast (PC), and is found to have more than double the phase-signal to phase-noise ratio (PNR) when compared with standard PC at reasonable repetition intervals (TRs). As TR decreases, this advantage increases exponentially, facilitating rapid scans with high PNR efficiency. Rapid switching between the two necessary steady states can be accomplished by the insertion of a single TR interval with no flow-encoding gradient. The technique is implemented in a 2DFT sequence and validated in a phantom study. Preliminary results indicate that further TR reduction may be necessary for high-quality cardiac images; however, images in more stationary structures, such as the descending aorta and carotid bifurcation, exhibit good signal-to-noise ratio (SNR) and PNR. Comparisons with standard-PC images verify the PNR advantage predicted by theory.

Aorta, Thoracic↗