Off‐resonance‐robust velocity‐selective magnetization preparation for non‐contrast‐enhanced peripheral MR angiography

Abstract

Purpose To develop a new velocity-selective (VS) excitation pulse sequence which is robust to field inhomogeneity and demonstrate its application to non-contrast-enhanced (NCE) peripheral MR angiography (MRA). Methods The off-resonance-robust VS saturation pulse is designed by incorporating 180° refocusing pulses into the k-space-based reference design and tailoring sequence parameters in a velocity region of interest. The VS saturation pulse is used as magnetization preparation for NCE peripheral MRA to suppress background tissues but not arterial blood based on their velocities. NCE peripheral MRA using the proposed VS preparation was tested in healthy volunteers and a patient with arterial stenosis. Results Calf angiograms obtained using the new VS preparation show more uniform background suppression than the reference VS preparation, as demonstrated by larger mean values and smaller standard deviations of artery-to-vein and artery-to-muscle contrast-to-noise ratios (71.0 ± 11.4 and 75.3 ± 12.1 versus 61.7 ± 22.7 and 58.5 ± 27.8). Two-station peripheral MRA using the new VS preparation identifies stenosis of the femoral and popliteal arteries in the patient, as validated by digital subtraction angiography. Conclusion NCE MRA using the new VS magnetization preparation can reliably provide high angiographic contrast in the lower extremities with significantly improved immunity to field inhomogeneity.

Characterization and suppression of stripe artifact in velocity‐selective magnetization‐prepared unenhanced MR angiography

“…Fourier‐based VS magnetization preparation concatenates multiple VS excitation steps each of which consists of a hard RF pulse, a unipolar gradient, a refocusing module, and a reversed unipolar gradient (Figure ) . The refocusing module denoted by the dotted grey box in Figure consists of 1 or multiple 180° rotations interleaved with additional unipolar gradients whereas the case of 2 180° pulses is shown in the figure.…”

“…Velocity‐selective (VS) magnetization‐prepared MRI has been recently introduced as a promising gadolinium‐free angiography approach . At the core of this approach, hereafter termed VS‐MRA, is Fourier‐based VS magnetization preparation pulse that excites magnetic spins selectively based on their velocities .…”

“…Unlike earlier flow‐sensitive preparation pulses used for subtractive angiography methods, the Fourier‐based VS preparation offers flexible velocity pass‐ and stop‐bands and sharp transition between the 2 bands and therefore enables positive angiographic contrast using single acquisition without subtraction. Although developed for renal angiography for its first application, VS‐MRA has shown great promise for peripheral, cerebral, and pedal angiography as well …”

Identification and reduction of image artifacts in non-contrast-enhanced velocity-selective peripheral angiography at 3T

“…An off‐resonance‐robust VS excitation pulse sequence can be designed by inserting 180 ° refocusing pulses in the middle of bipolar gradients, which are repeatedly interleaved with a train of rectangular radiofrequency (RF) pulses . From the excitation k‐space perspective , this applies the B 1 field at increments of the velocity Fourier variable (k v ) which is proportional to the first moment of the gradient waveform while erasing the undesired phase accrued by off‐resonance.…”

“…Figure a shows a refocused VS 90 ° excitation pulse sequence generated with the design parameters of velocity FOV (FOV v ) = 82 cm/s, full‐width‐half‐maximum excitation bandwidth = 18 cm/s and number of rectangular RF pulses = 5. A minor difference from the design described previously is the application of Malcolm‐Levitt (MLEV)‐4 phase cycling for the refocusing pulses instead of iterative numerical optimizations for further enhancement of B 0 and B 1 immunity.…”