Velocity-selective (VS) magnetization-prepared non-contrast-enhanced peripheral MR angiography (MRA) has shown great promise at 1.5T but might be challenging at 3T due to the sensitivity of VS excitation to B₀ and B₁ variation which tends to be large in the pelvis and legs. The purpose of this study is to identify image artifacts caused by B₀ and B₁ offsets and to propose strategies to suppress them with validations in human subjects including a patient with peripheral artery disease.

A refocused VS excitation pulse sequence has shown to reduce off-resonance sensitivity but at the cost of increased B1 sensitivity [1]. The effects of both B₀ and B₁ offsets can be further reduced by increasing the number of 180° pulses (N_refoc) in each velocity encoding step and weighting the refocusing pulses by MLEV phase cycling schemes [2,3]. Figure 1 contains VS pulse sequences with double refocusing (N_refoc = 2) and quadruple refocusing (N_refoc = 4). The Bloch simulations show that both designs well preserve the pass-band M_z profile over a wide range of B₀ and B₁ offsets (Figs. 1b, 1c, 1d and 1f). The stopband signal, however, remains to vary in proportion to the B₁ scale that directly affects the rectangular RF pulses.

Imperfect 180° refocusing causes image artifacts in stationary tissues since the unipolar gradients preceding and following the refocusing pulse fail to rephrase the spins. The residual modulation of longitudinal magnetization at the end of the sequence would be periodic with a period of λ_G defined as [(γ/2π)∫G_uni(τ)dτ]^-1 (where G_uni(τ) is unipolar gradient) as seen in the simulation (1st row in Fig. 2). As a simple solution to suppress the stripe artifact, we apply another VS preparation pulse sequence with the same excitation profile as the original sequence except being shifted in the spatial dimension by half λ_G (2nd row in Fig. 2). This additional VS sequence can be obtained by adding an RF phase waveform that is proportional to the area of a gradient waveform and the desired amount of shifting (i.e. 0.5 λ_G) to the original RF waveform B₁(t). The simulated M_z after the two successive VS preparations show that the stripe artifact and overall background signal level are significantly reduced (3rd row of Fig. 2).

In-vivo experiments were performed on four thighs and four calves of healthy subjects using a 3 T MR scanner (Tim-TRIO; Siemens Medical Solutions). Single, double and quadruple refocused VS preparation pulse sequences were tested for comparisons. For each of the 3 VS preparations, single preparation and two preparations with shifted excitation profiles were applied. Quadruple-refocused VS-MRA was performed in a patient who was referred for digital subtraction angiography (DSA).

Figure 3 shows representative pelvis angiograms reformatted through curved maximum-intensity-projection (MIP) of 3D raw data as well as B₀ and B₁ maps reformatted through average intensity projection on the same volume of interest as used for the MIP operation of angiograms. Single refocused VS preparation (Fig. 3a) yields mild (open arrowheads) and severe (solid arrowheads) signal loss in the left iliac arteries due to low B₁ field (arrowheads in Fig. 3h). Whereas, both double and quadruple refocused VS preparations well avoid the arterial signal loss in the same regions of low B₁ field (Figs. 3c and 3e) and suppress the stripe artifacts as well as the variation in background signal through two successive preparations with spatially shifted excitation profiles (Figs. 3d and 3f). Figure 4 shows good correlation of VS-MRA with DSA in a 76-year-old female patient with stenosis in the femoral arteries.

In calf MRA, the relative contrast ratio is 0.69±0.14 and 0.68±0.15 for double and quadruple refocused VS preparations, respectively and is 0.85±0.08 and 0.83±0.07 when additional preparations with shifted excitation profiles are applied. In pelvis MRA, the relative contrast ratio is increased from 0.77±0.11 and 0.78±0.10 (one preparation) to 0.89±0.06 and 0.86±0.07 (two preparations) for double and quadruple refocused designs, respectively.

We have identified image artifacts associated with peripheral VS-MRA at 3T, including signal loss in the arteries, stripe artifact and spatial variation in the background signal, and proposed a strategy to reduce the artifacts. The arterial signal loss can be avoided by using double or quadruple refocused excitation, which is immune to a wide range of B₀ and B₁ inhomogeneity. The stripe artifact and background signal variation can be suppressed by successive application of two VS preparations with excitation profiles spatially shifted by half the period of the stripes, as validated by increased mean and decreased standard deviation of relative contrast ratio.