Combined renal and peripheral MRA with a new technique at 3.0 T

Introduction: The proliferation and increased use of 3.0 T MR Systems offers both potential advantages and difficulties in the implementation of unenhanced MRA techniques. The increased signal-to-noise ratio (SNR) that scales linearly with the field-strength can be utilized in decreasing acquisition time, increasing spatial resolution or a combination of both. The implementation of unenhanced techniques can often be problematic with increased specific absorption rates, increased magnetic susceptibility effects and radiofrequency (RF) field inhomogeneity at higher field strengths. Irrespective of the magnetic field strength, an unenhanced MRA technique should incorporate fast acquisition times and relatively flow independence. We propose a novel unenhanced MRA technique for Renal MRA and MRA of the Lower Extremities employing the quiescent interval single shot (QISS) MRA sequence. Materials and Methods: To perform an initial feasibility study followed by technical optimization of quiescent interval single shot (QISS) magnetic resonance angiography for renal artery and lower extremity evaluation. The study was performed with IRB approval. Seven healthy volunteers underwent combined unenhanced Renal and Peripheral Lower Limb Angiography. The QISS technique acquires data using a modified ECG-triggered, fat suppressed, 2D balanced steady-state free precession pulse sequence incorporating slice selective saturation and a quiescent interval (QI) for maximal enhancement of inflowing blood (Fig.5). The QISS technique for unenhanced Renal MRA was investigated with both a spoiled gradient echo, spgr (FLASH) and balanced steady-state free precession, bSSFP (TruFISP) readout and with ECG and pulse gating for both techniques. Similarly, for its implementation in the peripheries, bSSFP and spoiled gradient echo, spgr readouts were both evaluated at 3.0 T. The effect of increased field strength, increased field inhomogeneity and dielectric effect was initially evaluated with a pulsatile flow. Renal MRA Protocol: Saggital oblique slab orientation was used to acquire right and left renal arteries to prevent in-plane saturation in orthogonally orientated renal vessels using a breath hold technique. Imaging parameters were as follows: QISS bSSFP technique consists of two saggital oblique slabs for evaluation of each renal artery. Each slab consists of 20 slices; 15mm slice thickness (1.2 mm effective slice thickness, 0.6 mm slice overlap), TE 1.4ms, TI 350ms, flip angle 44° time, delay (TD) TD=0 for pulse gating, FOV read 400mm, FOV phase 65%, matrix 250 x 384, voxel size 1 x 1 x 3, bandwidth 685 Hz/Px.. The QISS spgr sequence parameters consisted of two saggital oblique slabs for each renal artery. Each slab consisted of 20 slices; 3mm slice thickness (2.4 mm effective slice thickness, 0.6 mm slice overlap), TE 1.5, flip angle of 25°, delay (TD) TD=0 for pulse gating, FOV read 400mm, FOV phase 65%, matrix 250 x 384, voxel size 1 x 1 x 3, bandwidth 685 Hz/Px. Both techniques employed a parallel imaging (PI) factor of two, 5/8 partial Fourier using center-to-out trajectory.