Parallel Imaging

One recent option that can be used quite successfully with CE MRA is parallel imaging (e.g. sensitivity encoding or SENSE [58]; and simultaneous acquisition of spatial harmonics or SMASH [5961]). This approach requires the use of phased array coils, a sensitivity "reference" scan, and special ized software (SENSE, Philips Medical Systems, Best, The Netherlands; ASSET, General Electric Medical Systems, Waukesha, WI, USA; GRAPPA and SENSE, Siemens Medical Solutions, Erlangen, Germany). Each element of a phased array coil has its own sensitivity profile. Parallel imaging exploits the known sensitivities of each phased array coil element to reduce the density of the acquired k-space data, thereby reducing the imaging scan time. Effectively, this creates undersampled, heavily aliased datasets that can then be unfolded using spatial sensitivity maps. Individual sensitivity mapping of each coil element is determined by performing a preliminary reference scan. The subsequent CE MRA time can be reduced by a factor of 2 or more (acceleration factor or AF). The main compromise concerns the SNR, which is diminished by the square root of the acceleration factor plus a small additional amount related to the geometry of the coils and FOV. Parallel imaging strategies can be implemented to provide improved spatial resolution (Figs. 7,15) or to reduce scan durations for patients with limited breath holding capacity (Fig. 16).

Parallel Magnets
Fig. 15a-c. Fibromuscular dysplasia. Characteristic "string of beads" appearance (arrowheads) of fibromuscular dyslasia can be seen in both renal arteries on this optimized breath hold 3D CE MRA (a, coronal MIP; b, axial sub-volume MIP; c, coronal VR projection) using parallel imaging
Magnetic Parallels

Fig. 16a-c. A fast 7-second renal 3D CE MRA performed during free breathing in a patient unable to hold his breath. Although the images are slightly blurred, the high-grade severe stenosis (arrow) of the right renal artery is clearly seen (a, coronal MIP; b, coronal VR; c, oblique sub-volume MIP). Review of the thinner sub-volume MIP is essential to identify continuity of the renal artery lumen across the lesion and to determine that there is a high-grade stenosis, rather than occlusion of the right renal artery

Fig. 16a-c. A fast 7-second renal 3D CE MRA performed during free breathing in a patient unable to hold his breath. Although the images are slightly blurred, the high-grade severe stenosis (arrow) of the right renal artery is clearly seen (a, coronal MIP; b, coronal VR; c, oblique sub-volume MIP). Review of the thinner sub-volume MIP is essential to identify continuity of the renal artery lumen across the lesion and to determine that there is a high-grade stenosis, rather than occlusion of the right renal artery

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