Externally calibrated parallel imaging for 3D multispectral imaging near metallic implants using broadband ultrashort echo time imaging.

To develop an externally calibratedparallel imaging technique for three-dimensional multispectral imaging(3D-MSI) in the presence of metallic implants.A fast, ultrashort echo time (UTE) calibrationacquisition is proposed to enable externally calibratedparallel imaging techniques near metallic implants. The proposed calibrationacquisition uses a broadband radiofrequency (RF) pulse to excite the off-resonance induced by the metallic implant, fully phase-encoded imaging to prevent in-plane distortions, and UTE to capture rapidly decaying signal. The performance of the externally calibratedparallel imaging reconstructions was assessed using phantoms and in vivo examples.Phantom and in vivo comparisons to self- calibratedparallel imaging acquisitions show that significant reductions in acquisition times can be achieved using externally calibratedparallel imaging with comparable image quality. Acquisition time reductions are particularly large for fully phase-encoded methods such as spectrally resolved fully phase-encoded three-dimensional (3D) fast spin-echo (SR-FPE), in which scan time reductions of up to 8 min were obtained.A fully phase-encoded acquisition with broadband excitation and UTE enabled externally calibratedparallel imaging for 3D-MSI, eliminating the need for repeated calibrationregions at each frequency offset. Significant reductions in acquisition time can be achieved, particularly for fully phase-encoded methods like SR-FPE. Magn Reson Med 77:2303-2309, 2017. © 2016 International Society for Magnetic Resonance in Medicine.