Volumetric and multispectral DWI near metallic implants using a non‐linear phase Carr‐Purcell‐Meiboom‐Gill diffusion preparation

Abstract

Purpose DWI near metal implants has not been widely explored due to substantial challenges associated with through‐slice and in‐plane distortions, the increased encoding requirement of different spectral bins, and limited SNR. There is no widely adopted clinical protocol for DWI near metal since the commonly used EPI trajectory fails completely due to distortion from extreme off‐resonance ranging from 2 to 20 kHz. We present a sequence that achieves DWI near metal with moderate b‐values (400–500 s/mm2) and volumetric coverage in clinically feasible scan times. Theory and Methods Multispectral excitation with Cartesian sampling, view angle tilting, and kz phase encoding reduce in‐plane and through‐plane off‐resonance artifacts, and Carr‐Purcell‐Meiboom‐Gill (CPMG) spin‐echo refocusing trains counteract T2* effects. The effect of random phase on the refocusing train is eliminated using a stimulated echo diffusion preparation. Root‐flipped Shinnar‐Le Roux refocusing pulses permits preparation of a high spectral bandwidth, which improves imaging times by reducing the number of excitations required to cover the desired spectral range. B1 sensitivity is reduced by using an excitation that satisfies the CPMG condition in the preparation. A method for ADC quantification insensitive to background gradients is presented. Results Non‐linear phase refocusing pulses reduces the peak B1 by 46% which allows RF bandwidth to be doubled. Simulations and phantom experiments show that a non‐linear phase CPMG pulse pair reduces B1 sensitivity. Application in vivo demonstrates complementary contrast to conventional multispectral acquisitions and improved visualization compared to DW‐EPI. Conclusion Volumetric and multispectral DW imaging near metal can be achieved with a 3D encoded sequence.

Diffusion-weighted MRI of the spinal cord in cervical spondylotic myelopathy after instrumented fusion

“…Koch et al provided the first demonstration of MSI principles in DW-MRI (16), leveraging a fusion of 2D-MSI (17) and split-blade (18) Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) (19) DW techniques (20). More recently, Lee et al (21) proposed a 3D-MSI approach by developing carefully matched MSI-specific radiofrequency pulses for diffusion sensitization. In the present report, we utilize the 2D approach of Koch et al ( 16), which has recently been demonstrated in preliminary applications for spinal cord imaging (22), assessment of soft tissue pathology near total hip replacements (23), and quantitatively assessed utilizing standardized phantoms (24).…”