Body Diffusion Weighted Imaging Using Non-CPMG Fast Spin Echo

Abstract

SS-FSE is a fast technique that does not suffer from off-resonance distortions to the degree that EPI does. Unlike EPI, SS-FSE is ill-suited to diffusion weighted imaging (DWI) due to the Carr-Purcell-Meiboom-Geill (CPMG) condition. Non-CPMG phase cycling does accommodate SS-FSE and DWI but places constraints on reconstruction, which are resolved here through parallel imaging. Additionally, improved echo stability can be achieved by using short duration and highly selective DIVERSE radiofrequency pulses. Here, signal-to-noise ratio (SNR) comparisons between EPI and nCPMG SS-FSE acquisitions and reconstruction techniques give similar values. Diffusion imaging with nCPMG SS-FSE gives similar SNR to an EPI acquisition, though apparent diffusion coefficient values are higher than seen with EPI. In vivo images have good image quality with little distortion. This method has the ability to capture distortion-free DWI images near areas of significant off-resonance as well as preserve adequate SNR. Parallel imaging and DIVERSE refocusing RF pulses allow shorter ETL compared to previous implementations and thus reduces phase encode direction blur and SAR accumulation.

Isotropic resolution diffusion tensor imaging of lumbosacral and sciatic nerves using a phase‐corrected diffusion‐prepared 3D turbo spin echo

“…In the case of DW ss‐EPI, remaining geometric distortions and/or slice profile effects can lead to the incorrect estimation of the DTI metrics in the sciatic nerves. Trends of higher diffusivity metrics measured with DW TSE‐based methods compared with DW ss‐EPI have already been observed .…”

Slice profile effects on nCPMG SS‐FSE

“…First, image reconstruction is made more complex as the quadrature component of the signal receives a 61 modulation throughout the echotrain that leads to FOV=2 aliasing in the image space. This issue has been previously discussed at length in the original publication (3) and subsequent work (5). Second, the refocusing RF pulses must provide a consistent flip angle above 120 from one echo to the next throughout the echotrain as reported in Ref.…”

“…Each image was reconstructed based on the split echo hybrid‐SENSE reconstruction as described in Ref. .…”

“…GðsðtÞÞ ¼ GðtÞ; [5] which is written here as fB 1 ðsÞ; GðsÞg. The difference between the DIVERSE pulse generation process and the Time-Optimal VERSE (TO-VERSE) algorithm proposed in Ref.…”

“…Both minimum echo time (TE ¼ 30.4 ms) as well as repetition time (TR ¼ 800 ms) were used in a multi-slice sequence with slice interleaving using a skip factor of 4 (i.e., in a 12 slice sequence, slice were acquired with the order of [1,5,9,2,6,10,3,7,11,4,8,12]). Images were acquired by doubling the field-of-view to better visualize the intermediate aliasing caused by any instability in the nCPMG echotrain that arises for low refocusing flip angles.…”