Joint k-b space diffusion-weighted image reconstruction and apparent diffusion coefficient fitting for diffusion-weighted turbo-spin-echo imaging

Background: Diffusion-weighted (DW) turbo-spin-echo (TSE) imaging offers improved geometric fidelity compared to single-shot echo-planar-imaging (EPI). However, it suffers from low signal-to-noise ratio (SNR) and prolonged acquisition times, thereby restricting its applications in diagnosis and MRI-guided radiotherapy (MRgRT).

Purpose: To develop a joint k-b space reconstruction algorithm for concurrent reconstruction of DW-TSE images and the apparent diffusion coefficient (ADC) map with enhanced image quality and more accurate quantitative measurements.

Methods: The joint k-b reconstruction model was formulated as an optimization problem subject to a self-consistency condition comprising the exponential decay relationship between DW images and the ADC map. The objective function included a data fidelity term confirming an agreement between the reconstructed images and measured k-space data, incorporating the mono-exponential decay relationship between DW images and ADC map as a constraint, along with spatial regularization terms on both amplitude and phase images. The optimization problem was solved using the alternating-direction method of multipliers (ADMM). We evaluated the performance of the joint reconstruction (JR) algorithm for DW-TSE in a phantom study and patient studies on five brain and head/neck cancer patients. Image distortion, accuracy, and repeatability of ADC measurements were assessed and compared with those from conventional Fourier-transformed (FFT)-based reconstruction and magnitude-based ADC fitting methods, as well as with the clinically used DW-EPI technique.

Results: The proposed joint k-b reconstruction method demonstrated improved SNR and enhanced accuracy of ADC measurements in DW-TSE compared to the conventional method. Additionally, JR-DW-TSE with fewer averages at high b-values provided better image quality than the conventional FFT-reconstructed DW-TSE with full averages.

Conclusion: The proposed joint k-b reconstruction method for DW-TSE has the potential to deliver distortion-robust diffusion-weighted MRI (DWI) for clinical applications, which is particularly crucial for MRgRT.