Abstract
In prostate Diffusion Weighted MRI, differences in susceptibility values exist at the interface between the prostate and rectal-air. This can result in off-resonance magnetic field leading to geometric distortions including signal stretching and signal pile-up in the reconstructed images. Using a set of EPI data acquired with blip-up and blip-down phase encoding gradient directions, model based reconstruction has recently been proposed that can correct these distortions by using a B0 field estimated from a separate B0 scan. However, change in the size of the rectal air region across time can occur that can result in a mismatch of the B0 field to the EPI scan. Also, the measured B0 field itself can be erroneous in regions of low Signal to Noise ratio around the prostate rectal air interface. In this work, using a set of single shot EPI data acquired with blip-up and blip-down phase encoding gradient directions, a novel joint model based reconstruction is proposed that can account for changes in the off resonance effects between the B0 and EPI scans. For ten prostate patients, using a measured B0 field as an initial B0 estimate, on a 5-point scale (1-5) image quality scores evaluated by an experienced radiologist, the proposed framework achieved scores of 3.50 ± 0.85 and 3.40 ± 0.51 for b-values of 0 and 500 s/mm2, respectively compared to 3.40 ± 0.70 and 3.30 ± 0.67 for model based reconstruction. The proposed framework is also capable of estimating a distortion corrected EPI image even without an initial B0 field estimate in situations where a separate B0 scan cannot be obtained due to time constraint.
Highlights
Prostate diffusion MRI scans are used for tumour detection within a multi-parametric MRI protocol
For patient 1, results for the proposed joint reconstruction without an initial B0) Ed2own ( (B0) field estimate are shown in Fig. 1 for data acquired at bvalues of 0 and 500 s/mm2
The variation in B0 at the prostate rectal air interface resulted in significant artifacts
Summary
Prostate diffusion MRI scans are used for tumour detection within a multi-parametric MRI protocol. In some patients the diffusion scans are of limited use due to geometric distortions including signal pile-up and signal drop out at the interface between prostate and rectalair These distortions occur due to susceptibility differences at the interface creating an additional off-resonance field (called B0 field) so that the position-frequency relationship is changed. This results in signal stretching in areas within the image where the gradient of the B0 field has the same polarity as the phase encoding gradient. Several techniques have been proposed to correct for both signal pile-up and stretching distortions in diffusion EPI images These techniques acquire the same slice twice with opposite phase encoding gradient directions, resulting in two data sets, blip-up and blip-down [1,2,3]. The model based distortion correction techniques may benefit from an update of the B0 field estimation as part of the reconstruction problem
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