Abstract
Multi-parametric MRI is increasingly used for prostate cancer detection. Improving information from current sequences, such as T2-weighted and diffusion-weighted (DW) imaging, and additional sequences, such as magnetic resonance spectroscopy (MRS) and chemical exchange saturation transfer (CEST), may enhance the performance of multi-parametric MRI. The majority of these techniques are sensitive to B0-field variations and may result in image distortions including signal pile-up and stretching (echo planar imaging (EPI) based DW-MRI) or unwanted shifts in the frequency spectrum (CEST and MRS). Our aim is to temporally and spatially characterize B0-field changes in the prostate. Ten male patients are imaged using dual-echo gradient echo sequences with varying repetitions on a 3 T scanner to evaluate the temporal B0-field changes within the prostate. A phantom is also imaged to consider no physiological motion. The spatial B0-field variations in the prostate are reported as B0-field values (Hz), their spatial gradients (Hz/mm) and the resultant distortions in EPI based DW-MRI images (b-value = 0 s/mm2 and two oppositely phase encoded directions). Over a period of minutes, temporal changes in B0-field values were ≤19 Hz for minimal bowel motion and ≥30 Hz for large motion. Spatially across the prostate, the B0-field values had an interquartile range of ≤18 Hz (minimal motion) and ≤44 Hz (large motion). The B0-field gradients were between −2 and 5 Hz/mm (minimal motion) and 2 and 12 Hz/mm (large motion). Overall, B0-field variations can affect DW, MRS and CEST imaging of the prostate.
Highlights
Introduction ce an us criProstate cancer (PCa) is the second largest cause of male cancer deaths in the UK (Caul and Broggio 2016) making PCa assessment a necessity
Improving information from current sequences, such as T2-weighted and diffusionweighted (DW) imaging, and additional sequences, such as magnetic resonance spectroscopy (MRS) and chemical exchange saturation transfer (CEST), may enhance the performance of multi-parametric MRI. The majority of these techniques are sensitive to B0 -field variations and may result in image distortions including signal pile-up and stretching (echo planar imaging (EPI) based DW-MRI) or unwanted shifts in the frequency spectrum (CEST and MRS)
The SNR increases for the 3D sequence and the range of measured B0 -field values reduces within the prostate
Summary
Introduction ce an us criProstate cancer (PCa) is the second largest cause of male cancer deaths in the UK (Caul and Broggio 2016) making PCa assessment a necessity. Echo planar imaging (EPI) based DW-MRI sequences are an integral part of mpMRI due to their high tumour contrast and short acquisition time (Kirkham et al2013, Metens et al 2012). They often exhibit shift, shears and geometric distortions in the phase encoding (PE) direction caused by a combination of low bandwidth in the PE direction and the presence of off-resonance effects, such as B0 field inhomogeneities and susceptibility differences at the tissue-air interfaces
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