About the dependence of Gaussian diffusion and Kurtosis parameters on SNR in prostate DWIs

Purpose / Introduction Diffusion Weighted Imaging (DWI) plays an increasingly important role in diagnosis of prostate cancer, that is the second most common malignancy in men worldwide[1]. The evaluation of signal-to-noise ratio (SNR) is an important step to estimate image quality and the reliability of diffusion-derived parameters, in particular when strong diffusion weightings are employed. In order to avoid DWI signal to fall below the noise floor, Descoteaux et al.[2] suggest SNR>10. However, it is not easy to choose the background region of interest (ROI) on prostate DWI, since there is no visible background air if the FOV is small. Therefore, in literature, the noise is evaluated in different prostate background regions, such as air in rectum[3], obturator internus muscle[4] or air outside subject body. In this work we aim to evaluate the dependence of mean diffusivity (MD), fractional anisotropy (FA), apparent kurtosis (K) and K-derived apparent diffusion coefficient (ADCK) parameters on SNR. Moreover, we show how SNR depends on the b-values, on the number of signal averages (NSA) and also on the choice of the background-ROI. Subjects and Methods This study was realized on 5 healthy volunteers, following the written informed consent. All MRI examinations were performed with a 3T Siemens Biograph mMR. DWIs were acquired with a Multi-Band Accelerated EPI Pulse Sequences (TE=86ms,TR=3000ms,vox.size=1.8x1.8x3mm3,multiband acceleration factor=2), along 6 directions and with 8 different b-values (0,100,250,500,1000,1500,2000,2500 s/mm2). For each subject, a high-resolution T2-weighted image was acquired with a Turbo Spin-Echo sequence (TE=112ms,TR=4310ms,voxel size=0.6x0.6x3mm3). ROIs were drawn in central gland (CG), rectum, obturator internus muscle and air on DWIs. SNRs were calculated by dividing the mean signal in CG by the standard deviation (SD) in background; SNR was evaluated for each background-ROI, for each directions and in function of the NSA. MD, FA, K and ADCK maps were calculated.Results SNR is dependent on the NSA and also on the choice of the background-ROI: the highest SNR is the one evaluated considering the SD of air area. Muscle-ROI provides the worst prostate SNR, instead rectum-ROI manifests an intermediate trend. ADCK and MD value increase with NSA, while K values decrease with NSA, according to Russell Glenn et al[5].Discussion / Conclusion This work demonstrated that the choice of background region and the number of repetitions deeply influence the SNR calculation. Our results show that in order to obtain a SNR>10, 4 averages for low b-values (0,100,250,500s/mm^2) and 6 averages for high b-values (1000,1500,2000,2500s/mm^2) are necessary to extract reliable diffusion parameters.References [1] Ferlay, J. et al., Int. J. Cancer, (2015): E359–E386. [2] Descoteaux M. et al., IEEE Trans. Med. Imaging (2009); 28: 269-286. [3] Mazaheri Y. et al., Acad Radiol. (2013); 20(8): 1041–1047. [4] Kaji Y. et al., J. Magn. Reson. Imaging (2007);25:517–526. [5] Russell Glenn G. et al., Magnetic Resonance Imaging (2015); 33:124-133.