Minimum SNR and acquisition for bias-free estimation of fractional anisotropy in diffusion tensor imaging — a comparison of two analytical techniques and field strengths Youngseob Seo a,b, ⁎ , Zhiyue J. Wang a,b , Michael C. Morriss a,b , Nancy K. Rollins a,b a Department of Radiology, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA b Department of Radiology, Children's Medical Center Dallas, Dallas, TX 75235, USA Received 11 November 2011; revised 4 March 2012; accepted 1 April 2012 Abstract Although it is known that low signal-to-noise ratio (SNR) can affect tensor metrics, few studies reporting disease or treatment effects on fractional anisotropy (FA) report SNR; the implicit assumption is that SNR is adequate. However, the level at which low SNR causes bias in FA may vary with tissue FA, field strength and analytical methodology. We determined the SNR thresholds at 1.5 T vs. 3 T in regions of white matter (WM) with different FA and compared FA derived using manual region-of-interest (ROI) analysis to tract-based spatial statistics (TBSS), an operator-independent whole-brain analysis tool. Using ROI analysis, SNR thresholds on our hardware– software magnetic resonance platforms were 25 at 1.5 T and 20 at 3 T in the callosal genu (CG), 40 at 1.5 and 3 T in the anterior corona radiata (ACR), and 50 at 1.5 T and 70 at 3 T in the putamen (PUT). Using TBSS, SNR thresholds were 20 at 1.5 T and 3 T in the CG, and 35 at 1.5 T and 40 at 3 T in the ACR. Below these thresholds, the mean FA increased logarithmically, and the standard deviations widened. Achieving bias-free SNR in the PUT required at least nine acquisitions at 1.5 T and six acquisitions at 3 T. In the CG and ACR, bias-free SNR was achieved with at least three acquisitions at 1.5 T and one acquisition at 3 T. Using diffusion tensor imaging (DTI) to study regions of low FA, e.g., basal ganglia, cerebral cortex, and WM in the abnormal brain, SNR should be documented. SNR thresholds below which FA is biased varied with the analytical technique, inherent tissue FA and field strength. Studies using DTI to study WM injury should document that bias-free SNR has been achieved in the region of the brain being studied as part of quality control. © 2012 Elsevier Inc. All rights reserved. Keywords: Diffusion tensor imaging; SNR threshold; Bias-free FA; Manual ROI analysis; Tract-based spatial statistics 1. Introduction Diffusion tensor imaging (DTI) provides information about white matter (WM) microstructure and integrity not possible with conventional magnetic resonance (MR) imaging [1–3]. Numerous studies have reported alterations in diffusion tensor metrics with disease states which, when affecting the cerebral WM, usually cause fractional anisot- ropy (FA) to fall [4,5]. Prospective studies using DTI as a potential biomarker for disease or response to therapy are often conducted at multiple centers or at a single institution on different MR platforms at different times. Whether tensor data acquired at different field strengths at different sites can be compared continues to be debated. Problems with reproducibility of FA measurements have led to calls for standardization of sequence parameters, although less attention has been paid to quality control issues which may also impact the reproducibility of FA measurements. DTI is most often acquired with parallel imaging to decrease scan time and motion. Moreover, the noise within accelerated images is nonhomogeneous with higher signal Available online at www.sciencedirect.com Magnetic Resonance Imaging 30 (2012) 1123 – 1133 Abbreviations: ACR, anterior corona radiata; CG, callosal genu; DTI, diffusion tensor imaging; FA, fractional anisotropy; g-factor, geometric factor; PRIDE, Philips Research Image Development Environment; PUT, putamen; ROI, region of interest; SD, standard deviation; SNR, signal-to- noise ratio; TBSS, tract-based spatial statistics; WM, white matter. ⁎ Corresponding author. Tel.: + 1 214 456 1481; fax: + 1 214 456 6015. E-mail address: Youngseob.Seo@UTSouthwestern.edu (Y. Seo). 0730-725X/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2012.04.015