Quantitative quality assurance in a multicenter HARDI clinical trial at 3T

Abstract

A phantom-based quality assurance (QA) protocol was developed for a multicenter clinical trial including high angular resolution diffusion imaging (HARDI). A total of 27 3T MR scanners from 2 major manufacturers, GE (Discovery and Signa scanners) and Siemens (Trio and Skyra scanners), were included in this trial. With this protocol, agar phantoms doped to mimic relaxation properties of brain tissue are scanned on a monthly basis, and quantitative procedures are used to detect spiking and to evaluate eddy current and Nyquist ghosting artifacts. In this study, simulations were used to determine alarm thresholds for minimal acceptable signal-to-noise ratio (SNR). Our results showed that spiking artifact was the most frequently observed type of artifact. Overall, Trio scanners exhibited less eddy current distortion than GE scanners, which in turn showed less distortion than Skyra scanners. This difference was mainly caused by the different sequences used on these scanners. The SNR for phantom scans was closely correlated with the SNR from volunteers. Nearly all of the phantom measurements with artifact-free images were above the alarm threshold, suggesting that the scanners are stable longitudinally. Software upgrades and hardware replacement sometimes affected SNR substantially but sometimes did not. In light of these results, it is important to monitor longitudinal SNR with phantom QA to help interpret potential effects on in vivo measurements. Our phantom QA procedure for HARDI scans was successful in tracking scanner performance and detecting unwanted artifacts.

Keywords: HARDI; Multicenter; Phantom; QA; SNR.

Fig. 1

Apparatus for reproducible positioning of the BIRN phantom. Left: adapter ring allowed use of standard holder for cylindrical phantoms for Siemens scanners. Center: detail of adapter rings. Right: GE scanners used standard holders for spherical phantoms.

Fig. 2

Phantom images with different severities of spiking artifact ranging from severe (a) to minor (b).

Fig. 3

Percent signal ghosting (PSG) over time from 3 scanners. Examples of mild (scanner #22, 1–10), severe (scanner #25, 11–17) and normal (scanner #23, 18–23) levels of ghosting. One box represents one QA dataset for PSG values from all slices and volumes. For each box, the central mark is the median, the edges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data values not considered outliers and the outlier points are plotted individually.

Fig. 4

Eddy current distortion parameters indicate values among all slices and diffusion-weighted volumes from qualifying phantom scans. The bulls-eye is the median. The edges of the box are the 25th and 75th percentiles, and the outlier points are plotted individually.

Fig. 5

Variation of eddy current distortion parameters over time for 3 selected scanners. (a) Skyra (#11), (b) Trio (#25) and (c) GE Discovery MR750W (#2). In scanner #2, there was a reduction in eddy current distortion between months 2 and 3 associated with a change in head coil. For each box, the central mark is the median, the edges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data values not considered outliers, and the outlier points are plotted individually.

Fig. 6

(a) Correlation in SNR between phantom and volunteer scans. (b) Volunteer scan-rescan reproducibility using a Bland-Altman plot. The dotted lines represent the 95% confidence intervals.

Fig. 7

Comparison of SNR values. (a) SNR values of qualifying scans across 27 MR scanners. (b) Example of variation over time of SNR for one Trio scanner, #17. Straight lines show the thresholds.

Fig. 8

Temporal mean and standard deviation of MD and FA for each site. The labeling for the sites is the same as in Fig. 4.