A standard system phantom for magnetic resonance imaging

Abstract

Purpose A standard MRI system phantom has been designed and fabricated to assess scanner performance, stability, comparability and assess the accuracy of quantitative relaxation time imaging. The phantom is unique in having traceability to the International System of Units, a high level of precision, and monitoring by a national metrology institute. Here, we describe the phantom design, construction, imaging protocols, and measurement of geometric distortion, resolution, slice profile, signal‐to‐noise ratio (SNR), proton‐spin relaxation times, image uniformity and proton density. Methods The system phantom, designed by the International Society of Magnetic Resonance in Medicine ad hoc committee on Standards for Quantitative MR, is a 200 mm spherical structure that contains a 57‐element fiducial array; two relaxation time arrays; a proton density/SNR array; resolution and slice‐profile insets. Standard imaging protocols are presented, which provide rapid assessment of geometric distortion, image uniformity, T1 and T2 mapping, image resolution, slice profile, and SNR. Results Fiducial array analysis gives assessment of intrinsic geometric distortions, which can vary considerably between scanners and correction techniques. This analysis also measures scanner/coil image uniformity, spatial calibration accuracy, and local volume distortion. An advanced resolution analysis gives both scanner and protocol contributions. SNR analysis gives both temporal and spatial contributions. Conclusions A standard system phantom is useful for characterization of scanner performance, monitoring a scanner over time, and to compare different scanners. This type of calibration structure is useful for quality assurance, benchmarking quantitative MRI protocols, and to transition MRI from a qualitative imaging technique to a precise metrology with documented accuracy and uncertainty.

Multi-site, multi-platform comparison of MRI T1 measurement using the system phantom

“…Across all measurements, reported temperature of either the MRI room or of the bulk water in the phantom ranged from 17.1°C to 23.3°C. Previous research demonstrated that the T 1 of NiCl 2 solutions vary by ± 4% over this experimental range [ 20 ]. Therefore, we expect that the variation of T 1 due to temperature is negligible compared to other sources of measurement error (see S1 Fig for additional details).…”

“…The focus of this study was the NiCl 2 array (previously called the T 1 array) in the ISMRM/NIST system phantom. The NiCl 2 array was chosen since it has a smaller temperature and field dependence than other available reference arrays [ 20 ]. The NiCl 2 array contains 14 spheres that are doped with varying concentrations of NiCl 2 to achieve a progression of T 1 values from approximately 20 ms to 2000 ms at 1.5 T. The reference T 1 times at 1.5 T and 3 T were determined using the NMR-based relaxation time measurement service provided by NIST.…”

“…Incorrect temperature measurement (e.g., measuring the temperature of the room rather than the temperature of the phantom) did not require reacquisition of the data. Temperature changes are not expected to impact our study, as T 1 times for NiCl 2 are known to be relatively insensitive to temperature over the range 16°C to 26°C, and the 10 highest NiCl 2 concentration spheres have less than ± 4% variation over this range [ 20 ].…”

“…For each of these spheres, the regions of interest were defined as the collection of pixels within each sphere and well-separated from the boundary. Previous publications describe the details of the ROI identification algorithm [ 18 ] and [ 20 ]. In brief, we applied a gradient filter to the measured image, then thresholded the result to define a binary image of region edges.…”

“…The purpose of this study was to determine the variability in T 1 measurements on a variety of MRI systems to ascertain the feasibility of applying diagnostic threshold T 1 measurements across multiple clinical sites. We used the International Society of Magnetic Resonance in Medicine/National Institute of Standards and Technology (ISMRM/NIST) system phantom [ 20 ] to assess variations of T 1 measurements across MRI systems at 1.5 tesla (T) (two vendors, with number of MRI systems n = 9) and 3 T (three vendors, n = 18).…”