QA measurements were obtained for a minimum of 33 months and a maximum of 38 months. 144 monthly OAI, 129 monthly ACR and 16 annual ACR measurements were included in the analysis. Uniformly high quality artifact-free study images were obtained from all four MR facilities. Over the three year period, key criteria for quantitative cartilage morphometry were found to be well within target specifications. All QA performance criteria for MR system stability were met with two exceptions: Knee coil signal uniformity and signal levels varied substantially over time and 3.0mm and 5.0mm slice thickness was consistently larger than expected.
The inside end-to-end length of the ACR phantom was measured superior-inferior (SI) on a central sagittal slice. The longitudinal variation of the inner length was generally within ±0.5mm (±0.51 pixel), except when the phantom was mispositioned (, ). However the length was consistently measured to be slightly smaller than the nominal value of 148.0mm.
Longitudinal measurements of ACR phantom geometry.
Example longitudinal variation in ACR (A) and OAI (B) phantom length (Site A).
The inside diameter of the ACR phantom was measured anterior-posterior (AP), right-left (RL) and along both diagonals on two axial slices, one at isocenter and one at +50mm along the SI axis. The ACR phantom nominal inner diameter was 190.0mm and longitudinal variation was generally within ±0.5mm (±0.51 pixel) (, ). The inner diameter and inner length measurement over time had ≤0.2mm (±0.56 pixel) standard deviations for all sites. Inner diameter longitudinal stability was most accurate and consistent using the RL and two diagonal axis measurements and had <1.0mm variation with most measurements within 0.5mm of the nominal value. The AP axis measurements had greater variability due to the occasional presence of an air bubble.
Example longitudinal variation in ACR (A) and OAI (B) phantom diameter (Site B).
The slice thickness was determined using:
where “top” and “bottom” are the measured wedge lengths visible at 50% of the maximum signal intensity on the first axial slice. The slice thickness measurements were found to vary only slightly over time <0.2mm (), with variations resulting from errors in phantom alignment and/or slice placement. However, the slice thickness on all four systems averaged 3.68mm for 3mm slices (monthly ACR) using a TSE acquisition and 5.52mm for 5mm slices (annual ACR) using a SE acquisition, which was 22% and 10% larger than the nominal value, respectively.
Longitudinal variation of slice thickness.
Ghost levels were assessed using a large region of interest (ROI) in the phantom center and four small ROIs located above, below, left and right outside the phantom. Ghost level was defined as the mean noise signal in the phase encode direction minus the mean noise in the frequency encode direction expressed as a percentage of the mean signal inside the phantom. Ghost levels were typically ≤0.2%. A service call was made anytime they exceeded 0.5%.
The slice position wedge offset difference was measured on the first and last axial slices (±50mm) and had a maximum allowed offset of ±5.0mm. The wedge offset was always <±2.0mm and was influenced by a combination of z-gradient amplitude calibration and z-gradient non-uniformity.
Low contrast visibility was measured on four low-contrast disks located in axial slices 8–11 (+20mm to +50mm). Each disk had a different thickness, with thicker disks having lower signal. Each disk contained ten ‘spokes’ of three holes each and the hole diameter changed from spoke to spoke. The low contrast object systematically had 38–40 of 40 spokes visible for the PD, T1W and T2W acquisitions and >30 spokes visible for the IW acquisition, if the phantom was aligned properly and the slice prescription was placed through the test objects. These values comply with ACR recommendations for 3T performance.
High contrast spatial resolution was measured on the first axial slice and the smallest objects (0.9mm) were always visualized.
Image uniformity was measured in the center of the phantom using a large ROI (85% of phantom area) and two small ROIs located inside the larger ROI that identify the minimum and maximum signal. Image uniformity:
varied (77.5%–89.5%), with an average across all four sites of 85.5%. SNR of the head coils varied (53.6–91.9), with an average across all four sites of 73.9. Both signal uniformity and SNR decreases were observed as head coils aged with often dramatic improvements when the coil was replaced. Head coil SNR increased by ~44% around December 2005 because the number of averages in the MR acquisition doubled. This change was made to decrease the influence of the noise when measuring phantom diameters.
ACR Cross-Site Calibration
All OAI ACR phantoms were scanned at one site on one day using the Annual ACR acquisitions. In addition to standard quantitative analyses, the variation of phantom inner diameter as a function of angle and slice location was also assessed [4
]. Similar measurements were performed at each OAI site using the site phantom within a week of the central measurements.
When measured at one site, the inner lengths and diameters were consistent for all phantoms. However, when each phantom was measured at its individual OAI site, small differences were observed between the local and central measurements (, ) due to differences in gradient amplitude calibrations.
Central site measurement results for individual ACR phantoms.
Central site measurement results for individual ACR phantoms. The axial inner diameter (A) nominal value is 190.0mm. The sagittal inner length (B) nominal value is 148.0mm.
The inside end-to-end length of the OAI phantoms were measured SI on the mid-phantom sagittal slice. Inner lengths did not vary by more than ±0.25mm (±0.55 pixel; ). Typical sagittal inner lengths measured at R60 and L60 were indistinguishable when measured one month apart. Combined R60, L60 measurements resulted in standard deviations of ≤0.2mm (<0.55 pixel) for all sites ().
Longitudinal measurement results for the OAI phantom geometry.
The inner diameters of the OAI phantoms were measured AP, RL and along both diagonals on an axial slice located at isocenter. Typical diameters measured at R60 and L60 were highly correlated and varied by ≤0.25mm (≤0.68 pixel). The largest inner diameter changes were along the RL axis due to the fill port location, until November 2005, when the phantom was rotated to align the port location with any air bubbles. Thereafter, the largest changes were along the AP axis. All sites had <±0.25mm (<±0.68 pixel) longitudinal variation in inner diameter (, ), although one or more episodic changes of ~0.5mm were observed. Both R60 and L60 locations had only small inner diameter standard deviations (≤0.12mm, ≤0.33 pixel), with R60 at Site D ≤0.3mm (≤0.82 pixel), and Site C R60 and L60 at ≤0.2mm (≤0.55 pixel). Site B experienced small seasonal changes (0.28%) measured in both phantom diameter and length ( and ) which is greater than expected from only thermal expansion/contraction of the acrylic OAI phantom.
Three-dimensional spherical volumes were computed by summing the area contained within the exterior surface of the inner compartment for each slice of the DESS acquisition as determined from automated edge detection. The spherical volumes varied very little over time (, ) and variations were further reduced in November 2005 when the phantom was rotated to place the fill ports vertically. Longitudinal variation ranged from 0.24%–0.72% CV, with an overall RMS CV of 0.46%.
Example longitudinal variation in OAI phantom spherical volume (Site B).
SNRs for both outer and inner phantom regions were systematically and substantially lower for L60 than R60 by 10–15% at two sites for the majority of the quadrature transmit/receive knee coils (). There were exceptions at the other two sites where the R60-L60 SNR difference was very small and SNR remained stable (sagittal) or increased (axial) over time. Knee coil SNR generally trended downwards over time. When the SNR dropped precipitously or gradually decreased to 85–90% of the initial value, the coil was replaced by one of the two on-site backups and was subsequently sent for replacement.
Longitudinal SNR from OAI phantom.
Ghost levels were measured on the central axial image along the phase encode axis in the same manner as for the ACR phantom and were found to be <0.2%. Falsely high automated analysis results were reported when the phantom fill ports contained fluid. Upon manual review, no ghost problems were identified.
T2 relaxation times were computed on the central sagittal image by fitting a single exponential to the signal intensities from the second through seventh echo times on a pixel-by-pixel basis. The spatial variation of the T2 value () was assessed by dividing the outer compartment into quadrants on the central sagittal image. For all MR systems, except Site C, the right and left region T2 values were indistinguishable (). The T2 values of the anterior and posterior halves had complex relationships however. The T2 value longitudinal change ranged 2.3%–18.9%. The absolute T2 value was significantly lower at Site A than that of the same solution measured on the other MR systems. In addition, Site A had T2 value longitudinal variations well outside the expected range (13.3% and 18.8% CV). At Site B, small periodic changes in T2 value were observed (~3.5% CV).
Spatial and longitudinal variation for the OAI phantom outer compartment T2 relaxation times.
Example outer compartment T2 values for OAI phantom (Site D). The OAI phantom was dropped and replaced in August 2004. The knee coil was replaced in August 2005.
OAI Cross-Site Calibration
All OAI phantoms were scanned on one day at one site (). The same acquisitions and quantitative evaluations were performed as for the monthly OAI QA evaluations. Similar measurements were then performed within one week at each OAI site using the site phantom.
Central site measurement results for individual OAI phantoms.
Phantom inner length and diameter were consistent when measured at one site, and measurements performed at each site were consistent with those made at one site (). T2 values varied slightly during the single site measurement, with the Site B phantom having a significantly lower value (3.1%). The original Site A outer compartment solution was lost in shipping, hence cross-calibration and comparison to historic measured values was not possible. T2 measurements made at each site were systematically lower than those measured during cross-calibration, possibly due to temperature differences or, more likely, variations in coil excitation/refocus angle performance.