Table 1.
MRI system details.
Table 2.
Inversion recovery (IR) measurement protocols.
Table 3.
Variable flip angle (VFA) measurement protocols.
Fig 1.
An example coronal slice of the ISMRM/NIST system phantom through the NiCl2 array and resulting segmentation.
(A) The shortest inversion time image used for identification of sample spheres and (B) the segmentation with sample sphere centers identified.
Fig 2.
Inversion recovery measurements at 1.5 T.
The inversion recovery (IR) measurements at 1.5 T both over- and underestimated the T1,NMR. The circles represent the within group means, and the error bars are 95% confidence intervals about these means. The IR measurements, especially in the range of physiological T1 values (~250 ms for adipose tissue to 1800 ms for grey matter) are biased approximately 5% high. Both vendors exhibited this bias; there are no significant differences between them throughout the entire range of T1 times spanned by the ISMRM/NIST phantom array (Table 4).
Fig 3.
Inversion recovery measurements at 3 T.
At 3 T, the inversion recovery (IR) measurements generally overestimated the T1,NMR. The circles represent the within group means, and the error bars are 95% confidence intervals about these means. There were no differences between vendors C and D. By contrast, vendor E is biased almost 10% higher than vendors C and D for T1 values in the physiologically relevant range. Please see Table 5 for tests of significance.
Table 4.
ANOVA comparison for IR, VFA at 1.5 T.
Table 5.
ANOVA comparison for IR, VFA at 3 T.
Fig 4.
Variable flip angle measurements at 1.5 T.
The variable flip angle (VFA) measurements at 1.5 T had a broader range of deviations than the IR measurements (Fig 2), and again both over- and underestimated the T1,NMR. The circles represent the within group means, and the error bars are 95% confidence intervals about these means. There were significant (95% CI) differences between Vendors A & B for the two shortest T1 relaxation times; however, the T1 relaxation time of those spheres is below those values typically measured in the body.
Fig 5.
Variable flip angle measurements at 3 T.
At 3 T, the variable flip angle (VFA) measurements had a much broader range of deviations than the IR measurements (Fig 3). The circles represent the within group means, and the error bars are 95% confidence intervals about these means. Vendors C and D and D and E are significantly (95% CI) different for many spheres; p-values are given in Table 5. Vendors C and E generally overestimated the T1,NMR, while vendor D underestimated it. Finally, we observe a pattern in the vendor D deviation: The greatest deviation (largest underestimation) is for samples with T1 relaxation times 260 ms, 368 ms, 514 ms, which are located in the “chin” of the phantom (Fig 1, sample spheres 5–7).
Fig 6.
Reported tissue properties at 3 T.
Physiological values of normal and diseased tissue from [7–9]. Unless otherwise noted by a superscript, the reference is [9].
Fig 7.
Impact of vendor differences in T1 measurement.
Here, we have plotted the reported T1 of low grade glioma and glioblastomas [8] and an estimate for each vendor system of the diagnostic range for low grade glioma and glioblastomas based on the bias and dispersion of that system. The challenge is to define a diagnostic criterion based on T1 to distinguish low grade glioma from glioblastoma that would be suitable across vendor systems. If T1 relaxation time is measured using IR (A), the overestimate of values by vendor E is small compared to the range of physiological values, and as a result, T1 measured by IR could be a reliable measure across vendor systems. However, if the VFA method is used (B), the underestimate of T1 on vendor D could inaccurately diagnose a glioblastoma as a low-grade glioma, an incorrect determination with serious impacts to patient management.