Jeff Snyder^{1}, Peter Seres^{1}, Robert W Stobbe^{1}, Justin G Grenier^{1}, Penelope Smyth^{2}, Gregg Blevins^{2}, and Alan H Wilman^{1}

^{1}Biomedical Engineering, University of Alberta, Edmonton, AB, Canada, ^{2}Department of Medicine, Division of Neurology, University of Alberta, Edmonton, AB, Canada

A fast reconstruction method for PD-T2w T2 maps is presented and compared to the previous L2 norm minimization technique using Bland-Altman analysis in a five patient multiple sclerosis data set acquired at 3 T. The new (subtraction) technique was in excellent agreement with the L2 norm method (confidence intervals of -1.1 to +1.2 ms), with average single slice reconstruction times of 0.6 s compared to 134 s. The speed of T2 map production allowed accurate (based on sequence simulation via Bloch equations) inline T2 maps directly on the MRI console.

$$$ S_n (B1^+, T2)=\frac{S_{PD}-S_{T2}}{S_{PD}} =1-\frac{S_{T2}}{S_{PD}} .$$$ (1)

S

$$$ I_n=\frac{I_{PD}-I_{T2}}{I_{PD}} =1-\frac{I_{T2}}{I_{PD}} .$$$ (2)

The algorithm begins by iterating through each pixel in I

The two methods were compared using Bland-Altman analysis (12,13) of a five multiple sclerosis patient data set. All images were acquired using a Siemens Prisma 3 T scanner with an 80 mT/m gradient set and a 64-channel head and neck array for reception. Standard clinical PD-T2 parameters included: 16 echoes, TE1 = 10 ms, TE2 = 93 ms, 10 ms echo spacing, TR = 2500 ms, flip angle = 165°, 0.94 x 0.94 mm2 in-plane resolution with 3 mm slices (50 total) and TA = 2 min 43 s. A B1

A meta-analysis of each pixel in the five patient data set (2243585 pixels) is illustrated in the Bland-Altman plot in Figure 2. The confidence interval (CI) varies from -1.2 to 1.1 ms, indicating that 95% of all differences in T2 value between the old and new method are within this range. A significant concentration occurs near 0 (blue in 2b) with infrequent outliers (light colored). Statistics for Bland-Altman analysis for each subject are shown in Table 1.

The fast reconstruction of the subtraction method map allowed evaluation of reconstruction on the MRI console, shown in Figure 3 for the non-MS subject. Computation time took 50 s for the 50 slice set, and DICOM images were then included within the study for further analysis.

The authors gratefully acknowledge funding from the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council of Canada.

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Figure 1: Calculated
T2 maps for Patient #2 for a) the L2 norm minimization method and b) the
subtraction method, with the difference between methods shown in d). The brain mask, c), shows the region where
the differences were calculated. Outside
the brain mask region, both methods have difficulty assessing the T2 (typically
due to CSF and scalp regions). The
difference image in d) is scaled from 0.5 ms to 2 ms, with differences larger
than 2 ms clipped as red.

Figure 2:
Bland-Altman plot for each pixel in the five patient data set, displayed in
histogram format. The colorbar denotes
the number of pixels. Note the
concentration of values around the mean of -0.04 ms.

Table 1: Comparison of Bland-Altman statistics for the
difference between the two fitting methods.

Figure 3: Illustrative slice from the T2 map produced
directly on the scanner and included in DICOM series.

DOI: https://doi.org/10.58530/2022/0432