Improved brain MR imaging from a compact, lightweight 3T scanner with high performance gradients.
Norbert Campeau1, Yunhong Shu1, Joshua D Trzasko1, Erin M Gray1, Thomas K.F. Foo2, Matt A Bernstein1, and John Huston1

1Mayo Clinic, Rochester, MN, United States, 2GE Global Research, Niskayuna, NY, United States


A compact, low-cryogen 3T MRI scanner with high-performance gradients capable of simultaneously achieving 80 mT/m and 700 T/m/s was compared to a 60-cm, whole body 3T system (50 mT/m, 200T/m/s) for 5 routine brain MR imaging sequences in 9 clinical patients, graded by two neuroradiologists. The compact 3T system performed equally well to a standard whole-body scanner in terms of motion artifacts, and performed better in terms of signal-to-noise ratio, lesion conspicuity, gray/white contrast, susceptibility artifacts and overall exam quality.


The purpose of this NIH-funded initiative was to design, build, and evaluate a high-performance, low-cryogen, compact 3T (C3T) MRI system with high-performance gradients, capable of scanning heads, extremities and infants1,2,3. The reduced size, low liquid helium (<12 L) and light weight of the compact 3T system offers ease of siting. The initial goal was to show parity with a whole body system in terms of image quality and acquisition time. We previously reported findings of superior imaging with the compact 3T compared to a whole body system when evaluating a sagittal T2-weighted 3D FLAIR sequence in 16 human volunteers4. This work now extends the evaluation to other brain MRI sequences (T1 MP-RAGE, 3D T2 FLAIR, T2 FSE, GRE EPI, and DWI) obtained in 9 previously-unreported, patient subjects undergoing routine clinical imaging of the brain.


Under an IRB-approved protocol, 9 clinical patients undergoing a head MR examination consented to be scanned both on a standard, 60-cm bore, whole-body MR system with 50 mT/m, 200 T/m/s gradients (GE Discovery MR750, GE Healthcare, Waukesha, WI) and the compact 3T (C3T) with 80 mT/m 700 T/m/s gradients using an 8-channel receiver coil (In-vivo, Gainesville, FL). Imaging sequences evaluated included sagittal T1-weighted 3D MP-RAGE, sagittal T2-weighted 3D FLAIR, axial T2 FSE, axial GRE EPI, and DWI. Parameters used for these pulse sequences are summarized in Table 1. The C3T used real-time gradient pre-emphasis5 and frequency shifting to compensate additional concomitant fields arising from the asymmetric design of the transverse gradient coils. The 3D T2 FLAIR and T1 MP-RAGE sequences used a self-calibrating data-driven parallel imaging (ARC)6 acceleration factor of R = 2. The imaging studies were reviewed by two board-certified neuroradiologists with 19 and 27 years of experience. They were graded on a five-point ordinal scale from -2 to +2, with +2 indicating strong preference for the compact system and -2 indicating strong preference for the whole-body system, +1 and -1 representing preference for C3T or whole-body system respectively, and 0 representing no system preference. Each pair of images was comparatively evaluated (non-blinded) using the following attributes: signal-to-noise ratio (SNR), lesion conspicuity, motion artifact, gray/white matter contrast, susceptibility artifacts, cerebellar folia conspicuity, and overall exam quality. Statistical analysis was performed using a one-sided Wilcoxon signed rank tests7. The null hypothesis of the left-sided test was that the C3T performed equally to or better than the standard whole-body 3T MR system; and the right-sided test used the reverse of this hypothesis.


Figure 1 shows comparison images from the two scanners obtained in the same patient with small vessel ischemic disease. Table 2 shows summary of the results for left and right tailed Wilcoxon signed-rank statistics for all evaluated parameters across the five imaging sequences. Presuming a 5% significance level, the right-sided test results indicate that the C3T outperformed the 60-cm bore whole body system in terms of overall quality for all of the evaluated sequences, except the T1-weighted MP-RAGE. All left-sided tests also failed to reject the null hypothesis, indicating that the compact 3T system performed as good as the whole-body system for all five evaluated sequences. Histograms showing pooled results for overall quality comparison between the C3T and body systems are presented in Figure 2, showing comparison results for the five evaluated imaging sequences. The significance results of the right-tailed test are overlaid onto these histograms in Figure 2.


There is strong statistical evidence that the C3T provides equal or better image quality for the five of the most commonly performed brain pulse sequences evaluated across this patient sample set. The unchanged incidence of motion artifact also suggests that the compact system offers at least equivalent patient comfort. The high slew-rate gradient system of the C3T scanner allows for an echo spacing reduction of 25% compared to the whole-body system. For a long echo train length sequence such as 3D T2 FSE, this echo space reduction can be used to acquire higher resolution images while maintaining similar readout duration and SNR (Figure 3).


The compact, low-cryogen 3T MRI scanner with high-performance gradients provided equal or better image quality for five standard clinical brain MR imaging sequences in a small patient cohort.


This work was supported in part by NIH grants BRP-R01-EB010065 and U01 EB024450.


[1] Lee S-K, Mathieu J-B, Graziani D, Piel J, Budesheim E, Fiveland E, et al. Peripheral nerve stimulation characteristics of an asymmetric head-only gradient coil compatible with a high-channel-count receiver array. Magn Reson Med. 2016; 76:1939-1950.

[2] Mathieu J-B, Lee S-K, Graziani D, Lin J, Budesheim E, Piel JE, et al. Development of a Dedicated Asymmetric Head-only Gradient Coil for High-Performance Brain Imaging with a High PNS Threshold. Proceedings of the ISMRM Annual Meeting, Toronto: 2015, p. 1019.

[3] Weavers PT, Shu Y, Tao S, Huston J, Lee S-K, Graziani D, et al. Technical Note: Compact three-tesla magnetic resonance imager with high-performance gradients passes ACR image quality and acoustic noise tests. Med Phys 2016;43:1259–64. doi:10.1118/1.4941362.

[4] Weavers PT, Campeau NG, Shu Y, Tao S, Trzasko JD, Gray EM, Foo TK, Bernstein MA, Huston J. Improved T2-weighted 3D FLAIR from a compact, lightweight 3T scanner with high performance gradients. Proceedings of the ISMRM Annual Meeting, Honolulu, Hawaii:2017.

[5] Tao S, Weavers PT, Trzasko JD, Shu Y, Huston J, Lee S, et al. Gradient pre-emphasis to counteract first-order concomitant fields on asymmetric MRI gradient systems. Magn Reson Med. 2017, 77:2250-2262.

[6] Brau ACS, Beatty PJ, Skare S, Bammer R. Comparison of reconstruction accuracy and efficiency among autocalibrating data-driven parallel imaging methods. Magn Reson Med 2008;59:382–95.

[7] Siegel S. Nonparametric Statistics for the Behavioral Sciences. McGraw-Hill; 1956.


Table 1. Pulse sequence parameters of the five evaluated pulse sequences are listed for both the compact and whole body scanner systems.

Table 2. Summary of left- and right-tailed Wilcoxon Signed-Rank statistics for all evaluated parameters across all imaging sequences. These results provide strong statistical evidence that the compact 3T provides equal or better image quality for the five specific brain pulse sequences evaluated across this patient sample set. Under the presumed 5% significance level, the null hypothesis was rejected for all sequences except for the T1 weighted MP-RAGE. No significance was also observed for the left-tailed tests of these metrics, suggesting equivalent performance by both systems. (* denotes statistically significant results p < 0.05)

Figure 1. Comparison images between the compact 3T (top row) and whole body 3T scanner (bottom row) in a patient with chronic small vessel ischemic changes. The compact 3T system demonstrates better SNR, lesion conspicuity, gray/white matter contrast, and overall quality. Note the susceptibility artifact differences on the diffusion image, more marked on the whole body system (arrow).

Figure 2. Histograms summarizing the radiologic evaluation of all seven criteria across all patients for both readers. P-values of the right-tailed Wilcoxon Signed-Rank test are shown, with null hypothesis that the whole-body (WB) system performed better than or equal to the compact 3T system (C3T).

Figure 3. Improved SNR and increased image resolution on the compact 3T scanner. Advantages of the high gradient slew-rate available with the compact 3T system over the conventional whole body system, include the ability to acquire images with reduced echo spacing. This allows acquisition of higher resolution images with less T2 signal decay from later echoes.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)