Comparing signal-to-noise ratio for prostate imaging at 7T and 3T

Abstract

Background: In MRI, the signal-to-noise ratio (SNR) theoretically increases with B0 field strength. However, because of attenuation of the radiofrequency (RF) fields at 7T, it is not certain if this SNR gain can be realized for prostate imaging. Purpose/Hypothesis: To investigate the SNR gain in prostate imaging at 7T as compared with 3T. It is expected that SNR will improve for prostate imaging at 7T compared with 3T. Study Type: Prospective. Subjects: Four healthy volunteers and one prostate cancer patient. Field Strength/Sequence: All subjects were scanned at 3T and at 7T using optimal coil setups for both field strengths. For all volunteers, proton density-weighted images were acquired for SNR analysis and actual flip angle imaging (AFI) (Formula presented.) maps were acquired for correction of measured SNR values. In the patient, a T2-weighted (T2w) image was acquired at 3T and at 7T. Assessment: SNR was calculated in the prostate region for all volunteers. SNR was normalized for flip angle, receiver bandwidth, and voxel volume. SNR was also calculated for different sensitivity encoding (SENSE) acceleration factors. Statistical Testing: SNR values are represented as the arithmetic mean of SNR values in the prostate. Estimated SNR in the T2w image is calculated as the arithmetic mean of the signal intensity (SI) divided by the standard deviation of the SI in a specified zone. Tumor-to-tissue contrast is calculated as (SItumor+SIzone)/(SItumor-SIzone). Results: An increase in SNR ranging from 1.7-fold to 2.8-fold was measured in the prostate at 7T in comparison to 3T for four volunteers. At 7T, it is possible to achieve a 4-fold SENSE acceleration in the left-right direction with similar SNR to a nonaccelerated 3T image. T2w imaging was done at 3T and 7T in one patient, where improved tumor-to-tissue contrast was demonstrated at 7T. Data Conclusion: SNR improves for prostate imaging at 7T as compared with 3T. Level of Evidence: 2. Technical Efficacy: Stage 1. J. Magn. Reson. Imaging 2018.