Journal
JOURNAL OF MAGNETIC RESONANCE IMAGING
Volume 27, Issue 3, Pages 643-648Publisher
JOHN WILEY & SONS INC
DOI: 10.1002/jmri.21253
Keywords
cardiac MRI; RF nonuniformity; saturated double angle method; high-field MRI; prescan calibration
Funding
- NHLBI NIH HHS [R21 HL 079987, R01 HL 074332] Funding Source: Medline
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Purpose: To measure and characterize variations in the transmitted radio frequency (RF) (B1+) field in cardiac magnetic resonance imaging (MRI) at 3 Tesla. Knowledge of the B1+ field is necessary for the calibration of pulse sequences, image-based quantitation, and signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) optimization. Materials and Methods: A variation of the saturated double-angle method for cardiac B1+ mapping is described. A total of eight healthy volunteers and two cardiac patients were scanned using six parallel short-axis slices spanning the left ventricle (LV). B1+ profiles were analyzed to determine the amount of variation and dominant patterns of variation across the LV. A total of five to 10 measurements were obtained in each volunteer to determine an upper bound of measurement repeatability. Results: The amount of flip angle variation was found to be 23% to 48% over the LV in inid-short-axis slices and 32% to 63% over the entire LV volume. The standard deviation (SD) of multiple flip angle measurements was < 1.4 degrees over the LV in all subjects, indicating excellent repeatability of the proposed measurement method. The pattern of in-plane flip angle variation was found to be primarily unidirectional across the LV, with a residual variation of <= 3% in all subjects. Conclusion: The in-plane B1+ variation over the LV at 3T with body-coil transmission is on the order of 32% to 63% and is predominantly unidirectional in short-axis slices. Reproducible B1+ measurements over the whole heart can be obtained in a single breathhold of 16 heartbeats.
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