Measurement and characterization of RF nonuniformity over the heart at 3T using body coil transmission.
OBJECTIVE
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.


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 mid-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 < or =3% in all subjects.


CONCLUSIONS
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.
Sung
Kyunghyun
K
Magnetic Resonance Engineering Laboratory, Ming Hsieh Department of Electrical Engineering-Systems, University of Southern California, Los Angeles, CA 90089-2564, USA. kyunghsu@sipi.usc.edu
Nayak
Krishna S
KS
D006321:Heart / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D011846:Radio Waves