B compensation in 3T cardiac imaging using short 2DRF pulses

Abstract

The purpose of this study was to determine if tailored 2DRF pulses could be used to compensate for in-plane variations of the transmitted RF field at 3T. Excitation pulse profiles were designed to approximate the reciprocal of the measured RF transmit variation where the variation over the left ventricle was approximated as unidirectional. A simple 2DRF pulse design utilizing three subpulses was used, such that profiles could be quickly and easily adapted to different regions of interest. Results are presented from phantom and in vivo cardiac imaging. Compared with conventional slice-selective excitation, the average flip angle variation over the left ventricle (measured as the standard deviation divided by the mean flip angle) was reduced with P < 0.001 and the average reduction was 41% in cardiac studies at 3T. The spatial distribution of the radio frequency (RF) field used for excitation and reception has an important influence on image quality in magnetic resonance imaging (MRI). Inhomogeneous RF transmission (B + 1 ) produces nonuniform flip angles, causing spatially dependent tissue contrast and signal intensity. This is a critical source of error when quantifying NMR parameters from image data. B + 1 nonuniformity is influenced by several factors including the distance from the RF transmit coil, conductivity, tissue dielectric constant, and factors related to the body size and RF wavelength. Methods for reducing B + 1 inhomogeneity are highly desirable in high field imaging (1-4) and imaging with surface coil transmission (5). In high-field cardiac imaging (≥ 3T), B + 1 inhomogeneity on the order of 30-50% across the imaging volume has been predicted and observed (1,(6)(7)(8)(9).One method to address B + 1 inhomogeneity is single transmit channel RF shimming using 2D or 3D tailored RF excitation pulses. Homogeneous tissue excitation can be achieved by tailoring the excitation with a spatial profile that is approximately equal to the reciprocal of the B using the tailored RF pulses have been proposed for neurologic applications (10,11). Diechmann et al. (10) described an RF excitation pulse that performs rapid 2D compensation and was effective for structural 3D brain imaging. Small tip-angle 3D tailored RF pulses designed by Saekho et al. (11) included slice selection and were also promising in neuro imaging. Both pulse designs, however, are limited to a small family of variation patterns and not fully based on B + 1 measurement. In 3T cardiac imaging, the patterns of B + 1 variation across the heart are different from the typical patterns in brain imaging. More severe and diverse patterns of variation are expected because the chest is of comparable dimension to the RF wavelength at 3T (1,6-9). Tailored RF pulse designs based on actual measurements are needed for robust B + 1 compensation. Until recently, this approach has been limited by (a) the lack of time-efficient and volumetric methods for B + 1 mapping, and (b) long duration of appropriate tailored RF pulses. The saturated double angle method (SDAM)...

Quantification of myocardial perfusion using CMR with a radial data acquisition: comparison with a dual-bolus method

“…Both spatial excitation (flip angle) variations and an imperfect slice profile likely played a role in the findings of a net flip angle for the blood region of 8.5 ± 2.2 degrees, instead of the 14° that was designated in the radial perfusion sequence. This nearly 40% reduction in flip angle is similar to that found previously in the heart at 3T [22]. A 23-48% variation in flip angle across the heart has also been reported at 3T [23].…”

Simultaneous B0‐ and B1+‐Map acquisition for fast localized shim, frequency, and RF power determination in the heart at 3 T

“…A critical component of the SDAM method is the saturation or reset pulse because of its sensitivity to B 0 and B 1 + inhomogeneities. In this study, an 8‐ms BIR‐4 pulse was applied and has successfully been used at 3 T (12–14). Recently, it has been shown that a tailored hard‐pulse train results in better saturation and less RF power deposition compared to an 8‐ms BIR‐4 saturation pulse (22).…”

“…Applying SDAM, Sung and Nayak (13) reported a flip angle variation ranging from 31 to 66% over the entire left ventricle volume. As the variation was predominantly unidirectional in short‐axis (SA) slices, they proposed to use 3‐ms‐long two‐dimensional RF pulses to reduce the average flip angle variation in SA slices over the left ventricle by 41% (14). Sung and Nayak (13) reported a flip‐angle distribution from 34° to 63° across the left ventricle, for a nominal flip angle of 60°.…”

MRI artifacts and correction strategies

“…As a result, the reconstructed images will have shading artifacts, variable contrast and lower SNR. These artifacts can be corrected using RF pulses that do not depend on field homogeneity, such as adiabatic pulses [32][33][34], or by directly compensating for the nonuniformity when designing the pulse [35]. Angiography results demonstrating the improvement in image quality using adiabatic pulses are shown in Figure 6.…”