Evaluation of a subject specific dual-transmit approach for improving B1 field homogeneity in cardiovascular magnetic resonance at 3T

Abstract

Background: Radiofrequency (RF) shading artifacts degrade image quality while performing cardiovascular magnetic resonance (CMR) at higher field strengths. In this article, we sought to evaluate the effect of local RF (B1 field) shimming by using a dual-source-transmit RF system for cardiac cine imaging and to systematically evaluate the effect of subject body type on the B1 field with and without local RF shimming.

Methods: We obtained cardiac images from 37 subjects (including 11 patients) by using dual-transmit 3T CMR. B1 maps with and without subject-specific local RF shimming (exploiting the independent control of transmit amplitude and phase of the 2 RF transmitters) were obtained. Metrics quantifying B1 field homogeneity were calculated and compared with subject body habitus.

Results: Local RF shimming across the region encompassed by the heart increased the mean flip angle (μ) in that area (88.5 ± 15.2% vs. 81.2 ± 13.3%; P = 0.0014), reduced the B1 field variation by 42.2 ± 13%, and significantly improved the percentage of voxels closer to μ (39% and 82% more voxels were closer to ± 10% and ± 5% of μ, respectively) when compared with no RF shimming. B1 homogeneity was independent of subject body type (body surface area [BSA], body mass index [BMI] or anterior-posterior/right-left patient width ratio [AP/RL]). Subject specific RF (B1) shimming with a dual-transmit system improved local RF homogeneity across all body types.

Conclusion: With or without RF shimming, cardiac B1 field homogeneity does not depend on body type, as characterized by BMI, BSA, and AP/RL. For all body types studied, cardiac B1 field homogeneity was significantly improved by performing local RF shimming with 2 independent RF-transmit channels. This finding indicates the need for subject-specific RF shimming.

Figure 1

B₁maps of an axial slice across the heart without (A) and with (B) local RF shimming. The values are expressed as percentage of the intended flip angle experienced. The rectangular box corresponds to the region being shimmed. An elliptical region of interest (ROI) was manually drawn circumscribing the heart. Metrics characterizing B₁ field homogeneity were calculated within this ROI.

Figure 2

Location of six manually drawn regions of interest (ROIs) on the let ventricle. The myocardium in the short-axis view of the near-basal location in b-SSFP cine acquisition is shown. The measurements were made in systole.

Figure 3

Variation of the mean percentage flip angle with subject body habitus. The variation of μ with body mass index (A), body surface area (B), and anterior-posterior/right-left (AP/RL) ratio (C) is shown. Because no clear pattern of variation can be discerned, the mean flip angle does not appear to depend on body type. RF, radiofrequency; W/o, without.

Figure 4

Nomogram plotting the coefficient of signal variation (C_v) across the region of interest for all body types. An average decrease of 42 ± 13% is seen (P < 0.000001; paired Student t test), corresponding to a more uniform B₁ field in case of subject-specific radiofrequency (RF) shimming. Changes in C_v with body mass index (BMI) (A), body surface area (BSA) (B), and anterior-posterior/right-left ratio (AP/LR) (C) are shown. As with the mean flip angle, a reduction in C_v is not dependent on body type. The number of subjects with 1a) BMI < 25 kg/m²:14; 1b) BMI = 25.1–30 kg/m²:9; 1c) BMI =30.1–35 kg/m²:7; 1d) BMI > 35 kg/m²:7; 2a) BSA < 1.9 m²: 11; 2b) BSA = 1.9–2.1 m²: 11; 2c) BSA> 2.1 m²:15; 3a) AP/RL < 0.6:8; 3b) AP/RL = 0.6–0.7:22 ; 3c) AP/RL >0.7:7. W/o, without.

Figure 5

Changes in the coefficient of signal variation (C_v) with and without (W/o) radiofrequency (RF) shimming for different body types. Also shown is the variation of the differences in C_v values (ΔC_v) for different body types – varying body mass indexes (A, B), body surface areas (C, D), and anterior-posterior/right-left ratios (AP/RL) (E, F) are shown. No significant relationship is found between C_v (and ΔC_v) and body type, either with or without local RF shimming.

Figure 6

Cumulative histogram plottingthe total number of voxels that fall within a given range from the mean flip angle in the selected region of interest (ROI). The dotted vertical lines indicate the ± 5% and ± 10% limits. In the case of subject-specific local radiofrequency (RF) shimming, 88 ± 12% of the voxels inside the ROI fall within ± 10% of the mean value. This value decreases significantly when no RF shim is used. Abs, absolute; W/o, without.

Figure 7

Plot showing the power amplitude ratio (expressed in dB) with variation of the relative transmit phase in all 37 subjects. A wide variation is seen in both the relative phase and amplitude in all subjects. Changes with body mass index (BMI) (A), body surface area (BSA) (B), and anterior-posterior/right-left ratio (AP/LR) (C) are shown. No clear pattern of variation is found between the spread of amplitude/phase and subject body type.

Figure 8

The Image Integral Uniformity Values. The Image Integral Uniformity (IU) was calculated for each patient image obtained with and without local RF shimming. A significant reduction in inhomogeneity is noticed when local RF shimming is employed (23.7 ± 11.8 vs. 16.7 ± 6.9). This reduction is statistically significant (p = 0.013; paired Student’s t-test) and highlights the beneficial effects of local RF shimming on image signal homogeneity.

Figure 9

Representative balanced steady-state free-precession images of the left ventricle. The 4-chamber (A) and short-axis (B) views from subjects with various body types, shown in increasing order of BSA (Left to Right) is shown. Without (W/o) radiofrequency (RF) shimming, B₁ field inhomogeneity is observed (top row) in both A and B; this inhomogeneity is reduced with RF shimming (bottom row). BMI, body mass index; BSA, body surface area; AP/RL, anterior-posterior/right-left ratio.