Local Multi-Channel RF Surface Coil versus Body RF Coil Transmission for Cardiac Magnetic Resonance at 3 Tesla: Which Configuration Is Winning the Game?

Abstract

Introduction: The purpose of this study was to demonstrate the feasibility and efficiency of cardiac MR at 3 Tesla using local four-channel RF coil transmission and benchmark it against large volume body RF coil excitation.

Methods: Electromagnetic field simulations are conducted to detail RF power deposition, transmission field uniformity and efficiency for local and body RF coil transmission. For both excitation regimes transmission field maps are acquired in a human torso phantom. For each transmission regime flip angle distributions and blood-myocardium contrast are examined in a volunteer study of 12 subjects. The feasibility of the local transceiver RF coil array for cardiac chamber quantification at 3 Tesla is demonstrated.

Results: Our simulations and experiments demonstrate that cardiac MR at 3 Tesla using four-channel surface RF coil transmission is competitive versus current clinical CMR practice of large volume body RF coil transmission. The efficiency advantage of the 4TX/4RX setup facilitates shorter repetition times governed by local SAR limits versus body RF coil transmission at whole-body SAR limit. No statistically significant difference was found for cardiac chamber quantification derived with body RF coil versus four-channel surface RF coil transmission. Our simulation also show that the body RF coil exceeds local SAR limits by a factor of ~2 when driven at maximum applicable input power to reach the whole-body SAR limit.

Conclusion: Pursuing local surface RF coil arrays for transmission in cardiac MR is a conceptually appealing alternative to body RF coil transmission, especially for patients with implants.

Fig 1. Photographs and simulation setups of the used RF coil configurations.

(a) local four-channel TX/RX RF surface coil array (4TX/4RX) and (b) its EMF simulation setup loaded with the truncated human voxel model Duke. The feeding ports of the RF coil are marked in red. (c) basic circuit diagram of the 4TX/4RX RF coil. (d) photograph of the four-channel RX-only RF surface coil (4RX). (e) EMF simulation setup of the body RF coil loaded with the voxel model Duke. The detuned four-channel receive-only surface RF coil (4RX) is included to examine possible field distortions [23]. (f) photograph of the 32-channel RX-only RF surface coil. In configuration (d) and (f) the body RF coil (BC) transmits.

Fig 2. Simulated local SAR_10g distributions at maximum applicable input power.

Maximum projection images for voxel model Duke (top) and Ella (bottom) in coronal (1^st and 3^rd row) and axial orientation (2^nd and 4^th row) are shown. Formations of SAR hotspots can be seen for every transmission setup, with the dominating SAR hotspots being marked by arrows. Note: At the elbow twice the SAR is allowed to avoid a bias in favor of the 4TX/4RX configuration. Immoderate whole-body SAR limits lead to exceedance of local SAR limits (1^st column). For 4TX/4RX both transmit phase settings Φ₁ (3^rd column) and Φ₂ (4^th column) yield a local SAR_10g hotspot located underneath the shared middle conductor of the loop elements, where the currents can add up and the distance between the RF coil array and the body is minimal (1 cm/2 cm for the anterior/posterior part).

Fig 3. Simulated excitation fields at specified SAR limits.

(a) B₁⁺-fields for Duke (top) and Ella (bottom) are derived from EMF simulations for all transmission setups. To guide the eye the orthogonal slices and the borders of the heart are highlighted. 1^st column: BC/4RX scaled to whole-body SAR limit. 2^nd column: BC/4RX scaled to local SAR_10g limit. 3^rd and 4^th column: Transmission with 4TX/4RX RF coil (Φ₁ and Φ₂) scaled to local SAR_10g limit. The data shown in column 1, 3 and 4 reflect the maximum achievable excitation fields to stay within the safety limits governed by the IEC guidelines [2]. (b) Normalized histograms of the simulated excitation fields obtained for all voxels covering the heart of the human voxel model Duke (top) and Ella (bottom). The mean values of the B₁⁺-distributions are added as colored tick marks on the x-axes. The width of the curves reflects the B₁⁺-homogeneity within the heart. The red and blue curves are shifted horizontally by the factor δ ≈ 0.7, which represents the RF input power difference of the body RF coil when operating at whole-body and local SAR limit.

Fig 4. Validation of EMF simulations.

Comparison of simulated (1^st row) and measured (2^nd row) B₁⁺-maps of a mid-axial slice through a torso phantom (top) filled with a uniform myocardial tissue mimicking solution. Regions with angle-to-noise-ratios lower than 1% were discarded using thresholding. 3^rd row: Absolute difference maps (B₁⁺_simulation - B₁⁺_measurement) demonstrating a good agreement between simulations and experiments.

Fig 5. Cardiac images derived from 2D SSFP CINE.

Shown are end-diastolic phases of the cardiac cycle using standard cardiac views (denoted in the left line) of a healthy subject. The employed transmission regime is outlined on top of the figure for each column. Each image was windowed individually and its SNR within the heart is provided. Column 1, 3, 5 and 6 were acquired with the maximum flip angle allowed by SAR limits governed by the IEC guidelines [2]. Column 2 and 4 were derived by applying local SAR_10g limits for the body RF coil, which results in 30% reduced flip angle compared to whole-body SAR limit (1^st and 3^rd column). Please note the agreement in image quality obtained with the BC/4RX, BC/32RX and the 4TX/4RX RF coil configurations.

Fig 6. Quantitative analysis of 2D SSFP CINE images for a four-chamber view.

Assessment of in vivo performance of the transmission regimes illustrated for a four-chamber view of the heart of a healthy subject. The employed transmission regimes are outlined on top of the figure for each column. 1^st row: Measured B₁⁺-maps obtained with a 2D Bloch-Siegert technique. The borders of the heart are highlighted with a bold line. 2^nd row: Flip angle maps for SSFP CINE at specified SAR limit, which were derived from the B₁⁺-maps. 3^rd row: 2D SSFP CINE images of the same four-chamber view obtained at end-diastole using maximum flip angles at the specified SAR limit (denoted on top). 4^th row: Normalized signal intensity profiles along the lines drawn through the four-chamber view shown above. The green arrows indicate the position of the septum. The 1^st and 3^rd column represents the maximal applicable flip angle allowed by the IEC guidelines, i.e. the whole-body SAR limit. The 2^nd and 4^th column show the situation if local SAR_10g limits were applied for the body RF coil. The data shown in the 5^th and 6^th column were acquired with the 4TX/4RX RF coil with phase setting Φ₁ and Φ₂ at local SAR_10g limits.

Fig 7. Analysis of achieved SSFP-flip angles inside the heart of the scanned cohort.

The bar height reflects the mean FA thus the transmit efficiency. The error bars show the standard deviation thus the transmit homogeneity within the ROI. (a) and (b) The flip angle bars are grouped for each subject of a cohort of 12 subjects. Gender and BMI are provided. Averaging over the cardiac views (4CV, 3CV, 2CV, SAX) was performed. The data show the FA variability between individual subjects, which is minor for the body RF coil but more pronounced for the 4TX/4RX RF coil. (c) Mean flip angle resolved for different cardiac views. For each cardiac view FA data was averaged over all 12 subjects. The 4TX/4RX RF coil configuration yielded a B₁⁺-efficiency which outperformed the body RF coil when operated at local SAR_10g limit for all standard cardiac views except the two-chamber view. The last bar group shows the flip angle averaged over all subjects and all cardiac views. The 4TX/4RX RF coil configuration using phase setting Φ₂ yielded a B₁⁺-efficiency which is superior to that obtained for phase setting Φ₁ while having similar B₁⁺-homogeneity.

Fig 8. Effects of transmit efficiency on minimal TR_SSFP.

Shown are flip angle maps and 2D SSFP CINE images of an apical short axis view of the heart. Based on the FA-maps (top row) a target FA of 60° inside the ROI (blue contour) was set for the 2D SSFP CINE technique (bottom row). TR_SSFP was set to minimum so that SAR reached the denoted limits. When operating the body RF coil at whole-body/local SAR limit, the B₀ pass band of 2D SSFP CINE exhibited a width of 212 Hz/120 Hz. Severe banding artifacts can be seen for BC/4RX @ local SAR limit. When using the 4TX/4RX RF coil (Φ₃) a pass band of 263 Hz was achieved for 2D SSFP. This improvement helps to reduce SSFP related banding artifacts across the heart. The denoted SNR_rel is relative to the SNR obtained with the SSFP protocol (BW_RX = 1002 Hz/pixel) used for LV chamber quantification.

Fig 9. Effects of transmission regime on left ventricle quantification.

Left ventricle (LV) cardiac chamber quantification in a cohort of 11 subjects obtained for all transmission regimes were examined by evaluating a stack of short axis views ranging from apex to base. Bland-Altman plots of (a) LV end-diastolic volume (EDV), (b) LV end-systolic volume (ESV), (c) LV ejection fraction (EF) and (d) LV mass. No statistically significant differences were found between the reference (BC/32RX) and the BC/4RX setup (circles), the 4TX/4RX with phase setting Φ₁ (squares) or the 4TX/4RX with phase setting Φ₂ (stars). Data points of the same subject were marked with identical colors.