Effects of transmission regime on left ventricle quantification.

<p>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 <b>Φ</b>₁ (squares) or the 4TX/4RX with phase setting <b>Φ</b>₂ (stars). Data points of the same subject were marked with identical colors.</p

Simulated local SAR_10g distributions at maximum applicable input power.

<p>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).</p

SAR and B₁⁺-values derived from EMF simulation.

<p>SAR and B₁⁺-values derived from EMF simulation.</p

Cardiac images derived from 2D SSFP CINE.

<p>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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0161863#pone.0161863.ref002" target="_blank">2</a>]. 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.</p

Photographs and simulation setups of the used RF coil configurations.

<p>(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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0161863#pone.0161863.ref023" target="_blank">23</a>]. (f) photograph of the 32-channel RX-only RF surface coil. In configuration (d) and (f) the body RF coil (BC) transmits.</p

Effects of transmit efficiency on minimal TR_SSFP.

<p>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 (<b>Φ</b>₃) 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.</p

Validation of EMF simulations.

<p>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.</p

Simulated excitation fields at specified SAR limits.

<p>(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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0161863#pone.0161863.ref002" target="_blank">2</a>]. (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 <i>δ</i> ≈ 0.7, which represents the RF input power difference of the body RF coil when operating at whole-body and local SAR limit.</p

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

<p>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 <b>Φ</b>₁ and <b>Φ</b>₂ at local SAR_10g limits.</p

Synopsis of the characteristics of the 7.0 T whole body MR system used.

<p>Synopsis of the characteristics of the 7.0 T whole body MR system used.</p