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Quantitative Evaluation of Regional RF shimming on a...

Abstract
MRI NetForum Team Philips Healthcare • Netherlands

Quantitative Evaluation of Regional RF shimming on a Wide Aperture Dual-Channel Multi-Transmit 3.0T: Implications



R. Krishnamurthy(1), A. Pednekar(2), M. Kouwenhoven(3), P. Harvey(3), C. Arena(4), B. Cheong(4), M. Kouwenhoven(3), and R. Muthupillai(4)
(1)Bioengineering, Rice University, Houston, Texas, United States, (2)Philips Healthcare, Houston, Texas, United States, (3)Philips Healthcare, Best, Netherlands, (4)Diagnostic and Interventional Radiology, St. Luke's Episcopal Hospital, Houston, Texas, United States

Introduction

At 3.0T and higher field strengths, the wavelength of the radio-frequency (RF) excitation becomes comparable to or smaller than the size of the human body. As a result, B1 field is non-uniform across the slice, and is an important cause of artifacts and degradation of image quality. Unlike Bo inhomogeneities, the loss of contrast experienced due to B1 inhomogeneity is irrecoverable, and is a problem that needs to be addressed during the excitation process. Recently multiport parallel RF transmission systems have been proposed as a means for improving B1 homogeneity [1-3]. The purpose of this study is to quantitatively evaluate the performance of a dual channel multi-transmit system to RF shim a region of interest, in a series of subjects.


Materials and Methods

Eleven normal subjects (8 male, 49 ± 16 yrs) were imaged on a wide-aperture 3.0T Ingenia (Philips Healthcare). All data acquisition was VCG gated. A combination of 16 channels from the table-top integrated digital posterior coil and 16 channels from the digital anterior coil were used for signal reception. A two channel multi-transmit system with independent RF control was used
for excitation. All subjects provided written informed consent.

B1 maps of the axial plane across the heart were generated using a saturation-recovery, dual flip angle method described previously [4-6]. The acquisition parameters were: TR/TE = 1000/5.7 ms; Nominal flip angle = 30°/60°; acquired voxel size = 5*10*10 mm3; and readout EPI factor: 11. Based on the acquired complex B1 map, the amplitude and phase settings of the two transmit channels were set independently to minimize the B1 variation within the prescribed volume of interest (Volume shim). The B1 maps were then acquired with and without volume RF shimming
for direct comparison.

On the B1 maps generated with and without volume RF shim settings, ROIs circumscribing the heart were drawn using a custom built software (MATLAB™, The MathWorks, Natick, MA). The B1 maps were scaled as a percent of the prescibed flip angle. The following metrics were used for quantitative evaluation of RF homogeneity: 1) The ratio of standard deviation (σ) to mean (μ)of the pixels within the ROI, where a lower ratio corresponds to more uniformity. 2) The fraction of the total number of pixels that fall within a specific percent of the mean. A higher count at a given
threshold corresponds to a more uniform B1 field distribution.


Representative magnitude images                        Plot showing the σ/μ variation of the               
(A and B), and B1 maps (C and D).                      flip angle distribution across the ROI.
Images A and C are without volume                     An average drop of 48 +/- 12 is seen,
RF shimming. ROI selected is shown                    corresponding to a more uniform B1 map
in red in all images.                                            in the case of a volume RF shim.
The σ/μ ratio revealed better RF homogeneity in each subject with subject-specific volume
Plot showing the mean number of voxels                 Representative image of a short
in the selected ROI that falls within a                       axis balanced FFE image. The B1 field
given flip angle from the mean.                               inhomogencity can be seen when volume
Volume RF shim performs better with                      RF shimming is not used (arrow),
96.9% of voxels in the ROI falling within                  which is reduced using volume RF shimming
+/- 10% of the mean flip.                                       RF shimming (B).


Results

The σ/μ ratio revealed better RF homogeneity in each subject with subject-specific volume shimming (Figure 2). The average σ/μ for the 11 subjects improved from 0.116 ± 0.03
without patient adaptive RF shimming to 0.058 ± 0.01 for patient adaptive volume shimming (p < 0.0001, paired Student’s t-test). This reduction corresponded to a mean increase in B1 homogeneity of 48 ± 12% with volume RF shimming. The total number of voxels that lie within a fraction of the mean flip angle was also evaluated (Figure 3). With volume RF shimming, 97% of the voxels lie within ± 10% of the mean flip angle across the ROI compared to only 76 % of the voxels without. Also, the mean value of the B1 map from volume RF shimming was closer to the prescribed flip angle (i.e., 100 %) - 85.7 ± 11.5% with volume RF shimming vs. 79.2 ± 12.7% without (p < 0.005). A representative image demonstrating the benefit of volume RF shimming using a multi-transmit system is shown in Figure 4. Note the substantial shading artifact seen near the anterior chest wall and RV without volume RF shimming.

Conclusions

The results from the study show the following: (a) At 3.0T even across a small region as the heart, effective flip angles can be in excess of 20% of the prescribed flip angle in over 25% of the pixels without RF shimming,; (b) Patient specific, volume RF shimming using a two-channel multi-transmit system is effective in both reducing the flip angle variation over a prescribed region of interest, e.g., heart, as well as help attain a flip angle that is closer to the prescribed flip angle. B1 shimming is an important component to be considered in all quantitative magnetic resonance imaging.


References

1. Roschmann, Med. Phys, 14(6), 1987.
2. U.Katscher et al., NMR in Biomed,19, 393-400, 2006.
3. Sung et al., JMRI, 27:643-648.
4. Cunningham et al., MRM, 55:1326-1333.
5. M.Schar, MRM, 63:419-426.
6. Harvey P.R., et al Proc. ISMRM 2010, 1486.


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Abstract
Ingenia 3.0T
dStream, MultiTransmit
 

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