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

Krishnamurthy R, Pednekar A, Kouwenhoven M, Cheong B, Muthupillai R - J Cardiovasc Magn Reson (2013)

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Radiology, Texas Heart Institute at St, Luke's Episcopal Hospital, 6720 Bertner Avenue, Houston, TX 77030, USA.

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.

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Changes in the coefficient of signal variation (Cv) with and without (W/o) radiofrequency (RF) shimming for different body types. Also shown is the variation of the differences in Cv values (ΔCv) 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 Cv (and ΔCv) and body type, either with or without local RF shimming.
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Figure 5: Changes in the coefficient of signal variation (Cv) with and without (W/o) radiofrequency (RF) shimming for different body types. Also shown is the variation of the differences in Cv values (ΔCv) 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 Cv (and ΔCv) and body type, either with or without local RF shimming.

Mentions: 3. The Cv values revealed better RF homogeneity in each subject with subject-specific local RF shimming (Figures 4 and 5). The average Cv for the 37 subjects improved by 42.2 ± 13% with RF shimming (from 0.13 ± 0.03 to 0.07 ± 0.02; P < 0.0001; paired Student t test). Figure 5 shows the variation of Cv, both with and without local RF shimming, and the change in Cv (between with and without local RF shimming) with body type. No relationship could be found between these factors. Like the mean flip angle, Cv values were also compared to the height and weight of the subjects individually, and no significant interrelationship could be found (not shown in Figure 5).


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

Krishnamurthy R, Pednekar A, Kouwenhoven M, Cheong B, Muthupillai R - J Cardiovasc Magn Reson (2013)

Changes in the coefficient of signal variation (Cv) with and without (W/o) radiofrequency (RF) shimming for different body types. Also shown is the variation of the differences in Cv values (ΔCv) 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 Cv (and ΔCv) and body type, either with or without local RF shimming.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750927&req=5

Figure 5: Changes in the coefficient of signal variation (Cv) with and without (W/o) radiofrequency (RF) shimming for different body types. Also shown is the variation of the differences in Cv values (ΔCv) 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 Cv (and ΔCv) and body type, either with or without local RF shimming.
Mentions: 3. The Cv values revealed better RF homogeneity in each subject with subject-specific local RF shimming (Figures 4 and 5). The average Cv for the 37 subjects improved by 42.2 ± 13% with RF shimming (from 0.13 ± 0.03 to 0.07 ± 0.02; P < 0.0001; paired Student t test). Figure 5 shows the variation of Cv, both with and without local RF shimming, and the change in Cv (between with and without local RF shimming) with body type. No relationship could be found between these factors. Like the mean flip angle, Cv values were also compared to the height and weight of the subjects individually, and no significant interrelationship could be found (not shown in Figure 5).

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Radiology, Texas Heart Institute at St, Luke's Episcopal Hospital, 6720 Bertner Avenue, Houston, TX 77030, USA.

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.

Show MeSH
Related in: MedlinePlus