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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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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.
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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.

Mentions: On the B1 maps generated (as a percentage of intended flip angle) with and without local RF shimming, the region-of-interest (ROI) circumscribing the heart (Figure 1) was manually drawn using a custom-built software program (MATLAB™; The Mathworks, Natick, Massachusetts). The following metrics were used for quantitative evaluation of RF homogeneity: 1) The mean of the percentage of the intended flip angle within the ROI (μ); 2) the coefficient of signal variation (Cv), defined as the ratio of the standard deviation (σ) to the mean (μ) of the voxels within the ROI, where a lower ratio indicates more uniformity; and 3) a cumulative histogram that shows the fraction of the total number of voxels that fall within a specific value of the mean (expressed as a percentage); a higher count at a given threshold corresponds to a more uniform B1 field distribution. Also, the variation of μ and Cv with metrics that characterized the subject’s body type was studied.


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)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

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.
Mentions: On the B1 maps generated (as a percentage of intended flip angle) with and without local RF shimming, the region-of-interest (ROI) circumscribing the heart (Figure 1) was manually drawn using a custom-built software program (MATLAB™; The Mathworks, Natick, Massachusetts). The following metrics were used for quantitative evaluation of RF homogeneity: 1) The mean of the percentage of the intended flip angle within the ROI (μ); 2) the coefficient of signal variation (Cv), defined as the ratio of the standard deviation (σ) to the mean (μ) of the voxels within the ROI, where a lower ratio indicates more uniformity; and 3) a cumulative histogram that shows the fraction of the total number of voxels that fall within a specific value of the mean (expressed as a percentage); a higher count at a given threshold corresponds to a more uniform B1 field distribution. Also, the variation of μ and Cv with metrics that characterized the subject’s body type was studied.

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