- Workshop presentation
- Open Access
Improvement in B1+-homogeneity of 3T cardiac MRI in swine with dual-source parallel RF excitation
© Herzka et al; licensee BioMed Central Ltd. 2013
- Published: 30 January 2013
- Radio Frequency
- Flip Angle
- Radio Frequency Power
- Radio Frequency Pulse
- Animal Size
Conventional MRI scanners up to a magnetic field strength of 3T use an integrated birdcage quadrature coil to generate a radio frequency (RF) excitation field (B1+). At 3T, Sung et al. observed a flip angle variation ranging from 31 to 66% over the entire left ventricle (LV) in humans as well as a flip-angle distribution from 34° to 63, for a nominal flip angle of 60°. This not only demonstrates that the B1+ field over the LV is inhomogeneous but also that the average flip angle (RF power setting) can be 20% lower than desired. Such erroneous flip angles may lead to local signal reduction, artifacts, failure of magnetization preparation pulses, and eventually to biased quantitative measures. Recently, it has been shown that multi-channel transmit systems can be used to reduce these inhomogeneities in humans by the use of RF-shimming. Here, we quantify improvements in B1+-homogeneity at 3T when using dual-source parallel RF excitation, and correlate results with animal size.
Swine (N=22) were imaged repeatedly and at different times as part of other imaging studies. Animal weight varied between ~25 and 125 kg. A 3.0T MR system (Achieva TX, Philips Healthcare, Best, The Netherlands) and a 32-channel cardiac array were used. B1+-mapping was carried out before and after RF-shimming using a cardiac-triggered, breath-hold, saturated dual-angle method . Data were acquired axially, during breath-hold, in diastole. A volume manually drawn over the heart, primarily through the ventricles was used to localize the shimming. N=44 independent imaging sessions were analyzed. An elliptical ROI was drawn using the magnitude image as a guide, aiming to cover the whole heart. Pixel values where extracted from the B1+-map and represent the percent of the desired flip angle achieved. Animal size was characterized by drawing an additional elliptical ROI encompassing the whole animal.
The use of localized RF shimming with dual sources significantly increases the effective flip angle. These improvements should have significant effects in SNR and the predictability of image quality since current cardiac imaging involves high flip angle RF pulses whose performance can be affected by B1+ heterogeneity. Here we show that not only does the degree of loss due to dielectric effects depend on animal size, so does the magnitude of the improvement in B1+ homogeneity.
This work was funded in part by AHA-11SDG5280025.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.