1096 Simultaneous B0- and B1-map acquisition in a breath-hold for localized shim, frequency and RF power determination
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BioMed Central
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Meeting abstract
1096 Simultaneous B0- and B1-map acquisition in a breath-hold for localized shim, frequency and RF power determination Michael Schär*1, Evert-Jan PA Vonken2 and Matthias Stuber2 Address: 1Johns Hopkins University and Philips Medical Systems, Baltimore, MD, USA and 2Johns Hopkins University School of Medicine, Baltimore, MD, USA * Corresponding author
from 11th Annual SCMR Scientific Sessions Los Angeles, CA, USA. 1–3 February 2008 Published: 22 October 2008 Journal of Cardiovascular Magnetic Resonance 2008, 10(Suppl 1):A221
doi:10.1186/1532-429X-10-S1-A221
Abstracts of the 11th Annual SCMR Scientific Sessions - 2008
Meeting abstracts – A single PDF containing all abstracts in this Supplement is available here. http://www.biomedcentral.com/content/pdf/1532-429X-10-S1-info.pdfThis abstract is available from: http://jcmr-online.com/content/10/S1/A221 © 2008 Schär et al; licensee BioMed Central Ltd.
Introduction
Methods
Cardiac MR at 3 T is challenged by the increased inhomogeneity of the static magnetic field B0, especially when using balanced steady-state free precession sequences. One solution is to determine localized second order shim corrections and a localized on-resonance frequency F0 based on an acquired B0-map [1]. Furthermore, both numerical simulations [2] and measurements [3] have shown that also the transmit radiofrequency (RF) field B1 in the heart is more inhomogeneous at 3 T as compared to 1.5 T. Conventional methods to measure B1-maps, such as the dual-TR method [4], are rather lengthy. Recently, the saturated double angle method (SDAM) to acquire a B1-map covering the heart within a single breath-hold was introduced [5]. Applying SDAM, Sung et al reported a flip angle distribution from 34° to 63° across the entire left ventricle (LV) for a nominal flip angle of 60° [6]. This suggests that not only the B1 field over the LV is inhomogeneous by ± 30%, but that the average flip angle (power setting) is about 20% lower than the requested 60°. However, inadequate power settings may lead to signal reduction, changes in contrast, and eventually to biased quantitative measures.
The SDAM B1-map acquisition method was adapted to additionally acquire a second image for each slice with a longer echo time to simultaneously generate a B0-map (Figure 1).
Purpose To combine the acquisition of the B0- and the B1-map into one single breath-hold for fast determination of localized shim values, F0, and RF power settings.
As the measured signal in a multi-slice acquisition depends on the excitation profile of the applied pulse shape as well as on the RF excitation angle, the integral of the signal along the excitation profile was simulated based on Bloch-equation simulations of two different pulse shapes (sinc-Gaussian SG100 (tBW = 2.14) and Optex1 (tBW = 9.56)) and for a range of RF excitation angles from 0°–180°. This was used to calculate a correction lookup table to determine the local B1-field. The proposed method was tested in a phantom, in the head
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