Metzger GJ, Snyder C, Akgun C, et al. Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA. Magnetic Resonance in Medicine
(Impact Factor: 3.57).
02/2008; 59(2):396-409. DOI: 10.1002/mrm.21476
High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B(1) (B(1) (+)) shimming to minimize B(1) (+) losses resulting from destructive interferences. B(1) (+) shimming was performed by altering the input phase of individual RF channels based on relative B(1) (+) phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B(1) (+) coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B(1) (+) shimming provided a gain in transmit efficiency of 4.2 +/- 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B(1) (+) shimming decreased B(1) (+) nonuniformity within the prostate from (24 +/- 9%) to (5 +/- 4%). This study demonstrates the tremendous impact of fast local B(1) (+) phase shimming on ultrahigh magnetic field body imaging.
Available from: onlinelibrary.wiley.com
- "It is possible that the change in phase of the RF field resulting from head motion may also affect the phase of the EPI signal. Although this dynamic RF phase change was not obvious in the data acquired here and to our knowledge has not been reported in the literature, it may become more significant at higher field strengths and could be investigated using B1 phase maps (e.g., [Metzger et al., 2008]). It is also important to note that for this study, phase and magnitude images were reconstructed from the raw data using a customized image reconstruction method. "
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ABSTRACT: Field inhomogeneities caused by variations in magnetic susceptibility throughout the head lead to geometric distortions, mainly in the phase-encode direction of echo-planar images (EPI). The magnitude and spatial characteristics of the distortions depend on the orientation of the head in the magnetic field and will therefore vary with head movement. A new method is presented, based on a phase informed model for motion and susceptibility (PIMMS), which estimates the change in geometric distortion associated with head motion. This method fits a model of the head motion parameters and scanner hardware characteristics to EPI phase time series. The resulting maps of the model fit parameters are used to correct for susceptibility artifacts in the magnitude images. Results are shown for EPI-based fMRI time-series acquired at 3T, demonstrating that compared with conventional rigid body realignment, PIMMS removes residual variance associated with motion-related distortion effects. Furthermore, PIMMS can lead to a reduction in false negatives compared with the widely accepted approach which uses standard rigid body realignment and includes the head motion parameters in the statistical model. The PIMMS method can be used with any standard EPI sequence for which accurate phase information is available. Hum Brain Mapp, 2012. © 2012 Wiley Periodicals, Inc.
Available from: Xiaotong Zhang
- "Possible local overheating due to the latter poses a serious safety concern at UHF. Consequently, when using a multiple element transmit coil which has been recognized as a powerful tool for inhomogeneity compensation and has been widely utilized at UHF –, energy deposition in the body must be carefully controlled and kept under international safety guidelines . Although attempts to address this overheating concern have been made by constraining shimming or parallel transmission RF pulse design solutions with worst-case conditions derived from electromagnetic (EM) simulations –, in some situations this may excessively limit the achievable SNR and contrast at UHF. "
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ABSTRACT: Elevated Specific Absorption Rate (SAR) associated with increased main magnetic field strength remains as a major safety concern in ultra-high-field (UHF) Magnetic Resonance Imaging (MRI) applications. The calculation of local SAR requires the knowledge of the electric field induced by radiofrequency (RF) excitation, and the local electrical properties of tissues. Since electric field distribution cannot be directly mapped in conventional MR measurements, SAR estimation is usually performed using numerical model-based electromagnetic simulations which, however, are highly time consuming and cannot account for the specific anatomy and tissue properties of the subject undergoing a scan. In the present study, starting from the measurable RF magnetic fields (B1) in MRI, we conducted a series of mathematical deduction to estimate the local, voxel-wise and subject-specific SAR for each single coil element using a multi-channel transceiver array coil. We first evaluated the feasibility of this approach in numerical simulations including two different human head models. We further conducted experimental study in a physical phantom and in two human subjects at 7T using a multi-channel transceiver head coil. Accuracy of the results is discussed in the context of predicting local SAR in the human brain at UHF MRI using multi-channel RF transmission.
Available from: PubMed Central
- "The next sub-section discusses some details of RF coils for high-field experiments. Alternatively, one can use B1-shimming to improve the efficiency of an SE fMRI experiment by controlling the spatial pattern of thermal energy deposition to optimize acquisition in a specific region of interest or to remove “hot spots” where excess B1 is applied [10, 93, 94]. Finally, on-going pulse sequence development is exploring alternatives to the standard Hahn spin echo approach  in order to acquire T2-weighted images while minimizing the power used by the pulse sequence. "
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ABSTRACT: In the last decade, dozens of 7 Tesla scanners have been purchased or installed around the world, while 3 Tesla systems have become a standard. This increased interest in higher field strengths is driven by a demonstrated advantage of high fields for available signal-to-noise ratio (SNR) in the magnetic resonance signal. Functional imaging studies have additional advantages of increases in both the contrast and the spatial specificity of the susceptibility based BOLD signal. One use of this resultant increase in the contrast to noise ratio (CNR) for functional MRI studies at high field is increased image resolution. However, there are many factors to consider in predicting exactly what kind of resolution gains might be made at high fields, and what the opportunity costs might be. The first part of this article discusses both hardware and image quality considerations for higher resolution functional imaging. The second part draws distinctions between image resolution, spatial specificity, and functional specificity of the fMRI signals that can be acquired at high fields, suggesting practical limitations for attainable resolutions of fMRI experiments at a given field, given the current state of the art in imaging techniques. Finally, practical resolution limitations and pulse sequence options for studies in human subjects are considered.
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