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Power Deposition (power + deposition)

Distribution by Scientific Domains

Medical Sciences	100%

Selected Abstracts

Parallel transmit and receive technology in high-field magnetic resonance neuroimaging

INTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 1 2010
Andrew G. Webb
Abstract The major radiofrequency engineering challenges of high-field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B1 field, while remaining within regulatory guidelines for tissue power deposition and (2) to receive the signal with the maximum signal-to-noise and the greatest flexibility in terms of utilizing the benefits of parallel imaging. Borrowing from developments in electromagnetic hyperthermia, the first challenge has been met by the use of transmit arrays, in which the input power to each element of the array can be varied in terms of magnitude and phase. Optimization of these parameters, as well as the form of the applied RF pulse, leads to very homogeneous B1 fields throughout the brain. The design of large receive arrays, using impedance-mismatched preamplifiers and geometrical overlap for interelement isolation, has resulted in significant sensitivity improvements as well as large acceleration factors in parallel imaging. © 2010 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 20, 2,13, 2010 [source]

A simple low-SAR technique for chemical-shift selection with high-field spin-echo imaging

MAGNETIC RESONANCE IN MEDICINE, Issue 2 2010
Dimo Ivanov
Abstract We have discovered a simple and highly robust method for removal of chemical shift artifact in spin-echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin-echo echo-planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice-select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin-echo echo-planar imaging. Moreover, the method can be generally applied to any sequence involving slice-selective excitation and at least one slice-selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B0). Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source]

Spin-echo MRI using ,/2 and , hyperbolic secant pulses,

MAGNETIC RESONANCE IN MEDICINE, Issue 1 2009
Jang-Yeon Park
Abstract Frequency-modulated (FM) pulses have practical advantages for spin-echo experiments, such as the ability to produce a broadband , rotation, with an inhomogeneous radiofrequency (RF) field. However, such use leads to a nonlinear phase of the transverse magnetization, which is why FM pulses like the hyperbolic secant (HS) pulse are not commonly used for multislice spin-echo magnetic resonance imaging (MRI). Here, a general theory and methods are described for conventional spin-echo imaging using a , HS pulse for refocusing. Phase profiles produced by the HS pulse are analytically described. The analysis is extended to yield the specific relationships between pulse parameters and gradients, which must be satisfied to compensate the nonlinear phase variation produced with a spin-echo sequence composed of ,/2 and , HS pulses (the ,/2 HS , , HS sequence). The latter offers advantages for multislice spin-echo MRI, including excellent slice-selection and partial compensation for RF inhomogeneity. Furthermore, it can be implemented with a shorter echo time and lower power deposition than a previously described method using a pair of , HS pulses. Magn Reson Med 61:175,187, 2009. © 2008 Wiley-Liss, Inc. [source]

Method for reduced SAR T1, -weighted MRI

MAGNETIC RESONANCE IN MEDICINE, Issue 6 2004
Andrew J. Wheaton
Abstract A reduced specific absorption rate (SAR) version of the T1, -weighted MR pulse sequence was designed and implemented. The reduced SAR method employs a partial k -space acquisition approach in which a full power spin-lock pulse is applied to only the central phase-encode lines of k -space, while the remainder of k -space receives a low-power spin-lock pulse. Acquisition of high- and low-power phase-encode lines are interspersed chronologically to minimize average power deposition. In this way, the majority of signal energy in the central portion of k -space receives full T1, -weighting, while the average SAR of the overall acquisition can be reduced, thereby lowering the minimum safely allowable TR. The pulse sequence was used to create T1, maps of a phantom, an in vivo mouse brain, and the brain of a human volunteer. In the images of the human brain, SAR was reduced by 40% while the measurements of T1, differed by only 2%. The reduced SAR sequence enables T1, -weighted MRI in a clinical setting, even at high field strengths. Magn Reson Med 51:1096,1102, 2004. © 2004 Wiley-Liss, Inc. [source]

Strategy for Safe Performance of Magnetic Resonance Imaging on a Patient with Implantable Cardioverter Defibrillator

PACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 1 2006
CLAAS PHILIP NAEHLE
Clinically indicated magnetic resonance imaging (MRI) of the brain was safely performed at 1.5 T on a patient with an implantable cardioverter defibrillator (ICD). The ICD was reprogrammed to detection only, and imaging hardware and protocols were modified to minimize radiofrequency power deposition to the ICD system. The integrity of the ICD system was verified immediately post-MRI and after 6 weeks, including an ICD test with induction of ventricular fibrillation. This case demonstrates that in exceptional circumstances, in carefully selected patients, and using special precautions, an MRI exam of the brain may be possible in patients with ICDs. [source]