Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

In Magnetic Resonance Imaging (MRI) equipment, a set of shim coils are designed to generate specific magnetic fields, thus eliminating harmonic components of magnetic field to obtain a high level homogeneous magnetic field within the region of interesting (ROI). In the electromagnetic design process, in order to produce the desired magnetic field, the deviation between the calculated magnetic field of shim coil and the theoretical magnetic field is treated as a kind of traditional objective functions to optimize the distribution of current density on the surface of shim coil skeleton. However, such function is ill-posed because of the overdetermined or underdetermined system of equations. The regularization method is commonly used to solve such problem by constructing the regularization term. This article proposes a new iterative optimization method for the design of shim coils in MRI. Based on the boundary element method (BEM), the discretized stream functions can be obtained by discretizing the surface of coil skeleton using a set of triangular elements. As the regularization term, the second derivative stream function is included in the minimization of the deviation between calculated magnetic fields and target magnetic fields. The distribution of coil which meets the design requirements can be obtained by using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. At last, the cubic spline interpolation is used to make lines as smooth as possible to be processed. In this article, the proposed method was employed to design two kinds of room temperature shim coils for cylindrical and/or biplanar MRI shim coil system. The simulation results demonstrate that the proposed method is effective and practical.

In the development of a magnetic resonance imaging spectrometer, the equipment fault detection methods are mainly reliant on visual inspection of reconstructed images or k-space data, combined with observation of the output waveforms via an oscilloscope. However, when using the above methods, it may be quite difficult to determine minor design flaws that would produce image ghost or other problems. This article presents a fault detection method that is based on acquisition and analysis of the output waveforms from the spectrometer. While a sequence is running, the spectrometer outputs, including the digital gate and the gradients, are sampled using a data acquisition card. The acquired data is then processed using a high-performance graphic processing unit to allow the feature points, which are the endpoints of the waveform segments in this design, to be extracted. The processing operation is composed of data filtering, differencing, and clustering. Finally, the extracted feature points are compared with the predefined feature points of the sequence to determine any design errors. This method has been used to solve image ghost problems in our home-built spectrometer.

An assessment of radiofrequency (RF) power deposition independent of the information provided by MRI scanners is thus desirable. We developed a novel scanner-independent RF dosimeter based on measurements of the resistance of a thermistor that dissipates the RF power during scanning. With the RF dosimeter, the RF power deposition for four MRI sequences with specific absorption rate (SAR) values (0.1-3.3 W/kg) was measured on five different scanners and the correlation between the RF dosimeter reading and the SAR levels calculated by the scanners was investigated. The novel RF dosimeter showed a linear relationship between the RF power deposition and the scanner-reported whole-body averaged SAR for each scanner. However, there was a variability in the reading among different scanners. The RF dosimeter readings were 9.7 and 9.5 mW on GE 1.5 T (SAR=2.6 W/kg), 3.6 and 3.7 mW on Philips 1.5 T (SAR=3.3 W/kg), 9.5 and 8.6 mW on Siemens 3 T (SAR=3.0 W/kg), and 4.7 and 3.9 mW on Philips 3 T (SAR=2.6 W/kg), respectively. The scanner-independent RF dosimeter developed in this study can play a significant role in checking the accuracy of scanners’ SAR values as a standardized method for measuring the RF power deposition for MR safety.

A low-field nuclear magnetic resonance (NMR) instrument is an important tool for investigating a wide variety of samples under different conditions. In this paper, we describe a system constructed primarily with commercially available hardware and control software, capable of single-pulse NMR experiments. Details of the construction of the main B₀ magnet are also included. The operating frequency for demonstration is 460 kHz (10 mT), however, the range of the hardware spans 700 Hz (16 μT) to 25 MHz (0.6 T). Tip angle optimizations are used to find the most narrow usable pulse width for this configuration, and the T₁ of water is measured by single-pulse-saturation-recovery (SPSR) to demonstrate the potential for this system as a relaxometer. Discussions of resonator construction and efficiency, power requirements and programming strategies that would increase the utility of this system are also included. Construction of any low-field NMR system will depend on experimental interests, budget and engineering resources. A survey of other low-field NMR systems from the literature is included to aid the novice or experienced magnetic resonance scientist in consideration of how a low-field spectrometer could be constructed and used in the lab.