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Magnetic-resonance-imaging (MRI) techniques offer powerful techniques for studying the inside of the human body, but the engineering involved in these systems is quite complex. MRI systems require high-power fields across a range of frequencies from static to megahertz; interactions between the electromagnetic (EM) fields and the human body further complicate MRI system design. Safe, reliable MRI systems call for RF coils capable of homogeneous fields with low specific absorption rate (SAR), but measuring fields inside the body is not practical. This report describes a workflow for using computer-aided-engineering (CAE) simulation to model the behavior of these high-power coils in a realistic therapeutic environment, including the use of accurate voxel models, bioheat solvers, and a specialized spin-ensemble simulator. Simulation results will be compared where possible to measurements from phantoms and human subjects.

Safety and efficiency are important when designing any MRI device. During an MRI scan, the patient is subjected to significant energy from RF fields, typically at frequencies on the order of tens or hundreds of megahertz. These fields deposit energy in the body, which results in heating. If the temperature rises too much, significant tissue damage can occur.

The new generation of ultra-high-field devices, with B-fields of 7 T, 9.4 T, or higher, introduces a new set of problems for MRI designers to consider. High-field devices produce images with much better resolutions than their low-field counterparts (Fig. 1).1 But they also operate at higher frequencies and produce higher power absorption in the human body; while the signal-to-noise ratio (SNR) improves linearly over frequency, the SAR is proportional to the square of the frequency, and this can increase heating effects substantially.

1. This is a comparison of the images from low- and high-field MRI systems, showing a cross-section of a hippocampus.

Higher frequencies also represent shorter wavelengths: For a typical 7-T device, these will be around 13 cm, comparable to the size of structures in the body. For such short wavelengths, interference has a major effect on the quality of the images. Figure 2 shows a possible result of such interference, with a large shadow obscuring the view of the brain. These shadows can hide clinically relevant details, so it is important that these be reduced as much as possible.

2. This is an MRI scan degraded by interference.

To reduce interference and improve patient safety, while still experiencing the benefits of high-field MRI, radiographers use multichannel coils that allow very precise control of the RF fields. Computer simulation can be used to calibrate these coils to prevent crosstalk between separate parts of the coil, and to ensure that the coils produce homogeneous fields in the area of interest. Simulation offers a faster and less expensive means of checking a design than repeatedly fabricating and testing prototype coils and making incremental changes in each generation and can also provide additional insight into the functional mechanisms of a system, allowing automatic optimization and tuning schemes.

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