Radiofrequency heating studies on anesthetized swine using fractionated dipole antennas at 10.5 T

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Excerpt

Ultrahigh field (UHF) MRI yields higher signal to noise ratio (SNR) and increased sensitivity 1 than conventional MRI. Despite these advantages, various safety issues limit the use of these systems. Radiofrequency (RF) heating is one of those major safety issues. Previous studies have evaluated the thermal effects of RF heating with simulations and experiments 3. As a result, diverse engineering solutions were proposed to control and mitigate the specific absorption rate (SAR) 5 and temperature rise in UHF applications 9.
Multichannel excitation is one example of such an engineering solution used to overcome RF excitation problems. Previous studies demonstrated the value of multichannel excitation for improving image quality 5 as well as SAR reduction/control 5. The advances in multichannel excitation methods consist of both software and hardware‐based solutions. Software solutions typically include novel pulse design algorithms implemented to control desired imaging parameters such as transmit B1 homogeneity, flip angle homogeneity 1, or various safety factors such as global SAR peak local 10‐g average SAR 5 and/or temperature rise 9. In contrast, hardware‐based solutions typically focus on optimizing the transmit antenna arrays for RF excitation. For example, novel coil elements/arrays are used to alleviate RF safety issues in UHF‐MRI 2.
Among many transmit coil designs, the dipole antenna is a promising transmit element that is suitable specifically for UHF applications 15. Originally proposed in a previous work 16, different dipole antenna designs were used in UHF studies to address various imaging problems 17. In addition to its simplicity, dipole elements have unique field patterns that make them an interesting element to use simultaneously with existing designs 6.
In a typical UHF imaging scenario, it is critical to mitigate image inhomogeneity by controlling either the transmit B1 (B1+) distribution or the flip angle distribution in a target organ of interest. For that purpose, the B1 maps of individual coil elements can be measured in a subject specific manner using various techniques 20. Then, these measured maps can be used to generate a homogenous B1 field or flip angle distribution in a region of interest by using RF shimming or pulse design algorithms. Unfortunately, it is not a simple task to measure the SAR or temperature using similar subject‐specific solutions. Different techniques have been proposed to estimate SAR from B1 measurements. However, the accuracy of these techniques in estimating local SAR and temperature in vivo is limited 22. In contrast, many electromagnetic (EM) and thermal simulation studies have been performed in the past to assess the thermal effects of RF excitation in MRI. These studies helped us gain better understanding of the risks associated with RF heating.
To predict the temperature increase in a subject during an MR scan, one needs validated EM and thermal simulation tools as well as realistic anatomical models. Therefore, in vivo validation of such simulation tools and models is crucial. In this work, we conducted RF heating experiments at 447 MHz (ie, 10.5 Tesla (T) proton Larmor frequency) on anesthetized animals in a controlled RF safety lab environment. We measured temperature in anesthetized swine using fluoroscopic probes and compared our results with the simulated solutions obtained from a digital model of the same swine with specific arrangement of the dipole antennas in each case. For our studies, we used a four‐channel fractionated dipole array 15 that is placed on the neck/upper back region. Electromagnetic and thermal simulations were performed along with in vivo experiments with different RF excitation patterns at 10.5 T.

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