Experimental and numerical assessment of MRI‐induced temperature change and SAR distributions in phantoms and in vivo

Abstract

It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate induced by the radiofrequency fields in MRI experiments. Magn Reson Med 63:218 -223, 2010.

Assessing the Electromagnetic Fields Generated By a Radiofrequency MRI Body Coil at 64 MHz: Defeaturing Versus Accuracy

“…Different applications may require different levels of field accuracy in specific locations. An accurate assessment of the electromagnetic field inside the phantom is important when evaluating SAR levels as overall safety of the patient [9][10][11][12][13][14][15][16] or the RFinduced heating in patients with conductive medical devices that are fully implanted in the body [22] like deep brain stimulators [8,17,18] or pacemakers [1,20,21]. Conversely, an accurate representation of ||E ⃗ || in the space between the coil and the load is important when evaluating safety in patients with conductive medical devices that are partially implanted or in contact with the skin [7,23].…”

“…The term "defeaturing" was used to indicate that the birdcage model systems were implemented using different driving methods, while maintaining the same geometrical model and the same numerical algorithm. Five numerical models were implemented: one specific (i.e., S2) [11,13,20,28], two generic (i.e., G32 [9,10,32], G16 [30,31,33]), and two hybrid (i.e., H16 [15,35], H16 fr-forced [37]). The assessment of accuracy vs. defeaturing was performed by comparing each numerical model with a physical coil in terms of frequency response, as well as ||E ⃗ || and ||B ⃗ ||.…”

Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects

“…MRI-based methods for measuring the RF magnetic fields have received much attention and advancement in recent years due to the increasing utility and prevalence of, first, receive arrays and, more recently, transmit arrays in MRI. Because changes in temperature and even absolute temperature can be measured with MR-based techniques, in some cases this can be used to evaluate the SAR distribution (96)(97)(98). With some careful study design, acquisition, and analysis, DC or RF current densities (99,100) and even RF electrical (101) and magnetic (02) properties of the sample can be measured in some circumstances.…”

An improved MRI guided ultrasound system for superficial tumor hyperthermia

“…The MRI parameters that can be applied to temperature measurement include proton resonance frequency (PRF), spinlattice relaxation time and water molecular diffusion coefficient, etc. PRF method is based upon the linear relationship between the shift of temperature and of proton phase [2] . Because of its temperature sensitivity and short scanning-time, it becomes a popular method in MRI-based temperature measurement.…”