Calculations ofB 1 distribution, specific energy absorption rate, and intrinsic signal-to-noise ratio for a body-size birdcage coil loaded with different human subjects at 64 and 128 MHz

Abstract

A numerical model of a female body is developed to study the effects of different body types with different coil drive methods on radio-frequency magnetic (B 1 ) field distribution, specific energy absorption rate (SAR), and intrinsic signal-to-noise ratio (ISNR) for a body-size birdcage coil at 64 and 128 MHz. The coil is loaded with either a larger, more muscular male body model (subject 1) or a newly developed female body model (subject 2), and driven with two-port (quadrature), four-port, or many (ideal) sources. Loading the coil with subject 1 results in significantly less homogeneous B 1 field, higher SAR, and lower ISNR than those for subject 2 at both frequencies. This dependence of MR performance and safety measures on body type indicates a need for a variety of numerical models representative of a diverse population for future calculations. The different drive methods result in similar B 1 field patterns, SAR, and ISNR in all cases.

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].…”

“…models were shown to well replicate the homogeneity of the magnetic field, electric field, and SAR distribution inside the ASTM phantom [26], the cylindrical phantom [35], and the human body models [10]. Lastly, a hybrid model [15,[35][36][37] includes both the multiport excitation and presence of lumped elements, the convenience of such an approach, over a specific model, is the independence of the frequency response from the loading conditions [22].…”

“…There are several studies that compared the different approaches [10,26,35] as well as studies that compared the models against measurements [17,27,30,36]. However the comparison between simulated and measured electric and magnetic field magnitude (||E ⃗ || and ||B ⃗ ||, respectively) has been mostly performed along one-dimensional lines through the isocenter of the coil.…”

“…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 SAR for transmit coil arrays

“…Though at least one initial attempt to consider SAR while optimizing magnetic field homogeneity with numerical calculations looks promising (13), the general application of such an approach is yet uncertain as local SAR levels and locations are patient specific (14,15) and it is not practical to create patient-specific numerical models of every patient before MRI imaging. The results in this article demonstrate that the superposition of electric and magnetic fields is valid when using current source driving methods, a fundamental step toward future developments of considering SAR for transmit arrays.…”