Specific absorption rate studies of the parallel transmission of inner-volume excitations at 7T

Abstract

Purpose: To investigate the behavior of whole-head and local specific absorption rate (SAR) as a function of trajectory acceleration factor and target excitation pattern due to the parallel transmission (pTX) of spatially tailored excitations at 7T.

Materials and methods: Finite-difference time domain (FDTD) simulations in a multitissue head model were used to obtain B(1) (+) and electric field maps of an eight-channel transmit head array. Local and average SAR produced by 2D-spiral-trajectory excitations were examined as a function of trajectory acceleration factor, R, and a variety of target excitation parameters when pTX pulses are designed for constant root-mean-square excitation pattern error.

Results: Mean and local SAR grow quadratically with flip angle and more than quadratically with R, but the ratio of local to mean SAR is not monotonic with R. SAR varies greatly with target position, exhibiting different behaviors as a function of target shape and size for small and large R. For example, exciting large regions produces less SAR than exciting small ones for R >or=4, but the opposite trend occurs when R <4. Furthermore, smoother and symmetric patterns produce lower SAR.

Conclusion: Mean and local SAR vary by orders of magnitude depending on acceleration factor and excitation pattern, often exhibiting complex, nonintuitive behavior. To ensure safety compliance, it seems that model-based validation of individual target patterns and corresponding pTX pulses is necessary.

Figure 1

Quantitative B₁⁺ maps (nT/ampere) of the center transverse slice of the head derived from FDTD-simulated fields. Each map exhibits severe inhomogeneity. S_p(r) is the B₁⁺ field that arises when channel p is driven with a 1-ampere peak-to-peak 300-MHz sinusoid.

Figure 2

Birdcage mode simulation. For p = 1, …, 8, transmission channel p is driven with a 3-ms, 2.6-ampere, 100%-duty-cycle rectangular pulse with phase equal to 45(p — 1) degrees, producing a 90° flip in the center of the head (center transverse slice shown along with 1D profile and SAR statistics). Center brightening is evident, as well as signal loss left and right of center.

Figure 3

SAR as a function of acceleration factor, R, and flip angle, θ (fixed excitation quality). Target: 28 mm × 28 mm centered square with in-box flip angle θ, ε₁ = 15 ± 2%, ε_tot = 40 ± 1%. Top row: R = 4 excitations. Bottom row: mean SAR and maximum 1g SAR as a function of (R, θ). For fixed θ, SAR grows rapidly with R; for fixed R, SAR grows quadratically with θ.

Figure 4

Local SAR and θ as a function of R (fixed mean SAR and excitation quality). Target: 28 mm × 28 mm centered square with ε₁ = 15 ± 2% and ε_tot = 40 ± 1%. For each R, target flip angle, θ, is varied until mean SAR equals 0.15 ± 0.01 W/kg. Left: to achieve fixed mean SAR, θ must decrease radically with R. Only a 2° flip is achievable for R = 6 whereas a 36° flip is achievable using an R = 1 unaccelerated trajectory. Right: maximum 1g and 10g SAR are not monotonic with R, e.g., the R = 4 excitation has 1.3 times lower local SAR than the R = 1 one.

Figure 5

SAR as a function of R and shift along x (fixed excitation quality). Target: 15°, 28 mm × 28 mm square whose center x₀ varies along x with ε₁ = 15 ± 1% and ε_tot = 40 ± 1%. Top row: R = 4 excitations. Second and third rows: local 1g SAR maps due to R = 1 and R = 5 pulses. Each spatial location (x, y) shown equals max_z SAR_1-gram(x, y, z), i.e., the maps have been collapsed along the z axis to efficiently display 3D spatial SAR data. Bottom row: mean SAR and maximum 1g SAR as a function of (R, x₀). For R ≤ 4, SAR increases as |x₀| increases, whereas for R > 4, SAR decreases. For R = 1, local SAR seems to vary strongly across space with excitation position, whereas for R = 5, it seems to scale by only a multiplicative constant.

Figure 6

SAR as a function of R and shift along y (fixed excitation quality). Target: 15°, 28 mm × 28 mm square whose center y₀ varies along y with ε₁ = 15 ± 1% and ε_tot = 40 ± 1%. Top row: R = 4 excitations. Bottom row: mean SAR and maximum 1g SAR as a function of (R, y₀).

Figure 7

SAR as a function of R and ε₁ (fixed ε_tot). Target: 15°, 28 mm × 28 mm square with varying ε₁ and ε_tot = 40 ± 2%. Top row: R = 4 excitations; in-box flip angle decreases with ε₁. Bottom row: mean SAR and maximum 1g SAR as a function of (R, ε₁). For R ≠ 6, SAR decreases fairly regularly with ε₁. For R = 6, SAR is actually higher when ε₁ = 15% than when ε₁ = 10%.

Figure 8

SAR as a function of R and ε_tot (fixed ε₁). Target: 15°, 28 mm × 28 mm centered square with varying ε_tot and ε₁ = 15 ± 1%. Top row: R = 4 excitations. Outside of the square, artifacts increase with ε_tot. Bottom row: mean SAR and maximum 1g SAR as a function of (R, ε_tot). For all R, SAR generally decreases smoothly with ε_tot.

Figure 9

SAR as a function of R and target rotation angle φ (fixed excitation quality). Target: 15°, 44 mm × 28 mm centered rectangle with varying φ, with ε₁ = 15±1% and ε_tot = 40±2%. Top row: R = 4 excitations. Bottom row: mean SAR and maximum 1g SAR as a function of (R, φ). For R ≤ 4, SAR is relatively constant, whereas for R > 4, SAR varies.

Figure 10

SAR as a function of R and target size N (fixed excitation quality). Target: 15° centered square of varying size with ε₁ = 15 ± 1% and ε_tot = 40 ± 2%. Top row: R = 4 excitations. Middle row: mean SAR and maximum 1g SAR as a function of (R, N). Bottom row: for each R, data from middle row is scaled so that SAR(R, N = 12 mm) is unity. For R ≤ 4, SAR increases rapidly with N, whereas for R > 4, exciting larger regions reduces energy deposition.

Figure 11

SAR as a function of R and target smoothness (fixed excitation quality). Target: 15°, 44 mm × 44 mm square smoothed with various M mm × M mm Gaussian kernels with ε₁ = 15 ± 1% and ε_tot = 40 ± 1%. The base case of no smoothing is when M = 4 mm (pixel resolution is 4 mm, so an M = 4 mm window is simply a one-pixel unit-energy kernel). Top row: R = 4 excitations. Bottom row: mean SAR and maximum 1g SAR as a function of (R, M). In general, SAR generally decreases smoothly with M.