Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T

Abstract

Ultra-high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B /√P) in comparison with Tx-only arrays, which are usually larger to fit receive (Rx)-only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx-only/Rx-only RF system, a single-row eight-element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx-only array, which could also be used in combination with an 18-channel Rx-only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B /√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.

Decoupling of a tight‐fit transceiver phased array for human brain imaging at 9.4T: Loop overlapping rediscovered

“…For the gapped loops and a phantom size of 18 cm, k m at a 45 º angle measures ∼ ‐0.1 (Fig. C), which represents a very strong coupling and thus suggests that a decoupling circuit must be used to cancel it .…”

“…Each loop has 10 capacitors (100C series, American Technical Ceramics, Huntington Station, NY) distributed along its length and ranging from 5.6 pF to 6.2 pF. Figure C presents the schematic of the individual surface loops including the tuning and matching capacitors, the cable trap, and the home‐built T/R switch unit (‐0.25 dB of insertion loss, ‐40 dB of isolation) connected to each loop and placed inside the coil to minimize losses.…”

“…To decrease radiation losses , the array is shielded with double sided Kapton foil located 4 cm away from the loops. The distance to the shield was chosen based on previously published data for UHF human head size arrays . The performance of the overlapped array was compared with that of a gapped array with a very similar geometry and holder size (loop size: 10 cm length, 7.7 cm width, 1 cm gap between the loops), which was constructed previously .…”

“…Our results show that at 400 MHz overlapping can be successfully used for simultaneous cancellation of both the mutual inductive and resistive coupling. We also compared Tx‐ and Rx‐performance of the array with that of a common gapped array of similar geometry and demonstrated that overlapping improves both Tx‐ and Rx‐performance. Finally, we demonstrated that at UHF, overlapping does not compromise parallel Rx performance, which is a common issue at lower ( ≤ 3T) magnetic fields .…”

A human cerebral and cerebellar 8‐channel transceive RF dipole coil array at 7T

“…However, most of the 8‐channel single‐row coil arrays could not provide together whole‐brain coverage and sufficient transmit field, especially in the region of the cerebellum. Recently, a close fitting single‐row coil array built with loop coils, was shown at 9.4T . At similar B efficiency at the cerebral cortex, the dipole coil array demonstrated a slightly higher SAR 10g,max which lead to 17% lower value.…”