Endorectal coils (ERC) can improve the SNR inside the prostate at 7.0T, and are primarily advantageous when used in receive-only mode in combination with an external surface array (ESA) [1]. When used for field transmission alone or in combination with an ESA, ERC can provide higher B1+ fields in the prostate at the expense of field uniformity compared to ESA-only excitation. The ability to dynamically control the tuning state of the ERC would permit the coil to be detuned (i.e. turned off) during transmit for more controlled/homogeneous B1+ performance or tuned (i.e. turned on) for combined transmit for peak B1+ performance. Here, we develop an actively-tuned ERC that can be used both as a transmitter and a receive-only coil in combination with an ESA for improved prostate MRI capabilities at 7.0T.

Choke inductors and PIN diodes were added to a rigid two-channel ERC [2] to enable tuning/detuning capabilities (Figure 1.a). The ERC was tuned to 7.0T (297.2MHz) by applying a 100mA DC bias, for use as a transmitter and/or receiver (Figure 1.b). By blocking the current, the PIN diodes act as open circuits and the ERC becomes “invisible” to the ESA-only excitation (Figure 1.c). The physical implementation of the actively-tuned ERC and the sterilizable housing are shown in Figure 1.e. The hardware components, connections and switching of the ERC between tuned/detuned states are explained in Figure 2.

Fourteen channels of a 16-channel combined loop-dipole transceiver array (six-loops, eight-dipoles) were used as the ESA. The remaining two ESA channels (two-loops) were terminated with 50Ω to accommodate the ERC channels.

The ERC was modeled both in a tuned and detuned state with the ESA to determine the excitation scheme in different operating modes (i.e. ESA alone or ERC+ESA). EM-field distributions of each coil element were computed using an FDTD solver in SEMCAD software (SPEAG, Zurich, Switzerland) to investigate shim-dependent B1+ and SAR distributions. Peak 1g-averaged SAR around the rectum and 10g-averaged SAR inside the pelvis were calculated using virtual-observation points [3].

A consented healthy subject was scanned under an IRB approved protocol. Scanning was performed on a whole-body Magnetom 7.0T system (Siemens Healthcare, Erlangen, Germany) using the ESA combined with the actively-tuned ERC.

Simulated transmit performance metrics of the ESA+ERC combined and ESA-only excitations are shown in Figure 3. Depending on the amplitude/phase settings, 1g-averaged SAR around the rectum or 10g-averaged SAR around the skin can be the limiting factor for RF safety (Figure 3.a). The coefficient of variation (CV) of the B1+ magnitude inside the prostate is plotted in Figure 3.b, and demonstrates, as expected, that the ESA-only excitation can provide a more uniform field distribution (i.e. excitation scheme 2). ESA+ERC combined excitation can provide >40% higher B1+ SAR efficiency compared to the ESA alone (Figure 4.b). Using a B1+ shim with a tradeoff between homogeneity and efficiency (i.e. Scheme 1, Shim B), uniformity of the ESA+ERC excitation can be improved at the expense of transmit efficiency (Figure 4) compared to a pure efficiency solution (scheme 1, shim A). Anatomical turbo spin-echo MRI of the prostate at 0.4mm2 in-plane resolution are shown in Figure 5. Images in Figure 5.a-b are acquired with ESA-only (i.e. Scheme 2) and Figure 5.c is acquired with ESA+ERC (i.e. Scheme 1, Shim B) combined excitation.

Traditionally, DC bias is used to detune receive-only coils during RF transmission, but here this approach was not applicable because high RF currents during ERC transmission might activate the PIN diodes and force the coil into “detuned” state even if the intention was to have it “tuned”. Therefore an “inverse” logic is employed, and the ERC is actively-tuned by applying a DC bias. Series PIN diodes reduced the SNR of the ERC by 0.2dB.

In receive-only mode, the ERC is virtually “invisible” to the ESA-excitation; without accomplishing this, coupling can cause unpredictable field inhomogeneities close to the ERC. Combined ESA+ERC excitation can improve the B1+ transmit and SAR efficiencies compared to ESA-only excitation. Achieving high B1+ SAR efficiency is crucial for enabling SAR-intensive pulse sequences (i.e. turbo spin-echo, spectroscopy). Dynamically alternating between different shim solutions for individual RF pulses [4] can improve performance. For example, Scheme 1 (combined ERC+ESA) would be used for B1+ insensitive pulses such as adiabatic refocusing [5, 6] while other pulses including lipid suppression, water suppression and excitation could use Scheme 2 (ESA-only). The current in vivo imaging only demonstrated that the physical realizing of the coil works in MRI, however exploiting and characterizing the advantages of dynamic switching within specific acquisition protocols is currently under investigation.