The cervical spine presents a challenging target for 7T RF coils. Previous designs have used transmit elements primarily to the posterior of the neck [1,2], or have wrapped elements all the way around the neck [3]. Previous simulations by one of us have shown that by transmitting with only 6 out of 8 elements of a wrap around array a relatively uniform excitation can be achieved in the posterior-most 3/4 of a neck sized phantom [4]. We present here a 6 channel transmit-receive cervical spine coil constructed like a cervical collar, wrapping around the back of the neck. Phantom and in vivo data are presented, and the coil is compared to a commercially available 7T 4-channel cervical spine coil (RAPID Biomedical. Rimpar, Germany)

The 6-channel array was etched on pyralux flexible circuit board. The dimension of each element was about 7cm × 7cm. Each element incorporates 5 fixed and one trimmer capacitor (Fig 1a). Neighboring elements were capacitively decoupled. On each element, a lattice balun was designed for impedance match, with a cable trap on each cable to minimize common mode currents (Fig.1b). Preamp decoupling was achieved by adjusting the coax cable length. The coil was covered with foam and can be wrapped on the patient’s neck like a cervical collar (Fig. 2). There is a gap between coils 1 and 6 which is bridged with a Velcro strap. A minimum distance of 1 cm between the coil surface and the patient was maintained by a layer of foam for safety.

The coil was interfaced to a 7T scanner with 8 channel parallel transmit (Siemens, Erlangen Germany) using an in-house built transmit-receive interface. Safe power limits were determined for in vivo scanning on the parallel transmit system using a sequential heating technique [5] on a tissue-equivalent gel phantom (ε_r=52, σ=0.56) using MR thermometry. All volunteer measurements were conducted according to our institution’s IRB. Phases to the elements were chosen to create constructive interference at the center of the object. In vivo B₁⁺ maps were obtained with a turbo-FLASH sequence with preparation pulse [6], and SNR maps were generated based on GRE measurements obtained with RF excitation and without (TR/TE/BW=200/4.1/300, FOV/Matrix/Slice=220mm/256/3 mm) using the Kellman method [7]. The 6 channel TxRx cervical spine array was also compared to the Rapid Biomedical cervical spine array which has a form-fitting housing around the back of the neck with 4 transmit-receive elements and was used in single transmit mode [1].

For the constructed 6 channel cervical spine array, the unloaded & loaded Q values were 121 & 20 respectively and the average coupling between adjacent elements was -25 dB. S₁₁ of -20 dB or better was achieved on all elements. A power limit of 3 Watts was applied based on the MR thermometry measurements. With the 6-channel array, a 90-degree flip angle could be achieved in the center of the object with a single-channel equivalent 123 volt 500 μs hard pulse (50 volt pulse per channel). For comparison, with the same volunteer in the Rapid cervical spine coil a 200 volt 500 μs hard pulse was required. B₁⁺ maps obtained
from the in-vivo experiment are shown in Figure 3. SNR plots for root sum of squares reconstruction in the volunteer are shown in Figure 4. The average off-diagonal noise correlation was 22.2%. High resolution images of the cervical spinal cord were obtained with a 2D gradient echo sequence (TR/TE/BW=500/5.5/261, FOV/Matrix/Slice=189mm/832/5 mm, 5 slices, acquisition=5:12) (Fig. 5).Compared with the Rapid cervical spine coil, the B₁⁺ efficiency and B₁⁺ uniformity in the transverse plane was improved by the 6 channel coil, especially at the posterior-most 3/4 of the neck. The SNR in the spinal cord was similar for the 6 channel array and the Rapid Biomedical coil. Grey and white matter regions in the spinal cord are clearly depicted. The Rapid Biomedical coil has slightly better coverage in the head-foot direction. Although the 6 channel coil was used here with a parallel transmit system for initial tests, it could in principle be driven with a power splitter and fixed phases to the coil elements to enable use on a single channel system. For clinical applications, extended coverage in the head-foot direction would be desirable, which might be possible with a more complicated coil former and longer coil elements. A 6 element wrap-around transmit receive array provides a practical and high performing solution for cervical spine imaging at 7T. The small channel count may limit accelerated imaging applications, but it performs well for standard anatomical imaging.