Introduction

We are preparing to perform a clinical study using 7T abdominal MRI [1,2,3] and MRS to monitor the response to chemotherapy in patients with liver metastases. This is part of the EU FET-OPEN project: “Non-invasive chemistry imaging” (NICI). In Cambridge, we will scan using our recently-installed 7T Terra scanner (Siemens). This abstract describes our work to take a 7T cardiac TEM coil previously used on the Magnetom 7T system in Oxford (VB17, Step 2.3 software) and prepare it for use on the 7T Terra in Cambridge (VE12U software). This is the first custom pTx-capable coil that we have set up on the Terra. We believe our experience may help others who are setting up custom coils for 7T Terra systems. We also show our first abdominal 7T images.

Material and Methods

All experiments were performed on a 7T Terra whole-body MR system with an 8-channel parallel transmit TEM array coil [4]. We adapted the coil file from the VB17 format (with known safe per-channel power limits) for use in VE12. To assure correct understanding, we developed a set of Matlab code to dynamically plot values from the RFSWDHistoryListNew.log file while we changed parameters in the VE12U coil files and VOP supervision [6] files one at a time. We are happy to share this tool on request to other sites, subject to the usual legal formalities. We ran FID pulse sequences with either excitation in TrueForm mode, or excitation as a full pTx pulse sequence that happened to output the exact same RF waveform as the scan in TrueForm mode, and a pTx sequence in CP- RF shim, and a pTx sequence exciting a single channel. In each case, we adjusted the voltage and duty cycle to determine the point where the scanner lookahead and online SAR supervision cut off. Having selected our coilfile parameters, we performed a heating test using pork loin and Luxtron 3100 fibre-optic temperature probes.

We then recruited a healthy volunteer in accordance with ethical regulations. We scanned the volunteer in a transverse slice covering the liver to compare performance in these modes. B₁⁺ maps were acquired using a turbo-FLASH B₁-mapping pulse sequence (‘tfl_map’). Images were acquired in “TrueForm” mode, i.e. with a fixed CP⁺ RF shim setting, and with “Patient Specific” RF shimming manually set to a fixed CP⁺⁺ RF shim. FLASH images were acquired with a radial trajectory over a 350 x 350 x 8 mm³ slice and reconstructed to 0.9 x 0.9 x 8 mm³ resolution. TR and TE were 1105 and 3 ms respectively and an excitation input voltage of 143.9 V. FLASH images were also acquired with a Cartesian trajectory over the same slice, with 1.8 x 1.8 x 8 mm³ resolution. TR was 1000 ms, TE 10 ms with an excitation voltage of 179.9 V. Both radial and Cartesian acquisitions were then repeated with static pTx in a fixed CP⁺⁺ RF shim, with 1000 ms TR and TE of 10 ms.

Results

On the Terra, SAR limits are enforced by a combination of a coil file and a VOP file [5], over 6 min (LT), 10 s and 1 s averaging intervals. Table 1 shows an overview of the differences between VB17 Step 2.3 and VE12U configuration, together with the various configurations that we tested in phantom scans. Figure 1A shows the effect of the “k-factor”, or ratio of power on the patient protection VOP and power on the coil protection VOP. The impact of adjusting the k-factor was tested in both TrueForm/CP⁺ and Patient Specific shim/pTx modes. Our “default” patient protection VOPs (“”) were an 8x8x8 array, all set to zero, except the k,k,kth elements were 0.65x8/50. This means that gives a hypothetical total power if all 8 channels deposited the same as the channel with maximum power deposition.
Figure 2 shows the relative B₁⁺ maps plotted for each single channel in the volunteer.
Figure 3 shows the human in vivo results. Notice how the pattern of B₁⁺ dropouts (black lines) shifts in CP⁺ vs CP⁺⁺ RF shim setting, confirming a successful static pTx implementation.

Discussion and Conclusion

Our results demonstrate the feasibility for abdominal imaging at the 7T Terra using an 8-channel TEM coil built for cardiac imaging on the Magnetom 7T platform. We investigated how to translate coil RF safety limits from a Magnetom 7T system to the Terra system. Coil file L clearly indicates the limits where the scanner is able to successfully shutdown a scan. We report that we scanned a healthy volunteer using a matched protocol in a single slice across the liver to compare the in vivo performance of two modes. To our knowledge, these are the first liver pTx images presented from a 7T Terra scanner. We are now working to build a robust, optimised protocol ready for the NICI patient study.

Acknowledgements

*:SS and BD contributed equally to this work. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 801075. BD is funded by Gates Cambridge Trust. CTR is funded by a Sir Henry Dale Fellowship from the Wellcome Trust and the Royal Society [098436/Z/12/B]. This study was funded by the NIHR Cambridge Biomedical Research Centre and MRC Clinical Research Infrastructure Award for 7T. We are grateful for support to understand the VOP SAR supervision from Rene Gumbrecht, Siemens Healthcare.