Methods

An intermediate interface needs to enable 1) RF transmission: If the scanner supports one transmit port and the RF coil has multiple connectors, a multiplexer circuit can separate the proton and X-nuclear signals. A diplexer circuitry is an efficient solution to achieve frequency multiplexing. The diplexer consists of a high-pass arm for proton operation and a low-pass arm to support X-nuclear operation (Fig.1A). Otherwise, if the scanner supports distinct proton and X-nuclear transmit ports and RF coil has a single connector, the diplexer circuit can be used to combine proton and X-nuclear signals and route them to the coil connector. With strategically choice of the low-pass filter leg cut-off frequency, the diplexer can transmit a wide range of X-nuclear signals. 2) RF reception: If the MR scanner requires distinct proton and X-nuclear receive ports and the RF coil has one connector, multiplexing is required to route the signals to appropriate ports. The same is true if the coil has multiple connectors and the scanner has distinct receive ports. Electronically-controlled single pole double throw (SPDT) switches (Fig.1B) provide an efficient solution to re-route receive signals to the appropriate ports. 3) Routing DC signals from the MR scanner to the coil to achieve proper de-tuning during RF transmission and reception, ensuring patient safety and proper coil performance. A three-port bias tee (Fig.1C) can be used to combine and transmit RF and DC signals.

Specific Interface Box Implementation

We recently upgraded from a Trio (Siemens) to a Prisma (Siemens) platform, which includes RF architecture incompatible with our multi-nuclear coils^1,2,3. With the components and configurations described above, we designed an intermediate interface for operation of multiple legacy multi-nuclear coils on the Prisma platform. The specific design included 1) Two transmit cables and a diplexer circuit to combine separate proton and X-nuclear signals from the scanner and route them to the coil transmit plug. 2) Two sets of six non-magnetic broadband (10MHz-1GHz) SPDT switches routing the proton and X-nuclear receive signals from a single coil port for each of up to 12 coil elements to the corresponding, but separate proton and X-nuclear scanner ports. The switch control logic was generated by a network of MOSFET inverters, which were controlled by three PIN bias lines derived from the scanner. Coil files were modified to provide the appropriate bias, while no hardware modifications were required to the coils themselves. Design included minimizing the number of bias lines consumed by the SPDT switch logic for coil detuning, transmit/receive switch control. 3) DC routing circuits to pass bias from the scanner to the coil for the aforementioned functions. Interface circuitry was designed with Eagle software (CadSoft) (Fig 2A,2B) and housed in a 3-D printed enclosure (Fortus 360mc, Stratasys) fitting conveniently into the patient table (Fig 2C). Insertion loss was characterized on the bench, and practical performance was assessed through GRE images acquired on a Prisma system using several legacy multi-nuclear coils including a sodium proton body array (Stark Contrast) and a house-built sodium proton unilateral breast array⁴.

Results

The insertion loss introduced by the diplexer circuitry and additional cabling and connectors was 0.5dB in the proton transmit path and less than 0.3dB at phosphorus and sodium frequencies. Reflection in the proton, sodium, and phosphorous transmit paths were all less than -15dB. The SPDT receive switches and cabling added approximately 0.6dB insertion loss to received signals, having negligible effect on SNR because it occurred after amplification. Proton and sodium GRE images (Fig.3,4) indicate that the interface enabled legacy coils to be utilized on the current scanner platform by properly routing transmit, receive and PIN bias signals to the multi-nuclear coils.

Discussion and Conclusions

We describe a general framework for interfacing multinuclear RF coils to clinical scanners and constructed a specific interface enabling successful operation of legacy multi-nuclear coils on a new system. The interface provides compatibility without arduous hardware modifications to individual coils. At our center, the interface enabled the operation of six different phosphorous or sodium multi-nuclear multi-channel arrays. While our interface was customized for our case, the same principles could be applied to support other coil/platform combinations.

Acknowledgements

No acknowledgement found.