Frequency-agnostic inexpensive modular FDM receiver design
Edwin Eigenbrodt1 and Mary Preston McDougall2

1Texas A&M University, College Station, TX, United States, 2Biomedical Engineering, Texas A&M University, College Station, TX, United States

### Methods

A block diagram of the receiver chain is shown in Fig. 1 and a photograph of the completed portable six-channel receiver system is shown in Fig. 2. Multiple received signals are multiplexed into a single signal line and then digitized with a GE ICS-1650 four-channel high-speed digitizer card capable of a sampling rate of 250 MS/s with a dynamic range of 12 bits. The preamplifier is a Miteq AU1647 amplifier with a noise figure of ~1.3 followed by a variable attenuator (Minicircuits, ZX73-2500-S), mixer (Minicircuits ZX05-1L-S), another amplifier (Minicircuits ZFL-500LN+) and finally a bandpass filter (several filters from KR Filters depending on IF frequency). The attenuator is adjustable from -3 dB to -65 dB of attenuation to take full advantage of the dynamic range of the digitizer. The signal is power combined with the adjacent channel which has been mixed to a different frequency than the first channel. The filter removes the mixing products to assure that mixing products from one channel did not appear in the bandwidth of the other channel. The filter before the power combiner needs a very sharp cutoff to prevent the addition of noise from one channel into signal from another. The number of channels that can be digitized on a single signal line is restricted only by the bandwidth of the digitizer card, its dynamic range, and the availability of filters at the frequencies of interest (if trying to maintain the theme of off-the-shelf products only). The cost of the digitizer card was ~$5000 and the cost of each channel was about ~$1000. Using the multiplexing approach, the total cost of a 16 channel receiver would break down to about \$1300 per channel in this case. All imaging and spectroscopy data were acquired on a 4.7T Varian INOVA system. The only interface required to the scanner were two trigger lines. Six-channel images were acquired from a previously reported mouse array [4] (TR=250ms, TE=20ms, SW=50kHz, Navg=1, FOV=10cm, Npts=128) and the SNR of each image was compared to single-channel acquisitions from each array element with the Varian. The same procedure was followed for two-channel 13C spectral acquisition (TR=3s, SW=10000Hz, Navg =64). The coils used are shown in Fig. 3. The only necessary change in the receiver chain between 1H and 13C acquisitions was a change in the LO frequency in the receiver control GUI. In addition, it is worth noting that the six-channel mouse coil used isolation preamplifiers while the 13C coils used only the amplification provided by the receiver, emphasizing the flexibility of the receiver to handle a variety of coil configurations.

### Results & Discussion

The two-channel 13C spectrum and six-channel 1H image comparisons between the FDM receiver and the Varian are shown in Figs. 4 and 5, respectively. In its current form, the SNR of the FDM receiver is approximately 80% of the Varian. It is anticipated that the two will be comparable, however, after switching to a less noisy switch in the first stage. The proposed design will scale straightforwardly to sixteen frequency-independent receiver channels with inexpensive, off-the-shelf products, making multi-channel non-proton spectroscopy a viable reality for research groups.

### Acknowledgements

No acknowledgement found.

### References

[1] Iannotti et al., Proc. ISMRM. 2009. [2] Wei et al., Proc. ISMRM 2006 [3] Pavan et al., Proc. ISMRM 2010 [4] Chiang et al., Proc. ISMRM 2015.