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
Synopsis
Here
we describe a six channel inexpensive FDM receiver, agnostic to the nuclei of
interest or magnetic field strength, implemented straightforwardly using
off-the-shelf products, portable, and easily used in conjunction with any
system with a single trigger line. The architecture is straightforwardly
scalable to 16 channels at a cost of approximately $1300 per channel. This work
describes the receiver architecture and the capabilities are demonstrated by
acquiring six channel images from a previously reported mouse array coil,
two-channel 13C spectra, and comparing the SNR of the receiver to
the Varian Inova system.Purpose
Despite
great potential, non-1H MRS studies have not been adopted into
standard clinical use, largely due to the fact that non-1H nuclei
have limited signal-to-noise ratio (SNR) due to their lower Larmor frequency
and lower relative abundance. Exploiting the increased SNR provided by array coils
is a natural direction to turn in addressing this; however it is highly unusual
for scanners to be equipped with multi-channel multi-nuclear receivers due to
cost, complexity, and lack of any current widespread clinical adoption of
second-nuclei studies. Frequency domain multiplexing (FDM) recently has been
investigated as a low cost alternative to [expensive] multi-channel receivers
[1,2], and has even been applied to non-1H nuclei [3]. Here we
describe a six channel inexpensive FDM receiver, agnostic to the nuclei of
interest or magnetic field strength, implemented straightforwardly using
off-the-shelf products, portable, and easily used in conjunction with any
system with a single trigger line. In addition, the architecture is
straightforwardly scalable to 16 channels at a cost of approximately $1300 per
channel. This work describes the receiver architecture and the capabilities are
demonstrated by acquiring six channel images from a previously reported mouse
array coil, two-channel 13C spectra, and comparing the SNR of the
receiver to the Varian Inova system.
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.