Ali Attaran^{1}, Kyle Gilbert^{1}, Blaine A. Chronik^{1}, and Ravi S. Menon^{1}

Low-profile preamplifiers were developed to be used in multichannel coil arrays. Optimizing the combination of preamplifier input impedance and coil impedance for high-density receive arrays indicates that the highest SNR is obtained when using high input-impedance preamplifiers connected directly to high-impedance coils.

**Introduction:**

The noise
figure of a receiver strongly depends on the first stage, the noise figure of
subsequent devices will be divided by the gain of the previous stages and become
less important in practice; therefore, the noise figure of the MRI receiver
chain can be minimized by designing a very low noise preamplifier. Since the transistor performance determines the
overall performance, a GaAs enhancement-mode pseudomorphic high electron
mobility transistor (ePHEMT) should be utilized in the first stage of the
preamplifier. To
achieve the lowest noise figure, F_{min},
the reflection coefficient of the source (Γ_{s})
should be as close as possible to the optimum reflection coefficient (Γ_{opt})
required by the transistor. In this case, the second term of the
equation tends to zero, and the minimum noise
figure can be obtained: NF=F_{min}+(4R_{n }/ Z_{0})(abs(Γ_{s}-Γ_{opt})^{2} / ((abs(1+Γ_{opt})^{2}abs(1-Γ_{s})^{2}), where 4R_{n} / Z_{0}
is the normalized noise resistance. Typically,
an impedance much higher than 50 Ω is required for FETs to produce high Γ_{opt}
and a low F_{min}. In addition, both the high impedance of the coil and the high input
impedance of the preamplifier will serve to reduce the current flowing in the
coil; therefore, for multichannel receive arrays, the coupling between coils is
reduced. ADS simulations were performed to determine the noise circles versus
available gain and find out the Γ_{opt} required by the transistor. In addition to geometric
decoupling [4], M_{12} can also be minimized by using high-impedance
coils in conjunction with high-input-impedance preamplifiers, thereby limiting I_{2}: V_{out} = V_{sig}+(R_{1}+j(ωL_{1}-1/ωC_{1t}))I_{1}+jωM_{12}I_{2} (Fig. 1). SNR was measured on the scanner for 50 Ω and 500 Ω input-impedance
preamplifiers in conjunction with a receive coil tuned to either 5 Ω,
50 Ω, or 500 Ω. The experimental setup is shown in Fig. 2.

- C. L. Lim, P. Serano and J. L. Ackerman, “Pre-amplifiers for a 15-Tesla magnetic resonance imager,” RF and Microwave Conference (RFM), 2013 IEEE International, Penang, 2013, pp. 295-299.doi: 10.1109/RFM.2013.6757270.
- K. M. Huber, M. Hemmerlein, S. Biber, R. Oppelt, and K. Wicklow, “A Preamplifier for 7T MRI with Extended Dynamic Range and Integrated Cable Trap”, Proc. Intl. Soc. Mag. Reson. Med. 16, 2008.
- A. Reykowski, S. M. Wright, and J. R. Porter, “Design of Matching Networks for Low Noise Preamplifiers”, Magn. 1995.
- P. B. Roemer, W. A. Edelstein, C. E. Hayes, S. P. Souza, and O. M. Mueller, “The NMR phased array”. Magn Reson Med, 16: 192–225, 1990.

Fig. 1. Circuit model of two adjacent coils

Fig.
2. A schematic view of the test setup. The transmit coil is on top with tuning and matching
capacitors, an active-detuning circuit, and a balun. The receive coil
is placed closer to the phantom to reduce the path loss and to improve SNR. The receive coil has tuning and
matching capacitors, an active-detuning circuit, and a low-noise amplifier. All data was acquired on a 7T MRI scanner

Fig.
3. Implemented test setup

Table1. SNR Comparison