## 2 Materials and Methods
Coarse tuning and matching was accomplished using AVX high-voltage capacitors, with the fine tuning being achieved by changing the length of the coaxial cable between the duplexer and the capacitors. A series of silicone oils (Schumacer Racing, Northampton, UK) of different viscosities from 300 cP to 30,000 cP were used to give samples of different effective transverse relaxation times. A clinical MRI scanner (1.5 T MAGNETOM Avanto, Siemens, Erlangen, Germany) was used to provide a reference value to demonstrate the suitability of this system to replace high-field clinical imaging. The effective transverse relaxation time, \(\text{T}_{2}^{\text{eff}}\), is most commonly measured by the CPMG pulse sequence [17] where an RF excitation pulse tips the magnetization by 90\({}^{\circ}\) into the transverse plane where the phase becomes scrambled due to molecular interactions. After, a delay \(\tau\) an RF pulse is applied, which flips the magnetization by 180\({}^{\circ}\), causing an effective rewinding of the phase generating a so-called spin echo \(\tau\) later. This process is repeated to produce a train of echoes with an exponential envelope of time constant \(\text{T}_{2}^{\text{eff}}\). In very homogeneous field, the decay of \(\text{T}_{2}^{\text{eff}}\) is minimal since there is little loss of phase coherence due to Brownian motion through field gradients. However, for less homogeneous magnetic fields this results in a parameter that is highly dependent on diffusion, field gradient magnitude, and the echo time (which is equal to 2\(\tau\)). An alternative method reported in [18] measures the integral of the echoes as a function of increasing echo times (\(\text{T}_{\text{E}}\)) to avoid a contribution from molecular diffusion; however, such an approach is too lengthy for clinical applications of inhomogeneous low-field systems such as this. The signal-to-noise ratio (SNR) is improved by undertaking this experiment and averaging the results, and the signal is averaged linearly, whilst the noise scales with the square root of the number of averages; hence, the SNR is dependent on \(NA/\sqrt{NA}\). The value of \(\text{T}_{2}^{\text{eff}}\) is then calculated by determining the modulus of the real and imaginary components of the spin echoes before taking the integral of each of the echoes (to improve SNR); a monoexponential is then fitted to the timecourse of the decay using a Levenberg Marquardt least squares minimization revealing \(\text{T}_{2}^{\text{eff}}\) as the time constant of the decay. In our case, this is undertaken within the previously mentioned custom MATLAB (Mathworks, Natick, MA, USA) GUI used to run the console. Figure 4 shows the data for 5000 cP silicon oil from a CPMG sequence using the solenoid coil on the CAPIBarA system. The MR signal acquired from 256 averages of a CPMG experiment with RF pulse durations is set to 5 \(\upmu\)s for a frequency of 804.25 kHz. The signal is acquired with a dwell time of 10 \(\upmu\)s for 8 points over the center of 250 echoes that are 800 \(\upmu\)s apart. Each average is collected with a repetition time of 1200 ms, giving a total experimental time of 5 min. For the planar coil, the CPMG sequence parameters are similar except for the pulse duration, which is increased to 67.5 \(\upmu\)s for a frequency of 764 Khz; the dwell time is increased to 25 \(\upmu\)s and the number of averages is increased to 1536, giving a total experiment time of 30 min. The size of the sensitive volume is dependent on several parameters, including the transmission bandwidth, receiver chain bandwidth (which in turn is dependent on the acquisition time) and the homogeneity of the magnetic field. The more homogeneous the field, the larger this volume will be, whilst shorter dwell times and shorter transmit pulses will also increase the size of this volume. The narrowest bandwidth of these will dictate the overall bandwidth of the measurement. In this case, the RF pulse is limiting and excites a bandwidth of 50 kHz for the solenoid and 3.7 kHz for the planar coil.