MRI engineers, researchers, radiologist and all medical center interested in a low field MRI setup.Very low field Magnetic Resonance (VLF MRI) is a complementary imaging modality of conventional MRI¹. VLF MRI based on inductive coils has many advantages: It is a low cost MRI (<50 000$), light weight, transportable (<100 Kg) silent and applicable to patients with metallic implants and pacemakers, claustrophobics and premature babies. Moreover, a new kind of image contrast appears at this range of field that could help in depicting cancer^2-3. However, VLF MRI is inherently limited in signal-to-noise Ratio (SNR) and requires important averaging time to obtain images with sufficient SNR for a relevant diagnosis. Many methods were employed in order to improve NMR signal and/or detectivity, i.e., Pre-polarization method⁴ and/or the use of SQUID or atomic magnetometers^5-6. Our laboratory has developed Nitrogen cooled superconducting-magnetoresistive hybrid sensors⁷ competitive to low Tc SQUIDs above 100kHz. These sensors are untuned and can be coupled through low resistive room temperature flux transformers to a room temperature antenna presenting a high filling factor. They are very robust and accept strong RF pulses with a very short recovery time compared to tuned RF coils, which allow measurements of broad signals. We have already proved the efficiency of using these sensors on a small VLF MRI setup⁸. In the present work, a VLF whole head MRI system is developed and characterized in order to study the effectiveness of mixed sensors in such a simple clinical system without employing pre-polarization technique or magnetic shielding room.A VLF MRI setup and a spectrometer were constructed (figure 1). An adjusted three coils Maxwell configuration is adopted to generate the main magnetic field. Coils are designed in order to minimize enclose feelings of claustrophobes (cylinder diameter of 83 cm). Inductive coils are water cooled providing an adjustable magnetic field up to 12 mT. Experiments are usually done at B0=8 mT (353 kHz). Double split saddle coils are used to generate gradient fields along frequency and phase (X and Y) directions and circular Maxwell coils are used to generate the gradient field along Z direction. Gradient coils are used not only for 3D imaging but also for correcting external inhomogeneity (shimming). To test the homogeneity of the static field, we used a container of 200x200x200 mm³ volume filled with doped water (T1 ≈ 120 ms; T2 ≈ 70 ms) simulating the average relaxation time of a brain. A 90◦ pulse is applied to obtain a Free Induction Decay (FID). The NMR time-domain signal is Fourier transformed to obtain the magnitude of the resulting spectrum. To test gradients linearity, we have imaged a sample, presented in figure 3a, filled with same doped water. The linearity of the gradients is directly measured on the image profile. We performed 1D images along X and Y directions. Then the position of the signal with the actual position of doped water in the phantom are compared. We have used, for these measurements a tuned coil (Q = 50) placed around the sample.

The Fourier transform of the FID signal is presented in figure 2. Achieved homogeneity is about 115 ppm for 200x200x200 mm³ volume.

Then, gradients linearity was verified. We obtained around 3% of error along both X and Y directions (figure 3). Adjustable gradients strength was also studied. A gradient of 500 µT/m was sufficient to get a 2 mm resolution.

Homogeneity of 115 ppm gives the resolution bandwidth (1 point = 40 Hz). Thus, a bandwidth of only 4 kHz will be sufficient for imaging a 200 mm length sample. This fact will compensate partially the loss in SNR due to low polarization at low field. In another hand, although gradients strength allows a good image resolution, those gradients (less than 1 mT/m) imply longer readout sequence limiting the possibility of reducing time’s sequences.A VLF MRI brain setup was constructed with an adjustable main magnetic field (<12 mT). A field homogeneity of 115 ppm was achieved on a 200x200x200 mm³ volume with this simple and open configuration. Future work will focus on reducing 3D imaging time sequence in order to be ready to combine mixed sensors with brain VLF MRI setup.