Contrast-enhanced MRI relies on changing the relaxivity of tissue, either by reduction in T₁ allowing an enhanced signal on T₁-weighted images, or by a reduction in T₂^* resulting in negative contrast on T₂ or T₂^* weighted images to track the uptake of the contrast agent. At ultra-low field (ULF), normal biological T₁ values are significantly shorter compared to those at high field, and the ULF regime is generally immune to T₂^* effects suggesting that conventional contrast agents will not be of tremendous utility for ULF imaging. We describe here a new approach in ULF imaging by using saline as contrast medium. Saline is biologically safe, and with the use of a small, strong pre-polarizing permanent magnet, enables contrast-enhanced MRI at 0.0065 T.Imaging was performed at 0.0065 T using a custom-built electromagnet-based MRI scanner¹ (Fig.1). Saline hyperpolarization was performed using a 1.3 T permanent magnet placed in the Faraday cage of the scan room, 1 m away from the NMR coil (Fig.1). The permanent magnet is a 5.5 cm deep cylinder of diameter 5.5 cm, with a 2 cm axial hole (K&J Magnetics, Pipersville, USA). It was placed inside a custom-built steel shielded box, designed by simulation with COMSOL Multiphysics (Burlington, USA) to minimize the effect of the permanent magnet on the homogeneity of our scanner. A 10 mL plastic syringe was modified to form a cylinder with luer-lock connectors on both ends and was placed inside of the permanent magnet. One end was connected to a 60 mL syringe filled with water and placed on an infusion pump outside the Faraday cage. The other end was connected to a PE50 capillary which in turn connected to our phantom located in the imaging coil. The phantom itself consists of a modified 60 mL syringe allowing for continuous flow (with input from the 1.3 T magnet side, and output to a waste container/jar). Constant flow of 20 mL per minute was started a few seconds before the acquisition began. A 17 s b-SSFP sequence with 50% undersampling and number of average NA=5 for a 2×2×10 mm³ spatial resolution was used as a reference before repeating the same acquisition while injecting hyperpolarized water at a flow rate of 20 mL/min. The same scan with NA=20 was used as a reference. Data were processed using MATLAB (MathWorks, Natick, USA) with scripts written in house.Simulations with COMSOL showed that the contribution to the local magnetic field due to the permanent magnet were efficiently removed using our shielded box (Fig.2). The impact of the permanent magnet on T₂^* was, however, strong (T₂^* dropped to 52 ms and was 690 ms without the 1.3 T magnet), but did not affect too strongly image quality. Figure 3 shows the image with NA=5 and no hyperpolarized water, whereas b) shows the results for contrast-enhanced imaging. The enhanced water jet was overlaid on the reference image with NA=20 c) and the result is shown in d).Our results show that we are able to perform contrast-enhanced MR imaging at ultra-low magnetic field by using a strong permanent magnet placed close to our detection setup to hyperpolarize water. We show that the presence of a permanent magnet inside the Faraday cage affects T₂^* but not enough to prevent good imaging quality. The transfer to in vivo applications remains challenging for two main reasons. The first limitation comes from the weak imaging gradient strength of our MRI scanner (~ 1mT/m), which prevents from either better spatial resolution or faster duty cycles and thus shorter acquisition times. The second limitation comes from the short T₁ of hyperpolarized water (a few seconds) in our ULF regime. A combination of stronger gradients and optimized NMR coils for receive operation would certainly help, as well as an efficient way to inject hyperpolarized water in this setting (in particular by significantly reducing the dead volume in the 1m tubing to the our phantom).We have shown that hyperpolarized water can be used to perform contrast-enhanced MRI at ultra-low magnetic field. This result, once optimized and tested in vivo, would open opportunities for MR angiography in a cost-effective portable MRI scanner using fully biocompatible saline. Of particular interest is the possible early diagnostic of acute phase of stroke.