The medical imaging field can be divided in 2 imaging modalities: on one hand, those techniques which generate the imaging information from outside the body (basically all x-ray combinations) and those which send the image information from inside the body (nuclear medicine, US and MRI). The advantage of the latest is that they are able to give functional information (better contrast), however they lack of image resolution compared with CT, for example. The combination of resolution and functional information can be found in hybrid equipments like PET/CT, widely extended commercially. Recently, the big advance in MRI had achieved spatial resolution close to the CT, keeping the better contrast and functional information. The handicap of the hybrid PET/MRI is the influence of the magnetic field, inevitable for the MRI equipment, in the detectors ring. This problems seems to be solved with the new SiPM in high fields ca. 1T [1], however it is still a complex and expensive equipment [2]. There are different ways to combine MRI and PET, but all have the same approach: to insert the PET detectors ring in the MRI, so that it is necessary to adapt the PET part to the MRI. In low field MRI, it can be combined the other way around; adapt the MRI to PET by locating the PET ring outside the MRI. This has some restrictions since the MRI could attenuate and scatter the photons on their way to the detectors ring. The MF-UNAL has recently acquired an earth field MRI equipment (Magritek) [3], also known as ultra-low field MRI (ULF-MRI). It works in a range of 10 to 100 μT. This equipment has, of course, a very low signal to noise ratio (SNR) as drawback, but it is very versatile and can be combined with other imaging techniques, which would be otherwise affected by the high magnetic fields. In this project, it is purposed to develop a low field MRI which can be adapted for nuclear medicine. It is purposed to develop the permanent magnets array based on previous simulations. A very good estimation of the attenuation and scattering can be achieved by simulation software like GATE [4] or GEANT4 [5]. On the other hand, the ULF-MRI can be easily combined with gamma camera or PET in order to check the simulation. This verification will help to develop other MRI compatible devices, like those with permanent magnets, and then, to study different permanent magnets configurations.