PEDRI imagers normally use low-field magnets with magnetic fields of about 0.01 tesla (as opposed to the 1 tesla or so used in clinical MRI systems). This is because the ESR frequency is 659 times the NMR frequency in a given magnetic field. So, in a 1 tesla magnet, the ESR frequency would be 28 GHz, well into the microwave part of the electromagnetic spectrum, which would cook any biological sample being imaged! Reducing the magnetic field to 0.01 tesla brings the ESR frequency to a more manageable 280 MHz.