The objective of this research project is to demonstrate the technical feasibility of an ultra-low-cost form of magnetic resonance imaging (MRI) called Prepolarized MRI for human extremity imaging. Conventional MRI systems cost between 1 million dollars and 3 million dollars. We propose a novel Prepolarized MRI system that uses two inexpensive electromagnets rather than one expensive superconducting magnet. We aim to demonstrate that a complete dedicated extremity Prepolarized MRI scanner imaging can be constructed for less than 50,000 dollars while maintaining excellent image quality. Over the last five years, we have made excellent progress towards these goals, including our first in vivo human wrist images recorded earlier this month. The total capital cost of our 0.43 T Prepolarized MRI wrist imaging system was less than 35,000 dollars, excluding the MRI data acquisition console. Our principal aim is to expand the size of the imaging system to 31 cm to accommodate knees, feet, and elbows. Once basic imaging is achieived on the 31-cm-bore system, we plan several hardware and software improvements to increase the SNR and contrast flexibility. We also plan a monthly study of normal human knee cartilage conspicuity. Two Stanford Radiologists will compare the normal images with gold-standard images obtained from a 0.5 T GE extremity scanner, and gauge whether the previous month's engineering changes actually improved image conspicuity. We hope at the end of this four year grant to have constructed a 0.5 T Prepolarized MRI system with SNR and contrast comparable to a conventional 0.5 T MRI scanner for less than 50,000 dollars in capital costs. If successful, our research could ultimately lead to significantly greater availability of high-quality Prepolarized MRI scanners, which could be sold for less than the cost of an ultrasound scanner.