Breast cancer is the second leading cause of death in American women. For that reason the American Cancer Society recommends annual x-ray mammograms for women at risk. However, x-ray mammography has its drawbacks. The repeated x-ray dose may increase the risk of cancer, and x-ray mammography is notoriously non-specific, with five women sent to biopsy for every confirmed malignancy. Magnetic Resonance (MR) imaging has been extremely successful at detecting breast cancer when used with a contrast agent such as Gadolinium-DTPA. However, no one recommends that MR imaging be used in routine screening for breast cancer because it costs more than ten times as much as x-ray mammography. The long-range goal of this research is to develop a more economical method of MR imaging useful for breast cancer screening. This method will be cheaper because the magnet and imaging hardware will be tailored to the size of the breast rather than the whole body, and also because the static magnetic field will be about 100 times weaker than usual (i.e., 0.01 Tesla rather than 1.5 Tesla). The cost advantage of reducing the static field is that ordinary electromagnets can be used rather than expensive superconducting magnets. The disadvantage of low field strength is that MR signal is drastically reduced. The novelty of this system is that it will employ """"""""Overhauser enhancement"""""""" to regain much of the signal lost by reducing the static field. Overhauser enhancement is made possible by the presence of unpaired electrons that are magnetically coupled to the nuclei contributing to the MR image. When the electrons are irradiated at their resonant frequency, the MR signal can be enhanced by up to 50 times, depending on the power with which the electrons are irradiated. The limitation of using Overhauser enhancement in people is that the absorbed power must be kept below prescribed safety levels. The absorbed power is proportional to the square of the resonant frequency. One way to reduce absorbed power while maintaining strong enhancement is to reduce the strength of the static magnetic field while the electrons are irradiated, and then return it to a higher value to acquire the MR image. The focus of this proposal is to experimentally determine the degree of field reduction that maximizes Overhauser enhancement while keeping the absorbed radio frequency power below human safety limits. This will determine the feasibility of using Overhauser enhanced imaging for breast cancer detection, and indicate the degree of field reduction that produces optimum results.
| Lee, J N (1996) Feasibility of overhauser-enhanced MR imaging with nitroxide free radicals. J Magn Reson Imaging 6:261-3 |
