NOVEL APPROACHES TO LOW-COST MRI

This paper presents a combination of speculative approaches, some rela ted to earlier work and some apparently novel, which show great promis e in providing a new class of MRI machines that would be considerably less expensive. This class would have advantages and disadvantages as compared to existing MRI, over and above that of low cost. The disadva ntages include the apparent inability to perform classic spectroscopy, and limited flexibility in the area of selective excitation. The adva ntages include a fundamental immunity to inhomogeneity and susceptibil ity problems, the ability to create a wide class of machines that are designed for specific anatomy related applications, the ability to des ign open machines for physician access, and improved capability for hi gh speed imaging. Generic to all of the methods presented are a pulsed polarizing field and an oscillatory read-out bias field. The pulsed f ield initially polarizes the magnetic moments. Since it is not on duri ng the readout operation it has negligible homogeneity requirements si nce changes in the field amplitude will merely shade the image intensi ty. During readout a relatively low blas field is used. To enable the use of a relatively inhomogeneous blas field, an oscillatory field is used that has a zero average value. This prevents any long-term buildu p of phase errors due to a frequency error associated with inhomogenei ty. Thus the average bias frequency will be determined solely by the f requency rather than the amplitude of the bias field. Three methods ar e described, all including the above features. The first two involve i maging in the laboratory frame, while the third involves imaging in th e rotating frame. The second approach requires no RF excitation and th e third approach uses RF blas and gradient signals. Some approaches to slice selection are described.