Stuart G. Mirell

NMR IMAGING WITH MAGNETIC LENSES

A unique prototype magnet system for regional NMR imaging is described. The prototype demonstrates the feasibility of a lightweight iron core magnet using magnetic lenses and well suited to single organ imaging, particularly with regard to applications utilizing NMR contrast agents targeted to a specific organ. The 0.3 scale prototype system incorporates opposed contoured axially symmetric pole pieces. Fields in the range of 0.4T are achieved with alnico magnet elements while lower field strengths using ceramic magnet elements have facilitated the lens design. Corrective magnetic lenses permit an overall magnetic field 1000 times less uniform than that of conventional NMR imaging magnets. This far less stringent uniformity requirement translates into a full scale system of only 2 tons while maintaining high field strength and resolution. The (electro)magnetic lenses flatten the field to 1 part in 50,000 only in the pre-selected thin discshaped volume to be imaged. In this manner, uniformity comparable to conventional NMR imagers is achieved. The lenses are the physical equivalent of the terms in a Legendre polynomial representation of the magnetic field. The degrees of freedom afforded by controlling lens currents permit a nearly constant field solution in a disc-shaped planar segment co-axial to and at any selected position between the vertically oriented poles. In full scale, this segment would be 20 cm in diameter and 0.4 cm in thickness with a patient aperture of 40 cm permitting tomographic visualization of any region of the head or body. Such a system would be low in cost, mobile, negligible in fringe field and ideal for imaging regional NMR contrast agent uptake.

Determination of organ area. Application to thyroid

A method has been developed for the quantification of a projected organ area using a standard gamma camera with pinhole collimation and computer processing of two vertically displaced images. Formulae have been derived expressing the uniquely determined quadratic relationship between object area and aperture distance with respect to projective image areas. This technique is extremely simple and its successful application to thyroid imaging is described.