Cost and Performance of Low Field Magnetic Resonance Imaging
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COST AND PERFORMANCE OF LOW FIELD MAGNETIC RESONANCE IMAGING LAWRENCE E. CROOKS*, CHING YAO**, MITSUAKI ARAKAWA*, JAMES D. HALE*, JOSEPH W. CARLSON*, PAUL LICATO**, JOHN HOENNINGER*, JEFFREY WATTS* AND LEON KAUFMAN* * University of California, San Francisco - Radiologic Imaging Laboratory, South San Francisco, CA ** Diasonics, Inc., South San Francisco, CA ABSTRACT Magnetic Resonance Imaging technology has advanced on many fronts. Two important areas are the improvement of signal-to-noise levels and the understanding of imaging pulse sequence optimization. Based on these advances, low cost imagers using very low field permanent magnets are providing images of diagnostic quality. INTRODUCTION Magnetic Resonance Imaging (MRI) has changed the way diagnostic radiology is practiced. The accurate depiction of anatomy and non-invasiveness of the technique make it the modality of choice in an increasing number of applications. Nevertheless, cost is a continuing impediment to wider diffusion of the modality. The major component of cost is the magnet, both as a direct cost and because of its impact on siting costs. Today, superconducting magnets are used in most of the commercial systems. The cost of these units is dependent somewhat on field strength, and siting costs are strongly dependent on field strength [1]. The most effective way to reduce the cost of a superconducting magnet is to make it smaller, since this reduces the cost of the cryostat. A smaller magnet also reduces siting costs because the field is more contained. In any event, cryostat costs limit the lowest achievable cost with any superconducting magnet, and physical size can only be reduced to a point consistent with the need to place a body in the aperture. Other components are also significant in determining system cost, and these include computers, gradient coils and power supplies, RF electronics, installation and warranty. Thus, if drastic cost reduction is desired, subsystems other than the magnet also need to be considered. As operating field strength changes, so does the magnet technology that provides the lowest cost alternative. Roughly, permanent and air core resistives are the lowest cost options below about 1 Kgauss, air core resistives are to be favored between 1 and 2 Kgauss and superconducting magnets are lowest above 2-2.5 Kgauss. From the point of view of installation, permanent magnets (if not too heavy) are attractive in that they do not require electrical power and cooling water, services needed for resistive magnets. In an intergrated approach to cost reduction, the effects of lower field strengths cascade through the system. Reducing field strength impacts not just available magnet types, but also gradient systems and siting. If some speed is sacrificed in computer data flow times, then further savings are possible. An associated benefit of low frequency operation is that increased coil efficiency reduces the power needed to excite the nuclei. This reduces the cost of the RF transmitter. We are investigating the use of very low field
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