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Noninvasive room temperature instrument to measure liver iron stores

The work of Brittenham [1], Fischer [2], and others has shown that biomagnetic susceptibility measurements are a uniquely effective, noninvasive method to monitor liver iron in patients with iron overload diseases. However, existing instruments, based on superconducting quantum interference devices (SQUIDs), are too expensive for widespread clinical use. We are developing a prototype biomagnetic susceptometer based on roomtemperature magnetic sensors. This new instrument can potentially make noninvasive liver iron measurements available to patients worldwide. In developing this room-temperature biosusceptometer, we face two main technical challenges. First, we need to make sure that the noise in the magnetic sensing system is low enough to measure the very weak magnetic fields produced by the magnetic susceptibility response of liver iron. Second, we need to configure the applied-field coils and magnetic sensors so as to maximize the signal from the liver, while minimizing errors due to the response of other body tissues. In previous experiments, we achieved an instrumental noise level equivalent to a liver-iron concentration of 30 μg Fe /g of wet tissue, in magnetic susceptibility measurements on a liversized phantom [3]. This noise level, comparable to that of existing SQUID instruments [1,2], is small compared with the normal liver iron concentration of 200-500 μg/g (wet), and much smaller than other sources of error typically encountered in liver iron measurements [1,2]. However, these initial results were achieved using an applied magnetic field that reached well out from the sensor unit, so that our magnetic susceptibility measurements sampled a relatively large volume of material. We were concerned that, in measurements on human subjects, this large sampled volume would include too large a contribution from the lung, increasing the errors in the liver iron measurement. We have now redesigned the sensor unit so as to minimize the error due the lung, without increasing the errors due to instrumental noise. In order to optimize the sensor design, we used a numerical body model to estimate the magnetic susceptibility responses from each of the tissues surrounding the liver, as a function of the geometry of the appliedfield coil and magnetic sensors. In addition we improved the magnetic sensors so that we can make magnetic susceptibility measurements with lower noise, using a smaller sampled volume. This paper presents some of the results from our numerical modeling and sensor noise improvements.