Applications of Magnetic Tracers in the Human Lung and Gastrointestinal Tract
The use of ferrimagnetic material as a tracer represents a noninvasive approach in physiological measurements. Concerning the lung, magnetopneumography, MPG, is a unique technique to estimate retained dust in the lung, using the magnetizable dust fraction as a tracer. First a strong constant magnetic field is briefly applied to the thorax. After the field has been removed, the weak remanent field of the magnetized particles is measured outside the body. The sampled data include information not only about the amount and distribution, but also about the random rotation, of the tracer- particles. In the present work, the MPG method was used in vivo, and developed through model studies toward improved accuracy. Furthermore, applications of the MPG principle (and instrument) in the field of gastrointestinal motility were developed and evaluated. Such applications may constitute future alternatives to clinically established radiological methods, especially for subjects for whom radiation is of particular concern.
Previous investigations have shown large inter-individual variations of lung dust retention among arc welders with similar fume exposure history. Here, repeated MPG measurements were used to monitor the retention build-up in a group of eight, previously unexposed, trainees during a welding course. Already after 30 days of exposure there were inter-individual differences in estimated retention of more than a factor of two. A first-order retention model was suggested, including estimated individual dust exposure profiles.
The inverse problem of finding the amount and distribution of the source of a measured magnetic field has no unique solution. Anatomical differences of lung and chest shapes are important sources of error when total amounts of lung dust are estimated and inter-individual variations are studied. Multipole analysis involves estimation of a number of coefficients in a truncated sum; one of these represents the total dipole moment, which is proportional to the total amount of dust. In comparison, the commonly used mean value method showed much larger bias error than the multipole methods. Multipole orders up to and including octupoles were needed to describe the field from a magnetized thorax. This model is well suited when measuring total dust load in subjects where dust translocation is of concern, e.g. in studies of human lung dust clearance.
Gastric emptying measurements were carried out magnetically on 16 healthy male volunteers. The early part of the mean emptying curve decreased slightly faster than the curve of a corresponding previous scintigraphic study. Rotations (due to intraluminal movements) were studied by measuring the field decay outside the abdomen after magnetization. After a low activity phase of almost linear flux density decay, the mean rotation gradually increased, and the decay became bi-exponential. Moreover, by measuring field maps repeatedly after ingestion, the transit of a swallowed small permanent magnet was followed through the gut.
It is concluded that measurements of magnetic fields from ferrimagnetic tracers prove a valuable and promising tool in estimations of retained dust in the lungs, as well as in studies of gastric motility.