Hendryk Richert, Olaf Kosch, and Peter Gornert 4.2.1
The human digestive system, particularly the small intestine, is a difficult region to access with established investigative methods. Using different methods such as X-radiography, magnetic resonance imaging (MRI) or computed tomography (CT), it is easy to visualize the morphology of the gastrointestinal (GI) tract. Unfortunately, motility cannot be investigated satisfactorily with any of these established methods as they either deal with ionizing radiation (X-ray and scintigraphy) or they are too slow to register movements.
A relatively new investigative method uses a small capsule camera which yields information about the inner surface of the small intestine (Lewis and Swain, 2002). This camera supplies a picture every 500 ms for approximately 6 h, and using this instrument it is possible to identify abnormalities of the surface, especially internal bleeding. The main disadvantage of this method is that it requires a 6-h video evaluation session. A second disadvantage is that the exact position of the camera -and therefore the recorded abnormalities - cannot be determined. Therefore, there is a vital need for a method which is able to investigate the digestive motility and then to provide both the nature and the location of abnormalities.
Magnetic monitoring might be that method. Here, the patient simply swallows a small magnetic marker, which then moves on its natural path through the human digestive system as a small piece of indigestible material. During the total period of investigation the marker is tracked magnetically without using any electromagnetic radiation. Magnetic monitoring has been used successfully for several years, with investigations first being conducted at centers where biomagnetic signal sensing equipment, such as Superconducting Quantum Interference Device (SQUID) was available (Weitschies et al., 1994). These SQUID sensors (see also Section 2.2) allow the detection of extremely weak magnetic signals, but they must be cooled with liquid helium or nitrogen. Parallel to the SQUID activities, prototype systems based on fluxgate and magnetoresistive sensors have been developed. These can be used under room temperature conditions and provide exact information about the human GI tract (Andra et al., 2000; Richert, 2003).
Magnetic monitoring is based on the evaluation of the stray field which is generated by a magnetic marker and can be measured with appropriate magnetic field sensors. The evaluation on a PC delivers both the three-dimensional position and orientation of the marker. Magnetic monitoring can be used for investigations of human digestive motility as well as for the evaluation of medicaments dissolving and absorbing processes inside the GI tract (Weitschies et al., 2001a,b).
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