Friday, February 12, 2010

Biomagnetism and Medicalphysicsweb

Medicalphysicsweb is an excellent website for news and articles related to medical physics. Several articles that have appeared recently are related to the field of biomagnetism, a topic Russ Hobbie and I cover in Chapter 8 of the 4th edition of Intermediate Physics for Medicine and Biology. I have followed the biomagnetism field ever since graduate school, when I made some of the first measurements of the magnetic field of an isolated nerve axon. Below I describe four recent articles from medicalphysicsweb.

A February 2 article titled “Magnetomometer Eases Cardiac Diagnostics” discusses a novel type of magnetometer for measuring the magnetic field of the heart. In Section 8.9 of our book, Russ and I discuss Superconducting Quantum Interference Device (SQUID) magnetometers, which have long been used to measure the small (100 pT) magnetic fields produced by cardiac activity. Another way to measure weak magnetic fields is to determine the Zeeman splitting of energy levels of a rubidium gas. The energy difference between levels depends on the external magnetic field, and is measured by detecting the frequency of optical light that is in resonance with this energy difference. Ben Varcoe, of the University of Leeds, has applied this technology to the heart by separating the magnetometer from the pickup coil:
The magnetic field detector—a rubidium vapour gas cell—is housed within several layers of magnetic shielding that reduce the Earth’s field about a billion-fold. The sensor head, meanwhile, is external to this shielding and contained within a handheld probe.
I haven’t been able to find many details about this device (such as if the pickup coils are superconducting or not, and why the pickup coil doesn’t transport the noise from the unshielded measurement area to the shielded detector), but Varcoe believes the device is a breakthrough in the way researchers can measure biomagnetic fields.

Another recent (February 10) article on medicalphysicsweb is about the effect of magnetic resonance imaging scans on pregnant women. As described in Chapter 18 of Intermediate Physics for Medicine and Biology, MRI uses a radio-frequency magnetic field to flip the proton spins so their decay can be measured, resulting in the magnetic resonance signal. This radio-frequency field induces eddy currents in the body that heat the tissue. Heating is a particular concern if the MRI is performed on a pregnant woman, as it could affect fetal development.
Medical physicists at Hammersmith Hospital, Imperial College London, UK, have now developed a more sophisticated model of thermal transport between mother and baby to assess how MRI can affect the foetal temperature (Phys. Med. Biol. 55 913)… This latest analysis takes account of heat transport through the blood vessels in the umbilical cord, an important mechanism that was ignored in previous models. It also includes the fact that the foetus is typically half a degree warmer than the mother – another key piece of information overlooked in earlier work.
Russ and I discuss these issues in Sec. 14.10: Heating Tissue with Light, where we derive the bioheat equation. The authors of the study, Jeff Hand and his colleagues, found that under normal conditions, fetal heating wasn’t a concern, but if exposed to 7.5 minutes of continuous RF field (unlikely during MRI) the temperature increase could be significant.

In a January 27 article, researchers from the University of Minnesota (Russ’s institution) use magnetoencephalography to diagnose post-traumatic stress disorder.
Post-traumatic stress disorder (PTSD) is a difficult condition to diagnose definitively from clinical evidence alone. In the absence of a reliable biomarker, patients’ descriptions of flashbacks, worry and emotional numbness are all doctors have to work with. Researchers from the University of Minnesota Medical School (Minneapolis, MN) have now shown how magnetoencephalography (MEG) could identify genuine PTSD sufferers with high confidence and without the need for patients to relive painful past memories (J. Neural Eng. 7 016011).
The magnetoencephalogram is discussed in Sec. 8.5 of Intermediate Physics for Medicine and Biology. The data for the Minnesota study was obtained using a 248-channel SQUID magnetometer. The researchers analyzed data from 74 patients with post-traumatic stress disorder known to the VA hospital in Minneapolis, and 250 healthy controls. The authors claim that the accuracy of the test is over 90%.

Finally, a February 8 article describes a magnetic navigation system installed in Taiwan by the company Stereotaxis.
The Stereotaxis System is designed to enable physicians to complete more complex interventional procedures by providing image guided delivery of catheters and guidewires through the blood vessels and chambers of the heart to treatment sites. This is achieved using computer-controlled, externally applied magnetic fields that govern the motion of the working tip of the catheter or guidewire, resulting in improved navigation, shorter procedure time and reduced x-ray exposure.
The system works by having ferromagnetic material in a catheter tip, and an applied magnetic field that can be adjusted to “steer” the catheter through the blood vessels. We discuss magnetic forces in Sec. 8.1 of Intermediate Physics for Medicine and Biology, and ferromagnetic materials in Sec. 8.8.

Although I believe medicalphysicsweb is extremely useful for keeping up-to-date on developments in medical physics, I find that often their articles either have specialized physics concepts that the layman may not understand or, more often, don’t address the underlying physics at all. Yet, one can’t understand modern medicine without mastery of the basic physics concepts. My browsing through medicalphysicsweb convinced me once again about the importance of learning how physics can be applied to medicine and biology. Perhaps I am biased, but I think that studying from the 4th edition of Intermediate Physics for Medicine and Biology is a great way to master these important topics.

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