Friday, December 26, 2008

A Gift For the Readers of Intermediate Physics for Medicine and Biology

The holiday season is a time when we often exchange gifts. My gift to the readers of the 4th Edition of Intermediate Physics for Medicine and Biology is a new homework problem. It belongs to the chapter on magnetic resonance imaging, and specifically to Section 18.12 (this section about functional MRI has no homework problems associated with it, so the new one fills the gap), but it also draws heavily on Section 8.1 about the magnetic force on a current, sometimes called the Lorentz force. The purpose of the problem is to determine if you can use MRI and the Lorentz force to detect nerve activation.

Section 18.12

Problem 37 1/2 Suppose your median nerve, having a radius of 2 mm, carries a current density of 10 Amps per square meter over a length of 10 millimeters. (Assume all the axons are simultaneously active, so the current density is uniformly distributed throughout the nerve).

a) You are having a magnetic resonance image taken, and the steady uniform magnetic field has a strength of 4 Tesla and is directed perpendicular to the nerve. Calculate the magnitude and direction of the magnetic force on the nerve.

b) Assume the nerve is held in position by an elastic force equal to the product of k and s, where k is the spring constant of 400 Newtons per meter and s is the distance the nerve is displaced from its equilibrium position. Calculate the displacement of the nerve experiencing the force found in part a.

c) Finally, assume that a magnetic field gradient of 36 milliTesla per meter is present, so that when the nerve moves the distance calculated in part b, it enters a region of different magnetic field strength. Calculate the change in magnetic field that the nerve experiences because of its motion. Calculate the change in resonance angular frequency (assuming you are imaging protons). If the gradient and current last for 15 milliseconds, what is the change in phase of the MRI signal?

Where did I come up with this problem? It is based on a paper that Peter Basser and I recently published, titled "Mechanical Model of Neural Tissue Displacement During Lortenz Effect Imaging," which appeared in the January 2009 issue of the journal Magnetic Resonance in Medicine (Volume 61, Pages 59-64). The mechanical problem is somewhat more complicated than described in part b of the above problem, but the calculated displacement is similar to what Basser and I find for the more accurate calculation. If you solve the new homework problem correctly, you should obtain a very small displacement, implying a phase shift too small to measure with current technology. The conclusion is that nerve action currents are unlikely to be measurable using this method.

Do you want the solution to the problem? Send me an email ( and I will be happy to supply it.

Happy Holidays!

Friday, December 19, 2008

Benjamin Franklin, Biological Physicist

I recently finished reading H. W. Brands' biography The First American: The Life and Times of Benjamin Franklin. (Actually, I listened to the book on tape while walking my dog Suki each evening.) I enjoy history and biography, but was not expecting to find any biological physics in the book.

Wrong. Late in Franklin's life, during his years in France, he played a role in a bizarre episode related to biomagnetism. Brands writes:
"Friedrich Anton Mesmer had studied medicine at Vienna during the period when Franklin's electrical experiments were becoming known on the European continent. Like many of Franklin's readers from the Poor Richard days, Mesmer believed in astrology; having learned from Franklin how lightning carried celestial energy to earth, he easily concluded that electricity provided an invisible but pervasive fluid that linked the stars to human lives. Unfortunately for both his scientific theory and his medical practice, electricity was unpleasant to patients, sometimes violently so. But Mesmer was resourceful, and substituting magnetism for electricity as the invisible transmitter, he developed a flourishing practice stroking patients with magnets. In time he dispensed with the magnets, replying simply on his own powers of persuasion to release the therapeutic effects of 'animal magnetism'...

In March 1784 King Louis appointed a committee of the Paris faculty of medicine to investigate; the distinguished members included Joseph Ignace Guillotin, who would add a word to several languages by his advocacy of the use of a swift and thereby comparatively humane decapitation machine. The doctors decided they needed help from the Academy of Sciences, whereupon Louis added five members, including the great chemist Lavoisier--who would meet his end at the device endorsed by Dr. Guillotin--and the eminent American, Dr. Franklin...

Franklin and the commissioners filed their report [on Mesmer's activities], with his name heading the list of signatures. A public version was hurried into print, and twenty thousand copies were snatched up. The report declared the claims of animal magnetism unproven; such mitigation of symptoms as appeared were due to the customary causes of self-delusion and ordinary remission."
I am not too surprised that Mesmer was able to fool so many for so long in the 1700's, long before Faraday, Maxwell, and others provided a complete understanding of electricity and magnetism, and well before the importance of well controlled, double-blind medical studies was appreciated. What is disturbing about this tale is that similar hoaxes go on today, at a time when we know so much more about bioelectricity and biomagnetism, and put much more effort into conducting careful clinical trials (for specific examples, see Bob Park's book Voodoo Science). Such non-scientific activities have a striking similarity to Mesmer's claims.

How do we combat such nonsense? Knowledge and education is the only way I know. Hopefully readers of the 4th Edition of Intermediate Physics for Medicine and Biology will be in position to more effectively detect and expose non-scientific claims. Like Franklin, we must encourage educational and civic measures designed to separate exciting and important scientific developments from unfortunate and unsupportable scientific frauds.

Friday, December 12, 2008

The FDA Approves Transcranial Magnetic Stimulation for Treatment of Depression

In Chapter 8 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I describe transcranial magnetic stimulation. This technique is a classic example of how fundamental physics can be applied to solve a medical problem. A pulse of current is passed through a coil held near the head. The changing magnetic field produced by this current induces an electric field that excites neurons, thus allowing noninvasive, painless stimulation of the brain. Magnetic simulation was invented in the 1980s by Tony Barker and his colleagues in Great Britain. One of my projects when I worked at the National Institutes of Health in the late 1980s and early 1990s was to calculate the electric field induced in the head by different coil geometries. I had the pleasure of working with leading neurologists like Mark Hallett, Leo Cohen, and others in the intramural program of the National Institute of Neurological Disorders and Stroke, and with engineer Peter Basser (my May 30, 2008 entry to this blog tells the story of how Basser subsequently developed magnetic resonance diffusion tensor imaging).

For many years the main use of magnetic stimulation was as a diagnostic tool to assess diseases of the central nervous system, or as a research method to study how the brain changes over time. But another less well understood use of this technique is for therapy. Transcranial magnetic stimulation made news this fall when the Food and Drug Administration approved its use for treating depression (for more information about this decision, see the brain stimulant: stimulation blog or an article in IEEE Spectrum Online). Magnetic stimulation was first applied as a psychiatric treatment with the goal of improving on or replacing electroconvulsive therapy. The gist of both methods is to stimulate neurons in the brain, and thereby affect brain function. One major difference between electroconvulsive therapy and transcranial magnetic stimulation is that magnetic stimulation does not require a siezure. Electroconvulsive therapy is performed under sedation because in that case the brain must undergo a siezure in order to benefit the patient. The method is effective, but has severe side effects. Magnetic stimulation, on the other hand, is safe but less effective.

On October 8 of this year Neuronetics, a medical device company in Malvern, Pennsylvania, obtained FDA clearance for its Neurostar TMS Therapy for the Treatment of Depression. A recent double blind multisite study, whose results were published in the journal Biological Psychiatry (Volume 62, Pages 1208-1216, 2007), showed a statistically significant improvement in symptoms in patients treated with transcranial magnetic stimulation who had not responded to more traditional treatments for depression, compared to a control group. Despite these promising results, I think the effectiveness of magnetic stimulation as a psychiatric therapy is still an open question. I hope it proves successful, because it would be gratifying to me personally to have worked on a technique that could benefit so many people, but I'll need to see some confirmation of the initial success before I consider it as a proven treatment. Nevertheless, it certainly makes an interesting case study in the application of physics to medicine and biology.

Friday, December 5, 2008

Is Cell Phone Electromagnetic Radiation Dangerous?

Is electromagnetic radiation from cell phones dangerous? This point is debated in a point/counterpoint article in the December, 2008 issue of the journal Medical Physics. (See my January 11, 2008 entry to this blog for more about point/counterpoint articles). This readable, if somewhat testy, exchange between scientists highlights many of the crucial issues in this debate.

Arguing for the proposition that cell phones are dangerous is Dr. Vini Khurana, a neurosurgeon at the Canberra Hospital in Australia. Khurana appeared on Larry King Live (May 27, 2008) and claimed that cell phones could cause cancer. He writes that "there is a statistically significant elevated odds (about twofold) of developing a glioma or acoustic neuroma on the same side of the head preferred for cell phone use over a duration of exposure [greater than or equal to] 10 years." He cites the BioInitiative Report: A Rationale for a Biologically-based Public Exposure Standard for Electromagnetic Fields to support his claim.

Arguing against the proposition is Dr. John Moulder of the Medical College of Wisconsin. Moulder, an expert on the biological effects of exposure to non-ionizing radiation, contends that "the current evidence for a causal association between cancer and exposure to radiofrequency (RF) energy is weak and unconvincing." He cites a study published in the New England Journal of Medicine titled Cellular-Telephone Use and Brain Tumors (344:79-86, 2001) to support his claim.

Readers of the 4th Edition of Intermediate Physics for Medicine and Biology will be familiar with this topic from Chapter 9, Section 10: Possible Effects of Weak External Electric and Magnetic Fields. Our discussion there was centered on the question of 60 Hz power line fields and health hazards, but these considerations also apply to cell phone frequencies.

Where do I stand on this issue? I am skeptical of these claims of cell-phone-induced brain cancer. The key point is that these fields are non-ionizing. Cell phones operate at a frequency of about 850 MHz. The energy of a photon of that frequency is 3.5 millionths of an electron volt (0.0000035 eV). Energies of an electron volt or more are needed to break most chemical bonds. X-ray photons, and even ultraviolet photons, have that much energy, but photons at cell phone frequencies do not have nearly enough energy to break bonds, and most cancers are caused by the breaking of bonds in a DNA molecule. Moreover, the thermal energy of particles at body temperature is about 0.03 electron volts, which is roughly ten thousand times more energy than an 850 MHz photon has. In other words, all the molecules in your body are bouncing around and colliding with your DNA molecules with energies vastly larger than the energy of a cell phone photon. Perhaps there are a whole lot of photons, what then? That is another way of saying that the electromagnetic radiation heats the tissue (like in a microwave oven). Strong electric fields in the MHz and GHz range can heat tissue, that is the main way they can interact with your body, but slight heating doesn't cause cancer.

Physicist Bob Park, author of the weekly newsletter What's New, makes a similar case against the danger of biological effects of nonionizing radiation in an editorial published by the Journal of the National Cancer Institute. Khurana's response to these arguments is that "no known mechanism does not equate to no mechanism". Perhaps. But without a plausible mechanism, one expects the epidemiological evidence to be compelling and unambiguous. I suggest you read the point/counterpoint article and cited references, and then decide for yourself. One point I am certain about: you need a good understanding of basic physics and its application to medicine and biology in order to sort out complex issues such as these. The 4th Edition of Intermediate Physics for Medicine and Biology is one place to gain that knowledge.