Friday, October 30, 2009

Hobbie and Roth, Back in the Saddle Again

In the November, 2009 issue of the American Journal of Physics, Russ Hobbie and I published Resource Letter MP-2: Medical Physics. Our resource letter “provides a guide to the literature on the uses of physics for the diagnosis and treatment of disease." Think of it (along with Ratliff’s Resource Letter MPRT-1: Medical Physics in Radiation Therapy discussed in the August 28, 2009 entry to this blog) as an updated bibliography to the 4th Edition of Intermediate Physics for Medicine and Biology. Together, these two publications provide over 300 citations to the best and most recent books, articles, and websites about medical physics. We even slipped a mention of this blog into the list of references.

Friday, October 23, 2009

Felix Bloch

One hundred and four years ago today, Felix Bloch (1905-1983) was born in Zurich, Switzerland. Bloch received his PhD in physics in 1928 from the University of Leipzig working under Werner Heisenberg, and then immigrated to the United States after Hitler came to power in Germany. He worked for a time at Los Alamos on the Manhattan Project, and had a long career in the Physics Department at Stanford University.

Bloch is most familiar to readers of the 4th edition of Intermediate Physics in Medicine and Biology because of his contributions to our understanding of nuclear magnetic resonance. He shared the 1952 Nobel Prize with Edward Purcell for “their development of new ways and methods for nuclear magnetic precision measurements”. In Chapter 18 on Magnetic Resonance Imaging, Russ Hobbie and I present the Bloch Equations (Eq. 18.15), which govern the magnetization of a collection of spins in a static magnetic field. Essentially all of MRI begins with the Bloch equations, so they are part of the essential toolkit for any medical physicist. Bloch’s most cited paper is “Nuclear Induction” (Physical Review, Volume 70, Pages 460-474, 1946). The abstract is reproduced below:
"The magnetic moments of nuclei in normal matter will result in a nuclear paramagnetic polarization upon establishment of equilibrium in a constant magnetic field. It is shown that a radiofrequency field at right angles to the constant field causes a forced precession of the total polarization around the constant field with decreasing latitude as the Larmor frequency approaches adiabatically the frequency of the r-f field. Thus there results a component of the nuclear polarization at right angles to both the constant and the r-f field and it is shown that under normal laboratory conditions this component can induce observable voltages. In Section 3 we discuss this nuclear induction, considering the effect of external fields only, while in Section 4 those modifications are described which originate from internal fields and finite relaxation times."
At the moment, you can download a reprint of this historic article at

Bloch also appears in Chapter 15 of Intermediate Physics for Medicine and Biology, because of his contribution to the development of the Bethe-Bloch formula (Eq. 15.58) governing the stopping power of a charged particle by interaction with a bound electron. He is also known for his fundamental contributions to solid state physics, including his seminal calculation of the electron wave function in a periodic potential, derived when he was only 23. You can download a Biographical Memoir about Bloch by Robert Hofstadter at

I have an indirect connection to Felix Bloch. When in graduate school at Vanderbilt University in the 1980s, I had several classes from Ingram Bloch, who—if I recall correctly—was Felix’s cousin. At that time, Ingram Bloch was teaching many of the graduate classes, so I took classical mechanics, two semesters of quantum mechanics, and general relativity from him. I remember spending days working on his infamous “take-home” exams. They weren’t easy. With two physicists in the family, the Blochs made quite an impact on 20th century physics.

P.S. Right now, has the 4th Edition of Intermediate Physics for Medicine and Biology on sale at 40% off. I have no control over if and when amazon reduces prices on books, so the price may go back up anytime.

P.P.S. Last night I finished Steven Strogatz's book The Calculus of Friendship: What a Teacher and a Student Learned About Life While Corresponding About Math, mentioned in the July 3rd entry to this blog. In a word, the book is charming.

Friday, October 16, 2009

The Klein-Nishina Formula

In Chapter 15 of the 4th Edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I present the Klein-Nishina formula (Eq. 15.17).
“The inclusion of dynamics, which allows us to determine the relative number of photons scattered at each angle, is fairly complicated. The quantum-mechanical result is known as the Klein-Nishina formula.”
At first glance, Eq. 15.17 doesn’t look quantum-mechanical, because it does not appear to contain Planck’s constant, h. However, closer inspection reveals that the variable x in the equation, defined on the previous page (Eq. 15.15), does indeed contain h. Russ and I don’t derive the Klein-Nishina formula, nor do we give much background about it. Yet, this equation played an important role in the development of quantum mechanics, and specifically of quantum electrodynamics.

In the book Nishina Memorial Lectures: Creators of Modern Physics, the Nobel Prize winning physicist Chen Ning Yang wrote a chapter about The Klein-Nishina Formula & Quantum Electrodynamics.
“One of the greatest scientific revolutions in the history of mankind was the development of Quantum Mechanics. Its birth was a very difficult process, extending from Planck’s paper of 1900 to the papers of Einstein, Bohr, Heisenberg, Schrodinger, Dirac, and many others. After 1925-1927, a successful theory was in place, explaining many complicated phenomena in atomic spectra. Then attention moved to higher energy phenomena. It was in this period, 1928-1932, full of great ideas and equally great confusions, that the Klein-Nishina formula played a crucial role. The formula was published in 1929, in the journals Nature and Z. Physik. It dealt with the famous classical problem of the scattering of light rays by a charged particle….”
Oskar Klein and Yoshio Nishina derived their formula starting from the Dirac equation, which is a relativistic version of Schrodinger’s equation for an electron, including the effect of spin . During the summer of 1928, Klein and Nishina performed the lengthy calculations necessary to derive their formula. They would work independently during the day, and then compare results each evening (as Russ and I say, the calculation is “fairly complicated”). The final result was published in the German journal Zeit. f. Phys. ("Streuung von Strahlung durch freie Elektronen nach der neuen relativistischen Quantendynamik von Dirac," Volume 52, Pages 853-868, 1929). I don’t read German, so I can’t enjoy the original.

Later, the theory of Quantum Electrodynamics (QED) was developed to more completely describe the quantum mechanical interactions of electrons and photons. For an elementary introduction to this subject, see Richard Feynmann’s book QED. (I have to admit, although I took several semesters of quantum mechanics in graduate school, I never really mastered quantum electrodynamics.) When the problem of the scattering of light by electrons was reexamined using QED, the result was identical to the Klein-Nishina formula derived earlier. To learn more about how these results were obtained, see The Road to Stueckelberg's Covariant Perturbation Theory as Illustrated by Successive Treatments of Compton Scattering, by J. Lacki, H. Ruegg, and V. Telegdi ( But beware, the paper is quite mathematical and not for the faint of heart.

Who were the two men who derived this formula? Oskar Klein (1894-1977) was a Swedish theoretical physicist. He is known for the Kaluza-Klein theory, the Klein-Gordon equation, and the Klein paradox. Yoshio Nishina (1890–1951) was a Japanese physicist. He was a friend of Niels Bohr, and a close associate of Albert Einstein. The crater Nishina on the Moon is named in his honor. During World War II he was the head of the Japanese atomic program.

Let me share one last anecdote about Klein, Nishina, and Paul Dirac that I find amusing. Gosta Ekspong tells the story in his chapter The Klein-Nishina Formula, in the book The Oskar Klein Memorial Lectures.
“When Dirac paid a short visit to Copenhagen in 1928, he met Klein and Nishina. The three of them were once conferring in the library of the Bohr Institute. Dirac was a man of few words, so when the remark came from Nishina that he had found an error of sign in the new Dirac paper on the electron, Dirac drily answered: “But the result is correct.” Nishina, in an attempt to be helpful, said: “There must be two mistakes,” only to get Dirac’s reply that “there must be an even number of mistakes.”

Friday, October 9, 2009

Steven Chu, Biological Physicist

Readers of the 4th edition of Intermediate Physics for Medicine and Biology may wish to see examples of physicists who have contributed to biology. One excellent example is Steven Chu, who until recently was professor of physics and professor of molecular and cellular biology at the University of California, Berkeley. Chu describes his biological physics research on his Berkeley website:
"We apply single molecule techniques such as fluorescence resonance energy transfer, atomic force microscopy and optical tweezers, we study enzyme activity, and protein and RNA folding at the single bio-molecule level. Systems being studied include how the ribosome reads m-RNA and manufactures proteins, how vesicles fuse into the cell wall at the synapse of neurons, how cells adhere to each other via adhesive molecules, and how RNA molecules fold into active enzymes."
If you want to hear Chu talk about his biological physics research, watch this video on YouTube. Some of his best known biological physics papers, published while on the faculty at Stanford, are:
Steven Chu exemplifies how physicists can contribute to our understanding of biology.

....Oh, did I forget to mention something? Chu is best known for his work on the "development of methods to cool and trap atoms with laser light", for which he shared the Nobel Prize in Physics in 1997. He is currently Secretary of Energy in the Obama administration, and is leading the US effort to move away from fossil fuels and toward alternative energy sources, thereby combating global warming.

Who says we don’t have wonderful role models anymore?

Friday, October 2, 2009

Are Static Magnetic Fields Dangerous?

Are static magnetic fields dangerous? This question has recently taken on added importance because a European directive is limiting a worker’s exposure to the strong static magnetic field in a magnetic resonance imager, thereby impeding research with MRI. A recent article by Denis Le Bihan on asserts that
“those limits could end up preventing the technique from being used - just when European scientists are starting to lead the world in ultra-high-field (UHF) MRI magnet research. The initially proposed limits will immediately put the brakes on progress and, moreover, be a big blow to companies that make MRI scanners and magnets, such as Siemens, Philips, Bruker and Magnex. These firms could end up being unable to meet the growing global demand for clinical UHF MRI scanners, the high fields from which could boost the potential of MRI for healthcare and biomedical sciences, particularly for neurological applications.”
Physicist Bob Park, the author of Voodoo Science and the weekly newsletter What’s New, writes
MAGNETIC FIELDS: THE PRECAUTIONARY PRINCIPLE IN ACTION. According to Denis Le Bihan at the CEA-Saclay Centre, a European directive to prevent workers from being exposed to high magnetic fields could severely impact research into Ultrahigh-Field MRI which shows great promise particularly in neurological applications. It is particularly frustrating that limits on static magnetic fields resulted from the paranoia surrounding EMF, which was associated with everything from power lines to cell phones, Wi-Fi, Bluetooth, and other wireless devices. As I pointed out in an editorial in the Journal of the National Cancer Institute eight years ago, ‘there will always be some who will argue that the issue has not been completely settled. In science, few things ever are.’ "
Are these limits justified? Based on my knowledge of biomagnetism, I think not. There are few known mechanisms by which a static magnetic field can have a significant biological impact, except in unusual cases such as a person with a ferromagnetic medical implant, or in some animals (such as magnetotactic bacteria) that are believed to sense magnetic fields, presumably by the presence of ferromagnetic or superparamagnetic nanoparticles (magnetosomes). Russ Hobbie and I discuss the possible effects of weak magnetic fields in Chapter 9 of the 4th edition of Intermediate Physics for Medicine and Biology.

Denis Le Bihan is a leader in the field of MRI, known for his development of diffusion weighted imaging. He is director of NeuroSpin, a French institute aimed at developing and using ultra high field Magnetic Resonance to understand the brain. I knew Denis when we were both working at the National Institutes of Health in the 1990s. He was a close collaborator with my friend Peter Basser, and together they developed diffusion tensor imaging. (Incidentally, one of their early papers on this topic just received its 1000th citation in the citation index!) Let us hope that Le Bihan's important research is not interrupted unnecessarily by misguided government regulations.