Hello to the Medical Physics 2 Class at Ball State University

Russ Hobbie and I would like to thank those instructors and students who use the 4th edition of  Intermediate Physics for Medicine and Biology as the textbook for their class. Also, we greatly appreciate those careful readers who find errors in our book and inform us about them. Without our dear readers, all the work preparing the 4th edition would be pointless.

Special thanks go to Dr. Ranjith Wijesinghe, Assistant Professor of Physics and Astronomy at Ball State University in Muncie, Indiana. This semester, Ranjith is teaching APHYS 316 (Medical Physics 2) using Intermediate Physics for Medicine and Biology. As he prepares his class lectures, Ranjith emails me all the mistakes he finds in our book, which I dutifully add to the errata. I can keep track of what the class is covering by the location of the errors Ranjith finds. In mid January the class was studying Fourier series, and he found a missing “sin” in Eq. 11.26d. By early February they were analyzing images, and Ranjith noticed some missing text in the figure associated with Problem 12.7. Then in mid February they began studying ultrasound, and eagle-eyed Ranjith emailed me that the derivative in Eq. 13.2 should be a partial derivative. I’m expecting some newly-discovered typo in Chapter 14 next week.

Electric Fields of the Brain:
The Neurophysics of EEG,
by Paul Nunez.

Ranjith is an old friend of mine. We were graduate students together at Vanderbilt University in the late 1980s, and both worked in the lab of John Wikswo. I took care of the crayfish (which have some giant axons that are useful for studying action currents) and Ranjith looked after the frogs (whose sciatic nerve is an excellent model for analyzing the compound action potential). After leaving Vanderbilt, Ranjith was a postdoc at Tulane University with Paul Nunez, an expert in electroencephalography and author of the acclaimed textbook Electric Fields of the Brain: The Neurophysics of EEG. While a member of Nunez’s group, Ranjith coauthored several papers, including “EEG Coherency.1. Statistics, Reference Electrode, Volume Conduction, Laplacians, Cortical Imaging, and Interpretation at Multiple Scales” in the journal Electroencephalography and Clinical Neurophysiology (Volume 103, Pages 499–515, 1997). According to Google Scholar, this landmark paper has been cited 277 times, which is quite an accomplishment (and is more citations than my most cited paper has).

I hope Ranjith keeps on sending me errors he finds, and I encourage other careful readers to do so too. And a big HELLO! to Ball State students taking Medical Physics 2. The true measure of a textbook is what the students think of it. I hope you all find it useful, and best of luck to you as the end of the semester approaches. Don’t give Dr. Wijesinghe too hard a time in class. If he finishes early one day and you have a few minutes to spare, ask him for some old stories from graduate school. He has a few, if he will tell you!

Allan Cormack

This Monday (February 23) will mark the 85th anniversary of the birth of Allan Cormack (1924–1998), who won the 1979 Nobel Prize in Physiology or Medicine (along with Godfrey Hounsfield) for the “development of computer assisted tomography.”

Imagining the Elephant:
A Biography of Allan MacLeod Cormack,
by Christopher Vaughan.

Last year Christopher Vaughan published a book titled Imagining the Elephant: A Biography of Allan MacLeod Cormack. A book review by Reginald Greene in the December 18 issue of the New England Journal of Medicine (Volume 359, Pages 2735–2736) states that

This brief book is a fascinating biography. The author, Christopher Vaughan, warmly sketches Cormack as a quietly gregarious man, traces his Scottish parentage and antecedents, follows his schooling and family life in South Africa, and mines the origins of his research into CT [Computed Tomography] at the University of Cape Town, latter at Cambridge University, and during his subsequent years in the United States at Tufts University and at the Harvard University Cyclotron Laboratory.

I haven’t read Vaughan's book yet, but it’s high on my list of things to do. You can learn more about Cormack online at the website published by the American Physical Society. For those of you who prefer to go straight to the original source, take a look at Cormack’s two highly cited papers, both in the Journal of Applied Physics: “Representation of a Function by Its Line Integrals, with Some Radiological Applications” (Volume 34, Pages 2722–2727, 1963), and “Representation of a Function by Its Line Integrals, with Some Radiological Applications. II” (Volume 35, Pages 2908–2913, 1964). Warning: these papers are highly mathematical. For those who would rather not wade through the math (and shame on you for that attitude!), I recommend looking at Section 4 (An Experimental Test) of the second paper, to see perhaps the first CT scan ever made, of an aluminum phantom in air. Or, see Chapter 12 of the 4th edition of Intermediate Physics for Medicine and Biology for a discussion of the numerical algorithms underlying tomography.

Allan Cormack is a role model for all physicists (or physics students) who hope to make important contributions to medicine.

Image Gently

The December 7, 2007 entry of this blog addressed a controversy over the safety of computed tomography scans, particularly for children. In response to these concerns, the Society for Pediatric Radiology, the American Association of Physicists in Medicine, the American College of Radiology, and the American Society of Radiologic Technologists have banded together to establish the Alliance for Radiation Safety in Pediatric Imaging. Its “image gently” website states that

The Alliance for Radiation Safety in Pediatric Imaging—the Image Gently Alliance—is a coalition of health care organizations dedicated to providing safe, high quality pediatric imaging nationwide. The primary objective of the Alliance is to raise awareness in the imaging community of the need to adjust radiation dose when imaging children.

The ultimate goal of the Alliance is to change practice.

The Alliance has chosen to focus first on computed tomography (CT) scans. The dramatic increase in the number of pediatric CT scans performed in the United States in the past five years and the rapid evolution, change and availability of CT technology and equipment well justify this Alliance strategy.

Image Gently offers reasonable recommendations to parents, pediatricians, radiologic technologists, and medical physicists about the risks and benefits of CT scans. While asserting that “there’s no question: CT helps us save kids’ lives!,” it nevertheless provides specific suggestions for reducing radiation dose, such as: “child size the kVp and mA”; “one scan (single phase) is often enough”; and “scan only the indicated area”. Image gently offers such calm, science-based advice on a subject often dominated by emotion and a misunderstanding of risk assessment. You won’t find much physics at the image gently website, but you will benefit from a case study in how to use physics to help patients without scaring them (or hurting them) in the process.

After exploring the image gently website, if you want to know more about how computed tomography works, or about the biological effects of radiation, see Chapter 16 in the 4th edition of Intermediate Physics for Medicine and Biology.

Darwin Day

The Origin of Species,
by Charles Darwin.

Thursday, Feb 12, is Darwin Day: the 200th anniversary of Charles Darwin’s birth. This year is special because it also marks the sesquicentennial of Darwin’s masterpiece The Origin Of Species.

Although Darwin Day is primarily a time to celebrate biology, physics plays two important roles in Darwin’s theory of evolution. First, physics constrains evolution. Natural selection has produced an amazing variety and diversity of organisms, but each and every one obeys the laws of physics. You can dream up all sorts of organisms in your imagination, but some just won't work. Readers of the 4th edition of Intermediate Physics for Medicine and Biology will learn about several of these constraints. For instance, in Chapter 2 Russ Hobbie and I discuss scaling (see my blog entry from August 8, 2008 for an earlier discussion of scaling). One can imagine a giant spider a hundred feet high with thin spider legs, but physics won’t allow this: the spider would be crushed under its own weight (weight scales as the volume, but the strength of the legs scale as the cross-sectional area, so the larger the spider the more difficult it would be to support the weight). In Chapter 4 we show that diffusion is an effective way to transport molecules over short distances, but is a poor method over long distances. One can envision a three-story high single cell—a giant amoeba—but if that cell depends on diffusion to obtain oxygen and get rid of carbon dioxide, it will not survive. So, physics limits biological evolution, and these limitations provide important insights into why animals are designed the way they are.

The second role of physics in the study of evolution comes from the interplay between evolution and astronomy. A famous example is the idea proposed by physicist Luis Alverez that an asteroid slammed into the earth 65 million years ago, leading to the death of the dinosaurs and many other species. I’d like to highlight a different example, in part because it’s new and less familiar, and in part because it’s been developed by researchers in the Department of Physics at the University of Kansas, my undergraduate alma mater (go jayhawks!). Professor Adrian Melott and his colleagues have proposed that gamma-ray bursts may have caused other mass extinctions. In the January 2004 issue of the International Journal of Astrobiology (Volume 3, Pages 55–61), Melott et al. write

Gamma-ray bursts (GRBs) produce a flux of radiation detectable across the observable universe. A GRB within our own galaxy could do considerable damage to the Earth’s biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth has experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRBs may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRBs in terms of terrestrial effects is a nearly impulsive energy input of the order of 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition, a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB.

On Darwin Day, as you celebrate Charles Darwin and his theory of evolution by natural selection, remember that a knowledge of physics as well as biology is crucial to understanding this important idea. The 4th edition of Intermediate Physics in Medicine and Biology is a good place to obtain the necessary physics background.