The Eighteenth Elephant

I know that there are very few people out there interested in reading a blog about physics applied to medicine and biology. But those few (those wonderful few) might want to know of ANOTHER blog about physics applied to medicine and biology. It is called The Eighteenth Elephant. The blog is written by Professor Raghuveer Parthasarathy at the University of Oregon. He is a biological physicist, with an interest in teaching “The Physics of Life” to non-science majors. He also leads a research lab that studies many biological physics topics, such as imaging and the mechanical properties of membranes. If you like my blog about the 4th edition of Intermediate Physics for Medicine and Biology, you will also like The Eighteenth Elephant. Even if you don’t enjoy my blog, you still might like Parthasarathy’s blog (he doesn’t constantly bombard you with links to the amazon.com page where you can purchase his book).

One of my favorite entries from The Eighteenth Elephant was from last April. I’ve talked about animal scaling of bones in this blog before. A bone must support an animal’s weight (proportional to the animal’s volume), its strength increases with its cross-sectional area, and its length generally increases with the linear size of an animal. Therefore, large animals need bones that are thicker relative to their length, in order to support their weight. I demonstrate this visually by showing my class pictures of bones from different animals. Parthasarathy doesn’t mess around with pictures; he brings a dog femur and an elephant femur to class! (See the picture here, its enormous.) How much better than showing pictures! Now, I just need to find my own elephant femur….

Be sure to read the delightful story about 18 elephants that gives the blog its name.

Hexagons and Cellular Excitable Media

Two of my favorite homework problems in the 4th edition of Intermediate Physics for Medicine and Biology are Problems 39 and 40 in Chapter 10. Russ Hobbie and I ask the student to analyze a cellular excitable medium (often called a cellular automaton), which provides much insight into propagation of excitation in cardiac tissue. I’ve discussed these problems before in this blog. I’m always amazed how well you can understand cardiac arrhythmias using such a simple model that you could teach it to third graders.

When Time Breaks Down,
The Three-Dimensional Dynamics of
Electrochemical Waves and Cardiac Arrhythmias,
by Art Winfree.

I learned about cellular excitable media from Art Winfree’s book When Time Breaks Down. To the best of my knowledge, the idea was first introduced by James Greenberg and Stuart Hastings in their paper “Spatial Patterns for Discrete Models of Diffusion in Excitable Media” (SIAM Journal on Applied Mathematics, Volume 34, pages 515–523, 1978), although they performed their simulations on a rectangular grid rather than on a hexagonal grid as in the homework problems from IPMB. Winfree, with his son Erik Winfree and Herbert Seifert, extended the model to three dimensions, and found exotic “organizing centers” such as a “linked pair of twisted scroll rings” (“Organizing Centers in a Cellular Excitable Medium,” Physica D: Nonlinear Phenomena, Volume 17, Pages 109–115, 1985).

Predrawn hexagon grids to use with
homework problems about
cellular automata.

I imagine that students may have a difficult time with our homework problems, not because the problems themselves are difficult, but because they don’t have easy access to predrawn hexagon grids. It would be like trying to play chess without a chessboard. When I assign these problems, I provide my students with pages of hexagon grids, so they can focus on the physics. I thought my blog readers might also find this useful, so now you can find a page of predrawn hexagons on the book website. Or, if you prefer, you can find hexagon graph paper for free online here.

In the previous blog entry I mention a paper I published in the Online Journal of Cardiology in which I extended the cellular excitable medium to account for the virtual electrodes created when stimulating cardiac tissue. This change allowed the model to predict quatrefoil reentry. I concluded the paper by writing

This extremely simple cellular excitable medium—which is nothing more than a toy model, stripped down to contain only the essential features—can, with one simple modification for strong stimuli, predict many interesting and important phenomena. Much of what we have learned about virtual electrodes and deexcitation is predicted correctly by the model (Efimov et al., 2000; Trayanova, 2001). I am astounded that this simple model can reproduce the complex results obtained by Lindblom et al. (2000). The model provides valuable insight into the essential mechanisms of electrical stimulation without hiding the important features behind distracting details.

“Virtual Electrodes Made Simple.”

Unfortunately, the Online Journal of Cardiology no longer exists, so the link in my previous blog entry does not work. You can download a copy of this paper at my website. It contains everything except the animations that accompanied the figures in the original journal article. If you want to see the animations, you can look at the article archived here.

Naked to the Bone

Naked to the Bone:
Medical Imaging in the
Twentieth Century,
by Bettyann Kevles.

I recently finished reading Bettyann Kevles’ excellent book Naked to the Bone: Medical Imaging in the Twentieth Century. This fine history covers medical imaging in much the same way that Kirk Jeffrey’s Machines in our Hearts analyzes the development of pacemakers and defibrillators. Both books are outstanding examples of insightful writing about the history of modern technology. In Naked to the Bone, Kevles examines many topics that Russ Hobbie and I describe in the 4th edition of Intermediate Physics for Medicine and Biology. In fact, Naked to the Bone is a valuable resource for readers interested in the history of medical physics, and serves as a great supplement to the last eight chapters of IPMB. In her introduction, Kevles writes

Naked to the Bone tells the history of medical imaging from Roentgen’s discovery [of x-rays] in 1895 to the present, as imaging affected our entire culture. While this book traces the technological developments and their consequences in medicine, it also explores the impact that this new way of seeing has had upon society at large. Citizens of the twentieth century often sensed that their world differed in kind from what came before, and that science and technology are responsible for that difference…

The book falls naturally into two parts, corresponding roughly in time with the two halves of the century. The first part traces the history of the single technology of X-ray imaging: the second, the array of new competing technologies that arose after World War II when television and computers began to contribute to medical imaging.

In the first part, the emphasis is on the refinement of the technology of the X-ray and the immediate consequences of its discovery. As the machines improved, physicians gradually pushed back the veil in front of the internal organs, revealing first the living skeleton, then the stomach, intestines, gall bladder, lungs, heart, and brain….

Part II deals with the second stage of the imaging revolution. Thomas Hughes suggests in American Genesis that the convergence of two new technologies can cause a revolution. This is precisely what happened when X-rays met computers and produced CT, MRI, PET, and ultrasound. Each of these scanners reconstructs cross-sectional slices of the interior of the body, or creates three-dimensional volume images.

Kevles reviews the development of X-ray imaging in detail. Its use become ubiquitous in modern society. In Problem 8 of Chapter 16 in IPMB, Russ and I analyze the fluoroscopy units used in shoe stores in the early twentieth century.

During the 1930s and 1940s it was popular to have an x-ray flouroscope unit in shoe stores to show children and their parents that shoes were properly fit. These marvellous units were operated by people who had no concept of radiation safety and aimed the bean of x rays upward through the feet and right at the reproductive organs of the children!

Kevles describes the same thing (one can hardly avoid sarcasm when describing these devices).

All over the world, people who grew up between World War I and the 1960s recall the joy of standing inside the [“Foot-O-Scope” fluoroscope x-ray] machine, pressing the appropriate button (usually labeled “Man,” “Woman,” and “Child” although the X-ray dosage was identical) and staring at their wriggling toe bones. The Foot-O-Scope signaled the acceptance of X-ray machines in everyday life. Present in local shoe stores everywhere, they suggested that X-rays were safe and cheap enough so that just about anyone who shopped for shoes could see beyond the skin barrier.

Kevles’ book explores in detail the many contributors to the development of computed tomography. Russ and I just hint at this complex history in Chapter 16 of IPMB (Medical Use of X Rays).

The Nobel Prize acceptance speeches [Cormack (1980); Hounsfield (1980)] are interesting to read. A neurologist, William Oldendorf, had been working independently on the problem but did not share in the Nobel Prize [See DiChiro and Brooks (1979), and Broad (1980)].

Before reading Naked to the Bone, I didn’t realize that EMI, the British music publishing company associated with the Beatles, was also the company that Hounsfield worked for when he developed computed tomography, and that sold the first CT scanner in 1972, starting the tomography revolution.

The invention of magnetic resonance imaging (Chapter 18 in IPMB) was similarly complicated and controversial. Kevles tells the story of the contributions of Raymond Damadian, Paul Lauterbur, Peter Mansfield, and others (Lauterbur and Mansfield shared the Nobel Prize, but Damadian was left out). She then describes the development of positron emission tomography (discussed by Russ and I in our Chapter 17) and ultrasound imaging (our Chapter 13).

One disadvantage of Naked to the Bone is that it was written nearly 20 years ago, and a lot has happened in medical imaging in the last two decades. I would love to read an up-dated twentieth anniversary edition. Particularly interesting to me were some of the predictions made in the epilogue.

Looking ahead, it is easier to imagine an exhibition in the Imaging Museum of the future than to foresee machines in future imaging centers. Radiographs on glass, the original X-ray technology, have already disappeared. Film is likely to follow soon, replaced by versions that have been digitized for easy storage and electronic telecommunication. The use of CT will probably diminish but not disappear; its speed guarantees it a place in emergency medicine. MRI has a clear path before it, for while machines are costly, they last a long time and the upgrades they need are largely a matter of software programs. Ultrasound, the cheapest method of all, offers excellent images of organs that defy other imaging approaches and will probably occupy more space in the future. PET is a different story: full of promise for over a generation, and excellent for specialized procedures, it continues to run into regulatory obstacles that arise like dragon’s teeth even as others are overcome.

She then offers this insight about what might have happened if CT had been invented just after MRI, instead of just before it.

Looking back at the competition that accompanied the linkage of computers with imaging technologies, the timing suggests that, but for its few years’ head start, CT could never have competed with MRI. But it is hard to see how, without CT as a precedent, the chemical laboratory’s small-scale nuclear magnetic resonance technology would ever have, on its own, have become body-imaging MRI.

Kevles is particularly interested in how these imaging technologies impacted modern art. For me, having little background in art, these chapters were not my favorites. But perhaps Kevles liked them best, because she concludes her epilogue on this topic. As usual, I will give her the last word

Artists will most likely continue to extrapolate and elaborate on the remarkable technologies already available. For as a civilization, perhaps even as a species, we like to look, like to look through, and like to look at and through ourselves. In black and white and in color, in two-dimensional slices or in three-dimensional volumes, in frozen instants or moving sequences, the X-ray and its daughter technologies seem to satisfy an innate curiosity to see ourselves naked to the bone.

Our Job is to Find Stupid and Get Rid of It

This week I have been on vacation, including a trip to Kansas City to see relatives and a visit to the Grand Canyon. So, I don’t have much time for updating this blog. My work on the 5th edition of Intermediate Physics for Medicine and Biology has slowed to a crawl, and I need to get back to it next week.

This week I will simply suggest you watch and listen to the inspiring Boston University 2014 commencement address by my friend Kevin Kit Parker.

My favorite quote from the address is the title of this blog entry. Parker is with the Harvard School of Engineering and Applied Sciences. Academically speaking, he and I are brothers; we share a common PhD advisor, John Wikswo of Vanderbilt University. Parker obtained his PhD about ten years after I received mine, and I met him when I was on the faculty at Vanderbilt for a few years in the late 1990s.

Parker is known both for his science, and for being a scientist/soldier. You can learn more about his experiences in an interview that aired on the TV show 60 Minutes.