Russ Hobbie and I discuss the ear and hearing in Chapter 13 of the 4th edition of Intermediate Physics for Medicine and Biology. Last Wednesday, Russ attended a colloquium at the University of Minnesota titled “Bionic Hearing: The Science and the Experience,” presented by Ian Shipsey of Purdue University. The talk was about cochlear implants, at topic we mention briefly in Section 7.10 on Electrical Stimulation. You can download the entire powerpoint presentation from the colloquium. Shipsey’s story is itself inspirational. On his website he writes “I had cochlea implant surgery in November 2002 at the Riley Hospital for Children in Indianapolis, IN. The surgeon was Professor Richard Miyamoto. The device was activated in late December. I am now able to hear my daughter for the first time and my wife for the first in 12 years.”
When I worked at NIH in the 1990s, I used to attend the Neural Prosthesis Program workshops held in Bethesda every fall. I recall listening to the researchers each year report on how they were developing these incredible devices to restore hearing. From those workshops, I gained a great appreciation for cochlear implants, and I have come to view them as a prototypical example—along with the cardiac pacemaker—of how physics and engineering can contribute to medicine.
The December 7, 2007 entry to this blog was titled “Is Computed Tomography Safe?” As is often the case with such a difficult question, the answer is yes and no. No—there are risks associated with any exposure to ionizing radiation, so no procedure is entirely safe. Yes—in most cases the risks are small enough that the benefits outweigh the risks. In order to answer this question more precisely, a large scale study with nearly one million patients was conducted over three years. The conclusions were reported in the August 27 issue of the New England Journal of Medicine. The abstract of Fazel et al.’s paper “Exposure to Low-Dose Ionizing Radiation from Medical Imaging Procedures” (NEJM, Volume 361, Pages 849–857, 2009) is reproduced below.
Background: The growing use of imaging procedures in the United States has raised concerns about exposure to low-dose ionizing radiation in the general population.
Methods: We identified 952,420 nonelderly adults (between 18 and 64 years of age) in five health care markets across the United States between January 1, 2005, and December 31, 2007. Utilization data were used to estimate cumulative effective doses of radiation from imaging procedures and to calculate population-based rates of exposure, with annual effective doses defined as low (less than 3 mSv), high (greater than 20 to 50 mSv), or very high (greater than 50 mSv).
Results: During the study period, 655,613 enrollees (68.8%) underwent at least one imaging procedure associated with radiation exposure. The mean (±SD) cumulative effective dose from imaging procedures was 2.4±6.0 mSv per enrollee per year; however, a wide distribution was noted, with a median effective dose of 0.1 mSv per enrollee per year (interquartile range, 0.0 to 1.7). Overall, moderate effective doses of radiation were incurred in 193.8 enrollees per 1000 per year, whereas high and very high doses were incurred in 18.6 and 1.9 enrollees per 1000 per year, respectively. In general, cumulative effective doses of radiation from imaging procedures increased with advancing age and were higher in women than in men. Computed tomographic and nuclear imaging accounted for 75.4% of the cumulative effective dose, with 81.8% of the total administered in outpatient settings.
Conclusions: Imaging procedures are an important source of exposure to ionizing radiation in the United States and can result in high cumulative effective doses of radiation."
To help put this study in context, the NEJM published an accompanying editorial by Michael Lauer (“Elements of Danger—The Case of Medical Imaging,” Volume 361, Pages 841–843). Lauer writes that
Because the use of ionizing radiation carries “an element of danger in every . . . procedure,” we need to adopt a new paradigm for our approach to imaging. Instead of investing so many resources in performing so many procedures, we should take a step back and design and execute desperately needed large-scale, randomized trials to figure out which procedures yield net benefits. This approach would require leadership and courage on the part of our profession, our opinion leaders, and the research enterprise, but were we to insist that all, nearly all, procedures be studied in well-designed trials, we could answer many critical clinical questions within a short time. Because we will continue to be uncertain of the magnitude of harm, an accurate understanding of the magnitude of benefit is a moral imperative.
In Chapter 16 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the risk of radiation. While we do not provide a final answer regarding the safety of CT, we do outline many of the important issues one must examine in order to make an informed decision. The safety of computed tomography and other diagnostic imaging procedures will continue to be a crucial question of interest to readers of Intermediate Physics for Medicine and Biology. I will try to keep you posted as new information becomes available.
P.S. Thanks to Russ Hobbie for calling my attention to this paper. He reads the New England Journal of Medicine more than I do.
Problem 21.5The differential form of Ampere’s law, derived in Problem 21, provides a relationship between the current density J and the magnetic field B that allows you to measure biological current with magnetic resonance imaging [see, for example, Scott, G. C., M. L. G. Joy, R. L. Armstrong, and R. M. Henkelman (1991) “Measurement of Nonuniform Current Density by Magnetic Resonance,” IEEE Transactions on Medical Imaging, Volume 10, Pages 362–374]. Suppose you use MRI and find the distribution of magnetic field to be
Bx = C (y z2 – y x2) By = C (x z2 – x y2) Bz = C 4 x y z
where C is a constant with the units of T/m3. Determine the current density. Assume the current varies slowly enough that the displacement current can be neglected.
To solve this problem, you need the result of Problem 21 in Chapter 8, which asks the reader to derive the differential form of Ampere’s law from the integral form given in the book by Eq. 8.11. If I were teaching a class from the book, I would assign both Problems 21 and 21.5, and expect the student to solve them both. But for readers of this blog, I will tell you the answer to Problem 21 (ignoring displacement current), so you will have the relationship needed to solve the new Problem 21.5: curl B = μ0J. The curl is introduced in Section 8.6. If you don’t have the 4th edition of Intermediate Physics for Medicine and Biology handy, take a look in a math handbook for information about how to calculate the curl (or see Schey’s book Div, Grad, Curl, and All That).
The story of how you measure B using MRI is interesting, but a bit too complicated to describe in detail here. In brief, a magnetic resonance imaging device has a strong static magnetic field about which nuclear spins (such as those of hydrogen) precess. The magnetic field produced by the current density modifies the static magnetic field, causing a phase shift in this precession. This phase shift is detected, and the magnetic field can be deduced from it. Technically, this method allows one to determine the component of the magnetic field that is parallel to the static field. Obtaining the other components requires rotating the object and repeating the procedure. See Chapter 18 for more about MRI.
Send me an email (roth@oakland.edu) if you would like the answer to the new Problem 21.5.
My friend Ranjith Wijesinghe and I have a poster later today about the “Effect of Peripheral Nerve Action Currents on Magnetic Resonance Imaging.” We analyzed if the magnetic field of action currents can be used to generate an artifact in an MRI, allowing direct imaging of biocurrents in the brain. There has been a lot of interest, and many publications, on this topic recently, but we conclude that the magnetic fields are just too small to have a measureable effect.
Last night, I got to hear Earl Bakken give a talk on “The History of Short-Term and Long-Term Pacing.” Bakken is a giant in the history of artificial pacemakers, and is the founder of Medtronic Corportion based in Minneapolis. He talked about the early years when Medtronic was a small electronics laboratory in a garage. He recommended a 10-minute video on YouTube, which he said told his story well. He also quoted one of my favorite books, Machines in our Hearts, a wonderful history of pacemakers and defibrillators. Tonight a social is being held at the Bakken Museum, “the only museum of its kind in the country, [where you can] learn about the history of electricity and magnetism and how it relates to medicine.” For a guy like me, this is great stuff.
So far, the presentations are fascinating and inspirational. I must admit, the students who attend these conferences always stay the same age as I grow older. I don’t think these meetings used to be this exhausting for me. As that old Garth Brookssong says, “the competition’s getting younger.” They are also getting more diverse. The speaker who welcomed us to the Bakken talk said that in just a few years, Americans will be a minority within the IEEE Engineering in Medicine and Biology Society. This is not difficult to believe, after seeing researchers from so many countries attending this year.
As I survey all the research presented at this meeting, I am proud that so much of the underlying science is described in Intermediate Physics for Medicine and Biology. I am more convinced than ever that Russ Hobbie and I have written a book that will be of great value to future biomedical engineers.
I am an emeritus professor of physics at Oakland University, and coauthor of the textbook Intermediate Physics for Medicine and Biology. The purpose of this blog is specifically to support and promote my textbook, and in general to illustrate applications of physics to medicine and biology.
