Category: Journal Club

Ok, this one is a little bit out of my field, but the topic intrigued me. Found the article through a posting at ScienceDaily.

I’m always a little bit sceptical about claims on the effects of EM fields on tissue and the brain in particular. Mostly because many of the studies that show an effect use conditions that the majority of people aren’t exposed to in real life for significant periods of time. And there really isn’t a lot of energy in low frequency EM radiation (in the radio wave and low frequency microwave end of the spectrum) to do much more than re-arrange electron configurations. Maybe that’s enough though. Who knows. It’s a lot like extrapolating the effects of low level and chronic radiation exposure when all you’ve got is data from high level and acute exposures.

The article (Lai H, Singh N P, “Magnetic Field-Induced DNA Strand Breaks in Brain Cells of the Rat“, Environ Health Perspect, publication pending, doi:10.1289/ehp.6355) has been accepted for publication, but hasn’t been published yet. The PDF of the pre-publication version of the article is available here for the time being.

Going to try to read this one over the weekend and see what interesting tidbits it contains.

Abstract:

In previous research, we found that rats acutely (2 hrs) exposed to a 60-Hz sinusoidal magnetic field at intensities of 0.1 – 0.5 mT showed increases in DNA single and double strand breaks in their brain cells. Further research showed that these effects could be blocked by pretreating the rats before exposure with the free radical scavengers melatonin and N-tert-butyl-α-phenylnitrone, suggesting the involvement of free radicals. In the present study, effects of magnetic field exposure on brain cell DNA in the rat were further investigated. We found that: (1) Exposure to a 60-Hz magnetic field at 0.01 mT for 24 hrs caused a significant increase in DNA single and double strand breaks. Prolonging the exposure to 48 hrs caused a larger increase. This indicates that the effect is cumulative. (2) Treatment with Trolox (a vitamin E analog) or 7-nitroindazole (a nitric oxide synthase inhibitor) blocked magnetic field-induced DNA strand breaks. These data further support a role of free radicals on the effects of magnetic fields. (3) Treatment with the iron-chelator deferiprone also blocked the effects of magnetic field on brain cell DNA, suggesting the involvement of iron. (4) Acute magnetic field exposure increased apoptosis and necrosis of brain cells in the rat. We hypothesize that exposure to a 60-Hz magnetic field initiates an iron-mediated process (e.g., the Fenton reaction) that increases free radical formation in brain cells, leading to DNA strand breaks and cell death. This hypothesis could have an important implication on the possible health effects associated with exposure to extremely-low frequency magnetic fields in the public and occupational environments.

Extensive use of simulations with a few experiments thrown in for good measure. Upshot of this article is that the standard 15-20% energy windows most people use aren’t optimal for Ga-67 imaging. What I also found interesting was that the optimum windows changed depending on the task being performed (detection vs estimation). Results were based on SNR calculations, but did not look at any changes in image quality. Only planar images were examined in this paper. It would be interesting to see how the modified windows affect Ga-67 SPECT imaging. With 5/8″ crystals coming into vogue now, it would also be interesting to see if the optimum windows change with a thicker crystal. I’ll have to file that away on my list of projects to get to. That might make an interesting subject for a summer student to tackle.

Nuclear medicine is one of the areas I specialize in, so this week’s article is in that area. El Fakhri G, Moore SC, Kijewski MF, “Optimization of Ga-67 imaging for detection and estimation tasks: Dependence of imaging performance on spectral acquisition parameters“, Med Phys 29, 1859-1866 (2002).

Ga-67 is commonly used for tumour imaging, localization and staging. It has three photopeaks, 93, 185 and 300 keV, although commonly only the 93 and 185 keV photopeaks are used for imaging. This article examines ways to optimize the energy windows to maximize SNR and to take advantage of all three photopeaks.

Abstract:

We have compared the use of two (93 and 185 keV) and three (93, 185, and 300 keV) photopeaks for Ga-67 tumor imaging and optimized the placement of each energy window. Methods: The bases for optimization and evaluation were ideal and Bayesian signal-to-noise ratios (SNR) for the detection of spheres embedded in a realistic anthropomorphic digital torso phantom and ideal SNR for the estimation of their size and activity concentration. Seven spheres of radii ranging from 1 to 3 cm, located at several sites in the torso, were simulated using a realistic Monte Carlo program. We also calculated the ideal SNR for the detection from simple phantom acquisitions. Results: For detection and estimation tasks, the optimum windows were identical for all sphere sizes and locations. For the 93 keV photopeak, the optimal window was 84-102 keV for the detection and 87-102 keV for estimation; these windows are narrower than the 20% window often used in the clinic (83-101 keV). For the 185 keV photopeak, the optimal window was 170-220 keV for the detection and 170-215 keV for estimation; these are substantially different than the 15% window used in our clinic (171-199 keV). For the 300 keV photopeak, the optimal window for detection was 270-320 keV, and for estimation, 280-320 keV. Using the three optimized, rather than only the two lower-energy, windows yielded a 9% increase in the SNR for the detection of the 3 cm diam sphere (a 12% increase for a 2 cm diam sphere) and a 7% increase in the SNR for estimation of its size. For the acquired phantom data, detection also increased by 9%-12% when using three, rather than two, energy windows. © 2002 American Association of Physicists in Medicine.

The idea behind this paper is relatively easy to get. Traditionally, x-ray imaging is examined using the ‘light as particle’ method of thinking. It works, and the math is easy. But nobody really examines x-ray imaging from the ‘light as a wave’ point of view. Recently though, there have been a number of articles looking at phase imaging for x-ray systems. This article (Wu X, Liu H, “Clinical implementation of x-ray phase-contrast imaging: Theoretical foundations and design considerations“, Med Phys 30, 2169-2179 (2003)) is one of them.

It’s a topic that I’ve been peripherally interested for a while. I’ve always wondered what x-ray imaging physics might look like formulated from the ‘light as wave’ perspective.

One thing I found interesting was that the refractive portion of the refractive index for tissue (δ, Eq 1 & 2) was much larger than the absorptive portion (β, Eq 1), the implication being that it ought to be relatively easy to do phase based imaging.

One of the interesting things in this paper is that the authors extend the theory of phase contrast imaging to real-world x-ray machines as opposed to specialized micro-focus x-ray units or monochromatic x-rays from a synchrotron (Section II.C) and end up predicting things that previous treatments did not (A Pogany, D Gao, S Wilkins, “Contrast and resolution in imaging with a microfocus x-ray source“, Rev Sci Instrum 68, 2774-2782 (1997). Mammography machines fit the resolution requirements for observing phase contrast, but an x-ray tube with a smaller focal spot is needed. Still, the requirements aren’t something you find in a run-of the mill x-ray unit. However, the fact that phase contrast imaging can be done with polychromatic beams is exciting.

Theory is tested by simulating a mammography imaging system to find optimal values for source-object and object/detector distances. Experiments also illustrate the exciting prospects of phase contrast imaging in mammography (Section IV).

A well written paper, with some very interesting and promising results. Some of the more complicated math has been glossed over, but details can be found in another paper by the same authors.

This is an effort to get me to read more of the journal articles I find. More often than not, I run across an interesting article, skim through it, put it in my Read This Soon pile, and then it gets forgotten about.

So I think what I shall try to do is when I run across an interesting article (interesting to me, hopefully some others), I shall post it here along with the abstract and article reference. Then after I’ve read the article, I’ll try to write up a short blurb of my thoughts on it.

So the first journal club article is one from Medical Physics.

Wu X, Liu H, “Clinical implementation of x-ray phase-contrast imaging: Theoretical foundations and design considerations”, Med Phys 30, 2169-2179 (2003)

Abstract:

Theoretical foundation and design considerations of a clinical feasible x-ray phase contrast imaging technique were presented in this paper. Different from the analysis of imaging phase object with weak absorption in literature, we proposed a new formalism for in-line phase-contrast imaging to analyze the effects of four clinically important factors on the phase contrast. These are the body parts attenuation, the spatial coherence of spherical waves from a finite-size focal spot, and polychromatic x-ray and radiation doses to patients for clinical applications. The theory presented in this paper can be applied widely in diagnostic x-ray imaging procedures. As an example, computer simulations were conducted and optimal design parameters were derived for clinical mammography. The results of phantom experiments were also presented which validated the theoretical analysis and computer simulations.©2003 American Association of Physicists in Medicine.