Category: Journal Club

Hope whoever was reading was amused by the last journal club article.

Figuring out how to measure CT does is getting more and more difficult, with the increasing popularity of multi-slice helical CT scanners that can now acquire upwards of 64 slices per gantry rotation and with beam widths approaching 20 or 30 mm. Current methods of measuring CT dose (CTDI) date back to when single slice scanners were the only thing available. So perhaps it’s time for something better and more accurate. As long as it’s easy to do and doesn’t require much more in the way of extra equipment.

Dixon RL, “A new look at CT dose measurement: Beyond CTDI”, Med Phys (30), 1272-1280 (2003)

Abstract:

Equations are derived for generating accumulated dose distributions and the dose line integral in a cylindrical dosimetry phantom for a helical CT scan series from the single slice dose profiles using convolution methods. This exposition will better clarify the nature of the dose distribution in helical CT, as well as providing the medical physicist with a better understanding of the physics involved in dose delivery and the measurement process. Also addressed is the concern that as radiation beam widths for multi-slice scanners get wider, the current methodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamber (CTDI₁₀₀) may no longer be adequate. It is shown that this measurement would underestimate the equilibrium dose and dose line integral by about 20% in the center of the body phantom, and by about 10% in the center of the head phantom for a 20 mm nominal beam width in a multi-slice scanner. Rather than making the ion chamber even longer to collect the broad scatter tails of the single slice profile, an alternative to the CTDI method is suggested which involves using a small volume ion chamber, and scanning a length of phantom long enough to establish dose equilibrium at the location of the chamber. With a modern CT scanner, such a scan length can be covered in 15 s or less with a helical or axial series, so this method is not significantly more time-consuming than the long chamber method. The method is demonstrated experimentally herein. ©2003 American Association of Physicists in Medicine.

This paper comes out of the Western Research Lab at Compaq: Characterization of Organic Illumination Systems. Making organic things emit light is a very trendy topic now, with a good deal of research going into OLEDs (organic light emitting diodes) and similar devices. This paper from way back in 1989 is an interesting read on some early investigations on the subject. They do some very interesting investigations on materials you find in most kitchens.

Abstract:

Recent anecdotal reports of novel principles of illumination have stressed qualitative aspects. This note presents a quantitative study of an organic illumintation system, characterizing the temperature and current-flow properties of the system as functions of time and device parameters. Theoretical and practical implications of these measurements are discussed.

One of the main reasons to read papers is to learn things. Preferably new things. This is one of those papers where you can pick up a few ideas for analyzing or looking at data or measuring something that you may not have initially thought of. This was certainly the case for me while I read this paper.

There are two effects of additional filtration: reduced patient entrance exposure (from removal of low-energy x-rays) and increased tube loading (from increased technique to compensate for the radiation removed by the additional filtration).

The change in mAs for varying amounts of added filtration and different projections was examined under three conditions: constant patient entrance exposure, constant patient exit exposure and constant film optical density. If I was doing the same study, I probably only would have thought to look at entrance exposure (since it is easiest to measure) or CR exposure index (we use digital systems here).

Maintaining constant entrance exposure required the highest increase in mAs for a given amount of filtration, while maintaining constant film density required the lowest increase. To avoid increasing the amount of image noise with added filtration, I think the ideal parameter to maintain would be constant optical density (or receptor exposure for digital systems). Since maintaining constant optical density is more labour intensive to measure, constant exit exposure is probably an easier parameter to work with (and already measured by phototimer systems). Both methods resulted in similar increases in mAs.

Increased technique also affects image quality due to focal spot blooming when the tube current is increased. However this is normally a very small effect, which was confirmed by the authors. There is also the potential for increased motion blur when exposure time must be increased, but with the constant exit exposure/optical density methods, the increase in exposure time was small enough so that motion was not a problem.

Even with 4.0 mm added filtration, the increase in mAs should be well within the range of x-ray units to accomodate by increasing mA and/or time without adversely affecting image quality.

Overall I thought this was a well done paper that clearly showed that the impact of added filtration was minimal and easily accomodated by most x-ray systems.

The researchers used a pair of homemade Helmholtz coils to produce magnetic fields and exposed rats to a 0.01 mT field (Earth’s magnetic field averages about 0.05 mT (50 µT) with significant regional variations) for 24 and 48 hours. The rats were sacrificed, the brains extracted and studied for single and double strand DNA breaks and apoptosis/necrosis using gel electrophoresis techniques.

Statistically significant differences were found in the number of DNA strand breaks between the exposed rats and control rats. Free radical production with NO and Fe+ ions were theorized to be the cause of the DNA strand breaks. Cell apoptosis was also found to increase following exposure. Experiments where the rats were administered free radical scavengers showed no difference between exposed and control rats. What remains to be seen, and what was not addressed in the paper is what kinds of health effects these breaks might have. This would probably be a difficult study to perform, since it would require studying a large number of rats over a long term to detect what is probably a very small effect.

Also not studied was the effect of DNA repair. The authors have clearly established free radical production as one mechanism of DNA damage in brain cells. But as is well known, cells are very good at repairing DNA damage. How much of the damaged cells would be repaired after 24 or 48 hours post-exposure? How significant would the loss of the cells be if the DNA could not be repaired and the cell died? What types of cells were most likely to incur damage? Since Fe+ free radicals were thought to be most likely involved, the authors felt that cells with higher Fe uptake would be more susceptible to damage. Authors also mentioned increased risk of neurodegenerative diseases, ALS, Alzheimer’s and Parkinson’s reported in previous literature due to occupational magnetic field exposure.