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Radiation Effects from Perfusion CT

Department of Radiology, Medical University of South Carolina, 169 Ashley Avenue, Charleston, SC 29425. e-mail: freyd@musc.edu

Editor:

In their recent article in the June 2004 issue of Radiology (1), Dr Hoeffner and colleagues correctly note that because of the limited field of view irradiated, the effective dose from cerebral perfusion computed tomography (CT) is similar to that of routine CT brain examination. Dr Hoeffner and colleagues give the effective doses as 2.0–3.4 mSv for the perfusion study and 1.5–2.5 mSv for the routine brain study. These values are similar to those used at our institution.

Effective dose is designed to represent an overall radiation detriment, compared with the equivalent whole-body irradiation (2). By using the recommended risk value of 5% per sievert according to the National Council on Radiation Protection and Measurements (3), the risk of a fatal cancer from the study would be about 1 in 7500. This could be considered a reasonable risk for the valuable information obtained.

Another aspect of the perfusion study, however, is the actual dose to the irradiated volume. This is much higher for perfusion study than it is for standard brain CT. By using the data from Huda et al (4) and Boone et al (5), one can estimate the dose to irradiated tissue as approximately 300 mGy for perfusion study and 32 mSv for routine brain CT. The biologic effects of 300 mGy on a small volume of tissue are not well understood, but in a recent article, Hall et al (6) describe deficits in cognitive function in infants exposed to brain irradiation for the treatment of cutaneous hemangioma. In that study, defects were significant for individuals exposed to 250 mGy. While one would expect the adult brain to be less affected by radiation than would the infant brain, radiologists should be aware that the radiation dose to the irradiated brain from perfusion study is high.

REFERENCES

1. Hoeffner EG, Case I, Jain R, et al. Cerebral perfusion CT: technique and clinical application. Radiology 2004; 231:632-644.[Abstract/Free Full Text]
2. McCollough CH, Schueler BA. Calculation of effective dose. Med Phys 2000; 27:828-837.[CrossRef][Medline]
3. National Council on Radiation Protection and Measurements. Limitation of exposure to ionizing radiation NUCRP report 116. Bethesda, Md: National Council on Radiation Protection and Measurements, 1993.
4. Huda W, Lieberman KA, Chang J, et al. Patient size and x-ray technique factors in head computed tomography examinations. I. Radiation doses. Med Phys 2004; 31:588-594.
5. Boone JM, Cooper VN, 3rd, Nemzek WR. Monte Carlo assessment of computed tomography dose to tissue adjacent to the scanned volume. Med Phys 2000; 27:2393-2407.[CrossRef][Medline]
6. Hall P, Adami HO, Dimitrios A, et al. Effect of low doses of ionising radiation in infancy on cognitive function in adulthood: Swedish population based cohort study. BMJ 2004; 328:19.[Abstract/Free Full Text]

Dr Hoeffner and colleagues respond:

Department of Radiology, University of Michigan Health System, 1500 East Medical Center Drive, UH B2 B403, Ann Arbor, MI 48109. e-mail: hoeffner@umich.edu

We thank Drs Frey and Rumboldt for their interest in our article on cerebral perfusion CT (1).

We agree with them that the actual dose to the irradiated volume of brain is much higher for perfusion CT than for standard head CT. By using the perfusion CT technique from our article (1) and the ImPACT CT Patient Dosimetry Calculator available over the Internet (www.impactscan.org), however, we estimated the dose to the irradiated brain tissue as 110 mGy. While this is still a high value, it is less than that calculated by Drs Frey and Rumboldt. Doses in this range were associated with a decrease in high school attendance in the study by Hall et al (2), but it was not until doses exceeded 250 mGy that a statistically significant decrease in cognition of technical instruction occurred. The work of Hall et al also suggests that radiation to the frontal part of the brain has a more severe effect on cognitive function than does radiation to the posterior areas of the brain (2).

Only a small 2-cm-thick section of brain is imaged with most dynamic enhanced perfusion CT examinations, although it usually does include portions of the frontal lobes. While the radiologist should be mindful of the relatively high radiation dose from perfusion CT, most of the patients undergoing such examinations are adults with cerebrovascular diseases that often place them at high risk of stroke. In this setting, we believe that the benefits of perfusion CT examination likely outweigh the risk from radiation.

REFERENCES

1. Hoeffner EG, Case I, Jain R, et al. Cerebral perfusion CT: technique and clinical applications. Radiology 2004; 231:632-644.
2. Hall P, Adami HO, Trichopoulos D, et al. Effect of low doses of ionising radiation in infancy on cognitive function in adulthood: Swedish population based cohort study. BMJ 2004; 328:1-5.

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