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. 2013 Sep;268(3):927–928. doi: 10.1148/radiol.13130791

Radiation Dose of Second-Generation 320–Detector Row CT

Xue-jiang Qian 1,✉
PMCID: PMC6604792  PMID: 23970515

Editor

I read with interest the article by Dr Chen and colleagues in the April 2013 issue of Radiology (1) regarding the radiation with a second-generation 320–detector row computed tomographic (CT) scanner. Although the evaluation of the second-generation 320–detector row CT scanner was a prospective study, the comparison was conducted with the first-generation 320–detector row CT unit retrospectively. Because it is not realistic to control all of the characteristics between the two cohorts, the conclusion cannot be supported strongly by the results.

In general, the radiation dose of CT angiography is influenced by kilovolt, exposure time, and scanning length. In the study, the kilovolt and exposure time with the second-generation 320–detector row CT unit was significantly lower because of the use of iterative reconstruction and x-ray generators and the higher temporal resolution of the newer scanner. However, the scanning length of the newer scanner, which was determined by the enrolled patients (2), was also shorter than that of the first-generation scanners. The conclusion that the new technique can lead to at least a 75% reduction in radiation dose compared to that with the first-generation 320–detector row CT scanner is doubtful, as a shorter scanning length can also decrease patient dose effectively.

In addition, the radiation dose was also underestimated. In general, the effective dose E is calculated according to the following equation: E = k ´ dose-length product. The conversion factor k in this study was 0.014 mSv/mGy × cm. According to an updated investigation (3), it should be 0.029 mSv/mGy × cm for 320–detector row CT. The radiation dose, therefore, should have been up to 2 mSv.

Disclosures of Conflicts of Interest: No relevant conflicts of interest to disclose.

Footnotes

Recent eLetters to the Editor are available at radiology.rsna.org. eLetters that are no longer posted under ‘‘Recent eLetters’’ can be found as a link in the related article or by browsing through past Tables of Contents.

References

  • 1.Chen MY, Shanbhag SM, Arai AE. Submillisievert median radiation dose for coronary angiography with a second-generation 320–detector row CT scanner in 107 consecutive patients. Radiology 2013;267(1):76–85. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Khan A, Nasir K, Khosa F, Saghir A, Sarwar S, Clouse ME. Prospective gating with 320-MDCT angiography: effect of volume scan length on radiation dose. AJR Am J Roentgenol 2011;196(2):407–411. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Einstein AJ, Elliston CD, Arai AE, et al. Radiation dose from single-heartbeat coronary CT angiography performed with a 320–detector row volume scanner. Radiology 2010;254(3):698–706. [DOI] [PMC free article] [PubMed] [Google Scholar]
Radiology. 2013 Sep;268(3):928. doi: 10.1148/radiol.13130791a

Response

Marcus Y Chen 1,✉, Sujata M Shanbhag 1, Andrew E Arai 1

Dr Qian raised three concerns about the radiation dose of coronary CT angiography with the second-generation 320–detector row CT scanner (1): (a) the adequacy of the comparison cohort, (b) doubts about the radiation reduction because the scan length was not the same between groups, and (c) whether the correct k factor was used for estimated effective radiation dose.

The two cohorts were well matched for all factors that affect radiation exposure (table 1). The sample size and the consecutive patient design provide good power to detect differences between groups. Most centers, including the National Institutes of Health, could not afford to keep two generations of scanners for a randomized simultaneous comparison, so we had to rely on two consecutive series of patients.

The 13% difference in scan length (table 2) was enabled by an improved cone-beam reconstruction algorithm that reduced cranial and caudal conical truncation of volumes. Faster gantry rotation time, lower kilovolt, automatic exposure control, iterative reconstruction, and shorter scan length all helped reduce overall radiation dose.

The use of a different k factor would change the absolute estimate of effective radiation dose proportionately for both groups of patients but would not change the statistical differences between groups. The 0.014 mSv/mGy × cm k factor is recommended in guidelines (2,3) but may change in the future. Thus, achieving lower radiation exposure with the second-generation 320–detector row CT scanner is a robust conclusion. The absolute biologically relevant radiation dose in populations is difficult to estimate and a problem for the field in general. Radiation dose in an individual patient is even more difficult to estimate.

Disclosures of Conflicts of Interest: M.Y.C. Financial activities related to the present article: study was funded by the Intramural Research Program of the National Heart, Lung, and Blood Institute of the National Institutes of Health (HL006138-02). Financial activities not related to the present article: none to disclose. Other relationships: none to disclose. S.M.S. Financial activities related to the present article: study was funded by the Intramural Research Program of the National Heart, Lung, and Blood Institute of the National Institutes of Health (HL006138-02). Financial activities not related to the present article: none to disclose. Other relationships: none to disclose. A.E.A. Financial activities related to the present article: study was funded by the Intramural Research Program of the National Heart, Lung, and Blood Institute of the National Institutes of Health (HL006138-02). Financial activities not related to the present article: none to disclose. Other relationships: none to disclose.

References

  • 1.Chen MY, Shanbhag SM, Arai AE. Submillisievert median radiation dose for coronary angiography with a second-generation 320–detector row CT scanner in 107 consecutive patients. Radiology 2013;267(1):76–85. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Halliburton SS, Abbara S, Chen MY, et al. SCCT guidelines on radiation dose and dose-optimization strategies in cardiovascular CT. J Cardiovasc Comput Tomogr 2011;5(4):198–224. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Abbara S, Arbab-Zadeh A, Callister TQ, et al. SCCT guidelines for performance of coronary computed tomographic angiography: a report of the Society of Cardiovascular Computed Tomography Guidelines Committee. J Cardiovasc Comput Tomogr 2009;3(3):190–204. [DOI] [PubMed] [Google Scholar]

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