Elsevier

European Journal of Radiology

Volume 84, Issue 12, December 2015, Pages 2347-2358
European Journal of Radiology

Radiation protection issues in dynamic contrast-enhanced (perfusion) computed tomography

https://doi.org/10.1016/j.ejrad.2014.11.011Get rights and content

Highlights

  • We summarize the most recent recommendations of the ICRP on radiation effects.

  • We present dose distributions for three representative DCE-CT protocols.

  • We estimate stochastic radiation risks of patients using most recent risk models.

  • We discuss aspects to be considered in the process of justification and optimization.

Abstract

Dynamic contrast-enhanced (DCE) CT studies are increasingly used in both medical care and clinical trials to improve diagnosis and therapy management of the most common life-threatening diseases: stroke, coronary artery disease and cancer. It is thus the aim of this review to briefly summarize the current knowledge on deterministic and stochastic radiation effects relevant for patient protection, to present the essential concepts for determining radiation doses and risks associated with DCE-CT studies as well as representative results, and to discuss relevant aspects to be considered in the process of justification and optimization of these studies.

For three default DCE-CT protocols implemented at a latest-generation CT system for cerebral, myocardial and cancer perfusion imaging, absorbed doses were measured by thermoluminescent dosimeters at an anthropomorphic body phantom and compared with thresholds for harmful (deterministic) tissue reactions. To characterize stochastic radiation risks of patients from these studies, life-time attributable cancer risks (LAR) were estimated using sex-, age-, and organ-specific risk models based on the hypothesis of a linear non-threshold dose–response relationship.

For the brain, heart and pelvic cancer studies considered, local absorbed doses in the imaging field were about 100–190 mGy (total CTDIvol, 200 mGy), 15–30 mGy ( 16 m Gy) and 80–270 mGy (140 mGy), respectively. According to a recent publication of the International Commission on Radiological Protection (ICRP Publication 118, 2012), harmful tissue reactions of the cerebro- and cardiovascular systems as well as of the lenses of the eye become increasingly important at radiation doses of more than 0.5 Gy. The LARs estimated for the investigated cerebral and myocardial DCE-CT scenarios are less than 0.07% for males and 0.1% for females at an age of exposure of 40 years. For the considered tumor location and protocol, the corresponding LARs are more than 6 times as high. Stochastic radiation risks decrease substantially with age and are markedly higher for females than for males.

To balance the diagnostic needs and patient protection, DCE-CT studies have to be strictly justified and carefully optimized in due consideration of the various aspects discussed in some detail in this review.

Introduction

Technical innovations in multi-detector computed tomography (CT) allow for larger volume coverage in shorter scan times and have thus stimulated the application of dynamic contrast-enhanced (DCE) CT techniques in clinical practice. Since the temporal change of CT densities (in Hounsfield units, HU) upon administration of an extracellular CT contrast agent (CA) is related to the regional blood supply and the extravasation of the CA, DCE-CT is a valuable tool for rapid and non-invasive characterization of tissue microcirculation.1 It has been established in medical care for improved diagnosis and treatment of cerebral ischemia and infarction [1], [2], [3], [4] and is increasingly investigated in clinical trials to define its role in the diagnosis, management, and prognosis of patients with coronary artery disease (CAD, [5], [6], [7]) and cancer [8], [9]. The most recent state of the art is summarized in this special issue.

As compared to DCE magnetic resonance imaging (MRI), the CT technique offers the major methodological advantage of a direct and almost linear relationship between the CA-induced density increase and the local CA concentration. Moreover, it has practical advantages in that the examination is fast, commonly available and better applicable for critically ill and intensive care patients than MRI. But there are also two serious and interconnected shortcomings: First, the relatively small CA-induced increase in CT densities in most (in particular ischemic) tissues as compared to the noise level of the acquired density–time curves and, second, the exposure of patients to ionizing radiation.

Although the effective dose resulting from a DCE-CT study is typically less than about 30 mSv, local doses in the examined body region are rather high and may result in harmful radiation damages when the examination is repeated several times or combined with other high-dose angiographic or interventional procedures. Imanishi et al. reported on temporary bandage-shaped hair loss occurring in three patients with cerebrovascular disorders who underwent several CT perfusion studies and two angiographies of the head within a few days [10]. In the United States, approximately 385 patients from six hospitals were exposed to excess radiation during CT brain perfusion scans using inadequate protocols. Some of these patients reported obvious signs of excessive radiation exposure following their scans, such as hair loss or skin redness, which called attention to the problem [11]. But even if radiation exposures are not high enough to produce obvious signs of radiation injury, it can place patients at increased risk for long-term radiation effects, in particular cancer [11].

It is thus the aim of this review article (i) to briefly summarize the current knowledge on deterministic and stochastic radiation effects relevant for radiation protection of patients, (ii) to present the essential concepts for estimating radiation doses and risks associated with DCE-CT studies as well as representative results, and (iii) to discuss in detail the various aspects that have to be considered in the process of justification and optimization of DCE-CT studies as summarized in Fig. 1.

Section snippets

Biological effects of ionizating radiation

According to how the tissue response relates to the radiation dose, radiological protection deals with two types of adverse health effects. (i) Stochastic radiation effects (cancer or heritable effects due to cell transformation), which may be observed as a statistically detectable increase in the incidences of these effects occurring long after radiation exposure in the affected individuals or their offspring. (ii) Deterministic radiation effects (harmful tissue reactions due to cell killing),

Fundamental dose quantities

Both stochastic and deterministic radiation effects are related to the energy deposited by ionizing radiation in a specified organ or tissue, T. Therefore, the fundamental physical dose quantity is the absorbed dose DT (given in the unit Gray, 1 Gy = 1 J/kg), which is defined as the radiation energy absorbed in a small volume element of tissue divided by its mass. The absorbed dose is per definition a local dose quantity and should be used to characterize deterministic radiation effects [24].

For

Justification of DCE-CT studies

Current radiation protection recommendations and directives [45], [46], [47] require that

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    all new types of practice with a specified clinical objective involving exposure of patients to ionizing radiation (e.g., DCE-CT for the diagnosis of cerebral ischemia) shall be justified in advance before being generally adopted (generic justification); if a type of practice is not justified in general, it can be justified in special circumstances based on a documented case-by-case decision or in clinical

Optimization of DCE-CT studies

Once a specific imaging procedure has been justified for an individual patient, it has to be optimized, which means that patient exposure is kept as low as reasonably achievable (ALARA principle) consistent with obtaining the required medical information, taking into account economic and societal factors [45], [46], [47]. The primary objective of optimizing a DCE-CT study is to set up the examination protocol in such a way that the microcirculatory tissue parameter(s) justified in advance can

Protective shielding

DCE-CT examinations can result in substantial radiation doses to radiosensitive tissues near the body surface, such as the lenses of the eye, thyroid, female breast, or testicles. These tissues can either be exposed by low-energy scattered photons due to their proximity to the imaged body region (e.g., the lenses of the eye and the thyroid in case of brain scans) or by primary and scattered photons when they lie in the imaged body region (e.g. the female breast in case of heart scans). A

Conclusion

DCE-CT studies are increasingly used in medical care and clinical trials to improve diagnosis and therapy management of the most common life-threatening diseases: stroke, CAD and cancer. As exemplified by representative DCE-CT protocols, local absorbed doses in the scanned body regions can be rather high and can result in harmful radiation damages (note: the threshold for the cerebro- and cardiovascular systems may be as low as 0.5 Gy) when the examination is repeated several times or is

Acknowledgements

We thank C. Klingele, E. Klotz, and A. Schegerer for helpful discussions.

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      Using the weighted CT dose index (CTDIw), that combines absorbed doses measured at the periphery and center of a standard CT dosimetry phantom in the ratio of 2:1, as a surrogate for patient dose, Hamberg et al. reported exponents of 2.5 ± 0.1 and 2.8 ± 0.1 for the head and abdomen, respectively [21]. The somewhat greater discrepancy in the head figure to our result is due to the fact that the dosimetry head phantom does not account for the strong absorption and hardening of X-rays by the skull [17,22]. As easy-to-use quantitative tools for spectral optimization of CA-enhanced CT images, two characteristic parameters, RDE and RED, were derived that relate the fitted CNR-D curves plotted in Fig. 5 for different voltages to each other in vertical and horizontal direction, respectively.

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