Radiation protection issues in dynamic contrast-enhanced (perfusion) computed tomography
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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Evaluation of radiation exposure for patients undergoing computed tomography perfusion procedure for acute ischemic stroke
2022, Radiation Physics and ChemistryPeak skin and eye lens radiation dose from brain perfusion CT: CTDI<inf>vol</inf> and Monte Carlo based estimations
2020, European Journal of RadiologyCitation Excerpt :Doses have to be put in perspective: hair loss and skin damage are reported for single exposures over 2 Gy (early transient erythema) and 3 Gy (temporary epilation), which are typically above the cumulative CTDIvol of a head perfusion exam which is in the range of 132–433 mGy [2,6]. Some caution is also required as there is some variability in sensitivity among patients [7]. On top of this, the dose to the eye lens is of concern.
Patient dose in brain perfusion imaging using an 80-slice CT system
2019, Journal of NeuroradiologyCitation Excerpt :Li et al. [27] and Solano et al. [28] utilizing 128-slice CT systems as well, and a tube load of 150 and 80 mAs, respectively, reported higher CTDIvol values, than the mean value of our study. Brix et al. [29] using a 192-slice CT system and a tube load of 250 mAs, reported a CTDIvol value of 196 mGy which is 64% higher than our value, although they used a tube voltage of 70 kVp. Lin et al. [30] using a 256-slice system and a tube load of 80 mAs, reported a CTDIvol value of 128.2 mGy, which is 7% higher than our value.
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2019, Zeitschrift fur Medizinische PhysikComparison of patient dose from routine multi-phase and dynamic liver perfusion CT studies taking into account the effect of iodinated contrast administration
2019, European Journal of RadiologyCitation Excerpt :However, determination of absorbed dose to each exposed radiosensitive organ and organ dose-based calculation of effective dose is considered more reliable method to quantify radiation risk from CT exposures [10–12]. There are few publications reporting organ doses from liver CT examinations derived by Monte Carlo simulation of CT exposures on mathematical anthropomorphic phantoms [4,5]. Organs/tissues in mathematical phantoms are customarily represented with standard density and elemental composition to mimic corresponding human tissues.
Spectral optimization of iodine-enhanced CT: Quantifying the effect of tube voltage on image quality and radiation exposure determined at an anthropomorphic phantom
2016, Physica MedicaCitation Excerpt :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.