The impact of iodine contrast agent on radiation dose of heart and blood: a comparison between coronary CT angiography and cardiac calcium scoring CT

Abstract

Background

Iodine contrast agent (CA) is widely used in cardiac computed tomography (CT). The CA can increase the organ radiation doses due to the photoelectric effect.

Purpose

To investigate the impact of CA on radiation dose in cardiac CT by comparing the radiation dose between contrast coronary CT angiography (CCTA) and non-contrast calcium scoring CT (CSCT).

Material and Methods

Radiation doses were computationally calculated for 30 individual patients who received CSCT and CCTA in the same exam session. The geometry and acquisition parameters were modeled in the simulations based on individual patient CT images and acquisitions. Doses in the presence and absence of CA were obtained in the aorta, left ventricle (LV), right ventricle (RV), and myocardial tissue (MT). The dose values were normalized by size-specific dose estimate (SSDE). The dose enhancement factors (DEF_SSDE) were calculated as the ratio of doses in CCTA over doses in CSCT.

Results

Compared to the CSCT scans, doses increase in the CCTA scans in the aorta (DEF_SSDE = 2.14 ± 0.20), LV (DEF_SSDE = 1.78 ± 0.26), and RV (DEF_SSDE = 1.31 ± 0.22). A linear relation is observed between the local CA concentrations and the dose increase in the heart; DEF_SSDE = 0.07*I(mg/mL) + 0.80 (R² = 0.8; p < 0.01). The DEF_SSDE in the MT (DEF_SSDE = 0.96 ± 0.08) showed no noticeable impact of CA on the dose in this tissue. In addition, patient variability in the dose distributions was observed.

Conclusion

A linear causal relation exists between local CA concentration and increase in radiation dose in cardiac CT. For the same CT exposure, dose to the heart is on average 55% higher in contrast cardiac CT.

Keywords

Intravenous contrast agents computed tomography cardiac radiation safety dosimetry

Introduction

Based on the report of the World Health Organization, ischemic heart disease was the major cause of death in 2019, responsible for 16% of the world's total deaths. The performance of cardiac computed tomography (CT) has been emphasized in many guidelines as a necessary step for the detection and exclusion of coronary artery disease (CAD) (1 –3), resulting in an increase in the number of cardiac CT in diagnostics (4). As the patients are exposed to ionizing radiation during CT scans, awareness about patient radiation dose in CT interventions is necessary. Following the guidelines of the Society of Cardiovascular Tomography (5), CT operators must be aware of the risks from ionizing radiation and minimize the radiation exposure to patients based on the ALARA principle.

Up to recent years, the impact of iodine contrast agents (CA) on radiation dose in CT imaging was unknown. Recently, researchers have reported the radiobiological impact of CA on increasing the DNA double-strand breaks (6 –10). Moreover, a study has compared the computed tomography dose index (CTDI_vol) and dose length product (DLP) of adults receiving contrast and non-contrast CT and reported an increase in the CT scanner dose while using contrast (11). In addition, a few studies have used computational methods to investigate the impact of CA on the radiation dose in CT (12 –16). Conventional patient dosimetry in CT is obtained by methods such as converting DLP values from CT acquisition to effective dose, using experimental phantoms, or commercial tabulated CT dosimetry software such CT-Expo (17) and NCICT (18). None of these conventional dosimetry methods include the impact of CA on radiation dose.

The aim of the present study was to evaluate the impact of iodine CA on radiation dose in coronary CT angiography (CCTA). We compare the radiation dose in CCTA with a non-contrast cardiac calcium scoring CT (CSCT). Dose simulations are performed by using individual patient images as 3D geometrical models and including exposure parameters based on individual patient CT acquisitions.

Material and Methods

Acquisition model

Institutional ethical approval was obtained for this retrospective study. The CSCT and CCTA scans were performed with a Revolution CT scanner (GE Healthcare, Milwaukee, Wisconsin, USA) in the same scan session. For the CCTA scans, the contrast agent Iomeprol 350 mg I/mL (Iomeron, Bracco, Milan, Italy) was administered with a mean volume of 61 ± 7 mL (range = 50–70 mL) at the rate of 5 mL/s. All the scans were obtained with an electrocardiogram (ECG)-gated acquisition. All the images were reconstructed with the standard convolutional kernel and ASiR-V 60% on the same full chest field of view (FOV). The acquisition parameters for CSCT and CCTA scans are listed in Table 1.

Table 1.

The acquisition parameters of CSCT and CCTA scans.

Scan type	Tube voltage (kVp)	Beam collimation (cm)	Tube current (mA) (mean ± SD)	Rotation time (ms)	Filter	Scan mode	CTDI_vol (mGy) (mean ± SD)
CSCT	120	16	216 ± 46	0.28	Small bowtie	Axial	4.11 ± 0.88
CCTA	100	16	642 ± 304	0.28	Small bowtie	Axial	7.61 ± 3.61

CCTA, coronary computed tomography angiography; CSCT, calcium scoring computed tomography; CTDI_vol, computed tomography dose index volume.

Patient group

We retrospectively selected 30 consecutive patients (15 men, 15 women; mean age = 55 ± 15 years; age range = 26–81 years) who received CSCT and CCTA scans in the same exam session as a part of their routine diagnostic procedure. The CT data of these patients were obtained from the picture archiving and communication system (PACS). Patients with pacemakers and/or heart stents and large patients that exceeded the FOV of 32 cm were not included in this study.

Dosimetry calculation

All the individual acquisitions were modeled in CT-specific Monte Carlo (MC) software ImpactMC (AB–CT Advanced Breast-CT GmbH, Germany) which has been validated (19) and used in numerous clinical studies (20 –24). The patient CT data and acquisition details including the tube voltage, mean tube current, bow-tie filter information, and collimation were modeled. The simulations were performed for 10E9 photons for CSCT and CCTA scans. The relationship between CT values and local iodine concentrations was defined in the simulations similar to an earlier phantom study (12), considering the 120-kVp tube voltage of CSCT scans and 100-kVp tube voltage of CCTA scans. The following materials were considered: air, bone, and water/iodine mixtures in the range of 0–20 mg I/mL (interval = 0.2 mg I/mL). The outcomes of the simulations were 3D dose volumes that were used for data analysis. The absorbed radiation doses and CT values in the aorta, left ventricle, right ventricle, and myocardial tissue were obtained by placing circular regions of interest (ROIs) with diameters of 12.5 mm, 15.6 mm, 15.6 mm, and 6 mm in each region, respectively (Fig. 1a). The same ROIs were used to investigate the dose in the non-contrast CSCT. The mean dose values were normalized by the size-specific dose estimate (SSDE) values that were calculated based on the AAPM report 2011 (25) by using the CTDI_vol and effective diameter of the patients. The normalization of the dose values over SSDE reduces the dependency of the results to the patient and scanner-related factors to a substantial extent and allows comparing the SSDE-normalized dose of contrast CCTA scans (D_{SSDE, I}) with the SSDE-normalized dose of non-contrast CSCT scans (D_{SSDE, 0}). The SSDE-based Dose Enhancement Factor (DEF_SSDE, formula 1) was defined as the ratio of D_{SSDE, I} (with contrast) at each ROI in CCTA scans over D_{SSDE, 0} in the same ROI (without contrast) in CSCT scans. Furthermore, the relationship between the CA concentration and DEF_SSDE was obtained by linear regression for all the regions combined.

DE F_{SSDE} = \frac{D_{S S D E, I}}{D_{S S D E, 0}}

(1)

D_{SSDE, I:} SSDE-normalized dose of CCTA; D_{SSDE, 0:} SSDE-normalized dose of CSCT.

Results

Compared to the CSCT scans, the radiation doses increase in the CCTA scans, and this increase is related to the local iodine concentrations (Table 2). Table 2 shows the CT values, the local iodine concentrations, and the mean DEF_SSDE in the aorta, left ventricle, right ventricle, and myocardial tissue in the CCTA scans. The DEF_SSDE was in the range of 0.96–2.14 in the myocardial tissue and aorta, respectively. The DEF_SSDE >1 in the aorta, left ventricle, and right ventricle shows an increase in the radiation dose in CCTA compared to CSCT. The highest increase in the dose was observed in the aorta (114%), followed by the left ventricle (78%) and right ventricle (31%). The DEF close to one in the myocardial tissue indicates no increase in the dose in this tissue.

Table 2.

The CT values, the average local iodine concentrations, and the average DEF_SSDE in the aorta, left ventricle, right ventricle, and myocardial tissue in the CCTA scans.

ROI	CT value	Local iodine concentration (mg I/mL)	DEF_SSDE
Aorta	512 ± 68	17.9 ± 2.5	2.14 ± 0.20
Left ventricle	477 ± 104	16.6 ± 3.8	1.78 ± 0.26
Right ventricle	255 ± 83	7.5 ± 3.0	1.31 ± 0.22
Myocardial tissue	85 ± 14	2.4 ± 0.5	0.96 ± 0.08

Values are given as mean ± SD.

CCTA, coronary computed tomography angiography; DEF_SSDE, SSDE-based dose enhancement factor; ROI, region of interest; SD, standard deviation; SSDE, size-specific dose estimate.

Fig. 1b shows the relation between the local CA concentrations and DEF_SSDE. The relation between the local CA concentrations and radiation dose is strongly linear: DEF_SSDE = 0.07*I (mg/mL) + 0.80 (R² = 0.8; P < 0.01). Fig. 2 illustrates parametric dose maps of four patients for non-contrast (CSCT) and contrast (CCTA) cardiac CT. The effect of patient anatomy and local iodine concentrations on the organ doses is observed in Fig. 2.

Discussion

The results of this study showed that using CA in cardiac CT increases the radiation dose in the heart. The increase in the heart dose was linearly correlated to the local iodine concentration. The linear correlation between absorbed radiation dose and iodine concentration for the abdomen and thyroid has also been reported in previous studies (12,26). The results of this study showed that for the same CT scanner dose, the heart radiation dose increases by an average of 55% in contrast-enhanced cardiac CT (CCTA scan) compared to non-contrast cardiac CT (CSCT scan). The highest average increase in the dose is seen in the aorta (114%), followed by the left ventricle (78%) and right ventricle (31%). For tissues with a high iodine uptake such as blood, the radiation dose is highly dependent on the iodine content in the tissue at the time of the scan (Fig. 1b). Factors such as cardiac output, contrast administration, patient anatomy and physiology, cardiovascular diseases, and scan timing influence the iodine content in the tissues at the time of acquisition. The results of this study showed no noticeable impact of CA on radiation dose in the myocardial tissue with a low iodine uptake of 2.4 mg I/mL (DEF_SSDE close to 1).

Although the impact of CA on increasing the radiation dose has been reported by several studies, the risk associated with this increase is not yet fully understood. The iodine-induced dose increase in the heart would have a minor impact on the full body dose (effective dose) as the heart represents a limited tissue waiting factor (w_T = 0.009) (27). However, the effective dose is not a good indicator for the radiation risk assessment for a single patient (28). The associated risk depends on the range of secondary electrons produced by the photoelectric effect of X-rays in the iodine. If these electrons escape the blood pool and reach the surrounding tissue, the risk of DNA double-strand break increases in the surrounding tissues. A dosimetry study in micro-dimensional scale has reported the impact of CA on increasing the radiation dose to the endothelium of blood vessels and potential increase in the risk of radiation-induced cardiovascular diseases (13). In addition, the iodine-induced increase in the radiation dose could be a matter of concern for other radio-sensitive organs with dense and small vascularization such as parenchymal tissue. If the iodine-induced secondary electrons can escape the vessels <100 µm, they could induce risk for the surrounding tissue (29).

The present study has some limitations. These include using data from one center and one type of CT scanner, and the difference between the tube voltages in contrast and non-contrast CT scans. In addition, a simplified approach was taken to investigate the results in two-dimensional ROIs rather than in full segmented volumes. However, we believe that this simplification has a minor impact on the obtained linear relation between the heart dose and iodine concentration. Moreover, a single brand of iodine CA was used in this study. However, we do not expect different results for assorted brands of iodine-based CA. The iodine-induced increase in the dose is related to its electron density and it is a physics event rather than a chemical or pharmacological event.

In conclusion, this study shows that CA increases the radiation dose in the heart with a linear correlation. Although CA is widely used in cardiac CT, its impact on radiation dose is not yet considered in patient dosimetry. Considering the use for CA in many diagnostic procedures, it is reasonable to include the CA in patient dosimetry calculations.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Research Foundation-Flanders (FWO) (no. 1S52720N).

ORCID iD

Mahta Mazloumi

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