Ionizing Radiation in Medical Imaging and Efforts in Dose Optimization

Medical-related radiation is the largest source of controllable radiation exposure to humans and it accounts for more than 95% of radiation exposure from man-made sources. Its direct benefits in modern day medical practices are beyond doubt but risks-benefits ratios need to be constantly monitored as the use of ionizing radiation is increasing rapidly. From 1980 to 2006, the per-capita effective dose from diagnostic and interventional medical procedures in the United States increased almost six fold, from 0.5 to 3.0mSv, while contributions from other sources remained static (NCRP report no 160, 2009). This chapter will review radiation exposure from medical imaging initially starting from a historical viewpoint as well as discussing innovative technologies on the horizon. The challenges for the medical community in addressing the increasing trend of radiation usage will be discussed as well as the latest research in dose justification and optimization.link_to_OA_fulltex

Coronary CT angiography : radiation dose measurements and image quality assessments

Prospective ECG-triggering is regarded as one of the most effective approaches for reduction of radiation dose to patients during coronary CT angiography (CCTA). This study was conducted to investigate the diagnostic performance of prospective ECG-triggered CCTA with regard to the image quality and dose reduction, based on a multi-centre research. The research was performed in four stages, with stage 1 investigating the different CT scanning protocols and conventional angiography procedures with corresponding radiation dose measurements; stage 2 focused on the analysis of radiation dose in patients undergoing different generations of multislice CT scanners; stage 3 conducted a survey among medical specialists and radiographers with the aim of obtaining opinions regarding the benefits and difficulties in performing prospective ECG-triggered CCTA; stage 4 analysed image quality and radiation dose in patients undergoing single-source and dual-source 64-slice CT coronary angiography with use of prospective ECG-triggering.Stage 1 is a pilot study conducted on an anthropomorphic phantom. In this experiment, the radiation dose was compared between the invasive coronary angiography (ICA) and CCTA. These imaging protocols for ICA included the standard angular projection views with different magnifications. These ICA protocols were compared with several CT protocols including prospective and retrospective ECG gating. In addition, tube current modulation was applied in retrospective gating protocol. The radiation dose was also measured at the selected radiosensitive organs including breast and thyroid gland. Although ICA produced lower radiation dose than CCTA, application of modified techniques in both CCTA and ICA is recommended in clinical practice for further radiation dose reduction.Stage 2 involved a retrospective analysis of radiation dose in patients undergoing prospective ECG-triggered CCTA with different CT generations including single-source 64-slice CT (SSCT), dual-source 64-slice CT (DSCT), dual-source 128-slice CT and 320-slice CT based on several hospitals in Perth, Western Australia and Kuala Lumpur, Malaysia. A total of 164 patients undergoing prospective ECG-triggered CCTA with different types of CT scanners were studied. The analysis showed that the mean effective dose was estimated at 6.8 mSv, 4.2 mSv, 4.1 mSv, and 3.8 mSv, corresponding to 128-slice DSCT, 64-slice DSCT, 64-slice SSCT and 320-slice CT scanners, respectively. A positive relationship was found between effective dose and body mass index (BMI) in this study. Low radiation dose was achieved in prospective ECG-triggered CCTA, regardless of any CT scanner generation. BMI is identified as the major factor that has a direct impact on the effective dose associated with prospective ECG-triggered CCTA.A well-designed survey was performed in stage 3 among specialists and radiographers from 6 national health institutions in Malaysia in order to explore the opinion concerning the benefits and difficulties in performing prospective ECG triggered CCTA. In total, 53 responses were received (85%), comprising specialists (21%) and radiographers (79%). Across all the respondents, the main benefits of prospective triggering were agreed as: radiation dose reduction, image quality improvement and patients’ output increases. On the other hand, the issue of heart rate was agreed by all respondents as a main challenge when performing prospective triggered CCTA. The remaining challenges such as difficulty in obtaining cardiac functional assessments, diagnostic accuracy concerns and data processing management issue have been seen to vary according to the groups of respondents and the scanner type. Radiation dose reduction seems to be the main benefit, which is most agreed upon, while the issue of the heart rate is seen as the main challenge in prospective ECG-triggered CCTA.Finally, stage 4 is a comparative study consisting of quantitative and qualitative analysis, and it was conducted to investigate the image quality and radiation dose performance between retrospective gated and prospective ECG triggered CCTA with use of 64-slice SSCT and DSCT. The SSCT component was performed in the Royal Perth Hospital, Western Australia, while the DSCT component was conducted in the National heart Institute, Kuala Lumpur, Malaysia. A total of 209 patients who underwent CCTA with suspected coronary artery disease (CAD) scanned with SSCT (n=95) and DSCT (n=114) scanners using prospective ECG-triggering and retrospective ECG-gating protocols. The image was qualitatively assessed by two experienced observers, while quantitative assessment was performed by measuring the image noise, the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR).A total of 2,087 coronary artery segments were evaluated. Both DSCT and SSCT resulted in good image quality, regardless of prospective or retrospective gating protocols. Although radiation dose calculated between DSCT (6.5 mSv) and SSCT (6.2 mSv) showed no significant difference, the effective dose in prospective triggering was significantly lower than that in retrospective gating protocol. The results indicated that in the retrospective gating protocol, the effective dose with DSCT (18.2 mSv) was also significantly lower than that in SSCT (28.3 mSv). This study confirmed that prospective ECG triggered CCTA reduces radiation dose significantly compared to retrospective ECG-gating CCTA, while maintaining good image quality.In summary, the results of this project show that coronary CT angiography with prospective ECG-triggering is a reliable diagnostic technique with resultant very low radiation dose, but still maintaining diagnostic images. With widespread use of coronary CT angiography in the diagnosis of coronary artery disease, increased awareness of radiation dose associated with coronary CT angiography is of paramount importance, and application of dose-reduction strategies is highly recommended for routine clinical practice

My future and I:cardiovascular risk stratification of asymptomatic individuals

In the coming decades, a continuing increase in the number of cases of coronary heart disease (CHD) is expected. This is caused by, amongst others, the increasing prevalence of obesity and diabetes, and the rising numbers of elderly citizens. The morbidity and mortality toll of CHD is high. In many cases, a coronary event occurs acutely, without earlier signs suggesting CHD. So how can we identify individuals in the asymptomatic population at high risk of CHD, and prevent coronary events? Cardiovas- cular risk estimation in the general population is based on determining risk factors such as hypertension and smoking. Risk factor levels can be used to calculate a risk-scoring algorithm, like the European SCORE, and guide medical therapy. Unfortunately, risk factor based algorithms are neither highly sensitive nor specific. Accurate identification of asymptomatic indi- viduals who will develop a coronary event is challenging

Cardiovascular magnetic resonance guided electrophysiology studies

Catheter ablation is a first line treatment for many cardiac arrhythmias and is generally performed under x-ray fluoroscopy guidance. However, current techniques for ablating complex arrhythmias such as atrial fibrillation and ventricular tachycardia are associated with suboptimal success rates and prolonged radiation exposure. Pre-procedure 3D CMR has improved understanding of the anatomic basis of complex arrhythmias and is being used for planning and guidance of ablation procedures. A particular strength of CMR compared to other imaging modalities is the ability to visualize ablation lesions. Post-procedure CMR is now being applied to assess ablation lesion location and permanence with the goal of indentifying factors leading to procedure success and failure. In the future, intra-procedure real-time CMR, together with the ability to image complex 3-D arrhythmogenic anatomy and target additional ablation to regions of incomplete lesion formation, may allow for more successful treatment of even complex arrhythmias without exposure to ionizing radiation. Development of clinical grade CMR compatible electrophysiology devices is required to transition intra-procedure CMR from pre-clinical studies to more routine use in patients

Optimisation of CT protocols for cardiac imaging using three-dimensional printing technology

Objective: This thesis investigates the application of 3D-printing technology for optimising coronary CT angiography (CCTA) protocols using iterative reconstruction (IR) as a dose optimisation strategy. Methods: In phase one, a novel 3D-printed cardiac insert phantom for the Lungman phantom was developed. The attenuation values of the printed phantom were compared to CCTA patients and Catphan® 500 images. In phase two, the printed phantom was scanned at multiple dose levels, and the datasets were reconstructed using different IR strengths. The image quality characteristics were measured to determine the dose reduction potential. In phase three, the influence of IR strengths with low-tube voltage for dose optimisation studies was investigated. The printed phantom and the Catphan® 500 were scanned at different tube currents and voltages. The results were compared to the patient datasets to measure the agreement between the phantoms and patient datasets. Results: In phase one, the attenuation values were consistent between the printed phantom, patient and Catphan® 500 images. In phase two, the results showed that decreasing dose levels had significantly increased the image noise (p<0.001). The application of various IR strengths had yielded a stepwise improvement of noise image quality with a dose reduction potential of up to 40%. In phase three, the results showed a significant interaction between the effects of low-tube voltage and the IR strengths on image quality (all p<0.001) but not the attenuation values. The mean differences were small between the patient-phantom datasets. The optimised CT protocols allowed up to 57% dose reduction in CCTA protocols while maintaining the image quality. Conclusions: The 3D-printed cardiac insert phantom can be used to evaluate the effect of using IR on dose reduction and image quality. This thesis proposes and validates a new method of developing phantoms for CCTA dose optimisation studies

Image-guided Radiotherapy to Manage Respiratory Motion: Lung and Liver.

Organ motion as a result of respiratory and cardiac motion poses significant challenges for the accurate delivery of radiotherapy to both the thorax and the upper abdomen. Modern imaging techniques during radiotherapy simulation and delivery now permit better quantification of organ motion, which in turn reduces tumour and organ at risk position uncertainty. These imaging advances, coupled with respiratory correlated radiotherapy delivery techniques, have led to the development of a range of approaches to manage respiratory motion. This review summarises the key strategies of image-guided respiratory motion management with a focus on lung and liver radiotherapy