| Summary: | 博士 === 國立陽明大學 === 生物醫學影像暨放射科學系 === 103 === By nature of computed tomography (CT), CT involves larger radiation doses than the more common, conventional x-ray imaging procedures; esp. for pediatric population. Since the inception of CT in the 1970s, its use has increased rapidly. With advances in the multi-detector computed tomography generation by generation, the improvement in spatial and temporal resolution has allowed non-invasive evaluation of intrathoracic vessels, airways, cardiac anatomy and coronary arteries. Short scan time largely eliminates the need for anesthesia for the child and respiratory motion artifact for the adult during image acquisition. Although technologic innovation often heralds new or improved diagnostic benefits, it is prudent to approach these benefits with an understanding of additional costs and risks, and for CT this includes the radiation dose.
Coronary artery bypass grafting (CABG) is frequently performed to restore myocardial perfusion in patients with severe three-vessel disease or left main coronary artery stenosis. Although conventional coronary angiography is the current standard for the detection of graft patency and stenosis, it is associated with a small but non-negligible risk of complications such as stroke, myocardial infarction, pericardial tamponade, patient discomfort, and costs of hospital stay. Based on the recent guideline, cardiovascular computed tomography angiography (CCTA) using 64-slice CT is considered ‘‘appropriate’’ for symptomatic patients with prior coronary bypass surgery. Thus, CCTA assessment of CABG and anastomosis following surgery is expected to be a noninvasive alternative, and may demonstrate clinical benefit in selected patients. However, it still associates with the cost of high radiation and contrast doses. Because wider scan coverage is required to access the entire heart and graft course of CCTA following CABG, the radiation dose is approximately double as compared to the standard coronary CT angiography.
Congenital heart disease (CHD) has an incidence of between 4 and 50 per 1,000 live births and the leading cause of death for children. Although invasive catheter-directed cardiac angiography is currently the reference standard for the assessment of complex cardiovascular anomalies in children, it has the potential to impart high doses of radiation and contrast medium to pediatric patients due to extended fluoroscopic and cine evaluation. Other disadvantages include patient discomfort, bad availability, costs of hospital stay and a small but non-negligible risk of complications such as stroke, femoral artery occlusion, dissection, pseudoaneurysm formation, retroperitoneal hemorrhage, and serious sequelae such as limb growth discrepancy in infants and young children. Cardiac catheterization may also require general anesthesia or sedation. Consequently, pediatric CCTA has evolved to play a key role in evaluating complex CHD as a result of noninvasiveness and technical developments. Because the wider scan coverage required to access the entire heart, airway, aortopulmonary arteries and large venous structure over the chest and upper abdomen for comprehensive evaluation of CHD, pediatric CCTA exposes patients to a much higher radiation dose, approximately triple than that of routine chest CT examination.
Radiation exposure with CCTA has become an important issue due to the increased risk of cancer induction, esp. for wide-coverage CCTA. Although the link between medical radiation exposure and future cancer risk is still controversial, wide-coverage CCTA examination parameters should be considered and set in accordance with the as-low-as-reasonably-achievable (ALARA) strategy without compromise of diagnostic image quality. CCTA protocols for CHD and CABG follow-up are still under development even though the number of examinations is rapidly increasing. The recent introduction of the 256-slice CT scanner (Brilliance iCT; Philips Medical Systems, Eindhoven, Netherlands) with 270 ms gantry rotation and an 80 mm detector array has allowed a larger z-axial coverage and improved temporal resolution. We believe that the modern 256-slice CT may help in solving some ALARA issues. The purposes in the present thesis study are: 1) to investigate the image quality, radiation dose, and graft vessel accessibility for patients with prior CABG surgery using a 256-slice CT scanner; 2) to evaluate optimal temporal windows and dose-reducing strategy for CABG imaging with 256-slice CT; 3) to investigate the image quality and radiation dose resulting from various pediatric CCTA techniques, including non-electrocardiogram (ECG)-gated, retrospectively ECG-gated helical and prospectively ECG-gated axial (PGA) protocols, performed with 64-slice and 256-slice CT for 1-year-old infant; 4) to compare the image quality and radiation dose between different tube voltage settings and scan modes of 256-slice pediatric CCTA
The results of studies show that the PGA scan protocol on 256-slice CT was the preferable choice in balancing radiation exposure and image quality in pediatric CCTA and CCTA for CABG follow-up and better than older models of CT scanners. Hopefully, a wide-coveraged cardiovascular angiography examination that adheres to the ALARA principle could also maintain image quality for diagnostic use with 256-slice CT.
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