Cardiac-Specific Conversion Factors to Estimate Radiation Effective Dose From Dose-Length Product in Computed Tomography.

OBJECTIVES: This study sought to determine updated conversion factors (k-factors) that would enable accurate estimation of radiation effective dose (ED) for coronary computed tomography angiography (CTA) and calcium scoring performed on 12 contemporary scanner models and current clinical cardiac protocols and to compare these methods to the standard chest k-factor of 0.014 mSv·mGy-1cm-1. BACKGROUND: Accurate estimation of ED from cardiac CT scans is essential to meaningfully compare the benefits and risks of different cardiac imaging strategies and optimize test and protocol selection. Presently, ED from cardiac CT is generally estimated by multiplying a scanner-reported parameter, the dose-length product, by a k-factor which was determined for noncardiac chest CT, using single-slice scanners and a superseded definition of ED. METHODS: Metal-oxide-semiconductor field-effect transistor radiation detectors were positioned in organs of anthropomorphic phantoms, which were scanned using all cardiac protocols, 120 clinical protocols in total, on 12 CT scanners representing the spectrum of scanners from 5 manufacturers (GE, Hitachi, Philips, Siemens, Toshiba). Organ doses were determined for each protocol, and ED was calculated as defined in International Commission on Radiological Protection Publication 103. Effective doses and scanner-reported dose-length products were used to determine k-factors for each scanner model and protocol. RESULTS: k-Factors averaged 0.026 mSv·mGy-1cm-1 (95% confidence interval: 0.0258 to 0.0266) and ranged between 0.020 and 0.035 mSv·mGy-1cm-1. The standard chest k-factor underestimates ED by an average of 46%, ranging from 30% to 60%, depending on scanner, mode, and tube potential. Factors were higher for prospective axial versus retrospective helical scan modes, calcium scoring versus coronary CTA, and higher (100 to 120 kV) versus lower (80 kV) tube potential and varied among scanner models (range of average k-factors: 0.0229 to 0.0277 mSv·mGy-1cm-1). CONCLUSIONS: Cardiac k-factors for all scanners and protocols are considerably higher than the k-factor currently used to estimate ED of cardiac CT studies, suggesting that radiation doses from cardiac CT have been significantly and systematically underestimated. Using cardiac-specific factors can more accurately inform the benefit-risk calculus of cardiac-imaging strategies.

Clinical outcomes following a strategy of optimized medical management and selective "downstream" procedures following coronary computed tomography angiography.

BACKGROUND: Prior studies have shown a consistent relationship between coronary artery calcium (CAC) scores or the degree of coronary stenoses on coronary computed tomography angiography (CCTA) and all-cause mortality. Whether CCTA-targeted therapy, including intensive medical management, stress testing and/or invasive coronary angiography (ICA), can lead to a substantial reduction in adverse outcomes is not yet known. METHODS: We retrospectively analyzed 691 patients (55±13 years, male=63%) from a single medical practice who underwent a CAC scan and CCTA and were followed for a mean of 2.9±1.0 years. Of these, 416 (60%) patients were asymptomatic. All changes in medications, coronary risk factors (including lipids profiles), downstream testing, revascularization procedures, and clinical events (myocardial infarction and death) were recorded. RESULTS: Among our patients cohort 279 (40%) had no coronary artery disease. The most severe stenosis was <50% in 314 (46%) patients, 50-70% in 76 (11%) patients, and >70% in 22 (3%) patients. A high frequency of medical therapy was employed for those patients with any degree of stenosis, while stress testing was primarily applied for patients with >50% stenosis and ICA was primarily performed in those with >70% stenosis. Only two non-cardiovascular deaths and no cardiovascular deaths occurred during the follow-up period. CONCLUSIONS: Our findings support the hypotheses that among patients undergoing CCTA, comprehensive medical management, including targeted percutaneous coronary interventions and increasingly intensive medical therapy with progressively worse CCTA findings, can reduce event rates among patients with abnormal CCTA studies.

A rare case of a coronary vascular ring arising from an anomalous single coronary artery.

We report a rare finding of a pair of coronary artery anomalies noted on computed tomography coronary angiography in an asymptomatic 75-year-old woman. This patient had a single coronary artery that bifurcated into a left main coronary artery (with an unfavorable interarterial course) and a right coronary artery. This gave rise to a vascular ring around the atrioventricular groove. We propose that the presence of this vascular ring may have been protective against the potentially ischemic course of the single-origin coronary artery passing between the aorta and pulmonary trunk.

Comparison of radiation dose and image quality of triple-rule-out computed tomography angiography between conventional helical scanning and a strategy incorporating sequential scanning.

Triple-rule-out computed tomographic angiography (TRO CTA), performed to evaluate the coronary arteries, pulmonary arteries, and thoracic aorta, has been associated with high radiation exposure. The use of sequential scanning for coronary computed tomographic angiography reduces the radiation dose. The application of sequential scanning to TRO CTA is much less well defined. We analyzed the radiation dose and image quality from TRO CTA performed at a single outpatient center, comparing the scans from a period during which helical scanning with electrocardiographically controlled tube current modulation was used for all patients (n = 35) and after adoption of a strategy incorporating sequential scanning whenever appropriate (n = 35). Sequential scanning was able to be used for 86% of the cases. The sequential-if-appropriate strategy, compared to the helical-only strategy, was associated with a 61.6% dose decrease (mean dose-length product of 439 mGy × cm vs 1,144 mGy × cm and mean effective dose of 7.5 mSv vs 19.4 mSv, respectively, p <0.0001). Similarly, a 71.5% dose reduction occurred among the 30 patients scanned with the sequential protocol compared to the 40 patients scanned with the helical protocol using either strategy (326 mGy × cm vs 1,141 mGy × cm and 5.5 mSv vs 19.4 mSv, respectively, p <0.0001). Although the image quality did not differ between the strategies, a nonstatistically significant trend was seen toward better quality in the sequential protocol than in the helical protocol. In conclusion, approaching TRO CTA with a diagnostic strategy of sequential scanning, as appropriate, can offer a marked reduction in the radiation dose while maintaining the image quality.

The presence, characterization and prognosis of coronary plaques among patients with zero coronary calcium scores.

Patients with coronary artery calcium (CAC) scores of zero are generally considered not to have atherosclerosis. Recent studies involving computed tomography coronary angiography (CTCA) challenge this assumption. This goal of the present study is to assess the frequency, morphology, location, and the prognosis of patients with plaque detected on CTCA and zero CAC. 1,119 patients (51 ± 12 years, 52% male) with a zero CAC score during CTCA study were retrospectively identified. The CTCA studies were assessed for the presence, morphology, location and severity of all coronary plaques. All-cause mortality was assessed. The prevalence of coronary plaque was 13% (147 patients). Among the 212 plaques identified 154 (73%) were non-calcified, 28 (13%) were calcified, and 30 (14%) were of mixed morphology. Notably, ≥70% stenosis was noted among only 0.4% of all patients. ROC analysis revealed that coronary artery disease risk factors did not add to the prediction of plaque among our patients. Over a mean follow-up of 2.5 ± 0.6 years there were 4 deaths (0.4%), all in patients without coronary plaque on CTCA. The presence of coronary plaque is not uncommon among patients with zero CAC scores. These plaques were rarely associated with hemodynamically significant stenoses and were associated with an excellent prognosis. Clinical factors do not appear to be useful in predicting which patients with zero CAC scores have undetected coronary plaque.

Comparison of image quality and radiation dose of coronary computed tomographic angiography between conventional helical scanning and a strategy incorporating sequential scanning.

Radiation dose from coronary computed tomographic angiography may be decreased using a sequential scanning protocol rather than a conventional helical scanning protocol. We compared radiation dose and image quality from coronary computed tomographic angiography in a single center between an initial period during which helical scanning with electrocardiographically controlled tube current modulation was used for all patients (n = 138) and after adoption of a strategy incorporating sequential scanning whenever appropriate (n = 261). Using the sequential-if-appropriate strategy, sequential scanning was employed in 86.2% of patients. Compared to the helical-only strategy, this strategy was associated with a 65.1% dose decrease (mean dose-length product [DLP] 305.2 vs 875.1 and mean effective dose 14.9 vs 5.2 mSv, respectively), with no significant change in overall image quality, step artifacts, motion artifacts, or perceived image noise. For the 225 patients undergoing sequential scanning, the DLP was 201.9 +/- 90.0 mGy x cm; for patients undergoing helical scanning under either strategy, the DLP was 890.9 +/- 293.3 mGy x cm (p <0.0001), corresponding to mean effective doses of 3.4 and 15.1 mSv, respectively, a 77.5% decrease. Image quality was significantly greater for the sequential studies, reflecting the poorer image quality in patients undergoing helical scanning in the sequential-if-appropriate strategy. In conclusion, a sequential-if-appropriate diagnostic strategy decreases dose markedly compared to a helical-only strategy, with no significant difference in image quality.

Congenital left ventricular diverticulum diagnosed by 64-detector CT imaging.

A congenital left ventricular (LV) diverticulum or aneurysm is a rare cardiac malformation. It is a developmental anomaly that occurs during embryogenesis. Most congenital LV aneurysms and diverticula are asymptomatic or may cause systemic embolization, heart failure, valvular regurgitation, ventricular wall rupture, ventricular tachycardia or sudden cardiac death. Diagnosis is established by imaging studies such as echocardiography, magnetic resonance imaging or left ventriculography, visualizing the structural changes. We report the case of a 28-year-old male referred for the evaluation of atypical chest pain who was found to have an LV diverticulum affecting the inferoposterior wall of his LV.