R. Loader

A comparison of radiation doses between state-of-the-art multislice CT coronary angiography with iterative reconstruction, multislice CT coronary angiography with standard filtered back-projection and invasive diagnostic coronary angiography

Objective To accurately compare the radiation dose between prospectively gated cardiac multidetector CT (with and without iterative reconstruction) and diagnostic invasive coronary angiography using the latest International Commission on Radiological Protection 103 (ICRP) tissue weightings. Design, setting and patients A retrospective analysis of consecutive patients presenting to a university teaching hospital for investigation of coronary artery disease. Radiation doses for each technique were calculated using computational Monte Carlo modelling of a standard Cristy phantom rather than the application of previously published conversion factors. While these have frequently been used in other studies, they are based on out-dated ICRP tissue weightings (ICRP 60) and are for the whole chest rather than for structures irradiated in cardiac imaging. In order to allow a comparison, doses were calculated and expressed in terms of effective dose in millisieverts (mSv). Results From a population presenting for angiography within a clinical service, the median radiation dose from cardiac CT with standard filtered back-projection (84 patients, 5.4 mSv) was comparable with the dose from invasive diagnostic coronary angiography (94 patients, 6.3 mSv). The dose for cardiac CT using iterative reconstruction was significantly lower (39 patients, 2.5 mSv). Conclusion The median effective dose from cardiac CT with standard filtered back-projection was comparable with the effective dose from invasive coronary angiography, even with application of the most contemporary ICRP tissue weightings and use of cardiac specific volumes. Cardiac CT scanning incorporating iterative reconstruction resulted in a significant reduction in the effective dose.

Image Quality Assessment of Standard- and Low-Dose Chest CT Using Filtered Back Projection, Adaptive Statistical Iterative Reconstruction, and Novel Model-Based Iterative Reconstruction Algorithms

OBJECTIVE The purpose of this article is to compare image quality between filtered back projection (FBP), adaptive statistical iterative reconstruction (ASIR), and model-based iterative reconstruction (MBIR) at standard dose and two preselected low-dose scans. SUBJECTS AND METHODS Thirty patients (16 men and 14 women; mean age, 67 years) were prospectively recruited. Patients underwent three scans (one standard-dose scan and two low-dose scans at noise indexes [NIs] of 33, 60, and 70, respectively). All three scans were reconstructed with FBP, ASIR, and MBIR. Objective and subjective image qualities were compared. Dose-length products and effective doses for each scans were recorded. Mean image noise and attenuation values were compared between different reconstruction algorithms using repeated-measures analysis of variance and paired Student t tests. The interobserver variation between the two radiologists for subjective image quality and lesion assessment was estimated by using weighted kappa statistics. RESULTS Objective image analysis supports significant noise reduction with low-dose scans using the MBIR technique (p < 0.05). There was no significant change in mean CT numbers between different reconstructions (p > 0.05). Subjective analysis reveals no significant difference between image quality and diagnostic confidence between low-dose MBIR scans compared with standard-dose scans reconstructed using ASIR (p > 0.05). Average effective doses were 3.7, 1.2, and 0.9 mSv for standard scans at NIs of 33, 60, and 70, respectively. CONCLUSION MBIR shows superior noise reduction and improved image quality. Substantial dose reduction can be achieved by increasing the NI parameters as tested in this study without affecting image quality and diagnostic confidence.

Cardiac CT: are we underestimating the dose? A radiation dose study utilizing the 2007 ICRP tissue weighting factors and a cardiac specific scan volume

AIM To calculate the effective dose from cardiac multidetector computed tomography (MDCT) using a computer-based model utilizing the latest International Commission on Radiation Protection (ICRP) 103 tissue-weighting factors (2007), to compare this dose with those calculated with previously published chest conversion factors and to produce a conversion factor specific for cardiac MDCT. MATERIALS AND METHODS An observational study of 152 patients attending for cardiac MDCT as part of their usual clinical care in a university teaching hospital. The dose for each examination was calculated using the computer-based anthropomorphic ImPACT model (the imaging performance assessment of CT scanners) and this was compared with the dose derived from the dose-length product (DLP) and a chest conversion factor. RESULTS The median effective dose calculated using the ImPACT calculator (4.5 mSv) was significantly higher than the doses calculated with the chest conversion factors (2.2-3 mSv). CONCLUSION The use of chest conversion factors significantly underestimates the effective dose when compared to the dose calculated using the ImPACT calculator. A conversion factor of 0.028 would give a better estimation of the effective dose from prospectively gated cardiac MDCT.

Practical dosimetry methods for the determination of effective skin and breast dose for a modern CT system, incorporating partial irradiation and prospective cardiac gating

OBJECTIVE For CT coronary angiography (CTCA), a generic chest conversion factor returns a significant underestimate of effective dose. The aim of this manuscript is to communicate new dosimetry methods to calculate weighted CT dose index (CTDIw), effective dose, entrance surface dose (ESD) and organ dose to the breast for prospectively gated CTCA. METHODS CTDIw in 32 cm diameter Perspex phantom was measured using an adapted technique, accounting for the segmented scan characteristic. Gafchromic XRCT film (International Speciality Products, New Jersey, NJ) was used to measure the distribution and magnitude of ESD. Breast dose was measured using high sensitivity metal oxide semiconductor field-effect transistors and compared to the computer based imaging performance assessment of CT scanners (ImPACT) dosimetry calculations. RESULTS For a typical cardiac scan the mean ESD remained broadly constant (7-9 mGy) when averaged over the circumference of the Perspex phantom. Typical absorbed dose to the breast with prospectively gated protocols was within the range 2-15 mGy. The subsequent lifetime attributable risk (LAR) of cancer incidence to the breast was found at 0.01-0.06 for a 20-year-old female. This compares favourably to 100 mGy (LAR ~0.43) for a retrospectively gated CTCA. CONCLUSIONS Care must be taken when considering radiation dosimetry associated with prospectively gated scanning for CTCA and a method has been conveyed to account for this. Breast doses for prospectively gated CTCA are an order of magnitude lower than retrospectively gated scans. Optimisation of cardiac protocols is expected to show further dose reduction.

A phantom-based evaluation of three commercially available patient organ shields for computed tomography X-ray examinations in diagnostic radiology

Three commercially available in-plane patient organ shields (barium eye, bismuth eye and bismuth breast) for computed tomography (CT) examinations were evaluated to determine their effectiveness for dose reduction. Absorbed doses were measured using metal oxide semiconductor field effect transistor dosemeters fastened to a Kyoto CT Torso phantom. Resultant images were visually compared with those minus shielding by an experienced radiologist. Approximate dose reductions of 21, 38 and 50 % were achieved by the barium eye, bismuth eye and bismuth breast shields, respectively, at a cost of increased image noise and streak artefacts. Shielded images produced varied levels of image artefact, particularly those resulting from the eye shields. Measured dose reductions were not consistent with the potential dose savings stated by the manufacturers of the shields. When evaluating the breast shield, similar dose reduction was achieved without shield-induced artefact by simply reducing the X-ray tube current.

Diagnosing venous thromboembolism in pregnancy

OBJECTIVE We report the imaging outcomes of all pregnant patients referred for suspected thromboembolism over a 43-month period. METHODS We identified 168 patients who underwent ventilation/perfusion (VQ) single-photon emission CT (SPECT), CT pulmonary angiography (CTPA) or a Doppler ultrasound scan of the lower legs, as well as a control group of 89 non-pregnant age- and sex-matched patients who underwent VQ SPECT during the same period. Imaging outcomes were recorded, and radiation doses were calculated for individual patients. RESULTS VQ SPECT and CTPA were equally likely to diagnose pulmonary embolism (PE) in about one patient out of every seven patients investigated. One in three CTPA scans was of suboptimal quality. A Doppler ultrasound examination of the legs will find deep venous thrombosis much less often, in about 1 patient out of every 15 patients investigated. The prevalence of PE in pregnant patients (as diagnosed by VQ SPECT) was similar to that in the non-pregnant, age- and sex-matched control group. The effective dose and the absorbed radiation dose to the maternal breast were lower with VQ SPECT. The foetal dose is comparable for both VQ SPECT and CTPA. CONCLUSION VQ SPECT and CTPA provide a similar diagnostic yield for diagnosing PE during pregnancy, but VQ SPECT does so with a lower radiation dose to the mother (effective dose and breast dose). ADVANCES IN KNOWLEDGE Ours is the first report of the diagnostic performance of VQ SPECT, rather than planar VQ scans, in pregnancy in a routine clinical setting.

Ultra-low dose CT scanning in clinical practice? A preliminary study in 30 patients

Tuesday 11 September Proffered papers 3: Dose managment - Presenting Author: Dr Varut Vardhanabhuti

114 Radiation doses trends from cardiac CT using a cardiac specific conversion factor: system understanding & an optimisation strategy significantly reduces the dose to the patients in a clinical service

Background CT coronary angiography CTCA now has an established role in the investigation of patients with chest pain. Under the IRMER regulations radiation doses to patients should be kept as low as reasonably practical (ALARP). Previous publications have used a chest conversion factor to calculate the effective dose (mSv) from CTCA. We have previously demonstrated that chest conversion factors significantly under-estimate the effective dose to the patient when applied to CTCA and have calculated a cardiac specific conversion factor of 0.028 mSv (mGy.cm)-1. Our department follows the ALARP ethos and has implemented new technologies together with physician training to reduce the radiation dose from CTCA. We aimed to investigate what impact the implementation of new technologies has had on the radiation dose of CTCA. Method All patients who were coded as attending for a cardiac CT scan on the PACS and CRIS systems were included in the analysis. Scan indication included: rule out coronary artery disease, CABG assessment, pre-EP studies and problem solving. CT scanning between September 2007 and August 2010 was included; the total dose for the whole examination is used including the scout and non-enhanced scan (calcium score). Scans were performed on a Lightspeed VCT or HD750 (GE Healthcare). To calculate the effective dose a conversion factor was applied to the dose length product of each examination. The DLP is the radiation dose in one CT slice multiplied by the length of the scan. A cardiac specific conversion factor was used rather than a chest conversion factor (0.014) which significantly underestimates the effective dose from CTCA. Data was transformed and expressed as a geometric mean with 99% CI. For each analysis period all scans were included; retrospective, prospective, low kV and zero padding. Results In the 3-year period 1736 scans were performed. The mean radiation dose in the first 6 months of the study (retrospective gating) was 29.6 mSv; using the accepted conversion factor at the time the mean dose was 14.9 mSv. In March 2008 prospective ECG gating was installed; this resulted in a halving of the mean radiation dose to 13.6 mSv. In March 2009 the scanner parameters was set to zero padding and 100 KV reducing the dose to 7.4 mSv. For the final 6 months the mean radiation dose for a cardiac scan was 5.9 mSv; this Abstract 114 figure 1 incorporates scans performed with standard filtered back projection, iterative reconstruction, high definition scanning and retrospective ECG gating for a variety of differing clinical scenarios.Abstract 114 Figure 1 Effective dose (mSv) by protocol period. Conclusion The introduction of dose saving strategies and appropriate physician training has lead to a significant reduction in the radiation dose from cardiac CT. As CTCA programmes become established in hospitals around the UK it is important that clinicians have the appropriate training and experience to keep the radiation dose to the patients as low as reasonably practical.Abstract 114 Table 1 Scanning protocol Retrospective Gating—dose modulation Prospective gating Zero padding—100 kV Final 6 months Number of Patients 150 489 636 461 Mean Effective Dose (mSv) 29.6 13.6 7.4 5.9 CIs (99%) (mSv) 33 14.9 8 6.5 26.6 12.5 6.8 5.3