Though capable of multiple applications with high diagnostic accuracy, a major concern of cardiac CT has been the relatively high radiation exposure required for retrospective imaging, which is the most common method to obtain images.Various methods for reducing radiation dose are discussed.
by Dr Matthew Budoff
Modern cardiac CT in 2009 is capable of multiple applications with high diagnostic accuracy. Due to thinner slice acquisition (now submillimetre), more detectors (64-320) and faster gantry rotations (down to 330 milliseconds), many of the obstacles that previously plagued non-invasive imaging (CT and magnetic resonance imaging [MRI]) have been overcome. While with MRI it remains challenging to achieve adequate coronary artery imaging (accuracies of 72% for obstructive coronary artery disease [CAD]) , the diagnostic accuracy of CT has continued to improve. It is now possible to achieve sensitivities of 95% and specificities consistently above 80% for obstructive disease (as compared to invasive angiography) . Thus, common applications include ruling out CAD in patients with low to intermediate probability of obstruction, including borderline treadmill and stress imaging studies, atypical chest pain and chest pain syndromes in younger men and women.
Furthermore, calcium scoring (non-contrast studies) has received enough validation to now warrant use in asymptomatic persons at intermediate to high Framingham risk . A high score signifies a high risk patient, and a low score identifies a low risk patient. Taylor et al demonstrated that “coronary artery calcification provides substantial, cost-effective, independent prognostic value in predicting incident CHD that is incremental to measured coronary risk factors”. The final, and potentially most commonly used application is in acute chest pain syndromes. Emergency department CAD assessment (with the potential also to evaluate pulmonary embolism and aortic dissection simultaneously in patients presenting with chest pain) has been shown to be more cost- effective and expeditious than nuclear testing . Ongoing multicentre trials will evaluate prognostic implications of functional testing vs. CTA.
However, a major concern of cardiac CT has been the relatively high radiation exposure required for retrospective imaging, which is the most common method to obtain these images) . Many different methods of radiation reduction have been reported, all of which can result in complementary reductions in radiation exposure. The first to be introduced was dose modulation, which allows the power (milliamperes) to be turned down during the phases of the cardiac cycle when motion-free imaging is not possible (all of systole and end-diastole). The ideal “pulsing window”, when the current becomes maximal, is as short as possible, typically focused around the 70% phase of the cardiac cycle. The time of least motion occurs between 40% and 80% of the R-R interval (middiastole) . With dose modulation, the dose is reduced by 18 to 47% depending on the patient’s heart rate . Slower heart rates will lead to more effective use of ose-modulation and lower radiation doses in current scanners, with the exception of the dual source scanner, where slower heart rates will increase radiation exposure.
Reducing tube voltage
Another methodology to reduce the dose is to reduce tube voltage. Tube kVp affects the peak photon energy and affects image contrast. New studies suggest that protocols utilising 100 or 80 kVp may be effective in thin patients for reducing dosage, without degrading diagnostic accuracy . Tube voltage has a more dramatic effect on radiation dosage, which varies approximately with the square of the kVp. Budoff et al demonstrated that the use of 100 kVp instead of 120 kVp reduced radiation dose by 42% when using prospective triggering and 40% using retrospective imaging, (p<0.001) . Hausleiter et al showed a 53-64% reduction in estimated radiation dose using both reduced kVp and dose modulation . With 80 kVp, the radiation dose reductions may be as great as 70-80%. Of course, thin body habitus are required for low kVp use.
Limiting craniocaudal coverage length
Another technique to reduce dose is to limit the craniocaudal coverage length. It has been shown that a 30% reduction in radiation dose can be achieved by employing a calcium score to determine the superior and interior borders of the field of view, with the superior border at 1 cm above the visualised top of the coronary arteries and the inferior border at 1 cm below the posterior descending artery . The saving in radiation dose was 4 mSv, offsetting the dose delivered by calcium scoring (1.2 mSv). The average percentage reduction in radiation was 30 % (SD + 10) with a median of 22 %. This method of dose reduction is incremental to dose modulation as well as reducing the mA or kV during image acquisition. Multipurpose examinations (such as evaluating for pulmonary embolism or aortic dissection) have long Z axis acquisitions and thus a higher radiation dose. To do a “triple rule out” examination, both scan length and scan time are affected, and there is a consequent marked increase in radiation exposure. The full length of the chest may be 25-30 cm, while coverage of just the coronary arteries can typically be done in 12 cm, thus leading to 50-70% increases in radiation doses for these coronary, aortic and pulmonary combined scans.
Another simple technique is to use the lowest milliamperes (mA) during the scan. Radiation dose is approximately proportional to mA. It is important to reduce mA to the lowest necessary to make diagnostic images, without overexposing the patient; this is termed the ‘As Low As Reasonably Achievable’ [ALARA] principle. Typical doses are 350 mA for small patients, 450 mA for medium sized patients and 550 mA or higher for large patients, scaled to maximum mA of the X-ray tube for the heaviest patients.
While increased detectors are desirable to reduce collimation artifacts, this leads to increasing radiation doses. Increasing the number of detector-rows and reducing detector size tends to increase the radiation dose due to the increasing surface area of lead collimators (which can only be so thin while still being effective) in comparison to detector area. More lateral detectors require more photons, as there is less efficient delivery of dose due to scatter. Thus, 16 row scanners have lower patient radiation doses than 64 slice scanners, despite longer scan times. Subsequently, doses from 64 row scanners will be lower than 256 and 320 row scanners. Dual source scanners have double the exposure (two X-ray sources), but half the scan time, due to higher pitch or table speeds, so the net radiation is similar to 64 slice scanning.
However, the greatest reductions of dose with cardiac CT will come from changing the acquisition from retrospective (continuous imaging) to prospective (step and shoot) imaging. Prospective imaging was first introduced with Electron Beam CT angiography as early as 1995, and with calcium scoring with MDCT since 1998. However, MDCT angiography was only carried out using retrospective protocols, i.e., leaving the X-ray beam on continuously during the heart cycle, until two years ago. Prospective gating turns off the X-ray tube, moves the patient, then turns on the X-ray tube for only a short time during mid-diastole to obtain images. The X-ray beam is triggered by the ECG and is turned on only during the times of slowest coronary motion, which is usually 70-75% of the cardiac cycle. Recent studies demonstrate low radiation (1-3 mSev) with similar or superior image quality using prospective imaging . Budoff et al showed a 90% reduction in estimated radiation dose using a combination of reduced kVp (100) and prospective triggering together . A majority of patients can undergo prospective triggering with marked radiation dose reduction, if ejection fraction information or multiple phase acquisition is not necessary. Future directions to reduce radiation will include more sensitive detectors, which require fewer photons to create images, and potentially flat plate technology, which would improve image quality without requiring increased radiation doses.
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Matthew J Budoff MD FACC FAHA
Los Angeles Biomedical Research Institute at Harbor-UCLA
1124 W. Carson Street, RB2
Torrance, CA 90502, USA
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