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Radiation dose from coronary CT angiography: Five years of progress
Journal of Cardiovascular Computed Tomography (2010) 4, 365–374
Review Article
Radiation dose from coronary CT angiography: Five years of progress Gilbert L. Raff, MD, FACC, FSCCT* William Beaumont Hospital, 3601 W 13 Mile Road, Royal Oak, MI 48073, USA KEYWORDS: Angiography; Computed tomography; Coronary artery dose reduction; Radiation
Abstract. Radiation doses from coronary CT angiography have been scrutinized as a consequence of rising concern over cumulative lifetime radiation dose from diagnostic imaging and potential cancer risk. In response to this, the past 5 years have witnessed progressive refinements in CT technology and new dose reduction protocols, including electrocardiography-based tube current modulation, lower peak tube voltage, prospective or axial scanning, high-pitch spiral scanning, and iterative CT data reconstruction. As a direct result, compared with radiation exposure levels initially reported from 64-detector coronary CT angiography without dose modulation (range, 16–20 mSv), doses have decreased by approximately 50% every 2 years since 2005. Recent high-pitch spiral scan studies have documented doses %1 mSv. In routine clinical practice, registries show somewhat higher radiation dose levels, but nonetheless a similar rate of improvement with marked dose reduction enabled by dissemination of updated CT scanner technology. The current challenge is to continue the past rate of progress by incorporating research into practice and to facilitate improved technology. Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction In 1965, Gordon Moore, the founder of Intel Corporation, famously predicted that the number of transistors on a computer chip would follow an exponential rise, doubling every 2 years (Fig. 1A). Amazingly, this exponential growth has continued for .40 years. By contrast, for the past 5 years radiation exposure from coronary CT angiography has followed an inverse exponential path; doses reported by research studies have declined by approximately 50% every 2 years (Fig. 1B blue line). It is unclear how long this can
Conflict of interest: The author reports major research funding from Siemens and Blue Cross Blue Shield of Michigan. * Corresponding author. E-mail address: [email protected] Submitted August 18, 2010. Accepted for publication September 4, 2010.
be sustained, but current research reports document doses from coronary CT angiography that are lower than conventional calcium scoring. The results from multicenter clinical practice surveys are also encouraging; although doses are considerably higher than research reports, the rate of change is similar (Fig. 1B red line). The objective of this review is to explore the evidence on radiation dose from coronary CT in published research, to define the gap between research and clinical practice, and to consider how this gap can be narrowed. The cancer risk from diagnostic medical radiation engenders concern and controversy.1–7 Although this concern encompasses medical imaging in general, computed tomography lies at the center of the debate, given the rise in the use of CT imaging. In response to particularly intense scrutiny over coronary CT angiography, vigorous research and development have resulted in the clinical release of 3 generations of novel scanner technology in 5 years, each
1934-5925/$ - see front matter Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2010.09.002
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Journal of Cardiovascular Computed Tomography, Vol 4, No 6, November/December 2010 (eg, in persons with prior coronary bypass surgery). Therefore, research studies present what is ideally possible rather than what necessarily occurs in practice. However, there are now also large-scale surveys of clinical practices that include all types of scans that use a variety of techniques; some also include all ancillary phases of the examination. This review compares these clinical surveys with research findings.
Early data: 64-slice helical scanning without dose modulation
Figure 1 (A) Number of transistors per computer chip by year. Source: http://en.wikipedia.org/wiki/Moore’s_law, accessed August 18, 2010. (B) Radiation dose by year. Blue symbols indicate research reports by publication date; blue line, linear regression of reported research doses; red squares, multicenter clinical reported doses listed by year data collected; red line, linear regression of reported clinical survey doses.
incorporating improved dose reduction capability. Clinical studies have followed, testing protocols devised to optimize the use of new hardware and software. Clinical studies on new radiation reduction methods often are somewhat idealized, in that for uniformity, generally only the doses attendant to the actual coronary CT angiogram are reported. In fact, the scan protocol also includes other elements, such as a topogram, test bolus or monitoring scan, and, optionally, calcium scoring. Collectively, these elements can add 1–2 mSv of exposure. Clinical practice also includes a heterogeneous array of CT examinations, including ones in which the scan range encompasses the full thorax
The modern era of widespread clinical use of coronary CT angiography began in 2004 after the introduction of 64-slice multidetector scanners, whose improved temporal and spatial resolutions facilitated the ease and reliability of interpretation. The earliest dose figures were mostly from accuracy trials without a focus on radiation dose per se; acquisition protocols were primarily retrospectively electrocardiogram (ECG)–gated helical scans without current dose reduction methods.8–12 Helical or spiral scanning of the early generation 64-slice scanners involves continuous motion of the patient through the scanner bore with continuous image acquisition from a rotating gantry containing a single x-ray tube and opposing detector row (Fig. 2A). Such scanners contain either 64 rows of 0.5- or 0.625-mm detectors, or 32 rows of 0.625-mm detectors with double sampling of each detector per rotation. Initially, imaging was performed with maximal tube current and voltage (generally 330 mA and 120 kV, respectively) throughout the cardiac cycle, leading to relatively high radiation doses (Fig. 2B). Reconstructions from any point during the cardiac cycle (retrospectively) are enabled by pixel data assignment to time by the ECG (gating). Advantages of this technique include maximal flexibility in finding ideal reconstruction phases in instances of irregular heart rhythms and the ability to reconstruct high resolution cine loops for analysis of ventricular function. But, for most coronary imaging, only data acquired during diastole is used for coronary interpretation. Thus, exposed occurred during the entire cardiac cycle but image data from most of the cycle was not used for coronary reconstruction. The reported estimated dose reported from studies without current modulation showed an average dose of 15.7 mSv with a range from 9 to 21.4 mSv (Table 1).8–13
ECG-guided tube current modulation ECG-guided tube current modulation was the first widely available innovation in dose reduction in coronary imaging, and most clinical imaging today incorporates its use. Tube current is reduced (from 100% to 20% of maximal milliAmpere in early studies) outside of a specified ECG-gated acquisition window that is adjusted according to heart rate and heart rate stability (Fig. 3). Radiation dose varies approximately linearly with tube
Raff
Progress in radiation dose from CCTA
367 Table 2). Over the next 3 years other investigators confirmed that current modulation with heart rate–guided narrowing of the acquisition window reduced dose from a mean of 13.8 mSv to a mean of 7.7 mSv (244%; range, 4.2–13.4 mSv).14–16
Tube voltage reduction
Figure 2 (A) Retrospective or spiral scan mode. Source: Society for Cardiovascular Computed Tomography. Permission granted. (B) Retrospective scanning without dose modulation. In this mode there is continuous exposure, denoted by blue highlighting of electrocardiographic trigger panel. In this instance complete coverage of the heart requires 6 heart beats. This is related to the width of the detector array. Source: James P. Earls, MD. Permission granted.
current, so a reduction of 80% outside of the acquisition window results in a dose reduction inversely proportional to the size of the acquisition window. If acquisition heart rates are %60 beats/min, most current scanners can use a narrow (optimal) acquisition window, resulting in maximal dose reduction and preserved image quality, whereas higher rates require a larger acquisition window to insure that a phase without coronary motion can be reconstructed. Initially, current reduction was also limited to 80% by the technology of the x-ray tube, specifically how fast it could reliably ramp tube current up and down. Improvements were introduced about 1 year later in which tube current could be reduced by 96% outside the acquisition window. In 2006 Hausleiter et al13 reported a large single center retrospective study of 599 patients undergoing clinical 64slice scanning which showed an overall dose of 11.0 6 4.1 mSv. A subset analysis of 50 patient scans without dose modulation were compared with 50 scans with ECG-based tube current modulation and showed a dose reduction from 14.8 6 1.8 mSv to 9.4 6 1.0 mSv (236%;
Another dose reduction method in wide clinical use today is weight- or body mass guided reduction in tube voltage. Standard tube output in cardiac scanning is 120 kV. Because absorbed radiation dose varies approximately with the square of tube voltage, a reduction of voltage from 120 kV to 100 kV theoretically would reduce dose by 40%. In the study previously cited, another subset analysis of 30 patients with both tube current modulation and voltage reduction to 100kV showed that dose was reduced from 14.8 6 1.8 mSv to 5.4 6 1.1 mSv (264%; Table 2). Since then, multiple studies have confirmed the magnitude of this reduction, from a mean of 12.7 mSv to a mean of 5.9 mSv (253%; range, 4.4–7.8 mSv).14,17–20 Analyses of image quality support the preservation of interpretability at lower voltage, although image noise is increased. The preservation of interpretative quality is in part due to an increase in peak iodine photon absorption at 100 kV, resulting in an increase in the contrast-to-noise ratio. In clinical usage today, 100 kV is generally restricted to patients with a body mass index %30 kg/m2 or a weight under ,85 to 95 kg.
Prospective ECG-triggered scanning Improvement in the ability to rapidly change tube output as well as wider detector arrays led to the introduction of prospective ECG-triggered coronary scanning (also known as sequential, axial, or ‘‘step and shoot’’ acquisition; Fig. 4A). During sequential scanning the x-ray tube is turned entirely off outside of the acquisition window. The table advances in steps triggered by the ECG at a preset point in diastole, rather than continuously as in retrospectively gated helical scanning. Because of this step and shoot process, imaged anatomical slices do not overlap as they do in helical scanning, which contributes to dose reduction (Fig. 4B). There are disadvantages to sequential scanning: acquisition generally allows fewer phases of image reconstruction to deal with coronary motion, and originally no functional information was available because only diastole was acquired. Initially, even more rigid control of heart rate was required to avoid blurring (,62 beats/min). At higher heart rates, an optional wider acquisition window can be used (‘‘padding’’), but this does increase dose.21 Recently, new scanners with higher temporal resolution (75 milliseconds) have become available that allow prospective scanning at heart rates up to 75 beats/min.22,23 Additional end-systolic and end-diastolic images are now an option on some scanners to routinely calculate ejection fraction.
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Table 1
Radiation doses from coronary CT angiography
Author
Year
Raff et al8
2005 2005 2005 2005 2006 2006 2006 2006 2006 2006 2007 2007 2007 2007 2007 2008
Mollet et al9 Nikolaou et al10 Pugliese et al11 Hausleiter et al13
Muhlenbruch et al12 Husmann49 Hausleiter et al46 Raff et al47
Patient, total no. 70 52 72 35 599 150 51 41 1965 4995
Earls et al24 Gutstein et al14
2008 2008
203 118
Halliburton50 Hirai et al25 Murayama et al26 Leschka et al17
2008 2008 2008 2008 2008 2008 2008 2008 2008
284 60 173 80
2009 2009 2009 2009 2009 2009 2009 2009 2009 2009 2009 2009
60 200 321 312 30 25 399 514 42 70 200
2009 2009 2009 2010 2010 2010 2010 2010 2010 2010 2010
200 436 60 50 100 101 685 50 86 103 14
Shuman51 Stolzmann52 Stolzmann53 Weustink et al15 Weustink et al15 Weustink et al15 Arnoldi et al16 Arnoldi et al16 Baumiller54 Bischoff et al18 Blankstein et al28 Dewey et al34 Efstathopoulos et al29 Einstein et al30 Hein55 Herzog56 Herzog57 Hoe et al35 Pflederer et al19 Weustink58 Zhao et al31 Achenbach et al36 Alkhadi et al37 Bamberg59 Bischoff60 Carrascosa61 DeFrance et al33 Feuchtner62 Hausleiter
100 80 130 301
Method A Men, retro, 120 kV Women, retro, 120 kV Men, retro, 120 kV Women, retro, 120 kV Retro, 120 kV Men, retro, 120 kV Women, retro, 120 kV Retro, 100 or 120 kV Retro, 120 kV, no EKG mod Retro, 120 kV, no EKG mod Men, retro, 120 kV Women, retro, 120 kV Pro Registry pro or retro Registry retro baseline
Retro, 120 kV Wide window, 120 kV Wide window, 120 kV Single source, retro Retro, 120 kV Retro, 120 kV Retro, 120 kV, 330 mA Retro, 120 kV, 330 mA Retro, 100 or 120 kV Retro, 120 kV, 20% pulsing Pro, 120 kV Low HR, usual window Inter HR, usual window High HR, usual window Retro, 120 kV, wide window Retro, 120 kV, wide window Single source, 120 kV Retro, 120 kV Retro, 100 or 120 kV 320 Slice, single beat 256 Slice, retro, 120 kV Retro, 100 or 120 kV Retro, 100 or 120 kV Invasive angiography Pro, 100 or 120 kV 320 Slice, pro, 3 beats 320 Slice, pro, 3 beats Retro, 120 kV Retro, wide window Retro, 100 or 120 kV High-pitch, 100 kV Pro, 100 kV Retro, diastolic Retro, 100 or 120 kV Invasive, angiography Retro, 100–135 kV Retro, 120 kV High pitch, 100 or 120 kV
Dose, mSv 13 18 15.2 21.4 9 15 20 11.0 14.8 14.8 13.6 17.3 2.1 12 21.0 21.0 18.4 12.8 12.8 10.9 20.0 21.1 8.9 8.9 18.1 8.8 2.6 18.7 14.7 11.3 9 9 10.7 14 13.4 3.2 13.4 15.1 7.6 8.5 2.1 19.5 19.5 12.7 14.2 14.6 0.87 1.4 9.4 11.2 6.9 16.9 13.4 2.0
Method B
Dose, mSv
Difference, %
Retro, 120 kV with EKG mod Retro, 100 kV
9.4 5.4
236 264
Registry after 1 year of training Pro, 120 kV Narrow window, 120 kV Narrow window, 100 kV Dual source, retro Pro, 120 kV Pro, 120 kV Retro, 100 kV, 330 mA Retro, 100 kV, 220 mA Pro, 100 or 120 kV Retro, 120 kV, 4% pulsing Pro, 100 kV Optimal window Optimal window Optimal window Retro, 120 kV, narrow window Pro, 120 kV Dual source, 120 kV Retro, 100 kV Pro, 100 or 120 kV Invasive angiography 256 Slice, pro, 120 kV Pro, 100 or 120 kV Pro, 100 or 120 kV Pro, 100 or 120 kV
9.9
253
2.8 7.3 4.4 11.7 4.1 4.3 6.7 4.4 4.4 7.8 1.2 6.8 13.4 4.2 7 3 10.9 6 3.2 8.5 3.2 3.4 3.4 2.1
283 243 266 7 279 280 225 251 276 211 254 264 29 263 222 266 2 253 290 262 276 278 263 275
320 Slice, pro, 2 beats 320 Slice, pro, 1 beat Retro, 100 kV Retro, optimal window Pro, 100 or 120 kV
13.0 5.7 7.8 10.7 2.2
233 271 239 225 285
0.9 5.0 3.6 3.4 6.9 7.1
236 247 268 251 259 247
High-pitch, 100 kV Retro, systolic Pro, 100 or 120 kV Pro or retro, 100–120 kV Helical, pro, 100–135 kV Retro, 100 kV
Research data were listed by year published; multicenter clinical data were listed by year collected. Retro, retrospective gated scanning; pro, prospective triggered scanning; HR, heart rate; inter, intermediate.
30 21 (70) 25.662.3 57.068.2 122.0617.7 537.8650.7† 0.260 22.061.8† 50.0610.4† 7.061.9 9.262.5 5.461.1† 50 36 (72) 26.664.7 57.567.2 123.9611.8 551.0658.2† 0.260 38.363.1† 37.768.6 6.762.3 9.262.8 9.461.0†
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Figure 3 ECG-based tube current modulation. Tube current is reduced from 100% of maximal tube current to 20% outside of a prespecified window timed by the previous ECG R wave. Current level is denoted by the green line. Source: Society for Cardiovascular Computed Tomography. Permission granted.
CTDI, CT dose index; ECG, electrocardiographic. *P , 0.025 for comparison with 16-slice CT scanning protocol with 120 kV without ECG-dependent dose modulation. † P , 0.025 for comparison with 64-slice CT scanning protocol with 120 kV without ECG-dependent dose modulation.
50 34 (68) 26.462.9 57.865.3 124.067.7 387.6618.9* 0.21 60.02* 19.461.0* 36.969.4* 8.862.9 11.963.7 5.060.3* No. of patients Male sex, n (%) Body mass index, kg/m2, mean 6 SD Heart rate, beats/min, mean 6 SD Scan length, mm, mean 6 SD Tube current, mA, mean 6 SD Pitch, mean 6 SD CTDIvol, Gy, mean 6 SD Image noise, HU, mean 6 SD Contrast-to-noise ratio, mean 6 SD Signal-to-noise ratio, mean 6 SD Dose estimate, mSv, mean 6 SD
30 20 (67) 26.963.2 61.3111.3 128.2611.8 510.0640.3 0.1860.01 42.1 63.6 29.366.9 7.363.1 11.1 63.9 10.661.2
50 33 (66) 27.564.5 60.769.5 125.969.2 304.5642.3* 0.1860.01 25.262.9* 28.366.8 8.1 63.4 11.964.3 6.460.9*
50 34 (68) 26.263.2 60.1 610.4 125.9612.5 870.0655.6 0.260 58.866.3 39.2610.2 6.462.1 8.962.5 14.861.8
100 kV with dose modulation 120 kV without dose modulation 100 kV with dose modulation 120 kV with dose modulation 120 kV without dose modulation
16-Slice CT
Subgroup of patients studied with different scanning protocols Table 2
120 kV with dose modulation
Progress in radiation dose from CCTA
64-Slice CT
Raff
Because of clinically practical dose reduction of .50%, above and beyond the reduction with the use of 100 kV, prospective coronary scanning has generated a great deal of interest in the cardiac CT community. At least 11 papers have been published in the past 3 years directly comparing retrospective with prospective acquisition with a wide variety of scanners, showing an average estimated dose with retrospective of 15.6–3.7 mSv with prospective imaging (276%; range. 2.2–6.9 mSv).16,24–33
Wide detector array scanning Wide detector scanners with 256 or 320 detector rows include 16 cm longitudinal coverage per gantry rotation. These enable single-beat coverage of the heart at low, stable heart rates. Because of the brevity of scan time, radiation is significantly reduced with 2 studies reporting doses of 3.2 mSv and 5.7 mSv.34,35 However, when heart rates are higher, 2 or 3 beats may be required to complete imaging, and dose is considerably greater, 13.0 mSv and 19.5 mSv, respectively.25
High-pitch dual-source spiral scanning Recently, studies have reported coronary CT angiographic acquisitions with doses at or near 1 mSv (mean, 1.3 mSv; range, 0.87–2.0 mSv) using new scanner technology that incorporates a novel high-pitch spiral technique (pitch 3.4 compared with approximately 0.2 in conventional scanners).22,36–39 Pitch is the ratio of the table speed to the effective detector width, so patients move through the scanner rapidly enough to complete imaging in a single heartbeat. The technique depends on a combination of
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Journal of Cardiovascular Computed Tomography, Vol 4, No 6, November/December 2010 ‘‘optimum’’ image solution is found. The end point of this process is an image with less noise, which can be applied either to improve image quality in obese patients, for example, or to provide diagnostic quality at a lower tube output, which reduces dose. This method has been successfully applied in abdominal and thoracic imaging, and sub-1 mSv coronary CT angiography has been reported.41 There are potential drawbacks; overprocessing of images can result in scans with artifacts or smoothing that obscures subtle lesions. For this reason ASIR percentage is adjustable (Fig. 6). Accuracy studies have not yet confirmed the reliability of this new technology.
Multicenter clinical research In 2009, 2 studies published large-scale multicenter data on coronary CT angiogram radiation doses acquired in clinical practice. In both cases data were collected in 2007; hence, these studies reflect the CT equipment and methods widely in use at that time rather than those reported in contemporaneous 2009 research reports. The Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice (PROTECTION I) was
Figure 4 (A) Prospective or spiral scan mode. Source: Society for Cardiovascular Computed Tomography. Permission granted. (B) Prospectively triggered scanning. In this mode there is intermittent table motion, triggered by the ECG R wave of the oncoming beat. Prospective mode is facilitated by a wider detector array. In this case imaging is completed in 3 heart beats. Source: James P. Earls. Permission granted.
dual x-ray tubes, more rapid gantry rotation (280 milliseconds), and proprietary reconstruction algorithms (Fig. 5). At present, reliable heart rates %60 beats/min are necessary to avoid motion artifacts, so high-dose b-blockers are required for most patients.
Iterative reconstruction Adaptive statistical iterative reconstruction (ASIR) is a novel reconstruction technique with the potential for reducing coronary CT angiographic dose by reducing image noise.23,40–45 Conventional CT image reconstruction relies on an algorithm called filtered back projection for 3-dimensional reconstruction of raw pixel data. ASIR uses the initial filtered back projection to create a model reconstruction, and then repeated reconstruction iterations are produced until an
Figure 5 High-pitch spiral scanning. In this mode 2 x-ray tubes and detector arrays rotate around the patient during continuous table motion at a high pitch. Scanning is accomplished during a single heart beat. Source: James P. Earls. Permission granted.
Raff
Progress in radiation dose from CCTA
371
Figure 6 Adaptive statistical iterative reconstruction. Iterative reconstruction uses a feedback loop to reduce image noise. To avoid overprocessing of images, the extent of image data subject to iteration can be adjusted. FBP indicates femoral blood pressure. Source: James P. Earls, MD. Permission granted.
an international observational trial conducted at 50 sites, including university and community hospitals. A total of 1665 patient scans were surveyed; the median effective dose for all patients (reporting only the dose from the coronary angiographic image) was 12 mSv (25%–75% range, 8–18 mSv; Fig. 1A). Substantial variation was seen between sites, with a range of median doses from 5 to 30 mSv. Multivariate analysis showed that the use of ECGguided dose modulation, reduced (100 kV) tube voltage, and prospective ECG-triggered scanning were all significant independent predictors of reduced dose.46 The Advanced Cardiovascular Imaging Consortium, a collaborative quality improvement registry sponsored by Blue Cross Blue Shield of Michigan, reported dose levels from a total of 4995 scans performed at 15 hospital sites, 620 of which were performed during a control period from July to August 2007, and 835 from a postintervention period 1 year later.47 The median effective dose from the control period was 21 mSv (25%–75% range, 12–26 mSv). There are notable differences in the way doses were reported in that study; the dose from all phases of the examination were included (eg, topogram, monitoring doses, calcium scoring) as well as full-thorax studies (coronary bypass graft and ‘‘triple rule out’’ scans). Importantly, none of the scans during the control period used 100-kV tube voltage or prospective gating, and most were done without ECG-guided tube current modulation. During an
8-month interventional period, a ‘‘best-practice’’ algorithm was used to train physicians and staff at member sites (Table 3). Equipment for prospective scanning was not available at any site during the study. At the end of the intervention, data were collected from 835 scans between May and June 2008. The median dose had fallen from 21.4 to 9.8 mSv (25%–75% range, 6–16 mSv), a 53% reduction. Importantly, a semiquantitative image quality scale showed revealed no deterioration of image quality during this dose reduction. Multivariable analysis confirmed the findings of PROTECTION I: ECG-guided current modulation and use of 100 kV scanning were independent predictors of lower dose. Furthermore, scan volume per site and patient heart rate were also independent predictors of reduced dose. The investigators concluded that best practice guidelines and collaborative registries hasten the adoption of dose reduction methods, and efforts are under way to extend this approach beyond the Michigan registry. The Consortium Investigators recently reported 1-year follow-up data from the third quarter of 2009, showing that the Consortium centers, comprising 34 contributing sites, had maintained a reduced median dose range of 9.9 mSv. However, during the fourth quarter of 2009 median dose fell to 8.4 mSv, coincident with an increase in the use of prospective or highpitch spiral scan methods from 24% to 56% of all studies.48 This finding suggests that clinical implementation of novel low-dose imaging protocols is increasing.
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Table 3
Best-practice model for acquisition guidelines
Journal of Cardiovascular Computed Tomography, Vol 4, No 6, November/December 2010
Instructions to the patient and ordering physicians: At the time of scheduling, the patient should be instructed to avoid solid food or caffeine within 6 hours of the scan, but to take oral fluids liberally to avoid dehydration, and to continue taking b-blocking drugs; these measures encourage heart rate stability. Medical history: If the patient has taken b-blockers before arrival, subsequent doses should be adjusted. Nursing assessment: Vital signs, including heart rate, heart rate variability, and blood pressure, are assessed and monitored during the premedication period. Administration of b-blockers: For most current retrospective, gated-acquisition protocols, including dual-source scanners, heart rate control reduces radiation dose by allowing a more narrow acquisition window. For patients with baseline heart rates .65 beats/min, systolic blood pressure .90 mm Hg, and body mass index .18: administer 100 mg of oral metoprolol or comparable dose equivalent 30 minutes to 1 hour before the procedure, or comparable intravenous doses with telemetric monitoring. For patients with baseline heart rates .50 beats/min but ,65 beats/min and blood pressure .90 mm Hg, administer 50 mg of oral metoprolol to block heart rate acceleration during scan. Nitroglycerin administration: If blood pressure is .90 mm Hg, nitroglycerin generally improves image quality, yielding a lower frequency of repeat scans. Protocol parameters: Because radiation dose is directly proportional to z axis scan length, the field of view should be consistently restricted to midpulmonary artery to the diaphragm; extended field of view triple rule-out scans should be limited to patients with clinical likelihood of either a pulmonary embolus or aortic dissection, based on generally accepted diagnostic criteria. A simple rule is used to reduce scan voltage from the standard 120 kVp; 100 kVp may be substituted in patients with a body weight of %85 kg and a body mass index ,30 kg/m2, subject to physician discretion. Tube current modulation by electrocardiographic pulsing should be used in all patients unless atrial fibrillation or frequent premature contractions are present. Acquisition window: For scanners with adjustable acquisition windows during electrocardiographic pulsing, the following adjustments are recommended: Heart rate ,65 beats/min: 65%–75% Heart rate 66–70 beats/min: 60%–80% Heart rate .70 beats/min: 35%–80% Highly variable heart rates or atrial fibrillation may preclude the use of electrocardiographic dose modulation, but this should be weighed against the patient’s age and the suitability of other diagnostic options. If scanner model allows tube current adjustment, the lowest available tube current outside the window should be used (eg, 5% of maximal); this is equally applicable in obese patients because the data during systole will not be used. It was agreed that calcium scoring should be done only if specifically ordered by the referring physician as opposed to on all patients because this adds approximately 0.9–2.0 mSv of radiation.
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Conclusions Scanner manufacturers, researchers, and clinicians performing coronary CT angiography have taken the challenge of radiation dose reduction seriously. There has been extraordinary research and clinical progress over the past 5 years, with research and clinical doses falling by as much as 50% every 2 years. Our challenge is to extend this trend by incorporating new research advances into clinical practice and by facilitating continued technologic advances.
15.
16.
17.
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Review Article
Radiation dose from coronary CT angiography: Five years of progress Gilbert L. Raff, MD, FACC, FSCCT* William Beaumont Hospital, 3601 W 13 Mile Road, Royal Oak, MI 48073, USA KEYWORDS: Angiography; Computed tomography; Coronary artery dose reduction; Radiation
Abstract. Radiation doses from coronary CT angiography have been scrutinized as a consequence of rising concern over cumulative lifetime radiation dose from diagnostic imaging and potential cancer risk. In response to this, the past 5 years have witnessed progressive refinements in CT technology and new dose reduction protocols, including electrocardiography-based tube current modulation, lower peak tube voltage, prospective or axial scanning, high-pitch spiral scanning, and iterative CT data reconstruction. As a direct result, compared with radiation exposure levels initially reported from 64-detector coronary CT angiography without dose modulation (range, 16–20 mSv), doses have decreased by approximately 50% every 2 years since 2005. Recent high-pitch spiral scan studies have documented doses %1 mSv. In routine clinical practice, registries show somewhat higher radiation dose levels, but nonetheless a similar rate of improvement with marked dose reduction enabled by dissemination of updated CT scanner technology. The current challenge is to continue the past rate of progress by incorporating research into practice and to facilitate improved technology. Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction In 1965, Gordon Moore, the founder of Intel Corporation, famously predicted that the number of transistors on a computer chip would follow an exponential rise, doubling every 2 years (Fig. 1A). Amazingly, this exponential growth has continued for .40 years. By contrast, for the past 5 years radiation exposure from coronary CT angiography has followed an inverse exponential path; doses reported by research studies have declined by approximately 50% every 2 years (Fig. 1B blue line). It is unclear how long this can
Conflict of interest: The author reports major research funding from Siemens and Blue Cross Blue Shield of Michigan. * Corresponding author. E-mail address: [email protected] Submitted August 18, 2010. Accepted for publication September 4, 2010.
be sustained, but current research reports document doses from coronary CT angiography that are lower than conventional calcium scoring. The results from multicenter clinical practice surveys are also encouraging; although doses are considerably higher than research reports, the rate of change is similar (Fig. 1B red line). The objective of this review is to explore the evidence on radiation dose from coronary CT in published research, to define the gap between research and clinical practice, and to consider how this gap can be narrowed. The cancer risk from diagnostic medical radiation engenders concern and controversy.1–7 Although this concern encompasses medical imaging in general, computed tomography lies at the center of the debate, given the rise in the use of CT imaging. In response to particularly intense scrutiny over coronary CT angiography, vigorous research and development have resulted in the clinical release of 3 generations of novel scanner technology in 5 years, each
1934-5925/$ - see front matter Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2010.09.002
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Journal of Cardiovascular Computed Tomography, Vol 4, No 6, November/December 2010 (eg, in persons with prior coronary bypass surgery). Therefore, research studies present what is ideally possible rather than what necessarily occurs in practice. However, there are now also large-scale surveys of clinical practices that include all types of scans that use a variety of techniques; some also include all ancillary phases of the examination. This review compares these clinical surveys with research findings.
Early data: 64-slice helical scanning without dose modulation
Figure 1 (A) Number of transistors per computer chip by year. Source: http://en.wikipedia.org/wiki/Moore’s_law, accessed August 18, 2010. (B) Radiation dose by year. Blue symbols indicate research reports by publication date; blue line, linear regression of reported research doses; red squares, multicenter clinical reported doses listed by year data collected; red line, linear regression of reported clinical survey doses.
incorporating improved dose reduction capability. Clinical studies have followed, testing protocols devised to optimize the use of new hardware and software. Clinical studies on new radiation reduction methods often are somewhat idealized, in that for uniformity, generally only the doses attendant to the actual coronary CT angiogram are reported. In fact, the scan protocol also includes other elements, such as a topogram, test bolus or monitoring scan, and, optionally, calcium scoring. Collectively, these elements can add 1–2 mSv of exposure. Clinical practice also includes a heterogeneous array of CT examinations, including ones in which the scan range encompasses the full thorax
The modern era of widespread clinical use of coronary CT angiography began in 2004 after the introduction of 64-slice multidetector scanners, whose improved temporal and spatial resolutions facilitated the ease and reliability of interpretation. The earliest dose figures were mostly from accuracy trials without a focus on radiation dose per se; acquisition protocols were primarily retrospectively electrocardiogram (ECG)–gated helical scans without current dose reduction methods.8–12 Helical or spiral scanning of the early generation 64-slice scanners involves continuous motion of the patient through the scanner bore with continuous image acquisition from a rotating gantry containing a single x-ray tube and opposing detector row (Fig. 2A). Such scanners contain either 64 rows of 0.5- or 0.625-mm detectors, or 32 rows of 0.625-mm detectors with double sampling of each detector per rotation. Initially, imaging was performed with maximal tube current and voltage (generally 330 mA and 120 kV, respectively) throughout the cardiac cycle, leading to relatively high radiation doses (Fig. 2B). Reconstructions from any point during the cardiac cycle (retrospectively) are enabled by pixel data assignment to time by the ECG (gating). Advantages of this technique include maximal flexibility in finding ideal reconstruction phases in instances of irregular heart rhythms and the ability to reconstruct high resolution cine loops for analysis of ventricular function. But, for most coronary imaging, only data acquired during diastole is used for coronary interpretation. Thus, exposed occurred during the entire cardiac cycle but image data from most of the cycle was not used for coronary reconstruction. The reported estimated dose reported from studies without current modulation showed an average dose of 15.7 mSv with a range from 9 to 21.4 mSv (Table 1).8–13
ECG-guided tube current modulation ECG-guided tube current modulation was the first widely available innovation in dose reduction in coronary imaging, and most clinical imaging today incorporates its use. Tube current is reduced (from 100% to 20% of maximal milliAmpere in early studies) outside of a specified ECG-gated acquisition window that is adjusted according to heart rate and heart rate stability (Fig. 3). Radiation dose varies approximately linearly with tube
Raff
Progress in radiation dose from CCTA
367 Table 2). Over the next 3 years other investigators confirmed that current modulation with heart rate–guided narrowing of the acquisition window reduced dose from a mean of 13.8 mSv to a mean of 7.7 mSv (244%; range, 4.2–13.4 mSv).14–16
Tube voltage reduction
Figure 2 (A) Retrospective or spiral scan mode. Source: Society for Cardiovascular Computed Tomography. Permission granted. (B) Retrospective scanning without dose modulation. In this mode there is continuous exposure, denoted by blue highlighting of electrocardiographic trigger panel. In this instance complete coverage of the heart requires 6 heart beats. This is related to the width of the detector array. Source: James P. Earls, MD. Permission granted.
current, so a reduction of 80% outside of the acquisition window results in a dose reduction inversely proportional to the size of the acquisition window. If acquisition heart rates are %60 beats/min, most current scanners can use a narrow (optimal) acquisition window, resulting in maximal dose reduction and preserved image quality, whereas higher rates require a larger acquisition window to insure that a phase without coronary motion can be reconstructed. Initially, current reduction was also limited to 80% by the technology of the x-ray tube, specifically how fast it could reliably ramp tube current up and down. Improvements were introduced about 1 year later in which tube current could be reduced by 96% outside the acquisition window. In 2006 Hausleiter et al13 reported a large single center retrospective study of 599 patients undergoing clinical 64slice scanning which showed an overall dose of 11.0 6 4.1 mSv. A subset analysis of 50 patient scans without dose modulation were compared with 50 scans with ECG-based tube current modulation and showed a dose reduction from 14.8 6 1.8 mSv to 9.4 6 1.0 mSv (236%;
Another dose reduction method in wide clinical use today is weight- or body mass guided reduction in tube voltage. Standard tube output in cardiac scanning is 120 kV. Because absorbed radiation dose varies approximately with the square of tube voltage, a reduction of voltage from 120 kV to 100 kV theoretically would reduce dose by 40%. In the study previously cited, another subset analysis of 30 patients with both tube current modulation and voltage reduction to 100kV showed that dose was reduced from 14.8 6 1.8 mSv to 5.4 6 1.1 mSv (264%; Table 2). Since then, multiple studies have confirmed the magnitude of this reduction, from a mean of 12.7 mSv to a mean of 5.9 mSv (253%; range, 4.4–7.8 mSv).14,17–20 Analyses of image quality support the preservation of interpretability at lower voltage, although image noise is increased. The preservation of interpretative quality is in part due to an increase in peak iodine photon absorption at 100 kV, resulting in an increase in the contrast-to-noise ratio. In clinical usage today, 100 kV is generally restricted to patients with a body mass index %30 kg/m2 or a weight under ,85 to 95 kg.
Prospective ECG-triggered scanning Improvement in the ability to rapidly change tube output as well as wider detector arrays led to the introduction of prospective ECG-triggered coronary scanning (also known as sequential, axial, or ‘‘step and shoot’’ acquisition; Fig. 4A). During sequential scanning the x-ray tube is turned entirely off outside of the acquisition window. The table advances in steps triggered by the ECG at a preset point in diastole, rather than continuously as in retrospectively gated helical scanning. Because of this step and shoot process, imaged anatomical slices do not overlap as they do in helical scanning, which contributes to dose reduction (Fig. 4B). There are disadvantages to sequential scanning: acquisition generally allows fewer phases of image reconstruction to deal with coronary motion, and originally no functional information was available because only diastole was acquired. Initially, even more rigid control of heart rate was required to avoid blurring (,62 beats/min). At higher heart rates, an optional wider acquisition window can be used (‘‘padding’’), but this does increase dose.21 Recently, new scanners with higher temporal resolution (75 milliseconds) have become available that allow prospective scanning at heart rates up to 75 beats/min.22,23 Additional end-systolic and end-diastolic images are now an option on some scanners to routinely calculate ejection fraction.
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Table 1
Radiation doses from coronary CT angiography
Author
Year
Raff et al8
2005 2005 2005 2005 2006 2006 2006 2006 2006 2006 2007 2007 2007 2007 2007 2008
Mollet et al9 Nikolaou et al10 Pugliese et al11 Hausleiter et al13
Muhlenbruch et al12 Husmann49 Hausleiter et al46 Raff et al47
Patient, total no. 70 52 72 35 599 150 51 41 1965 4995
Earls et al24 Gutstein et al14
2008 2008
203 118
Halliburton50 Hirai et al25 Murayama et al26 Leschka et al17
2008 2008 2008 2008 2008 2008 2008 2008 2008
284 60 173 80
2009 2009 2009 2009 2009 2009 2009 2009 2009 2009 2009 2009
60 200 321 312 30 25 399 514 42 70 200
2009 2009 2009 2010 2010 2010 2010 2010 2010 2010 2010
200 436 60 50 100 101 685 50 86 103 14
Shuman51 Stolzmann52 Stolzmann53 Weustink et al15 Weustink et al15 Weustink et al15 Arnoldi et al16 Arnoldi et al16 Baumiller54 Bischoff et al18 Blankstein et al28 Dewey et al34 Efstathopoulos et al29 Einstein et al30 Hein55 Herzog56 Herzog57 Hoe et al35 Pflederer et al19 Weustink58 Zhao et al31 Achenbach et al36 Alkhadi et al37 Bamberg59 Bischoff60 Carrascosa61 DeFrance et al33 Feuchtner62 Hausleiter
100 80 130 301
Method A Men, retro, 120 kV Women, retro, 120 kV Men, retro, 120 kV Women, retro, 120 kV Retro, 120 kV Men, retro, 120 kV Women, retro, 120 kV Retro, 100 or 120 kV Retro, 120 kV, no EKG mod Retro, 120 kV, no EKG mod Men, retro, 120 kV Women, retro, 120 kV Pro Registry pro or retro Registry retro baseline
Retro, 120 kV Wide window, 120 kV Wide window, 120 kV Single source, retro Retro, 120 kV Retro, 120 kV Retro, 120 kV, 330 mA Retro, 120 kV, 330 mA Retro, 100 or 120 kV Retro, 120 kV, 20% pulsing Pro, 120 kV Low HR, usual window Inter HR, usual window High HR, usual window Retro, 120 kV, wide window Retro, 120 kV, wide window Single source, 120 kV Retro, 120 kV Retro, 100 or 120 kV 320 Slice, single beat 256 Slice, retro, 120 kV Retro, 100 or 120 kV Retro, 100 or 120 kV Invasive angiography Pro, 100 or 120 kV 320 Slice, pro, 3 beats 320 Slice, pro, 3 beats Retro, 120 kV Retro, wide window Retro, 100 or 120 kV High-pitch, 100 kV Pro, 100 kV Retro, diastolic Retro, 100 or 120 kV Invasive, angiography Retro, 100–135 kV Retro, 120 kV High pitch, 100 or 120 kV
Dose, mSv 13 18 15.2 21.4 9 15 20 11.0 14.8 14.8 13.6 17.3 2.1 12 21.0 21.0 18.4 12.8 12.8 10.9 20.0 21.1 8.9 8.9 18.1 8.8 2.6 18.7 14.7 11.3 9 9 10.7 14 13.4 3.2 13.4 15.1 7.6 8.5 2.1 19.5 19.5 12.7 14.2 14.6 0.87 1.4 9.4 11.2 6.9 16.9 13.4 2.0
Method B
Dose, mSv
Difference, %
Retro, 120 kV with EKG mod Retro, 100 kV
9.4 5.4
236 264
Registry after 1 year of training Pro, 120 kV Narrow window, 120 kV Narrow window, 100 kV Dual source, retro Pro, 120 kV Pro, 120 kV Retro, 100 kV, 330 mA Retro, 100 kV, 220 mA Pro, 100 or 120 kV Retro, 120 kV, 4% pulsing Pro, 100 kV Optimal window Optimal window Optimal window Retro, 120 kV, narrow window Pro, 120 kV Dual source, 120 kV Retro, 100 kV Pro, 100 or 120 kV Invasive angiography 256 Slice, pro, 120 kV Pro, 100 or 120 kV Pro, 100 or 120 kV Pro, 100 or 120 kV
9.9
253
2.8 7.3 4.4 11.7 4.1 4.3 6.7 4.4 4.4 7.8 1.2 6.8 13.4 4.2 7 3 10.9 6 3.2 8.5 3.2 3.4 3.4 2.1
283 243 266 7 279 280 225 251 276 211 254 264 29 263 222 266 2 253 290 262 276 278 263 275
320 Slice, pro, 2 beats 320 Slice, pro, 1 beat Retro, 100 kV Retro, optimal window Pro, 100 or 120 kV
13.0 5.7 7.8 10.7 2.2
233 271 239 225 285
0.9 5.0 3.6 3.4 6.9 7.1
236 247 268 251 259 247
High-pitch, 100 kV Retro, systolic Pro, 100 or 120 kV Pro or retro, 100–120 kV Helical, pro, 100–135 kV Retro, 100 kV
Research data were listed by year published; multicenter clinical data were listed by year collected. Retro, retrospective gated scanning; pro, prospective triggered scanning; HR, heart rate; inter, intermediate.
30 21 (70) 25.662.3 57.068.2 122.0617.7 537.8650.7† 0.260 22.061.8† 50.0610.4† 7.061.9 9.262.5 5.461.1† 50 36 (72) 26.664.7 57.567.2 123.9611.8 551.0658.2† 0.260 38.363.1† 37.768.6 6.762.3 9.262.8 9.461.0†
369
Figure 3 ECG-based tube current modulation. Tube current is reduced from 100% of maximal tube current to 20% outside of a prespecified window timed by the previous ECG R wave. Current level is denoted by the green line. Source: Society for Cardiovascular Computed Tomography. Permission granted.
CTDI, CT dose index; ECG, electrocardiographic. *P , 0.025 for comparison with 16-slice CT scanning protocol with 120 kV without ECG-dependent dose modulation. † P , 0.025 for comparison with 64-slice CT scanning protocol with 120 kV without ECG-dependent dose modulation.
50 34 (68) 26.462.9 57.865.3 124.067.7 387.6618.9* 0.21 60.02* 19.461.0* 36.969.4* 8.862.9 11.963.7 5.060.3* No. of patients Male sex, n (%) Body mass index, kg/m2, mean 6 SD Heart rate, beats/min, mean 6 SD Scan length, mm, mean 6 SD Tube current, mA, mean 6 SD Pitch, mean 6 SD CTDIvol, Gy, mean 6 SD Image noise, HU, mean 6 SD Contrast-to-noise ratio, mean 6 SD Signal-to-noise ratio, mean 6 SD Dose estimate, mSv, mean 6 SD
30 20 (67) 26.963.2 61.3111.3 128.2611.8 510.0640.3 0.1860.01 42.1 63.6 29.366.9 7.363.1 11.1 63.9 10.661.2
50 33 (66) 27.564.5 60.769.5 125.969.2 304.5642.3* 0.1860.01 25.262.9* 28.366.8 8.1 63.4 11.964.3 6.460.9*
50 34 (68) 26.263.2 60.1 610.4 125.9612.5 870.0655.6 0.260 58.866.3 39.2610.2 6.462.1 8.962.5 14.861.8
100 kV with dose modulation 120 kV without dose modulation 100 kV with dose modulation 120 kV with dose modulation 120 kV without dose modulation
16-Slice CT
Subgroup of patients studied with different scanning protocols Table 2
120 kV with dose modulation
Progress in radiation dose from CCTA
64-Slice CT
Raff
Because of clinically practical dose reduction of .50%, above and beyond the reduction with the use of 100 kV, prospective coronary scanning has generated a great deal of interest in the cardiac CT community. At least 11 papers have been published in the past 3 years directly comparing retrospective with prospective acquisition with a wide variety of scanners, showing an average estimated dose with retrospective of 15.6–3.7 mSv with prospective imaging (276%; range. 2.2–6.9 mSv).16,24–33
Wide detector array scanning Wide detector scanners with 256 or 320 detector rows include 16 cm longitudinal coverage per gantry rotation. These enable single-beat coverage of the heart at low, stable heart rates. Because of the brevity of scan time, radiation is significantly reduced with 2 studies reporting doses of 3.2 mSv and 5.7 mSv.34,35 However, when heart rates are higher, 2 or 3 beats may be required to complete imaging, and dose is considerably greater, 13.0 mSv and 19.5 mSv, respectively.25
High-pitch dual-source spiral scanning Recently, studies have reported coronary CT angiographic acquisitions with doses at or near 1 mSv (mean, 1.3 mSv; range, 0.87–2.0 mSv) using new scanner technology that incorporates a novel high-pitch spiral technique (pitch 3.4 compared with approximately 0.2 in conventional scanners).22,36–39 Pitch is the ratio of the table speed to the effective detector width, so patients move through the scanner rapidly enough to complete imaging in a single heartbeat. The technique depends on a combination of
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Journal of Cardiovascular Computed Tomography, Vol 4, No 6, November/December 2010 ‘‘optimum’’ image solution is found. The end point of this process is an image with less noise, which can be applied either to improve image quality in obese patients, for example, or to provide diagnostic quality at a lower tube output, which reduces dose. This method has been successfully applied in abdominal and thoracic imaging, and sub-1 mSv coronary CT angiography has been reported.41 There are potential drawbacks; overprocessing of images can result in scans with artifacts or smoothing that obscures subtle lesions. For this reason ASIR percentage is adjustable (Fig. 6). Accuracy studies have not yet confirmed the reliability of this new technology.
Multicenter clinical research In 2009, 2 studies published large-scale multicenter data on coronary CT angiogram radiation doses acquired in clinical practice. In both cases data were collected in 2007; hence, these studies reflect the CT equipment and methods widely in use at that time rather than those reported in contemporaneous 2009 research reports. The Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice (PROTECTION I) was
Figure 4 (A) Prospective or spiral scan mode. Source: Society for Cardiovascular Computed Tomography. Permission granted. (B) Prospectively triggered scanning. In this mode there is intermittent table motion, triggered by the ECG R wave of the oncoming beat. Prospective mode is facilitated by a wider detector array. In this case imaging is completed in 3 heart beats. Source: James P. Earls. Permission granted.
dual x-ray tubes, more rapid gantry rotation (280 milliseconds), and proprietary reconstruction algorithms (Fig. 5). At present, reliable heart rates %60 beats/min are necessary to avoid motion artifacts, so high-dose b-blockers are required for most patients.
Iterative reconstruction Adaptive statistical iterative reconstruction (ASIR) is a novel reconstruction technique with the potential for reducing coronary CT angiographic dose by reducing image noise.23,40–45 Conventional CT image reconstruction relies on an algorithm called filtered back projection for 3-dimensional reconstruction of raw pixel data. ASIR uses the initial filtered back projection to create a model reconstruction, and then repeated reconstruction iterations are produced until an
Figure 5 High-pitch spiral scanning. In this mode 2 x-ray tubes and detector arrays rotate around the patient during continuous table motion at a high pitch. Scanning is accomplished during a single heart beat. Source: James P. Earls. Permission granted.
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Figure 6 Adaptive statistical iterative reconstruction. Iterative reconstruction uses a feedback loop to reduce image noise. To avoid overprocessing of images, the extent of image data subject to iteration can be adjusted. FBP indicates femoral blood pressure. Source: James P. Earls, MD. Permission granted.
an international observational trial conducted at 50 sites, including university and community hospitals. A total of 1665 patient scans were surveyed; the median effective dose for all patients (reporting only the dose from the coronary angiographic image) was 12 mSv (25%–75% range, 8–18 mSv; Fig. 1A). Substantial variation was seen between sites, with a range of median doses from 5 to 30 mSv. Multivariate analysis showed that the use of ECGguided dose modulation, reduced (100 kV) tube voltage, and prospective ECG-triggered scanning were all significant independent predictors of reduced dose.46 The Advanced Cardiovascular Imaging Consortium, a collaborative quality improvement registry sponsored by Blue Cross Blue Shield of Michigan, reported dose levels from a total of 4995 scans performed at 15 hospital sites, 620 of which were performed during a control period from July to August 2007, and 835 from a postintervention period 1 year later.47 The median effective dose from the control period was 21 mSv (25%–75% range, 12–26 mSv). There are notable differences in the way doses were reported in that study; the dose from all phases of the examination were included (eg, topogram, monitoring doses, calcium scoring) as well as full-thorax studies (coronary bypass graft and ‘‘triple rule out’’ scans). Importantly, none of the scans during the control period used 100-kV tube voltage or prospective gating, and most were done without ECG-guided tube current modulation. During an
8-month interventional period, a ‘‘best-practice’’ algorithm was used to train physicians and staff at member sites (Table 3). Equipment for prospective scanning was not available at any site during the study. At the end of the intervention, data were collected from 835 scans between May and June 2008. The median dose had fallen from 21.4 to 9.8 mSv (25%–75% range, 6–16 mSv), a 53% reduction. Importantly, a semiquantitative image quality scale showed revealed no deterioration of image quality during this dose reduction. Multivariable analysis confirmed the findings of PROTECTION I: ECG-guided current modulation and use of 100 kV scanning were independent predictors of lower dose. Furthermore, scan volume per site and patient heart rate were also independent predictors of reduced dose. The investigators concluded that best practice guidelines and collaborative registries hasten the adoption of dose reduction methods, and efforts are under way to extend this approach beyond the Michigan registry. The Consortium Investigators recently reported 1-year follow-up data from the third quarter of 2009, showing that the Consortium centers, comprising 34 contributing sites, had maintained a reduced median dose range of 9.9 mSv. However, during the fourth quarter of 2009 median dose fell to 8.4 mSv, coincident with an increase in the use of prospective or highpitch spiral scan methods from 24% to 56% of all studies.48 This finding suggests that clinical implementation of novel low-dose imaging protocols is increasing.
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Table 3
Best-practice model for acquisition guidelines
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Instructions to the patient and ordering physicians: At the time of scheduling, the patient should be instructed to avoid solid food or caffeine within 6 hours of the scan, but to take oral fluids liberally to avoid dehydration, and to continue taking b-blocking drugs; these measures encourage heart rate stability. Medical history: If the patient has taken b-blockers before arrival, subsequent doses should be adjusted. Nursing assessment: Vital signs, including heart rate, heart rate variability, and blood pressure, are assessed and monitored during the premedication period. Administration of b-blockers: For most current retrospective, gated-acquisition protocols, including dual-source scanners, heart rate control reduces radiation dose by allowing a more narrow acquisition window. For patients with baseline heart rates .65 beats/min, systolic blood pressure .90 mm Hg, and body mass index .18: administer 100 mg of oral metoprolol or comparable dose equivalent 30 minutes to 1 hour before the procedure, or comparable intravenous doses with telemetric monitoring. For patients with baseline heart rates .50 beats/min but ,65 beats/min and blood pressure .90 mm Hg, administer 50 mg of oral metoprolol to block heart rate acceleration during scan. Nitroglycerin administration: If blood pressure is .90 mm Hg, nitroglycerin generally improves image quality, yielding a lower frequency of repeat scans. Protocol parameters: Because radiation dose is directly proportional to z axis scan length, the field of view should be consistently restricted to midpulmonary artery to the diaphragm; extended field of view triple rule-out scans should be limited to patients with clinical likelihood of either a pulmonary embolus or aortic dissection, based on generally accepted diagnostic criteria. A simple rule is used to reduce scan voltage from the standard 120 kVp; 100 kVp may be substituted in patients with a body weight of %85 kg and a body mass index ,30 kg/m2, subject to physician discretion. Tube current modulation by electrocardiographic pulsing should be used in all patients unless atrial fibrillation or frequent premature contractions are present. Acquisition window: For scanners with adjustable acquisition windows during electrocardiographic pulsing, the following adjustments are recommended: Heart rate ,65 beats/min: 65%–75% Heart rate 66–70 beats/min: 60%–80% Heart rate .70 beats/min: 35%–80% Highly variable heart rates or atrial fibrillation may preclude the use of electrocardiographic dose modulation, but this should be weighed against the patient’s age and the suitability of other diagnostic options. If scanner model allows tube current adjustment, the lowest available tube current outside the window should be used (eg, 5% of maximal); this is equally applicable in obese patients because the data during systole will not be used. It was agreed that calcium scoring should be done only if specifically ordered by the referring physician as opposed to on all patients because this adds approximately 0.9–2.0 mSv of radiation.
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Conclusions Scanner manufacturers, researchers, and clinicians performing coronary CT angiography have taken the challenge of radiation dose reduction seriously. There has been extraordinary research and clinical progress over the past 5 years, with research and clinical doses falling by as much as 50% every 2 years. Our challenge is to extend this trend by incorporating new research advances into clinical practice and by facilitating continued technologic advances.
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