- Email: [email protected]
How to use a prospective gated technique for cardiac CT
Journal of Cardiovascular Computed Tomography (2009) 3, 45–51
Questions in Cardiovascular CT
How to use a prospective gated technique for cardiac CT James P. Earls, MD* Fairfax Radiological Consultants PC, 2722 Merrilee Drive, Suite 230, Fairfax, VA 22031, USA KEYWORDS: Cardiac; Coronary; Computed tomography (CT); CT angiography; Prospective triggering
Abstract. A prospective, electrocardiographically gated technique was recently adapted for use with coronary and cardiac multidetector row CT studies. The most widely available form of prospective gating uses ‘‘step-and-shoot’’ axial data acquisition, an incrementally moving table, adaptive electrocardiographic triggering, an improved image reconstruction algorithm, and multiphase reconstruction capability. Studies have shown a 77%–87% effective radiation dose reduction compared with retrospective gating and equal to significantly improved image quality. Comparison with conventional angiography has proven it to be as accurate as retrospective gating for coronary stenosis detection. The technique is not applicable to all patients because there are some restrictions for clinical use, including a limited number of reconstructed phases and a maximum scan heart rate of 68–75 beats/min. However, with careful patient selection and effective heart rate control, prospective gating can be used in a high percentage of cardiac CT examinations. This article reviews the scanning and patient selection protocols for prospective gating and discusses how it may be used in clinical practice. Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction Prospective gating substantially reduces the dose of cardiac computed tomograpic angiography (CCTA) examinations compared with the more commonly performed retrospectively gated technique.1–10 The technique uses axial images and an incrementally moving table to cover the heart with minimal overlap of axial slices. Initial clinical evaluation found an 83% reduction in effective radiation dose and significantly improved image quality as compared with CCTA examinations performed with a helical retrospective gating technique.2,3 Subsequent studies have confirmed radiation dose reductions of 77%–87%.4–10 Conflict of interest: Dr. Earls is a member of the speaker’s bureau for GE Healthcare. * Address correspondence to James P. Earls. MD. 5553 Rockpointe Drive, Clifton VA 20124 E-mail address: [email protected] Submitted September 3, 2008. Accepted for publication October 25, 2008.
Cardiac CT has recently come under scrutiny because of its potentially high effective radiation dose.11,12 The effective radiation dose in coronary CT angiography studies using 64-row multidetector CT (MDCT) ranges widely, depending on the technique used, with the reported effective radiation dose using a retrospectively gated technique range from 5.7 to 36.5 mSv.13–17 A recent analysis concluded that the use of retrospectively gated 64-row CCTA, with a mean dose of approximately 15 mSv, is associated with a non-negligible increase in the lifetime attributable risk of cancer incidence associated with radiation exposure and that this is considerably greater for women, younger patients, and for combined cardiac and aortic scans.11 Prospective gating for CT is not new, being used by Dr. Godfrey Hounsfeld as early as 1980 with conventional single-slice CT.18 He recognized that CT image synchronization with diastole was optimal for imaging the heart. Cardiac imaging with electron beam CT also uses prospective data acquisition triggered by the electrocardiogram (ECG).
1934-5925/$ -see front matter Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2008.10.013
46
Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Scanning technique
Selection of mA
The prospective gating technique uses a combined ‘‘step-and-shoot’’ axial data acquisition and an incrementally moving table with prospective, adaptive ECG triggering. This method takes advantage of the large 40-mm (64 ! 0.625 mm) volume coverage available with the 64-row MDCT scanner that enables complete coverage of the heart in 2 to 3 steps. With use of this technique, the table is stationary during image acquisition. It then moves to the next position for another scan initiated by the subsequent cardiac cycle. The result is very little overlap between the scans, significant reduction in radiation dose, and more robust and adaptive ECG gating (Fig. 1). With the use of a 64-detector system, the scan is prescribed by using 3–5 incremental 64 ! 0.625-mm (40-mm) image groups that require 2–4 incremental table translations of 35 mm, which allow for 5 mm of overlap. The minimum interscan delay is approximately 0.6–1.0 second; this normally requires skipping a cardiac cycle between acquisitions of successive image groups (ie, one image acquisition per 2 cardiac R-R cycles). The minimum scan time at each axial location is 230 msec (180 degrees plus a fan angle), which translates to an effective temporal resolution of 175 msec with the half-scan weighting. The technique was recently adopted for use with both dual-source CT and the new 320-row MDCT.7,9 The detector width determines the number of steps to cover the heart and complete an examination. The dual-source CT has a narrower detector array (32 ! 0.6 mm 5 19.2 mm per acquisition); thus, it takes more incremental steps to cover the heart and complete an examination than the 320-row system (320 ! 0.5 mm 5 160 mm) which covers the heart in a single acquisition.
The tube output (in mA) has a linear relation to effective radiation dose. We manually select mA that ranges from 300 to 800 depending on the patient’s body mass index and chest circumference. Use of higher mA reduces image noise, making studies more visually appealing and somewhat easier to read. This can be useful in heavily calcified vessels, in studies with intracoronary stents, and in obese patients. However, it is tempting to overuse (at the expense of unnecessary radiation exposure) the higher available tube capacity available on newer MDCT systems. The mA should be individually tailored to each study based on the patient’s size, chest circumference, and estimated muscle and breast mass. With use of a prospective technique, a higher mA can at times be used to overcome the challenges of coronary calcium, stents, or high body mass index, while still keeping the total effective dose to a minimum (Fig. 2).
Selection of kVp The radiation dose varies with the square of the kilovoltage; therefore, relatively small reductions in voltage will result in disproportionately larger reduction in overall effective dose.19,20 On the dual-source CT, Gutstein et al7 recently reported the mean effective dose using prospective technique was 3.1 mSv when using 120 kVp, but it was only 1.5 mSv when 100 kVp was used. However, one must be careful in lowering the kVp in CCTA examinations. Although the image signal improves with lower tube voltage, the image noise also increases. In our clinical practice, we may lower the tube voltage to 100 kVp for patients with low body mass index and children, but we routinely use 120 kVp for most other patients.
Figure 1 Comparison of 3 commonly used CCTA scanning techniques. In standard retrospective gating, the tube output (in mA) is constant throughout the acquisition. In retrospective gating with ECG modulation, the mA peaks in diastole and is reduced substantially in other parts of the cardiac cycle. With prospective gating, the tube current is ‘‘off’’ for most of the scan period and is triggered by the electrocardiogram ‘‘on’’ only for a short period during diastole.
Earls
Prospective gated technique for cardiac CT
47
Figure 2 Prospectively gated CCTA performed in a 73-year-old man with a calcium score of 3215. A high mA (750) was used to generate images with little artifact from the patient’s extensive coronary calcium. Despite the use of a tube output of 750 mA, the effective radiation dose was only 2.5 mSv for the study. Volume-rendered (upper left) and maximal intensity projection (upper right) views of the coronary arteries depict the extensive calcium. Curved planar reformations of the right coronary artery (bottom left) and the left anterior descending artery (bottom right) depict widely patent coronary lumens without calcium artifacts.
Use of addition tube-on time (‘‘padding’’) Unlike retrospectively gated MDCT in which data are available throughout the entire cardiac cycle, prospectively gated studies have a limited number of cardiac phases available for reconstruction. We routinely select middiastole (75%) for data acquisition for all subjects. Additional ‘‘padding’’ of the time the x-ray tube is ‘‘on’’ is used to generate additional cardiac phases, the number of which depends on the amount of perceived beat-to-beat variability. Padding technique turns the x-ray tube on before and leaves it on after the minimum required 230 msec. Padding allows the reconstruction to adapt to minor heart rate variations and to produce consistent image quality, because the reconstruction window can be modified retrospectively to ensure identical cardiac phase from scan to scan (Fig. 3). Available padding options with current software ranges from 0 to 200 msec. In patients with stable heart rates, who show minimal heart rate variability, we normally select 0 msec (no additional on-time) to minimize dose. As the heart rate variability increases, we select padding in increments of
25 msec up to a maximum of 200 msec. If heart rate variability exceeds 10 beats/min, we generally will not use a prospective technique and will revert to retrospective gating. Use of padding increases the effective dose to the patient. For coronary examinations, the effective dose increases linearly from 2.15 mSv with 0 msec of padding to 7.89 mSv with maximum padding of 200 msec.21 In practice, we generally use limited amounts of padding; in our recent large clinical series of 2000 cases we recorded an average padding of 34 msec per examination.
Tips for clinical use Although all patients, especially women and younger patients, benefit from a reduced effective radiation dose, not every patient is optimally suited to the technique. Use of prospective gating is usually limited to patients with relatively low and stable heart rates. Currently, there are 8 published reports on the use of prospective gating; all use an upper heart rate limitation for scanning patients which range from a low of 65 beats/min to a high of 75 beats/ min.3–10 We limit use of prospective gating to patients who
48
Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Figure 3 Use of temporal padding to achieve diagnostic images. In this CCTA examination on a 51-year-old man with a prior left circumflex coronary artery stent, 100 msec of temporal padding was used because of a moderate amount (,10 beats/min) of observed heart rate variability. The target phase was 75% (mid-diastole). The 100 msec of padding allowed for reconstruction of additional phases from 62% to 86% of the cardiac cycle (A). Curved planar reconstruction of the left anterior descending coronary artery at the 75% phase was nondiagnostic; however, reconstruction at the 83% phase clearly depicted a widely patient left anterior descending coronary artery (B). Similarly, curved planar reconstruction of the left circumflex artery at the 75% phase was nondiagnostic, but reconstruction at the 62% phase clearly depicted a widely patent stent (C).
also show ,10 beats/min of heart rate variability during a short observation period before scanning. A final limitation is that prospective gating cannot be used if an analysis of global or regional myocardial function is desired, because data is not available throughout the entire cardiac cycle. We do not limit the application of the technique based on age or coronary calcium score.
Higher heart rates There are several reasons for the upper heart rate limitation of 65–75 beats/min. With retrospectively gated 64-MDCT, it is recognized that increasing heart rate decreases the diagnostic accuracy of coronary angiography.22 A heart rate of %65 beats/min results in significantly improved image quality for the left anterior descending, circumflex, and right coronary arteries.23 At heart rates
.64–68.5 beats/min, reconstruction of data during end-systole or early diastole improves image quality compared with the usually performed mid-diastole.24 However, because prospective gating limits the number of cardiac phases acquired, end-systolic or early diastolic phases may not be available. If 75%, or mid-diastole, was initially targeted, then there may not be optimal image quality. In theory, at higher heart rates, end-systolic or early diastolic phases could be targeted, rather than using the middiastolic phase. In addition, use of additional ‘‘padding’’ of tube on time can be used to broaden the acquisition window to include end-systole or early diastole. However, data on either of these two techniques is not available. Rather than adapting prospective technique to higher heart rates, reducing the heart rates with b-blockade into the acceptable window will likely result in better results. Our heart rate control protocol calls for use of oral
Earls
Prospective gated technique for cardiac CT
49
Figure 3
b-blocker, or calcium channel blocker when b-blockers are contraindicated, if the resting HR is .55 beats/min (Fig. 4). We recently reviewed 2000 clinical patients presenting for cardiac MDCT scans.21 Without b-blocker use, approximately 50% of the patients would not have been eligible
(continued).
for use of prospective gating because they had rates of .70 beats/min. Following our heart rate control protocol, we gave 74% of patients b-blockers and 2% calcium channel blockers for heart rate reduction. An additional 14% of patients, with a
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Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Figure 4
Flowchart depicting our b-blocker and technique selection protocol for CCTA.
heart rate of 55–59 beats/min, received a small dose of metoprolol to help minimize heart rate variability. After patients complete the protocol, their eventual heart rate and variability determined whether they would be scanned with prospective or retrospective gating (assuming a cardiac function analysis had not been requested). In our recent series of 2000 clinical patients, at the time of their diagnostic scan, 95.0% patients meet the heart rate criteria of ,70 beats/ min, 93.7% meet the criteria of ,10 beats/min of observed heart rate variability, and 90.5% met both criteria. As a result, we were able to use the prospective gated technique in 92.7% of coronary CTA examinations and in 82.9% of CCTA examinations that included coronary bypass grafts.
Heart rate variability As discussed earlier, a second limitation for use of prospective gating is heart rate variability. Heart rate variability was independently correlated with overall image quality for CCTA, and heart rates are less variable and image quality improved in patients receiving b-blockers.25,26 If we observe .10 beats/min of variability before scanning, we will usually not use prospective technique and revert to
retrospective gating. To minimize variability, we have found that use of b-blockers is useful even in some patients with heart rates ,60 beats/min. We routinely give 12.5–25 mg of oral metoprolol to patients with heart rates of 55–59 beats/min approximately 30 minutes before scanning.
Cardiac function Because prospective technique acquires data only during a limited portion of the cardiac cycle, it cannot be used to evaluate cardiac function. Both quantitative and qualitative functions, either global or regional, require images be reconstructed throughout the entire cardiac cycle. If the clinical scenario or referring physician calls for cardiac function, then retrospective gating must be used. Clinically, we have found only a small number of cases require function, however, this depends on many factors, and other sites use function much more frequently than we have.
Summary Prospectively gated cardiac CT is now available on most cardiac-capable MDCT systems. This technique affords
Earls
Prospective gated technique for cardiac CT
.80% effective radiation dose reduction compared with retrospectively gated helical techniques. With careful patient preparation, a prospectively gated technique can be used in a large percentage of routine CCTA cases not requiring analysis of cardiac function. We believe this has great promise to become a commonly used method for coronary CTA.
References 1. Hsieh J, Londt J, Vass M, Li J, Tang X, Okerlund D: Step-and-shoot data acquisition and reconstruction for cardiac x-ray computed tomography. Med Phys. 2006;33:4236–48. 2. Earls J, Urban B, Berman E, Curry C, Lane J, Jennings R: Prospectively gated coronary CT angiography: a comparison with helical CT angiography. Presented at the 2006 Radiological Society of North America (RSNA) annual meeting, Chicago, IL, Nov. 29, 2006. 3. Earls JP, Berman EL, Urban BA, Curry CA, Lane JL, Jennings RS, McCulloch CC, Hsieh J, Londt JH: Prospectively gated transverse coronary CT angiography versus retrospectively gated helical technique: improved image quality and reduced radiation dose. Radiology. 2008; 246:742–53. 4. Husmann L, Valenta I, Gaemperli O, Adda O, Treyer V, Wyss CA, Veit-Haibach P, Tatsugami F, von Schulthess GK, Kaufmann PA: Feasibility of low-dose coronary CT angiography: first experience with prospective ECG-gating. Eur Heart J. 2008;29:191–7. 5. Shuman WP, Branch KR, May JM, Mitsumori LM, Lockhart DW, Dubinsky TJ, Warren BH, Caoldwell JH: Prospective versus retrospective ECG gating for 64-detector CT of the coronary arteries: comparison of image quality and patient radiation dose. Radiology. 2008;248:431–7. 6. Hirai N, Horiguchi J, Fujioka C, Kiguchi M, Yamamoto H, Matsuura N, Kitagawa T, Teragawa H, Kohno N, Ito K: Prospective versus retrospective ECG-gated 64-detector coronary CT angiography: assessment of image quality, stenosis, and radiation dose. Radiology. 2008;248:424–30. 7. Gutstein A, Wolak A, Lee C, Dey D, Ohba M, Suzuki Y, Cheng V, Gransar H, Suzuki S, Friedman J, Thomson LE, Hayes S, Pimentel R, Paz W, Slomka P, Berman DS: Predicting success of prospective and retrospective gating with dual-source coronary computed tomography angiography: development of selection criteria and initial experience. J Cardiovasc Comput Tomogr. 2008;2:81–90. 8. Steigner ML, Otero HJ, Cai T, Mitsouras D, Nallamshetty L, Whitmore AG, Ersoy H, Levit NA, Di Carli MF, Rybicki FJ. Narrowing the phase window width in prospectively ECG-gated single heart beat 320-detector row coronary CT angiography [published ahead of print July 29, 2008]. Int J Cardiovasc Imaging. 9. Rybicki FJ, Otero HJ, Steigner ML, Vorobiof G, Nallamshetty L, Mitsouras D, Ersoy H, Mather RT, Judy PF, Cai T, Coyner K, Schultz K, Whitmore AG: De Carli MF: Initial evaluation of coronary images from 320-detector row computed tomography. Int J Cardiovasc Imaging. 2008;24:535–46. 10. Scheffel H, Alkadhi H, Leschka S, Plass A, Desbiolles L, Guber I, Krauss T, Gruenenfelder J, Genoni M, Luescher TF, Marincek B, Stolzmann P: Low-dose CT coronary angiography in the step-andshoot mode: diagnostic performance. Heart. 2008;94:1132–7. 11. Einstein AJ, Henzlova MJ, Rajagopalan S: Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA. 2007;298:317–23.
51 12. Brenner DJ, Hall EJ: Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357:2277–84. 13. Leber AW, Knez A, Becker A, Becker C, von Ziegler F, Nikolaou K, Rist C, Reiser M, White C, Steinbeck G, Boekstegers P: Accuracy of multidetector spiral computed tomography in identifying and differentiating the composition of coronary atherosclerotic plaques: a comparative study with intracoronary ultrasound. J Am Coll Cardiol. 2004;43: 1241–7. 14. Mollet NR, Cademartiri F, van Mieghem CA, Runza G, McFadden EP, Baks T, Serruys PW, Krestin GP: de Feyter PJ: High-resolution spiral computed tomography coronary angiography in patients referred for diagnostic conventional coronary angiography. Circulation. 2005; 112:2318–23. 15. Raff GL, Gallagher MJ, O’Neill WW, Goldstein JA: Diagnostic accuracy of noninvasive coronary angiography using 64-slice spiral computed tomography. J Am Coll Cardiol. 2005;46:552–7. 16. Francone M, Napoli A, Carbone I, Cavacece M, Nardis PG, Lanciotti K, Visconti S, Bertoletti L, De Castro E, Catalano C, Passariello R: Noninvasive imaging of the coronary arteries using a 64-row multidetector CT scanner: initial clinical experience and radiation dose concerns. Radiol Med. 2007;112:31–46. 17. Hausleiter J, Meyer T, Hermann F, Gerber TC, McCollough CH, Hadamitzky M, Martinoff S, Achenbach S. International prospective multicenter study on radiation dose estimates of cardiac CT angiography in daily practice. Presented at the 57th annual scientific session of the American College of Cardiology, Chicago IL, March 3, 2008. 18. Hounsfield G: Computed medical imaging. Science. 1980;210:22–8. 19. Abada HT, Larchez C, Daoud B, Sigal-Cinqualbre A, Paul JF: MDCT of the coronary arteries: feasibility of low-dose CT with ECG-pulsed tube current modulation to reduce radiation dose. AJR Am J Roentgenol. 2006;186(Suppl):S387–90. 20. Hausleiter J, Meyer T, Hadamitzky M, Huber E, Zankl M, Martinoff S, Kastrati A, Schomig A: Radiation dose estimates from cardiac multislice computed tomography in daily practice. Circulation. 2006;113: 1305–10. 21. Earls J: Prospective gating: results in 2000 clinical patients. Presented at Stanford University’s 10th Annual International Symposium on Multidetector-Row CT, Las Vegas, NV, May 2008. 22. Shim SS, Kim Y, Lim SM: Improvement of image quality with b-blocker premedication on ECG-gated 16-MDCT coronary angiography. AJR Am J Roentgenol. 2005;184:649–54. 23. Cademartiri F, Mollet NR, Runz G, Belgrano M, Malagutti P, Meijboom BW, Midiri M, Feyter PJ, Krestin GP: Diagnostic accuracy of multislice computed tomography angiography is improved at low heart rates. Int J Cardiovasc Imaging. 2006;22:101–5. 24. Leschka S, Wildermuth S, Boehm T, Desbiolles L, Husmann L, Plass A, Koepfli P, Schepis T, Marincek B, Kaufmann PA, Alkadhi H: Noninvasive coronary angiography with 64-section CT: effect of average heart rate and heart rate variability on image quality. Radiology. 2006;241:378–85. 25. Wintersperger BJ, Nikolaou K, von Ziegler F, Johnson T, Rist C, Leber A, Flohr T, Knez A, Reiser MF, Becker CR: Image quality, motion artifacts, and reconstruction timing of 64-slice coronary computed tomography angiography with 0.33-second rotation speed. Invest Radiol. 2006;41:436–42. 26. Hoffmann MHK, Shi H, Manzke R, Schmid FT, De Vries L, Grass M, Brambs HJ, Aschoff AJ: Noninvasive coronary angiography with 16-detector row CT: effect of heart rate. Radiology. 2005; 234:86–97.
Questions in Cardiovascular CT
How to use a prospective gated technique for cardiac CT James P. Earls, MD* Fairfax Radiological Consultants PC, 2722 Merrilee Drive, Suite 230, Fairfax, VA 22031, USA KEYWORDS: Cardiac; Coronary; Computed tomography (CT); CT angiography; Prospective triggering
Abstract. A prospective, electrocardiographically gated technique was recently adapted for use with coronary and cardiac multidetector row CT studies. The most widely available form of prospective gating uses ‘‘step-and-shoot’’ axial data acquisition, an incrementally moving table, adaptive electrocardiographic triggering, an improved image reconstruction algorithm, and multiphase reconstruction capability. Studies have shown a 77%–87% effective radiation dose reduction compared with retrospective gating and equal to significantly improved image quality. Comparison with conventional angiography has proven it to be as accurate as retrospective gating for coronary stenosis detection. The technique is not applicable to all patients because there are some restrictions for clinical use, including a limited number of reconstructed phases and a maximum scan heart rate of 68–75 beats/min. However, with careful patient selection and effective heart rate control, prospective gating can be used in a high percentage of cardiac CT examinations. This article reviews the scanning and patient selection protocols for prospective gating and discusses how it may be used in clinical practice. Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction Prospective gating substantially reduces the dose of cardiac computed tomograpic angiography (CCTA) examinations compared with the more commonly performed retrospectively gated technique.1–10 The technique uses axial images and an incrementally moving table to cover the heart with minimal overlap of axial slices. Initial clinical evaluation found an 83% reduction in effective radiation dose and significantly improved image quality as compared with CCTA examinations performed with a helical retrospective gating technique.2,3 Subsequent studies have confirmed radiation dose reductions of 77%–87%.4–10 Conflict of interest: Dr. Earls is a member of the speaker’s bureau for GE Healthcare. * Address correspondence to James P. Earls. MD. 5553 Rockpointe Drive, Clifton VA 20124 E-mail address: [email protected] Submitted September 3, 2008. Accepted for publication October 25, 2008.
Cardiac CT has recently come under scrutiny because of its potentially high effective radiation dose.11,12 The effective radiation dose in coronary CT angiography studies using 64-row multidetector CT (MDCT) ranges widely, depending on the technique used, with the reported effective radiation dose using a retrospectively gated technique range from 5.7 to 36.5 mSv.13–17 A recent analysis concluded that the use of retrospectively gated 64-row CCTA, with a mean dose of approximately 15 mSv, is associated with a non-negligible increase in the lifetime attributable risk of cancer incidence associated with radiation exposure and that this is considerably greater for women, younger patients, and for combined cardiac and aortic scans.11 Prospective gating for CT is not new, being used by Dr. Godfrey Hounsfeld as early as 1980 with conventional single-slice CT.18 He recognized that CT image synchronization with diastole was optimal for imaging the heart. Cardiac imaging with electron beam CT also uses prospective data acquisition triggered by the electrocardiogram (ECG).
1934-5925/$ -see front matter Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2008.10.013
46
Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Scanning technique
Selection of mA
The prospective gating technique uses a combined ‘‘step-and-shoot’’ axial data acquisition and an incrementally moving table with prospective, adaptive ECG triggering. This method takes advantage of the large 40-mm (64 ! 0.625 mm) volume coverage available with the 64-row MDCT scanner that enables complete coverage of the heart in 2 to 3 steps. With use of this technique, the table is stationary during image acquisition. It then moves to the next position for another scan initiated by the subsequent cardiac cycle. The result is very little overlap between the scans, significant reduction in radiation dose, and more robust and adaptive ECG gating (Fig. 1). With the use of a 64-detector system, the scan is prescribed by using 3–5 incremental 64 ! 0.625-mm (40-mm) image groups that require 2–4 incremental table translations of 35 mm, which allow for 5 mm of overlap. The minimum interscan delay is approximately 0.6–1.0 second; this normally requires skipping a cardiac cycle between acquisitions of successive image groups (ie, one image acquisition per 2 cardiac R-R cycles). The minimum scan time at each axial location is 230 msec (180 degrees plus a fan angle), which translates to an effective temporal resolution of 175 msec with the half-scan weighting. The technique was recently adopted for use with both dual-source CT and the new 320-row MDCT.7,9 The detector width determines the number of steps to cover the heart and complete an examination. The dual-source CT has a narrower detector array (32 ! 0.6 mm 5 19.2 mm per acquisition); thus, it takes more incremental steps to cover the heart and complete an examination than the 320-row system (320 ! 0.5 mm 5 160 mm) which covers the heart in a single acquisition.
The tube output (in mA) has a linear relation to effective radiation dose. We manually select mA that ranges from 300 to 800 depending on the patient’s body mass index and chest circumference. Use of higher mA reduces image noise, making studies more visually appealing and somewhat easier to read. This can be useful in heavily calcified vessels, in studies with intracoronary stents, and in obese patients. However, it is tempting to overuse (at the expense of unnecessary radiation exposure) the higher available tube capacity available on newer MDCT systems. The mA should be individually tailored to each study based on the patient’s size, chest circumference, and estimated muscle and breast mass. With use of a prospective technique, a higher mA can at times be used to overcome the challenges of coronary calcium, stents, or high body mass index, while still keeping the total effective dose to a minimum (Fig. 2).
Selection of kVp The radiation dose varies with the square of the kilovoltage; therefore, relatively small reductions in voltage will result in disproportionately larger reduction in overall effective dose.19,20 On the dual-source CT, Gutstein et al7 recently reported the mean effective dose using prospective technique was 3.1 mSv when using 120 kVp, but it was only 1.5 mSv when 100 kVp was used. However, one must be careful in lowering the kVp in CCTA examinations. Although the image signal improves with lower tube voltage, the image noise also increases. In our clinical practice, we may lower the tube voltage to 100 kVp for patients with low body mass index and children, but we routinely use 120 kVp for most other patients.
Figure 1 Comparison of 3 commonly used CCTA scanning techniques. In standard retrospective gating, the tube output (in mA) is constant throughout the acquisition. In retrospective gating with ECG modulation, the mA peaks in diastole and is reduced substantially in other parts of the cardiac cycle. With prospective gating, the tube current is ‘‘off’’ for most of the scan period and is triggered by the electrocardiogram ‘‘on’’ only for a short period during diastole.
Earls
Prospective gated technique for cardiac CT
47
Figure 2 Prospectively gated CCTA performed in a 73-year-old man with a calcium score of 3215. A high mA (750) was used to generate images with little artifact from the patient’s extensive coronary calcium. Despite the use of a tube output of 750 mA, the effective radiation dose was only 2.5 mSv for the study. Volume-rendered (upper left) and maximal intensity projection (upper right) views of the coronary arteries depict the extensive calcium. Curved planar reformations of the right coronary artery (bottom left) and the left anterior descending artery (bottom right) depict widely patent coronary lumens without calcium artifacts.
Use of addition tube-on time (‘‘padding’’) Unlike retrospectively gated MDCT in which data are available throughout the entire cardiac cycle, prospectively gated studies have a limited number of cardiac phases available for reconstruction. We routinely select middiastole (75%) for data acquisition for all subjects. Additional ‘‘padding’’ of the time the x-ray tube is ‘‘on’’ is used to generate additional cardiac phases, the number of which depends on the amount of perceived beat-to-beat variability. Padding technique turns the x-ray tube on before and leaves it on after the minimum required 230 msec. Padding allows the reconstruction to adapt to minor heart rate variations and to produce consistent image quality, because the reconstruction window can be modified retrospectively to ensure identical cardiac phase from scan to scan (Fig. 3). Available padding options with current software ranges from 0 to 200 msec. In patients with stable heart rates, who show minimal heart rate variability, we normally select 0 msec (no additional on-time) to minimize dose. As the heart rate variability increases, we select padding in increments of
25 msec up to a maximum of 200 msec. If heart rate variability exceeds 10 beats/min, we generally will not use a prospective technique and will revert to retrospective gating. Use of padding increases the effective dose to the patient. For coronary examinations, the effective dose increases linearly from 2.15 mSv with 0 msec of padding to 7.89 mSv with maximum padding of 200 msec.21 In practice, we generally use limited amounts of padding; in our recent large clinical series of 2000 cases we recorded an average padding of 34 msec per examination.
Tips for clinical use Although all patients, especially women and younger patients, benefit from a reduced effective radiation dose, not every patient is optimally suited to the technique. Use of prospective gating is usually limited to patients with relatively low and stable heart rates. Currently, there are 8 published reports on the use of prospective gating; all use an upper heart rate limitation for scanning patients which range from a low of 65 beats/min to a high of 75 beats/ min.3–10 We limit use of prospective gating to patients who
48
Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Figure 3 Use of temporal padding to achieve diagnostic images. In this CCTA examination on a 51-year-old man with a prior left circumflex coronary artery stent, 100 msec of temporal padding was used because of a moderate amount (,10 beats/min) of observed heart rate variability. The target phase was 75% (mid-diastole). The 100 msec of padding allowed for reconstruction of additional phases from 62% to 86% of the cardiac cycle (A). Curved planar reconstruction of the left anterior descending coronary artery at the 75% phase was nondiagnostic; however, reconstruction at the 83% phase clearly depicted a widely patient left anterior descending coronary artery (B). Similarly, curved planar reconstruction of the left circumflex artery at the 75% phase was nondiagnostic, but reconstruction at the 62% phase clearly depicted a widely patent stent (C).
also show ,10 beats/min of heart rate variability during a short observation period before scanning. A final limitation is that prospective gating cannot be used if an analysis of global or regional myocardial function is desired, because data is not available throughout the entire cardiac cycle. We do not limit the application of the technique based on age or coronary calcium score.
Higher heart rates There are several reasons for the upper heart rate limitation of 65–75 beats/min. With retrospectively gated 64-MDCT, it is recognized that increasing heart rate decreases the diagnostic accuracy of coronary angiography.22 A heart rate of %65 beats/min results in significantly improved image quality for the left anterior descending, circumflex, and right coronary arteries.23 At heart rates
.64–68.5 beats/min, reconstruction of data during end-systole or early diastole improves image quality compared with the usually performed mid-diastole.24 However, because prospective gating limits the number of cardiac phases acquired, end-systolic or early diastolic phases may not be available. If 75%, or mid-diastole, was initially targeted, then there may not be optimal image quality. In theory, at higher heart rates, end-systolic or early diastolic phases could be targeted, rather than using the middiastolic phase. In addition, use of additional ‘‘padding’’ of tube on time can be used to broaden the acquisition window to include end-systole or early diastole. However, data on either of these two techniques is not available. Rather than adapting prospective technique to higher heart rates, reducing the heart rates with b-blockade into the acceptable window will likely result in better results. Our heart rate control protocol calls for use of oral
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Prospective gated technique for cardiac CT
49
Figure 3
b-blocker, or calcium channel blocker when b-blockers are contraindicated, if the resting HR is .55 beats/min (Fig. 4). We recently reviewed 2000 clinical patients presenting for cardiac MDCT scans.21 Without b-blocker use, approximately 50% of the patients would not have been eligible
(continued).
for use of prospective gating because they had rates of .70 beats/min. Following our heart rate control protocol, we gave 74% of patients b-blockers and 2% calcium channel blockers for heart rate reduction. An additional 14% of patients, with a
50
Journal of Cardiovascular Computed Tomography, Vol 3, No 1, January/February 2009
Figure 4
Flowchart depicting our b-blocker and technique selection protocol for CCTA.
heart rate of 55–59 beats/min, received a small dose of metoprolol to help minimize heart rate variability. After patients complete the protocol, their eventual heart rate and variability determined whether they would be scanned with prospective or retrospective gating (assuming a cardiac function analysis had not been requested). In our recent series of 2000 clinical patients, at the time of their diagnostic scan, 95.0% patients meet the heart rate criteria of ,70 beats/ min, 93.7% meet the criteria of ,10 beats/min of observed heart rate variability, and 90.5% met both criteria. As a result, we were able to use the prospective gated technique in 92.7% of coronary CTA examinations and in 82.9% of CCTA examinations that included coronary bypass grafts.
Heart rate variability As discussed earlier, a second limitation for use of prospective gating is heart rate variability. Heart rate variability was independently correlated with overall image quality for CCTA, and heart rates are less variable and image quality improved in patients receiving b-blockers.25,26 If we observe .10 beats/min of variability before scanning, we will usually not use prospective technique and revert to
retrospective gating. To minimize variability, we have found that use of b-blockers is useful even in some patients with heart rates ,60 beats/min. We routinely give 12.5–25 mg of oral metoprolol to patients with heart rates of 55–59 beats/min approximately 30 minutes before scanning.
Cardiac function Because prospective technique acquires data only during a limited portion of the cardiac cycle, it cannot be used to evaluate cardiac function. Both quantitative and qualitative functions, either global or regional, require images be reconstructed throughout the entire cardiac cycle. If the clinical scenario or referring physician calls for cardiac function, then retrospective gating must be used. Clinically, we have found only a small number of cases require function, however, this depends on many factors, and other sites use function much more frequently than we have.
Summary Prospectively gated cardiac CT is now available on most cardiac-capable MDCT systems. This technique affords
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Prospective gated technique for cardiac CT
.80% effective radiation dose reduction compared with retrospectively gated helical techniques. With careful patient preparation, a prospectively gated technique can be used in a large percentage of routine CCTA cases not requiring analysis of cardiac function. We believe this has great promise to become a commonly used method for coronary CTA.
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