- Email: [email protected]
Radiation Dose and Image Quality of Prospective Triggering With Dual-Source Cardiac Computed Tomography
Radiation Dose and Image Quality of Prospective Triggering With Dual-Source Cardiac Computed Tomography Ron Blankstein, MDa,b,*, Amar Shah, MDa, Rodrigo Pale, MDa, Suhny Abbara, MDa, Hiram Bezerra, MDa, Michael Bolen, MDa, Wilfred S. Mamuya, MD, PhDa, Udo Hoffmann, MDa, Thomas J. Brady, MDa, and Ricardo C. Cury, MDa,c Prospectively triggered (PT) cardiac computed tomography (CT), whereby radiation is administered only at a predefined phase of the cardiac cycle, has been shown to substantially decrease radiation dose. The aim of our study was to assess the use of this technique in a clinical population using dual-source cardiac CT. Of 312 consecutive patients referred for a dual-source cardiac computed tomographic examination, PT was used in 42 patients for whom, based on physician judgment, it was decided to minimize radiation, whereas retrospective gating was used for 188 patients (coronary artery bypass grafting and pulmonary vein studies were excluded). Kilovolt and milliampere per second were chosen for each patient based on assessment of body habitus and effective radiation dose was calculated. Analysis of nonevaluable vessels was based on clinical readings. For each study, image quality (IQ) was rated on a subjective IQ score and contrast-to-noise and signal-tonoise ratios were calculated. Of the 42 PT examinations (mean age 44.3 years, body mass index 27.8 kg/m2, 62% men), 28 were referred for coronary evaluation, 11 for aortic disease with/without coronaries, and 3 for other reasons (i.e., suspected mass and congenital disease). Average heart rate was 64.5 beats/min. Average radiation dose of all 42 PT scans was 3.2 ⴞ 1.6 vs 13.4 ⴞ 7.8 mSv for the 188 non-PT scans. There was no significant difference in IQ score and contrast-to-noise and signal-to-noise ratios between the 2 groups. Furthermore, the incidence of limited right coronary artery evaluation and of limitations related to right coronary artery motion did not differ between PT and non-PT scans. In conclusion, in selected patients, prospective triggering with dual-source cardiac CT is feasible and results in a dramatic decrease of radiation dose without compromising IQ. Future advances in cardiac CT may further improve this technique, thus allowing for wider use. © 2009 Elsevier Inc. All rights reserved. (Am J Cardiol 2009;103:1168 –1173) In dual-source computed tomography (DSCT) 2 x-ray sources and detector systems perpendicular to each other are simultaneously used to collect data. As a result the temporal resolution is halved because only 90° of rotation (compared with the typical 180°) is needed to create a single image. We hypothesized that the improved temporal resolution of DSCT allows for adequate visualization of all coronary arteries with a single phase, thus permitting the use of prospective electrocardiographic triggering, whereby radiation is administered only at predefined time points of the cardiac cycle.1–3 Therefore, the purpose of this study was to assess the feasibility of prospective triggering in patients referred for a cardiac dual-source computed tomographic examination and to identify whether use of this technique a
Cardiac MR PET CT Program, Department of Radiology and Division of Cardiology, Massachusetts General Hospital and Harvard Medical School, and bNoninvasive Cardiovascular Imaging Program, Department of Medicine and Radiology, Brigham and Women’s Hospital, Boston, Massachusetts; and cBaptist Cardiac and Vascular Institute, Miami, Florida. Manuscript received August 21, 2008; revised manuscript received and accepted December 21, 2008. Dr. Blankstein has received support from Grant 1T32 HL076136 from the National Institutes of Health, Bethesda, Maryland. *Corresponding author: Tel: 617-724-5351; fax: 617-725-4152. E-mail address: [email protected] (R. Blankstein). 0002-9149/09/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.amjcard.2008.12.045
could achieve a lower radiation dose without compromising image quality (IQ). Methods Our study population included 312 consecutive patients referred for a cardiac computed tomographic examination. Of these, prospective triggering was used in 42 patients for whom, based on physician judgment, (1) use of prospective triggering was feasible (e.g., low, regular heart rate) and (2) minimizing radiation exposure was warranted (e.g., younger age). Of the 270 nonprospectively triggered (non-PT) PT scans, all scans performed for evaluation of bypass grafts (n ⫽ 26) or pulmonary veins (n ⫽ 56) were excluded. The remainder non-PT comparison group consisted of 188 patients. Cardiac CT (CCT) was performed on a dual-source 64slice computed tomographic scanner (Siemens Healthcare: Definition, Florsheim, Germany) with a gantry rotation time of 330 ms and standard detector collimation of 0.6 mm. A flying focus along the z-axis (z-sharp technology) was used to acquire 64 overlapping 0.6-mm slices using 2 32-detector rows. The resulting temporal resolution was 83 ms. Figure 1 displays some key parameters associated with dual-source computed tomographic prospective triggering. Scans using prospective triggering (Siemens Healthcare: Sequential Scanning) were obtained at 65% of the RR www.AJConline.org
Methods/Prospective Triggering With Cardiac CT
1169
Figure 1. Prospective triggering: explanation of key parameters.
interval. In this mode, there is no table movement during data acquisition, yet the gantry with its 2 x-ray sources and detectors continues to rotate. The resulting coverage in the z-axis is 19.2 mm per acquisition, which is also equal to the table movement that occurs between acquisitions. For all cardiac computed tomographic scans, contrast timing was determined with the use of a test bolus of contrast 20 ml. Initial image acquisition timing was then determined by adding 2 to 4 seconds to the time of peak contrast enhancement in the ascending aorta. A physician was present for each study and selected the kilovolt and milliampere per second based on assessment of patient’s body habitus. Beta blockers were not routinely administered according to our standard dual-source computed tomographic clinical protocol. For retrospectively gated CCT, raw data from 5% to 95% of the cardiac cycle were used to reconstruct data at 10% intervals with 0.75 mm slice thickness and 0.4-mm overlap using a medium smooth reconstruction kernel (B26f). For PT scans, raw data (65% phase) were used to reconstruct a single dataset using the same settings described earlier. Electrocardiographic editing was used whenever possible to decrease artifacts related to ectopic heart beats. Axial and double-oblique images viewed in thin-slab maximal intensity projections and multiplanar reformation settings were used for image analysis. Effective radiation dose was calculated by multiplying the dose–length product of the cardiac scan provided by the scan console (not including test bolus or calcium scoring) by a constant (k ⫽ 0.017 mSv/mGy/cm). IQ was determined based on 3 methods. Per-vessel analysis of nonevaluable vessels: For each patient who underwent CCT for coronary evaluation, clinical readings were used to identify any vessels that were deemed to have limited or nonevaluable segments.
Table 1 Baseline characteristics of patients in retrospective gating and prospective triggering groups Variable
Coronary evaluation Aorta ⫾ coronary evaluation Other (congenital/mass) Age (yrs) Women Height (inches) Weight (lbs) BMI (kg/m2)
Prospective Triggering (n ⫽ 42)
Retrospective Gating (n ⫽ 188)
p Value
28 (67%) 11 (26%)
159 (85%) 25 (13%)
0.02 (trend)
3 (7%) 44.3 ⫾ 13.7 16 (38%) 67 ⫾ 7.5 180.5 ⫾ 48.9 27.8 ⫾ 4.7
4 (2%) 56.5 ⫾ 13.7 74 (39%) 67.0 ⫾ 4.3 190.6 ⫾ 41.9 29.8 ⫾ 6.0
⬍0.001 NS NS NS 0.06
Values are numbers of patients (percentages) or means ⫾ SDs. BMI ⫽ body mass index.
The advantage of this method is that it identifies only limitations that are thought to have an impact on clinical interpretation. IQ score: Subjective IQ was determined retrospectively by use of a subjective scale (1 ⫽ poor; 2 ⫽ significantly decreased; 3 ⫽ mildly decreased; 4 ⫽ excellent). Each study was independently reviewed by 2 blinded readers (cardiologist and radiologist) who have each interpreted ⬎500 coronary computed tomographic scans. The average of the 2 scores was used to represent the study IQ. Contrast-to-noise and signal-to-noise ratios: Image noise was derived from the SD of the density values (in Hounsfield units) within a large region of interest in the left ventricle. Signal-to-noise ratio was defined as the ratio of the mean density of the contrast-filled left ventricular chamber divided by image noise. Contrast-to-noise ratio was
1170
The American Journal of Cardiology (www.AJConline.org) Table 3 Image quality assessment
Table 2 Scan parameters and effective radiation dose Scan Parameters
Prospective Triggering (n ⫽ 42)
Retrospective Gating (n ⫽ 188)
p Value
Tube current modulation Use of  blockers Average heart rate PAC, PVC, or sinus arrhythmia during scan acquisition Mean tube voltage (kV) 80 100 120 140 Tube currents* (mAs) Average pitch Scan length (cm) Radiation DLP CTDI volume Effective radiation dose (mSv)
N/A 9 (21.4%) 64.5 ⫾ 1.6 8 (19%)
184 (98%) 20 (10.6%) 68.0 ⫾ .9 34 (18%)
— 0.057 0.10 NS
114.3 4 (9.5%) 6 (14.3%) 30 (71.4%) 2 (4.8%) 200 ⫾ 41* N/A 17.2 ⫾ 4.5
115.1 0 (0%) 51 (27.1%) 132 (70.2%) 5 (2.7%) 320 ⫾ 67* 0.30 ⫾ .07 17.7 ⫾ 4.8
NS — — —
188 ⫾ 97 10.6 ⫾ 4.3 3.2 ⫾ 1.6
796 ⫾ 466 45.3 ⫾ 22.5 13.4 ⫾ 7.8
⬍0.001 ⬍0.001 ⬍0.001
NS* — NS
* For prospective triggering, tube current (milliamperes per second) is calculated as (total milliamperes ⫻ exposure time); for retrospective triggering, tube current/rotation is calculated as (total milliamperes ⫻ gantry rotation time). For further explanation on how to compare these 2 parameters, refer to Figure 1. CTDI ⫽ computed tomographic dose index; DLI ⫽ dose–length product; PAC ⫽ premature atrial contraction; PVC ⫽ premature ventricular contraction.
defined as the difference between the mean density of the contrast-filled left ventricular chamber and the mean density of the left ventricular wall, which was divided by image noise. This method, which has been previously described,4 is relevant for a wide range of cardiac computed tomographic studies, regardless of whether or not evaluation of the coronary arteries was performed. Data analysis was performed using STATA IC 10.0 (STATA Corp. LP, College Station, Texas). All continuous variables were expressed as mean ⫾ SD, and categorical variables were expressed as percentage. Differences in continuous variables were assessed using Student’s unpaired t tests. Differences in dichotomous variables were assessed using chi-square test. Kappa statistic was used to test for interobserver agreement of IQ assessment. A p value ⬍0.05 was considered statistically significant. Results For the PT group (n ⫽ 42) and the non-PT comparison group (n ⫽ 188), all scans were successfully completed without any complications. Table 1 presents the type of examination performed and the baseline characteristics of each group. Patients who were selected to have PT scans were younger (mean age 44.3 vs 56.5 years, p ⬍0.001) and were more likely to be referred for evaluation of noncoronary structures (i.e., aorta/masses). There was a trend toward lower body mass index in patients selected to have PT scans (body mass index 27.8 vs 29.8 kg/m2, p ⫽ 0.06).
IQ
Prospective Triggering (n ⫽ 42)
Retrospective Triggering (n ⫽ 188)
p Value
Mean IQ score 1 2 3 4 Nonevaluable vessels (e.g., ⱖ1 segment nonevaluable) Left main coronary artery Left anterior descending coronary artery Right coronary artery Left circumflex coronary artery Measurements Noise Contrast-to-noise Signal-to-noise
2.93 1 (2.4%) 8 (19.0%) 23 (54.8%) 10 (23.8%)
2.86 8 (4.3%) 39 (21.0%) 102 (54.8%) 37 (19.9%)
NS NS (trend)
0 (0%) 3 (7.7%)
8 (4.5%) 35 (19.3%)
0.18 0.08
5 (12.8) 1 (2.5%)
29 (16.0%) 23 (12.7%)
0.61 0.06
52.5 ⫾ 18.6 5.7 ⫾ 2.7 8.0 ⫾ 3.2
45.0 ⫾ 12.8 6.4 ⫾ 2.7 8.9 ⫾ 3.3
0.002 0.12 0.13
Of the 42 PT scans, 28 patients were referred for coronary evaluation, 11 for aortic disease with/without coronaries (larger coverage), and 3 for other reasons (i.e., suspected mass and congenital disease); 21.4% of patients were given -blocker medications before scanning. Average heart rate was 64.5 beats/min, and although all patients were in sinus rhythm, 19% had premature atrial contractions, premature ventricular contractions, or significant sinus arrhythmia during scan acquisition. Table 2 presents a comparison of scan acquisition parameters and resulting effective radiation doses for the PT and non-PT groups. Among the 188 retrospectively gated scans, tube current modulation was used 98% of the time and average window width was 24%. There was a trend toward more frequent use of  blockers with PT scan and a weak trend toward a lower average heart rate for PT scans (64.5 vs 68.0 beats/min, p ⫽ 0.10). There was no significant difference in the kilovolt or milliampere per second selected for PT and non-PT scans and no significant difference in average scan length for the 2 groups. The average radiation dose of the 42 PT scans was 3.2 ⫾ 1.6 versus 13.4 ⫾ 7.8 mSv for the 188 non-PT scans. As expected, a lower effective dose was achieved with a lower kilovolt and smaller coverage. When excluding the 11 studies with larger coverage, the average dose for PT scans was 2.8 mSv. Use of 100 kV (n ⫽ 6) and 80 kV (n ⫽ 4) resulted in average doses of 1.3 and 0.7 mSv, respectively. Table 3 present results of the IQ assessment between the PT and non-PT groups. The interobserver agreement for IQ scores between the 2 readers was excellent (kappa 0.85). Mean IQ score was 2.9 for the PT and non-PT scans. Furthermore, there was no significant difference in contrastto-noise or signal-to-noise between the PT and non-PT groups (signal-to-noise 8.0 vs 8.9, p ⫽ 0.13; contrast-to-
Methods/Prospective Triggering With Cardiac CT
1171
Table 4 Comparison of radiation dose and image quality: subgroup analysis Subgroup Analysis
Prospective Triggering (n ⫽ 42)
No. Low vs high heart rate Heart rate ⬍70 beats/min Heart rate ⱖ70 beats/min p value (low vs high heart rate) Low vs high kV 80 100 120 140 p value (100 vs 120 kV)
DLP
Retrospective Triggering (n ⫽ 188)
IQ
No.
29 13 —
189 186 NS
3.12 2.5 0.01
105 81 —
4 6 30 2 —
37.5 104 214 358 0.002
2.75 3.25 2.85 3.5 NS
— 51 140 5 —
DLP 804 786 NS — 502 896 1,186 ⬍0.001
p Value (prospective triggering vs nonprospective triggering) IQ
DLP
IQ
3.04 2.63 ⬍0.001
⬍0.001 ⬍0.001 —
NS NS —
— 2.99 2.83 2.4 NS
— ⬍0.001 ⬍0.001 0.03 —
— NS NS 0.07 —
Abbreviations as in Table 2.
Figure 2. A 32-year-old woman with a history of chest pressure and dyspnea on exertion was referred for CCT for evaluation of anomalous coronary arteries after her echocardiogram suggested possible abnormal origins of the coronary arteries. Computed tomographic study showed that all 3 coronary arteries had a normal origin from the ascending aorta and were free of coronary artery disease. The effective radiation dose was 2.4 mSv.
noise 5.7 vs 6.4, p ⫽ 0.12). Table 4 presents the radiation dose and IQ for patients with low (⬍70) and high (ⱖ70) heart rate and a subgroup analysis for scans using different kilovolts. There was no significant difference in the number of nonevaluable vessels between the PT and non-PT groups. Of the 39 PT scans evaluating coronary arteries, evaluation of the right, left anterior descending, and left circumflex coronary arteries was limited in 5 and 3 cases and 1 case, respectively. The most common reasons for these limitations included motion, ectopy, and calcium. Incidences of limited right coronary artery evaluation (irrespective of the
reason) were 12.8% and 16.0% for the PT and non-PT groups, respectively (p ⫽ 0.62). Although right coronary artery limitations related to motion were more commonly observed for PT scans (3 of 5 limited vessels) than for non-PT scans (6 of 29 limited vessels), this difference was not statistically significant. Discussion In 1 of the first studies evaluating the feasibility of PT CCT using DSCT, we observed that compared with retrospective gating, prospective electrocardiographic triggering
1172
The American Journal of Cardiology (www.AJConline.org)
Table 5 Suggested considerations for use of prospective triggering in cardiac computed tomography Use Prospective Use Triggering Retrospective Gating Considerations related to indication(s)/ patient characteristics Is radiation dose a concern for this patient? Young age, especially women Need to assess LV or RV function or wall motion abnormalities? Known CAD or high pretest probability of significant CAD? High CAC score (i.e., CAC ⱖ400) Previous abnormal stress test result Evaluation for suspected anomalous coronary arteries (i.e., young patients with low risk of obstructive CAD; main interest is only proximal vessel visualization) Technical considerations related to likelihood of diagnostic quality scan Is it possible to achieve a low heart rate? ⬍60 for single-source scanner; ⬍65 for DSCT Is there evidence of significant ectopy (frequent PACs or PVCs) on prescan monitoring? If calcium scoring is performed, is there evidence of motion artifacts for RCA?
Yes
No
No
Yes
No
Yes
Yes
No
Yes
No
No
Yes
No
Yes
CAC ⫽ coronary artery calcium; CAD ⫽ coronary artery disease; LV ⫽ left ventricular; RCA ⫽ right coronary artery; RV ⫽ right ventricular.
resulted in a decrease in patient radiation dose of 73%. In our selected patient population, this lower radiation was achieved without compromising IQ (Figure 2). The lower radiation dose that results from the use of prospective triggering can be attributed to 2 mechanisms. First, with prospective triggering the tube current is only turned on during a small predefined phase of the cardiac cycle. The second reason for the decreased radiation is that, in contrast to retrospective helical acquisition, there is no overlap between segments. On the contrary, when dualsource computed tomographic retrospective gating is employed, with a typical pitch of 0.2 to 0.5, each level is effectively scanned 2 to 5 times. Given our experience and the results of other centers, it is reasonable to conclude that, rather than using this technique on all patients, it is important to identify which patients who require CCT are appropriate candidates for prospective triggering. Suggested considerations for the use of prospective triggering are presented in Table 5. We recommend the use of prospective triggering for patients who are most susceptible to the potential harmful
effects of ionizing radiation (i.e., younger patients, particularly women). Prospective triggering may be ideally suited for patients with an appropriate indication for CCT but who have a low pretest probability of coronary artery disease. However, in patients with potentially obstructive coronary artery disease, a retrospective gating technique that enables assessment of images from different phases of the cardiac cycle may be preferred. Another consideration is that when selecting patients for PT scans it is important to select patients who are expected to have diagnostic IQ, namely patients with a slow heart rate who do not have frequent ectopy. Furthermore, it is important to remember that prospective triggering should not be used when functional data (i.e., wall motion abnormalities) are needed. Importantly, aside from evaluation of coronary arteries, there are other important noncoronary indications for CCT that could benefit from prospective triggering. Several such indications include evaluation of congenital heart disease or evaluation of diseases of the aorta.5 Our study has some limitations. Scoring of IQ was based on an arbitrary scale and is thus, by nature, a subjective process. Second, because all patients who underwent PT scanning were specifically selected, the results of our study are not applicable to all patients who present for CCT. Nevertheless, because milliampere per second, kilovolt, and scan length were similar for the PT and non-PT groups, the remarkable decrease of radiation dose should apply to all patients who are being considered for CCT. Although it is clear that prospective triggering results in a substantial decrease of radiation dose, whether this has an effect on the diagnostic accuracy of CCT remains unknown. Scheffel et al6 recently showed that, in selected patients who also underwent invasive angiography, use of prospective triggering resulted in excellent diagnostic accuracy. However, whether prospective triggering has improved diagnostic accuracy over retrospective triggering remains unknown. Answering this question will require randomized blinded prospective studies comparing PT and retrospective gating scans with invasive angiographic scans. A limitation of prospective triggering is that an irregular heart rate often results in misregistration artifacts (also known as “stair-step artifacts”). Future studies should address whether scanners that have a longer coverage in the z-dimension, albeit with the possible compromise of decreased temporal resolution, may help to improve IQ by decreasing or eliminating such artifacts. Future techniques that improve temporal and spatial resolutions stand to improve IQ for cardiac computed tomographic scans regardless of the method of data acquisition. Such advancements should ultimately result in lower radiation dose. In addition, better algorithms for arrhythmia recognition and rejection of ectopic beats are expected to improve the IQ of PT CCT. 1. 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 tech-
Methods/Prospective Triggering With Cardiac CT nique: improved image quality and reduced radiation dose. Radiology 2008;246:742–775. 2. 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;29:191–197. 3. Shuman WP, Branch KR, May JM, Mitsumori LM, Lockhart DW, Dubinsky TJ, Warren BH, Caldwell 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– 437. 4. Hausleiter J, Meyer T, Hadamitzky M, Huber E, Zankl M, Martinoff S, Kastrati A, Schomig A. Radiation dose estimates from cardiac multi-
1173
slice computed tomography in daily practice: impact of different scanning protocols on effective dose estimates. Circulation 2006;113: 1305–1310. 5. ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 appropriateness criteria for cardiac computed tomography and cardiac magnetic resonance imaging. A report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group. J Am Coll Radiol 2006;3: 751–771. 6. 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-and-shoot mode: diagnostic performance. Heart 2008;94:1132–1137.
Cardiac MR PET CT Program, Department of Radiology and Division of Cardiology, Massachusetts General Hospital and Harvard Medical School, and bNoninvasive Cardiovascular Imaging Program, Department of Medicine and Radiology, Brigham and Women’s Hospital, Boston, Massachusetts; and cBaptist Cardiac and Vascular Institute, Miami, Florida. Manuscript received August 21, 2008; revised manuscript received and accepted December 21, 2008. Dr. Blankstein has received support from Grant 1T32 HL076136 from the National Institutes of Health, Bethesda, Maryland. *Corresponding author: Tel: 617-724-5351; fax: 617-725-4152. E-mail address: [email protected] (R. Blankstein). 0002-9149/09/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.amjcard.2008.12.045
could achieve a lower radiation dose without compromising image quality (IQ). Methods Our study population included 312 consecutive patients referred for a cardiac computed tomographic examination. Of these, prospective triggering was used in 42 patients for whom, based on physician judgment, (1) use of prospective triggering was feasible (e.g., low, regular heart rate) and (2) minimizing radiation exposure was warranted (e.g., younger age). Of the 270 nonprospectively triggered (non-PT) PT scans, all scans performed for evaluation of bypass grafts (n ⫽ 26) or pulmonary veins (n ⫽ 56) were excluded. The remainder non-PT comparison group consisted of 188 patients. Cardiac CT (CCT) was performed on a dual-source 64slice computed tomographic scanner (Siemens Healthcare: Definition, Florsheim, Germany) with a gantry rotation time of 330 ms and standard detector collimation of 0.6 mm. A flying focus along the z-axis (z-sharp technology) was used to acquire 64 overlapping 0.6-mm slices using 2 32-detector rows. The resulting temporal resolution was 83 ms. Figure 1 displays some key parameters associated with dual-source computed tomographic prospective triggering. Scans using prospective triggering (Siemens Healthcare: Sequential Scanning) were obtained at 65% of the RR www.AJConline.org
Methods/Prospective Triggering With Cardiac CT
1169
Figure 1. Prospective triggering: explanation of key parameters.
interval. In this mode, there is no table movement during data acquisition, yet the gantry with its 2 x-ray sources and detectors continues to rotate. The resulting coverage in the z-axis is 19.2 mm per acquisition, which is also equal to the table movement that occurs between acquisitions. For all cardiac computed tomographic scans, contrast timing was determined with the use of a test bolus of contrast 20 ml. Initial image acquisition timing was then determined by adding 2 to 4 seconds to the time of peak contrast enhancement in the ascending aorta. A physician was present for each study and selected the kilovolt and milliampere per second based on assessment of patient’s body habitus. Beta blockers were not routinely administered according to our standard dual-source computed tomographic clinical protocol. For retrospectively gated CCT, raw data from 5% to 95% of the cardiac cycle were used to reconstruct data at 10% intervals with 0.75 mm slice thickness and 0.4-mm overlap using a medium smooth reconstruction kernel (B26f). For PT scans, raw data (65% phase) were used to reconstruct a single dataset using the same settings described earlier. Electrocardiographic editing was used whenever possible to decrease artifacts related to ectopic heart beats. Axial and double-oblique images viewed in thin-slab maximal intensity projections and multiplanar reformation settings were used for image analysis. Effective radiation dose was calculated by multiplying the dose–length product of the cardiac scan provided by the scan console (not including test bolus or calcium scoring) by a constant (k ⫽ 0.017 mSv/mGy/cm). IQ was determined based on 3 methods. Per-vessel analysis of nonevaluable vessels: For each patient who underwent CCT for coronary evaluation, clinical readings were used to identify any vessels that were deemed to have limited or nonevaluable segments.
Table 1 Baseline characteristics of patients in retrospective gating and prospective triggering groups Variable
Coronary evaluation Aorta ⫾ coronary evaluation Other (congenital/mass) Age (yrs) Women Height (inches) Weight (lbs) BMI (kg/m2)
Prospective Triggering (n ⫽ 42)
Retrospective Gating (n ⫽ 188)
p Value
28 (67%) 11 (26%)
159 (85%) 25 (13%)
0.02 (trend)
3 (7%) 44.3 ⫾ 13.7 16 (38%) 67 ⫾ 7.5 180.5 ⫾ 48.9 27.8 ⫾ 4.7
4 (2%) 56.5 ⫾ 13.7 74 (39%) 67.0 ⫾ 4.3 190.6 ⫾ 41.9 29.8 ⫾ 6.0
⬍0.001 NS NS NS 0.06
Values are numbers of patients (percentages) or means ⫾ SDs. BMI ⫽ body mass index.
The advantage of this method is that it identifies only limitations that are thought to have an impact on clinical interpretation. IQ score: Subjective IQ was determined retrospectively by use of a subjective scale (1 ⫽ poor; 2 ⫽ significantly decreased; 3 ⫽ mildly decreased; 4 ⫽ excellent). Each study was independently reviewed by 2 blinded readers (cardiologist and radiologist) who have each interpreted ⬎500 coronary computed tomographic scans. The average of the 2 scores was used to represent the study IQ. Contrast-to-noise and signal-to-noise ratios: Image noise was derived from the SD of the density values (in Hounsfield units) within a large region of interest in the left ventricle. Signal-to-noise ratio was defined as the ratio of the mean density of the contrast-filled left ventricular chamber divided by image noise. Contrast-to-noise ratio was
1170
The American Journal of Cardiology (www.AJConline.org) Table 3 Image quality assessment
Table 2 Scan parameters and effective radiation dose Scan Parameters
Prospective Triggering (n ⫽ 42)
Retrospective Gating (n ⫽ 188)
p Value
Tube current modulation Use of  blockers Average heart rate PAC, PVC, or sinus arrhythmia during scan acquisition Mean tube voltage (kV) 80 100 120 140 Tube currents* (mAs) Average pitch Scan length (cm) Radiation DLP CTDI volume Effective radiation dose (mSv)
N/A 9 (21.4%) 64.5 ⫾ 1.6 8 (19%)
184 (98%) 20 (10.6%) 68.0 ⫾ .9 34 (18%)
— 0.057 0.10 NS
114.3 4 (9.5%) 6 (14.3%) 30 (71.4%) 2 (4.8%) 200 ⫾ 41* N/A 17.2 ⫾ 4.5
115.1 0 (0%) 51 (27.1%) 132 (70.2%) 5 (2.7%) 320 ⫾ 67* 0.30 ⫾ .07 17.7 ⫾ 4.8
NS — — —
188 ⫾ 97 10.6 ⫾ 4.3 3.2 ⫾ 1.6
796 ⫾ 466 45.3 ⫾ 22.5 13.4 ⫾ 7.8
⬍0.001 ⬍0.001 ⬍0.001
NS* — NS
* For prospective triggering, tube current (milliamperes per second) is calculated as (total milliamperes ⫻ exposure time); for retrospective triggering, tube current/rotation is calculated as (total milliamperes ⫻ gantry rotation time). For further explanation on how to compare these 2 parameters, refer to Figure 1. CTDI ⫽ computed tomographic dose index; DLI ⫽ dose–length product; PAC ⫽ premature atrial contraction; PVC ⫽ premature ventricular contraction.
defined as the difference between the mean density of the contrast-filled left ventricular chamber and the mean density of the left ventricular wall, which was divided by image noise. This method, which has been previously described,4 is relevant for a wide range of cardiac computed tomographic studies, regardless of whether or not evaluation of the coronary arteries was performed. Data analysis was performed using STATA IC 10.0 (STATA Corp. LP, College Station, Texas). All continuous variables were expressed as mean ⫾ SD, and categorical variables were expressed as percentage. Differences in continuous variables were assessed using Student’s unpaired t tests. Differences in dichotomous variables were assessed using chi-square test. Kappa statistic was used to test for interobserver agreement of IQ assessment. A p value ⬍0.05 was considered statistically significant. Results For the PT group (n ⫽ 42) and the non-PT comparison group (n ⫽ 188), all scans were successfully completed without any complications. Table 1 presents the type of examination performed and the baseline characteristics of each group. Patients who were selected to have PT scans were younger (mean age 44.3 vs 56.5 years, p ⬍0.001) and were more likely to be referred for evaluation of noncoronary structures (i.e., aorta/masses). There was a trend toward lower body mass index in patients selected to have PT scans (body mass index 27.8 vs 29.8 kg/m2, p ⫽ 0.06).
IQ
Prospective Triggering (n ⫽ 42)
Retrospective Triggering (n ⫽ 188)
p Value
Mean IQ score 1 2 3 4 Nonevaluable vessels (e.g., ⱖ1 segment nonevaluable) Left main coronary artery Left anterior descending coronary artery Right coronary artery Left circumflex coronary artery Measurements Noise Contrast-to-noise Signal-to-noise
2.93 1 (2.4%) 8 (19.0%) 23 (54.8%) 10 (23.8%)
2.86 8 (4.3%) 39 (21.0%) 102 (54.8%) 37 (19.9%)
NS NS (trend)
0 (0%) 3 (7.7%)
8 (4.5%) 35 (19.3%)
0.18 0.08
5 (12.8) 1 (2.5%)
29 (16.0%) 23 (12.7%)
0.61 0.06
52.5 ⫾ 18.6 5.7 ⫾ 2.7 8.0 ⫾ 3.2
45.0 ⫾ 12.8 6.4 ⫾ 2.7 8.9 ⫾ 3.3
0.002 0.12 0.13
Of the 42 PT scans, 28 patients were referred for coronary evaluation, 11 for aortic disease with/without coronaries (larger coverage), and 3 for other reasons (i.e., suspected mass and congenital disease); 21.4% of patients were given -blocker medications before scanning. Average heart rate was 64.5 beats/min, and although all patients were in sinus rhythm, 19% had premature atrial contractions, premature ventricular contractions, or significant sinus arrhythmia during scan acquisition. Table 2 presents a comparison of scan acquisition parameters and resulting effective radiation doses for the PT and non-PT groups. Among the 188 retrospectively gated scans, tube current modulation was used 98% of the time and average window width was 24%. There was a trend toward more frequent use of  blockers with PT scan and a weak trend toward a lower average heart rate for PT scans (64.5 vs 68.0 beats/min, p ⫽ 0.10). There was no significant difference in the kilovolt or milliampere per second selected for PT and non-PT scans and no significant difference in average scan length for the 2 groups. The average radiation dose of the 42 PT scans was 3.2 ⫾ 1.6 versus 13.4 ⫾ 7.8 mSv for the 188 non-PT scans. As expected, a lower effective dose was achieved with a lower kilovolt and smaller coverage. When excluding the 11 studies with larger coverage, the average dose for PT scans was 2.8 mSv. Use of 100 kV (n ⫽ 6) and 80 kV (n ⫽ 4) resulted in average doses of 1.3 and 0.7 mSv, respectively. Table 3 present results of the IQ assessment between the PT and non-PT groups. The interobserver agreement for IQ scores between the 2 readers was excellent (kappa 0.85). Mean IQ score was 2.9 for the PT and non-PT scans. Furthermore, there was no significant difference in contrastto-noise or signal-to-noise between the PT and non-PT groups (signal-to-noise 8.0 vs 8.9, p ⫽ 0.13; contrast-to-
Methods/Prospective Triggering With Cardiac CT
1171
Table 4 Comparison of radiation dose and image quality: subgroup analysis Subgroup Analysis
Prospective Triggering (n ⫽ 42)
No. Low vs high heart rate Heart rate ⬍70 beats/min Heart rate ⱖ70 beats/min p value (low vs high heart rate) Low vs high kV 80 100 120 140 p value (100 vs 120 kV)
DLP
Retrospective Triggering (n ⫽ 188)
IQ
No.
29 13 —
189 186 NS
3.12 2.5 0.01
105 81 —
4 6 30 2 —
37.5 104 214 358 0.002
2.75 3.25 2.85 3.5 NS
— 51 140 5 —
DLP 804 786 NS — 502 896 1,186 ⬍0.001
p Value (prospective triggering vs nonprospective triggering) IQ
DLP
IQ
3.04 2.63 ⬍0.001
⬍0.001 ⬍0.001 —
NS NS —
— 2.99 2.83 2.4 NS
— ⬍0.001 ⬍0.001 0.03 —
— NS NS 0.07 —
Abbreviations as in Table 2.
Figure 2. A 32-year-old woman with a history of chest pressure and dyspnea on exertion was referred for CCT for evaluation of anomalous coronary arteries after her echocardiogram suggested possible abnormal origins of the coronary arteries. Computed tomographic study showed that all 3 coronary arteries had a normal origin from the ascending aorta and were free of coronary artery disease. The effective radiation dose was 2.4 mSv.
noise 5.7 vs 6.4, p ⫽ 0.12). Table 4 presents the radiation dose and IQ for patients with low (⬍70) and high (ⱖ70) heart rate and a subgroup analysis for scans using different kilovolts. There was no significant difference in the number of nonevaluable vessels between the PT and non-PT groups. Of the 39 PT scans evaluating coronary arteries, evaluation of the right, left anterior descending, and left circumflex coronary arteries was limited in 5 and 3 cases and 1 case, respectively. The most common reasons for these limitations included motion, ectopy, and calcium. Incidences of limited right coronary artery evaluation (irrespective of the
reason) were 12.8% and 16.0% for the PT and non-PT groups, respectively (p ⫽ 0.62). Although right coronary artery limitations related to motion were more commonly observed for PT scans (3 of 5 limited vessels) than for non-PT scans (6 of 29 limited vessels), this difference was not statistically significant. Discussion In 1 of the first studies evaluating the feasibility of PT CCT using DSCT, we observed that compared with retrospective gating, prospective electrocardiographic triggering
1172
The American Journal of Cardiology (www.AJConline.org)
Table 5 Suggested considerations for use of prospective triggering in cardiac computed tomography Use Prospective Use Triggering Retrospective Gating Considerations related to indication(s)/ patient characteristics Is radiation dose a concern for this patient? Young age, especially women Need to assess LV or RV function or wall motion abnormalities? Known CAD or high pretest probability of significant CAD? High CAC score (i.e., CAC ⱖ400) Previous abnormal stress test result Evaluation for suspected anomalous coronary arteries (i.e., young patients with low risk of obstructive CAD; main interest is only proximal vessel visualization) Technical considerations related to likelihood of diagnostic quality scan Is it possible to achieve a low heart rate? ⬍60 for single-source scanner; ⬍65 for DSCT Is there evidence of significant ectopy (frequent PACs or PVCs) on prescan monitoring? If calcium scoring is performed, is there evidence of motion artifacts for RCA?
Yes
No
No
Yes
No
Yes
Yes
No
Yes
No
No
Yes
No
Yes
CAC ⫽ coronary artery calcium; CAD ⫽ coronary artery disease; LV ⫽ left ventricular; RCA ⫽ right coronary artery; RV ⫽ right ventricular.
resulted in a decrease in patient radiation dose of 73%. In our selected patient population, this lower radiation was achieved without compromising IQ (Figure 2). The lower radiation dose that results from the use of prospective triggering can be attributed to 2 mechanisms. First, with prospective triggering the tube current is only turned on during a small predefined phase of the cardiac cycle. The second reason for the decreased radiation is that, in contrast to retrospective helical acquisition, there is no overlap between segments. On the contrary, when dualsource computed tomographic retrospective gating is employed, with a typical pitch of 0.2 to 0.5, each level is effectively scanned 2 to 5 times. Given our experience and the results of other centers, it is reasonable to conclude that, rather than using this technique on all patients, it is important to identify which patients who require CCT are appropriate candidates for prospective triggering. Suggested considerations for the use of prospective triggering are presented in Table 5. We recommend the use of prospective triggering for patients who are most susceptible to the potential harmful
effects of ionizing radiation (i.e., younger patients, particularly women). Prospective triggering may be ideally suited for patients with an appropriate indication for CCT but who have a low pretest probability of coronary artery disease. However, in patients with potentially obstructive coronary artery disease, a retrospective gating technique that enables assessment of images from different phases of the cardiac cycle may be preferred. Another consideration is that when selecting patients for PT scans it is important to select patients who are expected to have diagnostic IQ, namely patients with a slow heart rate who do not have frequent ectopy. Furthermore, it is important to remember that prospective triggering should not be used when functional data (i.e., wall motion abnormalities) are needed. Importantly, aside from evaluation of coronary arteries, there are other important noncoronary indications for CCT that could benefit from prospective triggering. Several such indications include evaluation of congenital heart disease or evaluation of diseases of the aorta.5 Our study has some limitations. Scoring of IQ was based on an arbitrary scale and is thus, by nature, a subjective process. Second, because all patients who underwent PT scanning were specifically selected, the results of our study are not applicable to all patients who present for CCT. Nevertheless, because milliampere per second, kilovolt, and scan length were similar for the PT and non-PT groups, the remarkable decrease of radiation dose should apply to all patients who are being considered for CCT. Although it is clear that prospective triggering results in a substantial decrease of radiation dose, whether this has an effect on the diagnostic accuracy of CCT remains unknown. Scheffel et al6 recently showed that, in selected patients who also underwent invasive angiography, use of prospective triggering resulted in excellent diagnostic accuracy. However, whether prospective triggering has improved diagnostic accuracy over retrospective triggering remains unknown. Answering this question will require randomized blinded prospective studies comparing PT and retrospective gating scans with invasive angiographic scans. A limitation of prospective triggering is that an irregular heart rate often results in misregistration artifacts (also known as “stair-step artifacts”). Future studies should address whether scanners that have a longer coverage in the z-dimension, albeit with the possible compromise of decreased temporal resolution, may help to improve IQ by decreasing or eliminating such artifacts. Future techniques that improve temporal and spatial resolutions stand to improve IQ for cardiac computed tomographic scans regardless of the method of data acquisition. Such advancements should ultimately result in lower radiation dose. In addition, better algorithms for arrhythmia recognition and rejection of ectopic beats are expected to improve the IQ of PT CCT. 1. 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 tech-
Methods/Prospective Triggering With Cardiac CT nique: improved image quality and reduced radiation dose. Radiology 2008;246:742–775. 2. 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;29:191–197. 3. Shuman WP, Branch KR, May JM, Mitsumori LM, Lockhart DW, Dubinsky TJ, Warren BH, Caldwell 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– 437. 4. Hausleiter J, Meyer T, Hadamitzky M, Huber E, Zankl M, Martinoff S, Kastrati A, Schomig A. Radiation dose estimates from cardiac multi-
1173
slice computed tomography in daily practice: impact of different scanning protocols on effective dose estimates. Circulation 2006;113: 1305–1310. 5. ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 appropriateness criteria for cardiac computed tomography and cardiac magnetic resonance imaging. A report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group. J Am Coll Radiol 2006;3: 751–771. 6. 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-and-shoot mode: diagnostic performance. Heart 2008;94:1132–1137.