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Feasibility of dual-source cardiac CT angiography with high-pitch scan protocols
Journal of Cardiovascular Computed Tomography (2009) 3, 236–242
Original Research Article
Feasibility of dual-source cardiac CT angiography with high-pitch scan protocols Jo¨rg Hausleiter, MDa,*, Bernhard Bischoff, MDa, Franziska Hein, MDa, Tanja Meyer, MDa, Martin Hadamitzky, MDa, Carsten Thierfelder, PhDb, Thomas Allmendinger, PhDb, Thomas G. Flohr, PhDb, Albert Scho¨mig, MDa, Stefan Martinoff, MDc a
Klinik fu¨r Herz- und Kreislauferkrankungen, Deutsches Herzzentrum Mu¨nchen; Klinik an der TU Mu¨nchen, Lazarettstrasse 36, 80636 Munich, Germany; bComputed Tomography, Siemens Healthcare, Forchheim, Germany and c Institut fu¨r Radiologie und Nuklearmedizin, Deutsches Herzzentrum Mu¨nchen; Klinik an der TU Mu¨nchen, Munich, Germany KEYWORDS: Coronary artery disease; Dual-source CT; High pitch; Radiation dose
BACKGROUND: Cardiac CT angiography (CCTA) has become a frequently used diagnostic tool in clinical practice, but concern remains about the radiation exposure. Because of the second x-ray acquisition system, dual-source CT systems might allow for high-pitch CT data acquisition and thus for examination of the whole heart during a single heart beat, with the potential for radiation dose reduction. OBJECTIVE: We assessed the feasibility of a high-pitch scan mode with a dual-source CT system. METHODS: High-pitch modes were used in patients undergoing CCTA with a dual-source CT system. Diagnostic image quality for cardiac structures and coronary arteries was assessed. Radiation dose was estimated from the scanner-generated dose-length product (DLP). RESULTS: CCTAwas performed in 14 patients during a single heart beat applying a pitch value of 3.4. Mean heart rate during examination was 56.4 6 8.1 beats/min. Diagnostic image quality for the assessment of larger cardiac structures was obtained in all patients, whereas diagnostic image quality could be achieved in 82% of all coronary segments. With a mean DLP of 145 6 47 mGy ! cm, the resulting estimated radiation dose was 2.0 6 0.7 mSv. CONCLUSIONS: This proof-of-concept study shows the ability of dual-source CT scanners to scan the whole heart during one single heart beat at low radiation dose. Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction Conflict of interest: Drs Thierfelder, Allmendinger, and Flohr are employees of Siemens Medical Solutions, Forchheim, Germany. Dr. Hausleiter reports receiving research grants and speaker honoraria from Siemens Medical Solutions unrelated to the current study. The remaining authors have no conflicts of interest concerning the topic of this study. * Corresponding author. E-mail address: [email protected] Submitted February 17, 2009. Accepted for publication May 31, 2009.
Cardiac CT angiography (CCTA) has become a frequently used diagnostic tool in clinical practice for the evaluation of the coronary arteries.1–3 Despite continuous improvements in CT technology, concerns remain about the patients’ radiation exposure. When performing 64-slice CCTA, the effective radiation dose has been shown to
1934-5925/$ -see front matter Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2009.05.012
Hausleiter et al Table 1
High-pitch cardiac CT angiography
237
Patient characteristics (n 5 14) Values
Age, mean 6 SD, y Male sex, n (%) Height, mean 6 SD, m Weight, mean 6 SD, kg Body mass index, mean 6 SD, kg/m2 Heart rate, mean 6 SD, beats/min Sinus rhythm, n (%)
59.7 6 10.8 11 (78.6) 1.74 6 0.09 81.1 6 16.5 26.4 6 4.4 52.6 6 5.5 14 (100)
approximate 12 mSv, depending on the scanning technique, patient-related factors, as well as the CT system.4 Recently, new scan protocols have been introduced into CCTA, including 100 kVp and sequential scan protocols, which will allow for considerable reductions in radiation exposure. With single-source CT systems, the spiral pitch (table feed per rotation divided by the collimated beam width) is limited to a maximum value of 1.5 for gapless data acquisition in the z-axis. At higher pitch values data gaps occur which will result in image artifacts. Compared with conventional single-source CT systems, the dual-source CT (DSCT) configuration with 2 radiation tubes and 2 detectors arranged in a 90-degree angle allows for both high temporal resolution5,6 and high-pitch data acquisition. The high temporal resolution is achieved because only a 90degree rotation of the system is necessary for image reconstruction, whereas conventional systems need at least a 180-degree rotation of the x-ray gantry. In the high-pitch scan mode, the second tube detector system of a DSCT system is being used to fill the data gaps; accordingly, the pitch can be increased to values .3.6 Thus, CCTA during a single heart beat would become possible with high-pitch data acquisition. This short, subsecond exposure time to ionizing radiation would also result in a considerable reduction in estimated radiation dose compared with spiral imaging Table 2 patients
Scanning characteristics in high-pitch scans in 14 Values
Administration of intravenous metoprolol, n (%) 9 (64) Metoprolol dose, mean 6 SD, mg 5.0 6 4.4 Contrast dye volume, mean 6 SD, mL 58 6 16 Tube voltage 120 kVp, n (%) 12 (86) 100 kVp, n (%) 2 (14) Tube current, mean 6 SD, mAs 427 6 28 Pitch value, mean 6 SD 3.4 6 0.0 Scan length, mean 6 SD, mm 154 6 57 Scan time, mean 6 SD, ms 1085 6 380 Volume CT dose index, mean 6 SD, mGy 6.8 6 1.4 Dose-length product, mean 6 SD, mGy ! cm 145 6 47 Dose estimate, mean 6 SD, mSv 2.0 6 0.7
Figure 1 Three-dimensional reconstruction of the heart and left internal mammary artery bypass-graft (arrows) in volume-rendering technique in a 62-year-old male patient with a heart rate of 56 beats/min.
with a lower pitch. Accordingly, the aim of this study was to prove the concept that CCTA during one heart beat is feasible with DSCT systems applying high-pitch modes for CT data acquisition.
Methods Patients We examined 14 selected patients undergoing CCTA for examination of the cardiac anatomy without need to evaluate the coronary arteries. The indications comprised the visualization of (1) the left atrium and pulmonary vein ostia before a planned electrophysiologic procedure, (2) the ascending aorta, and (3) aortocoronary bypass grafts for patency. Patients were included if the heart rate was below 60 beats/min without atrial or ventricular premature beats during preparation and if the renal and thyroid functions were normal. All patients gave written informed consent.
Image acquisition All patients were examined with the use of a 64-slice DSCT (SOMATOM Definition; Siemens Medical Solutions, Forchheim, Germany) with a prospectively triggered high-pitch spiral scan technique. Before image acquisition, patients with a heart rate .60 beats/min or a nonstable sinus rhythm received up to 2 doses of 5 mg metoprolol
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Figure 2 Curved-planar maximum intensity projections of the left anterior descending artery (A), the left circumflex artery (B), and the right coronary artery (C), as well as 3-dimensional reconstruction of the heart in volume-rendering technique (D) in a patient scanned with a heart rate of 53 beats/min.
intravenously to lower and stabilize heart rate. In addition, coronary vasodilatation was achieved in all patients with a systolic blood pressure of R100 mmHg by the administration of nitroglycerine 0.8 mg sublingually. The volume of contrast medium that was injected with a flow rate of 6 mL/s was individually adjusted. For patients undergoing CCTA before a planned electrophysiologic procedure a contrast volume of 50 mL was used, whereas a volume of 87 6 11 mL was injected in the remaining patients to account for the longer scan ranges. The contrast bolus was followed by a saline chaser bolus at the same flow rate. Image acquisition was started usually 5 seconds after the measured time to peak enhancement in the ascending aorta preceeding the CCTA scan. The scanner setting was as follows: We used a collimation of 32 ! 0.6 mm. Twelve patients were scanned with a tube voltage of 120 kVp and a tube current time product per rotation of 438 mAs, 2 patients with 100 kVp and 362 mAs, respectively. Applying a pitch value of 3.4, the scan was started at 33% 6 10% of the R-R interval in the caudocranial scan direction. From the obtained data, transaxial images (slice thickness, 0.6 mm; increment, 0.3 mm) were recontructed applying a standard kernel (B26f).
intensity projections (slice thickness: 5 mm), axial slices (slice thickness: 0.6 mm), and additional curved multiplanar reformations if needed. Disagreements between the first 2 readers were solved by a third experienced CCTA reader. When assessing diagnostic quality of cardiac anatomy, the image quality of the left atrium with the pulmonary veins, the ascending aorta and of the bypass grafts was either rated diagnostic or nondiagnostic. The cardiac anatomy was rated nondiagnostic if the left atrium, the ascending aorta, or the bypass grafts could not be clearly delineated because of motion or reconstruction artifacts, image discontinuity, steps in organ contour, or because of image or organ distortions. The coronary artery tree was segmented according to the 15-segment model.7 Segments were included in the analysis of image quality if they had a diameter of R1.5 mm. Each coronary segment was classified as either diagnostic or nondiagnostic. Nondiagnostic image quality was defined by severe vessel blurring or discontinuity of the vessel contour because of motion or by severe calcifications, which did not allow the exclusion of obstructive coronary lesions.
Image quality
The collected parameters relevant to radiation dose included the volume CT dose index and dose-length product (DLP), which were both obtained from the CT scan protocol. The calculation of the effective dose is based on
Diagnostic image quality was assessed by 2 experienced CCTA readers by evaluation of 3 orthogonal maximum
Estimation of radiation dose
Hausleiter et al
High-pitch cardiac CT angiography
239 region (k 5 0.014 mSv ! [mGy ! cm]21 averaged between male and female models).
Statistical analysis Continuous variables were expressed as mean and standard deviation. Categorical variables were expressed as frequencies or percentages. All statistical analyses were performed with the SPSS software (version 16.0.1; SPSS Inc, Chicago, IL).
Results
Figure 3 Curved-planar maximum intensity projection of the left anterior descending of a 75-year-old male patient with a stent (arrows) in the left anterior descending artery with a heart rate of 57 beats/min.
a method proposed by the European Working Group for Guidelines on Quality Criteria in CT,8 deriving radiation dose estimates from the product of the DLP and an organ weighting factor for the chest as the investigated anatomic
A total of 14 patients underwent high-pitch, dual-source CCTA. Tables 1 and 2 summarize patient and scanning characteristics. The mean heart rate was 52.6 6 5.5 beats/ min after injecting b-blocker in 9 patients. The duration of the scan was only 1085 6 380 milliseconds, initiated at 33% 610% of an R-R interval. Covering a scan length of 154 6 57 mm with a volume CT dose index of 6.8 6 1.4 mGy, the resulting DLP was 145 6 47 mGy ! cm. This corresponds to an estimated radiation dose of 2.0 6 0.7 mSv. Excluding patients with long scan ranges (aorta or bypass graft imaging), the estimated radiation dose for CCTA was 1.8 6 0.3 mSv. Cardiac anatomy including the left atrium was displayed with a diagnostic image quality in all patients. Furthermore, the examined sections of the ascending aorta had good diagnostic image quality in all patients, allowing for a
Figure 4 Curved-planar maximum intensity projections of the left anterior descending artery (A), the left circumflex artery (C), and the right coronary artery (D), as well as 3-dimensional reconstruction of the heart in volume-rendering technique (B) in a patient scanned with a heart rate of 43 beats/min. The arrows indicate coronary segments with motion artifacts in the mid-section of the scan.
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Figure 6 Three-dimensional reconstruction of the heart in volume-rendering technique in a 58-year-old male patient with a heart rate of 55 beats/min. The scan was performed applying a tube voltage of 100 kVp, resulting in an estimated radiation dose of only 0.9 mSv. Figure 5 CCTA scan duration for a typical scan length of 120 mm in a patient with a heart rate of 60 beats/min with a current DSCT system in the high-pitch mode. In the mid section, the graph depicts the motion velocities of the coronary arteries depending on the corresponding phase of cardiac cycle. The * marks the early diastole of the cardiac cycle and thus a phase with high coronary artery motion. In this early diastolic phase most motion artifacts were observed because of scan duration in the high-pitch mode with the current DSCT system. In the lower section, the CCTA scan duration of the next-generation DSCT system in the high-pitch mode is displayed. Because of the shortened scan duration, the early diastolic phase of the cardiac cycle with the high motion velocities of coronary arteries can be avoided.
definite evaluation of the aortic wall. One patient was scanned to assess patency of bypass grafts after coronary artery bypass grafting. Figure 1 displays a 3-dimensional reconstruction of the left internal mammary artery that was anastomosed to the distal left anterior descending coronary artery. The entire course of the bypass graft is depicted with good diagnostic image quality. Assessing the image quality of the coronary arteries, 142 (82%) of 173 coronary segments with a diameter R 1.5 mm were rated as showing diagnostic image quality. Figure 2 displays a high-pitch CCTA scan showing good image quality taken of a 41-year-old man with a heart rate of 53 beats/min. Figure 3 shows the CCTA of the left anterior descending artery of a 75-year-old man with a stent in the left anterior descending artery with a heart rate of 57 beats/min. Among the 31 coronary segments deemed with a nondiagnostic image quality, motion artifacts were considered the primary reason for nondiagnostic image quality. Assessing the location of nondiagnostic segments relative to the caudocranial scan, the majority of nondiagnostic coronary segments (21 of 31; 67.7%) were located in the
mid section of the cardiac scan range (Fig. 4). This mid section of the cardiac scan range corresponds to the early diastole of the cardiac cycle in this setting.
Discussion This small study was performed to show the proofof-concept that high-pitch CCTA during a single heart beat is feasible with DSCT technology. The current available DSCT technology with a detector width of 19 mm and a gantry rotational speed of 330 milliseconds allows for a maximum table speed of 198 mm/s at a pitch value of 3.4. Accordingly, a typical cardiac scan with a length of 120 mm lasts approximately 0.6 second. Within this short acquisition time, it is possible to obtain cross-sectional images of the cardiac anatomy with diagnostic image quality despite continuous cardiac motion. All studied patients had a sufficient diagnostic image quality, eg, to assess the left atrium before planned electrophysiologic procedures. Furthermore, 82% of coronary segments were deemed diagnostic for the evaluation of coronary artery stenosis. This percentage is surprisingly high, considering the configuration of the current DSCT, with which the scan time lasted .0.6 second. However, this study also clearly shows that current DSCT scanners are not suitable for the assessment of the coronary arteries with this high-pitch scan mode in daily practice. With conventional low-pitch spiral or sequential scan techniques, most studies showed that usually .97% of coronary arteries are deemed diagnostic with the use of current DSCT scanners.9 Because of the scan time of .0.6 second and a scan start in late systole, most motion artifacts were observed in the mid range of the CCTA scan, which corresponds to the early diastole of the cardiac cycle, when coronary motion is usually high (see also
Hausleiter et al
High-pitch cardiac CT angiography
Fig. 5). Thus, an even shorter scan time needs to be established with the next generation of DSCT systems which will have a larger detector and a further increased rotational speed. This will allow for a higher table speed during high-pitch data acquisition (up to 430 mm/s), resulting in an even shorter scan time of ,0.4 second for a cardiac CCTA. However, future studies will need to prove that this high-pitch scan mode will allow for high coronary diagnostic image quality that is comparable with conventional low-pitch spiral or sequential scan techniques. A variety of effective strategies have been introduced to reduce radiation exposure of CCTA, including the electrocardiographic (ECG)–controlled modulation of the tube current, a reduced tube voltage of 100 kVp, and the ECG-triggered sequential scan mode. Although some strategies have been applied more frequently than others, the median dose of CCTA approximated 12 mSv with an interquartile range between 8 and 18 mSv in the observational PROTECTION I study.4 By contrast, the current study shows an estimated radiation dose of only 2.0 6 0.7 mSv for high-pitch CCTA despite an overall scan length of 154 6 57 mm. Compared with currently available protocols for CCTA as applied in the recent PROTECTION I study, the high-pitch scan mode would result in a tremendously reduced radiation exposure if routinely used for CCTA. This reduction in radiation dose is particularly remarkable compared with other new technologies such as the 320-detector systems that also allow for CCTA image acquisition from a single heart beat. A first study reported a mean radiation dose of 6.5 6 1.5 mSv for 40 consecutive patients undergoing 320-detector CCTA with prospective ECG gating.10 The high-pitch scan mode can also be performed with a 100-kVp tube voltage in nonobese patients, which might even allow for performing CCTA scans with an radiation exposure of ,1 mSv (Fig. 6). Finally, it is conceivable that the volume of contrast medium needed for CCTA will be reduced with the high-pitch scan protocol as a result of the shortened scan duration. The current proof-of-concept study only investigated patients with a low and stable sinus rhythm. Accordingly, we cannot provide any data on the performance of this high-pitch scan mode in patients with higher heart rates or arrhythmias. Considering the scan duration of .600 milliseconds for a CCTA with the first-generation DSCT scanner, it is conceivable that the frequency of motion artefacts will increase at higher or more variable heart rates. However, it is also conceivable that CCTA image acquisition will be less susceptible to motion artefacts with the next-generation DSCT scanners because of the shortened scan duration as discussed earlier (see Fig. 5). In summary, this proof-of-concept study shows the ability of DSCT scanners to scan the whole heart with diagnostic image quality during one single heart beat with a high-pitch mode. Although we demonstrated that this highpitch mode is feasible with first-generation DSCT scanners,
241 this mode was developed and is intended for use with the next-generation DSCT systems. These CT systems will have a further reduced gantry rotation time and wider detectors (2 ! 64 detector elements), which is expected to shorten scan time and to further improve the assessment of coronary arteries. Accordingly, a systematic evaluation of next-generation DSCT systems is needed to evaluate the diagnostic performance of high-pitch CT scanning of the coronary arteries.
References 1. Budoff MJ, Achenbach S, Blumenthal RS, Carr JJ, Goldin JG, Greenland P, Guerci AD, Lima JA, Rader DJ, Rubin GD, Shaw LJ, Wiegers SE: Assessment of coronary artery disease by cardiac computed tomography: a scientific statement from the American Heart Association Committee on Cardiovascular Imaging and Intervention, Council on Cardiovascular Radiology and Intervention, and Committee on Cardiac Imaging, Council on Clinical Cardiology. Circulation. 2006;114:1761–91. 2. Fox K, Garcia MA, Ardissino D, Buszman P, Camici PG, Crea F, Daly C, De Backer G, Hjemdahl P, Lopez-Sendon J, Marco J, Morais J, Pepper J, Sechtem U, Simoons M, Thygesen K, Priori SG, Blanc JJ, Budaj A, Camm J, Dean V, Deckers J, Dickstein K, Lekakis J, McGregor K, Metra M, Osterspey A, Tamargo J, Zamorano JL: Guidelines on the management of stable angina pectoris: executive summary: the Task Force on the Management of Stable Angina Pectoris of the European Society of Cardiology. Eur Heart J. 2006;27:1341–81. 3. Hendel RC, Patel MR, Kramer CM, Poon M, Carr JC, Gerstad NA, Gillam LD, Hodgson JM, Kim RJ, Lesser JR, Martin ET, Messer JV, Redberg RF, Rubin GD, Rumsfeld JS, Taylor AJ, Weigold WG, Woodard PK, Brindis RG, Douglas PS, Peterson ED, Wolk MJ, Allen JM: 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, American College of Radiology, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, American Society of Nuclear Cardiology, North American Society for Cardiac Imaging, Society for Cardiovascular Angiography and Interventions, and Society of Interventional Radiology. J Am Coll Cardiol. 2006; 48:1475–97. 4. Hausleiter J, Meyer T, Hermann F, Hadamitzky M, Krebs M, Gerber TC, McCollough C, Martinoff S, Kastrati A, Scho¨mig A, Achenbach S: Estimated radiation dose associated with cardiac CT angiography. JAMA. 2009;301:500–7. 5. Achenbach S, Ropers D, Kuettner A, Flohr T, Ohnesorge B, Bruder H, Theessen H, Karakaya M, Daniel WG, Bautz W, Kalender WA, Anders K: Contrast-enhanced coronary artery visualization by dual-source computed tomography: initial experience. Eur J Radiol. 2006;57:331–5. 6. Flohr TG, McCollough CH, Bruder H, Petersilka M, Gruber K, Suss C, Grasruck M, Stierstorfer K, Krauss B, Raupach R, Primak AN, Kuttner A, Achenbach S, Becker C, Kopp A, Ohnesorge BM: First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol. 2006;16:256–68. 7. Austen WG, Edwards JE, Frye RL, Gensini GG, Gott VL, Griffith LS, McGoon DC, Murphy ML, Roe BB: A reporting system on patients evaluated for coronary artery disease. Report of the Ad Hoc Committee for Grading of Coronary Artery Disease, Council on Cardiovascular Surgery, American Heart Association. Circulation. 1975; 51(4 Suppl):5–40.
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8. Menzel H, Schibilla H, Teunen D: European guidelines for quality criteria for computed tomography. Brussels, Belgium: European Commission; 2000. 9. 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. 10. Hoe J: First experience with 320 MSCT coronary angiography using volume prospective ECG gating to reduce radiation dose. J Cardiovasc Comput Tomogr. 2008;2(Suppl):S21.
Original Research Article
Feasibility of dual-source cardiac CT angiography with high-pitch scan protocols Jo¨rg Hausleiter, MDa,*, Bernhard Bischoff, MDa, Franziska Hein, MDa, Tanja Meyer, MDa, Martin Hadamitzky, MDa, Carsten Thierfelder, PhDb, Thomas Allmendinger, PhDb, Thomas G. Flohr, PhDb, Albert Scho¨mig, MDa, Stefan Martinoff, MDc a
Klinik fu¨r Herz- und Kreislauferkrankungen, Deutsches Herzzentrum Mu¨nchen; Klinik an der TU Mu¨nchen, Lazarettstrasse 36, 80636 Munich, Germany; bComputed Tomography, Siemens Healthcare, Forchheim, Germany and c Institut fu¨r Radiologie und Nuklearmedizin, Deutsches Herzzentrum Mu¨nchen; Klinik an der TU Mu¨nchen, Munich, Germany KEYWORDS: Coronary artery disease; Dual-source CT; High pitch; Radiation dose
BACKGROUND: Cardiac CT angiography (CCTA) has become a frequently used diagnostic tool in clinical practice, but concern remains about the radiation exposure. Because of the second x-ray acquisition system, dual-source CT systems might allow for high-pitch CT data acquisition and thus for examination of the whole heart during a single heart beat, with the potential for radiation dose reduction. OBJECTIVE: We assessed the feasibility of a high-pitch scan mode with a dual-source CT system. METHODS: High-pitch modes were used in patients undergoing CCTA with a dual-source CT system. Diagnostic image quality for cardiac structures and coronary arteries was assessed. Radiation dose was estimated from the scanner-generated dose-length product (DLP). RESULTS: CCTAwas performed in 14 patients during a single heart beat applying a pitch value of 3.4. Mean heart rate during examination was 56.4 6 8.1 beats/min. Diagnostic image quality for the assessment of larger cardiac structures was obtained in all patients, whereas diagnostic image quality could be achieved in 82% of all coronary segments. With a mean DLP of 145 6 47 mGy ! cm, the resulting estimated radiation dose was 2.0 6 0.7 mSv. CONCLUSIONS: This proof-of-concept study shows the ability of dual-source CT scanners to scan the whole heart during one single heart beat at low radiation dose. Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved.
Introduction Conflict of interest: Drs Thierfelder, Allmendinger, and Flohr are employees of Siemens Medical Solutions, Forchheim, Germany. Dr. Hausleiter reports receiving research grants and speaker honoraria from Siemens Medical Solutions unrelated to the current study. The remaining authors have no conflicts of interest concerning the topic of this study. * Corresponding author. E-mail address: [email protected] Submitted February 17, 2009. Accepted for publication May 31, 2009.
Cardiac CT angiography (CCTA) has become a frequently used diagnostic tool in clinical practice for the evaluation of the coronary arteries.1–3 Despite continuous improvements in CT technology, concerns remain about the patients’ radiation exposure. When performing 64-slice CCTA, the effective radiation dose has been shown to
1934-5925/$ -see front matter Ó 2009 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2009.05.012
Hausleiter et al Table 1
High-pitch cardiac CT angiography
237
Patient characteristics (n 5 14) Values
Age, mean 6 SD, y Male sex, n (%) Height, mean 6 SD, m Weight, mean 6 SD, kg Body mass index, mean 6 SD, kg/m2 Heart rate, mean 6 SD, beats/min Sinus rhythm, n (%)
59.7 6 10.8 11 (78.6) 1.74 6 0.09 81.1 6 16.5 26.4 6 4.4 52.6 6 5.5 14 (100)
approximate 12 mSv, depending on the scanning technique, patient-related factors, as well as the CT system.4 Recently, new scan protocols have been introduced into CCTA, including 100 kVp and sequential scan protocols, which will allow for considerable reductions in radiation exposure. With single-source CT systems, the spiral pitch (table feed per rotation divided by the collimated beam width) is limited to a maximum value of 1.5 for gapless data acquisition in the z-axis. At higher pitch values data gaps occur which will result in image artifacts. Compared with conventional single-source CT systems, the dual-source CT (DSCT) configuration with 2 radiation tubes and 2 detectors arranged in a 90-degree angle allows for both high temporal resolution5,6 and high-pitch data acquisition. The high temporal resolution is achieved because only a 90degree rotation of the system is necessary for image reconstruction, whereas conventional systems need at least a 180-degree rotation of the x-ray gantry. In the high-pitch scan mode, the second tube detector system of a DSCT system is being used to fill the data gaps; accordingly, the pitch can be increased to values .3.6 Thus, CCTA during a single heart beat would become possible with high-pitch data acquisition. This short, subsecond exposure time to ionizing radiation would also result in a considerable reduction in estimated radiation dose compared with spiral imaging Table 2 patients
Scanning characteristics in high-pitch scans in 14 Values
Administration of intravenous metoprolol, n (%) 9 (64) Metoprolol dose, mean 6 SD, mg 5.0 6 4.4 Contrast dye volume, mean 6 SD, mL 58 6 16 Tube voltage 120 kVp, n (%) 12 (86) 100 kVp, n (%) 2 (14) Tube current, mean 6 SD, mAs 427 6 28 Pitch value, mean 6 SD 3.4 6 0.0 Scan length, mean 6 SD, mm 154 6 57 Scan time, mean 6 SD, ms 1085 6 380 Volume CT dose index, mean 6 SD, mGy 6.8 6 1.4 Dose-length product, mean 6 SD, mGy ! cm 145 6 47 Dose estimate, mean 6 SD, mSv 2.0 6 0.7
Figure 1 Three-dimensional reconstruction of the heart and left internal mammary artery bypass-graft (arrows) in volume-rendering technique in a 62-year-old male patient with a heart rate of 56 beats/min.
with a lower pitch. Accordingly, the aim of this study was to prove the concept that CCTA during one heart beat is feasible with DSCT systems applying high-pitch modes for CT data acquisition.
Methods Patients We examined 14 selected patients undergoing CCTA for examination of the cardiac anatomy without need to evaluate the coronary arteries. The indications comprised the visualization of (1) the left atrium and pulmonary vein ostia before a planned electrophysiologic procedure, (2) the ascending aorta, and (3) aortocoronary bypass grafts for patency. Patients were included if the heart rate was below 60 beats/min without atrial or ventricular premature beats during preparation and if the renal and thyroid functions were normal. All patients gave written informed consent.
Image acquisition All patients were examined with the use of a 64-slice DSCT (SOMATOM Definition; Siemens Medical Solutions, Forchheim, Germany) with a prospectively triggered high-pitch spiral scan technique. Before image acquisition, patients with a heart rate .60 beats/min or a nonstable sinus rhythm received up to 2 doses of 5 mg metoprolol
238
Journal of Cardiovascular Computed Tomography, Vol 3, No 4, July/August 2009
Figure 2 Curved-planar maximum intensity projections of the left anterior descending artery (A), the left circumflex artery (B), and the right coronary artery (C), as well as 3-dimensional reconstruction of the heart in volume-rendering technique (D) in a patient scanned with a heart rate of 53 beats/min.
intravenously to lower and stabilize heart rate. In addition, coronary vasodilatation was achieved in all patients with a systolic blood pressure of R100 mmHg by the administration of nitroglycerine 0.8 mg sublingually. The volume of contrast medium that was injected with a flow rate of 6 mL/s was individually adjusted. For patients undergoing CCTA before a planned electrophysiologic procedure a contrast volume of 50 mL was used, whereas a volume of 87 6 11 mL was injected in the remaining patients to account for the longer scan ranges. The contrast bolus was followed by a saline chaser bolus at the same flow rate. Image acquisition was started usually 5 seconds after the measured time to peak enhancement in the ascending aorta preceeding the CCTA scan. The scanner setting was as follows: We used a collimation of 32 ! 0.6 mm. Twelve patients were scanned with a tube voltage of 120 kVp and a tube current time product per rotation of 438 mAs, 2 patients with 100 kVp and 362 mAs, respectively. Applying a pitch value of 3.4, the scan was started at 33% 6 10% of the R-R interval in the caudocranial scan direction. From the obtained data, transaxial images (slice thickness, 0.6 mm; increment, 0.3 mm) were recontructed applying a standard kernel (B26f).
intensity projections (slice thickness: 5 mm), axial slices (slice thickness: 0.6 mm), and additional curved multiplanar reformations if needed. Disagreements between the first 2 readers were solved by a third experienced CCTA reader. When assessing diagnostic quality of cardiac anatomy, the image quality of the left atrium with the pulmonary veins, the ascending aorta and of the bypass grafts was either rated diagnostic or nondiagnostic. The cardiac anatomy was rated nondiagnostic if the left atrium, the ascending aorta, or the bypass grafts could not be clearly delineated because of motion or reconstruction artifacts, image discontinuity, steps in organ contour, or because of image or organ distortions. The coronary artery tree was segmented according to the 15-segment model.7 Segments were included in the analysis of image quality if they had a diameter of R1.5 mm. Each coronary segment was classified as either diagnostic or nondiagnostic. Nondiagnostic image quality was defined by severe vessel blurring or discontinuity of the vessel contour because of motion or by severe calcifications, which did not allow the exclusion of obstructive coronary lesions.
Image quality
The collected parameters relevant to radiation dose included the volume CT dose index and dose-length product (DLP), which were both obtained from the CT scan protocol. The calculation of the effective dose is based on
Diagnostic image quality was assessed by 2 experienced CCTA readers by evaluation of 3 orthogonal maximum
Estimation of radiation dose
Hausleiter et al
High-pitch cardiac CT angiography
239 region (k 5 0.014 mSv ! [mGy ! cm]21 averaged between male and female models).
Statistical analysis Continuous variables were expressed as mean and standard deviation. Categorical variables were expressed as frequencies or percentages. All statistical analyses were performed with the SPSS software (version 16.0.1; SPSS Inc, Chicago, IL).
Results
Figure 3 Curved-planar maximum intensity projection of the left anterior descending of a 75-year-old male patient with a stent (arrows) in the left anterior descending artery with a heart rate of 57 beats/min.
a method proposed by the European Working Group for Guidelines on Quality Criteria in CT,8 deriving radiation dose estimates from the product of the DLP and an organ weighting factor for the chest as the investigated anatomic
A total of 14 patients underwent high-pitch, dual-source CCTA. Tables 1 and 2 summarize patient and scanning characteristics. The mean heart rate was 52.6 6 5.5 beats/ min after injecting b-blocker in 9 patients. The duration of the scan was only 1085 6 380 milliseconds, initiated at 33% 610% of an R-R interval. Covering a scan length of 154 6 57 mm with a volume CT dose index of 6.8 6 1.4 mGy, the resulting DLP was 145 6 47 mGy ! cm. This corresponds to an estimated radiation dose of 2.0 6 0.7 mSv. Excluding patients with long scan ranges (aorta or bypass graft imaging), the estimated radiation dose for CCTA was 1.8 6 0.3 mSv. Cardiac anatomy including the left atrium was displayed with a diagnostic image quality in all patients. Furthermore, the examined sections of the ascending aorta had good diagnostic image quality in all patients, allowing for a
Figure 4 Curved-planar maximum intensity projections of the left anterior descending artery (A), the left circumflex artery (C), and the right coronary artery (D), as well as 3-dimensional reconstruction of the heart in volume-rendering technique (B) in a patient scanned with a heart rate of 43 beats/min. The arrows indicate coronary segments with motion artifacts in the mid-section of the scan.
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Figure 6 Three-dimensional reconstruction of the heart in volume-rendering technique in a 58-year-old male patient with a heart rate of 55 beats/min. The scan was performed applying a tube voltage of 100 kVp, resulting in an estimated radiation dose of only 0.9 mSv. Figure 5 CCTA scan duration for a typical scan length of 120 mm in a patient with a heart rate of 60 beats/min with a current DSCT system in the high-pitch mode. In the mid section, the graph depicts the motion velocities of the coronary arteries depending on the corresponding phase of cardiac cycle. The * marks the early diastole of the cardiac cycle and thus a phase with high coronary artery motion. In this early diastolic phase most motion artifacts were observed because of scan duration in the high-pitch mode with the current DSCT system. In the lower section, the CCTA scan duration of the next-generation DSCT system in the high-pitch mode is displayed. Because of the shortened scan duration, the early diastolic phase of the cardiac cycle with the high motion velocities of coronary arteries can be avoided.
definite evaluation of the aortic wall. One patient was scanned to assess patency of bypass grafts after coronary artery bypass grafting. Figure 1 displays a 3-dimensional reconstruction of the left internal mammary artery that was anastomosed to the distal left anterior descending coronary artery. The entire course of the bypass graft is depicted with good diagnostic image quality. Assessing the image quality of the coronary arteries, 142 (82%) of 173 coronary segments with a diameter R 1.5 mm were rated as showing diagnostic image quality. Figure 2 displays a high-pitch CCTA scan showing good image quality taken of a 41-year-old man with a heart rate of 53 beats/min. Figure 3 shows the CCTA of the left anterior descending artery of a 75-year-old man with a stent in the left anterior descending artery with a heart rate of 57 beats/min. Among the 31 coronary segments deemed with a nondiagnostic image quality, motion artifacts were considered the primary reason for nondiagnostic image quality. Assessing the location of nondiagnostic segments relative to the caudocranial scan, the majority of nondiagnostic coronary segments (21 of 31; 67.7%) were located in the
mid section of the cardiac scan range (Fig. 4). This mid section of the cardiac scan range corresponds to the early diastole of the cardiac cycle in this setting.
Discussion This small study was performed to show the proofof-concept that high-pitch CCTA during a single heart beat is feasible with DSCT technology. The current available DSCT technology with a detector width of 19 mm and a gantry rotational speed of 330 milliseconds allows for a maximum table speed of 198 mm/s at a pitch value of 3.4. Accordingly, a typical cardiac scan with a length of 120 mm lasts approximately 0.6 second. Within this short acquisition time, it is possible to obtain cross-sectional images of the cardiac anatomy with diagnostic image quality despite continuous cardiac motion. All studied patients had a sufficient diagnostic image quality, eg, to assess the left atrium before planned electrophysiologic procedures. Furthermore, 82% of coronary segments were deemed diagnostic for the evaluation of coronary artery stenosis. This percentage is surprisingly high, considering the configuration of the current DSCT, with which the scan time lasted .0.6 second. However, this study also clearly shows that current DSCT scanners are not suitable for the assessment of the coronary arteries with this high-pitch scan mode in daily practice. With conventional low-pitch spiral or sequential scan techniques, most studies showed that usually .97% of coronary arteries are deemed diagnostic with the use of current DSCT scanners.9 Because of the scan time of .0.6 second and a scan start in late systole, most motion artifacts were observed in the mid range of the CCTA scan, which corresponds to the early diastole of the cardiac cycle, when coronary motion is usually high (see also
Hausleiter et al
High-pitch cardiac CT angiography
Fig. 5). Thus, an even shorter scan time needs to be established with the next generation of DSCT systems which will have a larger detector and a further increased rotational speed. This will allow for a higher table speed during high-pitch data acquisition (up to 430 mm/s), resulting in an even shorter scan time of ,0.4 second for a cardiac CCTA. However, future studies will need to prove that this high-pitch scan mode will allow for high coronary diagnostic image quality that is comparable with conventional low-pitch spiral or sequential scan techniques. A variety of effective strategies have been introduced to reduce radiation exposure of CCTA, including the electrocardiographic (ECG)–controlled modulation of the tube current, a reduced tube voltage of 100 kVp, and the ECG-triggered sequential scan mode. Although some strategies have been applied more frequently than others, the median dose of CCTA approximated 12 mSv with an interquartile range between 8 and 18 mSv in the observational PROTECTION I study.4 By contrast, the current study shows an estimated radiation dose of only 2.0 6 0.7 mSv for high-pitch CCTA despite an overall scan length of 154 6 57 mm. Compared with currently available protocols for CCTA as applied in the recent PROTECTION I study, the high-pitch scan mode would result in a tremendously reduced radiation exposure if routinely used for CCTA. This reduction in radiation dose is particularly remarkable compared with other new technologies such as the 320-detector systems that also allow for CCTA image acquisition from a single heart beat. A first study reported a mean radiation dose of 6.5 6 1.5 mSv for 40 consecutive patients undergoing 320-detector CCTA with prospective ECG gating.10 The high-pitch scan mode can also be performed with a 100-kVp tube voltage in nonobese patients, which might even allow for performing CCTA scans with an radiation exposure of ,1 mSv (Fig. 6). Finally, it is conceivable that the volume of contrast medium needed for CCTA will be reduced with the high-pitch scan protocol as a result of the shortened scan duration. The current proof-of-concept study only investigated patients with a low and stable sinus rhythm. Accordingly, we cannot provide any data on the performance of this high-pitch scan mode in patients with higher heart rates or arrhythmias. Considering the scan duration of .600 milliseconds for a CCTA with the first-generation DSCT scanner, it is conceivable that the frequency of motion artefacts will increase at higher or more variable heart rates. However, it is also conceivable that CCTA image acquisition will be less susceptible to motion artefacts with the next-generation DSCT scanners because of the shortened scan duration as discussed earlier (see Fig. 5). In summary, this proof-of-concept study shows the ability of DSCT scanners to scan the whole heart with diagnostic image quality during one single heart beat with a high-pitch mode. Although we demonstrated that this highpitch mode is feasible with first-generation DSCT scanners,
241 this mode was developed and is intended for use with the next-generation DSCT systems. These CT systems will have a further reduced gantry rotation time and wider detectors (2 ! 64 detector elements), which is expected to shorten scan time and to further improve the assessment of coronary arteries. Accordingly, a systematic evaluation of next-generation DSCT systems is needed to evaluate the diagnostic performance of high-pitch CT scanning of the coronary arteries.
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