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A new method for measuring coronary artery diameters with CT spatial profile curves
Radiography (2007) 13, 44e50
A new method for measuring coronary artery diameters with CT spatial profile curves Ryoichi Shimamoto a, Jun-ichi Suzuki a,*, Tadashi Yamazaki a, Taeko Tsuji a, Yuki Ohmoto a, Toshihiro Morita a, Hiroshi Yamashita a, Junko Honye a, Ryozo Nagai a, Masaaki Akahane b, Kuni Ohtomo b a
Department of Cardiovascular Medicine, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan b Department of Radiology, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan Received 12 July 2005; accepted 16 October 2005 Available online 2 December 2005
KEYWORDS Multi-detector row CT; CT spatial profile curve; Coronary artery diameters; Intravascular ultrasound; Conventional coronary angiography; Hounsfield number
Abstract Purpose Coronary artery vascular edge recognition on computed tomography (CT) angiograms is influenced by window parameters. A noninvasive method for vascular edge recognition independent of window setting with use of multi-detector row CT was contrived and its feasibility and accuracy were estimated by intravascular ultrasound (IVUS). Methods Multi-detector row CT was performed to obtain 29 CT spatial profile curves by setting a line cursor across short-axis coronary angiograms processed by multi-planar reconstruction. IVUS was also performed to determine the reference coronary diameter. IVUS diameter was fitted horizontally between two points on the upward and downward slopes of the profile curves and Hounsfield number was measured at the fitted level to test seven candidate indexes for definition of intravascular coronary diameter. The best index from the curves should show the best agreement with IVUS diameter. Results Of the seven candidates the agreement was the best (agreement: 16 11%) when the two ratios of Hounsfield number at the level of IVUS diameter over that at the peak on the profile curves were used with water and with fat as the background tissue. These edge definitions were achieved by cutting the horizontal
Abbreviations: MDCT, multi-detector row computed tomography; IVUS, intravascular ultrasound; Conventional CAG, conventional coronary angiography. * Corresponding author. Laboratory Room-213, Department of Cardiovascular Medicine, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan. Tel.: þ81 3 3815 5411x33079; fax: þ81 3 3814 0021. E-mail address: [email protected] (J.-i. Suzuki). 1078-8174/$ - see front matter ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radi.2005.10.002
Method for measuring coronary artery diameters
45
distance by the curves at the level defined by the ratio of 0.41 for water background and 0.57 for fat background. Conclusions Vascular edge recognition of the coronary artery with CT spatial profile curves was feasible and the contrived method could define the coronary diameter with reasonable agreement. ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Introduction New mechanical engineering has successfully introduced computed tomography (CT) innovations. Spiral multi-detector row CT scanner with fast gantry rotation system has realized three-dimensional delineation of the coronary artery tree.1e4 Sixteen detector rows5 can provide isotropic high spatial resolution and can shorten scanning time6,7 with reduction of doses of radiation and contrast media.8,9 It is now the clinicians’ turn to apply this technology to fruitful use. A cross-sectional plane perpendicular to the axis of the running course of the coronary arteries, i.e., a coronary short-axis plane, provides an adequate view not only to describe severity of intravascular configurations but also to determine whether the atherosclerotic lesions are concentric or eccentric.10e12 Multi-planar reconstruction can provide a contiguous short-axis view along the coronary axis from a raw CT datum set.13,14 Thus CT observation of the coronary arteries is congruent with that of intravascular ultrasound (IVUS). And therefore, IVUS was used as the reference standard in the current study. Evaluation of coronary artery diameters should be based on accurate vascular edge recognition.15 When the recognition is done on CT display, what
should be emphasized is that window setting influences observers’ inspection, that is to say, measurements of coronary intravascular diameters on CT display depend on window center and width. CT spatial profile curves obtained by setting a cursor line across the short-axis coronary angiogram do not depend on window parameters (Fig. 1a,b). If a method with use of CT spatial profile curves is established to determine the intra-arterial edges, the method independent of display parameters might be of great clinical use.16 Accordingly the aim in the current study was to evaluate feasibility and accuracy of CT spatial profile curves for recognizing the edge of the coronary arteries.
Subjects and methods Subjects Twenty-nine CT spatial profile curves (19 curves from the left anterior descending arteries and 10 curves from the right coronary arteries) were obtained from seven patients (seven males; 64.0 8.5 years old; mean weight, 58.1 5.3 kg) who were admitted to the hospital to undergo both catheter coronary angiography and IVUS. Spiral multi-detector row CT was performed to obtain
Figure 1 Coronary short-axis CT angiogram and spatial profile curve. (a) Cursor line is set to intersect coronary artery and to pass the center of short-axis coronary angiogram reformatted by using multi-planar reconstruction. Length of cursor line is shown to be 1.1 cm. (b) CT spatial profile curve is automatically displayed with software installed at offline workstation. Vertical axis represents CT value in Hounsfield units and horizontal axis shows spatial distance in millimeter from starting edge through ending edge of cursor.
46 CT coronary angiograms within three days of the IVUS examination (either before or after). All gave written informed consent and the protocol for coronary CT angiography was approved by the institutional review board.
Spiral multi-detector row CT examination A three-dimensional datum set with a scan range of 120 mm was obtained with use of 16 detector row spiral CT scanner (Aquilion, Toshiba Medical Systems Co., Tokyo) with retrospective ECG gated reconstruction during a single breath-hold for 30 s. Imaging parameters used were gantry rotation time of 500 ms, collimated slice thickness of 0.5 mm and table feed of 4 mm/s, i.e., helical pitch of 4.0. Field of view of 200 mm 200 mm and matrix of 512 512 resulted in spatial resolution of 0.4 mm 0.4 mm in the transaxial plane. Contrast medium (iopamidol, Iopamiron 370 syringe, Schering AG, Berlin, Germany) was injected via the cubital vein at 2.5 cc/s. Total injected dose was less than 100 cc.
Measurements with intravascular ultrasound IVUS was performed within three days of CT examination (either before or after) with use of commercially available equipment (INSIGHT, Boston Scientific Co., Natick, MA, USA) with 40 MHz transducer (Atlantis Pro, Boston Scientific Co., Natick, MA, USA). Short-axis ultrasonograms perpendicular to the axis of the coronary artery were obtained at the site of the branch diverging from the main coronary arteries (Fig. 2a). The diverging branches were used as the markers to identify the
R. Shimamoto et al. common site or common section of the coronary artery and to define the common direction for measurements of intravascular diameter on the short-axis CT angiograms obtained with use of multi-planar reconstruction (Fig. 2b). Intravascular diameters of the coronary arteries were measured on short-axis ultrasonograms from leading edge to leading edge.
Coronary artery short-axis images with multi-planar reconstruction Ten sets of transaxial three-dimensional raw data obtained with datum acquisition window at each 10% cardiac cycle were transferred to the offline workstation where image processing softwares such as multi-planar reconstruction were available. Thus each three-dimensional datum set was reconstructed at each 10% cardiac cycle. Image quality obtained at 70 or 80% cardiac cycle was the most stable, and therefore, a datum set reconstructed at these cardiac phases was used thereafter for all analyses. Multi-planar reconstructed CT images in the common short-axis coronary artery planes to IVUS were obtained, and on these images the common intravascular diameters were identified by recognizing the direction of the diverging branches as the anatomical markers (Fig. 2a,b).
Spatial-Hounsfield unit curves, i.e., CT spatial profile curves Spatial-Hounsfield unit curves, i.e., CT spatial profile curves were obtained by setting the straight cursor line across the short-axis coronary CT angiograms processed with multi-planar reconstruction
Figure 2 Common cross-sectional plane to IVUS (a) and short-axis coronary CT angiogram (b). Twig diverging from coronary artery is an anatomical marker to accord direction for setting cursor line of spatial profile curve with that for measuring coronary diameter on IVUS.
Method for measuring coronary artery diameters (Fig. 1). The cursor line was set on each short-axis coronary CT angiogram in 29 multi-planar reconstructed planes from seven patients. The directions of the cursors were perpendicular to those of diverging branches and coronary plaques were respected in order not to be involved. The curves were automatically displayed by the software installed at the workstation (Virtual Place Advance, Aze Ltd., Tokyo, Japan). The vertical axis represented CT value in Hounsfield units and the horizontal axis indicated length or distance from the starting point of the cursor of the straight line across the coronary artery in millimeters (Fig. 1b).
Analysis on spatial profile curves On the spatial profile curve obtained by setting the cursor line in the common direction and in the common plane to IVUS, the reference diameter measured by IVUS was inserted horizontally by fitting its length into the profile curve in parallel with the horizontal axis of the curve. CT Hounsfield value was measured on the curve at the fitted level of the IVUS diameter [H(IVUS)] and at the peak of the profile curve [H(peak)]. As the background, CT value was also measured at the region of the myocardium [H(myocardium)] and the subepicardial fat [H(fat)] (Fig. 3).
Figure 3 Schematic representation of spatial profile curve. IVUS diameter represents diameter measured by IVUS with common direction for cursor setting to obtain spatial profile curve. IVUS diameter is horizontally fitted at Hounsfield level on spatial profile curve where IVUS diameter becomes the same value for horizontal distance between two points on upward slope and downward slope of spatial profile curve. [H(IVUS)] represents Hounsfield value at a level where IVUS diameter is fitted on spatial profile curve. [H(peak)], [H(myocardium)], [H(water)] and [H(fat)] represent Hounsfield value at peak of curve, myocardium, water (¼0) and subepicardial fat, respectively.
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Candidate indexes from spatial profile curves to define the vascular edge Each reference diameter directly measured by IVUS was fitted into each CT spatial profile curve and the Hounsfield value [H(IVUS)] was measured at the fit level on each profile curve. The mean and standard deviation of candidate indexes that were used for edge recognition were calculated by use of the above-obtained [H(IVUS)]. The seven indexes were as follows: (1) H(IVUS) H(water) i.e., CT Hounsfield value at the wedge level on the profile curve with water (Hounsfield value of water: 0) as the background; (2) H(IVUS) H(myocardium) i.e., CT Hounsfield value at the wedge level with myocardium as the background; (3) H(IVUS) H(fat) i.e., CT Hounsfield value at the wedge level with sub-epicardial fat as the background; (4) [H(IVUS) H(water)]/[H(peak) H(water)] ¼ H(IVUS)/H(peak) i.e., a ratio of CT Hounsfield value at the wedge level over that at the peak on the profile curve with water as the background; (5) [H(IVUS) H(myocardium)]/[H(peak) H(myocardium)] i.e., a ratio with myocardium as the background; (6) [H(IVUS) H(fat)]/[H(peak) H(fat)] i.e., a ratio with fat tissue as the background; (7) [H(peak)] [H(IVUS)] i.e., a difference between Hounsfield value at the peak and that at the wedge; where H(peak), H(myocardium), H(fat), and H(IVUS) represent CT value in Hounsfield units at the peak of the profile curves, that of the myocardium, and that of the fat, and again Hounsfield value corresponds to the wedge level where IVUS diameter fits the profile curves, respectively (Fig. 3). H(water) represents Hounsfield number of water, i.e., H(water) ¼ 0. The best index that defines the diameter most accurately should show the best agreement with the reference IVUS diameter.
Diameter measurement on CT profile curves with each index With use of each value of [H(IVUS)] obtained on each profile curve, the mean value [Mean(k), k: candidate number, k ¼ 1, 2, 3, 4, 5, 6, 7] of each index was calculated and the following seven
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R. Shimamoto et al.
formulae were obtained. Then from each formula the Hounsfield value to define the coronary diameter on the curves [H(k), k: candidate number, k ¼ 1, 2, 3, 4, 5, 6, 7] was calculated. (1) H(1) H(water) ¼ Mean(1) i.e., H(1) ¼ Mean(1) þ H(water) ¼ Mean(1); (2) H(2) H(myocardium) ¼ Mean(2) i.e., H(2) ¼ Mean(2) þ H(myocardium); (3) H(3) H(fat) ¼ Mean(3) i.e., H(3) ¼ H(fat) þ Mean(3); (4) [H(4) H(water)]/[H(peak) H(water)] ¼ Mean(4) i.e., H(4) ¼ [Mean(4)] [H(peak) H(water)] þ H(water) ¼ [Mean(4)] [H(peak)]; (5) [H(5) H(myocardium)]/[H(peak) H(myocardium)] ¼ Mean(5) i.e., H(5) ¼ Mean(5) [H(peak) H(myocardium)] þ H(myocardium); (6) [H(6) H(fat)]/[H(peak) H(fat)] ¼ Mean(6) i.e., H(6) ¼ Mean(6) [H(peak) H(fat)] þ H(fat); (7) [H(peak)] [H(7)] ¼ Mean(7) i.e., H(7) ¼ H(peak) Mean(7). Horizontal distance was measured between two points on the upward and downward slopes of each profile curve at the Hounsfield level that corresponded to each H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7) value. These distances measured on profile curves and corresponding IVUS diameters were used to calculate agreement for seven candidate formulae.
Statistical analysis All values were expressed as the mean standard deviation (S.D.). Agreement between the CT expected value and reference IVUS value was expressed as the absolute difference of the paired two measured values over the mean of the two.
Results As the background tissue, Hounsfield values of the tissue of the myocardium and that of the sub-epicardial fat were 90 6 and 111 8, respectively. Table 1 summarizes the mean and the standard deviation of each index calculated by using CT Hounsfield values corresponding to IVUS diameters. Table 2 represents agreement between the diameter expected on spatial profile curves by each candidate index and the diameter actually
Table 1
Scatter range converted into Hounsfield value
Mean(1) Mean(2) Mean(3) Mean(4) Mean(5) Mean(6) Mean(7)
Mean
S.D.
130 40 241 0.41 0.15 0.57 182
75 73 75 0.19 0.28 0.14 64
Mean(1), Mean(2), Mean(3), Mean(4), Mean(5), Mean(6), and Mean(7) were calculated from spatial profile curves by use of the formula of [H(IVUS) H(water)], [H(IVUS) H(myocardium)], [H(IVUS) H(fat)], [H(IVUS) H(water)]/ [H(peak) H(water)], [H(IVUS) H(myocardium)]/[H(peak) H(myocardium)], [H(IVUS) H(fat)]/[H(peak) H(fat)] and [H(peak) H(IVUS)], respectively. One standard deviation of each index was converted into scatter range in Hounsfield units.
measured by IVUS. The absolute difference between the diameter indicated by CT and that measured by IVUS over the mean of the paired two values was calculated for each candidate index. Two cases gave the best agreement of 16 11%: when the diameter was horizontally cut by the curves at the Hounsfield value of 0.41 of the peak with water as the background and at 0.57 with fat as the background tissue (Fig. 4).
Discussion Two promising indexes Of the seven candidate indexes defined with CT spatial profile curves whose clinical feasibility and accuracy were evaluated in determination of coronary artery diameter, the best agreement was obtained by using the two indexes, i.e., the ratio of Hounsfield number corresponding to coronary artery diameter over that of the peak of the profile curve with water as the background and the ratio with the sub-epicardial fat as the background. Evaluation of the coronary artery diameter with the former ratio required measuring the horizontal distance cut by the profile curve at Hounsfield number of 0.41 of the peak of the profile curve. Identical results for the diameter were found when measuring the diameter at 0.57 of the profile curve peak with fat as the background. The diameters determined by these two ratios had the best agreement with IVUS diameters.
Clinical feasibility Clinical accuracy in coronary intravascular edge recognition with use of the above-mentioned two
Method for measuring coronary artery diameters Table 2
49
Agreement between CT expected diameter and reference IVUS diameter
H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7)
Agreement
H(1) ¼ 130 H(2) ¼ H(myocardium) þ 39 H(3) ¼ H(fat) þ 244 H(4) ¼ 0.41 H(peak) H(5) ¼ 0.15 [H(peak) H(myocardium)] þ H(myocardium) H(6) ¼ 0.57 [H(peak) H(fat)] þ H(fat) H(7) ¼ H(peak) 182
Mean
S.D.
0.20 0.20 0.21 0.16 0.17 0.16 0.19
0.13 0.13 0.14 0.11 0.12 0.11 0.12
Each CT expected diameter was measured on each spatial profile curve at the Hounsfield value of H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7). Agreement between CT expected diameter and reference IVUS diameter was calculated by following formulae: absolute difference of the paired two measured diameters over the mean of the two.
ratios was completely identical. As a matter of fact, both showed the same value of agreement of 16 11%. Although the latter ratio required two measurements of Hounsfield value of the subepicardial fat tissue and that of the curve peak, the former needed only one measurement at the peak. All procedures for the former method were to obtain spatial profile curve by setting a line cursor across the short-axis CT angiogram and to measure the horizontal distance cut by the profile curve at the Hounsfield level of 0.41 of the peak.
Clinical accuracy In comparison with IVUS as the reference standard, the value of agreement from the most promising
and simpler method proposed in the current study was 16% in determination of coronary artery diameter. If a given diameter of coronary artery is 2.5 mm, the mean value of error with the current method is calculated as 0.4 mm [¼2.5 0.16 mm]. This almost corresponds to one spatial resolution (0.4 mm in the transaxial planes). Although this agreement is enough to detect moderate to severe stenotic legions, this is not necessarily sufficient to identify mildly stenotic lesions that have the possibility to develop into acute coronary syndrome with jump-up phenomenon. In the future with evolution in spatial resolution the agreement with this method will reach the clinical use in all types of lesions.
Advantages of window independence When referred to CT profile curves it is apparent that inspective recognition of intravascular edge of the coronary arteries depends on window center and level, i.e., upward and downward parts of the curves do not indicate vertical lines but gradual slopes. This implies display parameters vary the subjectively optical level for the edge recognition along these slopes. Window dependent method requires window standardization. CT spatial profile curves are independent of window setting, and therefore, indexes defined using these curves will be unaffected by window parameters.
Figure 4 Scheme for representation of how to measure coronary diameter with two promising ratios on spatial profile curve. Coronary artery diameter is obtained by measuring distance between two intersections made by spatial profile curve and horizontal line at Hounsfield level of peak value multiplied by 0.41 [H(peak) 0.41]. Identical diameter is also obtained at Hounsfield level of peak value multiplied by 0.57 with sub-epicardial fat as background [{H(peak) H(fat)} 0.57] þ H(fat).
Conclusion Using CT spatial profile curves, coronary diameter could be obtained by measuring a horizontal length between two points on the curve at 41% Hounsfield level of the peak. This method was independent of window parameters, clinically feasible, and showed agreement of 16% with reference standard.
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References 1. Nieman K, Oudkerk M, Rensing BJ, van Ooijen P, Munne A, van Geuns RJ, et al. Coronary angiography with multi-slice computed tomography. Lancet 2001;357:599e603. 2. Achenbach S, Giesler T, Ropers D, Ulzheimer S, Derlien H, Schulte C, et al. Detection of coronary artery stenoses by contrast-enhanced, retrospectively electrocardiographicallygated, multislice spiral computed tomography. Circulation 2001;103:2535e8. 3. Schroeder S, Kopp AF, Baumbach A, Meisner C, Kuettner A, Georg C, et al. Noninvasive detection and evaluation of atherosclerotic coronary plaques with multislice computed tomography. J Am Coll Cardiol 2001;37:1430e5. 4. Maruyama T, Yoshizumi T, Tamura R, Takashima S, Toyoshima H, Konishi I, et al. Comparison of visibility and diagnostic capability of noninvasive coronary angiography by eight-slice multidetector-row computed tomography versus conventional coronary angiography. Am J Cardiol 2004;93:537e42. 5. Mollet NR, Cademartiri F, Nieman K, Saia F, Lemos PA, McFadden EP, et al. Multislice spiral computed tomography coronary angiography in patients with stable angina pectoris. J Am Coll Cardiol 2004;43:2265e70. 6. Nieman K, Cademartiri F, Lemos PA, Raaijmakers RT, Pattynama PMT, de Feyter PJ. Reliable noninvasive coronary angiography with fast submillimeter multislice spiral computed tomography. Circulation 2002;106:2051e4. 7. Ropers D, Baum U, Pohle K, Anders K, Ulzheimer S, Ohnesorge B, et al. Detection of coronary artery stenoses with thin-slice multi-detector row spiral computed tomography and multiplanar reconstruction. Circulation 2003; 107:664e6. 8. Kuettner A, Kopp AF, Schroeder S, Rieger T, Brunn J, Meisner C, et al. Diagnostic accuracy of multidetector computed tomography coronary angiography in patients with angiographically proven coronary artery disease. J Am Coll Cardiol 2004;43:831e9.
R. Shimamoto et al. 9. Achenbach S, Ropers D, Hoffmann U, MacNeill B, Baum U, Pohle K, et al. Assessment of coronary remodeling in stenotic and nonstenotic coronary atherosclerotic lesions by multidetector spiral computed tomography. J Am Coll Cardiol 2004;43:842e7. 10. Fayad ZA, Fuster V. Clinical imaging of the high-risk or vulnerable atherosclerotic plaque. Circ Res 2001;89: 305e16. 11. Schartl M, Bocksch W, Koschyk DH, Voelker W, Karsch KR, Kreuzer J, et al. Use of intravascular ultrasound to compare effect of different strategies of lipid-lowering therapy on plaque volume and composition in patients with coronary artery disease. Circulation 2001;104:387e92. 12. Kapadia SR, Nissen SE, Ziada KM, Rincon G, Crowe TD, Boparai N, et al. Impact of lipid abnormalities in development and progression of transplant coronary disease: a serial intravascular ultrasound study. J Am Coll Cardiol 2001; 38:206e13. 13. Fayad ZA, Fuster V, Nikolaou K, Becker C. Computed tomography and magnetic resonance imaging for noninvasive coronary angiography and plaque imaging: current and potential future concepts. Circulation 2002;106:2026e34. 14. Achenbach S, Moselewski F, Ropers D, Ferencik M, Hoffmann U, MacNeill B, et al. Detection of calcified and noncalcified coronary atherosclerotic plaque by contrastenhanced, submillimeter multidetector spiral computed tomography: a segment-based comparison with intravascular ultrasound. Circulation 2004;109:14e7. 15. Kopp AF, Schroeder S, Kuettner A, Heuschmid M, Georg C, Ohnesorge B, et al. Coronary arteries: retrospectively ECG-gated multi-detector row CT angiography with selective optimization of the image reconstruction window. Radiology 2001;221:683e8. 16. Shimamoto R, Suzuki J-i, Nishikawa J-i, Fujimori Y, Nakamura F, Shin WS, et al. Measuring the diameter of coronary arteries on MR angiograms using spatial profile curves. AJR 1998;170:889e93.
A new method for measuring coronary artery diameters with CT spatial profile curves Ryoichi Shimamoto a, Jun-ichi Suzuki a,*, Tadashi Yamazaki a, Taeko Tsuji a, Yuki Ohmoto a, Toshihiro Morita a, Hiroshi Yamashita a, Junko Honye a, Ryozo Nagai a, Masaaki Akahane b, Kuni Ohtomo b a
Department of Cardiovascular Medicine, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan b Department of Radiology, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan Received 12 July 2005; accepted 16 October 2005 Available online 2 December 2005
KEYWORDS Multi-detector row CT; CT spatial profile curve; Coronary artery diameters; Intravascular ultrasound; Conventional coronary angiography; Hounsfield number
Abstract Purpose Coronary artery vascular edge recognition on computed tomography (CT) angiograms is influenced by window parameters. A noninvasive method for vascular edge recognition independent of window setting with use of multi-detector row CT was contrived and its feasibility and accuracy were estimated by intravascular ultrasound (IVUS). Methods Multi-detector row CT was performed to obtain 29 CT spatial profile curves by setting a line cursor across short-axis coronary angiograms processed by multi-planar reconstruction. IVUS was also performed to determine the reference coronary diameter. IVUS diameter was fitted horizontally between two points on the upward and downward slopes of the profile curves and Hounsfield number was measured at the fitted level to test seven candidate indexes for definition of intravascular coronary diameter. The best index from the curves should show the best agreement with IVUS diameter. Results Of the seven candidates the agreement was the best (agreement: 16 11%) when the two ratios of Hounsfield number at the level of IVUS diameter over that at the peak on the profile curves were used with water and with fat as the background tissue. These edge definitions were achieved by cutting the horizontal
Abbreviations: MDCT, multi-detector row computed tomography; IVUS, intravascular ultrasound; Conventional CAG, conventional coronary angiography. * Corresponding author. Laboratory Room-213, Department of Cardiovascular Medicine, Faculty of Medicine and Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 1138655, Japan. Tel.: þ81 3 3815 5411x33079; fax: þ81 3 3814 0021. E-mail address: [email protected] (J.-i. Suzuki). 1078-8174/$ - see front matter ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radi.2005.10.002
Method for measuring coronary artery diameters
45
distance by the curves at the level defined by the ratio of 0.41 for water background and 0.57 for fat background. Conclusions Vascular edge recognition of the coronary artery with CT spatial profile curves was feasible and the contrived method could define the coronary diameter with reasonable agreement. ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Introduction New mechanical engineering has successfully introduced computed tomography (CT) innovations. Spiral multi-detector row CT scanner with fast gantry rotation system has realized three-dimensional delineation of the coronary artery tree.1e4 Sixteen detector rows5 can provide isotropic high spatial resolution and can shorten scanning time6,7 with reduction of doses of radiation and contrast media.8,9 It is now the clinicians’ turn to apply this technology to fruitful use. A cross-sectional plane perpendicular to the axis of the running course of the coronary arteries, i.e., a coronary short-axis plane, provides an adequate view not only to describe severity of intravascular configurations but also to determine whether the atherosclerotic lesions are concentric or eccentric.10e12 Multi-planar reconstruction can provide a contiguous short-axis view along the coronary axis from a raw CT datum set.13,14 Thus CT observation of the coronary arteries is congruent with that of intravascular ultrasound (IVUS). And therefore, IVUS was used as the reference standard in the current study. Evaluation of coronary artery diameters should be based on accurate vascular edge recognition.15 When the recognition is done on CT display, what
should be emphasized is that window setting influences observers’ inspection, that is to say, measurements of coronary intravascular diameters on CT display depend on window center and width. CT spatial profile curves obtained by setting a cursor line across the short-axis coronary angiogram do not depend on window parameters (Fig. 1a,b). If a method with use of CT spatial profile curves is established to determine the intra-arterial edges, the method independent of display parameters might be of great clinical use.16 Accordingly the aim in the current study was to evaluate feasibility and accuracy of CT spatial profile curves for recognizing the edge of the coronary arteries.
Subjects and methods Subjects Twenty-nine CT spatial profile curves (19 curves from the left anterior descending arteries and 10 curves from the right coronary arteries) were obtained from seven patients (seven males; 64.0 8.5 years old; mean weight, 58.1 5.3 kg) who were admitted to the hospital to undergo both catheter coronary angiography and IVUS. Spiral multi-detector row CT was performed to obtain
Figure 1 Coronary short-axis CT angiogram and spatial profile curve. (a) Cursor line is set to intersect coronary artery and to pass the center of short-axis coronary angiogram reformatted by using multi-planar reconstruction. Length of cursor line is shown to be 1.1 cm. (b) CT spatial profile curve is automatically displayed with software installed at offline workstation. Vertical axis represents CT value in Hounsfield units and horizontal axis shows spatial distance in millimeter from starting edge through ending edge of cursor.
46 CT coronary angiograms within three days of the IVUS examination (either before or after). All gave written informed consent and the protocol for coronary CT angiography was approved by the institutional review board.
Spiral multi-detector row CT examination A three-dimensional datum set with a scan range of 120 mm was obtained with use of 16 detector row spiral CT scanner (Aquilion, Toshiba Medical Systems Co., Tokyo) with retrospective ECG gated reconstruction during a single breath-hold for 30 s. Imaging parameters used were gantry rotation time of 500 ms, collimated slice thickness of 0.5 mm and table feed of 4 mm/s, i.e., helical pitch of 4.0. Field of view of 200 mm 200 mm and matrix of 512 512 resulted in spatial resolution of 0.4 mm 0.4 mm in the transaxial plane. Contrast medium (iopamidol, Iopamiron 370 syringe, Schering AG, Berlin, Germany) was injected via the cubital vein at 2.5 cc/s. Total injected dose was less than 100 cc.
Measurements with intravascular ultrasound IVUS was performed within three days of CT examination (either before or after) with use of commercially available equipment (INSIGHT, Boston Scientific Co., Natick, MA, USA) with 40 MHz transducer (Atlantis Pro, Boston Scientific Co., Natick, MA, USA). Short-axis ultrasonograms perpendicular to the axis of the coronary artery were obtained at the site of the branch diverging from the main coronary arteries (Fig. 2a). The diverging branches were used as the markers to identify the
R. Shimamoto et al. common site or common section of the coronary artery and to define the common direction for measurements of intravascular diameter on the short-axis CT angiograms obtained with use of multi-planar reconstruction (Fig. 2b). Intravascular diameters of the coronary arteries were measured on short-axis ultrasonograms from leading edge to leading edge.
Coronary artery short-axis images with multi-planar reconstruction Ten sets of transaxial three-dimensional raw data obtained with datum acquisition window at each 10% cardiac cycle were transferred to the offline workstation where image processing softwares such as multi-planar reconstruction were available. Thus each three-dimensional datum set was reconstructed at each 10% cardiac cycle. Image quality obtained at 70 or 80% cardiac cycle was the most stable, and therefore, a datum set reconstructed at these cardiac phases was used thereafter for all analyses. Multi-planar reconstructed CT images in the common short-axis coronary artery planes to IVUS were obtained, and on these images the common intravascular diameters were identified by recognizing the direction of the diverging branches as the anatomical markers (Fig. 2a,b).
Spatial-Hounsfield unit curves, i.e., CT spatial profile curves Spatial-Hounsfield unit curves, i.e., CT spatial profile curves were obtained by setting the straight cursor line across the short-axis coronary CT angiograms processed with multi-planar reconstruction
Figure 2 Common cross-sectional plane to IVUS (a) and short-axis coronary CT angiogram (b). Twig diverging from coronary artery is an anatomical marker to accord direction for setting cursor line of spatial profile curve with that for measuring coronary diameter on IVUS.
Method for measuring coronary artery diameters (Fig. 1). The cursor line was set on each short-axis coronary CT angiogram in 29 multi-planar reconstructed planes from seven patients. The directions of the cursors were perpendicular to those of diverging branches and coronary plaques were respected in order not to be involved. The curves were automatically displayed by the software installed at the workstation (Virtual Place Advance, Aze Ltd., Tokyo, Japan). The vertical axis represented CT value in Hounsfield units and the horizontal axis indicated length or distance from the starting point of the cursor of the straight line across the coronary artery in millimeters (Fig. 1b).
Analysis on spatial profile curves On the spatial profile curve obtained by setting the cursor line in the common direction and in the common plane to IVUS, the reference diameter measured by IVUS was inserted horizontally by fitting its length into the profile curve in parallel with the horizontal axis of the curve. CT Hounsfield value was measured on the curve at the fitted level of the IVUS diameter [H(IVUS)] and at the peak of the profile curve [H(peak)]. As the background, CT value was also measured at the region of the myocardium [H(myocardium)] and the subepicardial fat [H(fat)] (Fig. 3).
Figure 3 Schematic representation of spatial profile curve. IVUS diameter represents diameter measured by IVUS with common direction for cursor setting to obtain spatial profile curve. IVUS diameter is horizontally fitted at Hounsfield level on spatial profile curve where IVUS diameter becomes the same value for horizontal distance between two points on upward slope and downward slope of spatial profile curve. [H(IVUS)] represents Hounsfield value at a level where IVUS diameter is fitted on spatial profile curve. [H(peak)], [H(myocardium)], [H(water)] and [H(fat)] represent Hounsfield value at peak of curve, myocardium, water (¼0) and subepicardial fat, respectively.
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Candidate indexes from spatial profile curves to define the vascular edge Each reference diameter directly measured by IVUS was fitted into each CT spatial profile curve and the Hounsfield value [H(IVUS)] was measured at the fit level on each profile curve. The mean and standard deviation of candidate indexes that were used for edge recognition were calculated by use of the above-obtained [H(IVUS)]. The seven indexes were as follows: (1) H(IVUS) H(water) i.e., CT Hounsfield value at the wedge level on the profile curve with water (Hounsfield value of water: 0) as the background; (2) H(IVUS) H(myocardium) i.e., CT Hounsfield value at the wedge level with myocardium as the background; (3) H(IVUS) H(fat) i.e., CT Hounsfield value at the wedge level with sub-epicardial fat as the background; (4) [H(IVUS) H(water)]/[H(peak) H(water)] ¼ H(IVUS)/H(peak) i.e., a ratio of CT Hounsfield value at the wedge level over that at the peak on the profile curve with water as the background; (5) [H(IVUS) H(myocardium)]/[H(peak) H(myocardium)] i.e., a ratio with myocardium as the background; (6) [H(IVUS) H(fat)]/[H(peak) H(fat)] i.e., a ratio with fat tissue as the background; (7) [H(peak)] [H(IVUS)] i.e., a difference between Hounsfield value at the peak and that at the wedge; where H(peak), H(myocardium), H(fat), and H(IVUS) represent CT value in Hounsfield units at the peak of the profile curves, that of the myocardium, and that of the fat, and again Hounsfield value corresponds to the wedge level where IVUS diameter fits the profile curves, respectively (Fig. 3). H(water) represents Hounsfield number of water, i.e., H(water) ¼ 0. The best index that defines the diameter most accurately should show the best agreement with the reference IVUS diameter.
Diameter measurement on CT profile curves with each index With use of each value of [H(IVUS)] obtained on each profile curve, the mean value [Mean(k), k: candidate number, k ¼ 1, 2, 3, 4, 5, 6, 7] of each index was calculated and the following seven
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formulae were obtained. Then from each formula the Hounsfield value to define the coronary diameter on the curves [H(k), k: candidate number, k ¼ 1, 2, 3, 4, 5, 6, 7] was calculated. (1) H(1) H(water) ¼ Mean(1) i.e., H(1) ¼ Mean(1) þ H(water) ¼ Mean(1); (2) H(2) H(myocardium) ¼ Mean(2) i.e., H(2) ¼ Mean(2) þ H(myocardium); (3) H(3) H(fat) ¼ Mean(3) i.e., H(3) ¼ H(fat) þ Mean(3); (4) [H(4) H(water)]/[H(peak) H(water)] ¼ Mean(4) i.e., H(4) ¼ [Mean(4)] [H(peak) H(water)] þ H(water) ¼ [Mean(4)] [H(peak)]; (5) [H(5) H(myocardium)]/[H(peak) H(myocardium)] ¼ Mean(5) i.e., H(5) ¼ Mean(5) [H(peak) H(myocardium)] þ H(myocardium); (6) [H(6) H(fat)]/[H(peak) H(fat)] ¼ Mean(6) i.e., H(6) ¼ Mean(6) [H(peak) H(fat)] þ H(fat); (7) [H(peak)] [H(7)] ¼ Mean(7) i.e., H(7) ¼ H(peak) Mean(7). Horizontal distance was measured between two points on the upward and downward slopes of each profile curve at the Hounsfield level that corresponded to each H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7) value. These distances measured on profile curves and corresponding IVUS diameters were used to calculate agreement for seven candidate formulae.
Statistical analysis All values were expressed as the mean standard deviation (S.D.). Agreement between the CT expected value and reference IVUS value was expressed as the absolute difference of the paired two measured values over the mean of the two.
Results As the background tissue, Hounsfield values of the tissue of the myocardium and that of the sub-epicardial fat were 90 6 and 111 8, respectively. Table 1 summarizes the mean and the standard deviation of each index calculated by using CT Hounsfield values corresponding to IVUS diameters. Table 2 represents agreement between the diameter expected on spatial profile curves by each candidate index and the diameter actually
Table 1
Scatter range converted into Hounsfield value
Mean(1) Mean(2) Mean(3) Mean(4) Mean(5) Mean(6) Mean(7)
Mean
S.D.
130 40 241 0.41 0.15 0.57 182
75 73 75 0.19 0.28 0.14 64
Mean(1), Mean(2), Mean(3), Mean(4), Mean(5), Mean(6), and Mean(7) were calculated from spatial profile curves by use of the formula of [H(IVUS) H(water)], [H(IVUS) H(myocardium)], [H(IVUS) H(fat)], [H(IVUS) H(water)]/ [H(peak) H(water)], [H(IVUS) H(myocardium)]/[H(peak) H(myocardium)], [H(IVUS) H(fat)]/[H(peak) H(fat)] and [H(peak) H(IVUS)], respectively. One standard deviation of each index was converted into scatter range in Hounsfield units.
measured by IVUS. The absolute difference between the diameter indicated by CT and that measured by IVUS over the mean of the paired two values was calculated for each candidate index. Two cases gave the best agreement of 16 11%: when the diameter was horizontally cut by the curves at the Hounsfield value of 0.41 of the peak with water as the background and at 0.57 with fat as the background tissue (Fig. 4).
Discussion Two promising indexes Of the seven candidate indexes defined with CT spatial profile curves whose clinical feasibility and accuracy were evaluated in determination of coronary artery diameter, the best agreement was obtained by using the two indexes, i.e., the ratio of Hounsfield number corresponding to coronary artery diameter over that of the peak of the profile curve with water as the background and the ratio with the sub-epicardial fat as the background. Evaluation of the coronary artery diameter with the former ratio required measuring the horizontal distance cut by the profile curve at Hounsfield number of 0.41 of the peak of the profile curve. Identical results for the diameter were found when measuring the diameter at 0.57 of the profile curve peak with fat as the background. The diameters determined by these two ratios had the best agreement with IVUS diameters.
Clinical feasibility Clinical accuracy in coronary intravascular edge recognition with use of the above-mentioned two
Method for measuring coronary artery diameters Table 2
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Agreement between CT expected diameter and reference IVUS diameter
H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7)
Agreement
H(1) ¼ 130 H(2) ¼ H(myocardium) þ 39 H(3) ¼ H(fat) þ 244 H(4) ¼ 0.41 H(peak) H(5) ¼ 0.15 [H(peak) H(myocardium)] þ H(myocardium) H(6) ¼ 0.57 [H(peak) H(fat)] þ H(fat) H(7) ¼ H(peak) 182
Mean
S.D.
0.20 0.20 0.21 0.16 0.17 0.16 0.19
0.13 0.13 0.14 0.11 0.12 0.11 0.12
Each CT expected diameter was measured on each spatial profile curve at the Hounsfield value of H(k) (k ¼ 1, 2, 3, 4, 5, 6, 7). Agreement between CT expected diameter and reference IVUS diameter was calculated by following formulae: absolute difference of the paired two measured diameters over the mean of the two.
ratios was completely identical. As a matter of fact, both showed the same value of agreement of 16 11%. Although the latter ratio required two measurements of Hounsfield value of the subepicardial fat tissue and that of the curve peak, the former needed only one measurement at the peak. All procedures for the former method were to obtain spatial profile curve by setting a line cursor across the short-axis CT angiogram and to measure the horizontal distance cut by the profile curve at the Hounsfield level of 0.41 of the peak.
Clinical accuracy In comparison with IVUS as the reference standard, the value of agreement from the most promising
and simpler method proposed in the current study was 16% in determination of coronary artery diameter. If a given diameter of coronary artery is 2.5 mm, the mean value of error with the current method is calculated as 0.4 mm [¼2.5 0.16 mm]. This almost corresponds to one spatial resolution (0.4 mm in the transaxial planes). Although this agreement is enough to detect moderate to severe stenotic legions, this is not necessarily sufficient to identify mildly stenotic lesions that have the possibility to develop into acute coronary syndrome with jump-up phenomenon. In the future with evolution in spatial resolution the agreement with this method will reach the clinical use in all types of lesions.
Advantages of window independence When referred to CT profile curves it is apparent that inspective recognition of intravascular edge of the coronary arteries depends on window center and level, i.e., upward and downward parts of the curves do not indicate vertical lines but gradual slopes. This implies display parameters vary the subjectively optical level for the edge recognition along these slopes. Window dependent method requires window standardization. CT spatial profile curves are independent of window setting, and therefore, indexes defined using these curves will be unaffected by window parameters.
Figure 4 Scheme for representation of how to measure coronary diameter with two promising ratios on spatial profile curve. Coronary artery diameter is obtained by measuring distance between two intersections made by spatial profile curve and horizontal line at Hounsfield level of peak value multiplied by 0.41 [H(peak) 0.41]. Identical diameter is also obtained at Hounsfield level of peak value multiplied by 0.57 with sub-epicardial fat as background [{H(peak) H(fat)} 0.57] þ H(fat).
Conclusion Using CT spatial profile curves, coronary diameter could be obtained by measuring a horizontal length between two points on the curve at 41% Hounsfield level of the peak. This method was independent of window parameters, clinically feasible, and showed agreement of 16% with reference standard.
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