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Virtual Endoscopy of Coronary Arteries Using Contrast-Enhanced ECG-Triggered Electron Beam CT Data Sets

¹ Department of Radiology, Hiroshima University, School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan
² Second Department of Internal Medicine, Hiroshima University, School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan.

Received July 13, 1999; accepted after revision November 1, 1999.

 
Address correspondence to T. Nakanishi.

Introduction

Top Introduction Materials and Methods Results Discussion References

 
Application of virtual endoscopy to vascular structures has been limited to the aorta and its branch orifices [1,2,3]. The coronary artery is a promising target for virtual endoscopic imaging because invasive coronary angioscopy has contributed to the assessment of atheromatous plaques and has potential application for the assessment of ischemic heart disease [4]. Electron beam CT is capable of providing high-resolution spatial and temporal images and ECG-triggered scaning for coronary artery imaging [5, 6]. The present study investigated coronary endoscopic images created from electron beam CT data sets of both a phantom model and patients with ischemic heart disease.

Materials and Methods

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A phantom and patients were evaluated with an electron beam CT scanner (Imatron C-150; Imatron, South San Francisco, CA). A phantom for cineangiography was scanned in an axial direction similar to the scanning of the human heart. The phantom was constructed from a barium cast of the coronary artery of an adult pig embedded in polyester resin. The difference in the CT number between the cast and the resin was approximately 1400 H. In addition, the diameter of the proximal coronary cast was 6 mm. To investigate the effectiveness of CT data sets as a coronary artery evaluation tool, source phantom images from four different scanning protocols were obtained as follows: 1.5-mm thickness contiguous sections, 3.0-mm thickness sections with 1-mm overlap, 3.0-mm thickness contiguous sections, and 6.0-mm thickness contiguous sections. These four different scanning protocols were performed with the following parameters: field of view, 26 cm; scan time, 100 msec per slice; number of slices, 40 covering the entire phantom.

The subjects consisted of 22 patients (17 men and five women; age range, 46-81 years; mean age, 64 years) who underwent catheter coronary angiography showing no complete obstruction in any coronary branch. Image acquisition was done on inspiratory breath-holds, triggered to the ECG, with one acquisition after every QRS complex at 80% of the R-R wave interval. We used the following parameters: acquisition time, 100 msec per section; section thickness, 1.5 mm; field of view, 26 cm; acquisition matrix, 512 x 512. Thus, the spatial resolution of the axial plane was approximately 0.5 mm per pixel, and the spatial resolution in the longitudinal direction was 1.5 mm per pixel.

An 18- or 22-gauge angiographic catheter was placed in the antecubital vein. An 80- to 100-ml injection of nonionic contrast agent (iopamidol, 370 mgI/ml) (Iopamiron 370; Nippon Shering, Osaka, Japan) was administered with a power injector at a rate of 2-3 ml/sec, followed by a 23- to 30-sec scanning delay.

Virtual endoscopic views were created from the patient data sets with Navigator (General Electric Medical Systems, Milwaukee, WI). Before creating the virtual endoscopic views, a survey of coronary calcification, showing apparently higher attenuation than vessel lumen, was performed with multiplanar reconstruction. The first step was to apply a lower threshold "black in white" algorithm value setting of 70-120 H for the patient data sets and 300 H for the phantom data sets. Interactive changing thresholds were performed when segmentation artifacts were observed in the proximal coronary artery branch. The second step was to add an upper threshold individually optimized value that had been adjusted to an attenuated calcified portion to differentiate coronary calcification from the vessel inner lumen. The quality of the virtual endoscopic images was evaluated in terms of visualization of coronary orifices, percentage of patients in whom coronary arteries could be observed, and a measurement of fly-through distance for each coronary branch.

Results

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The phantom examination revealed that a 1.5-mm contiguous thickness or a 3-mm thickness with a 1-mm overlap scan produced good volumetric data sets for coronary artery evaluation. Other scanning methods, including 3-mm contiguous scans, produced inadequate source images with the result being poor coronary vessel images. A fly-through within the coronary barium cast of the phantom could be easily performed in every field from the main branch to the small side branches in data sets involving a 1.5-mm contiguous thickness or a 3-mm thickness with a 1-mm overlap scan. In addition, the coronary orifice was clearly visualized in all branches (Fig. 1A,1B), where virtual endoscopic views clearly showed a round configuration that corresponded to the images taken from the phantom. When using the patient data sets, a fly-through was not achieved for at least one branch in three patients (14%) as a result of dense calcification and small coronary branch diameters. The mean and standard deviation values for CT attenuation at the left coronary orifice, the right coronary orifice, and at the mid left ventricular level were 205 ± 40, 212 ± 52, and 208 ± 59 H, respectively. A total of 21 (95%) of 22 patients showed attenuation levels of more than 130 H at all three regions. Visualization of the coronary orifices was possible in 17 (89%) of 19 patients in the right coronary artery, 18 (95%) of 19 in the left main coronary artery, and 18 (95%) of 19 in the left circumflex artery. Typical coronary artery orifice images created from the patient data sets are shown in Figure 2A,2B. In addition, the fly-through distance was 23.5 ± 11.9 mm for the right coronary artery, 29.4 ± 12.8 mm for the left main-left anterior descending arteries, and 22.5 ± 11.4 mm for the left circumflex artery. A fly-through to the lower end of the scanning field was possible in only two vessels. Calcified plaque was successfully visualized by adjusting the upper and lower threshold settings (Fig. 3A,3B).

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —Virtual endoscopic images created from phantom data sets. Left coronary orifice was viewed inside from left main coronary artery.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —Virtual endoscopic images created from phantom data sets. Conversely, viewing from aortic root shows virtually two orifices represented as left anterior descending artery and left circumflex artery.

View larger version (141K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —56-year-old man with ischemic heart disease. Human virtual view shows slight irregularity (arrows) of left main coronary artery.

View larger version (136K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —56-year-old man with ischemic heart disease. Human virtual view also shows two holes representing orifices of left anterior descending artery and left circumflex artery.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —64-year-old man with ischemic heart disease. Multiplanar reconstruction image created from contrast-enhanced ECG-triggered 1.5-mm contiguous thickness sections clearly shows focal calcification (arrow).

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —64-year-old man with ischemic heart disease. Virtual endoscopic image clearly shows shape and site of calcified coronary plaque from perspective of lumen.

Discussion

Top Introduction Materials and Methods Results Discussion References

 
Vascular disorders have been successfully evaluated with virtual endoscopy since additional information on projectional images of the vessel orifices could be obtained [1,2,3]. To our knowledge, the use of endoscopic views for coronary arteries has not been reported.

Recent reports have shown the feasibility of electron beam CT data sets for coronary artery stenosis with several postprocessing methods including multiplanar reconstruction, three-dimensional surface display, and maximum intensity projection [5, 6]. Previous coronary artery evaluation studies involving electron beam CT reportedly used 1.5-mm collimation gapless [5] or 3-mm with 1-mm overlap scanning [6]. To reduce any partial volume averaging effects, a thinner section is desirable because the diameter of the coronary artery is small and differentiation of the vessel inner lumen and coronary artery wall is essential. It is especially important to render calcification as a wall structure.

Direct visualization of vessel orifices by CT angiography is fundamentally difficult because the projectional nature of these images simulates the appearance of conventional angiograms. Virtual endoscopic images could show the coronary artery orifices.

One advantage associated with virtual endoscopy is that it enables an evaluation of the inner surface of the coronary artery images. Visualization of coronary artery plaque can be established by viewing from the inside, especially when coronary artery calcification is so severe that it precludes viewing from the outside [5]. A characterization of the atheromatous plaque is essential to properly determine treatment. Virtual endoscopic imaging might have potential applications for showing disorders of the coronary artery surface such as ulceration, intimal flap, and irregularity [4].

Practical disadvantages include a relatively short fly-through distance covering the proximal coronary artery tree. However, this difference depends largely on source image quality because the phantom data sets enable a long fly-through to the periphery. Because an apparent difference exists between the phantom and patient data sets, the fly-through distance depends on the image quality of a given data set. Vessel contrast determined by CT number is considered to be a major contributing factor to the quality of the endoscopic images. Although endoscopy does not provide a quantitative method for evaluating vessel stenosis, the images obtained provide unique information about the vessel lumen. Further research is needed to improve source data sets and postprocessing methods.

References

Top Introduction Materials and Methods Results Discussion References

 

1. Davis CP, Ladd ME, Romanowski BJ, et al. Human aorta: preliminary results with virtual endoscopy based on three-dimensional MR imaging data sets. Radiology 1996;199 : 37-40[Abstract/Free Full Text]
2. Kimura F, Shen Y, Date S, Azemoto S, Mochizuki T. Thoracic aortic aneurysm and aortic dissection: new endoscopic mode for three-dimensional CT display of aorta. Radiology 1996;198 : 573-578[Abstract/Free Full Text]
3. Marro B, Galanaud D, Valery CA, et al. Intracranial aneurysm: inner view and neck identification with CT angiography virtual endoscopy. J Comput Assist Tomogr 1997;21 : 587-589[Medline]
4. Mizuno K, Miyamoto A, Satomura K, et al. Angioscopic coronary macromorphology in patients with acute coronary disorders. Lancet 1991;337 : 809-812[Medline]
5. Nakanishi T, Ito K, Imazu M, Yamakido M. Evaluation of coronary artery stenoses using electronbeam CT and multiplanar reformation. J Comput Assist Tomogr 1997;21 : 121-127[Medline]
6. Moshage WE, Achenbach S, Seese B, Bachmann K, Kirchgeorg M. Coronary artery stenoses: three-dimensional imaging with electrocardiographically triggered, contrast agent—enhanced, electron-beam CT, Radiology 1995;196 : 707-714[Abstract/Free Full Text]

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