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Neuroradiology |
1 From the Department of Radiology, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860-8556, Japan (T.H., K.O., M.Y., Y.Y.); Departments of Radiology (T.H.) and Neurosurgery (S.U.), Amakusa Medical Center, Kumamoto, Japan; and Department of Radiology, University of Occupational and Environmental Health, School of Medicine, Kitakyushu, Japan (Y.K.). Received July 26, 2003; revision requested October 9; revision received December 28; accepted January 30, 2004. Address correspondence to T.H. (e-mail: toshinor@beige.ocn.ne.jp).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Results of a total of 68 volume-rendered 3D digital angiographic examinations in 56 patients with intracranial aneurysms were retrospectively evaluated in comparison with results of digital subtraction angiography regarding the appearance of a pseudostenosis phenomenon. The phenomenon was analyzed by two radiologists in consensus with regard to frequency, location, percentage stenosis, and angle between the axis of the vessel with pseudostenosis and the axis of rotational angiography. The phenomenon’s effect on aneurysm evaluation was also analyzed. For assessing the physical properties of the phenomenon, a phantom study was performed with different lengths of tubing and different angles to the axis of rotational angiography.
RESULTS: The pseudostenosis was observed at 23 (34%) of 68 3D digital angiographic examinations and in 53 (5%) of 1161 segments. The percentage stenosis ranged from 3% to 85% (mean, 18% ± 16.2 [standard deviation]). The arterial segments with pseudostenosis included 15 (25%) of 61 C6 segments, 10 (16%) of 61 M1 segments, and six (10%) of 60 A1 segments. Angles between the two axes ranged from 86° to 93° (mean angle, 90° ± 1.6). Pseudostenosis affected delineation of the shape and size of two middle cerebral artery aneurysms. At phantom analysis, the phenomenon was most obvious at an angle of 90° and with the longest phantom.
CONCLUSION: The pseudostenosis phenomenon on volume-rendered 3D digital angiograms was relatively frequently observed in some segments of the intracranial arteries and affected the delineation of middle cerebral artery aneurysms. This phenomenon was associated with the angle to the axis of rotational angiography and the length of the vessel.
© RSNA, 2004
Index terms: Aneurysm, intracranial, 17.73 • Angiography, comparative studies, 17.124, 17.1249, 17.127 • Cerebral angiography, 17.124, 17.1249, 17.127 • Cerebral blood vessels, stenosis or obstruction, 17.124, 17.127 • Phantoms • Test objects
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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We have encountered cases of a pseudostenosis phenomenon in the intracranial arteries at 3D angiography. Our definition of a pseudostenosis is a stenosis that is seen at 3D angiography but is revealed not to be a real stenosis at two-dimensional (2D) DSA. To our knowledge, this pseudostenosis phenomenon has not been previously described. Thus, the purpose of our study was to assess the frequency, location, and effect on image interpretation of the pseudostenosis phenomenon at volume-rendered 3D digital angiography performed for the evaluation of intracranial arteries and to assess the physical characteristics of the phenomenon by using a phantom.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Patients who had undergone endovascular coil embolization or surgical clipping before angiography were excluded from this study because the presence of coils or clips may result in artifacts. A total of 56 patients (38 women, 18 men; age range, 37–88 years; mean age, 64.3 years) were included, and the results of 68 angiographic examinations (61 internal carotid and seven vertebral angiographic examinations) were assessed. Written informed consent to perform imaging examinations was obtained from patients or from their relatives. The institutional review board of Kumamoto University School of Medicine did not require its approval or patient informed consent for this retrospective study.
The indications for angiography included evaluation of subarachnoid hemorrhage seen at computed tomography (CT) (n = 46) and evaluation of intracranial aneurysm suspected to be present on the basis of magnetic resonance angiographic findings (n = 10). The angiographic examination was performed the day the subarachnoid hemorrhage occurred (day 0) in 26 patients, within 1–3 days after the hemorrhage occurred in 16 patients, and within 4–10 days after the hemorrhage occurred in four patients. All patients were imaged after local anesthesia and sedation had been induced.
2D DSA imaging and display.—Intraarterial DSA was performed with a 1024 x 1024 matrix. All catheterizations were performed with a standard diagnostic catheter by using a transfemoral approach and the Seldinger technique. Selective three- or four-vessel angiography was performed in the anteroposterior and lateral projections. A total of 5–6 mL of iopamidol (Iopamiron 300; Schering, Osaka, Japan) was injected into the internal carotid or the vertebral artery at a flow rate of 3.0–4.0 mL/sec by using a power injector (Auto Enhance A-50; Nemoto Kyorindo, Tokyo, Japan).
Because the 3D digital angiograms enable observation and analysis of an aneurysm from multiple views, we used two or three limited projections at 2D DSA when rotational angiography would be performed in a selected vessel that was known or suspected to contain an aneurysm. In 12 patients who had an aneurysm in the anterior communicating artery, DSA with manual compression of the contralateral carotid artery was performed to obtain more detailed information about the relationship between the aneurysm and the parent artery. A radiologist (T.H. or K.O.) selected six images for each projection and printed them on hard-copy film.
3D digital angiography and image display.—After 2D DSA examinations were performed, rotational digital angiography was performed with a field of view of 17 x 17 cm by using the frontal plane of a biplanar C-arm. The counterbalanced ceiling-suspended C-arm rotated in a continuous 180° (–90° to +90°) arc with the path of the x-ray tube. The isocenter for the rotational field was the area of interest in the patient’s head. One hundred and twenty digital angiograms with a 512 x 512 matrix were obtained with the 180° C-arm sweep; the C-arm rotated at a speed of 30° per second. The total 180° rotation was accomplished in 8 seconds. A total of 24–32 mL of iopamidol was injected into the internal carotid or the vertebral artery at a flow rate of 3.0–4.0 mL/sec by using the same power injector used at 2D DSA. The injection began 2.0 seconds before the acquisition of the first opacified image so that complete filling of the selected arteries during rotational angiography would be achieved. Rotational digital angiography was performed when an aneurysm was confirmed or suspected to be present at 2D DSA.
The images acquired at rotational digital angiography were transferred to a workstation (3D-RA; Philips Medical Systems) for generating volume-rendered 3D digital angiograms. A 128 x 128 x 128 3D angiogram that showed all the vessels included in the field of view was automatically displayed within 5 minutes after all images were transferred to the workstation. The 256 x 256 x 256 angiograms were generated after regions of interest were selected. Then, images of the aneurysm and parent artery were also reconstructed with a multiplanar reconstruction technique. One radiologist (T.H.) measured the diameter of each cerebral aneurysm on multiplanar reconstruction images at a workstation. The maximum diameter of each aneurysm was classified as large (>12 mm), medium (5–12 mm), small (
3 but <5 mm), or very small (<3 mm).
Volume-rendered display was the only method used for evaluating 3D digital angiograms in this study. The reviewer (T.H.) determined the threshold of vessel images in each case by interactively observing the angiograms at the workstation. When demonstration of the small vessels or arteries ("perforators") penetrating the brain parenchyma in the region of interest was required, the threshold was changed according to their visibility.
Assessment of angiograms.—The assessed intracranial vessels included segments of the internal carotid artery (C1 [the terminal segment], C2 [the first part of the cisternal segment], C3 [the carotid knee segment], C4 [the cavernous segment], C5 [the vertical segment], and C6 [the carotid canal segment of the petrous portion]), the middle cerebral artery (M1 [the sphenoidal segment], M2 [the insular segment], and M3 [the opercular segment]), the anterior cerebral artery (A1 [the segment from the origin of the anterior cerebral artery to the anterior communicating artery], A2 [the infracallosal segment], and A3 [the supracallosal segment]), the posterior cerebral artery (P1 [the segment from the origin of the posterior cerebral artery to the posterior communicating artery], P2 [the ambient cistern segment], and P3 [the quadrigeminal segment]), the vertebral artery (V1 [the intracranial segment] and V2 [the C1-to-C2 cervical segment]), and the basilar artery.
Thus, 12 and six arterial segments were evaluated at the internal carotid and the vertebral angiographic examinations, respectively. When fetal-type circulation in the posterior cerebral artery was seen, the P1 segment was defined as the proximal portion of the posterior cerebral artery. When a hypoplastic A1 was seen, that A1 segment was excluded from further analysis.
In assessing the volume-rendered 3D digital angiograms, the 2D DSA images were used as the standard of reference. Two radiologists (T.H., K.O.) analyzed the pseudostenosis phenomenon together in consensus. For each arterial segment, they visually assessed the 3D digital angiograms in conjunction with the corresponding 2D DSA images on hard-copy films and documented the presence of the phenomenon and its location. When the phenomenon was present, they also measured the maximum percentage stenosis. Diameters were measured on the 2D DSA and 3D digital angiographic hard-copy films by using a commercially available vernier caliper with 0.1-mm divisions. To maintain uniformity in choosing the location of the vessel wall, the outermost margin of the vessel was selected on the angiograms. The percentage stenoses were measured by comparing the diameter of the maximally stenosed region with the diameter of a nearby normal segment of the vessel.
With regard to the angle between the axis of the vessel with pseudostenosis and the axis of rotational angiography, one radiologist (T.H.) measured the segment that was judged to contain the pseudostenosis at the 3D workstation. Visual estimation was used to construct an axis line for the vessel with the pseudostenosis. The angle between this line and the axis line of rotational angiography was then determined. The angles were assigned positive values on the basis of the axis line of rotational angiography.
Two radiologists (T.H., K.O.) also evaluated whether the phenomenon affected aneurysm depiction and delineation. Aneurysm depiction meant the presence or absence of aneurysm. Aneurysm delineation represented the visualization of the aneurysm neck, the shape of the aneurysm, and the relationship between the aneurysm and the parent artery.
Phantom Study
The results of our clinical study motivated us to create an in vitro model of intracranial arteries. Nonpulsatile phantoms that consisted of 3-mm-inner-diameter Teflon tubes filled with nonionic contrast material (iopamidol [300 mg of iodine per milliliter]) in a water bath were used. First, three 60-mm-long tubes were placed with three different angles (80°, 85°, and 90°) to the axis of rotational angiography and imaged with rotational digital angiography. Then, three different-length tubes (60, 40, and 20 mm) were imaged at an angle of 90°.
The unsubtracted rotational images were transferred to the same workstation used for generating 3D images, and 256 x 256 x 256 images were generated. Then, one radiologist (T.H.) assessed the appearance of the tubes and measured the diameter of the area that appeared maximally stenosed and the diameter of a nearby normal area of the tube.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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The pseudostenosis was observed at 23 (34%) of 68 3D digital angiographic examinations and in 53 (5%) of 1161 segments. The pseudostenosis was observed in 15 (25%) of 61 C6 segments, 10 (16%) of 61 M1 segments, one (14%) of seven V2 segments, six (10%) of 60 A1 segments, six (5%) of 122 A3 segments, nine (4%) of 244 M3 segments, one (4%) of 26 P2 segments, two (3%) of 61 A2 segments, and three (2%) of 122 M2 segments. The maximum percentage stenosis of the pseudostenosis ranged from 3% to 85% (mean, 18% ± 16.2 [standard deviation]). The angles between the two axes ranged from 86° to 93° (mean angle, 90° ± 1.6).
A total of 80 intracranial aneurysms were observed. The locations and sizes of the aneurysms are shown in the Table. The aneurysms were located in the internal carotid artery (n = 26), middle cerebral artery (n = 25), anterior cerebral artery (n = 23), vertebral artery (n = 3), basilar artery (n = 2), and posterior cerebral artery (n = 1). Ten aneurysms were very small, 25 were small, 38 were medium, and seven were large. The pseudostenosis affected delineation of two small aneurysms in the middle cerebral artery (2% of all aneurysms and 8% of middle cerebral artery aneurysms). In one M1– M2 aneurysm, the aneurysm neck appeared to be smaller at 3D digital angiography (Fig 1). In the other M1 aneurysm, the aneurysm shape was irregular and its relationship to the anterior temporal artery was unclear at 3D digital angiography (Fig 2). There were no cases in which the pseudostenosis affected aneurysm detection at 3D digital angiography.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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On the basis of the results of our clinical study, we speculated that the pseudostenosis phenomenon is associated with the angle between the axis of the vessel and the axis of rotational angiography. Because the range of the angle was very limited at patient imaging, we used a very narrow range of angles to conduct a phantom study. In the phantom study, the pseudostenosis was reproducible and more obvious when the longitudinal axis of the vessel phantom was perpendicular or nearly perpendicular to the axis of rotational angiography. Thus, results of the clinical and phantom studies indicated that the pseudostenosis phenomenon is associated with geometric factors of the angle between the axis of the vessel and the axis of rotational angiography. The phantom study results also suggest that the length of the vessel may affect the phenomenon.
Although the exact reasons for this artifact are unknown, the possible explanations are as follows: First, there may have been a physical problem regarding the geometric difference in x-ray absorption for contrast material–filled structures. When a vessel has an angle perpendicular to the axis of rotational angiography and has some degree of length, a shortage of image spatial information caused by superimposition of the vessel itself during rotational image acquisition may occur. This shortage of information is considered to be most obvious in areas perpendicular to the axis of rotational angiography and tangential to the image intensifier. This theory may also explain why this phenomenon was most obvious in the central portion of the phantom tubes.
Second, the range of rotational image acquisition might have been small. Although rotational digital angiography was performed with a continuous 180° acquisition, more acquisitions might be required to prevent this phenomenon. Further investigations of this matter need to be performed. Third, equipment vibration and patient motion during rotational acquisition may affect the image data, especially data in vessels that are perpendicular to the axis of rotational angiography. Although equipment vibration was fairly restricted in the system used in this study and patient motion during rotational acquisition was limited, these factors might have influenced the outcome.
Fourth, contrast material that fills the vessels during the rotation is not homogeneous because of pulsatile blood flow and contrast material dilution. Hemodynamic parameters such as inhomogeneous and incomplete filling of the aneurysmal sac with contrast material might be associated with the artifact (10). Because relatively large amounts of contrast material and an adequate infusion speed were used in this study, we believe this heterogeneity of contrast material is negligible.
Fifth, vessel deformity similar to the pseudostenosis phenomenon at 3D digital angiography may be observed at 3D CT angiography (11). This vessel deformity artifact is caused by the lower spatial resolution of the z-axis at 3D CT angiography (11). Because each voxel is isotropic at 3D digital angiography, it is thought that the mechanism of the vessel deformity at 3D digital angiography is different from the mechanism of the vessel deformity at 3D CT angiography. Although the x-ray sensitivity of the image intensifier used in rotational digital angiography is much lower than that of CT detectors, it is unknown whether this lower sensitivity of the image intensifier causes the artifact. Finally, imaging parameters and postprocessing techniques such as the image matrix and reconstruction method may have to be considered.
In this study, the pseudostenosis phenomenon affected the delineation of two small middle cerebral artery aneurysms. This artifact may not only lead to a misdiagnosis of vessel caliber but may also affect aneurysm delineation. Therefore, it is important to know the predominant location and mechanism of this artifact. When evaluating intracranial aneurysms at 3D digital angiography, reviewers need to carefully assess the proximal portion of the anterior and middle cerebral arteries and to perform combined evaluation with 2D DSA and/or rotational angiographic images. Although we did not evaluate intracranial steno-occlusive disease in this study, this artifact may cause overestimation of vessel caliber in steno-occlusive disease.
There were some limitations in this study. First, a pulsatile phantom was not used in assessing the physical characteristics of the phenomenon. Because our results were obtained with a nonpulsatile and nonflowing phantom, we believe that the peudostenosis phenomenon is not related to flow phenomena but rather is caused by the geometric factors of angle to the axis of rotation and vessel length. Second, we did not evaluate this artifact at 3D DSA in this study. With regard to this artifact, further investigation must be performed to determine whether the subtraction technique affects the phenomenon.
Third, the volume-rendering technique was the only 3D display method used in this study. Because the volume-rendering technique may be the best technique for evaluating intracranial aneurysms at 3D digital angiography (9), we did not evaluate other display techniques in this study. Fourth, the results were acquired in a relatively small group of patients, and few arteries and aneurysms in the posterior fossa were included in this study. However, these results warrant further studies in a larger group of patients and phantoms to investigate the mechanism of this phenomenon.
In conclusion, the incidence of a pseudostenosis phenomenon on volume-rendered 3D digital angiograms of the intracranial arteries was relatively high. The pseudostenosis phenomenon was observed in vessels that were perpendicular or nearly perpendicular to the axis of rotational angiography and had some degree of length. This artifact affected delineation of some middle cerebral artery aneurysms. When evaluating intracranial arteries and aneurysms at volume-rendered 3D digital angiography, knowledge of this artifact may be required and careful attention should be paid to certain areas in the intracranial arteries. When this phenomenon is observed at volume-rendered 3D digital angiography, the use of a slight tilt to the axis of the vessel that has this artifact may resolve the problem.
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FOOTNOTES |
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Abbreviations: DSA = digital subtraction angiography, 3D = three-dimensional, 2D = two-dimensional
Author contributions: Guarantor of integrity of entire study, T.H.; study concepts, T.H., M.Y.; study design, T.H.; literature research, T.H.; clinical studies, T.H., K.O., S.U.; data acquisition, K.O.; data analysis/interpretation, T.H., K.O.; statistical analysis, T.H.; manuscript preparation and definition of intellectual content, T.H.; manuscript editing, T.H., Y.K., Y.Y.; manuscript revision/review and final version approval, T.H., Y.K.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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