Intravascular ultrasound imaging: a current perspective

A snake-based method for segmentation of intravascular ultrasound images and its in vivo validation

Image segmentation for detection of vessel walls is necessary for quantitative assessment of vessel diseases by intravascular ultrasound. A new segmentation method based on gradient vector flow (GVF) snake model is proposed in this paper. The main characteristics of the proposed method include two aspects: one is that nonlinear filtering is performed on GVF field to reduce the critical points, change the morphological structure of the parallel curves and extend the capture range; the other is that balloon snake is combined with the model. Thus, the improved GVF and balloon snake can be automatically initialized and overcome the problem caused by local energy minima. Results of 20 in vivo cases validated the accuracy and stability of the segmentation method for intravascular ultrasound images.

Endoneurosonography: technique and equipment, anatomy and imaging, and clinical application

OBJECTIVE

To evaluate the usefulness of transendoscopic ultrasound in neurosurgery, we studied two new sonoprobes measuring 6 and 8 French in diameter in 20 fresh specimens. The application and indication are discussed in the first clinical series of 75 patients.

METHODS

Sonocatheters (ALOKA, Meerbusch, Germany) 1.9 mm (6 French) and 2.4 mm (8 French) in diameter were introduced into the working channel of an endoscope. The preparations were done in nonfixed skulls in a surgical simulation-setting laboratory. Based on these experiences with imaging possibilities, intraoperative transendoscopic ultrasound was applied in 75 patients and a variety of lesions. It was used for imaging (41 patients), targeting (18 patients), and neuronavigation (16 patients) in neuroendoscopy.

RESULTS

The sonoprobe adds a transverse scan at the tip of the probe to the anterior endoscopic view. This axial scan to the longitudinal axis of the endoscope is geometrically comparable with radar scanning. Three probes working with 10, 15, and 20 MHz were used, resulting in a short penetration with a radius of 3 cm. The orthogonal scanning plane had limitations, which were documented. We observed precise imaging of well known anatomic structures and, moreover, achieved an additional dimension in endoscopy. The axial scan presents the anatomic landmarks like a map at the tip of the endoscope where the endoscope is represented as a spot. The real-time imaging and representation of the tip of the endoscope showed a capacity for navigation. This preclinical study rectified clinical application. The real-time imaging of this technique showed the ability of the navigation of endoscopes to detect more overall movements, such as blood flow or change of ventricle size during endoscopy. The primary benefit in this first clinical series was witnessed in difficult endoscopy cases and complex lesions, but benefit was also observed in cases in which vision through the endoscope alone was obscured. The main limitation was the result of little penetration depth and lack of anterior scanning.

CONCLUSION

Application of transendoscopic ultrasound is appropriate in neurosurgery. Training is necessary to understand the imaging and the geometry of scans because this technique does not scan along the axis of the endoscope. Further development to overcome the current limits of this technique and more clinical experience are needed.

New techniques to evaluate esophageal function

Classical techniques like videofluoroscopy, stationary manometry and ambulatory 24-hour pH-metry are routinely used in the clinic to study patients with dysphagia, chest pain and reflux-related symptoms. Although many patients can be accurately diagnosed and their therapy successfully guided with these techniques, in many other patients, non-obstructive dysphagia or chest pain cannot be attributed to clear fluoroscopic or manometric abnormalities. Furthermore, ambulatory 24-hour pH-metry often shows a poor association between spontaneous acid reflux events and esophageal or extraesophageal symptoms, particularly in patients 'on' treatment. Non-obstructive dysphagia can be assessed with high-resolution manometry to detect segmental disturbances of peristalsis, increase in pressure gradient across the lower esophageal sphincter (LES) or abnormal axial movement of the LES during esophageal spasm. Impedance evaluation of bolus transit is a non-radiological method that can evaluate the functional relevance of manometric abnormalities. Patients with non-cardiac chest pain that do not respond to proton pump inhibitor therapy can be further assessed with intraluminal high-frequency ultrasound to detect sustained esophageal contractions of the longitudinal muscle layer. Impedance planimetry, with multimodal esophageal stimulation, may contribute to evaluate the sensitivity to mechanical, thermal and chemical stimuli. Finally, patients with persistent symptoms of gastroesophageal reflux in spite of adequate treatment with proton pump inhibitors may still have weakly acidic reflux and/or bile reflux associated with their symptoms. These types of refluxates can now be detected with combinations of pH-impedance or pH-Bilitec monitoring. This review will describe the available new techniques to evaluate patients with non-obstructive dysphagia, non-cardiac chest pain and persistent gastroesophageal reflux symptoms.

Regional material property alterations in porcine femoral arteries with atheroma development

We have developed a novel methodology that permits assessment of regional vascular mechanical property alterations in the presence of atheroma in vivo employing a Yucatan miniswine model with induced lesions. Femoral arteries were imaged with intravascular ultrasound. Image data were segmented and, following three-dimensional reconstruction, underwent finite element and sensitivity analysis with optimization to identify regions with altered vascular mechanical properties. All regions were compared to histological analysis. In 12 animals with 8 weeks of endothelial cell denudation and high cholesterol diet (induced atherosclerosis), the elastic modulus initially decreased with early lesion development and then increased with increasing fibrosis-(elastic modulus-all values x10(4)Pa-mean+/-SEM) histologically normal (non-denuded control segment) elements 9.73+/-0.01, fatty elements 9.53+/-0.01, fibrofatty elements 9.41+/-0.03, and fibrous elements 9.68+/-0.02 (all p<0.001 vs. normal elements). Wall thickness, however, increased with atheroma formation. These data demonstrate decreasing vascular material properties with early lesions, followed by an increase as lesions progress. This methodology permits determination of areas with early atheroma development, follow atheroma progression, and potentially evaluate interventions aimed at decreasing atheroma load and normalizing vascular material properties.

Porcine carotid arterial material property alterations with induced atheroma: an in vivo study

OBJECTIVE

A novel methodology has been developed to evaluate regional alterations in arterial wall material properties with induced atheroma in an animal model.

METHODS

Atheromatous lesions (fatty, fibro-fatty, and fibrous) were induced in the carotid arteries of a Yucatan miniswine model by endothelial cell denudation and high cholesterol diet. The images at base line and 8 weeks after denudation were obtained using intravascular ultrasound (IVUS) imaging along with hemodynamic data. Finite element analysis (FEA) along with optimization was employed to assess regional alterations in elastic modulus in the presence of atheroma confirmed by histology.

RESULTS

In animals with 8 weeks of induced atherosclerosis, the elastic modulus increased-(elastic modulus-all values x 10(4) Pa, mean+/-S.D.) normal elements (9.34+/-0.36) compared to abnormal elements (9.52+/-0.36) (p<0.05 versus normal elements). Wall thickness increased with atheroma formation. These data demonstrate stiffening vascular wall elastic modulus with lesion progression. This is different from the behavior of femoral arteries, where the elastic modulus decreases with early stages of atheroma development followed by an increase as lesions progress.

CONCLUSIONS

This methodology permits determination of areas with early atheroma development, follow atheroma progression, and potentially evaluate interventions aimed at decreasing atheroma load and normalizing vascular material properties.

Motor and sensory function of the esophagus: revelations through ultrasound imaging

Catheter based high frequency intraluminal ultrasound (HFIUS) imaging is a powerful tool to study esophageal sensory and motor function and dysfunction in vivo in humans. It has provided a number of important insights into the longitudinal muscle function of the esophagus. Based on the ultrasound images and intraluminal pressure recordings, it is clear that there is synchrony in the timing as well as the amplitude of contraction between the circular and the longitudinal muscle layers of the esophagus in normal subjects. On the other hand, in patients with spastic disorders of the esophagus, there is an asynchrony of contraction related to the timing and amplitude of contraction of the two muscle layers during peristalsis. Achalasia, diffuse esophageal spasm, and nutcracker esophagus (spastic motor disorders of the esophagus) are associated with hypertrophy of the circular as well as longitudinal muscle layers. A sustained contraction of the longitudinal muscle of the esophagus is temporally related to chest pain and heartburn and may very well be the cause of symptoms. Longitudinal muscle function of the esophagus can be studied in vivo in humans using dynamic ultrasound imaging. Longitudinal muscle dysfunction appears to be important in the motor and sensory disorders of the esophagus.

Sensory and motor function of the esophagus: lessons from ultrasound imaging

Catheter-based high-frequency intraluminal ultrasound imaging is a powerful tool to study esophageal sensory and motor function and dysfunction in vivo in humans. It can be combined with manometry, pH, and impedance measurement techniques to determine the relationships between different physiologic parameters. High-frequency intraluminal ultrasound imaging has provided a number of important insights regarding the longitudinal muscle function of the esophagus. On the basis of the ultrasound images and intraluminal pressure recordings, it seems that there is synchrony in the timing and the amplitude of contraction between the circular and longitudinal muscle layers. A sustained contraction of the longitudinal muscle layer is temporally related to esophageal chest pain and heartburn. The biomechanics of the esophageal wall and its relationship to sensory and motor function can be studied in humans in vivo by using high-frequency intraluminal ultrasound much more precisely than has previously been possible. Achalasia, diffuse esophageal spasm, and nutcracker esophagus are associated with hypertrophy of circular and longitudinal muscle layers. Finally, high-frequency intraluminal ultrasound imaging is the only technique that can detect reflux-related distention of the esophagus and its role in esophageal symptoms. Future approaches to display and quantify ultrasound image data are discussed. The principles of high-frequency intraluminal ultrasound described here are also applicable to study of the motor and sensory function of the other regions of the gastrointestinal tract.

Time dependent changes of arterial distensibility induced by cholesterol and balloon injury in rabbits: an in vivo intravascular ultrasound study

The aim of this study was to validate in vivo measurement of intravascular ultrasound (IVUS) for the analysis of structural and functional vessel wall alterations in a chronic animal model. Furthermore, the relation between functional and structural alteration of the vessel wall should be investigated. Fifteen cholesterol-fed rabbits (1%) and 15 control rabbits underwent balloon injury of the abdominal aorta. Immediately before and after balloon traumatization as well as 2 and 6 weeks later IVUS depiction of 10 aortal vessel segments was performed (n = 1,100 measurements). In vivo IVUS measurements and morphometric analysis of the neointimal area of same aortal segments showed a high correlation (n = 148, r = 0.844, p < 0.001). Plaque area determined by morphometry revealed larger areas than the evaluation by IVUS (0.162 +/- 0.138 vs. 0.130 +/- 0.126 mm2, p < 0.001). Before balloon traumatization, pulsatility of the aortal vessel segments was less in cholesterol-fed rabbits (0.067 vs. 0.090, p < 0.01) and neointimal index higher (0.003 vs. 0). Investigation using IVUS 2 and 6 weeks after balloon traumatization demonstrated a continuous loss of arterial distensibility and an increase of neointimal index, being more pronounced in the cholesterol-fed group. As demonstrated by IVUS the loss of distensibility preceded the atherosclerotic alterations. Our investigation suggests using IVUS in this animal model is a reliable setting for long-term investigation of characteristics of the vessel wall. We could demonstrate that altered function of the vessel wall precedes the structural atherosclerotic vessel wall alterations.

Quantitative assessment of diffuse coronary artery disease using a three-dimensional reconstruction method compared with intravascular ultrasound images

BACKGROUND

It can be difficult to estimate the degree of stenosis in patients with diffuse coronary artery disease (CAD), because of the lack of a normal reference segment. If the size of normal coronary lumen has a direct relation to size of distal myocardial bed, it could be used to estimate the 'normal' cross-sectional area of coronary lumen. Accordingly, we could estimate the degree of stenosis of coronary arteries with diffuse disease by comparing them with calculated 'normal' areas of lumen.

OBJECTIVE

To assess the validity of the above hypothesis.

METHOD

Fourteen subjects without coronary atherosclerosis (group A) and 16 patients with CAD (group B) underwent simultaneous bidirectional coronary arteriography. Using these coronary arteriograms, we determined the relationship between cross-sectional area of coronary lumen measured by using a computerized edge-detection system and summed distal branch length calculated by using our computerized three-dimensional reconstruction method.

RESULTS

For group A, we found a close correlation between area of lumen and branch length (r= 0.948). However, for group B, there were some segments for which the measured area of lumen was clearly smaller than that expected from the relationship for group A. From this relationship for group A, we calculated the stenosis ratios of 22 segments and, to confirm their accuracy, we compared the stenotic ratios with those measured on intravascular ultrasound images. The stenotic ratio of each segment of stenotic coronary artery calculated by our method agreed significantly well with the results obtained from the ultrasound measurements (r= 0.980).

CONCLUSIONS

These observations validate a novel approach to quantifying diffuse CAD using clinical arteriograms.

Three-dimensional automatic quantitative analysis of intravascular ultrasound images

Intravascular ultrasound (IVUS) has established itself as a useful tool for coronary assessment. The vast amount of data obtained by a single IVUS study renders manual analysis impractical for clinical use. A computerized method is needed to accelerate the process and eliminate user-dependency. In this study, a new algorithm is used to identify the lumen border and the media-adventitia border (the external elastic membrane). Setting an initial surface on the IVUS catheter perimeter and using active contour principles, the surface inflates until virtual force equilibrium defined by the surface geometry and image features is reached. The method extracts these features in three dimensions (3-D). Eight IVUS procedures were performed using an automatic pullback device. Using the ECG signal for synchronization, sets of images covering the entire studied region and corresponding to the same cardiac phase were sampled. Lumen and media-adventitia border contours were traced manually and compared to the automatic results obtained by the suggested method. Linear regression results for vessel area enclosed by the lumen and media-adventitia border indicate high correlation between manual vs. automatic tracings (y = 1.07 x -0.38; r = 0.98; SD = 0.112 mm(2); n = 88). These results indicate that the suggested algorithm may potentially provide a clinical tool for accurate lumen and plaque assessment.

In vitro validation of the luminal measurement of a novel catheter based moulding technique

OBJECTIVE

To investigate a modified angioplasty balloon catheter, which uses a novel balloon polymer to produce luminal moulds.

DESIGN

The catheter was tested in polyurethane phantoms of diameter 1.5 to 4.0 mm. Inflations were to 1.4 atmospheres for 20 seconds at 37 degrees C. The moulds were viewed by reinflating the balloon to 0. 34 atmospheres and quantified using macrophotography and caliper measurement.

RESULTS

Evidence of systematic error was found with lumen diameters </= 2.0 mm, accuracy being 0.32 to 0.80 mm and precision 0.23 to 0.24 mm. However, between 2.5 and 4.0 mm the accuracy of measuring luminal diameters was 0.01 mm, the precision 0. 06 mm, and the absolute mean error 0.05 mm. The results for percentage diameter stenosis were -1.15%, 0.86%, and 1.21%, and for lesion length they were 0.20, 0.60, and 0.41 mm, respectively.

CONCLUSIONS

The prototype catheter shows a high degree of accuracy and precision in phantoms of diameter 2.5 to 4.0 mm-the range within which most interventional work is performed. Further work on this technique as a method of direct three dimensional moulding of the coronary artery lumen is warranted.

Intravascular ultrasound imaging before and after balloon angioplasty for pulmonary artery stenosis

Previous reports regarding intravascular ultrasound (IVUS) imaging of the pulmonary arteries in children and its application to balloon pulmonary angioplasty are limited. This study was designed to compare findings of IVUS imaging and those of angiography of the pulmonary artery before and after the balloon angioplasty procedure. Thirty patients had significant pulmonary artery stenosis and underwent balloon angioplasty. In all, of 34 branch pulmonary arteries were dilated. All patients underwent both angiography and IVUS imaging at the time of balloon angioplasty. The mean age at balloon angioplasty was 5.7 +/- 4.0 yr. One echo-dense layer on IVUS was detected in 9% of the 34 stenotic vessels, and a two- or three-layered vascular wall pattern in 91%. The thickness of intima-medial layer (inner and middle layers) was greater than normal in 91% of stenotic vessels. After balloon angioplasty, intimal flaps and aneurysm were observed at 29 and 28 locations, respectively. Of these locations, the intimal flaps were detected by angiography in 44% and by IVUS in 100%; the aneurysm was detected by angiography in 61% and by IVUS in 93%. Media rupture was observed at 26 locations, and the change was detected only by IVUS. The present study suggests that intimal and medial changes in the pulmonary artery can be detected more precisely by IVUS than by angiography.

[Assessment of saphenous vein graft wall characteristics with intravascular ultrasound imaging]

The wall characteristics of saphenous vein grafts (SVGs) after coronary artery bypass grafting (CABG) were assessed with intravascular ultrasound (IVUS) imaging. Twenty-seven SVGs in 25 patients who had undergone CABG were studied with IVUS in vivo. Ten SVGs excised from 6 patients who had undergone reoperation were studied with IVUS in vitro. The results were compared with the histopathologic findings. IVUS images of four SVGs studied within 1 month after CABG showed a thin and single-layer structure, which was almost identical to the IVUS features of a normal, fresh SVG. In five SVGs studied between 3 months and 1 year after CABG, the wall thickness was increased, and there were hypoechoic areas. In sixteen SVGs studied more than 2 years after CABG, two patterns were noted on IVUS imaging. One pattern was a triple-layer structure of alternating hyperechoic, hypoechoic, and hyperechoic bands seen in the angiographically normal sections of the SVGs. The second pattern was a heterogeneous, echogenic monolayer found in the angiographically irregular and diseased sections of the SVGs. Comparison of the in vitro IVUS images with the histopathology of the SVGs showed a strong correlation between the triple-layer structure and fibrointimal proliferation, and between the heterogeneous monolayer and atherosclerosis. In conclusion, IVUS is useful for assessing SVGs and for long-term qualitative monitoring of vein graft disease after CABG.

Altered compliance and residual strain precede angiographically detectable early atherosclerosis in low-density lipoprotein receptor deficiency

BACKGROUND

This study was performed to detect changes in vascular biomechanical properties early in atherogenesis.

METHODS AND RESULTS

Age- and weight-matched LDL-receptor deficient Watanabe hypercholesterolemic male rabbits (Group I: n = 11) and normal rabbits (Group II: n = 11) were studied. Fasting plasma lipoprotein concentrations, aortic angiography and intravascular ultrasound, in vivo aortic compliance evaluation, ex vivo aortic residual strain measurements, aortic lipid content and histopathology were determined. Plasma cholesterol was increased 9.8 fold and aortic cholesterol content was increased from 20 to 43 fold in Group I compared to Group II, respectively (P < .00005). Angiography revealed no stenoses in either group, whereas intravascular ultrasound and histological studies of Group I showed small circumferential plaques with < 10% cross-sectional area involvement. The residual strain in Group I was significantly increased in the ascending thoracic aorta (22.1 +/- 6.9% versus 10.4 +/- 3.2% in Group II, P < .0001), descending thoracic aorta (15.7 +/- 7.2% versus 4.8 +/- 1.3% in Group II, P < .0001), and abdominal aorta (18.0 +/- 4.8% versus 8.3 +/- 6.3% in Group II, P < .005). Changes in residual strain were inversely correlated with the aortic cholesterol content in the ascending thoracic aorta (r = -.72; P = -.001), descending thoracic aorta (r = -.95; P < .001), and abdominal aorta (r = -.51; P = .019).

CONCLUSIONS

Early atherosclerosis in LDL-receptor deficient rabbits, undetectable by angiography yet observed by intravascular ultrasound imaging and histology, is associated with marked changes in ex vivo residual strain. Alterations in vascular biomechanical properties, associated with changes in cholesterol content, may have physiologic consequences and may be useful in detecting and quantitating early atherosclerosis.

Highly localized arterial remodeling in patients with coronary atherosclerosis: an intravascular ultrasound study

BACKGROUND

Preservation of luminal area and symmetry in the presence of irregular plaques necessitates local expansion of the artery wall.

METHODS AND RESULTS

Cross-sectional dimensions of coronary arteries in 65 patients were measured with the use of intravascular ultrasound. A total of 104 arterial segments were studied, of which 88 had atherosclerosis; 16 served as nonatherosclerotic control segments. Three features of atherosclerotic arterial segments were classified: (1) plaque formation, (2) lumen shape, and (3) shape of arterial external elastic lamina. With our intravascular ultrasound-based three-level classification system, we identified three patterns that accounted for 89% of all atherosclerotic arterial segments: (1) concentric plaque with a circular lumen and a circular external elastic lamina (n= 17), (2) eccentric plaque with a circular lumen and an oval external elastic lamina (n=35), and (3) eccentric plaque with an oval lumen and a circular external elastic lamina (n=26). A circular lumen was preserved in 66% of all atherosclerotic arterial segments. Arterial segments with a circular lumen in the presence of an eccentric plaque had a significantly larger lumen area than the other two main groups (P<.05).

CONCLUSIONS

With our intravascular ultrasound-based classification, we provided information regarding the local remodeling response in the coronary artery wall. In a majority of cases, a circular lumen is maintained. Failure of this highly localized response to be operative may contribute to the development of stenotic lesions at a specific site in the artery.

Decreasing common carotid artery intimal thickness during hypolipidemic therapy

It has been demonstrated in recent years that ultrasound can be used to measure common carotid artery intimal thickness; an increase in intimal thickness is regarded as an early stage of atherosclerosis. This study was designed to establish whether or not intimal thickness can be modulated by therapy. Twenty-nine patients with familial hyperlipoproteinemias had follow-up ultrasound of the common carotid artery after twenty-nine months of comprehensive therapy. In 21 patients with familial hypercholesterolemia, intimal thickness decreased from 0.83 to 0.68 mm (P < 0.01), in 9 with familial combined hyperlipoproteinemia, the decrease was from 0.77 to 0.74 mm (a decrease was seen in only 50% of patients). With the group taken as a whole, the larger decrease was observed in patients treated with statins while the reduction was less marked in those administered fibrates. The authors found a decrease in common carotid artery intimal thickness following hypolipidemic therapy in patients with hyperlipoproteinemias. Their impression is that this was a manifestation of atherosclerosis regression.

Contribution of inadequate arterial remodeling to the development of focal coronary artery stenoses. An intravascular ultrasound study

BACKGROUND

Adaptive remodeling occurs to compensate for the accumulation of atherosclerotic plaque. Lumen reduction depends on the relative rates of plaque deposition and adaptive remodeling responses. Intravascular ultrasound permits detailed, high-quality, cross-sectional imaging of the coronary arteries in vivo.

METHODS AND RESULTS

Preintervention intravascular ultrasound was used to study 603 focal, new, nonostial significant coronary artery stenoses in patients with chronic stable angina. Measurements of the target lesion of the external elastic membrane (EEM), lumen, and plaque plus media (P&M; P&M = EEM - Lumen) cross-sectional areas (CSAs) were compared with a proximal reference segment (most normal-looking cross section within 10 mm proximal to the lesion but distal to any side branch). Inadequate remodeling was defined as lesion/ reference EEM CSA that exceeded the upper limits of normal arterial tapering (lesion/reference EEM CSA ratio < or = 0.78 or a 21% reduction in EEM CSA per 10-mm length). Overall, the lesion/reference EEM CSA ratio was 1.00 +/- 0.22; 15% of lesions had inadequate remodeling, and 37% of the 603 lesions had less plaque than expected. This represented a lesion-specific response. The only predictor of inadequate remodeling was the arc of superficial lesion calcium.

CONCLUSIONS

Inadequate remodeling is present in at least 15% of chronic, focal, new coronary arterial stenoses in patients with stable angina. The magnitude of arterial remodeling appears to be a lesion-specific response.

Intracoronary ultrasound before coronary interventions: a prospective comparison of two different catheters

Intravascular ultrasound (IVUS) provides unique information about the coronary arterial wall that can be used to guide transcatheter therapy. In this prospective study, two different IVUS systems were compared with respect to feasibility of imaging before intervention and angiographic changes induced by the simple advancement of the catheter across the lesion. Eighty-five patients (mean age 59 +/- 10 yr, 11 female) were studied with IVUS before intervention. In 34 patients, a 4.8 F (1.6-mm) IVUS catheter was used (Group I), whereas in the remaining 51 patients, a 3.5 F (1.2-mm) IVUS catheter was used (Group II). Quantitative angiography was performed before and after the IVUS study to determine potential changes in lumen diameter. Clinical and angiographic characteristics were similar in the two groups. A successful IVUS interrogation of the target lesion was obtained more frequently in Group II (45/51 (88%) vs. 19/34 (56%) patients, P < 0.01). After the IVUS study, a change in minimal lumen diameter was seen in Group I (baseline 0.84 +/- 0.2 vs. final 1.17 +/- 0.2 mm, P < 0.001) and Group II patients (baseline 0.80 +/- 0.3 vs. final 1.03 +/- 0.4 mm, P < 0.01). In the 64 lesions successfully crossed, the absolute gain in lumen diameter was significantly higher in Group I (0.40 +/- 0.2 vs. 0.23 +/- 0.2 mm, P < 0.05). In addition, an inverse correlation was found between baseline minimal lumen diameter and the absolute lumen gain induced by the IVUS study in Group I (r = -0.47, P < 0.05) but not in Group II patients (r = -0.16, NS). Neither angiographic nor echogenic lesion characteristics were associated with the change in lumen diameter. When multivariate analysis was applied, catheter size was the only independent predictor of lumen gain induced by IVUS after adjustment. Thus, the advancement of IVUS catheters across severe coronary lesions induces significant angiographic changes consistent with plaque remodeling and a Dotter effect. The use of smaller catheters not only allows a higher number of lesions to be studied before intervention, but also lessens the mechanical disruption of the plaque, yielding a more accurate and veracious picture of baseline plaque characteristics.

The effect of vascular curvature on three-dimensional reconstruction of intravascular ultrasound images

OBJECTIVE

To characterize the effect of vessel curvature on the geometric accuracy of conventional three-dimensional reconstruction (3DR) algorithms for intravascular ultrasound image data.

BACKGROUND

A common method of 3DR for intravascular ultrasound image data involves geometric reassembly and volumetric interpolation of a spatially related sequence of tomographic cross sections generated by an ultrasound catheter withdrawn at a constant rate through a vascular segment of interest. The resulting 3DR is displayed as a straight segment, with inherent vascular curvature neglected. Most vascular structures, however, are not straight but curved to some degree. For this reason, vascular curvature may influence the accuracy of computer-generated 3DR.

METHODS

We collected image data using three different intravascular ultrasound catheters (2.9 Fr, 4.3 Fr, 8.0 Fr) during a constant-rate pullback of 1 mm/sec through tubing of known diameter with imposed radii of curvature ranging from 2 to 10 cm. Image data were also collected from straight tubing. Image data were digitized at 1.0-mm intervals through a length of 25 mm. Two passes through each radius of curvature were performed with each intravascular ultrasound catheter. 3DR lumen volume for each radius of curvature was compared to that theoretically expected from a straight cylindrical segment. Differences between 3DR lumen volume of theoretical versus curved (actual) tubes were quantified as absolute percentage error and categorized as a function of curvature. Tubing deformation error was quantified by quantitative coronary angiography (QCA).

RESULTS

Volumetric errors ranged from 1% to 35%, with an inverse relationship demonstrated between 3DR lumen volume and segmental radius of curvature. Higher curvatures (r < 6.0 cm) induced greater lumen volume error when compared to lower curvatures (r > 6.0 cm). This trend was exhibited for all three catheters and was shown to be independent of tubing deformation artifacts. QCA-determined percentage diameter stenosis indicated no deformation error as a function of curvature. Total volumetric error contributed by tubing deformation was estimated to be 0.05%.

CONCLUSIONS

Catheter-dependent geometrical error arises in three-dimensionally reconstructed timed linear pullbacks of intravascular ultrasound images due in part to uniplanar vascular curvature. Three-dimensional reconstruction of timed linear pullbacks is robust for vessels with low radii of curvature; however, careful interpretation of three-dimensional reconstructions from timed linear pullbacks for higher radii of curvature is warranted. These data suggest that methods of spatially correct three-dimensional reconstruction of intravascular ultrasound images should be considered when more pronounced vascular curvature is present.

Distortion of intravascular ultrasound images because of nonuniform angular velocity of mechanical-type transducers

The purpose of this study was to quantify nonuniform rotation in a current mechanical intravascular ultrasound (IVUS) instrument and its effect on arc, area, and diameter measurements. The accurate reconstruction of IVUS two-dimensional images is dependent on uniform rotation of the catheter tip. Prior investigations suggested that bends in the catheter driveshaft may be responsible for poor torque transmission, nonuniform rotation, and consequent errors in IVUS measurements. Eight 30 MHz mechanically driven IVUS catheters were evaluated in a model simulating the catheter course through the aorta and coronary ostium in a clinical study. Angular velocity and posi-ion profiles of the transducer, image angle, and diameter and area measurement errors were obtained from each catheter by imaging a vascular phantom with eight equispaced echogenic markers from concentric and eccentric positions. Six catheters also were tested for comparison in a simple curvature model. Rotational error was found in all catheters tested and worsened in the aortic model. Maximal angular error, defined as the largest angle between actual and presumed transducer direction, increased when measured in the aortic model as compared with the simple curvature model (17 +/- 12 degrees to 45 +/- 25 degrees; p < 0.05). Angles of 45 degrees were misrepresented with a mean range of values of 26 to 63 degrees. With eccentric catheter placement, area and diameters had average maximal absolute errors of 26% +/- 7.8% and 23% +/- 10%, respectively. In conclusion, nonuniform rotation of mechanical IVUS transducers constitutes a significant potential source of error in IVUS measurement of arcs of calcification, and lumen shape, area, and diameter.

Intravascular imaging of serial changes of disease in saphenous vein grafts after coronary artery bypass grafting

To clarify the structural changes of saphenous vein grafts after coronary artery bypass grafting, intravascular ultrasound and angioscopic images were obtained from 23 grafts in vivo and 5 grafts and 3 new veins in vitro; the images were compared with histologic findings. Intravascular ultrasound demonstrated a single-layered appearance at new veins and all of the angiographically normal grafts within 6 months after surgery. A triple-layered appearance that might be related to the remarkably proliferative and degenerated intima was revealed histologically at 73.3% of the normal sites of grafts between 5 and 10 years after operation. In 83.3% of the stenoses at several years after operation, angioscopy showed yellow atheromatous plaques, often with a friable surface; a heterogeneous, lucent echo pattern was revealed on intravascular ultrasound. Thus intravascular ultrasound and angioscopy may be used to identify the morphologic changes of graft at different points after implantation more precisely than conventional angiography.

Intravascular and intracardiac ultrasound: a tool of the future

Intravascular ultrasonography has the capability to visualize the vessel wall and perivascular structures, to identify the spatial distribution and composition of atherosclerotic plaque, and to measure accurately vessel and wall dimensions. This article discusses relevant principles of instrumentation, issues pertaining to the validation and interpretation of intravascular ultrasound images, and the current and potential applications of intravascular (and intracardiac) imaging. Emphasis is placed on uses that are likely to be of interest to the critical care physician.

Intracoronary ultrasound: insights into mechanisms and results of coronary interventions

Intracoronary ultrasound imaging is a modality which allows in vivo cross-sectional visualization of coronary arteries similar to that obtained by pathology. Compared with coronary angiography, intracoronary ultrasound provides more detail on plaque morphology and topography and more accurate quantification of lumen and plaque area. Thus, it has evolved into a valuable research tool. For example, intracoronary ultrasound imaging has increased understanding of the mechanisms of action of balloon angioplasty and new interventions such as atherectomy and laser treatment. It may prove to have clinical utility by helping to individualize device selection and sizing and by assessing treatment results more accurately. Coronary imaging may be performed at low risk. Future developments will include smaller catheters, combined ultrasound and therapeutic catheters, and three-dimensional reconstruction of images.

Intravascular ultrasound appearance of normal and mildly diseased coronary arteries: correlation with histologic specimens

Intravascular ultrasound imaging was performed in vitro on six histologically normal and 104 minimally diseased arteries in patients aged 13 to 83 years. This study tested the hypothesis that normal coronary arteries produce a three-layer image that corresponds to the histologic layers of intima, media, and adventitia. The results showed a very good correlation between area of the echolucent ultrasound layer with the media and the inner echogenic layer with intimal area. In addition, a three-layer appearance was consistently seen when the internal elastic membrane was present with or without intimal hyperplasia. If the internal elastic membrane was absent, a three-layer appearance was still seen if the collagen content of the media was low. However, a two-layer appearance was observed when there was absence of the internal elastic membrane as well as a high collagen content of the media.

Influence of intimal thickening on coronary blood flow responses in orthotopic heart transplant recipients. A combined intravascular Doppler and ultrasound imaging study

BACKGROUND

Intravascular ultrasound imaging detects epicardial intimal thickening in the majority of heart transplant recipients with angiographically normal epicardial coronary arteries. Although coronary artery vasoreactivity is abnormal after cardiac transplantation, intimal thickening does not appear to affect conduit vessel responses. However, the effect of intimal thickening on both conduit and resistance vessel responses, as measured by changes in volumetric coronary blood flow (CBF), is unknown.

METHODS AND RESULTS

Epicardial coronary artery conductance and microvascular resistance vessel responses were studied after intracoronary adenosine and nitroglycerin administration in 36 orthotopic heart transplant recipients 1 month to 7 years after transplantation. Sequentially measured coronary flow average peak velocity ([APV, cm/s] 0.018 in Doppler guide wire) and epicardial luminal cross-sectional area ([CSA, mm2] 4.3F 30-MHz ultrasound catheter) data were obtained at baseline and during peak hyperemia after administration of 12 to 18 micrograms IC adenosine and 150 to 200 micrograms IC nitroglycerin. Volumetric CBF (mL/min) was calculated as CBF = APV (cm/s) x CSA (mm2) x 60 seconds/1 min x 1 cm2/100 mm2 x 0.5. Measurements were made from a discrete position in the proximal left anterior descending (LAD) artery (n = 22), mid-LAD artery (n = 7), proximal circumflex artery (n = 6), and proximal right coronary artery (n = 1). Intimal thickening was present in 19 of 32 patients (60%). Both adenosine and nitroglycerin increased APV (from 18.9 +/- 4.9 to 56.0 +/- 11.5 cm/s for adenosine and from 20.2 +/- 5.3 to 49.1 +/- 11.5 cm/s for nitroglycerin; both P < .05). Coronary flow velocity reserve was significantly higher for adenosine compared with nitroglycerin (3.1 +/- 0.6 versus 2.5 +/- 0.7, respectively; P < .001). Epicardial luminal CSA was unchanged during adenosine hyperemia compared with baseline (17.4 +/- 3.8 versus 17.3 +/- 4.0 mm2, respectively; P = NS) but was significantly greater during nitroglycerin hyperemia compared with baseline (18.7 +/- 3.8 versus 17.3 +/- 4.0 mm2, 6.2 +/- 3.6% change; P < .05). Baseline CBF was similar before drug administration. Hyperemic adenosine and nitroglycerin CBF responses (297 +/- 99 and 276 +/- 87 mL/min, respectively; P = NS) and CBF reserve (3.0 +/- 0.7 and 2.7 +/- 0.7, respectively; P = NS) were not significantly different. Importantly, intimal thickening did not diminish resting or hyperemic APV, coronary flow velocity reserve, luminal CSA, CBF, or CBF reserve responses.

CONCLUSIONS

In this study of angiographically normal heart transplant recipients, epicardial intimal thickening does not diminish conduit and resistance vessel responses during endothelial-independent vasodilator administration.

Atherosclerosis in angiographically "normal" coronary artery reference segments: an intravascular ultrasound study with clinical correlations

OBJECTIVES

This study evaluated the magnitude, patterns and clinical correlates of atherosclerosis in angiographically "normal" reference segments in patients undergoing transcatheter therapy for symptomatic coronary artery disease.

BACKGROUND

Pathologic studies indicate that the extent of coronary atherosclerosis is underestimated by visual analysis of angiographically normal coronary artery segments. Intravascular ultrasound allows detailed, high quality cross-sectional imaging of the coronary arteries in vivo.

METHODS

Intravascular ultrasound was used to study angiographically normal coronary artery reference segments in 884 patients evaluated for transcatheter therapy for symptomatic native coronary artery disease. The reference segment was the most visually normal intravascular ultrasound cross section within 10 mm proximal to the target lesion but distal to any major side branch. Results are presented as mean value +/- 1 SD.

RESULTS

Only 60 (6.8%) of 884 angiographically normal reference segments were normal by intravascular ultrasound. Reference segment percent cross-sectional narrowing measured 51 +/- 13% and correlated poorly with the target lesion percent cross-sectional narrowing (r = 0.166, p < 0.0001). Reference segments contained less calcific and dense fibrotic plaque and proportionately more soft plaque elements. Independent predictors of reference segment percent cross-sectional narrowing were male gender, patient age, diabetes mellitus, hypercholesterolemia and presence of multivessel disease. Independent predictors of reference segment calcification were patient age and serum creatinine levels. Reference segment percent cross-sectional narrowing in 723 patients undergoing transcatheter therapy was similar to that in patients studied for diagnostic purposes; however, reference segment arc of calcium was greater in treated patients (43 +/- 81 vs. 25 +/- 57, p = 0.015). Reference segment disease was not an independent predictor of subsequent angiographic restenosis or clinical events within 12 months of follow-up.

CONCLUSIONS

Atherosclerosis is ubiquitous in angiographically normal coronary artery reference segments. Reference segment disease parallels the severity of target lesion disease and is associated with many of the conventional risk factors for coronary artery disease. Because of its sensitivity in detecting atherosclerosis in angiographically normal reference segments, intravascular ultrasound should enhance the study of risk factors for atherosclerosis and the results of therapies to control disease progression.

Angiographic, intravascular ultrasound and functional findings early after orthotopic heart transplantation

Accelerated graft atherosclerosis is responsible for increased mortality and morbidity among heart transplant recipients. The aim of this in-vivo study was to evaluate coronary atherosclerotic vessel alterations and endothelial function. Seventeen patients (14 males; mean age 49.3 years; range 24 to 69) were studied an average of 11 weeks (range 5 to 21) after heart transplantation because of coronary artery disease (n = 8), dilative cardiomyopathy (n = 7), mitral valve replacement (n = 1) and left atrial metastases of a leiomyosarcoma (n = 1). Mean age of the donor hearts (9 males) was 29 years (range 12 to 55). All recipients underwent biplane ventriculography and coronary angiography. In this study population, a total of 120 coronary segments (main stem, 21; left anterior descending artery, 85; circumflex artery, 14) were analyzed by intravascular ultrasound (20 MHz, 3.5F). In 13 patients, acetylcholine was infused into the proximal left anterior descending artery (10(-8) to 10(-5) M) to evaluate vasomotion within this segment. Regional contraction abnormalities were documented in 2 patients. Nine segments angiographically showed non-critical stenoses (5 patients). Intravascular ultrasound detected 52 cross-sectional areas with a three-layer pattern indicating intimal thickening. Mean circumferential extension of intimal proliferation was 192 degrees, mean intimal thickness 0.35 mm. Only 5 segments of the sonographically pathological cross-sectional areas showed angiographical evidence of atherosclerotic lesions. Intracoronary administration of acetylcholine at doses of 10(-8) and 10(-7) M resulted in vasoconstriction of the examined coronary segment in only 2 patients; the intracoronary application of acetylcholine at doses of 10(-6) and 10(-5) M revealed coronary vasoconstriction in 10 of the total of 13 patients. Using intravascular ultrasound, coronary artery lesions in heart transplant recipients can already be depicted at a very early stage. The abnormal response to acetylcholine in most of the heart recipients is independent of the extent of atherosclerotic vessel abnormalities documented by ultrasound or angiography.

Intravascular ultrasound in the management of acute coronary artery dissection: a case report

Crossing total occlusions is frequently difficult. The guidewire may enter a false lumen, thereby preventing successful balloon dilatations. We present a case of an acute arterial dissection following attempted angioplasty of a totally occluded right coronary artery. With an intravascular ultrasound probe in the false lumen, we were able to visualize a second guidewire and direct its passage into the true arterial lumen. This allowed for successful balloon dilatation and stent deployment restoring vessel patency.

Incidence and severity of transplant coronary artery disease early and up to 15 years after transplantation as detected by intravascular ultrasound

OBJECTIVES

The purpose of this study was to quantify the severity of transplant coronary artery disease and to assess lesion characteristics early and up to 15 years after heart transplantation by using intracoronary ultrasound.

BACKGROUND

Intravascular ultrasound has the ability to measure the components of the arterial wall and has been shown to be a sensitive method for detection of transplant coronary artery disease.

METHODS

A total of 304 intracoronary ultrasound studies were performed in 174 heart transplant recipients at baseline and up to 15 (mean 3.3 +/- 0.2) years after transplantation. Mean intimal thickness and an intimal index were calculated, and lesion characteristics (eccentricity, calcification) were assessed for all coronary sites imaged (mean 3.0 +/- 0.1 sites/study). The Stanford classification was used to grade lesion severity.

RESULTS

Compared with findings in patients studied at baseline (< 2 months after transplantation, n = 50), mean intimal thickness (0.09 +/- 0.02 vs. 0.16 +/- 0.02 mm, p < 0.01), intimal index (0.07 +/- 0.01 vs. 0.14 +/- 0.02, p < 0.01) and mean severity class (1.5 +/- 0.2 vs. 2.3 +/- 0.2, p < 0.01) were significantly higher at year 1 (n = 52) after transplantation. Thereafter, all three variables further increased over time and reached highest values between years 5 and 15. Calcification of lesions was detected in 2% to 12% of studies up to 5 years after transplantation, with a significant increase to 24% at years 6 to 10 (p < 0.05).

CONCLUSIONS

Severity of transplant coronary artery disease appeared to progress with time after transplantation in this cross-sectional study. This characteristic was most prominent during the 1st 2 years after transplantation, whereas calcification of plaques occurred to a significant extent only later in the process. These data may serve as a reference for comparison of intravascular ultrasound findings in other studies of patients with transplant coronary artery disease.

Angiographic changes (Dotter effect) produced by intravascular ultrasound imaging before coronary angioplasty

Intravascular ultrasound (IVUS) imaging of the coronary arteries has recently been introduced for the study of coronary lesions, but the angiographic effects produced by an IVUS examination before coronary angioplasty are unknown. Accordingly, the feasibility of and the potential angiographic changes caused by IVUS study (4.8F catheter) of severe coronary lesions was prospectively studied. Thirty consecutive coronary lesions were analyzed before intervention (29 patients, mean age 61 +/- 9 years, 5 women and 24 men). Before and after IVUS examination, intracoronary nitroglycerin 0.2 mg, was administered and two orthogonal angiographic views obtained. In 17 (57%) lesions the transducer of the IVUS catheter (radiopaque) could be gently advanced for precise location at the lesion site, and in every case the ultrasonic images revealed that the catheter was wedged into the plaque. In the remaining 13 lesions only the catheter tip but not the transducer could be located at the lesion site. Baseline minimal luminal diameter was similar in the crossed lesions and in lesions that prevented complete advancement of the IVUS catheter (0.86 +/- 0.2 vs 0.82 +/- 0.2 mm, difference not significant). Lesion characteristics could not predict the feasibility of the IVUS study. No complications resulted from the IVUS study. Quantitative angiography (automatic edge-detection system) revealed a significant increment in minimal luminal diameter (0.84 +/- 0.2 vs 1.16 +/- 0.3 mm, p < 0.001) and minimal luminal cross-sectional area (0.67 +/- 0.4 vs 1.09 +/- 0.5 mm2, p < 0.01) after passage of the IVUS catheter.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo diagnosis of pulmonary atherosclerosis. Role of intravascular ultrasound

The application of intravascular ultrasound (IVUS) in diagnosing pulmonary artery atherosclerosis is described. To the best of our knowledge, this is the first description of the use of this technique in the recognition of pulmonary atherosclerosis. We describe a patient with long-standing primary pulmonary hypertension with evidence of pulmonary artery calcification seen on a chest x-ray film. The IVUS imaging of the pulmonary arteries identified intimal disruption with complex atherosclerosis. Thus, IVUS can be a useful tool in the diagnosis of pulmonary artery atherosclerosis.

Acute and follow-up intravascular ultrasound findings after balloon dilation of coarctation of the aorta

BACKGROUND

The study objective was to examine the vascular wall changes caused by balloon dilation of coarctation of the aorta (CoA) acutely and at short-term follow-up using intravascular ultrasound imaging. Intravascular ultrasound has been valuable in assessing the vessel wall changes in coronary and peripheral arteries after balloon dilation, often with more detail than angiography.

METHODS AND RESULTS

Intravascular ultrasound imaging, using 4.8F, 20-MHz or 6.2F, 12.5-MHz catheters on either Diasonics or HP scanners, was performed in 17 patients during balloon angioplasty for native (n = 12) and recurrent (n = 5) CoAs. Nine patients were also studied at the time of follow-up cardiac catheterization 28.1 +/- 18.0 months after angioplasty. Immediately after dilation, the mean pressure gradient across the CoA decreased from 42.9 +/- 16.4 to 9.0 +/- 5.4 mm Hg (P < .001) and the mean diameter of the coarcted segment increased from 4.4 +/- 1.9 to 7.9 +/- 2.4 mm (P < .001). An intimal tear or flap was noted by ultrasound in 12 of the 12 native CoAs and 4 of the 5 recoarctations. In contrast, only 6 of the native CoAs and 2 of the recoarctations had an intimal flap or dissection detected by angiography. At follow-up, the residual pressure gradient did not significantly change from that measured immediately after dilation, but the CoA diameter increased from 7.8 +/- 1.5 to 9.9 +/- 2.3 mm (P < .01). No aneurysms were detected. Four of the 9 patients showed ultrasonic and angiographic evidence of healing and remodeling with diminution in size or disappearance of the intimal tears.

CONCLUSIONS

There is a high incidence of intimal tears and dissections immediately after balloon angioplasty for native and recurrent CoAs. Intravascular ultrasound is more sensitive than angiography in detecting the vascular wall changes. Even significant intimal tears are not necessarily associated with aneurysm formation, and many decrease in size or disappear at short-term follow-up.

Vascular mechanics for the cardiologist

Many common problems in clinical cardiology are due to disturbances in vascular mechanics. The terminology and basic principles of vascular mechanics, including fundamentals of the relation of stress and strain, are described in this review. Approaches to measuring vessel wall stiffness and the mechanical basis for vascular catastrophes are introduced.

Determinants of coronary compliance in patients with coronary artery disease: an intravascular ultrasound study

OBJECTIVES

The aim of this study was to elucidate determinants of coronary compliance in patients with coronary artery disease.

BACKGROUND

Intravascular ultrasound potentially enables in vivo evaluation of coronary artery compliance.

METHODS

Twenty-seven patients (mean age [+/- SD] 57 +/- 11 years, three women) undergoing coronary angioplasty were studied with intravascular ultrasound imaging. A mechanical intravascular ultrasound system (4.8F, 20 MHz) was used. A total of 58 different coronary segments (proximal to the target angiographic lesion) were studied. Of these, 35 were located in the left anterior descending, 9 in the left main, 8 in the left circumflex and 6 in the right coronary arteries. During intravascular ultrasound imaging, 22 segments (38%) appeared normal, but 36 (62%) had plaque (24 fibrotic, 3 lipidic and 9 calcified). Systolic-diastolic changes in area (delta A) and pressure (delta P) with respect to vessel area (A) were used to study normalized compliance (Normalized compliance = [delta A/A]/delta P [mm Hg-1 x 10(3)]).

RESULTS

Lumen area and plaque area were 12.6 +/- 5.7 and 3 +/- 3 min2, respectively. Plaque was concentric (more than two quadrants) at 10 sites, but the remaining 26 plaques were eccentric. Compliance was inversely related to age (r = -0.34, p < 0.05) but was not related to other clinical variables. Compliance was greater in the left main coronary artery (3.9 +/- 2.1 vs. 1.8 +/- 1.2 mm Hg-1, p < 0.05) and in coronary segments with normal findings on ultrasound imaging (2.9 +/- 1.9 vs. 1.6 +/- 1.1 mm Hg-1, p < 0.01). Moreover, at diseased coronary segments compliance was lower in calcified plaques than in other types of plaques (1.2 +/- 0.7 vs. 2.3 +/- 1.6 mm Hg-1, p < 0.01) but was similar in concentric and eccentric plaques (1.6 +/- 1.5 vs. 1.6 +/- 0.9 mm Hg-1). Plaque area (r = -0.38, p < 0.01) was inversely correlated with compliance. On multivariate analysis, only age and plaque area were independently related to compliance.

CONCLUSIONS

Intravascular ultrasound may be used to evaluate compliance in patients with coronary artery disease. Compliance is reduced with increasing age and is mainly determined by the arterial site and by the presence, size and characteristics of plaque on intravascular ultrasound imaging.

Arterial imaging with a new forward-viewing intravascular ultrasound catheter, I. Initial studies

BACKGROUND

Intravascular ultrasound (IVUS) of arteries is limited by the inability of current instruments to visualize beyond the catheter tip. We have developed a prototype 4-mm-diameter forward-viewing IVUS catheter (Cardiovascular Imaging Systems, Sunnyvale, Calif) that has the ability to provide B-mode cross-sectional ultrasound data for a distance of up to 2 cm distal to the catheter tip.

METHODS AND RESULTS

To study the utility of this device, a 20-MHz forward-viewing IVUS catheter was used to examine 13 arterial segments (5 human femoral arteries, 1 human carotid artery, 7 canine arteries) in vitro and 1 phantom. After imaging, all data were compared with histology (Histo). In all cases, the IVUS catheter provided forward-viewing images corresponding to the arterial geometry and demonstrated vascular landmarks and atherosclerotic lesions. There was a good correlation between Histo-determined luminal diameters (LD) and IVUS-determined diameters for a distance of 14 mm ahead of the catheter tip: IVUS LD = 1.0 Histo LD + 1.3 (r = .87).

CONCLUSIONS

These preliminary data suggest that a forward-viewing IVUS catheter is feasible, accurate, and useful for evaluation of arterial geometry distal to the catheter tip.

Arterial imaging with a new forward-viewing intravascular ultrasound catheter, II. Three-dimensional reconstruction and display of data

BACKGROUND

Current intravascular ultrasound (IVUS) catheters provide transverse imaging at the level of the ultrasound transducer. This limits imaging to large-diameter segments without critical atherosclerotic narrowings. We have developed a prototype 20-MHz forward-viewing IVUS catheter that provides two-dimensional sector imaging distal to the catheter tip. A present limitation of this technique is that the catheter must be manually rotated to obtain multiple longitudinal views required to integrate the segment into a three-dimensional matrix. To overcome this, we have developed an algorithm that reconstructs these multiple two-dimensional forward-viewing IVUS images into a three-dimensional matrix for more complete depiction of the segment distal to the ultrasound catheter. This algorithm allows display and multidimensional slicing of the three-dimensional reconstruction. METHODS AND RESULTS. To test our algorithms, five arterial segments (three canine aortas, two human femoral arteries) were evaluated in vitro. In each segment, 36 forward-viewing longitudinal slices were collected, digitized, processed, and reoriented to produce a three-dimensional reconstruction (3DR) matrix. The matrix data were sliced into parallel transverse sections and compared with morphometric interpretation of histological sections (Histo). As a result, image data could be reconstructed for a distance of 2.0 cm ahead of the catheter. 3DR easily demonstrated wall and luminal morphology and provided transverse IVUS images comparable to the histological specimens. A good correlation was noted between Histo- and 3DR-determined luminal diameters (LD) and luminal areas: 3DR LD = 1.4 Histo LD-0.4, r = .86; 3DR LD = 0.7 +/- 0.20 cm (mean +/- SD); and Histo LD = 0.7 +/- 0.13 cm.

CONCLUSIONS

These preliminary data demonstrate the feasibility of 3DR of forward-viewing IVUS data. This method allows rapid, detailed analysis of diseased arterial segments previously unavailable with standard IVUS and may permit better targeting of interventional techniques.

Intravascular ultrasound of coronary arteries in children. Assessment of the wall morphology and the lumen after Kawasaki disease

BACKGROUND

The long-term clinical issue in Kawasaki disease (KD) concerns the coronary artery lesion. Two-dimensional echocardiography and coronary angiography are routine examinations to evaluate the coronary lesions; however, these are not adequate to assess the wall morphology of the coronary artery (CA). Intravascular ultrasound imaging (IVUS), a new technology for the evaluation of the coronary artery lumen and wall morphology in vivo, was performed for patients after KD in their long-term follow-up, and we examined the new insights it gave.

METHODS AND RESULTS

IVUS was performed during cardiac catheterization in 20 subjects (10 patients after KD who still had coronary aneurysms or regressed coronary aneurysms, 2 after KD who had no coronary abnormal lesion, and 8 control patients with congenital heart disease and normal CA). We evaluated the wall structure at 10 to 15 sites of the CA in each patient. IVUS was performed with a commercially available ultrasound imaging catheter. Four sites of a CA aneurysm in KD demonstrated a markedly dilated lumen without thickened intima. One site of a CA aneurysm with calcification demonstrated an asymmetrical lumen by a dense echo with acoustic shadows. Twenty-two sites of a regressed CA aneurysm demonstrated a marked symmetrical or asymmetrical thickening of the intima with a dense echo, in which the size of the lumen was similar to that at a site near a regressed aneurysm. The sites of angiographically normal CA revealed normal structures and a thin intima in many instances. Nine of 28 sites in KD with a CA abnormal lesion, particularly near a coronary aneurysm or regressed aneurysm, demonstrated a mild thickening of the intima. All the 10 sites in KD without a CA abnormal lesion and all the 25 sites in patients with congenital heart disease with normal CA demonstrated a smooth intima.

CONCLUSIONS

This study demonstrated that the site of a regressed coronary aneurysm has a markedly thickened but smooth intima. The sites of angiographically normal CA after KD with or without a coronary lesion demonstrated normal IVUS findings in most instances but in some cases revealed a mild intimal thickening. IVUS is useful to evaluate the CA wall morphology and may contribute to the assessment of long-term CA sequelae and the possible development of arteriosclerotic changes in KD.

Three-dimensional intravascular ultrasonography: reconstruction of endovascular stents in vitro and in vivo

BACKGROUND

Intravascular ultrasonography displays an artery in loosely related cross-sectional images with limited axial information. However, intravascular ultrasound images are suited to three-dimensional reconstruction.

METHODS

A comprehensive intravascular ultrasound imaging system was used to reconstruct planar images in three-dimensions. This system consisted of a 25MHz transducer-tipped rigid probe (for in vitro studies) or a 25MHz transducer-tipped catheter within a 3.9F monorail imaging sheath (for in vivo studies), a motorized catheter pullback device that withdraws the transducer at 0.5mm/s, and an image processing computer that stacks 15 cross-sectional images/mm of stent axial length and then performs thresholding-based three-dimensional image rendering. We imaged 10 stents (4 Palmaz-Schatz, 3 Wiktor, 2 Strecker, and 1 Medinvent) in vitro after implantation in freshly harvested saphenous veins and 37 Palmaz-Schatz stents in vivo, 10 in native coronary arteries and 27 in vein grafts, 21 acutely and 18 on follow-up.

RESULTS

Three-dimensional reconstruction of images obtained with this system reproduced the geometry of each stent design. In vitro, images of the Palmaz-Schatz stents showed the expanded diamonds, the central articulation, and flaring of both ends of both halves of the stents. Images of the Wiktor stents showed the sinusoidal wave-shaped coils in their helical configuration. Images of the Strecker stents showed the interlocking-loop design with gaps between the terminal loops at either end of the stent. Images of the Medivent stent reproduced the woven texture formed by braiding the stent wires. Three-dimensional reconstruction of images obtained in vivo also reproduced the spatial geometry of the Palmaz-Schatz stent. However, reconstruction of in vivo images was limited by cardiac-cycle-linked vessel motion and torsion and the presence of echo-dense tissue that could not be separated completely from the stent itself.

CONCLUSIONS

Properly acquired intravascular ultrasound images can be used to reconstruct the spatial geometry of endovascular stents. Because stent spatial geometry is known and unambiguous, reconstruction of endovascular stents should be one of the tests of imaging systems designed to perform three-dimensional reconstruction of ultrasound images.