Li Wenguang

Three-dimensional reconstruction of intracoronary ultrasound images. Rationale, approaches, problems, and directions.

Although intracoronary ultrasonography allows detailed tomographic imaging of the arterial wall, it fails to provide data on the structural architecture and longitudinal extent of arterial disease. This information is essential for decision making during therapeutic interventions. Three-dimensional reconstruction techniques offer visualization of the complex longitudinal architecture of atherosclerotic plaques in composite display. Progress in computer hardware and software technology have shortened the reconstruction process and reduced operator interaction considerably, generating three-dimensional images with delineation of mural anatomy and pathology. The indications for intravascular ultrasonography will grow as the technique offers the unique capability of providing ultrasonic histology of the arterial wall, and the need for a three-dimensional display format for comprehensive analysis is increasingly recognized. Consequently, three-dimensional imaging is being rapidly implemented in the catheterization laboratories for guidance of intracoronary interventions and detailed assessment of their results. However exciting the prospects may be, three-dimensional reconstructions at present remain partially artificial because the true spatial position of the imaging catheter tip is not recorded, and shifts in its location and curves of the arterial lumen result in pseudoreconstructions rather than true reconstructions. In this report, we address the principles of three-dimensional reconstruction with a critical review of its limitations. Potential solutions for refinement of this exciting imaging modality are presented.

Detection and Characterization of Vascular Lesions by Intravascular Ultrasound: An In Vitro Study Correlated with Histology

High-frequency intravascular ultrasound (30 and 40 MHz) was applied to study 112 human vascular specimens. The ultrasound images were compared with histologic cross-sections. In 44 out of 58 of the histologically classified muscular arteries, a hypoechoic middle layer was seen in the vessel wall, giving it a three-layered appearance. In 10 arteries, fibrous degeneration of the muscular media resulted in a homogeneous appearance of the vessel walls, whereas atherosclerotic plaque precluded the visualization of the arterial media in four of the arteries. A three-layered appearance was seen in seven of nine histologically classified transitional arteries, and a homogeneous arterial wall was seen in two of the nine. None of the 33 elastic arteries, veins, venous bypass, and Goretex conduits showed a hypoechoic medial layer. Histologically proved fibrous intimal thickening was echographically detected in 32 of 48 specimens (67%). It was noted that these intimal lesions were easier to detect with 40 MHz than with 30 MHz transducers. Hypoechoic areas of lipid deposition were detected in 32 of 36 specimens (89%) and could be distinguished from fibrous plaques. Histologically evident calcium deposits were detected with intravascular ultrasound in 35 of 36 specimens (97%). Measurement of plaque area was only possible in cross sections with a three-layered appearance. Quantitative analysis showed a significantly larger lumen area measured from ultrasonic images (26.3 +/- 21.3 mm2) than from histologic cross-sections (21.8 +/- 16.6 mm2, p less than 0.001), probably because of tissue shrinkage during processing for histology. A significant correlation (r = 0.96, p less than 0.001) between ultrasonic and histologic measurements of lumen areas was observed, with and a negligible interobserver and intraobserver variability. Plaque area and medial thickness correlated well with histology (r = 0.87, p less than 0.001 and r = 0.93, p less than 0.001, respectively). It appears from this in vitro study that intravascular ultrasound is an accurate technique for detection and characterization of atherosclerotic lesions. Vessel lumen area can be measured in most instances, whereas plaque area and medial thickness can only be reliably assessed in muscular arteries in which the hypoechoic media serves as a reference, and shadowing by calcium or attenuation by fibrous plaque components is absent.

Validation of quantitative analysis of intravascular ultrasound images

This study investigated the accuracy and reproducibility of a computer-aided method for quantification of intravascular ultrasound. The computer analysis system was developed on an IBM compatible PC/AT equipped with a framegrabber. The quantitative assessment of lumen area, lesion area and percent area obstruction was performed by tracing the boundaries of the free lumen and original lumen.Accuracy of the analysis system was tested in a phantom study. Echographic measurements of lumen and lesion area derived from 16 arterial specimens were compared with data obtained by histology. The differences in lesion area measurements between histology and ultrasound were minimal (mean ± SD: −0.27±1.79 mm2, p>0.05). Lumen area measurements from histology were significantly smaller than those with ultrasound due to mechanical deformation of histologic specimens (−5.38±5.09 mm2, p<0.05). For comparison with angiography, 18 ultrasound cross-sections were obtainedin vivo from 8 healthy peripheral arteries. Luminal areas obtained by angiography were similar to those by ultrasound (−0.52±5.15 mm2, p>0.05). Finally, intra- and interobserver variability of our quantitative method was evaluated in measurements of 100in vivo ultrasound images. The results showed that variations in lumen area measurements were low (5%) whereas variations in lesion area and percent area obstruction were relatively high (13%, 10%, respectively).Results of this study indicate that our quantitative method provides accurate and reproducible measurements of lumen and lesion area. Thus, intravascular ultrasound can be used for clinical investigation, including assessment of vascular stenosis and evaluation of therapeutic intervention.

Displacement Sensing Device Enabling Accurate Documentation of Catheter Tip Position

The need for reproducible positioning of an intravascular ultrasound catheter tip before and after intervention in relation to the angiographic records, resulted in the development of a dedicated displacement sensing device that provides instantaneous information about the catheter tip position on the videoscreen. The relative distance information can be displayed together with the ultrasound image. The accuracy of this device was tested in vitro.

Semiautomatic frame-to-frame tracking of the luminal border from intravascular ultrasound

The authors describe a template-matching method which allows the measurement of the frame-to-frame changes in the luminal cross-sectional area from intravascular ultrasound images through the analysis of the regional wall displacement. The matching is performed by calculating a cross-correlation coefficient between the template and the test data. The optimal matching is determined using the minimum-cost algorithm. Comparison of the results from the template-matching method and those from manual tracing showed that the mean difference in the measure of the area change was very small. The beat-to-beat variations evaluated with data from the method were lower than with manual tracing. It is concluded that this method can be applied to wall compliance studies using intravascular ultrasound.<<ETX>>

Intravascular ultrasonic assessment of lumen geometry and distensibility of the angiographically normal artery: a correlation with quantitative angiography.

The feasibility of assessing lumen diameter and area using a 30‐MHz mechanically driven ultrasound imaging device was evaluated in vitro in phantoms and in vivo in eight human arteries (six iliac, two brachial). Ultrasound data were compared to angiographic data derived from the cardiovascular angiographic analysis system. In addition, the change of lumen area in a given cardiac cycle was determined in each patient. A close relation between ultrasound and angiography was observed in the phantom studies. In the first three patients there was disagreement; ultrasound images showed larger values compared to the angiographically derived values. Disagreement was related to the use of nominal measurements of the sheath supplied by the manufacturer as calibration. Data on the five other patients showed a close relation between the values derived with ultrasound and angiography. The arterial lumen area revealed a 5%± 2% change during one cardiac cycle. The intra‐ and interobserver variability test showed good correlation for the ultrasound study. This study demonstrates that intravascular ultrasound is an accurate and reproducible technique to measure vascular diameter, lumen area, and arterial wall distensibility in vivo.

In vivo measurement of regional large artery compliance by intravascular ultrasound under pentobarbital anesthesia.

Background: The presence of smooth muscle fibers on the wall of large arteries would suggest that arterial compliance might change in response to vasoactive substances. The purpose of this study is to determine the basal level of vasomotor tone in these arteries in a commonly used animal preparation and to learn whether the compliance of large conductance arteries can be altered in vivo by vasoactive agents. Methods: Proximal iliac arterial compliance was measured in 7 pentobarbital-anes thetized pigs, before and during local infusions of adenosine and norepinephrine. Luminal area was measured every forty milliseconds by means of a 30 MHz intravas cular ultrasound catheter and an automatic edge detection program. Simultaneous high- fidelity pressure measurements were obtained by means of a catheter-tipped pressure microtransducer positioned at the origin of the iliac artery. Linear regression analysis of the area/pressure relationship in two consecutive cardiac cycles (systolic phase only) was performed before and during adenosine and norepinephrine infusions. The slope of the area/pressure regression line was defined as an index of arterial compliance. Measurements after three minutes of infusions of adenosine (5-5000 μg/minute) and norepinephrine (0.001-10 μg/minute) were compared with the control measurements. Results: Even at the highest infusion rate, adenosine did not significantly increase arterial compliance as compared with baseline (25 ± 7 vs 19 ±4 mm2/mmHg x 10-3, respectively, P = ns). In contrast, norepinephrine decreased arterial compliance as compared with the second baseline control (13 ±3 vs 20 ±3 mm2/mmHg x 10-3, respectively, P < 0.01). Conclusions: In this animal model with pentobarbital anesthesia, arterial compliance may be modified more by the acute infusion of norepinephrine than by adenosine in large conductance arteries such as the proximal iliac. Thus, in this preparation, smooth muscle tone tends to be minimal and arterial compliance near maximal (ie, mostly a passive phenomenon). However, in response to norepinephrine, arterial compliance can decrease significantly as smooth muscle tone increases. Intravascular ultrasound allows continuous and accurate monitoring of these changes of arterial dimensions, suggesting that this technique may be useful in the evaluation of pharmacologically induced changes in the compliance of large arteries by vasoactive agents.

State of the art in ICUS quantitation

IntraCoronary UltraSound (ICUS) data are the basis of two-dimensional (2D) quantitative information and three-dimensional (3D) reconstruction. A method for semi-automatic 3D image quantification for volumetric study of series of echo slices has been developed. The semi-automatic contour detection method was tested in-vitro in tubular phantoms of known dimensions. Intra- and interobserver variability was evaluated in-vivo for area and volume measurements of diseased human coronary arteries.

Computer-Aided Intravascular Ultrasound Diagnostics

This paper describes the computerized methods for image enhancement, segmentation and three-dimensional (3-D) reconstruction of intravascular ultrasound images. To improve the visibility of the arterial wall when masked by blood scattering, a temporal averaging method was used during off-line analysis. The results showed that the reproducibility of the lumen area measurement was improved 2.4 times with the averaging method (intra-observer variation coefficient: single-frame: 8.8% vs averaged image: 3.6%). To reconstruct a 3-D image from a series of echographic slices, an automatic image segmentation technique based on temporal subtraction was developed. This method applies histogram processing and low-pass filtering techniques for data preprocessing, and dedicated techniques for data postprocessing. The feasibility of this method is demonstrated by 3-D reconstruction derived from a phantom object.

Detection of vascular morphology by high frequency intravascular ultrasonic imaging

This study was designed to validate the potential clinical utility of intravascular ultrasonic imaging in vitro and in vivo. In vitro studies were performed to assess the accuracy of dimensional and morphological information. In vitro images of human vessels (n = 75) demonstrated that lesion thickness determined echographically closely related with histological samples (r = 0.83). Morphologically, muscular and elastic arteries could be distinguished echographically based on the echogenicity of the arterial media. Close relation was also found in the morphological subtypes of atherosclerosis.Subsequently, intravascular ultrasound was used percutaneously in vivo in 20 patients to obtain images of the iliac and superficial femoral artery. High quality real-time images were obtained. Normal vessels were seen showing pulsatile circular images with a hypoechoic muscular media resulting in a typical three-layered appearance. Diseased arteries revealed non-obstructive and obstructive lumen. At the site of obstruction thinning of the muscular media was evident. Pulsation was not always present. Following dilatation of the obstructive lesion using balloon angioplasty the ultrasonic cross-sections changed drastically revealing plaque rupture, dissection, plaque-free wall rupture, rest stenosis and oedema.We conclude that intravascular ultrasonic imaging is a promising technique to document accurate dimensional and morphological characteristics of human vascular disease for guidance of therapeutic interventions.