Intracoronary ultrasound: current state of the art

Preintervention lesion remodelling affects operative mechanisms of balloon optimised directional coronary atherectomy procedures: a volumetric study with three dimensional intravascular ultrasound

AIMS

To classify atherosclerotic coronary lesions on the basis of adequate or inadequate compensatory vascular enlargement, and to examine changes in lumen, plaque, and vessel volumes during balloon optimised directional coronary atherectomy procedures in relation to the state of adaptive remodelling before the intervention.

DESIGN

29 lesion segments in 29 patients were examined with intravascular ultrasound before and after successful balloon optimised directional coronary atherectomy procedures, and a validated volumetric intravascular ultrasound analysis was performed off-line to assess the atherosclerotic lesion remodelling and changes in plaque and vessel volumes that occurred during the intervention. Based on the intravascular ultrasound data, lesions were classified according to whether there was inadequate (group I) or adequate (group II) compensatory enlargement.

RESULTS

There was no significant difference in patient and lesion characteristics between groups I and II (n = 10 and 19), including lesion length and details of the intervention. Quantitative coronary angiographic data were similar for both groups. However, plaque and vessel volumes were significantly smaller in group I than in II. In group I, 9 (4)% (mean (SD)) of the plaque volume was ablated, while in group II 16 (11)% was ablated (p = 0.01). This difference was reflected in a lower lumen volume gain in group I than in group II (46 (18) mm(3) v 80 (49) mm(3) (p < 0.02)).

CONCLUSIONS

Preintervention lesion remodelling has an impact on the operative mechanisms of balloon optimised directional coronary atherectomy procedures. Plaque ablation was found to be particularly low in lesions with inadequate compensatory vascular enlargement.

Classification of arterial plaque by spectral analysis of in vitro radio frequency intravascular ultrasound data

To test whether radio-frequency analysis of coronary plaques predicts the histological classification, r.f. data were collected using a 30 MHz intravascular ultrasound scanner. Two hundred ninety-nine regions-of-interest from eight postmortem coronary arteries were selected and identified by histology as falling into one of seven different tissue types. These are loose fibrous tissue (n = 78), moderate fibrous tissue (n = 27), dense fibrous tissue (n = 33), microcalcification (n = 14), calcified plaque (n = 55), lipid/fibrous mixture (n = 51) and homogeneous areas of lipid pool (n = 29). On the basis of a previous study, four spectral parameters were calculated for each of the regions-of-interest: maximum power (dB), mean power (dB), spectral slope (dB/MHz) over the bandwidth 18-35 MHz and the intercept of the spectral slope with the 0 Hz axis (dB). A minimum-distance classifier using the Mahalanobis (1948) distance was applied to the data. Following resubstitution of the training data into the classifier, the total correctly classified was 54%. The data were reclassified using three broader tissue groups: (1) calcified plaque, (2) lipid pool and (3) a mixed fibrous category, incorporating loose fibrous tissue, moderate fibrous tissue, dense fibrous tissue, lipid/fibrous mixture and microcalcification. The total correctly classified was 86%. Using "leave-one-out" cross-validation, the classification rates were 48% for seven tissue subgroups and 83% for three broader categories of tissue type.

Arterial remodelling of native human coronary arteries in patients with unstable angina pectoris: a prospective intravascular ultrasound study

OBJECTIVE

To investigate the use of intravascular ultrasound (IVUS) in detecting the presence of arterial remodelling in patients with unstable angina.

DESIGN

Prospective case study.

PATIENTS

60 of 95 consecutively admitted patients with unstable angina (41 male, 19 female), mean (SD) age 61.2 (8.1) years.

INTERVENTIONS

Qualitative and quantitative coronary angiography and IVUS.

MAIN OUTCOME MEASURES

Adaptive or constrictive remodelling (AR, CR) was considered present when the cross sectional area of the external elastic membrane at the lesion site was larger than the proximal cross sectional area or smaller than the distal cross sectional area, respectively.

RESULTS

22 of the 60 patients (37%) showed AR and 14 (23%) showed CR. No remodelling was seen in 24 patients (group NR). The plaque contained more thrombus and plaque rupture in group AR than in groups CR and NR (thrombus: 91% v 50% and 67%, respectively, p = 0.023; rupture: 73% v 29% and 42%, p = 0.020). AR was associated with a larger plaque cross sectional area (12.6 (SD 4.6) mm2 v 10.8 (6.3) and 9.2 (3.7) mm2, p = 0.001) and larger external elastic membrane cross sectional area (16.5 (5.8) mm2 v 13.2 (5.2) and 14.4 (3.6) mm2, p = 0.01 in group AR v groups CR and NR, respectively), while the plaque burden was larger in groups AR (74.9 (9.1)%) and CR (72.4 (16.6)%) than in group NR (66.2 (18.1)%, p = 0.005).

CONCLUSIONS

IVUS is capable of detecting adaptive and constrictive remodelling of target lesions and its relation to plaque morphology in unstable angina.

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.

Developments in cardiovascular ultrasound. Part 2: Arterial applications

Many of the changes resulting from arterial disease can be measured, using Doppler ultrasound for measurement of blood velocity and B-scan imaging for measurement of tissue structure and composition. Wall thickness, the degree of arterial narrowing and plaque volume can be measured using B-scan imaging, and 3D ultrasound can be used to improve the accuracy of measurements of plaque volume and for improved visualisation of complex arterial geometries. Measurement of the dynamic properties of the arterial wall permits estimation of wall elasticity and plaque motion. From the Doppler signal, measurements of blood velocity are used to estimate the degree of arterial narrowing and volumetric flow, although measurement errors can be large. Wall shear stress can be estimated by measuring the velocity gradient at the vessel wall. The problems of inadequate spatial resolution and interference from overlying tissue are largely removed when intravascular systems are used, and these have superior capability in the assessment of arterial structure and tissue composition. However, measurement of quantities relating to blood flow is more difficult using the intravascular approach, as the indwelling cather disturbs the blood flow pattern, and currently, assessment of flow and vessel cross-section are not performed at the same site.

Atherosclerotic coronary lesions with inadequate compensatory enlargement have smaller plaque and vessel volumes: observations with three dimensional intravascular ultrasound in vivo

OBJECTIVE

To compare vessel, lumen, and plaque volumes in atherosclerotic coronary lesions with inadequate compensatory enlargement versus lesions with adequate compensatory enlargement.

DESIGN

35 angiographically significant coronary lesions were examined by intravascular ultrasound (IVUS) during motorised transducer pullback. Segments 20 mm in length were analysed using a validated automated three dimensional analysis system. IVUS was used to classify lesions as having inadequate (group I) or adequate (group II) compensatory enlargement.

RESULTS

There was no significant difference in quantitative angiographic measurements and the IVUS minimum lumen cross sectional area between groups I (n = 15) and II (n = 20). In group I, the vessel cross sectional area was 13.3 (3.0) mm2 at the lesion site and 14.4 (3.6) mm2 at the distal reference (p < 0.01), whereas in group II it was 17.5 (5.6) mm2 at the lesion site and 14.0 (6.0) mm2 at the distal reference (p < 0.001). Vessel and plaque cross sectional areas were significantly smaller in group I than in group II (13.3 (3.0) v 17.5 (5.6) mm2, p < 0.01; and 10.9 (2.8) v 15.2 (4.9) mm2; p < 0.005). Similarly, vessel and plaque volume were smaller in group I (291.0 (61.0) v 353.7 (110.0) mm3, and 177.5 (48.4) v 228.0 (92.8) mm3, p < 0.05 for both). Lumen areas and volumes were similar.

CONCLUSIONS

In lesions with inadequate compensatory enlargement, both vessel and plaque volume appear to be smaller than in lesions with adequate compensatory enlargement.

A study of the quantitative and qualitative impact of catheter shaft angulation in a mechanical intravascular ultrasound system

Nonuniform rotation of mechanical intravascular ultrasound transducers may give rise to a geometric distortion of the ultrasound image known as the rotation angle artefact. This investigation studied the influence of different degrees and combinations of catheter shaft angulation on image morphology and the quantitative impact of the artefact using a circular perspex phantom and 3.5 F, 30 MHz Boston Scientific "Sonicath" catheters connected to a Hewlett Packard Sonos intravascular scanner. Major and minor diameters, cross-sectional area and circumference of the phantom lumen were measured and a "distortion index" calculated. Visually apparent geometric distortion was graded from 1 (absent) to 4 (severe). As expected, eccentric transducer location was associated much more frequently with identifiable distortion (70%) than was a concentric location (6%). Greater distortion occurred with increasing degrees of catheter shaft angulation, and was more pronounced in images from older catheters. The lumen area measurements in images in which no artefact was identified were accurate to within +/- 10% in 97% of cases, compared to only 81% of cases when an artefact was noted. The quantitative accuracy of an image in which geometric distortion is identified is thus not reliable. The direction of the quantitative error cannot be confidently predicted in any given case, although the mean lumen area tends to increase as the grade of distortion increases.

Articulation strut coronary restenosis in Palmaz-Schatz stents

We describe articulation strut coronary restenosis in two patients with Palmaz-Schatz 153 stents and discuss its aetiology, management and possible prevention.