Variance components analysis of carotid and femoral intima-media thickness measurements. REGRESS Study Group, Interuniversity Cardiology Institute of The Netherlands, Utrecht, The Netherlands. Regression Growth Evaluation Statin Study

B-mode ultrasound intima-media thickness (IMT) measurements of carotid and femoral arterial walls are used in atherosclerosis studies. In this study, the components contributing to IMT measurement variability in males with coronary artery disease were investigated by means of repeated B-mode ultrasound scans and repeated off-line video image analyses. For statistical analysis, a mixed-model analysis of variance was used. From sonographer data, it was shown that human subjects and their arterial wall segments contributed 75% of the total IMT measurement variability in this population. Inter-sonographer variance contributed 25%. The intra-sonographer variance was negligible (<1%). In off-line image analysis, variance components due to subjects and segments, inter-analyst variance, and residual fluctuation were 88%, < 1% and 11%, respectively. Intra-analyst variance was negligible (<1%). The major source of B-mode ultrasound IMT measurement variability finds its origin in subjects and their arterial walls. Although sonographers proved a lesser source of variability, in comparative studies they should enter a study well trained and should be randomly assigned to subjects. Follow-up examinations should preferably be done by the same sonographer. Off-line image analysis contributed little to IMT measurement variability.

B-mode ultrasound assessment of pravastatin treatment effect on carotid and femoral artery walls and its correlations with coronary arteriographic findings: a report of the Regression Growth Evaluation Statin Study (REGRESS)

OBJECTIVES

In this B-mode ultrasound study we assessed pravastatin treatment effects on carotid and femoral artery walls and investigated the correlations between the state and evolution of peripheral and coronary atherosclerosis.

BACKGROUND

The Regression Growth Evaluation Statin Study (REGRESS) was an 11-center, 2-year, double-blind, placebo-controlled, prospective study of 885 men with coronary artery disease (CAD) (total cholesterol 4 to 8 mmol/liter). The study primarily investigated pravastatin treatment effects on the coronary lumen. This report focuses on the 255 patients who participated in the REGRESS ultrasound study.

METHODS

Carotid and femoral artery walls were imaged at baseline and at 6, 12, 18 and 24 months. Pravastatin treatment effect was defined as the difference in progression of the combined intima-media thicknesses (IMT) between treatment groups.

RESULTS

Pravastatin treatment effects were highly significant (combined IMT: p = 0.0085; combined far wall IMT: p < 0.0001; common femoral artery far wall IMT: p = 0.004). Correlations between the IMTs of the arterial wall segments ranged from -0.17 to 0.81. Baseline correlations between IMT and percent coronary lumen stenoses ranged from 0.23 to 0.36. Baseline IMT correlated with the mean coronary segment diameter (r = -0.32, p = 0.001) and minimal coronary obstruction diameter (r = -0.27, p = 0.005). There were no individual correlations between IMT and coronary lumen variables (p > 0.30).

CONCLUSIONS

Pravastatin treatment effects on carotid and femoral artery walls were observed. B-mode ultrasound imaging studies of peripheral arterial walls could not describe the state and evolution of the coronary lumen in the individual patient, but proved to be a highly suitable tool for the assessment of antiatherosclerotic properties of agents.

Blood flow assessment with intravascular ultrasound catheters: the ideal tool for simultaneous assessment of the coronary haemodynamics and vessel wall?

We present the potentials of a novel method of intracoronary flow visualization and quantification that is based on conventional intravascular ultrasound (IVUS) imaging catheters. The quantification of flow is obtained from analysis of the rate of decorrelation of digitized radiofrequency ultrasound echo signals. Flow information is superimposed on the IVUS image using a colour scale. Integration of the blood velocity components normal to the scan plane permits calculation of the volume flow. Validation using IVUS and electromagnetic (EM) flowmeter recordings were obtained in vivo from instrumented pigs. IVUS flow (IVUS(f)) compared favourably to EM flow (EM(f)): IVUS(f)=1.0 EM(f)+5.72 cc/min, r2=0.98. Clinical results for the first five patients investigated are reported. A Doppler wire was used to measure the flow in four coronary arteries and one renal artery in baseline and hyperaemia conditions. IVUS flow and derived coronary flow reserve (CFR) demonstrated a very good agreement with the data derived from the combination of quantitative angiography and velocity when measured with the Doppler wire (DOP(f)): IVUS(f)=1.01 DOP(f)-20 cc/min, r2=0.90 and IVUS(cfr)=1.03 DOP(cfr)-0.03, r2=0.93. This demonstrates that simultaneous morphological and physiological assessment of coronary or peripheral arteries with one IVUS catheter is feasible. This method should be very useful for the evaluation of intermediate coronary stenoses or the results of revascularization procedures.

Intravascular imaging

Based on three-dimensional (3D) information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based in image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values which in turn may indicate sites prone to stenosis. With a decorrelation technique for radio frequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as animal experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain as a measure of local tissue hardness can be estimated in principle. Hard or soft plaques can be identified from the strain images independently of the echogenic contrast between plaque and vessel wall.

New developments in intravascular ultrasound imaging

IntraVascular Ultrasound Imaging (IVUS) has already been proposed in the early days of diagnostic ultrasound. Today, it has come under further full attention as a result of minimal invasive techniques. Not only excellent intravascular two-dimensional (2D) images are presently obtained, also three-dimensional (3D) reconstructed images show their diagnostic value. Based on 3D information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based on image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values, which in turn may indicate sites prone to stenosis. With a decorrelation technique for radiofrequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as in vivo experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain, used as a measure of local tissue hardness, can be estimated to identify hard or soft plaques independently of the echogenicity contrast between plaque and vessel wall.

Blood flow imaging and volume flow quantitation with intravascular ultrasound

Current intravascular ultrasound techniques produce real-time imaging of a vessel cross-section with a scan plane approximately normal to blood flow. When a cluster of randomly distributed blood particles moves across the ultrasound beam, the received echo signals decorrelate as a function of time. This phenomenon may be used to estimate blood velocities by measuring the decorrelation rate from a sequence of blood scattering signals. A decorrelation-based method for measuring local blood velocity and quantifying volume flow from cross-sectional radio frequency intravascular echo signals was developed. Serial in vitro measurements were performed with a flow phantom to test the principle of the proposed velocity estimation method. An in vivo pig experiment was carried out to study the feasibility of applying this method in clinical settings. Preliminary results of this study indicate that the proposed decorrelation method is able to extract cross-sectional velocity data and volumetric flow both in vitro and in vivo.

Decorrelation of intravascular echo signals: potentials for blood velocity estimation

When blood particles travel through an intravascular ultrasound imaging plane, the received echo signals decorrelate at a rate that is related to the flow velocity. In this paper, the feasibility of extracting blood velocity from the decorrelation function of radio frequency signals was investigated through theoretical analysis and computer simulation. A computer model based on the impulse response method was developed to generate the ultrasound field of a 30-MHz intravascular transducer. The decorrelation due to the scatterer displacement as well as other nonmotion related decorrelation sources were studied. The computer simulations show that the decorrelation function is linearly related to the lateral displacement. The monotonic relationship between correlation and displacement provides possibilities to estimate flow velocity with decorrelation measurements. Because of the complexity of the beam profile in the near field, assessment of local velocities requires detailed knowledge of the decorrelation at each axial beam position. Sources of signal decorrelation other than the lateral displacement may cause a bias in the decorrelation based velocity measurements. For localized decorrelation estimation, measurement variations in small range windows present a major challenge. An approach based on multiple decorrelation measurements should be adopted in order to reduce the variations. In conclusion, results of this study suggest that it is feasible to measure flow velocity by quantifying the decorrelation of intravascular ultrasound signals from blood.

Prediction of restenosis after coronary balloon angioplasty. Results of PICTURE (Post-IntraCoronary Treatment Ultrasound Result Evaluation), a prospective multicenter intracoronary ultrasound imaging study

BACKGROUND

Intracoronary ultrasound (ICUS) imaging is potentially suitable to identify lesions at high risk of restenosis after percutaneous transluminal coronary angioplasty (PTCA), but it has not been studied systematically.

METHODS AND RESULTS

We recruited 200 patients in whom ICUS studies were performed after successful PTCA and related their ICUS parameters to 6-month follow-up quantitative coronary angiography. This was performed in 164 patients (82%), yielding 170 lesions for analysis. The overall incidence of a > or = 50% diameter stenosis at follow-up (categorical restenosis) was 29.4%. Quantitative ICUS parameters were weakly but significantly related to follow-up minimal luminal diameter on quantitative coronary angiography (lumen area: R2 = .36, P = .0001; vessel area: R2 = .29, P = .0002; plaque area: R2 = -.18, P = .021; percent obstruction: R2 = -.15, P = .05), but categorical restenosis was not significantly related to these parameters (P = .63, .77, .38, and .08, respectively). There were no significant predictors of restenosis in ICUS parameters of plaque morphology: eccentric versus concentric (P = 1.0), plaque type (hard, soft, or calcific, P = .98), or the number of calcified quadrants (P = .41). There were no significant predictors of restenosis in two predefined types of vessel-wall disruptions: (1) rupture: presence (P = .79), depth (partial versus complete, P = .85), or extent in quadrants (P = .6), and (2) dissection: presence (P = .31), depth (P = .82), or extent (P = .38).

CONCLUSIONS

Qualitative ICUS parameters after PTCA did not predict restenosis. A larger lumen and vessel area and a smaller plaque area by ICUS were associated with a larger angiographic minimal lumen diameter at follow-up, but these parameters were not significantly related to categorical restenosis.

Quantitative three-dimensional intravascular ultrasound

Three-dimensional (3-D) intravascular ultrasound (IVUS) allows for the visualization of entire coronary segments, provides more detailed insights into the geometry of atherosclerotic plaques and facilitates serial studies. Automated quantitative 3-D IVUS methods reduce the analysis time and the subjectivity of boundary tracing, and permit complex IVUS studies. The 3-D IVUS approach is not restricted to research applications, but may be used as a valuable clinical tool. Evaluation of the coronary segment of interest before catheter-based coronary interventions provides information which may facilitate the selection of interventional devices. Moreover, 3-D IVUS allows for a careful assessment of the procedural results and potential post-procedural complications. ECG-gated image acquisition, automated contour detection, and approaches using data of both 3-D IVUS and biplane angiography represent the recent progress in this field. Three-dimensional IVUS will surely gain further importance and become a routine technique, if the interest and research effort is sustained.

Potentials of volumetric blood-flow measurement

Current intravascular ultrasound techniques produce real-time imaging of a vessel cross-section with a scan plane normal to blood flow. When randomly distributed blood particles travel through this ultrasound imaging plane, the received echo signals decorrelate as a function of time. The speed of such a decorrelation procedure is proportional to the flow velocity. This phenomenon provides a potential to estimate blood velocities by means of decorrelation analysis. In this paper, we present a method for measuring local blood velocity and quantifying volume flow directly from cross-sectional intravascular ultrasound data. This method is based on multiple decorrelation assessments with a sequence of radio frequency echo signals. The velocity measurement is obtained by comparing the measured decorrelation value with the prior knowledge of the beam characteristics of an intravascular ultrasound transducer. Volume flow is derived by integrating the cross-sectional area and its corresponding velocity vector over the vessel lumen. The decorrelation-based method was tested in vitro with a flow phantom. Measurements were also carried out in vivo in pig experiments to determine the usefulness of this method in clinical settings. Preliminary results of these experiments indicate that the proposed decorrelation method is able to extract cross-sectional velocity profiles and volumetric flow both in vitro and in vivo.

Influence of data processing on cyclic variation of integrated backscatter and wall thickness in stunned porcine myocardium

This study was performed to investigate the relationship between the cyclic variation of integrated backscatter and myocardial wall thickening in stunned myocardium. Different definitions of cyclic variation were evaluated to be able to compare with other studies. Ultrasound data were acquired from 10 open-chested Yorkshire pigs (25-33 kg) at baseline, during regional ischemia and during 30 min of stunning, using a broadband ultrasound transducer (3-7 MHz) sutured directly upon the left ventricular myocardial wall. Cyclic variation of integrated backscatter and myocardial wall thickening were calculated using three definitions obtained from the literature. Independent of the definition, cyclic variation of wall thickness and integrated backscatter were blunted during acute ischemia and returned transiently to or above baseline during the first minute of reperfusion, followed by a gradual decrease to a level under baseline during stunning. An early return of the cyclic variation of the integrated backscatter was not observed in pigs, independent of the data processing used. The relationship between integrated backscatter and wall thickness was maintained.

The relationship between myocardial integrated backscatter, perfusion pressure and wall thickness during isovolumic contraction: an isolated pig heart study

To investigate the independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter, experiments were designed in which integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isolated pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction and relaxation were realized by inserting a noncompressible water-filled balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), the myocardial wall thickness and the left ventricular pressure were determined simultaneously at various balloon volumes (5-25 mL). A quasistatic increase of balloon volume by 50% resulted in an average decrease of wall thickness of 6.5% (p < 0.01) and a mean increase in the integrated backscatter level of 1.1 dB (p < 0.01). Integrated backscatter levels increased statistically significant by 0.14 +/- 0.014 dB per percent decrease of wall thickness. Measurements of percentage end-systolic myocardial wall thickening ranged from -10% to +10%, mean 0.15 +/- 4.5% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -3.9 to +3.9 dB, mean 0.19 +/- 1.5 dB (NS from zero). In a second experiment, at a constant midrange balloon volume, the same parameters were determined simultaneously at various perfusion pressures (20-120 mmHg). An increase in perfusion pressure by 50% resulted in a small but statistically significant increase of 1.5% in myocardial wall thickness, which could be explained by an increase of intravascular volume. The integrated backscatter levels did not change statistically significantly. Measurements of percentage end-systolic myocardial wall thickening ranged from -8.9 to +7.8%, mean 0.13 +/- 4.0% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -1.8 to +4.2 dB, mean 0.37 +/- 1.3 dB (NS from zero). The magnitude of cyclic variation of integrated backscatter of myocardial tissue in a contractile state is reduced if myocardial muscle is prevented from normal thickening. In addition, changes in intravascular volume during the cardiac cycle have a negligible influence on the absolute backscatter level or its cyclic variation. We conclude, if only wall thickness and perfusion pressure are involved, that integrated backscatter is mainly determined by myocardial wall thickness.

Two decades of transesophageal phased array probes

After its introduction about two decades ago, transesophageal echocardiography (TEE) has rapidly evolved into an important diagnostic feature for the cardiologist, since it offers anatomic and hemodynamic information which cannot be obtained precordially. Part of this success was due to the developments in transducer technology which resulted in smaller probes with progressively better imaging qualities. A short review of past, recent and future developments of TEE phased array probes, in particular those at the Erasmus University in Rotterdam, will be given. Furthermore, this article discusses basic parameters of the transducer dictating image quality such as centre frequency, array aperture and focusing illustrated with several simulations. The simulations show that a poor design of the transducer will limit the resolution and will give artefacts in the two-dimensional image.

Interobserver reproducibility of qualitative and quantitative analysis of intravascular ultrasound images before and after peripheral balloon angioplasty

In this study, interobserver agreement on intravascular ultrasound data obtained before and after balloon angioplasty (PTA) of the superficial femoral artery was assessed. Two observers analyzed intravascular ultrasound cross-sections from 38 patients. Interobserver agreement was good for soft and hard lesions, dissection and vascular damage (kappa values 0.61, 0.67, 0.69 and 0.66, respectively); moderate for eccentric lesions (kappa 0.45); fair for media ruptures (kappa 0.25); and poor for lipid deposits and plaque ruptures (kappa 0.0 and 0.04, respectively). Differences for the arc of normal wall and hard lesion were not significant, but were for dissection. There were no significant interobserver differences between area measurements. The coefficient of variation for free lumen area and media-bounded area before PTA was 17.2% and 10.5% and after PTA 11.2% and 9.2%, respectively. This study identified the intravascular ultrasound parameters that are reproducible.

An in vitro evaluation of the line pattern of the near and far walls of carotid arteries using B-mode ultrasound

This in vitro study was executed to evaluate the double line pattern generated at both near and far walls of human carotid arteries using B-mode ultrasound. Therefore, extravascular (7.5 MHz) and intravascular (30 MHz) ultrasound imaging were performed at the same locations of the carotid artery. The thickness of the double line pattern of the extravascular image (7.5 MHz) was compared to the thickness of the intima-media complex seen on the corresponding intravascular image (30 MHz) and on the histologic section. At the far wall of the extravascular image, the measurements were executed at the leading edge of the echo. The data showed high correlation and agreement with the intravascular (r = 0.91, p < 0.001; mean(diff) = -0.01 and SDdiff = 0.12) and the histologic measurements (r = 0.87, p < 0.001; mean(diff) = -0.12 and SDdiff = 0.13). In addition, the results of the measurements of the intravascular image showed high correlations and agreement with the histologic data (r(near) = 0.86, p < 0.001; mean(diff) = -0.08 and SDdiff = 0.15, respectively, r(far) = 0.92, p < 0.001; mean(diff) = -0.12 and SDdiff = 0.12). For comparison with other studies, near wall measurements were also included. These had to be performed at the trailing edge of the echoes to be compatible with these studies. The results of the measurements of the extravascular image showed poor correlations and lack of agreement with those of the intravascular (r = 0.49, p = 0.03; mean(diff) = 0.09 and SDdiff = 0.25) and of the histologic (r = 0.37, p = 0.03; mean(diff) = 0.04 and SDdiff = 0.23) measurements. These results can easily be explained from the physical limitations of measuring at the trailing edges. We conclude that the double line pattern seen at the far wall of the extravascular image is representative of the intima-media complex.

Temporal correlation of blood scattering signals in vivo from radiofrequency intravascular ultrasound

One limitation encountered using high frequency intravascular ultrasound (IVUS) is the echogenicity of blood, which increases dramatically at frequencies of 20-40 MHz. Because of the higher velocity of moving blood particles, the echo pattern of flowing blood shows more variations in time than that of the wall. To investigate the time-varying characteristics of the blood scattering measurements were performed on the radiofrequency (RF) data collected in vivo from five pig experiments. After positioning the echo catheter inside the iliac artery, an M-mode sequence of 30 RF traces was acquired at a high pulse repetition rate (5 kHz). The RF correlation time was measured on the regions of blood and the arterial wall. Two processing techniques, temporal averaging and correlation, were tested for suppression of the blood echo intensity. The correlation time Tc measured in the blood region was approximately 1 ms, which was shorter than that measured in the wall region (Tc >> 6 ms). The correlation values calculated in a small window showed a large variation in the blood region while the wall region produced a constant high output. After processing eight consecutive RF traces (delta T = 200 microseconds), the temporal averaging method results in a 50% intensity reduction in the blood region. Using the correlation output as a weighting function, the blood echo intensity can be further reduced to only 10% of its original value. Application of the RF correlation processing to a cross-sectional image data demonstrates the feasibility of this technique to remove most of the blood echoes and enhance the image contrast of the luminal interface.

Transesophageal transducer technology: an overview

Early developments and basic principles in the field of Transesophageal Echocardiography (TEE) probe technology are summarized. Mechanical and electronical sector scanners are compared, and several probe characteristics and image parameters are discussed. A short review of recent developments in TEE is given.

Ultrasound backscatter at 30 MHz from human blood: influence of rouleau size affected by blood modification and shear rate

High frequency intravascular ultrasound may show a high intensity backscatter from blood which hampers the discrimination between lumen and arterial wall. In this study, the acoustic behaviour of blood at 30 MHz in relation to rouleau size was analyzed. In a Couette viscometer, high frequency (20-40 MHz) backscatter data from normal and modified blood samples from eight volunteers were obtained at shear rates from 0 to 1000 s-1. The acoustic behaviour of blood was quantified by the integrated backscatter power and the spectral slope of the backscatter coefficient. Backscatter from blood depended on rouleau size. At a shear rate of zero, both whole blood and rouleau-enhanced blood showed a 11-dB-higher integrated backscatter power than rouleau-suppressed blood, which itself was 10 dB higher than that of hemolysed blood, the latter showing a 6-dB-higher backscatter than saline. Platelets did not contribute to the backscatter power. Plasma and saline produced no detectable integrated backscatter power other than noise. The spectral slope of whole and rouleau-enhanced blood was small (1 and 0.5, respectively), whereas rouleau-suppressed blood and hemolysed blood (both with a slope of 3.3) behaved almost like a Rayleigh scattering medium (slope = 4). The backscatter from rouleau-suppressed blood showed no shear rate dependence. At low shear rates ( < 0.8 s-1 for integrated backscatter power and < 0.2 s-1 for the spectral slope), whole blood and rouleau-enhanced blood tended to the results from the static situation (no shear). At high shear rates ( > 80 s-1 for integrated backscatter power and >11 s-1 for spectral slope), these samples tended to the results of rouleau-suppressed blood. Ultrasound backscatter at 30 MHz from human blood is only caused by red blood cells. With increasing aggregate (rouleau) size, the integrated backscatter power increased by 11 dB, and the spectral slope decreased from 3.3 to 1.

Discrimination of intravascular lumen and dissections in single intravascular ultrasound images using subtraction, conventional averaging and saline flush

With current 30-MHz intravascular ultrasound systems, flowing blood may cause considerable backscatter which in real-time images is characterized by dynamic speckle. However, in a single intravascular ultrasound image (still-frame) the discrimination between arterial lumen and wall may be difficult due to the frozen intraluminal speckle, particularly in the presence of dissections. We compared subtraction, averaging and saline flush as methods to improve the discrimination between arterial lumen and wall in a single image. The real-time images served as gold standard. In 22 patients who underwent peripheral balloon angioplasty, ultrasound images obtained from 84 sites were examined. The sensitivity and specificity of detecting dissections were in the subtraction image 85% and 100%, in the averaged image 57% and 96%, and in the saline flush image 58% and 86%, respectively. Subtraction is a promising method to outline the irregular lumen in a single image.

Intravascular ultrasonography before and after intervention: in vivo comparison with angiography

PURPOSE

To compare the additional capacity of intravascular ultrasonography versus angiography to assess morphologic features and lumen dimension, 37 patients undergoing vascular intervention of the common iliac or superficial femoral artery were studied. A total of 181 ultrasonic cross sections were analyzed (94 before and 87 after intervention).

METHODS AND RESULTS

Before intervention intravascular ultrasonography distinguished normal cross sections (n = 17) from cross sections with a lesion (n = 77): soft (51%) versus hard (31%) lesions, and eccentric (75%) versus concentric (7%) lesions. After intervention intravascular ultrasonography documented dissection (43%), plaque rupture (10%), and internal elastic lamina rupture (8%). A good correlation between ultrasonography and angiography was found for the recognition of eccentric or concentric lesions and dissections. The degree of stenosis was assessed semiquantitatively by visual estimation of the degree of luminal narrowing from the angiograms and intravascular ultrasonic images and was categorized into four classes: (1) normal, (2) less than 50% stenosis, (3) 50% to 90% stenosis, and (4) greater than 90% stenosis. Intravascular ultrasonographic assessment of stenosis was in agreement with angiography in 78% of cases and showed more severe lesions in 22% before intervention. Similar data were observed after intervention, with 72% of results being in agreement and 28% of cases showing more severe lesions. The degree of stenosis was also quantitatively evaluated by computer-aided analysis of the intravascular ultrasonic images. The semiquantitative analysis by intravascular ultrasonography corresponded well with the quantitative analysis done by the computer-aided system. When both echography and angiography suggested that arteries were normal, quantitative intravascular ultrasonography identified lesions that occupied an average of 18% of the cross-sectional area of the vessel.

CONCLUSIONS

This in vivo study shows that intravascular ultrasonography is capable of documenting detailed morphologic features. Semiquantitative ultrasonic data correlate closely with those of angiography, albeit stenoses were assessed as more severe on ultrasonography.

Electrical nerve and muscle stimulation by radio frequency surgery: role of direct current loops around the active electrode

Tissue cutting by electrosurgery is often accompanied with stimulation of nerves and muscles, despite the high frequency of the alternating current being applied. The main source of this stimulation is thought to be the generation of low frequency current by the nonlinear sparking process. However, measurement of this low-frequency current, in the generator electrode's circuit, showed relatively small values, barely sufficient to support this hypothesis. In this study more powerful low frequency current could be identified, indeed also originating from the nonlinear sparking process. Local direct and low frequency currents, at a level of tens of milliamperes, appeared to be generated between different sites of the active electrode-tissue interface. Probably these local currents have not been noticed before as they cannot be detected in the outer chain of generator, electrodes, and connecting wires. This finding may explain why most measures, intended to prevent stimulation by modifying this outer chain, had only limited success.

The angle of incidence of the ultrasonic beam: a critical factor for the image quality in intravascular ultrasonography

The effects of the angle of incidence of the ultrasound beam on the image quality were studied in 21 pressurized arterial specimens examined with a 30 MHz intravascular ultrasonographic catheter. When the ultrasonographic catheter was in an eccentric position in the vessel lumen, the videodensity of the segments of the vessel wall with the least favorable angle of interrogation (a shift of 49 +/- 6 degrees from the tangent to the tissue surface) was 27% +/- 19% lower than the videodensity measured with the catheter in the center of the lumen. When the catheter was placed in a position that was not parallel to the long axis of the vessel, a further decrease was observed, especially in the vessel wall opposite the position of the catheter. An artificial dissection was induced in eight specimens. Dropouts that involved the dissection plane and the underlying structures were produced with positions of the echographic catheter inducing a narrow angle between ultrasound beam and dissection plane. These experimentally induced artifacts were compared with similar findings from the in vivo evaluation of peripheral and coronary arteries. The angle of incidence of the ultrasound beam is a major determinant of the image quality in intravascular ultrasonography. Angle-dependent artifacts occur with eccentric and noncoaxial positions of the ultrasonographic catheter and, in particular, with imaging of large intraluminal dissections. Awareness of this problem may prevent image misinterpretation and has relevance for future improvement of catheter technology and design.

Ultrasound myocardial integrated backscatter signal processing: frequency domain versus time domain

In the literature, different forms of measuring the ultrasound power returned by myocardial tissue are reported. Frequency domain methods will give the maximum frequency information, whereas time domain methods are limited in bandwidth, but more practical to realize. It was the purpose of this study to compare the various methods of signal processing. High frequency ultrasound signals from a pig's myocardium, digitally recorded during normal contractile performance, were analyzed by six different methods of signal processing to obtain estimates of backscatter power. The myocardial tissue characterization parameters studied were the integrated power as well as its cyclic variation during the cardiac cycle. A total number of 8109 ultrasound traces obtained in 16 pigs were processed. The study included three signal processing methods in the frequency domain: frequency compensated integrated backscatter calculated over both a large (4 MHz, method 1) as well as a small frequency bandwidth (2 MHz, method 2) and uncompensated integrated backscatter (method 3), and three methods in the time domain: high frequency signal squared and integrated (method 4), mean rectified signal level (method 5) and mean signal level after logarithmic compression and envelope detection (method 6). The random measurement variation (including beat-to-beat variation) was analyzed as well as the paired differences of the backscatter parameters obtained by the respective methods as compared with the only theoretically correct method in the time domain (method 4). The magnitudes of the random measurement variation expressed as a standard deviation (SD) were comparable (range 0.93-1.2 dB) except for method 6 (0.61 dB), where the measurement variation is decreased by the logarithmic compression.(ABSTRACT TRUNCATED AT 250 WORDS)