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Biomedical subjects

N Bom

Publications and source records attributed to N Bom.

At least 55 records · Page 3Linked to original sources
PubMed1996

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.

Animals↗
PubMed1996

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.

Angioplasty, Balloon↗
PubMed1996

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.

Adult↗
PubMed1995

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.

Blood↗
PubMed1995

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.

Aged↗
PubMed1995

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.

Echocardiography, Transesophageal↗
PubMed1993

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.

Aortic Dissection↗
PubMed1993

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)

Animals↗
PubMed1993

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.

Aged↗
PubMed1993

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.

Catheter Ablation↗
PubMed1992

Absorption and scatter of encapsulated gas filled microspheres: theoretical considerations and some measurements.

Albunex is an ultrasound contrast agent for use in echocardiology and other areas. It is capable of passing the lung circulation after intravenous injection. A theoretical model is developed for some acoustic properties, particularly the scatter and absorption, of this contrast agent, considering the individual microspheres as air bubbles surrounded by a thin shell. The attenuation, the sum of absorption and scatter, of this contrast medium is measured with five transducers to cover the frequency range from 700 kHz to 8.5 MHz. It is concluded that the model correlates well with these acoustic measurements. When Albunex is used intravenously the backscatter enhancement in the left ventricle is caused mainly by the microspheres with diameters between 5 and 8 microns.

Acoustics↗
PubMed1992

Electrical impedance of layered atherosclerotic plaques on human aortas.

Electrical impedance measurements were performed on 13 atherosclerotic human aortic segments at 67 measuring spots in order to determine whether or not on the basis of these data a distinction can be made between atherosclerotic lesions and normal tissue. Stenosis localization and guidance of interventional techniques could be among the applications of an impedance measuring technique implemented on a catheter system. The experimental results, obtained with a two-electrode measuring technique, show that the apparent resistivity of an atherosclerotic spot does not necessarily deviate much from the resistivity of normal tissue. This is clarified by histology which shows that the majority of lesions has a surface layer of connective, fibrous tissue having almost similar conducting properties as the normal arterial wall. For gaining a deeper understanding of the way in which the measured data come about, a physical model of an atherosclerotic lesion is presented and confronted with the data. Both experimental data and theoretical considerations lead to the conclusion that only when the superficial fibrous layer is absent or very thin in relation to the size of the measuring electrode, the measured resistivity at a lesion is much higher than at normal spots. This occurs as a consequence of the high ohmic properties of the calcified or lipid deposits in the atherosclerotic lesion.

Aortic Diseases↗
PubMed1991

Assessment of medial thinning in atherosclerosis by intravascular ultrasound.

This study investigated the in vitro (40 MHz) and in vivo (30 MHz) feasibility of intravascular ultrasound to document the influence of atherosclerotic lesions on the typical 3-layered appearance of muscular arteries. The in vitro images of 39 arteries were compared with the corresponding histologic sections. Media and lesion thickness were measured at the areas of minimal and maximal lesion thickness. The median media thickness was 0.8 mm in the absence of a lesion, decreasing to 0.3 mm in the area of maximal atherosclerosis. The ultrasonic data correlated closely with histologic measurements (0.6 and 0.3 mm, respectively). The in vivo study was performed in 29 patients undergoing coronary or peripheral vascular procedures. A total of 150 still-frames were selected for quantitative analysis. The median media thickness was 0.6 mm in the absence of a lesion, decreasing to 0.1 mm in the area with maximal atherosclerosis. This study revealed that intravascular ultrasound imaging accurately determines that media thickness of muscular arteries is inversely related to lesion thickness. In vitro data, verified with histology, can be translated to humans in vivo.

Arteries↗
PubMed1991

Assessment of arterial disease and arterial reconstructions by intravascular ultrasound.

Clinical application of intravascular ultrasound to assess arterial atherosclerotic disease was introduced in humans after extensive in vitro and in vivo animal studies. Real-time images, obtained with a 30 MHz element mounted on a 5 F catheter, consistently confirmed angiographic images, up till now considered to be the gold standard. In addition to these data, ultrasonic cross-sectional imaging provided information on the composition of atheroselectic lesions and the size and shape of the lumen. Based on the experimentally derived criteria for tissue characterization, a better insight into arterial morphology could be obtained, allowing improved planning of interventional or reconstructional procedures. Moreover intravascular ultrasound has proved valuable as a post-interventional procedure to monitor and assess the quality of interventional results. The ultrasound images are clearly superior to angiographic studies, albeit the ultrasonic information is an adjunct to angiography and, as yet, not a substitute. We present our initial experience with intravascular ultrasound obtained in patients with substantial peripheral arterial disease.

Angioplasty, Balloon↗
PubMed1991

Imaging of post-mortem coronary arteries by 30 MHz intravascular ultrasound.

Intravascular ultrasound cross-sectional imaging of the atherosclerotic artery may be employed to guide plaque ablation by spark erosion, atherectomy, laser irradiation or other means. To assess whether in the coronary artery the echolucent zone of the three-layered ultrasound appearance of muscular arteries might be a reliable boundary to halt transluminal ablative angioplasty, epicardial coronary arteries were scanned under pressure over a distance of 4-8 cm at 1 mm intervals. A 5.2F catheter with a 30 MHz transducer rotating at 600 RPM was used. In 419 images from four post-mortem hearts, aged at death 22, 31, 56 and 82 years, the presence of a distinct echolucent zone was scored as percentage of the wall circumference. The median scores were 0%, 0%, 10% and 75%, respectively. In the three youngest hearts, the three-layered appearance was largely absent. In the 82 year old heart, in contrast, the three-layered appearance could generally be identified. Within subjects, the presence of an echolucent zone was variable in location. Qualitatively, neither abundant medial elastin tissue nor a reduced medial thickness could explain the absence of an echolucent zone when the ultrasound image was compared to the corresponding microscopic section. The results show that in the 82 year old heart, 30 MHz intravascular ultrasound discrimination of coronary wall layers would be adequate to guide transluminal plaque ablation. In the younger hearts, guided ablation would fail due to the limited presence of a distinct echolucent zone as an ultrasound landmark of the media. Whether the presence or absence of an echolucent zone is related to age remains to be determined.

Adult↗