Structural evaluation of porcine heart valve prostheses with radiofrequency ultrasound.

BACKGROUND: Bioprosthetic heart valve use is limited by progressive degeneration. Early degenerative changes are often occult, making assessment of tissue integrity difficult. Ultrasound tissue characterization may detect alterations in tissue structure and allow early detection of leaflet degeneration. METHODS: Using a modified echocardiographic unit (Acuson), radiofrequency (RF) integrated backscatter amplitude (IBA) (integral/RF/dt) was measured in 38 leaflets from nine explanted and six control porcine valves. Regions of interest in each leaflet were studied using four ultrasound frequencies. Radiographic gray scale mean and leaflet thickness were measured at each region of interest. Percent collagen and mineral were calculated for each region of interest using color-image processing of histologic sections and compared to IBA. RESULTS: IBA values for control vs. explanted leaflets were (mean value+/-standard deviation): 8.2+/-4.69 dB vs. -4.7+/-4.64 dB at 7.0 MHz; -5.8+/-4.34 dB vs. -3.1+/-5.34 dB at 5.0 MHz; -3.8+/-3.38 dB vs. -2.1+/-3.18 dB at 3.5 MHz; and -9.0+/-4.58 dB vs. -7.1+/-4.25 dB at 2.5 MHz. Collagen content was 27.7+/-8.50% vs. 33.2+/-10.90%, mineral content was 0.1+/-0.10% vs. 2.1+/-4.30%, and radiographic gray scale mean was 150.6+/-1.96 vs. 145.3+/-5.14 for control vs. explanted leaflets, respectively. For control and explanted leaflets IBA, collagen content, mineral content, and radiographic gray scale mean were different (control vs. explanted P<0.05). Leaflet thickness was also noted to be different between the two groups. IBA was different among explanted leaflets with low, medium, and high mineral content. CONCLUSION: IBA was found to be a useful technique to differentiate normal from explanted porcine prosthetic valves in vitro. This method may be useful in the serial assessment of bioprosthetic leaflet degenerative properties in vivo.

A hypothesis regarding vascular acoustic emission accompanying arterial injury induced by balloon angioplasty.

Stress-induced structural damage is often accompanied by sound release. This behavior is known as acoustic emission (AE). We hypothesize that vascular injury such as that produced by balloon angioplasty is associated with AE. Postmortem human peripheral arterial specimens were randomly partitioned into test (n = 10) and control segments (n = 10). Test segments were inserted into a pressurization circuit and subjected to two consecutive hydrostatic pressurizations. Amplitude, frequency, and energy content of the AE signals released during pressurization were quantified. Test and matched control segments subsequently underwent identical histological processing. Pressure-induced tissue trauma was estimated via computerized histomorphometric analysis of the resulting slides (n = 100). Vascular acoustic emission (VAE) signals exhibited an amplitude range of +/- 5.0 mu bars and were observed to occur during periods of increasing intraluminal pressure. The VAE signal power within the monitored bandwidth was concentrated below 350 Hz. More than 25 times as much VAE energy was released during the first pressurization as during the second: 1,855 +/- 513.8 mJ vs. 73 +/- 44.9 mJ (mean +/- SEM, p < 0.006). Estimates of circumferential intimal wall stress at AE onset averaged 170 kPa, slightly below reported values of arterial tissue rupture strength. Histomorphometric estimates of tissue trauma was greater for the test than their matched control segments (p < 0.0001). These preliminary data suggest that detectable acoustic energy is released by vascular tissue subjected to therapeutic stress levels. Histological analysis suggest that the underlying source of sound energy may be related to tissue trauma, independent of histological preparation artifacts. From this preliminary work, we conclude that VAE may be a fundamental property accompanying vascular tissue trauma, which may have applications to improving balloon angioplasty outcomes.

Cannulation of the aortic branches using ultrasound guidance. An animal study.

Catheter placement by ultrasound may reduce radiation, improve positioning, and allow the use of echo contrast agents for diagnostic and therapeutic procedures. To evaluate its utility in the peripheral and coronary vascular beds, a preshaped 20 MHz Doppler catheter was inserted into the femoral artery for renal artery, or into the right carotid artery for left coronary artery cannulation in five dogs. Ultrasonic imaging of the vascular structure and catheter was provided by either transabdominal or transesophageal ultrasound. Using Doppler waveform polarity for retrograde guidance, the catheter was advanced to the region of the left renal or left coronary ostia. Ultrasonic emissions from the Doppler catheter were identified by color Doppler mode of the ultrasound machine and allowed the catheter tip to be identified within the beam width of the scanning transducer, providing the depth dimension. In the two animals in which left renal artery cannulation was attempted, the catheter was successfully manipulated into the ostium. In two of the three animals in which left coronary artery cannulation was attempted, the catheter was successfully manipulated into the ostium, followed by saline contrast injections revealing myocardial perfusion. In addition, in one animal, a Doppler flow wire was identified as it was advanced into the mid circumflex coronary. In conclusion, ultrasonically guided cannulation of aortic branches may be possible without x-ray, and this technique may lead to further use of ultrasound in diagnostic and therapeutic procedures.

In vitro identification of angioplasty-induced injury by use of vascular acoustic emissions.

BACKGROUND: We have developed a novel method of diagnosing stress-induced vascular injury. This approach uses the sound energy released from atherosclerotic arterial tissue during in vitro balloon angioplasty to characterize type and severity of induced trauma. METHODS AND RESULTS: Thirty-two postmortem human peripheral arterial specimens 1.0 cm long were subjected to in vitro balloon angioplasty with simultaneous acoustic emission monitoring. Specimens were examined before and after angioplasty to ascertain the extent of angioplasty-induced injury. Gross observation was used to identify dissection. A three-dimensional intravascular ultrasound reconstruction technique was used to estimate the luminal surface area of the specimen. Change in luminal surface area (postangioplasty minus preangioplasty) was used to quantify induced injury. The energy content and spectral distribution of the digitally acquired vascular acoustic emission (VAE) signals were computed. Comparisons of angioplasty-induced trauma with VAE signal characteristics were made. Dissection (mural laceration of variable depth) was observed in 15 of 32 specimens. Eleven showed no evidence of induced dissection, and 6 had preexisting intimal disruptions. The energy content of the VAE signals collected from specimens with dissection was greater than that obtained from those in which dissection was absent: 845 +/- 89.4 mJ (mean +/- SEM; n = 15) versus 128 +/- 40.8 mJ (n = 1 l; P < .001). Comparison of induced trauma and VAE signal energy demonstrated a proportional relationship (r = .87, P < .001, n = 32). CONCLUSIONS: VAE signals contain information characterizing type and severity of angioplasty-induced arterial injury. Because vascular injury is related to adverse procedural outcome, development of VAE technology as an adjunct to conventional diagnostic modalities may facilitate optimal balloon angioplasty delivery and postprocedural care.

Regional vascular mechanical properties by 3-D intravascular ultrasound with finite-element analysis.

A method employing intravascular ultrasound (IVUS) and simultaneous hemodynamic measurements, with resultant finite element analysis (FEA) of accurate three-dimensional IVUS reconstructions (3-DR), was developed to estimate the regional distribution of arterial elasticity. Human peripheral arterial specimens (iliac and femoral, n = 7) were collected postmortem and perfused at three static transmural pressures: 80, 120, and 160 mmHg. At each pressure, IVUS data were collected at 2.0-mm increments through a 20.0-mm segment and used to create an accurate 3-DR. Mechanical properties were determined over normotensive and hypertensive ranges. An FEA and optimization procedure was implemented in which the elemental elastic modulus was scaled to minimize the displacement error between the computer-predicted and actual deformations. The "optimized" elastic modulus (Eopt) represents an estimate of the component element material stiffness. A dimensionless variable (beta), quantifying structural stiffness, was computed. Eopt of nodiseased tissue regions (n = 80) was greater than atherosclerotic regions (n = 88) for both normotensive (Norm) and hypertensive (Hyp) pressurization: Norm, 9.3 +/- 0.98 vs. 3.5 +/- 0.30; Hyp, 11.3 +/- 0.72 vs. 8.5 +/- 0.47, respectively (mean +/- SE x 10(6) dyn/cm2; P < 0.01 vs. nondiseased). No differences in beta between nondiseased and atherosclerotic tissue were noted at Norm pressurization. With Hyp pressurization, beta of atherosclerotic regions were greater than nondiseased regions: 21.5 +/- 2.21 vs. 14.0 +/- 2.11, respectively (P < 0.03). This method provides a means to identify regional in vivo variations in mechanical properties of arterial tissue.

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.

Confirmation and initial documentation of thoracic and abdominal aortic helical flow. An ultrasound study.

Aortic helical flow may play an important role in plaque deposition, dissection formation, and organ perfusion. The authors have previously demonstrated, using in vitro flow models and transesophageal echocardiography, that helical flow begins in the mammalian aortic arch and continues into the descending thoracic aorta. The purpose of this study was to confirm thoracic aortic helical flow and document its extent into the abdominal aorta using direct measurements. Twelve mongrel dogs underwent surgery with exposure of the abdominal aorta up to the diaphragm. Six of the 12 underwent further thoracotomy with thoracic aorta exposure. Color Doppler ultrasound images were obtained using a 5 megaHz esophageal transducer, hand held, directly applied, and visually aligned for transverse aortic imaging. Helical flow was considered present with the appearance of red/blue hemicircles during a systolic wave when the aorta was imaged transversely. All six dogs that had thoracotomy showed clockwise thoracic aortic helical flow (along the direction of blood flow) at the retro left ventricular region. In all dogs, clockwise helical flow was demonstrated to and immediately beyond the renal arteries. In 11 of 12 dogs, clockwise helical flow was demonstrated 7 cm below the renal arteries. The study confirms the presence of helical flow in the thoracic aorta and documents its extent into the abdominal aorta below the level of the renal arteries. The teleologic flow pattern of mammals may extend to other classes of vertebrates and must be accounted for in studies of endothelial shear and flow separation. In addition, tangential velocities imparted by helical flow may affect organ perfusion.

Myocardial acoustics in pediatric allograft rejection.

BACKGROUND: Ultrasonographic tissue characterization is the assessment of physical properties of biologic tissue on the basis of quantitative analysis of its acoustic characteristics. Abnormalities in microscopic structure that occur with cardiac allograft rejection may result in characteristic alterations in myocardial acoustics. Ultrasonographic tissue characterization may allow noninvasive detection of rejection. METHODS: Findings in 22 pediatric heart transplant patients undergoing routine surveillance for rejection by endomyocardial biopsy were prospectively evaluated. Off-line ultrasonographic tissue characterization analysis was done on transthoracic echocardiograms obtained at each biopsy. Within patients, tissue characterization texture measures derived from the ultrasonographic image data were compared with histologic findings. Univariate multiple regression analysis was used to identify texture measures associated with acute allograft rejection in a subgroup (n = 8) with at least one biopsy-proven episode of moderate rejection. RESULTS: Measures of homogeneity (co-occurrence matrix correlation and heterogeneity (run-length nonuniformity) decreased with moderate rejection (p < 0.03). Homogeneity measures decreased if the patient had a previous episode of rejection. Several measures of heterogeneity (gray level difference and run-length statistics) were affected by the presence of edema. Run-length nonuniformity was the only measure that differentiated moderate rejection from edema. Discriminant analysis on all 22 patients correctly identified 96% of first rejection episodes (sensitivity 80%, specificity 64%), 93% of moderate and severe rejection episodes (sensitivity 71%; specificity 62%), and 69% of all rejection episodes (sensitivity 51%, specificity 91%). CONCLUSIONS: Histologic changes associated with moderate and severe pediatric allograft rejection as reflected by characteristic alterations in myocardial acoustics can be assessed with ultrasonographic tissue characterization. Histologic changes associated with transplantation itself (resolution of rejection and edema) also affect myocardial acoustics and must be taken into account in rejection surveillance.

Computation of vascular flow dynamics from intravascular ultrasound images.

Analysis of three-dimensional velocity profiles and wall shear stress distribution in a segment of an artery reconstructed from in vivo imaging data are presented in this study. Cross-sectional images of a segment of the abdominal aorta in dogs were obtained using intravascular ultrasound (IVUS) imaging employing a constant pull back technique. Simultaneous measurement of pressures distal and proximal to the vessel segment along with gated pulsed Doppler velocity measurements were also obtained. The three-dimensional geometry of the vascular segment was reconstructed from the IVUS images during peak forward flow phase, and a computational mesh was constructed from the data. A quasi-steady analysis of incompressible Newtonian fluid was performed with a finite difference general purpose computational analysis program FLOW3D. The velocity at the inlet and pressure at the outlet measured at the corresponding time (time referenced to ECG) were used to specify the boundary conditions for the computational flow model. The computed results compared favorably with previously reported results. The purpose of the present study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional reconstructions. The method can be potentially employed in analyzing the hemodynamics in the region of atherosclerotic plaques at various stages of development and the reactivity of the vessel in response to pharmacological and mechanical interventions.

Development of inherently echogenic liposomes as an ultrasonic contrast agent.

Ultrasonic contrast agents have been developed for improved assessment of blood flow and tissue perfusion. Many of these agents are not inherently acoustically reflective (echogenic), and nearly all are not suitable for tissue specific targeting. The purpose of this study was to develop acoustically reflective liposomes, which are suitable for antibody conjugation, without using gas or any other agent entrapment. Echogenic liposomes were prepared from phosphatidylcholine (PC), phophatidylethanolamine (PE), phosphatidylglycerol (PG), and cholesterol (CH), using a dehydration/rehydration method. The formulation was optimized for higher acoustic reflectivity by varying the lipid composition. Liposomes were imaged with a 20 MHz intravascular ultrasonic imaging catheter. Echogenicity levels were expressed using pixel gray scale. The presence of PE and PG at specific concentrations improved echogenicity due to their effects on liposomal morphology as confirmed by freeze-etch electron microscopy. The acoustic reflectivity of liposomes was retained when liposomes were treated with blood at room temperature and 37 degrees C under in vitro conditions. It was demonstrated that the liposomes were also acoustically reflective in vivo after they were injected into a miniswine model. We have developed echogenic liposomes that are stable and suitable for tissue specific targeting as a novel contrast agent. This new contrast agent can be used for ultrasonic image enhancement and/or treatment of targeted pathologic sites.

Accurate three-dimensional reconstruction of intravascular ultrasound data. Spatially correct three-dimensional reconstructions.

BACKGROUND: The geometrical accuracy of conventional three-dimensional (3D) reconstruction methods for intravascular ultrasound (IVUS) data (coronary and peripheral) is hampered by the inability to register spatial image orientation and by respiratory and cardiac motion. The objective of this work was the development of improved IVUS reconstruction techniques. METHODS AND RESULTS: We developed a 3D position registration method that identifies the spatial coordinates of an in situ IVUS catheter by use of simultaneous ECG-gated biplane digital cinefluoroscopy. To minimize distortion, coordinates underwent pincushion correction and were referenced to a standardized calibration cube. Gated IVUS data were acquired digitally, and the spatial locations of the imaging planes were then transformed relative to their respective 3D coordinates, rendered in binary voxel format, resliced, and displayed on an image-processing workstation for off-line analysis. The method was tested by use of phantoms (straight tube, 360 degrees circle, 240 degrees spiral) and an in vitro coronary artery model. In vivo feasibility was assessed in patients who underwent routine interventional coronary procedures accompanied by IVUS evaluation. Actual versus calculated point locations were within 1.0 +/- 0.3 mm of each other (n = 39). Calculated phantom volumes were within 4% of actual volumes. Phantom 3D reconstruction appropriately demonstrated complex morphology. Initial patient evaluation demonstrated method feasibility as well as errors if respiratory and ECG gating were not used. CONCLUSIONS: These preliminary data support the use of this new method of 3D reconstruction of vascular structures with use of combined vascular ultrasound data and simultaneous ECG-gated biplane cinefluoroscopy.

In vivo assessment of nonlinear myocardial deformation using finite element analysis and three-dimensional echocardiographic reconstruction.

In vitro data have shown that the myocardium exhibits nonlinear passive stress-strain relationship and a non-linear pressure-volume relationship. A finite element (FE) analysis and optimization algorithm was used on three-dimensional reconstructed left ventricular (LV) geometry using echocardiographic images, along with hemodynamic measurements, in seven closed-chest dogs to show a nonlinear stress-strain relationship in vivo. Our analysis included the computation of Poisson's ratio from the measured volumetric strain of the myocardium and a simulated pericardial pressure load ("equivalent pericardial pressure") applied to the epicardial surface of the reconstructed LV. LV geometry was reconstructed in three or four incremental time steps in diastasis and the myocardium was assumed to be homogeneous, isotropic, and linearly elastic during these short intervals in this initial study. Simultaneous LV chamber pressure and equivalent pericardial pressure were incorporated into the algorithm to predict actual LV expansion. Computations were performed iteratively at each interval to compute the optimized elastic modulus. By performing the FE analysis and optimization at each interval (a step-wise linear analysis approach), a linear relationship between the myocardial elastic modulus and LV chamber pressure was derived (r = .87 to .98). Such a linear relationship is equivalent to an exponential myocardial stress-strain relationship in vivo. Detailed measurement of nonhomogeneous regional deformation are becoming possible with the advent of sophisticated imaging techniques. The methodology described in this study, with appropriate modifications in the FE analysis and optimization algorithms, can be applied to assess the complex three-dimensional pressure-deformation characteristics in vivo.

A method for automatic edge detection and volume computation of the left ventricle from ultrafast computed tomographic images.

RATIONALE AND OBJECTIVES: Detection of endocardial and epicardial borders of the left ventricle (LV) using various imaging modalities is time-consuming and prone to interpretive error. An automatic border detection algorithm is presented that is used with ultrafast computed tomographic images of the heart to compute cavity volumes. METHODS: The basal-level slice is identified, and the algorithm automatically detects the endocardial and epicardial borders of images from the basal to the apical levels. From these, the ventricular areas and chamber volumes are computed. The algorithm uses the Fuzzy Hough Transform, region-growing schemes, and optimal border-detection techniques. The cross-sectional areas and the chamber volumes computed with this technique were compared with those from manually traced images using canine hearts in vitro (n = 8) and studies in clinical patients (n = 27). RESULTS: Though the correlation was good (r = .88), the algorithm overestimated the LV epicardial area by 4.8 +/- 6.4 cm2, though this error was not statistically different from zero (P > .05). There was no difference in endocardial areas (r = .95, P > .05). The algorithm tended to underestimate the end-diastolic volume (r = .94) and the end-systolic volume (r = .94), although these errors were not statistically different from zero (P > .05). The algorithm tended to underestimate the ejection fraction (r = .80), although this error was not statistically different from zero (P > .05). CONCLUSIONS: Automatic detection of myocardial borders provides the clinician with a useful tool for calculating chamber volumes and ejection fractions. The algorithm, with the corrections suggested, provides an accurate estimation of areas and volumes. This algorithm may be useful for contour border identification with ultrasound, positron-emission tomography, magnetic resonance imaging, and other imaging modalities in the heart, as well as other structures.

Atherosclerotic plaque evolution in the descending thoracic aorta in familial hypercholesterolemic patients. A transesophageal echo study.

We explored the concept that transesophageal echocardiography can be used as a tool to detect, characterize, and study plaque morphology in the descending thoracic aorta. The pattern of atherosclerotic plaques in the descending thoracic aorta in familial hypercholesterolemic (FH) patients was evaluated. Additionally, evolution of plaque characteristics as a result of therapy was analyzed. In a randomized prospective protocol, eight FH patients (five men and three women, aged 23 to 65 years [mean +/- SD, 42 +/- 14 years]) receiving standard therapy (n = 3; baseline low-density lipoprotein [LDL] cholesterol, 222 +/- 71 mg/dL, mean +/- SD) or LDL apheresis (n = 5; baseline LDL cholesterol, 262 +/- 51 mg/dL) were studied. Baseline and follow-up (mean, 12 months) transesophageal echocardiographic studies were performed. Measurements obtained were atherosclerotic plaque area (PA), aortic wall area (WA), total arterial area (TAA), and plaque-to-wall area ratio (PWR). LDL cholesterol decreased in both groups. The greatest severity of plaque was detected at 30 to 35 cm from the incisors (approximately 15 to 20 cm from the aortic arch). The smallest plaques were present at the arch and more distal descending aorta. In the control group, TAA, PA, and PWR did not change significantly (P = NS versus baseline). In the LDL-apheresis group, TAA increased (P < .05 versus baseline), PA decreased in three of five patients (P = NS versus baseline), and PWR fell (P < .05 versus baseline).(ABSTRACT TRUNCATED AT 250 WORDS)

Doppler catheter tip localization using color enhancement.

The objective of this research was to determine if the ultrasound emissions of the Doppler catheter can be used to locate its position in 3 dimensions by conventional echocardiography. A Doppler catheter has previously been shown to permit nonfluoroscopic retrograde catheterization of the aortic root and left ventricular chamber by using velocity waveform polarity for directional guidance. A significant difficulty in providing ultrasound catheter guidance, however, has been the inability to recognize the Doppler catheter tip, because each point at which a flexible catheter crosses the image plane can be misinterpreted as the catheter tip. Initial in vitro water bath trials were performed using the Doppler catheter attached to a standard velocimeter. Using a 5 MHz imaging transducer and color Doppler methods, the presence or absence of a banded color pattern which could demarcate the Doppler catheter tip was recorded at various angles in and out of the scanning plane. Using Doppler retrograde guidance and transesophageal echocardiography, color Doppler banded patterns, which could identify the Doppler catheter tip, were investigated in the dog aorta. In order to understand the physical mechanisms involved, a series of water bath trials were then conducted using the Doppler catheter attached to a velocimeter which was synchronized to the echo machine. Initial nonsynchronized water bath trials revealed distinct banded color patterns demarcating the Doppler catheter tip when it pointed in any direction within the beam width, except for a 40 degrees blind cone directly away from the imaging transducer.(ABSTRACT TRUNCATED AT 250 WORDS)

Feasibility of transcolonic and transgastric abdominal vascular ultrasound.

Abdominal vascular ultrasound is hampered by tissue attenuation. To improve imaging, the feasibility of real-time color Doppler transcolonic and transgastric abdominal vascular ultrasound was evaluated. A monoplane 5 MHz transesophageal probe was inserted via a proctoscope in five patients with normal colons. At 25 cm (end of proctoscope), the aortic bifurcation and iliac arteries were dynamically imaged with high resolution. In three patients undergoing abdominal surgery, the surgeon grasped the probe and advanced it further through the descending to the transverse colon to examine further vascular structures. High resolution two-dimensional imaging and color Doppler flow were obtained from the abdominal aorta, the origins and proximal portions of the superior mesenteric artery, and the renal and the inferior mesenteric arteries. Additionally, the inferior vena cava, left renal vein, and portal system were imaged. All vascular structures imaged were within 3 cm of the probe. In two additional patients undergoing abdominal surgery and transesophageal echocardiography, the surgeon grasped the probe in the stomach and placed it on various vascular structures. The results were similar and the celiac trunk was also imaged. This technique, when integrated with fiberoptics for guidance, could provide a method for abdominal vascular ultrasound with less tissue-imposed attenuation, providing high resolution imaging and allowing additional structure recognition. This has potential applications for studying patients with disease of the abdominal aorta or its branches.

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.

Automatic detection of myocardial contours in cine-computed tomographic images.

Quantitative evaluation of cardiac function from cardiac images requires the identification of the myocardial walls. This generally requires the clinician to view the image and interactively trace the contours. This method is susceptible to great variability that depends on the experience and knowledge of the particular operator tracing the contours. The particular imaging modality that is used may also add tracing difficulties. Cine-computed tomography (cine-CT) is an imaging modality capable of providing high quality cross-sectional images of the heart. CT images, however, are cluttered, i.e., objects that are not of interest, such as the chest wall, liver, stomach, are also visible in the image. To decrease this variability, investigators have developed computer-assisted or near-automatic techniques for tracing these contours. All of these techniques, however, require some operator intervention to confidently identify myocardial borders. The authors present a new algorithm that automatically finds the heart within the chest, and then proceeds to outline (detect) the myocardial contours. Information at each tomographic slice is used to estimate the contours at the next tomographic slice, thus allowing the algorithm to work in near-apical cross-sectional images where the myocardial borders are often difficult to identify. The algorithm does not require operator input and can be used in a batch mode to process large quantities of data. An evaluation and correction phase is included to allow an operator to view the results and selectively correct portions of contours. The authors tested the algorithm by automatically identifying the myocardial borders of 27 cardiac images obtained from three human subjects and quantitatively comparing these automatically determined borders with those traced by an experienced cardiologist.

Application of finite-element analysis with optimisation to assess the in vivo non-linear myocardial material properties using echocardiographic imaging.

An application of finite-element analysis with an optimisation technique to assess the myocardial material properties in diastasis in vivo is described. Using the data collected from an animal model, the three-dimensional geometry of the left ventricular chamber, at several times in diastole, was reconstructed. From the measurement of the ventricular chamber pressure during image acquisition, finite-element analysis was performed to predict the expansion during diastasis. Initially, by restricting the motion of the epicardial nodes and computing the reaction forces, an 'equivalent pericardial pressure' was determined and applied in subsequent analysis. The duration of diastasis was divided into three or four intervals and the analysis was performed at each interval to assess the material properties of the myocardium. Using such a step-wise linear approach, the non-linear material properties of the myocardium during passive expansion was determined. Our results demonstrated that the computed 'equivalent pericardial pressure' increased with and was smaller than the corresponding left ventricular chamber pressure. The passive myocardium exhibited a linear tangent modulus against chamber pressure relationship which is equivalent to an exponential stress/strain relationship, similar to those suggested by in vitro studies.