Intravascular ultrasound imaging for guidance of atherectomy and other plaque removal techniques

Intravascular Ultrasonography

The intravascular ultrasonographic findings in normal and diseased veins after transfemoral catheterization of 25 patients are presented. The iliac veins, the inferior and superior vena cava, the renal veins, the right atrium, both brachiocephalic veins, and the right internal jugular vein were studied. In 4 cases valves or valvelike structures were observed. Anatomic variants such as spurs and webs were seen in 3 and mural thrombi or postthrombotic wall changes in 4 patients. In one case a sphincter-like ostium venae cavae was observed. In 2 patients thin filaments within the right atrium, most likely a Chiari net, were seen. Two patients had an abnormal, stratified “artery-like” vessel wall structure. This new imaging modality has several potential applicabilities in the veins and may contribute new information about anatomy and function of the venous system. It provides cross-sectional in vivo visualization and the demonstration of motility of small intraluminal structures which cannot be revealed by traditional diagnostic methods. Because of the wide range of anatomic variation in the venous system, knowledge of its normal intravascular ultrasonographic appearance is a prerequisite for further clinical investigations.

Intravascular Ultrasonographic Appearance of Angiographically Normal Arteries Related to Age and the Occurrence of Vascular Disease

In 22 individuals (mean age 52 years) the ultrasonographic images of arteries defined as normal by arteriography were studied and related to the age and medical history of the patients. The series was divided into 2 groups: patients with clinical manifestation of atherosclerosis and patients without a history of arterial disease. The study included 6 young patients (mean age 14 years) referred for angiographic documentation of abolished intracranial circulation. A typical 3-layered appearance of the arterial wall was found in young healthy individuals as well as in adult and elderly subjects. There was no difference in the ultrasonograhic appearance of muscular and elastic arteries. In patients with extensive obstructive atherosclerosis affecting other parts of the arterial tree, a segment of the iliac artery can have the same appearance as seen in young healthy individuals. There are indications that severe hypertension can result in a thickening of the middle low-echogenic layer of the arterial wall. In patients with chronic renal insufficiency, small calcifications in the middle layer were a typical finding.

Angiographic and Intravascular Ultrasonographic Findings after Endovascular Stent Implantation

The study was an attempt to evaluate the benefit of intravascular ultrasound imaging (IVUS) as a supplement to follow-up angiography after endovascular stent implantation. A consecutive series of 15 patients underwent stent implantation in the peripheral or coronary arteries. Ten Palmaz stents, 3 Palmaz-Schatz stents and 2 Wallstents were used. After a period from 1 to 6 months (mean 3.2 months) follow-up angiography was performed. In 12 cases the angiography was combined with IVUS of the stent and the adjacent vessel segments. In one case IVUS failed due to the tortuous course of the vessel, in another case the stent was occluded, and in one case IVUS was considered too hazardous. In stents of diameter ≥5 mm, ultrasound (US) did not reveal more information concerning vessel and stent diameter, stent stenosis and intraluminal surface contact than angiography alone. Smaller stents were insufficiently visualized by conventional radiologic methods. In small stents only IVUS permitted an exact stent identification and differentiation between stent stenosis and stenosis of the native vessel. At US imaging artifacts, caused by the highly reflectant metallic stent struts, interfered with the native vessel wall and partly obscured its structural details.

Catheter-based intraluminal sonography

With the development of interventional and minimally invasive surgical techniques in the last decade, a strong interest in intraluminal sonography has arisen because of the need for better imaging information and management of the interventional procedure. High-resolution intraluminal sonography is a unique approach for the evaluation of a wide range of abnormalities within the luminal structures throughout the body. This imaging technique has been able to obtain information not available with even the most sophisticated percutaneous sonography, CT, or MRI. The uniqueness of this approach has led to extensive research, establishing a variety of clinical applications. These miniature catheter-based transducers have become important supplemental tools in the evaluation of the urinary and gastrointestinal tracts. Other areas need to be evaluated more thoroughly before efficacy is established, but the concept of using miniature transducers has shown promise in many areas of the body. This should lead to the provision of important information for decision making relative to patient care and surgical intervention. In the future, with projected technical progress, intraluminal sonography should substantially improve its diagnostic capabilities.

Intravascular ultrasound evaluation of peripheral arterial stent-grafts

RATIONALE AND OBJECTIVES

To evaluate neointimal hyperplasia, plaque distribution, and morphologic features of peripheral arterial stent-grafts with intravascular ultrasound (IVUS).

METHODS

Twenty-three patients with stenoses or occlusions of the pelvic or femoral arteries were treated with 31 stent-grafts. Angiography and IVUS of the stented artery were performed 13.9 +/- 9.7 months after stent implantation. Maximum in-stent restenosis was measured by IVUS. Plaque composition and lesion topography were also assessed.

RESULTS

The maximum in-stent restenosis was 53.2 +/- 26.5% for the femoral and 14.2 +/- 10.1 for pelvic arterial stent-grafts. Predilection sites of maximum neointimal tissue accumulation were the edges of the femoral stent-grafts. Only small amounts of neointimal hyperplasia were found in the stent-graft edges. No predilection site for maximum in-stent restenosis was found for the pelvic arterial stent-grafts.

CONCLUSIONS

Predilection sites of maximum in-stent restenosis were the edges of femoral stent-grafts in contrast to pelvic stent-grafts. Femoral stent-grafts showed significantly higher graded stenoses with IVUS than iliac stent-grafts. The authors' findings at IVUS did not change the treatment plan in these patients treated with stent-grafts.

Impact of residual plaque burden on clinical outcomes of coronary interventions

In this study, we summarize the role of residual plaque burden, as determined by intravascular ultrasound, on the development of restenosis following percutaneus coronary interventions. Several clinical trials have shown that the amount of residual plaque is a consistent and independent predictor of subsequent restenosis. The impact of residual plaque burden on late lumen loss is particularly augmented by negative vessel remodeling that is commonly seen after balloon angioplasty and atherectomy. However, early evidence suggests that the importance of plaque burden also applies in the context of stenting. The cotreatment of debulking may further improve the long-term outcome of stenting by maximizing an acute lumen gain with less vessel stretching, preventing stent edge problems and possibly reducing the cell source involved in the intimal hyperplastic process. Evaluation of residual plaque burden with on-line intravascular ultrasound could lead to definitive therapies via risk stratification of the treated segments.

2-D and 3-D endoluminal ultrasound: vascular and nonvascular applications

Endoluminal ultrasound using catheter-based transducers has been used for the evaluation of a wide range of abnormalities. To date, one of the most promising areas of clinical application is its use intravascularly for quantitating the degree of arterial stenosis and for monitoring the therapeutic effects of angioplasty in peripheral and coronary arteries. Uses in the gastrointestinal tract include quantification of the size and wall thickness of esophageal varices, distinguishing between various submucosal lesions and measuring the degree of fibrosis in scleroderma. In the genitourinary system, endoluminal ultrasound provides a unique intraoperative tool allowing the addition of a third dimension (depth) to endourological procedures. The indications for, and the use of, endoluminal ultrasound within the upper urinary tract can be expected to increase with more experience, and the procedure has become an important technique that yields information not available through other modalities. In the bronchotracheal tree, endoluminal ultrasound allows imaging and subsequent biopsy of lymph nodes and tumors that cannot be visualized at routine bronchoscopy. Three-dimensional (3-D) reconstruction of two-dimensional (2-D) ultrasound imaging is a new method in the evolution of intraluminal imaging. It provides information about spatial relationships of anatomic structures that cannot be evaluated using conventional 2-D imaging. Although still in its infancy, 3-D endoluminal ultrasound has the potential to become a dynamic tool in both the research and clinical areas.

Clinical feasibility of 0.018-inch intravascular ultrasound imaging device

OBJECTIVES

Intravascular ultrasound imaging (IVUS) is limited by the size of the imaging catheter. To facilitate imaging before and during interventions, a 30-MHz ultrasonic imaging device was developed that is the same dimension as a 0.018-inch guide wire. The purpose of this study was to evaluate the clinical feasibility of this device.

METHODS AND RESULTS

The imaging core was tested in 8 patients with the use of a monorail guiding sheath that was advanced through a 7F catheter. In addition, after coronary interventions, the standard guide wire was removed, the imaging core was placed inside a compatible balloon, and imaging was performed. In 4 patients, imaging was also performed with a standard 3.2F IVUS catheter. The lumen-plaque interface and the media-plaque interface were clearly visualized in all patients. There was no detectable loss in image quality between the new imaging device and the larger IVUS catheter, and measurements of lumen cross-sectional area were not statistically different.

CONCLUSIONS

Improvements in manufacturing technology have permitted the development of a mechanically rotating ultrasound imaging core 0.018 inches in diameter. It is compatible with current balloon catheters without degradation of image quality.

Are soft echoes really soft? Intravascular ultrasound assessment of mechanical properties in human atherosclerotic tissue

To examine the accuracy of intravascular ultrasound (IVUS) in assessing the biophysical properties of atherosclerotic plaque, 33 human iliac arteries were imaged with a 25 MHz IVUS transducer and classified into four groups on the basis of IVUS appearance: minimally diseased arterial wall, bright echogenic plaque with acoustic shadowing, bright echogenic plaque without shadowing, and hypoechogenic plaque (so-called "soft echoes"). The hardness of each plaque was assessed with an ultrasensitive compression ergonometer. The radial static stress-strain relations fit well (r > 0.98) to exponential curves, providing a compression stiffness constant (K) defined as the coefficient of the exponential power. K for bright echogenic plaque with shadowing was significantly greater than that of the other tissues. However, K among minimally diseased entire arterial wall, hypoechogenic plaque, and bright echogenic plaque without shadowing was not significantly different, but these tissues are not physically soft compared with adipose tissue. Therefore, tissue characterization by IVUS distinguishes calcified from noncalcified plaque and accurately predicts its biomechanical hardness. However, soft echoes, although less firm than calcium, do not necessarily correspond to soft tissue.

Comparison of balloon angioplasty and Simpson atherectomy for lesions in the femoropopliteal artery: angiographic and clinical results of a prospective randomized trial

PURPOSE

This study involves a prospective randomized trial comparing clinical and angiographic results of balloon angioplasty (BA) and Simpson directional atherectomy (DA) in patients with short lesions in the femoropopliteal artery causing symptoms of intermittent claudication.

MATERIALS AND METHODS

Thirty-five patients were treated with BA and 38 with DA. Procedural complications were seen in eight patients. Residual stenoses immediately after the procedure with between 30% and 50% diameter reduction (DR) were observed in three patients after BA and in five patients after DA. In all other patients, residual stenosis was less than 30% DR. Two study end-points during a 2-year follow-up were used: the angiographic occurrence of restenosis with a DR of 50% or greater or the recurrence of symptoms.

RESULTS

Clinical success after 2 years, according to the criteria of the Society for Vascular Surgery/International Society for Cardiovascular Surgery, was seen in 79% of the BA patients and 56% of the DA patients (P = .07). The 2-year primary angiographic patency rates were 67% in patients treated with BA and 44% in patients treated with DA (P = .06). The secondary angiographically determined patency rates were 80% and 65%, respectively (P = .15).

CONCLUSION

Simpson atherectomy is an interventional technique to treat arterial lesions in the femoropopliteal artery with an acceptably low complication rate. The clinical and angiographic results of DA and BA are comparable. DA should not be used to replace BA for routine treatment of short femoropopliteal lesions.

Comparison of intravascular ultrasonography and intraarterial digital subtraction angiography after directional atherectomy of short lesions in femoropopliteal arteries

PURPOSE

In this study a group of patients undergoing directional atherectomy for localized occlusive disease in the femoropopliteal arteries, the value of intravascular ultrasonography (IVUS) to improve the efficacy of plaque removal was evaluated. The findings obtained by IVUS were correlated with intraarterial digital subtraction angiography (IA DSA) performed during the procedure. In addition, the patency rates at follow-up in patients undergoing atherectomy with and without IVUS were compared.

METHODS

Forty patients were treated by atherectomy because of segmental lesions of the femoropopliteal arteries causing intermittent claudication. Twenty-two patients underwent atherectomy, guided by biplane IA DSA only, and 18 patients were also studied by IVUS. The groups were divided by means of consecutive presentation, IVUS being used in the second part of the study period. The median follow up was 16 months (range, 0 to 40 months). Variables, measured by IVUS during the procedure, were the minimal transverse luminal diameter (MTLD) and the free luminal area. Patency rates at follow-up were determined by regular color flow duplex examinations. Color-flow duplex criteria for occlusions were absence of arterial flow and, for stenosis, a ratio of peak systolic velocities at the diseased segment and a normal segment of 2.5 or greater.

RESULTS

Qualitative IVUS assessment prompted additional atherotome passages because of insufficient atheroma removal or nonaesthetic appearance of the vessel lumen in 15 of the 18 patients who underwent this examination. Only in four of these patients would abnormalities at IA DSA have been a reason for further attempts of atheroma removal. As for the quantitative findings during AT, after a first series of atherectomy passes the mean MTLD of the reference lesion resulted in an increase of the MTLD from a mean of 3.3 +/- 0.7 mm to 3.7 +/- 0.6mm (p = 0.001), and the free luminal area increased from a mean of 11.2 +/- 4.8 mm2 to 12.5 +/- 4.5 mm2 (p = 0.001). However the occurrence of restenosis during follow-up was comparable in patients monitored during the intervention by IVUS (1-year patency rate, 57%) and patients not studied by IA DSA only (1-year patency rate, 64%). In addition, the presence of an intimal dissection or a plaque rupture at IVUS examination did not predict restenosis.

CONCLUSIONS

The application of IVUS resulted in an improved luminal enlargement by directional atherectomy but not in a better 1-year patency rate.

Future challenges to coronary angioplasty: perspectives on intracoronary imaging and physiology

Several intravascular techniques have been developed with the purpose of achieving optimal guidance for treatment during coronary angioplasty (PTCA). Although the coronary angiographic technique is well established, there are still some inherent limitations. Due to intimal rupture, tears, dissection and thrombus following PTCA treatment, angiography does not allow exact delineation of the true borders of the vessel. Coronary angioscopy is currently the most sensitive method to detect coronary thrombus and can also be used to classify atheromatous plaques. Furthermore, coronary dissection can be detected with more accuracy than with angiography. One limitation associated with angioscopy is the need to occlude the vessel during imaging, which may create myocardial ischaemia. Furthermore, there is presently no method for quantifying angioscopic findings. Intravascular ultrasound produces a cross-sectional image of the vessel, which permits analysis of the layers of the vascular wall. Characterization and classification of various types of plaque can be made because thrombus, lipid, fibrous tissue and calcium have different ultrasonic echogenicity. Flow velocity measurement with the Doppler technique is an interesting approach to the physiological assessment of coronary stenoses. Coronary flow reserve can be estimated with this method and monitoring of the flow signal following angioplasty will aid in the diagnosis of flow-limiting complications. The trans-stenotic pressure gradient is a valuable measure of the haemodynamic importance of a coronary lesion. Trans-stenotic gradients during maximal hyperaemia obtained with a miniaturized pressure transducer yield reliable information regarding the severity of the stenosis, and the pressure values may be used to calculate the relative coronary flow reserve. In conclusion, all of these intracoronary diagnostic techniques will to some extent play a role in the future of coronary angioplasty. Safety, cost and complexity are some of the factors which will determine the growth potential of each method.

Predictors of restenosis: a morphometric and quantitative evaluation by intravascular ultrasound

Despite advances in catheter-based interventional techniques, restenosis remains a major complication of angioplasty. Recently, intravascular ultrasound imaging (IVUS) has provided new insight into plaque composition and geometric distribution inside the vessel. To investigate if IVUS-defined parameters can predict restenosis in patients after coronary angioplasty, we performed IVUS in 33 patients (33 lesions) after balloon angioplasty (n = 25) or directional atherectomy (n = 8). Qualitative analysis included assessment of plaque composition, plaque eccentricity, plaque fracture, and presence of dissection. In addition, minimal luminal diameter, percent diameter stenosis, percent area stenosis, plaque burden, and elastic recoil were quantitatively analyzed. Follow-up data were obtained 1, 2, and 6 months after angioplasty and were available for 30 patients. Angiographic restenosis occurred in 11 patients (group 1), and no restenosis occurred in 19 patients (group 2) by clinical (n = 10) or angiographic (n = 9) assessment. Plaque fracture was noted in 30% of group 1 patients and 74% of group 2 patients (p = 0.04). Major dissections were more frequent in group I than in group II (78% vs 10%, p = 0.009). Of the quantitative parameters analyzed, plaque burden was significantly higher in group 1 than in group 2 (0.50 +/- 0.05 vs 0.34 +/- 0.05, p = 0.0001). In 78% of the patients with plaque burden of > 0.40, restenosis developed. Thus, of the various parameters analyzed, the absence of plaque fracture, the existence of a major dissection, and greater plaque burden were associated with increased incidence of restenosis. Our results indicate that IVUS can identify a subset of patients in whom restenosis is likely to develop. Information about the morphologic features of the atheroma and its composition may be used to modify the interventional strategy and thus optimize lumen size and possibly reduce the chance of restenosis.

A new laser-ultrasound transducer for medical applications

A new type of probe is presented, which combines an ultrasound receiver constructed from piezoelectric polymer materials with a coaxial optical fibre for laser-induced ultrasound. The initial application of this probe, which may be further miniaturized, is in intra-arterial imaging and therapy. Overall diameters of the prototype probes presently range from 2.5 mm to 10.0 mm. Pulsed laser beams are introduced through the optical fibre to form a photoacoustic source within the material under investigation. Ultrasonic waves are induced in the material due to the photo-acoustic effect and received on return by a forward-looking piezoelectric polymer transducer. In principle, other high-power laser beams can also be introduced through the same optical fibre for therapeutic use. Initial development and characterization of this combined probe is reported in this paper, together with some results.

In vitro assessment of luminal dimensions of coronary arteries by intravascular ultrasound with and without application of echogenic contrast dye

To evaluate the impact and limitations of intracoronary ultrasound in the assessment of lumen, we examined 80 segments of 20 isolated coronary arteries with a mechanical ultrasound device (CVIS) comparing the results of ultrasound with the corresponding histological specimens. Ultrasound was performed with and without echogenic contrast dye (Laevovist, Schering AG, FRG). After application of contrast dye, correlation of luminal area between histology and ultrasound was improved from r = 0.85 to r = 0.89 (p = ns). Accuracy of lumen measurements was low in segments < 2.5 mm; only after application of contrast dye a relationship between ultrasound and histological measurements was found. In all cases in which a deviation of more than 20% between ultrasound and histology was observed, this deviation could be reduced by the application of contrast dye. There are considerable limitations in the accuracy of ultrasound measurements in the near field. Thus, further improvement of intracoronary ultrasound devices is mandatory. However, with the use of the currently available systems, additional application of echogenic contrast dye can improve accuracy of luminal measurements, especially in smaller size vessels.

Ultrasonic Doppler detection of laser-tissue interaction

Laser therapy for the safe and effective coagulation and ablation of tissues requires precise control of the amount of energy delivered to and absorbed by the volume of tissue of interest. We propose that an ultrasonic transducer pointing in the same direction as the laser fiber could be used to monitor the changes in ultrasonic properties caused by the absorption of light by the tissue. A modified 20 MHz pulsed Doppler was used to evaluate the ultrasonic effects of 35 exposures of beef liver and muscle to a high power diode laser in vitro in real time. We found two distinct levels of acoustic activity in the tissue. Type 1 activity consisted of slow variations in the phase and small changes in the amplitude of the echoes, while type 2 activity consisted of large and rapid fluctuations in amplitude and phase. We found that the residual increase in echogenicity of the tissue and the delay to the onset of type 2 activity were functions of laser power and tissue type and were correlated to lesion severity. We hypothesize that type 1 activity corresponds to motion (thermal expansion and contraction) of the tissue, and that type 2 activity corresponds to the creation of gas bubbles (vaporization) in the tissue. We conclude that the absorption of energy changes the acoustic properties of tissue during and after exposure and that Doppler signal processing can be used to differentiate various levels of laser-tissue interaction in real time.

Applications of coronary flow velocity during angioplasty and other coronary interventional procedures

Previous studies utilizing Doppler catheters to assess blood flow immediately following coronary artery interventions have failed to demonstrate significant improvement in proximal coronary artery velocities or vasodilator reserve. Measurement of blood flow velocity, flow reserve, and the phasic diastolic/systolic velocity ratio in the distal coronary artery has been performed following various interventional procedures utilizing a low profile (.018 in) Doppler angioplasty guidewire. Following balloon angioplasty in 38 patients, average peak velocity increased significantly from 19 +/- 12 to 35 +/- 16 cm/sec in the distal coronary artery. The diastolic/systolic flow ratio improved from 1.3 +/- 0.5 to 1.8 +/- 0.5 and coronary flow reserve remained unchanged. Similar improvement in distal mean velocities (200%) versus proximal mean velocities (90% increase), and improvement in phasic velocity patterns, total velocity integral, and peak diastolic velocity were also noted in a separate study of 29 patients, before and after balloon angioplasty. Following excimer laser angioplasty in 10 patients, average peak velocity in the distal coronary artery was noted to increase from 6.3 to 13.0 cm/sec following laser alone, with a further increase to 20.6 cm/sec following adjunctive balloon angioplasty. The diastolic/systolic flow ratio increased from a mean value of 1.1 to 2.0 following laser recanalization, with a further increase to 2.9 following adjunctive balloon angioplasty. Following directional coronary atherectomy only modest improvement in distal average peak velocity was noted (24.7 to 31.2 cm/sec), with no significant change in diastolic/systolic velocity ratio (1.78 vs 2.04) immediately following the procedure. Measurement of distal flow velocity parameters performed immediately following coronary interventions may prove useful in assessing the hemodynamic result of these interventions and may prove useful in clinical decision making.

Comparison of two mechanical ultrasound devices with and without echogenic contrast dye

To evaluate the impact and limitations of intracoronary ultrasound, 64 segments of 13 isolated coronary arteries were examined and 2 mechanical devices (device A, 30 MHz 5Fr [CVIS] and device B, 20 MHz 4.8Fr [Diasonics/Boston Scientific] were compared with the corresponding histologic specimens. Luminal dimensions were assessed with and without echogenic contrast dye (Laevovist, Schering AG, Germany). After application of contrast dye, correlation of luminal area between histology and ultrasound was improved from r = 0.62 to r = 0.77 (device B; p < 0.05) and from r = 0.82 to r = 0.88 (device A; p = NS). Low accuracy of lumen measurements in segments < 2.5 mm could be improved by application of contrast dye. The number of quadrants in which wall thickness measurements were impossible was significantly higher for device B (n = 56; 22%) than for device A (n = 28; 11%); p < 0.01. This may be due to the different ringdown diameters of both systems (B, 2.6 mm; A, 2.0 mm; p < 0.0001). In assessing wall thickness only in segments of > 2.5 mm, a reliable correlation between ultrasound and histology was found (A, r = 0.80; B, r = 0.60). Sensitivity of plaque (n = 51) detection was lower for device B (63%) than for device A (82%, p < 0.05), and measurements correlated with histology only for device A. There are considerable differences in the accuracy of ultrasound measurements between mechanical systems. Nevertheless, additional application of contrast dye can improve accuracy of luminal measurements, especially in smaller vessels.

Artifacts in intravascular ultrasound imaging: analyses and implications

The ability of an intravascular ultrasound catheter to give cross-sectional images of vessel walls and surrounding tissues, and the behavior of ultrasound in heterogeneous media, are at the origin of degradation of image quality. Qualitative and quantitative analyses of in vivo studies are then operator-dependent and are limited by artifacts. We investigated these limitations by an in vitro study on plexiglass phantoms and segments of fresh arteries. We used a 20 MHz transducer mounted on the tip of a 4.8 F catheter and an interventional ultrasound system. The ultrasound beam is reflected onto the rotating transducer at 600 rotations per minute (RPM), creating 360 degrees real-time images (10 images/second). We then observed, analyzed and interpreted the most specific reasons for image artifacts: geometric distortions, multiple echoes, the point spread function (PSF) of the imaging system, near-field effects, "petal-shaped" effect, and ultrasound speckle. Various practical implications have resulted from this study. Only a thorough knowledge of how to avoid some of the most obvious pitfalls will enable the user to obtain maximum benefits from intravascular ultrasound imaging, and to appreciate its limitations.

Intravascular ultrasound and intracardiac echocardiography: concepts for the future

Intravascular ultrasound imaging is a catheter-based method that has been shown to be better than contrast angiography in the detailed assessment of coronary and peripheral arterial atherosclerotic lesions, arterial dissections and clots, aortic and pulmonary arterial disorders, and the effects and complications of interventional therapy in various vascular beds. Evaluation of refined ultrasound catheters, combined imaging and therapeutic devices, and off-line 3-dimensional reconstruction capabilities indicates that intravascular ultrasound could provide guidance during various catheter-based therapeutic procedures. Early experience with intracardiac echocardiography suggests that this technique could evolve as a clinically useful method with diagnostic, monitoring, and guidance applications. Future directions for further development include designing of miniaturized imaging devices, "look-forward" imaging devices, low frequency devices for whole heart imaging, multifrequency ultrasound catheters and multifunction ultrasound consoles, and approaches to real-time 3-dimensional imaging, on-line tissue characterization, automated acoustic quantification and tissue characterization, the study of myocardial perfusion, and catheter-based acoustic myocardial biopsy. These developments could lead to the conversion of catheterization laboratories into integrated imaging, monitoring, and therapeutic stations. In addition, continuous monitoring of cardiac function could be possible in the critical care unit and in the operating room as well. Ongoing advances in catheter technology and image processing indicate that these concepts are within the realm of becoming reality.