The effect of vascular curvature on three-dimensional reconstruction of intravascular ultrasound images

In vivo comparative study of linear versus geometrically correct three-dimensional reconstruction of coronary arteries

Although conventional linear 3-dimensional (3D) reconstruction of coronary arteries by intravascular ultrasound has been widely used for the assessment of plaque volume and progression; the volumetric error (VE) that is produced has not been adequately studied. Linear and geometrically correct 3D reconstruction was applied in 16 coronary arterial segments from 9 patients. Using geometrically correct reconstruction as reference, VE was assessed in 1-mm-long arterial slices. Although for the entire length of the coronary arteries VEs for lumen, external elastic membrane (EEM), and intima-media volumes were minimal (lumen VE 0.4%, -0.8 to 1.8; EEM VE 0.3%, -0.9 to 1.9; intima-media VE 0.4%, -1.4 to 2.2), the VE in each arterial slice exhibited a large variation from -15.6% to 36.2% for lumen volume, from -12.9% to 33.1% for EEM volume, and from -17.2% to 46.7% for intima-media volume, suggesting that linear reconstruction over- or underestimates the true arterial volumes. Lumen VE, EEM VE, and intima-media VE were also significantly higher in curved arterial subsegments than in relatively straight arterial subsegments (p <0.05). In conclusion, in highly curved arterial subsegments, the VE that is produced by linearly stacking the intravascular ultrasound images may be not negligible. Geometrically correct reconstruction of coronary arteries provides more reliable arterial reconstructions and plaque volume measurements. It is anticipated that clinical application of this technique will contribute to more accurate follow-up of the progression of atherosclerosis and assessment of arterial remodeling.

Automated three-dimensional assessment of coronary artery anatomy with intravascular ultrasound scanning

BACKGROUND

Angiography allows the definition of advanced, severe stages of coronary artery disease, but early atherosclerotic lesions, which do not lead to luminal stenosis, are not identified reliably. In contrast, intravascular ultrasound scanning allows the precise characterization and quantification of a wide range of atherosclerotic lesions, independent of the severity of luminal stenosis.

METHODS

Three-dimensional (3-D) reconstruction of entire coronary segments is possible with the integration of sequential 2-dimensional tomographic images and allows volumetric analysis of coronary arteries.

RESULTS

Automated systems able to recognize lumen and vessel borders and to display 3-D images are becoming available.

CONCLUSION

These systems have the potential for on-line 3-D image reconstruction for clinical decision-making and fast routine volumetric analysis in research studies. This review describes 3-D intravascular ultrasound scanning acquisition, analysis, and processing, and the associated technical challenges.

Impact of vessel curvature on the accuracy of three-dimensional intravascular ultrasound: validation by phantoms and coronary segments

BACKGROUND

Three-dimensional intravascular ultrasound (IVUS) is used for volumetric assessment of arteriosclerotic plaque burden and restenotic tissue at follow-up after coronary interventions. However, the accuracy of these measurements, especially in tortuous vessels, is unclear.

METHODS

A commercially available electrocardiogram (ECG)-gated 3-dimensional-IVUS system was tested in volume-validated straight and curved hydrocolloid phantoms and in volume-validated coronary specimens. Catheter withdrawal (30 MHz, 3.2F) was triggered using standardized ECG source with 0.2-mm step intervals per cardiac cycle simulation.

RESULTS

On the basis of automated phantom volume measurements, IVUS overestimated true phantom volume (relative error = [measured V - true V]/true V x 100) by a median of 0.9%, 0.25%, and 1.96% for straight, mildly curved, and severely curved segments, respectively. The true volume of the coronary specimens was overestimated by a median of 5.79%.

CONCLUSION

A median percentage deviation of 3-dimensional-IVUS-measured volumes from the true volumes of less than 10% in phantoms and coronary artery segments can be achieved.

Pulsatile flow simulation in arterial vascular segments with intravascular ultrasound images

Previous studies have indicated a correlation between local variation in wall shear stress in arterial blood flow and atheroma development. The purpose of this study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional (3D) reconstruction, and to compare the computed wall shear stress in a compliant vascular segment model and the corresponding rigid walled model. Cross-sectional images of the segments of femoral and carotid arteries in five Yucatan miniswine were obtained using intravascular ultrasound (IVUS) imaging and the segment geometry was reconstructed at different times in the cardiac cycle. The actual measured wall motion from the reconstruction was employed to specify the moving boundaries for simulation of physiological distensibility. Velocity profiles and wall shear stress were computed using unsteady computational fluid dynamics analysis. The computed results revealed that the maximum wall shear stress in the compliant model was approximately 4-17 percent less than that in the rigid model if the wall motion is larger than 10 percent. Our analysis demonstrates that inaccuracies due to inflow velocity profile can be minimized by the extension of the model upstream. The phase angle between the diameter change and wall shear is affected by the local changes in geometry of the arteries. These simulations can be potentially used to analyze the effect of regional wall motion changes in the presence of atherosclerotic lesions on the local fluid dynamics and to correlate the same with subsequent growth of the lesions.

On the IVUS plaque volume error in coronary arteries when neglecting curvature

Plaque volume determined by common linear 3-D IVUS analysis systems will show under- or overestimation in curved vessel segments because these systems approximate the true 3-D transducer pull-back trajectory by a straight line. We developed a mathematical model that showed that the error is primarily dependent on the curvature of the pull-back trajectory and not on vessel tortuosity. Furthermore, we measured this error in vivo in the coronary arteries of 15 patients, comparing the plaque volume using a true 3-D reconstruction method with that of the linear approach. The in vivo plaque volume error ranged from 2.3% to -1.2% for 15 coronary segments with lengths ranging from 38.8 to 89.1 mm (62.2 +/- 13 mm). The volume error introduced by linear 3-D IVUS analysis systems is dependent on the curvature of the pull-back trajectory. The error measured in vivo was small and inversely related to segment length.

Evaluation of three-dimensional segmentation algorithms for the identification of luminal and medial-adventitial borders in intravascular ultrasound images

Intravascular ultrasound (IVUS) provides direct depiction of coronary artery anatomy, including plaque and vessel area, which is important in quantitative studies on the progression or regression of coronary artery disease. Traditionally, these studies have relied on manual evaluation, which is laborious, time consuming, and subject to large interobserver and intraobserver variability. A new technique, called active surface segmentation, alleviates these limitations and makes strides toward routine analyses. However, for three-dimensional (3-D) plaque assessment or 3-D reconstruction to become a clinical reality, methods must be developed which can analyze many images quickly. Presented is a comparison between two active surface techniques for three-dimensional segmentation of luminal and medial-adventitial borders. The force-acceleration technique and the neighborhood-search technique accurately detected both borders in vivo (r2 = 0.95 and 0.99, Williams' index = 0.67 and 0.65, and r2 = 0.95 and 0.99, WI = 0.67 and 0.70, respectively). However, the neighborhood-search technique was significantly faster and required less computation. Volume calculations for both techniques (r2 = 0.99 and r2 = 0.99) also agreed with a known-volume phantom. Active surface segmentation allows 3-D assessment of coronary morphology and further developments with this technology will provide clinical analysis tools.

Tissue characterization in intravascular ultrasound images

Intravascular ultrasound (IVUS) imaging permits direct visualization of vascular pathology. It has been used to evaluate lumen and plaque in coronary arteries and its clinical significance for guidance of coronary interventions is increasingly recognized. Conventional manual evaluation is tedious and time-consuming. This paper describes a highly automated approach to segmentation of coronary wall and plaque, and determination of plaque composition in individual IVUS images and pullback image sequences. The determined regions of plaque were classified in one of three classes: soft plaque, hard plaque, or hard plaque shadow. The method's performance was assessed in vitro and in vivo in comparison with observer-defined independent standards. In the analyzed images and image sequences, the mean border positioning error of the wall and plaque borders ranged from 0.13-0.17 mm. Plaque classification correctness was 90%.

The latest in ultrasound: three-dimensional imaging. Part II

INTRODUCTION

The three-dimensional (3D) reconstruction of ultrasound images has become a widespread option in ultrasound equipment. Specific softwares have become available and 3D reconstruction feasible since the early 1990s, particularly since 1994.

POSSIBLE CLINICAL APPLICATIONS

Several clinical applications are feasible in all parenchymatous organs (mainly the liver and prostate), hollow viscera (e.g. the bladder and gallbladder), peripheral vessels (supra-aortic trunks and limb vessels) and central (the aorta and iliac arteries) or cerebral vessels. Moreover, tumoral vessels in parenchymatous organs can be reconstructed, and even the fetus in the uterine cavity, with excellent detailing. The recent introduction of echocontrast agents and second harmonic imaging has permitted to study normal and abnormal peripheral, central and parenchymatous vessels, with similar patterns to those obtained with digital angiography. The spatial relationships between the vascular structures of the liver, kidney and placenta were studied with 3D ultrasound angiograms. The applications of this new technique include the analysis of vascular anatomy and the potential assessment of organ perfusion. THE LATEST APPLICATIONS--INTRAVASCULAR STUDIES: Some catheters with an ultrasound transducer in the tip have been tested for intravascular studies. Just like conventional transducers, they provide two-dimensional (2D) images which are then postprocessed into longitudinal 3D or volume reconstructions. The former resemble angiographic images and can be viewed 3D rotating the image along its longitudinal axis. Volume images, which are more complex and slower to obtain, can be rotated on any spatial plane and provide rich detailing of the internal vascular lumen. The clinical importance of intravascular ultrasound with 3D volume reconstructions lies in the diagnosis of vascular conditions and the assessment and monitoring of intravascular interventional procedures--e.g. to detect inaccurate deployment of intravascular stents and endoluminal grafts during the maneuver. Three-dimensional reconstructions involve geometric data assembly 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, resulting in the display of a straight segment. Therefore particular care is needed and there are some useful hints to avoid mistakes.

CONCLUSIONS

Three dimensional reconstructions of B-mode and color Doppler images are no longer a work in progress and their clinical importance and possible applications are both established and ever-increasing. On the other hand, independent of the different types of energy used, also computed tomography and magnetic resonance 3D reconstructions are very useful from a clinical viewpoint and they have become an established routine technique for both these methods. It is very likely that 3D volume reconstructions in ultrasound will find numerous applications in the near future. They may help to increase the diagnostic confidence and to facilitate diagnosis, intraprocedure monitoring in interventional radiology and follow-up and also to reduce the number of invasive examinations with iodinated contrast agents. This could result in cutting the cost and duration of the most expensive examinations. New, although invasive, applications can be hypothesized for intravascular or intraluminal catheters with an ultrasound transducer inside.