An evaluation of the potential and limitations of three-dimensional reconstructions from intravascular ultrasound images

Freehand 3-D Ultrasound Imaging: A Systematic Review

Two-dimensional ultrasound (US) imaging has been successfully used in clinical applications as a low-cost, portable and non-invasive image modality for more than three decades. Recent advances in computer science and technology illustrate the promise of the 3-D US modality as a medical imaging technique that is comparable to other prevalent modalities and that overcomes certain drawbacks of 2-D US. This systematic review covers freehand 3-D US imaging between 1970 and 2017, highlighting the current trends in research fields, the research methods, the main limitations, the leading researchers, standard assessment criteria and clinical applications. Freehand 3-D US systems are more prevalent in the academic environment, whereas in clinical applications and industrial research, most studies have focused on 3-D US transducers and improvement of hardware performance. This topic is still an interesting active area for researchers, and there remain many unsolved problems to be addressed.

The role of intravascular ultrasound in venous thromboembolism

Venous thromboembolism (VTE) remains a serious problem, and treatments surrounding this potentially life-threatening disease continue to evolve. Evidence-based guidelines purport the need for minimally invasive catheter-based procedures as part of the armamentarium to prevent and treat VTE. When the appropriate clinical scenarios arise, intravascular ultrasound (IVUS) becomes a necessary part of those procedures to provide alternative imaging that complements traditional venography. IVUS of the major axial veins provides a 360-degree two-dimensional gray scale ultrasound image of lumen and vessel wall structures. IVUS remains the criterion standard for venous imaging when contemplating catheter-based procedures from the common femoral vein to the inferior vena cava. Not only can precise location and size of these veins be determined by the IVUS probe from key landmarks and venous branches, but other important abnormalities can be visualized. These include external compression, acute and chronic thrombus, fibrosis, mural wall thickening, spurs, and trabeculations. Specific procedures that use IVUS include the treatment of venous obstruction and the placement of vena cava filters at the bedside. IVUS remains a vital part of accurately imaging the major axial veins when contemplating catheter-based procedures to prevent or treat VTE-related disorders.

Modelling of image-catheter motion for 3-D IVUS

Three-dimensional intravascular ultrasound (IVUS) allows to visualize and obtain volumetric measurements of coronary lesions through an exploration of the cross sections and longitudinal views of arteries. However, the visualization and subsequent morpho-geometric measurements in IVUS longitudinal cuts are subject to distortion caused by periodic image/vessel motion around the IVUS catheter. Usually, to overcome the image motion artifact ECG-gating and image-gated approaches are proposed, leading to slowing the pullback acquisition or disregarding part of IVUS data. In this paper, we argue that the image motion is due to 3-D vessel geometry as well as cardiac dynamics, and propose a dynamic model based on the tracking of an elliptical vessel approximation to recover the rigid transformation and align IVUS images without loosing any IVUS data. We report an extensive validation with synthetic simulated data and in vivo IVUS sequences of 30 patients achieving an average reduction of the image artifact of 97% in synthetic data and 79% in real-data. Our study shows that IVUS alignment improves longitudinal analysis of the IVUS data and is a necessary step towards accurate reconstruction and volumetric measurements of 3-D IVUS.

Reduction of stent artifacts using high-frequency harmonic ultrasound imaging

Tissue harmonic imaging (THI) has been shown to improve medical ultrasound (US) image quality in the frequency range from 2 to 10 MHz and might, therefore, also be advantageous in high-frequency US applications, like US biomicroscopy and intravascular US (IVUS). In this study, we compared high-frequency THI (40 MHz) with fundamental imaging (20 and 40 MHz) with a distorting reflective metal stent in the near fields of both a spherically-focused US biomicroscopy transducer (aperture 8 mm, focal distance 13 mm) and an unfocused elliptical IVUS element. Hydrophone measurements of the harmonic beam (40 MHz) of both transducers showed relatively low signal strength in the near field compared with both (20 and 40 MHz) fundamental beams. For the focused transducer, THI suppressed the second stent echo up to 14 dB compared with fundamental imaging. No significant reduction in stent artifact imaging was observed for the unfocused IVUS element.

Image-based cardiac gating for three-dimensional intravascular ultrasound imaging

Three-dimensional (3-D) intravascular ultrasound (US), or IVUS, provides valuable insight into the tissue characteristics of the coronary wall and plaque composition. However, artefacts due to cardiac motion and vessel wall pulsation limit the accuracy and variability of coronary lumen and plaque volume measurement in 3-D IVUS images. ECG-gated image acquisition can reduce these artefacts but it requires recording the ECG signal and may increase image acquisition time. The goal of our study was to reconstruct a 3-D IVUS image with negligible cardiac motion and vessel pulsation artefacts, by developing an image-based gating method to track 2-D IVUS images over the cardiac cycle. Our approach involved selecting 2-D IVUS images belonging to the same cardiac phase from an asynchronously-acquired series, by tracking the changing lumen contour over the cardiac cycle. The algorithm was tested with IVUS images of a custom-built coronary vessel phantom and with patient images. The artefact reduction achieved using the image-gating approach was > 86% in the in vitro images and > 80% in the in vivo images in our study. Our study shows that image-based gating of IVUS images provides a useful method for accurate reconstruction of 3-D IVUS images with reduced cardiac motion artefact.

A review on MR vascular image processing algorithms: acquisition and prefiltering: part I

Vascular segmentation has recently been given much attention. This review paper has two parts. Part I focuses on the physics of magnetic resonance angiography (MRA) generation and prefiltering techniques applied to MRA data sets. Part II of the review focuses on the vessel segmentation algorithms. The first section of this paper introduces the five different sets of receive coils used with the MRI system for magnetic resonance angiography data acquisition. This section then presents the five different types of the most popular data acquisition techniques: time-of-flight (TOF), phase-contrast, contrast-enhanced, black-blood, T2-weighted, and T2*-weighted, along with their pros and cons. Section II of this paper focuses on prefiltering algorithms for MRA data sets. This is necessary for removing the background nonvascular structures in the MRA data sets. Finally, the paper concludes with a clinical discussion on the challenges and the future of the data acquisition and the automated filtering algorithms.

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.

A pulsating coronary vessel phantom for two- and three-dimensional intravascular ultrasound studies

The evaluation of new techniques for 2-D and 3-D intravascular ultrasound (US) imaging (IVUS) often requires the use of a pulsating coronary phantom. This study describes the design, construction and evaluation of a phantom simulating the pulsation of a human coronary artery for IVUS studies. Polyvinyl alcohol (PVA) cryogel was used as a tissue mimic for the coronary vessel, which was incorporated in a custom-built assembly. The phantom was programmed to pulsate under servomotor control, to model the pulsation of a normal coronary artery and 2-D IVUS images were obtained using an IVUS imaging catheter. To evaluate the performance of the phantom, the lumen area variation of the phantom was determined and compared with the programmed pulsation waveforms. Our results showed that phantom pulsation correlated well with the programmed pulsation waveform (r = 0.97). The deviation of the least squares line from the line of identity was calculated to be < 4%.

Scanning techniques for three-dimensional forward-viewing intravascular ultrasound imaging

Intravascular ultrasound (US) imaging is a useful tool for assessing arterial disease and aiding treatment procedures. Forward-viewing intravascular US imaging could be of particular use in severely stenosed or totally occluded arteries, where the current side-viewing intravascular US systems are limited by their inability to access the site of interest. In this study, five 3-D forward-viewing intravascular scanning patterns were investigated. The work was carried out using scaled-up vessel phantoms constructed from tissue-mimicking material and a PC-controlled scanning and acquisition system. The scanning patterns were examined and evaluated with regard to the image quality of dense and sparse data sets, the accuracy of quantitative measurements of lumen dimensions and the potential for clinical use. The relative merits and drawbacks of the different patterns are discussed and a preferred scanning pattern is recommended.

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.

Validation of volume measurements in esophageal pseudotumors using 3D endoluminal ultrasound

The purpose of this study was to validate the accuracy and reliability of volume measurements using three-dimensional (3D) endoluminal ultrasound (ELUS) in canine pseudotumor esophageal specimens in vitro. Pseudotumors were created by injecting various volumes of US gel (0.1-1.0 ml) into canine esophageal specimens. A stepping-motor was used to pull either a 9, 12.5 or 20 MHz transducer through the lumen of the specimen at 1.5 mm/s. Images were downloaded to a LIFE computer system for 3D reconstruction. Volume measurements were made by two investigators and compared to spiral CT images. Averaging across all measurements, the average magnitude of error was 8.7% in individual US determinations and 11. 9% in CT measures. Volumes estimated from images spaced 0.5 and 1.0 mm apart, from images in the original and reconstructed planes, and from different scan frequencies, produced percentage errors that were not statistically significantly different from each other on ELUS. 3D ELUS can be used accurately and reproducibly to measure tumor volumes with a low mean percent in vitro.

Three-dimensional automatic quantitative analysis of intravascular ultrasound images

Intravascular ultrasound (IVUS) has established itself as a useful tool for coronary assessment. The vast amount of data obtained by a single IVUS study renders manual analysis impractical for clinical use. A computerized method is needed to accelerate the process and eliminate user-dependency. In this study, a new algorithm is used to identify the lumen border and the media-adventitia border (the external elastic membrane). Setting an initial surface on the IVUS catheter perimeter and using active contour principles, the surface inflates until virtual force equilibrium defined by the surface geometry and image features is reached. The method extracts these features in three dimensions (3-D). Eight IVUS procedures were performed using an automatic pullback device. Using the ECG signal for synchronization, sets of images covering the entire studied region and corresponding to the same cardiac phase were sampled. Lumen and media-adventitia border contours were traced manually and compared to the automatic results obtained by the suggested method. Linear regression results for vessel area enclosed by the lumen and media-adventitia border indicate high correlation between manual vs. automatic tracings (y = 1.07 x -0.38; r = 0.98; SD = 0.112 mm(2); n = 88). These results indicate that the suggested algorithm may potentially provide a clinical tool for accurate lumen and plaque assessment.

Three-dimensional reconstruction of the coronary artery wall by image fusion of intravascular ultrasound and bi-plane angiography

BACKGROUND

Intravascular ultrasound (IVUS) is becoming increasingly accepted for assessing coronary anatomy. However, its utility in visualizing and quantifying coronary morphology has been limited by its 2D tomographic nature. This study presents a 3D reconstruction technique that accurately preserves 3D geometric information.

METHODS AND RESULTS

Images obtained from manual IVUS pullbacks and continuous bi-plane angiography were fused, using angiography to reconstruct the transducer trajectory and aid in solving for the correct rotational orientation. A novel 3D active surface method automatically identified the luminal and medial-adventitial borders which, when superimposed on the transducer trajectory, could be surface-rendered for visualization and morphometry. Segmentation agreed well with manual assessment, and 3D luminal shape matched that of angiography when projected to 2D.

CONCLUSIONS

We conclude that this method provides an accurate reconstruction of the vessel's anatomy, which accounts for the true curvature of the vessel.

Reproducibility of volumetric quantification in intravascular ultrasound images

The reproducibility of volume measurements in intravascular ultrasound (IVUS) images derived from separate pull-back manoeuvres remains to be elucidated. Patients (n = 23) were imaged with IVUS prior to (first series) and following percutaneous transluminal angioplasty (PTA) (second series). In 15 patients, one matched vascular segment (3-4 cm in length), not subjected to PTA, was used for analysis of lumen, vessel and plaque volume using an automated contour analysis system. Volume measurements assessed by two independent observers and in the two separate series were compared. Interobserver differences in volume measurements were small (< or =0.4%), with low coefficients of variation (< or =1.7%) and high correlation coefficients (r = 1.00). Differences in volume measurements obtained in the two separate series were small (< or =2.6%), with low coefficients of variation (< or = 8.6%) and high correlation coefficients (r = 0.97-0.99). In conclusion, volume measurements derived from IVUS images are highly reproducible. Therefore, IVUS may be used to monitor the progression/regression of atherosclerotic plaque volume in a longitudinal study.

Dynamic imaging of coronary stent structures: an ECG-gated three-dimensional intracoronary ultrasound study in humans

Three-dimensional (3D) intracoronary ultrasound (ICUS) systems allow dynamic 3D reconstruction of coronary segments after stent deployment, but motion artifacts are frequently present. The use of an electrocardiographic-gated ICUS image acquisition workstation and a dedicated pullback device may overcome this problem. In the present study, we evaluated the potential of dynamic 3D reconstruction of intracoronary stents in 51 patients. Two different types of stent designs were investigated: (1) the Wallstent (mesh type; n = 36) and (2) the Cordis Coronary stent (coil type; n = 15). There was a tendency for imaging of the mesh stent type to be better than imaging of coil type stents (p = 0.06). Differences in the orientation of the stent struts (mesh:longitudinal; coil:transversal) most likely explain this difference. These in vivo observations were tested and confirmed in in vitro experiments. In conclusion, dynamic 3D ICUS reconstruction of the entire stent architecture in vivo was feasible for stents of mesh type, while stents of coil type were incompletely visualized.

Torsion measurement of catheters using polarized light in a single glass fibre

Several types of intravascular ultrasound (IVUS) catheters are connected to a motor at their proximal end, in order to let the catheter rotate around its length axis. However, the rotation of the distal axis tip does not follow exactly the rotation of the motor, since the catheter axis is not completely torsion-free and friction forces cause the tip to rotate in a shockwise manner. In the case of the IVUS catheter, continuous information concerning the exact rotation of the crystal is essential for 3D image reconstruction. We developed a simple method of measuring the tip rotation continuously, the TOMCAT method, using only a single optical fibre glued in a fixed position inside the rotating axis of the IVUS catheter, or any rotating axis or catheter in general. Our method does not require external electromagnetic fields or the presence of a non-rotating guiding catheter. The rotation of the distal tip is related to that of the proximal part by transporting polarized light through the fibre. We performed in vitro experiments using various types of optical fibres to test the TOMCAT method, and conclude that using a specific step-index monomode cylinder-symmetrical optical fibre the TOMCAT approach yields accurate results.