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.     

Quantitative measurements in IVUS images

IntraVascular UltraSound (IVUS) is a catheter-based technique which provides real-time high resolution tomographic images of both the lumen and arterial wall of a coronary segment, this in contrast to X-ray arteriography that provides a shadow image (luminogram) of the entire lumen. Nowadays the lumen and vessel parameters are measured manually, which is very time consuming and suffers from high inter- and intra-obser variability. With the continuing improvement in IVUS imaging, it is now feasible to develop and clinically apply automated methods of three-dimensional quantitative analysis of the coronary vessel morphology in an objective and reproducible way with automated contour detection techniques (QCU). Quantification, in 2D and 3D, as well as volume rendering for visualization of the IVUS images requires segmentation of the images (contour detection). The 3D contour detection system described in this article is based on the combination of contour detection in the transversal and sagital view. This article provides some of the basic principles of IVUS, the IVUS image quantification, the three-dimensional reconstruction and the contour detection and quantification in three-dimensional IVUS images.     

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%.     

血管内超声测量血流动力学参数的研究进展

血管内超声显像可在活体中观察血管壁、管腔及斑块形态、性质,甚至斑块成分和管壁功能状态,由此获得的冠状动脉的血流动力学参数(包括管壁的弹性模量和血管壁应力-应变分布等)对于正确诊断冠心病、制定最佳诊疗方案具有重要意义。本文就近年来血管内超声在冠状动脉血流动力学参数测量中的应用进行综述。     

Estimation of the transverse strain tensor in the arterial wall using IVUS image registration

Intravascular ultrasound (IVUS) elastography is an imaging technique that obtains the local mechanical properties of the artery wall and atherosclerotic plaques through strain measurements using IVUS. Knowledge of these mechanical properties may provide crucial information that can help in estimating plaque composition and its vulnerability. Here, we present a new method to estimate the transverse strain tensor of the arterial wall based on nonrigid image registration using IVUS images. This method registers a pair of images acquired at a vessel site under different levels of luminal pressure. The 2-D displacement field in the vessel cross-section is estimated from image registration; then the displacement field is used to calculate the 2-D local strain tensor. From the strain tensor, the strain in any direction in the cross-section can be obtained; here, the radial and circumferential strain distributions are presented. This strain estimation method has been validated with synthetic motion IVUS images and evaluated using the IVUS images of a polyvinyl alcohol cryogel phantom. The accuracy of the estimated strain and the ability of the method to overcome IVUS system noise are demonstrated.     

冠状动脉弹性参数测量的系统与方法

本文提出一种利用血管内超声技术测量血管弹性参数的方法.这种方法通过血管内超声成像(IVUS)得到冠状动脉的B型超声视频图像,然后通过图像处理技术提取血管壁轮廓,从而计算出截面积.获得一个心动周期以上的冠状动脉截面积曲线后,计算出相应的冠状动脉直径曲线,结合基于血管内压力测量技术检测到的血压曲线,计算出与冠状动脉弹性有关的参数.文中给出了利用该方法建立的系统所测量的实例及其相应结果.     

Invasive diagnostic imaging of coronary atherosclerosis

Invasive diagnostic imaging technique of coronary atherosclerosis has rapidly developed. For example, intravascular ultrasound(IVUS) is recognized as an essential device for percutaneous coronary intervention to evaluate the vessel wall, vascular lumen and coronary plaque morphologies because of its accuracy for quantitative analysis capability. Recently new imaging modalities such as radio-frequency signal analysis, elastography and contrast harmonic echography have been developed for the evaluation of histological characteristics. Also, optical coherence tomography(OCT), which provides approximately ten-times higher-resolutional cross-section images of the coronary arterial wall in comparison with IVUS, became available in clinical setting. In this article, we review the latest progress of the invasive diagnostic imaging of coronary atherosclerosis.     

Detection of cardiac cycle from intracoronary ultrasound

In this paper, we describe a method automatically to determine the phase of a cardiac cycle for each video frame of an intravascular ultrasound (IVUS) video recorded in vivo. We first review the principle of IVUS video and demonstrate the general applicability of our method. We show that the pulsating heart leads to phasic changes in image content of an IVUS video. With an image processing method, we can reverse this process and reliably extract the heart-beat phase directly from IVUS video. With the phase information, we demonstrate that we can build 3-D (3D) time-variant shapes and measure lumen volume changes within a cardiac cycle. We may also measure the changes of IVUS imaging probe off-center vector within a cardiac cycle, which serves as an indicator of vessel center-line curvature. The cardiac cycle extraction algorithm requires one scan of the IVUS video frames and takes O(n) time to complete, n being the total number of the video frames. The advantage of this method is that it requires no user interaction and no hardware set-up and can be applied to coronary scans of live beating hearts. The extracted heart-beat rate, compared with clinical recordings, has less than 1% error.     

Retrospective respiratory motion correction for navigated cine velocity mapping.

In general, high spatial and temporal resolutions in cine cardiac imaging require long scan times, making breath-hold acquisition impossible in many cases. To enable free-breathing cardiac imaging, methods such as navigator gating were developed to reduce image artifacts due to respiratory motion. Nevertheless, residual image blurring is seen in images acquired late in the cardiac cycle. Image blurring itself hampers accurate blood flow quantification, especially in vessels exhibiting high flows during diastole. In the present work, the navigator gating and slice tracking method was extended by using navigator information to correct for in-slice motion components throughout the cardiac cycle. For this purpose, a standard two-dimensional (2D) cine phase contrast sequence with navigator gating and slice position correction was used, and navigator information was recorded along with the raw k-space data. In postprocessing, in-plane motion components arising from respiration during the actual data acquisition were estimated and corrected according to the Fourier shift theorem. In phantom experiments, the performance of the correction algorithm for different slice angulations with respect to the navigator orientation was validated. In vivo, coronary flow measurements were performed in 9 healthy volunteers. The correction algorithm led to considerably improved vessel sharpness throughout the cardiac cycle in all measured subjects [increase in vessel sharpness: 16+/-11% (mean+/-SD)]. Furthermore, these improvements resulted in increased volume flow rates [16+/-13% (mean+/-SD)] after retrospective correction indicating the impact of the method. It is concluded that retrospective respiratory motion corrections for navigated cine two-dimensional (2D) velocity mapping can correct for in-plane motion components, providing better image quality for phases acquired late in the cardiac cycle. Therefore, this method holds promise in particular for free-breathing coronary flow quantification.     

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.     

磁共振黑血序列冠状动脉管壁成像评价粥样硬化斑块初步研究结果:与血管内超声对照研究

目的与血管内超声（IVUS）对照,探讨磁共振黑血序列冠状动脉管壁成像评价冠状动脉斑块的可行性及准确性。方法拟行IVUS检查及行IVUS检查而未行支架植入术的患者11例,分别于IVUS术前及术后10日内行磁共振检查。成像序列采用二维横截面、双反转恢复、呼吸导航心电门控触发、压脂TSE序列,采集自病变冠脉开口至中段无间隔连续扫描,测量以下数据：血管横截面积（CSA）、管腔CSA、斑块负荷、管壁信噪比（SNR）及对比噪声LL（CNR）。在IVUS上将靶冠脉白开口分为每5mm节段,与MRI管壁横截面一一对应。当冠状动脉管壁厚度≥0．5mm时考虑存在斑块。结果共9例病人,9支冠脉,37个层面纳入分析;2例病人被排除（因扫描时间长,病人无法坚持1,扫描成功率82％。37个层面中有20个有斑块。斑块层面的斑块负荷、SNR、CNR大于非斑块层面（分别为0．70±0．11vs0．58±0．14,1．95±0．39vs1．48±0．21,5．47±2．06vs2．99±0．78,P〈0．05）。MRI斑块层面血管CSA、管腔CSA及斑块负荷与IVUS比较,具有良好的相关性（13．66±4．52vs14．92±6．37,4．62±2．23vs6．03±3．85,0．63±0．13vs0．60±0．14,P〈0．05）。结论冠状动脉黑血管壁成像可以发现冠状动脉近中段的粥样硬化斑块,且能够相对评价狭窄段血管面积和管腔面积。     

OCT based four-dimensional cardiac imaging of a living chick embryo using an impedance signal as a gating for post-acquisition synchronization

Optical coherence tomography (OCT) is a non-invasive imaging modality with high spatial resolution suitable for early embryonic heart imaging. However, the most commonly used OCT systems cannot provide direct 4-D imaging due to acquisition speed limitations. We proposed a retrospective gating 4-D reconstruction method based on spectral domain OCT. A special circuit was designed to measure the impedance change of chick embryos in response to the heart beating. The impedance signal was acquired simultaneously with the OCT B-scan image sequence at several different locations along the heart. The impedance signal was used as a gating for 4-D reconstruction. The reconstruction algorithm includes cardiac period calculation, interpolation from multi-cardiac cycle image sequence into one cardiac cycle, and cardiac phase synchronization among the different locations of the heart. The synchronism of the impedance signal change with the heartbeat was verified. Using the proposed method, we reconstructed the cardiac outflow tract (OFT) of chick embryos at an early stage of development (Hamburger–Hamilton stage 18). We showed that the reconstructed 4-D images correctly captured the dynamics of the OFT wall motion.     

On the potential of the lagrangian estimator for endovascular ultrasound elastography: in vivo human coronary artery study

Diagnosis and prognosis of coronary artery atherosclerosis evolution currently rely on plaque morphology and vessel stenosis degree. Such information can accurately be assessed with intravascular ultrasound (IVUS) imaging. A severe complication of coronary artery atherosclerosis is thrombosis, a consequence of plaque rupture or fissure, which might lead to myocardial infarction and sudden ischemic death. Plaque rupture is a complicated mechanical process, correlated with the plaque morphology, composition, mechanical properties and with the blood pressure. Extracting information on the plaque local mechanical properties may reveal relevant features about plaque vulnerability. Accordingly, endovascular elastography (EVE) was introduced to complement IVUS for investigating coronary artery diseases. In this article, in vivo elastographic data are reported for three patients (patient 1, patient 2 and patient 3) who were diagnosed with severe coronary artery stenoses. Time-sequence radio-frequency (RF) data were acquired, in the minutes preceding angioplasty, using an ultrasound scanner working with a 30 MHz mechanical rotating single-element transducer. The elastograms of the radial strain and radial shear distributions within the vessel wall were computed from pairs of successive RF images using the Lagrangian estimator (LE). A hard atherosclerotic plaque (low radial strain and shear) was identified in patient 1. High radial strain and shear values in the plaque areas for patient 2 and patient 3 suggested the presence of lipid cores (soft materials), known to be prone-to-rupture sites when located close to the lumen. To conclude, EVE allowing radial strain and shear images is an improvement over existing EVE methods that may assist IVUS in preoperative vessel lesion assessments and in endovascular therapy planning.     

Suppression of motion artifacts in intravascular photoacoustic image sequences

Intravascular photoacoustic (IVPA) imaging is an image-based imaging modality for the assessment of atherosclerotic plaques. Successful application of IVPA for in vivo coronary arterial imaging requires one overcomes the challenge of motion artifacts associated with the cardiac cycle. We propose a method for correcting artifacts owing to cardiac motion, which are observed in sequential IVPA images acquired by the continuous pullback of the imaging catheter. This method groups raw photoacoustic signals into subsets corresponding to similar phases in the cardiac cycles. Thereafter, the sequential images are reconstructed, by representing the initial pressure distribution on the vascular cross-sections based on the clustered frames of signals by time reversal. Results of simulation data demonstrate the efficacy of this method in suppressing motion artifacts. Qualitative and quantitative evaluations of the method indicate an enhancement of the image quality. Comparison results reveal that this method is computationally efficient in motion correction compared with the image-based gating.     

Influence of coronary pulsation on volumetric intravascular ultrasound measurements performed without ECG-gating. Validation in vessel segments with minimal disease

Volumetric analysis of coronary arteries can be performed using intravascular ultrasound (IVUS) images selected at 1 mm intervals without ECG gating. However, there are few data regarding the influence of coronary pulsation on this volumetric analysis. We developed two models of consecutive area measurements consisting of duplicated area measurements from short coronary segments and virtual measurements based on a sine function. These models allowed the re-calculation of volumes using different sets of frames from the same simulated segments. The variability of the volume determinations was evaluated by its percent standard deviation [%SD = (SD/the mean value) × 100]. The relation of the variability to the extent of external elastic membrane (EEM) area change during the cardiac cycle (amplitude) and heart rates (frequency) were examined. In 58 short coronary segments of 15 patients, consecutive IVUS images were measured [%EEM area change: 12.3 ± 7.7 %, heart rate 78 ± 21 beats/min (bpm)]. In both models, %SD of the volume calculations was directly proportional to the %EEM area change and showed two peaks at heart rates of 60 ± 2 and 90 ± 2 bpm. In the model based on actual coronary measurements, the %SD of volume calculations of a segment with 10% EEM area change was 0.7% except for heart rates of 60 ± 2 and 90 ± 2 bpm. The variability of a volumetric analysis based upon measuring IVUS images at constant intervals without ECG gating is affected by coronary pulsation, extent of cross-sectional area changes, and heart rate. Despite these limitations, this method is feasible and provides reproducible volume measurements.     

Asymmetric pulsation of rat carotid artery bifurcation in three-dimension observed by ultrasound imaging

The arterial structure cyclically fluctuates in three-dimensions (3-D) caused by pulsatile blood flow. The evaluation of arterial wall motion and hemodynamics contributes to early diagnosis of carotid atherosclerosis. Ultrasound is one of the most appropriate imaging modalities to evaluate arterial wall motion in real time. Although many previous studies have discussed the mechanical properties of the carotid artery bifurcation (CAB) from the two-dimensional (2-D) view, the spatio-temporal variation of carotid artery geometry in 3-D has not yet been investigated in detail. In this study, the 3-D data set of CAB from rats was acquired using a high spatio-temporal resolution ultrasound imaging system with a 40 MHz probe using mechanical sector scanning. A total of 31 slices of cross-section images were stored and a spoke scan algorithm was implemented to radially scan the lumen area in polar coordinates based on a pre-tracked seed point. The boundary of the arterial lumen was segmented using intensity-threshold-based boundary detection and fitted by polynomial regression. Two operators, who were trained with the same protocol to minimize inter- and intra-operator variability, manually segmented the lumen boundary on systolic and diastolic phase from the gray-scale images. Finally, the 3-D lumen geometries of CAB during one cardiac cycle were constructed based on the segmented lumen boundaries. From this constructed 3-D geometry, we observed that the CAB geometry favorably expanded to the anterior/posterior direction, parallel to the sagittal plane; and the manually segmented geometry also confirmed the asymmetrical change in bifurcation geometry. This is the first study on visualization and quantification on the asymmetrical variation of the CAB geometry of a rat in 3-D during a whole cardiac cycle. This finding may be useful in understanding hemodynamic etiology of various cardiovascular diseases such as arterial stenosis and its complications, and also provides reference information for numerical simulation studies on arterial wall motion.     

Automatic coronary wall segmentation in intravascular ultrasound images using binary morphological reconstruction

Intravascular ultrasound (IVUS) image segmentation can provide more detailed vessel and plaque information, resulting in better diagnostics, evaluation and therapy planning. A novel automatic segmentation proposal is described herein; the method relies on a binary morphological object reconstruction to segment the coronary wall in IVUS images. First, a preprocessing followed by a feature extraction block are performed, allowing for the desired information to be extracted. Afterward, binary versions of the desired objects are reconstructed, and their contours are extracted to segment the image. The effectiveness is demonstrated by segmenting 1300 images, in which the outcomes had a strong correlation to their corresponding gold standard. Moreover, the results were also corroborated statistically by having as high as 92.72% and 91.9% of true positive area fraction for the lumen and media adventitia border, respectively. In addition, this approach can be adapted easily and applied to other related modalities, such as intravascular optical coherence tomography and intravascular magnetic resonance imaging.     

A comparison of prospective and retrospective respiratory navigator gating in 3D MR coronary angiography

A comparison between the prospective and retrospective respiratory navigator gating in MR coronary angiography was performed with eight normal subjects. A three-dimensional (3D) ECG-gated fast gradient echo pulse sequence was used for image data acquisition. The results show that the MR coronary angiography obtained using retrospective gating retains a considerable amount of motion artifacts. In this study, the images acquired using prospective navigator gating demonstrated significantly reduced motion artifacts (p = 0.009), improved vessel visibility (p = 0.021) with reduced imaging time (p = 0.013) compared to the images obtained using retrospective navigator gating.     

造影正常冠状动脉左主干的血管内超声研究

目的 探讨冠状动脉造影正常左主干的血管内超声特征.方法 选取冠状动脉造影显示单纯左前降支或左回旋支病变而左主干正常同时行血管内超声(IVUS)检查的76例患者.应用IVUS测量病变部位的斑块负荷,确定斑块的性质.同时确定左主干是否存在病变,若存在病变则确定病变性质;测量左主干的管腔直径和面积以及血管直径和面积;对存在动脉粥样硬化者,测量斑块负荷.结果 76例冠状动脉造影正常左主干患者中IVUS显示完全正常28例,内膜增生12例,有斑块36例,发现内膜斑片2例.对于存在斑块者偏心斑块为30例,向心斑块为6例;脂质斑块25例(占69.4%),纤维斑块4例(占11.1%),钙化斑块2例(占5.6%),混合斑块5例(占13.9%).IVUS显示女性左主干正常者的管腔直径为(5.32±0.68)mm,管腔面积为(23.34±5.27)mm2,男性左主干正常者的管腔直径为(5.90±0.50)mm,管腔面积为(27.75±4.47)mm2.男女管腔直径和管腔面积比较差别均有统计学意义(P值分别为0.042和0.048).内膜增生者血管直径为(5.90±0.47)mm,血管面积为(27.58±4.21)mm2;存在斑块者的管腔直径为(4.39±0.54)mm,管腔面积为(17.45±5.23)mm2,血管直径为(5.99±0.67)mm,血管面积为(26.61±6.27)mm2,直径狭窄百分比为(26.17±7.87)%,斑块负荷为(34.79±9.37)%.结论 IVUS能发现冠状动脉造影所无法显示的左主干病变,并且能精确地确定左主干病变的性质和严重程度.     

Online motion-gated dynamic three-dimensional echocardiography in the fetus--preliminary results

The aim of this study was to visualise the fetal heart in dynamic three dimensions (4-D) during an ultrasound (US) scan (online), rather than after (offline). With special pairing and sequential setting to minimise interference between two scanners, umbilical arterial Doppler waveforms (UADWs) from one scanner were used as an online motion gating source to trigger simultaneous 3-D cardiac structural data acquisition by another. Of 25 data sets from 10 fetuses, 18 were acquired in 15 to 30 s per set with > or = 50% Doppler waveforms efficiently converted to triggering signals. Of 15 valid 4-D data sets, 10 were reconstructed in 2 to 20 min, compared to over 2 h previously reported (mainly for offline gating). Fine structures (including chordae tendinae and trabecular muscles) were depicted in six sets. The main problems in degrading 4-D images were extensive shadowing (6) from bony structures during rigid mechanical scanning, and random motion artefacts (6) from prolonged setting-up time with a complex combination of several systems. Integration of these systems is, therefore, recommended.