ECG-Gated Cardiac CT

Coronary artery analysis: Computer-assisted selection of best-quality segments in multiple-phase coronary CT angiography

PURPOSE

The authors are developing an automated method to identify the best-quality coronary arterial segment from multiple-phase coronary CT angiography (cCTA) acquisitions, which may be used by either interpreting physicians or computer-aided detection systems to optimally and efficiently utilize the diagnostic information available in multiple-phase cCTA for the detection of coronary artery disease.

METHODS

After initialization with a manually identified seed point, each coronary artery tree is automatically extracted from multiple cCTA phases using our multiscale coronary artery response enhancement and 3D rolling balloon region growing vessel segmentation and tracking method. The coronary artery trees from multiple phases are then aligned by a global registration using an affine transformation with quadratic terms and nonlinear simplex optimization, followed by a local registration using a cubic B-spline method with fast localized optimization. The corresponding coronary arteries among the available phases are identified using a recursive coronary segment matching method. Each of the identified vessel segments is transformed by the curved planar reformation (CPR) method. Four features are extracted from each corresponding segment as quality indicators in the original computed tomography volume and the straightened CPR volume, and each quality indicator is used as a voting classifier for the arterial segment. A weighted voting ensemble (WVE) classifier is designed to combine the votes of the four voting classifiers for each corresponding segment. The segment with the highest WVE vote is then selected as the best-quality segment. In this study, the training and test sets consisted of 6 and 20 cCTA cases, respectively, each with 6 phases, containing a total of 156 cCTA volumes and 312 coronary artery trees. An observer preference study was also conducted with one expert cardiothoracic radiologist and four nonradiologist readers to visually rank vessel segment quality. The performance of our automated method was evaluated by comparing the automatically identified best-quality segments identified by the computer to those selected by the observers.

RESULTS

For the 20 test cases, 254 groups of corresponding vessel segments were identified after multiple phase registration and recursive matching. The AI-BQ segments agreed with the radiologist's top 2 ranked segments in 78.3% of the 254 groups (Cohen's kappa 0.60), and with the 4 nonradiologist observers in 76.8%, 84.3%, 83.9%, and 85.8% of the 254 groups. In addition, 89.4% of the AI-BQ segments agreed with at least two observers' top 2 rankings, and 96.5% agreed with at least one observer's top 2 rankings. In comparison, agreement between the four observers' top ranked segment and the radiologist's top 2 ranked segments were 79.9%, 80.7%, 82.3%, and 76.8%, respectively, with kappa values ranging from 0.56 to 0.68.

CONCLUSIONS

The performance of our automated method for selecting the best-quality coronary segments from a multiple-phase cCTA acquisition was comparable to the selection made by human observers. This study demonstrates the potential usefulness of the automated method in clinical practice, enabling interpreting physicians to fully utilize the best available information in cCTA for diagnosis of coronary disease, without requiring manual search through the multiple phases and minimizing the variability in image phase selection for evaluation of coronary artery segments across the diversity of human readers with variations in expertise.

Coping with real world data: Artifact reduction and denoising for motion-compensated cardiac C-arm CT

PURPOSE

Detailed analysis of cardiac motion would be helpful for supporting clinical workflow in the interventional suite. With an angiographic C-arm system, multiple heart phases can be reconstructed using electrocardiogram gating. However, the resulting angular undersampling is highly detrimental to the quality of the reconstructed images, especially in nonideal intraprocedural imaging conditions. Motion-compensated reconstruction has previously been shown to alleviate this problem, but it heavily relies on a preliminary reconstruction suitable for motion estimation. In this work, the authors propose a processing pipeline tailored to augment these initial images for the purpose of motion estimation and assess how it affects the final images after motion compensation.

METHODS

The following combination of simple, direct methods inspired by the core ideas of existing approaches proved beneficial: (a) Streak reduction by masking high-intensity components in projection domain after filtering. (b) Streak reduction by subtraction of estimated artifact volumes in reconstruction domain. (c) Denoising in spatial domain using a joint bilateral filter guided by an uncompensated reconstruction. (d) Denoising in temporal domain using an adaptive Gaussian smoothing based on a novel motion detection scheme.

RESULTS

Experiments on a numerical heart phantom yield a reduction of the relative root-mean-square error from 89.9% to 3.6% and an increase of correlation with the ground truth from 95.763% to 99.995% for the motion-compensated reconstruction when the authors' processing is applied to the initial images. In three clinical patient data sets, the signal-to-noise ratio measured in an ideally homogeneous region is increased by 37.7% on average. Overall visual appearance is improved notably and some anatomical features are more readily discernible.

CONCLUSIONS

The authors' findings suggest that the proposed sequence of steps provides a clear advantage over an arbitrary sequence of individual image enhancement methods and is fit to overcome the issue of lacking image quality in motion-compensated C-arm imaging of the heart. As for future work, the obtained results pave the way for investigating how accurately cardiac functional motion parameters can be determined with this modality.

Echocardiography as an indication of continuous-time cardiac quiescence

Cardiac computed tomography (CT) angiography using prospective gating requires that data be acquired during intervals of minimal cardiac motion to obtain diagnostic images of the coronary vessels free of motion artifacts. This work is intended to assess B-mode echocardiography as a continuous-time indication of these quiescent periods to determine if echocardiography can be used as a cost-efficient, non-ionizing modality to develop new prospective gating techniques for cardiac CT. These new prospective gating approaches will not be based on echocardiography itself but on CT-compatible modalities derived from the mechanics of the heart (e.g. seismocardiography and impedance cardiography), unlike the current standard electrocardiogram. To this end, echocardiography and retrospectively-gated CT data were obtained from ten patients with varied cardiac conditions. CT reconstructions were made throughout the cardiac cycle. Motion of the interventricular septum (IVS) was calculated from both echocardiography and CT reconstructions using correlation-based, deviation techniques. The IVS was chosen because it (1) is visible in echocardiography images, whereas the coronary vessels generally are not, and (2) has been shown to be a suitable indicator of cardiac quiescence. Quiescent phases were calculated as the minima of IVS motion and CT volumes were reconstructed for these phases. The diagnostic quality of the CT reconstructions from phases calculated from echocardiography and CT data was graded on a four-point Likert scale by a board-certified radiologist fellowship-trained in cardiothoracic radiology. Using a Wilcoxon signed-rank test, no significant difference in the diagnostic quality of the coronary vessels was found between CT volumes reconstructed from echocardiography- and CT-selected phases. Additionally, there was a correlation of 0.956 between the echocardiography- and CT-selected phases. This initial work suggests that B-mode echocardiography can be used as a tool to develop CT-compatible gating techniques based on modalities derived from cardiac mechanics rather than relying on the ECG alone.

Cardiac-gated computed tomography angiography in three alpacas with complex congenital heart disease

BACKGROUND

The prevalence of congenital heart disease is higher in camelids than in other domestic species and complex defects, often involving the great vessels, are more frequently encountered in llamas and alpacas than in other species. Some of these complex defects can be difficult to accurately characterize via echocardiography, the most commonly used diagnostic imaging technique to evaluate the heart in veterinary patients. Contrast-enhanced, electrocardiogram (ECG)-gated computed tomography (CT) has proven utility for the evaluation of human patients with certain congenital heart defects, including those with conotruncal septation defects and other abnormalities involving the formation of the great vessels.

METHODS

Three alpaca crias, 4 days, 5 weeks and 14 months of age were clinically evaluated and subjected to a complete color-flow Doppler echocardiogram and a contrast-enhanced ECG-gated CT.

RESULTS

These alpacas exhibited a variety of clinical findings including lethargy, failure to thrive, exercise intolerance, heart murmur, and/or respiratory difficulty. All three crias were subsequently diagnosed with complex cardiac defects including pulmonary atresia with a ventricular septal defect (VSD), a truncus arteriosus with a large VSD, and a double outlet right ventricle with a large VSD and aortic hypoplasia. In each case, the diagnosis was confirmed by postmortem examination.

CONCLUSION

Color flow echocardiographic evaluation identified all of the intra-cardiac lesions and associated flow anomalies but contrast-enhanced ECG-gated CT permitted more accurate assessment of the morphology of the extracardiac structures and permitted a more precise determination of the exact nature and anatomy of the great vessels.

Relationship between cardiac quiescent periods derived from seismocardiography and echocardiography

The seismocardiogram (SCG) is a measure of chest wall acceleration due to cardiac motion that could potentially supplement the electrocardiogram (ECG) to more reliably predict cardiac quiescence. Accurate prediction is critical for modalities requiring minimal motion during imaging data acquisition, such as cardiac computed tomography (CT) and magnetic resonance imaging (MRI). For seven healthy subjects, SCG and B-mode echocardiography were used to identify quiescent periods on a beat-by-beat basis. Quiescent periods were detected as time intervals when the magnitude of the velocity signals calculated from SCG and echocardiography were less than a specified threshold. The quiescent periods detected from SCG were compared to those detected from B-mode echocardiography. The quiescent periods of the SCG were found to occur before those detected by echocardiography. A linear relationship between the delay from SCG- to echocardiography-detected phases with respect to heart rate was found. This delay could potentially be used to predict cardiac quiescence from SCG-observed quiescence for use with cardiac imaging modalities such as CT and MRI.

The role of cardiac CT angiography in the pre- and postoperative evaluation of tetralogy of Fallot

Tetralogy of Fallot (TOF) is the first described and most common cyanotic congenital heart anomaly that generated the first successful surgical palliation procedure and definitive intracardiac repair. Classical TOF comprises the four typical features of right ventricular outflow tract obstruction (RVOTO), right ventricular hypertrophy (RVH), ventricular septal defect (VSD) and aortic dextroposition. Complex forms of the condition include TOF associated with absent pulmonary valve and TOF with pulmonary atresia (TOF-PA) with or without major aortopulmonary collateral arteries (MAPCAs). The pathophysiological understanding that the tetrad is basically the sequel of a singular abnormality of infundibular malformation, with anterior deviation of the infundibular septum, had major surgical consequences that improved survival. Diagnostic and functional imaging play a key role in the clinical and surgical management of patients with TOF. We revisit the role of traditionally employed imaging modalities (echocardiography and cardiac catheterisation) only briefly as these remain within the domain of cardiology practice. The emphasis of the present review is to outline the role of cardiac computed tomographic angiography (CTA) in the evaluation of TOF, with special reference to the technical considerations and best practice recommendations. Cardiovascular magnetic resonance imaging (CMR) is addressed only in passing, as this service is not currently available at our institution.

Characterization of cardiac quiescence from retrospective cardiac computed tomography using a correlation-based phase-to-phase deviation measure

PURPOSE

Accurate knowledge of cardiac quiescence is crucial to the performance of many cardiac imaging modalities, including computed tomography coronary angiography (CTCA). To accurately quantify quiescence, a method for detecting the quiescent periods of the heart from retrospective cardiac computed tomography (CT) using a correlation-based, phase-to-phase deviation measure was developed.

METHODS

Retrospective cardiac CT data were obtained from 20 patients (11 male, 9 female, 33-74 yr) and the left main, left anterior descending, left circumflex, right coronary artery (RCA), and interventricular septum (IVS) were segmented for each phase using a semiautomated technique. Cardiac motion of individual coronary vessels as well as the IVS was calculated using phase-to-phase deviation. As an easily identifiable feature, the IVS was analyzed to assess how well it predicts vessel quiescence. Finally, the diagnostic quality of the reconstructed volumes from the quiescent phases determined using the deviation measure from the vessels in aggregate and the IVS was compared to that from quiescent phases calculated by the CT scanner. Three board-certified radiologists, fellowship-trained in cardiothoracic imaging, graded the diagnostic quality of the reconstructions using a Likert response format: 1 = excellent, 2 = good, 3 = adequate, 4 = nondiagnostic.

RESULTS

Systolic and diastolic quiescent periods were identified for each subject from the vessel motion calculated using the phase-to-phase deviation measure. The motion of the IVS was found to be similar to the aggregate vessel (AGG) motion. The diagnostic quality of the coronary vessels for the quiescent phases calculated from the aggregate vessel (PAGG) and IVS (PIV S) deviation signal using the proposed methods was comparable to the quiescent phases calculated by the CT scanner (PCT). The one exception was the RCA, which improved for PAGG for 18 of the 20 subjects when compared to PCT (PCT = 2.48; PAGG = 2.07, p = 0.001).

CONCLUSIONS

A method for quantifying the motion of specific coronary vessels using a correlation-based, phase-to-phase deviation measure was developed and tested on 20 patients receiving cardiac CT exams. The IVS was found to be a suitable predictor of vessel quiescence. The diagnostic quality of the quiescent phases detected by the proposed methods was comparable to those calculated by the CT scanner. The ability to quantify coronary vessel quiescence from the motion of the IVS can be used to develop new CTCA gating techniques and quantify the resulting potential improvement in CTCA image quality.

Cardiac-Gated En Face Doppler Measurement of Retinal Blood Flow Using Swept-Source Optical Coherence Tomography at 100,000 Axial Scans per Second

PURPOSE

To develop and demonstrate a cardiac gating method for repeatable in vivo measurement of total retinal blood flow (TRBF) in humans using en face Doppler optical coherence tomography (OCT) at commercially available imaging speeds.

METHODS

A prototype swept-source OCT system operating at 100-kHz axial scan rate was developed and interfaced with a pulse oximeter. Using the plethysmogram measured from the earlobe, Doppler OCT imaging of a 1.5- × 2-mm area at the optic disc at 1.8 volumes/s was synchronized to cardiac cycle to improve sampling of pulsatile blood flow. Postprocessing algorithms were developed to achieve fully automatic calculation of TRBF. We evaluated the repeatability of en face Doppler OCT measurement of TRBF in 10 healthy young subjects using three methods: measurement at 100 kHz with asynchronous acquisition, measurement at 100 kHz with cardiac-gated acquisition, and a control measurement using a 400-kHz instrument with asynchronous acquisition.

RESULTS

The median intrasubject coefficients of variation (COV) of the three methods were 8.0%, 4.9%, and 6.1%, respectively. All three methods correlated well, without a significant bias. Mean TRBF measured at 100 kHz with cardiac-gated acquisition was 40.5 ± 8.2 μL/min, and the range was from 26.6 to 55.8 μL/min.

CONCLUSIONS

Cardiac-gated en face Doppler OCT can achieve smaller measurement variability than previously reported methods. Although further validation in older subjects and diseased subjects is required, precise measurement of TRBF using cardiac-gated en face Doppler OCT at commercially available imaging speeds should be feasible.

Detection of quiescent cardiac phases in echocardiography data using nonlinear filtering and boundary detection techniques

We describe an algorithm to detect cardiac quiescence within a heartbeat using nonlinear filtering and boundary detection techniques in echocardiography images. The motivation for detection of these quiescent phases is to provide improved cardiac gating to obtain motion-artifact-free images of the heart at cardiac computed tomography (CT). Currently, cardiac gating is provided through electrocardiography (ECG), which does not provide information about the instantaneous mechanical state of the heart. Our goal is to test if information about the actual mechanical motion of the heart obtained from B-mode echocardiographic data could potentially be used for gating purposes. The nonlinear filtering algorithm presented involves anisotropic diffusion to smoothen the homogeneous regions of the B-mode images while preserving image edges that represent myocardial boundaries. Following this, we detect the boundary of a particular region of interest (ROI) using a thresholding step. The positional changes of this ROI are then observed for quiescent phases over multiple cardiac cycles using the ECG's R-R interval. In a pilot study, seven subjects were imaged in the apical, four-chamber view, and quiescence of the interventricular septum was primarily observed in the diastolic region of the ECG signal. However, the position and length of quiescence vary across multiple heartbeats for the same individual and for different individuals as well. The center of quiescence for the seven patients ranged from 51 to 84 % and did not show a trend with heart rates, which ranged from 54 to 83 beats per minute. The gating intervals based on such analysis of echocardiographic signals could potentially optimize cardiac CT gating.

Seismocardiography-Based Detection of Cardiac Quiescence

Cardiac-computed tomography angiography (CTA) is a minimally invasive imaging technology for characterizing coronary arteries. A fundamental limitation of CTA imaging is cardiac movement, which can cause artifacts and reduce the quality of the obtained images. To mitigate this problem, current approaches involve gating the image based on the electrocardiogram (ECG) to predict the timing of quiescent periods of the cardiac cycle. This paper focuses on developing a foundation for using a mechanical alternative to the ECG for finding these quiescent periods: the seismocardiogram (SCG). SCG was used to determine beat-by-beat systolic and diastolic quiescent periods of the cardiac cycle for nine healthy subjects, and 11 subjects with various cardiovascular diseases. To reduce noise in the SCG, and quantify these quiescent periods, a Kalman filter was designed to extract the velocity of chest wall movement from the recorded SCG signals. The average systolic and diastolic quiescent periods were centered at 29% and 76% for the healthy subjects, and 33% and 79% for subjects with cardiovascular disease. Both inter and intrasubject variability in the quiescent phases were observed compared to ECG-predicted phases, suggesting that the ECG may be a suboptimal modality for predicting quiescence, and that the SCG provides complementary data to the ECG.

Motion based X-ray imaging modality

A new X-ray imaging method (patent pending) was developed to visualize function-related motion information. We modify existing X-ray imaging methods to provide four images without increasing the necessary measurement time or radiation dose. The most important of these images is a new "kinetic" image that represents motions inside the object or living body. The motion-based contrast of the kinetic image can help visualize details that were not accessible before. The broad range of the movements and high sensitivity of the method are illustrated by imaging the mechanics of a working clock and the chest of a living African clawed frog (Xenopus laevis). The heart, valves, aorta, and lungs of the frog are clearly visualized in spite of the low soft tissue contrast of the animal. The new technology also reconstructs a "static" image similar to the existing conventional X-ray image. The static image shows practically the same information as the conventional image. The new technology presents two more images which show the point-wise errors of the static and kinetic images. This technique gives a better estimation of errors than present methods because it is based entirely on measured data. The new technology could be used in imaging cardiopulmonary movements, nondestructive testing, or port security screening.

Separating arterial and venous-related components of photoplethysmographic signals for accurate extraction of oxygen saturation and respiratory rate

We propose an algorithm for separating arterial and venous-related signals using second-order statistics of red and infrared signals in a blind source separation technique. The separated arterial signal is used to compute accurate arterial oxygen saturation. We have also introduced an algorithm for extracting the respiratory pattern from the extracted venous-related signal. In addition to real-time monitoring, respiratory rate is also extracted. Our experimental results from multiple subjects show that the proposed separation technique is extremely useful for extracting accurate arterial oxygen saturation and respiratory rate. Specifically, the breathing rate is extracted with average root mean square deviation of 1.89 and average mean difference of -0.69.

Image artefact propagation in motion estimation and reconstruction in interventional cardiac C-arm CT

The acquisition of data for cardiac imaging using a C-arm computed tomography system requires several seconds and multiple heartbeats. Hence, incorporation of motion correction in the reconstruction step may improve the resulting image quality. Cardiac motion can be estimated by deformable three-dimensional (3D)/3D registration performed on initial 3D images of different heart phases. This motion information can be used for a motion-compensated reconstruction allowing the use of all acquired data for image reconstruction. However, the result of the registration procedure and hence the estimated deformations are influenced by the quality of the initial 3D images. In this paper, the sensitivity of the 3D/3D registration step to the image quality of the initial images is studied. Different reconstruction algorithms are evaluated for a recently proposed cardiac C-arm CT acquisition protocol. The initial 3D images are all based on retrospective electrocardiogram (ECG)-gated data. ECG-gating of data from a single C-arm rotation provides only a few projections per heart phase for image reconstruction. This view sparsity leads to prominent streak artefacts and a poor signal to noise ratio. Five different initial image reconstructions are evaluated: (1) cone beam filtered-backprojection (FDK), (2) cone beam filtered-backprojection and an additional bilateral filter (FFDK), (3) removal of the shadow of dense objects (catheter, pacing electrode, etc) before reconstruction with a cone beam filtered-backprojection (cathFDK), (4) removal of the shadow of dense objects before reconstruction with a cone beam filtered-backprojection and a bilateral filter (cathFFDK). The last method (5) is an iterative few-view reconstruction (FV), the prior image constrained compressed sensing combined with the improved total variation algorithm. All reconstructions are investigated with respect to the final motion-compensated reconstruction quality. The algorithms were tested on a mathematical phantom data set with and without a catheter and on two porcine models using qualitative and quantitative measures. The quantitative results of the phantom experiments show that if no dense object is present within the scan field of view, the quality of the FDK initial images is sufficient for motion estimation via 3D/3D registration. When a catheter or pacing electrode is present, the shadow of these objects needs to be removed before the initial image reconstruction. An additional bilateral filter shows no major improvement with respect to the final motion-compensated reconstruction quality. The results with respect to image quality of the cathFDK, cathFFDK and FV images are comparable. In conclusion, in terms of computational complexity, the algorithm of choice is the cathFDK algorithm.

Advances in cardiac magnetic resonance imaging and computed tomography

Imaging evaluation of the heart encompasses structural evaluation of the chambers, valves and coronary arteries, and functional evaluation, including assessment of perfusion, wall motion and myocardial viability. Magnetic resonance imaging is well established for the structural and functional evaluation of the heart, and benefits from direct multiplanar image acquisition and a lack of ionizing radiation. Magnetic resonance imaging assessment of myocardial viability after myocardial infarction appears to be helpful in predicting benefit from revascularization procedures. Magnetic resonance imaging continues to hold promise as the least invasive method of coronary artery evaluation, and continuing developments are improving image quality and decreasing examination time. The development of cardiac-gating techniques for multidetector computed tomography has the potential to provide widespread availability of cardiac computed tomography. Short examination times and straightforward scanning procedures promise a convenient method for the examination of cardiac structure and function. However, this convenience must be balanced against radiation dose and contrast-media requirements when determining the appropriate use of cardiac computed tomography. Computed tomography coronary-calcium scoring can aid in the prediction of significant coronary events in all but the lowest-risk patients. The high negative-predictive value of computed tomography coronary angiography may allow some patients to avoid cardiac catheterization, but its role in the assessment of patients with moderate coronary atherosclerosis remains unclear. New software tools can assist in the complex and tedious analysis of the large volumes of data produced by these examinations.

Detection of Cardiac Quiescence from B-Mode Echocardiography Using a Correlation-Based Frame-to-Frame Deviation Measure

Two novel methods for detecting cardiac quiescent phases from B-mode echocardiography using a correlation-based frame-to-frame deviation measure were developed. Accurate knowledge of cardiac quiescence is crucial to the performance of many imaging modalities, including computed tomography coronary angiography (CTCA). Synchronous electrocardiography (ECG) and echocardiography data were obtained from 10 healthy human subjects (four male, six female, 23–45 years) and the interventricular septum (IVS) was observed using the apical four-chamber echocardiographic view. The velocity of the IVS was derived from active contour tracking and verified using tissue Doppler imaging echocardiography methods. In turn, the frame-to-frame deviation methods for identifying quiescence of the IVS were verified using active contour tracking. The timing of the diastolic quiescent phase was found to exhibit both inter- and intra-subject variability, suggesting that the current method of CTCA gating based on the ECG is suboptimal and that gating based on signals derived from cardiac motion are likely more accurate in predicting quiescence for cardiac imaging. Two robust and efficient methods for identifying cardiac quiescent phases from B-mode echocardiographic data were developed and verified. The methods presented in this paper will be used to develop new CTCA gating techniques and quantify the resulting potential improvement in CTCA image quality.

Computed tomographic coronary angiography - is it ready as a screening tool for coronary artery disease?

Currently, there are no formal screening programmes for coronary artery disease (CAD). Computed tomographic coronary angiography (CTCA) has been suggested as a non-invasive and reliable method of atherosclerotic plaque assessment, with the potential for use in screening programmes. In this article, we briefly present the current understanding of atherosclerotic plaque formation, explain key technological aspects of CTCA and critique this method in the light of World Health Organisation (WHO) criteria for devising a screening programme. Current evolving and future insights are also considered. Overall, in our view, there is currently insufficient evidence to support the formal use of CTCA in a screening programme for CAD, although this viewpoint will undoubtedly evolve.

Evaluation of interpolation methods for surface-based motion compensated tomographic reconstruction for cardiac angiographic C-arm data

PURPOSE

For interventional cardiac procedures, anatomical and functional information about the cardiac chambers is of major interest. With the technology of angiographic C-arm systems it is possible to reconstruct intraprocedural three-dimensional (3D) images from 2D rotational angiographic projection data (C-arm CT). However, 3D reconstruction of a dynamic object is a fundamental problem in C-arm CT reconstruction. The 2D projections are acquired over a scan time of several seconds, thus the projection data show different states of the heart. A standard FDK reconstruction algorithm would use all acquired data for a filtered backprojection and result in a motion-blurred image. In this approach, a motion compensated reconstruction algorithm requiring knowledge of the 3D heart motion is used. The motion is estimated from a previously presented 3D dynamic surface model. This dynamic surface model results in a sparse motion vector field (MVF) defined at control points. In order to perform a motion compensated reconstruction, a dense motion vector field is required. The dense MVF is generated by interpolation of the sparse MVF. Therefore, the influence of different motion interpolation methods on the reconstructed image quality is evaluated.

METHODS

Four different interpolation methods, thin-plate splines (TPS), Shepard's method, a smoothed weighting function, and a simple averaging, were evaluated. The reconstruction quality was measured on phantom data, a porcine model as well as on in vivo clinical data sets. As a quality index, the 2D overlap of the forward projected motion compensated reconstructed ventricle and the segmented 2D ventricle blood pool was quantitatively measured with the Dice similarity coefficient and the mean deviation between extracted ventricle contours. For the phantom data set, the normalized root mean square error (nRMSE) and the universal quality index (UQI) were also evaluated in 3D image space.

RESULTS

The quantitative evaluation of all experiments showed that TPS interpolation provided the best results. The quantitative results in the phantom experiments showed comparable nRMSE of ≈0.047 ± 0.004 for the TPS and Shepard's method. Only slightly inferior results for the smoothed weighting function and the linear approach were achieved. The UQI resulted in a value of ≈ 99% for all four interpolation methods. On clinical human data sets, the best results were clearly obtained with the TPS interpolation. The mean contour deviation between the TPS reconstruction and the standard FDK reconstruction improved in the three human cases by 1.52, 1.34, and 1.55 mm. The Dice coefficient showed less sensitivity with respect to variations in the ventricle boundary.

CONCLUSIONS

In this work, the influence of different motion interpolation methods on left ventricle motion compensated tomographic reconstructions was investigated. The best quantitative reconstruction results of a phantom, a porcine, and human clinical data sets were achieved with the TPS approach. In general, the framework of motion estimation using a surface model and motion interpolation to a dense MVF provides the ability for tomographic reconstruction using a motion compensation technique.

Effect of contrast material on radiation dose in an adult cardiac dual-energy CT using retrospective ECG-gating

The purpose of this study was to determine the effect of contrast material and retrospective ECG-gating on radiation dose in an adult cardiac dual-energy computed tomography (DECT). Sixty-two patients underwent CT cardiac examination with a Somatom Definition Flash DECT using tube voltages of 100 kV without filter and 140 kV with a tin filter (mean effective mA s: arterial 72.39 and 62.94, venous 93.21 and 78.45, and late phase 134.5 and 118.2). The arterial and late phases were examined with retrospective ECG-gating, but gating was not used for the venous phase. Seventy milliliters (70 ml) iodinated contrast material (CM) was injected into the patient during examination. The effective doses (ED) were calculated from dose-length-product (DLP) and computed tomographic dose index volume (CTDIvol) using the latest k-factor (0.028). Pearson's correlation coefficient was used for statistical tests on continuous variables. Mean CTDIvol and DLP were lower in the late phase (10.15 ± 1.5 mGy and 202.9 ± 23 mGy cm) compared to the arterial phase (19.69 ± 3 mGy and 394 ± 90 mGy cm). Differences between the arterial and late phase were statistically significant (p = 0.005), and mean values for the late phase were 48.5% lower than mean values for the arterial phase. Mean CTDIvol and DLP were lower in venous (7.72 ± 1 mGy and 154.3 ± 17 mGy cm) compared to late phase (10.15 ± 1.5 mGy and 202.9 ± 23 mGy cm). The difference between venous and late phase was statistically significant (p < 0.001). The mean results for the venous phase were 24% lower than those for the late phase. This study shows that contrast material (CM) absorbs radiation significantly and increases dose by 48.5% in an adult cardiac dual-energy CT with retrospective ECG-gating. Care must be taken to determine the type, concentration, and volume of CM used for the scan. The dual-energy non-ECG-gated technique decreased radiation dose by 24% compared to the ECG-gated technique. ECG-gated cardiac examination should be limited to patients with strong clinical indications. SNR and HU increased with decreasing energy. The image noise values showed a negligible difference in the arterial and late phase datasets, and this did not affect the diagnostic quality of the image evaluation.

MDCT coronary angiography -- postprocessing, reading, and reporting: last but not least

Significant literature on MDCT coronary angiography (MDCT-CA) has emerged in the last decade concerning patient's selection, technical aspects of different generations of CT equipment, ECG gating, contrast material and beta-blockade administration, acquisition parameters, and radiation dose. However, the literature regarding postprocessing, reading, and reporting is not so extensive. This review highlights the main elements of MDCT-CA data analysis, thereby allowing the radiologist to take full advantage of this technology and enable a structured report to be generated, promoting best practice with high-quality results.

A trimodal system for the acquisition of synchronous echocardiography, electrocardiography, and seismocardiography data

A novel system was developed to acquire synchronous echocardiography, electrocardiography (EKG), and seismocardiography (SCG) data. The system was developed to facilitate the study of the relationship between the mechanical and electrical characteristics of the heart. The system has both a hardware and software component. The hardware component consists of an application-specific device designed and built to acquire both SCG and EKG signals simultaneously. The software component consists of a package developed to record and synchronize data from both the device and a clinical ultrasound machine. A feasibility test was performed by simultaneous acquisition of a synchronous dataset from a human subject.

A system for seismocardiography-based identification of quiescent heart phases: implications for cardiac imaging

Seismocardiography (SCG), a representation of mechanical heart motion, may more accurately determine periods of cardiac quiescence within a cardiac cycle than the electrically derived electrocardiogram (EKG) and, thus, may have implications for gating in cardiac computed tomography. We designed and implemented a system to synchronously acquire echocardiography, EKG, and SCG data. The device was used to study the variability between EKG and SCG and characterize the relationship between the mechanical and electrical activity of the heart. For each cardiac cycle, the feature of the SCG indicating Aortic Valve Closure was identified and its time position with respect to the EKG was observed. This position was found to vary for different heart rates and between two human subjects. A color map showing the magnitude of the SCG acceleration and computed velocity was derived, allowing for direct visualization of quiescent phases of the cardiac cycle with respect to heart rate.

Five levels of PACS modularity: integrating 3D and other advanced visualization tools

The current array of PACS products and 3D visualization tools presents a wide range of options for applying advanced visualization methods in clinical radiology. The emergence of server-based rendering techniques creates new opportunities for raising the level of clinical image review. However, best-of-breed implementations of core PACS technology, volumetric image navigation, and application-specific 3D packages will, in general, be supplied by different vendors. Integration issues should be carefully considered before deploying such systems. This work presents a classification scheme describing five tiers of PACS modularity and integration with advanced visualization tools, with the goals of characterizing current options for such integration, providing an approach for evaluating such systems, and discussing possible future architectures. These five levels of increasing PACS modularity begin with what was until recently the dominant model for integrating advanced visualization into the clinical radiologist's workflow, consisting of a dedicated stand-alone post-processing workstation in the reading room. Introduction of context-sharing, thin clients using server-based rendering, archive integration, and user-level application hosting at successive levels of the hierarchy lead to a modularized imaging architecture, which promotes user interface integration, resource efficiency, system performance, supportability, and flexibility. These technical factors and system metrics are discussed in the context of the proposed five-level classification scheme.

An active contour based method for analyzing cardiac quiescence from echocardiography

A semi-automated method for analyzing cardiac quiescence of anatomical cardiac features from two-dimensional echocardiographic cine data is presented. The method utilizes both active contour and optical flow techniques for feature identification and tracking. A curvature-based potential surface was used in the active contour calculations to attract the contour to regions of inflection on the image surface rather than the standard gradient-based surface that attracts the contour to strong edges. After identifying the feature in each frame, the frame-to-frame correlation matrix of the feature was calculated with correlation values corresponding to how well the feature matched between frames. Therefore prolonged regions of high correlation correspond to periods of cardiac quiescence. The location and duration of these periods were automatically identified from the correlation matrix by finding the largest region around each time index with a mean correlation above a specified threshold. In parallel, the position of the feature was calculated for each frame by finding the centroid of the pixel locations inside the contour. From this trajectory, the magnitude of the two-dimensional velocity was calculated. These methods were used to analyze the quiescence of the interventricular septum from an apical four-chamber echocardiogram performed on a human subject. Correlation-derived quiescent phases were observed to coincide with periods of the cardiac cycle with minimal velocity magnitude.

Automatic segmentation of the wire frame of stent grafts from CT data

Endovascular aortic replacement (EVAR) is an established technique, which uses stent grafts to treat aortic aneurysms in patients at risk of aneurysm rupture. Late stent graft failure is a serious complication in endovascular repair of aortic aneurysms. Better understanding of the motion characteristics of stent grafts will be beneficial for designing future devices. In addition, analysis of stent graft movement in individual patients in vivo can be valuable for predicting stent graft failure in these patients. To be able to gather information on stent graft motion in a quick and robust fashion, we propose an automatic method to segment stent grafts from CT data, consisting of three steps: the detection of seed points, finding the connections between these points to produce a graph, and graph processing to obtain the final geometric model in the form of an undirected graph. Using annotated reference data, the method was optimized and its accuracy was evaluated. The experiments were performed using data containing the AneuRx and Zenith stent grafts. The algorithm is robust for noise and small variations in the used parameter values, does not require much memory according to modern standards, and is fast enough to be used in a clinical setting (65 and 30s for the two stent types, respectively). Further, it is shown that the resulting graphs have a 95% (AneuRx) and 92% (Zenith) correspondence with the annotated data. The geometric model produced by the algorithm allows incorporation of high level information and material properties. This enables us to study the in vivo motions and forces that act on the frame of the stent. We believe that such studies will provide new insights into the behavior of the stent graft in vivo, enables the detection and prediction of stent failure in individual patients, and can help in designing better stent grafts in the future.

Multidetector CT of Aortic Dissection: A Pictorial Review

Aortic dissection is the most common acute emergency condition of the aorta and often has a fatal outcome. Outcome is determined by the type and extent of dissection and the presence of associated complications (eg, cerebral sequelae, aortic branch involvement, pericardial involvement, and visceral involvement), with early diagnosis and treatment being essential for improved prognosis. Aortic dissections are classified on the basis of the site of the intimal tear according to the Stanford classification system. Type A aortic dissection involves the ascending thoracic aorta and may extend into the descending aorta, whereas in a type B dissection the intimal tear is located distal to the left subclavian artery. Type A dissection typically requires urgent surgical intervention, whereas type B dissection can often be treated medically. Modern multidetector computed tomography (CT) is a fast, widely available imaging modality with high sensitivity and specificity. Multidetector CT allows the early recognition and characterization of aortic dissection as well as determination of the presence of any associated complications, findings that are essential for optimizing treatment and improving clinical outcomes.

Radiation dose and cancer risk in retrospectively and prospectively ECG-gated coronary angiography using 64-slice multidetector CT

This study aimed to estimate the radiation dose and cancer risk to adults in England, the USA and Hong Kong associated with retrospectively and prospectively electrocardiogram (ECG)-gated coronary computed tomography angiography (CTA) using currently practised protocols in Hong Kong. The doses were simulated using the ImPACT spreadsheet. For retrospectively ECG-gated CTA with pitches of 0.2, 0.22 and 0.24, the effective doses were 27.7, 23.6 and 20.7 mSv, respectively, for males and 23.6, 20.0 and 18.8 mSv, respectively, for females. For prospectively ECG-gated CTA, the effective dose was 3.7 mSv for both males and females. A table of lifetime attributable risks (LAR) of cancer incidence was set up for the English population for the purpose of estimating cancer risk induced by low-dose radiation exposure, as previously reported for US and Hong Kong populations. From the tables, the LAR of cancer incidence for a representative 50-year-old subject was calculated for retrospectively ECG-gated CTA to be 0.112% and 0.227% for English males and females, respectively, 0.103% and 0.228% for US males and females, respectively, and was comparatively higher at 0.137% and 0.370% for Hong Kong males and females, respectively; for prospectively ECG-gated CTA, the corresponding values were calculated to be 0.014% and 0.035% for English males and females, respectively, and 0.013% and 0.036% for US males and females, respectively, and again were higher at 0.017% and 0.060% for Hong Kong males and females, respectively. Our study shows that prospectively ECG-gated CTA reduces radiation dose and cancer risks by up to 87% compared with retrospectively ECG-gated CTA.

The influence of reconstruction algorithm and heart rate on coronary artery image quality and stenosis detection at 64-detector cardiac CT

Objective

We wanted to evaluate the impact of two reconstruction algorithms (halfscan and multisector) on the image quality and the accuracy of measuring the severity of coronary stenoses by using a pulsating cardiac phantom with different heart rates (HRs).

Materials and Methods

Simulated coronary arteries with different stenotic severities (25, 50, 75%) and different luminal diameters (3, 4, 5 mm) were scanned with a fixed pitch of 0.16 and a 0.35 second gantry rotation time on a 64-slice multidetector CT. Both reconstruction algorithms (halfscan and multisector) were applied to HRs of 40-120 beats per minute (bpm) at 10 bpm intervals. Three experienced radiologists visually assessed the image quality and they manually measured the stenotic severity.

Results

Fewer measurement errors occurred with multisector reconstruction (p = 0.05), a slower HR (p < 0.001) and a larger luminal diameter (p = 0.014); measurement errors were not related with the observers or the stenotic severity. There was no significant difference in measurements as for the reconstruction algorithms below an HR of 70 bpm. More nonassessable segments were visualized with halfscan reconstruction (p = 0.004) and higher HRs (p < 0.001). Halfscan reconstruction had better quality scores when the HR was below 60 bpm, while multisector reconstruction had better quality scores when the HR was above 90 bpm. For the HRs between 60 and 90 bpm, both reconstruction modes had similar quality scores. With excluding the nonassessable segments, both reconstruction algorithms achieved a similar mean measured stenotic severity and similar standard deviations.

Conclusion

At a higher HR (above 90 bpm), multisector reconstruction had better temporal resolution, fewer nonassessable segments, better quality scores and better accuracy of measuring the stenotic severity in this phantom study.

Interventional 4-D motion estimation and reconstruction of cardiac vasculature without motion periodicity assumption

Anatomical and functional information of cardiac vasculature is a key component of future developments in the field of interventional cardiology. With the technology of C-arm CT it is possible to reconstruct intraprocedural 3-D images from angiographic projection data. Current approaches attempt to add the temporal dimension (4-D) by ECG-gating in order to distinct physical states of the heart. This model assumes that the heart motion is periodic. However, frequently arrhytmic heart signals are observed in a clinical environment. In addition breathing motion can still occur. We present a reconstruction method based on a 4-D time-continuous motion field which is parameterized by the acquisition time and not the quasi-periodic heart phase. The output of our method is twofold. It provides a motion compensated 3-D reconstruction (anatomic information) and a motion field (functional information). In a physical phantom experiment a vessel of size 3.08 mm undergoing a non-periodic motion was reconstructed. The resulting diameters were 3.42 mm and 1.85 mm assuming non-periodic and periodic motion, respectively. Further, for two clinical cases (coronary arteries and coronary sinus) it is demonstrated that the presented algorithm outperforms periodic approaches and is able to handle realistic irregular heart motion.

Physics of cardiac imaging with multiple-row detector CT

Cardiac imaging with multiple-row detector computed tomography (CT) has become possible due to rapid advances in CT technologies. Images with high temporal and spatial resolution can be obtained with multiple-row detector CT scanners; however, the radiation dose associated with cardiac imaging is high. Understanding the physics of cardiac imaging with multiple-row detector CT scanners allows optimization of cardiac CT protocols in terms of image quality and radiation dose. Knowledge of the trade-offs between various scan parameters that affect image quality--such as temporal resolution, spatial resolution, and pitch--is the key to optimized cardiac CT protocols, which can minimize the radiation risks associated with these studies. Factors affecting temporal resolution include gantry rotation time, acquisition mode, and reconstruction method; factors affecting spatial resolution include detector size and reconstruction interval. Cardiac CT has the potential to become a reliable tool for noninvasive diagnosis and prevention of cardiac and coronary artery disease.

ACR practice guideline for the performance and interpretation of cardiac computed tomography (CT)

Cardiac computed tomography (CT) is an evolving modality that includes a variety of examinations to assess the anatomy and pathology of the cardiac chambers, valves, myocardium, coronary arteries and veins, pericardium, aortic root, and central great vessels. The development of multidetector CT scanners with increasing numbers of detector rows, narrow section thicknesses, increasing scanner speeds, the ability for electrocardiographic gating, and radiation dose modulation allows the performance of CT coronary arteriography. Computed tomography coronary arteriography enables the assessment of multiple types of cardiac pathology, including intraluminal coronary arterial plaque formation, coronary artery stenosis, congenital anomalies, coronary artery aneurysms, sequelae of cardiac ischemia, and the assessment of prior vascular interventions, while providing information about cardiac and valvular function. Noncardiac structures included in cardiac CT examinations must also be evaluated. This guideline attempts to maximize the probability of detecting cardiac abnormalities with cardiac CT. American College of Radiology requirements for physicians and personnel performing examinations are also addressed and will become applicable by July 1, 2008.