Cardiac CT exams are some of the most complex CT exams due to need to carefully time the scan to capture the heart during the quiescent cardiac phase and when the contrast bolus is at its peak concentration. We are interested in developing a robust and autonomous cardiac CT protocol, using deep learning approaches to extract contrast timing and cardiac phase timing directly from pulsed projections. In this paper, we present a new approach to generate large amounts of clinically realistic virtual data for training deep learning networks. We propose a five-dimensional cardiac model generated from 4D cardiac coronary CT angiography (CTA) data for synthetic contrast bolus dynamics and patient ECG profiles. We apply deep learning to segment seven heart compartments and simulate intravenous contrast propagation through each compartment to insert contrast bolus. Additional augmentation techniques by randomizing a bolus curve, patient ECG profile, acquisition timing, and patient motion are applied to increase the amount of data that can be generated. We demonstrate good performance of the deep learning segmentation network, examples of simulated bolus curves using a realistic protocol, and good correspondence between virtually generated projections and real projections from patient scans.
Maintaining image quality in the presence of motion is always desirable and challenging in clinical Cardiac CT imaging. Different image-reconstruction algorithms are available on current commercial CT systems that attempt to achieve this goal. It is widely accepted that image-quality assessment should be task-based and involve specific tasks, observers, and associated figures of merits. In this work, we developed an observer model that performed the task of estimating the percentage of plaque in a vessel from CT images. We compared task performance of Cardiac CT image data reconstructed using a conventional FBP reconstruction algorithm and the SnapShot Freeze (SSF) algorithm, each at default and optimal reconstruction cardiac phases. The purpose of this work is to design an approach for quantitative image-quality evaluation of temporal resolution for Cardiac CT systems. To simulate heart motion, a moving coronary type phantom synchronized with an ECG signal was used. Three different percentage plaques embedded in a 3 mm vessel phantom were imaged multiple times under motion free, 60 bpm, and 80 bpm heart rates. Static (motion free) images of this phantom were taken as reference images for image template generation. Independent ROIs from the 60 bpm and 80 bpm images were generated by vessel tracking. The observer performed estimation tasks using these ROIs. Ensemble mean square error (EMSE) was used as the figure of merit. Results suggest that the quality of SSF images is superior to the quality of FBP images in higher heart-rate scans.
An analysis of a task based simulation study of coronary artery imaging via computed tomography (CT). Evaluation of standard filtered backprojection (FBP) reconstruction and motion compensated reconstruction of a moving cylindrical vessel that contains a hyper-intense lesion. Multiple conditions are simulated including: varying rest times of the vessel and varying motion orientations. A reference image with no motion was used for all comparisons. The images were segmented and quantitative metrics for accurate segmentation were compared. The motion compensated images have consistent error metrics with respect to the static case for all rest times. The FBP reconstructions were visually inferior for shorter rest times and had significantly inferior metrics. This is the first demonstration of equivalent performance for a given task when the rest times are reduced well below the temporal aperture of the acquisition, using either advanced algorithms or different data acquisition such as multi-source geometries.
KEYWORDS: Liver, Tissues, Signal attenuation, Dual energy imaging, Monte Carlo methods, Biopsy, X-ray computed tomography, Imaging spectroscopy, Visualization, Medicine
Nonalcoholic steatohepatitis (NASH) is a liver disease that occurs in patients that lack a history of the well-proven association of alcohol use. A major symptom of NASH is increased fat deposition in the liver. Gemstone Spectral Imaging (GSI) with fast kVp-switching enables projection-based material decomposition, offering the opportunity to accurately characterize tissue types, e.g., fat and healthy liver tissue, based on their energy-sensitive material attenuation and density. We describe our pilot efforts to apply GSI to locate and quantify the amount of fat deposition in the liver. Two approaches are presented, one that computes percentage fat from the difference in HU values at high and low energies and the second based on directly computing fat volume fraction at each voxel using multi-material decomposition. Simulation software was used to create a phantom with a set of concentric rings, each composed of fat and soft tissue in different relative amounts with attenuation values obtained from the National Institute of Standards and Technology. Monte Carlo 80 and 140 kVp X-ray projections were acquired and CT images of the phantom were reconstructed. Results demonstrated the sensitivity of dual energy CT to the presence of fat and its ability to distinguish fat from soft tissue. Additionally, actual patient (liver) datasets were acquired using GSI and monochromatic images at 70 and 140 keV were reconstructed. Preliminary results demonstrate a tissue sensitivity that appears sufficient to quantify fat content with a degree of accuracy as may be needed for non-invasive clinical assessment of NASH.
The feasibility and utility of creating virtual un-enhanced images from contrast enhanced data acquired using a fast
switching dual energy CT acquisition, is explored. Utilizing projection based material decomposition data,
monochromatic images are generated and a Multi-material decomposition technique is applied. Quantitative and
qualitative evaluation is performed to assess the equivalence of Virtual Un-Enhanced (VUE) and True Un-enhanced
(TUE) for multiple tissue types and different organs in the abdomen. Ten patient cases were analyzed where a TUE
and a subsequent Contrast Enhanced (CE) acquisition were obtained using fast kVp-switching dual energy CT
utilizing Gemstone Spectral Imaging. Quantitative measurements were made by placing multiple Regions of Interest
on the different tissues and organs in both the TUE and the VUE images. The absolute Hounsfield Unit (HU)
differences in the mean values between TUE & VUE were calculated as well as the differences of the standard
deviations. Qualitative analysis was done by two radiologists for overall image quality, presence of residual contrast,
appearance of pathology, appearance and contrast of normal tissues and organs in comparison to the TUE. There is a
very strong correlation between the TUE and VUE images.
The clinical application of Gemstone Spectral ImagingTM, a fast kV switching dual energy acquisition, is explored in the
context of noninvasive kidney stone characterization. Utilizing projection-based material decomposition, effective
atomic number and monochromatic images are generated for kidney stone characterization. Analytical and experimental
measurements are reported and contrasted. Phantoms were constructed using stone specimens extracted from patients.
This allowed for imaging of the different stone types under similar conditions. The stone specimens comprised of Uric
Acid, Cystine, Struvite and Calcium-based compositions. Collectively, these stone types span an effective atomic
number range of approximately 7 to 14. While Uric Acid and Calcium based stones are generally distinguishable in conventional CT, stone compositions like Cystine and Struvite are difficult to distinguish resulting in treatment uncertainty. Experimental phantom measurements, made under increasingly complex imaging conditions, illustrate the impact of various factors on measurement accuracy. Preliminary clinical studies are reported.
Coronary CT Angiography (CTA) is limited in patients with calcified plaque and stents. CTA is unable to
confidently differentiate fibrous from lipid plaque. Fast switched dual energy CTA offers certain advantages. Dual
energy CTA removes calcium thereby improving visualization of the lumen and potentially providing a more
accurate measure of stenosis. Dual energy CTA directly measures calcium burden (calcium hydroxyapatite) thereby
eliminating a separate non-contrast series for Agatston Scoring. Using material basis pairs, the differentiation of
fibrous and lipid plaques is also possible.
Patency of a previously stented coronary artery is difficult to visualize with CTA due to resolution
constraints and localized beam hardening artifacts. Monochromatic 70 keV or Iodine images coupled with Virtual
Non-stent images lessen beam hardening artifact and blooming. Virtual removal of stainless steel stents improves
assessment of in-stent re-stenosis.
A beating heart phantom with 'cholesterol' and 'fibrous' phantom coronary plaques were imaged with dual
energy CTA. Statistical classification methods (SVM, kNN, and LDA) distinguished 'cholesterol' from 'fibrous'
phantom plaque tissue. Applying this classification method to 16 human soft plaques, a lipid 'burden' may be useful
for characterizing risk of coronary disease. We also found that dual energy CTA is more sensitive to iodine contrast
than conventional CTA which could improve the differentiation of myocardial infarct and ischemia on delayed
acquisitions.
These phantom and patient acquisitions show advantages with using fast switched dual energy CTA for
coronary imaging and potentially extends the use of CT for addressing problem areas of non-invasive evaluation of
coronary artery disease.
Hypodense metastases are not always completely distinguishable from benign cysts in the liver using conventional
Computed Tomography (CT) imaging, since the two lesion types present with overlapping intensity distributions
due to similar composition as well as other factors including beam hardening and patient motion. This problem
is extremely challenging for small lesions with diameter less than 1 cm. To accurately characterize such lesions,
multiple follow-up CT scans or additional Positron Emission Tomography or Magnetic Resonance Imaging exam
are often conducted, and in some cases a biopsy may be required after the initial CT finding. Gemstone
Spectral Imaging (GSI) with fast kVp switching enables projection-based material decomposition, offering the
opportunity to discriminate tissue types based on their energy-sensitive material attenuation and density. GSI
can be used to obtain monochromatic images where beam hardening is reduced or eliminated and the images
come inherently pre-registered due to the fast kVp switching acquisition. We present a supervised learning
method for discriminating between cysts and hypodense liver metastases using these monochromatic images.
Intensity-based statistical features extracted from voxels inside the lesion are used to train optimal linear and
nonlinear classifiers. Our algorithm only requires a region of interest within the lesion in order to compute
relevant features and perform classification, thus eliminating the need for an accurate segmentation of the lesion.
We report classifier performance using M-fold cross-validation on a large lesion database with radiologist-provided
lesion location and labels as the reference standard. Our results demonstrate that (a) classification using a single
projection-based spectral CT image, i.e., a monochromatic image at a specified keV, outperforms classification
using an image-based dual energy CT pair, i.e., low and high kVp images derived from the same fast kVp
acquisition and (b) classification using monochromatic images can achieve very high accuracy in separating
benign liver cysts and metastases, especially for small lesions.
With increasing longitudinal detector dimension available in diagnostic volumetric CT, step-and-shoot scan is
becoming popular for cardiac imaging. In comparison to helical scan, step-and-shoot scan decouples patient table
movement from cardiac gating/triggering, which facilitates the cardiac imaging via multi-sector data acquisition, as well
as the administration of inter-cycle heart beat variation (arrhythmia) and radiation dose efficiency. Ideally, a multi-sector
data acquisition can improve temporal resolution at a factor the same as the number of sectors (best scenario). In reality,
however, the effective temporal resolution is jointly determined by gantry rotation speed and patient heart beat rate,
which may significantly lower than the ideal or no improvement (worst scenario). Hence, it is clinically relevant to
investigate the behavior of effective temporal resolution in cardiac imaging with multi-sector data acquisition. In this
study, a 5-second cine scan of a porcine heart, which cascades 6 porcine cardiac cycles, is acquired. In addition to
theoretical analysis and motion phantom study, the clinical consequences due to the effective temporal resolution
variation are evaluated qualitative or quantitatively. By employing a 2-sector image reconstruction strategy, a total of 15
(the permutation of P(6, 2)) cases between the best and worst scenarios are studied, providing informative guidance for
the design and optimization of CT cardiac imaging in volumetric CT with multi-sector data acquisition.
Since the advent of multi-slice CT, helical scan has played an increasingly important role in cardiac imaging. With the
availability of diagnostic volumetric CT, step-and-shoot scan has been becoming popular recently. Step-and-shoot scan
decouples patient table motion from heart beating, and thus the temporal window for data acquisition and image
reconstruction can be optimized, resulting in significantly reduced radiation dose, improved tolerance to heart beat rate
variation and inter-cycle cardiac motion inconsistency. Multi-sector data acquisition and image reconstruction have been
utilized in helical cardiac imaging to improve temporal resolution, but suffers from the coupling of heart beating and
patient table motion. Recognizing the clinical demands, the multi-sector data acquisition scheme for step-and-shoot scan
is investigated in this paper. The most outstanding feature of the multi-sector data acquisition combined with the stepand-
shoot scan is the decoupling of patient table proceeding from heart beating, which offers the opportunities of
employing prospective ECG-gating to improve dose efficiency and fine adjusting cardiac imaging phase to suppress
artifacts caused by inter-cycle cardiac motion inconsistency. The improvement in temporal resolution and the resultant
suppression of motion artifacts are evaluated via motion phantoms driven by artificial ECG signals. Both theoretical
analysis and experimental evaluation show promising results for multi-sector data acquisition scheme to be employed
with the step-and-shoot scan. With the ever-increasing gantry rotation speed and detector longitudinal coverage in stateof-
the-art VCT scanners, it is expected that the step-and-shoot scan with multi-sector data acquisition scheme would play
an increasingly important role in cardiac imaging using diagnostic VCT scanners.
We have developed an adaptive filtering algorithm for cardiac CT scans with EKG-modulated tube current to optimize resolution and noise for different cardiac phases and to provide safety net for cases where end-systole phase is used for coronary imaging. This algorithm has been evaluated using patient cardiac CT scans where lower tube currents are used for the systolic phases. In this paper, we present the evaluation results. The results demonstrated that with the use of the proposed algorithm, we could improve image quality for all cardiac phases, while providing greater noise and streak artifact reduction for systole phases where lower CT dose were used.
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