KEYWORDS: Image segmentation, Image processing algorithms and systems, Lung, Medical imaging, Computed tomography, Magnetic resonance imaging, 3D image processing, Image processing, 3D metrology, Brain
Radiologists are required to read thousands of patient images every day, and any tools that can improve their workflow to help them make efficient and accurate measurements is of great value. Such an interactive tool must be intuitive to use, and we have found that users are accustomed to clicking on the contour of the object for segmentation and would like the final segmentation to pass through these points. The tool must also be fast to enable real-time interactive feedback. To meet these needs, we present a segmentation workflow that enables an intuitive method for fast interactive segmentation of 2D and 3D objects. Given simple user clicks on the contour of an object in one 2D view, the algorithm generates foreground and background seeds and computes foreground and background distributions that are used to segment the object in 2D. It then propagates the information to the two orthogonal planes in a 3D volume and segments all three 2D views. The automated segmentation is automatically updated as the user continues to add points around the contour, and the algorithm is re-run using the total set of points. Based on the segmented objects in these three views, the algorithm then computes a 3D segmentation of the object. This process requires only limited user interaction to segment complex shapes and significantly improves the workflow of the user.
KEYWORDS: Image segmentation, Lung, Medical imaging, Liver, Pathology, Computed tomography, Principal component analysis, 3D image processing, Emphysema, 3D metrology
Minkowski Functionals (MFs) are geometric measurements of 3D shapes, including volume, surface area, curvature
and Euler number. MFs can be used as texture descriptors for medical image analysis in the segmentation
of normal anatomy as well as in the detection/diagnosis of pathology. In this paper, we propose a method for
fast computation of MFs based on integral images, which offers significantly improved accuracy and efficiency
compared with previous works. In addition, MFs computed using our method are used in applications on image
segmentation and pathology detection. Our experiment results clearly demonstrate the potential of MFs in such
medical image analysis tasks.
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.
Spectral Computed Tomography (Spectral CT), and in particular fast kVp switching dual-energy computed tomography,
is an imaging modality that extends the capabilities of conventional computed tomography (CT). Spectral CT enables the
estimation of the full linear attenuation curve of the imaged subject at each voxel in the CT volume, instead of a scalar
image in Hounsfield units. Because the space of linear attenuation curves in the energy ranges of medical applications can
be accurately described through a two-dimensional manifold, this decomposition procedure would be, in principle, limited
to two materials. This paper describes an algorithm that overcomes this limitation, allowing for the estimation of N-tuples
of material-decomposed images. The algorithm works by assuming that the mixing of substances and tissue types in the
human body has the physicochemical properties of an ideal solution, which yields a model for the density of the imaged
material mix. Under this model the mass attenuation curve of each voxel in the image can be estimated, immediately
resulting in a material-decomposed image triplet. Decomposition into an arbitrary number of pre-selected materials can
be achieved by automatically selecting adequate triplets from an application-specific material library. The decomposition
is expressed in terms of the volume fractions of each constituent material in the mix; this provides for a straightforward,
physically meaningful interpretation of the data. One important application of this technique is in the digital removal of
contrast agent from a dual-energy exam, producing a virtual nonenhanced image, as well as in the quantification of the
concentration of contrast observed in a targeted region, thus providing an accurate measure of tissue perfusion.
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.
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