Digital Chest Phantom Research Articles

Design consideration of compact cardiac TOF-PET systems: a simulation study

Myocardial perfusion imaging (MPI) with PET plays a vital role in the management of coronary artery disease. High sensitivity systems can contribute to maximizing the potential value of PET MPI; therefore, we have proposed two novel detector arrangements, an elliptical geometry and a D-shape geometry, that are more sensitive and more compact than a conventional large-bore cylindrical geometry. Here we investigate two items: the benefits of the proposed geometries for cardiac imaging; and the effects of scatter components on cardiac PET image quality. Using the Geant4 toolkit, we modeled four time-of-flight (TOF) PET systems: an 80 cm diameter cylinder, a 40 cm diameter cylinder, a compact ellipse, and a compact D-shape. Spatial resolution and sensitivity were measured using point sources. Noise equivalent count rate and image quality were examined using an anthropomorphic digital chest phantom. The proposed geometries showed higher sensitivity and better count rate characteristics with a fewer number of detectors than the conventional large-bore cylindrical geometry. In addition, we found that the increased intensity of the scatter components was a big factor affecting the contrast in defect regions for such a compact geometry. It is important to address the issue of the increased intensity of the scatter components to develop a high-performance compact cardiac TOF PET system.

Simulation Study of High-sensitivity Cardiac-dedicated PET Systems with Different Geometries

Noninvasive quantification of myocardial blood flow with PET is a vital tool for detecting and monitoring of coronary artery disease. However, current standard cylindrical PET scanners are not optimized for cardiac imaging because they are designed mainly for whole-body imaging. In this study, we proposed two compact geometries, the elliptical geometry and the D-shape geometry, for cardiac-dedicated PET systems. We then evaluated their performance compared with a whole-body-size cylindrical geometry by using the Geant4 Monte Carlo simulation toolkit. In the simulation, an elliptical water phantom was scanned for 10-sec, and we calculated the sensitivity and the noise-equivalent count rate (NECR). Subsequently, a digital chest phantom was scanned for 30-sec and the coincidence data were reconstructed by in-house image reconstruction software. We evaluated the image noise in the liver region and the contrast recoveries in the heart region. Even with the limited number of detectors, the proposed compact geometries showed higher sensitivity than the whole-body geometry. The D-shape geometry achieved 47% higher NECR and 44% lower image noise compared with the whole-body cylindrical geometry. However, the contrasts in the hot area obtained by the proposed compact geometries were not as good as that obtained by the whole-body cylindrical geometry. There was no considerable difference in image quality between the elliptical geometry and the D-shape geometry. In conclusion, the compact geometries we have proposed are promising designs for a high-sensitivity and low-cost cardiac-dedicated PET system. A further study using a defect phantom model is required to evaluate the contrast of cold areas.

WE‐FG‐207B‐06: Plaque Composition Measurement with Dual Energy Computed Tomography

Purpose:To investigate the feasibility of characterizing arterial plaque composition in terms of water, lipid and protein or calcium using dual energy computed tomography. Characterization of plaque composition can potentially help distinguish vulnerable from stable plaques.Methods:Simulations studies were performed by the CT simulator based on ASTRA tomography toolbox. The beam energy for dual energy images was selected to be 80 kVp and 135 kVp. The radiation dose and energy spectrum for the CT simulator were carefully calibrated with respect to a 320‐slice CT scanner. A digital chest phantom was constructed using Matlab for calibration and plaque measurement. Pure water, lipid, protein or calcium was used for calibration and a mixture of different volume percentages of these materials were used for validation purposes. Non‐calcified plaque was simulated using water, lipid and protein with volumetric percentage range of 35%∼65%, 5%∼60% and 5%∼40%, respectively. Calcified plaque was simulated using water, lipid and calcium with volumetric percentage range of 50%∼80%, 8%∼45% and 3%∼13%, respectively. We employed iterative sinogram processing (ISP) to reduce the beam hardening effect in the simulation to improve the decomposition results.Results:The simulated known composition and dual energy decomposition results were in good agreement. Water, lipid and protein (calcium) mixtures were decomposed into water, lipid and protein (calcium) contents. The RMS errors of volumetric percentage for the water, lipid and protein (non‐calcified plaque) decomposition, as compared to known values, were estimated to be approximately 5.74%, 2.54%, and 0.95% respectively. The RMS errors of volumetric percentage for the water, lipid and Calcium (calcified plaque) decomposition, as compared to known values, were estimated to be approximately 7.4%, 8.64%, and 0.08% respectively.Conclusion:The results of this study suggest that the dual energy decomposition can potentially be used to quantify the water, lipid, and protein or calcium composition of a plaque with relatively good accuracy.Grant funding from Toshiba Medical Systems and Philips Medical Systems

A dual-view digital tomosynthesis imaging technique for improved chest imaging.

Digital tomosynthesis (DTS) has been shown to be useful for reducing the overlapping of abnormalities with anatomical structures at various depth levels along the posterior-anterior (PA) direction in chest radiography. However, DTS provides crude three-dimensional (3D) images that have poor resolution in the lateral view and can only be displayed with reasonable quality in the PA view. Furthermore, the spillover of high-contrast objects from off-fulcrum planes generates artifacts that may impede the diagnostic use of the DTS images. In this paper, the authors describe and demonstrate the use of a dual-view DTS technique to improve the accuracy of the reconstructed volume image data for more accurate rendition of the anatomy and slice images with improved resolution and reduced artifacts, thus allowing the 3D image data to be viewed in views other than the PA view. With the dual-view DTS technique, limited angle scans are performed and projection images are acquired in two orthogonal views: PA and lateral. The dual-view projection data are used together to reconstruct 3D images using the maximum likelihood expectation maximization iterative algorithm. In this study, projection images were simulated or experimentally acquired over 360° using the scanning geometry for cone beam computed tomography (CBCT). While all projections were used to reconstruct CBCT images, selected projections were extracted and used to reconstruct single- and dual-view DTS images for comparison with the CBCT images. For realistic demonstration and comparison, a digital chest phantom derived from clinical CT images was used for the simulation study. An anthropomorphic chest phantom was imaged for the experimental study. The resultant dual-view DTS images were visually compared with the single-view DTS images and CBCT images for the presence of image artifacts and accuracy of CT numbers and anatomy and quantitatively compared with root-mean-square-deviation (RMSD) values computed using the digital chest phantom or the CBCT images as the reference in the simulation and experimental study, respectively. High-contrast wires with vertical, oblique, and horizontal orientations in a PA view plane were also imaged to investigate the spatial resolutions and how the wire signals spread in the PA view and lateral view slice images. Both the digital phantom images (simulated) and the anthropomorphic phantom images (experimentally generated) demonstrated that the dual-view DTS technique resulted in improved spatial resolution in the depth (PA) direction, more accurate representation of the anatomy, and significantly reduced artifacts. The RMSD values corroborate well with visual observations with substantially lower RMSD values measured for the dual-view DTS images as compared to those measured for the single-view DTS images. The imaging experiment with the high-contrast wires shows that while the vertical and oblique wires could be resolved in the lateral view in both single- and dual-view DTS images, the horizontal wire could only be resolved in the dual-view DTS images. This indicates that with single-view DTS, the wire signals spread liberally to off-fulcrum planes and generated wire shadow there. The authors have demonstrated both visually and quantitatively that the dual-view DTS technique can be used to achieve more accurate rendition of the anatomy and to obtain slice images with improved resolution and reduced artifacts as compared to the single-view DTS technique, thus allowing the 3D image data to be viewed in views other than the PA view. These advantages could make the dual-view DTS technique useful in situations where better separation of the objects-of-interest from the off-fulcrum structures or more accurate 3D rendition of the anatomy are required while a regular CT examination is undesirable due to radiation dose considerations.

エネルギーサブトラクション技術を研究するためのディジタル胸部ファントムの開発

Digital chest phantoms continue to play a significant role in optimizing imaging parameters for chest X-ray examinations. The purpose of this study was to develop a digital chest phantom for studies on energy subtraction techniques under ideal conditions without image noise. Computed tomography (CT) images from the LIDC (Lung Image Database Consortium) were employed to develop a digital chest phantom. The method consisted of the following four steps: 1) segmentation of the lung and bone regions on CT images; 2) creation of simulated nodules; 3) transformation to attenuation coefficient maps from the segmented images; and 4) projection from attenuation coefficient maps. To evaluate the usefulness of digital chest phantoms, we determined the contrast of the simulated nodules in projection images of the digital chest phantom using high and low X-ray energies, soft tissue images obtained by energy subtraction, and "gold standard" images of the soft tissues. Using our method, the lung and bone regions were segmented on the original CT images. The contrast of simulated nodules in soft tissue images obtained by energy subtraction closely matched that obtained using the gold standard images. We thus conclude that it is possible to carry out simulation studies based on energy subtraction techniques using the created digital chest phantoms. Our method is potentially useful for performing simulation studies for optimizing the imaging parameters in chest X-ray examinations.

Spectral CT imaging of vulnerable plaque with two independent biomarkers

The purpose of this paper is to investigate the feasibility of a novel four-material decomposition technique for assessing the vulnerability of plaque with two contrast materials spectral computer tomography (CT) using two independent markers: plaque's inflammation and spotty calcification. A simulation study was conducted using an energy-sensitive photon-counting detector for k-edge imaging of the coronary arteries. In addition to detecting the inflammation status, which is known as a biological marker of a plaque's vulnerability, we use spotty calcium concentration as an independent marker to test a plaque's vulnerability. We have introduced a new method for detecting and quantifying calcium concentrations in the presence of two contrast materials (iodine and gold), calcium and soft tissue background. In this method, four-material decomposition was performed on a pixel-by-pixel basis, assuming there was an arbitrary mixture of materials in the voxel. The concentrations of iodine and gold were determined by the k-edge material decomposition based on the maximum likelihood method. The calibration curves of the attenuation coefficients, with respect to the concentrations of different materials, were used to separate the calcium signal from both contrast materials and different soft tissues in the mixtures. Three different materials (muscle, blood and lipid) were independently used as soft tissue. The simulations included both ideal and more realistic energy resolving detectors to measure the polychromatic photon spectrum in single slice parallel beam geometry. The ideal detector was used together with a 3 cm diameter digital phantom to demonstrate the decomposition method while a more realistic detector and a 33 × 24 cm2 digital chest phantom were simulated to validate the vulnerability assessment technique. A 120 kVp spectrum was generated to produce photon flux sufficient for detecting contrast materials above the k-edges of iodine (33.2 keV) and gold (80.7 keV). By performing simulations on a 3 cm diameter digital phantom, we successfully identified four materials that were simultaneously present in the mixture at different proportions and in multiple locations on the phantom. Quantitative analysis with a chest digital phantom showed that the results for iodine, gold and calcium were highly correlated with the known concentrations. The analysis revealed a potentially powerful technique for assessing a plaque's vulnerability with two independent markers. High correlation and low relative errors between calculated and known materials’ concentrations showed that the method is feasible. This technique can potentially have a high clinical impact.

SU‐C‐218‐01: Comparison of Single‐View and Dual‐View Digital Chest Tomosynthesis

Purpose: Digital tomosynthesis (DTS) has been applied to chest imaging to crudely separate objects at different depth levels. However, the spatial resolution in the posterior‐anterior (PA) direction is very poor. We implement a dual‐view DTS technique to improve the spatial resolution along the PA direction and compare the results with the single‐view DTS technique and the cone beam CT (CBCT) technique. Methods: Computer simulations and experiments were conducted with the CBCT, dual‐view DTS and single‐view DTS techniques using the same geometry. For simulations, clinical CT images obtained at 120 kVp were used to generate a digital chest phantom, with which the projection images were computed. For imaging experiments, an anthropomorphic chest phantom was scanned with a bench top experimental CBCT system. 300 projections were obtained for CBCT imaging using the FDK algorithm for reconstruction. 51 projections covering 60 degrees in the PA direction were used for single‐view DTS imaging using the iterative algorithm for reconstruction. 25 projection over 60 degrees in the PA direction and another 25 images over 60 degrees in the lateral direction were used for dual view DTS. Results: For sagittal slice images, the CBCT image closely resembled the original phantom image. The dual‐view DTS images were fairly accurate in depicting the shape and dimensions of the anatomy in the fulcrum, despite the presence of some artifacts. With the single‐view DTS images, the anatomy was poorly depicted in the PA direction. For coronal slice images, the single‐view DTS images appeared less blurred but were much more affected by artifacts from the off‐fulcrum objects, resulting in severe distortion. The dual‐view DTS images, while not free from artifacts, provides more accurate rendition of the anatomy. Conclusions: Our results show that the dual‐view DTS technique substantially improved the spatial resolution of the reconstructed images in the PA direction. This work was supported in part by grants CA104759, CA124585, and CA138502A1 from NIH‐NCI, a research grant CA00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Fractal-Feature Distance Analysis of Radiographic Image

We have conducted a fractal analysis of low-dose digital chest phantom radiographs and evaluated the relationship between the fractal-feature distance and the tube current-exposure time product. Chest phantom radiographs were obtained at various mAs values (0.5-4.0 mAs) and 140 kVp with a computed radiography system, and the reference images were acquired at 13 mAs. The lung field images were converted to binary images after processing them using the rolling-ball technique; a fractal analysis was conducted using the box-counting method for these binary images. The fractal-feature distances between the low-dose and reference images were calculated using the fractal dimension and the complexity. For all binary images of lung fields, the relationship between the length of the square boxes and the number of boxes needed to cover the positive pixels of the binary image was linear on a log-log scale (r > or = 0.99). For mAs > or = 3.0, the fractal-feature distances were almost constant, whereas for mAs < or = 2.5, they increased depending on the reduction in mAs values. We have shown that a binary image of the lung field obtained from a chest phantom radiograph can be analyzed by the box-counting method and that its fractal-feature distance grows as the radiation dose declines.

SU‐FF‐I‐31: Diagnostic Utility Evaluation of a ASi:H/CsI(Tl) Flat‐Panel Based Digital Radiography System

Rationale and objectives: The aim of this work is to quantitatively evaluate the diagnostic utility of our digital radiography system based on a aSi:H/CsI(Tl) flat‐panel. Materials and Methods: Two commercially available amorphous silicon flat‐panel imaging systems are tested together using a chest phantom. This phantom contains the low contrast objects in the lung, heart and subdiaphragm regions, and the spatial resolution pattern in the lung region. Digital chest phantom images are acquired under the same environment, e.g., exposure condition, a focal spot, and a source‐to‐image receptor distance and a stationary antiscatter grid. All images are post‐processed by each image processing unit. Results: The signal‐to‐noise ratio(SNR) of the nine low contrast objects in three chest regions and the spatial resolution are analyzed. No significant difference is showed in three different image processing units based on the same detector. Conclusions: The resulting SNRs are in the range of Rose Model for human observers to detect a contrast object signal. Finally, the simple procedure employed in this work eliminates observer subjectivity and can be applied for the detectability measurement of low contrast objects in other digital diagnostic modalities.

TU‐C‐I‐609‐03: Effects of Exposure and Structural Background On the Detection of Computer‐Simulated Nodules in Digital Chest Phantom Images: 2‐AFC Study

Purpose: To investigate how the exposure level and structural background affect the detection of computer‐simulated nodules in digital chest phantom images. Method and Materials: Images of an anthropomorphic chest phantom were acquired with an a‐Si/CsI flat‐panel based digital radiography system at 120 kVp and a tube current ranging from 0.25 to 32 mAs. The source‐to‐image distance (SID) was increased and the x‐ray beam filtered to further reduce the exposure levels. 1‐cm diameter computer‐simulated nodules with a contrast ratio of 2.68% were computer generated at various locations of the chest images. Two‐alternate forced choice (2‐AFC) experiments were performed to measure the ratio of correct observations as a function of the exposure level for various structural backgrounds present. For each exposure level and location, 30 pairs of 512×512 images were generated and displayed in pairs on a review workstation for reading. The ratio of correct observations versus exposure levels were computed from the reading scores for various locations. The plots along with the exposures corresponding to 90% accuracy were used to study the effects of structural backgrounds. Results: For all regions, the accuracy generally increases from 0.5 to 1.0 with the exposure. However, the threshold exposure for 90% accuracy varies with the location. In the abdomen region, the ratio of correct observations reaches 90% at a higher exposure level (∼ 0.4 mAs). In the lung region, the ratio reaches 90% at a much lower exposure level (∼ 0.0625 mAs). The difference in the threshold exposures may be due to the heavier attenuation in the abdomen region. Complexity of structural backgrounds may affect the threshold exposure and the level and consistency of the ratios measured. This work was supported in part by a research grant EB000117 from the NIBIB and a research grant CA104759 from the NCI.