Distribution Of Attenuation Coefficient Research Articles

Objective CT-based quantification of lung sequelae in treated patients with paracoccidioidomycosis.

This study presents methodology for objectively quantifying the pulmonary region affected by emphysemic and fibrotic sequelae in treated patients with paracoccidioidomycosis. This methodology may also be applied to any other disease that results in these sequelae in the lungs.Pulmonary high-resolution computed tomography examinations of 30 treated paracoccidioidomycosis patients were used in the study. The distribution of voxel attenuation coefficients was analyzed to determine the percentage of lung volume that consisted of emphysemic, fibrotic, and normal tissue. Algorithm outputs were compared with subjective evaluations by radiologists using a scale that is currently used for clinical diagnosis.Affected regions in the patient images were determined by computational analysis and compared with estimates by radiologists, revealing mean (± standard deviation) differences in the scores for fibrotic and emphysemic regions of 0.1% ± 1.2% and −0.2% ± 1.0%, respectively.The computational results showed a strong correlation with the radiologist estimates, but the computation results were more reproducible, objective, and reliable.

A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging

The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenuation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV–140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for converting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.

Classification of bones from MR images in torso PET-MR imaging using a statistical shape model

There have been exclusive features for hybrid PET/MRI systems in comparison with its PET/CT counterpart in terms of reduction of radiation exposure, improved soft-tissue contrast and truly simultaneous and multi-parametric imaging capabilities. However, quantitative imaging on PET/MR is challenged by attenuation of annihilation photons through their pathway. The correction for photon attenuation requires the availability of patient-specific attenuation map, which accounts for the spatial distribution of attenuation coefficients of biological tissues. However, the lack of information on electron density in the MR signal poses an inherent difficulty to the derivation of the attenuation map from MR images. In other words, the MR signal correlates with proton densities and tissue relaxation properties, rather than with electron density and, as such, it is not directly related to attenuation coefficients. In order to derive the attenuation map from MR images at 511keV, various strategies have been proposed and implemented on prototype and commercial PET/MR systems. Segmentation-based methods generate an attenuation map by classification of T1-weighted or high resolution Dixon MR sequences followed by assignment of predefined attenuation coefficients to various tissue types. Intensity-based segmentation approaches fail to include bones in the attenuation map since the segmentation of bones from conventional MR sequences is a difficult task. Most MR-guided attenuation correction techniques ignore bones owing to the inherent difficulties associated with bone segmentation unless specialized MR sequences such as ultra-short echo (UTE) sequence are utilized. In this work, we introduce a new technique based on statistical shape modeling to segment bones and generate a four-class attenuation map. Our segmentation approach requires a torso bone shape model based on principle component analysis (PCA). A CT-based training set including clearly segmented bones of the torso region of 20 clinical studies was designed. Using this training set, a bone atlas was trained taking advantage of PCA analysis. Our active shape segmentation technique uses the trained shape model to segment bones from user defined initial seed points. The segmentation algorithm was evaluated using 10 clinical datasets (aligned MR and CT pairs). The resulting attenuation maps were compared to corresponding attenuation maps derived from CT resulting in a mean relative difference less than 7%.

Ultrasound Transmission Tomography Imaging of Structure of Breast Elastography Phantom Compared to US, CT and MRI

Abstract The paper presents an analysis of the results of ultrasound transmission tomography (UTT) imaging of the internal structure of a breast elastography phantom used for biopsy training, and compares them with the results of CT, MRI and, conventional US imaging; the results of the phantom examination were the basis for the analysis of UTT method resolution. The obtained UTT, CT and MRI images of the CIRS Model 059 breast phantom structure show comparable (in the context of size and location) heterogeneities inside it. The UTT image of distribution of the ultrasound velocity clearly demonstrates continuous changes of density. The UTT image of derivative of attenuation coefficient in relation to frequency is better for visualising sharp edges, and the UTT image of the distribution of attenuation coefficient visualises continuous and stepped changes in an indirect way. The inclusions visualized by CT have sharply delineated edges but are hardly distinguishable from the phantom gel background even with increased image contrast. MRI images of the studied phantom relatively clearly show inclusions in the structure. Ultrasonography images do not show any diversification of the structure of the phantom. The obtained examination results indicate that, if the scanning process is accelerated, ultrasound transmission tomography method can be successfully used to detect and diagnose early breast malignant lesions. Ultrasonic transmission tomography imaging can be applied in medicine for diagnostic examination of women’s breasts and similarly for X-ray computed tomography, while eliminating the need to expose patients to the harmful ionising radiation.

Assessment of the summer–autumn bloom in the Bohai Sea using satellite images to identify the roles of wind mixing and light conditions

In shallow coastal ecosystems, summer–autumn phytoplankton blooms require thorough investigation because of their large variability in space and time. This variability results primarily from the complex interplay of different governing factors, e.g., wind mixing and light availability. To identify the effects of these governing factors on the summer–autumn blooms in the Bohai Sea, we first established a statistical approach to define the biological variables that characterize summer–autumn blooms, i.e., the bloom onset time (denoted as bloom onset), the bloom magnitude, and the chlorophyll-a (Chl-a) increase rate during the bloom period (denoted as Chl-a increase rate). These variables were quantified using daily Medium Resolution Imaging Spectrometer (MERIS) satellite scenes from 2003 to 2009. We observed a remarkable onshore–offshore gradient in all three biological variables. For example, bloom onsets were delayed in coastal areas compared to deep offshore waters. Spatially, the bloom onset exhibited a high correlation with wind mixing prior to the bloom period (R>0.6), whereas the Chl-a increase rate and bloom magnitude were highly correlated to the distribution of the light attenuation coefficient (Kd(min), R=−0.65 and R=0.69, respectively). Furthermore, the empirical orthogonal function (EOF) analysis revealed similar interannual variability in the Kd(min), bloom onset and Chl-a increase rate. We also observed similar interannual trends in the bloom magnitude and wind mixing during the bloom period. From the analysis above, we believe that light is a crucial limiting factor for summer–autumn phytoplankton dynamics in the Bohai Sea. Wind influenced Chl-a dynamics in many ways and at various scales. Spatially, weak wind speeds prior to bloom events tended to provide favorable light conditions for the development of blooms. Over the long-term, however, enhanced wind mixing tended to supply the surface layer with nutrients more effectively and thus promoted a higher bloom intensity.

Characterization of multiphase flow in bubble columns using KT-1 signature and fractal dimension

This work presents the analysis of phase fraction distribution in bubble column reactor using KT-1 signature and fractal dimension. The experiment was carried out using X-ray CT scanner at Leibniz University Hannover. Convolution back projection algorithm is used to obtain the cross-sectional attenuation coefficient distribution. Individual phase distributions of the three phases (air, water and PVC), across the column cross-section, have been obtained using dual energy X-ray tomography. This paper reports measurement of phase fraction distribution at a cross-section level located at 3.2m from the inlet. The effect of variation of PVC concentration on phase fraction distribution of air and PVC has been investigated. Analysis of reconstructed phase fraction using KT-1 signature and fractal dimension reveals interesting information regarding the flow regime transition and mixing phenomenon in the bubble column.

Simultaneous reconstruction of emission activity and attenuation coefficient distribution from TOF data, acquired with external transmission source

The simultaneous PET data reconstruction of emission activity and attenuation coefficient distribution is presented, where the attenuation image is constrained by exploiting an external transmission source. Data are acquired in time-of-flight (TOF) mode, allowing in principle for separation of emission and transmission data. Nevertheless, here all data are reconstructed at once, eliminating the need to trace the position of the transmission source in sinogram space. Contamination of emission data by the transmission source and vice versa is naturally modeled. Attenuated emission activity data also provide additional information about object attenuation coefficient values. The algorithm alternates between attenuation and emission activity image updates. We also proposed a method of estimation of spatial scatter distribution from the transmission source by incorporating knowledge about the expected range of attenuation map values. The reconstruction of experimental data from the Siemens mCT scanner suggests that simultaneous reconstruction improves attenuation map image quality, as compared to when data are separated. In the presented example, the attenuation map image noise was reduced and non-uniformity artifacts that occurred due to scatter estimation were suppressed. On the other hand, the use of transmission data stabilizes attenuation coefficient distribution reconstruction from TOF emission data alone. The example of improving emission images by refining a CT-based patient attenuation map is presented, revealing potential benefits of simultaneous CT and PET data reconstruction.

Multidetector Row Computed Tomography Quantification of Bronchiolitis Obliterans Syndrome After Hematopoietic Stem Cell Transplantation

To evaluate multidetector row computed tomographic indexes to quantify bronchiolitis obliterans syndrome (BOS) after hematopoietic stem cell transplantation by correlation with pulmonary function tests (PFT). Eighteen patients with BOS after hematopoietic stem cell transplantation and 18 age-matched healthy volunteers were included. They underwent multidetector row computed tomography including inspiration and expiration scans. The patients also underwent PFTs. Mean lung density (MLD), relative areas (RAs) of lung with attenuation coefficients below specific thresholds, and 9 percentiles of the distribution of attenuation coefficients were computed. These parameters and PFT results were correlated. Mean MLD on inspiration (MLDin), MLD on expiration (MLDex), and the difference between the 2 (ΔMLD) were -858.5 HU, -788.6 HU, and 69.9, respectively, for the patients and -815.9 HU, -691.8 HU, and 124.1, respectively, for the volunteers. These parameters showed significant differences between the 2 groups (P<0.001) and strongly correlated with patients' residual forced expiratory volume in 1 second (FEV1) (r=0.71, 0.92, and 0.85, P≤0.001, respectively). RA with attenuation coefficients <-800 HU (RA800e) and the 80th percentile (80the) on expiration correlated most highly with the patients' residual FEV1 (r=-0.94, P<0.001). RA with attenuation coefficients <-800 HU (RA800e) and the 80th percentile (80the) on expiration are valid indexes of the patients' residual FEV1 in BOS.

Comparison of two single-image phase-retrieval algorithms for in-line x-ray phase-contrast imaging

The attenuation of x-rays in a material forms the basis of x-ray radiography and tomography. By measuring the transmission of the x-rays over a large amount of raypaths, the three-dimensional (3D) distribution of the x-ray linear attenuation coefficient can be reconstructed in a 3D volume. In x-ray microtomography (μCT), however, the x-ray refraction yields a significant signal in the transmission image and the 3D distribution of the refractive index can be reconstructed in a 3D volume. To do so, several methods exist, on both a hardware and software level. In this paper, we compare two similar software methods, the modified Bronnikov algorithm and the simultaneous phase-and-amplitude retrieval. The first method assumes a pure phase object, whereas the latter assumes a homogeneous object. Although these assumptions seem very restrictive, both methods have proven to yield good results on experimental data.

Examining the mean vertical attenuation of scalar quantum irradiance (PAR) over the Louisiana–Texas shelf (northern Gulf of Mexico)

We examined freshwater and ocean circulation effects on the distribution of vertical quantum diffuse attenuation coefficients (Kq0) of photosyntheticaly available radiation (PAR) in waters of the northern Gulf of Mexico's Louisiana–Texas shelf. Mean Kq0 coefficients were estimated from 509 vertical profiles of PAR collected during 10 cruises spanning 30 months (1992–1994). Vertical profiles of density revealed that the shelf waters are divided into two periods: a stratified period with an upper layer 10m thick of turbid waters (0.06≤Kq0≤1.18m−1) and a lower layer of more transparent waters (0.01≤Kq0≤0.49m−1). The second or non-stratified period consists of a homogenous layer ∼55m thick and less turbid waters (0.03≤Kq0≤1.00m−1). Horizontally, the distribution of Kq0 reveals nearshore coastal or case 2 waters followed by offshore oceanic or case 1 waters that separate near the 70-m isobath regardless of time and place. The Kq0 distribution reflects the freshwater influx from the Mississippi and Atchafalaya Rivers which causes a turbid surface trapped river plume, the shelf wind-driven circulation, and ensuing mixing. To investigate Kq0 we used two regression models involving salinity, suspended particulate matter (SPM), chlorophyll-a (Chl), and water depth. The best statistical model explained 57% to 85% of the observed Kq0 variability and involved the reciprocal of water depth, salinity, and SPM. However, a more bio-optically relevant model involving salinity, SPM, and Chl, explained only 32% to 64% of the observed Kq0 variability. Estimates of Kq0 for the upper layer indicate compensation depths of 30–92m in waters deeper than 70m which help account for the presence of coral communities on submerged banks near the shelf edge. The observed temporal and spatial distribution of Kq0 agrees qualitatively with that of satellite-derived values of the diffuse attenuation coefficient, Kd(490) over this shelf.

Synthetic Aperture Technique Applied to Tissue Attenuation Imaging

AbstractThe attenuating properties of biological tissue are of great importance in ultrasonic medical imaging. Investigations performedin vitroandin vivoshowed the correlation between pathological changes in the tissue and variation of the attenuation coefficient. In order to estimate the attenuation we have used the downshift of mean frequency (fm) of the interrogating ultrasonic pulse propagating in the medium. To determine thefmalong the propagation path we have applied thefmestimator (I/Q algorithm adopted from the Doppler mean frequency estimation technique). The mean-frequency shift trend was calculated using Single Spectrum Analysis. Next, the trends were converted into attenuation coefficient distributions and finally the parametric images were computed. The RF data were collected in simulations and experiments applying the synthetic aperture (SA) transmit-receiving scheme. In measurements the ultrasonic scanner enabling a full control of the transmission and reception was used. The resolution and accuracy of the method was verified using tissue mimicking phantom with uniform echogenicity but varying attenuation coefficient.

Development of a prototype gantry system for preclinical x‐ray phase‐contrast computed tomography

To explore the potential of grating-based x-ray phase-contrast imaging for clinical applications, a first compact gantry system was developed. It is designed such that it can be implemented into an in-vivo small-animal phase-contrast computed tomography (PC-CT) scanner. The purpose of the present study is to assess the accuracy and quantitativeness of the described gantry in both absorption and phase-contrast. A phantom, containing six chemically well-defined liquids, was constructed. A tomography scan with cone-beam reconstruction of this phantom was performed yielding the spatial distribution of the linear attenuation coefficient μ and decrement δ of the complex refractive index. Theoretical values of μ and δ were calculated for each liquid from tabulated data and compared with the experimentally measured values. Additionally, a color-fused image representation is proposed to display the complementary absorption and phase-contrast information in a single image. Experimental and calculated data of the phantom agree well confirming the quantitativeness and accuracy of the reconstructed spatial distributions of μ and δ. The proposed color-fused image representation, which combines the complementary absorption and phase information, considerably helps in distinguishing the individual substances. The concept of grating-based phase-contrast computed tomography (CT) can be implemented into a compact, cone-beam geometry gantry setup. The authors believe that this work represents an important milestone in translating phase-contrast x-ray imaging from previous proof-of-principle experiments to first preclinical biomedical imaging applications on small-animal models.

A virtual sinogram method to reduce dental metallic implant artefacts in computed tomography-based attenuation correction for PET

Attenuation correction of PET data requires accurate determination of the attenuation map (mumap), which represents the spatial distribution of linear attenuation coefficients of different tissues at 511 keV. The presence of high-density metallic dental filling material in head and neck X-ray computed tomography (CT) scanning is known to generate streak artefacts in the resulting CT images and thus in the corresponding mumaps generated using CT-based attenuation correction. Consequently, an under/overestimation of activity concentration occurs in corresponding regions of the corrected PET images. The purpose of this study is to develop a simple yet practical approach for reduction of metallic dental implant artefacts in the generated mumaps. Currently available sinogram-based metal artefact reduction (MAR) algorithms operate directly on the raw sinograms. These usually consist of huge files stored in proprietary format not easily disclosed by the manufacturers and thus are not straightforward to read and manipulate. The proposed method uses the concept of virtual sinograms produced by forward projection of CT images in Dicom format for MAR. The projection data affected by metallic objects are detected in the sinogram space through segmentation of metallic objects in the CT image followed by forward projection of the metal-only image. Thereafter, the affected sinogram bins are replaced by interpolated values from adjacent projections using the spline interpolation technique. The algorithm was assessed using a polyethylene phantom containing materials simulating different tissues and a dedicated jaw phantom scanned before and after the insertion of metallic objects, where the corrected and noncorrected mumaps were compared with the artefact-free mumap. In addition, the Jaszczak and standard germanium phantoms including four metallic inserts were scanned on a PET/CT scanner to evaluate the impact of the MAR procedure on PET data through the comparison of uncorrected and corrected PET images to the actual activity concentrations in the phantoms. The proposed algorithm was also applied to head and neck CT images of 10 patients with metallic dental implants. The MAR method proved to be practical in a clinical setting and reduced substantially the visible metal induced artefacts. The mean relative error in regions close to metallic objects is reduced by approximately 90%. The statistical analysis of the Jaszczak and solid Ge-68 phantoms PET images did not reveal statistically significant differences between the corrected and artefact-free images (P>0.05). Moreover, the evaluation of clinical studies did not reveal statistically significant differences between the attenuation coefficients of the corrected mumaps and the expected theoretical values. The proposed MAR method provides a simple and convenient approach allowing correction for the presence of metal artefacts caused by dental implants without the need to manipulate the complex raw CT data. Further evaluation using a larger clinical PET/CT database is under way to evaluate the potential of the technique in a clinical setting.

Tissue Attenuation Estimation from Backscattered Ultrasound Using Spatial Compounding Technique - Preliminary Results

The pathological states of biological tissue are often resulted in attenuation changes. Thus, information about attenuating properties of tissue is valuable for the physician and could be useful in ultrasonic diagnosis. We are currently developing a technique for parametric imaging of attenuation and we intend to apply it for in vivo characterization of tissue. The attenuation estimation method based on the echoes mean frequency changes due to tissue attenuation dispersion, is presented. The Doppler IQ technique was adopted to estimate the mean frequency directly from the raw RF data. The Singular Spectrum Analysis technique was used for the extraction of mean frequency trends. These trends were converted into attenuation distribution and finally the parametric images were computed. In order to reduce variation of attenuation estimates the spatial compounding method was applied. Operation and accuracy of attenuation extracting procedure was verified by calculating the attenuation coefficient distribution using the data from the tissue phantom (DFS, Denmark) with uniform echogenicity while attenuation coefficient underwent variation.

Non-destructive analysis of micro texture and grain boundary character from X-ray diffraction contrast tomography

Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting new possibilities for mapping 3D grain shapes and crystallographic orientations in different classes of polycrystalline materials. X-ray diffraction contrast tomography (DCT) is a monochromatic beam imaging technique combining the principles of X-ray micro-tomography and three-dimensional X-ray diffraction microscopy (3DXRD). DCT provides simultaneous access to 3D grain shape, crystallographic orientation and attenuation coefficient distribution at the micrometer length scale. The microtexture of the material can be quantified in more detail by post-processing of the volume data provided by DCT. In particular one can determine the local crystallographic habit plane of the grain boundary by analysing the surface normal of the grain boundary with respect to the crystal orientation. The resulting five parameter description of the character of individual grain boundaries could previously be produced only by destructive characterization techniques. Statistical analysis of this kind of data can be expected to provide new insight into various physico-chemical processes, driven by the grain boundary energy (corrosion, coarsening).

SU-FF-T-192: Complete 3D QA for Rapid Arc Using BANG Polymer Gel and OCTOPUS-IQ Fast Laser CT Scanner

Purpose: To provide rapid 3D QA and treatment verification for Varian's Rapid Arc. Method and Materials: Our 3D QA system is composed of a fast laser CT scanner (OCTOPUS‐IQ, MGS Research, Inc., Madison, CT), re‐usable acrylic cylindrical inserts for self‐developing polymer gels (BANG gel, MGS Research, Inc.), and a Virtual Water solid pelvic phantom (Med‐Cal, Inc., Verona, WI). A single focused laser beam is moved at 10 Hz frequency across a rotating gel cylinder immersed in a refractive‐index matching liquid in an aquarium. The field of view is adjustable up to 250 mm, the maximum length of scan is 250 mm, and the spatial resolution is adjustable down to 0.25 mm. The 3D distribution of optical attenuation coefficients in the polymergel is reconstructed by a filtered back‐projection open‐source program written in Matlab (MathWorks, Inc., Natick, MA). Image registration and data analysis are performed using Matlab's Image Processing Toolbox. The gel phantom was scanned with a GE lightspeed 16 slice CT scanner. The gel'sCT DICOM images were transferred to Eclipse treatment planning system. A QA plan was then generated by projecting a patient's RapidArc plan onto the gel's phantom CT data set. The calculated dose was exported via DICOM for analysis. The gel phantom was then set up on the Linac and irradiated with Varian 2300iX equipped with RapidArc. Results: With isotropic voxel size 2.5 mm (equivalent to TPS grid resolution), it takes less than half hour to scan the entire irradiated gel volume. Gamma maps indicate agreement within 3% dose difference, 3mm distance to agreement level. Conclusions: Our study demonstrates excellent performance of our fast 3D QA system for Rapid Arc. The system is very robust, easy to use, and saves time for 3D QA and treatment verification for Rapid Arc and other complex 3D treatment techniques.

WE‐E‐304A‐01: Contrast Mechanism and Origin of Artifacts in Phase‐Sensitive X‐Ray Volumetric Imaging

Purpose: Phase‐sensitive x‐ray volumetric imaging has great potential for tissue contrast enhancement and radiation dose reduction, compared to conventional tissue‐attenuation based x‐ray volumetric imaging. In order to achieve quantitative and artifact‐free phase sensitive volumetric imaging, one should quantify the formed phase contrast and find the origin of artifacts formed in phase‐sensitive volumetric imaging. Method and Materials: We incorporated x‐ray Fresnel diffraction into x‐ray projection acquisitions, and reconstructed volumetric images from these phase‐sensitive projections by the filtered backprojection method. A formula for the reconstructed tissue's apparent attenuation coefficients was derived for quantitative analysis. Computer simulations of phase sensitive 3D tomography were performed, and reconstructed 3D phantom tomograms were analyzed, and the artifacts associated with phase‐contrast were identified. In order to eliminate these artifacts, we performed phase retrievals based on the phase‐attenuation duality method for each projection, and these phase‐retrieved projections were then used for new 3D tomographic reconstructions. Results: The apparent linear attenuation coefficients in 3D tomograms were found each being the sum of three parts: the tissue linear attenuation coefficient, the scaled 3D Laplacian of tissue refraction index, and the artifact depending on global distributions of tissue attenuation coefficients and refraction indices. The simulated high‐resolution 3D images of phantoms exhibit severe artifacts as the upward and downward density over‐shootings and the streaks. The phase retrieval was shown very effective for removing these artifacts and accurately reconstructing tissue attenuation coefficients in phase‐sensitive 3D tomography. Conclusion: We clarified the contrast mechanism in phase‐sensitive volumetric imaging, found the quantitative contribution of the phase contrast in reconstructed 3D images, and revealed the origin of severe artifacts. The need of the phase‐retrievals for quantitative and artifact‐free phase sensitive volumetric imaging was clearly demonstrated.Research supported in part by DoD Breast Cancer Research Program.

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.

Impact of Contamination from Scattered Photons in Singles-Mode Transmission Data on Quantitative Small-Animal PET Imaging

In previous work, we described and validated a method of scatter correction for singles-mode transmission data using experimental preinjection data acquired with a dedicated rodent PET scanner. In the current work, we investigated the impact that our method has on the quantitative accuracy of small-animal PET. This investigation had 3 stages. We first confirmed the general validity of our method by applying it to preinjection transmission data from a different imaging system (a larger dedicated primate scanner). For these data, we evaluated the accuracy of the reconstructed distributions of linear attenuation coefficients (mu-values). In the second stage, we applied our attenuation-map reconstruction and scatter correction procedure for postinjection transmission data acquired with the dedicated rodent scanner. For these studies, we investigated the quantitative accuracy of reconstructed emission images that use attenuation correction derived from postinjection transmission data. In the third stage, we compared our scatter correction method with 2 more commonly used alternatives (automated rescaling and segmentation of the attenuation-map images). For the primate scanner data, the average reconstructed mu-values with scatter correction were within 3% of the expected values for water and soft tissue, whereas uncorrected values were 19%-26% lower than their expected values. For the postinjection transmission studies, we found that the correct average mu-values and reconstructed activity concentrations consistent with well-counter measurements were obtained only when scatter correction and emission contamination correction were applied to the transmission data. We also found that our transmission scatter correction provides more accurate mu-values and better image quantification than either rescaling or segmentation. Using different imaging systems (primate and rodent) and different scanning protocols (before and after injection), we found that our transmission scatter correction is more accurate (for both reconstructed mu-values and activity concentrations) than the existing alternatives.

Characterizing anatomical variability in breast CT images

Previous work [Burgess et al., Med. Phys. 28, 419-437 (2001)] has shown that anatomical noise in projection mammography results in a power spectrum well modeled over a range of frequencies by a power law, and the exponent (beta) of this power law plays a critical role in determining the size at which a growing lesion reaches the threshold for detection. In this study, the authors evaluated the power-law model for breast computed tomography (bCT) images, which can be thought of as thin sections through a three-dimensional (3D) volume. Under the assumption of a 3D power law describing the distribution of attenuation coefficients in the breast parenchyma, the authors derived the relationship between the power-law exponents of bCT and projection images and found it to be betasection=betaproj-1. They evaluated this relationship on clinical images by comparing bCT images from a set of 43 patients to Burgess' findings in mammography. They were able to make a direct comparison for 6 of these patients who had both a bCT exam and a digitized film-screen mammogram. They also evaluated segmented bCT images to investigate the extent to which the bCT power-law exponent can be explained by a binary model of attenuation coefficients based on the different attenuation of glandular and adipose tissue. The power-law model was found to be a good fit for bCT data over frequencies from 0.07 to 0.45 cyc/mm, where anatomical variability dominates the spectrum. The average exponent for bCT images was 1.86. This value is close to the theoretical prediction using Burgess' published data for projection mammography and for the limited set of mammography data available from the authors' patient sample. Exponents from the segmented bCT images (average value: 2.06) were systematically slightly higher than bCT images, with substantial correlation between the two (r=0.84).