Accurate effective atomic number determination with polychromatic grating-based phase-contrast computed tomography.

The demand for quantitative medical imaging is increasing in the ongoing digitalization. Conventional computed tomography (CT) is energy-dependent and therefore of limited comparability. In contrast, dual-energy CT (DECT) allows for the determination of absolute image contrast quantities, namely the electron density and the effective atomic number, and is already established in clinical radiology and radiation therapy. Grating-based phase-contrast computed tomography (GBPC-CT) is an experimental X-ray technique that also allows for the measurement of the electron density and the effective atomic number. However, the determination of both quantities is challenging when dealing with polychromatic GBPC-CT setups. In this paper, we present how to calculate the effective atomic numbers with a polychromatic, laboratory GBPC-CT setup operating between 35 and 50\,kVp. First, we investigated the accuracy of the measurement of the attenuation coefficients and electron densities. For this, we performed a calibration using the concept of effective energy. With the reliable experimental quantitative values, we were able to evaluate the effective atomic numbers of the investigated materials using a method previously shown with monochromatic X-ray radiation. In detail, we first calculated the ratio of the electron density and attenuation coefficient, which were experimentally determined with our polychromatic GBPC-CT setup. Second, we compared this ratio with tabulated total attenuation cross sections from literature values to determine the effective atomic numbers. Thus, we were able to calculate two physical absolute quantities -- the electron density and effective atomic number -- that are in general independent of the specific experimental conditions like the X-ray beam spectrum or the setup design.

AHA classification of coronary and carotid atherosclerotic plaques by grating-based phase-contrast computed tomography.

OBJECTIVES: To evaluate the potential of grating-based phase-contrast computed-tomography (gb-PCCT) to classify human carotid and coronary atherosclerotic plaques according to modified American Heart Association (AHA) criteria. METHODS: Experiments were carried out at a laboratory-based set-up consisting of X-ray tube (40 kVp), grating-interferometer and detector. Eighteen human carotid and coronary artery specimens were examined. Histopathology served as the standard of reference. Vessel cross-sections were classified as AHA lesion type I/II, III, IV/V, VI, VII or VIII plaques by two independent reviewers blinded to histopathology. Conservative measurements of diagnostic accuracies for the detection and differentiation of plaque types were evaluated. RESULTS: A total of 127 corresponding gb-PCCT/histopathology sections were analyzed. Based on histopathology, lesion type I/II was present in 12 (9.5 %), III in 18 (14.2 %), IV/V in 38 (29.9 %), VI in 16 (12.6 %), VII in 34 (26.8 %) and VIII in 9 (7.0 %) cross-sections. Sensitivity, specificity and positive and negative predictive value were ≥0.88 for most analyzed plaque types with a good level of agreement (Cohen's kappa = 0.90). Overall, results were better in carotid (kappa = 0.97) than in coronary arteries (kappa = 0.85). Inter-observer agreement was high with kappa = 0.85, p < 0.0001. CONCLUSIONS: These results indicate that gb-PCCT can reliably classify atherosclerotic plaques according to modified AHA criteria with excellent agreement to histopathology. KEY POINTS: • Different atherosclerotic plaque types display distinct morphological features in phase-contrast CT. • Phase-contrast CT can detect and differentiate AHA plaque types. • Calcifications caused streak artefacts and reduced sensitivity in type VI lesions. • Overall agreement was higher in carotid than in coronary arteries.

Quantitative imaging using high-energy X-ray phase-contrast CT with a 70 kVp polychromatic X-ray spectrum.

Imaging of large and dense objects with grating-based X-ray phase-contrast computed tomography requires high X-ray photon energy and large fields of view. It has become increasingly possible due to the improvements in the grating manufacturing processes. Using a high-energy X-ray phase-contrast CT setup with a large (10 cm in diameter) analyzer grating and operated at an acceleration tube voltage of 70 kVp, we investigate the complementarity of both attenuation and phase contrast modalities with materials of various atomic numbers (Z). We confirm experimentally that for low-Z materials, phase contrast yields no additional information content over attenuation images, yet it provides increased contrast-to-noise ratios (CNRs). The complementarity of both signals can be seen again with increasing Z of the materials and a more comprehensive material characterization is thus possible. Imaging of a part of a human cervical spine with intervertebral discs surrounded by bones and various soft tissue types showcases the benefit of high-energy X-ray phase-contrast system. Phase-contrast reconstruction reveals the internal structure of the discs and makes the boundary between the disc annulus and nucleus pulposus visible. Despite the fact that it still remains challenging to develop a high-energy grating interferometer with a broad polychromatic source with satisfactory optical performance, improved image quality for phase contrast as compared to attenuation contrast can be obtained and new exciting applications foreseen.

Simulated cystic renal lesions: quantitative X-ray phase-contrast CT--an in vitro phantom study.

PURPOSE: To determine if grating-based x-ray phase-contrast computed tomography (CT) can allow differentiation of simulated simple, protein-rich, hemorrhagic, and enhancing cystic renal lesions in an in vitro phantom. MATERIALS AND METHODS: An in vitro phantom specifically designed to simulate simple, protein-rich, hemorrhagic, and enhancing renal cysts was scanned with an experimental grating-based phase-contrast CT setup consisting of a Talbot-Lau interferometer with a rotating anode x-ray tube and a single photon counting detector. Various combinations of serum and saline (100% and 0% to 0% and 100%), blood and saline, blood and serum (100% and 0% to 6.25% and 93.75% for both), and an iodinated contrast agent and saline (7.6-1.6 mg per milliliter of saline) were used to reproduce the chemical composition of the different types of cysts. A thickened solution of an iodinated contrast agent calibrated with a clinical multidetector CT scanner served as contrast agent-enhanced renal parenchyma (195 HU at 80 kVp, 400 mAs and 98 HU at 140 kVp, 200 mAs). Standard attenuation- and phase-contrast images were reconstructed from the raw projection data. Quantitative values for attenuation and phase contrast and image noise were determined. Contrast-to-noise ratios were calculated. Simulated lesions were assessed for visual differentiability by means of pairwise comparison of the attenuation- and phase-contrast images and both images simultaneously. RESULTS: Attenuation-contrast imaging showed large differences in Hounsfield units with increasing concentrations of iodine (118.9 HU for 1.6 mg/mL vs 331.4 HU for 7.6 mg/mL). Values for phase-contrast imaging were substantially distinguishable for saline, serum, and blood (7.9, 23.7, and 52.8 HU, respectively). Both imaging modalities combined allowed differentiation of all four types of simulated cysts (100% saline, 100% serum, 100% blood, and 1.6-7.6 mg of iodine per milliliter of saline) with one imaging acquisition. CONCLUSION: Grating-based phase-contrast CT allows differentiation of simulated simple, protein-rich, hemorrhagic, and enhancing renal cysts in an in vitro phantom through simultaneous assessment of their distinct attenuation- and phase-contrast signal.

Phase-contrast CT: qualitative and quantitative evaluation of atherosclerotic carotid artery plaque.

PURPOSE: To evaluate the potential of phase-contrast computed tomography (CT) for atherosclerotic plaque imaging in human carotid arteries in an experimental ex vivo study. MATERIALS AND METHODS: The study was approved by the institutional review board, and informed consent was obtained from the patients' relatives. Seven postmortem human carotid artery specimens were imaged at a laboratory setup by using a conventional x-ray tube and grating interferometer. After histologic processing, phase-contrast imaging and histopathologic data were matched. Characteristics of the necrotic core (NC) covered by a fibrous cap (FC), intraplaque hemorrhage (IPH), and calcifications (CAs) were established, and sensitivity, specificity, and accuracy of phase-contrast CT for plaque detection and the potential for accurate quantification were assessed. The Cohen κ and Pearson correlation coefficient R were used to determine the agreement between phase-contrast imaging and histopathologic findings for plaque characterization and correlation of quantitative plaque measurements, respectively. A difference with a P value of less than .05 was considered significant. RESULTS: Characteristic criteria were found in all analyzed plaque components. Applying these criteria, phase-contrast CT had a good sensitivity for the detection of the FC and NC, IPH, and CAs (all, >80%) and excellent specificity and accuracy (all, >90%), with good interreader agreement (κ ≥ 0.72, P < .0001). There were excellent correlations for quantitative measurements of FC, NC, and CAs between phase-contrast imaging and histopathologic findings (R ≥ 0.92). Interreader reproducibility was excellent, with an intraclass correlation coefficient of 0.98 or higher for all measurements. CONCLUSION: The results of this study indicate that ex vivo phase-contrast CT can help identify and quantify atherosclerotic plaque components, with excellent correlation to histopathologic findings. Although not yet applicable in vivo, phase-contrast CT may become a valuable tool to monitor atherosclerotic disease process noninvasively.

Regularized iterative integration combined with non-linear diffusion filtering for phase-contrast x-ray computed tomography.

Phase-contrast x-ray computed tomography has a high potential to become clinically implemented because of its complementarity to conventional absorption-contrast.In this study, we investigate noise-reducing but resolution-preserving analytical reconstruction methods to improve differential phase-contrast imaging. We apply the non-linear Perona-Malik filter on phase-contrast data prior or post filtered backprojected reconstruction. Secondly, the Hilbert kernel is replaced by regularized iterative integration followed by ramp filtered backprojection as used for absorption-contrast imaging. Combining the Perona-Malik filter with this integration algorithm allows to successfully reveal relevant sample features, quantitatively confirmed by significantly increased structural similarity indices and contrast-to-noise ratios. With this concept, phase-contrast imaging can be performed at considerably lower dose.

Quantitative X-ray phase-contrast computed tomography at 82 keV.

Potential applications of grating-based X-ray phase-contrast imaging are investigated in various fields due to its compatibility with laboratory X-ray sources. So far the method was mainly restricted to X-ray energies below 40 keV, which is too low to examine dense or thick objects, but a routine operation at higher energies is on the brink of realisation. In this study, imaging results obtained at 82 keV are presented. These comprise a test object consisting of well-defined materials for a quantitative analysis and a tooth to translate the findings to a biomedical sample. Measured linear attenuation coefficients ? and electron densities ?e are in good agreement with theoretical values. Improved contrast-to-noise ratios were found in phase contrast compared to attenuation contrast. The combination of both contrast modalities further enables to simultaneously assess information on density and composition of materials with effective atomic numbers Z? > 8. In our biomedical example, we demonstrate the possibility to detect differences in mass density and calcium concentration within teeth.

Assessment of grating-based X-ray phase-contrast CT for differentiation of invasive ductal carcinoma and ductal carcinoma in situ in an experimental ex vivo set-up.

OBJECTIVE: Limited contrast between healthy and tumour tissue is a limiting factor in mammography and CT of the breast. Phase-contrast computed tomography (PC-CT) provides improved soft-tissue contrast compared with absorption-based techniques. In this study, we assessed the technical feasibility of grating-based PC-CT imaging of the breast for characterisation of ductal carcinoma in situ (DCIS). METHODS: Grating-based PC-CT was performed on one breast specimen containing an invasive ductal carcinoma and DCIS using monochromatic radiation of 23 keV. Phase-contrast and absorption-based images were compared qualitatively and quantitatively with histopathology in a blinded fashion. RESULTS: Grating-based PC-CT showed improved differentiation of soft-tissue components. Circular structures of high phase-shift contrast corresponding to the walls of the dilated ductuli of the DCIS were visualised with a contrast-to-noise ratio (CNR) of 9.6 using PC-CT but were not detectable on absorption-based images (CNR = 0.27). The high phase-shift structures of the dilated ductuli were identifiable in the PC-CT volume data set allowing for 3D characterisation of DCIS. CONCLUSIONS: Our results indicate that unlike conventional CT, grating-based PC-CT may allow the differentiation between invasive carcinoma and intraductal carcinoma and healthy breast tissue and provide 3D visualisation of DCIS.

Quantitative differentiation of minimal-fat angiomyolipomas from renal cell carcinomas using grating-based x-ray phase-contrast computed tomography: An ex vivo study.

BACKGROUND: The differentiation of minimal-fat-or low-fat-angiomyolipomas from other renal lesions is clinically challenging in conventional computed tomography. In this work, we have assessed the potential of grating-based x-ray phase-contrast computed tomography (GBPC-CT) for visualization and quantitative differentiation of minimal-fat angiomyolipomas (mfAMLs) and oncocytomas from renal cell carcinomas (RCCs) on ex vivo renal samples. MATERIALS AND METHODS: Laboratory GBPC-CT was performed at 40 kVp on 28 ex vivo kidney specimens including five angiomyolipomas with three minimal-fat (mfAMLs) and two high-fat (hfAMLs) subtypes as well as three oncocytomas and 20 RCCs with eight clear cell (ccRCCs), seven papillary (pRCCs) and five chromophobe RCC (chrRCC) subtypes. Quantitative values of conventional Hounsfield units (HU) and phase-contrast Hounsfield units (HUp) were determined and histogram analysis was performed on GBPC-CT and grating-based attenuation-contrast computed tomography (GBAC-CT) slices for each specimen. For comparison, the same specimens were imaged at a 3T magnetic resonance imaging (MRI) scanner. RESULTS: We have successfully matched GBPC-CT images with clinical MRI and histology, as GBPC-CT presented with increased soft tissue contrast compared to absorption-based images. GBPC-CT images revealed a qualitative and quantitative difference between mfAML samples (58±4 HUp) and oncocytomas (44±10 HUp, p = 0.057) and RCCs (ccRCCs: 40±12 HUp, p = 0.012; pRCCs: 43±9 HUp, p = 0.017; chrRCCs: 40±7 HUp, p = 0.057) in contrast to corresponding laboratory attenuation-contrast CT and clinical MRI, although not all differences were statistically significant. Due to the heterogeneity and lower signal of oncocytomas, quantitative differentiation of the samples based on HUp or in combination with HUs was not possible. CONCLUSIONS: GBPC-CT allows quantitative differentiation of minimal-fat angiomyolipomas from pRCCs and ccRCCs in contrast to absorption-based imaging and clinical MRI.

Quantifying wood decomposition by insects and fungi using computed tomography scanning and machine learning.

Wood decomposition is a central process contributing to global carbon and nutrient cycling. Quantifying the role of the major biotic agents of wood decomposition, i.e. insects and fungi, is thus important for a better understanding of this process. Methods to quantify wood decomposition, such as dry mass loss, suffer from several shortcomings, such as destructive sampling or subsampling. We developed and tested a new approach based on computed tomography (CT) scanning and semi-automatic image analysis of logs from a field experiment with manipulated beetle communities. We quantified the volume of beetle tunnels in wood and bark and the relative wood volume showing signs of fungal decay and compared both measures to classic approaches. The volume of beetle tunnels was correlated with dry mass loss and clearly reflected the differences between beetle functional groups. Fungal decay was identified with high accuracy and strongly correlated with ergosterol content. Our data show that this is a powerful approach to quantify wood decomposition by insects and fungi. In contrast to other methods, it is non-destructive, covers entire deadwood objects and provides spatially explicit information opening a wide range of research options. For the development of general models, we urge researchers to publish training data.

Grating-based phase-contrast CT (PCCT): histopathological correlation of human liver cirrhosis and hepatocellular carcinoma specimen.

AIMS: To correlate signal intensities in grating-based phase-contrast CT (PCCT) images obtained at a synchrotron light source and a conventional X-ray source with tissue components in human liver cirrhosis and hepatocellular carcinoma (HCC) specimen. METHODS: Study approval was obtained by the institutional review board. Human specimen of liver cirrhosis and HCC were imaged at experimental grating-based PCCT setups using either a synchrotron radiation source or a conventional X-ray tube. Tissue samples were sectioned and processed for H&E and Elastica van Gieson staining. PCCT and histological images were manually correlated. Depending on morphology and staining characteristics tissue components like fibrosis, HCC, inflammation, connective tissue and necrosis were differentiated and visually correlated with signal intensity in PCCT images using a 5-point Likert scale with normal liver parenchyma as a reference. RESULTS: Grating-based PCCT images of human cirrhotic liver and HCC specimen showed high soft-tissue contrast allowing correlation with histopathological sections. Signal intensities were similar in both setups independent of the nature of the radiation source. Connective tissue and areas of haemorrhage displayed the highest signal intensities, fibrotic liver tissue the lowest. CONCLUSIONS: Grating-based PCCT provides comparable results for the characterisation of human specimen of liver cirrhosis and HCC using either a synchrotron light source or a conventional X-ray tube. Due to its high soft-tissue contrast and its applicability to conventional X-ray tubes grating-based PCCT holds potential for preclinical research and virtual histology applications.

Single spectrum three-material decomposition with grating-based x-ray phase-contrast CT.

Grating-based x-ray phase-contrast imaging provides three simultaneous image channels originating from a single image acquisition. While the phase signal provides direct access to the electron density in tomography, there is additional information on sub-resolutional structural information which is called dark-field signal in analogy to optical microscopy. The additional availability of the conventional attenuation image qualifies the method for implementation into existing diagnostic routines. The simultaneous access to the attenuation coefficient and the electron density allows for quantitative two-material discrimination as demonstrated lately for measurements at a quasi-monochromatic compact synchrotron source. Here, we investigate the transfer of the method to conventional polychromatic x-ray sources and the additional inclusion of the dark-field signal for three-material decomposition. We evaluate the future potential of grating-based x-ray phase-contrast CT for quantitative three-material discrimination for the specific case of early stroke diagnosis at conventional polychromatic x-ray sources. Compared to conventional CT, the method has the potential to discriminate coagulated blood directly from contrast agent extravasation within a single CT acquisition. Additionally, the dark-field information allows for the clear identification of hydroxyapatite clusters due to their micro-structure despite a similar attenuation as the applied contrast agent. This information on materials with sub-resolutional microstructures is considered to comprise advantages relevant for various pathologies.

Assessment of intraductal carcinoma in situ (DCIS) using grating-based X-ray phase-contrast CT at conventional X-ray sources: An experimental ex-vivo study.

BACKGROUND: The extent of intraductal carcinoma in situ (DCIS) is commonly underestimated due to the discontinuous growth and lack of microcalcifications. Specimen radiography has been established to reduce the rate of re-excision. However, the predictive value for margin assessment with conventional specimen radiography for DCIS is low. In this study we assessed the potential of grating-based phase-contrast computed tomography (GBPC-CT) at conventional X-ray sources for specimen tomography of DCIS containing samples. MATERIALS AND METHODS: GBPC-CT was performed on four ex-vivo breast specimens containing DCIS and invasive carcinoma of non-specific type. Phase-contrast and absorption-based datasets were manually matched with corresponding histological slices as the standard of reference. RESULTS: Matching of CT images and histology was successful. GBPC-CT showed an improved soft tissue contrast compared to absorption-based images revealing more histological details in the same sections. Non-calcifying DCIS exceeding the invasive tumor could be correlated to areas of dilated bright ducts around the tumor. CONCLUSIONS: GBPC-CT imaging at conventional X-ray sources offers improved depiction quality for the imaging of breast tissue samples compared to absorption-based imaging, allows the identification of diagnostically relevant tissue details, and provides full three-dimensional assessment of sample margins.

3D grating-based X-ray phase-contrast computed tomography for high-resolution quantitative assessment of cartilage: An experimental feasibility study with 3T MRI, 7T MRI and biomechanical correlation.

OBJECTIVE: Aim of this study was, to demonstrate the feasibility of high-resolution grating-based X-ray phase-contrast computed tomography (PCCT) for quantitative assessment of cartilage. MATERIALS AND METHODS: In an experimental setup, 12 osteochondral samples were harvested from n = 6 bovine knees (n = 2 each). From each knee, one cartilage sample was degraded using 2.5% Trypsin. In addition to PCCT and biomechanical cartilage stiffness measurements, 3T and 7T MRI was performed including MSME SE T2 and ME GE T2* mapping sequences for relaxationtime measurements. Paired t-tests and receiver operating characteristics (ROC) curves were used for statistical analyses. RESULTS: PCCT provided high-resolution images for improved morphological cartilage evaluation as compared to 3T and 7T MRI. Quantitative analyses revealed significant differences between the superficial and the deep cartilage layer for T2 mapping as well as for PCCT (P<0.05). No significant difference was detected for PCCT between healthy and degraded samples (P>0.05). MRI and stiffness measurements showed significant differences between healthy and degraded osteochondral samples. Accuracy in the prediction of cartilage degradation was excellent for MRI and biomechanical analyses. CONCLUSION: In conclusion, high-resolution grating-based X-ray PCCT cartilage imaging is feasible. In addition to MRI and biomechanical analyses it provides complementary, water content independent, information for improved morphological and quantitative characterization of articular cartilage ultrastructure.

High resolution laboratory grating-based X-ray phase-contrast CT.

The conventional form of computed tomography using X-ray attenuation without any contrast agents is of limited use for the characterization of soft tissue in many fields of medical and biological studies. Grating-based phase-contrast computed tomography (gbPC-CT) is a promising alternative imaging method solving the low soft tissue contrast without the need of any contrast agent. While highly sensitive measurements are possible using conventional X-ray sources the spatial resolution does often not fulfill the requirements for specific imaging tasks, such as visualization of pathologies. The focus of this study is the increase in spatial resolution without loss of sensitivity. To overcome this limitation a super-resolution reconstruction based on sub-pixel shifts involving a deconvolution of the image data during each iteration is applied. In our study we achieve an effective pixel size of 28 µm with a conventional rotating anode tube and a photon-counting detector. We also demonstrate that the method can upgrade existing setups to measure tomographies with higher resolution. The results show the increase in resolution at high sensitivity and with the ability to make quantitative measurements. The combination of sparse sampling and statistical iterative reconstruction may be used to reduce the total measurement time. In conclusion, we present high-quality and high-resolution tomographic images of biological samples to demonstrate the experimental feasibility of super-resolution reconstruction.

Qualitative and Quantitative Evaluation of Structural Myocardial Alterations by Grating-Based Phase-Contrast Computed Tomography.

OBJECTIVES: Grating-based phase-contrast computed tomography (gb-PCCT) relies on x-ray refraction instead of absorption to generate high-contrast images in biological soft tissue. The aim of this study was to evaluate the potential of gb-PCCT for the depiction of structural changes in heart disease. MATERIALS AND METHODS: Four human heart specimens from patients with hypertensive disease, ischemic disease, dilated heart disease, and cardiac lipomatosis were examined. The gb-PCCT setup consisted of an x-ray tube (40 kV, 70 mA), grating-interferometer, and detector, and allowed simultaneous acquisition of phase- and absorption-contrast data. With histopathology as the standard of reference, myocardium (MC), fibrotic scar (FS), interstitial fibrosis (IF), and fatty tissue (FT) were visually and quantitatively evaluated. Systematic differences in absorption- and phase-contrast Hounsfield units (HUabs and HUp) were assessed. RESULTS: Thirteen corresponding cross-sections were included, and MC, FS, IF, and FT were found in 13 (100%), 4 (30.8%), 7 (53.8%), and 13 (100%) cross-sections, respectively. Mean HUp/HUabs were 52.5/54.1, 86.6/69.7, 62.4/62.3, and -38.6/-258.9 for MC, FS, IF, and FT, respectively. An overlap in HUabs was observed for MC and IF (P = 0.84) but not for HUp (P < 0.01). Contrast-to-noise ratios were significantly higher in phase- than in absorption-contrast for MC/FT (35.4 vs 7.8; P < 0.01) and for MC/FS (12.3 vs 0.2; P < 0.01). CONCLUSIONS: Given its superior soft tissue contrast, gb-PCCT is able to depict structural changes in different cardiomyopathies, which can currently not be obtained by x-ray absorption-based imaging methods. If current technical limitations can be overcome, gb-PCCT may evolve as a powerful tool for the anatomical assessment of cardiomyopathy.

Tilted grating phase-contrast computed tomography using statistical iterative reconstruction.

Grating-based phase-contrast computed tomography (GBPC-CT) enables increased soft tissue differentiation, but often suffers from streak artifacts when performing high-sensitivity GBPC-CT of biomedical samples. Current GBPC-CT setups consist of one-dimensional gratings and hence allow to measure only the differential phase-contrast (DPC) signal perpendicular to the direction of the grating lines. Having access to the full two-dimensional DPC signal can strongly reduce streak artefacts showing up as characteristic horizontal lines in the reconstructed images. GBPC-CT with gratings tilted by 45° around the optical axis, combining opposed projections, and reconstructing with filtered backprojection is one method to retrieve the full three-dimensional DPC signal. This approach improves the quality of the tomographic data as already demonstrated at a synchrotron facility. However, additional processing and interpolation is necessary, and the approach fails when dealing with cone-beam geometry setups. In this work, we employ the tilted grating configuration with a laboratory GBPC-CT setup with cone-beam geometry and use statistical iterative reconstruction (SIR) with a forward model accounting for diagonal grating alignment. Our results show a strong reduction of streak artefacts and significant increase in image quality. In contrast to the prior approach our proposed method can be used in a laboratory environment due to its cone-beam compatibility.

Revising the lower statistical limit of x-ray grating-based phase-contrast computed tomography.

Phase-contrast x-ray computed tomography (PCCT) is currently investigated as an interesting extension of conventional CT, providing high soft-tissue contrast even if examining weakly absorbing specimen. Until now, the potential for dose reduction was thought to be limited compared to attenuation CT, since meaningful phase retrieval fails for scans with very low photon counts when using the conventional phase retrieval method via phase stepping. In this work, we examine the statistical behaviour of the reverse projection method, an alternative phase retrieval approach and compare the results to the conventional phase retrieval technique. We investigate the noise levels in the projections as well as the image quality and quantitative accuracy of the reconstructed tomographic volumes. The results of our study show that this method performs better in a low-dose scenario than the conventional phase retrieval approach, resulting in lower noise levels, enhanced image quality and more accurate quantitative values. Overall, we demonstrate that the lower statistical limit of the phase stepping procedure as proposed by recent literature does not apply to this alternative phase retrieval technique. However, further development is necessary to overcome experimental challenges posed by this method which would enable mainstream or even clinical application of PCCT.

Ex Vivo Assessment of Coronary Atherosclerotic Plaque by Grating-Based Phase-Contrast Computed Tomography: Correlation With Optical Coherence Tomography.

OBJECTIVES: The aim of this study was to determine the diagnostic accuracy of grating-based phase-contrast computed tomography (gb-PCCT) to classify and quantify coronary vessel characteristics in comparison with optical coherence tomography (OCT) and histopathology in an ex vivo setting. MATERIALS AND METHODS: After excision from 5 heart specimens, 15 human coronary arteries underwent gb-PCCT examination using an experimental imaging setup consisting of a rotating molybdenum anode x-ray tube, a Talbot-Lau grating interferometer, and a single photon counting detector. Subsequently, all vessels were imaged by OCT and histopathologically processed. Optical coherence tomography, gb-PCCT, and histopathology images were manually matched using anatomical landmarks. Optical coherence tomography and gb-PCCT were reviewed by 2 independent observers blinded to histopathology. Vessel, lumen, and plaque area were measured, and plaque characteristics (lipid rich, calcified, and fibrous) were determined for each section. Measures of diagnostic accuracy were derived, applying histopathology as the standard of reference. RESULTS: Of a total of 286 assessed cross sections, 241 corresponding sections were included in the statistical analysis. Quantitative measures derived from gb-PCCT were significantly higher than from OCT (P < 0.001) and were strongly correlated with histopathology (Pearson r ≥0.85 for gb-PCCT and ≥0.61 for OCT, respectively). Results of Bland-Altman analysis demonstrated smaller mean differences between OCT and histopathology than for gb-PCCT and histopathology. Limits of agreement were narrower for gb-PCCT with regard to lumen area, for OCT with regard to plaque area, and were comparable with regard to vessel area. Based on histopathology, 228/241 (94.6%) sections were classified as fibrous, calcified, or lipid rich. The diagnostic accuracy of gb-PCCT was excellent for the detection of all plaque components (sensitivity, ≥0.95; specificity, ≥0.94), whereas the results for OCT showed sensitivities of ≥0.73 and specificities of ≥0.66. CONCLUSIONS: In this ex vivo setting, gb-PCCT provides excellent results in the assessment of coronary atherosclerotic plaque characteristics and vessel dimensions in comparison to OCT and histopathology. Thus, the technique may serve as adjunct nondestructive modality for advanced plaque characterization in an experimental setting.

Ex vivo characterization of pathologic fluids with quantitative phase-contrast computed tomography.

PURPOSE: X-ray phase-contrast imaging (PCI) provides additional information beyond absorption characteristics by detecting the phase shift of the X-ray beam passing through material. The grating-based system works with standard polychromatic X-ray sources, promising a possible clinical implementation. PCI has been shown to provide additional information in soft-tissue samples. The aim of this study was to determine if ex vivo quantitative phase-contrast computed tomography (PCCT) may differentiate between pathologic fluid collections. MATERIALS AND METHODS: PCCT was performed with the grating interferometry method. A protein serial dilution, human blood samples and 17 clinical samples of pathologic fluid retentions were imaged and correlated with clinical chemistry measurements. Conventional and phase-contrast tomography images were reconstructed. Phase-contrast Hounsfield Units (HUp) were used for quantitative analysis analogously to conventional HU. The imaging was analyzed using overall means, ROI values as well as whole-volume-histograms and vertical gradients. Contrast to noise ratios were calculated between different probes and between imaging methods. RESULTS: HUp showed a very good linear correlation with protein concentration in vitro. In clinical samples, HUp correlated rather well with cell count and triglyceride content. PCI was better than absorption imaging at differentiating protein concentrations in the protein samples as well as at differentiating blood plasma from cellular components. PCI also allowed for differentiation of watery samples (such as lymphoceles) from pus. CONCLUSION: Phase-contrast computed tomography is a promising tool for the differentiation of pathologic fluids that appear homogenous with conventional attenuation imaging.