Respiratory-gated KES imaging of a rat model of acute lung injury at the Canadian Light Source.

In this study, contrast-enhanced X-ray tomographic imaging for monitoring and quantifying respiratory disease in preclinical rodent models is proposed. A K-edge imaging method has been developed at the Canadian Light Source to very accurately obtain measurements of the concentration of iodinated contrast agent in the pulmonary vasculature and inhaled xenon in the airspaces of rats. To compare the iodine and xenon concentration maps, a scout projection image was acquired to define the region of interest within the thorax for imaging and to ensure the same locations were imaged in each K-edge subtraction (KES) acquisition. A method for triggering image acquisition based on the real-time measurements of respiration was also developed to obtain images during end expiration when the lungs are stationary, in contrast to other previously published studies that alter the respiration to accommodate the image acquisition. In this study, images were obtained in mechanically ventilated animals using physiological parameters at the iodine K-edge in vivo and at the xenon K-edge post mortem (but still under mechanical ventilation). The imaging techniques were performed in healthy Brown Norway rats and in age-matched littermates that had an induced lung injury to demonstrate feasibility of the imaging procedures and the ability to correlate the lung injury and the quantitative measurements of contrast agent concentrations between the two KES images. The respiratory-gated KES imaging protocol can be easily adapted to image during any respiratory phase and is feasible for imaging disease models with compromised lung function.

Micro-computed tomography analysis of post space preparation in root canals filled with carrier-based thermoplasticized gutta-percha.

AIM: To determine whether post space preparation deviated from the root canal preparation in canals filled with Thermafil, GuttaCore or warm vertically compacted gutta-percha. METHODOLOGY: Forty-two extracted human permanent maxillary lateral incisors were decoronated, and their root canals instrumented using a standardized protocol. Samples were divided into three groups and filled with Thermafil (Dentsply Tulsa Dental Specialties, Johnson City, TN, USA), GuttaCore (Dentsply Tulsa Dental Specialties) or warm vertically compacted gutta-percha, before post space preparation was performed with a GT Post drill (Dentsply Tulsa Dental Specialties). Teeth were scanned using micro-computed tomography after root filling and again after post space preparation. Scans were examined for number of samples with post space deviation, linear deviation of post space preparation and minimum root thickness before and after post space preparation. Parametric data were analysed with one-way analysis of variance (anova) or one-tailed paired Student's t-tests, whilst nonparametric data were analysed with Fisher's exact test. RESULTS: Deviation occurred in eight of forty-two teeth (19%), seven of fourteen from the Thermafil group (50%), one of fourteen from the GuttaCore group (7%), and none from the gutta-percha group. Deviation occurred significantly more often in the Thermafil group than in each of the other two groups (P < 0.05). Linear deviation of post space preparation was greater in the Thermafil group than in both of the other groups and was significantly greater than that of the gutta-percha group (P < 0.05). Minimum root thickness before post space preparation was significantly greater than it was after post space preparation for all groups (P < 0.01). CONCLUSIONS: The differences between the Thermafil, GuttaCore and gutta-percha groups in the number of samples with post space deviation and in linear deviation of post space preparation were associated with the presence or absence of a carrier as well as the different carrier materials.

Measuring absorbed dose for i-CAT CBCT examinations in child, adolescent and adult phantoms.

OBJECTIVES: Design and construct child and adolescent head phantoms to measure the absorbed doses imparted during dental CBCT and compare with the absorbed dose measured in an adult phantom. METHODS: A child phantom was developed to represent the smallest patients receiving CBCT, usually for craniofacial developmental concerns, and an adolescent phantom was developed to represent healthy orthodontic patients. Absorbed doses were measured using a thimble ionization chamber for the custom-built child and adolescent phantoms and compared with measurements using a commercially available adult phantom. Imaging was performed with an i-CAT Next Generation (Imaging Sciences International, Hatfield, PA) CBCT using two different fields of view covering the craniofacial complex (130 mm high) or maxilla/mandible (60 mm high). RESULTS: Measured absorbed doses varied depending on the location of the ionization chamber within the phantoms. For CBCT images obtained using the same protocol for all phantoms, the highest absorbed dose was measured in all locations of the small child phantom. The lowest absorbed dose was measured in the adult phantom. CONCLUSIONS: Images were obtained with the same protocol for the adult, adolescent and child phantoms. A consistent trend was observed with the highest absorbed dose being measured in the smallest phantom (child), while the lowest absorbed dose was measured in the largest phantom (adult). This study demonstrates the importance of child-sizing the dose by using dedicated paediatric protocols optimized for the imaging task, which is critical as children are more sensitive to harmful effects of radiation and have a longer life-span post-irradiation for radiation-induced symptoms to develop than do adults.

Dose measurements for dental cone-beam CT: a comparison with MSCT and panoramic imaging.

To date there is a lack of published information on appropriate methods to determine patient doses from dental cone-beam computed tomography (CBCT) equipment. The goal of this study is to apply and extend the methods recommended in the American Association of Physicists in Medicine (AAPM) Report 111 for CBCT equipment to characterize dose and effective dose for a range of dental imaging equipment. A protocol derived from the one proposed by Dixon et al (2010 Technical Report 111, American Association of Physicist in Medicine, MD, USA), was applied to dose measurements of multi-slice CT, dental CBCT (small and large fields of view (FOV)) and a dental panoramic system. The computed tomography dose index protocol was also performed on the MSCT to compare both methods. The dose distributions in a cylindrical polymethyl methacrylate phantom were characterized using a thimble ionization chamber and Gafchromic™ film (beam profiles). Gafchromic™ films were used to measure the dose distribution in an anthropomorphic phantom. A method was proposed to extend dose estimates to planes superior and inferior to the central plane. The dose normalized to 100 mAs measured in the center of the phantom for the large FOV dental CBCT (11.4 mGy/100 mAs) is two times lower than that of MSCT (20.7 mGy/100 mAs) for the same FOV, but approximately 15 times higher than for a panoramic system (0.6 mGy/100 mAs). The effective dose per scan (in clinical conditions) found for the dental CBCT are 167.60 ± 3.62, 61.30 ± 3.88 and 92.86 ± 7.76 mSv for the Kodak 9000 (fixed scan length of 3.7 cm), and the iCAT Next Generation for 6 cm and 13 cm scan lengths respectively. The method to extend the dose estimates from the central slice to superior and inferior slices indicates a good agreement between theory and measurement. The Gafchromic™ films provided useful beam profile data and 2D distributions of dose in phantom.

Quantifying lung morphology with respiratory-gated micro-CT in a murine model of emphysema.

Non-invasive micro-CT imaging techniques have been developed to investigate lung structure in free-breathing rodents. In this study, we investigate the utility of retrospectively respiratory-gated micro-CT imaging in an emphysema model to determine if anatomical changes could be observed in the image-derived quantitative analysis at two respiratory phases. The emphysema model chosen was a well-characterized, genetically altered model (TIMP-3 knockout mice) that exhibits a homogeneous phenotype. Micro-CT scans of the free-breathing, anaesthetized mice were obtained in 50 s and retrospectively respiratory sorted and reconstructed, providing 3D images representing peak inspiration and end expiration with 0.15 mm isotropic voxel spacing. Anatomical measurements included the volume and CT density of the lungs and the volume of the major airways, along with the diameters of the trachea, left bronchus and right bronchus. From these measurements, functional parameters such as functional residual capacity and tidal volume were calculated. Significant differences between the wild-type and TIMP-3 knockout groups were observed for measurements of CT density over the entire lung, indicating increased air content in the lungs of TIMP-3 knockout mice. These results demonstrate retrospective respiratory-gated micro-CT, providing images at multiple respiratory phases that can be analyzed quantitatively to investigate anatomical changes in murine models of emphysema.

Implementation of dual- and triple-energy cone-beam micro-CT for postreconstruction material decomposition.

Micro-CT has become a powerful tool for small animal research, having the ability to obtain high-resolution in vivo and ex vivo images for analyzing bone mineral content, organ vasculature, and bone microarchitecture extraction. The use of exogenous contrast agents further extends the use of micro-CT techniques, but despite advancements in contrast agents, single-energy micro-CT is still limited in cases where two different materials share similar grey-scale intensity values. This study specifically addresses the development of multiple-energy cone-beam micro-CT, for applications where bone must be separated from blood vessels filled with a Pb-based contrast material (Microfil) in ex vivo studies of rodents and tissue specimens. The authors report the implementation of dual- and triple-energy CT algorithms for material-specific imaging using postreconstruction decomposition of micro-CT data; the algorithms were implemented on a volumetric cone-beam micro-CT scanner (GE Locus Ultra). For the dual-energy approach, extrinsic filtration was applied to the x-ray beam to produce spectra with different proportions of x rays above the K edge of Pb. The optimum x-ray tube energies (140 kVp filtered with 1.45 mm Cu and 96 kVp filtered with 0.3 mm Pb) that maximize the contrast between bone and Microfil were determined through numerical simulation. For the triple-energy decomposition, an additional low-energy spectrum (70 kVp, no added filtration) was used. The accuracy of decomposition was evaluated through simulations and experimental verification of a phantom containing a cortical bone simulating material (SB3), Microfil, and acrylic. Using simulations and phantom experiments, an accuracy greater than 95% was achieved in decompositions of bone and Microfil (for noise levels lower than 11 HU), while soft tissue was separated with accuracy better than 99%. The triple-energy technique demonstrated a slightly higher, but not significantly different, decomposition accuracy than the dual-energy technique for the same achieved noise level in the micro-CT images acquired at the multiple energies. The dual-energy technique was applied to the decomposition of an ex vivo rat specimen perfused with Microfil; successful decomposition of the bone and Microfil was achieved, enabling the visualization and characterization of the vasculature both in areas where the vessels traverse soft tissue and when they are surrounded by bone. In comparison, in single energy micro-CT, vessels surrounded by bone could not be distinguished from the cortical bone, based on grey-scale intensity alone. This work represents the first postreconstruction application of material-specific decomposition that directly takes advantage of the K edge characteristics of a contrast material injected into an animal specimen; the application of the technique resulted in automatic, accurate segmentation of 3D micro-CT images into bone, vessel, and tissue components. The algorithm uses only reconstructed images, rather than projection data, and is calibrated by an operator with signal values in regions identified as being comprised entirely of either cortical bone, contrast-enhanced vessel, or soft tissue; these required calibration values are observed directly within reconstructed CT images acquired at the multiple energies. These features facilitate future implementation on existing research micro-CT systems.

In vivo characterization of lung morphology and function in anesthetized free-breathing mice using micro-computed tomography.

Lung morphology and function in human subjects can be monitored with computed tomography (CT). Because many human respiratory diseases are routinely modeled in rodents, a means of monitoring the changes in the structure and function of the rodent lung is desired. High-resolution images of the rodent lung can be attained with specialized micro-CT equipment, which provides a means of monitoring rodent models of lung disease noninvasively with a clinically relevant method. Previous studies have shown respiratory-gated images of intubated and respirated mice. Although the image quality and resolution are sufficient in these studies to make quantitative measurements, these measurements of lung structure will depend on the settings of the ventilator and not on the respiratory mechanics of the individual animals. In addition, intubation and ventilation can have unnatural effects on the respiratory dynamics of the animal, because the airway pressure, tidal volume, and respiratory rate are selected by the operator. In these experiments, important information about the symptoms of the respiratory disease being studied may be missed because the respiration is forced to conform to the ventilator settings. In this study, we implement a method of respiratory-gated micro-CT for use with anesthetized free-breathing rodents. From the micro-CT images, quantitative analysis of the structure of the lungs of healthy unconscious mice was performed to obtain airway diameters, lung and airway volumes, and CT densities at end expiration and during inspiration. Because the animals were free breathing, we were able to calculate tidal volume (0.09 +/- 0.03 ml) and functional residual capacity (0.16 +/- 0.03 ml).

Fundamental image quality limits for microcomputed tomography in small animals.

Small-animal imaging has become increasingly more important as transgenic and knockout mice are produced to model human diseases. One imaging technique that has emerged is microcomputed tomography (micro-CT). For live-animal imaging, the precision in the images will be determined by the x-ray dose given to the animal. As a result, we propose a simple method to predict the noise performance of an x-ray micro-CT system as a function of dose and image resolution. An ideal, quantum-noise limited micro-CT scanner, assumed to have perfect resolution and ideal efficiency, was modeled. Using a simplified model, the coefficient of variation (COV) of the linear attenuation coefficient was calculated for a range of entrance doses and isotropic voxel sizes. COV calculations were performed for the ideal case and with simulated imperfections in efficiency and resolution. Our model was validated in phantom studies and mouse images were acquired with a specimen scanner to illustrate the results. A simplified model of noise propagation in the case of isotropic resolution indicates that the COV in the linear attenuation coefficient is proportional to (dose)(-1/2) and to the (isotropic voxel size)(-2) in the reconstructed volume. Therefore an improvement in the precision can be achieved only by increasing the isotropic voxel size (thereby decreasing the resolution of the image) or by increasing the x-ray dose. For the ideal scanner, a COV of 1% in the linear attenuation coefficient for an image of a mouse exposed to 0.25 Gy is obtained with a minimum isotropic voxel size of 135 microm. However, the same COV is achieved at a dose of 5.0 Gy with a 65 microm isotropic voxel size. Conversely, for a 68 mm diameter rat, a COV of 1% obtained from an image at 5.0 Gy would require an isotropic voxel size of 100 microm. These results indicate that short-term, potentially lethal, effects of ionizing radiation will limit high-resolution live animal imaging. As improvements in detector technology allow the resolution to improve, by decreasing the detector element size to tens of microns or less, high quality images will be limited by the x-ray dose administered. For the highest quality images, these doses will approach the lethal dose or LD50 for the animals. Approaching the lethal dose will affect the way experiments are planned, and may reduce opportunities for experiments involving imaging the same animal over time. Dose considerations will become much more important for live small-animal imaging as the limits of resolution are tested.

Comparison of image quality indicators among mammography facilities in Ontario.

OBJECTIVE: To compare the technical aspects of image quality of mammography machines in facilities across Ontario. METHODS: Eight mammographic physics consultants took measurements and calculated accreditation phantom scores, mean glandular dose, entrance exposure, average optical density, half value layer and the limiting resolution of the imaging chain for 100 mammography machines across Ontario. RESULTS: Of the 100 machines, 39 were affiliated with the Ontario Breast Screening Program (OBSP), and the remaining 61 were applying to become OBSP affiliates. All of the OBSP facilities and 32 of the applicants were accredited with the Mammography Accreditation Program (MAP) of the Canadian Association of Radiologists (CAR). All OBSP facilities had passing phantom scores, and 84% of the applicants with CAR accreditation and 79% of those without had passing phantom scores. The mean glandular dose was 1.5 (range 1.0-1.9, standard deviation [SD] 0.22) mGy for OBSP facilities, 1.3 (0.8-1.8, SD 0.29) mGy for CAR accredited and 1.4 (0.9-1.9, SD 0.24) mGy for nonaccredited applicants. Mean entrance exposures were 798 (540-1280, SD 135) mR, 717 (430-980, SD 153) mR and 770 (520-930, SD 116) mR for the OBSP, accredited and nonaccredited facilities respectively. There were no appreciable differences in the mean optical densities (approx. 1.62 OD), mean half-value layers (approx. 0.34 mm Al) or mean resolutions measured with the large focal spot (nominal size of 0.3 mm) either parallel (approx. 16 line pairs/mm) or perpendicular (approx. 14 line pairs/mm) to the cathode-anode axis of the x-ray tube, among the 3 types of facilities. CONCLUSION: Image quality is increased and variability is decreased in facilities participating in a province-wide screening program.