Micro-CT is a non-invasive imaging modality usually used to assess morphology in small animals. In our previous work, we have demonstrated that functional micro-CT imaging is also possible. This paper describes a dual micro-CT system with two fixed x-ray/detectors developed to address such challenging tasks as cardiac or perfusion studies in small animals. A two-tube/detector system ensures simultaneous acquisition of two projections, thus reducing scanning time and the number of contrast injections in perfusion studies by a factor of two. The system is integrated with software developed in-house for cardio-respiratory monitoring and gating. The sampling geometry was optimized for 88 microns in such a way that the geometric blur of the focal spot matches the Nyquist sample at the detector. A geometric calibration procedure allows one to combine projection data from the two chains into a single reconstructed volume. Image quality was measured in terms of spatial resolution, uniformity, noise, and linearity. The modulation transfer function (MTF) at 10% is 3.4 lp/mm for single detector reconstructions and 2.3 lp/mm for dual tube/detector reconstructions. We attribute this loss in spatial resolution to the compounding of slight errors in the separate single chain calibrations. The dual micro-CT system is currently used in studies for morphological and functional imaging of both rats and mice.

The authors describe a dual tube/detector micro-computed tomography (micro-CT) system that has the potential to improve temporal resolution and material contrast in small animal imaging studies. To realize this potential, it is necessary to precisely calibrate the geometry of a dual micro-CT system to allow the combination of projection data acquired with each individual tube/detector in a single reconstructed image. The authors present a geometric calibration technique that uses multiple projection images acquired with the two imaging chains while rotating a phantom containing a vertical array of regularly spaced metallic beads. The individual geometries of the imaging chains are estimated from the phantom projection images using analytical methods followed by a refinement procedure based on nonlinear optimization. The geometric parameters are used to create the cone beam projection matrices required by the reconstruction process for each imaging chain. Next, a transformation between the two projection matrices is found that allows the combination of projection data in a single reconstructed image. The authors describe this technique, test it with a series of computer simulations, and then apply it to data collected from their dual tube/detector micro-CT system. The results demonstrate that the proposed technique is accurate, robust, and produces images free of misalignment artifacts.

The purpose of this work is to investigate the use of dual-energy microcomputed tomography (CT) for the estimation of vascular, tissue, and air fractions in rodent lungs using a postreconstruction three material decomposition method. Using simulations, we have estimated the accuracy limits of the decomposition for realistic micro-CT noise levels. Next, we performed experiments involving ex vivo lung imaging in which intact rat lungs were carefully removed from the thorax, injected with an iodine-based contrast agent, and then inflated with different volumes of air (n ϭ 2). Finally, we performed in vivo imaging studies in C57BL/6 mice (n ϭ 5) using fast prospective respiratory gating in end inspiration and end expiration for three different levels of positive end expiratory pressure (PEEP). Before imaging, mice were injected with a liposomal blood pool contrast agent. The three-dimensional air, tissue, and blood fraction maps were computed and analyzed. The results indicate that separation and volume estimation of the three material components of the lungs are possible. The mean accuracy values for air, blood, and tissue were 93, 93, and 90%, respectively. The absolute accuracy in determining all fraction materials was 91.6%. The coefficient of variation was small (2.5%) indicating good repeatability. The minimum difference that we could detect in material fractions was 15%. As expected, an increase in PEEP levels for the living mouse resulted in statistically significant increases in air fractions at end expiration but no significant changes at end inspiration. Our method has applicability in preclinical pulmonary studies where changes in lung structure and gas volume as a result of lung injury, environmental exposures, or drug bioactivity would have important physiological implications. micro-computed tomography; small animal imaging; perfusion COMPUTED TOMOGRAPHY (CT) IS one of the most used imaging modalities for the evaluation of thoracic disorders. With the recently developed dual-energy CT technique (DECT), the clinical utility of CT in the pulmonary diseases could expand even more. DECT allows analysis of the chemical composition of tissues by means of dual-energy data acquisition and tissue decomposition (14). CT data analysis with DECT is based on differences in X-ray absorption of heavy elements with energy. In clinical use, many DECT studies are already focusing on the thorax (9,18,19,(23)(24)(25)(26)(27). In the thorax, the three materials most frequently analyzed are iodine, air, and soft tissue. Recently, researchers (26) have evaluated the feasibility of using DECT for pulmonary perfusion and ventilation in the lungs using both iodinated contrast agents and xenon to provide distributions representing the local perfusion and ventilation.More interestingly, a recent study (8) has abandoned xenon for ventilation imaging in CT because the use of xenon gas is logistically demanding and requires the use of special inhalators and pressurized gas bottles. Consequently, the authors have investigated the fe...

Purpose Micro-CT is currently used to provide anatomical information in preclinical studies. There is however significant interest in obtaining functional measurements on the same systems. We report here novel strategies for 4D Micro-CT with a focus on two applications: i) cardiac imaging based on retrospective gating and ii) pulmonary perfusion using multiple contrast injections/rotations paradigm. Methods A dual source micro-CT system is used for image acquisition with pulsed x-rays exposures (80 kVp, 70mA, 10 ms) and a sampling rate of 20 projections/sec. The cardiac micro-CT protocol involves the use of a liposomal blood pool contrast agent containing 123 mg I/ml delivered via a tail vein catheter in a dose of 0.01 ml/g body weight. Fast scanning of free breathing mice is achieved using retrospective gating. The ECG and respiratory signals are used to sort projections into 10 cardiac phases. The pulmonary perfusion protocol uses a conventional contrast agent (Isovue 370). The total volume of contrast agent is limited to 0.4 ml and is delivered by a micro-injector in 4 injections separated by 2 minute intervals to allow for clearance. Each injection is synchronized with the rotation of the animal, and each of the 4 rotations is started with an angular offset of 22.5° from the starting angle of the previous rotation. Both cardiac and perfusion protocols result in an irregular angular distribution of projections that causes significant streaking artifacts in reconstructions when using traditional filtered back projection algorithms. To alleviate these artifacts, we construct the point spread function (PSF) of our micro-CT system and analyze the distribution of the reconstructed data in the Fourier domain. This enables us to correct for irregular angular inconsistencies via deconvolution and also identify regions where data is missing. These regions are filled in with data from a high quality but temporally averaged image reconstructed with all available projections. We implemented parts of this algorithm on a graphics processing unit (GPU) to reduce run time. The proposed method was validated with numerical simulations and experimental micro-CT data. Results The simulations indicate that deconvolution successfully removes the streaking artifacts while preserving temporal information. 4D cardiac micro-CT in a mouse was performed with adequate image quality at isotropic voxel size of 88 microns and 10 ms temporal resolution. 4D pulmonary perfusion images were obtained in a mouse at 176 microns and 687 msec temporal resolution. Compared with FBP reconstruction, the streak reduction ratio is 70% and the contrast to noise ratio is 2.5 times greater in the deconvolved images. The radiation dose associated with the proposed methods is in the range of a typical micro-CT dose (0.17 Gy for the cardiac study and 0.21 Gy for the perfusion study). Conclusions The low dose 4D micro-CT imaging protocols presented here can be applied in high throughput longitudinal studies in a wide range of applications, including drug...

Purpose: Non-small cell lung cancers (NSCLC) are a heterogeneous group of carcinomas harboring a variety of different gene mutations. We have utilized two distinct genetically engineered mouse models of human NSCLC (adenocarcinoma) to investigate how genetic factors within tumor parenchymal cells influence the in vivo tumor growth delay after one or two fractions of radiation therapy (RT).Materials and Methods: Primary lung adenocarcinomas were generated in vivo in mice by intranasal delivery of an adenovirus expressing Cre-recombinase. Lung cancers expressed oncogenic KrasG12D and were also deficient in one of two tumor suppressor genes: p53 or Ink4a/ARF. Mice received no radiation treatment or whole lung irradiation in a single fraction (11.6 Gy) or in two 7.3 Gy fractions (14.6 Gy total) separated by 24 h. In each case, the biologically effective dose (BED) equaled 25 Gy10. Response to RT was assessed by micro-CT 2 weeks after treatment. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and immunohistochemical staining were performed to assess the integrity of the p53 pathway, the G1 cell-cycle checkpoint, and apoptosis.Results: Tumor growth rates prior to RT were similar for the two genetic variants of lung adenocarcinoma. Lung cancers with wild-type (WT) p53 (LSL-Kras; Ink4a/ARFFL/FL mice) responded better to two daily fractions of 7.3 Gy compared to a single fraction of 11.6 Gy (P = 0.002). There was no statistically significant difference in the response of lung cancers deficient in p53 (LSL-Kras; p53FL/FL mice) to a single fraction (11.6 Gy) compared to 7.3 Gy × 2 (P = 0.23). Expression of the p53 target genes p21 and PUMA were higher and bromodeoxyuridine uptake was lower after RT in tumors with WT p53.Conclusion: Using an in vivo model of malignant lung cancer in mice, we demonstrate that the response of primary lung cancers to one or two fractions of RT can be influenced by specific gene mutations.

Purpose: The use of preclinical rodent models of disease continues to grow because these models help elucidate pathogenic mechanisms and provide robust test beds for drug development. Among the major anatomic and physiologic indicators of disease progression and genetic or drug modification of responses are measurements of blood vessel caliber and flow. Moreover, cardiopulmonary blood flow is a critical indicator of gas exchange. Current methods of measuring cardiopulmonary blood flow suffer from some or all of the following limitations-they produce relative values, are limited to global measurements, do not provide vasculature visualization, are not able to measure acute changes, are invasive, or require euthanasia. Methods: In this study, high-spatial and high-temporal resolution x-ray digital subtraction angiography ͑DSA͒ was used to obtain vasculature visualization, quantitative blood flow in absolute metrics ͑ml/min instead of arbitrary units or velocity͒, and relative blood volume dynamics from discrete regions of interest on a pixel-by-pixel basis ͑100ϫ 100 m 2 ͒. Results: A series of calibrations linked the DSA flow measurements to standard physiological measurement using thermodilution and Fick's method for cardiac output ͑CO͒, which in eight anesthetized Fischer-344 rats was found to be 37.0Ϯ 5.1 ml/ min. Phantom experiments were conducted to calibrate the radiographic density to vessel thickness, allowing a link of DSA cardiac output measurements to cardiopulmonary blood flow measurements in discrete regions of interest. The scaling factor linking relative DSA cardiac output measurements to the Fick's absolute measurements was found to be 18.90ϫ CO DSA =CO Fick . Conclusions: This calibrated DSA approach allows repeated simultaneous visualization of vasculature and measurement of blood flow dynamics on a regional level in the living rat.

Purpose: Quantitative in vivo imaging of lung perfusion in rodents can provide critical information for preclinical studies. However, the combined challenges of high temporal and spatial resolution have made routine quantitative perfusion imaging difficult in small animals. The purpose of this work is to demonstrate 4D micro-CT for perfusion imaging in rodents at heartbeat temporal resolution and isotropic spatial resolution. Methods: We have recently developed a dual tube/detector micro-CT scanner that is well suited to capture first pass kinetics of a bolus of contrast agent used to compute perfusion information. Our approach is based on the paradigm that similar time density curves can be reproduced in a number of consecutive, small volume injections of iodinated contrast agent at a series of different angles. This reproducibility is ensured by the high-level integration of the imaging components of our system with a microinjector, a mechanical ventilator, and monitoring applications. Sampling is controlled through a biological pulse sequence implemented in LABVIEW. Image reconstruction is based on a simultaneous algebraic reconstruction technique implemented on a graphic processor unit. The capabilities of 4D micro-CT imaging are demonstrated in studies on lung perfusion in rats. Results: We report 4D micro-CT imaging in the rat lung with a heartbeat temporal resolution ͑approximately 150 ms͒ and isotropic 3D reconstruction with a voxel size of 88 m based on sampling using 16 injections of 50 L each. The total volume of contrast agent injected during the experiments ͑0.8 mL͒ was less than 10% of the total blood volume in a rat. This volume was not injected in a single bolus, but in multiple injections separated by at least 2 min interval to allow for clearance and adaptation. We assessed the reproducibility of the time density curves with multiple injections and found that these are very similar. The average time density curves for the first eight and last eight injections are slightly different, i.e., for the last eight injections, both the maximum of the average time density curves and its area under the curve are decreased by 3.8% and 7.2%, respectively, relative to the average time density curves based on the first eight injections. The radiation dose associated with our 4D micro-CT imaging is 0.16 Gy and is therefore in the range of a typical micro-CT dose. Conclusions: 4D micro-CT-based perfusion imaging demonstrated here has immediate application in a wide range of preclinical studies such as tumor perfusion, angiogenesis, and renal function. Although our imaging system is in many ways unique, we believe that our approach based on the multiple injection paradigm can be used with the newly developed flat-panel slip-ring-based micro-CT to increase their temporal resolution in dynamic perfusion studies.

Spectral CT imaging is expected to play a major role in the diagnostic arena as it provides material decomposition on an elemental basis. One fascinating possibility is the ability to discriminate multiple contrast agents targeting different biological sites. We investigate the feasibility of dual energy micro-CT for discrimination of iodine (I) and gold (Au) contrast agents when simultaneously present in the body. Simulations and experiments were performed to measure the CT enhancement for I and Au over a range of voltages from 40-to-150 kVp using a dual source micro-CT system. The selected voltages for dual energy micro-CT imaging of Au and I were 40 kVp and 80 kVp. On a massconcentration basis, the relative average enhancement of Au to I was 2.75 at 40 kVp and 1.58 at 80 kVp. We have demonstrated the method in a preclinical model of colon cancer to differentiate vascular architecture and extravasation. The concentration maps of Au and I allow quantitative measure of the bio-distribution of both agents. In conclusion, dual energy micro-CT can be used to discriminate probes containing I and Au with immediate impact in pre-clinical research.