Experimental microcomputed tomography study of the 3D microangioarchitecture of tumors

Ultra High-Resolution In vivo Computed Tomography Imaging of Mouse Cerebrovasculature Using a Long Circulating Blood Pool Contrast Agent

Abnormalities in the cerebrovascular system play a central role in many neurologic diseases. The on-going expansion of rodent models of human cerebrovascular diseases and the need to use these models to understand disease progression and treatment has amplified the need for reproducible non-invasive imaging methods for high-resolution visualization of the complete cerebral vasculature. In this study, we present methods for in vivo high-resolution (19 μm isotropic) computed tomography imaging of complete mouse brain vasculature. This technique enabled 3D visualization of large cerebrovascular networks, including the Circle of Willis. Blood vessels as small as 40 μm were clearly delineated. ACTA2 mutations in humans cause cerebrovascular defects, including abnormally straightened arteries and a moyamoya-like arteriopathy characterized by bilateral narrowing of the internal carotid artery and stenosis of many large arteries. In vivo imaging studies performed in a mouse model of Acta2 mutations demonstrated the utility of this method for studying vascular morphometric changes that are practically impossible to identify using current histological methods. Specifically, the technique demonstrated changes in the width of the Circle of Willis, straightening of cerebral arteries and arterial stenoses. We believe the use of imaging methods described here will contribute substantially to the study of rodent cerebrovasculature.

Noninvasive monitoring of mouse renal allograft rejection using micro-CT

Purpose

Acute renal graft rejection can only be definitively diagnosed by renal biopsy. However, biopsies carry a risk of renal transplant injury and loss. Micro-CT is widely used in preclinical studies of small animals. Here, we propose micro-CT could noninvasively monitor and evaluate renal location and function in a mouse kidney transplant model.

Methods

Orthotopic kidney transplantation was performed in a BALB/c -to- C57BL/6j or C57BL/6j-to- C57BL/6j mouse model. After optimizing imaging techniques, five mice were imaged with micro-CT and the findings were verified histologically.

Results

Micro-CT can monitor and evaluate renal location and function after orthotopic kidney transplantation. There were no mice deaths while renal transplants were failure.

Conclusion

We propose that graft micro-CT imaging is a new option that is noninvasive and specific, and can aid in early detection and follow-up of acute renal rejection. This method is potentially useful to improve posttransplant rejection monitoring.

Pushing the optical imaging limits of cancer with multi-frequency-band raster-scan optoacoustic mesoscopy (RSOM)

Angiogenesis is a central cancer hallmark, necessary for supporting tumor growth and metastasis. In vivo imaging of angiogenesis is commonly applied, to understand dynamic processes in cancer development and treatment strategies. However, most radiological modalities today assess angiogenesis based on indirect mechanisms, such as the rate of contrast enhancement after contrast agent administration. We studied the performance of raster-scan optoacoustic mesoscopy (RSOM), to directly reveal the vascular network supporting melanoma growth in vivo, at 50 MHz and 100 MHz, through several millimeters of tumor depth. After comparing the performance at each frequency, we recorded, for the first time, high-resolution images of melanin tumor vasculature development in vivo, over a period of several days. Image validation was provided by means of cryo-slice sections of the same tumor after sacrificing the mice. We show how optoacoustic (photoacoustic) mesoscopy reveals a potentially powerful look into tumor angiogenesis, with properties and features that are markedly different than other radiological modalities. This will facilitate a better understanding of tumor's angiogenesis, and the evaluation of treatment strategies.

Oral delivery of a potent anti-angiogenic heparin conjugate by chemical conjugation and physical complexation using deoxycholic acid

Angiogenesis, the formation of new blood vessels, plays a pivotal role in tumor progression and for this reason angiogenesis inhibitors are an important class of therapeutics for cancer treatment. Heparin-based angiogenesis inhibitors have been newly developed as one of such classes of therapeutics and possess a great promise in the clinical context. Taurocholate conjugated low molecular weight heparin derivative (LHT7) has been proven to be a potent, multi-targeting angiogenesis inhibitor against broad-spectrum angiogenic tumors. However, major limitations of LHT7 are its poor oral bioavailability, short half-life, and frequent parenteral dosing schedule. Addressing these issues, we have developed an oral formulation of LHT7 by chemically conjugating LHT7 with a tetrameric deoxycholic acid named LHTD4, and then physically complexing it with deoxycholylethylamine (DCK). The resulting LHTD4/DCK complex showed significantly enhanced oral bioavailability (34.3 ± 2.89%) and prolonged the mean residence time (7.5 ± 0.5 h). The LHTD4/DCK complex was mostly absorbed in the intestine by transcellular pathway via its interaction with apical sodium bile acid transporter. In vitro, the VEGF-induced sprouting of endothelial spheroids was significantly blocked by LHTD4. LHTD4/DCK complex significantly regressed the total vessel fractions of tumor (77.2 ± 3.9%), as analyzed by X-ray microCT angiography, thereby inhibiting tumor growth in vivo. Using the oral route of administration, we showed that LHTD4/DCK complex could be effective and chronically administered as angiogenesis inhibitor.

Imaging aspects of the tumor stroma with therapeutic implications

Cancer cells rely on extensive support from the stroma in order to survive, proliferate and invade. The tumor stroma is thus an important potential target for anti-cancer therapy. Typical changes in the stroma include a shift from the quiescence promoting-antiangiogenic extracellular matrix to a provisional matrix that promotes invasion and angiogenesis. These changes in the extracellular matrix are induced by changes in the secretion of extracellular matrix proteins and glucose amino glycans, extravasation of plasma proteins from hyperpermeable vessels and release of matrix modifying enzymes resulting in cleavage and cross-linking of matrix macromolecules. These in turn alter the rigidity of the matrix and the exposure and release of cytokines. Changes in matrix rigidity and vessel permeability affect drug delivery and mediate resistance to cytotoxic therapy. These stroma changes are brought about not only by the cancer cells, but also through the action of many cell types that are recruited by tumors including immune cells, fibroblasts and endothelial cells. Within the tumor, these normal host cells are activated resulting in loss of inhibitory and induction of cancer promoting activities. Key to the development of stroma-targeted therapies, selective biomarkers were developed for specific imaging of key aspects of the tumor stroma.

Potentiation of anti-angiogenic activity of heparin by blocking the ATIII-interacting pentasaccharide unit and increasing net anionic charge

Heparin, a potent anticoagulant used for the prevention of venous thromboembolism, has been recognized as a tumor angiogenesis inhibitor. Its limitation in clinical application for cancer therapy, however, arises from its strong anticoagulant activity, which causes associated adverse effects. In this study, we show the structural correlation of LHT7, a previously developed heparin-based angiogenesis inhibitor, with its influence on VEGF blockade and its decreased anticoagulant activity. LHT7 was characterized as having average seven molecules of sodium taurocholates conjugated to one molecule of low-molecular-weight heparin (LMWH). This study showed that the conjugation of sodium taurocholates selectively blocked interaction with antithrombin III (ATIII) while enhancing the binding with VEGF. This resulted in LHT7 to have negligible anticoagulant activity but potent anti-angiogenic activity. Following up on this finding, we showed that the bidirectional effect of sodium taurocholate conjugation was due to its unique structure, that is, the sterane core hindering the ATIII-binding pentasaccharide unit of LMWH with its bulky and rigid structural characteristics while the terminal sulfate group interacts with VEGF to produce stronger binding. In addition, we showed that LHT7 was localized in the tumor, especially on the endothelial cells. One explanation for this might be that LHT7 was delivered to the tumor via platelets.

Early changes of hepatic hemodynamics measured by functional CT perfusion in a rabbit model of liver tumor

BACKGROUND

Early detection and treatment of hepatocellular carcinoma is crucial to improving the patients' survival. The hemodynamic changes caused by tumors can be serially measured using CT perfusion. In this study, we used a CT perfusion technique to demonstrate the changes of hepatic hemodynamics in early tumor growth, as a proof-of-concept study for human early hepatocellular carcinoma.

METHODS

VX2 tumors were implanted in the liver of ten New Zealand rabbits. CT perfusion scans were made 1 week (early) and 2 weeks (late) after tumor implantation. Ten normal rabbits served as controls. CT perfusion parameters were obtained at the tumor rim, normal tissue surrounding the tumor, and control liver; the parameters were hepatic blood flow, hepatic blood volume, mean transit time, permeability of capillary vessel surface, hepatic arterial index, hepatic arterial perfusion and hepatic portal perfusion. Microvessel density and vascular endothelial growth factor were correlated.

RESULTS

At the tumor rim, compared to the controls, hepatic blood flow, hepatic blood volume, permeability of capillary vessel surface, hepatic arterial index, and hepatic arterial perfusion increased, while mean transit time and hepatic portal perfusion decreased on both early and late scans (P<0.05). Hepatic arterial index increased (135%, P<0.05), combined with a sharp increase in hepatic arterial perfusion (182%, P<0.05) and a marked decrease in hepatic portal perfusion (-76%, P<0.05) at 2 weeks rather than at 1 week (P<0.05). Microvessel density and vascular endothelial growth factor showed significant linear correlations with hepatic blood flow, permeability of capillary vessel surface and hepatic arterial index, but not with hepatic blood volume or mean transit time.

CONCLUSION

The CT perfusion technique demonstrated early changes of hepatic hemodynamics in this tumor model as proof-of-concept for early hepatocellular carcinoma detection in humans.

X-ray phase contrast for CO2 microangiography

We demonstrate a laboratory method for imaging small blood vessels using x-ray propagation-based phase-contrast imaging and carbon dioxide (CO(2)) gas as a contrast agent. The limited radiation dose in combination with CO(2) being clinically acceptable makes the method promising for small-diameter vascular visualization. We investigate the possibilities and limitations of the method for small-animal angiography and compare it with conventional absorption-based x-ray angiography. Photon noise in absorption-contrast imaging prevents visualization of blood vessels narrower than 50 µm at the highest radiation doses compatible with living animals, whereas our simulations and experiments indicate the possibility of visualizing 20 µm vessels at radiation doses as low as 100 mGy. Experimental computed tomography of excised rat kidney shows blood vessels of diameters down to 60 µm with improved image quality compared to absorption-based methods. With our present prototype x-ray source, the acquisition time for a tomographic dataset is approximately 1 h, which is long compared to the 1-20 min common for absorption-contrast micro-CT systems. Further development of the liquid-metal-jet microfocus x-ray sources used here and high-resolution x-ray detectors shows promise to reduce exposure times and make this high-resolution method practical for imaging of living animals.

The Characteristics of Vascular Growth in VX2 Tumor Measured by MRI and Micro-CT

Blood supply is crucial for rapid growth of a malignant tumor; medical imaging can play an important role in evaluating the vascular characterstics of tumors. Magnetic resonance imaging (MRI) and micro-computed tomography (CT) are able to detect tumors and measure blood volumes of microcirculation in tissue. In this study, we used MR imaging and micro-CT to assess the microcirculation in a VX2 tumor model in rabbits. MRI characterization was performed using the intravascular contrast agent Clariscan (NC100150-Injection); micro-CT with Microfil was used to directly depict blood vessels with diameters as low as 17 um in tissue. Relative blood volume fraction (rBVF) in the tumor rim and blood vessel density (rBVD) over the whole tumor was calculated using the two imaging methods. Our study indicates that rBVF is negatively related to the volume of the tumor measured by ultrasound (R = 0.90). rBVF in the tissue of a VX2 tumor measured by MRI in vivo was qualitatively consistent with the rBVD demonstrated by micro-CT in vitro (R = 0.97). The good correlation between the two methods indicates that MRI studies are potentially valuable for assessing characteristics or tumor vascularity and for assessing response to therapy noninvasively.

Three-dimensional characterization of the vascular bed in bone metastasis of the rat by microcomputed tomography (MicroCT)

Background

Angiogenesis contributes to proliferation and metastatic dissemination of cancer cells. Anatomy of blood vessels in tumors has been characterized with 2D techniques (histology or angiography). They are not fully representative of the trajectories of vessels throughout the tissues and are not adapted to analyze changes occurring inside the bone marrow cavities.

Methodology/Principal Findings

We have characterized the vasculature of bone metastases in 3D at different times of evolution of the disease. Metastases were induced in the femur of Wistar rats by a local injection of Walker 256/B cells. Microfil®, (a silicone-based polymer) was injected at euthanasia in the aorta 12, 19 and 26 days after injection of tumor cells. Undecalcified bones (containing the radio opaque vascular casts) were analyzed by microCT, and a first 3D model was reconstructed. Bones were then decalcified and reanalyzed by microCT; a second model (comprising only the vessels) was obtained and overimposed on the former, thus providing a clear visualization of vessel trajectories in the invaded metaphysic allowing quantitative evaluation of the vascular volume and vessel diameter. Histological analysis of the marrow was possible on the decalcified specimens. Walker 256/B cells induced a marked osteolysis with cortical perforations. The metaphysis of invaded bones became progressively hypervascular. New vessels replaced the major central medullar artery coming from the diaphyseal shaft. They sprouted from the periosteum and extended into the metastatic area. The newly formed vessels were irregular in diameter, tortuous with a disorganized architecture. A quantitative analysis of vascular volume indicated that neoangiogenesis increased with the development of the tumor with the appearance of vessels with a larger diameter.

Conclusion

This new method evidenced the tumor angiogenesis in 3D at different development times of the metastasis growth. Bone and the vascular bed can be identified by a double reconstruction and allowed a quantitative evaluation of angiogenesis upon time.

Three-dimensional analysis of tumour vascular corrosion casts using stereoimaging and micro-computed tomography

OBJECTIVE

In order to perform effective translational research for cancer therapy, we need to employ pre-clinical models which reflect the clinical situation. The purpose of this study was to quantitatively compare the vascular architecture of human colorectal cancer and experimental tumour models to determine the suitability of animal models for vascular studies and antivascular therapy.

METHODS

In this study we investigated the three-dimensional properties of colonic tumour vasculature in both human clinical tissues (normal mucosa control [n=20], carcinoma [n=20] and adenoma [n=6]) and murine colorectal xenografts (LS147T [n=6] and SW1222 [n=6]). Scanning Electron Microscope Stereoimaging (SEM) and X-ray Micro-Computed Tomography (Micro-CT) methods were employed for 3D analyses of the vascular corrosion casts from these tissues.

RESULTS

Morphological measurements showed that there were significant differences in the underlying morphology in the different tissues. Of the studied xenografts, LS147T is more consistently similar to the vascular architecture of the human carcinoma than SW1222. The only reversal of this is for the inter-vessel distance.

CONCLUSION

While SEM stereoimaging provided better surface detailed resolution of the corrosion casts, it was complimented by the fully 3D micro-CT method. Comparison made between the xenografts and clinical tumours showed that the LS147T xenografts shared many similarities with the clinical tumour vasculature. This study provides insight into how to select the most suitable pre-clinical models for translational studies of clinical cancer therapy.

The vascular network in the femoral head and neck after hip resurfacing

The purpose of this study was to determine whether femoral neck fracture after cemented resurfacing hip arthroplasty (RHA) arises from intraosseous vascularity around the femoral head-neck junction. We implanted a replica of the femoral head component into osteoarthritic femoral heads and compared the intraosseous vascularity network between the femoral heads with and without the RHA procedure using microangiography through a retinacular artery with micro-computed tomography. Our results showed no significant difference in the vascularity around the femoral head-neck junction between the groups with and without the RHA procedure. These results suggest that deterioration of the intraosseous vascular network around the head-neck junction after RHA was not severe enough to induce complete avascularity.

Rapid three-dimensional quantification of VEGF-induced scaffold neovascularisation by microcomputed tomography

Microcomputed tomography (micro-CT) is increasingly being used to analyze the three-dimensional structure and architecture of microvascular networks. Therefore we have evaluated a micro-CT analysis of VEGF-induced vessel ingrowth into a porous polyurethane scaffold through comparison with analyses by CD31 immunohistochemistry, vascular perfusion by intravital Lycopersicon esculentum lectin perfusion and vascular corrosion casting. Micro-CT scanning found a similar level of vascularisation within the VEGF treated scaffolds to that determined by the other analytical methods. However, although the relative increase in vascularisation (17 fold above PBS controls p<0.05) induced by VEGF determined by micro-CT was similar to the perfusion based analyses (20.1 and 10.4 fold for lectin perfusion and vascular corrosion respectively p<0.05), it differed substantially from that determined by CD31 immunohistochemistry (3.2 fold p<0.05). This difference was due to a large proportion of unperfused vessels in the PBS control that were not present in the VEGF group. The increase in perfusion probably resulted in part from an increase in average vessel diameter. Though this increase was detected by micro-CT, the actual diameters were overestimated by 60-90% most likely as a consequence of a merging effect for juxtaposed vessels. Thus whilst micro-CT gives an accurate three-dimensional quantification of the VEGF-induced increase in perfused vessels, resolution needs to be maximized for accurate sizing of a microvascular network's components.

In vivo micro-CT imaging of liver lesions in small animal models

Three-dimensional micro computed tomography (microCT) offers the opportunity to capture images liver structures and lesions in mice with a high spatial resolution. Non-invasive microCT allows for accurate calculation of vessel tortuosity and density, as well as liver lesion volume and distribution. Longitudinal monitoring of liver lesions is also possible. However, distinguishing liver lesions from variations within a normal liver is impossible by microCT without the use of liver- or tumor-specific contrast-enhancing agents. The combination of microCT for morphologic imaging with functional imaging, such as positron emission tomography (PET) or single photon emission tomography (SPECT), offers the opportunity for better abdominal imaging and assessment of structure discrepancies visible by functional imaging. This paper describes methods of current microCT imaging options for imaging of liver lesions compared to other imaging techniques in small animals.

Evaluation of angiogenesis using micro-computed tomography in a xenograft mouse model of lung cancer

Quantitative evaluation of lung tumor angiogenesis using immunohistochemical techniques has been limited by difficulties in generating reproducible data. To analyze intrapulmonary tumor angiogenesis, we used high-resolution micro-computed tomography (micro-CT) of lung tumors of mice inoculated with mouse Lewis lung carcinoma (LLC1) or human adenocarcinoma (A549) cell lines. The lung vasculature was filled with the radiopaque silicone rubber, Microfil, through the jugular vein (in vivo application) or pulmonary artery (ex vivo application). In addition, human adenocarcinoma lung tumor-bearing mice treated site-specifically with humanized monoclonal antibody (bevacizumab) against vascular endothelial growth factor. Quantitative analysis of lung tumor microvessels imaged with micro-CT showed that more vessels (mainly small, <0.02 mm(2)) were filled using the in vivo (5.4%) compared with the ex vivo (2.1%) method. Furthermore, bevacizumab-treated lung tumor-bearing mice showed significantly reduced lung tumor volume and lung tumor angiogenesis compared with untreated mice as assessed by micro-CT. Interestingly, microvascularization of mainly the smaller vessels (<0.02 mm(2)) was reduced after bevacizumab treatment. This observation with micro-CT was nicely correlated with immunohistochemical measurement of microvessels. Therefore, micro-CT is a novel method for investigating lung tumor angiogenesis, and this might be considered as an additional complementary tool for precise quantification of angiogenesis.

Low density contrast agents for x-ray phase contrast imaging: the use of ambient air for x-ray angiography of excised murine liver tissue

We report a new preparative method for providing contrast through reduction in electron density that is uniquely suited for propagation-based differential x-ray phase contrast imaging. The method, which results in an air or fluid filled vasculature, makes possible visualization of the smallest microvessels, roughly down to 15 microm, in an excised murine liver, while preserving the tissue for subsequent histological workup. We show the utility of spatial frequency filtering for increasing the visibility of minute features characteristic of phase contrast imaging, and the capability of tomographic reconstruction to reveal microvessel structure and three-dimensional visualization of the sample. The effect of water evaporation from livers during x-ray imaging on the visibility of blood vessels is delineated. The deformed vascular tree in a cancerous murine liver is imaged.

Micro-CT imaging with a hepatocyte-selective contrast agent for detecting liver metastasis in living mice

RATIONALE AND OBJECTIVES

Micro-computed tomography (CT) is a important tool for longitudinal imaging of tumor development. The detection and monitoring of tumors in the liver in live animals using micro-CT is challenging. We evaluated the feasibility of high-resolution micro-CT enhanced with a hepatocyte-selective contrast agent for detecting liver metastases in a live murine model.

MATERIALS AND METHODS

Hepatic metastases were induced in 10 BALB/C mice. Two mice each were randomly selected on days 3, 5, 7, 10, and 13 after CT26 colon adenocarcinoma cells were injected into the portal vein; micro-CT imaging was performed at 10 minutes and 4 hours after intravenous administration of a hepatocyte-selective contrast agent at a dose of 0.4 mL/mouse. The attenuation values of the normal liver and the tumors were obtained. The number of metastases was counted and their sizes were measured on the micro-CT images. Gross or histopathologic evaluation was performed for correlating the liver tumors with the micro-CT images.

RESULTS

A total of 74 separate tumor sites larger than 300 microm in diameter were detected on pathologic examination of the mice that were sacrificed 7 days after cell injection. On micro-CT, 66 of 74 tumors were detected (83.8%). The smallest tumor detected on micro-CT was 300 microm. There were eight false-negative readings on micro-CT. The sizes of the individual liver metastases measured by micro-CT and on the excised specimen were highly correlated (P < .001). The correlation between the CT scan measurement and the actual measurement was r = 0.8354 (P < .0001).

CONCLUSIONS

High-resolution micro-CT enhanced with a hepatocyte-selective contrast agent can be a promising tool for detecting liver metastases in a live murine model.

Imaging techniques to evaluate the response to treatment in oncology: current standards and perspectives

Response evaluation in solid tumours currently uses radiological imaging techniques to measure changes under treatment. Imaging requires a well-defined anatomical lesion to be viewed and relies on the measurement of a reduction in tumour size during treatment as the basis for presumed clinical benefit. However, with the development of anti-angiogenesis agents, anatomical imaging has became inappropriate as certain tumours would not reduce in size. Functional studies are therefore necessary and dynamic contrast enhanced magnetic resonance imaging (DCE-MRI), DCE-computed tomography (CT) and DCE-ultrasonography (US) are currently being evaluated for monitoring treatments. Diffusion-weighted MR imaging (DW-MRI) and magnetic resonance spectroscopy (MRS) are also capable of detecting changes in cell density and metabolite content within tumours. In this article, we review anatomical and functional criteria currently used for monitoring therapy. We review the published data on DCE-MRI, DCE-CT, DCE-US, DW-MRI and MRS. This literature review covers the following area: basic principles of the technique, clinical studies, reproducibility and repeatability, limits and perspectives in monitoring therapy. Anatomical criteria such as response evaluation criteria in solid tumours (RECIST) will require adaptation to employ not only new tools but also different complementary techniques such as functional imaging in order to monitor therapeutic effects of conventional and new anti-cancer agents.

The hepatic sinusoidal endothelial lining and colorectal liver metastases

Colorectal cancer (CRC) is a common malignant disease and the severe nature of cases in men and women who develop colorectal cancer makes this an important socio-economic health issue. Major challenges such as understanding and modeling colorectal cancer pathways rely on our understanding of simple models such as outlined in this paper. We discuss that the development of novel standardized approaches of multidimensional (correlative) biomolecular microscopy methods facilitates the collection of (sub) cellular tissue information in the early onset of colorectal liver metastasis and that this approach will be crucial in designing new effective strategies for CRC treatment. The application of X-ray micro-computed tomography and its potential in correlative imaging of the liver vasculature will be discussed.

Imaging of angiogenesis: clinical techniques and novel imaging methods

OBJECTIVE

A wide variety of antiangiogenic agents have been developed for the treatment of neoplasms. Imaging studies play an important role in assessing the effects of these treatments.

CONCLUSION

This review article introduces radiologists to features of these therapies and the most important clinical and preclinical imaging techniques for evaluating antiangiogenic agents.

μ-CT

While many radiologists are aware of the revolutionary development of computed tomography (CT)-scanners and their diagnostic implications, some researchers have focused on studying the potentials of mu-CT. The aim of this article is to give a brief overview of its physical properties and outline possible indications both for the real ex vivo and small animal mu-CT studies, as well as the modified mu-CT units used in dental practice.

Prospective respiratory-gated micro-CT of free breathing rodents

Microcomputed tomography (Micro-CT) has the potential to noninvasively image the structure of organs in rodent models with high spatial resolution and relatively short image acquisition times. However, motion artifacts associated with the normal respiratory motion of the animal may arise when imaging the abdomen or thorax. To reduce these artifacts and the accompanying loss of spatial resolution, we propose a prospective respiratory gating technique for use with anaesthetized, free-breathing rodents. A custom-made bed with an embedded pressure chamber was connected to a pressure transducer. Anaesthetized animals were placed in the prone position on the bed with their abdomens located over the chamber. During inspiration, the motion of the diaphragm caused an increase in the chamber pressure, which was converted into a voltage signal by the transducer. An output voltage was used to trigger image acquisition at any desired time point in the respiratory cycle. Digital radiographic images were acquired of anaesthetized, free-breathing rats with a digital radiographic system to correlate the respiratory wave form with respiration-induced organ motion. The respiratory wave form was monitored and recorded simultaneously with the x-ray radiation pulses, and an imaging window was defined, beginning at end expiration. Phantom experiments were performed to verify that the respiratory gating apparatus was triggering the micro-CT system. Attached to the distensible phantom were 100 microm diameter copper wires and the measured full width at half maximum was used to assess differences in image quality between respiratory-gated and ungated imaging protocols. This experiment allowed us to quantify the improvement in the spatial resolution, and the reduction of motion artifacts caused by moving structures, in the images resulting from respiratory-gated image acquisitions. The measured wire diameters were 0.135 mm for the stationary phantom image, 0.137 mm for the image gated at end deflation, 0.213 mm for the image gated at peak inflation, and 0.406 mm for the ungated image. Micro-CT images of anaesthetized, free-breathing rats were acquired with a General Electric Healthcare eXplore RS in vivo micro-CT system. Images of the thorax were acquired using the respiratory cycle-based trigger for the respiratory-gated mode. Respiratory gated-images were acquired at inspiration and end expiration, during a period of minimal respiration-induced organ motion. Gated images were acquired with a nominal isotropic voxel spacing of 44 microm in 20-25 min (80 kVp, 113 mAs, 300 ms imaging window per projection). The equivalent ungated acquisitions were 11 min in length. We observed improved definition of the diaphragm boundary and increased conspicuity of small structures within the lungs in the gated images, when compared to the ungated acquisitions. In this work, we have characterized the externally monitored respiratory wave form of free-breathing, anaesthetized rats and correlated the respiration-induced organ motion to the respiratory cycle. We have shown that the respiratory pressure wave form is an excellent surrogate for the radiographic organ motion. This information facilitates the definition of an imaging window at any phase of the breathing cycle. This approach for prospectively gated micro-CT can provide high quality images of anaesthetized free-breathing rodents.

Volumetric computed tomography (VCT): a new technology for noninvasive, high-resolution monitoring of tumor angiogenesis

Volumetric computed tomography (VCT) is a technology in which area detectors are used for imaging large volumes of a subject with isotropic imaging resolution. We are experimenting with a prototype VCT scanner that uses flat-panel X-ray detectors and is designed for high-resolution three-dimensional (3D) imaging. Using this technique, we have demonstrated microangiography of xeno-transplanted skin squamous cell carcinomas in nude mice. VCT shows the vessel architecture of tumors and animals with greater detail and plasticity than has previously been achieved, and is superior to contrast-enhanced magnetic resonance (MR) angiography. VCT and MR images correlate well for larger tumor vessels, which are tracked from their origin on 3D reconstructions of VCT images. When compared with histology, small tumor vessels with a diameter as small as 50 microm were clearly visualized. Furthermore, imaging small vessel networks inside the tumor tissue improved discrimination of vital and necrotic regions. Thus, VCT substantially improves imaging of vascularization in tumors and offers a promising tool for preclinical studies of tumor angiogenesis and antiangiogenic therapies.

Quantitative microcomputed tomography analysis of collateral vessel development after ischemic injury

Transgenic mouse models are increasingly being used to investigate the functions of specific growth factors or matrix proteins to design therapeutic strategies for controlling blood vessel growth. However, the available methodologies for evaluating angiogenesis and arteriogenesis in these models are limited by animal size, user subjectivity, the power to visualize the three-dimensional vessel networks, or the capability to employ a vigorous quantitative analysis. In this study, we employed contrast-enhanced microcomputed tomography imaging to assess collateral development after induction of hindlimb ischemia in the mouse. The morphological parameters vessel volume, connectivity, number, thickness, thickness distribution, separation, and degree of anisotropy were evaluated in control and surgery limbs 0, 3, and 14 days postsurgery. Results indicate that the vascular volume of the surgically manipulated limb was reconstituted as early as 3 days after femoral artery excision through development of a series of highly connected, small caliber, closely spaced, and isotropically oriented collateral vessels. Parametric analyses were completed to assess the sensitivity of the calculated morphological parameters to variations in image binarization threshold and voxel size. Images taken at the 36-μm voxel size were found to be optimal for evaluating collateral vessel formation, whereas 8- to 16-μm voxel sizes were needed to resolve smaller vascular structures. This study demonstrates the utility of microcomputed tomography as a robust method for quantitative, three-dimensional analysis of blood vessel networks. Whereas these initial efforts focused on the mouse hindlimb ischemia model, the developed techniques may be applied to a variety of model systems to investigate mechanisms of angiogenesis and arteriogenesis.

Diffraction enhanced imaging of articular cartilage and comparison with micro-computed tomography of the underlying bone structure

The goal of this study was to explore the role of diffraction enhanced X-ray imaging (DEI) for assessing changes in osteoarthritic cartilage and correlating the findings with concurrent changes in the underlying bone imaged using micro-computed tomography (microCT). DEI was used to image femoral head specimens at various beam energies. DEI utilizes a monochromatic, highly collimated beam, with an analyzer crystal that selectively weights out photons according to the angle they have been deviated with respect to the original direction. This provides images of very high contrast, with the rejection of X-ray scatter. The underlying bone was imaged using microCT and measures quantifying the bone structure were derived. Confirmation of cartilage degeneration was obtained from histology and polarized light microscopy. DEI allowed the visualization of articular cartilage and reflected the fibrillations and fissures in tissues from degenerated joints. The trabecular bone underlying the most degenerated articular cartilage showed increased bone volume fraction and more plate-like characteristics, compared with that underlying normal appearing cartilage. The histology and polarized light microscopy images reflected the DEI based features of cartilage architecture. These data reflect the ability of X-ray based emerging technologies to depict cartilage-bone interactions in joint degeneration.

In Vivo Respiratory-Gated Micro-CT Imaging in Small-Animal Oncology Models

Micro-computed tomography(micro-CT) is becoming an accepted research tool for the noninvasive examination of laboratory animals such as mice and rats, but to date, in vivo scanning has largely been limited to the evaluation of skeletal tissues. We use a commercially available micro-CT device to perform respiratory gated in vivo acquisitions suitable for thoracic imaging. The instrument is described, along with the scan protocol and animal preparation techniques. Preliminary results confirm that lung tumors as small as 1 mm in diameter are visible in vivo with these methods. Radiation dose was evaluated using several approaches, and was found to be approximately 0.15 Gy for this respiratory-gated micro-CT imaging protocol. The combination of high-resolution CT imaging and respiratory-gated acquisitions appears well-suited to serial in vivo scanning.