Lung injury may occur following acute inhalation of a toxic gas, or following chronic exposure to pulmonary irritants. The choice of imaging studies and the information sought in the patient with a history of inhalation of a toxic gas varies according to the time elapsed since the acute event. Experimental studies in animals have helped to define more clearly some of the very early radiographic changes that occur following lung injury. The abnormal chest radiograph in the subacute phase of inhalation injury can be considered an example of adult respiratory distress syndrome. Imaging studies may be of greatest value in the subacute phase of inhalation injury, defined as the patient's hospitalization following the first 24 hr after the acute event. The chronic sequelae of inhalation injury include bronchiolitis obliterans, bronchiectasis, and tracheal stenosis. Tracheal stenosis can occur as a result of chemical irritation of the tracheobronchial mucosa, and also as a result of tracheostomies performed during the patient's hospitalization.

The growing exposure to chemicals in our environment and the increasing concern over their impact on health have elevated the need for new methods for surveying the detrimental effects of these compounds. Today's gold standard for assessing the effects of toxicants on the brain is based on hematoxylin and eosin (H&E)-stained histology, sometimes accompanied by special stains or immunohistochemistry for neural processes and myelin. This approach is time-consuming and is usually limited to a fraction of the total brain volume. We demonstrate that magnetic resonance histology (MRH) can be used for quantitatively assessing the effects of central nervous system toxicants in rat models. We show that subtle and sparse changes to brain structure can be detected using magnetic resonance histology, and correspond to some of the locations in which lesions are found by traditional pathological examination. We report for the first time diffusion tensor image-based detection of changes in white matter regions, including fimbria and corpus callosum, in the brains of rats exposed to 8 mg/kg and 12 mg/kg trimethyltin. Besides detecting brain-wide changes, magnetic resonance histology provides a quantitative assessment of dose-dependent effects. These effects can be found in different magnetic resonance contrast mechanisms, providing multivariate biomarkers for the same spatial location. In this study, deformation-based morphometry detected areas where previous studies have detected cell loss, while voxel-wise analyses of diffusion tensor parameters revealed microstructural changes due to such things as cellular swelling, apoptosis, and inflammation. Magnetic resonance histology brings a valuable addition to pathology with the ability to generate brain-wide quantitative parametric maps for markers of toxic insults in the rodent brain.

CT and digital subtraction angiography (DSA) are ubiquitous in the clinic. Their preclinical equivalents are valuable imaging methods for studying disease models and treatment. We have developed a dual source/detector X-ray imaging system that we have used for both micro-CT and DSA studies in rodents. The control of such a complex imaging system requires substantial software development for which we use the graphical language LabVIEW (National Instruments, Austin, TX, USA). This paper focuses on a LabVIEW platform that we have developed to enable anatomical and functional imaging with micro-CT and DSA. Our LabVIEW applications integrate and control all the elements of our system including a dual source/detector X-ray system, a mechanical ventilator, a physiological monitor, and a power microinjector for the vascular delivery of X-ray contrast agents. Various applications allow cardiac- and respiratory-gated acquisitions for both DSA and micro-CT studies. Our results illustrate the application of DSA for cardiopulmonary studies and vascular imaging of the liver and coronary arteries. We also show how DSA can be used for functional imaging of the kidney. Finally, the power of 4D micro-CT imaging using both prospective and retrospective gating is shown for cardiac imaging.

BACKGROUND:Hyperpolarized (HP) (129)Xe magnetic resonance imaging (MRI) permits high resolution, regional visualization of pulmonary ventilation. Additionally, its reasonably high solubility (>10%) and large chemical shift range (>200 ppm) in tissues allow HP (129)Xe to serve as a regional probe of pulmonary perfusion and gas transport, when introduced directly into the vasculature. In earlier work, vascular delivery was accomplished in rats by first dissolving HP (129)Xe in a biologically compatible carrier solution, injecting the solution into the vasculature, and then detecting HP (129)Xe as it emerged into the alveolar airspaces. Although easily implemented, this approach was constrained by the tolerable injection volume and the duration of the HP (129)Xe signal.METHODS AND PRINCIPAL FINDINGS:Here, we overcome the volume and temporal constraints imposed by injection, by using hydrophobic, microporous, gas-exchange membranes to directly and continuously infuse (129)Xe into the arterial blood of live rats with an extracorporeal (EC) circuit. The resulting gas-phase (129)Xe signal is sufficient to generate diffusive gas exchange- and pulmonary perfusion-dependent, 3D MR images with a nominal resolution of 2×2×2 mm(3). We also show that the (129)Xe signal dynamics during EC infusion are well described by an analytical model that incorporates both mass transport into the blood and longitudinal relaxation.CONCLUSIONS:Extracorporeal infusion of HP (129)Xe enables rapid, 3D MR imaging of rat lungs and, when combined with ventilation imaging, will permit spatially resolved studies of the ventilation-perfusion ratio in small animals. Moreover, EC infusion should allow (129)Xe to be delivered elsewhere in the body and make possible functional and molecular imaging approaches that are currently not feasible using inhaled HP (129)Xe.

Introduction: Proper matching of ventilation (V) and pulmonary perfusion (Q) is essential for efficient gas exchange. Moreover, mismatches between V and Q and spatial heterogeneity in the V/Q ratio are hallmarks of virtually all pulmonary diseases. Unfortunately, currently available methods of imaging V and Q in small animals: 1) lack 3D resolution, which is needed to measure V/Q heterogeneity; 2) employ multiple modalities or contrast agents, which complicates data quantification; or 3) require hour-long image acquisitions, which prevent monitoring acute changes in V/Q. Potentially, these problems can be avoided using hyperpolarized (HP) Xe, which can generate lung images that reflect either ventilation or perfusion if delivered via the vasculature [1]. To exploit these unique properties, we previously introduced a method of continuously infusing Xe to the blood using an extracorporeal (EC) circuit [2]. Here we demonstrate that EC infusion can be used to generate 3D images of pulmonary perfusion.

We present a ventilator that enables high-resolution proton and hyperpolarized gas MR imaging of mice and rats. The design differs from previous approaches by eliminating the need for a custom pneumatic valve located near the trachea. This permits the system to be constructed from off-the-shelf components and reduces dead volumes sufficiently to make HP gas MRI feasible in the mouse. The constant-volume ventilator routinely ventilates mice and rats for period of time up to 6 hrs and maintains reproducible tidal volumes over extended image acquisition periods, as we demonstrate with high-resolution 3D lung images in the mouse using 1H, 3He and 129Xe. The ventilator is designed to deliver a constant tidal volume regardless of changes in airway resistance, which we demonstrate with 3He MR images acquired during severe broncho-constriction. While the images reveal clear airway narrowing, the 3He signal intensity remained within ±10% of baseline level. Finally, given the paucity of 3He and the high cost of enriched 129Xe, the ventilator has been designed to enable the recapture of these rare gases and we demonstrate a compact system to compress and store them for subsequent reprocessing. We expect that this constant-volume ventilator will be readily reproducible by other laboratories, which we facilitate by providing extensive parts lists, detailed wiring diagrams and complete plumbing schematics. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 39B: 78–88, 2011

Introduction: Dobutamine (DOB) stress in animal models of heart disease has been imaged so far using echocardiography and magnetic resonance imaging. The purpose of this study was to assess normal response to DOB stress in rats using anatomical and functional data using micro-computed tomography (CT). Methods: Ten normal adult male rats were first injected with a liposomal-based blood pool contrast agent and next infused with DOB via a tail vein catheter. Using prospective gating, 5 pairs of systole/diastole micro-CT images were acquired (a) pre-infusion baseline; (b) at heart rate plateau during infusion of 10 mu g/kg/min DOB; (c) at post-DOB infusion baseline; (d) at heart rate plateau during infusion of 30 mu g/kg/min DOB; and (e) after post-infusion return to baseline. Heart rate, peripheral and breathing distensions were monitored by oximetry. Micro-CT images with 88-mu m isotropic voxels were segmented to obtain cardiac function based on volumetric measurements of the left ventricle. Results: DOB stress increased heart rate and cardiac output with both doses. Ejection fraction increased above baseline by an average of 35.9% with the first DOB dose and 18.4% with the second dose. No change was observed in the relative peripheral arterial pressures associated with the significant increases in cardiac output. Discussion: Micro-CT proved to be a robust imaging method able to provide isotropic data on cardiac morphology and function. Micro-CT has the advantage of being faster and more cost-effective than MR and is able to provide higher accuracy than echocardiography. The impact of such an enabling technology can be enormous in evaluating cardiotoxic effects of various test drugs. (C) 2010 Elsevier Inc. All rights reserved.

Introduction: X-ray based digital subtraction angiography (DSA) is a common clinical imaging method for vascular morphology and function. Coronary artery characterization is one of its most important applications. We show that bi-plane DSA of rat coronary arteries can provide a powerful imaging tool for translational safety assessment in drug discovery. Methods: A novel, dual tube/detector system, constructed explicitly for preclinical imaging, supports image acquisition at 10 frames/s with 88-micron spatial resolution. Ventilation, x-ray exposure, and contrast injection ate all precisely synchronized using a biological sequence controller implemented as a LabVIEW application. A set of experiments were performed to test and optimize the sampling and image quality. We applied the DSA imaging protocol to record changes in the visualization of coronaries and myocardial perfusion induced by a vasodilator drug, nitroprusside. The drug was infused into a tail vein catheter using a peristaltic infusion pump at a rate of 0.07 mL/h for 3 min (dose: 0.0875 mg). Multiple DSA sequences were acquired before, during, and up to 25 min after drug infusion. Perfusion maps of the heart were generated in MATLAB to compare the drug effects over time. Results: The best trade-off between the injection time, pressure, and image quality was achieved at 60 PSI, with the injection of 150 ms occurring early in diastole (60 ms delay) and resulting in the delivery of 113 mu L of contrast agent. DSA images clearly show the main branches of the coronary arteries in an intact, beating heart. The drug test demonstrated that DSA can detect relative changes in coronary circulation via perfusion maps. Conclusions: The methodology for DSA imaging of rat coronary arteries can serve as a template for future translational studies to assist in safety evaluation of new pharmaceuticals. Although x-ray imaging involves radiation, the associated dose (0.4 Gy) is not a major limitation. (C) 2011 Elsevier Inc. All rights reserved.

Introduction: In previous work, we have shown that spatially resolved information on pulmonary perfusion can be obtained by imaging with intravenous injection of hyperpolarized (HP) Xe dissolved in saline (Fig. 1) [1]. Further work has demonstrated a potential of for absolute quantification [2] by fitting signal curves recorded during bolus injection to a simple model based on the Kety theory of diffusible tracers (Fig. 2) [3]. However, careful analysis showed that such fits required using an effective flip angle αeff that was considerably smaller than the value obtained from a separate flip-angle calibration scan. Similar observations were reported from brain perfusion experiments employing HP Xe inhalation [4]. Here, we demonstrate that such effects can be addressed by a refined model that accounts for bolus delay and dispersion effects. Methods: Three Sprague-Dawley rats (male, 316-342g; Charles River, Wilmington, MA) were prepared according to an IACUC-approved protocol including anesthesia by IP injections of pentobarbital/butorphanol and ventilation on a constant-volume ventilator [5]. HP Xe, enriched to 83% (Spectra Gases, Alpha, NJ) was produced in batches of ≈120 mL at P≈10% using a prototype commercial polarizer (model 9800, MITI, Durham, NC). Xe MR experiments used a 23.6MHz quadrature birdcage coil in a 2T, horizontal, 30cm clear-bore magnet (Oxford Instruments, Oxford, UK) and GE Excite console (GE Healthcare, Milwaukee, WI). HP Xe was dissolved in 30-40mL of half-concentrated saline and shaken for ≈20 s. Subsequently, 5 mL of the fluid was withdrawn into a syringe and injected over a period of 15 s into the rat’s tail vein while respiration was suspended. A total of 32 experiments with repetitive spectroscopic acquisitions for 30 s (α 3-33°, TR 125-250ms) were performed to study dynamics of the HP Xe resonances. To model signal dynamics we assumed constant injection starting at t = 0 and ending at t = tB and treated HP Xe as a diffusible tracer considering an arterial input depending on pulmonary perfusion, Q, and Xe exchange between the vascular and alveolar compartments, determined by the Ostwald solubility, L. Contributions to signal loss arise from relaxation and transport described by an apparent relaxation rate,

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 ms) and isotropic 3D reconstruction with a voxel size of 88 microm based on sampling using 16 injections of 50 microL each. The total volume of contrast agent injected during the experiments (0.8 mL) 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.

The global increase in asthma, chronic obstructive pulmonary disease, and other pulmonary diseases has stimulated interest in preclinical rat models of pulmonary disease. Imaging methods for study of these models is particularly appealing since the results can be readily translated to the clinical setting. Comprehensive understanding of lung function can be achieved by performing registered pulmonary ventilation and perfusion imaging studies in the same animal. While ventilation imaging has been addressed for small animals, quantitative pulmonary perfusion imaging has not been feasible until recently, with our proposed technique for quantitative perfusion imaging using multiple contrast‐agent injections and a view‐sharing radial imaging technique. Here, we combine the method with registered ventilation imaging using hyperpolarized 3He in an airway obstruction rodent model. To our knowledge, this is the first comprehensive quantitative assessment of lung function in small animals at high spatial resolution. Standard deviation of the log (V/Q) is used as a quantitative biomarker to differentiate heterogeneity between the control and treatment group. The estimated value of the biomarker lies within the normal range of values reported in the literature. The biomarker that was extracted using the imaging technique described in this work showed statistically significant differences between the control rats and those with airway obstruction. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.

Maintaining optimal blood glucose (Glu) and insulin (Ins) levels is essential for physiological studies in anesthetized mice. This study examined the temporal effects of varying levels of isoflurane (ISO) anesthesia on blood Glu and Ins levels in mice for 90 min. post‐induction. Male C57BL/6 mice were allowed to breath freely ISO at 1 (n=5), 1.5 (n=7), and 2.0% (n=8) in 100% O2 while body temperature was maintained at 37.1±0.4°C. ECG and respiratory flowrate were monitored. A 0.6μl tail vein blood aliquot was extracted at 5 min. intervals for Glu measurements. Ins was measured from 1ml arterial blood at 1 and 2% ISO in 20 mice euthanized at t=0, 20, 40, 60 and 80 min. Mean heart rates (±SD) were 532±39, 488±39, and 449±23 bpm at 1, 1.5, and 2% ISO respectively (p<0.0001). Glu concentrations (mg/dl) ranged between 155±28–196±76 [1% ISO], 162±43–239±38 [1.5% ISO], and 159±52–205±49 [2.0% ISO]. Ins values ranged between 3.5±0.3–4.2±0.6ng/dl [2% ISO], and 3.4±0.5–4.2±0.9ng/dl at 1% ISO. Glu values were significantly higher in the ISO=1.5 and 2% groups compared to the 1% group (p<0.0001). No significant temporal change was noted in Glu values at the 3 ISO doses. Ins concentrations did not differ significantly between groups. The results confirm that ISO levels ≥1.5% produce a mild hyperglycemic effect, prominent at later time periods post‐induction.Research supported by the Hellenic Bank and the Research Promotion Foundation.

A long circulating liposomal, nanoscale blood pool agent encapsulating traditional iodinated contrast agent (65 mg I/mL) was used for micro-computed tomography (CT) imaging of rats implanted with R3230AC mammary carcinoma. Three-dimensional vascular architecture of tumors was imaged at 100-micron isotropic resolution. The image data showed good qualitative correlation with pathologic findings. The approach holds promise for studying tumor angiogenesis and for evaluating anti-angiogenesis therapies.

Purpose To develop and demonstrate a method for regional evaluation of pulmonary perfusion and gas exchange based on intravenous injection of hyperpolarized xenon 129 (129Xe) and subsequent magnetic resonance (MR) imaging of the gas-phase 129Xe emerging in the alveolar airspaces. Materials and Methods Five Fischer 344 rats that weighed 200—425 g were prepared for imaging according to an institutional animal care and use committee—approved protocol. Rats were ventilated, and a 3-F catheter was placed in the jugular (n = 1) or a 24-gauge catheter in the tail (n = 4) vein. Imaging and spectroscopy of gas-phase 129Xe were performed after injecting 5 mL of half-normal saline saturated with 129Xe hyperpolarized to 12%. Corresponding ventilation images were obtained during conventional inhalation delivery of hyperpolarized 129Xe. Results Injections of 129Xe-saturated saline were well tolerated and produced a strong gas-phase 129Xe signal in the airspaces that resulted from 129Xe transport through the pulmonary circulation and diffusion across the blood-gas barrier. After a single injection, the emerging 129Xe gas could be detected separately from 129Xe remaining in the blood and was imaged with an in-plane resolution of 1 × 1 mm and a signal-to-noise ratio of 25. Images in one rat revealed a matched ventilation-perfusion deficit, while images in another rat showed that xenon gas exchange was temporarily impaired after saline overload, with recovery of function 1 hour later. Conclusion MR imaging of gas-phase 129Xe emerging in the pulmonary airspaces after intravenous injection has the potential to become a sensitive and minimally invasive new tool for regional evaluation of pulmonary perfusion and gas exchange. Supplemental material: http://radiology.rsnajnls.org/cgi/content/full/2522081550/DC1 © RSNA, 2009

Hyperpolarized (HP) (129)Xe yields high signal intensities in nuclear magnetic resonance (NMR) and, through its large chemical shift range of approximately 300 ppm, provides detailed information about the local chemical environment. To exploit these properties in aqueous solutions and living tissues requires the development of methods for efficiently dissolving HP (129)Xe over an extended time period. To this end, we have used commercially available gas exchange modules to continuously infuse concentrated HP (129)Xe into flowing liquids, including rat whole blood, for periods as long as one hour and have demonstrated the feasibility of dissolved-phase MR imaging with submillimeter resolution within minutes. These modules, which exchange gases using hydrophobic microporous polymer membranes, are compatible with a variety of liquids and are suitable for infusing HP (129)Xe into the bloodstream in vivo. Additionally, we have developed a detailed mathematical model of the infused HP (129)Xe signal dynamics that should be useful in designing improved infusion systems that yield even higher dissolved HP (129)Xe signal intensities.

Liver fibrosis is currently staged using needle biopsy, a highly invasive procedure with a number of disadvantages. Measurement of liver stiffness changes that accompany progression of the disease may provide a quantitative and noninvasive method to assess the health of the liver. The purpose of this study is to investigate the correlation between liver stiffness measured by radiation force induced shear waves and disease related changes in the liver. An additional aim is to present initial findings on the effects of liver viscosity on radiation force induced shear wave morphology. Liver fibrosis was induced in 10 rats using carbon tetrachloride (CCl(4)), while five rats acted as controls. Liver stiffness was measured in vivo in all rats after a treatment period of 8 weeks using a modified Siemens SONOLINE Antares scanner (Siemens Medical Solutions USA, Ultrasound Division, Issaquah, WA, USA). The spatial coherence of radiation force induced shear waves propagating in the viscoelastic rat liver decreased significantly with propagation distance, compared with shear waves in an elastic phantom and a finite element model of a purely elastic medium. Animals were sacrificed after imaging and liver samples were taken for histopathologic analysis and collagen quantification using picrosirius red staining and hydroxyproline assay. At the end of the treatment period, five rats had healthy livers (stage F0), while six had severe fibrosis (F3) and the rest had light to moderate fibrosis (F1 and F2). The measured liver stiffness for the F0 group was 1.5+/-0.1 kPa (mean+/-95% confidence interval) and for F3 livers was 1.8+/-0.2 kPa. In this study, liver stiffness was found to be linearly correlated with the amount of collagen in the liver measured by picrosirius red staining (r(2)=0.43, p=0.008). In addition, stiffness spatial heterogeneity was also linearly correlated with liver collagen content (r(2)=0.58, p=0.001) by picrosirius red staining. These results are consistent with those obtained by Salameh et al. (2007) and Yin et al. (2007b) using animal models of liver fibrosis and MR elastography. This suggests that stiffness measurement using acoustic radiation force can provide a quantitative assessment of the extent of fibrosis in the liver and can be potentially used for the diagnosis, management and study of liver fibrosis.

Small animal magnetic resonance microscopy (MRM) has evolved significantly from testing the boundaries of imaging physics to its expanding use today as a tool in noninvasive biomedical investigations. MRM now increasingly provides functional information about living animals, with images of the beating heart, breathing lung, and functioning brain. Unlike clinical MRI, where the focus is on diagnosis, MRM is used to reveal fundamental biology or to noninvasively measure subtle changes in the structure or function of organs during disease progression or in response to experimental therapies. High-resolution anatomical imaging reveals increasingly exquisite detail in healthy animals and subtle architectural aberrations that occur in genetically altered models. Resolution of 100 mu m in all dimensions is now routinely attained in living animals, and (10 mu m)(3) is feasible in fixed specimens. Such images almost rival conventional histology while allowing the object to be viewed interactively in any plane. In this review we describe the state of the art in MRM for scientists who may be unfamiliar with this modality but who want to apply its capabilities to their research. We include a brief review of MR concepts and methods of animal handling and support, before covering a range of MRM applications-including the heart, lung, and brain-and the emerging field of MR histology. The ability of MRM to provide a detailed functional and anatomical picture in rats and mice, and to track this picture over time, makes it a promising platform with broad applications in biomedical research.