Localized Oxygen Exchange Platform for Intravital Video Microscopy Investigations of Microvascular Oxygen Regulation.

Intravital microscopy has proven to be a powerful tool for studying microvascular physiology. In this study, we propose a gas exchange system compatible with intravital microscopy that can be used to impose gas perturbations to small localized regions in skeletal muscles or other tissues that can be imaged using conventional inverted microscopes. We demonstrated the effectiveness of this system by locally manipulating oxygen concentrations in rat extensor digitorum longus muscle and measuring the resulting vascular responses. A computational model of oxygen transport was used to partially validate the localization of oxygen changes in the tissue, and oxygen saturation of red blood cells flowing through capillaries were measured as a surrogate for local tissue oxygenation. Overall, we have demonstrated that this approach can be used to study dynamic and spatial responses to local oxygen challenges to the microenvironment of skeletal muscle.

Image-based design and 3D-metal printing of a rat hip implant for use in a clinically representative model of joint replacement.

The aim of this study was to obtain micro-computed tomography derived measurements of the rat proximal femur, to create parameterized rat hip implants that could be surgically installed in a clinically representative small animal model of joint replacement. The proximal femoral anatomy of N = 25 rats (male, Sprague-Dawley, 390-605 g) was quantified. Key measurements were used to parameterize computer-aided design models of monoblock rat femoral implants. Linear regression analysis was used to determine if rat hip dimensions could be predicted from animal weight. A correlation analysis was used to determine how implants could be scaled to create a range of sizes. Additive manufacturing (3D printing) was used to create implants in medical-grade metal alloys. Linear regressions comparing rat weight to femoral head diameter and neck-head axis length revealed a significant nonzero slope (P < .05). Pearson's correlation analysis revealed five significant correlations between key measurements in the rat femur (P < .05). Implants were installed into both cadaveric and live animals; iterative design modifications were made to prototypes based on these surgical findings. Animals were able to tolerate the installation of implants and were observed ambulating on their affected limbs postoperatively. Clinical significance: We have developed a preclinical rat hip hemiarthroplasty model using image-based and iterative design techniques to create 3D-metal printed implants in medical-grade metal alloys. Our findings support further development of this model for use as a low-cost translational test platform for preclinical orthopaedic research into areas such as osseointegration, metal-on-cartilage wear, and periprosthetic joint infection.

Vibration of osteoblastic cells using a novel motion-control platform does not acutely alter cytosolic calcium, but desensitizes subsequent responses to extracellular ATP.

Low-magnitude high-frequency mechanical vibration induces biological responses in many tissues. Like many cell types, osteoblasts respond rapidly to certain forms of mechanostimulation, such as fluid shear, with transient elevation in the concentration of cytosolic free calcium ([Ca2+ ]i ). However, it is not known whether vibration of osteoblastic cells also induces acute elevation in [Ca2+ ]i . To address this question, we built a platform for vibrating live cells that is compatible with microscopy and microspectrofluorometry, enabling us to observe immediate responses of cells to low-magnitude high-frequency vibrations. The horizontal vibration system was mounted on an inverted microscope, and its mechanical performance was evaluated using optical tracking and accelerometry. The platform was driven by a sinusoidal signal at 20-500 Hz, producing peak accelerations from 0.1 to 1 g. Accelerometer-derived displacements matched those observed optically within 10%. We then used this system to investigate the effect of acceleration on [Ca2+ ]i in rodent osteoblastic cells. Cells were loaded with fura-2, and [Ca2+ ]i was monitored using microspectrofluorometry and fluorescence ratio imaging. No acute changes in [Ca2+ ]i or cell morphology were detected in response to vibration over the range of frequencies and accelerations studied. However, vibration did attenuate Ca2+ transients generated subsequently by extracellular ATP, which activates P2 purinoceptors and has been implicated in mechanical signaling in bone. In summary, we developed and validated a motion-control system capable of precisely delivering vibrations to live cells during real-time microscopy. Vibration did not elicit acute elevation of [Ca2+ ]i , but did desensitize responses to later stimulation with ATP.

Development and Assessment of a Microcomputed Tomography Compatible Five Degrees-of-Freedom Knee Joint Motion Simulator.

Currently available knee joint kinematic tracking systems fail to nondestructively capture the subtle variation in joint and soft tissue kinematics that occur in native, injured, and reconstructed joint states. Microcomputed tomography (CT) imaging has the potential as a noninvasive, high-resolution kinematic tracking system, but no dynamic simulators exist to take advantage of this. The purpose of this work was to develop and assess a novel micro-CT compatible knee joint simulator to quantify the knee joint's kinematic and kinetic response to clinically (e.g., pivot shift test) and functionally (e.g., gait) relevant loading. The simulator applies closed-loop, load control over four degrees-of-freedom (DOF) (internal/external rotation, varus/valgus rotation, anterior/posterior translation, and compression/distraction), and static control over a fifth degree-of-freedom (flexion/extension). Simulator accuracy (e.g., load error) and repeatability (e.g., coefficient of variation) were assessed with a cylindrical rubber tubing structure and a human cadaveric knee joint by applying clinically and functionally relevant loads along all active axes. Micro-CT images acquired of the joint at a loaded state were then used to calculate joint kinematics. The simulator loaded both the rubber tubing and the cadaveric specimen to within 0.1% of the load target, with an intertrial coefficient of variation below 0.1% for all clinically relevant loading protocols. The resultant kinematics calculated from the acquired images agreed with previously published values, and produced errors of 1.66 mm, 0.90 mm, 4.41 deg, and 1.60 deg with respect to anterior translation, compression, internal rotation, and valgus rotation, respectively. All images were free of artifacts and showed knee joint displacements in response to clinically and functionally loading with isotropic CT image voxel spacing of 0.15 mm. The results of this study demonstrate that the joint-motion simulator is capable of applying accurate, clinically and functionally relevant loads to cadaveric knee joints, concurrent with micro-CT imaging. Nondestructive tracking of bony landmarks allows for the precise calculation of joint kinematics with less error than traditional optical tracking systems.

Marker-based technique for visualizing radiolucent implant components in radiographic imaging.

Radiography is the predominant imaging modality used for the in-vivo analysis of orthopaedic implants. A major disadvantage of radiography is that the articulating joint components that are composed of radiolucent polyethylene cannot be directly visualized. Current strategies attempt to circumvent this limitation by estimating component positions and simplifying the joint system, however, these approaches lead to a number of associated errors. Thus, this study provides a method to enable the visualization of the polyethylene component of total knee replacements in radiographic images. This was achieved through the repeatable insertion of markers and accompanying registration process, which were evaluated in this study for reproducibility and accuracy. An insertion guide was developed to insert tantalum beads into polyethylene tibial surface liners. The bead-inserted liners were micro-CT scanned to obtain 3D surface geometries. An in-vivo mimicking phantom RSA experiment was then used to test the 3D to 2D registration process. The guide positioned the beads consistently to ±0.21 mm. The 3D to 2D registration demonstrated a repeatability of -0.014 ± 0.008 mm. Registration of different bead-inserted tibial liners to the phantom revealed an average error of 0.026 ± 0.047 mm for this visualization method. This visualization approach provides greatly improved registration and inter-component measurements than current alternative strategies. This process is suitable for a number of other joints and would greatly benefit procedures that analyze component interactions and implant performance over time. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2017-2022, 2017.

Evaluation of the effect of custom burr holes on a surgeon's sense of screw fixation in revision porous metal cups.

BACKGROUND/AIMS: It is common practice to burr custom holes in revision porous metal cups for screw insertion. The objective of this study was to determine how different hole types affect a surgeon's sense of screw fixation. METHODS: Porous revision cups were prepared with pre-drilled and custom burred holes. Cups were held in place adjacent to synthetic bone material of varying density. Surgeons inserted screws through the different holes and materials. Surgeon subjective rating, compression, and torque was recorded. RESULTS: The torque achieved was greater (p = 0.002) for screws through custom holes than pre-fabricated holes in low and medium density material, with no difference for high density. Peak compression was greater (p = 0.026) through the pre-fabricated holes only in high density material. CONCLUSION: Use of burred holes affects the torque generated, and may decrease the amount of cup-acetabulum compression achieved.

Practical fabrication of microfluidic platforms for live-cell microscopy.

We describe a simple fabrication technique - targeted towards non-specialists - that allows for the production of leak-proof polydimethylsiloxane (PDMS) microfluidic devices that are compatible with live-cell microscopy. Thin PDMS base membranes were spin-coated onto a glass-bottom cell culture dish and then partially cured via microwave irradiation. PDMS chips were generated using a replica molding technique, and then sealed to the PDMS base membrane by microwave irradiation. Once a mold was generated, devices could be rapidly fabricated within hours. Fibronectin pre-treatment of the PDMS improved cell attachment. Coupling the device to programmable pumps allowed application of precise fluid flow rates through the channels. The transparency and minimal thickness of the device enabled compatibility with inverted light microscopy techniques (e.g. phase-contrast, fluorescence imaging, etc.). The key benefits of this technique are the use of standard laboratory equipment during fabrication and ease of implementation, helping to extend applications in live-cell microfluidics for scientists outside the engineering and core microdevice communities.

Quantification of mouse in vivo whole-body vibration amplitude from motion-blur using x-ray imaging.

Musculoskeletal effects of whole-body vibration on animals and humans have become an intensely studied topic recently, due to the potential of applying this method as a non-pharmacological therapy for strengthening bones. It is relatively easy to quantify the transmission of whole-body mechanical vibration through the human skeletal system using accelerometers. However, this is not the case for small-animal pre-clinical studies because currently available accelerometers have a large mass, relative to the mass of the animals, which causes the accelerometers themselves to affect the way vibration is transmitted. Additionally, live animals do not typically remain motionless for long periods, unless they are anesthetized, and they are required to maintain a static standing posture during these studies. These challenges provide the motivation for the development of a method to quantify vibrational transmission in small animals. We present a novel imaging technique to quantify whole-body vibration transmission in small animals using 280 µm diameter tungsten carbide beads implanted into the hind limbs of mice. Employing time-exposure digital x-ray imaging, vibrational amplitude is quantified based on the blurring of the implanted beads caused by the vibrational motion. Our in vivo results have shown this technique is capable of measuring vibration amplitudes as small as 0.1 mm, with precision as small as ±10 µm, allowing us to distinguish differences in the transmitted vibration at different locations on the hindlimbs of mice.

Metrology test object for dimensional verification in additive manufacturing of metals for biomedical applications.

Additive manufacturing continues to increase in popularity and is being used in applications such as biomaterial ingrowth that requires sub-millimeter dimensional accuracy. The purpose of this study was to design a metrology test object for determining the capabilities of additive manufacturing systems to produce common objects, with a focus on those relevant to medical applications. The test object was designed with a variety of features of varying dimensions, including holes, cylinders, rectangles, gaps, and lattices. The object was built using selective laser melting, and the produced dimensions were compared to the target dimensions. Location of the test objects on the build plate did not affect dimensions. Features with dimensions less than 0.300 mm did not build or were overbuilt to a minimum of 0.300 mm. The mean difference between target and measured dimensions was less than 0.100 mm in all cases. The test object is applicable to multiple systems and materials, tests the effect of location on the build, uses a minimum of material, and can be measured with a variety of efficient metrology tools (including measuring microscopes and micro-CT). Investigators can use this test object to determine the limits of systems and adjust build parameters to achieve maximum accuracy.

Quantification of in vivo implant wear in total knee replacement from dynamic single plane radiography.

An in vivo method to measure wear in total knee replacements was developed using dynamic single-plane fluoroscopy. A dynamic, anthropomorphic total knee replacement phantom with interchangeable, custom-fabricated components of known wear volume was created, and dynamic imaging was performed. For each frame of the fluoroscopy data, the relative location of the femoral and tibial components were determined, and the apparent intersection of the femoral component with the tibial insert was used to calculate wear volume, wear depth, and frequency of intersection. No difference was found between the measured and true wear volumes. The precision of the measurements was ±39.7 mm(3) for volume and ±0.126 mm for wear depth. The results suggest the system is capable of tracking wear volume changes across multiple time points in patients. As a dynamic technique, this method can provide both kinematic and wear measurements that may be useful for evaluating new implant designs for total knee replacements.

Evaluation of distal turbulence intensity for the detection of both plaque ulceration and stenosis grade in the carotid bifurcation using clinical Doppler ultrasound.

OBJECTIVES: To determine the interrelationship of stenosis grade and ulceration with distal turbulence intensity (TI) in the carotid bifurcation measured using conventional clinical Doppler ultrasound (DUS) in vitro, in order to establish the feasibility of TI as a diagnostic parameter for plaque ulceration. METHODS: DUS TI was evaluated in a matched set of ulcerated and smooth-walled carotid bifurcation phantoms with various stenosis severities (30, 50, 60 and 70 %), where the ulcerated models incorporated a type 3 ulceration. RESULTS: Post-stenotic TI was significantly elevated owing to ulceration in the mild and moderate stenoses (P < 0.001). TI increased with stenosis severity in both the ulcerated and non-ulcerated series, with a statistically significant effect of increasing stenosis severity (P < 0.001). Whereas TI in the mild and non-ulcerated moderate stenoses was less than 20.4 ± 1.3 cm s(-1), TI in the ulcerated moderate and severe models was higher than 25.6 ± 1.3 cm s(-1), indicating a potential diagnostic threshold. CONCLUSION: We report a two-curve relationship of stenosis grade and ulceration to distal TI measured using clinical DUS in vitro. Clinical DUS measurement of distal TI may be a diagnostic approach to detecting ulceration in the mild and moderately stenosed carotid artery. KEY POINTS: • Patients with carotid artery plaque ulcerations are at higher risk of stroke. • Clinical Doppler ultrasound is routinely used to detect carotid artery stenosis. • Doppler ultrasound turbulence intensity can detect ulceration in realistic flow models. • Turbulence intensity also increases with stenosis severity independent of ulceration. • Doppler ultrasound should help in assessing both stenosis severity and ulceration.

Micro-CT-compatible technique for measuring self-expanding stent forces.

PURPOSE: To develop and evaluate a technique for measuring the radial resistive force, chronic outward force, and dimensions of self-expanding stents. MATERIALS AND METHODS: A Mylar film was looped around the stent, threaded through two carbon fiber rods, and immersed in a 37 degrees C oil bath. A force gauge mounted on a micro-positioning stage was used to measure the applied forces. The apparatus containing the self-expanding nitinol stent (diameter, 40 mm; length, 80 mm) was placed inside a micro-computed tomographic (CT) scanner. At each stent deformation, the load was manually recorded from the force gauge and a micro-CT volume (isotropic voxel spacing, 0.15 mm) obtained. Stent diameter and length were measured from the images, and radial resistive force and chronic outward force were calculated for each deformation. RESULTS: The stress-strain curves indicate that the stents exert much smaller maximum outward forces (1.2 N/cm) than the force that is required to compress them (3.6 N/cm). The forces were measured with a precision of +/-3.3% (standard deviation of five repeated measurements). The stent's diameter was measured with precision better than 0.3% and accuracy of +/-0.1 mm. CONCLUSIONS: The authors have developed a radiographic technique that enables precise measurements of radial resistive force, chronic outward force, and the dimensions of self-expanding stents during deformation.

Clinical Doppler ultrasound for the assessment of plaque ulceration in the stenosed carotid bifurcation by detection of distal turbulence intensity: a matched model study.

The assessment of flow disturbances due to carotid plaque ulceration may provide added diagnostic information to Doppler ultrasound (DUS) of the carotid stenosis, and indicate whether the associated hemodynamics are a potential thromboembolic source. We evaluated the effect of ulceration in a moderately stenosed carotid bifurcation on distal turbulence intensity (TI) measured using clinical DUS in matched anthropomorphic models. Several physiologically relevant ulcer geometries (hemispherical, mushroom-shaped, and ellipsoidal pointing distally and proximally) and sizes (2-mm, 3-mm and 4-mm diameter hemispheres) were investigated. An offline analysis was performed to determine several velocity-based parameters from ensemble-averaged spectral data, including TI. Significant elevations in TI were observed in the post-stenotic flow field of the stenosed carotid bifurcation by the inclusion of ulceration (P < 0.001) in a region two common carotid artery diameters distal to the site of ulceration during the systolic peak and the diastolic phase of the cardiac cycle. Both the size and shape of ulceration had a significant effect on TI in the distal region (P < 0.001). Due to the use of a clinical system, this method provides the means to evaluate for plaque ulcerations in patients with carotid atherosclerosis using DUS.

In vitro Doppler ultrasound investigation of turbulence intensity in pulsatile flow with simulated cardiac variability.

An in vitro investigation of turbulence intensity (TI) associated with a severe carotid stenosis in the presence of physiological cardiac variability is described. The objective of this investigation was to determine if fluctuations due to turbulence could be quantified with conventional Doppler ultrasound (DUS) in the presence of normal physiological cycle-to-cycle cardiac variability. An anthropomorphic model of a 70% stenosed carotid bifurcation was used in combination with a programmable flow pump to generate pulsatile flow with a mean flow rate of 6 mL/s. Utilizing the pump, we studied normal, nonrepetitive cycle-to-cycle cardiac variability (+/-3.9%) in flow, as well as waveform shapes with standard deviations equal to 0, 2 and 3 times the normal variation. Eighty cardiac cycles of Doppler data were acquired at two regions within the model, representing either laminar or turbulent flow; each measurement was repeated six times. Turbulence intensity values were found to be 11 times higher (p < 0.001), on average, in the turbulent region than in the laminar region, with a mean difference of 24 cm/s. Twenty cardiac cycles were required for confidence in TI values. In conclusion, these results indicate that it is possible to quantify TI in vitro, even in the presence of normal and exaggerated cycle-to-cycle cardiac variability.

PIV-measured versus CFD-predicted flow dynamics in anatomically realistic cerebral aneurysm models.

Computational fluid dynamics (CFD) modeling of nominally patient-specific cerebral aneurysms is increasingly being used as a research tool to further understand the development, prognosis, and treatment of brain aneurysms. We have previously developed virtual angiography to indirectly validate CFD-predicted gross flow dynamics against the routinely acquired digital subtraction angiograms. Toward a more direct validation, here we compare detailed, CFD-predicted velocity fields against those measured using particle imaging velocimetry (PIV). Two anatomically realistic flow-through phantoms, one a giant internal carotid artery (ICA) aneurysm and the other a basilar artery (BA) tip aneurysm, were constructed of a clear silicone elastomer. The phantoms were placed within a computer-controlled flow loop, programed with representative flow rate waveforms. PIV images were collected on several anterior-posterior (AP) and lateral (LAT) planes. CFD simulations were then carried out using a well-validated, in-house solver, based on micro-CT reconstructions of the geometries of the flow-through phantoms and inlet/outlet boundary conditions derived from flow rates measured during the PIV experiments. PIV and CFD results from the central AP plane of the ICA aneurysm showed a large stable vortex throughout the cardiac cycle. Complex vortex dynamics, captured by PIV and CFD, persisted throughout the cardiac cycle on the central LAT plane. Velocity vector fields showed good overall agreement. For the BA, aneurysm agreement was more compelling, with both PIV and CFD similarly resolving the dynamics of counter-rotating vortices on both AP and LAT planes. Despite the imposition of periodic flow boundary conditions for the CFD simulations, cycle-to-cycle fluctuations were evident in the BA aneurysm simulations, which agreed well, in terms of both amplitudes and spatial distributions, with cycle-to-cycle fluctuations measured by PIV in the same geometry. The overall good agreement between PIV and CFD suggests that CFD can reliably predict the details of the intra-aneurysmal flow dynamics observed in anatomically realistic in vitro models. Nevertheless, given the various modeling assumptions, this does not prove that they are mimicking the actual in vivo hemodynamics, and so validations against in vivo data are encouraged whenever possible.

Doppler ultrasound compatible plastic material for use in rigid flow models.

A technique for the rapid but accurate fabrication of multiple flow phantoms with variations in vascular geometry would be desirable in the investigation of carotid atherosclerosis. This study demonstrates the feasibility and efficacy of implementing numerically controlled direct-machining of vascular geometries into Doppler ultrasound (DUS)-compatible plastic for the easy fabrication of DUS flow phantoms. Candidate plastics were tested for longitudinal speed of sound (SoS) and acoustic attenuation at the diagnostic frequency of 5 MHz. Teflon was found to have the most appropriate SoS (1376 +/- 40 m s(-1) compared with 1540 m s(-1) in soft tissue) and thus was selected to construct a carotid bifurcation flow model with moderate eccentric stenosis. The vessel geometry was machined directly into Teflon using a numerically controlled milling technique. Geometric accuracy of the phantom lumen was verified using nondestructive micro-computed tomography. Although Teflon displayed a higher attenuation coefficient than other tested materials, Doppler data acquired in the Teflon flow model indicated that sufficient signal power was delivered throughout the depth of the vessel and provided comparable velocity profiles to that obtained in the tissue-mimicking phantom. Our results indicate that Teflon provides the best combination of machinability and DUS compatibility, making it an appropriate choice for the fabrication of rigid DUS flow models using a direct-machining method.

A quality assurance phantom for the performance evaluation of volumetric micro-CT systems.

Small-animal imaging has recently become an area of increased interest because more human diseases can be modeled in transgenic and knockout rodents. As a result, micro-computed tomography (micro-CT) systems are becoming more common in research laboratories, due to their ability to achieve spatial resolution as high as 10 microm, giving highly detailed anatomical information. Most recently, a volumetric cone-beam micro-CT system using a flat-panel detector (eXplore Ultra, GE Healthcare, London, ON) has been developed that combines the high resolution of micro-CT and the fast scanning speed of clinical CT, so that dynamic perfusion imaging can be performed in mice and rats, providing functional physiological information in addition to anatomical information. This and other commercially available micro-CT systems all promise to deliver precise and accurate high-resolution measurements in small animals. However, no comprehensive quality assurance phantom has been developed to evaluate the performance of these micro-CT systems on a routine basis. We have designed and fabricated a single comprehensive device for the purpose of performance evaluation of micro-CT systems. This quality assurance phantom was applied to assess multiple image-quality parameters of a current flat-panel cone-beam micro-CT system accurately and quantitatively, in terms of spatial resolution, geometric accuracy, CT number accuracy, linearity, noise and image uniformity. Our investigations show that 3D images can be obtained with a limiting spatial resolution of 2.5 mm(-1) and noise of +/-35 HU, using an acquisition interval of 8 s at an entrance dose of 6.4 cGy.

Virtual angiography for visualization and validation of computational models of aneurysm hemodynamics.

It has recently become possible to simulate aneurysmal blood flow dynamics in a patient-specific manner via the coupling of three-dimensional (3-D) X-ray angiography and cmputational fluid dynamics (CFD). Before such image-based CFD models can be used in a predictive capacity, however, it must be shown that they indeed reproduce the in vivo hemodynamic environment. Motivated by the fact that there are currently no techniques for adequately measuring complex blood velocity fields in vivo, in this paper we describe how cine X-ray angiograms may be simulated for the purpose of indirectly validating patient-sperific CFD models. Mimicking the radiological procedure, a virtual angiogram is constructed by first simulating the time-varying injection of contrast agent into a precomputed, patient-specific CFD model. A time-series of images is then constructed by simulating the attenuation of X-rays through the computed 3-D contrast-agent flow dynamics. Virtual angiographic images and residence time maps, here derived from an image-based CFD model of a giant aneurysm, are shown to be in excellent agreement wiith the corresponding clinical images and residence time maps, but only when the interaction between the quasisteady contrast agent injection and the pulsatile flow are properly accounted for. These virtual angiographic techniques pave the way for validating image-based CFD models against routinely available clinical data, and provide a means of visualizing complex, 3-D blood flow dynamics in a clinically relevant manner. They also clearly show how the contrast agent injection perturbs the noraml blood flow patterns, further highlighting the potential utility of image-based CFD as a window into the true aneurysmal hemodynamics.

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.

Multisite trial of MR flow measurement: phantom and protocol design.

PURPOSE: To describe a portable, easily assembled phantom with well-defined bore geometry together with a series of tests that will form the basis of a standardized quality assurance protocol in a multicenter trial of flow measurement by the MR phase mapping technique. MATERIALS AND METHODS: The phantom consists of silicone polymer layers containing parallel straight and stenosed flow channels in one layer and a U-bend in a second layer, separated by hermetically sealed agarose slabs. The phantom is constructed by casting low melting-point metal in an aluminum mold precisely milled to the desired geometry, and then using the low melting-point metal core as a negative around which the silicone is allowed to set. By melting out the metal, the flow channels are established. The milled aluminum mold is reusable, ensuring faithful reproduction of the flow geometry for all phantoms thus produced. The agarose layers provide additional loading and static background signal for background correction. With the use of the described phantom, one can evaluate flow measurement accuracy and repeatability, as well as the influence of several imaging geometry factors: slice offset, in-plane position, and slice-flow obliquity. RESULTS: The new phantom is compact and portable, and is well suited for reassembly. We were able to demonstrate its facility in a battery of tests of interest in evaluating MR flow measurements. CONCLUSION: The phantom is a robust standardized test object for use in a multicenter trial. Such a trial, to investigate the performance of MR flow measurement using the phantom and the tests we describe, has been initiated.