High tibial osteotomy to neutral alignment improves medial knee articular cartilage composition

PURPOSE

The purpose of this study was to: (1) test the hypothesis that HTO improves articular cartilage composition in the medial compartment without adversely affecting the lateral compartment and patella, and; (2) explore associations between knee alignment and cartilage composition after surgery.

METHODS

3T MRI and standing radiographs were obtained from 34 patients before and 1-year after HTO. Articular cartilage was segmented from T2 maps. Mechanical axis angle (MAA), posterior tibial slope, and patellar height were measured from radiographs. Changes in T2 and radiographic measures were assessed using paired t tests, and associations were assessed using Pearson correlation coefficients.

RESULTS

The mean (SD) MAA before and after HTO was - 6.5° (2.4) and 0.6° (3.0), respectively. There was statistically significant shortening [mean (95%CI)] of T2 in the medial femur [- 2.8 ms (- 4.2; - 1.3), p < 0.001] and medial tibia [- 2.2 ms (- 3.3; - 1.0), p < 0.001], without changes in the lateral femur [- 0.5 ms (- 1.6; 0.6), p = 0.3], lateral tibia [0.2 ms (- 0.8; 1.1), p = NS], or patella [0.5 ms (- 1.0; 2.1), p = NS). Associations between radiographic measures and T2 were low. 23% of the increase in lateral femur T2 was explained by postoperative posterior tibial slope (r = 0.48).

CONCLUSION

Performing medial opening wedge HTO without overcorrection improves articular cartilage composition in the medial compartment of the knee without compromising the lateral compartment or the patella. Although further research is required, these results suggest HTO is a disease structure-modifying treatment for knee OA.

Extending the dynamic range of biomedical micro-computed tomography for application to geomaterials

BACKGROUND

X-ray computed tomography (CT) can non-destructively examine objects by producing three-dimensional images of their internal structure. Although the availability of biomedical micro-CT offers the increased access to scanners, CT images of dense objects are susceptible to artifacts particularly due to beam hardening.

OBJECTIVE

This study proposes and evaluates a simple semi-empirical correction method for beam hardening and scatter that can be applied to biomedical scanners.

METHODS

Novel calibration phantoms of varying diameters were designed and built from aluminum and poly[methyl-methacrylate]. They were imaged using two biomedical micro-CT scanners. Absorbance measurements made through different phantom sections were fit to polynomial and inversely exponential functions and used to determine linearization parameters. Corrections based on the linearization equations were applied to the projection data before reconstruction.

RESULTS

Correction for beam hardening was achieved when applying both scanners with the correction methods to all test objects. Among them, applying polynomial correction method based on the aluminum phantom provided the best improvement. Correction of sample data demonstrated a high agreement of percent-volume composition of dense metallic inclusions between using the Bassikounou meteorite from the micro-CT images (13.7%) and previously published results using the petrographic thin sections (14.6% 8% metal and 6.6% troilite).

CONCLUSIONS

Semi-empirical linearization of X-ray projection data with custom calibration phantoms allows accurate measurements to be obtained on the radiodense samples after applying the proposed correction method on biomedical micro-CT images.

Whole-body vibration of mice induces articular cartilage degeneration with minimal changes in subchondral bone

OBJECTIVE

Low-amplitude, high-frequency whole-body vibration (WBV) has been adopted for the treatment of musculoskeletal diseases including osteoarthritis (OA); however, there is limited knowledge of the direct effects of vibration on joint tissues. Our recent studies revealed striking damage to the knee joint following exposure of mice to WBV. The current study examined the effects of WBV on specific compartments of the murine tibiofemoral joint over 8 weeks, including microarchitecture of the tibia, to understand the mechanisms associated with WBV-induced joint damage.

DESIGN

Ten-week-old male CD-1 mice were exposed to WBV (45 Hz, 0.3 g peak acceleration; 30 min/day, 5 days/week) for 4 weeks, 8 weeks, or 4 weeks WBV followed by 4 weeks recovery. The knee joint was evaluated histologically for tissue damage. Architecture of the subchondral bone plate, subchondral trabecular bone, primary and secondary spongiosa of the tibia was assessed using micro-CT.

RESULTS

Meniscal tears and focal articular cartilage damage were induced by WBV; the extent of damage increased between 4 and 8-week exposures to WBV. WBV did not alter the subchondral bone plate, or trabecular bone of the tibial spongiosa; however, a transient increase was detected in the subchondral trabecular bone volume and density.

CONCLUSIONS

The lack of WBV-induced changes in the underlying subchondral bone suggests that damage to the articular cartilage may be secondary to the meniscal injury we detected. Our findings underscore the need for further studies to assess the safety of WBV in the human population to avoid long-term joint damage.

C57BL/6 mice are resistant to joint degeneration induced by whole-body vibration

OBJECTIVE

Whole-body vibration (WBV) platforms are commercially available devices that are used clinically to treat numerous musculoskeletal conditions based on their reported ability to increase bone mineral density and muscle strength. Despite widespread use, there is an alarming lack of understanding of the direct effects of WBV on joint health. Previous work by our lab demonstrated that repeated exposure to WBV using protocols that model those used clinically, induces intervertebral disc (IVD) degeneration and osteoarthritis-like damage in the knee of skeletally mature, male mice of a single outbred strain (CD-1). The present study examined whether exposure to WBV induces similar deleterious effects in a genetically different strain of mouse (C57BL/6).

DESIGN

Male 10-week-old C57BL/6 mice were exposed to vertical sinusoidal WBV for 30 min/day, 5 days/week, for 4 or 8 weeks using previously reported protocols (45 Hz, 0.3 g peak acceleration). Following WBV, joint tissues were examined using histological analysis and gene expression was quantified using real-time PCR (qPCR).

RESULTS

Our analyses show a lack of WBV-induced degeneration in either the knee or IVDs of C57BL/6 mice exposed to WBV for 4 or 8 weeks, in direct contrast to the WBV-induced damage previously reported by our lab in CD-1 mice.

CONCLUSIONS

Together with previous studies from our group, the present study demonstrates that the effects of WBV on joint tissues vary in a strain-specific manner. These findings highlight the need to examine genetic or physiological differences that may underlie susceptibility to the deleterious effects of WBV on joint tissues.

Receptor for hyaluronan mediated motility (RHAMM/HMMR) is a novel target for promoting subcutaneous adipogenesis

Hyaluronan, CD44 and the Receptor for Hyaluronan-Mediated Motility (RHAMM, gene name HMMR) regulate stem cell differentiation including mesenchymal progenitor differentiation. Here, we show that CD44 expression is required for subcutaneous adipogenesis, whereas RHAMM expression suppresses this process. We designed RHAMM function blocking peptides to promote subcutaneous adipogenesis as a clinical and tissue engineering tool. Adipogenic RHAMM peptides were identified by screening for their ability to promote adipogenesis in culture assays using rat bone marrow mesenchymal stem cells, mouse pre-adipocyte cell lines and primary human subcutaneous pre-adipocytes. Oil red O uptake into fat droplets and adiponectin production were used as biomarkers of adipogenesis. Positive peptides were formulated in either collagen I or hyaluronan (Orthovisc) gels then assessed for their adipogenic potential in vivo following injection into dorsal rat skin and mammary fat pads. Fat content was quantified and characterized using micro CT imaging, morphometry, histology, RT-PCR and ELISA analyses of adipogenic gene expression. Injection of screened peptides increased dorsal back subcutaneous fat pad area (208.3 ± 10.4 mm²versus control 84.11 ± 4.2 mm²; p < 0.05) and mammary fat pad size (45 ± 11 mg above control background, p = 0.002) in female rats. This effect lasted >5 weeks as detected by micro CT imaging and perilipin 1 mRNA expression. RHAMM expression suppresses while blocking peptides promote expression of PPARγ, C/EBP and their target genes. Blocking RHAMM function by peptide injection or topical application is a novel and minimally invasive method for potentially promoting subcutaneous adipogenesis in lipodystrophic diseases and a complementary tool to subcutaneous fat augmentation techniques.

Iodine potassium iodide improves the contrast-to-noise ratio of micro-computed tomography images of the human middle ear

High-resolution imaging of middle-ear geometry is necessary for finite-element modeling. Although micro-computed tomography (microCT) is widely used because of its ability to image bony structures of the middle ear, it is difficult to visualize soft tissues - including the tympanic membrane and the suspensory ligaments/tendons - because of lack of contrast. The objective of this research is to quantitatively evaluate the efficacy of iodine potassium iodide (IKI) solution as a contrast agent. Six human temporal bones were used in this experiment, which were obtained in right-left pairs, from three cadaveric heads. All bones were fixed using formaldehyde. Three bones (one from each pair) were stained in IKI solution for 2 days, whereas the other three were not stained. Samples were scanned using a microCT system at a resolution of 20 μm. Eight soft tissues in the middle ear were segmented: anterior mallear ligament, incudomallear joint, lateral mallear ligament, posterior incudal ligament, stapedial annular ligament, stapedius muscle, tympanic membrane and tensor tympani muscle. Contrast-to-noise ratios (CNRs) of each soft tissue were calculated for each temporal bone. Combined CNRs of the soft tissues in unstained samples were 6.1 ± 3.0, whereas they were 8.1 ± 2.7 in stained samples. Results from Welch's t-test indicate significant difference between the two groups at a 95% confidence interval. Results for paired t-tests for each of the individual soft tissues also indicated significant improvement of contrast in all tissues after staining. Relatively large soft tissues in the middle ear such as the tympanic membrane and the tensor tympani muscle were impacted by staining more than smaller tissues such as the stapedial annular ligament. The increase in contrast with IKI solution confirms its potential application in automatic segmentation of the middle-ear soft tissues.

Mechanical Forces Exacerbate Periodontal Defects in Bsp-null Mice

Bone sialoprotein (BSP) is an acidic phosphoprotein with collagen-binding, cell attachment, and hydroxyapatite-nucleating properties. BSP expression in mineralized tissues is upregulated at onset of mineralization. Bsp-null (Bsp(-/-)) mice exhibit reductions in bone mineral density, bone turnover, osteoclast activation, and impaired bone healing. Furthermore, Bsp(-/-) mice have marked periodontal tissue breakdown, with a lack of acellular cementum leading to periodontal ligament detachment, extensive alveolar bone and tooth root resorption, and incisor malocclusion. We hypothesized that altered mechanical stress from mastication contributes to periodontal destruction observed in Bsp(-/-) mice. This hypothesis was tested by comparing Bsp(-/-) and wild-type mice fed with standard hard pellet diet or soft powder diet. Dentoalveolar tissues were analyzed using histology and micro-computed tomography. By 8 wk of age, Bsp(-/-) mice exhibited molar and incisor malocclusion regardless of diet. Bsp(-/-) mice with hard pellet diet exhibited high incidence (30%) of severe incisor malocclusion, 10% lower body weight, 3% reduced femur length, and 30% elevated serum alkaline phosphatase activity compared to wild type. Soft powder diet reduced severe incisor malocclusion incidence to 3% in Bsp(-/-) mice, supporting the hypothesis that occlusal loading contributed to the malocclusion phenotype. Furthermore, Bsp(-/-) mice in the soft powder diet group featured normal body weight, long bone length, and serum alkaline phosphatase activity, suggesting that tooth dysfunction and malnutrition contribute to growth and skeletal defects reported in Bsp(-/-) mice. Bsp(-/-) incisors also erupt at a slower rate, which likely leads to the observed thickened dentin and enhanced mineralization of dentin and enamel toward the apical end. We propose that the decrease in eruption rate is due to a lack of acellular cementum and associated defective periodontal attachment. These data demonstrate the importance of BSP in maintaining proper periodontal function and alveolar bone remodeling and point to dental dysfunction as causative factor of skeletal defects observed in Bsp(-/-) mice.

Deficiency in acellular cementum and periodontal attachment in bsp null mice

Bone sialoprotein (BSP) is an extracellular matrix protein found in mineralized tissues of the skeleton and dentition. BSP is multifunctional, affecting cell attachment and signaling through an RGD integrin-binding region, and acting as a positive regulator for mineral precipitation by nucleating hydroxyapatite crystals. BSP is present in cementum, the hard tissue covering the tooth root that anchors periodontal ligament (PDL) attachment. To test our hypothesis that BSP plays an important role in cementogenesis, we analyzed tooth development in a Bsp null ((-/-)) mouse model. Developmental analysis by histology, histochemistry, and SEM revealed a significant reduction in acellular cementum formation on Bsp (-/-) mouse molar and incisor roots, and the cementum deposited appeared hypomineralized. Structural defects in cementum-PDL interfaces in Bsp (-/-) mice caused PDL detachment, likely contributing to the high incidence of incisor malocclusion. Loss of BSP caused progressively disorganized PDL and significantly increased epithelial down-growth with aging. Bsp (-/-) mice displayed extensive root and alveolar bone resorption, mediated by increased RANKL and the presence of osteoclasts. Results collected here suggest that BSP plays a non-redundant role in acellular cementum formation, likely involved in initiating mineralization on the root surface. Through its importance to cementum integrity, BSP is essential for periodontal function.

Damage of an Oxinium femoral head and polyethylene liner following 'routine' total hip replacement

We present a case of early retrieval of an Oxinium femoral head and corresponding polyethylene liner where there was significant surface damage to the head and polyethylene. The implants were retrieved at the time of revision surgery to correct leg-length discrepancy just 48 hours after the primary hip replacement. Appropriate analysis of the retrieved femoral head demonstrated loss of the Oxinium layer with exposure of the underlying substrate and transfer of titanium from the acetabular shell at the time of a reduction of the index total hip replacement. In addition, the level of damage to the polyethylene was extensive despite only 48 hours in situ. The purpose of this report is to highlight the care that is required at the time of reduction, especially with these hard femoral counter-faces such as Oxinium. To our knowledge, the damage occurring at the time of reduction has not been previously reported following the retrieval of an otherwise well-functioning hip replacement.

Design and construction of a multipath vessel phantom for interventional training

This short communication reports on the design and construction of a catheter manipulation skill enhancement phantom for use by residents and fellows outside the clinical environment. The phantom contains a variety of path trajectories and vessel diameter transitions, allowing trainees to manipulate catheters through vessel paths of varying difficulty. The multipath phantom, which is easy to construct and provides easily visualised paths, provides a simple, cost-effective training platform to facilitate and accelerate interventional training.

Development of a customized density—modulus relationship for use in subject-specific finite element models of the ulna

Assigning an appropriate density—modulus relationship is an important factor when applying inhomogeneous material properties to finite element models of bone. The purpose of this study was to develop a customized density—modulus equation for the distal ulna, using beam theory combined with experimental results. Five custom equations of the form E = aρb were used to apply material properties to models of eight ulnae. All equations passed through a point (1.85, Ec), where ρ = 1.85 g/cm3 represents the average density of cortical bone. For custom equations (1) to (3), Ec was predicted using beam theory, and the value of b was varied within the range reported in the literature. Custom equations (4) and (5) used other values of Ec from the literature, while keeping b constant. Results obtained from the custom equations were compared with those from other equations in the literature, and with experimental results. The beam theory analysis predicted Ec = 21 ± 1.6 GPa, and the three custom equations using this value tended to have the lowest errors. The power of the equations did not affect the results as much as the value used for Ec. Overall, a customized density—modulus relationship for the ulna was generated, which provided improved results over using previously reported density—modulus equations.

Use of co-registered high-resolution computed tomography scans before and after screw insertion as a novel technique for bone mineral density determination along screw trajectory

INTRODUCTION

Bone mineral density (BMD) is an important factor in the examination of the performance of bone instrumentation both in and ex vivo, and until now, there has not existed a reliable technique for determining BMD at the precise location of such hardware. This paper describes such a technique, using cadaveric human sacra as a model.

METHODS

Nine fresh-frozen sacra had solid and hollow titanium screws placed into the S1 pedicles from a posterior approach. High-resolution micro-computed tomography (CT) was performed on each specimen before and after screw placement. All images were reconstructed with an isotropic spatial resolution of 308 mum, reoriented, and the pre-screw and post-screw scans were registered and transformed using a six-degree rigid-body transformation matrix. Once registered, two points, corresponding to the center of the screw at the cortex and at the screw tip, were determined in each scan. These points were used to generate cylindrical regions of interest (ROI) with the same trajectory and dimensions as the screw. BMD measurements were obtained within each of the ROI in the pre-screw scan. To examine the effect of artefact on BMD measurements around the titanium screws, annular ROI of 1 mm thickness were created expanding from the surface of the screws, and BMD was measured within each in both the pre- and post-screw scans.

RESULTS

The registration process was accurate to 190 mum, with a precision of 189 mum and error in BMD measurement of +/-2% in repeated scans. BMD values in the cylindrical ROI corresponding to screw trajectories were not statistically different from side to side of each specimen (p=0.23). Metal artefact created significant differences in BMD values (p=0.001) and followed an exponential decay curve as distance from the screws increased, approaching a low value of approximately 20 mg HA cm(-3), but not disappearing completely.

SUMMARY

CT in the presence of metal creates artefact, making measured BMD values near implants unreliable. This technique is accurate for determination of BMD, non-destructive, and eliminates the problem of this metal artefact through the use of co-registered scans. This technique has applications both in vitro and in vivo.

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

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

In vivo small-animal imaging using micro-CT and digital subtraction angiography

Small-animal imaging has a critical role in phenotyping, drug discovery and in providing a basic understanding of mechanisms of disease. Translating imaging methods from humans to small animals is not an easy task. The purpose of this work is to review in vivo x-ray based small-animal imaging, with a focus on in vivo micro-computed tomography (micro-CT) and digital subtraction angiography (DSA). We present the principles, technologies, image quality parameters and types of applications. We show that both methods can be used not only to provide morphological, but also functional information, such as cardiac function estimation or perfusion. Compared to other modalities, x-ray based imaging is usually regarded as being able to provide higher throughput at lower cost and adequate resolution. The limitations are usually associated with the relatively poor contrast mechanisms and potential radiation damage due to ionizing radiation, although the use of contrast agents and careful design of studies can address these limitations. We hope that the information will effectively address how x-ray based imaging can be exploited for successful in vivo preclinical imaging.

Study of subchondral bone adaptations in a rodent surgical model of OA using in vivo micro-computed tomography

OBJECTIVE

To non-invasively investigate the changes to epiphyseal bone occurring in a longitudinal pre-clinical model of osteoarthritis (OA) using in vivo micro-computed tomography (micro-CT).

DESIGN

In vivo micro-CT images were acquired using a bench-top micro-CT scanner, which produces three-dimensional data with isotropic voxel spacing of 0.046 mm. Male rodents were scanned prior to surgical destabilization, consisting of anterior cruciate ligament transection and partial medial menisectomy (ACLX). Subsequent scans were performed every 4 weeks post-ACLX, for up to 5 months. Volumetric bone mineral density (vBMD) was measured in specific, anatomically segmented regions within each image. The ACLX rodent data were compared with the contralateral non-operated hind limb of the same animal, as well as a sham-operated group (SHAM) of animals, for each time point. End-point histology compared changes to cartilage and bone between the ACLX and control animals.

RESULTS

The micro-CT protocol produced sufficient spatial resolution and signal-to-noise ratio (SNR=19) to quantify subchondral bone pathology, with an acceptable entrance exposure to radiation (0.36 Gy). Significantly lower vBMD was measured in the ACLX group, vs SHAM rodents, at 1, 4, and 5 months post-surgery (P<0.05). Qualitative observations of ACLX joints revealed significant loss of cartilage, subchondral bone cysts, and calcification of tendon similar to changes found in humans.

CONCLUSIONS

This study demonstrates in vivo micro-CT as an effective method for investigating the development of rodent knee OA longitudinally. This method can be applied, in future pre-clinical trials, to non-destructively monitor the efficacy of pharmacological interventions.

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

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

4 T MRI of chondrocalcinosis in combination with three-dimensional CT, radiography, and arthroscopy: a report of three cases

OBJECTIVE

To describe 4 T MRI techniques in imaging chondrocalcinosis within the knee and examine the results together with those demonstrated using three-dimensional (3D) computed tomography, conventional radiography, and arthroscopy.

DESIGN AND PATIENTS

From a larger clinical imaging study of early osteoarthritis, knee arthroscopy patients were imaged using high-field MRI and high-resolution 3D CT prior to their surgery. Retrospective review of the imaging data diagnosed three patients with chondrocalcinosis. Fat-suppressed 3D spoiled gradient (3D SPGR) and two-dimensional fat-suppressed fast spin echo (FSE) imaging was performed at 4 T. The MR images, multi-planar reformatted CT (MPR-CT) and maximum intensity projection CT (MIP-CT) images, and radiographs were examined by a musculoskeletal radiologist for the presence and location of chondrocalcinosis. The findings from arthroscopy were also included.

RESULTS

MRI showed 16 sites of punctate hypointense regions from 18 articular surfaces and five of six menisci with similar signal characteristics. Both meniscal chondrocalcinosis and meniscal tears were clearly visible using the 3D SPGR sequence. Only three sites were demonstrated to have calcification using MPR-CT and MIP-CT revealed an additional three. In articular cartilage surfaces showing surface disruption, arthroscopy demonstrated 11 sites with crystal deposition. Arthroscopy also revealed five menisci with calcification present.

CONCLUSION

Our preliminary findings suggest that imaging chondrocalcinosis using spoiled gradient 4 T MRI is superior and complementary to the other imaging modalities in the detection of crystal deposition in both articular cartilage and menisci.

Fundamental image quality limits for microcomputed tomography in small animals

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

In vitro investigation of calcium distribution and tissue thickness in the human thoracic aorta

Atherosclerosis represents a significant cause of morbidity and mortality in the western world. Manifestations of atherosclerotic disease among the elderly include the development of raised lesions that often include calcified regions with material properties similar to bone. There is little information available about the amount and distribution of these calcified plaques within the human aorta, partly due to the difficulty in obtaining this information during in vivo studies. We report the results of a cadaveric investigation of thoracic aortic wall thickness, diameter and calcium content. A non-destructive x-ray imaging technique was used to obtain two-dimensional maps of total thickness and mineral content in excised thoracic aortic specimens. In a study of 39 individuals (23 male and 16 female, aged 20-92 years) we report a significant non-linear correlation between calcium burden and age, with calcium deposition most commonly occurring proximal to the ostia of major branching vessels. We also found a significant linear correlation between age and both total aortic wall thickness and aortic diameter. An improved understanding of the pathological changes in the ageing thoracic aorta may be useful in the development of strategies to reduce the undesirable vessel calcification associated with atherosclerosis.

Rapid small-animal dual-energy X-ray absorptiometry using digital radiography

Although dual-energy X-ray absorptiometry (DEXA) is an established technique for clinical assessment of areal bone mineral density (BMD), the spatial resolution, signal-to-noise ratio, scan time, and availability of clinical DEXA systems may be limiting factors for small-animal investigations using a large number of specimens. To avoid these limitations, we have implemented a clinical digital radiography system to perform rapid area DEXA analysis on in vitro rat bone specimens. A crossed step-wedge (comprised of epoxy-based materials that mimic the radiographic properties of tissue and bone) was used to calibrate the system. Digital radiographs of bone specimens (pelvis, spine, femur, and tibia from sham-ovariectomized [SHAM] and ovariectomized [OVX] rats) were obtained at 40 kilovolt peak (kVp) and 125 kVp, and the resulting areal BMD values were compared with those obtained with a clinical fan-beam DEXA system (Hologics QDR 4500). Our investigation indicates that the cross-wedge calibrated (CWC) DEXA technique provides high-precision measurements of bone mineral content (BMC; CV = 0.6%) and BMD (CV = 0.8%) within a short acquisition time (<30 s). Areal BMD measurements reported by the CWC-DEXA system are within 8.5% of those reported by a clinical fan-beam scanner, and BMC values are within 5% of the known value of test specimens. In an in vivo application, the CWC-DEXA system is capable of reporting significant differences between study groups (SHAM and OVX) that are not reported by a clinical fan-beam DEXA system, because of the reduced variance and improved object segmentation provided by the CWC-DEXA system.

The radiographic quantitation of aortic valve calcification: implications for assessing bioprosthetic valve calcification in vitro

Calcification of natural aortic and bioprosthetic heart valves is a poorly understood phenomenon that results in valvular obstruction and tissue failure. We describe a non-destructive quantitative computed microtomographic (QCT) technique for determining both calcium content and local calcium distribution within explanted valves. As a reference standard, a dual-energy x-ray absorptiometry (DEXA) system with an accuracy demonstrated to be within 1% of the true calcium mass of test material was used to obtain the total calcium content of 24 human aortic valve cusps recovered at autopsy from patients aged 51-80 years. These cusps were then scanned using our unique volume QCT scanner, with multiple x-ray projections acquired by rotating the explanted tissue through a single axis of rotation. A three-dimensional cross-sectional map was reconstructed for each cusp. Voxel size was 0.003 mm3 and a calibration phantom was used to calculate calcium content. The minimum detection limit for calcium mass was 1 mg within the whole cusp. The DEXA and QCT scans were compared with respect to total calcium content, which ranged from 0 to 15 mg. An excellent correlation between the two independent techniques was demonstrated with an r2 value of 0.94 (p < 0.001). Non-destructive microtomographic CT scanning provided excellent volumetric density measurements, with quantitative 3D images permitting an assessment of any individual area of the cusp for calcium content and spatial distribution. This new approach to valve tissue analysis allows for subsequent histologic assessment.

Bone mineral measurement of phalanges: comparison of radiographic absorptiometry and area dual X-ray absorptiometry

With a standard, image-intensifier-based, digital radiographic system, high-spatial-resolution images of the hand were acquired for analysis of phalangeal bone mineral density with dual x-ray absorptiometry (DXA). Results with phalangeal DXA had precision of plus or minus 0.67% and accuracy of 4.1% and correlated well with those with radiographic absorptiometry. This phalangeal DXA technique is potentially useful for clinical diagnosis of osteoporosis.

Flow patterns at the stenosed carotid bifurcation: effect of concentric versus eccentric stenosis

Carotid stenosis severity is a commonly used indicator for assessing risk of stroke. However, the majority of individuals with severe carotid artery disease never suffer a stroke, and strokes can occur even with only mild or moderate stenosis. This suggests local factors (other than stenosis severity) at or near the carotid artery bifurcation may be important in determining stroke risk. In this paper we investigate the effect of stenosis geometry on flow patterns in the stenosed carotid bifurcation, using concentrically and eccentrically stenosed anthropomorphic carotid bifurcation models having identical stenosis severity. Computational simulations and experimental flow visualizations both demonstrate marked differences in flow patterns of concentric and eccentric stenosis models for moderately and severely stenosed cases, respectively. In particular, we identify post-stenotic recirculation zone size and location, and spatial extent of elevated wall shear stress as key factors differing between the two geometries. As these are also rotid plaque more vulnerable to cerebral embolus prokey biophysical factors promoting thrombogenesis, we propose that the stenosed carotid bifurcation geometry--or the induced flow patterns themselves--may provide more specific indicators for those plaques that are vulnerable to enhanced thromboembolic potential, and hence, increased risk of ischemic stroke.

The stimulation of vertebral and tibial bone growth by the parathyroid hormone fragments, hPTH-(1-31)NH2, [Leu27]cyclo(Glu22-Lys26)hPTH-(1-31)NH2, and hPTH-(1-30)NH2

The native human parathyroid hormone, hPTH-(1-84), and certain carboxyl truncated analogs such as hPTH-(1-34) and even smaller fragments such as hPTH-(1-31)NH2, [Leu27]cyclo(Glu22-Lys26)hPTH-(1-31)NH2, and hPTH-(1-30)NH2 stimulate femoral trabecular and cortical bone growth in ovariectomized (OVX) rats. Here we show that when injected once daily for 6 weeks starting 2 weeks after OVX in doses of 1 or 2 nmol/100 g of body weight, hPTH-(1-31)NH2, [Leu27] cyclo(Glu22-Lys26)hPTH-(1-31)NH2, and hPTH-(1-34)NH2 prevented the loss of trabecular volume in the L5 vertebrae induced by OVX. In fact, by the end of the sixth week of injections (i.e., the eighth week after OVX) the fragments had increased the volume and trabecular thickness significantly above the values in vehicle-injected sham-operated rats. hPTH-(1-30)NH2 can stimulate vertebral bone growth as much as the larger fragments, but 10-25 times more of it was needed to do so. The same daily doses of hPTH-(1-31)NH2, [Leu27]cyclo(Glu22-Lys26)hPTH-(1-31)NH2, and hPTH-(1-34)NH2 also raised the trabecular volume and thickness in the L5 vertebrae of rats well above the values in vehicle-treated animals when the injections were started 9 weeks after OVX. This restoration of trabecular bone in the L5 vertebrae in estrogen-deprived animals was accompanied by a significant increase in the bone mineral density (BMD) of the L1-L4 vertebrae and tibias. However, there was no significant drop in the pelvic BMD in the estrogen-deprived animals and the effects of hPTH-(1-31)NH2, [Leu27]cyclo(Glu22-(Lys) hPTH-(1-31)NH2, and hPTH-(1-34)NH2 on the pelvic BMD were equivocal.

Three-dimensional computed tomographic reconstruction using a C-arm mounted XRII: image-based correction of gantry motion nonidealities

The image quality of 3D reconstructions produced using a C-arm mounted XRII depends on precise determination of the geometric parameters that describe the detector system in the laboratory frame of reference. We have designed a simplified calibration system that depends on images of a metal sphere, acquired during rotation of the gantry through 200 degrees. Angle-dependent shift corrections are obtained, accounting for nonideal motion in two directions: perpendicular to the axis of rotation and tangential to the circular trajectory (tau), and parallel to the axis of rotation (xi). Projection images are corrected prior to reconstruction using a simple shift-interpolation algorithm. We show that the motion of the gantry is highly reproducible during acquisitions within one day (mean standard deviation in tau and xi is 0.11 mm and 0.08 mm, respectively), and over 21 months (mean standard deviation in tau and xi is 0.10 mm and 0.06 mm, respectively). Reconstruction of a small-bead phantom demonstrates uniformity of the correction algorithm over the full volume of the reconstruction [standard deviation of full-width-half-maximum of the beads is approximately 0.25 pixels (0.13 mm) over the volume of reconstruction]. Our approach provides a simple correction technique that can be applied when trajectory deviations are significant relative to the pixel size of the detector but small relative to the detector field of view, and when the fan angle of the acquisition geometry is small (<20 degrees). A comparison with other calibration techniques in the literature is provided.

Quantitative angiographic blood-flow measurement using pulsed intra-arterial injection

A technique for quantitative blood-flow measurement using a novel pulsed injection of radiographic contrast agent is reported. A pressurized source of contrast agent is interrupted by a rotary valve at rates ranging from 1 to 30 Hz, producing well-defined boli at the end of a catheter. The position of these boli can be recorded by a digital radiographic system and analyzed by one of several previously reported techniques, to produce quantitative measurements of blood velocity and flow rate throughout the cardiac cycle. The contrast-agent flow wave form produced by the pulsed injector has been measured with an electromagnetic flow meter, for driving pressures ranging from 600 to 1500 kPa. Excellent modulation of the contrast agent is observed for injection frequencies up to 20 Hz, through catheters up to 100 cm in length. Preliminary in vitro angiographic flow measurements have been performed using an x-ray image intensifier, coupled to a linear photodiode array as the digital detector. Both constant flow and pulsatile human blood-flow wave forms were simulated within a 6.4-mm-diam straight tube and monitored with an electromagnetic flow meter. These experiments indicate that the pulsed injector can be used to provide estimates of arterial blood flow over the entire cardiac cycle (including reverse flow), to within about +/-11%, following injection of less than 10 ml of iodinated contrast agent.

Anthropomorphic carotid bifurcation phantom for MRI applications

Anthropomorphic carotid bifurcation flow phantoms that incorporate different stenotic geometries within the internal carotid artery have been developed. This technique produces high-fidelity, life-size vascular flow models that are compatible with magnetic resonance techniques. The models, in conjunction with a computer-controlled flow pump, address the need for a complex vascular geometry that can be used to verify magnetic resonance angiography (MRA) techniques that quantify stenosis severity and blood flow. Stenotic geometries, with up to 80% diameter reduction, have been fabricated in two different phantom materials. Plastic phantoms provide a durable, rigid geometry where the absolute dimensions of the model are well known. Agar gel phantoms provide tissue-like signal (T1, T2) up to the lumen boundary and are also compatible with ultrasound techniques. In this paper the technique to produce vascular flow phantoms is outlined and the compatibility of these phantoms with MRA techniques is demonstrated. J. Magn. Reson. Imaging 1999;10:533-544.

Characterization of common carotid artery blood-flow waveforms in normal human subjects

Knowledge of human blood-flow waveforms is required for in vitro investigations and numerical modelling. Parameters of interest include: velocity and flow waveform shapes, inter- and intra-subject variability and frequency content. We characterized the blood-velocity waveforms in the left and right common carotid arteries (CCAs) of 17 normal volunteers (24 to 34 years), analysing 3560 cardiac cycles in total. Instantaneous peak-velocity (Vpeak) measurements were obtained using pulsed-Doppler ultrasound with simultaneous collection of ECG data. An archetypal Vpeak waveform was created using velocity and timing parameters at waveform feature points. We report the following timing (post-R-wave) and peak-velocity parameters: cardiac interbeat interval (T(RR)) = 0.917 s (intra-subject standard deviation = +/- 0.045 s); cycle-averaged peak-velocity (V(CYC)) = 38.8 cm s(-1) (+/-1.5 cm s(-1)); maximum systolic Vpeak = 108.2 cm s(-1) (+/-3.8 cm s(-1)) at 0.152 s (+/-0.008 s); dicrotic notch Vpeak = 19.4 cm s(-1) (+/-2.9 cm s(-1)) at 0.398 s (+/-0.007 s). Frequency components below 12 Hz constituted 95% of the amplitude spectrum. Flow waveforms were computed from Vpeak by analytical solution of Womersley flow conditions (derived mean flow = 6.0 ml s(-1)). We propose that realistic, pseudo-random flow waveform sequences can be generated for experimental studies by varying, from cycle to cycle, only T(RR) and V(CYC) of a single archetypal waveform.

A three-dimensional cerebrovascular flow phantom

We have constructed a life-sized fully three-dimensional (3D) rigid flow-through model of the cerebral vasculature. Average vessel diameters and lengths, taken from published values in the literature, were used to describe the geometry of our phantom; numerically controlled machining techniques were used to fabricate the model. Inflow to the phantom is provided through two internal carotid arteries and two vertebral arteries. Outflow is provided through the anterior cerebral arteries, the middle cerebral arteries, and the posterior cerebral arteries. The phantom includes the circle of Willis, and aneurysms of variable size may be attached at different locations. We have tested the model for geometric accuracy using high-resolution MR and CT imaging protocols, and have found that measured and prescribed diameters agree to within better than 4%. Flow dynamics, including waveform shape and flow division between branches, also mimic that seen in vivo, with flows within 16% (on average) of the prescribed values. We present 3D magnetic resonance angiography, digital subtraction angiography, and computed rotational angiography images of the phantom under conditions that mimic physiological situations.

Techniques to alleviate the effects of view aliasing artifacts in computed tomography

Due to practical limitations in data acquisition, 3-D computed tomography systems must attempt to provide rapid reconstructions of acceptable quality from a limited number of views. The use of convolution backprojection (CBP) for image reconstruction from an inadequate number of projections, results in view aliasing artifacts. In this paper we investigate different post-processing methods of alleviating the effects of view aliasing artifacts. Two distinct methods and their variants are considered. The first, termed the intermediate view reprojection (IVR) method, involves estimating a set of intermediate views by reprojection, followed by a reconstruction using the augmented set of views. The second, termed the error-correction (EC) method, incorporates a correction on the initial reconstruction based on the projection-domain error. Suitable modifications and variants of the above methods are indicated. Of the methods discussed, the IVR method is simple, tends to reduce the effects of artifacts with less susceptibility to secondary effects, and is applicable to region-of-interest reconstructions.

An investigation of the flow dependence of temperature gradients near large vessels during steady state and transient tissue heating

Temperature distributions measured during thermal therapy are a major prognostic factor of the efficacy and success of the procedure. Thermal models are used to predict the temperature elevation of tissues during heating. Theoretical work has shown that blood flow through large blood vessels plays an important role in determining temperature profiles of heated tissues. In this paper, an experimental investigation of the effects of large vessels on the temperature distribution of heated tissue is performed. The blood flow dependence of steady state and transient temperature profiles created by a cylindrical conductive heat source and an ultrasound transducer were examined using a fixed porcine kidney as a flow model. In the transient experiments, a 20 s pulse of hot water, 30 degrees C above ambient, heated the tissues. Temperatures were measured at selected locations in steps of 0.1 mm. It was observed that vessels could either heat or cool tissues depending on the orientation of the vascular geometry with respect to the heat source and that these effects are a function of flow rate through the vessels. Temperature gradients of 6 degrees C mm(-1) close to large vessels were routinely measured. Furthermore, it was observed that the temperature gradients caused by large vessels depended on whether the heating source was highly localized (i.e. a hot needle) or more distributed (i.e. external ultrasound). The gradients measured near large vessels during localized heating were between two and three times greater than the gradients measured during ultrasound heating at the same location, for comparable flows. Moreover, these gradients were more sensitive to flow variations for the localized needle heating. X-ray computed tomography data of the kidney vasculature were in good spatial agreement with the locations of all of the temperature variations measured. The three dimensional vessel path observed could account for the complex features of the temperature profiles. The flow dependences of the transient temperature profiles near large vessels during the pulsed experiments were consistent with the temperature distributions measured in the steady state experiments and provided unique insights into the process of convective heat transfer in tissues. Finally, it was shown that even for very short treatment times (3-20 s), large vessels had significant effects on the tissue temperature distributions.

An analysis of the geometry of saccular intracranial aneurysms

BACKGROUND AND PURPOSE

Our goal was to characterize the geometry of simple-lobed cerebral aneurysms and to find the absolute size of these lesions from angiographic tracings.

METHODS

Measurements of angiographic neck width (N), dome height (H), dome diameter (D), and semi-axis height (S) were obtained from tracings of 87 simple-lobed lesions located at the basilar bifurcation (BB), middle cerebral (MCA), anterior communicating (AcomA), posterior communicating (PcomA), superior cerebellar (SCA), and posterior cerebral (PCA) arteries. The following ratios were analyzed as subgroups according to location and as a collective sample: dome diameter/dome height (D/H), dome height/neck width (H/N), dome diameter/neck width (D/N), and dome height/semi-axis height (H/S). Using the parent artery as a reference, aneurysm dimensions were normalized to absolute in vivo size. Estimations were validated using angiographic markers.

RESULTS

For the entire sample, mean ratios were D/H = 1.11, D/N = 1.91, and H/N = 1.86. For the H/S ratio, the value was 1.98 for BB, MCA, and PcomA lesions and significantly smaller for the AcomA subgroup, at 1.52. The average sizes (in mm) for these dimensions were N = 3.4 for MCA, 3.0 for AcomA, 3.1 for PcomA, and 6.5 for BB; D = 6.1 for MCA, 5.9 for AcomA, 5.3 for PcomA, and 11.7 for BB; H = 5.6 for MCA, 5.0 for AcomA, 5.3 for PcomA, and 11.3 for BB. On average, BB aneurysms were twice as large as aneurysms at other locations. Good correlations were found between the scaled values for D and N, H and N, and H and D.

CONCLUSION

These results have been used to characterize the typical simple-lobed aneurysm geometry and to provide a framework for the development of a method of assessment of treatment choice and outcome on the basis of lesion geometry.

Accuracy of computational hemodynamics in complex arterial geometries reconstructed from magnetic resonance imaging

PURPOSE

Combining computational blood flow modeling with three-dimensional medical imaging provides a new approach for studying links between hemodynamic factors and arterial disease. Although this provides patient-specific hemodynamic information, it is subject to several potential errors. This study quantifies some of these errors and identifies optimal reconstruction methodologies.

METHODS

A carotid artery bifurcation phantom of known geometry was imaged using a commercial magnetic resonance (MR) imager. Three-dimensional models were reconstructed from the images using several reconstruction techniques, and steady and unsteady blood flow simulations were performed. The carotid bifurcation from a healthy, human volunteer was then imaged in vivo, and geometric models were reconstructed.

RESULTS

Reconstructed models of the phantom showed good agreement with the gold standard geometry, with a mean error of approximately 15% between the computed wall shear stress fields. Reconstructed models of the in vivo carotid bifurcation were unacceptably noisy, unless lumenal profile smoothing and approximating surface splines were used.

CONCLUSIONS

All reconstruction methods gave acceptable results for the phantom model, but in vivo models appear to require smoothing. If proper attention is paid to smoothing and geometric fidelity issues, models reconstructed from MR images appear to be suitable for use in computational studies of in vivo hemodynamics.

Application of dynamic computed tomography for measurements of local aortic elastic modulus

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2 mm-1 (for a 40 mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately +/- 0.25 mm and allowed determination of the dynamic elastic modulus E(dyn) with a precision of +/- 8 kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E(dyn) of 373 +/- 29 kPa. This value was not statistically different (p < 0.05) from the values of 430 +/- 77 and 390 +/- 47 kPa obtained by uniaxial tensile testing and volumetric measurements respectively.

Elastic response of human iliac arteries in-vitro to balloon angioplasty using high-resolution CT

Previous angioplasty studies have used angiography and intravascular ultrasound to obtain vascular dimensions. These imaging methods do not always provide reliable measurements due to limitations in image orientation and resolution. In this study, high-resolution (0.1 x 0.1 x 0.5 mm) transverse CT slices were obtained from human common-iliac arteries in vitro to study their elastic response pre- and post-angioplasty. Seven iliacs from five patients were imaged over the physiological pressure range both pre- and post-angioplasty. Contrast was obtained with humidified air surrounding the artery. Angioplasty was done with 10 or 12 mm diameter Medi-Tech balloon catheters with a balloon pressure of 300 kPa held for 30 s. Lumen circumference, c, measured from the images, was plotted against pressure, P, and curve fitting showed c = A(1 - e(-KP)) + B where A, K, and B are fitting parameters. Six lesions appeared soft and were compressed, while one was calcified and partially lifted off the wall. Normalized changes in parameters B and K were much higher post-angioplasty in the calcified lesion, and were over 3 standard deviations from the means of the normalized changes in the six compressed lesions. Balloon/stenosed lumen diameter ratios greater than 1.2 produced a lumen area increase of 38.6 +/- 4.1%(S.D.)(n = 3); ratios less than 1.2 produced an increase of 4.4 +/- 5.1%(S.D.)(n = 4). There was no correlation between area increase and balloon/normal lumen diameter ratio (the value used clinically). Arteries with lesions containing stiffer plaques that tear from the artery wall during angioplasty appear more distensible over the physiological pressure range post-angioplasty.

The effect of storage time and repeated measurements on the elastic properties of isolated porcine aortas using high resolution x-ray CT

A Laboratory CT scanner with a resolution of (0.1 mm)3 was used to determine if storage up to 7 days in saline at 4 degrees C and (or) repeated measurements would alter the compliance, C, and incremental elastic modulus, Einc, of isolated porcine aortas. All specimens were obtained fresh, made pressure-tight, and then mounted in the scanner, with humidified air used to produce adequate x-ray contrast. The specimens were imaged at pressures of 4, 8, 12, 16, 20, and 24 kPa, and vessel measurements were then obtained with a computerized technique and analyzed. Seven thoracic aortas were studied on days 0, 3, 5, and 7, with a significant change (p < 0.05) in compliance first occurring after three imaging studies (i.e., day 5). Compliance of the fresh thoracic aortas (mean +/- SD) was 0.90 +/- 0.28 mm/kPa at 14.4 kPa and 0.85 +/- 0.31 mm/kPa at 22.5 kPa. Six thoracic aortas were studied only on days 0 and 6 with no intermediate measurements. They showed no change in either compliance (0.88 +/- 0.07 mm/kPa at 14.4 kPa and 0.64 +/- 0.09 mm/kPa at 22.5 kPa) or Einc (0.46 +/- 0.05 MPa at 14.4 kPa and 0.88 +/- 0.15 MPa at 22.5 kPa) from day 0 to day 6. Thus, number of measurements rather than time appears to be the important factor. Six abdominal aortas were studied similarly but on days 0, 3, and 6. No significant change occurred in compliance (0.15 +/- 0.06 mm/kPa at 14.4 kPa and 0.032 +/- 0.026 mm/kPa at 22.5 kPa) but Einc showed a change, possibly due to their viscoelastic properties. We conclude that this nondestructive CT measurement method is suitable for repeated studies on porcine thoracic aortas, but not abdominal aortas, if the measurement involves two consecutive imaging sessions separated by no more than 6 days.

Use of a C-arm system to generate true three-dimensional computed rotational angiograms: preliminary in vitro and in vivo results

PURPOSE

To evaluate the potential use of a C-arm mounted X-ray image intensifier (XRII) system to generate three-dimensional computed rotational angiograms during interventional neuroradiologic procedures.

METHODS

A clinical angiographic system was modified to allow collection of sufficient views during selective intraarterial contrast injections for CT reconstruction of a 15 x 15 x 15-cm3 volume. Image intensifier distortion and C-arm instabilities were corrected by using image-based techniques. The impact of the pulsatile nature of the vessels during image data acquisition and of the presence of bone on the 3-D reconstructions was investigated by generating 3-D reconstructions of an anesthetized 20-kg pig and of a human skull phantom.

RESULTS

A sequence of images sufficient for 3-D reconstruction was acquired in less than 5 seconds. Image intensifier distortion and C-arm instabilities were corrected to subpixel accuracy (0.035 mm and 0.07 mm, respectively). Both the intracranial vessels of the pig and the small, high-contrast structures in the skull were reconstructed with negligible artifacts.

CONCLUSIONS

Using a C-arm mounted XRII system, computed rotational angiography can provide true 3-D images of diagnostic quality.

Three-dimensional computed tomographic reconstruction using a C-arm mounted XRII: correction of image intensifier distortion

X-ray image intensifiers (XRIIs) have many applications in diagnostic imaging including acquisition of near-real-time projection images of the intracranial and coronary vasculature. Recently, there has been some interest in using this projection data to generate three-dimensional (3-D) computed tomographic (CT) reconstructions. The XRII and x-ray tube are rotated around the object, acquiring sufficient data for the simultaneous reconstruction of many transverse slices. Three-dimensional reconstructions are compromised, however, if the projection data is geometrically distorted in any way. Previous studies have shown the distortion in XRIIs to be substantial and to be highly angular dependent. In this paper, we present a global correction technique which provides a table of correction coefficients for an image acquired at any arbitrary angle about the patient. The coefficients are generated using a linear least-squares fit between the detected and known locations of a grid of small steel beads which is attached to the XRII (27 cm nominal diameter). We have performed corrections on 100 images obtained during rotation of the gantry through 200 degrees and find that a fifth-order polynomial provides optimum image distortion reduction (mean residual distortion of 0.07 pixels), however, fourth-order polynomials provide sufficient distortion reduction for our application (mean residual displacement of 0.1 pixels). Using sixth-order polynomials does not provide a statistically significant reduction in image distortion. The spatial distribution of residual distortion did not demonstrate any particular pattern over the face of the XRII. Image angle and coefficient angle must be known to within +/- 2 degrees in order to keep the mean residual distortion be approximately 0.5 pixels.

A real vessel phantom for imaging experimentation

Vascular phantoms are used to evaluate imaging techniques such as ultrasound (US), CT, and angiography. They are expected to mimic the vasculature, surrounding tissue, and blood, and therefore must meet specific requirements on the mimicking materials, with respect to x-ray attenuation and acoustic properties (velocity, attenuation). In the past, researchers have used a variety of vessel models, including walled (typically latex tube) and wall-less phantoms (obtained by moulding a lumen in a block of agar). These models lacked the exact geometry of human vessels as well as pathologic features such as plaques and calcifications. To overcome these disadvantages, this paper describes a real vessel phantom for US and x-ray studies. The phantom consists of an agar-filled acrylic box containing a formaldehyde fixed section of a real human vessel (obtained at autopsy) cannulated onto two acrylic tubes. This phantom was evaluated by comparing the images obtained with x-ray angiography, CT, and 3-D B-mode US. The images show good overall correlation based on the location of the geometrical features within the phantom, such as lumen, plaques, and calcifications. Discrepancies, artifacts, and difficulties were minor, and are discussed. The use of a real vessel, with its natural geometry and pathology, makes this phantom attractive for evaluation of imaging techniques including projection radiography, CT and US, and for extending its use to MR and US based flow studies.

X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: experimental verification

A novel in vitro radiographic technique using saline/iodine displacement, which can be used to study the bone-equivalent and soft-tissue-equivalent thicknesses within vessel walls, was applied to imaging of arterial specimens. Results concerning the accuracy and precision of the bone-equivalent and soft-tissue-equivalent thickness measurements obtained with this technique are reported and discussed. Planar radiographs of a phantom were obtained under two different conditions: (1) when it is immersed in an isotonic saline solution using a 45-kVp spectrum with no added filtration, and (2) when it is immersed in a concentrated iodine solution using a 100-kVp spectrum with 12.5-mm aluminum-added filtration. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves that are used to convert the radiographs into bone-equivalent and total-thickness images. A soft-tissue-thickness image is obtained from the subtraction of the bone-equivalent image from the total-thickness image. Thickness measurements obtained from these images yielded average accuracies of +/- 110 microns for both the bone-equivalent and the soft-tissue-equivalent images. The precision (one standard deviation) of the thickness measurements was +/- 60 and +/- 90 microns for the bone-equivalent and the soft-tissue-equivalent images, respectively. In conclusion, since calcified plaque can become as thick as 3-4 mm, the saline/iodine displacement technique has the potential to be a very useful technique for ex vivo studies of the progression of atherosclerosis because of its high accuracy and precision.

X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: technique optimization

An in vitro radiographic technique which uses saline/iodine displacement has been developed to study the thickness of bone-equivalent and soft-tissue-equivalent materials within atherosclerotic plaques in arterial specimens which have been cut open longitudinally and laid flat. Results concerning the optimization of the imaging parameters are presented and discussed. The technique consists of imaging arterial specimens under two different conditions: (1) when it is immersed in an isotonic saline solution, to estimate the calcium content, and (2) when it is immersed in a concentrated iodine solution, to estimate the total thickness of the specimen. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves which are used to convert the radiographs into bone-equivalent and soft-tissue-equivalent thickness images. The optimal spectral parameters were determined to be 45 and 100 kVp for the saline and the iodine images, respectively, with a significant amount of added filtration for both images. Inherent systematic inaccuracies due to (1) the nonidealities due to linear attenuation coefficient mismatch between tissue and calibration materials and (2) beam hardening due to heel effect are determined theoretically, and can be used to correct a set of bone-equivalent and the soft-tissue-equivalent images to within +/- 6 microns with an ideal, noise-free imaging system.

Geometric characterization of stenosed human carotid arteries

RATIONALE AND OBJECTIVES

The geometry of stenosed carotid bifurcations was analyzed to determine average representations for several stenosis grades.

METHODS

Film angiograms of 62 patients with internal carotid artery stenoses were digitized. Residual lumen boundaries were manually outlined. The outlines were processed with a computer to extract geometric measurements. The measurements were grouped according to stenosis grade and used to create average representations.

RESULTS

Accuracy and precision of the outlining technique were +/- 0.020 common carotid diameters (CCD) and +/- 0.025 CCD, respectively. Maximum narrowing of the internal carotid artery occurred at 0.3 CCD +/- 1.5 (mean +/- standard deviation) distal to the flow divider. The region of significant narrowing extended axially 1.2 CCD +/- 1.0. Poststenotic dilatations were observed, with enlargement of 1.3 +/- 0.7 times the normal diameter of the distal internal carotid artery. A tendency toward smaller bifurcation angles with increasing stenosis severity was observed.

CONCLUSION

Three-dimensional geometric models could be created for carotid bifurcations that were disease free (normal) and of arbitrary stenosis grade.

Further comments on the measurement of carotid stenosis from angiograms. North American Symptomatic Carotid Endarterectomy Trial (NASCET) Group

BACKGROUND AND PURPOSE

Three different methods for estimating the percentage of reduction in the diameter of the internal carotid artery (ie, stenosis) have been proposed in the literature. Further comparisons of the methods were carried out with the intent of recommending a current standard for determining the percentage of stenosis from angiograms.

METHODS

Angiograms from 112 patients were obtained. For each angiogram, stenosis was estimated in the manner of the European Carotid Surgery Trial (ECST method), the North American Symptomatic Carotid Endarterectomy Trial (NASCET method), and by a method using the common carotid artery lumen diameter (CC method).

RESULTS

Although there is much discrepancy among the estimates of stenosis arising from the three different methods for any particular patient, it is possible to predict (on average) the percentage of stenosis from one method to another. The relationship between the NASCET and CC methods is linear, with a mean ratio of distal internal carotid artery to common carotid diameter of 0.62 (SD of 0.11). The variability in the diameter of the common carotid artery lumen stabilizes only beyond 2.5 common carotid diameter units (approximately 20 to 30 mm by conventional angiography) proximal to the bifurcation. Unexpectedly, the relationships between both the ECST and NASCET methods and ECST and CC methods were parabolic (P < .001). The reasons underlying these departures from linearity are uncertain.

CONCLUSIONS

The comparability of our results with those reported in the literature regarding the CC and NASCET methods provides further evidence of the reproducibility of methods measuring anatomic features that can be visualized on an angiogram. Disease of the internal carotid artery is one of the important causes of ischemic symptoms. Measuring the narrowest portion of the internal artery relative to the normal portion of the same artery, well beyond the bulb, is a logical method. Moreover, benefits of carotid endarterectomy for patients with 70% to 99% stenosis as determined by the NASCET method have been well established in a clinical trial. Converting from the NASCET method to the CC method, given that the CC method is neither superior nor easier to calculate, is not recommended.

Effects of physiologic waveform variability in triggered MR imaging: theoretical analysis

One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k,t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.

Dual-energy x-ray imaging technique for in vitro tissue composition measurement

A dual-energy in vitro radiographic technique has been developed to study the thickness of tissue and bone within atherosclerotic plaques. Results concerning the accuracy and precision of the thickness measurements using this technique are presented and discussed. Planar radiographs of phantoms were obtained with a low-energy spectrum (45 kVp, no added filtration) and a high-energy spectrum (100 kVp, 2.88-mm copper-added filtration), and then decomposed into bone-equivalent and Lucite basis-material images. Thickness measurements from these images yielded average accuracies of +/- 750 microns for the Lucite images, and +/- 25 microns for the bone-equivalent images. The imprecision (one standard deviation) of the thickness measurements was +/- 192 and +/- 47 microns for the Lucite and the bone-equivalent images, respectively (for thin sections). Although the accuracy and precision of Lucite thickness measurements were not as good as those obtained with other techniques, such as the iodine displacement technique, the accuracy and precision of the bone thickness measurements are shown to be much better. The high accuracy and precision of the bone measurement makes dual energy a very appealing technique for analyzing the physical properties of calcified atherosclerotic plaques in excised arterial specimens.