CT Imaging of skeleton in small animals Massimo Marenzana
Introduction Osteoporosis is a disease in which bones become fragile and more likely to break. It can be defined as a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a concomitant increase in bone fragility and fracture risk. It occurs most frequently in postmenopausal women. To measure bone mass,, DEXA is the most common diagnostic tool. Microarchitectural deterioration of bone tissue is best measured by microct and pqct
X-ray absorptiometry How it works X-ray generation and detection electrons tungsten material X-rays are high frequency ray type: short wavelength λ ~ 1 nm highly penetrating and ionizing safety: ionizing radiation carcinogenic Unit: - Absorbed Dose [gray[ (Gy)] - Dose Equivalent [Sievert[ (Sv)] (=AD for X-rays) X - Effective Dose [Sievert[ (Sv)] X-rays DEXA absorptiometry (2D) ED = 0.1 3.6 Sv CT absorptiometry (3D) ED = 300 1000 Sv X-ray tube
X-ray generation and detection Interaction of X-rays with matter: decay low: I = I 0 exp(-μx) I = intensity of X-rays after passing through a thickness x μ = absorption constant for the material in mm^-1 (function of ρ and Z) CT scanners image the distribution of linear attenuation coefficients in a thin slice of tissue. Log A = ln (I 0 /I) Limits of fan beam image Computer X-ray source object ray bigger object Array of detectors (Linear/Matricial) Semiconductor detectors (CCD) plus scintillators (gadolinium oxysulfide or cesium iodide)
DEXA How it works 2D images are provided from an x-ray x beam that sends out 2 rays with different energies. As the rays pass through the patient, some are stopped by the bone mineral or by the other tissues in the body, such as fat, water, or protein. An x ray detector above the patient can tell how much energy got through, and this information is sent to a computer, which analyzes it and forms an image.
DEXA Became popular because most bone mineral density (BMD) of the hip, helps to predict whether a person will have a hip fracture. The risk of a fracture will double if the DEXA decreases by approximately 12%.
DEXA - Benefits Quantifies fat, muscle and bone mineral content Low radiation exposure Good predictor of bone strength (r = 0.92) (Thomsen, 2002) DEXA - Limitations It cannot differentiate between cancellous and cortical bone. It is insensitive to changes within cancellous bone such as thickening of trabeculae and connectivity Because of it is a 2D measure, the results should be corrected for body size and/or bone size.
The 2D problem Trabecular bone consists of a complicated 3D network of plates and rods, arranged in a lattice-like like network. Aging, osteoporosis, and therapeutic agents could influence the number of elements in this network, their dimensions and connectivity, thereby causing dramatic changes in bone strength. HOW DO WE MEASURE THIS?
CT Unlike, 2D histomorphometry and DEXA, CT-based measurements allows us to assess 3-D 3 D bone structure and geometry.
CT Unlike, 2D histomorphometry and DEXA, CT-based measurements allows us to assess 3-D 3 D bone structure and geometry. Limits of fan beam object plane X-ray source object ray thickness of fan beam => image thickness => voxel size TRADE OFF IN CT : object size resolution [voxel[ size]
Some CT catches Resolution Sample size trade off Beam Hardening Partial volume effect
CT generation of tomograms and 3D visualization Computer algorithms allow to generate a crosssectional image by mathematically integrating many side-view linear array X-ray absorption images taken at different angles (0-360 degrees) All CT scanner are supplied with computer stations to generate cross-sections, a process also called reconstruction. High resolution CT tomograms can be further elaborated by image processing software to obtain 3D visualization
pqct tomograms Proximal Metaphysis (5 mm from tibial plate) Diaphysis (1.5 mm from fibula inters.)
pqct High resolution CT for measuring cortical and quasi- histomorphometric trabecular bone parameters in humans. Reduced radiation exposure compared to conventional CT/QCT Limited to measuring small volumes and peripheral sites. Can be used for animal research to monitor bone changes in vivo
pqct image showing significantly reduced trabecular bone in patient with spinal cord injury Spinal Cord Injury Able-Bodied Shields R et al, 2006
MicroCT In 1994, Feldkamp pioneered the field of micro-computed computed tomography. Micro-CT allows us to assess 3-D 3 D trabecular bone structure and cortical bone strength. Scanning with micro-ct can be achieved at resolutions as low as 1 µm, allowing the determination of porosities and subtle remodeling events of cancellous and cortical bone. object source detector stepwise rotation
MicroCT 3D visualization X-ray transmission image and several reconstructed cross sections in different orientations Cross-section plane 3D visualization with virtual opening to show the internal microstructure. Longitudinal plane
MicroCT in Human metabolic bone diseases MicroCT can be use to assess bone microarchitecture in humans by extracting a bone biopsy from the iliac crest. Increased osteoclastic activity in postmenopausal osteoporosis and hypogonadism osteoporosis in males may lead to the complete removal of trabeculae. 3D measurements are highly correlated with 2D obtained by histomorphometry although microct provides slightly increased values for bone volume.
Human Bone Biopsies from Iliac Crest Young Adult Patient with glucocorticosteroid induced osteoporosis Patient with alcoholic osteoporosis Very thin trabeculae and plates Perforated trabeculae
Limitations in human studies Limited to bone biopsies from the iliac crest Can not measure trabecular bone at other sites, such as vertebrae and femoral neck or measure cortical bone strength Size of sample Reasonably good correlation between iliac crest trabecular bone volume from microct and vertebral bone strength (r= 0.6 to 0.7)
MicroCT-Animal Models Animal models are a useful for understanding the mechanisms of bone loss. In addition, they can be used to test the efficiency of new therapeutic strategies. The ovariectomized rat is considered by WHO and the FDA as the most suitable model of post menopausal osteoporosis. MicroCT changes in OVX models are very similar to humans with osteoporosis; they show a reduction of trabecular number and a conversion of trabecular plates into rods. In the same way, orchidectomy in the male rat can be use as a model for hypogonadic osteoporosis.
Rat Model for Postmenopausal Osteoporosis Rat Tibia after 16 weeks Ovariectomized Sham Yang et al 2003
Rat Model for Hypogonadic Osteoporosis Tibial metaphysis of a Control Wistar Rat.. Orchidectomised rat 16 weeks after surgery Note the reduction in bone mass and the marked conversion of trabeculae into pilars
Effect of propranolol on the Bone Response to sciatectomyinduced Unloading Contra-lateral Neurectomized Contra-lateral Neurectomized Metaphysis BV / TV 15-39%*** - 31%*** Midshaft Cortical width 0.20-12%*** - 11%*** BV / TV (%) 10 5 Contra-lateral Neurectomized Ct.Th (mm) 0.15 0 Vehicle Propranolol 0.10 Vehicle Propranolol N=8 N=8 N=8 N=8
Cortical Bone Strength Engineering Beam Theory MicroCT The further the material is distributed from a central axis, the stronger the section and the greater its ability to withstand breakage or collapse when subjected to loading. Bending Strength Ix refers denotes bone strength in the anterior-posterior plane Iy denotes the medial-lateral lateral bending strength Torsional Strength J and refer to the sum of two values Ix and Iy
microct quantitative parameters 3D and 2D Geometry (volume, size, area, CSMI) 3D Micro-Structure (trabecular thickness, shape, connectivity and distribution and cortical porosity) Volumetric bone mass density (vbmd( vbmd) upon appropriate calibration
microct advantages in small animal research True 3D micro-architectural parameters of the intact sample Increased precision over histological techniques Speed up experiments to results time Possibility of in vivo microct for longitudinal studies with last generation devices
Implant Research Titanium implant 3mm in diameter inside the bone
Next generation pqcts approach in vivo microct performances 3D rendering with front top part virtually removed (CTan+CTvol programs)
3D µangiography Vessels infused with contrast agent around a bone segment
3D µangiography cortical bone main metaphyseal vein
nanoct focal spot size of <400nm object details down to 150-200nm. object positioning / rotation with an accuracy of better than 100nm. x-ray detector: intensified CCD with single photon sensitivity. separated trabecula (0.2 x 2mm), 480nm pixel size
nanoct
Summary DXA measures bone mineral density of the skeleton and is a good predictor of bone strength. However, it does not measure trabecular or cortical bone structure. pqct provides 3D measures of cortical and trabecular bone at limited sites in humans and animals. microct/pqct allows detailed 3D measures of cortical and trabecular bone in animals but has limited use in humans. Combining 3D and 2D measures yields the best results in the diagnosis of osteoporosis.