Advances in dual-energy computed tomography imaging of radiological properties

Dual-energy computed tomography (DECT) has shown great potential in the reduction of uncertainties of proton ranges and low energy photon cross section estimation used in radiation therapy planning. The work presented herein investigated three contributions for advancing DECT applications. 1) A linear and separable two-parameter DECT, the basis vector model (BVM) was used to estimate proton stopping power. Compared to other nonlinear two-parameter models in the literature, the BVM model shows a comparable accuracy achieved for typical human tissues. This model outperforms other nonlinear models in estimations of linear attenuation coefficients. This is the first study to clearly illustrate the advantages of linear model not only in accurately mapping radiological quantities for radiation therapy, but also in providing a unique model for accurate linear forward projection modelling, which is needed by the statistical iterative reconstruction (SIR) and other advanced DECT reconstruction algorithms. 2) Accurate DECT requires knowledge of x-ray beam properties. Using the Birch-Marshall1 model and beam hardening correction coefficients encoded in a CT scanner’s sinogram header files, an efficient and accurate way to estimate the x-ray spectrum is proposed. The merits of the proposed technique lie in requiring no physical transmission measurement after a one-time calibration against an independently measured spectrum. This technique can also be used in monitoring the aging of x-ray CT tubes. 3) An iterative filtered back projection with anatomical constraint (iFBP-AC) algorithm was also implemented on a digital phantom to evaluate its ability in mitigating beam hardening effects and supporting accurate material decomposition for in vivo imaging of photon cross section and proton stopping power. Compared to iFBP without constraints, both algorithms demonstrate high efficiency of convergence. For an idealized digital phantom, similar accuracy was observed under a noiseless situation. With clinically achievable noise level added to the sinograms, iFBP-AC greatly outperforms iFBP in prediction of photon linear attenuation at low energy, i.e., 28 keV. The estimated mean errors of iFBP and iFBP-AC for cortical bone are 1% and 0.7%, respectively; the standard deviations are 0.6% and 5%, respectively. The achieved accuracy of iFBP-AC shows robustness versus contrast level. Similar mean errors are maintained for muscle tissue. The standard deviation achieved by iFBP-AC is 1.2%. In contrast, the standard deviation yielded by iFBP is about 20.2%. The algorithm of iFBP-AC shows potential application of quantitative measurement of DECT. The contributions in this thesis aim to improve the clinical performance of DECT

Hand X-ray absorptiometry for measurement of bone mineral density on a slot-scanning X-ray imaging system

Includes bibliographical references.Bone mineral density (BMD) is an indicator of bone strength. While femoral and spinal BMDs are traditionally used in the management of osteoporosis, BMD at peripheral sites such as the hand has been shown to be useful in evaluating fracture risk for axial sites. These peripheral locations have been suggested as alternatives to the traditional sites for BMD measurement. Dual-energy X-ray absorptiometry (DXA) is the gold standard for measuring BMD due to low radiation dose, high accuracy and proven ability to evaluate fracture risk. Computed digital absorptiometry (CDA) has also been shown to be very effective at measuring the bone mass in hand bones using an aluminium step wedge as a calibration reference. In this project, the aim was to develop algorithm s for accurate measurement of BMD in hand bones on a slot - scanning digital radiography system. The project assess e d the feasibility of measuring bone mineral mass in hand bones using CDA on the current system. Images for CDA - based measurement were acquired using the default settings on the system for a medium sized patient. A method for automatic processing of the hand images to detect the aluminium step wedge, included in the scan for calibration, was developed and the calibration accuracy of the step wedge was evaluated. The CDA method was used for computation of bone mass with units of equivalent aluminium thickness (mmA1). The precision of the method was determined by taking three measurements in each of 1 6 volunteering subjects and computing the root - mean - square coefficient of variation (CV) of the measurements. The utility of the method was assessed by taking measurements of excised bones and assessing the correlation between the measured bone mass and ash weight obtained by incinerating the bones. The project also assessed the feasibility of implementing a DXA technique using two detectors in a slot-scanning digital radiography system to acquire dual-energy X-ray images for measuring areal and volumetric BMD of the middle phalanx of the middle finger. The dual-energy images were captured in two consecutive scans. The first scan captured the low- energy image using the detector in its normal set-up. The second scan captured the high- energy image with the detector modified to include an additional scintillator to simulate the presence of a second detector that would capture the low-energy image in a two-detector system. Scan parameters for acquisition of the dual-energy images were chosen to optimise spectral separation, entrance dose and image quality. Simulations were carried out to evaluate the spectral separation of the low- and high-energy spectra

Quantifizierung struktureller Veränderungen im kortikalen Knochen durch Abschätzung von Dicke, Schallgeschwindigkeit und Porengrößenverteilung

Quantitative bone ultrasound (QUS) method has been introduced as a promising alternative for diagnosing osteoporosis and assessing fracture risk. The latest QUS technologies aim to quantitatively assess structural cortical bone characteristics, e.g, cortical porosity, cortical thickness (Ct.Th) and cortical speed of sound at cortical measurement regions. Large cortical pores and reduced Ct.Th in the tibia have been proposed as an indication of reduced hip strength and structural deterioration. In this work two novel ultrasound methods were studied using a conventional ultrasound transducer to measure cortical bone properties at the tibia. The first method is a refraction and phase aberration corrected multifocus (MF) imaging approach that measures Ct.Th and the compressional sound velocity traveling in the radial bone direction (Ct.ν11). The second method is a novel cortical backscatter (CortBS) method that assesses microstructural properties in cortical bone. Both methods were validated in silico on bone models, ex vivo on bone samples and in vivo on 55 postmenopausal women at the anteromedial tibia midshaft. The aim of this work was to study the precision, accuracy, and fragility fracture discrimination performance of CortBS and MF parameters in comparison to clinical High-resolution peripheral quantitative computed tomography (HR-pQCT) and Dual-energy X-ray absorptiometry (DXA) measurements. The results of the MF approach show precise and accurate estimation of Ct.Th and Ct.ν11. The comparison of the measured Ct.Th with reference thicknesses from HR-pQCT measurement have also shown accurate determination of Ct.Th (R2=0.94, RMSE=0.17 mm). Future simulation studies with real bone structures from HR-pQCT measurements should target the validation of accurate Ct.ν11 estimation. For the first time, CortBS assessed the distribution of cortical pore size and viscoelastic properties of cortical bone in vivo. The short- term in vivo precision was observed between 1.7% and 13.9%. Fragility fracture discrimination performance was retrieved using multivariate partial least squares regression. The combination of CortBS+MF showed superior fracture discrimination performance compared with DXA and similar fracture discrimination performance compared with HR-pQCT. Further clinical studies with larger cohort size should target the potential to demonstrate the ability of CortBS and MF parameters for individual fracture risk assessment. In conclusion, alteration in cortical microstructure and viscoelasticity caused by the aging process and the progression of osteoporosis can be measured by CortBS and MF. These methods have high potential to identify patients at high risk for fragility fractures.Die quantitative Knochenultraschallmethode (QUS) wurde als vielversprechende Alternative für die Diagnose von Osteoporose und die Bewertung des Frakturrisikos eingeführt. Die neuesten QUS-Technologien zielen darauf ab, strukturelle kortikale Knochenmerkmale, z. B. kortikale Porosität, kortikale Dicke (Ct.Th) und kortikale Schallgeschwindigkeit in kortikalen Messregionen quantitativ zu bewerten. Große kortikale Poren und eine verringerte Ct.Th in der Tibia wurden als Anzeichen für eine verringerte Festigkeit der Hüfte und eine strukturelle Verschlechterung vorgeschlagen. In dieser Arbeit wurden zwei neuartige Ultraschallmethoden unter Verwendung eines herkömmlichen Ultraschallwandlers zur Messung der Eigenschaften am kortikalen Knochen des Schienbeins untersucht. Bei der ersten Methode handelt es sich um einen brechungs- und phasenaberrationskorrigierten multifokalen (MF) Bildgebungsansatz, der Ct.Th und die Kompressionsschallgeschwindigkeit in radialer Knochenrichtung (Ct.ν11) misst. Die zweite Methode ist eine neuartige kortikale Rückstreumethode (CortBS), die die mikrostrukturellen Eigenschaften des kortikalen Knochens misst. Beide Methoden wurden in silico an Knochenmodellen, ex vivo an Knochenproben und in vivo an 55 postmenopausalen Frauen am anteromedialen Tibia-Mittelschaft validiert. Ziel dieser Arbeit war es, die Präzision, Genauigkeit und Fragilitätsfraktur-Diskriminierungsleistung von CortBS- und MF-Parametern im Vergleich zur klinischen hochauflösenden peripheren quantitativen Computertomographie (HR-pQCT) und Dualen-Energie-Röntgenabsorptiometrie (DXA) zu untersuchen. Die Ergebnisse des MF-Ansatzes zeigen eine präzise und genaue Schätzung von Ct.Th und Ct.ν11. Der Vergleich der gemessenen Ct.Th mit Referenzdicken aus HR-pQCT-Messungen hat ebenfalls eine genaue Bestimmung der Ct.Th gezeigt (R2=0,94, RMSE=0,17 mm). Zukünftige Simulationsstudien mit realen Knochenstrukturen aus HR-pQCT-Messungen sollten die genauen Schätzung der Ct.ν11 validieren. Zum ersten Mal hat CortBS die kortikale Porengrößenverteilung und die viskoelastischen Eigenschaften des kortikalen Knochens in vivo untersucht. Die kurzfristige In-vivo-Präzision lag zwischen 1,7% und 13,9%. Die Fragilitätsfraktur-Diskriminierungsleistung wurde mittels multivarianter Regression der partiellen kleinsten Quadrate bewertet. Die Kombination von CortBS+MF zeigte im Vergleich zur DXA eine überlegene Leistung bei der Frakturerkennung und eine ähnliche Leistung wie die bei HR-pQCT. Weitere klinische Studien mit größerer Kohortengröße sollten die Fähigkeit von CortBS- und MF-Parametern zur individuellen Frakturrisikobewertung nachweisen. Zusammenfassend lässt sich sagen, dass Veränderungen der kortikalen Mikrostruktur und Viskoelastizität, die durch den Alterungsprozess und das Fortschreiten der Osteoporose verursacht werden, mit CortBS und MF gemessen werden können. Diese Methoden haben ein hohes Potenzial zur Identifizierung von Patienten mit hohem Risiko für Fragilitätsfrakturen

Bone Densitometry using CT Imaging

Master'sMASTER OF SCIENC

Cancellous Bone Density Evaluation using Ultrasound Backscatter from an Imaging System: Exploring the Possibility for Fetal Bone Density Evaluation

Osteoporosis is a common chronic disease and a well known major source of morbility and mortality among the elderly. Low bone density also occurs in infants and small children during development and can be problematically excessive if the fetus experiences issues during pregnancy such as malnutrition, lack of vitamin D and smoking. Currently the only available methodologies for fetal bone density evaluation are Dual-energy X-ray Absorptiometry (DEXA) or Magnetic Resonance Imaging (MRI). Both are sensitive to movement artifacts. DEXA exposes the subjects to significant radiation so is not suggested during pregnancy. Quantitative MRI is noisy, expensive, slow (8-20 mins) and the effects of high field strengths on the developing fetus is unknown. Therefore, the goal of this study is to find a fast, accurate and non-ionizing method for the evaluation of fetal bone density. In this study, the quantitative ultrasound backscatter coefficient (BSC) was chosen to evaluate bone density using the B-mode ultrasound system. Compared with the speed of sound and ultrasound attenuation in the traditional ultrasound measurement for bone density, the backscatter method is more accessible to central sites such as the human spine and fetal femur bone. Additionally, it has a rapid path to commercialization with the potential to be added as a new feature in the current commercial ultrasound imaging systems for bone density evaluation. The contributions of this work are: 1. A simulation study was accomplished that compared backscatter coefficients from a single element transducer, a linear array transducer, and a curved array transducer with the change of trabecular thickness and trabecular spacing. An overall similar Pearson correlation (single: R = 0.94, SD = 10.84dB, linear: R = 0.92, SD = 6.6dB, curved: R=0.95, SD=6.89dB) between the BSC and porosity was found from three transducers, but the standard deviation (SD) was smaller from the two array probes. This improved standard deviation may result from the wider spatial range of the array transducers. 2. A simulation model using COMSOL for the fetal bone density evaluation was built based on the Biot’s poroelastic theory and the backscatter coefficient. The theoretical backscatter coefficient from the Biot model was calculated with the best available biomechanical parameters from the human femoral cancellous bone and the geometrical features of the fetal femur. This work also proposed a method for compensating the ultrasound signal attenuation from abdominal tissue, femur tissue, amniotic fluid between the probe and fetal femur. The result showed good correlation of BSC (R = -0.9970, P = 2.0058e^-04, SD = 10.21%) and apparent integrated backscatter (AIB) (R = -0.9469, P = 0.0146, SD = 10.62%) with the porosity. This suggests in vivo ultrasound bone evaluation could be implemented in the current commercial ultrasound B-mode systems. 3. An in vitro study was conducted that compared the backscatter coefficient (BSC), the apparent integrated backscatter (AIB) and the Spectrum Centroid Shift (SCS) from the fundamental backscatter signal and the second harmonics of the ultrasound imaging system. The result from the second harmonics (R : BSC = 0.7374, AIB = 0.6243, SCS =-0.6421) showed better correlation than the fundamental backscatter (R : BSC = 0.7055, AIB = 0.5393, SCS = -0.5858) with a gold standard bone mineral density obtained from DEXA scans of the same samples. An analysis from the Farran cylindrical model and the second harmonics of a rigid cylinder showed the second harmonics has less noise and showed better performance than the fundamental backscatter approach. In conclusion, the backscatter coefficient from ultrasound imaging showed good correlation in both the simulation studies and the in vitro study. It has the potential to be a convenient, fast, cheap methodology for adult and fetal bone density evaluation

Maximum-Likelihood Dual-Energy TomographicImage Reconstruction

Dual-energy (DE) X-ray computed tomography (CT) has shown promise for material characterization and for providing quantitatively accurate CT values in a variety of applications. However, DE-CT has not been used routinely in medicine to date, primarily due to dose considerations. Most methods for DE-CT have used the filtered backprojection method for image reconstruction, leading to suboptimal noise/dose properties. This paper describes a statistical (maximum-likelihood) method for dual-energy X-ray CT that accommodates a wide variety of potential system configurations and measurement noise models. Regularized methods (such as penalized-likelihood or Bayesian estimation) are straightforward extensions. One version of the algorithm monotonically decreases the negative log-likelihood cost function each iteration. An ordered-subsets variation of the algorithm provides a fast and practical version.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85934/1/Fessler172.pd

Study of bone-metal interface in orthopaedic application using spectral CT

This thesis investigates the diagnostic potential of MARS spectral photon counting computed tomography (CT) in assessing musculoskeletal disorders such as bone fractures and crystal arthritis. The hypothesis states that the high spatial resolution, quantitative material specific information and reduced metal artefacts of spectral photon counting CT makes the MARS spectral CT scanner a promising imaging tool to confirm or rule out a diagnosis. Being a new imaging modality, a protocol to scan samples with metal implants has to be optimised, before it can be implemented clinically for patient imaging. I contributed to optimising a protocol for imaging bone-implant specimens. Different biomaterials (titanium and stainless steel) used for fracture fixation were imaged. The artefacts were evaluated in both the energy and material domain. A bone analysis tool for measuring bone morphological parameters such as trabecular thickness and spacing was developed in collaboration with the Human Interface Technology Lab. Bone healing at the bone-metal interface was studied and the results were compared with plain radiographs, dual energy x-ray absorptiometry and clinical single and dual energy CT. The advantages of photon counting spectral CT in the early assessment of bone healing due to reduced artefacts was demonstrated. This thesis also investigated the potential of spectral photon counting CT to differentiate calcium crystals present in phantoms and osteoarthritic human meniscus samples. Our results show that MARS spectral CT can moderately discriminate calcium pyrophosphate (crystals inducing pseudogout) and calcium hydroxyapatite crystals. The results were compared with plain radiographs, polarised light microscopy and x-ray diffraction methods. In conclusion, this thesis demonstrated the clinical potential of MARS preclinical spectral photon counting CT scanner for the non-invasive and non-destructive imaging of bone-metal interfaces, for early assessment of bone healing, and for the detection and characterisation of articular crystals

Quantification of bone using a 3.0 tesla clinical magnetic resonance scanner

The work in this thesis examines the potential of using magnetic resonance imaging and spectroscopy (MRI & MRS) as a quantitative tool for diagnosing bone abnormalities at multiple skeletal sites, which could be used in conjunction with routine clinical imaging.MRI and MRS are routinely used in the clinical setting for the diagnosis of various types of diseases and abnormalities due to its advantages of providing excellent soft tissue contrast and also providing physiological and metabolic information. The use of MRI and MRS as a direct diagnostic tool for bone abnormalities is very limited at the moment due to issues of costs and standardisation. The aim in this thesis was to use the clinical 3.0 T MR scanner to acquire data from bone and bone marrow for identification of structural and chemical properties and to use those features to identify differences in bone strength and condition. The volunteers in this thesis were part of the high bone mass (HBM) study and they had additional acquisitions from dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT).MR acquisition protocols have been successfully optimised for each type of bone region and in-house software has also been created to process the acquired data and quantify various types of structural and chemical properties.The MR data from distal radius and tibia demonstrated good correlation with pQCT data (e.g. Figure 8-2 & Figure 8-3) and were also able to differentiate between HBM-affected and control populations (e.g. Figure 8-26). The MR data from lumbar vertebrae also demonstrated good correlation with DEXA data and some of the measurements were also able to differentiate between the HBM-affected and control populations.The combined results from this thesis demonstrate that both MRI and MRS are sensitive techniques for measurement of bone quantity and quality, and they are ready to be applied for clinical investigation as part of routine clinical imaging to identify bone strength in relation to abnormalities and treatments