Xenon-enhanced, dual-energy x-ray radiography has been proposed for imaging of lung ventilation. It is important to assess the ability of dual-energy subtraction to suppress anatomic noise associated with lung parenchyma. Anatomic noise in thoracic radiography obeys an inverse power law and there exist imaging phantoms that mimic this power law. Such phantoms are based on a random, tight packing of solid acrylic spheres and are not suitable for lung ventilation studies. We developed a phantom based on randomly-packed, hollow acrylic cylinders with inner diameters of 1.59 cm, wall thicknesses of 0.16 cm and lengths of 1.59, 1.27, 0.95, 0.64 or 0.32 cm. The number of segments of each length was chosen to approximately match the volume of space occupied by each set of segments. Measurements of the effective density of the packed cylinders yielded ~0.26 g cm-3. A randomly-packed-sphere phantom was also constructed as a reference. Both phantoms were imaged using a flat-panel detector at tube voltages of 50 kV to 150 kV. A power-law model (NPS ∝ κ/|u|β) was fit to the anatomic noise power spectra. The β-value of the cylinder phantom was within 1/5 of that of the sphere phantom, although both phantoms yielded power-law parameters ranging from 2.0 to 2.4, which is lower than that reported in the literature. The κ-value of the cylinder phantom was ~1.1 times that of the sphere phantom. We conclude that the cylinder-based clutter phantom, with some modifications, can be used to simulate the anatomic noise power spectrum in thoracic radiography.
Chronic obstructive pulmonary disease (COPD) is characterized by persistent airflow limitation resulting from emphysema and small airway disease. In our recent work we proposed xenon-enhanced dual-energy (XeDE) radiography for functional imaging of COPD. Using mathematical models, we showed that XeDE radiography has the potential to enable detecting functional abnormalities associated with early-stage COPD. The purpose of this study is to investigate the optimal exposure allocation for XeDE X-ray imaging of lung function by experiment and to validate the predictions of our theoretical model. Experiments were conducted using a custom-built chest phantom representing an adult female chest and containing a simulated ventilation defect. The phantom was imaged using a CsI/CMOS energy integrating X-ray detector (XINEOS-3030HS, Teledyne DALSA - Professional Imaging, Ontario, Canada) with a 151.8 µm pixel pitch. The low-energy (LE) and highenergy (HE) tube voltages were fixed at 60 kV and 140 kV respectively. We define the exposure allocation factor (f ) as the ratio of HE entrance exposure [Roentgens] to the LE entrance exposure. The value of (f ) was varied from 0.25 to 2 while keeping the total entrance exposure fixed at ∼60 mR. We used contrast-to-noise ratio (CNR) normalized by the square root of total entrance exposure as a figure of merit. Our theoretical model of CNR accounted for the contrast of ventilation defects, quantum noise and X-ray scatter. Quantum noise was calculated using cascaded system analysis accounting for the quantum efficiency, K fluorescence, optical collection efficiency, optical blur and noise aliasing. Our models of defect contrast, noise and CNR agreed well with experimental results. The theoretical and experimental results show that the optimal exposure allocation is f = 0.5 indicating that ∼2/3 of the total exposure should be allocated to LE image.
We propose two-dimensional (2D) dual-energy (DE) x-ray imaging of lung structure and function for the assessment of COPD, and investigate the resulting image quality theoretically using the human observer detectability index (d') as a figure of merit. We modeled the ability of human observers to detect ventilation defects in xenon enhanced DE (XeDE) images and emphysema in unenhanced DE images. Our model of d' accounted for the extent of emphysematous destruction and functional impairment as a function of defect/lesion contrast, spatial resolution, x-ray scatter, quantum and background anatomical noise power spectrum (NPS), and the efficiency of human observers. The effect of x-ray spectrum and exposure allocation factor on d' was also explored. Our results suggest that, the detectability is maximized for exposure allocation factors that minimize quantum NPS. The optimal combination of tube voltage was found to be ~50/140 kV or 60/140 kV depending on the task and patient at an x-ray exposure equal to that of a standard chest x-ray. In 2D DE x-ray imaging of COPD, the detectability is primarily limited by low contrast, x-ray scatter, and anatomic noise, the latter two of which reduce the detectability of individual defects by 30% and ~>90%, respectively.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.