Kwok Leung Lam

Studies of performance of antiscatter grids in digital radiography: Effect on signal-to-noise ratio

We developed a theoretical model which describes the improvement of signal-to-noise ratio (SNR) by a grid in digital radiography. The model takes into account the effects of spatial variations in the scatter-to-primary ratio and in the large-area contrast over an image with structured background on quantum noise, and the effects of noise in the imaging system such as electronic noise and digitization noise. Based on the theoretical model, we analyzed the effects of these factors on the SNR when a grid is employed. We performed experimental measurements to evaluate the improvement in the SNR by a grid when quantum noise is the dominant noise source. It was found that the measured SNR improvement factor due to quantum noise agreed closely with that determined from the measured transmission values of a grid, as predicted from our theoretical model. In order to evaluate the relative performance of grids with various geometric design parameters for digital radiographic systems, we employed Monte Carlo calculations and determined the transmission values of a number of grids under various scatter conditions. The calculated SNR improvement factor, due to quantum noise, correlated well with the measured improvement of the SNR by the grids. Our model predicts that the SNR improvement factor depends strongly on the local contrast ratio and also on the scatter-to-primary ratio. The SNR improvement factor is higher in the underpenetrated regions than in the well-penetrated regions of an image.

Effects of x-ray beam equalization on mammographic imaging

We employed an elastic reservoir filled with tissue-equivalent fluid as an x-ray beam filter for exposure equalization. The filter is designed to be placed between the upper compression plate and the breast. During compression, it will conform to the shape of the peripheral region of the breast and fill the space between the upper and lower compression plates, thereby reducing the variation in x-ray penetration between the thick and thin regions of the breast and compressing the dynamic range of the breast image. Results of a phantom study showed that the visibility of low-contrast features was improved in the peripheral region of the equalized breast phantom image. Experimental measurements indicated that the filter reduced scattered radiation in the peripheral region. This result was confirmed by a Monte Carlo simulation study. The radiation contrast, with the effect of beam hardening included, was also improved when equalization was employed.

Effect of daily localization and correction on the setup uncertainty: Dependences on the measurement uncertainty, re-positioning uncertainty and action level

With the advent of commercial image-guided radiotherapy, daily correction of the setup uncertainty is feasible. It is beneficial to understand the dependence of the probability density function (pdf) of the corrected setup variation on the action level, localization uncertainty and re-positioning uncertainty so that an appropriate action level is used. Also, that pdf can be used in treatment planning to incorporate setup variation directly in the planning process to generate treatment plans more robust to setup variations. We have found an analytical expression of the pdf of the corrected setup variation assuming normal distributions for the uncertainties. Using the second moment of that pdf as a metric, we have explored the dependence of the metric on the action level for the following cases: (1) the uncertainties in measurement and re-positioning are less than the initial setup uncertainty, (2) the uncertainties in measurement and re-positioning are on the order of the initial setup uncertainty, (3) the uncertainty in measurement is the least and (4) the uncertainty in re-positioning is the least. An optimal action level exists in case 3. We have also found that an action level of [Formula: see text] works well in practice where micro(p) is the mean of the re-positioning uncertainty, sigma(p) is the standard deviation of the re-positioning uncertainty and sigma(m) is the standard deviation of the localization uncertainty. In typical clinical situations, the distribution of the corrected setup variation can be closely approximated by a normal distribution.

Digital Characterization Of Clinical Mammographic Microcalcifications: Applications In Computer-Aided Detection

As part of our ongoing effort to develop an automated computer scheme for the detection and analysis of microcalcifications in digital mammograms, we have analyzed the physical characteristics of microcalcifications from a data base of 39 clinical mammograms in patients undergoing breast biopsy. A signal-extraction method was developed for determination of the size, contrast, and signal-to-noise ratio (SNR) of each microcalcification from unprocessed mammograms. The average power spectrum of the microcalcifications thus extracted was compared to that of the mammographic background. Based on an analysis of these characteristics, we designed a new type of spatial filter, obtained as the difference between a matched filter and a box-rim filter, that can selectively preserve the frequency content of microcalcifications while suppressing the low-frequency background and high-frequency noise. The SNR of the microcalcifications is thereby enhanced. Signal-extraction tests that make use of the size, contrast, local frequency content, and clustering properties of microcalcifications were employed for further discrimination between true signals and normal mammographic structures or artifacts. In order to evaluate the potential clinical utility of our approach, we applied the program to 20 clinical mammograms that contained subtle clustered microcalcifications. These mammograms were not included in the data base mentioned above. The automated computer detection scheme provided a true-positive cluster detection rate of 90% at a false-positive detection rate of one-half cluster per image. These results demonstrate the feasibility of using computer methods to aid radiologists in screening of mammograms for subtle microcalcifications.

Dynamic digital subtraction evaluation of regional pulmonary ventilation with nonradioactive xenon.

A method of evaluating pulmonary ventilation with a 57-cm image intensifier/television (II/TV) digital chest system is reported. With this method, the patient inhales a mixture of xenon and oxygen gases while dynamic imaging of the chest is done. Images of the airways and ventilated portions of the lungs are obtained by subtraction of images acquired before and after the xenon-oxygen mixture is administered. The feasibility of the method was evaluated by studies with xenon-filled tubes, an airway phantom, and a ventilation phantom. The results indicate that tubes larger than 3.2 mm in diameter are detectable at a xenon concentration of 41%, and that gas flow and flow distribution can be examined after image subtraction. If background subtraction is incomplete because of motion, the visibility of small airways is reduced greatly, although unventilated regions can still be delineated. The initial evaluation of this technique included imaging a healthy volunteer during xenon inhalation.

Optical image processing with liquid-crystal display for image intensifier/television systems.

We have studied the effect of real-time optical image processing (OIP) in an image intensifier/television (II-TV) radiographic imaging system by using a liquid-crystal display (LCD) placed between the II and the TV camera. The LCD compresses the dynamic range of the transmitted image by modulating the spatial distribution of the light intensity of the image from the output phosphor of the II. The degree of dynamic-range compression can be designed so that the dependence of the signal-to-noise ratio (SNR) of the LCD-TV system on x-ray intensity matches that of the quantum noise. We measured the physical properties of an LCD and evaluated its capability for OIP. Our experimental results demonstrate that it is feasible to use an LCD to compress the dynamic range and to improve the SNR of the image. The advantages of implementing OIP with an LCD in image acquisition systems in which a TV camera is used are discussed.

Technical note: Acquisition of CT models for radiotherapy applications with reduced tube heating

The potential or changing computed tomography (CT) protocols to provide data sets that generate high quality digitally reconstructed radiographs (DRRs) from scans with very low tube currents is demonstrated. DRRs were generated from CT data acquired with slice thickness of 1, 3, and 5 mm, using high current to reduce noise in axial images. These DRRs were compared to one generated from a CT scan acquired using 1 mm aperture and very low (10 mA) current. The DRR generated via this technique is comparable to that generated with high current and 1 mm aperture, and higher resolution than from the 3 and 5 mm CT scans.