Kish Chakrabarti

Evaluation of high‐resolution and mobile display systems for digital radiology in dark and bright environments using human and computational observers

— As digital display systems replace film traditionally used for reading radiographic images, resource-intensive acceptance testing must be performed to ensure that quality meets and maintains desired specifications. If machine observers can replace human readers, whose performances are highly variable, the results will be more consistent and less costly. To be effective, however, the automated observers must track human performance. An approach for a model observer, validated with human readers, for the evaluation of the visibility of low-contrast small targets in high-resolution and mobile displays under different ambient illumination, will be described. The displays were tested using CDMAM-like digital phantoms containing disks of varying diameters and contrasts on a flat background. For this task, we find the best indicator of display performance to be the displays ability to represent small luminance contrast, not resolution or pixel size. The results confirm that high-resolution systems perform better under low illumination while illuminance has a minor impact on the mobile-display performance. Finally, the results show that the machine observer tracks the performance of human readers. Machine observers with proper validation can replace humans in the acceptance testing procedures, saving the testers both time and money.

Evaluation of various mammography phantoms for image quality assessment in digital breast tomosynthesis

We investigated the appropriateness of four different mammography phantoms for image quality evaluation in Digital Breast Tomosynthesis (DBT). We tested the CIRS BR3D phantom, the ACR Prototype FFDM Accreditation Phantom, the Penn anthropomorphic breast phantom and the Quart mam/digi EPQC phantom. This work discusses the advantages and shortcomings of each phantom and concludes that none of them, in their current form, can be considered to be adequate as an image quality evaluation phantom for DBT.

Evaluation of clinical full field digital mammography with the task specific system-model-based Fourier Hotelling observer (SMFHO) SNR.

PURPOSE The purpose of this work is to evaluate the performance of the image acquisition chain of clinical full field digital mammography (FFDM) systems by quantifying their image quality, and how well the desired information is captured by the images. METHODS The authors present a practical methodology to evaluate FFDM using the task specific system-model-based Fourier Hotelling observer (SMFHO) signal to noise ratio (SNR), which evaluates the signal and noise transfer characteristics of FFDM systems in the presence of a uniform polymethyl methacrylate phantom that models the attenuation of a 6 cm thick 20/80 breast (20% glandular/80% adipose). The authors model the system performance using the generalized modulation transfer function, which accounts for scatter blur and focal spot unsharpness, and the generalized noise power spectrum, both estimated with the phantom placed in the field of view. Using the system model, the authors were able to estimate system detectability for a series of simulated disk signals with various diameters and thicknesses, quantified by a SMFHO SNR map. Contrast-detail (CD) curves were generated from the SNR map and adjusted using an estimate of the human observer efficiency, without performing time-consuming human reader studies. Using the SMFHO method the authors compared two FFDM systems, the GE Senographe DS and Hologic Selenia FFDM systems, which use indirect and direct detectors, respectively. RESULTS Even though the two FFDM systems have different resolutions, noise properties, detector technologies, and antiscatter grids, the authors found no significant difference between them in terms of detectability for a given signal detection task. The authors also compared the performance between the two image acquisition modes (fine view and standard) of the GE Senographe DS system, and concluded that there is no significant difference when evaluated by the SMFHO. The estimated human observer efficiency was 30 ± 5% when compared to the SMFHO. The results showed good agreement when compared to other model observers as well as previously published human observer data. CONCLUSIONS This method generates CD curves from the SMFHO SNR that can be used as figures of merit for evaluating the image acquisition performance of clinical FFDM systems. It provides a way of creating an empirical model of the FFDM system that accounts for patient scatter, focal spot unsharpness, and detector blur. With the use of simulated signals, this method can predict system performance for a signal known exactly/background known exactly detection task with a limited number of images, therefore, it can be readily applied in a clinical environment.

Testing optimum viewing conditions for mammographic image displays.

The viewbox luminance and viewing room light level are important parameters in a medical film display, but these parameters have not had much attention. Spatial variations and too much room illumination can mask real signal or create the false perception of a signal. This presentation looks at how scotopic light sources and dark-adapted radiologists may identify more real diseases.

SU‐FF‐I‐41: Effect of Room Setting On Object Dectectability for Two Different AMLCD Displays

Purpose: The purpose of this study was to estimate the effect of viewing room setup on the readers ability to detect subtle details in radiographicimages displayed on two different active matrix liquid crystal displays (AMLCD). Method and Materials: Two viewing rooms were used in the study: one of them having a standard setup with white ceiling, light walls and furniture, and other monitors. The other room was a specially designed with black walls and ceiling and no other equipment except the monitors under study. Two different monitors, a standard IBM flat panel T‐221 and an experimental high‐bright IBM flat panel T‐221 were used in this study. In order to evaluate the effect of the different viewing room setups on the results of image reading, a computer generated 11×11 test pattern with two circular objects in every square cell were used. Each object of the pattern is defined by two parameters: diameter (number of pixels used for its generation) and contrast (number of digital driving levels). Five medical physicists were requested to read these contrast‐detail patterns at three different room illuminances (0, 5 and 20 lux) and four different backgrounds surrounding the entire test pattern (0%, 5%, 20% and 50% of max luminance) on both monitors. Results: At all conditions (room illuminance and surround) and on both flat panels, reader performance was significantly better in the specially designed dark room than in the typical viewing room. Conclusion: Higher diagnostic quality is realized in the room specially designed to minimize reflections.

SU‐FF‐I‐73: Comparison of the Effects of Viewing Conditions and Viewing Angle On Object Dectectability for Different AMLCD Displays

Purpose: The ability to interpret images displayed on active matrix liquid crystal displays (AMLCD) can be influenced by factors such as display luminance, surrounding background, room illuminance and viewing angle. We have been investigating how these parameters influence reader scores with images featuring both small objects and low contrast as typically seen in mammography. We are in a position to make some comparisons between the results obtained with displays from two different manufacturers. Method and Materials: Reader studies were conducted using a computer generated contrast detail phantom alternately presented against a display background of selected luminance levels. Luminance was also measured at different viewing angles and at four selected room illuminance levels. Image scoring was performed at each combination of background level, viewing angle and room illuminance level. Results:Image scoring performance was interpreted using k values, which reflect the contrast and diameter of the objects detected in the images. The best image scoring results were obtained when viewing angles were kept small, and also when room illuminance was at the level of 5 – 10 lux. Better scoring results were also obtained when the image background luminance was adjusted to 5 – 20 % of maximum. These results differ from what had previously been found when evaluating displays from a different manufacturer. In this case the best scoring results were obtained at zero background and the room illuminance did not seem to have a significant effect on the results obtained when kept in the range of 0 – 20 lux. Conclusion: The results support the view that while it is advisable to keep the viewing angle to a minimum, it may be advisable also to adjust the room illuminance and monitor background luminance to specific levels which may be best suited for a particular AMLCD display.

TU-FF-A3-04: Impact of Room Illuminance On Black Level Luminance and Contrast Detection for Off-Axis Viewing On High Resolution Normal and High-Bright Flat Panel Displays

Purpose: Flat panel display image quality is known to be highly dependent on viewing angle. The contribution of room illuminance to black level luminance is well understood for CRTs, but, is not clearly understood for flat panel displays. We have investigated the effect of room illuminance and off axis viewing on displayed image quality. Two high resolution 9.2 megapixel flat panels were evaluated under various room illuminance levels and viewing angles — a standard IBM T221. Method and Materials: Black level luminance was measured at 0, ±15, ±30 and ±45 degrees with room illuminance levels of 0, 2, 5, 10 and 50 lux. Luminance was measured with a Minolta LA‐100a spot meter and illuminance levels were measured with an International Light Luminance Meter IL 1400a. The luminance target (3% of total display area) was centered on the display. Reader studies using a computer generated contrast detail phantom were conducted at each viewing angle and room illuminance level. Five readers read the target images. Specular reflections were minimized for all measurements and reader observations. Results: Minimization of specular reflections in the viewing room minimized the change in measured black level luminance. As room illuminance levels increased and viewing angles became more acute black level luminance increased. Reader results are presented as k‐values and a correlation with room illuminance and black level luminance is demonstrated. The optimal viewing condition was shown to be 0 degrees with a room illuminance of 2 lux. Substantial degradation of measured k‐value and black level luminance occurred at other viewing conditions. Conclusion:Visual perception of small targets is substantially impacted by both viewing angle and room illuminance. The results of these measurements help to explain the observed variations in 5 and 50 lux data presented at the Pittsburgh AAPM meeting.

Digital mammography and the Mammography Quality Standards Act.

B OTH WIDE and narrow band gap materials are used for x-ray imaging in digital mammography. In computed radiography, those wide band gap materials ~ are selected when x-rays can efficiently produce defect centers as F type centers, which are electron-trapped anion vacancies and hole centers such as H centers and Vk-centers. Appropriate laser light ata later time stimulates the trapped charges, and their recombination at the luminescence centers results in emission of light producing x-ray imaging. In some materials 2 double doping by impurity ions also provides stable trapped charges, enabling one to perform read out of the imaging plateat a later time. Efforts are still under way to use the photostimulable phosphors in mammography, in which bigh detected quantum efficiency (DQE) and resolution and faster decay of the materials are important. Bandyapadhyay and Chakrabarti 3 and ChakrabartP have recently reported photostimulable emission in single crystals of copper doped alkali halides, which seem to possess some of these desirable characteristics. Some rare earth doped wide band gap phosphors that are widely used as screen phosphors in conventional screen-film systems are utilized with CCD arrays in scanning techniques for full breast imaging. Evidently, the choice of materials that have emissions involving parity forbidden f-f transitions has caused huge disadvantages in this attempt; however, efforts are under way to grow these materials in nanocrystaline 5 form, which may both improve DQE and decrease the decay time because of wave function mixing removing the parity restriction, thus facilitatieg the use of these materials for slot scanning.