LCDs are replacing CRTs as primary diagnostic softcopy displays in clinics. They exhibit higher spatial noise than CRTs, which can interfere with diagnosis and reduce the efficiency especially when subtle abnormalities are presented. We have reported recently a study on the LCD spatial noise1. A high quality CCD camera was used to acquire images from the LCD. Noise properties were estimated from the camera images. Then an error diffusion based operation was applied to compensate for the display spatial noise. This paper presents the noise estimation and compensation results on five different LCDs using same processing protocol. These five different LCDs vary in terms of matrix size, pixel size, pixel structure and vendors. The purpose of this work is to demonstrate that the LCD spatial noise estimation and compensation scheme we proposed earlier is valid, robust and necessary for different medical grade LCDs used in clinics
This paper presents results of psychophysical evaluations of an LCD with respect to its spatial noise. Spatial noise is quantified using a high-resolution CCD camera and compensated for using an error diffusion based algorithm. Psychophysical evaluation is performed in order to explore the dependence of human contrast sensitivity on display spatial noise. This evaluation uses the two-alternative forced choice (2-AFC) method. Gaussian-shaped objects, which simulate lung nodules, serve as stimuli. Three types of test images are used: Images containing simulated noise amounting to twice the amount of spatial noise present, images containing the original spatial noise and images compensated for the spatial noise. The probability of correct response and the detectability index, d', calculated indicates that spatial noise compensation leads to a lower contrast threshold.
This paper presents a few results of an initial study to compare softcopy displays with 8 bits contrast resolution with those of 11 bits contrast resolution. Particular interest was paid to failure to detect subtle objects and appearance of artifacts like false contours as a result of improper quantization. Objects like squares, discs and Gaussian nodules were simulated at different amplitudes on uniform backgrounds. Another approach was to place simple objects like disks and Gaussian objects into a clinical image. The study concluded that indeed subtle objects can be missed and artifacts such as false contours can occur, dependent on signal amplitude and noise. A comprehensive observer study is under development to confirm and refine these results.
This paper presents the outcome of an initial study to compare softcopy displays (Liquid Crystal Displays) with 8 bits contrast resolution with those of 11 bits contrast resolution. Of particular interest was decreased detection of objects and appearance of artifacts like false contours as a result of quantization. The study was based on simulation of objects like squares, discs and Gaussian nodules at different amplitudes on uniform backgrounds. We also placed simple objects like disks and Gaussian objects into a clinical image. These objects were displayed on the two different types of displays to untrained observers. These objects were also analyzed quantitatively in their digital form with a computer program made for display and analysis. The study found that subtle objects can be missed and artifacts such as false contours can occur, dependent on signal amplitude and noise. A comprehensive observer study is necessary to confirm and refine these results.
This paper presents results of physical as well as psychophysical evaluations of an LCD with respect to its spatial noise. Spatial noise is quantified using a high-resolution CCD camera and a method is developed to compensate for it. This compensation method is based on a spatial noise map, derived from the CCD camera images, and on the application of an error diffusion algorithm. This method of noise compensation reduces the spatial noise by about a factor of 2. Psychophysical evaluation is performed in order to explore the dependence of human contrast sensitivity on display spatial noise. This evaluation uses the two-alternative forced choice (2-AFC) method. Aperiodic Gaussian-shaped objects, which simulate lung nodules, serve as stimuli. The detectability index, d', calculated indicates that spatial noise compensation leads to a lower contrast threshold.
Physical characteristics necessary to calculate the Detective Quantum Efficiency of a clinically used flat panel imager for full-breast digital mammography are presented. Objective quantities such as modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) have been evaluated. The X-ray photon fluence was determined using Half-Value-Layer (HVL) techniques. At an X-ray beam characterized by 28 kVp, Mo-anode and Mo filter as well as beam hardening by 5 cm Lucite, the detector is practically linear with x-ray exposure at least up to 33 mR. At an exposure of 33 mR and close to zero spatial frequency the DQE is in the vicinity of 60%.
Recent developments in Liquid Crystal Display (LCD) technology suggest that they will replace the Cathode Ray Tube (CRT) as the most common softcopy display in the medical arena. But LCDs are far from ideal for medical imaging. One of the problems they possess is spatial noise. This paper presents some work we have conducted recently on spatial noise of high resolution LCDs. The purpose of this work is to explore the properties of spatial noise and the method to reduce them. A high quality CCD camera is used for physical evaluation. Spatial noise properties are analyzed and estimated from the camera images via signal modeling and processing. Noise compensation algorithm based on error diffusion is developed to process images before they are displayed. Some initial results shown in this paper suggest that LCD spatial noise can be eliminated via appropriate processing.
This paper discusses the issue of calibration for the growing number of electronic displays used in the filmless and electronic radiology departments. It concentrates on CRT and LCD displays as these are the most matured electronic display systems available at this time. It is shown that grayscale calibration is necessary and useful to optimally display the information contained in the various digital images in diagnostic radiology. In addition properties and drawbacks of four prevalent standards for display function have been discussed.
The paper presents methodologies for characterizing liquid crystal displays (LCDs) and the image quality of two new high-performance monochrome LCDs, a 2- and a 5-million-pixel display. The systems' image quality is described by on-axis characteristic curves, luminance range and contrast, luminance and contrast as a function of viewing angle, diffuse and specular reflection coefficients, color coordinates, luminance uniformity across the display screen, temporal response time and temporal modulation transfer function (MTF), spatial MTF, spatial noise power spectra and signal-to-noise ratios.
The LCDs are equipped with an internal photosensor that maintains a desired maximum luminance and calibration to a given display function. The systems offer aperture and temporal modulation to place luminance levels with more than 12-bit precision on a desired display function and achieve very uniform contrast distribution over the luminance range. The LCDs have image quality that is superior in many respects to high-performance and high-resolution cathode-ray-tube (CRT) displays, except for the temporal MTF and the spatial noise. Spatial noise appears to be comparable to CRT display systems with P4 or P104 phosphor screens.
Four methods for near real-time measurement of the modulation transfer function (MTF) of electronic displays are presented. The methods are based on measuring the display’s response to an edge, periodic bar-patterns, line and whitenoise stimuli. Although all the methods yield practically the same result, they require different data acquisition time and different degrees of human intervention while analyzing the acquired data. The paper presents a comparison between the four methods in context of the time required to implement each and cites implementation issues that need to be addressed in order to achieve real time data analysis and presentation.
This paper presents the results of initial physical and psycho-physical evaluations of the noise of high resolution LCDs. 5 LCDs were involved, having 4 different pixel structures. Spatial as well as temporal noise was physically measured with the aid of a high-performance CCD camera. Human contrast sensitivity in the presence of spatial noise was determined psycho-physically using periodic stimuli (square-wave patterns) as well as aperiodic stimuli (squares). For the measurements of the human contrast sensitivity, all LCDs were calibrated to the DICOM 14 Grayscale Standard Display Function (GSDF). The results demonstrate that spatial noise is the dominant noise in all LCDs, while temporal noise is insignificant and plays only a minor part. The magnitude of spatial noise of LCDs is in the range between that of CRTs with a P104 and that of CRTs with a P45. Of particular importance with respect to LCD noise is the contribution of the pixel structure to the Noise Power Spectrum, which shows up as sharp spikes at spatial frequencies beyond the LCDs’ Nyquist frequency. The paper does not offer any clues about the importance of these spikes on the human contrast sensitivity.
This paper discusses display parameters such as display function, contrast, dynamic range, veiling glare and spatial resolution of displays useful in digital radiology. After a review of the traditional display in diagnostic radiology, namely the film-lightbox, based on the film-screen combination, the paper concentrates on the Active Matrix Liquid Crystal Flat Panel Display (AM-LCD). The AM-LCD will most likely mature and may become the display of choice in the near future, replacing the Cathode Ray Tube Display (CRT), which is presently the dominating softcopy display. A comparison between pertinent performance characteristics of AM-LCD and CRT demonstrates that spatial resolution (Modulation Transfer Function or MTF) and veiling glare for the AM-LCD are already superior to those of the CRT.