Hartwig R. Blume

DQE(f) of four generations of computed radiography acquisition devices.

Measurements were made of the MTF(f), NPS(f), and DQE(f) of four generations of computed radiography (CR) imaging plates and three generations of CR image readers. The MTF generally showed only a minor change between generations of plates and readers, but the DQE(f) has improved substantially from a very early plate/reader combination to a more recent one. The DQE in the more recent plate/reader combination is 1.3X greater at low frequencies and about 3X greater at high frequencies than the much earlier versions. Thus there has been substantial improvement in the imaging performance obtainable with CR since some of the early observer studies which indicated poorer performance with CR than with screen-film.

Introduction to Grayscale Calibration and Related Aspects of Medical Imaging Grade Liquid Crystal Displays

Consistent presentation of digital radiographic images at all locations within a medical center can help ensure a high level of patient care. Currently, liquid crystal displays (LCDs) are the electronic display technology of choice for viewing medical images. As the inherent luminance (and thereby perceived contrast) properties of different LCDs can vary substantially, calibration of the luminance response of these displays is required to ensure that observer perception of an image is consistent on all displays. The digital imaging and communication in medicine (DICOM) grayscale standard display function (GSDF) defines the luminance response of a display such that an observer’s perception of image contrast is consistent throughout the pixel value range of a displayed image. The main purpose of this work is to review the theoretical and practical aspects of calibration of LCDs to the GSDF. Included herein is a review of LCD technology, principles of calibration, and other practical aspects related to calibration and observer perception of images presented on LCDs. Both grayscale and color displays are considered, and the influence of ambient light on calibration and perception is discussed.

Characterization of high-resolution liquid crystal displays for medical images

The main subjects of the paper are the methodology of characterizing liquid crystal displays (LCDs) and the properties of a three-million-pixel monochrome display system. The system is characterized by display function and dynamic range as a function of viewing angle, spatial luminance uniformity, flicker, peak-to-peak temporal modulation transfer, spatial modulation transfer function (MTF), spatial noise power spectra, and single-pixel signal-to-noise ratios. The evaluated LCD has image quality that, in most respects, is superior to CRT monitors of comparable addressable pixel matrix. In particular, the LCD has perfect spatial modulation transfer. For the evaluated monochrome display system, the general limitation of liquid crystal display (LCD) drivers to 8-bits of grayscale precision is overcome by spatial as well as temporal modulation techniques. The architecture of the control electronics of the system is presented, and as part of it, the implementation of the modulation techniques. The spatial or a combination of spatial and temporal modulation techniques increases the precision with which luminance levels can be defined to 9.58 or 11.58 bits, respectively. The conformance of the calibrated display with the DICOM Standard Display Function is demonstrated without and with application of the modulation techniques. Excellent conformance is achieved for the combination of spatial and temporal modulation.

Comparison of the physical performance of high-resolution CRT displays and films recorded by laser image printers and displayed on light-boxes and the need for a display standard

Performance figures are reported on absolute luminance, luminance uniformity, characteristic display function, internal scatter, dynamic range, distortion, modulation transfer and its spatial uniformity, and temporal and spatial noise of two commercial ORT display systems as well as films printed by laser image recorders and displayed on light-boxes. One of the ORT displays has a matrix of nominally 2000 x 2000 pixels, the other of 1000 x 1500 pixels. The laser image recorders cover a matrix of 3500 x 2200 pixels. When comparing equal pixel matrices of hard versus soft copy displays, typically hard copies facilitate greater information transfer than soft copies due to a presentation with higher absolute luminance, greater perceived dyiiainic range, and better spatial resolution. Perceived dynamic range and resolution are partially degraded in the ORT displays by internal scatter. Soft and hard copy displays are about equivalent in terms of luminance non-uniformity, noise, and geometrical distortion. All displays differ in their characteristic display functions and thus in perceived contrast resolution. A display function standard is proposed to the industry by which mismatches between hard and soft copy presentations can be nimized as well as means for maintaining standardized performance.

ACR/NEMA proposal for a gray-scale display function standard

The proposed standard defines a gray-scale display function for monochrome image presentation devices, such as cathode-ray-tube (CRT) monitor/display-controller systems and digital laser image printers. The display function is based on perceptual linearization. It is defined for the luminance range of 0.05 to 4000 cd/m2. The standard provides a mathematical formula for the standard display function as well as a table of the luminance levels of the just-noticeable differences (JNDs) of a standard target. The standard facilitates similarity in gray-scale between different image display devices independent of their luminance. The standard does not eliminate the use of application-specific display functions, but rather assures their effectiveness through the standard display function. To realize conformance of an image presentation device with the display function standard, standard test patterns are defined. The patterns are used to measure the luminance of the display or optical density of a print as a function of digital input. By proper interpolation of the inverted measured characteristic function of the display system and inserting the desired standard luminance values for every digital input, a transformation can be computed so that the display system may conform with the display function standard. The standard also defines the dynamic range of an image presentation device as the number of JNDs that theoretically can be realized for a given digitization resolution and the standard target. Interpolation processes and potential conformance measures for the standard are discussed. The proposed display function standard is essential for the proper realization of image presentation services according to the DICOM Standard. The relation of the standard display function to DICOM-defined look-up tables is outlined.

Reversible And Irreversible Image Data Compression Using The S-transform and Lempel-Ziv Coding

Views are expressed on desirable characteristics of image data compression schemes for PACS, in particular, that such schemes should not only enable reversible and irreversible compression but also provide user-friendly features and compatibility with PACS requirements. Among the image conversion schemes for data compression, hierarchical picture decomposition meets most of the desirable features and requirements, such as ease of composing a pictorial index of a patient file, adaptation of transmission to the variable resolution requirements in PACS, quasi-instant transmission of low-resolution pictures for teleradiology, and ease of implementing spatial pyramid filters. One of the simplest image conversion schemes which fulfills these requirements is the S-transform. Among the coding methods, the Lempel-Ziv scheme offers features which could be advantageous for PACS applications, such as that the code is always optimal - even though it is derived from a single-pass operation, that it may have fixed length, and that the code table does not need to be transmitted. Results of reversible and irreversible data compression with the S-transform, Lempel-Ziv coding, and quantizer functions are presented for computed radiographs and magnetic resonance images. The results are assessed and compared with S-transform/Huffman coding as well as differential pulse code modulation with Huffman or Lempel-Ziv coding.

Chest imaging within the radiology department by means of photostimulable phosphor computed radiography: A review

Photostimulable phosphor computed radiography has been clinically used outside of Japan for more than 8 years. Results of at least 35 quantitative or semiquantitative studies have been published so far in which the clinical utility of computed radiography (CR) is compared with that of conventional screen/film radiography (FR) for the study of the adult chest within the radiology department. The results can be summarized as follows: CR is superior to FR in the mediastinum, retrocardiac region, and subdiaphragmatic recesses, as well as in the evaluation of coronary artery calcifications. CR is reported to be generally superior or equivalent in the detection and evaluation of pulmonary nodules and larger pulmonary opacities. Equivocal results have been reported for pathologies requiring the inspection of fine details, such as interstitial infiltrates or pneumothorax. The studies indicate that image processing algorithms dedicated to the delineation of specific anatomies or pathologies improve clinical performance.

Noise of CRT display systems

This paper is an attempt to correlate CRT noise (measured physically) with contrast sensitivity obtained from the detection of square-wave patterns presented to human observers on a CRT. It presents the results of both physical and psychophysical evaluations of the noise of high resolution CRTs at a luminescence level of 65 ft-L. Temporal as well as spatial noise was physically measured with the aid of a photomultiplier-based evaluation system. Human contrast sensitivity was determined physchophysically using square-wave patterns of frequencies ranging from 0.47 lp/deg to 14.9 lp/deg. Even though the set of data is not very extensive, the results indicate a correlation between physical and psychophysical evaluation: for the luminance levels under consideration, the detection of human observers seems to be limited by the spatial noise (phosphor granularity); the temporal noise plays a role only at low luminance levels.

Optimizing the display function of display devices

This paper discusses the optimization of the display function of an image display with respect to properties of the human eye/brain system. The optimization minimizes the contrast information loss through the display/human observer system and is based on the threshold contrast curve or the curve of just noticeable differences, JND, which one can acquire by psychophysical experiments or physical measurements. It was found that, given the luminance dynamic range of an image display system, the optimum display function is the inverse of the scaled visual response function which is approximately independent of the absolute values of the threshold contrast. In particular, in good approximation, the optimum display function is independent of the spatial frequencies of the displayed image. With an analytical model of the threshold contrast curve, it is shown how the perceived dynamic range depends on factors such as the display device noise, internal scatter and the maximum luminance. The optimum display function based on the threshold contrast curve was employed with a CRT monitor. Preliminary results indicate that an improvement in the overall contrast resolution of clinical images can be achieved compared to operating the CRT with its original display function. The optimum display function guarantees maximum utilization of the contrast information transfer capabilities of a display device uniformly over the available display range. Its suitability as standard for image displays is reiterated. Often, however, for given display tasks, the best display function considering intrinsic image noise, perception requirements, and the desired certainty of perceiving a given contrast detail may be different from the optimum display function. A look-up table should then adapt the display function from the standard optimum state to a task-specific function.

Image quality assessment of monochrome monitors for medical soft copy display

Soft-copy presentation of medical images is becoming part of the medical routine as more and more health care facilities are converted to digital filmless hospital and radiological information management. To provide optimal image quality, display systems must be incorporated when assessing the overall system image quality. We developed a method to accomplish this. The proper working of the method is demonstrated with the analysis of four different monochrome monitors. We determined display functions and veiling glare with a high-performance photometer. Structure mottle of the CRT screens, point spread functions and images of stochastic structures were acquired by a scientific CCD camera. The images were analyzed with respect to signal transfer characteristics and noise power spectra. We determined the influence of the monitors on the detective quantum efficiency of a simulated digital x-ray imaging system. The method follows a physical approach; nevertheless, the results of the analysis are in good agreement with the subjective impression of human observers.