Jiahua Fan

Use of a Human Visual System Model to Predict Observer Performance with CRT vs LCD Display of Images

This Project evaluated a human visual system model (JNDmetrix) based on just noticeable difference (JND) and frequency-channel vision-modeling principles to assess whether a Cathode ray tube (CRT) or a liquid crystal display (LCD) monochrome display monitor would yield better observer performance in radiographic interpretation. Key physical characteristics, such as veiling glare and modulation transfer function (MTF) of the CRT and LCD were measured. Regions of interest from mammographic images with masses of different contrast levels were shown once on each display to six radiologists using a counterbalanced presentation order. The images were analyzed using the JNDmetrix model. Performance as measured by receiver operating characteristic (ROC) analysis was significantly better overall on the LCD display (P = 0.0120). The JNDmetrix model predicted the result (P = 0.0046) and correlation between human and computer observers was high (r2 (quadratic) = 0.997). The results suggest that observer performance with LCD displays is superior to CRT viewing, at least for on-axis viewing.

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.

Evaluation of and compensation for spatial noise of LCDs in medical applications.

Recent developments in liquid crystal display (LCD) technology suggest that this technology will replace the cathode ray tube (CRT) as the most popular softcopy display technology in the medical arena. However, LCDs are far from ideal for medical imaging. One of the principal problems they possess is spatial noise contamination, which requires accurate characterization and appropriate compensation before LCD images can be effectively utilized for reliable diagnosis. This paper presents some work we have conducted recently on characterization of spatial noise of high resolution LCDs. The primary purpose of this work is to explore the properties of spatial noise and propose a method to reduce it. A high quality CCD camera was used for physical evaluation. Spatial noise properties were analyzed and estimated from the camera images via signal modeling and processing. A noise compensation algorithm based on error diffusion was developed to process images before they were displayed. Results shown in this paper suggest that LCD spatial noise can be effectively reduced via appropriate processing.

Differential Use of Image Enhancement Techniques by Experienced and Inexperienced Observers

Full-field digital mammography (FFDM) systems are currently being used to acquire mammograms in digital format, but digital displays are less than ideal compared to traditional film-screen display. Certain physical properties of softcopy displays [e.g., modulation transfer function (MTF)] are less than optimal compared to film. We developed methods to compensate for some of these softcopy display deficiencies, based on careful physical characterization of the displays and image-processing software. A series of 100 FFDM and 60 digitized images was shown to six observers—half experienced (mammographers) and half inexperienced (radiology residents). The observers had to decide if a mass or microcalcification cluster was present and classify it as benign or malignant. A window could be activated that brought the image detail within the window to full resolution and corrected for the nonisotropic MTF of the Cathode Ray Tube (CRT) display. Experienced readers had better diagnostic performance and took less time to view the images. Experienced readers used window/level more than inexperienced readers, but inexperienced readers used magnification and the MTF compensation tool more often. Use of the magnification and the MTF tool increased reader decision confidence. Experienced and inexperienced readers use image-processing tools differently, with certain tools increasing reader confidence. Understanding how observers use image-processing tools may help in the development of better and more automated user interfaces.

Influence of 8‐bit vs. 11‐bit digital displays on observer performance and visual search: A multi‐center evaluation

— Medical-grade monochrome monitors typically display 8 bits of data. This study determined if 11-bit displays could improve observer performance and decrease use of window/level. 8- and 11-bit displays from three manufacturers were used at three sites. Six radiologists at each site viewed 100 DR chest images (half with a pulmonary nodule) on both displays. Decisions, confidence, nodule location, viewing time, and window/level use were recorded. There was no significant difference in ROC Az as a function of bit depth. The average Az with 8 bits was 0.8284 and with 11 bits was 0.8253. There was a significant difference in viewing time favoring the 11-bit displays. Window/level use did not differ. Eye position was recorded on a subset of images at one site. Cumulative dwell times for each decision category were lower with the 11-bit than with the 8-bit display. When tested with t-tests for paired observations, the TP (t = 1.452, p = 0.1507), FN (t = 0.050, p = 0.9609), and FP (t = 0.042, p = 0.9676) were not statistically significant. The difference in the TN decisions was statistically significant (t = 1.926, p = 0.05). 8-bit displays will not impact negatively diagnostic accuracy, but using 11-bit displays may improve workflow efficiency.

Minimization of over-ranging in helical volumetric CT via hybrid cone beam image reconstruction—Benefits in dose efficiency

Diagnostic computed tomography (CT) images are usually acquired in both helical and axial scans in the clinical applications using cone beam volumetric CT. In addition to faster patient throughput, a helical scan in volumetric CT can provide better image quality because of the satisfaction of data sufficiency condition, and thus has been performed far more frequently so far in the clinic. However, the first and last images in a helical scan are usually prescribed at the locations that are half helical turn indented from the starting and ending points of the scan. Due to such an indention, the dose efficiency of helical scan deteriorates with increasing detector dimension along z direction. To improve the dose efficiency of helical scan in volumetric CT, a hybrid helical cone beam filtered backprojection (CB-FBP) algorithm is presented here to reconstruct helical images beyond the conventional indented image zone. The hybrid algorithm is a combination of the ray-wise three-dimensional (3D) weighted CB-FBP algorithms that are recently proposed for helical and axial CB image reconstructions. Through the hybridization, the ray-wise 3D weighting becomes dependent on both helical pitch and image slice location. Phantom study shows that the conventional indented image zone in helical scan can be extended substantially by using the hybrid algorithm. Consequently, the dose efficiency of volumetric CT in helical scan can be improved significantly. It is believed that, with increasing detector dimension along z direction in cone beam volumetric CT, the hybrid algorithm will become more attractive in clinical applications.

Real-time MTF evaluation of displays in the clinical arena

Abstract not available.

The liquid crystal display (LCD) for medical imaging in comparison with the cathode ray tube display (CRT)

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.

Performance evaluation of LCD displays

This paper presents measurements of display function, spatial resolution (MTF), grayscale precision and spatial noise of three monochrome LCDs. Each of these LCDs features a different method to increase the grayscale precision: Frame Rate Modulation, Sub-Pixel Modulation and Aperture Modulation. A CCD camera was used for the evaluation. It imaged a small portion of the LCD, usually with over-sampling of between 115:1 and 8:1 CCD pixels per LCD pixel. The evaluated systems have image quality that in many respects is superior to CRT displays. Most impressive is the spatial resolution. The MTF of the systems investigated is almost unity. The typically 8 bits grayscale precision can be significantly increased by temporal as well as spatial modulation techniques. It appears that the aperture modulation technique alone can achieve a precision of 10.8 bits or 1800 distinct luminance levels. Spatial noise was evaluated in terms of single pixel signal-to-noise ratio and in terms of the spatial noise power spectrum. Single pixel signal-to-noise ratios for one LCDs were in the order of 100:1, and for another one the spatial noise power density of the normalized NPS at spatial frequencies below the LCD Nyquist frequency of 2.4 lp/mm was about 3.1E-5 mm2, values which are in the order of those from high performance CRTs.

Image quality evaluation of a LightSpeed CT750 HD computed tomography system

With the advancement of Computed Tomography technology, improving image quality while reducing patient dose has been a big technical challenge. The recent CT750 HD system from GE Healthcare provides significantly improved spatial resolution and the capability to reduce dose during routine clinical imaging. This paper evaluates the image quality of this system. Spatial resolution, dose reduction, noise, and low contrast detectability have been quantitatively characterized. Results show a quantifiable and visually discernable higher spatial resolution for both body and cardiac scanning modes without compromise of image noise. Further, equivalent image quality performance with up to 50% lower dose has been achieved.