Kunal Gandhi

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.

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.

Spatial noise and threshold contrasts in LCD displays

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.

25.2: Characterization of Physical Image Quality of 2- and 5-Million-Pixel Monochrome AMLCDs for Medical Imaging

The physical image quality of two new high-performance monochrome active matrix liquid crystal displays (AMLCDs), a 2- and a 5-million-pixel display, is reviewed. The performance is illustrated by examples (because of space limitations) of on-axis and off-axis characteristic curves, luminance range and contrast, luminance uniformity across the display screen, temporal modulation transfer function (MTF), spatial MTF, spatial noise power spectra and spatial 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 uniform contrast distribution over the luminance range for a narrow viewing angle perpendicular to the LCD face. The image quality of the LCDs is compared with that of high-performance high-resolution cathode-ray-tube (CRT) displays.

DQE evaluation of a full-field digital mammography system

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%.

Why should you calibrate your display

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.

Display of mammograms on a CRT

In most radiological imaging workstations the monochrome cathode ray tube (CRT) is the electronic display of choice. The CRT offers the best performance; it is the most highly developed, mature and reliable display in common use. Unfortunately, because of limited spatial resolution and because of limited dynamic range, the efficacy of soft-copy versus hard-copy diagnosis has been challenged.

Correction of the spatial noise of LCD using error diffusion

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 today.