Spatial Noise Power Spectra Research Articles

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

AbstractThe 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.

8.3: Characterization of Physical Image Quality of a 3‐Million‐Pixel Monochrome AMLCD: DICOM Conformance, Spatial MTF, and Spatial NPS

AbstractA high degree of conformance of a 3‐million‐pixel monochrome LCD with the DICOM Standard Display Function is achieved by spatial and temporal modulation. The spatial modulation transfer function (MTF) is found to be very close to 1 up to the Nyquist frequency. Visual comparisons of the LCD and high‐resolution monochrome CRTs with P45 phosphors indicate that the spatial noise levels are comparable. The measured spatial noise power spectra (NPS) on the other hand are higher than the actual noise powers appear to be. The measured noise levels are affected by the systematic structures of the display which we have not been able to eliminate totally from the noise power spectra.

Wiener noise power spectra of radiological television systems using a digital oscilloscope

Measurement of spatial noise power spectra from television based radiographic and fluoroscopic systems is essential to the understanding of their operation and optimization. However, conventional methods require acquisition and processing of large numbers of complete images, thus confining such measurements to special applications where accessible frame buffers already exist or elaborately equipped laboratories. We have developed a method which only requires storage of single TV lines or point scans. A digital oscilloscope captures these point scans and a laboratory microcomputer facilitates manipulation of the data to separate out different components of the noise power spectra. The x-ray dependent component of the noise power spectrum so produced is not the ordinary Wiener spectrum. However, it is shown that reconstruction of the full Wiener spectrum from this is possible subject only to the requirement that the x-ray noise spectrum at the output of the imaging system is circularly symmetric.

Measurement of the spatial Wiener spectrum of nonstorage imaging devices.

All previous methods for measuring image noise spectra require a noise realization, a static image, typified as a photograph which can be scanned to create the Wiener spectrum. We wished to analyze the spatial noise power spectrum at the output phosphor of a continuously irradiated imaging device, an x-ray image intensifier (XRII), which is incapable of image storage and thus the image is continually changing as a function of both time and space. Our new method utilizes a pair of slits to measure the relative Wiener spectrum of the temporally changing components of the image (i.e., x-ray quantum and XRII gain noises). By measuring the modulation transfer function and the Wiener spectrum of the same XRII on the same apparatus it was possible to demonstrate the spatial frequency dependence of the detective quantum efficiency. Adaptations of the method should permit the measurement of Wiener spectra of fluoroscopic television systems directly from the TV monitor.