Biomedical subjects

L N Loo

Publications and source records attributed to L N Loo.

7 recordsLinked to original sources

PubMed1984

A comparison of physical image quality indices and observer performance in the radiographic detection of nylon beads.

Although various models have been proposed in an attempt to predict the usefulness of a radiographic image in terms of its physical characteristics, no previous work has shown whether a single physical image quality index, such as a signal-to-noise ratio, can reliably predict the performance of a human observer over a broad range of image characteristics. We studied the relationship between physical and visual image quality for the task of detecting nylon beads in radiographs. Thirty-seven imaging cases with different combinations of physical image characteristics were considered; these included variations in object size and magnification, X-ray beam quality, screen-film system, screen-film contact, film density and illumination, and viewing distance. For each imaging case, visual image quality was quantified in terms of observer performance in a 2AFC visual detection experiment. Physical image quality indices were calculated according to eight different models of the detection process; these indices combined data regarding object size and attenuation, screen-film system MTF, film gradient, noise Wiener spectrum, and visual system response. The results of this work indicate that, for the conditions studied, human observer detection performance most closely resembles that of a sub-optimal statistical decision process.

Form Perception↗

PubMed1984

Digital image processing: effect on detectability of simulated low-contrast radiographic patterns.

Detection studies of simulated low-contrast radiographic patterns were performed with a high-quality digital image processing system. The original images, prepared with conventional screen-film systems, were processed digitally to enhance contrast by a "windowing" technique. The detectability of simulated patterns was quantified in terms of the results of observer performance experiments by using the multiple-alternative forced-choice method. The processed images demonstrated a significant increase in observer detection performance over that for the original images. These results are related to the displayed and perceived signal-to-noise ratios derived from signal detection theory. The improvement in detectability is ascribed to a reduction in the relative magnitude of the human observer's "internal" noise after image processing. The measured dependence of threshold signal contrast on object size and noise level is accounted for by a statistical decision theory model that includes internal noise.

Computers↗

PubMed1982

X-ray tube focal spot sizes: comprehensive studies of their measurement and effect of measured size in angiography.

Thirty-two focal spot sizes of four x-ray tubes were measured by the pinhole, star pattern, slit, and root-mean-square (RMS) methods under various exposure conditions. The modulation transfer functions (MTFs) and line spread functions (LSFs) were also determined. The star pattern focal spot sizes agreed with the effective sizes calculated from the frequencies at the first minimum of the MTF within 0.04 mm for large focal spots and within 0.01 mm for small focal spots. The focal spot size determined by the slit method was approximately equal to the width of the LSF at the cutoff level of 0.15 +/- 0.06 of the peak value. The RMS method provided the best correlation between the measured focal spot sizes and the corresponding image distributions of blood vessels. The pinhole and slit methods tended to overestimate the focal spot size, but the star pattern method tended to underestimate it. For approximately 90% of the focal spots, the average of the star and slit (or pinhole) focal spot sizes agreed with the RMS focal spot size within +/- 0.1 mm.

Angiography↗

PubMed1982

The energy-dependent behavior of noise Wiener spectra in their low-frequency limits: comparison with simple theory.

The Wiener spectra of five screen-film systems were studied as a function of monoenergetic x-ray energy. These spectra, in their low-frequency limits, were seen to depend linearly on x-ray energy. The Wiener spectra also decreased in magnitude as the incident x-ray energy exceeded that of the screen phosphor K absorption edge. The agreement of these experimental findings with simple theoretical predictions is discussed.

Models, Theoretical↗

PubMed1981

Effect of crossover exposure on radiographic image quality of screen-film systems.

Modulation transfer functions (MTFs), Wiener spectra, and H & D curves of conventional and anticrossover screen-film systems were measured in an evaluation of the potential clinical use of anticrossover systems. The relative speed of anticrossover (3M) systems was reduced by 40%, compared with conventional (Kodak OG) systems. The anticrossover system improved both resolution and noise properties of screen-film systems (55% improvement of MTF at 2 cycles/mm and 40% reduction of radiographic mottle at low spatial frequencies) in a way that a combination of slow, sharp screens with conventional film may not accomplish, even when the speeds of different systems are matched. Detail visibility, in phantom images and radiographs of the hand, was considerably enhanced with anticrossover film compared with that of conventional film.

Humans↗

PubMedDate not supplied

Validity of computer simulation of blood vessel imaging in angiography.

The computer-simulation technique for imaging of vessels in angiography is being recognized as a useful tool for the study of the effect of physical parameters on the vessel image. We have examined the validity of this technique by comparing the computed image distribution directly with experimental results. The imput x-ray pattern of a blood vessel phantom, the LSF of the focal spot, and the LSF of a screen-film system were measured. From the data obtained, the final image distributions of the phantom were computed and compared with the experimentally determined distributions. A high-precision device for the alignment and positioning of the vessel phantom was employed in the experiments. Results indicate that computer simulation can correctly predict the image distribution of the blood vessel phantom.

Angiography↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 4. Effect of unsharp masking on the detectability of simple patterns.

The signal-to-noise ratios (SNRs) of simple radiologic patterns processed by the unsharp-masking technique were calculated on the basis of a statistical decision theory model that includes both the observer's visual transfer function and a noise component internal to his eye-brain system. For a variety of processing parameters, the contrast-detail diagrams predicted from this SNR agreed qualitatively with experimental results obtained in observer performance studies. Unsharp masking with a large mask and a large weighting factor improved the detection of square objects to a level comparable with that achieved by the overall contrast enhancement technique using a factor of 4. However, unsharp masking with a small mask and a large weighting factor can substantially degrade the detectability of these objects. The potential practical utility of the unsharp-masking technique is discussed.

Humans↗