Brent S. Baxter

Factors Affecting the Measurement of Size and CT Number in Computed Tomography

Computed tomography scanners may be used to make anatomic size measurement; however, manipulating the viewer controls (especially the window center) can have a significant influence on the apparent size of structures in the image. A special phantom was constructed to study the effect of window center adjustments on the apparent size of lone cylindrical objects (no variation within slice thickness) and also for spherical objects (maximum variation within slice thickness). Using this phantom, the authors were able to identify several commonly encountered situations in which both the apparent size and CT numbers may be seriously in error. It was found that for cylindrical objects larger than about one transverse resolution element and aligned with the scanner axis, CT numbers may be determined accurately; however, the apparent diameter changes by several millimeters as the window center setting is changed. CT numbers for spheres are inaccurate when the diameter is comparable to or less than the slice thickness and, as a result, diameters of spheres cannot be measured accurately with fixed window center settings. Diameters of spheres can be measured accurately (+/- 1 mm) if the sphere is centered in the slice and the full width at half maximum of the CT number profile is used.

Photoreceptor contributions to contrast sensitivity: Applications in radiological diagnosis

Electrophysiological and psychophysical observations are described which appear to have important implications in diagnostic image interpretation. While the interpretation of a complex radiographic image depends upon processing by higher-order neural centers, this interpretation process is influenced by the sensitivity of the visual system. Attention is given to two mechanisms which operate at the input stages of the visual system and which directly affect visual sensitivity: (1) the effects of background illumination on cone photoreceptor sensitivity, and (2) the image-forming properties of the eye (including small involuntary eye movements). The authors have constructed a model of the preprocessing occurring in the eye, by which light patterns viewed by a subject are transformed into patterns of neural activity of the cone photoreceptors in the subjects retina. The desensitization of cone photoreceptors by steady background illumination was studied using intracellular techniques. Background intensities desensitize both cone photoreceptors and the photopic visual system in a similar manner. This finding is consistent with the hypothesis that photopic visual threshold may be set at the very input to the visual system by the sensitivity of the cone photoreceptors. These two mechanisms appear to account for much of the visual system desensitizations caused by both uniform and stylized nonuniform backgrounds.

The effect of extraneous light on lesion detectability: A demonstration

Ravindra H, Normann RA, Baxter B. The effect of extraneous light on lesion detectability: a demonstration.

PACS Data Base Design

A PACS database must manage three different types of data; structured data in the form of patient/exam identification information, and unstructured data in the form of text and images. Queries based on the content of text documents as well as the content of images must be suported, in addition to those based on standard, well structured keys such as name, age and sex. We model the PACS database as three logically distinct databases, each supporting one of these data types, with mapping structures relating all three. Several design issues which have a bearing, both on these models and on physical implemtations, are discussed. Because image database systems are the least understood at this time, most of the paper focusses on the them. We briefly discuss current trends in structured and text databases, without discussing commercial systems, and then present some current methods of implementing image database systems.

Application of a three-dimensional display in diagnostic imaging.

An autostereoscopic viewing device for tomographic scans that allow s a physician to examine multiple computed tomography sections with each section properly positioned in three dimensions has been constructed and tested. Images produced on the device allow the observer to utilize both motion parallax and stereoscopic depth cues as if viewing a real three-dimensional (3D) object. These .3D images can be very striking because of the ease with which one can form a true impression of depth relationships. We describe operating principles of the viewing device and the appearance of images produced on it. Stereo photographs made from 3D images displayed on the device are included to illustrate potential applications and problems.

An American Association Of Physicists In Medicine (AAPM) Standard Magnetic Tape Format For Digital Image Exchange

This paper will describe the details of a standard magnetic tape format for digital image exchange; The topics which will be covered include: key-value pairs, required keys, comments, pixel location, and an overview of the software needed to utilize this standard. It will be illustrated by showing: How the standard would be used on a simple image; How additional descriptive information may be added to further annotate this image (the electronic lab notebook); How a tape is created, How images are added to an existing tape; and, How an image is selected and extracted for the tape. A separate paper will describe the philosophy for this standard, motivating the features of the standard, while this paper will focus on the information necessary for implementation of software to support its use.

Three-Dimensional Viewing Device For Examining Internal Structure

A new 3-D viewing system will be described which allows a physician to simultaneously examine multiple CT or B-scan ultrasound scans in their proper orientation in all three dimensions. Several persons can view the display at the same time and there is no need for special glasses. Test images and line drawings displayed on a prototype viewing device exhibit both stereoscopic and motion parallax depth cues characteristic of a real three dimensional object. Operating principles of the device will be outlined and a new 3-D figure/ground illusion will be described.

Digital Separation of Primary and Scatter Components of Chest Radiographs

This article describes a technique for digital separation of the primary and scatter components of a radiographic image. The method involves mathematical modeling of the process whereby an antiscatter grid reduces scatter patterns in film radiographs. Two superimposable radiographs (one taken with and the other without an intervening antiscatter grid) are applied to the model. Performance characteristics of the grid (primary and scatter transmittance factors) are also determined and used in the model. Radiographs of a humanoid chest phantom are processed. Scatter/primary separation appears to be accurate to within 15%. Film images that are quantitatively faithful to the calculated primary and scatter fields are included.

Characteristics Of A Protocol For Exchanging Digital Image Information

This paper will discuss requirements for a protocol for exchanging digital image information between users of possibly dissimilar equipment. The need for simplicity, flexibility and ease of data access will be outlined and several examples will be presented which illustrate how practical issues such as data block sizes and hardware compatibility impact these requirements. An additional requirement for users who must exchange physical media, perhaps on a one time basis through the mail, is that the data format contain sufficient descriptive information so the data may be utilized without reference to a published document. A companion paper describes one implementation of a standard magnetic tape format which meets these requirements.

Can We Predict Visual Performance Using a Model of the Human Eye

This paper describes an approach for analyzing radiographic tumor detection which is based on the light adaptation properties of photoreceptor cells (the cones) in the retina. Factors which affect the sensitivity of these cells will be discussed, changes in sensitivity due to light adaptation will be demonstrated and preliminary results from experiments dealing with the effect of age and radiologic training will be presented. A computer model of these processes has been constructed that appears to be useful for predicting the sensitivity of the visual system for detecting low contrast patches on a variety of background intensity distributions. Predictions of observer performance based on calculations from this model will provide a means for evaluating the clinical usefulness of image processing/enhancement schemes.