Mixing errors that result in an incorrect concentration can produce undesirable changes in film sensitivity. Unless the solution is replaced, film sensitivity will gradually decrease. In radiographic film processors, the replenishment of the developer solution is automatic. When a sheet of film enters the processor, it activates a switch that causes fresh solution to be pumped into the development tank. The replenishment rate can be monitored by means of flow meters mounted in the processor.
The appropriate replenishment rate depends on the size of the films being processed. A processor used only for chest films generally requires a higher replenishment rate than one used for smaller films. CONTENTS If the developer solution becomes contaminated with another chemical, such as the fixer solution, abrupt changes in film sensitivity can occur in the form of either an increase or decrease in sensitivity, depending on the type and amount of contamination.
Developer contamination is most likely to occur when the film transport rollers are removed or replaced. It is a gradual process during which more and more film grains are developed, resulting in increased film density. The development process is terminated by removing the film from the developer and placing it in the fixer. To some extent, increasing development time increases film sensitivity, since less exposure is required to produce a specific film density. In most radiographic film processors, the development time is usually fixed and is approximately seconds.
However, there are two exceptions. So-called rapid access film is designed to be processed faster in special processors. Some but not all mammographic films will produce a higher contrast when developed for a longer time in an extended cycle processor. An increase in temperature speeds up the development process and increases film sensitivity because less exposure is required to produce a specific film density. The temperature of the developer is thermostatically controlled in an automatic processor.
Specific processing temperatures are usually specified by the film manufacturers. The spectral sensitivity is a characteristic of film that must be taken into account in selecting film for use with specific intensifying screens and cameras. In general, the film should be most sensitive to the color of the light that is emitted by the intensifying screens, intensifier tubes, cathode ray tubes CRTs , or lasers. A basic silver bromide emulsion has its maximum sensitivity in the ultraviolet and blue regions of the light spectrum.
For many years most intensifying screens contained calcium tungstate, which emits a blue light and is a good match for blue sensitive film. Although calcium tungstate is no longer widely used as a screen material, several contemporary screen materials emit blue light. Several image light sources, including image intensifier tubes, CRTs, and some intensifying screens, emit most of their light in the green portion of the spectrum.
Film used with these devices must, therefore, be sensitive to green light. Silver bromide can be made sensitive to green light by adding sensitizing dyes to the emulsion. Users must be careful not to use the wrong type of film with intensifying screens. If a blue-sensitive film is used with a green-emitting intensifying screen, the combination will have a drastically reduced sensitivity.
Many lasers produce red light. Devices that transfer images to film by means of a laser beam must, therefore, be supplied with a film that is sensitive to red light. Darkrooms in which film is loaded into cassettes and transferred to processors are usually illuminated with a safelight. A safelight emits a color of light the eye can see but that will not expose film.
Although film has a relatively low sensitivity to the light emitted by safelights, film fog can be produced with safelight illumination under certain conditions. The safelight should provide sufficient illumination for darkroom operations but not produce significant exposure to the film being handled.
This can usually be accomplished if certain factors are controlled. These include safelight color, brightness, location, and duration of film exposure. The color of the safelight is controlled by the filter.
The filter must be selected in relationship to the spectral sensitivity of the film being used. An amber-brown safelight provides a relatively high level of working illumination and adequate protection for blue-sensitive film; type 6B filters are used for this application.
However, this type of safelight produces some light that falls within the sensitive range of green-sensitive film. A red safelight is required when working with green-sensitive films. Type GBX filters are used for this purpose. Selecting the appropriate safelight filter does not absolutely protect film because film has some sensitivity to the light emitted by most safelights.
Therefore, the brightness of the safelight bulb size and the distance between the light and film work surfaces must be selected so as to minimize film exposure.
Since exposure is an accumulative effect, handling the film as short a time as possible minimizes exposure. The potential for safelight exposure can be evaluated in a darkroom by placing a piece of film on the work surface, covering most of its area with an opaque object, and then moving the object in successive steps to expose more of the film surface. The time intervals should be selected to produce exposures ranging from a few seconds to several minutes. After the film is processed, the effect of the safelight exposure can be observed.
Film is most sensitive to safelight fogging after the latent image is produced but before it is processed. CONTENTS In radiography it is usually possible to deliver a given exposure to film by using many combinations of radiation intensity exposure rate and exposure time. Since radiation intensity is proportional to x-ray tube MA, this is equivalent to saying that a given exposure in milliampere-seconds can be produced with many combinations of MA and time.
This is known as the law of reciprocity. In effect, it means that it is possible to swap radiation intensity in milliamperes for exposure time and produce the same film exposure. When a film is directly exposed to x-radiation, the reciprocity law holds true. That is, mAs will produce the same film density whether it is exposed at 1, mA and 0. However, when a film is exposed by light, such as from intensifying screens or image intensifiers, the reciprocity law does not hold.
With light exposure, as opposed to direct x-ray interactions, a single silver halide grain must absorb more than one photon before it can be developed and can contribute to image density. This causes the sensitivity of the film to be somewhat dependent on the intensity of the exposing light. This loss of sensitivity varies to some extent from one type of x-ray film to another. The clinical significance is that MAS values that give the correct density with short exposure times might not do so with long exposure times.
Each type of film is designed and manufactured to have specified sensitivity speed and contrast characteristics. The loss of sensitivity can usually be compensated for by increasing exposure but the loss of contrast cannot be recovered.
The contrast of some films might increase with over processing, up to a point, and then decrease. A major problem with over processing is that it increases fog base plus fog density which contributes to a decrease in contrast.
CONTENTS The first step in processing quality control is to set up the correct processing conditions and then verify that the film is being correctly processed. Processing Conditions. A specification of recommended processing conditions temperature, time, type of chemistry, replenishment rates, etc.
Processing Verification. After the recommended processing conditions are established for each type of film, a test should be performed to verify that the film is producing the design sensitivity and contrast characteristics as specified by the manufacturer.
These specifications are usually provided in the form of a film characteristic curve that can be compared to one produced by the processor being evaluated. Variations in processing conditions can produce significant differences in film sensitivity. One objective of a quality control program is to reduce exposure errors that cause either underexposed or overexposed film. Processors should be checked several times each week to detect changes in processing.
This is done by exposing a test film to a fixed amount of light exposure in a sensitometer, running the film through the processor, and then measuring its density with a densitometer. It is not necessary to measure the density of all exposure steps. Only a few exposure steps are selected, as shown in the figure below, to give the information required for processor quality control.
The density values are recorded on a chart see the second figure below so that fluctuations can be easily detected. A Processor Quality Control Chart. This is a measure of the base plus fog density. A low density value is desirable. An increase in the base plus fog density can be caused by over processing a film. CONTENTS A single exposure step that produces a film density of about 1 density unit above the base plus fog value is selected and designated the "speed step.
The density of this step is a general indication of film sensitivity or speed. Abnormal variations can be caused by any of the factors affecting the amount of development. This is the contrast index. This value is recorded on the chart to detect abnormal changes in film contrast produced by processing conditions. If abnormal variations in film density are observed, all possible causes, such as developer temperature, solution replenishment rates, and contamination, should be evaluated.
If more than one processor is used for films from the same imaging device, the level of development by the different processes should be matched. Bending unprocessed film can produce artifacts or "kink marks," which can appear as either dark or light areas in the processed image. Handling film, especially in a dry environment, can produce a build-up of static electricity; the discharge produces dark spots and streaks. Artifacts can be produced during processing by factors such as uneven roller pressure or the accumulation of a substance on the rollers.
This type of artifact is often repeated at intervals corresponding to the circumference of the roller. Online Textbook Table of Contents. The Two Steps in the Formation of a Film Image The General Relationship between Film Density Shades of Gray and Exposure The specific relationship between the shades of gray or density and exposure depends on the characteristics of the film emulsion and the processing conditions.
A Film Processor When a film is inserted into a processor, it is transported by means of a roller system through the chemical developer. Reducer Chemical reduction of the exposed silver bromide grains is the process that converts them into visible metallic silver. Activator The primary function of the activator, typically sodium carbonate, is to soften and swell the emulsion so that the reducers can reach the exposed grains. This is in general agreement with such other estimates and measurements of surface temperatures involved in solid friction that have been attempted.
The effect of temperature on a silver bromide grain has been considered in terms of the Gurney and Mott theory of the latent image. While gross uncertainties exist in the values of the activation energies involved in such a calculation, it at least seems plausible that temperatures of the order of magnitude expected could produce a permanent latent image. Lester I. Zimmerman J. Mather J. Anna Joyce Reardon J. Sheppard and E. Wightman J. You do not have subscription access to this journal.
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Cited by links are available to subscribers only. Figure files are available to subscribers only. Equations are available to subscribers only. Allow All Cookies. This will result in a greater difference in attenuation between the different parts of the subject, leading to higher contrast. A higher kVp will make the x-ray beam more penetrating. Spatial resolution refers to the ability to differentiate small structures. In terms of hardware, the fundamental spatial resolution improves with a smaller focal spot.
The size and quality of the detectors directly affect the spatial resolution. Another way to enhance spatial resolution is to improve the sampling of detector units by deflecting the focal spot on the x-ray tube anode along longitudinal and fan angle direction 2—4. In MRI, spatial resolution is defined by the size of the imaging voxels. Since voxels are three-dimensional rectangular solids, the resolution is frequently different in the three different directions.
The size of the voxel and therefore the resolution depends on matrix size, the field-of-view, and the slice thickness. It features full three- dimensional 3-D capabilities, excellent soft-tissue contrast and high spatial resolution. Furthermore, MRI allows functional, diffusion and perfusion imaging to be performed. To get a useful picture, the amount of signal from the thing being imaged should be greater than the noise.
A higher SNR means a better and more useful image more signal than there is noise …. To improve the SNR:. We show that, without cortical constraints, electrode EEG systems have optimal spatial resolution near 22— 37 cm 3 , while electrode systems have 6— 8 cm 3. These results emphasize the benefits of more electrodes, but also the need for methods of measuring local skull conductivity.
Axial also called longitudinal resolution is the minimum distance that can be differentiated between two reflectors located parallel to the direction of ultrasound beam. Mathematically, it is equal to half the spatial pulse length. Axial resolution is high when the spatial pulse length is short. Factors which improve temporal resolution Investigating the mathematical relationship between the above factors demonstrates that frame rate, and hence temporal resolution is inversely proportional to the number of focal points examined, the number of beamlines in the field, and the depth of field setting 3.
The frequency with which these pulses are emitted determines the maximum Doppler shifts obtainable. When a high PRF is chosen, it is assumed that high-velocity flow is of interest.
It is also possible to adjust by lowering or elevating the baseline of the ultrasound image to reduce aliasing; doing this will adjust the PRF. Aliasing can be remedied by reducing the frequency of the ultrasound or increasing the PRF.
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