Free Radiology Flashcards about RADT Unit 3
Misalignment of the tube-part-IR relationship results in. 2 and 3 only . Unopened boxes of radiographic film should be stored away from radiation and. 2 and 3. kVp, distance, beam restriction, anatomic part, grid, film/screen combination, there is an inverse relationship; as tissue thickness, average atomic number, misalignment of the tube or film under 40% and static results. The exposure to the background area of the receptor, or film, (shown here as This relationship shows that as the proportion of scattered radiation in the . Because of the presence of the lead strips, grids attenuate part of the primary radiation. . Misalignment of the x-ray tube focal spot with respect to the focal point of the.
What is it called when the x-ray photon intensity is reduced as the photon passes through material? What is the density of an unexposed and processed film?
What device is used to overcome severe variation in patient anatomy or tissue density, providing more uniform radiographic density?
What test is performed to check the correctness of the developing parameters? What is the purpose of the thin layer of lead that is often located behind the rear intensifying screen in an IR? What will result if developer replenishment is inadequate? What can be used to determine the sensitivity of a particular film emulsion?
What is a wire-mesh test performed to evaluate? What accessory is used to measure the thickness of body parts in order to determine optimal selection of exposure factors? What is a focal-spot size of 0. What is the device that is used to give film emulsion a predetermined exposure in order to test its response to processing called? What is the relationship between the height of a grid's lead strips and the distance between them referred to?
What material is film base currently made of? What happens to the image when a slow screen-film system is used with a fast screen-film AEC system? For a scatter factor value of 4, the scatter-primary ratio is 3. The background area exposure is, therefore, composed of one unit of primary and three units of scattered radiation. The object area receives only the three units of scattered radiation. With respect to image contrast, the scatter factor, S, is also the contrast reduction factor.
The figure below shows the general relationship between contrast and scatter factor. The value of the scatter factor is primarily a function of patient thickness, field size, and x-ray beam spectrum as determined by the KV.
In examinations of relatively thick body sections, contrast reduction factors of 5 or 6 are common. Most objects within the body are penetrated to some extent.
Therefore, contrast is reduced by both object penetration and scattered radiation. Since scattered radiation robs an x-ray image of most of its contrast, specific actions must be taken to regain some of the lost contrast.
Several methods can be used to reduce the effect of scattered radiation but none is capable of restoring the full image contrast.
The use of each scatter reduction method usually involves compromises, as we will see below. This is, in turn, determined by the thickness of the patient and the area or field size being exposed.
Increasing the field size increases the total amount of scattered radiation and the value of the scatter contrast-reduction factors.
Therefore, one method of reducing scattered radiation and increasing contrast is to reduce the field size with x-ray beam collimators, cones, or other beam-limiting devices, as illustrated below. This method is limited by the necessity to cover a specific anatomical region. However, in most situations, contrast can be improved by reducing the field size to the smallest practical value. This separation is known as an air gap. Scattered radiation leaving a patient's body is more divergent than the primary x-ray beam.
Therefore, scattered radiation spreads out of the primary beam area. The reduction of scattered radiation in proportion to primary radiation increases with air-gap distance.
Several factors must be considered when using this method of scatter reduction. Patient exposure is increased because of the inverse-square effect. The use of an air gap introduces magnification. Therefore, a larger receptor size is required to obtain the same patient area coverage. If the air gap is obtained by increasing the tube-to-receptor distance, the x-ray equipment must be operated at a higher output to obtain adequate receptor exposure.
Contrast Improvement by Using an Air Gap Also, increasing the separation distance between the patient and the receptor increases focal spot blurring. It is usually necessary to use relatively small focal spots with an air-gap technique. One common use of the air gap is in magnification mammography. Since an air gap is produced by separating the breast from the receptor to produce magnification, it can be used for scatter reduction.
The usual procedure is to remove the grid and rely on the air gap in magnification mammography. The grid is placed between the patient's body and the receptor, as shown below.
It is constructed of alternate strips of an x-ray-absorbing material, such as lead, and a relatively non-absorbing interspace material, such as fiber, carbon, or aluminum. Under normal operating conditions, the grid strips are aligned with the direction of the primary x-ray beam.
In most grids, the interspaces are angled so as to align with a specific point in space. These are designated focused grids. The focal point of the grid should coincide with the focal spot of the x-ray tube, which is the source of the primary radiation.
Free Radiology Flashcards about RADT
In an unfocused grid, the interspaces and strips are parallel and are not aligned with a single point in space. Because the x-ray beam direction is aligned with the grid, much of the primary radiation passes through the interspaces without encountering the lead strips.
Scattered radiation, on the other hand, leaves the patient's body in a direction different from that of the primary beam, as shown in the second figure below. Since scattered radiation is not generally lined up with the grid strips, a large portion of it is absorbed by the grid. The ideal grid would absorb all scattered radiation and allow all primary x-rays to penetrate to the receptor.