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312 midterm review
| Question | Answer |
|---|---|
| What effect does increased kVp have on the speed and energy of the electrons in the x-ray tube? | Increasing the kilovoltage will cause an increase in the speed and energy of the electrons across the x-ray tube; more electrons will meet the target per second resulting in an increase in x-ray production |
| State the inverse square law | The inverse square law states that the intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance |
| What is the relationship between distance and density/IR exposure | X-ray exposure is inversely proportional to the square of the distance. As a result, if distance is increased, film density and image receptor exposure will decrease |
| Why does beam restriction reduce scatter radiation production? | Proper beam restriction will keep the total amount of tissue irradiated to a minimum. As the beam is restricted, fewer primary photons are emitted from the tube and collimator, and therefore fewer scattered photons will be created. |
| What are the two principal factors that affect the amount of scatter produced? | The principal factors that affect the amount of scatter produced are kilovoltage and the irradiated material |
| How does the atomic number of a material affect the amount of scatter produced? | Higher-atomic-number materials have a greater number of electrons within each atom. Photons have a greater chance of striking an electron and interacting with these materials, thereby creating more scatter. |
| How does a collimator restrict the primary beam field size? | A collimator has sets of lead shutters at right angles to one another that move in opposing pairs. Each set moves symmetrically from the center of the field and functions to restrict the beam. |
| What is PBL? | Positive beam limitation devices are collimators with automatic controls. When an IR is placed in the bucky tray; sensing devices determine size of IR & sensing devices activate an electric motor that drives the collimator lead shutters into position. |
| How does beam restriction affect patient dose? | As the beam is restricted, fewer primary photons are emitted from the tube and collimator, therefore fewer scattered photons will be created. The decrease in the number of primary and scattered photons will result in a decrease in the dose to the patient. |
| How does tissue density affect attenuation? | The density of the absorbing material describes how tightly the atoms of a given substance are packed together. The denser the absorbing material, the greater will be the attenuation. |
| Why does a grid improve contrast? | A grid is a device consisting of a series of lead strips that is used to improve the contrast of the radiographic image. It does this by absorbing scatter radiation before it can reach the image receptor. |
| As a general rule, when should a grid be used? | As a general rule of thumb, a grid is employed when body part thickness exceeds 10 cm, tissue opacity is primarily soft tissue and bone, and when kVp is above 60. |
| How is a grid constructed? | A grid is a thin, flat, rectangular device that is made by placing a series of radiopaque lead strips side by side and separating the strips by an interspace material, which is radiolucent. |
| Define grid ratio | Grid ratio is defined as the ratio of the height of the lead strips to the distance between the strips |
| What type of grid pattern has lead strips running in only one direction? | Grids with lead strips running in only one direction are called linear grids |
| How is a focused grid designed? | Focused grids are designed so that the central grid strips are parallel and as strips move away from the central axis they become more inclined. The focused design will result in a grid with lead strips designed to match the divergence of the x-ray beam. |
| As the ability of a grid to clean up scatter increases, what is the effect on patient dose and image receptor exposure? | The more efficient a grid is at absorbing scatter, the less exposure will be received by the image receptor and requires compensation. This is generally done with increasing mAs, which in turn results in a greater patient dose. |
| How does an off-level grid error occur? | An off-level grid error occurs when the tube is angled across the long axis of the grid strips. This can be the result of improper tube or grid positioning |
| How does the air-gap technique improve contrast? | The air-gap technique involves placing the patient at a greater OID, thus creating an air gap between the patient and IR. Moving the patient away from the image receptor, reduces scatter reaching IR, resulting in improved contrast. |
| How is digital radiography spatial resolution controlled? | Spatial resolution is increased with smaller pixel sizes. Pixel size is dependent upon the sampling frequency, which is expressed in pixels/mm. |
| Name as many digital radiography artifacts as you know | CR artifacts include such problems as scratches, light spots, white lines, fogging, quantum mottle, algorithm artifacts, histogram analysis errors, nonparallel collimation, and poor grid alignment. |
| How are density/IR exposure adjustments made for changes in kilovoltage? | The 15 percent rule is used as a guide to maintain the same IR exposure: a15 percent increase in kilovoltage causes a doubling of exposure to the image receptor, and a 15 percent decrease in kilovoltage causes a halving of exposure to the image receptor. |
| How do variations in the anatomical part affect density/IR exposure? | There is an inverse relationship between tissue thickness/type and density/IR exposure. In other words, as tissue thickness, average atomic number of the tissue, and/or tissue density increases, film density and IR exposure decreases. |
| What are the relationships to density/IR exposure of grid ratio, frequency, interspace material, and grid use? | Grids with high ratios, grids with low frequency, and grids with dense interspace material; moving grids; and improperly used grids (incorrect focal distance, etc.) all reduce image receptor exposure and film density. |
| What is the controlling factor of subject contrast and how does it affect it? | Kilovoltage peak is the primary controller of subject contrast. As kVp increases, a wider range of photon energies is created, which leads to a wider range of densities on the film. This results in an overall lower contrast. |
| How do variations in the anatomical part affect contrast? | Increasing the amount of irradiated tissue, the atomic number, or the tissue density will result in a decrease in contrast. |
| What is the effect of a grid on contrast? | Grids improve contrast by removing scatter radiation before it reaches the image |
| What is recorded detail? | Recorded detail is the degree of geometric sharpness or accuracy of the structural lines actually recorded in the image |
| How is resolution measured? | Resolution is measured by counting line pairs per millimeter, a resolution tool is composed of pairs of lines a set distance from one another. The point at which the viewer can discern the closest pair of lines from each other represents the lp/mm |
| How do the SID and the OID affect recorded detail? | Recorded detail is improved when SID increases and degraded when it decreases. Recorded detail is improved when OID decreases and degraded whenever it increases. |
| What is the relationship between focal spot size and recorded detail? | As the focal spot decreases in size, recorded detail increases, and vice versa |
| What is the difference between umbra and penumbra? | The umbra is the distinctly sharp area of a shadow. The penumbra is the imperfect, unsharp shadow surrounding the umbra. It is also referred to as the edge gradient. |
| What are the methods that can be used to reduce the possibility of motion? | The best method of reducing motion is patient communication. Reducing exposure time (with a corresponding increase in mA to maintain sufficient mAs and density) and immobilization will also help to eliminate motion. |
| What is the difference between size and shape distortion? | Size distortion is the misrepresentation of the size of an object and can only be magnification in conventional radiography. Shape distortion is the misrepresentation of the actual shape of the structure being examined by unequal magnification. |
| How do the SID and the OID affect size distortion? | The greater the SID and the smaller the OID, the less the size distortion |
| What is the magnification factor formula? | M=SID/SOD |
| What is the difference between elongation and foreshortening? | Elongation projects the object so it appears to be longer than it really is, whereas foreshortening projects it shorter |
| How does the alignment of the anatomical part affect shape distortion? | The long axis of the anatomical part, or object, is intended to be positioned perpendicular to the central ray and parallel to the image receptor. When this does not occur, distortion may occur. |
| How do the direction and degree of angulation affect shape distortion? | Angulation refers to the direction & degree the tube is moved from its normal position perpendicular to the IR. The angulation of the tube is designed to cause a controlled or expected amount of shape distortion to avoid superimposition. |
| How can shape distortion be used to advantage? | Shape distortion is used primarily to avoid superimposition of anatomical structures |
| What is the basic principle of a fixed kVp exposure system? | In a fixed kilovoltage system, the kVp is held constant for a given range of subject densities and contrasts while the mAs is varied to achieve an appropriate image receptor exposure. |
| Shape distortion in a projected image is controlled by: | alignment |
| Calculating 400 mA and 0.05 seconds will provide a result of _____mAs | 20mAs |
| What is the primary controlling factor for the quantity or total amount of x-rays delivered from the x-ray tube? | mAs |
| An x-ray beam with ______ average energy will be capable of penetrating through more different types of tissue | higher |
| Using a higher ______ increases the quality and penetrating power of the x-ray beam | kVp |
| As ______ is increased, a greater number of electrons are driven across to the anode with greater force: | kilovoltage |
| If a duration of 0.05 seconds was selected for a particular exposure, what milliamperage would be necessary to produce 30mAs? | 600 |
| What are the 2 primary factors that control the degree of size distortion? | SID and OID |
| Types of shape distortion include: | elongation and foreshortening |
| What is the radiographic contrast between two adjacent details measuring 3.2 and 1.8? | 1.78 |
| With an SID of 40 inches, and an OID of 10 inches, what is the factor of magnification? | 1.33 times |
| In an image if a measurement changes equally in both length and width we know that ______ has occurred | size distortion |
| The smaller the collimated field size, the _____ the subject contrast present | higher |
| In general, the use of a grid is needed for body parts larger than _____ | 10-13 cm |
| There are 2 primary variables that control the amount of tissue exposed to x-rays, the size of the patient and the: | size of the x-ray field |
| The 3 primary causes of scatter radiation include: large body part thicknesses of soft tissue, high levels of kVp, and: | large field sizes |
| Grid ____ is the number of lead strips counted per inch when scanning transversely across the grid | frequency |
| The ______ the grid ratio, the more severe grid cut-off from misalignment of the IR/part/tube appears on the image | higher |
| What is the primary method for preventing scatter radiation from being produced? | collimation |
| What is the critical factor in a grid's efficiency? | grid ratio |
| The purpose of a grid is to restore _____ in the remnant x-ray beam signal | subject contrast |
Created by:
lrkelley