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Terms in this set (91)

a. Density - overall darkening of a film in response to light or x-ray photons. In digital world, it is the brightness of the image on a monitor. On a viewbox, darker areas have "greater radiographic density". Judged by the human eye. Proper density is necessary in order to present adequate visibility of detail. Impacted by patient size, mAs, kVp, distance, beam modification (collimation, filtration, grids), film/screen combinations and processing.
i. Milliampere Seconds: the greater amount of x-ray photons generated, the better the image receptor exposure and film density on the radiograph. mAs is the chief controlling factor of exposure and density.
ii. Patient factors - cell composition, relative atomic number, thickness and cell density. Bone is thick and doesn't allow for easy passage of x-ray photons, so its image appears lighter on the image (thus resulting in a decreased radiographic density).
iii. Kilovoltage peak - strength of photons produced - low energy wouldn't pass through dense body tissue. The "penetrating ability" of an x-ray beam. This setting determines the highest energy level or "peak" possible for photons within that beam. As kVp increases, image receptor increases, but not proportionately. It's the 15% rule. Increasing kVp 15% will double the image receptor exposure. Decreasing kVp by 15% will half the image receptor exposure. The kVp and mAs relationship is: when we increase kVp by 15%, half the mAs to get the same exposure. To decrease kVp 15%, double the mAs.
iv. Distance - intensity increases as the distance decreases (flashlight close to wall is more intense than farther away). Inverse relationship. The farther the distance the photons have to travel, the less chance they have to reach the image receptor. Reduced image receptor exposure.
1. Inverse square law: the intensity of a beam of radiation is inversely proportional to the square of the distance from the source. I1/I2 = D22/D12.
2. Exposure maintenance formula: a direct square law - mAs1/mAs2 = d12/d22
v. Beam modification - anything that changes the nature of the radiation beam
1. Primary beam - can be adjusted by changing filtration and beam limitation.
a. Filtration - use of attenuation material - usually aluminum - between the tube and the patient. Removes very-low-energey nondiagnostic photons to decrease patient exposure.
b. Half-value layer - the amount of attenuating material required to reduce the intensity of a beam to one half the original value. Mm Al/Eq.
c. Beam limitation - confining the beam to the area of interest, reducing exposure. Also affects radiographic quality. Reduces scatter radiation which is often responsible for "fog" - an unwanted radiographic density. Subtracting photons from the remnant beam.
i. Diaphragms, cones and cylinders
ii. Aperture collimators - most familiar to radiographers. Adjustable pair of lead shutter leaves to create various sized rectangular fields.
iii. Positive beam limitation - automatic collimation - electronic interlock system that automatically collimates the beam to the size of the image receptor placed in the Bucky.
vi. Grids - devices used to remove as many scattered photons exiting the patient as possible before they reach the film. Thin lead strips interspersed with spacing material. Place between the patient and the IR. Increased lead increases the ability to remove scatter. Increased lead also requires increased exposure factor settings. grid ratio - 5:1 to 16:1. The higher the grid, the less exposure or film density.
vii. Film/screen combinations - spectral matching - matching of a specific color light and radiographic film that is sensitive to the same color. Essential to good radiographic quality. A fast system requires less radiation, but slower systems produce sharper images. Relative speed of 100. Range from 50 to 1200. The higher the speed number, the less radiation required.
viii. Processing errors
1. Underdevelopment
2. Overdevelopment
3. Temperature of chemicals in film emulsion can cause chemical fog or insufficient chemical activity
4. Requires rigorous quality control to ensure consistent results.
- visual difference between adjacent radiographic densities. A comparison of all of the various densities represented on a radiograph. Objects need to be distinguishable from each other. Factors that impact it are: patient factors, kVp, mAs, beam modification, film/screen combinations, contrast media and processing.
i. kVp - control of the penetrating ability of the beam allows us to manipulate contrast. Body parts have optimal kVp levels to allow for contrast. kVp is the chief controlling factor of contrast. Increasing kVp lowers contrast. A higher beam tends to penetrate everything in its path, producing a wider range of gray tones.
1. High contrast - few gray tones
2. Low contrast - many gray tones
ii. Patient factors - tissues attenuate the beam to differing degrees. Abdominal organs are of about the same density, so distinguishing them can be difficult.
iii. Milliampere seconds - Secondary influence on contrast. No incrase in mAs or density can compensate for inadequate penetration.
iv. Beam modification - filtration, collimation, grids - scatter control. Removal of fog removes some gray tones, so contrast will be higher.
v. Film/screen combinations - screens and film are manufactured to produce a particular scale of contrast. Specialty systems include mammographic, chest, and extremity film/screen systems. The faster the system, the higher the contrast.
vi. Contrast media - Attenuates the beam to a different degree than the surrounding tissue. Barium and iodine compounds and air. Structures can be visualized easily. Introduces an additional subject density to the body, so other technical factors like kVp must also be adjusted.
vii. Processing - underdevelopment and overdevelopment change the range of visible densities.
sharpness of borders and structure details on an image (aka sharpness of detail, definition and resolution) - impacted by motion, object unsharpess, focal spot size, SID, OKD and material unsharpness.
i. Motion - most common cause of image unsharpness. Voluntary or involuntary patient motion.
1. Voluntary - use communication - instructions to patient, suspension of patient respiration during exposure, immobilization devices and short exposure times.
2. Involuntary - use shortest exposure time possible.
3. Vibration of tabletop during a table grid exposure - use short exposure times
ii. Object unsharpess - adjust factors we have control over - focal spot size, SID and OID.
iii. Focal spot size - Use a small focal spot size when fine detail is required as what is necessary for small bones. Use a large focal spot for most general radiographic exams.
iv. Source-to-image distance - If object is close to the IR, if beam is close, the penumbra increases. If farther away, the penumbra decreases causing the image to be sharper. The greater the SID, the better the recorded detail. Most common is 40 inches but chest radiography use a 72-inch SID
v. Object-to-image distance - moving the object farther from the IR, penumbra increase and sharpness decreases. The smaller the OID is the better the recorded detail will be. Use knowledge of anatomy and positioning to get the object as close as possible to the IR.
vi. Material unsharpness - the equipment also contributes to unsharpness. Films and screens can affect ability to represent an image accurately. Choose them carefully to provide adequate recorded detail. Faster systems produce greater unsharpness but decrease patient dose.