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42 Cards in this Set
- Front
- Back
X-ray beam Quantity Factors |
1. mA 2. Exposure time 3. mAs* 4. kVp * 5. SID* 6. Filtration* * indicates prime factor |
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X-ray beam quality factors |
1. kVp 2. Filtration |
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4 prime factors of radiographic exposure: |
1. Milliamperage (mA) 2. Exposure time 3. kVp 4. SID (source image receptor distance) |
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Exposure |
A broad term used to describe the x-rays that the patient is exposed to, the amount of xrays in the primary beam, and also the amount of xrays that reach the image receptor. |
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Miliamperage |
1. Controls radiographic density 2. Controls quantity of xrays produced (rate of exposure is photons produced per second) 3. Controlled by adjusting the filament heat. 4. Quantity of exposure is directly proportional to mA. |
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Exposure Time |
1. Controls radiographic density. 2. Controls quantity of x-rays produced. 3. Controlled by adjusting the timer in x-ray circuit. 4. Controls duration of exposure. 5. Quantity of exposure is directly proportional to exposure time. |
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Primary controller of radiographic density |
mAs |
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Primary controller of the penetrating of xrays |
kVp |
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Kilovoltspeak kVp |
1. Controls radiographic contrast. 2. Controls xray penetrating 3. Controls the quantity AND quality of the x-ray beam. 4. Increased kVp results in increased quantity of photons. 5. Increased kVp results in increased penetration of the body part. |
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Primary controller of radiographic contrast |
kVp |
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Source - image receptor distance (SID) |
1. Affects the density and intensity of the x-ray beam. 2. Quantity of the exposure is inversely proportional to the square of the distance. (More distance = less exposure). 3. Each dimension of the radiation field is proportional to the square of the SID. Therefore, the field area is proportional to the square of the SID and the radiation intensity is inversely proportional to the square of the SID. (Size of field bigger as distance increases, intensity lower as distance increases). |
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Inverse square law |
Original intensity ÷ new intensity = new distance squared ÷ original distance squared
***intensity or quantity of xrays is reduced 4X when SID doubles and is increased 4X when SID is halved. |
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Common SID |
40 inches for everything except lateral and oblique cervical and chest which are 72 inches |
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4 primary factors that directly affect how the xray image looks: |
1. density 2. Contrast 3. Distortion 4. Spatial resolution **density and contrast are considered photographic properties. **distortion and recorded detail are considered geometric properties |
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Density |
A photographic property that refers to the overall blackness or darkness of the radiographic image. Density affects the visibility of detail. |
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Overexposed |
An image that is too dark. |
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Underexposed |
An image that is too light. (Not enough mAs). |
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Primary control for varying density: |
mAs |
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Tissue density |
The mass density or atomic number of the body part being xrayed. Lower density body parts are darker (fat) and higher density body parts appear lighter (bone) |
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Radiographic Density vs Tissue Density |
Inversely related, more dense tissues will appear lighter on the image. |
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Brightness |
The term used in digital imaging in place of density. |
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Contrast |
A photographic property defined as the difference in radiographic density between adjacent portions of the image. Too much contrast makes the image too black and white (short scale). Too little contrast creates a flat grey appearance making it difficult to differentiate. |
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Visibility of detail is controlled by |
Density and contrast |
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Optimal Contrast |
Varies depending on the body part. Contrast is primarily controlled by the kVp. |
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Subject Contrast |
The range of differences in the intensity of the xray beam after it has been attenuated by the patient. It is affected by kVp and tissue density. * long scales of contrast (high kVp 100 to 120) are desired for structures with high subject Contrast like the chest **short scales (low kVp 75 to 90)of contrast are desirable when the subject Contrast is low (ie the abdomen with many structures of similar density) |
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Contrast is directly influenced by: |
1. Subject Contrast 2. Fog (decreases contrast) 3. Collimation (too wide increases scatter producing fog and decreasing contrast). |
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Radiographic Distortion |
A geometric property referring to differences between the actual subject and it's radiographic image. Can be either size distortion or shape distortion. |
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Size distortion |
When the part is magnified as a result of the relationship between the SID (source - image receptor distance) and the OID (object-image receptor distance).
* controlled by positioning the body part as close to the IR as possible and using the longest SID practical. * Larger SID'S are needed for greater OID's |
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Radiographic images can never be smaller than the actual body size |
True because the body part is always slightly above the IR and magnified slightly. |
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Shape Distortion |
The result of unequal magnification.
The least shape distortion occurs when the plane of the subject is parallel to the plane of the IR. Only the central ray is truly perpindicular so the least distortion is at the center of the image. |
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Foreshortening |
Projects the body part so it appears shorter than it really is. This usually occurs when the body part is not correctly aligned. |
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Elongation |
Projects the object so it appears longer than it really is. This distortion occurs when either the IR or the xray tube is not aligned with the body part. |
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Spatial Resolution |
Previously called recorded detail is a geometric property that refers to the sharpness of the image. Also sometimes called: resolution, sharpness, definition, or detail. In Optimum resolution, the edge sharpness of structures in the image are crisp and accurately rendered. Poor resolution tends to appear fuzzy. |
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Factors that affect spatial resolution: |
1. Patient motion 2. OID 3. SID 4. Focal spot size *the last 3 are geometric factors |
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Factors Affecting Size Distortion |
1. OID 2. SID |
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Factors Affecting Shape Distortion |
1. Alignment of a. Central ray b. Body part c. Image receptor And 2. CR angular ion A. Direction B. Degree |
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Umbra |
The actual anatomic area, body part, or structure shown in the radiographic image. |
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Penumbra |
Describes the "unsharp edges" of the umbra, or body part. Also known as "blur" or "geometric unsharpness". The goal is to reduce the penumbra as much as possible |
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Effects of Image Geometry on Recorded Detail |
Decreased OID = Decreased penumbra = greater recorded detail. Increased SID = decreased penumbra = greater recorded detail. Smaller focal spot have smaller penumbra and greater recorded detail. Magnification results in image unsharpness. |
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Quantum mottle |
A grainy or mottled image appearance caused by insufficient photons from a kVp or mAs that was set too low. |
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Ch 7 summary |
Part 1 |
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Ch 7 summary 2 |
Part 2 |