Principles of Radiographic Imaging - Carlton
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| Negative effects of scatter radiation | Not part of useful beam, unwanted density on the radiograph, overall graying effect (fog), contributes to patient and operator dose
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| Factors contributing to Compton scatter | High kVp values, tissue volume, field size, and patient thickness
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| Increase kVp | Decreases patient dose (less photoelectric interactions, more Comptons), longer scale of contrast, gray haze of entire image (scatter_
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| Decrease in kVp | Decreased transmission through patient, increased photoelectric absorption, decreased Compton scatter
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| Decrease in kVp | Increases patient dose, increased photoelectric absorption, to maintain image density/exposure a decrease in kVp is accompanied by an increase in mAs
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| Decrease in kVp | Shorter scale of contrast, fewer grays
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| Field Size = Collimation | increased field size increases the volume of tissue irradiated, resulting in increased scatter, longer scale of contrast in image
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| Field Size | Use the smallest field size possible, decreases scatter production
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| The Trade Off | By decreasing the field size fewer photons reach the IR so the exposure is decreased, you must increase mAs to maintain exposure
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| Patient Thickness | Thicker body parts produce more scatter, more atoms, more photon interactions, resulting in an increase scatter production
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| Decreasing Part Thickness | Compression devices can be used to improve spatial resolution and contrast. Results in lower patient dose, brings tissue closer to field, mammography primarily
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| Beam Restricting Devices | 1. Reduce patient dose 2. Limits scatter produced 3. Enhances contrast 4. Visibility of detail
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| Variable Aperture Collimator | Function-reduces field size and controls shape of field (automatically)
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| Upper Stage | (1st) reduces off focus radiation
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| Lower Stage | (2nd) decreases penumbra
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| Positive Beam Limitating Device | PBL- Automatically collimates when a cassette is placed in the bucky
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| Beam Filtration | Removes low energy radiation, placed between tube part and collimator, above 70 kVp--- 2.5mm Al required (federal regulation)
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