RAD141 - Chap 2a - Image Quality
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| What are the 3 exposure factors for film-screen imaging? | kilovoltage (kV), milliamperage (mA), and exposure time (ms)
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| What is another term for exposure factors? | technique factors
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| What does the kV setting control? | the energy (penetrating power) of the x-ray beam
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| What does the mA setting control? | the qty or # of x-rays produced
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| What does the ms setting control | the duration of the exposure in milliseconds
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| What are the 4 quality factors? | density, contrast, resolution, and distortion
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| What is density? | the amount of "blackness" on the processed film image
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| How is density controlled? | primary controlling factor is mAs -> linear relationship; source image receptor distance (SID) -> inverse square relationship; also, kV, part thickness, chemical development time/temp, grid ratio, and film-screen speed
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| How can film image density be adjusted? | for under or overexposure, a min change in mAs of 25% to 30% is required to make a visible difference
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| What is the anode heel effect? | the intensity of the radiation emitted from the cathode end of the x-ray tube is greater than that at the anode end because of the ange of the anode (rays must travel through more anode material before exiting and are attenuated more)
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| How can the anode heel effect be applied? | patient s/b positioned so that the thicker portion of the part is at the cathode end; thinner part at the anode
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| What are compensating filters? | sometimes used to filter out a portion of the primary beam toward the tin (less dense) part of the body; ex - wedge filters
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| Describe the relationship of density to exposure | adequate density must be visible to accurately demonstrate structures being radiographed; too little density -> underexposed; too much density -> overexposed
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| What is contrast? | the difference in density on adjacent areas of a radiographic image; greater the difference -> higher contrast; lower the difference in densities -> lower contrast
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| What is long- and short-scale contrast? | total range of all optical densities; more shades of gray but less pure black and white is low-contrast, long-scale (desirable for chest x-rays)
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| How is contrast controlled? | primary factor -> kV; higher kV produces less variation in attenuation -> lower contrast; also, scatter radiation -> irradiation of thick body parts produces greater scatter radiation -> decr image contrast
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| What is the effect of kV on density? | higher kV causes more x-ray energy to reach the IR, increasing density; a 15% incr in kV increases film density similar to doubling the mAs
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| What is the relationship between kV and mAs in regards to density? | a 15% incr in kV -> incr film density similar to doubling mAs; as kV is incr, mAs can be significantly decr -> less radiation to the patient
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| What is scatter radiation? | radiation that has been changed in direction and intensity as a result of interaction with patient tissue
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| How can scatter be reduced? | through close collimation of the x-ray field and use of a grid
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| When should grids be used? | for body parts over 10 cm
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| How should kV be determined? | by balancing optimal image contrast and lowest possible patient does; highest kV and lowest mAs that yields sufficient diagnostic information
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| What is resolution? | the recorded sharpness of structures on the image; demonstrated by clarity or sharpness of fine structural lines and borders of tissues or structures on the image
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| What are other terms for resolution? | detail, recorded detail, image sharpness, definition
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| How is resolution measured? What is a typical measurement? | as lines pairs per millimeter (lp/mm); typically 5-6 lp/mm
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| What is a lack of visible sharpness or resolution called? | blur or unsharpness
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| What factors control resolution? | geometric (focal spot size, SID, OID), film-screen system, and motion
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| What is the relationship of focal spot size on resolution? | small focal spot -> less geometric unsharpness (greater resolution) -> less penumbra
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| What is penumbra? | unsharp edges of the projected image; some penumbra is always present, even with use of the small focal spot
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| What is a resolution fctor in film-screen systems? | film-screen speed; the faster the film-screen speed -> shorter the exposure time -> reduces potential for patient motion & reduces dose -> but image is less shar than a slower system
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| What is the effect of motion on resolution? | the greatest deterrent to image sharpness; voluntary and involuntary; voluntary movement can be minimized by controlled breathing and patient immobilization
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| How can voluntary & involuntary motion be distinguised on an image? | voluntary motion -> generalized blurring of linked structures; involuntary motion -> localized unsharpness (blurring)
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| What can the technologist due to reduce motion or the effects of motion? | through explanation of the procedure & breathing instructions; high mA and short exposure time
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| What is the effect of OID on resolution? | the closer the object being radiographed is to the image receptor, the less the magnification and shape distortion and the better the detail or resolution
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| What is the effect of SID on resolution? | an increase in SID will increase resolution by decreasing geometric unsharpness
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| What is distortion? | the misrepresentation of object size or shape as projected onto radiographic recording media; can be size or shape distortion; a degree of magnification and/or distortion always esists as a result of OID & divergence of the x-ray beam
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| What is X-ray beam divergence? | x-rays originate from the focal spot of the x-ray tube & diverges as the travel to the IR
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| How is the field size of the x-ray beam limited? | by a collimator (adjustable lead collimators); the collimator and shutters absorb the x-rays on the periphery controlling the size of the x-ray beam
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| What is the CR? | central ray; the center point of the x-ray beam which strikes the IR 90 degrees to the plane of the IR; theoretically no divergence here -> least amount of distortion
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| Where is divergence greatest? | at the outermost portions of the x-ray beam; further increased when x-ray field is large and SID is short
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| What are the 4 primary controlling factors of distortion? | SID, OID, object image receptor alignment, and central ray alignment/centering
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| What is the effect of SID on distortion? | at a greater SID, less magnification occurs than at a shorter SID (ex - chest radiographs are obtained at a minimum SID of 72 inches rather than the typical 40-48 inches for most other exams)
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| What is the standard SID? | 40 inches; becoming more common to use 44 or 48 inches as the min to improve resolution & decrease distortion; additionally incr SID reduces entrance (skin) dose (by 12-13% when SID incr from 40 to 48 inches) even when necessary incr mAs is considered
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| What is the effect of OID on distortion? | the closer the object being radiographed to the IR, the less the magnification and shape distortion and the better the detail or resolution
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| How does object image receptor alignment effect distortion? | if the object plane is not parallel to the plane of the image receptor, distortion occurs; the greater the angle of inclination of the object or the IR, the greater the amt of distortion
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| Where is improper object alignment most obvious? | at joints or ends of bony structures; when joints are parallel to the IR, joints are open (good!); when joints not parallel -> joints not open (bad!)
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| What is the effect of CR alignment on distortion? | ecause the CR has no divergence, the least possible distortion occurs at the central ray; distortion increases as the angle of divergence increases from thecenter of the x-ray beam to the outer edges
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| What is the CR angle? | normally, CR is aligned perpendicular to IR plane; for certain body parts, a specific CR angle is required -> CR is angled from the vertical in a cephalic or caudad direction to utilize distortion intentionally & not superimpose anatomic structures
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