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RAD141-Chap 2a

RAD141 - Chap 2a - Image Quality

What are the 3 exposure factors for film-screen imaging? kilovoltage (kV), milliamperage (mA), and exposure time (ms)
What is another term for exposure factors? technique factors
What does the kV setting control? the energy (penetrating power) of the x-ray beam
What does the mA setting control? the qty or # of x-rays produced
What does the ms setting control the duration of the exposure in milliseconds
What are the 4 quality factors? density, contrast, resolution, and distortion
What is density? the amount of "blackness" on the processed film image
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
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
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)
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
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
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
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
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)
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
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
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
What is scatter radiation? radiation that has been changed in direction and intensity as a result of interaction with patient tissue
How can scatter be reduced? through close collimation of the x-ray field and use of a grid
When should grids be used? for body parts over 10 cm
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
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
What are other terms for resolution? detail, recorded detail, image sharpness, definition
How is resolution measured? What is a typical measurement? as lines pairs per millimeter (lp/mm); typically 5-6 lp/mm
What is a lack of visible sharpness or resolution called? blur or unsharpness
What factors control resolution? geometric (focal spot size, SID, OID), film-screen system, and motion
What is the relationship of focal spot size on resolution? small focal spot -> less geometric unsharpness (greater resolution) -> less penumbra
What is penumbra? unsharp edges of the projected image; some penumbra is always present, even with use of the small focal spot
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
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
How can voluntary & involuntary motion be distinguised on an image? voluntary motion -> generalized blurring of linked structures; involuntary motion -> localized unsharpness (blurring)
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
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
What is the effect of SID on resolution? an increase in SID will increase resolution by decreasing geometric unsharpness
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
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
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
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
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
What are the 4 primary controlling factors of distortion? SID, OID, object image receptor alignment, and central ray alignment/centering
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)
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
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
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
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!)
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
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
Created by: debmurph