Unit 4 Flashcards
Define attenuation
Reduction in total number of X-ray photons after passing through matter
Amount of attention is determined by:
Amount of material irradiated
Type of material irradiated (density of part and atomic number)
Lowest attenuation to highest:
Air
fat
Muscle
Bone
Subject density
Image receptor exposure is altered by changes in the amount or type of tissue in the patient
Atomic number and thickness
Subject contrast
Image contrast is affected by the degree of differential absorption resulting from differing absorption characteristics of body tissues
Abdomen has what contrast
Low
Chest has what contrast
High
Subject detail
Recorded detail is dependent on a structured position within the body and placement in relationship to film
Close to IR to reduce penumbra
Subject distortion
May not be accurately represented on a radiograph
Additive
Increase attenuation
5-15% increase in kV
Destructive
Decrease attenuation
25-50% decrease in mAs
Tumors
Additive
Pneumonia
Additive
Ascites
Additive
Anorexia nervosa
Destructive
Emphysema
Destructive
Pneumothorax
Destructive
Osteoporosis
Destructive
Star pattern
Metal template with star pattern
Easy to use, most practical, requires low exposure
Wisconsin focal spot test tool
Metal template with vertical and horizontal line pairs
Good for ongoing QC check to see if any drastic change in size
Pinhole camera
Pin sized hole in metal plate
Difficult to use and requires excessive exposure
Slit camera
Standard tool used to measure effective spot size
Slit cut in metal plate
Advantages of magnification radiography
Scatter clean up- no g rid
High kV utilization
Disadvantages of magnification radiography
Increased patient dose
Decreased FOV
Decreased resolution
For best consistency in radiographs
Measurement of part
Technique chart
Establishing a technique chart
Phantom images Select optimal phantom image (what radiologist will read) Extrapolate technique chart Phantom testing for different sized pts. Clinical trials Clinical fine tuning Ongoing fine tuning
Advantages of Fixed kV
Decrease patient dose Uniform contrast Wider latitude Low tube heating Extended tube life Decrease time/ less motion
Disadvantages of fixed kV
Use high kV low contrast more scatter
Small increments in mAs may not be possible
Variable kV systems
KV varies according to body part thickness
mAs value specified for each body part
Advantages of variable kV
Permits small increments changes
Higher contrast images less scatter
Disadvantages of variable kV
Varying contrast
High patient dose
Increased tube wear
Problems with phototiming
Poor positioning Pathology Combination Minimum response time Back up time must be set
