Screen Film Radiographic Technique Chapter 13 Flashcards
Prime Exposure Factors
kVp
mA
Exposure Time
Source to Image Distance (SID)
kVp
Controls image contrast Primary control of quality Controls energy and beam penetrability Higher quality means higher energy Greater kVp=more Compton=more scatter=reduced contrast. ie....Chest exam, high kVp, low contrast
mA
Controls image density or brightness
controls number of x rays that are produced
if time does not change, changing the mA will change quantity of x rays proportionately. Double exposure=Double Patient Dose
No change in energy, just quantity
x ray systems are often identified by their maximum mA
Exposure Time
Reducing time reduces motion blur
mAs= mA x time (seconds)
Time is decreased, mA must be increased to retain the same OD/brightness
Falling Load Generators
This type of generator only allows the tech to select mAs
not mA and time separate
Highest mA and shortest time is automatically used for each exposure
Distance
effects exposure and intensity
no effect on quality of radiation
Standard SIDs are used to ensure continuity 40, 44, 72 inches
More SID means:
Less magnification and blur, increased spatial resolution
More mAs must be used with a longer SID
Direct Square Law
Allows rad tech to calculate new mAs after a change in SID
New mAs/old mAs= (SID) suared new SID/ (SID) squared old SID
Direct not inverse relationship
Half the distance= 1/4 mAs
Double the distance= 4x’s the mAs
Imaging system characteristics
Focal spot size
Filtration
High voltage generation
Focal spot size
Tubes have Large and Small Focal spots
Large focal spots are used for general radiography, thick dense objects such as abdomen and pelvis.
Small focal spots are used for detail radiography
Large focal spots
Produce more mAs
shorter time can be used
Small focal spots
produce better record detail
High mAs is not needed
Filtration
Inherent
Added
Compensating
Filtration increases beam quality
Filtration works like kVp: both increase beam quality
Result:
increased scatter
Decreased image contrast
Inherent filtration
2.5 mm Al equivalent is required
Metal or glass envelope provides 0.5 mm Al equivalent
Collimator provides normally 1.0 mm Al equivalent
Another 1 mm Al equivalent is added to meet the minimum requirement
Added filtration
Normally not necessary to change filtration of an imaging system, however, some tubes are equipped to do so.
Higher filtration can or would be used for exams with High Subject Contrast such as extremities, joint, chest
Adding filtration decreases patient dose
Compensating filters
Mounted on the collimator for specific exams, slides in or is magnetic
Wedge shaped filters are useful for spine studies, cross table hips, leg length studies
High voltage generation
x ray quantity and quality are greatly influenced by the type of generator
Single phase, three phase, and high frequency
Half and full wave rectified generators (Single phase)
Only emit x rays half the time
Half wave generators produce the SAME quality as full wave
BUT
half wave produce HALF the quantity of x rays.
Full wave
has NO dead time
Three phase and high frequency generators
produce higher quantity and quality
Most units today are high frequency
Technique selection factors
Patient factors
Image quality factors
Exposure technique factors
Patient factors include
habitus
body thickness
body composition
pathology
Sthenic- Average, strong, active
Hyposthenic- thin but healthy
Hypersthenic- overweight, large frame
Asthenic-small, frail
Body thickness
Thicker patients require more x radiation to penetrate them
Thickness should be measured with calipers
Vary kVp or mAs according to patient thickness
Body Composition
Same thickness doesn’t equal same technique
Mass density of the part must be estimated
Soft tissue radiography uses low kVp and high mAs, because they are low subject contrast.
Extremities use low kVp because they are small
Chests use high kVp, low mAs, they are high contrast, however our technique gives us a low and long scale of contrast.
Pathology
Destructive pathology causes tissue to be more radiolucent
Constructive pathology causes tissue to be more radiopaque
Radiolucent (destructive) pathology
Active tuberculosis Atrophy Bowel obstruction Cancer Degenerative arthritis Emphysema Osteoporosis Pneumothorax
Radiopaque (constructive) pathology
Aortic aneurysm Ascites Atelectasis Cirrhosis Hypertrophy Metastases Pleural effusion Pneumonia Sclerosis
Image Quality Factors
Optical density
Contrast
Detail
Distortion
Optical density/brightness
Degree of blackening of the finished radiograph
Black= OD of 3 or greater
Clear= OD of 0.2 or less
High OD caused by overexposure
Low OD caused by underexposure
What are the two major factors controlling OD?
mAs and SID
SID is usually fixed, making mAs the primary controlling factor of OD
How much of a change is mAs is necessary to see a visible change in OD?
30% change is mAs is required to produce a visible change.
What is the normal rule to adjust mAs?
Usually mAs is halved or doubled.
If a significant change in mAs is NOT required,
repeat examination is probably not required.
Does kVp change OD?
Yes kVp also changes OD, however, it also effects contrast, that’s why mAs is normally used if OD is the ONLY factor that needs to be adjusted.
What is the rule in changing kVp to produce a visible change in OD?
A 4% change in kVp is needed to produce a visible change in OD.
What is the 15% rule?
Changing the kVp 15% is equal to halving or doubling the mAs for changing OD.
Recorded Detail= Spatial Resolution
Spatial Resolution is the ability to image small objects that have high contrast.
Visibility of Detail= Contrast Resolution
Contrast Resolution is the ability to distinguish between and to image similar tissue.
Contrast
The degree of difference between light and dark areas of a radiograph.
What is the major factor in controlling radiographic contrast?
kVp is the major factor.
What is the function of contrast?
to make anatomy more visible
Low Contrast=Long Scale
High kVp
More shades of grey
High Contrast=Short Scale
Low kVp
Black and white
What is the relationship between contrast and kVp?
The relationship between kVp and contrast is inverse.
How is kVp adjusted on an image?
If an image is too low contrast kVp must be lowered 15% (but to achieve the same OD, mAs must be raised/doubled)
If an image is too high contrast, kVp must be raised 15% ( but to achieve the same OD, mAs must be halved)
Detail
Sharpness of appearance of small structures on the radiograph.
What is image detail?
Image detail is evaluated by recorded detail and the visibility of image detail.
Recorded detail/Sharpness of detail
Best measured by spatial resolution
How is spatial resolution visible?
structural lines and borders, amount of blur in the image.
How is spatial resolution controlled?
Spatial resolution is controlled by:
focal spot size
SID
OID
Also effected by motion and blur from the IR
How is the best detail achieved?
Smallest focal spot
Longest SID
Lowest OID
Visibility of detail
Best measured by contrast resolution
Ability to see detail on the radiograph
Affected by Fog, and Scatter
Distortion/ Magnification
Misrepresentation of an objects size or shape
Poor part or tube alignment can cause elongation or foreshortening.
How is distortion/magnification controlled?
Primarily by patient positioning.
Exposure technique charts
Variable kVp
Fixed kVp
High kVp
Automatic Exposure/AEC
Provides consistency for patients, from one technologist or room to another.
Variable kVp chart
mAs remains constant and kVp is varied according to part and thickness. Provides shorter contrast scale results in higher patient dose results in lower exposure latitude kVp is varied 2 kVp per cm
Fixed kVp chart
kVp is set for each body part, mAs value is changed for each body part
provides lower patient dose
provides greater exposure latitude
High kVp chart
Usually greater than 100 kVp
Used for chest and barium studies
Generally low contrast for chest, high for abdomens
provides lower patient dose
Automatic Exposure Control/ AEC
Radiation intensity is measured with photocell or ionization chamber between patient and IR.
Technologist must position properly and select appropriate cells
Technologist must select OD and backup time
APR- Anatomically Programmed Radiography
APR uses microprocessor to select kVp, mAs, backup time, OD
