Screen Film Radiographic Technique Chapter 13 Flashcards by Kathleen Lindsay

Prime Exposure Factors

kVp
mA
Exposure Time
Source to Image Distance (SID)

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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

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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

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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

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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

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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

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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

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Imaging system characteristics

Focal spot size
Filtration
High voltage generation

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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

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Large focal spots

Produce more mAs

shorter time can be used

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Small focal spots

produce better record detail

High mAs is not needed

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Filtration

Inherent
Added
Compensating

Filtration increases beam quality
Filtration works like kVp: both increase beam quality

Result:
increased scatter
Decreased image contrast

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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

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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

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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

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High voltage generation

x ray quantity and quality are greatly influenced by the type of generator
Single phase, three phase, and high frequency

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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.

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Full wave

has NO dead time

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Three phase and high frequency generators

produce higher quantity and quality

Most units today are high frequency

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Technique selection factors

Patient factors
Image quality factors
Exposure technique factors

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Patient factors include

habitus
body thickness
body composition
pathology

Sthenic- Average, strong, active
Hyposthenic- thin but healthy
Hypersthenic- overweight, large frame
Asthenic-small, frail

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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

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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