UNIT 1: Technical Conversions and Exposure Systems Flashcards
What is the purpose of exposure systems?
Provides a consistent method of image production
• Holds most variable constant while changing only one
• Provides consistent subject contrast and exposure from study to study
• Allows radiologists to see patterns in pathology easier
• Provides lower patient doses by reducing repeats
What is Anatomically Programmed Radiography (APR)?
Preprogrammed technique based on anatomical region (body part)
-A type of technique chart in which the user selects a body part and projection from a menu on the console, and the kVp and mAs are automatically present for the exposure
What is the device used to measure patient thickness?
Caliper
What are the 4 types of technique charts?
- Fixed kVp
- Variable kVp
- Pediatric
- High kVp
When are high kVp charts used?
For chest images and Barium studies
What is included in a technique chart?
-AEC cells to activate
-kVp
-mAs
-Grid use
-Patient size
-FSS
What are the characteristics of Variable kVp charts?
Variable kVp (Fixed mAs)
• Finer adjustments made per patient thickness
• Shorter scale of contrast
• Increased patient dose
• Increased tube wear
What are the characteristics of a Fixed kVp chart?
Fixed kVp (Variable mAs)
• Decreased patient dose
• Longer exposure latitude( means more room for error)
• Longer contrast scale (Optimum kVp)
• More scatter
• Reduced x-ray tube wear
• Decreased time settings - reduce motion
Which type of technique chart uses optimum kVp?
Fixed kVp chart
What is optimum kVp?
Maximum kVp that will produce images with appropriate subject contrast
What are the benefits of using the optimum kVp?
- Minimizes Skin Dose (ESE)
- Allows for reduced mAs
- Facilitates Long Scale Subject Contrast
What is Exposure Latitude? How does this affect Radiographer practices?
-The range of technical parameters that produce a diagnostic image
-More room for error
Steps of creating a technique chart
1. Measure thickness of phantom using caliper
2. Produce 5-7 test images on phantom
3. Reject analysis
4. Extrapolate technique chart
5. Clinical trials
6. Clinical fine tuning
7. Ongoing fine tuning
Extrapolating Fixed kVp chart
mAs doubles and halves for every 5cm
Ex:
Variable kVp technique chart equation to find kVp
2 kVp x part thickness + 40cm
Extrapolating Variable kVp chart
For every 1cm of patient thickness, kVp increases or decreases 2
EX:
Two classifications of image quality
Photographic and Geometric
Photographic quality factors are
the factors that can either enhance or degrade the anatomic details visible on the image
What factors affect photographic quality
-Contrast
-IR exposure/density
Contrast is
the ability to distinguish different shades of gray
Is contrast controlled by kVp in digital or film screen systems?
Film screen systems
Long scale and Short scale contrast: High kVp vs Low kVp
-High kVp = more uniform penetration of the body = Long Scale Contrast
-Low kVp = more photons absorbed = Short Scale Contrast
How does contrast work in digital systems?
The computer uses various mathematical functions to process the data received from the digital IR and then displays that data with the appropriate contrast range for that specific body part
Density is
the degree of blackening on the radiograph in film screen systems
For digital system, the term we use in place of density is
IR exposure
IR exposure is
the amount or intensity of photons that reach the IR
IR exposure does or does not directly correspond to the brightness level on the computer monitor
does NOT
The contrast and brightness can be manipulated by using
Post processing functions
Window width vs Window level
-Window width: a user can define the number of gray shades (gray scale) to be displayed. Changes the displayed CONTRAST (the number of shades of gray displayed) on the viewing monitor
-Window level: the center gray shade of the scale to be displayed. Changes the level of BRIGHTNESS (center shade of gray) displayed on the viewing monitor
What are the Geometric Quality Factors?
-Detail
-Distortion
The controlling factor for detail is
focal spot size
Penumbra
Influencing the spread of photons at the edge of the object’s projected shadow. Image blur, unsharpness.
Distortion is
the misrepresentation of the size or shape of an object
Distortion is controlled by
patient positioning
Magnification is
A distortion of size
Shape distortion is due to
Part angulation
What are the 2 types of part distortion?
-Elongation: is a type of shape distortion that occurs from angulation of the central ray, tube and IR are not aligned
-Foreshortening is caused by angulation of the body part, part and IR are not aligned
The controlling factor for shape distortion
is patient positioning
What patient factors affect image quality?
-Anatomic part thickness
-Body composition
-Underlying pathology
For Anatomic part thickness, with other variables held constant, thicker parts need more
-kVp to penetrate the part
-mAs to provide the number of photons needed to transverse the patient and reach the IR
For Body Composition: Sthetnic, Hypersthenic, Hyposthenic, Asthenic
-Sthenic to Hypersthenic patient tend to have higher ratios of muscle and tend to be thicker in measurement
-Hyposthenic and Asthenic patients tend to be thinner and have less muscle/ fat
Photons will be attenuated more in the following order:
- Bone
- Muscle
- Fat
- Lung / air
A pathologic process that increases subject density is called an
Additive Condition
An additive condition results in
radiopacity (the “white” area on a conventional radiograph)
Radiopacity vs Radiolucent
-Radiopacity: will result in a lighter or white area on the radiography
-Radiolucent: (blacker) compared to surrounding tissues
A destructive condition results in
radiolucency (shows up black on radiograph)
List of the some of the factors that may be found on a technique chart:
• kVp
• mA
• Exposure time (s)
• SID
• Grids
• Screens
• Speed class
• Focal-spot size
• Filtration
• Image Receptor
• AEC cells
Phantoms are
used to develop technique charts before a chart is tested in the clinical (on patients) environment
Caliper
a measuring device used to measure the thickness of the anatomic area under examination
The caliper must be placed at the location of the central ray entrance to the exit point at the thickest or thinnest part?
Thickest
Steps for establishing a technique chart:
- Measure thickness of phantom using a caliper
- Produce 5-7 test images using phantom body part
- Radiologist performs reject analysis
4.Extrapolate Technique chart using Baseline image selected by Reject analysis - Perform Clinical trials
- Perform clinical fine tuning
- Perform ongoing fine tuning and maintenance of the chart
Two main types of technique charts:
Fixed kVp and Variable kVp
Variable kVp Charts
• Vary kVp by body part thickness, mAs remains constant
• Produces higher contrast images = perceived increase in resolution
• Tends to increase patient dose
• Allows smaller adiustments based on part thickness
Fixed kVp Charts
-mAs varies
-Optimal kVp= maximum level that produces images with contrast within acceptance levels
Advantages:
• Ensures sufficient penetration of the body part.
• Sufficient penetration permits visualization through the area of interest
• Produces lower contrast and patient dose
• Uses lower mA and time-reducing x-ray tube wear
• Lengthened exposure latitude
Automatic Exposure Control (AEC)
• Uses ionizing chamber or solid-state detectors to terminate exposure time when a preset intensity has been reached.
• Eliminates the need to set exposure TIME only - mA and kVp must still be set manually.
• Location of ionization chamber is most important - therefore accurate positioning is critical when using AEC.
• A Technique chart using AEC should contain all factors except for time and addition of cell locations.
What is dose creep
Dose creep is when a radiographer uses higher exposure than what is actually needed so that they can avoid image noise and having to do repeats
Fixed kVp vs Variable kVp
-Fixed kVp chart: reduces patient dose, has a longer scale contrast, increased latitude, higher overall kVp, longer tube life
-Variable kVp chart: reduces scatter production, has a shorter scale contrast, improved visibility of detail, consistent image quality