Positioning Flashcards
Agenda
Image Evaluation - Analog (film) - Digital Speed Class Technical Factors - kVp - ESE Exposure Indicators Exposure Indicator Errors
ANALOG IMAGE EVALUATION
Radiographic Density
Radiographic Contrast
Image Blur
Distortion
Radiographic Density
- Overall blackening of the image present on film.
- Anything that affects the quantity of radiation that strikes the screen/film impacts the density present in the image.
- Changed by the processing conditions that converted the latent image into the manifest image.
Radiographic Contrast
- Radiographic contrast is the differences among densities seen in an image.
- Anything that alters the quantity or quality of radiation that strikes the screen/film receptor affects the contrast present in an image.
- The processing conditions that convert the latent image into a manifest image also impact radiographic contrast.
Blur
Major causes:
- Motion
- Changes in a geometric factor
- Motion is the predominant cause of the blur seen in radiographic images.
- Assessing blur caused by the technical factor selected is best done using specific test phantoms.
Distortion
Image distortion is demonstrated as follows:
- Size distortion
- SID
- OID
- Shape distortion
- Foreshortening
- Elongation
- Beam-Part-Receptor Alignment
Digital Images are Evaluated Based on
- Brightness
- Contrast
- Image Blur
- Exposure Indicator
- Image Noise
Image Brightness
- The brightness of the digital image is equivalent to the term density that was applied to the analog image.
Image Contrast
- Contrast is determined by the diference in adjacent densities contained within the image.
Image Blur
- Image blur results from motion occurring during the exposure or may be due to the characteristics of the digital image receptor used.
Exposure Indicator
- The exposure indicator is a vendor specific value that provides the technologist with an indication of the accuracy of their exposure settings for that given image.
Noise
- Noise in the digital image has a speckled, grainy appearance. Indicates photon starvation.
Digital Exposure Principles
Fundamental principles are similar
- For optimal exposure to the IR
- Consideration to mAs, kV, body habitus, SID
- For scatter control
- -Collimation
- -Grid selection
- Detail/Spatial resolution
- -smallest focal spot size
- -longest sid
Speed Class
In F/S radiography, speed determined by:
- size of crystals contained in screen and film
- thickness of crystal layer in screen and film
- The slower the speed, the more exposure required
In digital radiography, speed class refers to the operational exposure level at which the digital system is operated.
- vendor specific; influences by radiologist preferences and exposure received by the IR
- same system can operate as a 100, 400 or 800 speed system
SNR
Optimal ranges for appropriate SNR:
DR systems
- 400 - 800 speed class
CR systems
- 300 speed class
Operate at higher speed class when increased noise is acceptable
- net effect: lower total dose to patient.
kV and Digital Systems
Higher kV values can be used compared to F/S
- With F/S, changes in kV change scale of contrast
Contrast scale not an issue
- Processing algorithms have more impact on contrast
Film
mA=quantity
kVp=quality
IP
mA=data
kVp-penetration
kVp Levels and Digital Image Receptors
Higher kVp Levels and Digital Image Receptors=Reduced ESE to pt.
Knee example
At 85 kV pt ese was reduced to 50% of exposure at 65 kV
kV and Digital Receptors
Increasing kV is a viable way to decrease a pts ese
Increasing kV has the potential to increase the amount of noise in a image
BUT, there is a limit. High kV =
- higher transmission of xray photons
- lower mAs
- decrease in signal strength
- risk of noise (quantum mottle)
Dose Creep
Lack of visual feedback may lead to dose creep
Creeping dose problem
Under exposed images = incrase noise
Radiologists complain: techs get “joy” note
Tech know digital algorithms will “fix” almost anything: increase exposure to avoid criticism
- “when in doubt, burn it out”
- “it’s ok since we repeat fewer images with digital”
Is true about fewer repeats
- 5% repeat rate w F/S, 2% w/digital
- Most repeats in digital radiography are due to positioning errors
Exposure Indicators
How does one evaluate exposure accuracy?
DR systems
- Exposure hitting the IR
- Does Area Product (DAP)
CR systems
- Receptor exposure determined from histogram analysis
-Typically found combined with the collimator housing
DAP
- Entrance dose (cGy) x field area (cm^2)
- Measured by detector inside or on collimator
- Commonly employed with cassette-less systems
- Detector interface with generator
- Can be part of patient’s image record
- Is expressed in rads or Gy/cm^2
- Smaller field size results in lower DAP, lower risk or injury to skin, because a smaller amount of tissue is exposed
- Problem using DAP to evaluate exposure accuracy?
- Is useless unless standardization and consistency in collimating.
The Problem with DAP
Vendor Specific Exposure Indicators
Exposure indicators indicate the amount of radiation
It is assumed that the IR exposure is proportional to the patient exposure
- agfa: lgm
- fuji, philips, konica: s # (sensitivity)
- kodak & philips DR- EI (exp index)
- Canon Dr - REX # (reached exp)
exposure indicator is a nice tol but is not comparable between modalities or vendors
ea vendor has:
- diff scales
- diff algorithms
- diff calibrations
standardization is neded
agfa system - CR
lgm
- directly proportional to the incident exposure
- 2.2 considered optimal exp level
- +/- 0.3 is considered acceptable range of 1.9 - 2.5
fuji, philips, konica system - CR
S number
- inversely proportional to amount of radiation stiking IP
- S values
–under exp is indicated by a high s# generaly < 500
–s# under 75 overexp
each change or 200 represents a doubling or halving or exp
acceptable s # guide
ad grid and non 200-300
distal non extremities 75-125
kodak - CR
EI
range 1700-2300
opt range 1800-1900
every 300 = double/half
Flat-panel exp indices
Philips - EI
- Measures the speed class of operation
- Increasing the exp decreases the EI value
Siemens - EXI
- directly related to the exp level
- increasing the exp increases the EXI val
Canon - REX
- directly related to the exp level
- increasing the exp increases the REX val
in addition to the final image, vendor’s software could show an “exp based view.”
Miscalculating exp indicator
- exp field recognition error
- unexpected exp variations recorded on the receptor.
Miscalculating exp indicator
- exp factors acceptable
- exp field recognized
- exp indicator correct
Miscalculating exp indicator
- exp factors acceptable
- exp field NOT recognized
- exp indicator unacceptable
Eval exp indicator errors
- exp field recog error
- unexpected structures contained in exp field
- excessive scatter striking receptor
Altering the exp indicator
any medication a technologist administers to a patient must be ducumented in the pts chart and incld the folowing info:
- person who administers the medication
- dose
- administration route
- administration time
- pts radiations exp
- may be recorded in the pts perm electronic med rec
Causes of suboptimal images
- excessive mottle/noise
a. >50% below the optimal receptor exp - low con
a. >200% overexp of plate
b. background buildup on plate
c. grid cutoff
d. incorrect menu for anatomy imaged
e. excessive scatter hitting IR - dark or light images
a. rescaling error due to histogram analysis error
b. inappropriate exp field alignment or collimation leading to exp recognition failure - motion
