Positioning

Question 1

Agenda

Answer 1

Image Evaluation
- Analog (film)
- Digital
Speed Class
Technical Factors
- kVp
- ESE
Exposure Indicators
Exposure Indicator Errors

Question 2

ANALOG IMAGE EVALUATION

Answer 2

Radiographic Density
Radiographic Contrast
Image Blur
Distortion

Question 3

Radiographic Density

Answer 3

• 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.

Question 4

Radiographic Contrast

Answer 4

• 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.

Question 5

Blur

Answer 5

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.

Question 6

Distortion

Answer 6

Image distortion is demonstrated as follows:

• Size distortion
• SID
• OID
• Shape distortion
• Foreshortening
• Elongation
• Beam-Part-Receptor Alignment

Question 7

Digital Images are Evaluated Based on

Answer 7

• Brightness
• Contrast
• Image Blur
• Exposure Indicator
• Image Noise

Question 8

Image Brightness

Answer 8

• The brightness of the digital image is equivalent to the term density that was applied to the analog image.

Question 9

Image Contrast

Answer 9

• Contrast is determined by the diference in adjacent densities contained within the image.

Question 10

Image Blur

Answer 10

• Image blur results from motion occurring during the exposure or may be due to the characteristics of the digital image receptor used.

Question 11

Exposure Indicator

Answer 11

• 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.

Question 12

Noise

Answer 12

• Noise in the digital image has a speckled, grainy appearance. Indicates photon starvation.

Question 13

Digital Exposure Principles

Answer 13

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

Question 14

Speed Class

Answer 14

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

Question 15

SNR

Answer 15

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.

Question 16

kV and Digital Systems

Answer 16

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

Question 17

Film

Answer 17

mA=quantity

kVp=quality

Question 18

IP

Answer 18

mA=data

kVp-penetration

Question 19

kVp Levels and Digital Image Receptors

Answer 19

Higher kVp Levels and Digital Image Receptors=Reduced ESE to pt.

Question 20

Knee example

Answer 20

At 85 kV pt ese was reduced to 50% of exposure at 65 kV

Question 21

kV and Digital Receptors

Answer 21

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)

Question 22

Dose Creep

Answer 22

Lack of visual feedback may lead to dose creep

Question 23

Creeping dose problem

Answer 23

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

Question 24

Exposure Indicators

Answer 24

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

Question 25

DAP

Answer 25

- 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.

Question 26

The Problem with DAP | Vendor Specific Exposure Indicators

Answer 26

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

Question 27

agfa system - CR

Answer 27

lgm - directly proportional to the incident exposure - 2.2 considered optimal exp level - - +/- 0.3 is considered acceptable range of 1.9 - 2.5

Question 28

fuji, philips, konica system - CR

Answer 28

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

Question 29

kodak - CR

Answer 29

EI range 1700-2300 opt range 1800-1900 every 300 = double/half

Question 30

Flat-panel exp indices

Answer 30

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."

Question 31

Miscalculating exp indicator

Answer 31

- exp field recognition error | - unexpected exp variations recorded on the receptor.

Question 32

Miscalculating exp indicator

Answer 32

- exp factors acceptable - exp field recognized - exp indicator correct

Question 33

Miscalculating exp indicator

Answer 33

- exp factors acceptable - exp field NOT recognized - exp indicator unacceptable

Question 34

Eval exp indicator errors

Answer 34

- exp field recog error - unexpected structures contained in exp field - excessive scatter striking receptor

Question 35

Altering the exp indicator

Answer 35

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

Question 36

Causes of suboptimal images

Answer 36

1. excessive mottle/noise a. >50% below the optimal receptor exp 2. 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 3. dark or light images a. rescaling error due to histogram analysis error b. inappropriate exp field alignment or collimation leading to exp recognition failure 4. motion