1. Technical Factors or Prime Factors
Bushong Ch 15

2. What is “technique” ? How does it affect the “image”
PRIME FACTORS
What is “technique” ?
How does it affect the “image”

3. Exposure Factors – 3 or 4 The four prime exposure factors are:
Voltage = kVp*
Current = mA*
Exposure time = seconds or fractions of a sec*
Source-to-image distance = SID

4. PRIME FACTORS
KVP
MAS
DISTANCE

5. kVp Kilovolts controls how fast the electrons are sent across the tube
Controls, quality, penetrability & contrast
Increasing kVp also increases scattered photons reducing image quality
Does kVp influence OD?

6. kVp
Low kVp (50 – 60)
Short scale
High contrast
“Bone work”

7. kVp
High kVp (90 – 120)
Long scale
Low contrast
“Chest images”

8. mA
Determines the number of photons, radiation quantity, OD & patient dose
Changing mA does not change the kinetic energy of e-
Available mA stations are usually 50, 100, 200, 300, 400 & 600

9. Exposure Time mA X s = mAs mAs controls OD
Should be kept as short as possible, for most examinations. To minimize the risk of patient motion
mA X s = mAs
mAs controls OD
mAs determines the number of photons in the primary beam

10. Distance Affects exposure of the IR because of the Inverse Square Law
SID largely determines the intensity of photons at the IR
Distance has no effect on radiation quality

11. INTENSITY IS SPREAD OUT…

12. Inverse Square Law Formula
Distance #2 - Squared
Intensity #1
Distance #1 - Squared
Intensity #2

13. Direct Square Law
New mAs = New distance 2
Old mAs Old distance 2

14. Focal-Spot Changes

15. Tube voltage (kVp) Determines the maximum energy in the beam
spectrum and affects the quality of the output spectrum
Efficiency of x-ray production is directly related to tube voltage

16. Influencing factors: kVp
15% rule:
15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
15% rule will always change the contrast of the image because kV is the primary method of changing image contrast.
Remember :
15% change ( ) KVP has the same effect as doubling or ½ the MAS on density

17. kVp Changes
The kVp setting must be changed by at least 4% to produce visual changes an image

18. 4% kVp Changes

19. Radiographic Technique
Technique charts are based on the “average patient”
The thicker the part the more x-radiation is required to penetrate. Calipers should be used
Keep in mind not only the measurement but the type of tissue you need to penetrate (fat vs muscle)

20. Technique In general, Soft tissue = low kVp and high mAs
Extremity (soft tissue & bone) = low kVp
Chest (high subject contrast) = high kVp
Abdomen (low subject contrast) = middle kVp

21. Pathology Can appear with increased radiolucency or radiopacity
Some pathology is destructive causing tissue to be radiolucent
Others can be additive causing tissue to be radiopaque

22. Technique selection – Fixed kVp
For each anatomic part there is an optimum kVp
mAs is varied based on part thickness or pathological condition

23. Image Quality
Bushong Ch. 16

24. Objectives Image Quality – Factors Geometric Factors Subject Factors
Artifacts

25. Image Quality
Is the exactness of the representation of the patient’s anatomy
3 major factors affecting image quality that is under the control of the technologist: Image Receptor selection/use, Geometric factors & Subject factors.

26. Judging Image Quality
The most important characteristic of radiographic quality are: Spatial Resolution, Contrast Resolution, Noise & Artifacts

28. Main Factors Affecting Recorded Detail
kVp & mAs
Motion
Object Unsharpness
SID (Source to Image Distance)
OID (Object to Image Distance)
Material Unsharpness/ Film Screen Combo
Focal Spot Size
MTF (modulation transfer function)

30. Recorded Detail Other names: - detail -sharpness of detail -definition
-resolution
-degree of noise
- visibility of detail

34. Resolution
Is the ability to image two separate objects and visually distinguish one from the other.
Spatial resolution is the ability to image small objects that have high subject contrast. Ex: bone-soft tissue interface, breast calcifications, calcified lung nodule
Conventional radiography has excellent spatial resolution

35. RESOLUTION TEST
TOOLS
LINE PAIRS/ MM
Depicts how well you can see the differences in structures
More lines=more detail

36. Measuring Resolution for an x-ray imaging system

39. SMPTE Test Pattern
In 1985 the Society of Motion Picture and Television Engineers (SMPTE) published a recommended practice (RP-122).
Specifications for Medical Diagnostic Imaging Test Patterns for Television Monitors and Hard-copy Cameras.

40. SMPTE Test Pattern

41. Focal Spot Size Smaller x-ray beam width will produce a sharper image.
Fine detail = small focal spot (i.e. small bones)
General radiography uses large focal spot
Beam from penlight size flashlight vs. flood light beam

42. Focal spot size of the cathode

43. Line-focus principle

44. Modulation Transfer Function
The ability of a system to record available spatial frequencies.
The sum of the components in a recording system cannot be greater than the system as a whole.
When any component’s function is compromised because of some type of interference, the overall quality of the system is affected.

45. Contrast Resolution
Is the ability to distinguish anatomic structures of similar subject contrast. Ex: liver-spleen, gray matter-white matter
Magnetic Resonance Imaging has the highest contrast resolution
Computed Tomography is excellent as well

46. MRI CT

48. The contrast of an object is expressed relative to its surrounding background.  That is what determines its visibility.

49. Radiographic Contrast
Is the product of image receptor contrast and subject contrast

54. “Noise” Borrowed from electrical engineering
Audio noise = hum or flutter heard from a stereo
Video noise = “snow” on a TV
Radiographic noise = random fluctuation on the OD of the image

55. QUANTUM MOTTLE Not enough PHOTONS – can create a mottled or grainy image - MORE COMMON IN CR SYSTEMS

56. Radiographic noise

57. Radiographic Noise Four components:
Film graininess, structure mottle, quantum mottle & scatter radiation

58. Radiographic Noise
Film graininess – distribution & size of the silver halide grains in the emulsion
Structure mottle – speed of the intensifying screen. Phosphor size & DQE/CE
Not under the control of the technologist

59. Image Noise Speckled background on the image
Caused when fast screens and high kVp techniques are used. Noise reduces image contrast
The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE)
The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)

60. Quantum Mottle
An image produced with just a few x-rays will have more quantum mottle.
The use of very fast intensifying screens or not enough mAs or kVp will increase quantum mottle

61. Quantum mottle

64. Screen Speed
Efficiency of a screen in converting x-rays to light is Screen Speed.

65. Speed Fast image receptors Slow image receptors
? Noise ? Spatial resolution
? Contrast resolution
Slow image receptors
See pg 274 for answers

66. Speed Low noise = fast or slow speed?
High contrast resolution = fast or slow speed?
Reduced spatial resolution = fast or slow speed?

67. PARALLAX – each emulsion has an image single image overlaped edges edge sharp less sharp

68. Other Film Factors Characteristic Curve
Is used to describe the relationship between OD and radiation exposure

69. What is the useful OD range?
Characteristic curve
of radiographic film

71. The latitude of an image receptor is the exposure range over which it responds with diagnostically useful OD.
Depending on the manufacturing characteristics radiographic film will respond differently to radiation exposure

73. F/S vs Digital Dynamic Range

74. Unexposed film Appears like a frosted glass window
ODs of unexposed film are due to base density and fog density
Base density – tint added to the base to reduce eye strain and crossover. Has a densitometer value of approximately 0.1

75. CROSSOVER
Reducing crossover by adding a dye to the base

76. Unexposed film
Fog Density – inadvertent exposure of film during storage, chemical contamination, improper processing, radiation exposure, etc.
Fog density contributes to reduction of radiographic contrast
Fog density should not exceed 0.1
Base + fog OD = 0.1 to 0.3

78. 2 principal characteristics of any image are Spatial & Contrast Resolution
Spatial resolution
Resolution is the ability to image two separate objects and visually distinguish one from the other
Spatial resolution is the ability to image small objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung nodules)
Determined by focal-spot size and other factors that contribute to blur
Diagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm.

79. Other factors affecting the finished radiograph
The concentration of processing chemicals
The degree of chemistry agitation during development
Development time
Development temperature

81. Image Quality in Digital
Matrix size is determined by . . .
Receptor size (Field of View: FOV)
Pixel size
CR - Sampling frequency
DR - DEL size

85. Spatial Resolution determined by:
􀁹 Pixel size.
CR- sampling frequency
DR – DEL size
􀁹 There are relationships between
Pixel size
Receptor size
Matrix size
􀁹 pixel size = larger matrix
􀁹 receptor size = larger matrix
Spatial resolution is not related the amount of exposure

87. Sampling Frequency The sampling frequency is the rate at
which the laser extracts the image data
from the plate.

89. Signal Sampling Frequency Good sampling under sampling

90. Nyquist Frequency The Nyquist Frequency will be ½ of the
sampling frequency.
A plate that is scanned using a sampling frequency of 10 pixels per millimeter would not be able to demonstrate more than 5 line pairs per millimeter based upon the Nyquist Frequency.
The Nyquist Frequency allows the
determination of the spatial resolution for
a given sampling frequency.

97. Geometric Factors
Producing high quality radiographs. Technologists must maximize geometric conditions
Three principal geometric conditions affect radiographic quality: Magnification, Distortion & Focal-spot blur.
Review table 16-4, pg. 295 for summary

98. Object Unsharpness
Main problem is trying to image a 3-D object on a 2-D film.
Human body is not straight edges and sharp angles.
We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID

99. Magnification
All image on the radiograph are larger than the object they represent.
For most exams minimizing magnification is desired. There are a few exams where some magnification can be helpful
TUBE CLOSE TO THE PART (SID),
PART FAR FROM THE CASSETTE (OID)

100. Minimizing Magnification
Large SID: use as large a source-to-image receptor distance as possible
Small OID: place the object as close to the Image receptor as possible
In terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.

101. Size Distortion & SID Major influences: SID & OID
As SID , magnification 
Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present
Must note deviations from standard SID

102. The position of the tube (SID) to IR Will influence how the structures appear on the image The farther away – the less magnified ↑SID ↓ MAGNIFICATION

103. SID
Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy”
-On a radiograph called Penumbra
Penumbra (fuzziness) obscures true border – umbra
Farther the flashlight from object = sharper borders. Same with radiography.

106. OID Object to Image Distance
The closer the object to the film, the sharper the detail.
OID , penumbra , sharpness 
OID , penumbra , sharpness 
Structures located deep in the body, radiographer must know how to position to get the object closest to the film.

107. Size Distortion & OID
If source is kept constant, OID will affect magnification
As OID , magnification 
The farther the object is from the film, the more magnification

108. The position of the structure in the body will influence how magnified it will be seen on the image
The farther away – the more magnified

112. Minimal magnification small OID
Magnification - large OID

114. 40” SID VS 72” SID

116. Magnification Factor MF = source-to-object distance SID MF = SOD
source-to-image receptor distance
MF = source-to-object distance
SOD difficult to measure accurately
*usually an estimated value
SID
MF = SOD

117. Finding SOD
SID – OID = SOD
SID = 100 cm
OID = 7 cm
What is the SOD?

118. Finding magnification of the heart on a lateral CXR
SID – OID = SOD
SID = 72 inches
OID = 8 inches (estimated)
What is the SOD?
What is the Mag Factor?

119. Distortion Misrepresentation of the true size or shape of an object
-MAGNIFICATION (size distortion)
-TRUE DISTORTION (shape distortion)
Shape distortion: unequal magnification of different portions of the same object

121. Shape Distortion Depends on: Object thickness Object position
Object shape

122. Object Thickness
Thick objects have more OID and are more distorted than thinner structures

124. Object Position
If the object plane and the image plane are parallel, the image is not distorted
CR perpendicular
to the part

125. Position Distortion
Foreshortened = anatomy at an incline to the CR displays smaller than true size

126. D & E = shape distortion (foreshortening of part)

127. Position Distortion
Elongation: anatomy at an incline and lateral to the central axis
Could be
foreshortened as well

128. Elongation Foreshortened Normal

129. Position Distortion
Spatial distortion = anatomy positioned at various OIDs but superimposed, only one can be seen

130. Position Distortion – Irregular Anatomy
Anatomy or objects can cause considerable distortion when imaged off the central axis

131. Focal-Spot Blur Dependent on the size of the effective focal spot
Smaller the effective focal spot = less blur and better spatial resolution
Focal-spot blur is the most important factor in determining spatial resolution

132. Focal Spot Size Smaller x-ray beam width will produce a sharper image.
Fine detail = small focal spot (i.e. small bones)
General radiography uses large focal spot
Beam from penlight size flashlight vs. flood light beam

133. ANODE
ANODE

134. Focal spot size – determined by filament in cathode & surface area used at anode

135. THE SMALLER THE BEAM TOWARDS THE PATIENT - THE BETTER THE DETAIL OF THE IMAGE PRODUCED

136. Focal-spot blur is caused by the effective size of the focal spot, which is larger to the cathode side of the image.

137. Focal-spot blur is small when the object-to-image receptor distance (OID) is small.

138. Image Quality Subject Factors
Patient thickness
Effective atomic number
Object shape
Subject contrast
Tissue mass density
kVp

139. Object shape Objects with structure having a form that
coincides with the
x-ray beam has
maximum detail

140. Patient Motion Can be voluntary or involuntary
Best controlled by short exposure times
Use of careful instructions to the pt.
Suspension of pt. respiration
Immobilization devices

144. Blurring of image due to patient movement during exposure.

145. Questions?