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Technical Factors or Prime Factors Bushong Ch 15 1.

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Presentation on theme: "Technical Factors or Prime Factors Bushong Ch 15 1."— Presentation transcript:

1 Technical Factors or Prime Factors Bushong Ch 15 1

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

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 3

4 PRIME FACTORS KVP MAS DISTANCE 4

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? 5

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

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

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 8

9 Exposure Time 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 9

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 10

11 INTENSITY IS SPREAD OUT… 11

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

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

14 Focal-Spot Changes 14

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 15

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 16

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

18 4% kVp Changes 18

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) 19

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 20

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 21

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

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

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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)

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

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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

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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

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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

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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

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

65 Speed Fast image receptors –? Noise? Spatial resolution –? Contrast resolution Slow image receptors –? Noise? Spatial resolution –? Contrast resolution –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

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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

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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

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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

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

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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

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

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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.

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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.

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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

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112 Minimal magnification small OID Magnification - large OID

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114 40” SID VS 72” SID

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116 Magnification Factor 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

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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

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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

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

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144 Blurring of image due to patient movement during exposure.

145 Questions?


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