Radiographic Quality. Factors Affecting Optical Density: Optimum film/screen image density: 0.25 to 2.5 Milliamperage or ma (primary beam quantity)

Transcription

1 The Density Unit: Factors Affecting Optical Density: Optimum film/screen image density: 0.25 to 2.5 Milliamperage or ma (primary beam quantity) Primary controlling factor for density Direct and proportional relationship Radiographic Quality Photographic Properties Geometric Properties Density Contrast Distortion Definition Shape Size Factors Affecting Optical Density: Exposure Time Has a direct and proportional relationship on density How accurate is the timer? Why is time not the primary controlling factor for density? Foreshortening Elongation Magnification 1

2 Factors Affecting Optical Density: Reciprocity Law Requires calibrated equipment ma time mas Dose mr mr mr Which of the above techniques would be best employed for a small child? The X-ray Emission Spectrum: Quantity or Intensity of X-ray Photons With 2.5 mm of Filtration At 90 kvp X-ray Photon Energy (kev) Factors Affecting Optical Density: 30% Rule Apply the 30% Rule to the following technique in order to obtain a minimal visible increase in image density. 120 kvp and 50 mas 50 x 0.3 = = 65 or 50 x 1.3 = kvp and 65 mas ma vs. Film/Screen Density: Quantity or Intensity of X-ray Photons 200 ma 100 ma With 2.5 mm of Filtration At 90 kvp X-ray Photon Energy (kev) The X-ray Emission Spectrum: ma vs. Film/Screen Density: Quantity or Intensity of X-ray Photons With No Filtration At 90 kvp X-ray Photon Energy (kev) 100 ma 200 ma 2

3 Factors Affecting Optical Density: Kilovoltage Peak or kvp Primarily a measurement of beam quality but does affect beam quantity (mr) to a lesser extent If the kvp is increased, will more electrons be produced at the filament? Will more x-rays be produced at the anode? Optimum kvp: Patient IR 10 mas at 40 kvp 20 mas at 40 kvp Factors Affecting Optical Density: 15% Rule Increasing the kvp by 15% will double film density and reducing it by 15% will reduce the density by 50% To maintain i the density, increase the kvp by 15% and reduce the mas by 50% or..reduce the kvp by 15% and double the mas. kvp vs. Film/Screen Density: Quantity or Intensity of X-ray Photons What happened to the quantity of x-rays produced? What will happen to density? X-ray Photon Energy (kev) Factors Affecting Optical Density: 15% Rule Example How would you change the following technique if you wanted to use kvp to help reduce motion but maintain the same density? 80 kvp 100 ma 0.2 s 80 kvp x 1.15 = (s) x 0.5 = 0.1 New Technique: 92 kvp 100 ma 0.1 s kvp vs. Film/Screen Density: kvp & 1.0 mas 75 kvp & 0.6 mas 3

4 Factors Affecting Optical Density: Intensifying Screens Direct and proportional relationship Relative Speed (RS) Class System: (must be paired with appropriate film): Screen Speed Descriptive Name 50 to 80 Extremity or Detail 200 to 300 Medium 400 Regular 600 to 800 High Speed SID & the Direct Square Law: This formula is employed to maintain beam intensity or density at a new distance I 1 D 2 1 I 2 D 2 2 I 1 = old mr or density I 2 = new mr or density D 1 = old distance D 2 = new distance SID & the Inverse Square Law: The intensity (mr) of the beam is inversely proportional to the square of the distance between the source and the image receptor. This formula is employed to determine changes in beam intensity or in density at a new distance. I 1 D 2 2 I 2 D 1 2 I 1 = old mr or density I 2 = new mr or density D 1 = old distance D 2 = new distance SID & the Direct Square Law: The following versions of this formula can be employed to maintain beam intensity or density at a new distance new mr = old mr new SID 2 old SID new density = old density new SID 2 old SID new mas = old mas new SID 2 old SID SID & the Inverse Square Law: The following version of this formula may be employed to determine changes in beam intensity or density. new mr = old mr old SID 2 new SID new density = old density old SID 2 new SID Collimation vs. Film/Screen Density: Screen Film Tight Collimation 4

5 Collimation vs. Film/Screen Density: Collimation vs. Film/Screen Density: Screen Film Screen Film No Collimation No Collimation Collimation vs. Film/Screen Density: Collimation vs. Film/Screen Density: Screen Film Screen Film No Collimation No Collimation Collimation vs. Film/Screen Density: Collimation vs. Film/Screen Density: Screen Film Screen Film No Collimation No Collimation 5

6 Collimation vs. Film/Screen Density: Proper Collimation No Collimation Factors Affecting Optical Density: Grid Ratio (GR or r) Height (H or h) of the lead strips divided by the distance (D) between GR = H/D or r = h/d With all other grid construction factors constant, the higher the GR, the greater the scatter clean-up Higher GRs also require more accuracy in their use and result in a higher patient dose A 16:1 GR may remove up to 97% of the scatter produced Factors Affecting Optical Density: Radiographic Grids Invented by Gustav Bucky in 1913 Refined by Hollis Potter in 1920 by adding sidto-side motion to blur the grid lines It become known loosely as a Potter-Bucky Diaphragm Grid Ratio Calculation: H = mm mm GR GR = = H/D H/D GR GR = = 4.0/0.5 24/4 GR GR = = 8:1 6:1 D D = = mm Lines/Inch H = mm mm GR = 4.0/ /1 GR = 16:1 12:1 120 Lines/Inch D D = 0.25 = 2 mm mm Factors Affecting Optical Density: Grid Anatomy Consists of alternating lead and inter space material The lead strips are approximately 0.05 mm in width Aluminum Interspacing Approximately 0.33 mm in width Physically holds lead strips together Acts as a filtering device due to its relatively high Z# Factors Affecting Optical Density: Grid Frequency (GF) The number of lead strips per inch GF varies between 50 and 200 lines/inch The most common range is 85 to 103 lines/inch Digital image receptors are more sensitive and require a range of 103 to 200 lines/inch 6

7 Factors Affecting Optical Density: Lead Content An important indicator of grid performance For grids with equal GRs, the one with fewer lead strips will be more efficient at removing scatter by virtue of a higher h lead content 72 SID Parallel, Linear Grid: Source Patient Grid Screen/ Film Grid Frequency: 72 Focused, Linear Grid: H = mm GR = 8:1 6:1 60 Lines/Inch D D = = mm H = mm GR = 8:1 6:1 120 Lines/Inch 4 mm Which of of these 8:1 grids ratio is grids the most would D = 0.5 mm Which Which of these of these 8:1 provide 12:1 efficient? ratio ratio grids the grids Which best would would visibility one provide provide most of the detail? the best likely visibility of detail? best Best to visibility produce efficiency? of an detail? artifact? Highest pt. dose? 72 SID Source Note canting of lead strips. Patient Grid Screen/ Film Types of Grids: Crosshatch or Crossed Grid: Linear Crosshatch Parallel Focused Removes Secondary and Scatter in Both Directions 7

8 Off-center Grid Cutoff: Magnified Grid Lines: 40 SID Screen/Film Even loss of density across the IR. Properly Positioned: 15:1 GR Off Level Grid Cutoff: 40 SID Patient Grid Screen/Film Even loss of density across the IR. Off-center Grid Cutoff: 15:1 GR Off Level Grid Cutoff: 10:1 GR 8

9 Parallel Grid Off-focus Cutoff: Upsiddown Grid Cutoff: Source SID Patient Grid Screen/ Film Loss of density Off-Distance on the lateral Grid edges Cut-off the image. Focal Range is infinity 40 SID Patient Grid Screen/Film Severe loss of density along lateral edges. Focused Grid Off-focus Cutoff: Upsiddown Grid Cutoff: 6:1 GR SID Source 34 to 44 Focal Range Patient Grid Screen/ Film Loss of density Off-Distance on the lateral Grid edges Cut-off the image. 15:1 Focused Grid: Off-focus Cutoff Upsiddown Grid Cutoff: 15:1 GR 34 to 44 Focal Range; Actual SID = 30 9

10 Upsiddown Grid Cutoff: 6:1 Crossed The Moire Effect: Notice the improved scale of contrast on this properly positioned image. Upsiddown Grid Cutoff: 6:1 Crossed The Moire Effect: This grid was made with two focused, linear grids turned 90 degrees apart and placed on top of each other. Notice how there is a loss of density on all four edges but not in the middle. Why? Upsiddown Grid Cutoff: 6:1 Crossed Filtration vs. Radiographic Density: Here is a magnified view. Can you see the squares? 10

11 Filtration vs. Radiographic Density: What happened to the to the beam average energy of the intensity? primary beam? How about image density? Quantity or Intensity of 2.5 mm X-ray Photons 3.5 mm X-ray Photon Energy (kev) The Anode Heel Effect vs. Density: Stator Anode (+) Focusing Cup Anode Heel Engage the Rotor: Thermionic Emission (-) The Anode Heel Effect vs. Density: The Anode Heel Effect vs. Density: Stator Anode (+) Target Cathode (-) Stator Anode Heel Anode (+) Focusing Cup e ē- e (-) ē-e Close the Circuit Filament Focusing Cup The Anode Heel Effect vs. Density: The Anode Heel Effect vs. Density: Stator Anode (+) Focusing Cup Stator Anode (+) Focusing Cup Anode Heel (-) Anode Heel (-) Percent of Primary X-ray Beam Intensity 11

12 The Anode Heel Effect vs. Density: Wedge Filter: Stator Anode (+) Focusing Cup Anode Heel (-) Percent of Primary X-ray Beam Intensity Off-focus Radiation: Wedge Filter: Factors Affecting Optical Density: Compensating Filters Used on body parts of unequal thicknesses These are NOT radiation protection devices Usually made of aluminum but some are plastic Techniques will have to be increased by 8 to 10 kvp Factors Affecting Optical Density: Patient Factors: Additive Diseases Generally require a 7.5% increase in kvp to compensate 12

13 Additive Disease: Ascites Destructive Disease: COPD L Normal Ascites Normal COPD Additive Disease: Paget s Disease Destructive Disease: COPD L Paget s Disease Normal Normal COPD Factors Affecting Optical Density: Patient Factors: Destructive Diseases Generally require a 5% decrease in kvp to compensate Destructive Disease: Osteoporosis Normal Osteoporosis 13

14 Factors Affecting Optical Density: Patient Factors: Casts Wet plaster: double the mas and increase the kvp by 15% Dry plaster: double the mas or increase the kvp by 15% Fiberglass: increase mas by 30% or increase kvp by 5% Plaster/fiberglass: increase mas by 50% or increase kvp by 7.5% The Contrast Unit: Patient Factors to Consider: Casts Radiographic Contrast: Subject Contrast The difference in the thickness and atomic numbers of the structures that comprise the body part of interest This is the basis for optimum kvp Radiographic Contrast The difference between adjacent densities on a radiograph Patient Factors: Casts Radiographic Contrast: Primary controlling factor for both subject and radiographic contrast As kvp increases, PE & CI both go down but PE goes down faster h l i i i h l The net result is an image with a longer scale of contrast Fiberglass 14

15 Short vs. Long Scale Contrast: Collimation vs. Film/Screen Contrast: Low kvp = High Contrast = Short Scale = Few Shades of Gray High kvp = Low Contrast = Long Scale = Many Shades of Gray Tight Collimation Screen Film Short vs. Long Scale Contrast: Collimation vs. Film/Screen Contrast: Screen Film 50 kvp 100 kvp No Collimation Air-Gap vs. Contrast: Collimation vs. Film/Screen Contrast: What What happens happens to to contrast? density? 1 OID 4 OID Screen Film 60 kvp at 10 mas 60 kvp at 10 mas No Collimation 15

16 Grids vs. Film/Screen Contrast: Patient What would happen to contrast if you changed to a larger GR? 12:1 Grid GR Primary Beam Exit Beam Screen/ Film Recorded Detail or Definition: May also be referred to as resolution Measured in line pairs per mm or lp/mm Recorded Detail vs. Visibility of Detail Short vs. Long Scale Dark vs. Light Radiographs Is a geometrically sharp image possible to achieve? Filtration vs. Film/Screen Contrast: Quantity or Intensity of X-ray Photons What happened to the average energy of the primary beam? 2.5 mm 3.5 mm What will happen to contrast? Factors That Influence Detail: Motion Single most detrimental factor Voluntary vs. involuntary X-ray Photon Energy (kev) Motion vs. Recorded Detail: The Recorded Detail Unit: 16

17 Factors That Influence Detail: Rare Earth (RE) Intensifying Screens Primarily made from combinations of the elements Lanthanum and Gadolinium They may also be made from Barium and Yttrium compounds Screen X-ray Absorption Efficiency RE screens absorb 60% of the exit beam photoelectrically Screen X-ray Conversion Efficiency RE phosphors convert 20% of the exit beam into light photons Screens vs. Recorded Detail: Factors That Influence Detail: Screens should be cleaned monthly with an electrostatic cleaning solution Screen Thickness vs. Recorded Detail: Crystal Size vs. Recorded Detail: 17

18 Reflective Layer vs. Recorded Detail: Wire Mesh Test: Wire Mesh Test: 18

19 Image Crossover vs. Recorded Detail: 19

20 Film Anticrossover Layer: Factors That Influence Detail: Geometric factors which influence detail: 1. Focal Spot Size Primary controlling factor for detail Only variable that exclusively affects detail No Image Crossover: Focal Spot Size vs. Recorded Detail: Factors That Influence Detail: Geometric factors which influence detail: 1. Focal Spot Size 2. OID 3. SID Focal Spot Size vs. Detail: 20

21 Focal Spot Size vs. Detail: Factors That Influence Detail: Geometric factors which influence detail: 3. SID Increase SID, decrease penumbra and improve recorded detail Factors That Influence Detail: Geometric factors which influence detail: 2. OID Most critical geometric factor that affects detail Decrease OID, decrease penumbra and improve recorded detail SID vs. Recorded Detail: OID vs. Recorded Detail: The Distortion Unit: 21

22 Radiographic Distortion: Can distortion ever be totally eliminated? Radiographic Distortion: Shape Distortion Elongation vs. Foreshortening May lead to displacement How to Prevent Shape Distortion: Ensure that the part is parallel to cassette The CR must be perpendicular to cassette Ensure proper tube angle Can shape distortion be used to your advantage? Radiographic Distortion: Factors That Affect Size Distortion or Magnification: OID is the most critical factor SID can help compensate for OID A Comprehensive Review of Image Production & Evaluation Part II Presented by: y John Fleming M.Ed., RT(R), (MR), (CT) St. Petersburg College Office: (727) flemingj@spcollege.edu 22

23 Radiographic Film: Film Base Made of polyester (1960s) Very resistant to age and warping Tinted blue Dimensional Stability Silver Halide Crystal Lattice: I- Ag+ Ag+ Radiographic Film: Film Emulsion Gelatin Silver Halide Crystals Silver Bromide Silver Iodide Sensitivity Specks Silver Sulfide (Ag + + S - = AgS) Silver Halide Crystal: Br- Sensitivity Speck (AgS) I- Br- Br- Br- Br- Br- Ag+ Ag+ Br- Br- Br- Br- Br- I- Br- Br- Br- Duplitized Film Anatomy: T-Coat, Supercoat, or Protective Coating Emulsion Film Base Emulsion T-Coat, Supercoat, or Protective Coating Cross-Section of a Silver Halide Crystal: Sensitivity Speck (AgS) Br- I- Br- Br- Br- Br- Ag+ Ag+ Ag+ Ag+ Br- Br- Br- Br- I- Br- 23

24 Latent Image Formation: Latent Image Formation: Br- Br Br- I- I Br Latent Image Formation: X-ray Br- Br- I- Latent Image Formation: Br I Br Latent Image Formation: Latent Image Formation: Br I Br S S S Br Br I 24

25 Latent Image Formation: Manifest Image Formation: Br Br S S I S S..... I S Br S Br Latent Image Formation: S S S Br Ag+ Ag+ I Ag+ Br Manifest Image Formation: eē..... Latent Image Formation: S S..... S Br I Br Manifest Image Formation: eē

26 Manifest Image Formation: eē Manifest Image Formation: Manifest Image Formation: eē Radiographic Film: Film Storage Ideal temperature: < 68 degrees F Ideal humidity: 40 to 60% Manifest Image Formation: 26

27 H & D or Characteristic Curves: 3.0 Density Shoulder A D-Max 1.0 D-Min Toe Straight Line Portion B Log Relative Exposure H & D or Characteristic Curves: 4.0 A 3.0 Density 2.0 B Log Relative Exposure Radiographic Film: Characteristic/Sensitometric/H & D Curves Film Speed = 1 + Base + Fog Film Contrast -Average Gradient vs Gamma Film Latitude 27

28 Automatic Film Processors: Automatic Film Processors: Developer Tank (alkaline) Solvent: dissolves chemicals for use Developing or Reducing Agents: electron donors; produces manifest image Activator: softens gelatin Preservative: prolongs chemical life Restrainer: prevents unexposed crystals from being developed Hardener: prevents excessive swelling of emulsion Feed Rack: Automatic Film Processors: Fixer Tank (acidic) Solvent: dissolves chemicals for use Activator: stops action of developing agents Clearing Agents or Hypo: dissolves unexposed crystals Hardener: stiffens and shrinks the emulsion Preservative: prolongs chemical life Wash Tank Dryer System: 110 to 150 degrees F Feed Rack: 28

29 Feed Rack: Developer Deep Rack Assembly Detection Rollers To Fixer Feed Tray Planetary Rollers Squeegee Rll Rollers Guide Shoe Solar Roller Guide Shoe Feed Rack: Guide Shoe Marks vs Pi Lines: Travel of Film Guide Shoe Marks: Run parallel to the travel of the film (Scratches) Pi Lines: Run perpendicular to the travel of the film (Deposits on rollers) Feed Rack Artifact: Guide Shoe Marks: 29

30 The Dryer System: Temperatures range from 110 to 150 F 30

31 AEC Devices: The Phototimer Grid Patient The photomultiplier tube converts light into Primary Beam an electric signal that is eventually sent to the Exit Beam exposure switch which terminates the exposure. Image Receptor Fluorescent Screen Light Photons Photomultiplier Tube Electrons Capacitor Thyratron Exposure Switch Relay

32 AEC Devices: The Phototimer Note: Optimum kvp must be maintained for all AEC exposures One of two AEC devices AKA photomultiplier tube or photomat How it works: Make an exposure The exit beam passes through the cassette and strikes a fluorescent screen The screen emits light in proportion to the number of x-rays it receives The light photons strike a photocathode Ionization Chamber AEC: AEC Devices: The Phototimer An electric current is generated which is stored in the capacitor The capacitor discharges through the thyratron (thyristor) after a maximum charge is achieved The thyratron sets the maximum charge that the capacitor can hold It is initially set by a service specialist and is calibrated for each type of cassette The density control button adjusts the thyratron The relay opens the exposure switch terminates the exposure Ionization Chamber AEC: Ionization Chamber AEC: AEC Devices: Ionization Chamber Patient Note that the image receptor is below the ionization chamber and the grid. Ionization Chamber Image Receptor Grid Exposure Switch Capacitor Thyratron Relay 32

33 AEC Devices: Ionization Chamber Patient Exit Beam Ionization Chamber Image Receptor Primary Beam Grid The exit beam ionizes the gas found within the ionization chamber and creates an electric signal that is stored in the capacitor. Electrons Exposure Switch A Shortcut to Solving Technique Problems: Capacitor Thyratron Relay AEC Devices: Ionization Chamber Patient Exit Beam Ionization Chamber Image Receptor Primary Beam Grid Electrons The capacitor will discharge after a maximum charge has been met. The electric signal that is generated is eventually sent to the exposure switch which terminates t the exposure. Exposure Switch Screen Conversion Formula: old mas new mas = OR new RS old RS new mas = old mas x old RS new RS Capacitor Thyratron Relay AEC Devices: Ionization Chamber Most commonly employed AEC Exit beam strikes a gas filled ion chamber Ionizations occur that create an electric charge The electric charge is attracted to a (+) anode where it is stored in a capacitor The capacitor discharges through the thyratron after a maximum charge is met The relay causes the exposure switch to open and terminate the exposure Screen Conversion Formula: Which of the following combinations would produce the greatest optical film density? Screen mas New mas New Screen a NC b x 200/50 = c x 40/50 = d x 400/50 =

34 Sample Technique Problem: Which of the following combinations would produce the greatest optical film density? Screen mas Conversion a = 4 b = 6 c = 5 d = 5 Sample Technique Problem: Which of the following combinations would produce the greatest optical film density? SID mas Conversion a = 5 b = 7 c = 6 d = 5 SID & the Direct Square Law: This formula is employed to maintain beam intensity or density at a new distance I 1 D 2 1 I 2 D 2 2 OR new mas = old mas new SID 2 old SID Grid Conversion Factors (GCF): Non Grid to Grid Adjustments: Grid Ratio GCF NG 1 5:1 2 6:1 3 8:1 or 10:1 4 12:1 5 16:1 6 SID & the Direct Square Law: Which of the following combinations would produce the greatest optical film density? SID mas New mas New SID a x (40/60) 2 = b NC c x (40/72) 2 = d NC Grid Conversion Factors (GCF): Employ the following formula to MAINTAIN the original optical film density when making grid to grid changes. new mas new GCF old mas = old GCF OR new mas = old mas x new GCF old GCF 34

35 Grid Conversion Factors (GCF): Which of the following combinations would produce the greatest optical film density? Radiograph Quality Assurance: Causes of Poor Radiographic Quality Improvement of Suboptimal Images GR mas New mas New GR a. 8:1 100 NC = 100 8:1 b. 12:1 80 x 4/5 = 64 8:1 c. 8:1 115 NC = 115 8:1 d. 16:1 150 x 4/6 = 100 8:1 Sample Technique Problem: Which of the following combinations would produce the greatest optical film density? GR mas Conversion a. 8: = 6 b. 12: = 4 c. 8: = 7 d. 16: = 6 Sample Technique Problem: Which of the following combinations would produce the greatest optical film density? mas GR SID Conversion a : =9 b : = 8 c : = 8 d : = 8 35