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X-ray Projection Imaging

X-ray Projection Imaging

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X-ray Projection Imaging

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  1. X-ray Projection Imaging

    Module 10
  2. Fundamental Knowledge Characteristics of projection imaging systems that determine capabilities/limitations in producing an x-ray image Detector types used to acquire an x-ray image How radiation is detected by each Attributes of each for recording info
  3. Concise Syllabus Radiography Concepts Geometry Radiographic Contrast Scatter & Scatter Reduction Artifacts & Image Degradation Radiographic Detectors Intensifying Screen & Film Computed Radiography (CR) Direct Digital Radiography (DR) Indirect Digital Radiography (DR)
  4. Radiography Concepts Geometry Radiographic Contrast Scatter & Scatter Reduction Artifacts & Image Degradation
  5. Geometry Distance Magnification Inverse-Square Law 1) Find x.
  6. Projection Imaging 2-D projection of 3-D patient Geometric distortion X-ray beam diverges from focal spot Image magnification Intensity ↓ w/ distance from focal spot Inverse square law X-rays differentially attenuated by anatomical structures in patient Small fraction pass through patient to form image on receptor e-µx
  7. Distance Source-to-Image Receptor Distance (SID) Source-to-Object Distance (SOD) Object-to-Image Receptor Distance (OID) SID SOD Object OID receptor Image
  8. Similar Triangles Similar triangles have same angles as each other but not necessarily the same side lengths Since ΔABC similar to Δabc: a b SOD B A c SID C
  9. Magnification M = SID SID SOD SOD SID SOD Object Object Object OID OID OID Image Image SID same SOD ↑ Object magn. ↓ SID ↑ SOD same Object magnified Image
  10. Inverse Square Law Intensity @ sphere’s surface Area = 4πr2 = I Source strength r Energy 2x as far from source spread over 4x area →¼ intensity
  11. Radiographic Contrast Subject (AKA X-ray Beam) Invisible image emerging from patient due to different attenuation through body Components: Technique factors-photon energy spectrum Physical contrast (i.e. object contrast) Scatter Object Intrinsic anatomic contrast, contrast agents If object physically different, absorbs more/less than surrounding tissue Absorbs less, ↑ radiation hits detector, casts negative shadow, darker Absorbs more, ↓ radiation hits detector, radiograph lighter 3 factors that affect object contrast: Chemical composition (Z) Physical density Thickness Detector Desire  all subject contrast recorded Detector dynamic range/latitude important factor Digital radiography (including CR) much wider latitude than screen-film
  12. Physical Characteristics of Contrast-Producing Materials
  13. Photon Energy & Contrast Absorption too strong 100 Contrast among soft tissue by Z Mass attenuation coefficient Soft tissue/bone contrast by Z Bone 1 Contrast only by density Soft tissues 0.01 10 30 50 100 500 Photon Energy (keV)
  14. Photon Energy Spectrum vs. X-ray Penetration Contrast not only consideration when selecting photon energy Photon Energy αPenetration As amount of penetration ↓, amount of radiation req’d from tube ↑ ↑ Tube loading Subject Contrast = I0 Body Penetration Object Penetration ↓ Contrast ↓ Dose I2 I1 receptor
  15. Scatter & Scatter Reduction Scatter-to-Primary Ratio Scatter Fraction Collimation Anti-Scatter Grids Air Gap
  16. Scattered Radiation X-ray projection imaging based on concept that x-rays travel in straight lines from source Scattered x-rays not aligned w/ original primary x-ray Scattered x-rays not reaching receptor: No effect on image May ↑ radiation dose to nearby staff Scattered x-rays reaching receptor: Creates density where it does not belong ↓ contrast Even if displayed contrast ↑ by windowing & leveling, scatter source of noise ↓ SNR
  17. X-ray beam X-ray beam Air Air Bone Bone Tissue Tissue IT X-ray intensity I1 I0 I2 I3 I1 I0 I2 I3 Position Position W/ scatter W/out scatter
  18. Scatter-to-Primary Ratio (SPR) Describes amount of scatter detected Energy deposited (in a specific location) in detector by scattered photons SPR = SPR ↑ as irradiated tissue volume ↑ Also described by Scatter Fraction (F) F = F =
  19. SPR for Various FOV sizes & Patient thicknesses Patient thickness Square FOV Typical 30 x 30 cm abdominal radiograph for 25-cm thick patient: SPR ~ 4.5 Scatter Fraction = 82% i.e. 82 % of image information useless!
  20. Collimation To ↓ scatter : Collimate beam to smallest size possible that will still encompass necessary anatomy
  21. Antiscatter Grids Widely used in: Radiography Fluoroscopy Mammography X-ray transparent interspace regions alternated w/ x-ray opaque septa Aligned w/ x-ray focal spot X-rays emanating from focal spot Primary radiation High probability passing through to reach detector X-ray emanating from patient Scatter radiation Higher probability of striking absorbing grid septa Septa commonly Pb (Z=82, ρ = 11.3 g/cm3) Interspace state-of-the-art carbon fiber (Z=6, ρ = 1.8 g/cm3) Air ideal, but Pb very soft  req’s support Al (Z=13, ρ = 2.7 g/cm3) used in lower cost grids Will absorb appreciable # of lower energy primary photons
  22. Grid Parameters Grid Ratio (fundamental descriptor) Ratio of height/width of interspace material Radiography 6, 8, 10, 12, 14 Mammography ~5 As GR ↑→ SPR ↓ As GR ↑ → Dose ↑ Grid Frequency # of grid septa per cm = Placed between patient & detector
  23. Bucky Factor Relative ↑ in mAs req’d when using a grid Grid slightly ↓ primary & substantially ↓ secondary radiation striking detector Must ↑ technique (by Bucky factor) to achieve same film OD Radiation dose penalty for using grid Typically ~ 3 - 8 for abdominal radiography Less of issue for digital radiography Much larger dynamic range  less need to ↑ mAs
  24. Bucky Factor 30 cm H2O 30x30 cm FOV 12:1 10:1 Bucky Factor 8:1 6:1 Grid Ratio kVp
  25. Grid Types Parallel Pb strips arranged parallel to each other & perpendicular to incident radiation Short Dimension Allows for crosswise placement AKA “decubitus” grid Long Dimension (standard) Focused Pb strips angled towards focal spot Crossed Parallel or focused strips arranged @ right angles Often used in mammography Stationary Grid lines often seen on image Moving Grid moves w/ reciprocating motion during exposure Perpendicular to long axis of septa Blurs grid lines so that they are not seen in image AKA Bucky grid Long Short
  26. Decubitus (Short Dimension) Grid Improved image quality More uniform density on decubitus a& BE air contrast studies Ease of positioning w/ reduced cut-off Recommended for use in: trans-lateral views of skull, spine, hips emergency room surgery Allow portable crosswise chest radiography on large patients
  27. When are Grids Unnecessary? Patient thickness Extremities, Pediatric
  28. Air Gap Moving patient farther from detector ↓ probability scatter will hit detector Probability α for extended (not point) source Disadvantages: ↑Magnification ↓Anatomical coverage ↑Focal spot blurring  ↓spatial resolution
  29. Scan-Slot System Gold standard for scatter ↓ technique Image produced by scanning a small slot across FOV Scatter ↓ by: ↓ FOV being scanned Pre-patient beam-defining slit ↓ Scatter allowed to reach receptor Post-patient slot aligned w/ pre-patient slit No grid req’d ↑ Dose efficiency Disadvantages: Significant ↑ scan times ↑ Motion artifacts Tube loading ↑ Scan times Narrow aperture req’s↑ tube current ↑ Mechanical complexity  ↑ service concerns Alignment of pre-, post-slots Scanning motion hardware Focal Spot Collimator/Shutters Beam-Defining Fore Slit Primary Radiation Scattered Radiation Patient Table Scatter-Eliminating Aft Slit Cassette
  30. Artifacts & Image Degradation Geometrical Distortion Focal Spot: Blur & Penumbra Grid: Artifacts & Cutoff Motion Superposition
  31. Geometrical Distortion Divergent nature of x-rays produces unequal magnification over the image receptor Focal Spot X-ray Beam ‘Central Ray’ Best resolution/least distortion occurs near central ray Spheres Image Receptor Central ray of x-ray beam centered near the trapezium bone Image represents differences between true anatomical locations & those imaged
  32. Focal Spot: Blur & Penumbra SID SOD Object OID penumbra Penumbra AKA focal spot blur
  33. ↓ Focal Spot Blurring As magnification ↑, focal spot blurring ↑ spatial resolution ↓ As focal spot size ↓, blurring ↓ spatial resolution ↑ ↓ focal spot size SID SID SOD SID SOD SOD Object OID OID OID ↓ magnification
  34. Grid Cutoff Loss of primary radiation caused by improper alignment of grid
  35. Quiz: What is the cause of grid Cut-off? A B C Tube Anode Focus of Grid Tube Anode Focus of Grid Tube Anode Grid Parallel Grid Grid Film Width of Pb shadow Film Width of Pb shadow Film Complete Cutoff Width of Pb shadows @ different distances from center of film Tube Anode D E Focus of Grid Tube Anode Focused grid placed outside focal range Tilted Grid Laterally centered tube Inverted grid Parallel grid Grid E Grid C Film Width of Pb shadows Film Width of Pb shadow B D A
  36. Grid Artifacts Grid lines from low frequency stationary grid Cutoff
  37. Motion Artifacts Detail of lateral chest radiograph shows double images of a vascularclip, a pacemaker wire, and a prosthetic aortic valve.
  38. Superposition 3-D objects projected onto 2-D image causes superposition of anatomical features Tomography (either conventional or computed) reduces contribution of overlying structures
  39. Radiographic Detectors Intensifying Screen & Film Computed Radiography (CR) Direct Digital Radiography (DR) Indirect Digital Radiography (DR)
  40. Intensifying Screen & Film Phosphors Film Screen/Film Systems Latent Image Formation Chemical Processing Characteristic Curve Spatial & Contrast Resolution Artifacts
  41. Screens Intensifying screens made w/ phosphors Substance that gives off UV/visible light when struck w/ x-rays Usually blue, green Visible light exposes film Greatly ↓ dose req’d to expose film (up to dose!) Film alone x-ray absorption efficiency ~ 1% High Z compounds High x-ray absorption efficiency Trade-off between ↓ resolution (w/ screen) & ↑ dose (w/out screen)
  42. Screens Reduce Dose Film X-rays LSF Phosphor grain High binder Exposure Same OD Low X-rays Screen
  43. Screens Introduce Blur X-ray X-ray Thin phosphor screen Thick phosphor screen Light Spread Light Spread
  44. Screen Speed Ranges from 100 – 800 Screen w/ 200 speed rating would be 2x fast as 100 speed, requiring ½ x-ray exposure to produce same amount of light Fast screen Used to ↓exposure time or penetrate extremely thick tissue Slow screens (high definition) made for high definition images where exposure time not critical Medium / Regular most common speed used Provide good resolution w/ relatively low exposures
  45. Intensifying Screen Desirables High x-ray absorption efficiency in diagnostic energy range High conversion efficiency of x-rays to light Transparent to that light Emission spectrum of light released must match film spectral sensitivity Low refractive index-light not internally reflected Robust & Low Cost Relatively short fluorescence decay time Min. light loss due to lateral diffusion in phosphor layer
  46. Phosphor Compounds Originally Calcium Tungstate (CaWO4) Relatively low absorption efficiency for W K-edge 69.5 keV > most diagnostic x-ray energies Rare earth used since 70’s K-edges 35 – 50 keV Lanthanide Series Gd2O2S:Tb-common Gadolinium oxysulfide: terbium LaOBr:Tm Lanthanum oxybromide: thulium YTaO4:Nb Yttrium tantalate: niobium
  47. Comparison: Screen Types CaWO4 (high speed) Gd2O4S (Lanex reg.) X-ray Photon Absorption (%) BaSrSO4 (X-Omatic reg.) Primary Relative Intensity Scatter X-ray Energy (keV)
  48. Film Functions as recording, display & storage device Optical Density (OD) Structure Base Clear plastic sheet ~ 150 µm thick Physical support for emulsion Emulsion Light sensitive Small silver halide crystals (grains) suspended in water-soluble gelatin 95% AgBr 15% AgI Ag slightly + charged, Br slightly - charged ~ 10 µm thick OD 0* .5 1 1.5 2 2.5 3 0.1% 1% 100% 10% Light Penetration *Film OD never == 0 Base not absolutely transparent Fog: Density not associated w/ exposure Base Emulsion
  49. Photographic Process Density produced by converting Ag+ ions into metallic Ag Expose film to light Forms invisible latent image Each grain has structural defect (sensitivity speck) Acts as electron traps Chemical Processing (Development) Converts latent image to visible image in form of range of densities (shades of gray) Micrograph of emulsion layer Silver halide in gelatin on base
  50. Latent Image Formation Film Grain Ag+ Sensitivity speck Light photon Br- Br- electron Ag+ Br + γ → Br+ + e- e- → electron trap Ag+ → electron trap Ag+ + e-→ Ag Ag+ + Ag+→ Ag atom Latent Image Center
  51. Development Development Final developed grain Developer Supplies electrons that migrate to sensitized grains Converts other Ag ions to black metallic Ag Fixer Stops development Clears undeveloped silver halide grains from film Wash Washes fixer solution out of emulsion electrons electrons
  52. Film Speed AKA Sensitivity Determines amount of exposure to produce: OD = 1 + (base + fog) High Speed ↓Patient exposure ↓Tube loading Low Speed ↓Image noise Density 1 Low Sensitivity (Speed) Film Relative Exposure 2 ¼ ½ 1 4 High Sensitivity (Speed) Film 1
  53. Film Characteristic Curve AKA “H & D” Curve Plot of OD vs. log of the relative exposure Latitude Under-exposed Over-exposed
  54. QuizWhich curve represents the faster film? Optical Density OD = 1 + B + F Exposure (mR)
  55. Latitude & Contrast Latitude Range of exposures that give ODs in useful range Contrast Depends on gradient of H & D curve “Steepness” of slope Relationship High Contrast →↓Latitude Harder to achieve consistent proper exposures w/ low-latitude screen-film systems B A Latitude A OD Useful Range Latitude B Exposure (mR)
  56. Screen-Film Systems Sheet of film sandwiched between 2 intensifying screens held in a light-tight cassette Single Emulsion Film uses single screen Dual Emulsion film uses 2 screens Protective layer Phosphor layer Reflective layer
  57. Reciprocity Law Relationship between exposure & OD should remain constant regardless of exposure rate Reciprocity Law For very long or very short exposures this relationship breaks down Reciprocity Law Failure Reciprocity Law obeyed OD Reciprocity Law Failure Exposure Time (sec)
  58. Artifacts Pressure Abrasions & Scratches Fingerprints Static electricity Material in cassette Fog Light leak
  59. Artifacts – Crossover Exposure Escape of light from one emulsion to the other in dual emulsion film ↑Film speed ↓Film resolution X-rays Desired exposure screen emulsion base Crossover exposure
  60. Cast Source of artifact?
  61. Patient Motion Source of artifact?
  62. Double Exposure Source of artifact?
  63. Poor Screen-Film Contact Sources of blur in S-F systems
  64. Clothing Source of artifact?
  65. Hair
  66. Debris/Scratches
  67. Scratches from Rollers in Automatic Processor
  68. Developer Spots
  69. Water Spot
  70. Fixer Retention Chemicals not completely washed off Over time film will discolor
  71. Crimping/Creasing
  72. Light Fog
  73. Static Electricity
  74. Computed Radiography (CR) Storage Phosphors Latent Image Formation Image Digitization Pre-Processing (e.g. Gain & Bad-Pixel Correction) Imaging Characteristics Artifacts X-rays Storage Phosphor IP Storage of x-ray energy Laser Storage Phosphor, IP Readout process Light Photomultiplier, A/D Converter Conversion to electrical charges
  75. Storage Phosphors Photostimulable Phosphors (PSP) Traditional scintillators emit light nearly instantaneously upon irradiation PSP emit some instantly, but fraction is trapped Trapped energy released via stimulation of HeNe laser Material BaFBr:Eu (Barium Fluorobromide doped w/ Europium)
  76. Latent Image Formation Energy band theory Electron environment in certain crystalline materials can be represented by 2energy bands: Valence Band Conduction Band Separated by a forbidden energy zone Energy Gap Conduction band Trap Increasing Energy  Energy Gap Refers to energy of crystals atomic e-s Valence band Outer shell e-s occupy valence band When e-sleave atom they join conduction band Normally energy gap cannot be occupied by e-s Impurities/defects in lattice structure may generate intermediate energy levels Can trap e-s w/in energy gap
  77. X-ray Absorption → Latent Image Process 1: X-ray absorption causes e- from valence band to be elevated to conduction band if sufficient energy given to overcome energy gap C Process 2: e- can be collected directly e.g. a-Se detectors in direct digital radiography Laser stimulation Increasing Energy  PSL* X-ray energy excites e- Process 3: Or e- may immediately drop back to valence band accompanied by emission of visible/UV light radiation e.g. intensifying screens, fluoro, indirect digital radiography V Process 4: Or e- dropping from conduction band gets trapped in intermediate energy level Stays there until stimulated to release rest of energy by emitting light by: Heat e.g. thermoluminescence, TLD radiation monitors Light e.g. photoluminscence, PSL in CR *Note: process greatly oversimplified
  78. Readout/Digitization Unexposed Imaging Plate 1st readout doesn’t release all trapped e- in latent image; rest released w/ exposure to very bright light
  79. Pre-Processing-Shading Corrections “Raw” data req’scompensation for variations in light-guide response to uniform IP exposure e.g. PSL intensity @ edge of light guide < @ center Over time subtle deposits of material on light guide produce fixed variations in light intensity Introduce static noise patterns Reproduced @ every scan
  80. Preprocessing: Identification & Scaling Raw data must be properly identified prior to post-processing for specific anatomical display Find collimator borders for 1 or more exposures Produce histogram From which examination-specific distribution used for determination of min & max useful signals Scaling (under- & over- exposure compensation) Similar presentation of output data →independent of incident exposure Identify histogram shape- Unchanged if exposures fall w/in detector’s dynamic range Internal amplification used if: Under exposed (↑) or Over exposed (↓) Collimation areas determined Histogram Collimation Border Shift & Subtract Shape dependent on study type, positioning, & technique
  81. Image Post-Processing Raw Data Contrast Enhancement Contrast Reduction (Smoothing) Edge enhancement
  82. Imaging Characteristics Exposure Latitude Extremely wide compared w/ screen-film Linear characteristic curve Allow wide range of exposure info to be captured Wide dynamic range allows use under broad range of exposure conditions e.g. bedside radiography
  83. Characteristic Curve Dynamic range Screen-film systems have limited tolerance for radiation exposure Steep & tight curve Dynamic range ~ 16:1 Digital detector Signal response Curve for digital detectors less steep & covers wider range Optimal signal response will occur over a wider exposure range Dynamic range ~ 10,000:1 Screen-film Dose [µGy]
  84. Exposure Latitude Film-Screen Exposure Variations CR
  85. Properly exposed CR abdomen Under exposed CR abdomen
  86. In some cases, particularly in image areas w/ little or no attenuation, overexposure of patient & digital detector can result in saturation &loss of image info beyond the linear operating range of the detector, as shown for lung areas &un-collimated areas adjacent to the patient anatomy Over exposed CR
  87. Background Radiation
  88. Moiré Artifact Stationary Grid Grid lines & scanning laser are parallel
  89. Wrong Processing Algorithm
  90. Image Noise Due to quantum mottle Prevalent in underexposed images Due to detector variations Prevalent at very high exposures In either case, patient care compromised Inability to achieve optimal image Needless radiation overexposure to the patient
  91. Detector efficiency in converting incident x-ray energy into image signal Calculated by comparing SNR(out) to SNR(in) as a function of spatial frequency High DQE  ↓radiation needed to achieve same image quality DQE
  92. Radiation Exposure ↓Retakes due to wider exposure latitude CR DQE > Screen-Film DQE Can either have higher image quality for same dose, or Same image quality for lower dose Exposure ↓ w/ CR limited to certain clinical indications & cannot be applied unrestrictedly Some incidental finding might be masked by ↑ image noise in low-exposure images
  93. Spatial Resolution Pixel size = 200 µm 2.5 - 5 lp/mm limiting spatial frequency Matrix ~ 2000 x 2500 Screen-Film Depends on screen thickness Fast Thick Req’s less dose ↓ resolution 6-9 lp/mm Slow (Detail) Thin Req’s more dose to achieve same quality ↑ resolution 10-15 lp/mm
  94. MTF Comparison Resolution depends on: Scattering of x-ray w/in phosphor Mainly depends on scattering of stimulating laser light w/in phosphor layer HR Modulation Screen-film ST Spatial Frequency (LP/mm) HR: High Res. IP ST: Standard Res. IP
  95. Contrast Resolution What CR may lack in inherent spatial resolution, makes up in ability to manipulate image contrast & brightness Screen-Film What you see is what you get [Hot light for very dark areas]
  96. Scan direction  CR Artifacts Gridlines Failure of reciprocating Bucky Inappropriate grid frequency Grid position vs. readout scan lines Latent image Failure to erase properly Extreme overexposure Storing improperly Double exposure Linear Dust on light source Damaged IPs - Scratches, moisture, lint, dirty Incorrect histogram shape identification Collimator borders not correctly found Wrong examination Poor patient positioning Excessive scatter Highly attenuating objects such as prostheses Extreme under- or over-exposure Inappropriate kVp Subscan direction 
  97. Double Exposures
  98. Exposure Factors Over exposure Under exposure
  99. Debris Linear white artifact is superimposed over the head of the fibula (white arrow). Ahair was found in the CR cassette Prevented light emitted by photosensitive phosphor plate during reading process from reaching detector Dust or debris was present on light guide as CR plate was fed through CR reader. Resultant artifact is a thin white line (white arrows) that spans the length of radiograph
  100. Debris? Foreign Body? It is difficult to distinguish debris on this CR imaging plate from foreign bodies (arrows). Technologist must open cassette &inspect the plate or re-expose the cassette.
  101. Bad IP Cleaning Artifact
  102. Mechanical: CR Transport Failure
  103. Failure to Determine Collimator Boundaries Exposure Data Recognizer (EDR) failure Inability of the software to determine collimation boundaries. Causes: Operator fails to follow collimation rules or Interferences prevent software from detecting collimation boundaries Results in Incorrect histogram analysis & Inappropriate rescaling Can re-collimate manually
  104. Missing Lines/Pixels Missing lines or pixels in CR can indicate memory problems, digitization problems, or communication errors
  105. Direct Digital Radiography (DR) Semiconductor & Thin-Film Transistor Image Formation & Readout Pre-Processing (e.g. Gain & Bad-Pixel Correction) Imaging Characteristics Artifacts X-rays Photoconductor (aSe) Conversion of x-ray energy to electrical charges TFT array Readout Process
  106. Band Theory of Solids Instead of having discrete e- energies (as in free atoms) Available energy states for e-s form bands Conduction req’s e-s in the conduction band Insulators e-s in valence band separated by large gap from conduction band Conductors Valence band overlaps conduction band Semiconductors Small enough gap between bands that thermal/other excitations can bridge gap Conduction Conduction Conduction Valence Valence Valence Conductor Insulator Semiconductor
  107. Semiconductors 0 K No e- in conduction band 300 K Conduction Conduction 1.09 eV Valence Valence Current small compared w/ doped semiconductors under same conditions
  108. TFT Photosensitive array made up of small detector elements (dexels) 100 – 200 µm Each dexel contains photodiode that: Absorbs e- & Generates electrical charges & Stores signal until readout Electronic switch containing 3 connections: Gate TFT on/off switch Source Attached to storage capacitor Drain Conductor line running along each array column Similar technology in LCDs
  109. Image Formation + + + + + + + Electric field → Screen-Film X-rays absorbed & directlygenerate electronic signal Under influence of external electric field: Holes/e-s (depending upon applied field polarity) drift towards a pixel electrode Collected on a pixel capacitor Variations in surface charge correspond to incident x-ray pattern e-s/holes travel along electric field lines: No lateral movement of charge → Narrow PSF ~ 1μm
  110. Direct Digital Radiography X-rays Top Electrode Dielectric Layer Photoconductor Electron Blocking Layer Charge Collection Electrode TFT Programmable high voltage power supply Signal Storage Capacitor Glass Substrate
  111. Readout During exposure: TFT in off mode while charge collected @ signal storage capacitor After exposure: + pulse applied to TFT Sequentially activates each dexel, row by row Accumulated charge in each dexel flows through drain line Charge is: Amplified Converted to proportional voltage Voltage level is digitized Gray scale value for each dexel Erase cycle Removes any residual charge to prepare for next exposure
  112. Image Pre-Processing Non-Uniformity Corrections Correction LUT stores a reference image of detector array response to a uniform x-ray exposure Variations in array response @ different energy/exposure levels Due to slight differences in gain of each dexel & charge amplifier output, or Bad Pixels Manufacturing defects may manifest as individual bad pixels or entire bad pixel columns or rows Dark Field Reference Images a-Se generates small dark current @ room temp Acquired before & after each exposure 2 reference images averaged Simulates background noise during exposure Crystal Amorphous
  113. Imaging Characteristics: Resolution Indirect Processes Direct Processes Computed Radiography Fluorescent Phosphor Photodiode Array Cesium Iodide Photodiode Array a-Se TFT Array X-ray X-ray X-ray X-ray Surface reflector Micro-plated electrode Photostimulable phosphor imaging plate Dielectric layer Intensifying screen CsI a-Se Imaging plate moved to reader Photodiode array w/ a-Si TFT & storage capacitor Electrode collection array w/ a-Si TFT & storage capacitor Photodiode array w/ a-Si TFT & storage capacitor PSP IP Laser Signal profile Signal profile Signal profile Signal profile
  114. Speed Considerations X-ray X-ray Direct conversion: High resolution maintained even @ higher speed Direct conversion: High speed & resolution Line spread functions
  115. Imaging Characteristics: MTF a-Se a-Si CR
  116. Indirect Digital Radiography (DR) Phosphor, Photodiodes & Thin-Film Transistor Image Formation & Readout Pre-Processing (e.g. Gain & Bad-Pixel Correction) Imaging Characteristics Artifacts X-rays Scintillator Conversion of x-ray energy to electrical charges Light Photoconductor (aSi) TFT array Readout Process
  117. Phosphor, Photodiode, & TFT Indirect-conversion systems based on TFT arrays are constructed by adding a-Si photodiode circuitry & a scintillator (phosphor) as top layer of TFT Phosphor Unstructured e.g. Gd2O2S X-ray interactions more likely to occur toward front of scintillation layer Light has to propagate large distances to reach TFT array Structured e.g. CsI Grown in columnar crystals Act as light pipes to ↓ lateral spread of light Photodiode Converts light into current or voltage
  118. Image Formation & Readout X-rays strike phosphor Visible light emitted α Incident x-ray energy Visible light photons converted into electric charge by photodiode array Readout electronics Charge collected at each photodiode converted into digital value
  119. Imaging Characteristics
  120. Speed Considerations X-ray X-ray Indirect conversion: low speed, high resolution Indirect conversion: high speed, low resolution Line spread functions
  121. Artifacts-LUT Errors LUT: curve that maps pixel values to monitor brightness Raw data bit depth typically 12-14 bits During preprocessing LUT applied Bits beyond display range discarded 10-12 bits used for viewing & storage Info loss can lead to clipping Window/Level adjusts LUT interactively If image clipped, no amount of adjustment can display lost info
  122. Clipping Lateral radiograph of dog’s tibia w/ LUT appliedduring acquisition that caused clipping. (A) Image as presented to viewing software. (B) After the radiograph is re-windowed using the display software. It is obvious the soft tissues were clipped and are no longer available.
  123. Artifacts - Halo Radiolucent halo around metal/areas where large density differences between adjacent objects Simulate loosening of orthopedic devices Mimic pneumothorax Wide dynamic range → single radiograph captures > dynamic range that can be viewed on monitor Window/level or To view all anatomic areas at once → image processing Mathematical data manipulation to min. range of displayed pixel values But maintain high contrast Unsharp Masking (Edge Enhancement) Make copy of original Blur it Subtract from original Pixel values along edges are not real e.g. bone/soft tissue interface, blurring averages pixel values After subtraction, edge enhancement causes halo artifact
  124. Halo Cropped, lateral radiograph of a dog’s stifle that has had unsharp masking applied to improve the display. (A) Black halos (black arrows) present around distal part of bone screw &femoral condyle. (B) Multi-frequency processing* applied to improve the display. No black halos are noted. Multi-frequency processing: Decompose image into frequency components Higher frequency (sharp edges &noise) Lower frequency (overall density) Low-frequency components can then be removed Image reassembled w/ minimal halo artifact or noise enhancement
  125. Artifacts – Under Exposure When receptor does not get enough X-ray input resultant image is: Grainy Noisy Mottled Pixilated Adjusting LUT gives appearance of proper exposure Subtle findings may be missed Display size ↓, apparent sharpness ↑ Zoom factor ↑, noise becomes objectionable ↑ Exposure/dose req’d to see finer detail
  126. Artifacts – Over Exposure Even w/ wide dynamic range, as exposure ↑ eventually detector system will saturate Each pixel in overexposed are set to max. value Structure margins, esp. thin structures no longer viewable CR Saturated areas appear uniform DR Calibration mask becomes visible
  127. Over Exposure 2radiographs of canine L-spine acquired from a flat panel detector (FPD) system. Radiograph overexposed to the point of detector saturation. Mid-abdominal organs not seen & soft tissues dorsal to spine are completely black Calibration mask is visible, revealing the plank-like arrangement of FPD system (B) Properly exposed lateral radiograph of L-spine of same dog.
  128. Artifacts - Calibration Mask Uniform X-ray exposure applied to digital detector → Allpixel responses should be identical X-ray field inherently not uniform Heel effect Inverse square law Calibration mask compensates Acquire a flood field Set each pixel sensitivity to be identical If anything in x-ray beam during calibration, artifacts result
  129. Calibration Mask Errors Piece of electrical tape placed on X-ray table. Calibration mask created. Flood-field radiograph made w/out moving table. No artifact because calibration mask successfully compensated for tape. Table moved to right & 2nd flood-field radiograph made. Pair of black & white images of tape now visible. Black image is correction made by calibration mask & white image is actual piece of tape. FPD system calibrated w/ small spot of contrast medium on collimator face. Dark artifact (↑) caused by spot being burned into calibration mask. Indistinct borders ofartifact meant the offending object was not on detector panel.
  130. Artifacts - RF Interference Detectors are shielded from extraneous RF fields Periodic pattern artifacts can occur: RF source placed in close proximity e.g. some AEC detectors Break in shielding of cables Example (between white arrows) caused by breakdown in RF shielding in FPDsystem. Artifacts from RF interference can take many forms but typically generate a repeatable pattern in the radiograph
  131. Artifacts – Ghost Images Analogous to ghost images produced by certain film/screen systems because of afterglow in the scintillating screen Screen stimulated, light emitted Intensity ↓ w/ time Cassette immediately reloaded w/ fresh film, afterglow from previous exposure can be burned into film Particularly w/indirect DR Stimulated photodiodes trap charge Release of that charge can persist after readout
  132. Ghost Images Ghost image of lead L marker adjacent to equine patella from a FPD system. Radiograph w/ L marker in non-collimated portion of X-ray beam was made seconds before this image. Ghost L occurs because photodiodes around marker record a very high radiation dose relative to photodiodes beneath marker. For a short period of time after 1stexposure, level of charge retained by photodiodes differs between the 2areas. 2ndexposure was made, negative images of L marker appeared.
  133. Processing Algorithm (A) Inappropriate processing of pediatric digital radiography image by vendor-supplied parameters suitable for adults (B) Image processed by parameters modified by customer. Images acquired on GE DR system
  134. Clinical Application Demonstrate how variations in each of the fundamental characteristics of projection imaging system affect detected information in producing an image. Give examples of how each detector type performs in imaging a specific body part or view, & describe how attributes of each detector type influence resulting image.
  135. Clinical Problem-Solving What is the difference in exposure class between CR & DR systems? How does this difference affect patient dose? Describe some of the common artifacts seen in a portable chest x-ray image, & explain how these can be minimized. Describe how distance to patient & detector affect patient dose. Describe how transition from film to digital detector systems eliminates some artifacts & creates possibility of others. What are properties of a detector system that determines its suitability for pediatric procedures?
  136. End