Advanced Imaging

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Advanced Imaging

• CT
• Computed Tomography

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No Shadows
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Computed Tomography (CT)

• Body section linked w/ computer images
• Tomography- Greek word section
• CT Imaging accomplished in 3 steps
• Scanning (data acquisition)
• Processing (image reconstruction)
• Image display
• CT- now in 3 planes

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It All In How You Slice IT
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How CT Scanners Work

• Test to ensure scanner working properly
• Patient placed in gantry and positioned correctly
  for exam
• Tech sets technical factors at control console
• Scanning can begin
• Detectors collect attenuated beam

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• Detectors convert x-ray photons into analog
  signals
• Analog signals changed to digital (numerical)
  signal
• Computer performs the image reconstruction
  process
• Image displayed
• Image stored see picture p. 6 in CT book

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The Gantry

• X-ray tube, detectors, collimators, mechanics
  that provide motion
• Angled 30 degrees in 2 directions
• Central opening- aperture
• Table ( couch) linked to gantry

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The Computer

• 2 types software
• Operating system
• Manages hardware
• Applications
• Manages preprocessing, image reconstruction, post
  processing
• Operating console
• Networking and PACS
• Storage/ review

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Image Acquisition

• CT tube
• Detectors (sometimes called scintillators)
• Analog-to-Digital Convertor(ADC)- so data can be
  processed and stored
• Array Processor
• Specialized computer
• Reconstructs images from raw data
• Anatomical and contrast raw data placed in proper
  location on the image
• Millions of calculations in less than a second
• PACS

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Image Reconstruction

• Digital images - numerical representations of
  objects recognized and processed by computers
• CT reconstructed using algorithms
• Matrix- image - rows and columns of tiny blocks
  called pixels
• Matrix size rowsX columns
• Pixel - 2D representation of 3D volume of tissue
  (Voxel)

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Post- Processing Manipulations

• Hounsfield numbers (inventor of 1st CT scanner)
• Shades of gray assigned CT numbers( based on
  attenuation properties)
• Dense (bone) 1000
• Baseline WATER 0
• Air - 1000
• Snopek, p. 152
• Window Width (WW)- CT s range gray scale
• Controls contrast
• Wide window-low contrast-chest
• Narrow window-high contrast-bone

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• Window Level (WL)- sets center CT of WW
• Controls brightness
• Determined by tissue density most abundant in
  area
• Adjusted by radiographer to enhance structures
• Pitch- determines amount anatomy covered
• Ratio between table speed and slice thickness
• Determined by radiologist

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Image Display, Manipulation, Storage, Recording
and Communication

• Image can be displayed on cathode ray tube, LCD
  screen
• Imaging manipulation
• Re-formatted into various planes
• Smoothing, edge enhancement, gray-scale
  manipulation, 3-D processing
• Communication made possible by DICOM digital
  imaging and communication in medicine

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Image Digitization

• Convert analog to digital
• 3 steps
• Scanning- image transparency w/ grid overlay
  (pixels)
• Sampling- measures brightness in each pixel
• Quantization- sampled pixel assigned integer (
  signal dependent)

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• Characteristics of Digital Things
• Easily manipulated / processed by computer
• Electronically transported to sites
• Easily stored
• Accurately reproduced

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Why digitize?

• Image Enhancement
• Image Restoration
• Image Analysis
• Image Detection
• Pattern Recognition
• Geometric Transformation
• Data Compression

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Helpful CT Terms

• Spatial Resolution
• Contrast Resolution
• Region of Interest (ROI)
• Scan (FOV)
• Reconstructed (FOV)

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CT Advantages

• No anatomic superimposition
• Increased contrast resolution
• ( distinguish 1 soft tissue from another)
• MPR- multiplanar reconstruction
• Manipulation of data
• Viewing CT Images
• Pt right placed to viewers left like x-ray
• Axial scans viewed like viewer facing pt looking
  at foot end

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Limitations of CT

• 1. Higher dose than radiography
• 2. Difficult to image tissue with surrounding
  bone ( skull area)
• 3. Metallic objects produce streak artifacts

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Basically 3 CT Scanning Methods

• 1. Localizer- single projection scan obtained.
  Generates image used to position the
  cross-sectional slices.
• 2. Conventional CT- characterized by an
  increment of pt. table after x-ray tube rotation(
  slice-by-slice)
• 3. Helical or Spiral CT- characterized by
  continuous pt. table motion during x-ray tube
  rotation.

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• Localizer can be in AP or Lateral position, done
  w/ Conventional CT
• Most CT done w/ helical CT
• speed
• ease of use
• ability to reconstruct images

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Evolution of CT

• Clinical CT introduced early 1970s
• 5 Generations, then Spiral ( Helical)
• Scan method- x-ray tube and type detectors
• Detectors- devices that measure x-ray beam
  attenuation
• FIRST GENERATION
• Pencil beam, 1 detector, rotate-translate
• Parallel beam 180 degree
• 4.5 min per slice ( only head CT)

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• SECOND GENERATION
• Fan beam w/ multiple detectors
• Rotate-translate
• 180 degree
• 15 sec/ slice
• THIRD GENERATION
• Fan beam w/ multiple detectors
• Rotate-rotate design
• 360 degrees
• Decreased scan time

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• FOURTH GENERATION
• Developed in 1980s
• Rotating fan beam w/ 360 degree stationary ring
  of detectors
• Rotate- stationary geometric design
• FIFTH GENERATION
• EBCT (Electron Beam Computed Tomography)
• Alternate design- electron gun/ does NOT use an
  x-ray tube
• Mainly cardiac

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• Spiral or Helical
• Continuous tube and detectors rotate
• Patient steadily advanced thru gantry
• Scan times significantly reduced
• Prevents breathing artifacts
• Slip ring replaced high tension cables

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• Generation 1-4
• Slice by slice Scanners
• High tension cables
• Scan, delay, scanner reset, pt repositioned, next
  slice
• Limitations
• Longer exposure times
• Missing anatomy (breathing)
• Inaccurate reformatting (breathing)
• Few slices/ max contrast time

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• 5th generation, spiral considered volume scanning
• Volume of data in short time
• Slip ring technology
• 64 slices/ tube rotation
• Advantages
• Multiplanar reconstruction (MPR)
• Short scan times
• Artifacts reduced (breathing not problem)
• Decreased amount contrast needed
• Improved spatial resolution
• Improved reconstructed image quality

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• Disadvantages
• Cost
• Large volume cases (review/ archive)
• CT system Components ( 3 major)
• Gantry
• Computer
• Operator Console

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CT DOSE

• Factors that affect CT dose
• Slice thickness
• Noise
• Resolution detector efficency
• Reconstruction algorithms, collimation and
  filtration

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• Collimator assembly (p. 156)
• Reduces pt dose
• Improves image quality
• 2 collimators
• Prepatient- at x-ray tube-restricts beam exiting
  CT tube
• Post patient- at detector which shape and limit
  beam

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Minimizing Radiation Dose

• To Patient
• Decrease mAs
• Decrease anatomical coverage
• Increase slice thickness
• Increase table increment
• Increase pitch
• Decrease dose repeat by making pt. comfortable
• To CT Technologist
• Maintain distance from beam
• Close scan room door to control room
• Wear lead aprons, gloves and drop down shields

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• CT Artifacts- anomalies
• User error, design defect, improper system
  maintenance, system failure, normal physical
  phenomena, voluntary or involuntary pt. motion,
  pt. prep mistakes
• Radiation Protection
• Technical factor selection
• Technical adjustments for children (lower KVP and
  MAS values)
• Scatter reduction

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CT Advantages

• Demonstration of Anatomy
• Ease of performance
• Elimination of patient discomfort
• Noninvasive procedure
• Reduction in Hospital
• May be performed on an outpatient basis

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CT with Contrast- What it Visualizes

• Brain vascular system
• Blood Brain Barrier (BBB) , Bontrager, p. 735
• Brain tissue has natural barrier
• Some substances wont pass normally
• Contrast appearing outside normal vascular system
  indicates problem
• Thoracic CT- mediastinum structures
• Vessel assessment/ aneurysm

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CTA ( p 361-363 Patient Care Book)

• Use spiral/ helical scanners
• Differentiate vessels from bone and soft tissue
• Less risky than angio but lt image resolution for
  small vessels
• High volume contrast remote location tech
  potential problems
• Abdominal organs opacified by contrast