Medical Imaging Physics - University of Denvermysite.du.edu/~balzar/Imaging-13.pdf · May 6, 2008...
Transcript of Medical Imaging Physics - University of Denvermysite.du.edu/~balzar/Imaging-13.pdf · May 6, 2008...
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Medical Imaging Physics 13May 6, 2008
Medical Imaging Physics Spring Quarter
Week 7-1
X-Rays
Davor [email protected]
www.du.edu/~balzar
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Medical Imaging Physics 13May 6, 2008
Outline• Midterm
• Radiology, CT
• Reading assignment:CSG D 16; http://www.sprawls.org/ppmi2/
• HomeworkEssay questionsDue Thursday, May 8
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Medical Imaging Physics 13May 6, 2008
Radiation Interaction with Matter
• X or Gamma rays (photons) can interact with matter:
Penetration w/o interactionAbsorption and deposition of all energy
• Photoelectric effectScattering and partial deposition of energy
• Compton scattering
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Medical Imaging Physics 13May 6, 2008
Interactions
• Photoelectric (photon-electron) effect:Photon transfers all its energy to a bound electron (K or L)Electron is ejected from the atom and gets quickly absorbed close to the origination pointThe vacancy is filled with another electron
• Characteristic radiation (x-rays or visible light)• It’s called Fluorescent Radiation
• Compton scatteringPhoton bounces off an electron and changes its directionSimilar to partially inelastic collision of billiard ballsPart of the photon energy transfers to the electron
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Medical Imaging Physics 13May 6, 2008
Secondary Interactions
• Electron interactionsEjected electron interacts with matter and loses its energy
• Elevated energy state of atoms• Ionization of atoms (33.4 eV per ionization of one “atom” of air)• Very important: 50-keV x-ray photon undergoing a photoelectric
interaction– The initial interaction of the photon ionizes one atom– The resulting energetic electron ionizes approximately 1,500 additional
atoms
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Medical Imaging Physics 13May 6, 2008
Attenuation
• Linear attenuation coefficientThe fraction of photons interacting per 1-unit thickness of material
• Mass attenuation coefficient
Area mass is the amount of material behind a 1-unit surface area )(Density / (µ)t Coefficienn AttenuatioLinear )(µ/t Coefficienn Attenuatio Mass ρρ =
Area Mass (g/cm2) = Thickness (cm) x
Density (g/cm3)
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Medical Imaging Physics 13May 6, 2008
Overall Picture
• PE effect attenuates much stronger because all photon energy si deposited
• Compton Predominant at higher energiesIn tissue: above 30 keV
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Medical Imaging Physics 13May 6, 2008
X-Ray Imaging
• Two different ways:Projecting a (large) shadow image on the receptor
• Radiography and fluoroscopyScanning with a thin x-ray beam and reconstructing 3-D image
• Computed Tomography (CT)
• In general, larger penetration requires higher energies• Different issues define image quality (contrast)
Scattered radiationCharacteristics of the receptor and display systemCharacteristics of body parts and radiation
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Medical Imaging Physics 13May 6, 2008
Contrast
• X-ray imaging contrast:Either atomic number (Z) or density difference
Material Effective Atomic Number (Z)
Density (g/cm3)
Water 7.42 1.0Muscle 7.46 1.0
Fat 5.92 0.91Air 7.64 0.00129
Calcium 20.0 1.55Iodine 53.0 4.94Barium 56.0 3.5
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Medical Imaging Physics 13May 6, 2008
Mammography
• The setup:
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Medical Imaging Physics 13May 6, 2008
Calcium Effects
• Calcium is a source of contrast
Important for imaging bonesEven more important in calcifications that are associated with some pathological conditionsUsed x-ray energy will depend on the size
• Imaging bones requires high energy
• Smaller calcifications in breasts require lower energy
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Medical Imaging Physics 13May 6, 2008
Other Contrast Media
• Iodine and BariumK edge (I) = 33 keVK edge (Ba) = 37 keV
• The highest contrast achieved with a slightly higher x-ray energy
Maximizes PE effect
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Medical Imaging Physics 13May 6, 2008
Radiography
• Chest imaging is challengingHigh contrast between lungs and the mediastinumHigh penetration neededSome objects that need to be identified (calcifications) are very small
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Medical Imaging Physics 13May 6, 2008
Radiography
• Different receptors usedFilm – dynamic range has to be highDigital imaging – overcomes some problems with digital processing (local enhancements of contrast,…)
• Sensitivity of different techniques
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Medical Imaging Physics 13May 6, 2008
Imaging Media
• FilmTraditionally usedProper exposure is crucialGood detail – long exposures neededImage has to be developed
• Intensifying screen and image intensifierIncrease the efficiency and shorten exposure timeCalcium tungstate fluoresces in blue and additionally exposes the film Different materials are used lately (rare-earth elements)
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Medical Imaging Physics 13May 6, 2008
Imaging Media
• Digital detectorsPixels define the imageManipulation and storage simplerBased on photodiodes, scintillation probes, CCDs, storage phosphors,…
• Digital systemsDo not have limitations on exposure time, as defined by film speedWide dynamic exposure rangeMuch more sensitive than image screen systems
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Medical Imaging Physics 13May 6, 2008
Fluoroscopy
• Radiography furnishes information at a particular point in time
• Fluoroscopy: continuous stream of imagesRadiation passing through the patient directed to the fluorescent screenImage intensifier and camera are usually used
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Medical Imaging Physics 13May 6, 2008
Fluoroscopy
• Image intensifierLayer of CsI emits visible light in proportion to x-ray intensity incident on the screenLight ejects electrons from photocathode surfaceElectrons are accelerated toward the anode and form an image on output screenSignificant brightness gain (minification and electronic gain)
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Medical Imaging Physics 13May 6, 2008
Next Time
• Computed Tomography
• Radiation Doses