University of Wisconsin Diagnostic Imaging Research

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University of Wisconsin Diagnostic Imaging
Research
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Lecture 1 Introduction (1/2) History, basic
principles, modalities

• Class consists of
• Deterministic Studies
• - distortion
• - impulse response
• - transfer functions
• All modalities are non-linear and space variant
  to some degree.
• Approximations are made to yield a linear,
  space-invariant system.
• Stochastic Studies
• SNR (signal to noise ratio) of the resultant
  image
• - mean and variance

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Course Objectives

• Learn basics of 2D to n-dimensional system theory
  and signal processing
• Emphasis on duals between space and frequency
  domain
• Emphasis on intuitive understanding
• Understand underlying physics of medical imaging
  modalities
• Study the deterministic and stochastic
  descriptions of medical imaging systems
• Theory is applicable beyond medical imaging

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Prerequisites and Postrequisites

• System Theory
• ECE 330, BME/MP 573
• Statistics Helpful but Not Required
• Mean and variance of stochastic processes
• ECE 331, BME/MP 574, ECE 730
• Other Courses
• Microscopy of Life
• BME 568/ MP 568 MRI ( less math)

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Wilhelm Röntgen, Wurtzburg

• Nov. 1895 Announces X-ray discovery
• Jan. 13, 1896 Images needle in patients hand
• X-ray used presurgically
• 1901 Receives first Nobel Prize in Physics
• Given for discovery and use of X-rays.

Radiograph of the hand of Röntgens wife, 1895.
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Röntgens Setup

• Röntgen detected
• No reflection
• No refraction
• Unresponsive to mirrors or lenses
• His conclusions
• X-rays are not an EM wave
• Dominated by corpuscular behavior

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Projection X-Ray
attenuation coefficient
Measures line integrals of attenuation
Film shows intensity as a negative ( dark areas,
high x-ray detection

• Disadvantage Depth information lost
• Advantage Cheap, simple

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Sagittal
Coronal
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Early Developments

• Intensifying agents, contrast agents all
  developed within several years.
• Creativity of physicians resulted in significant
  improvements to imaging.
• - found ways to selectively opacify regions of
  interest
• - agents administered orally, intraveneously, or
  via catheter

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Later Developments

• More recently, physicists and engineers have
  initiated new developments in technology, rather
  than physicians.
• 1940s, 1950s
• Background laid for ultrasound and nuclear
  medicine
• 1960s
• Revolution in imaging ultrasound and nuclear
  medicine
• 1970s
• CT (Computerized Tomography)
• - true 3D imaging
• (instead of three dimensions projected down
  to two)
• 1980s
• MRI (Magnetic Resonance Imaging)
• PET ( Positron Emission Tomography)
• 2000s
• PET/ CT

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Computerized Tomography (CT)
Result

• 1972 Hounsfield announces findings at British
  Institute of Radiology
• Hounsfield, Cormack receive Nobel Prize in
  Medicine
• (CT images computed to actually display
  attenuation coefficient m(x,y))
• Important Precursors
• 1917 Radon Characterized an image by its
  projections
• 1961 Oldendorf Rotated patient instead of
  gantry

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First Generation CT Scanner

• Acquire a projection (X-ray)
• Translate x-ray pencil beam and detector across
  body and record output
• Rotate to next angle
• Repeat translation
• Assemble all the projections.

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Reconstruction from Back Projection
1.Filter each projection to account for sampling
data on polar grid 2. Smear back along the line
integrals that were calculated by the detector.
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Modern CT Scanner
From Webb, Physics of Medical Imaging
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Computerized Tomography (CT), continued
Current technology
Early CT Image
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Inhalation
Exhalation
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Nuclear Medicine

• - Grew out of the nuclear reactor research of
  World War II
• Discovery of medically useful radioactive
  isotopes
• 1948 Ansell and Rotblat Point by point
  imaging of thyroid
• 1952 Anger First electronic gamma camera

• Radioactive tracer is selectively taken up by
  organ of interest
• Source is thus inside body!
• This imaging system measures function
  (physiology)
• rather than anatomy.

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Nuclear Medicine, continued

• Very specific in imaging physiological function -
  metabolism
• - thyroid function
• - lung ventilation inhale agent
• Advantage Direct display of disease process.
• Disadvantage Poor image quality ( 1 cm
  resolution)
• Why is resolution so poor?
• Very small concentrations of agent used for
  safety.
• - source within body
• Quantum limited
• CT 109 photons/pixel
• Nuclear 100 photons/pixel
• Tomographic systems
• SPECT single photon emission computerized
  tomography
• PET positron emission tomography

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Combined CT / PET Imaging
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Necessary Probe Properties

• Probe can be internal or external.
• Requirements
• Wavelength must be short enough for adequate
  resolution.
• bone fractures, small vessels lt 1 mm
• large lesions lt 1 cm
• Body should be semi-transparent to the probe.
• transmission gt 10-1 - results in contrast
  problems
• transmission lt 10 -3 - results in SNR
  problems
• ? gt 10 cm - results in poor resolution
• ? lt .01Å - negligible attenuation
• Standard X-rays .01 Å lt ? lt .5 Å
• corresponding to 25 kev to 1.2 Mev
  per photon

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Necessary Probe Properties Transmission vs. ?
Graph Medical Imaging Systems Macovski, 1983
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Probe properties of different modalities

• NMR
• Nuclear magnetic moment ( spin)
• Makes each spatial area produce its own signal
• Process and decode
• Ultrasound
• Not EM energy
• Diffraction limits resolution
• resolution proportional to ?