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University of Wisconsin Diagnostic Imaging Research

University of Wisconsin Diagnostic Imaging Research. 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.

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University of Wisconsin Diagnostic Imaging Research

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  1. University of Wisconsin Diagnostic Imaging Research

  2. 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

  3. Wilhelm Röntgen, Wurtzburg Nov. 1895 – Announces X-ray discovery Jan. 13, 1896 – Images needle in patient’s 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 Röntgen’s wife, 1895.

  4. Röntgen’s 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

  5. Projection X-Ray Disadvantage: Depth information lost Advantage: Cheap, simple attenuation coefficient Measures line integrals of attenuation Film shows intensity as a negative ( dark areas, high x-ray detection

  6. Sagittal Coronal

  7. 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

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

  9. Computerized Tomography (CT) 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 Result:

  10. 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.

  11. 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.

  12. Modern CT Scanner From Webb, Physics of Medical Imaging

  13. Computerized Tomography (CT), continued Current technology Early CT Image

  14. Inhalation Exhalation

  15. 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.

  16. 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 proton emission computerized tomography PET: positron emission tomography

  17. Combined CT / PET Imaging

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

  19. Necessary Probe Properties: Transmission vs. λ Graph: Medical Imaging Systems Macovski, 1983

  20. 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 toλ

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