Lucas Parra CCNY City College of New York BME I5000 Biomedical Imaging Lecture 2 X Ray Imaging Lucas C Parra parra ccny cuny edu some slides inspired by lecture notes of Andreas H Hilscher at Columbia University 1 Lucas Parra CCNY City College of New York Schedule 1 Introduction Spatial Resolution Intensity Resolution Noise 2 X Ray Imaging Mammography Angiography Fluoroscopy 3 Intensity manipulations Contrast Enhancement Histogram Equalization 4 Computed Tomography 5 Image Reconstruction Radon Transform Filtered Back Projection 6 Positron Emission Tomography 7 Maximum Likelihood Reconstruction 8 Magnetic Resonance Imaging 9 Fourier reconstruction k space frequency and phase encoding 10 Optical imaging Fluorescence Microscopy Confocal Imaging 11 Enhancement Point Spread Function Filtering Sharpening Wiener filter 12 Segmentation Thresholding Matched filter Morphological operations 13 Pattern Recognition Feature extraction PCA Wavelets 14 Pattern Recognition Bayesian Inference Linear classification 2 Lucas Parra CCNY City College of New York Biomedical Imaging Imaging Modality X Ray Single Photon Emission Comp Tomography SPECT Positron Emission Tomography PET Year R ntgen 1895 Nobel 1901 Kuhl Edwards Wavelength Energy 3 100 keV 150 keV 1963 1953 Computed Axial Tomography CAT 1972 Magnetic Resonance Imaging MRI Ultrasound Inventor 1973 19401955 Brownell Sweet 150 keV Hounsfield Cormack Nobel 1979 Lauterbur Mansfield Nobel 2003 keV many GHz MHz Physical principle Measures variable tissue absorption of X Rays Radioactive decay Measures variable concentration of radioactive agent SPECT with improved SNR due to increased number of useful events Multiple axial X Ray views to obtain 3D volume of absorption Space and tissue dependent resonance frequency of kern spin in variable magnetic field Measures echo of sound at tissue boundaries 3 Lucas Parra CCNY City College of New York X Ray Discovery Wilhelm Conrad Roentgen 1845 1923 in 1896 and the first radiogram of his hand 1895 4 Lucas Parra CCNY City College of New York Early X Ray Schematic presentation of how it works Detection Fluorescent screen Interaction with tissue Absorption Scatter Generation X Ray tube 5 Lucas Parra CCNY City College of New York X Ray Generation Energy X ray are high energy electromagnetic radiation above 3x1016 Hz and below 10 nm c 8 c 3 10 m s Energy in the keV range e E h 15 h 4 136 10 eV s 5 150 kV 6 Lucas Parra CCNY City College of New York X Ray Generation Tube X ray vacuum tube accelerates electrons emitting form a heated cathode towards anode When electrons impact on anode x rays are emitted This leads to two forms of radiation 1 Bremsstrahlung or breaking radiation 2 Characteristic radiation 7 Lucas Parra CCNY City College of New York X Ray Generation Radiation Bremsstrahlung Maxwell s equations imply that accelerated or de accelerated charges emit radiation Characteristic radiation Electrons ionize atoms in anode Radiation is emitted from heavy elements when their electrons subsequently make transitions between the lower atomic energy levels K and L level to fill that gap Figures from http hyperphysics phy astr gsu edu hbase hframe html 8 Lucas Parra CCNY City College of New York X Ray Generation Tube Design Rotating anode typically Tungsten is used to increase surface area and reduce heating 9 Lucas Parra CCNY City College of New York X Ray Generation Tube Design Due to finite size of focal spot on the anode the image of a disk has a penumbra This leads to blur in the final image i e reduced spatial resolution The goal is to reduce effective focal spot 10 Lucas Parra CCNY City College of New York X Ray Interaction with tissue EM radiation Interaction of EM waves with tissue X ray interact primarily through 1 Photoelectric absorption 2 Compton scattering 11 Lucas Parra CCNY City College of New York X Ray Interaction Scatter and Absorption Interactions as photon progressively loose energy decreasing energy 1 Pair production 2 Compton scattering 3 Photoelectric absorption 4 Inelastic scattering 5 Non ionizing absorption ionizing and therefore carcinogenic bad 12 Lucas Parra CCNY City College of New York X Ray Interaction Scatter and Absorption Secondary photons after Compton scatter may scatter again and will eventually be absorbed 1 3 4 2 5 13 Lucas Parra CCNY City College of New York X Ray Interaction Scatter and Absorption Attenuation coefficient is dominated by Compton scatter and photoelectric absorption Likelihood of photo electic effect Compton effect pair production Total likelihood of x ray absorption 14 Lucas Parra CCNY City College of New York X Ray Interaction with tissue Parameters Likelihood of events are dominated by atomic number Z photon energy E h electron density e and mass density 15 Lucas Parra CCNY City College of New York X Ray Interaction Attenuation Coefficient Likelihood of scatter and absorption events depend on photon energy Material cm at 60keV Iodine used as Air 0 Water 0 21 contrast medium Muscle 0 2 in angiography Fat 0 18 1 Bone Lead used for x ray shielding 0 38 Notice contrast between bone and soft tissue Lower energies give better soft tissue contrast Attenuation coefficient is sometimes given as a density to factor out the effect of mass density 16 Lucas Parra CCNY City College of New York X Ray Interaction with tissue This first Angiography image of 1896 demonstrates well the contrast of due to high and low Z Post mortem injection of mercury compounds Haschek and Lindenthal of Vienna 1896 17 Lucas Parra CCNY City College of New York X Ray Interaction Transmission Imaging Spatially dependent attenuation coefficient and narrow parallel x ray beams give negative image of summed attenuation x z I0 x z z x g x Ideal detector measuring log intensity combines attenuation linearly I x g x log dz x z I0 18 Lucas Parra CCNY City College of New York X Ray Interaction Transmission Imaging X ray imaging measures the intensity of light not absorbed or scattered by tissue which is quantified by the cumulative attenuation coefficient likelihood of I z dz I z absorption per unit length z dz I z dz I z I z z d z d I z I z dz I z lim I z z dz dz dz 0 z I z I 0 exp dz z 0 19 Lucas Parra CCNY City College of New York X Ray Detectors Photographic film Converts x ray into chemical process Phosphor screen Converts X ray to visible light Image intensifier Increases light intensity to detect low dose in real time Digital detectors CCD Direct detection and digitization of x ray to improves