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X-Ray Imaging, 1! Medical Imaging Modali.es • Recall: based on absorbed or emi+ed signal. • Categorize by Radia2on & Imaging Source External source Internal source X-‐ray radiography SPECT X-‐ray CT PET Ultrasound Op.cal Mixed source MRI, fMRI Op.cal fluores -‐cence X-‐Rays • Wilhelm Conrad Röntgen – 11/8/1895: discovers X-‐rays. – 11/22/1895: X-‐rays Mrs. Röntgen’s hand. – 1901: Nobel Prize in physics • Basic idea: X-‐rays Basics X-‐ray: EM waves of wavelength λ and frequency f: c = λ f Energy of photon characterized by wavelength. e = 1.24 keV / λ (nm) Energy sufficient to trigger ionizing radia1on. Exposure of pa.ent important to determine & limit. X-‐Ray – Atomic Structure Review • Atom comprised of nucleus and electrons revolving around it in orbital shells. • Protons and neutrons comprising the nucleus provide mass to the atom. • Each electron shell has a characteris1c energy level represen.ng binding energy of electron. X-‐Ray – Atomic Structure Review • K shell binding energy > L shell > M > N … • An electron can be ejected or transferred to another shell depending on energy exchange caused by interac.on of atom with quantum. • Total energy is preserved. Energy lost by electron due to change in path is emi\ed in form of X-‐ray electron. X-‐ray–ma\er Interac.on #1 • Rayleigh Sca+ering – elas.c collision of photon with ma\er; slight change of direc.on of photon travel. Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray–ma\er Interac.on #2 • Photoelectric absorp1on -‐ • Photon transfers all of its energy to inner (K or L) shell electron, subsequently ejected from atom (lee). • Empty shell resul.ng from ejec.on filled from upper orbits, resuling in characteris.c X-‐rays. Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray–ma\er Interac.on #3 • Compton Sca+ering –photon and outer shell -‐ valence electrons interact, with negligible binding energy. • The electron is ejected, and photon sca\ered with reduced energy. Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray–ma\er Interac.on #4 • Pair produc1on-‐ high energy photon interacts w. nucleus. Photon’s energy transformed into negatron and positron. • Positron interacts w. electron → annihila.on of pair. • Annihila.on of positron-‐electron pair → two photons. Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray – ma\er interac.on summary • Most important effects are Compton sca+ering and photoelectric absorp1on µ/ρ (cm2/g) 1.0 Compton Sca\ering Total Mass A\enua.on Coefficient Photoelectric Absorp.on Sca\ering Rayleigh Sca\ering 0 0 100 Photon Energy (keV) 500 X-‐Ray Genera.on • Heated tungsten cathode (e-‐ source), copper anode oeen used. – K-‐ and L-‐shell binding energies: 70 & 11 keV. • Electrons oeen interact with several nuclei before stopping – X-‐ray photons -‐ polychroma.c energy: Bremsstrahlung. • Special filtering needed: beam hardening. Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray 2D Projec.on Imaging X-ray Source 3-D Object or Patient Anti-scatter Grid X-ray Screen Film X-ray Screen 2-D Projection Image X-‐ray 2D Projec.on Imaging -‐ limita.ons • Focal spot: non-‐zero area → loss of resolu.on • A\enua.on es.ma.on complicated -‐ polychroma1c energy. • Image intensifier artefacts VignePng: shading artefact Pincushion distor2on: Curved screen, magnifica.on S-‐distor1on: external EM fields affect electron trajectory. X-‐ray Fluoroscopy and Angiography • Fluoroscopy: imaging for real-‐.me x-‐ray viewing of pa.ent. • Basic idea: x-‐ray source, image intensifier, CC television • Needs: Real-‐.me (intraop) imaging, Aligning pa.ent w.r.t. imaging system. Possible moun.ng on C-‐arm + Contrast agent, subtrac.on = angiography Reproduced from The Essen.al Physics of Medical Imaging – JT Bushberg, 2nd ed. X-‐ray Mammography • Need: high res, high contrast and low radia.on dose. • Entails mono-‐energe1c and low-‐energy source . • For .ssue 3-‐6 cm thick, 17-‐25 keV works “best”. • Use molybdenum or rhodium, not tungsten. • Filtra1on and an1sca+er grids reduce sca\er and dose. Reproduced from Dhawan & Basics of X-‐ray and Mammographic Systems Univ. Washington lecture X-‐Ray CT • Tomography by slice stacking • Why? 3D Shape important: Tumors, broken bones. • Same principle: measure a\enua.on along path. • Assume 3D object to be a stack of 2D slices. y x z X-Y Slices X-‐Ray CT • Tomography by slice stacking • Consider X-‐ray source-‐detector pair, parallel to x-‐ axis, transla.ng along y to cover en.re 2D slice. • Output intensity via sum of a+enua1ons along x. • Take a series of these x-‐projec1ons along y. y x µ(x,y; z) z Iin(x; y,z) µ15 µ12 µ22 µ42 µ52µ62µ72 µ82µ92 µ11 Iout(x; y,z) Image Reconstruc.on for CT • Idea: from 1D projec1ons obtained at different angles around the selected 2D slice, a reconstruct 2D image. • The original reconstruc.on method is called filtered backprojec1on. • A series of 2D images, obtained at varying z values, are stacked together to form a volume. Computed Tomography: Concept History: Computed Tomography • The breakthrough: – acquiring many projec.ons around the object enables the reconstruc.on of the 3D object. History: Computed Tomography (Cont’d) • 1917: Johann Radon: mathema.cal framework. • 1963: Allan Cormack: tomographic image reconstruc.on, • 1972: Godfrey Hounsfield: first CT system, reconstruc.on. • 1979 Hounsfield & Cormack: Nobel Prize in Medicine. Radon Cormack Hounsfield 4 Genera.ons of CT Scanner 1. Translate-‐rotate pencil beam geometry. 2. Translate-‐rotate fan beam geometry. 3. Rotate-‐only geometry and offset-‐mode geometry. 4. Detector ring around the object (720+ detectors), & divergent X-‐ray fan beam. Ring of Detectors Source Rotation Path Source X-rays Object Spiral CT • Why Spiral CT? Need for high-‐res, e.g. neurosurgery. • Normally, high-‐res=long scan .mes; • want fast scan, low dose. • Classical CT: keep pa1ent sta1onary while source-‐ detector ring gantry is translated along z. • Spiral CT, bed moved while gantry is rotated. CT Chest Images A\enua.on coefficients of several .ssues expressed in Hounsfield units.
