Contents. X-ray and Computed Tomography. Characterization of X-rays. Production of X-rays

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1 J. E. Wilhjelm Ørsted TU Technical University of enmark, Bldg. 348, K-2800 Kongens Lyngby, enmark. X-ray and Computed Tomography Contents History and characterization of X-rays Conventional (projection) X-ray Attenuation, contrast and resolution (Fluoroscopy and examples) CT scanning Principle Image presentation techniques Examples of images from the VH project Ted Meyer: Structural Abnormalities (1992).Oil on canvas, 3' by 4'. Key points of history of X-ray The effect of ignoring what can't be seen 1895 Wilhelm Conrad öntgen discovered X-ray 1896 After being public, X-rays were all over the world within weeks 1896 GE and Siemens begins selling X-ray equipment 1896 Fluoroscopy appears 1904 Hazards of radiation is described 1924 Introduction of "tolerance dose" 1925 The meeting where they could not eat meat 1956 First reconstruction algorithm 1958 First model CT scanner without computer 1968 Hounsfield's method for CT patented 1972 Hounsfield's method for CT demonstrated in the US 1989 First spiral CT scanner enters the market Mihran K. Kassbian ( ) and his X-ray dermatitis hands. Courtesy of the American College of adiology. Characterization of X-rays Electromagnetic radiation (photons) 10 pm < λ < 10 nm c 0 = km/s ν = c 0 /λ [Hz] Ionizing The energy of the individual photon: E = hν [J], where Planck's number, h = Js High kinetic energy e - Production of X-rays Metal Photon Heat elative intensity Electron's energy loss Bremsstrahlung Bremsestråling Photon energy (kev) Electrons moving from one shell to another Characteristic radiation :23

Image presentation techniques Examples of images from the VH project Ted Meyer: Structural Abnormalities (1992).Oil on canvas, 3' by 4'.

2 Electron volts 1 ev is the energy increase that an electron experiences, when accelerated over a potential difference of 1 V. 1 ev = q e ΔV = J In medical imaging: 20 kev < E < 150 kev - problem Calculate the frequency and energy for monochromatic x-rays with λ = 1 nm Answer: ν = E = Contents X-ray tube with rotating anode History and characterization of X-rays Conventional (projection) X-ray Attenuation, contrast and resolution Vacuum Stator Fluoroscopy and examples CT scanning Principle Image presentation techniques Examples of images from the VH project Cathode Anode ( kv) Anode Fokus Protective shield of lead Exposure: milliseconds to a few seconds kv Principle of X-ray system Primary radiation (fotoelectric absorption) Film X-ray tube (røntgenrør) Al-filter (removes low energy radiation) Collimator (Primærblænde) Object Secondary radiation (Compton scattering) Grid (sekundærblænde) Screens (skærme) Lambert-Beer's law for monocromatic radiation I = exp(- l l) I l = linear attenuation coefficient ( l = l ( ν )) l increases with atomic number l l :23

and characterization of X-rays Conventional (projection) X-ray Attenuation, contrast and resolution Vacuum Stator Fluoroscopy and examples CT scanning Principle Image presentation techniques Examples

3 Attenuating material with inhomogeniety ( - problem) Now write I 1 relative to I 2 and I 3 l 1 Film I 1 / I 2 = exp(( 2-1 )l 2 ) 2 I 2 = I 1 / I 3 = exp( 1 l 3 ) 1 I 1 = I 3 = l 2 l 3 efine contrast as K = (I 1 - I 2 ) / (I 1 + I 2 ) X-ray tube Modulation transfer function Example: K > 0.03 gives I 1 / I 2 > 1.06 Using the same argument for I 1 and I 3 gives that lδ 1 > 0.06 or 1 Δl > 0.06 must be fulfilled to see a difference on the film. Pb "raster" K 0 0, lp/mm Contents History and characterization of X-rays Conventional (projection) X-ray Attenuation, contrast and resolution Fluoroscopy and examples CT scanning Principle Image presentation techniques Examples of images from the VH project Invention of Computed Tomograhpy abbreviated CT or CAT 1972: G.N. Hounsfield, scientist in Middlesex, England Announced computed axial transverse scanning Presented cross-sectional images of the head showing tissues inside the brain as separate structures of gray matter, white matter, CSF, and bone Pathologic processes such as blood clots, tumors, and infarcts could be easily seen r. Hounsfield's discovery completely revolutionized the practice of medicine: Structures inside the human body that had never been imaged before, could now be visualized. (CSF = Cerebrospinal fluid) :23

06 must be fulfilled to see a difference on the film. Pb "raster" 0.

4 evelopment of computed tomograhpy Basic principle of computed tomograhpy (algebraic reconstruction) Acquisition speed: In the early 70's: Several minutes to acquire a single slice through the brain I r1 = exp(- 11 l - 12 l ) Today: The newest scanners can image the entire body in 1 to 2 minutes I r2 = exp(- 21 l - 22 l) I c2 = exp(- 12 l - 22 l) I c1 = exp(- 11 l - 21 l) ecording scheme for CT scanning ecording scheme for CT scanning Attenuation Note: We actually record intensity, so curve should be inverse Attenuation ecording scheme for CT scanning ecording scheme for CT scanning Attenuation :23

21 22 I r2 = exp(- 21 l - 22 l) I c2 = exp(- 12 l - 22 l) I c1 = exp(- 11 l - 21 l) ecording scheme for CT scanning ecording scheme for CT scanning

5 (assuming 128 lines) Imaging a point target Either 128 equations with 128 unknowns 1. Problems with noise 2. Huge calculation times or Linear superposition of filtered back projections (LSFBP) Imaging a point target Back projection - point target The resulting image Explaining the process in terms of a transfer function istance Object space H Image space istance :23

Huge calculation times or Linear superposition of filtered back projections (LSFBP) Imaging a

6 Pre-filtering (before back projection) Some questions to Matlab demo Measured signal "Inverse impulse response" What do we see on the thorax image? What is the orientation of this image? f r * g r Show Matlab demo ifferent implementations CT Spiral Scanner X-ray source and detectors rotate X-ray source rotate, but detectors are stationary What can be seen on the CT images? Anatomical photograph CT image CT Images based on The Visible Human Project (Show movie) :23

f r * g r Show Matlab demo ifferent implementations CT Spiral Scanner X-ray source and detectors rotate X-ray source

7 What can be seen on the CT images? What can be seen on the CT images? Anatomical photograph CT image Anatomical photograph CT image Advantages - disadvantages (Tomography) Advantages: Can image the entire body isadvantages: Ionizing radiation Equipment is expensive Expensive in acquisition and use Not good in distinguishing soft tissues :23

(Tomography) Advantages: Can image the entire body isadvantages: Ionizing radiation

CT scanning. By Mikael Jensen & Jens E. Wilhjelm Risø National laboratory Ørsted DTU. (Ver. 1.2 4/9/07) 2002-2007 by M. Jensen and J. E.

CT scanning. By Mikael Jensen & Jens E. Wilhjelm Risø National laboratory Ørsted DTU. (Ver. 1.2 4/9/07) 2002-2007 by M. Jensen and J. E. 1 Overview CT scanning By Mikael Jensen & Jens E. Wilhjelm Risø National laboratory Ørsted DTU (Ver. 1.2 4/9/07) 2002-2007 by M. Jensen and J. E. Wilhjelm) As it can be imagined, planar X-ray imaging has