2-2 x rays

2-2 x rays - BENG101 Foundations of Biomedical Imaging Fall 2009 X-rays Introduction X-rays discovered by William Rontgen in 1895 Tube emitting

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
BENG101 Foundations of Biomedical Imaging Fall 2009 X-rays
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Introduction X-rays discovered by William Rontgen in 1895 Tube emitting photons caused a fluorescent screen to glow, (even when tube placed inside a box!) Tube emitting new type of radiation, called X-rays (mysterious nature) Different media had different attenuation and it could be captured on film First X-ray picture of his wife’s hand (December 1895)
Background image of page 2
What are X-Rays? X-ray photons have high energies and short wavelengths. ( E = hf) Wavelength:10 -10 m (Angstrom), Energy: KeV (eV: 1.6x10 -19 J) EM Spectrum
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
X-ray Generation X-rays generated by X-ray tube Cathode releases electrons by thermal excitation Accelerated towards anode (e.g. tungsten) by high voltage Electrons hit anode and release energy (“Bremsstrahlung” – breaking radiation) E E max = qU (q electric charge of electron, U is voltage, e.g. 100 Kv Emax = 100 Kev
Background image of page 4
X-ray Generation Also, electrons energy can release orbital electron from its shell (e.g. K shell). Leaves a hole filled by electron from higher energy (e.g. L shell) which emits a photon of a specific energy , E = E L –E K , yielding characteristic peaks in the X-ray spectrum. X-ray source parameters: Amount of electrons generated by cathode – current x on time: mAs (6-100) Energy of emitted X-rays from anode – applied voltage (50 – 125 kV) Incident at anode, 1 joule = 1kVmAs, less that 1% are X-rays, rest is heat.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Interaction of Photons with Matter X-rays are “ionizing” radiation – can release electron from atom. Lower energy photons (radiation) are non-ionizing, but can raise electron to higher energy shell (“excitation”). Rayleigh Scattering Photon absorbed by atom and immediately released as new photon in new direction. Non-ionizing process and not a major role for X-rays (except mammography at lower voltages) Photoelectric Absorption Photon absorbed by atom and energy excites electron which escapes nucleus in same direction as incoming photon. Compton Scattering Only part of energy used to eject electron, photon loses energy and emitted in a different direction than incoming photon. Pair Production If energy at least 1.2 MeV, photon transformed into electron-positron pair. Positron then meets another electron and they annihilate creating two 511 KeV photons in opposite directions. Used in Nuclear medicine. At higher energies may cause Nuclear reactions (not for medical applications!)
Background image of page 6
Interaction of X-ray beam with tissue X-ray beam undergoes an exponential decay through tissue: d out in II e μ = d = x out –x in μ is the Linear Attenuation coefficient Valid for homogenous medium and single energy photons For inhomogeneous medium For inhomogeneous medium & spectrum of energies - Since μ is f(E,x) and I in is f(E) (,) () xx out out in in E x dx E x dx out in e E e d E μμ σ −− −∞ == ∫∫ x out x in xd x out in e =
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Attenuation Coefficient in Lead and Aluminum
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/30/2010 for the course BENG 101 taught by Professor Silva,g during the Fall '08 term at UCSD.

Page1 / 29

2-2 x rays - BENG101 Foundations of Biomedical Imaging Fall 2009 X-rays Introduction X-rays discovered by William Rontgen in 1895 Tube emitting

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online