em spectrum x-ray tube x-ray production
TRANSCRIPT
1
TT Liu, BE280A, UCSD Fall 2006
Bioengineering 280APrinciples of Biomedical Imaging
Fall Quarter 2006X-Rays Lecture 1
TT Liu, BE280A, UCSD Fall 2006
EM spectrum
Suetens 2002
TT Liu, BE280A, UCSD Fall 2006
X-Ray Tube
Suetens 2002
Tungsten filament heated to about 2200 C leading to thermionicemission of electrons.
Usually tungsten is used for anodeMolybdenum for mammography
TT Liu, BE280A, UCSD Fall 2006
X-Ray Production
Prince and Links 2005
Collisional transfers
Radiative transfers
2
TT Liu, BE280A, UCSD Fall 2006
X-Ray Spectrum
Prince and Links 2005
Lowerenergyphotons areabsorbed byanode, tube,and otherfilters
TT Liu, BE280A, UCSD Fall 2006
Interaction with Matter
Photoelectric effectdominates at low x-rayenergies and high atomicnumbers.
Typical energy range for diagnostic x-rays is below 200keV.The two most important types of interaction are photoeletricabsorption and Compton scattering.
Compton scatteringdominates at high x-rayenergies and low atomicnumbers, not much contrast
http://www.eee.ntu.ac.uk/research/vision/asobania
TT Liu, BE280A, UCSD Fall 2006
Interaction with Matter
Photoelectric absorption Compton Scattering
Pair Production
TT Liu, BE280A, UCSD Fall 2006
Attenuation
!
Iout
= Iinexp("µd)
d
For single-energy x-rays passing through a homogenous object:
Linear attenuation coefficient
3
TT Liu, BE280A, UCSD Fall 2006
Attenuation
5
10 50 100 150
1
0.1
AttenuationCoefficient
500
BoneMuscleFat
Adapted from www.cis.rit.edu/class/simg215/xrays.ppt
Photon Energy (keV)
Photoelectric effectdominates
Compton ScatteringdominatesMore Attenuation
Less Attenuation
TT Liu, BE280A, UCSD Fall 2006
Half Value Layer
Values from Webb 2003
2.84.51502.33.91001.23.0500.41.830
HVLBone (cm)
HVL,muscle(cm)
X-rayenergy(keV)
In chest radiography, about 90% of x-rays are absorbed by body.Average energy from a tungsten source is 68 keV. However,many lower energy beams are absorbed by tissue, so averageenergy is higher. This is referred to as beam-hardening, andreduces the contrast.
TT Liu, BE280A, UCSD Fall 2006
Attenuation
!
Iout
= Iinexp " µ(x)dx
xin
xout
#( )For an inhomogenous object:
Integrating over energies
!
Iout
= "(E)0
#
$ exp % µ(E,x)dxxin
xout
$( )dE
Intensity distribution of beam
TT Liu, BE280A, UCSD Fall 2006
X-Ray Imaging Chain
Suetens 2002
Reduces effects of Compton scattering
4
TT Liu, BE280A, UCSD Fall 2006
X-ray film
Flexible base~ 150 µm
Emulsion withsilver halide crystalsEach layer~ 10 µm
Silver halide crystals absorb optical energy. After development,crystals that have absorbed enough energy are converted tometallic silver and look dark on the screen. Thus, parts in theobject that attenuate the x-rays will look brighter.
TT Liu, BE280A, UCSD Fall 2006
Intensifying Screen
http://learntech.uwe.ac.uk/radiography/RScience/imaging_principles_d/diagimage11.htmhttp://www.sunnybrook.utoronto.ca:8080/~selenium/xray.html#Film
TT Liu, BE280A, UCSD Fall 2006
X-Ray Examples
Suetens 2002 TT Liu, BE280A, UCSD Fall 2006
X-Ray w/ Contrast Agents
Suetens 2002
Angiogram using an iodine-basedcontrast agent.K-edge of iodine is 33.2 keV
Barium SulfateK-edge of Barium is 37.4 keV
5
TT Liu, BE280A, UCSD Fall 2006
Contrast
Bushberg et al 2001 TT Liu, BE280A, UCSD Fall 2006
Contrast
TT Liu, BE280A, UCSD Fall 2006
!
A = N0exp("µx)
B= N0exp("µ(x + z))
CS
=A"B
A
=N0exp("µx) "N
0exp("µ(x + z))
N0exp("µx)
=1"exp("µz)
Subject Contrast
Bushberg et al 2001