1TT Liu, BE280A, UCSD Fall 2006Bioengineering 280APrinciples of Biomedical ImagingFall Quarter 2006X-Rays Lecture 1TT Liu, BE280A, UCSD Fall 2006EM spectrumSuetens 2002TT Liu, BE280A, UCSD Fall 2006X-Ray TubeSuetens 2002Tungsten filament heated to about 2200 C leading to thermionicemission of electrons.Usually tungsten is used for anodeMolybdenum for mammographyTT Liu, BE280A, UCSD Fall 2006X-Ray ProductionPrince and Links 2005Collisional transfersRadiative transfers2TT Liu, BE280A, UCSD Fall 2006X-Ray SpectrumPrince and Links 2005Lowerenergyphotons areabsorbed byanode, tube,and otherfiltersTT Liu, BE280A, UCSD Fall 2006Interaction with MatterPhotoelectric 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 contrasthttp://www.eee.ntu.ac.uk/research/vision/asobaniaTT Liu, BE280A, UCSD Fall 2006Interaction with MatterPhotoelectric absorptionCompton ScatteringPair ProductionTT Liu, BE280A, UCSD Fall 2006Attenuation! Iout= Iinexp("µd)dFor single-energy x-rays passing through a homogenous object:Linear attenuation coefficient3TT Liu, BE280A, UCSD Fall 2006Attenuation510 50 100 15010.1AttenuationCoefficient500BoneMuscleFatAdapted from www.cis.rit.edu/class/simg215/xrays.ppt Photon Energy (keV)Photoelectric effectdominatesCompton ScatteringdominatesMore AttenuationLess AttenuationTT Liu, BE280A, UCSD Fall 2006Half Value LayerValues from Webb 20032.84.51502.33.91001.23.0500.41.830HVLBone (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 2006Attenuation! Iout= Iinexp "µ(x)dxxinxout#( )For an inhomogenous object:Integrating over energies! Iout="(E )0#$exp %µ(E,x)dxxinxout$( )dEIntensity distribution of beamTT Liu, BE280A, UCSD Fall 2006X-Ray Imaging ChainSuetens 2002Reduces effects of Compton scattering4TT Liu, BE280A, UCSD Fall 2006X-ray filmFlexible base~ 150 µmEmulsion withsilver halide crystalsEach layer~ 10 µmSilver 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 2006Intensifying Screenhttp://learntech.uwe.ac.uk/radiography/RScience/imaging_principles_d/diagimage11.htmhttp://www.sunnybrook.utoronto.ca:8080/~selenium/xray.html#FilmTT Liu, BE280A, UCSD Fall 2006X-Ray ExamplesSuetens 2002TT Liu, BE280A, UCSD Fall 2006X-Ray w/ Contrast AgentsSuetens 2002Angiogram using an iodine-basedcontrast agent.K-edge of iodine is 33.2 keVBarium SulfateK-edge of Barium is 37.4 keV5TT Liu, BE280A, UCSD Fall 2006ContrastBushberg et al 2001TT Liu, BE280A, UCSD Fall 2006ContrastTT Liu, BE280A, UCSD Fall 2006! A = N0exp("µx)B = N0exp("µ(x + z))CS=A " BA=N0exp("µx) " N0exp("µ(x + z))N0exp("µx)=1" exp("µz)Subject ContrastBushberg et al
View Full Document
Unlocking...