5: Emission (Computed) Tomography

Introduction au cours

Fund BioImag 20135-15: Emission (Computed) TomographyWhat is a tracer ?Why is collimation necessary and what are its consequences ?How are the effects of attenuation taken into account ?What is the principle of x-ray detection ?scintillation followed by amplificationHow can scattered photons be eliminated ? After this course youUnderstand the reason for collimation in imaging g-emitting tracers and its implication on resolution/sensitivity Understand the implications of x-ray absorption on emission tomography Understand the basic principle of radiation measurement using scintillation Are familiar with the principle/limitations of photomultiplier tube amplificationUnderstand the use of energy discrimination for scatter correction Fund BioImag 20135-25-1. Emission computed tomographyHuman brain examples

uncontrolled complex partial seizures left temporal lobe has less blood flow than rightindicates nonfunctioning brain areas causing the seizures 99mTc-HMPAO SPECT of Brain(Sagittal Views)

Alzheimers DiseaseDementia with Lewy Bodies

Control

patient with AIDScontrast enhanced CT scan (left) ring enhancing lesion with surrounding edema toxoplasma abscess or CNS lymphoma. thallium SPECT scan (right) intense uptake of radiotracer by the lesion lymphoma Thallium SPECT - CNS LymphomaFund BioImag 20135-3Emission computed tomographyHeart and rodent examples

perfusion of heart muscleorange, yellow: good perfusionblue, purple: poor perfusion

SPECT Image of a small mouse (25 gr) injected with 10 millicuries of 99mTc-MDP.

Fund BioImag 20135-4What is Emission Computed Tomography ?Until now: CT and x-ray imaging measure attenuation of incident x-rayEmission tomography: X-rays emitted by exogenous substance (tracer) in body are measured

What is a tracer ? Exogenously administered substance (infused into blood vessel) that(a) alters image contrast (CT, MRI)(b) has a unique signal (g emitting)-> Emission computed tomographyTypical tracers for emission tomographyhalf-life and photon energies-Two issues:How to determine directionality of x-rays ?Absorption is undesirableInjection of radioactive tracer[h][keV]99mTc6140201Tl7370123I13159133Xe0.0881Fund BioImag 20135-5What are the basic elements needed for g-emitter imaging ?CollimatorPhotomultipliersLight guideScintillating crystalSeptag-cameraPosition logic electronics

w. collimator boardThyroid scannerWhole body scintigraphy

Fund BioImag 20135-6

5-2. How can directionality of x-rays be established ? Collimation Problem: Photon detection alone does not give directionality Solution:

Collimation establishes direction of x-rayCollimatorThick (lead or tungsten) with thin holesSelect rays orthogonal to crystal

Signal S(y)Consider one detector, assuming perfect collimation (and neglecting attenuation, see later)xyCT(x,y): tissue radioactivityRadon transform Reconstruction as in CTseptaImpact of collimation on resolution

Line of incidence (LOI)Fund BioImag 20135-7Crystal scintillant (NaI)DHow does collimation affect resolution ? Its never perfect tdawpenetratingscatteringPerfect collimation, i.e. resolution ?geometricd/a 0mcollimatort Impossible to achieve (Why?)septaDSource

Collimator resolution:Two objects have to be separated by distance >Rb

Septa penetration < 5% occurs when t=t5%:Price of collimation (resolution) ?Sensitivity !

(ae: imperfect septal absorption)Fund BioImag 20135-85-3. How to deal with attenuation of the emitted x-rays ?result of x-ray absorption in tissue

Signal measured from a homogeneous sphere (CT(x,y)=constant)

D

Intensity distortion:Cause ?

Attenuation T

depends on object dimension and source location (D=f(object))

Photon energym=f(En)

5 cm10 cm15 cm99mTc in H2OT=0.46T=0.21T=0.10Consider point source:Attenuation depends on location of source in tissueFund BioImag 20135-9What are the basic steps in attenuation correction ?Corrected imageAttenuation correction procedureEstimated object geometry and estimated m(x,y) or measured m(x,y) Transmission loss : T(projection)=f((object), projection)Attenuation correction A(x,y)=1/T(x,y)Corrected Ccorr(x,y)=A(x,y) C(x,y)

lungheartA(x,y) of thoraxProblem is prior knowledge needed for A (i.e. m(x,y))Measured

ABCD

(x,y):Attenuation correctionAttenuation correction rarely applied!Fund BioImag 20135-10How to simplify attenuation correction ?by measuring at 1800 using geometric meanProblem: Spatial dependence of correctionSignal (S1)xyCT(x,y): tissue radioactivitySignal (S2)x1x2D = x1 + x2Solution: Geometric mean of the two 1800 opposite signals:Measure at 1800 simultaneously and take the geometric mean

attenuation correction depends only on dimension of object along the measured Radon transform

(D = x1 + x2)

Signal=sum of all activity along x:

Consider point source:Fund BioImag 20135-11PhotomultiplierTube5-4. What is the principle of x-ray detection ?Collimation, followed by scintillation and amplificationelectronsLightEl. SignalScintillator crystale.g. Tl-doped SodiumIodide (NaI)-energy # scintillation photons Signal

Fund BioImag 20135-12What is Scintillation ?

T = energy transfer efficiency from excited ion to luminescence centre qA= quantum efficiency of luminescence centreWe-h = energy required to create one electron-hole pairConduction bandActivation band (Doped)Valence bandActivator ground stateGapSequence of events in scintillation crystalAtom ionized by Compton interaction Electron-hole pair Hole ionizes activator, electron falls into activatorActivator is deactivated by emission of Photons (10-7 sec)

Efficiency of scintillatorsFund BioImag 20135-13What elements characterize scintillation materials ? Overview of some crystals

Requirements for scintillatorHigh yieldGood linearitySmall time constant tTransparent for scintillation light lgood mechanical properties Refraction index close to 1.5ZeffRefr. IndexYield732.1513%511.85100%596679%Most of the energy of the x-ray is lost as heat (to lattice), seee.g. NaI(140keV)=40140=5600 photons at l400nmE400nm[keV]=hc/l = 1.2/l [nm]=1.2/400 keV=3eV