5: Emission (Computed) Tomography

1. 5: Emission (Computed) Tomography • What 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 amplification • How can scattered photons be eliminated ? • After this course you • Understand 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 amplification • Understand the use of energy discrimination for scatter correction

2. 5-1. Emission computed tomographyHuman brain examples • uncontrolled complex partial seizures • left temporal lobe has less blood flow than right • indicates nonfunctioning brain areas causing the seizures 99mTc-HMPAO SPECT of Brain (Sagittal Views) Thallium SPECT - CNS Lymphoma patient with AIDS contrast 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 Control Alzheimer’s Disease Dementia with Lewy Bodies

3. Emission computed tomographyHeart and rodent examples • perfusion of heart muscle • orange, yellow: good perfusion • blue, purple: poor perfusion SPECT Image of a small mouse (25 gr) injected with 10 millicuries of 99mTc-MDP.

4. What is Emission Computed Tomography ? • Two issues: • How to determine directionality of x-rays ? • Absorption is undesirable • Until now: CT and x-ray imaging measureattenuation of incident x-ray • Emission tomography: X-rays emitted by exogenous substance (tracer) in body are measured Injection of radioactive tracer - • What is a tracer ? • Exogenously administered substance (infused into blood vessel) that • (a) alters image contrast (CT, MRI) • (b) has a unique signal (gemitting) • -> Emission computed tomography Typical tracers for emission tomography half-life and photon energies

5. What are the basic elements needed for g-emitter imaging ? Position logic electronics Photomultipliers g-camera Light guide Scintillating crystal Collimator Septa w. collimator board Thyroid scanner Whole body scintigraphy

6. y x 5-2. How can directionality of x-rays be established ? Collimation Consider one detector, assuming perfect collimation (and neglecting attenuation, see later) Problem: Photon detection alone does not give directionality Solution: Line of incidence (LOI) Signal S(y) Collimation establishes direction of x-ray CT(x,y): tissue radioactivity Radon transform  Reconstruction as in CT Impact of collimation on resolution Collimator Thick (lead or tungsten) with thin holes Select rays orthogonal to crystal septa

7. D Crystal scintillant (NaI) Source D How does collimation affect resolution ? It’s never perfect … Perfect collimation, i.e. resolution ? d/a→ 0 mcollimatort→  Impossible to achieve (Why?) w septa a geometric Collimator resolution: Two objects have to be separated by distance >R t d b penetrating scattering (ae: imperfect septal absorption) Septa penetration < 5% occurs when t=t5%: Price of collimation (resolution) ? Sensitivity !

8. 5-3. 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) • Attenuation T • depends on object dimension and source location (D=f(object)) • Photon energym=f(En) Intensity distortion:Cause ? Consider point source: Attenuation depends on location of source in tissue 5 cm 10 cm T=0.46 D 15 cm T=0.21 T=0.10 99mTc in H2O

9. What are the basic steps in attenuation correction ? µ(x,y):Attenuation correction Measured Corrected image A(x,y) of thorax • Attenuation correction procedure • Estimated object geometry and estimated m(x,y) or measured m(x,y) • Transmission loss : T(projection)=f(µ(object), projection) • Attenuation correction A(x,y)=1/T(x,y) • Corrected Ccorr(x,y)=A(x,y) C(x,y) heart lung A B Attenuation correction rarely applied! Problem is prior knowledge needed for A (i.e. m(x,y)) D C

10. y x How to simplify attenuation correction ?by measuring at 1800 using geometric mean Problem:Spatial dependence of correction D = x1 + x2 x1 x2 Signal=sum of all activity along x: Signal (S2) Consider point source: Signal (S1) Measure at 1800 simultaneously and take the geometric mean → attenuation correction depends only on dimension of object along the measured Radon transform CT(x,y): tissue radioactivity Solution: Geometric mean of the two 1800 opposite signals: (D = x1 + x2)

11. 5-4. What is the principle of x-ray detection ?Collimation, followed by scintillation and amplification Scintillator crystal e.g. Tl-doped SodiumIodide (NaI) Photomultiplier Tube γ Light electrons El. Signal -energy  # scintillation photons  Signal

12. What is Scintillation ? • Sequence of events in scintillation crystal • Atom ionized by Compton interaction → Electron-hole pair • Hole ionizes activator, electron falls into activator • Activator is deactivated by emission of Photons (10-7 sec) Conduction band Activation band (Doped) Efficiency of scintillators Gap Activator ground state Valence band T = energy transfer efficiency from excited ion to luminescence centre qA= quantum efficiency of luminescence centre We-h = energy required to create one electron-hole pair

13. What elements characterize scintillation materials ? Overview of some crystals ph/keV Requirements for scintillator High yield Good linearity Small time constant t Transparent for scintillation light l good mechanical properties Refraction index close to 1.5 Most of the energy of the x-ray is lost as heat (to lattice), see e.g. NaI(140keV)=40∙140=5600 photons at l400nm <20keV or <120eV/keV E400nm[keV]=hc/l = 1.2/l [nm]=1.2/400 keV=3eV

14. How is the scintillation light converted to an electrical signal ? Photomultiplier tube (PMT) -Noiseless amplification Photocathode Dynode Anode Pulse Height Analyzer 300V 450V 600V 750V 900V 1200V PMTs are bulky How to increase resolution beyond PMT dimensions ? 1 cm Scintillator crystal

15. γ Scintillation light Signal Location, Energy How to improve the spatial resolution of PMT ? (Anger, 1964) NaI-crystal (40 cm x 40 cm) Light-guide Photomultiplier Looks familiar ? (see center of mass 1st year Physics) I3 I2 I1 x1 x2 x3

16. Most scattering is by Compton 5-5. How to discriminate scattered photons ? Tc-99m spherical phantom (w. holes) 140 keV Measure Ef→ identify severely scattered photons 104 keV primary scatter total Intensity

17. γ γ What processes contribute to the Scintillation light spectrum ? scintillation signal depends on x-ray energy NaI (Tl) scintillation peak for Cs-37: 662 keV crystal γ γ γ max. energy of the recoil electron (i.e. 662 keV photon scattered by 1800) Scintillation light intensity energy of 662 keV photon scattered by 1800 photons that lose all energy (i.e. Compton events + final photoelectric event) • light from Compton events • secondary photons escaped from crystal

18. SPECT summarySinglePhotonEmissionComputedTomography • Measurement of single photon emitters • injected into subject • Collimation ensures x-ray directionality ( backprojection) • Absorption is undesirable • Photon energies comparable to CT SPECT-CT ECG-triggered Pancreatic tumor (mouse)