lecture 5: tomographic nuclear systems: spect · project groups 1.rodriguez, dones 2.romig,...
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Lecture 5: Tomographic nuclear systems: SPECT
Field trip this saturday at 11 AM at UWMC– meet in main hospital lobby at 11 AM– if you miss the 'boat', page me at 540-4950– should take ~1 to 1.5 hours, depending
No class next weekExam will will be emailed to class email list Wednesday afternoonSubmit to class website - closes at 10 PM Nov 1st.Next homework
– Read chp 7 Seutens– Find 2 medical images of abnormal anatomy or physiology
(pathology) formed using ultrasound (AKA 'sonography'). Placethese images in a document. Write 1-2 brief sentences describingeach image. Write 1-2 brief sentences describing differencesbetween the images. Write 1-2 sentence what the image valuesrepresent physically.
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Project Groups
1. Rodriguez, Dones2. Romig, Martinez, Sung, Harmelin3. Pham, Wong, Legesse4. Braddock, Mauro5. Jeddi, Miller, Zhang6. Morrow, Dahl, Clayton7. Rundgren, Lam
Deadlines:Nov 1: Outline due 30% of mark for projectNov 29: Final report due 50% of mark for projectDec 6: Class presentation 20% of mark for project
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X-ray Projection Imaging
yx z
source
detector
f(x,y,z)
p(x,z) = ∫ f(x,y,z) dyx
z
‘overlay’ of all information (nonquantitative)
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X-ray Tomographic Imaging
orbiting source + detectordata for all angles
f(x,y,z)
true cross-sectional image
‘Tomo’ + ‘graphy’ = Greek: ‘slice’ + ‘picture’
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Nuclear Medicine Tomographic Imaging:single photon emission computed tomography (SPECT)
Underlying Principles of SPECT
Scintillation site for !-ray
parallel to collimator holes
Rotating NaI(Tl) Detector Module with Pb/W Collimator in Front
Patient
Radioisotope decay
by ! emission
+ !Tc99m
Tc99
tracer tracer
Absorbed in collimator
Emitted !-rays
Direction of Rotation
Signals to electronics
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Nuclear Medicine Imaging
Projection Images
Tomographic Images
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Basic SPECT
2D planarprojection
Sinogram
Axial level of sinogram
Angle of above projection
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Early Clinical SPECT
GE 400T Rotating Anger Camera (ca. 1981)
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Modern Clinical Systems
GE Millenium VG Philips Cardio 60 Siemens e.camVariable Angle
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Conventional Anger Camera
PMTs coupled to large, continuousNaI(Tl) crystal
Spatial resolution 3–4 mm FWHM
Energy resolution 8–10% FWHM
Mature technology (DoB ~1957)
Large-area, >40cm x 40cm typical
Simple and cost-effective
SPRINT II camera module
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Ideal Photon Detection
Absorbed Photon
Photon Absorption
NOTE: at 30cm cylinder centerSPECT (µ=0.153/cm): I/Io = 0.10
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Attenuation Effects
Attenuation 0.15/cm (140 keV photons in water)Reconstructions of disks should be uniform
Attenuation causes a distortion that increases with object size
5 cm dia.
20 cm
10 cm
30 cm
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Possible Attenuation Artifacts
Breast Attenuation
Diaphragmatic Attenuation
Tc-99m Sestimibi Myocardial Perfusion Images
Activity in myocardium should be uniform but isn’tCould be interpreted as abnormal
Shouldbe
uniformintensit
y
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TCT/ECT Acquisition Geometry
Detector 1 - Transmission and emission 65cm focal distance fanbeam
collimator
Detectors 2&3 - Emission onlyParallel hole collimator
Transmission SourceAm-241 (60 keV photons)#1
#2#3
Transmission line source
This is one configuration; Many others are in use
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Photon Scatter
NOTES:-Additive-Loss in resolution and contrast-Typical scatter fractions. Tl:1.0, Tc:0.3-0.4, PET: 0.15-0.40
Ideal Photon Detection
Scattered Photon
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Camera Energy Spectrum140 keV
Typical Anger camera has from 8–10% FWHM energyresolution at 140 keV
15–20%Energywindow
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Collimation Systems
The collimation system is the heart of the SPECT instrument – it’s the front-endand has the biggest impact on SNR
Its function is to form an image by determining the direction along whichgamma-rays propagate
Ideally, a lens similar to that used for visible photon wavelengths would be usedfor high efficiency – not feasible at gamma-ray wavelengths
Absorbing collimation typically used
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Parallel Hole Collimation
Photon absorbed bycollimator channel
Photon reachesdetector
Detector
Collimator
Source
h
L
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Typical Parallel Hole Collimator
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Parallel Hole Collimation
FL
h!"
2
2
16#
Efficiency Intrinsic Resolution (FWHM)
dL
hhR !+"
Essentially solid-angleefficiency of single channel
Single collimator channel
Response is trapezoidalbecoming triangular at largerdistances
Response
X distance
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Resolution vs. Depth
h = 1mm
h = 2mm
L = 30mm L = 20mm L = 10mm
Point source depths: 10, 15, 20, 25, 30, 35 cm
Parallel Hole Collimator
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Sad Facts About Absorbing Collimation
In the best case efficiency is only a few photons detected for 10,000 emitted
Performance of absorbing collimation decreases rapidly with increasing energyPhotoelectric absorption coefficient changes as ~ Z4 / E3
Typical collimator for 140 keV Tc-99m – 9mm FHWM @ 10cm, 2.3e-4
Typical for 364 keV (I-131) – 12mm FWHM @ 10cm, 1e-4
Radionuclides with even a small fraction of higher energy radiation will result in severelycompromised imaging performance
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Alternatives for Increasing Efficiency
• Surround patient with morecameras
• Use fan-beam or cone-beamcollimation to trade FOV size forefficiency at given resolution