Transcript What are SPECT basics? - Siegfried Schrei, Molecular

What are SPECT basics?
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Anger camera
 Hal O. Anger invented the
scintillation camera in 1958
 Established basic design:
 NaI(Tl) crystal
 PMT array
 Position weighted signals
Hal O. Anger
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Overview
POSITION
SIGNALS
ENERGY
SIGNAL
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PULSE
HEIGHT
ANALYZER
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X
Y
Z
PMT ARRAY
NaI(Tl)
Crystal
COLLIMATOR
Image Display
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Scintillation camera components
Detector
Collimator
 NaI(Tl) crystal
 Low energy
 Photomultiplier tube (PMT) array
 Medium energy
 Analog-to-digital converters
(ADCs)
 High energy
 Axial shields (coincidence
imaging)
 Pinhole
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Scintillation camera components
Computer(s)
Patient Table
 Acquisition
 Pallet
 Processing
 Accessories
 Acquisition & processing
 Physicians viewing
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Nal(TI) Scintillator
 Sensitive material for gamma ray
detection
 Large rectangular (40 x 50 cm),
thin (9.5 mm) crystal*
 Converts gamma ray energy into
visible light (Total absorption of a
140 keV gamma ray yields 5000
photons)
 Fragile: Sensitive to trauma and
temperature changes
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Nal(TI) Crystal
Advantages
Disadvantages
 85% sensitivity @ 140 keV
 Hygroscopic (requires hermetic
seal)
 Moderate energy resolution
(9-10% @ 140 keV)
 Moderate cost
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 Limiting component in count rate
performance (200 nSec
scintillation decay time)
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PMT array
Side
View
PMTs are arranged in a close-packed
array to cover the crystal surface
PMT Cross Sections
Circular
Hexangonal
Square
FOV
30 x 40 cm
40 x 55 cm
8
3" PMTs
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55
2" PMTs
60
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Analog position electronics
Position-based
Signal Weights
Position
Signal
(x or y)
X/Z
Y/Z
Weighted
Sum
Normalized
Position
Signal
(x or y)
Normalization
Energy
Signal (Z)
Total
Sum
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Pulse
Height
Analyzer
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PULSE HEIGHT ANALYZER
POSITION
SIGNALS
ENERGY
SIGNAL
Y
Z
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X
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PMT ARRAY
NaI(Tl)
Crystal
COLLIMATOR
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Image Display
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Collimation
 Purpose: To project gamma ray distribution onto the detector
 Basic design
 Distance performance
 Spatial resolution vs. count sensitivity
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Collimator design
Image forming aperture of the scintillation camera.
Limiting component in spatial resolution & count sensitivity.
Collimators are fabricated from lead.
25 mm
1.2 mm
Gamma rays that hit the septa are absorbed.
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Collimator performance
Count sensitivity
 ~ 1/5,000 gamma rays are
transmitted
 Requires short holes with large
diameters
 Inverse relationship with
resolution
Spatial resolution
 6 - 12 mm FWHM @ 10 cm
 Requires long holes with small
diameters
 Distance dependent
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Spatial resolution
Dependence on source to collimator distance
5 cm
10 cm
15 cm
20 cm
25 cm
30 cm
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Energy correction
Before energy correction
After energy correction
 Corrects for the difference in energy responses within and between
PMTs
 Digitize local spectra (e.g. @ 64 x 64 locations)
 Set local photopeak windows
 Event must fall within local window
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Linearity correction
New location
x = x’ + Dx’
y = y’ + Dy’
Event location is estimated as
x’,y’
Before linearity correction
After linearity correction
 Image a known rectangular hole pattern
 Calculate x & y correction offsets
 Interpolate values over entire field
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Linearity correction
Before correction
After correction
Correcting the mispositioning of events (spatial linearity) has a profound
effect on field uniformity.
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Uniformity correction
Energy & linearity correction
Energy, linearity & uniformity correction
After energy and linearity corrections are performed, residual nonuniformities are corrected using a reference flood image.
The high count reference flood image is used to regionally weight
events.
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Scintillation camera performance specifications
 Field uniformity (2% - 4%)
 Intrinsic spatial resolution (3.5-5.5 mm)
 System spatial resolution at 10 cm (8-12 mm)
 Energy resolution (9-10%)
 Multi-energy window spatial registration (< 2 mm)
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Spatial resolution
Count profile
Ideal point
FWHM
Image of point with real system
pixels
 Specifies amount of image blur
 Quantified by the full-width-at-half maximum (FWHM) of the point or
line spread function
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Clinical Applications
Planar & SPECT
 Cardiac
 Whole-body bone
 Renal
 Gastric
 Hepatobiliary
 Thyroid
 Pulmonary
 Brain
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Acquisition types
 Static
 Dynamic
 Whole-body
 SPECT
 Gated SPECT
 Dynamic SPECT
 Whole-body SPECT
 Coincidence imaging
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SPECT Reconstruction Algorithms
 FBP (Filtered Back Projection)
 Iterative reconstruction (MLEM, OSEM)
 2D
 3D
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Iterative reconstruction process
Estimated Reconstruction Volume
Estimated Projections
Acquired (measured) Projections
Estimate
Projections
Update
Image
Estimate
Compare
Projections
If the correct physical model for the collimation is used in estimating the
projections, then the feedback of the iterative process drives the
convergence with implicit recovery of resolution. Enhancement filtering
of projection data is not required.
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Bone SPECT comparison
FBP
Flash 3D
2D - OSEM
e.cam 3/8”
Hx: 36-year-old female. Indication staging for osteosarcoma
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Myocardial perfusion SPECT
FBP
Flash 3D
2D Iterative
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GATED SPECT
R-R interval
R-Wave
Gates
Gate 1 Gate 2 Gate 3 Gate 4 Gate 5 Gate 6 Gate 7 Gate 8
Gate 1
Aufsummierte Datensatz
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Clinical software
 Organ specific software
(cardiac, renal, gastric, pulmonary, brain, etc.)
 Cardiac quantification software
 Cedars Sinai QGS, QPS, QBS
 4D-MSPECT (Univ. of MI)
 Emory Cardiac Toolbox
 3D display software
 Image fusion software
 CT-based attenuation correction
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e.soft
Innovative Workflow Concept
Schedule
Acquisition
Data Aquisition
Quality Control
Processing
Physician
Review
Printing
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Administrative Data
Automatic Reconstruction
Automatic Processing
Data Display
HARDCOPY Activity
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e.soft
Fully Automated Data Distribution
Administrative Data
Data Aquisition
Archiving
Printing
Quality Control
Automatic Reconstruction
PACS
Automatic Processing
Data Display
e.station
HARDCOPY Activity
Workflow Complete
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OEM
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