what are spect basics?

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1 Siemens Medical Solutions Molecular Imaging

What are SPECT basics?



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



Overview

PULSEHEIGHT

ANALYZERPOSITIONSIGNALS

ENERGYSIGNAL

X Y Z

Image Display

COLLIMATOR

NaI(Tl)Crystal

. . . . . . .

PMT ARRAY



Scintillation camera components

Detector

NaI(Tl) crystal

Photomultiplier tube (PMT) array

Analog-to-digital converters (ADCs)

Collimator

Low energy

Medium energy

High energy

Axial shields (coincidence imaging)

Pinhole



Scintillation camera components

Computer(s)

Acquisition

Processing

Acquisition & processing

Physicians viewing

Patient Table

Pallet

Accessories



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



Nal(TI) Crystal

Advantages

85% sensitivity @ 140 keV

Moderate energy resolution (9-10% @ 140 keV)

Moderate cost

Disadvantages

Hygroscopic (requires hermetic seal)

Limiting component in count rate performance (200 nSec scintillation decay time)



PMT array

FOV 3" PMTs 2" PMTs

30 x 40 cm 28 6040 x 55 cm 55 120

PMT Cross SectionsPMTs are arranged in a close-packed array to cover the crystal surface

SideView

Circular

Hexangonal

Square



Analog position electronics

WeightedSum

TotalSum

PositionSignal (x or y)

EnergySignal (Z)

Normalization

NormalizedPositionSignal (x or y)

X/ZY/Z

Position-basedSignal Weights

PulseHeightAnalyzer



POSITIONSIGNALS

ENERGYSIGNAL

PULSE HEIGHT ANALYZER

X

Y

Z

Image Display

COLLIMATOR

NaI(Tl)Crystal

. . . . . . .

PMT ARRAY

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

1

Y

X



Collimation

Purpose: To project gamma ray distribution onto the detector

Basic design

Distance performance

Spatial resolution vs. count sensitivity



Collimator design

25 mm

1.2 mm

Collimators are fabricated from lead.

Gamma rays that hit the septa are absorbed.

Image forming aperture of the scintillation camera.Limiting component in spatial resolution & count sensitivity.



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



Spatial resolution Dependence on source to collimator distance

5 cm

10 cm

15 cm

20 cm

25 cm

30 cm



Energy correction

Corrects for the difference in energy responses within and betweenPMTs

Digitize local spectra (e.g. @ 64 x 64 locations) Set local photopeak windows Event must fall within local window

Before energy correction After energy correction



Linearity correction

Event location is estimated as x’,y’

New locationx = x’ + Dx’y = y’ + Dy’

Image a known rectangular hole pattern

Calculate x & y correction offsets

Interpolate values over entire field

Before linearity correction After linearity correction



Linearity correction

Correcting the mispositioning of events (spatial linearity) has a profound effect on field uniformity.

Before correction After correction



Uniformity correction

After energy and linearity corrections are performed, residual non-uniformities are corrected using a reference flood image.

The high count reference flood image is used to regionally weight events.

Energy & linearity correction Energy, linearity & uniformity correction



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)



Spatial resolution

Ideal point

Image of point with real system

pixels

Count profile

FWHM

Specifies amount of image blur

Quantified by the full-width-at-half maximum (FWHM) of the point or line spread function



Clinical ApplicationsPlanar & SPECT

Cardiac

Whole-body bone

Renal

Gastric

Hepatobiliary

Thyroid

Pulmonary

Brain



Acquisition types

Static

Dynamic

Whole-body

SPECT

Gated SPECT

Dynamic SPECT

Whole-body SPECT

Coincidence imaging



SPECT Reconstruction Algorithms

FBP (Filtered Back Projection)

Iterative reconstruction (MLEM, OSEM)

2D 3D



Iterative reconstruction process

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.

Estimated Reconstruction Volume Acquired (measured) ProjectionsEstimated Projections

Estimate Projections

Compare Projections

Update Image Estimate



Bone SPECT comparison

FBP Flash 3D 2D - OSEM

e.cam 3/8”Hx: 36-year-old female. Indication staging for osteosarcoma



Myocardial perfusion SPECT

FBP

Flash 3D

2D Iterative



GATED SPECT

Aufsummierte Datensatz

Gates Gate 7 Gate 8 Gate 1

R-Wave

Gate 1 Gate 2 Gate 3 Gate 4 Gate 5 Gate 6

R-R interval



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



PhysicianReview

Schedule

Acquisition

Processing

Administrative Data

Data Aquisition

Quality Control

Automatic ReconstructionAutomatic Processing

Data Display

HARDCOPY ActivityPrinting

Innovative Workflow ConceptInnovative Workflow Concept

SYNGO

e.so

fte.

soft



Fully Automated Data DistributionFully Automated Data Distribution

Archiving

Printing

e.station

OEM WorkstationWorkflow Complete

PACS

e.so

fte.

soft

Administrative Data

Data Aquisition

Quality Control

Automatic Reconstruction

Automatic Processing

Data Display

HARDCOPY Activity

SYNGO

what are spect basics?

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