nuclear medicine: tomographic imaging – spect, spect-ct and pet-ct

Nuclear Medicine: Nuclear Medicine: Tomographic Imaging – Tomographic Imaging –

SPECT, SPECT-CT and PET-SPECT, SPECT-CT and PET-CTCT

Katrina CockburnKatrina Cockburn

Nuclear Medicine PhysicistNuclear Medicine Physicist

Image Acquisition TechniquesImage Acquisition Techniques

StaticStatic -- (Bones, Lungs)(Bones, Lungs) DynamicDynamic -- (Renography)(Renography) GatedGated -- (Cardiac)(Cardiac) TomographyTomography

SPECTSPECT PETPET

List ModeList Mode -- (Cardiac)(Cardiac)

Problems with Planar ImagingProblems with Planar Imaging

Planar imagingPlanar imaging 2D representation of 3D 2D representation of 3D

Distribution of activityDistribution of activity No depth informationNo depth information Structures at different Structures at different

depths are superimposeddepths are superimposedLoss of contrastLoss of contrast


Planar imagingPlanar imaging 2D representation of 3D Distribution of activity2D representation of 3D Distribution of activity No depth informationNo depth information Structures at different depths are superimposedStructures at different depths are superimposed

Loss of contrastLoss of contrast





Image contrast 2:1

Object Contrast 4:1

Single Photon Emission Computed Single Photon Emission Computed TomographyTomography

Collect multiple planar Collect multiple planar images at several images at several angles around the angles around the patientpatient Typically 64-128 views Typically 64-128 views

over 360over 360°° Can be 32-64 views Can be 32-64 views

over 180over 180°°

Single Photon Emission Computed Single Photon Emission Computed TomographyTomography

Image ReconstructionImage Reconstruction 2D images of selected 2D images of selected

planes within the 3D objectplanes within the 3D object Better ContrastBetter Contrast Lower Spatial ResolutionLower Spatial Resolution

Normal reconstruction Normal reconstruction techniques are Filtered techniques are Filtered Back Projection or Back Projection or Iterative ReconstructionIterative Reconstruction

Back ProjectionBack Projection

Back Project Back Project each planar each planar image onto image onto three three dimensional dimensional image matriximage matrix

3 3

3

3

3

3

33

6

6 6

6



3 36

1

1

1

1

1

1

2

2

2



3 36

1

1

1

1

1

1

2

2

2

3

3

6

22 3 2

3 4 3

2 3 2



3 3

3

3

3

3

33

6

6 6

6

4 4

4 4

6 6

6

6

8


More views – More views – better better reconstructionreconstruction

1/r blurring, even 1/r blurring, even with infinite number with infinite number of viewsof views

Filtered Back ProjectionFiltered Back Projection

Filter planar views Filter planar views prior to back prior to back projectionprojection

Correction of 1/r Correction of 1/r blurring requires blurring requires ‘Ramp’ Filter‘Ramp’ Filter Gives increasing weight to Gives increasing weight to

higher spatial frequencieshigher spatial frequencies Amplifies NoiseAmplifies Noise

SPECT FIlters

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Frequency (fraction of Nyquist)

Ramp

Shepp-Logan

ModifiedShepp_Logan

Hanning

Hamming

Butterworth


In PracticeIn Practice Use modifications Use modifications

of Ramp Filterof Ramp Filter Compromise Compromise

between Noise between Noise and Spatial and Spatial ResolutionResolution

SPECT FIlters

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Frequency (fraction of Nyquist)

Ramp

Shepp-Logan

ModifiedShepp_Logan

Hanning

Hamming

Butterworth

Modified Ramp FilterModified Ramp Filter

Multiplication of the Multiplication of the ramp filter by another ramp filter by another functionfunction Often a gaussian Often a gaussian

shape shape Width of the gaussian Width of the gaussian

affects the “roll off” of affects the “roll off” of the rampthe ramp

Problems with Filtered Back Problems with Filtered Back ProjectionProjection

Back projection is mathematically correct, Back projection is mathematically correct, but real life images require Filtered Back but real life images require Filtered Back ProjectionProjection

Back Projection can introduce noise and Back Projection can introduce noise and streaking artefactsstreaking artefacts Not good with attenuation correctionNot good with attenuation correction

Filtered Back Projection can reduce noise Filtered Back Projection can reduce noise and artefacts, but may degrade resolutionand artefacts, but may degrade resolution

Iterative ReconstructionIterative Reconstruction

NOT a new techniqueNOT a new technique Pre-dates Filtered Back ProjectionPre-dates Filtered Back Projection Computationally IntensiveComputationally Intensive

Long Reconstruction TimesLong Reconstruction Times Requires fast computers for reconstructionRequires fast computers for reconstruction Takes around 1 min for a 16-frame gated 128 Takes around 1 min for a 16-frame gated 128

x 128 matrix cardiac scanx 128 matrix cardiac scan

What is Iterative Reconstruction?What is Iterative Reconstruction?

Iteration is process of successively better Iteration is process of successively better “guesses”“guesses”

The image processing computer creates The image processing computer creates an image by refining the expected an image by refining the expected projections in comparison to those projections in comparison to those recordedrecorded

This form of IR is known as “Maximum This form of IR is known as “Maximum Likelyhood Expectation Maximisation” Likelyhood Expectation Maximisation” (MLEM)(MLEM)

MLEM AlgorithmMLEM Algorithm

Benefits of IRBenefits of IR

More accurate More accurate modelling of modelling of emission/detectionemission/detection

Can include Can include attenuation attenuation correction and other correction and other information from information from MR, CT etcMR, CT etc

Lower noiseLower noise

Image FusionImage Fusion

““Unclear Medicine” Unclear Medicine” images can be images can be registered to CTregistered to CT Reduces attenuation Reduces attenuation

artefactsartefacts Allows localisation of Allows localisation of

“fuzzy blob” images“fuzzy blob” images Can improve Can improve

diagnostic accuracydiagnostic accuracy

Attenuation CorrectionAttenuation Correction

X-Ray imaging essentially provides an X-Ray imaging essentially provides an attenuation “map”attenuation “map” Images formed by different attenuation Images formed by different attenuation

patterns patterns NM imaging does not need attenuationNM imaging does not need attenuation

In fact do not want it!In fact do not want it! Hybrid imaging (e.g.SPECT-CT) takes Hybrid imaging (e.g.SPECT-CT) takes

attenuation map of CT images and uses to attenuation map of CT images and uses to correct for attenuation in 3D NM images correct for attenuation in 3D NM images

““Jordan”Jordan”

6 x 500ml saline bags 6 x 500ml saline bags strapped to torso strapped to torso phantom (3 each phantom (3 each side) to simulate side) to simulate breast attenuationbreast attenuation

Positioned to cover Positioned to cover anterior LVanterior LV

Normal Perfusion: “Jordan”Normal Perfusion: “Jordan”

FBP FBPSC IR IRSC IRAC IRACSC

Resolution RecoveryResolution Recovery

Resolution worsens Resolution worsens with increasing with increasing distance from the distance from the collimatorcollimator

If we can model how If we can model how this happens, we can this happens, we can build this into our build this into our Iterative projectionsIterative projections

Resolution RecoveryResolution Recovery

Better modelling Better modelling means better means better imagesimages

Fewer counts Fewer counts needed to get needed to get acceptable acceptable imagesimages Shorter Shorter

acquisitionsacquisitions Lower dosesLower doses

NM Imaging: The PET CameraNM Imaging: The PET Camera

PET camera invented in the 1970sPET camera invented in the 1970s Positron Camera 1959Positron Camera 1959

Early positron study (1953)Early positron study (1953)

Why use positron emitters?Why use positron emitters?

Many of the positron emitters occur in biological Many of the positron emitters occur in biological molecules (C, N, O, etc.)molecules (C, N, O, etc.)

Many have small molecular weights relative to Many have small molecular weights relative to the biological molecules they may be used to the biological molecules they may be used to label (e.g., F) even if they aren’t found there label (e.g., F) even if they aren’t found there naturally.naturally.

PET isotopes can be attached to biologically PET isotopes can be attached to biologically interesting molecules with no or minimal impact interesting molecules with no or minimal impact on the behaviour of those molecules in the body.on the behaviour of those molecules in the body.

Positron Emission TomographyPositron Emission Tomography

PET isotopes emit positrons rather than PET isotopes emit positrons rather than gamma raysgamma rays

Coincidence ImagingCoincidence Imaging Better Spatial Resolution (Typically 4mm)Better Spatial Resolution (Typically 4mm)

Requires Dedicated EquipmentRequires Dedicated Equipment Limited AvailabilityLimited Availability

AnnihilationAnnihilationconservation of momentum:

before: system at rest; momentum ~ 0

after: two photons created; must have same energy and travel in opposite direction.

conservation of energy

before: 2 electrons, each with a rest mass of 511keV

after: 2 photons, each with 511keV.

decay via positronemission

electron/positronannihilation

annihilationphoton

annihilationphoton

detector

Coincidence ImagingCoincidence Imaging

line of response(LOR)

Detector ArrayDetector Array

Coincidence Imaging - Detector Coincidence Imaging - Detector RingRing

Types of Coincidence EventTypes of Coincidence Event

2D to 3D Imaging2D to 3D Imaging

Stack multiple rings behind each otherStack multiple rings behind each other Allows for true 3D imaging Allows for true 3D imaging Shorter imaging time so better throughput Shorter imaging time so better throughput

and fewer motion artefactsand fewer motion artefacts

Time of Flight (TOF) PETTime of Flight (TOF) PET

Because we are “timing” Because we are “timing” the arrivals of the photons, the arrivals of the photons, we can tell how far apart we can tell how far apart they arethey are All photons travel at the All photons travel at the

speed of lightspeed of light Simultaneous equation to Simultaneous equation to

work out point of originwork out point of origin Makes “line of response” Makes “line of response”

more like a pointmore like a point

PET Camera CrystalsPET Camera Crystals

NaI has too poor stopping power for 511keVNaI has too poor stopping power for 511keV BGO is main material usedBGO is main material used Siemens patented LSOSiemens patented LSO*this table was provided by Siemens…*this table was provided by Siemens…

nuclear medicine: tomographic imaging – spect, spect-ct and pet-ct

Documents

projectionfilter planar

different depths

projectionback projection

ramp filtergives

real life images

object contrast

tomographic imaging

gaussian shape width