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Nuclear Medicine:
Tomographic Imaging –
SPECT, SPECT-CT and PETCT
Katrina Cockburn
Nuclear Medicine Physicist
Image Acquisition Techniques
 Static
 Dynamic
 Gated
 Tomography
-
(Bones, Lungs)
(Renography)
(Cardiac)
-
(Cardiac)
 SPECT
 PET
 List Mode
Problems with Planar Imaging
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Problems with Planar Imaging
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Problems with Planar Imaging
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Problems with Planar Imaging
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Problems with Planar Imaging
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Problems with Planar Imaging
Image contrast 2:1
 Planar imaging
 2D representation of 3D
Distribution of activity
 No depth information
 Structures at different
depths are superimposed
 Loss of contrast
Object Contrast 4:1
Single Photon Emission Computed
Tomography
 Collect multiple planar
images at several
angles around the
patient
 Typically 64-128 views
over 360°
 Can be 32-64 views
over 180°
Single Photon Emission Computed
Tomography
 Image Reconstruction
 2D images of selected
planes within the 3D object
 Better Contrast
 Lower Spatial Resolution
 Normal reconstruction
techniques are Filtered
Back Projection or
Iterative Reconstruction
Back Projection
 Back Project
3
6
3
3
3
6
6
3
3
3
6
3
each planar
image onto
three
dimensional
image matrix
Back Projection
 Back Project
3
6
3
1
2
1
1
2
1
1
2
1
each planar
image onto
three
dimensional
image matrix
Back Projection
 Back Project
3
6
3
3
21
2
2
3
1
2
6
31
2
4
31
3
21
23
1
2
each planar
image onto
three
dimensional
image matrix
Back Projection
 Back Project
3
6
3
3
4
6
4
3
6
6
8
6
6
3
4
6
4
3
3
6
3
each planar
image onto
three
dimensional
image matrix
Back Projection
 Back Project
3
6
3
3
4
6
4
3
6
6
8
6
6
3
4
6
4
3
3
6
3
each planar
image onto
three
dimensional
image matrix
Back Projection
 More views –
better
reconstruction
 1/r blurring, even
with infinite number
of views
Filtered Back Projection
 Filter planar views
prior to back
projection
 Correction of 1/r
blurring requires
‘Ramp’ Filter
 Gives increasing weight to
higher spatial frequencies
 Amplifies Noise
SPECT FIlters
1
Ramp
0.8
Shepp-Logan
0.6
Modified
Shepp_Logan
0.4
Hanning
0.2
Hamming
0
0
0.2
0.4
0.6
0.8
-0.2
Frequency (fraction of Nyquist)
1
Butterworth
Filtered Back Projection
 In Practice
 Use modifications
of Ramp Filter
 Compromise
between Noise
and Spatial
Resolution
SPECT FIlters
1
Ramp
0.8
Shepp-Logan
0.6
Modified
Shepp_Logan
0.4
Hanning
0.2
Hamming
0
0
0.2
0.4
0.6
0.8
-0.2
Frequency (fraction of Nyquist)
1
Butterworth
Modified Ramp Filter
 Multiplication of the
ramp filter by another
function
 Often a gaussian
shape
 Width of the gaussian
affects the “roll off” of
the ramp
Filtered Back Projection
Problems with Filtered Back
Projection
 Back projection is mathematically correct,
but real life images require Filtered Back
Projection
 Back Projection can introduce noise and
streaking artefacts
 Not good with attenuation correction
 Filtered Back Projection can reduce noise
and artefacts, but may degrade resolution
Iterative Reconstruction
 NOT a new technique
 Pre-dates Filtered Back Projection
 Computationally Intensive
 Long Reconstruction Times
 Requires fast computers for reconstruction
 Takes around 1 min for a 16-frame gated 128
x 128 matrix cardiac scan
What is Iterative Reconstruction?
 Iteration is process of successively better
“guesses”
 The image processing computer creates
an image by refining the expected
projections in comparison to those
recorded
 This form of IR is known as “Maximum
Likelyhood Expectation Maximisation”
(MLEM)
MLEM Algorithm
Benefits of IR
 More accurate
modelling of
emission/detection
 Can include
attenuation
correction and other
information from
MR, CT etc
 Lower noise
Image Fusion
 “Unclear Medicine”
images can be
registered to CT
 Reduces attenuation
artefacts
 Allows localisation of
“fuzzy blob” images
 Can improve
diagnostic accuracy
Attenuation Correction
 X-Ray imaging essentially provides an
attenuation “map”
 Images formed by different attenuation
patterns
 NM imaging does not need attenuation
 In fact do not want it!
 Hybrid imaging (e.g.SPECT-CT) takes
attenuation map of CT images and uses to
correct for attenuation in 3D NM images
“Jordan”
 6 x 500ml saline bags
strapped to torso
phantom (3 each
side) to simulate
breast attenuation
 Positioned to cover
anterior LV
Normal Perfusion: “Jordan”
FBP
FBPSC
IR
IRSC
IRAC
IRACSC
Resolution Recovery
 Resolution worsens
with increasing
distance from the
collimator
 If we can model how
this happens, we can
build this into our
Iterative projections
Resolution Recovery
 Better modelling
means better
images
 Fewer counts
needed to get
acceptable
images
 Shorter
acquisitions
 Lower doses
NM Imaging: The PET Camera
 PET camera invented in the 1970s
 Positron Camera 1959
Early positron study (1953)
Why use positron emitters?
 Many of the positron emitters occur in biological
molecules (C, N, O, etc.)
 Many have small molecular weights relative to
the biological molecules they may be used to
label (e.g., F) even if they aren’t found there
naturally.
 PET isotopes can be attached to biologically
interesting molecules with no or minimal impact
on the behaviour of those molecules in the body.
Positron Emission Tomography
 PET isotopes emit positrons rather than
gamma rays
 Coincidence Imaging
 Better Spatial Resolution (Typically 4mm)
 Requires Dedicated Equipment
 Limited Availability
Annihilation
annihilation
photon
annihilation
photon
conservation of momentum:
before: system at rest; momentum ~ 0
electron/positron
annihilation
b-
g
b+
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
g
after: 2 photons, each with 511keV.
decay via positron
emission
Coincidence Imaging
line of response
(LOR)
detector
Detector Array
Coincidence Imaging - Detector
Ring
Types of Coincidence Event
2D to 3D Imaging
 Stack multiple rings behind each other
 Allows for true 3D imaging
 Shorter imaging time so better throughput
and fewer motion artefacts
Time of Flight (TOF) PET
 Because we are “timing”
the arrivals of the photons,
we can tell how far apart
they are
 All photons travel at the
speed of light
 Simultaneous equation to
work out point of origin
 Makes “line of response”
more like a point
PET Camera Crystals
 NaI has too poor stopping power for 511keV
 BGO is main material used
 Siemens patented LSO
*this table was provided by Siemens…