Transcript Nuclear Medicine Physics

Nuclear Medicine Physics
SPECT/CT, PET/CT, PET/MR
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia
A note of thanks to
Z. J. Cao, Ph.D.
Medical College of Georgia
And
Sameer Tipnis, Ph.D.
G. Donald Frey, Ph.D.
Medical University of South Carolina
for
Sharing nuclear medicine presentation content
SPECT vs PET
PET
SPECT
(Step-and-shoot acquisition)
2015
(Simultaneous acquisition)
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
SPECT vs PET imaging
Attribute
SPECT
PET
Detection
Single s
Coincident s
Radionuclides
99mTc, 67Ga, 111In
18F, 82Rb, 13N,
E
70 – 300 keV
511 keV
Spatial res.
 10 – 12 mm
 5 - 6 mm
Atten.Correction
No / Yes*
Yes
* Possible with SPECT/CT or transmission source
systems
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
SPECT & PET
• SPECT – 2 views from opposite sides
 Res.  collimator res., which degrades rapidly with
increasing distance from collimator face
• PET – Simultaneous acquisition
 Res.  detector width; is max in center of ring
• SPECT sensitivity ~ 0.02%
 Huge losses due to absorptive collimators
• PET sensitivity- 2D ~ 0.2%; 3D ~ 2% or higher
 High sensitivity due to ACD (electronic collimation)
 Allows higher frequency filters / higher spatial resolution
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
SPECT/CT, PET/CT, PET/MR
• Combine a PET scanner
(or gamma camera) with a CT
(or MR) scanner on the same
gantry to share the same
patient table
• Acquire CT (or MR) and PET
(or SPECT) images sequentially
(or simultaneously) with minimum patient
movement and then use software to fuse
the images
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Why PET/CT, PET/MR, and
SPECT/CT?
• Fusion of PET (or SPECT) and CT (or
MR) images provides a clear
background for tumors and hence
better tumor localization.
• The CT (not MR) image provides a
patient-specific attenuation map for
PET (or SPECT) attenuation
compensation.
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PET/CT scanner
• First PET/CT scanner: 1998
• Currently all PET scanners have CT.
SPECT/CT scanner
Introduced in early 2000’s
PET/CT image fusion
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SPECT/CT images
Oncology
From CT image to attenuation map
• Attenuation coefficient m depends on photon
energy.
• The average energy of CT x-rays is ~70 keV
while the photons are 511 keV in PET or 140
keV in SPECT.
• CT image is segmented to bone, muscle, and
lungs.
• Apply different scaling factors (e.g. m511/m70 for
PET) to tissues, e.g. 0.495 to muscle and
0.406 to bone, to convert the CT image to m
map.
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Artifacts in PET (or SPECT)
caused by CT
 Since CT image is used for PET (or SPECT)
attenuation compensation, CT image artifacts
may cause artifacts in PET (or SPECT)
reconstructed image.
 CT artifacts may be caused by
• metal, patient movement, or image truncation,
• contrast
• misalignment between CT and PET (or SPECT) due
to respiration and other patient motions.
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Respiration misalignment in PET/CT
PET comp
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Misalignment in SPECT/CT
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Misalignment in SPECT/CT
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Head motion in PET
Corrected
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PET/MR
• Functional and anatomic image fusion, MR
provides better soft tissue contrast than CT
 better lesion localization particularly in
brain imaging
• No ionizing radiation to the patient from MR
so radiation dose is cut down to about half
• It is difficult to obtain patient-specific MRbased attenuation map for PET
reconstruction.
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Simultaneous PET/MR
• PET inserted in the MR ring so that the
data acquisition is truly simultaneous, e.g.
Siemens Biograph mMR (PET + 3T MR)
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Sequential PET/MR
• PET separated from MR but sharing the
same patient table. Data acquisition is
sequential, e.g. Philips TF Ingenuity
(PET + 3T MR)
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Main technical challenges of
PET/MR
• Reduce the interference between PET and MR
components.
 PMT replaced by avalanche photodiodes, or
 PMT installed far away from the MR ring using
long optical fibers
 PET electronics shielded from RF and gradient
fields
 MR coils shielded from the metal PET insert
• Patient-specific, MR-based attenuation map for PET
attenuation compensation?!?!
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PET/MR human brain
PET
MRI
fused
PET/MR human brain
PET/CT
PET/MR
PET/MR
PET/MR
w.o. m comp trans-based MR-based
PET/MRI breast cancer
MRI
fused
PET
Attenuation Correction:
A Review
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation correction
• s traveling smaller paths through pt (nearer to
camera) have less attenuation compared to those
from deeper in pt
 Inaccurate representation for the same amount of
radioactivity
 AC allows correct representation of the
distributed radioactivity
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation correction
• SPECT: m for AC can be assumed or measured
 Chang (assumed), Measured - Gd rods (older) or CT
(new)
 CT can be non-diagnostic (low power) or fully diagnostic
• PET: m for AC is measured
 CT (fully diagnostic)
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation in SPECT
D1
a
I1 = I0e-ma
t
I0 = I1e+ma
Probability of detection / correct intensity I0, dependent
on the depth at which  originates  need to know “a”
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Chang’s AC method
• Image first reconstructed without AC
• Contours of image used to estimate t for each
projection, m assumed to be constant
• ACF determined for every projection
• Average ACF determined for each pixel (x,y)
from all projections
• Reconstructed image corrected pixel-by-pixel
• Works well for area with approximately constant
attenuation like head, abdomen but not for areas
like chest / thorax
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Uniform phantom with evenly
distributed 99mTc
Low counts
in center
2015
Chang
method
Proper AC
Nuclear Medicine Physics for Radiology Residents
Chang
method
Overcorre
ction
Sameer Tipnis, PhD, DABR
SPECT-CT
AC in SPECT CT
• Accurate / realistic m-map obtained for each
projection using CT
• m values used in Chang’s algorithm to correct
pixel-by-pixel
• AC here is more realistic (since m is not assumed
to be constant)
• Current SPECT/CT systems use this method
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation in PET
b
D1
D2
a
P1 = e-ma
t
P2 = e-mb
P = P1 × P2 = e-ma × e-mb
= e-m(a+b)
= e-mt
Probability of detection dependent only on the total
thickness
Attenuation Correction in PET/CT
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation Correction
Without
AC
With
AC
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation in PET
• For each LOR, probability of detection
dependent only on the total thickness
• Every point along LOR has SAME ACF
(No depth dependance)
• Attenuation more severe than in SPECT
but much easier to correct
• By far the most important data
correction in PET
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
PET/CT co-registration
•
•
2015
After acquisition, check PET / CT registration
•
Scanners have manual QC tool for checking co-reg
•
If unsatisfactory, adjust co-reg using QC tool and
repeat PET reconstruction
Proper co-registration of PET and CT data is critical for
proper diagnosis
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Attenuation Correction
Proper
registration
Misregistration
Lateral walls of
myocardium in
the PET data,
corrected with
the lower
attenuation of
lung tissue
Lateral walls
of
myocardium
corrected with
the higher
attenuation of
heart tissue
(Ref: Attenuation correction of PET cardiac data with low-dose average CT in
PET/CT, Tinsu Pan et al, Med. Phys. 33, October 2006)
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Typical PET / CT imaging
(1)
(2)
512 x512 128 x128 120 kVp  511 keV
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
• October 7, 2015 -- Researchers at the University of California,
Davis (UC Davis) have received a five-year, $15.5 million grant to
develop what they are calling the world's first total-body PET
scanner.
 National Cancer Institute and will fund the Explorer project, led by Simon
Cherry, PhD, distinguished professor of biomedical engineering and Ramsey
Badawi, PhD, a professor of radiology.
 The total-body PET scanner would image an entire body all at once, and it
would acquire images much faster or at a much lower radiation dose by
capturing almost all of the available signal from radiopharmaceuticals. … the
design would line the entire inside of the PET camera bore with multiple rings of
PET detectors.
 … such a total-body PET design could reduce radiation dose by a factor of 40 or
decrease scanning time from 20 minutes to 30 seconds
http://www.auntminnie.com/index.aspx?sec
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