PET/CT and Fusion Issues
Acknowledgements
Jon A. Anderson
Department of Radiology
The University of Texas Southwestern
Medical Center at Dallas
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American Associate of Physicists in Medicine
2003 Annual Meeting
Why Are We Excited About
PET/CT?
Tammy Pritchett, RT, CNMT (UTSW)
Michael Viguet, CNMT, (UTSW)
Dana Mathews, MD (UTSW)
Thomas Lane, PhD (UTSW)
Jonathan Frey (Siemens)
Jim McCann (Siemens)
James Bland (CPS)
Ken Halliday (CTI)
Alex Ganin, PhD (GE)
Jeff Simer (GE)
Johann Fernando, PhD (Philips)
and many others
PET Fundamentals: Ideal Case
Annihilation
Event
Positron-emitting
Nucleus
• Use of sequential CT and PET in the same scanner
to obtain detailed anatomical and functional
images yields
γ2
Detector Ring
e+
– “automatic” and reliable co-registration of the two
images for improved diagnostic performance,
– reduced acquisition time for PET study to enable better
patient compliance and higher patient throughput,
– lower noise in the attenuation correction step of the
PET data processing
– “knock your socks off” images that referring physicians
can readily appreciate and use.
γ1
Two Events (
) in Detector
at the Same Time* Define
Line of Response (LOR)
Used in Reconstruction
*Same Time = within 6-12 ns (typ)
PET Fundamentals: Real Case
RANDOMS
with false
LOR
γ2
suppress
with small
coincidence
time and
collimation
γ1
to single-row vs multi-detector CT)
2D
3D
γ2
γ1
collimators
2D
γ2
γ3
scatter
γ1
SCATTERS
with misplaced
LOR
suppress with energy
resolution and collimation
Sensitivity
TRUES
with correct
LOR
2D and 3D PET (Distinctly similar
3D
Plane
Collimators, restricted
span between rings, lower
sensitivity, reduced scatter
No collimators, larger
span between rings,
high sensitivity, higher
scatter
1
Corrections for Quantitative
Studies (All PET is Quantitative)
Attenuation Correction Factors
(ACFs)
− ∫ µ ds
Raw Sinogram Data (T +S+ R)
P1 = e
γ1
Remove Randoms
γ2
Normalize Detector Responses
−
P2 = e
Correct for Deadtime
s2
Correct for Attenuation
T Sinogram
Ready for Reconstruction
How Big is the Attenuation
Correction Factor (ACF)?
d = 23 cm
→ ACF ≈ 9
Attenuation Correction Factor in Soft Tissue
Attenuation Correction Factor
140
120
100
80
60
40
d = 35 cm
→ ACF ≈ 28
20
0
0
10
20
30
40
50
∫ µ ds
s2
Pcoincidence = P1 • P2
s1
Correct for Scatter
s1
60
Path Length [cm]
Basic Arrangement of a PET/CT
Siemens Biograph
(#6)
HR+ PET (BGO)
Somatom
Emotion CT
(single slice)
−
Probability of both photons escaping depends
on the integral of the absorption coefficient
along the entire LOR -- exactly the data from
transmission CT!
=e
∫ µ ds
s1+ s2
ACF is inverse of
this probability.
In A Sense, We Have Had PET/CT
For A Long Time, but ….
Corrections to PET scans for
attenuation and possibly
scatter require an attenuation
map (µ-map, AKA CT image)
at 511 keV obtained by
measuring transmission of 511
or 662 keV photons from
isotopic sources.
- obtained sequentially to
emission scan
- acquired on same time
scale (requires about
1/3 of total imaging
time, say 3 minutes
out of 8 minute scan)
PET µ-map images are CT scans that
- are made at 511 keV
- have low spatial resolution
- visualize bone, tissue, and contrast
differently than x-ray CT
Current Implementations
(August 2003)
CPS
(marketed by CTI and Siemens)
Reveal RT/Reveal XVI
Biograph LSO/Biograph Sensation 16
GE
Discovery LS
Discovery ST
PET
CT
apologies to any vendor
that was inadvertently
omitted!
Philips GEMINI
2
Available Technology
Manufacturer
CPS
(CTI and
Siemens)
GE
Philips
Product
CT
Reveal RT
Biograph
LSO
Siemens
Reveal XVI
Biograph
Sensation 16
Siemens
Somatom Emotion
(2 Slice)
Crystal Technology
PET
Detectors
Reveal
ECAT
ACCEL
BGO
LSO
GSO
bismuth germanate
lutetium oxyorthosilicate:Ce
gadolinium oxyorthosilicate:Ce
Decay Time ns 300
40
60
Attenuation cm 1.05
Length
1.16
1.43
Material
LSO
Somatom Sensation
(16 Slice)
Discovery LS
GE Lightspeed
(4, 8, 16 slice)
Advance NXI BGO
Discovery ST
GE Lightspeed
(4, 8 slice)*
Discovery ST
GEMINI
Philips MX8000 Allegro
(2 slice)
GSO
Relative
Light Out
%
15%
75%
25%
Energy
Resolution
%
10.2
10
8.5
(single crystal)
DS Bailey et al in Valk et al Positron Emission Tomography (Springer, 2003)
*16 expected by end of 2003
PET Configurations
Product
Crystal Size
and
(#)
Axial
Field of
View
mm
Transverse
Field of
View
PET Performance
#
Image
Planes
Axial
Sampling
Ring
and
(Port)
Diameter
mm
cm
3.4
82.7
(70)
Product
Sensitivity
(Trues)
Mode
Axial
Resolution
Axis/10 cm
27**
3D
kcps/kBq/mL
Transverse
Resolution
Axis/10 cm
Energy
Resolution
PET
Xmis’n
mm
mm
%
5.8/7.1*
6.3/7.4*
25
(recently
announced
15 w new
electronics)
No
cm
cm
6.45 x 6.45
Reveal RT
x 25
Biograph LSO
(9216)
Reveal XVI
Biograph
Sensation 16
16.2
58.5
Discovery LS
4 x 8 x 30
(12096)
15.2
55
35
4.25
92.7
(70/59)*
Discovery LS
5.4**
31**
2D
3D
4.8/5.4**
6.0/6.3**
4.8/5.4**
4.8/5.4**
20
Yes
Ge-68
Discovery ST
6.3 x 6.3 x30
(10080)
15.7
70
47
3.27
88.6
(70)
Discovery ST
8.1**
35**
2D
3D
6.2/6.8**
6.2/6.8**
6.2/6.8**
6.2/6.8**
17
No
4 x 6 x 20
(17864)
18
90
2
90
(70/63)*
25**
3D
5.0/6.1*
4.9/5.5*
16
Yes
Cs-137
GEMINI
47
Reveal XVI
Biograph
Sensation 16
* CT port/ PET port
CT Configuration
Product
Generator
Anode Heat
Rating
Minimum
Slice
Thickness
Field of
View
kW
MHU
mm
cm
Reveal RT
Biograph LSO
40
3.5
1
50
Reveal XVI
Biograph
Sensation 16
60
5.3
1
50
Discovery LS
53.2
6.3
.625
50
Discovery ST
53.2
6.3
.625
50
60
6.5
0.5
50
GEMINI
Reveal RT
Biograph LSO
GEMINI
(t+s), 410 LLD
* NEMA 2001
** NEMA 1994
Noise Equivalent Count Rate:
NECR
Noise equivalent count rate (NECR) is used to compare
the effective count rate performance of systems and to
determine the optimal operating regime of particular
combinations of hardware and data collection strategy
• It represents the count rate in a perfect system (no
randoms or scatters) that would have the same SNR as
the measured count rate in the system
• Under certain
T2
assumptions, can
NEC =
be calculated as:
(T + S + nfR)
n = 2 for direct subtraction of randoms and 1 if
variation reduction scheme is applied to randoms;
f = fraction of field of view filled by phantom
3
Bone Differs From Other Tissues
When Changing µ70keV →µ511keV
Specific Issues for PET/CT
• Derivation of proper attenuation corrections
from CT image
• Recognition of PET artifacts due to
– Attenuation correction model failures
– Patient motion
– CT artifacts
Mass Attenuation Coefficients for Principal Tissue Types
10.00
Soft Tissue Total
Soft Tissue PE
Adipose Tissue Total
Bone Total
Bone PE
2
µ/ρ [cm /g]
Adipose Tissue PE
CT Energy
70 keV
1.00
• QA and test procedures
• Design features of PET/CT facilities
PET Energy
511 keV
0.10
Photoelectric
Component
of Attenuation
Coefficient
Compton Component
Accounts for Almost All of
Attenuation Coefficient
0.01
10
100
1000
Energy [keV]
Conclusions Regarding the
Relation of µ70keV to µ511keV
Initial Scheme for Converting
µ70keV →µ511keV
1) The attenuation coefficient at 511 keV can be
obtained from that at 70 keV by simple
multiplication by a constant, independent of tissue
type, for all tissues having attenuation dominated
by Compton scatter at CT energies (≈ 70 keV).
2) For materials having substantial contribution from
photoelectric absorption, the scaling factor will be
dependent on the nature of the material.
1) Adjust resolution in CT to agree
with resolution of PET
2) Convert CT numbers to µ70 keV,
µ = ((CT/1000)+1)*µH2O,70 keV
3) Scale all µ values corresponding to
CT values below ≈ 200 − 300 by a
factor of µH2O,511 keV/µH2O,70 keV
4) Scale all µ values corresponding to
CT values above ≈ 200 − 300 by a
factor of µbone,511 keV/µbone,70 keV
CT + 1024
Histogram of CT image, showing
separation of tissue types [Kinahan
et al.]
Used in Siemens/CTI Scanners
“hybrid segmentation”
Kinahan et al.,Med Phys 25, 2046-2053 (1998)
A Second Algorithm for
Converting µ70keV →µ511keV
Comparison of Attenuation
Correction Methods
0.20
µPET= [((CT/1000)+1)*µH2O,80 keV]*
µH2O,511 keV
µH2O,80 keV
Equivalent to
formula in
Kinahan for
CT# ≤ 0 HU
CT# > 0 HU:
µbone,511 keV- µH2O,511 keV
µPET= µH2O,511 keV + (CT/1000)*µH2O,80 keV* µ
bone,80 keV- µH2O,80 keV
Tissue is treated as a mixture of air and water below 0 HU and a
mixture of water and bone above 0 HU. “GE Approach”
Burger et al. EJNM 29 922-927(2002)
Attenuation Coefficient at 511 keV
0.18
CT# ≤ 0 HU:
Exact forms depend
on choice of
parameters; for this
plot
Hybrid Segmentation Approach
Mixture Approach
0.16
0.14
Hybrid
Segmentation
0.12
0.10
threshold=200 HU
PET/CT<200 = 0.5
PET/CT>200 = 0.41
0.08
0.06
Mixture
0.04
0.02
0.00
-1000
-500
0
CT Number (HU)
500
1000
threshold = 0 HU
µH2O,80 keV = 0.184 cm-1
µbone,80 keV = 0.428 cm-1
µH2O,511 = 0.096 cm-1
µbone,511 keV = 0.172 cm-1
4
Principal Artifacts Specific to
PET/CT
Failure of the Attenuation
Correction Model
• Artifacts due to fundamental failure of the
attenuation correction model
• Artifacts due to patient motion between the
CT and PET scans
• Truncation artifacts due to patient extending
outside the CT field of view
The Origin of Contrast and Metal
Artifacts in PET/CT
• There is no way to intrinsically deconvolve the measured
attenuation coefficients µ into
µ
µ = ∑   ∗ ρ i
i  ρ i
so we don’t know what we’ve got or how much,
only what the total attenuation is at CT energies
• Materials not in the model will scale differently and give
the wrong attenuation correction
• Incorrect attenuation values give incorrect activities,
too high or too low
Artifacts from Oral Contrast
CT
AC
Mass Attenuation Coefficients for Tissue and Contrast Materials
10.00
Soft Tissue
Bone
Barium Sulfate
Sodium Iodide
CT Energy
70 keV
NonAC
2
µ/ρ [cm /g]
1.00
PET Energy
511 keV
Bolus of contrast in bowel
generates apparent area of
high uptake on attenuationcorrected image
0.10
The mass attenuation coefficients for contrast materials
are significantly larger than those for tissues (including
bone) due to the large photoelectric component; the
scaling to 511 keV is different and the attenuation
correction will be overestimated!
0.01
10
100
1000
Energy [keV]
Artifacts from Hardware:
Mediports
Non-attenuationcorrected image
reveals little
contrast wrt
surrounding tissues
Artifacts from Metal: Orthopedic
Hardware
Intense activity shown on
PET/CT (SUV = 6) is associated
with metallic hardware having
CT# > 3000HU
Hot spot on PET
Correlates to Mediport
placement
No anomalous
uptake in nonattenuation corrected
image
5
Artifacts from Metal: Orthopedic
Hardware, continued
Artifacts from Patient Motion
Artifacts arise because the CT and PET scans are taken
- at different times (i.e. sequentially)
- on different time scales (seconds for CT, minutes for PET)
Voluntary movements: Patient shifts position between CT scan
and PET scan; principally movements of head and arms
Attenuation Corrected PET
Non-attenuation Corrected PET
Breathing Artifact
Involuntary movements: CT catches snapshot of patient
position, but PET averages over minutes
- CT is not normally done with breath hold (contrary to
normal CT practice), but with shallow breathing
- PET artifact not seen on conventional PET because both
emission and transmission averaged over similar periods
Quality Assurance Program:
• QA for PET --
Breathing during helical
CT acquisition can lead
to “floating liver” artifact
(variously called banana
or mushroom artifact)
which can then be
reflected in PET scan.
– Daily check per mfg with line or volume source to
assure that system performance is not drifting
– Period Physics check per standard PET practice
• QA for CT -– Daily air cals and image quality check per mfg
– Period Physics check per standard CT practice
• Registration check -- check for proper
registration of PET and CT images on periodic
basis
Examples: QA Procedures
CPS (Siemens-CTI):
Obtained with volume
phantom (1.5 mCi
68Ge); summary report
presented and logged;
sinogram viewer for
details.
Gantry Offset Correction/Check:
Phantom Geometry
GE: Obtained with rotating line source in
gantry. Report generated with fan-sum
views and comparisons to previous trends.
6
Registration Test
Stability of Registration: An
Example
The reproducibility of the
acquisition process was
better than σ = 0.1 mm
(registration repeated
without removing the
dual line phantom)
Registration Stability
1
0.5
CT
FUSION
PET
In our case, Siemens/CT Gantry Offset Procedure could be used to
numerically evaluate x (horizontal) y(vertical) and z(axial)
registration without modifying machine calibration. Results can
also be evaluated visually or with other programs
Examples of PET Testing Tools
Provided for PET/CT Machines
• CPS (Siemens, CTI)
– Uniformity and Sensitivity test software shipped with
machine, based on volume 68Ge source used for daily
QC
• GE
– Will provide physicist with testing software (NEMA
NU-2 2001)
Correction [mm]
0
Z-900 mm
-1
-1.5
X
-2
Y
-2.5
-3
2/23
3/5
3/15
3/25
4/4
Workflow at the PET Center
(FDG Whole Body Scans)
(You may get different answers from different folks!)
NEMA NU-2 2001
Spatial resolution
Scatter fraction, count losses, randoms
Sensitivity
{now part of scatter fraction}
NEMA NU-2 1994
Spatial resolution (xverse,axial)
Scatter fraction
Sensitivity
Deadtime,count-rate losses
Uniformity
Scatter correction accuracy
Count-rate correction accuracy
Attenuation correction accuracy
{now part of image quality test}
Count rate correction accuracy
{now part of image quality test}
Image quality (scatter, AC accuracy)
94 Scatter fraction test
94 Count rate tests
Pt instruction and prep
30-60 min
Injection of Pt
Workload Estimation
PET Facility Throughput Example:
1 Hour Uptake, 30 Minute Scan
Assay of dose
Uptake of pharmaceutical
Have Pt empty bladder
Transport Pt to scanner
10 min
Position Pt
5-30 min
Scan
Release Pt
Many of these tests were developed to compare classes of machines rather
than to provide practical acceptance test procedures. NU2-94 may be
more appropriate for brain scanners and NU2-01 for WB oncology scans.
Receive doses
*
*
*
* steps with highest
technologist exposure
QA Check of Scan
4/24
The stability of the
system over the first 45
days was characterized by
σ = 0.5 mm or better for
all 3 axes.
What Tests are Important?
15
Patient Number
Arrival of patient
4/14
Date
• Philips
– Will ship NEMA NU-2 2001 software with machine
The reproducibility of the
registration procedure
(multiple replacement of
the phantom) was better
than σ = 0.4 mm.
-0.5
13
11
9
7
Phase
5
Uptake
Scanning
3
1
8:00
9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00
Time of Day
Read study
Print for file and referring; distribute to PACS
#pts/day = (Twork - Tuptake)/Tscan_rm
# uptake rooms = Tuptake/Tscan_rm
7
Effective Dose Comparison:
Conventional PET vs PET/CT
Whole Body Scan, HR+ in 2D mode
12 mCi 18F FDG
1.3 rem
transmission scan
(average exposure of
56 mR on axis, free air,
6 bed scan, 8 min/bed,)
Whole Body Scan, Biograph
10 mCi 18F FDG
1.1 rem
WB CT scan
(130 kVp, 120 mAs, 5 mm, p = 1.5)
1.3 rem
Total
2.4 rem
The
End
8