Introduction
PET Scanner Performance,
Quality Assurance and
Acceptance Testing
Quality Control
Procedures required to ensure that the distribution of radiation
emitted from a patient is accurately reflected in the measured
raw data.
Facets
Brad Kemp, PhD
Department of Radiology
Mayo Clinic
– quality assurance:
– verify scanner is operating properly
– identify problems prior to scanning patients
– acquisition of corrections to compensate for known
imperfections in data measurement
TU-A-319-01
Review: Types of Coincident Events
Data Acquisition
Sinogram
Sort parallel LORs
True
Scatter
Angle
Radial Distance
Randoms
Gamma ray (Singles)
Line of Response (LOR)
Prompts = True + Scatter + Randoms
1
Block Detectors
Block Detectors
2D array of crystals attached to 4 PMTs
2D array of crystals attached to 4 PMTs
Crystal element
B A
D C
z
PMT
x
X=
Light guides define the pattern of light
distribution in the block
Determines light output (sharing) to PMT's
System Correction: Normalization
Purpose: Corrects for the variations in efficiency in LORs in
each slice of the sinogram
• A uniformity correction
• For 2D: Direct Measurement using a low activity source
• For 3D: Indirect Measurement using a uniform phantom
• Acquired quarterly or after system maintenance
B+D
A+ B+C + D
Z=
C+D
A+ B+C + D
Each crystal produces a unique combination of signals in the PMTs
Compare signals X and Z to preset values in a 2D lookup table
- map X and Z to a single crystal
System Correction: Normalization
Sinogram of a Uniform Cylinder
Before
Normalization
After
Normalization
2
Blank Scan
Quality Assurance
Quality Assurance Test Requirements
• Well defined regimen of measurements
– Quick and easy to conduct
• On stand-alone PET systems, used with transmission
images to create attenuation correction factors
• Acquired daily - good source of QA data
• Emission scan of uniform cylinder
Good blank
Bad block
Miscoded event
– Sensitive to modes of failure of the scanner
– Preferably quantitative, not qualitative
Quantitative Analysis of Blank Scans
• The quantitative analysis of blank scans is an important
aspect of the QA of a PET scanner
Overview: Daily QA
Daily QA analysis calculates a global and slice normalized mean
square error (NMSE) by using a reference scan; the reference scan is
defined as the blank scan acquired immediately after normalization
• Quantitative analysis of blank scans:
- validates system calibration
- provides information on crystal, block or module (bucket)
efficiencies with respect to system average
- monitors system stability
- ensures validity of normalization
3
Overview: Advanced QA
Advanced QA analysis computes the bias in efficiency for each
crystal, block and module. Those crystals, blocks and modules with
a bias greater than a preset limit are listed in a table. Deviation of
module bias with respect to the reference scan is also shown.
Quality Assurance
Detector and Electronics Characterization
– Singles mode detector calibration
• Crystal map
• PMT gain adjustment
• Energy map
– Coincidence timing calibration
Did the engineer calibrate the scanner properly?
Crystal Map
Verification of Crystal Map
QA of crystal maps created during calibration of the scanner
Purpose:
Map the position of the detected event to a specific crystal
Obtained quarterly or after detector maintenance
Count distribution in a block
Crystal Map Look Up Table
4
PMT Gain Adjustment
Coincidence Timing Calibration
Purpose:
Purpose:
Balances the gain characteristics of the PMTs in a block
Compensates for PMT gain drift with temperature, age, …
Acquired weekly (daily) or after detector maintenance
Adjust for timing delays so events from all blocks are
timestamped equivalently
Acquired weekly or after detector maintenance
0.0
Update Gains
Update Gains
Block Bias (%)
-1.0
-2.0
Morning
-3.0
Afternoon
-4.0
γ
γ
-5.0
Coincidence ?
Yes
Register
Event
-6.0
28-Feb
7-Mar
14-Mar
21-Mar
28-Mar
Date
Coincidence Timing Calibration
Daily Quality Assurance
Blank scan created with incorrect CTC calibration
Can the Scanner be Used Today?
Potential Problems
• System stability, drifts
• Detector module / PMT/ preamp failure
• Loose cables, connectors
• Inoperable gantry motors, source loader
Daily QA Regimen
• Automatic PMT gain adjustment
• Blank scan
• QA will detect but not prevent these problems
5
Update Gains and Block Bias
Example: NMSE and Block Bias
Advanced QA analysis can be used to assess the stability of the
system and the effectiveness of daily Update Gains
QA must be sensitive to modes of failure of the scanner
Update Gains
Block Bias (%)
-2.0
Morning
-3.0
Afternoon
-4.0
-5.0
-6.0
28-Feb
7-Mar
14-Mar
21-Mar
28-Mar
6
Global NMSE
Slice NMSE
Block Bias
0.008
4
2
0.006
0
-2
0.004
-4
Block Bias (%)
Update Gains
Normalized Mean Square Error
0.0
-1.0
0.010
0.002
-6
Date
0.000
-8
10-Dec-02
29-Jan-03
20-Mar-03
9-May-03
28-Jun-03
17-Aug-03
Date
Example: Blank Scan Count Rate
The acquisition count rate is used as an estimate of
system sensitivity.
Daily QA: Loose Cable
Loose Cable
Better
800
Acquisition Rate (kcps)
700
600
500
400
300
200
100
0
Nov-01
Feb-02
May-02
Sep-02
Dec-02
Mar-03
Jun-03
Oct-03
Date
6
QA Schedule
Detector and Electronics
Characterization
– Crystal map
– PMT gain
– Coincidence timing
Frequency
Quarterly
Weekly (daily)
Weekly
System Corrections
Normalization
– Scanner calibration
– Blank Scan
–
Quarterly
Quarterly (weekly)
Daily
NEMA NU 2-2001
NEMA NU 2-2001
PET Performance Measurements
National Electrical Manufacturers Association.
NEMA Standards Publication NU 2-2001:
Performance Measurements of Positron
Emission Tomographs
Phantoms for NU 2-2001
NEMA NU 2-2001
Acceptance Testing
Can we use our new scanner?
Annual QA
Is the scanner still performing within specification?
Three phantoms:
Scatter phantom
203x700mm phantom with activity
in line source
Sensitivity phantom
Image quality phantom
7
NEMA NU 2-2001
• Measures performance of PET scanners under conditions
that attempt to represent whole body studies
– Phantom of greater length
– Out of field activity
• Standardizes oblique LOR manipulation in 3D acquisitions
NEMA NU 2-2001
Performance Measures:
• Spatial Resolution (Transaxial, axial)
• Sensitivity
• Scatter Fraction
• Count Losses
• Count Rate correction accuracy
• Daube-Witherspoon ME, et al. PET Performance
Measurements Using the NEMA NU 2-2001 Standard.
J Nucl Med 2002;43:1398-1409.
Spatial Resolution
• Spatial resolution of a system represents its ability to
distinguish between two points of radioactivity
• F-18 point sources in air at six locations:
– (0,1), (0,10) and (10,0) cm
– Center of axial FOV and ¼ axial FOV from center
• Reconstruct: image pixel < ⅓ expected FWHM
• Profile width ~ 2 times FWHM
• Report FWHM and FWTM in radial, tangential and axial
directions
• Image Quality
Sensitivity
• Sensitivity of a scanner represents its ability to detect
annihilation radiation
• Rate of true coincidence counts per unit radioactivity
(expressed in cps/kBq) in absence of attenuating media
• Rationale: need material around source to ensure
annihilation of positrons, but this material also attenuates
the annihilation photons
• Based on technique by Bailey DL, Jones T, et al. Eur J
Nucl Med 1991;18:374-379.
8
Sensitivity
•Successive measurements with a 700 mm line source of F-18
surrounded by nested, known absorbers
Sensitivity
• Measure at radial locations of 0 and 10cm
• Report system sensitivity and slice sensitivity profile
•The count rate with no absorber is extrapolated from these measures
Count Rate, R (cps)
1.5E+05
R0
R0
System Sensitivity =
Activity
1.0E+05
Axial sensitivity (cps/kBq)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
5.0E+04
10
20
30
40
Slice
0.0E+00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Sleeve Thickness (cm)
Intrinsic Scatter Fraction
• Scatter fraction is a measure of the relative system
sensitivity to scatter
SF =
Scatter
Scatter + Trues
• Use 203 mm diameter polyethylene cylinder of length 700 mm,
with activity located in a line source of diameter 2.3mm that is
4.5mm off axis
• Measured with low activity (Randoms:Trues = 1%) to avoid
random coincidences, deadtime and pulse pileup.
Count Rate Performance
• Measurement of count rate performance gives an
indication of scanner performance as a function of activity
• Use 700 mm long polyethylene cylinder
• Measured with high initial activity of F-18
3D: 800 MBq; 2D: 5 GBq
• Acquire data until randoms and deadtime losses are
negligible (14 to 18 hrs)
9
Count Rate Performance
Count Rate Performance
40
• Calculate Noise Equivalent Count Rate
35
R
NECR (1R)
NECR (2R)
30
RTrues + RScatter + k ⋅ RRandoms
NECR (kcps)
RNEC =
2
Trues
RNEC: Figure of merit relating scanner performance to image
SNR after randoms and scatter corrections.
25
20
15
10
For NEMA, k = 1 (calculated Randoms)
5
0
• Report peak NEC and effective activity concentration at peak
0
5
10
15
20
25
30
Activity concentration (kBq/cc)
Image Quality Measurement
• Standardized imaging situation that simulates a clinical
whole body imaging condition
• Phantom consists of a torso phantom with hot and cold
lesions in a warm background
• Scatter phantom abutted to image quality phantom
Image Quality Measurement
•
•
•
•
•
•
Hot spheres: 10, 13, 17, 22 mm id
Cold spheres: 28, 37 mm id
Lung insert
Activity in hot spheres 8 and 4 times that of background
Activity in background 5.3 kBq/ml
Simulated acquisition 100cm in 60min
60 min
Tacq =
∆z ≈ 7 min
100 cm
• Repeat acquisition three times
• Reconstruct using clinical protocol
10
Image Quality Measurement
• Report image contrast and SNRs for hot and cold lesions,
residual error in lung, variability in background
• Visual inspection for artifacts
NEMA and LSO
• LSO is inherently radioactive
• Background radiation gives rise to Randoms, some Trues
• Implications for NEMA: cannot obtain Randoms:Trues
ratio of 1%
• For count rate measure – acquire delayed events
• For sensitivity – acquire blank scan to estimate Trues
• Watson CC, et al. NEMA NU 2 Performance Tests for
Scanners with Intrinsic Radioactivity. J Nucl Med
2004;45:822-826.
Summary
•
•
•
•
Review of PET physics
Need for QA regimen
Calibrations and corrections required for PET imaging
NEMA NU 2-2001 Performance Standard procedures
11