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1
PET Scanner Performance, Quality Assurance and
Acceptance Testing
PET Scanner Performance, Quality Assurance and
Acceptance Testing
Brad Kemp, PhDDepartment of Radiology
Mayo Clinic
TU-A-319-01
IntroductionIntroduction
Quality ControlProcedures required to ensure that the distribution of radiationemitted from a patient is accurately reflected in the measured raw data.
Facets– quality assurance:
– verify scanner is operating properly– identify problems prior to scanning patients
– acquisition of corrections to compensate for known imperfections in data measurement
PET ReviewPET Review
γγγγ
γγγγ
Coincidence ? Register EventYes
Review: Types of Coincident EventsReview: Types of Coincident Events
True Scatter Randoms
Gamma ray (Singles)
Line of Response (LOR)
Prompts = True + Scatter + Randoms
2
Radial DistanceA
ngle
Sinogram
Data AcquisitionData Acquisition
Sort parallel LORs
Radial Distance
Ang
le
Data AcquisitionData Acquisition
2D array of crystals attached to 4 PMTs
Light guides define the pattern of lightdistribution in the block
Determines light output (sharing) to PMT's
Block DetectorsBlock Detectors
Crystal element
2D array of crystals attached to 4 PMTs
PMT
x
z
DCBADBX
++++=
DCBADCZ
++++=
Each crystal produces a unique combination of signals in the PMTsCompare signals X and Z to preset values in a 2D lookup table
- map X and Z to a single crystal
Block DetectorsBlock Detectors
B AD C
3
System Correction: NormalizationSystem 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
System Correction: NormalizationSystem Correction: Normalization
BeforeNormalization
Sinogram of a Uniform Cylinder
AfterNormalization
Quality AssuranceQuality Assurance
Quality Assurance Test Requirements
• Well defined regimen of measurements
– Quick and easy to conduct
– Sensitive to modes of failure of the scanner
– Preferably quantitative, not qualitative
Blank ScanBlank Scan• 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
Bad block Miscoded eventGood blank
4
Quantitative Analysis of Blank ScansQuantitative Analysis of Blank Scans
• The quantitative analysis of blank scans is an important aspect of the QA of a PET scanner
• 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
Overview: Daily QAOverview: 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
Overview: Advanced QAOverview: Advanced QAAdvanced 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 AssuranceQuality 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?
5
Crystal MapCrystal Map
Purpose: Map the position of the detected event to a specific crystalObtained quarterly or after detector maintenance
Count distribution in a block Crystal Map Look Up Table
QA of crystal maps created during calibration of the scanner
Verification of Crystal MapVerification of Crystal Map
Purpose:Balances the gain characteristics of the PMTs in a blockCompensates for PMT gain drift with temperature, age, …Acquired weekly (daily) or after detector maintenance
PMT Gain AdjustmentPMT Gain Adjustment
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
28-Feb 7-Mar 14-Mar 21-Mar 28-Mar
Date
Blo
ckB
ias
(%)
MorningAfternoon
Update GainsUpdate Gains Update GainsUpdate Gains
Coincidence Timing CalibrationCoincidence Timing Calibration
Purpose:Adjust for timing delays so events from all blocks are
timestamped equivalentlyAcquired weekly or after detector maintenance
γγγγ
γγγγ
Coincidence ? Register EventYes
6
Coincidence Timing CalibrationCoincidence Timing Calibration
Blank scan created with incorrect CTC calibration
Daily Quality AssuranceDaily Quality Assurance
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
Update Gains and Block BiasUpdate Gains and Block Bias
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
28-Feb 7-Mar 14-Mar 21-Mar 28-Mar
Date
Blo
ckB
ias
(%)
MorningAfternoon
Advanced QA analysis can be used to assess the stability of the system and the effectiveness of daily Update Gains
Update GainsUpdate Gains Update GainsUpdate Gains
Example: NMSE and Block BiasExample: NMSE and Block Bias
0.000
0.002
0.004
0.006
0.008
0.010
10-Dec-02 29-Jan-03 20-Mar-03 9-May-03 28-Jun-03 17-Aug-03
Date
Nor
mal
ized
Mea
nSq
uare
Erro
r
-8
-6
-4
-2
0
2
4
6
Bloc
kBi
as(%
)
Global NMSESlice NMSEBlock Bias
QA must be sensitive to modes of failure of the scanner
7
Example: Blank Scan Count RateExample: Blank Scan Count Rate
0
100
200
300
400
500
600
700
800
Nov-01 Feb-02 May-02 Sep-02 Dec-02 Mar-03 Jun-03 Oct-03
Date
Acqu
isiti
onR
ate
(kcp
s)
The acquisition count rate is used as an estimate of system sensitivity.
Daily QA: Loose CableDaily QA: Loose Cable
Loose Cable Better
QA ScheduleQA Schedule
Detector and Electronics Characterization Frequency
– Crystal map Quarterly– PMT gain Weekly (daily)– Coincidence timing Weekly
System Corrections– Normalization Quarterly– Scanner calibration Quarterly (weekly)– Blank Scan Daily
NEMA NU 2-2001NEMA NU 2-2001
PET Performance Measurements
National Electrical Manufacturers Association. NEMA Standards Publication NU 2-2001: Performance Measurements of Positron
Emission Tomographs
8
NEMA NU 2-2001NEMA NU 2-2001
Acceptance TestingCan we use our new scanner?
Annual QAIs the scanner still performing within specification?
Phantoms for NU 2-2001Phantoms for NU 2-2001
NEMA NU 2-2001Three phantoms:
Scatter phantom 203x700mm phantom with activity
in line source
Image quality phantom
Sensitivity phantom
NEMA NU 2-2001NEMA 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
• Daube-Witherspoon ME, et al. PET Performance Measurements Using the NEMA NU 2-2001 Standard. J Nucl Med 2002;43:1398-1409.
NEMA NU 2-2001NEMA NU 2-2001
Performance Measures:• Spatial Resolution (Transaxial, axial)
• Sensitivity
• Scatter Fraction
• Count Losses
• Count Rate correction accuracy
• Image Quality
9
Spatial ResolutionSpatial 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
Spatial ResolutionSpatial Resolution
Transaxial Resolution
FWHM @ 1cm FWHM @ 10cm
Philips Allegro 5.5 mm 5.6 mm
GE DST 6.2 6.7
CTI HiREZ 6.5 7.54.6 5.8
SensitivitySensitivity
• 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.
SensitivitySensitivity
•Successive measurements with a 700 mm line source of F-18 surrounded by nested, known absorbers
•The count rate with no absorber is extrapolated from these measures
0.0E+00
5.0E+04
1.0E+05
1.5E+05
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Sleeve Thickness (cm)
Cou
ntR
ate,
R(c
ps) R0
ActivityRySensitivitSystem 0=
10
SensitivitySensitivity
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 10 20 30 40
Slice
Axia
lsen
sitiv
ity(c
ps/k
Bq)
• Measure at radial locations of 0 and 10cm• Report system sensitivity and slice sensitivity profile
SensitivitySensitivity
Sensitivity(cps/kBq)
GE DST 2D 2.03D 9.3
Philips Allegro 4.4
CTI HiRez 4.5
Intrinsic Scatter FractionIntrinsic Scatter Fraction
• Scatter fraction is a measure of the relative system sensitivity to scatter
TruesScatterScatterSF
+=
• Use 203 mm diameter polyethylene cylinder of length 700 mm, with activity located in a line source of diameter 2.3mm that is4.5mm off axis
• Measured with low activity (Randoms:Trues = 1%) to avoid random coincidences, deadtime and pulse pileup.
Intrinsic Scatter FractionIntrinsic Scatter Fraction
Scatter Fraction LLD(keV)
GE DST 2D 19% 3753D 44% 375
Philips Allegro 40% 410
CTI HiRez 36% 425
11
Count Rate PerformanceCount 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)
Count Rate PerformanceCount Rate Performance
• Calculate Noise Equivalent Count Rate
RandomsScatterTrues
TruesNEC RkRR
RR⋅++
=2
• Report peak NEC and effective activity concentration at peak
RNEC: Figure of merit relating scanner performance to image SNR after randoms and scatter corrections.
For NEMA, k = 1 (calculated Randoms)
Count Rate PerformanceCount Rate Performance
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30
Activity concentration (kBq/cc)
NE
CR
(kcp
s)
NECR (1R)NECR (2R)
Count Rate PerformanceCount Rate Performance
Count Rate Performance
Peak NECR Activity(kcps) (kBq/ml)
CTI HiREZ 85 21
GE DST 2D 84 493D 63 12
Philips Allegro 30 9
12
Image Quality MeasurementImage 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 MeasurementImage 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
• Repeat acquisition three times• Reconstruct using clinical protocol
min7cm100
min60≈∆= zTacq
Image Quality MeasurementImage 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 LSONEMA and LSO
• LSO is inherently radioactive• Background radiation gives rise to Randoms, some Trues
• Implications for NEMA: cannot obtain Randoms:Truesratio 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 Med2004;45:822-826.
13
SummarySummary
• Review of PET physics• Need for QA regimen• Calibrations and corrections required for PET imaging• NEMA NU 2-2001 Performance Standard procedures
PET Advanced QC SoftwarePET Advanced QC SoftwareTwo adjacent direct slices (ie adjacent rings, rows in block)