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)