Uniformity testing of gamma cameras at high count-ratesNICK BATESUNIVERSITY OF SURREYROYAL SURREY COUNTY HOSPITALNATIONAL PHYSICAL LABORATORYUK

Background• Imaging patients after therapeutic

administration of radiopharmaceutical causes significant dead-time effects in gamma cameras

• Gamma cameras behave as paralysabledetectors so the count-rate can saturate at high activities

• To deal with this, manufacturers have implemented high count-rate modes. Some switch in automatically, others operate separately from “normal mode”

• GE SPECT/CT cameras have a “fast mode” setting that must be set-up for each radionuclide

Input Count Rate

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Gamma camera

Background• Prior to carrying out patient imaging, appropriate

Quality Assurance should be completed.• Flood-field uniformity is a standard test of overall

camera performance• Response of the detector to a uniform flux of radiation• Ideally equal image intensity over entire field of view• Non-uniformity indicates detector drift / inaccurate

corrections

• The stability of uniformity with time and activity in “fast mode” is unknown…

• As part of the set-up for the SELIMETRY study, RSCH has investigated the dependence of “fast mode” uniformity on source activity and the stability over time

Initial Set-up and Testing• GE engineers enabled “fast mode” after preliminary testing revealed

paralysis of detector in “normal mode” at ~1.5GBq source activity (~25kcps)

• A 60M count uniformity correction map was acquired using a 500MBq point source of I-131 positioned >5xUFOV distance from the uncollimated detector head.• This activity was chosen as it gave a count-rate of ~70kcps • For Tc-99m fast mode, GE recommend acquiring calibrations at 45-

100kcps, so we assumed the same would apply for I-131• Uniformity, energy peak position and FWHM was assessed by

acquiring 20M count flood-field with sources ranging from 50MBq (8kcps) to 700MBq (94kcps).

Initial Uniformity Results

y = -0.1675x + 13.844R² = 0.994

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y = 9E-05x2 + 0.0086x + 8.5638R² = 0.9988

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Initial Uniformity Results• Uniformity degrades linearly either side of the count-rate used for calibrations.• Uniformity becomes clinically unacceptable (>3%) within 10kcps of calibration

count-rate• Peak position does not change significantly with count-rate• Peak FWHM increased with count rate (8.6% at 8kcps to 10.2% at 94kcps)

Repeat testing• Suggestion from other hospitals (Oxford) to acquire uniformity map at lower

count-rate• ~140MBq source used ~20kcps• 60M count uniformity correction map as before• Prepared 7 sources (10, 20, 50, 100, 140, 400, 700MBq)

• Acquired individually and in combination to study dependence of uniformity on count-rate in more detail

Repeat Testing Results: Uniformity

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Repeat Testing Results: Uniformity

y = -0.0001x3 + 0.014x2 - 0.405x + 6.1714R² = 0.9797

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Repeat Testing Results: Peaking

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y = 0.0002x2 + 0.004x + 8.6525R² = 0.9936

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Observable Changes

Centres of PMTs have lower counts at higher count-rates

Increased count rate

20kcps (140MBq) 100kcps (890MBq)

Observable Changes

• Event pile-up towards centre of PMTs at very high rates

• Streaking between centres of PMTs – more prominent horizontally

Observable Changes20kcps (140MBq)

Decreased count rate

1.5kcps (10MBq)

Centres of PMTs have higher counts at lower count-rates

Possible causes• Events outside energy window contribute to

dead-time• Count-rate dependent spatial linearity

distortions?• Amplifier saturation?• GE factory response:

• Significant scatter downward shift in energy peak…?

• Tests have been conducted with increased event threshold; no effect

• Suggesting adding copper behind source to reduce scatter

• What about patient scatter?!

Stability with time

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Measured Predicted based on count-rate

• Possibly random variation versus predicted values?

• No signficant trend in uniformity seen over 1 month time-scale

• Longer-term stability still to be tested

• 1 month stipulation for SELIMETRY study seems sensible

Conclusion• Image uniformity degrades at count-rates above and below that used

for calibrations• Unusual artefacts in images at high count rates which pile up towards

centres of PMT, with streaks between the tubes• Multiple uniformity calibrations or list-mode data processing may be

required• Uniformity at calibrated count-rate is stable over ~1 month timescale• Experiments may need to be repeated with Cu behind source to reduce

scatter • Similar experiments are planned for “normal” mode, and with other

radionuclides• Has anyone else studied this?

• Thoughts from the audience would be greatly appreciated