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Quantification of Ventilation Activity in V/Q Scanning

Quantification of Ventilation Activity in V/Q Scanning. Michelle Lax Lincoln County Hospital. Background. We wanted to accurately determine the ventilation activity inhaled for a VQ scan. WHY? for accurate reporting of administered activity to check we aren’t exceeding ARSAC limits

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Quantification of Ventilation Activity in V/Q Scanning

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  1. Quantification of Ventilation Activity in V/Q Scanning Michelle Lax Lincoln County Hospital

  2. Background • We wanted to accurately determine the ventilation activity inhaled for a VQ scan. • WHY? • for accurate reporting of administered activity • to check we aren’t exceeding ARSAC limits • to determine when to delay perfusion scan

  3. Estimation of Ventilation Activity • Technigas is inhaled for vent part of V/Q scan • ARSAC limit = 40MBq (eff. dose = 0.6mSv). • GM tube to monitor patient up to 100cps • 10sec posterior ventilation image • HR sensitivity = 70cps/MBq • GP sensitivity = 120cps/MBq • Round down to 50 and 100 cps/MBq • to account for patient attenuation • therefore estimating higher activity

  4. Example 10sec vent posterior image = 12 kcnts (1200cps) GP approx. sensitivity = 100cps/MBq  Estimated Activity = 12 MBq

  5. Example 10sec vent posterior image = 8 kcnts (800cps) HR approx. sensitivity = 50cps/MBq  Estimated Activity = 16MBq Estimated value used to determine whether the perfusion scan can commence straight away Rule of thumb: perf cnts (100MBq) should be 3 - 5 times vent cnts

  6. BUT…... 1. We need to account for attenuation differences between anterior and posterior need to use geometric mean 2. We need to calculate patient specific sensitivity

  7. Theory of Vent Activity Program • Protocol_tool • Query: counts, duration, date, time of ventilation and perfusion ant and post images NB: ACQTIME in seconds from 00:00hrs • Convert to counts per second • Calc geometric mean of vent and perf cnts • Ventgeometric mean = (Ventant x Ventpost) • (Perf&Vent)geometric mean = (Perfant x Perfpost)

  8. Theory of Vent Activity Program • Decay correct Ventgeo to time of Perf scan (ask operator: Were both completed? Were they on the same day? Which was first?) • (Perf&Vent)geo-Ventdecay corr= Perfonly • Calculate camera sensitivity for this patient (ask operator for perf activity, including residual activity) • Sensitivity (cps/MBq) = Perfonlycnts / Perf Act • Vent Act = Vent cnts / Sensitivity

  9. Analysis of Ventilation Activity • No significant difference in vent activity between using posterior counts or geometric mean (p=0.8) • A significant difference in vent activity between using camera or patient sensitivity (p<0.001) • Comparing vent activity calculated from program to estimated vent activity: • % Activity Increase = 39.3  14.2 % • Smallest Error: 9  12.75 MBq (+3.75 MBq) • Largest Error: 24  45.65 MBq (+21.65 MBq) (Mean Activity Increase = 10 MBq)

  10. Conclusions • A simple program has been written to calculate accurate ventilation activities. • Found we had been underestimating ventilation activity. • Changed action level for delaying Perfusion scan to when estimated ventilation activity (from 10sec posterior counts) = 25MBq.

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