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Non-Imaging Devices in Nuclear Medicine

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Non-Imaging Devices in Nuclear Medicine

  1. 1. Non-imaging devices in Nuclear Medicine Pawitra Masa-at 4937092 SIRS/M March 23, 2007
  2. 2. Type of non-imaging device <ul><li>Gas-filled detectors </li></ul><ul><ul><li>Radionuclide dose calibrators </li></ul></ul><ul><li>Scintillation detector </li></ul><ul><ul><li>Gamma well counter </li></ul></ul><ul><ul><li>The thyroid uptake probe </li></ul></ul><ul><ul><li>Liquid scintillation detector </li></ul></ul>
  3. 3. Radionuclide dose calibrators
  4. 4. Gas-Filled Detectors I ; Ionization chamber region P ; Proportional region GM ; Geigur-Mueller region
  5. 5. <ul><li>In the ionization chamber region </li></ul><ul><ul><li>The number of ion pairs collected by the electrodes is equal to the number of ion pair produced by the radiation in the detector. </li></ul></ul><ul><ul><li>There is no change in the number of ion pairs collected as the voltage increase. </li></ul></ul>Ionization Chamber
  6. 6. Ionization Chamber HV + - Negative ion Positive ion 1234 Electrometer The response is proportional to ionization rate (activity, exposure rate)
  7. 7. Dose calibrators ( Activity meter ) <ul><li>A radionuclide calibrator is in essence a well-type gas ionization chamber into the well of which a radioactive material is introduced for measurement. </li></ul><ul><li>The activity of the material is measured in terms of the ionization current produced by the emitted radiations which interact in the gas. </li></ul><ul><li>The chamber is sealed, usually under pressure, and has two co-axial cylindrical electrodes maintained at a voltage difference derived from a suitable supply, the axial space constituting the well. </li></ul>
  8. 8. Dose calibrators <ul><li>In the associated electrometer, the ionization current is converted to a voltage signal, which is amplified, processed and finally displayed, commonly in digital form in units of activity - becquerels (Bq) or curies (Ci). </li></ul><ul><li>This is possible since for a given radionuclide, assuming a fixed geometry and a linear response, ionization current is directly proportional to activity. </li></ul>
  9. 9. Dose calibrators SC97 Proportionality between the number of photons emitted and the ionization current Well-shaped ionization chamber filled with a gas of high atomic number (e.g. Xenon) and kept under pressure
  10. 10. Dose calibrators <ul><li>The response of the detector will depend on: </li></ul><ul><li>Radionuclide (energy and abundance of photons). </li></ul><ul><li>Geometry of the detector. </li></ul><ul><li>Geometry of the source. </li></ul><ul><li>The condition of the instrument (QC). </li></ul>
  11. 11. ACTIVITY MEASUREMENT Setting Measured activity Tc-99m 1.00 Co-57 1.19 In-111 2.35 Tl-201 1.76 Ga-67 1.12 I-123 2.19 I-131 1.43 Measured activity/True activity of Tc.99m if the indicated settings are used
  12. 12. Geometric efficiency The quotient: number of photons reaching the detector over the number of photons emitted from the sample Increasing geometric efficiency
  13. 13. SAMPLE HOLDER (reproducible geometry)
  14. 14. Quality control of the dose calibrator. <ul><li>should include: </li></ul><ul><li>Test of precision and accuracy </li></ul><ul><li>Test of linearity of activity response </li></ul><ul><li>Test of reproducibility (Constancy test) </li></ul><ul><li>Check of background </li></ul>
  15. 15. Sealed sources for calibration of activity meters <ul><li>Long half-life </li></ul><ul><li>Range of photon energies </li></ul><ul><li>Range of activities </li></ul><ul><li>Calibrated within 5% </li></ul>Co-57, Ba-133, Cs-137, Co-60
  16. 16. Sealed sources for calibration of activity meters 1.9 5.27 y 1173, 1332 Co-60 7.4 30 y 662 Cs-137 9.3 10.7 y 81, 356 Ba-133 185 271 d 122 Co-57 Activity (MBq) Half-life Photon energy (keV) Radionuclide
  17. 17. Measurement of precision and accuracy Source (sealed) : Cs-137 or Co-57 Procedure: Select settings for the radionuclide and adjust background. Insert source in holder and make 10 measurements. Data analysis : To assess precision , calculate for each source (i) the percentage difference between the measured activity A i and their mean A mv . (+/-5%) To assess accuracy , calculate the percentage difference between the mean activity and the certified activity. (+/- 10%).
  18. 18. Measurement of reproducibility Measure the activity of a sealed reference source e.g. every morning. Use Tc-99m settings.
  19. 19. Measurement of reproducibility <ul><li>Check of Reproducibility. Part of control chart. The Cs-137 Source used had amean measured activity of 4.55 MBq (123 uCi) on 1 April. </li></ul><ul><li>The limits of accepability indicated correspond to +/- 5% of the expected activity. </li></ul><ul><li>The initial discrepant result on 17 May arose from failure to allow sufficient time for the reading to stabilize </li></ul>
  20. 20. Measurement of linearity Use a radionuclide with short half-life e.g. Tc-99m Make repeated measurements during several half-lives.
  21. 21. Type of non-imaging device <ul><li>Gas-filled detectors </li></ul><ul><ul><li>Radionuclide dose calibrators </li></ul></ul><ul><li>Scintillation detector </li></ul><ul><ul><li>Gamma well counter </li></ul></ul><ul><ul><li>The thyroid uptake probe </li></ul></ul><ul><ul><li>Liquid scintillation detector </li></ul></ul>
  22. 22. <ul><ul><ul><li>Gamma well counter </li></ul></ul></ul><ul><ul><ul><li>The thyroid uptake probe </li></ul></ul></ul>
  23. 23. <ul><li>Scintillation detectors can be used as a part of both non - imaging and imaging devices . </li></ul><ul><li>From the non-imaging devices, scintillation well counters and thyroid probes are used . </li></ul>
  24. 24. The gamma well counter <ul><li>The gamma well counter consists of a scintillation detector with a hole in the center, for a sample to be placed inside for increasing the geometric efficiency and hence the counting efficiency of the counter, and other associated electronics . </li></ul><ul><li>Well counters are used namely for in vitro measurements of different samples . They are usually available with automatic sample changers and are mostly programmable with computers . </li></ul><ul><li>Their major advantage is high detection efficiency which is from 50% to 70%  for 140 keV gamma photons . </li></ul><ul><li>  </li></ul>
  25. 25. Examples of use of sample counters RIA 125 I Kidney clearance 51 Cr Vitamin B12 deficiency 57 Co, 58 Co Ferrokinetic studies 59 Fe Total body water 3 H Blood volume 125 I, 51 Cr, 99m Tc Biomedical research 3 H, 14 C
  26. 26. Gamma counter HV Ampl. PHA Timer Scaler Rate- meter Gain Base Window Voltage Detector Sample Lead shield PM-tube
  27. 27. Scintillation detector Amplifier PHA Scaler Proportionality between the signal and the energy absor- bed in the detector
  28. 28. Pulse height analyzer UL LL Time Pulse height (V) The pulse height analyzer allows only pulses of a certain height (energy) to be counted. counted not counted
  29. 29. Pulse-height distribution NaI(Tl)
  30. 30. The T hyroid P robe <ul><li>The thyroid probe is a scintillation counter used for measuring radioacitivity above the thyroid gland to assess the uptake of 131I after its oral administration . </li></ul><ul><li>In contrast to well counter the thyroid probe must be equipped with collimator , which limits the field of view . </li></ul><ul><li>This is a cylindrical barrel made of lead and it covers all the detector including PM tube . It prevents the gamma radiations from other organs to reach the detector . </li></ul>
  31. 31. Probe system HV Ampl. PHA Timer Scaler Rate- meter Gain Base Window Voltage Recorder Collimator PM D
  32. 32. Quality control of any scintillation detector system <ul><li>should include tests such as: </li></ul><ul><li>Energy calibration </li></ul><ul><li>Energy resolution and energy response </li></ul><ul><li>Sensitivity </li></ul><ul><li>Counting precision </li></ul><ul><li>Background count rate </li></ul><ul><li>Linearity of activity response </li></ul>
  33. 33. Window setting Energy window setting depends on the energy resolution of the detector and the photon energies
  34. 34. Gamma counter Different design of the detector
  35. 35. COUNT LOSSES ( LINEARITY OF ACTIVITY RESPONSE) <ul><li>Decaying source method </li></ul><ul><li>Graded source method </li></ul>
  36. 36. Liquid scintillation counter
  37. 37. Liquid scintillation counter <ul><li>In the beta-counter or liquid scintillation counter , the sample is dissolved in an organic scintillation solution. Due to the resulting 100% counting geometry and the absence of any detector window,this means that the counter has excellent properties in detecting radionuclides of low activity emitting low energy beta-particles, such as H3 and C14. </li></ul><ul><li>The light photons from the sample are collected by two photomultipliers in coincidence. </li></ul><ul><li>This arrangement will reduce the background due to thermal noise and only true scintillation events will be analysed and counted. </li></ul>
  38. 38. Liquid scintillation counter <ul><li>The main problem in liquid scintillation counting is the varying counting efficiency due to quenching of scintillation events. </li></ul><ul><li>This process is caused by chemical contamination of the sample and/or a coloured sample. This means that the counting efficiency has to be determined for every sample. </li></ul><ul><li>Therefore a quality control of the instrument must include a control of the correction methods. Otherwise the QC methods will be the same as for any scintillation counter. </li></ul><ul><li>The sources needed for QC of a liquid scintillation counter include calibrated sources of H3 and C14 with different counting efficiencies as well as a background sample. </li></ul>
  39. 39. Liquid scintillation counter PM PM Coinc Ampl PHA Scaler Timer No window 100% geometric efficiency Sample mixed with scintillation solution
  40. 40. Liquid scintillation counter <ul><ul><li>Counting efficiency </li></ul></ul><ul><ul><li>Quenching </li></ul></ul><ul><ul><li>Sample preparation </li></ul></ul><ul><ul><li>Window setting </li></ul></ul><ul><ul><li>Reproducibility </li></ul></ul><ul><ul><li>Background </li></ul></ul>Operational considerations
  41. 41. Thank you Pawitra Masa-at 4937092 SIRS/M March 23, 2007

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