1. Nuclear Medicine Physics Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia Radiation Detectors

2. A note of thanks to Z. J. Cao, Ph.D. Medical College of Georgia And Sameer Tipnis, Ph.D. G. Donald Frey, Ph.D. Medical University of South Carolina for Sharing nuclear medicine presentation content

3. Basic principle Radiation enters a medium, deposits energy Energy deposition produces ionizations, scintillations Signal converted to electrical current/pulses Current/pulses amplified (original current/pulses generally small) Amplified current is measured or amplified pulses are counted/sorted by their energies, and recorded Display of radiation level or energy spectrum 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

4. Radiation Detectors in NM Survey meters (gas-filled detector) Ionization chambers (IC) Geiger Müeller (GM) Dose calibrator (gas-filled detector) Well counter (scintillation detector) Thyroid probe (scintillation detector) Miniature  -probe (scintillation)

5. Survey meters (IC) Gas-filled detectors GM chamber “pancake” (GM) Dose calibrators (IC) 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

6. Gas-filled Detectors Basic gas-filled detector consists of gas medium positive and negative charged electrodes Shapes Versatile: cylindrical, flat, well- type 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

7. Radiation ionizes gas molecules to produce +/- ion pairs Electric field draws e - to anode, generates signal Signal characteristics depend on the applied voltage Gas-filled Detectors 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

8. Gas-filled detectors How it works?.

9. IC region Current pulse (signal) produced by radiation Signal strength is proportional to energy deposited Used for measuring “amount” of radiation (i.e., exposure, air kerma) Ionization Chamber Region S1S1 S2S2 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

10. Ionization Chambers Gas used Survey meter: air Dose calibrator: Argon (10 – 20 atmospheres, less in PET) Low efficiency (gas has low density) Air chambers are temperature/pressure sensitive Fairly rugged, not easily saturated with radiation 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

11. Ionization Survey Meters Can be used to accurately measure: Exposure (measure of ionization in air, C/kg) C/kg SI units Roentgen (R) traditional units 1R = 2.58 x 10 -4 C/kg 33 ev deposited per ion pair created Air Kerma (absorbed dose in air) Kerma: kinetic energy released in media Gray SI units 1 Gy = 33.7 C/kg 1 R of exposure = 0.00869 Gy of absorbed dose [AIR] 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

12. Dose calibrator Measure activity only Select correct isotope button Drop a sample to the bottom to avoid position effect Quality control is regulated by NRC or Agreement State Every patient dose must be assayed before administration

13. Dose calibrator Radionuclide selection Ion chamber well Shielded syringe transport

14. Dose calibrator quality control n Constancy: daily, using Cs-137 (660 keV, 30 y) and Co-57 (122 keV, 9 mo) for all nuclide settings, error < 10% Linearity: quarterly, using 300  mCi Tc-99m, down to 10  Ci or lineators, error < 10% n Accuracy: yearly, using Cs-137 and Co-57, error < 5% Geometry: upon installation, using 1 mCi Tc-99m with different volumes, error < 10% Syringes (1ml, 3ml, 5ml, 10ml) Vial (10ml)

15. Dose calibrator - daily constancy test When performing the constancy test, one must check every setting that might be used that day starting officially at 12:01 AM. A QC was performed on the dose calibrator at 7:00 AM today. If the technologist is called in for a lung V/Q study at 12:15 AM tomorrow morning, she/he must check the constancy of Tc-99m and Xe-133 settings even though the elapsed time is less than 24 hours.

16. 16 Linearity test using lineators Lineators: a set of lead sleeves with summed thickness to mimic physical decay

17. Linearity test using lineators The whole test must be done within 5 min. The initial ratios of decay among the sleeves are verified and calibrated by the physical decay method. The initial ratios are then used to compare with the ratios obtained from later tests. 17

18. What if a test fails? If deviation is out of the limit, obligation is to record value, note repair and recalibration, retest, and record new values. Until repair and recalibration are accomplished, every measured dose must be corrected mathematically.

19. GM region High voltage applied to anode Iniitial ionizations produced by radiation and secondary ionizations produced by accelerating electrons Signal strength is independent of energy deposited Used for measuring “presence” of radiation Geiger-Müller Region S 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

20. GM Counters Sealed pressurized chambers for maximum detection efficiency Not possible to identify energy Extremely sensitive to radiation Can be saturated (“zero” reading if radiation flux too high) Typical applications is detection of trace radiation (contamination) 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

21. 21 Quality control of survey meter Checking battery: before each use Checking the reference source: before each use Calibration: before initial use and every year

22. Scintillation Detectors 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR (Main detectors in NM imaging, including gamma cameras)

23. Scintillation Detectors Two main components - Scintillator Radiation deposits energy in scintillator causing light flashes (fluorescence) Photomultiplier tube (PMT) Used to detect fluorescence from scintillator and amplify the signal NM – Inorganic solid scintillator (e.g. NaI(Tl)) and PMT 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

24. Operation Radiation + scintillator produce fluorescence proportional to energy Light strikes PMT photocathode, ejecting e - e - successively accelerated towards 8 – 12 dynodes Signal amplified ~ 10 6 -10 7 Signal read out and processed 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

25. Energy resolution ∆E E 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

26. Energy resolution Energy resolution plays an important role in scatter rejection / image quality  High energy resolution   Image quality ∆E E 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

27. Well-counters (NaI(Tl)) Daily wipe tests Scintillation Detectors 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

28. Well counter NaI(Tl) Measure small amount of radioactivity (< 1  Ci for daily wipe tests) Main components:  single NaI(Tl) crystal (4.5×5 cm or 1.6×3.8 cm) with a hole for sample  PMT  preamplifier  amplifier  PHA  readout device

29. Thyroid probe (NaI(Tl)) Scintillation Detectors 2015Nuclear Medicine Physics for Radiology ResidentsSameer Tipnis, PhD, DABR

30. Thyroid probe Measure thyroid uptake of I-131 in-vivo  5×5 cm NaI(Tl) with 15 cm long conical collimator  pointing to neck and thigh (bkg)  calibration phantom with known activity for calculating uptake  1 – 2 cm difference in depth  10 – 40% difference in count rate

31. Thyroid probe Thyroid uptake neck phantom

32. 32 QC of well counter and thyroid probe Constancy: before each use, using a Cs-137 source Chi-square: quarterly, using a Cs-137 source Do a series of counts have a Poisson distribution? Energy resolution: quarterly, using a Cs-137 source

33. Miniature  probe  99m Tc-colloid injected before surgery  99m Tc-colloid is concentrated in the sentinel lymph nodes.  Detecting sentinel lymph nodes using the  probe in surgery  Probe: 5 × 10 mm, high directional sensitivity, light, easy to operate  Also detecting other isotopes, e.g. I-131