Nuclear Medicine Physics Radiation Detectors Jerry Allison Ph
- Slides: 33
Nuclear Medicine Physics • Radiation Detectors Jerry Allison, Ph. D. Department of Radiology Medical College of Georgia
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
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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
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 g-probe (scintillation)
Gas-filled detectors Survey meters (IC) Dose calibrators (IC) GM chamber “pancake” (GM) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Gas-filled Detectors Basic gas-filled detector consists of gas medium positive and negative charged electrodes Shapes 2015 Versatile: cylindrical, flat, welltype Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Gas-filled Detectors Radiation ionizes gas molecules to produce +/- ion pairs Electric field draws e- to anode, generates signal Signal characteristics depend on the applied voltage 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Gas-filled detectors How it works? .
Ionization Chamber Region IC region Current pulse (signal) produced by radiation Signal strength is proportional to energy deposited Used for measuring S 2 S 1 “amount” of radiation (i. e. , exposure, air kerma) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Ionization Survey Meters Can be used to accurately measure: 2015 Exposure (measure of ionization in air, C/kg) C/kg SI units Roentgen (R) 1 R = 2. 58 x 10 -4 C/kg 33 ev deposited per ion pair created traditional units Air Kerma (absorbed dose in air) Kerma: kinetic energy released in media Gray 1 Gy = 33. 7 C/kg SI units 1 R of exposure = 0. 00869 Gy of absorbed dose [AIR] Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
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
Dose calibrator Ion chamber well Radionuclide selection Shielded syringe transport
Dose calibrator quality control Constancy: daily, using Cs-137 (660 ke. V, 30 y) and Co-57 (122 ke. V, 9 mo) for all nuclide settings, error < 10% Linearity: quarterly, using 300 m. Ci Tc-99 m, down to 10 Ci or lineators, error < 10% Accuracy: yearly, using Cs-137 and Co-57, error < 5% Geometry: upon installation, using 1 m. Ci Tc-99 m with different volumes, error < 10% Syringes (1 ml, 3 ml, 5 ml, 10 ml) Vial (10 ml)
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-99 m and Xe-133 settings even though the elapsed time is less than 24 hours.
Linearity test using lineators Lineators: a set of lead sleeves with summed thickness to mimic physical decay 16
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
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.
Geiger-Müller Region 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 S Used for measuring “presence” of radiation 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
GM Counters Sealed pressurized chambers for 2015 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) Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Quality control of survey meter Checking battery: before each use Checking the reference source: before each use Calibration: before initial use and every year 21
Scintillation Detectors (Main detectors in NM imaging, including gamma cameras) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
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. Na. I(Tl)) and PMT 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Operation Radiation + scintillator 2015 produce fluorescence proportional to energy Light strikes PMT photocathode, ejecting ee- successively accelerated towards 8 – 12 dynodes Signal amplified ~ 106 -107 Signal read out and processed Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Energy resolution ∆E E 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Energy resolution ∆E E Energy resolution plays an important role in scatter rejection / image quality High energy resolution Image quality 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Scintillation Detectors Well-counters (Na. I(Tl)) 2015 Daily wipe tests Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Well counter Na. I(Tl) Measure small amount of radioactivity (< 1 Ci for daily wipe tests) Main components: § single Na. I(Tl) crystal (4. 5× 5 cm or 1. 6× 3. 8 cm) with a hole for sample § PMT § preamplifier § PHA § readout device
Scintillation Detectors Thyroid probe (Na. I(Tl)) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, Ph. D, DABR
Thyroid probe Measure thyroid uptake of I-131 in-vivo § 5× 5 cm Na. I(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
Thyroid probe Thyroid uptake neck phantom
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 32
Miniature g probe § § 99 m. Tc-colloid injected before surgery 99 m. Tc-colloid is concentrated in the sentinel lymph nodes. § Detecting sentinel lymph nodes using the g probe in surgery § Probe: 5 × 10 mm, high directional sensitivity, light, easy to operate § Also detecting other isotopes, e. g. I-131
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