ความร พ นฐานและเทคน คการบ าร งร กษา เคร องโดสคาล เบรเตอร (Dose Calibrator) ส ว ทย ป ณณช ยยะ

Transcription

1 ความร พ นฐานและเทคน คการบ าร งร กษา เคร องโดสคาล เบรเตอร (Dose Calibrator) ส ว ทย ป ณณช ยยะ

2 Principle & Maintenance Technique of Dose Calibrator Contents : 1. General principle 2. Basic system operation 3. Well-type Ionization chamber detector 4. Isotope factor 5. Electrometer 6. Dose calibrator structure 7. Maintenance Technique

3 General principle Dose calibrators are also known as; - Radioisotope Calibrators - Radionuclide Calibrators - Curie Meters - Activity meters Radioisotope dose calibrator is essential Nuclear Instrument in preparation of radiopharmaceuticals for clinical investigation. Their primary application is the measurement of the dose administered to a patient in nuclear imaging or nuclear medicine. เคร องโดสคาล เบรเตอร ร นเก า เม อป พ.ศ. 2520

4 Dose Calibrator An instrument used for measurement a quantity of known radioisotope in term of activity (A) dn = number of spontaneous nuclear transformations dt = time interval Unit of activity Non-SI unit : Ci (Curie) 1 Ci = 3.7 x dps (disintegration per second; dps) SI unit : Bq (Becquerel) 1 Bq = 1 dps 1 Ci = 37 GBq

5 Radioisotope activity Radioactivity Rate of disintegration λ = decay constant... (1)... (2) Refer to the symbol of Activity (A)... (3) By substituting, at t = T 1/2 in equation (3) From equation (3) ;, called half-life

6 Radiopharmaceutical products Medical isotope production Radioactive drugs Nuclear Research Reactor Radionuclide Generator Cyclotron, Linac I-125, I-131, Mo-99, Sr-90 Tc-99m, In-113m, Y-90 PET isotopes C-11, N-13, O-15, F-18

7 Dose calibrator is a nuclear medicine facility to assure the accurate radiopharmaceutical dosages before administration into patients. Dose calibrator Radiopharmacist Radiological doctor Technician (Engineer) Patients Clinical investigation Quality Assurance & Quality Control HA (Hospital Accreditation) IAEA

8 Quality control of instrument Four quality control procedures required for the dose calibrator are; Constancy (Precision, Repeatability, Stability) Linearity Accuracy (Error) Geometry Three factors concerned in quality control of the instrument in laboratory are; Specification : features, performance, supporting function Maintenance : planning, skill & experience, equipment Operation : operating steps, skill & experience, technical supports Laboratory Operator Service Engineer

9 Specification determination General Performance : Measuring range : 0.01 µci to 40 Ci (.0004 MBq to 1500 GBq) - 40 Ci (Tc-99m) - 10 Ci (F-18) Response time : high activity (1-2 s) low activity ( s) Accuracy : overall 0.3 µci or ± 3% Linearity : low activity (± 1%) [Detector + Electrometer] high activity (± 2%) Repeatability : short term (24 hr) 0.3% (1mCi) long term (1yr) 1% (exclusive of background)

10 General Performance (continue) Energy range : 25 kev 3 MeV (Photons) Isotope factor : Isotope selector : - standard Factory set - routine - user define Zero adjust : manual / auto (background) Detector : well type ionization chamber Detector shielding : lead (Pb) Detector well liner: Plastic liner (Contamination prevention) Sample holder : Vial/Syringe Dipper (geometry) Approvals: ETL to UL and ANSI N No M90 IEC , IEC and IEC ,

11 Error factors in measurement : Geometric variation Response time Linearity Long-term drift Radionuclides impurity Type of radionuclides Warm-up and setting Operating environment Human error or Instrument error or External error

12 Basic system operation Bias voltage Block Diagram of Radioisotope Dose Calibrator

13 Well-type Ionization chamber detector Principle of radiation detection process Energy (radiation) β - emitters γ - emitters β + - emitters Interaction Energy absorbing medium Air, Gas ion-pair, e-h pair Electronic charges (signal) Radiation interaction with matters Charged particle Ionization Bremsstrahlung Excitation Photon Photoelectric effect Compton effect Pair production

14 Basic gas filled detector Ionization chamber Proportional counter Geiger Muller counter

15 Characteristic of gas filled detector Current mode Secondary ionization Pulse mode detection

16 Ionization chamber region Primary ionization Principle of operation Plateau curve of ionization chamber Operating voltage range (Bias) : V Ion pairs produced by the primary ionizing particle There is no multiplication of ions Current mode operation

17 Ion current generation Average number of ion pairs Amount of ion current Where : E = Energy deposited (ev) w = Energy required to create one ion pair i = i = Q t N ave q t Coulomb/sec (A) Where : q = electron charge (1.6 x C) t = time (s)

18 Comparison of current and pulse mode measuring circuit Current mode Radiation induced current pulse Average current flows in detector is measured Average current proportion to radiation exposure rate Dose rate, Activity measurement Pulse mode Radiation induced current pulse Voltage developed on the detector at each pulse (Q/C) is measured Voltage pulse height proportion to energy Energy spectrum measurement

19 Saturated ion current in chamber Saturated region Electrometer Bias Basic components The saturated ion current : HV Ion current At saturation regions : R = Exposure rate C/kg.s M = Mass contained in the active volume kg Ion current depends only geometry of source and detector.

20 Current mode of ion chamber Ω Typical ionization current in most application are extremely small ( < A ), the leakage current between anode and cathode must be blocked by guard ring.

21 Detection Efficiency 2 pi-geometry 4 pi- geometry Cylindrical-type ionization chamber Well-type ionization chamber

22 Behavior of ionization current for displacement of source position

23 Well-type ionization chamber Plastic liner Detector features: Detecting gas : Pressurized Ar Geometry : near 4π Wall : steel, aluminum, brass Collecting electrode : thin foil Cu High gas volumes : >10 3 cm 3 Excellent long term stability Chamber pressure: 2-20 atm

24 Typical data for well type ion chamber A chamber with a 10 4 cm 3 active volume, the saturation current produced by 1µCi of Co-60 is order of A, about 5 times the background current. Raising the gas pressure to 20 atm will increase the ion current by a factor 20, but the total background also arising. Chamber Gas Pressure: 149 kpa gauge (21.6 psig) at 20 C or 250 kpa absolute (36.3 psia) at 20 C. (Exempt) IATA regulation Exempts Gases of Division 2.2 from Dangerous Goods Regulations when transported at pressure less than 200 kpa gauge (29 psig) at 20 C. Device is shipped standard goods. Pressure units 1 atm = psi, 1 bar = 14.5 psi, 1 kpa = psi

25 Detector response of activity measurement from a radionuclide Detection processes : α- particle : cannot penetrate the chamber wall (cannot be detected) β + - radiation : annihilation produce 2 γ-photons of 511 kev (positron) (easily detected) β - - radiation : Bremsstrahlung produce low energy photon emission (can be detected) X, γ - radiation : photon produce electron particles by photoelectric effect, Compton effect or pair production (E>1.02 MeV)

26 Mechanism of radiation detection Depend on type of radiation and photon energy Photon interaction photoelectric effect compton effect pair production Photo e - recoil e - e + and e - Charged particles

27 Energy dependence of ion chamber Energy response : up to 3 MeV Efficiency : Interaction at different energy

28 Relationship between radiation exposure and activity The measured exposure is converted into activity by the equation A is the calculated activity, Ẋ is the measured exposure rate, d is the distance between the source and the detector Γ is the specific gamma constant From Equation above, the impact of the distance (d) in determining the activity. The Inverse Square Law states that the calculated activity is proportional to the square of the distance between the source and detector. The dipper that comes with the dose calibrator is designed to minimize discrepancies of the physical placement of the dose.

29 Converting current to activity Each radionuclide has different gamma energies emitted with specific probabilities. As a result, each radionuclide will generate different currents within the dose calibrator per decay. The unique current produced by a radioactive source is dependent on the specific gamma constant; in other words, the amount of radiation (related to radioactivity) and the energy of the photons. Higher activities generate more photons which, in turn generate more current. The chamber s response is different for 1Bq of 99m Tc (140 kev) and for 1Bq of 131 I (364 kev). For the dose calibrator display to read one millicurie (mci) for both isotopes, a conversion factor (isotope factor) must be applied.

30 Isotope factor The conversion factor can be accomplished by: using adjustable resistors to regulate the amplifier gain (analog method) multiplying the digital output with an isotope specific calibration factor (digital method) Example of Radioisotopes list Calibration number (R A ) depends on : decay mode energy dependence source container

31 Electrometer Electrometer is an electrical instrument for measuring very small electric charge or electrical potential difference without loading the signal source Diagram of Dolezalek electrometer and ionization chamber, configured for activity measurement quadrant electrometer (Lord kelvin, 1880)

32 Modern electrometer A modern electrometer is a highly sensitive electronic voltmeter whose input impedance is so high. The actual value of input resistance for modern electronic electrometers is around Ω. Modern electrometers based on vacuum tube or solid-state technology (MOSFET and IC) can be used to make voltage and charge measurements with very low leakage currents, down to 1 femto-ampere Sub-miniature Tetrode electron tube MOSFET (IGFET) Field Effect Transistor Electrometer Amplifier (Integrated circuit)

33 Universal Electrometer Electrometer for extremely low current measurement in reference class dose calibration

34 Basic operation of electrometer amplifier The current input (I s ) is converted into voltage output (V o ) based on ideal characteristic of operational amplifier I s = I f V o = -I f R f Current to Voltage Converter FET input Low noise amplifier Very high resistance feedback

35 Extremely low current electrometer amplifier for ion chamber

36 Dose Calibrator Structure Display and Unit conversion Block diagram of dose calibrator

37 Dose calibrator configuration Factory preset/ User defined Isotope selector Functional diagram of analog type dose calibrator

38 Dose calibrator evolution Obsolete model Modern model Analog Technology Free-air ionization chamber Battery HV bias Analog display Manual adjust Digital Technology Gas pressurized ionization chamber DC-DC converter HV bias Digital display Software adjust Diagnostic check

39 Modern Dose calibrator Block diagram of Multi-chamber dose calibrator

40 Modern Dose Calibrator Detector: Gas Ionization chamber Current mode Well counter mode Energy spectrum Detector: NaI(Tl) Scintillator Pulse mode Dose calibrator/well counter

41 Maintenance technique Type of maintenance: Preventive maintenance Corrective maintenance (adjust, repair) Predictive maintenance (failure data evaluation)

42 Routine maintenance Operating environment Humidity, Temperature, Area cleaning, Warm up (power on > 1 hr) Contamination check Sample holder (Both the chamber well and vial/syringe dipper), Background surveying Determination of error Constancy (Reproducibility) check, Abnormal function and Abnormal display observation

43 Quality Assurance Testing NRC regulation 10 CFR (1/1/2003) Constancy: Starting at installation and at least once each day before measuring patient dosages (±5 percent). Dial Value Setting: at receipt of isotope activity in container other than a plastic syringe (±10% from decay corrected calibration activity) Linearity: at installation and at least quarterly thereafter (±5 percent). Geometry: at installation (±5 percent). Accuracy: at installation and at least annually thereafter (±5 percent).

44 Standard Calibration source NIST-traceable radionuclide Ba-133, 250µCi Co-57, 5 mci, Cs-137, 200 µci Standard sources For constancy or reproducibility test (every day) For accuracy test (Energy range kev), (quarterly or annually)

45 Dial Value Setting Dial Value Setting must be determined when the source container is changed Number photons will decrease of the 30 kev photon by 18%, and of the 80 kev photon by 4% with increasing of 0.8 mm in container wall thickness. (Effect of container wall) I = Ie µ x 0 x µ = attenuation coefficient of container material x = thickness of container wall

46 Linearity test Linearity is the proportionality of measurement result to the activity measured over an activity range of dose calibrator Testing method Decay method Shield method Lineator : Lead lined tube (Designed to attenuate Tc-99m) Tc-99m vial or syringe When Mo-99 decays, high energy gamma photons (~750 kev) are produced which can "breakthrough" a lead wall with a thickness sufficient to totally absorb the lower energy Tc-99m gamma (~140 kev)

47 Geometry Independence Test If any correction factors are greater than 1.05 or less than 0.95, it will be necessary to make a correction table.

48 Repair or replacement System must be repaired when: Reproducibility check : if errors exceed ± 10 % Accuracy check : if error exceed ± 10 % Abnormal functions appeared A percent error of ± 5.0% may indicate a need for repair or adjustment. A percent error of ± 10.0% requires repair or replacement of the unit. After repair or replace of the dose calibrator, the quality assurance testing must be repeated as a new installation

49 Action for accuracy test error Biodex Medical system

50 Common trouble in dose calibrator Detector terminal insulation leakage Electrometer amplifier defects: leakage on feedback resistor (R f ) surface leakage on PCB at MOSFET located bad contact of reed switch of activity range selector amplifier offset drift bad contact in connection cable HV bias unstable in DC-DC converter loss of HV supply Power supply defects : unstable in system supply ripple in power supply EMI/RFI interference Voltage drop

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