ความรู้พืนฐานและเทคนิคการบํารุงร้ ักษา เครื่องโดสคาลิเบรเตอร์ (Dose Calibrator)

สุวิทย์ ปุณณชยยะั Principle & Maintenance Technique of Dose Calibrator

Contents :

1. General principle 2. Basic system operation 3. Well-type chamber detector 4. Isotope factor 5. 6. Dose calibrator structure 7. Maintenance Technique General principle

Dose calibrators are also known as;

- Radioisotope Calibrators - 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 เครื่องโดสคาลิเบรเตอร์ รุ่นเก่า to a patient in nuclear imaging or เม ื่อปี พ.ศ. 2520 nuclear medicine. 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 1010 dps (disintegration per second; dps)  SI unit : Bq () 1 Bq = 1 dps

1 Ci = 37 GBq Radioisotope activity

Radioactivity Rate of disintegration

...... (1) λ = decay constant

...... (2)

Refer to the symbol of Activity (A)

...... (3)

By substituting , at t = T1/2 in equation (3)

From equation (3) ; , called “half-life” Radiopharmaceutical products

Medical isotope production Radioactive drugs

Nuclear Research Radionuclide Cyclotron , Reactor Generator Linac

I-125, I-131, Mo-99, Tc-99m , In-113m, PET isotopes Sr-90 Y-90 C-11, N-13, O-15, F-18 Clinical investigation Dose calibrator is a nuclear medicine facility to assure the accurate radiopharmaceutical dosages before administration into patients.

Quality Assurance & Quality Control  Dose calibrator  Radiopharmacist  Radiological doctor  Technician (Engineer)  Patients

HA (Hospital Accreditation) IAEA 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 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 (10-100 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) General Performance (continue)

Energy range : 25 keV – 3 MeV (Photons) Isotope factor : 0.01 – 999 Isotope selector : - standard Factory set - routine - user define Zero adjust : manual / auto (background) Detector : well – type Detector shielding : lead (Pb) Detector well liner: Plastic liner (Contamination prevention) Sample holder : Vial/Syringe Dipper (geometry)

Approvals: ETL to UL 60601-1 and ANSI N42.13-2004 No. 601-1-M90 IEC 60601-1, IEC 60601-1-4 and IEC 60601-1-2, Error factors in measurement :  Geometric variation  Response time  Linearity  Long-term drift  impurity  Type of radionuclides  Warm-up and setting  Operating environment

Human error or Instrument error or External error Basic system operation

Bias voltage

Block Diagram of Radioisotope Dose Calibrator Well-type Ionization chamber detector

Principle of radiation detection process

Interaction Energy -pair, e-h pair (radiation) Energy

absorbing Electronic charges β - emitters (signal) γ - emitters medium β+ - emitters Air, Gas

Radiation interaction with matters

Charged particle Photon  Ionization  Photoelectric effect  Bremsstrahlung  Compton effect  Excitation  Pair production Basic gas filled detector

Ionization chamber Geiger Muller counter Characteristic of gas filled detector

Secondary Current ionization mode Pulse mode detection Ionization chamber region

Primary ionization

Principle of operation Plateau curve of ionization chamber

 Operating voltage range (Bias) : 90 – 450 V  Ion pairs produced by the primary ionizing particle  There is no multiplication of  Current mode operation Ion current generation

Average number of ion pairs

Where : E = Energy deposited (eV) w = Energy required to create one ion pair Amount of ion current

Q Coulomb/sec (A) i = t Nq× i = ave t Where : q = electron charge (1.6 x 10-19 C) t = time (s) 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 Saturated ion current in chamber

Saturated region

Electrometer

Bias

HV Basic components Ion current The saturated ion current :

R = Exposure rate C/kg.s M = Mass contained in the active volume kg

At saturation regions : Ion current depends only geometry of source and detector. Current mode of ion chamber

1016 Ω

Typical ionization current in most application are extremely small ( < 10-12A ), the leakage current between anode and cathode must be blocked by guard ring. Detection Efficiency

≈2 pi-geometry ≈4 pi- geometry

Cylindrical-type Well-type ionization ionization chamber chamber Behavior of ionization current for displacement of source position Well-type ionization chamber

Plastic Detector features: liner  Detecting gas : Pressurized Ar  Geometry : near 4π  Wall : steel, aluminum, brass  Collecting electrode : thin foil Cu  High gas volumes : >103 cm3  Excellent long term stability  Chamber pressure: 2-20 atm Typical data for well – type ion chamber

 A chamber with a 104 cm3 active volume, the saturation current produced by 1µCi of Co-60 is order of 10-13 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 3.2.2.4 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 = 14.69 psi, 1 bar = 14.5 psi, 1 kPa = 0.145 psi 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) Mechanism of radiation detection Depend on type of radiation and photon energy

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-

-

photoelectric effect Photo e- Charged Photon compton effect recoil e- interaction particles + - pair production e and e Energy dependence of ion chamber

Energy response : up to 3 MeV Efficiency : Interaction at different energy 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. 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 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 99mTc (140 keV) and for 1Bq of 131I (364 keV).  For the dose calibrator display to read one millicurie (mCi) for both isotopes, a conversion factor (isotope factor) must be applied. 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 (RA) depends on :  decay mode  energy dependence  source container 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, quadrant electrometer configured for activity measurement (Lord kelvin, 1880) 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 is around 1014Ω. 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 MOSFET (IGFET) Electrometer Amplifier electron tube Field Effect Transistor (Integrated circuit) Universal Electrometer

Electrometer for extremely low current measurement in reference class dose calibration Basic operation of electrometer amplifier

The current input (Is) is converted into voltage output (Vo) based on ideal characteristic of operational amplifier

Is = If

Vo = -If Rf

Current to Voltage Converter

 FET input  Low noise amplifier  Very high resistance feedback Extremely low current electrometer amplifier for ion chamber Dose Calibrator Structure

Display and Unit conversion

Block diagram of dose calibrator Dose calibrator configuration

Factory preset/ User defined

Isotope selector

Functional diagram of analog type dose calibrator Dose calibrator evolution Obsolete model Modern model

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

Block diagram of Multi-chamber dose calibrator Modern Dose Calibrator

Detector:  Gas Ionization chamber  Current mode

Detector:  NaI(Tl) Scintillator Well counter mode  Pulse mode Energy spectrum

Dose calibrator/Well counter Maintenance technique

Type of maintenance:  Preventive maintenance  Corrective maintenance (adjust, repair)  Predictive maintenance (failure data evaluation) 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 Quality Assurance Testing

NRC regulation 10 CFR 35.60 (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). 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 100 – 500 keV), (quarterly or annually) 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)

−µx Ix = Ie0

µ = attenuation coefficient of container material x = thickness of container wall 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) 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. 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 Action for accuracy test error

Biodex Medical system Common trouble in dose calibrator  Detector terminal insulation leakage  Electrometer amplifier defects:

 leakage on feedback resistor (Rf ) 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 จบการบรรยาย ขอบคุณคร ับ