Instrumentation review

Scintillation Counter

Instrument that detects radiation using a crystal or other material that emits light when struck by ionizing radiation

Measured during a QC test on a dose calibrator

Accuracy, Linearity, Geometry, and Constancy

Frequency of testing Accuracy of a dose calibrator

Yearly

Frequency of testing Linearity on a dose calibrator

Quarterly (every 3 mo)

Frequency of testing Geometry on a dose calibrator

Installation or after moving the dose calibrator

Measured by a QC test on a scintillation detector

Constancy, Calibration, Chi-Square, Energy Resolution

Radiation Source used to test a surgical gamma probe

Co-57 due to its energy (122 keV) and half-life (270 days)

Radionuclide to be used with a collimator that has thinner septa between its holes

P-32, because it is a Beta emitter and not used in diagnostic imaging (24, 305 keV)

Do before returning a package with radioactive materials

Wipe test/survey to ensure no surface contamination

Limit for surface exposure rate at which you must notify the NRC and final delivery carrier of radioactive materials

200 mR/hr

Limit for exposure rate at 1 meter away from surface at which you must notify the NRC and final delivery carrier of radioactive materials

10 mR/hr

Measuring a window of a an energy level (eg 10% for 159 keV)

5% below and 5% above

• In this case, 151.05 and 166.95 keV

Formula for sensitivity

(Source cpm - background cpm) / Source activity in uCi

Composition and function of a PLES phantom

Two lead bars of the same width and spacing. Can take two images at 90^o of each other to achieve linearity over an entire FOV.

NRC time limit on personnel radiation exposure records

Indefinite

Orthogonal hole transmission phantom

Produces a “mesh-like” QC image when used with a scintillation camera

Camera movement that improves Spatial Resolution during SPECT imaging

Elliptical and/or Body Contour orbits

Distance at which exposure rate (mR/hr) is measured according to transport index

1 meter from the surface of a package

Maximum radiation dose that one would receive in an unrestricted area

2 mrem/hr

Formula for measuring exposure rate on a GM meter

Meter reading x Adjustment setting (knob)

Frequency of surveying areas where therapeutic radiopharm’s are prepared or administered

End of Day whenever a therapeutic dose has been prepared/administered

• Therapeutic doses are quite high and require written directive as well as a daily survey

Frequency of calibrating a survey meter (GM meter, scintillation probe, ionization chamber)

Yearly

Difference between a Direct Radiation Survey and a Wipe Check Radiation survey

Direct surveys provide an instant reading, while a Wipe Check will need to go in a well counter to get a reading

NRC limit for the yearly occupational radiation dose of the lens/eye

15,000 mRem/year (15 rem/year)

If a worker receives close to the limit some occupational radiation dose in one year, what should be done?

Review ALARA protocols and dedicate as many protective measures as possible during work hours

Done before daily use of the GM meter

Check the batteries

Average energy needed to create an ion pair in an ionization chamber

34 eV per pair (20 - 45 eV depending on type of gas in chamber)

Geiger Effect

Ionization of the entire fixed volume of gas in a detector chamber due to secondary ionization

• This occurs in the Geiger-Muller region of the voltage region chart (highest applied voltage and collected charge)

Pulses produced by the collection of ions in the Proprtional Region vs those produced by an instrument operating in the Ionization Region

The Proportional Region of the gas chamber operates at a higher current, so the pulses from this region tend to be larger and more ions are created

Region at which gas amplification occurs in the chamber of a radiation detector

Proportional Region of the gas curve

Pulse Height

Spectrometer representation of an amount of radiation detected in the chamber of a radiation detector. It is dependent on the number of ions produced by interaction of the gas and incoming radiation

Jaszczak Phantom

20 cm plexigalss cylinder with rods and spheres inserted. Used to determine SPECT system performance.

Full Width at Half Maximum (FWHM)

Measure of Spatial Resolution, based on possible positions of the source when the count rate is half of its maximum

• A smaller FWHM is indicative of a higher resolution, so a gamma camera can better differentiate between two radioactive sources

Cause of ring artifacts in tomographic ring construction

Abnormalities of the detector OR rotational mechanics in SPECT imaging

• These are usually seen with uncorrected non-uniformities in the flood field or defects in the collimator

Frequency of measuring the Activity Linearity of a dose calibrator

Quarterly (every 3 mo)

Time for retaining records of surveys

3 years

Best reseolution for a parallel-hole collimator

At the surface of the collimator. Greater distance between the PHC and your object degrades the test image.

Frequency of checking Center of Rotation (COR) for SPECT imaging

At least weekly, until the COR system is shown to be several over a time frame of several months

Best collimator for high resolution images of a small organ, gland or joint

Pinhole Collimator

• The PC is a large, hollow cone with a single aperture at the point. This allows it to magnify images of small organs, etc.

Activity needed to evaluate intrinsic uniformity of a planar imaging camera (using Tc 99m)

200 uCi

• An intrinsic flood test is performed without a collimator, and at a distance of at least 5x the largest camera dimension from the camera face

Two most important Quality Assurance measurements for a scintillation camera system

• Field Uniformity to measure detector performance

• Spatial Resolution to measure image quality

Light generated by a NAI crystal in a scintillation detector

A result of the photon energy absorbed in the crystal from output from a photomultiplier tube (PMT)

• Scintillator material creates light when met with gamma rays. This light is converted to an electrical signal by the PMT

Basic function of ANY collimator

Limit access of photons to the instrument, thereby “filtering” the gamma rays that will pass through to the scintillator crystal

True/False: All survey meters/radiation detectors (that we use) contain gas detecors

True!

Common cause of field non-uniformity

Misadjusted photopeak

Geometry

Measurement of the sensitivity of the dose calibrator to different source configurations

• Correction factors are used when comparing dose activities in different sample volumes or syringe sizes

Limit for activity levels of multiple radioactive materials in a package shipped back to a pharmacy

The lowest activity of any of the materials in the package

Scintillation

Light emitted by some material after it absorbs ionizing radiation