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32 Cards in this Set

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Radiation detectors are encountered in what three distinct capacities?
1. Monitor and control radiation risk to personnel (ie. survey meter, area monitor, extremity monitor, personnel dosimeter
2. Veriphy and monitor radiopharmaceutical dose (ie. dose calibrator, automatic dose injection system.
3) Measure, locate or visualize the relative uptake of radiopharmaceutical administered to patient (well counter, thyroid uptake probe or surgical probe.
An ideal radiation detector will have what 2 properties?
High detection efficiency
linear response with increasing activity
What is "current mode?"
Mode where individual radiation interactions are not distinguishable but the detector outputs a current proportional to the amount of energy. AKA energy- or charge-integrating detector.

Ideal for surveying applications
What is "pulse mode?"
Mode where detector counts photon events. Type of detector used in well counter and scintillation camera

Drawback: deadtime= time needed to process interaction during which no other events are being recorded
what is "cumulative dose mode?"
Mode where detector collects and stores dose. Typical of dosimetry badge
What factors effect detection efficiency?
How do they relate to total efficiency?
Inherent efficiency of detector- based on energy of incident photon and linear attenuation coefficient (u) of transducer material. = number of photons detected/photons incident

Efficiency resulting from the geometry of the radiation detector in relation to the source. The closer the detector is to the source, the better it is as registering an event (think inverse square law)= #photons incident on detector/# of photons emitted from source

Total efficiency = intrinsic x geometric (always <1)
What's the difference between paralyzble vs. non paralyzable deadtime?
Non paralizable means that deadtime (t) is not influenced by a second interaction.

paralyzable means that if a second interaction occurs before deadtime is completed, it will prolong the deadtime. Therefore this detector will experience greater...
Non paralizable means that deadtime (t) is not influenced by a second interaction.

paralyzable means that if a second interaction occurs before deadtime is completed, it will prolong the deadtime. Therefore this detector will experience greater counting losses with increased activity
How do we resolve adjacent photopeaks?
What is FWHM?
Set photopeak breadth = the energy spectrum we're interested in.

FWHM= full width at half maximum. It means you measure the width of peak at half its height
Set photopeak breadth = the energy spectrum we're interested in.

FWHM= full width at half maximum. It means you measure the width of peak at half its height
What are the 3 primary categories of detectors in Nuclear medicine?
1. Gas-Filled detectors
2. Sold-State detectors
3. Scintillation Detectors
How does a gas filled detector work?
Gas contained in a vessel which is connected to two electrodes. When a photon interacts with gas it can excite or ionize the gas resulting in ion pairs which move towards electrodes when a voltage is applied. the current generated is proportional ...
Gas contained in a vessel which is connected to two electrodes. When a photon interacts with gas it can excite or ionize the gas resulting in ion pairs which move towards electrodes when a voltage is applied. the current generated is proportional to rate of ionization.

Not all interactions=ionizations (it takes approximately 34 eV/ion pair)
How do Recombination and secondary ionizations effect detection efficiency?
Recombination= ions of opposite charge rejoin before they can be accelerated to their respective electrode (weak field applied) Lowers detection efficiencey

Secondary ionizations= ions gain sufficient kinetic energy to generate additoinal ions (strong field is applied) increases efficiency
Depending on voltage applied, what are the three types of gas detectors?
Ionization detector- least sensitive- used in current mode used for accurate estimates of deposited energy
Proportional counter- more sensitive- used in pulse mode for gamma ray spectrometry
Geiger-Mueller detector- super sensitive 2/2 avalanche...
Ionization detector- least sensitive- used in current mode used for accurate estimates of deposited energy
Proportional counter- more sensitive- used in pulse mode for gamma ray spectrometry
Geiger-Mueller detector- super sensitive 2/2 avalanche of ionizations- used in pulse mode to detect low radiation fields. Prone to saturation
Describe solid state detector physics
similar to gas chamber except in the sold state the ionizations release electrons (conduction band) and electron vacancies (valance band). The energy difference between the two bands= band gap.

These are more efficient than gas because they are super dense
Average energy expended to generate ion pair = 3-5 eV. (34 eV in gas detector) higher detection efficiency
= better energy resolution
What's the utility of using compound semiconductors over pure semiconductors.
overcome need for elaborate cooling
How do scintillation crystals work?
Primary photon interacts with NaI (compton or photoelectric effect) resulting in release of a primary compton or photoelectric electron. These electrons travel short distances in the crystal and excite additonal electrons. These 2ndary electrons m...
Primary photon interacts with NaI (compton or photoelectric effect) resulting in release of a primary compton or photoelectric electron. These electrons travel short distances in the crystal and excite additonal electrons. These 2ndary electrons migrate to sites of impurity (activation centers) and releasing a burst of scintillation photons with characteristic energies
What's the big problem with NaI crystal?
It's hygroscopic: usually transparent but looses transparency in response to water or humidity and becomes "yellowed" and decreases its efficiency.
What are the properties of a good crystal?
high density and atomic number = better efficiency
high light output=better resolution= smaller FWHM
Shorter scintillation decay=better resolution

deadtime= sum of physical deadtime and electronic deadtime
How does a photomultiplyer tube work?
for every 5 scintillation photons entering a PMT impacting the phtocathode, an electron is emitted. That electron is accelerated towards a dynode which releases 5 more electrons which are further acclerated to the next dynode etc. etc resulting in electron multiplication on the order of 1-10 million
What is a single channel analyzer?
Single channel analyzer (simple counting system) analyzes a single upper and lower threshold values (channels), if amplified signal falls within this range, it is counted.
Single channel analyzer (simple counting system) analyzes a single upper and lower threshold values (channels), if amplified signal falls within this range, it is counted.
What is a multi channel analyzer
Multi channel analyzer (spectrometer) = analyzes multiple channels and separates them into histogram
Multi channel analyzer (spectrometer) = analyzes multiple channels and separates them into histogram
What is coincidence detection? What kind of imaging do we see this principle employed?
2 detectors which sense photons and determine if they occured within a short time interval (coincidence window). if occurs within the window they are said to have occurred simultaneously

PET imaging- annihilation
How do thermoluminescent (TLD) Dosimeters work?
How do optically-stimulated (OSL) dosimeters work?
TLD's are scintillation crystal that trap and store energy in the conduction band. After being heated, the electrons fall back to the valence band and emit photons proportional to amount of exposed radiation. Need to be "erased" before next use by heating in oven (aka annealing)

OSL's (AlO3) work the same way except they are read out by shining a green light instead of heating. Added benefit of being able to be read multiple times
Other types of dosimeters?
Real time digital dosimeter and film badge dosimeter
How frequently should radiation contamination surveys be conducted in areas where unsealed radioactive sources have been handled?

What devices are used?
daily

Ionization and G-M detectors
QA: How frequently do survey meters need to be calibrated?
Annually and after any repair
done at 1/3 and 2/3 of the full activity scale of instrument
QA: How frequently must a survey meter zero-level be calibrated to zero
daily
battery operated meters should have batteries checked daily
QA: how frequently must the efficiency, and energy resolution of a well counter be checked?

How about background reading and constancy test?
Installation, annually and after repair

Daily
QA: How frequently is a dose calibrator's accuracy checked?

How about constancy testing?
installation, annually and after repair

Daily
QA: How is Dose Calibrator Linearity measured? WHen should it be measured?
Decay Method- take Tc99m and let it decay and measure over 3-4 days for linearity measurement
Attenuator Sleeve Method- place Tc99m in a cylinder of concentric attenuators and measure

Installation, QUARTERLY, and after repair
QA: When should the energy resolution of the thyroid probe be evaluated?

When should pulse height analyzer and constancy be checked?
annually

Daily
QA: For intra-operative Probes when should constancy be checked?
Before each use

everything else follows thyroid probe protocol
QA: What are the acceptable limits of QA testing?
+/- 5%