40- Radiation Detection Instrumentation In Nuclear Medicine Practice

Front 1

Radiation detectors are encountered in what three distinct capacities?

Back 1

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.

Front 2

An ideal radiation detector will have what 2 properties?

Back 2

High detection efficiency
linear response with increasing activity

Front 3

What is "current mode?"

Back 3

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

Front 4

What is "pulse mode?"

Back 4

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

Front 5

what is "cumulative dose mode?"

Back 5

Mode where detector collects and stores dose. Typical of dosimetry badge

Front 6

What factors effect detection efficiency?
How do they relate to total efficiency?

Back 6

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)

Front 7

What's the difference between paralyzble vs. non paralyzable deadtime?

Back 7

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

Front 8

How do we resolve adjacent photopeaks?
What is FWHM?

Back 8

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

Front 9

What are the 3 primary categories of detectors in Nuclear medicine?

Back 9

1. Gas-Filled detectors
2. Sold-State detectors
3. Scintillation Detectors

Front 10

How does a gas filled detector work?

Back 10

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)

Front 11

How do Recombination and secondary ionizations effect detection efficiency?

Back 11

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

Front 12

Depending on voltage applied, what are the three types of gas detectors?

Back 12

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

Front 13

Describe solid state detector physics

Back 13

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

Front 14

What's the utility of using compound semiconductors over pure semiconductors.

Back 14

overcome need for elaborate cooling

Front 15

How do scintillation crystals work?

Back 15

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

Front 16

What's the big problem with NaI crystal?

Back 16

It's hygroscopic: usually transparent but looses transparency in response to water or humidity and becomes "yellowed" and decreases its efficiency.

Front 17

What are the properties of a good crystal?

Back 17

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

Front 18

How does a photomultiplyer tube work?

Back 18

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

Front 19

What is a single channel analyzer?

Back 19

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.

Front 20

What is a multi channel analyzer

Back 20

Multi channel analyzer (spectrometer) = analyzes multiple channels and separates them into histogram

Front 21

What is coincidence detection? What kind of imaging do we see this principle employed?

Back 21

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

Front 22

How do thermoluminescent (TLD) Dosimeters work?
How do optically-stimulated (OSL) dosimeters work?

Back 22

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

Front 23

Other types of dosimeters?

Back 23

Real time digital dosimeter and film badge dosimeter

Front 24

How frequently should radiation contamination surveys be conducted in areas where unsealed radioactive sources have been handled?

What devices are used?

Back 24

daily

Ionization and G-M detectors

Front 25

QA: How frequently do survey meters need to be calibrated?

Back 25

Annually and after any repair
done at 1/3 and 2/3 of the full activity scale of instrument

Front 26

QA: How frequently must a survey meter zero-level be calibrated to zero

Back 26

daily
battery operated meters should have batteries checked daily

Front 27

QA: how frequently must the efficiency, and energy resolution of a well counter be checked?

How about background reading and constancy test?

Back 27

Installation, annually and after repair

Daily

Front 28

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

How about constancy testing?

Back 28

installation, annually and after repair

Daily

Front 29

QA: How is Dose Calibrator Linearity measured? WHen should it be measured?

Back 29

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

Front 30

QA: When should the energy resolution of the thyroid probe be evaluated?

When should pulse height analyzer and constancy be checked?

Back 30

annually

Daily

Front 31

QA: For intra-operative Probes when should constancy be checked?

Back 31

Before each use

everything else follows thyroid probe protocol

Front 32

QA: What are the acceptable limits of QA testing?

Back 32

+/- 5%