DDT 7: Diagnostics Therapy 3- Gamma Cameras

Question 1

radiation detectors

Answer 1

tools used to detect exposure of radiation - limited info on type of exposure or the kind of damage that has occured

Question 2

simplest form of radiation detector and function

Answer 2

a Film Badge – these indicate cumulative exposure to radiation by the degree of blackening of photographic film!

Question 3

what does film badge consist of?

Answer 3

It is typical for a single badge to contain a series of filters of different thicknesses and of different materials (the precise choice may be determined by the environment to be monitored).

Question 4

2 common forms of radiation detectors

Answer 4

The Geiger-Muller Tube.

Scintillation Counters.

Question 5

geiger muller tube

Answer 5

consists of a metal tube filled with gas (helium, neon, argon) and an electrode wire in it
encased in a glass tube and is sealed by a thin window at one end/

Question 6

what happens when alpha particles beta particles or gamma rays enter the geiger muller tube

Answer 6

When a or b-particles or g-rays enter the tube, they cause ionization of some of the gas molecules

Question 7

PET

Answer 7

Positron Emission Tomography

Question 8

what is PET

Answer 8

imaging technique that uses gamma rays formed during positron-electron annihilation (pair production) to construct images of the form and function of various organs within the body.

Question 9

specific purpose of PET scan

Answer 9

look at functionality of specific organs but not the anatomy

Question 10

how are positrons produced

Answer 10

pair production
beta plus decay
other various ways

Question 11

what happens when a positron encounters or comes in contact with an electron?

Answer 11

positron-electron annihilation occurs, producing TWO identical 511 keV g-rays travelling in exactly opposite directions (i.e. 180 degrees to each other).

Question 12

what detects coincident gamma rays

Answer 12

These coincident g-rays are detected by scintillation counters at exactly (almost) the same time.

Question 13

what direction are photons emitted

Answer 13

180* degrees to each other

Question 14

how does emitting photons at a specific angle help with diagnostic imaging in PET scans

Answer 14

Since the photons are emitted at 180o to each other, it is possible to localise their source by projecting back along their line of coincidence

Question 15

what does reconstruction surface determine?

Answer 15

Reconstruction software determines the angular and linear coincidence events.

Question 16

How does positron-electron annihilation contribute to imaging?

Answer 16

By measuring the activity level (number of annihilation events) as a function of position, an image can be reconstructed – similar to that in CT

Question 17

What radionuclides are used to generate the positrons used in PET?

Answer 17

a short-lived radioactive tracer isotope which decays by emitting a positron

Question 18

how are radionuclides are being introduced to the body?

Answer 18

chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation).

Question 19

why is there a waiting period in the PET scan?

Answer 19

while the metabolically active molecule become concentrated in tissues of interest; then the subject is placed in the imaging scanner

Question 20

name radionuclide and waiting period

Answer 20

(most commonly fluorodeoxyglucose (FDG), waiting period of an hour

Question 21

what is the average half life of the radionuclide in PET scan?

Answer 21

quite short

Question 22

what compounds can be used to incorporate radionuclides in the body?

Answer 22

such as glucose, water or ammonia and then injected into the body to trace where they are distributed

Question 23

what information are PET scanners good at getting?

Answer 23

information on metabolism or molecular biology processes

Question 24

what other diagnostic imaging technique are they usually integrated with and why?

Answer 24

integrated with CT scan to get anatomical and metabolic information

Question 25

how are images from PET scans and CT scans integrated?

Answer 25

The two sets of images are ‘co-registered’ or can be superimposed digitally so that areas of abnormality on the PET scan can be perfectly correlated with anatomy on the CT scan

Question 26

how do geiger muller tubes work?

Answer 26

Thin mica window allows radiation to enter the tube Causes ionisation to occur within the metal tube filled with inert gas. positively charged high voltage wire electrode attracts electrons from ionisation positively charged ions moved to walls of tube (cathode) - causing more ionisation forms a short intense pulse of a current

Question 27

what needs to occur after the detection of radiation and why?

Answer 27

quenching/dead time - 1 particle = 1 discharge | no further detection until discharge is stopped

Question 28

scintillation counters

Answer 28

Defined as the production of small flashes of visible light from certain materials (scintillators) as a result of the absorption of high energy radiation (e.g. g-rays

Question 29

what does scintillation counter consist of?

Answer 29

consists of a transparent crystal (e.g. sodium iodide (NaI)) which fluoresces when struck by ionising radiation.

Question 30

what does the transparent crystal produce and how is it detected and amplified?

Answer 30

The visible flash (single or multiple photons) produced by the crystal is detected and amplified by a Photomultiplier tube (PMT).

Question 31

why is the intensity of the scintillation flash can be used to determine the original photon energy?

Answer 31

Since the number of emitted photons per MeV of incident energy is fairly constant, the intensity of the scintillation flash can be used to determine the photon energy

Question 32

series of events that occur in scintillation counters

Answer 32

incident photon enter scintillator and emits a visible photon visible photon meets photocathode and releases photoelectron photoelectron strikes first dynode - releases additional electrons strikes another dynode of higher voltage generates more electrons - so on reach the anode, accumulation of charge is registered as a pulse of current

Question 33

what is the dynode coated with and what does it form

Answer 33

This dynode is coated with a material which emits several additional electrons for every electron that strikes it.

Question 34

where are scintillation counters commonly used

Answer 34

gamma cameras

Question 35

what does gamma camera consist of?

Answer 35

A typical gamma camera might consist of a large flat crystal of Sodium Iodide a bank of PMT tubes arranged in a hexagon fashion behind the crystal.

Question 36

what does collimator consist of?

Answer 36

The collimator typically consists of a thick sheet of lead with thousands of adjacent holes drilled through it

Question 37

purpose of components of collimator?

Answer 37

This allows spatial discrimination to the source of the detected photons (i.e. relative position of the source of the photons).

Question 38

how are gamma cameras used?

Answer 38

A radiopharmaceutical is administered to the patient is preferentially absorbed into the organ to be studied The Gamma Camera detects the intensity of the radioactive activity within the organ under study and ‘plots’ the intensity as a function of position

Question 39

function of gamma camera

Answer 39

Such imaging techniques allow for comprehensive studies of organ function

Question 40

how are abnormalities detect in images?

Answer 40

Abnormalities lead to an increase in blood supply to the diseased region, which increases the amount of uptake of the tracer in the area - clearly visible as ‘hotspots’ in the above diagrams.

Question 41

what does nuclear medicine studies used for specific organ function

Answer 41

Many diagnostic nuclear medicine studies use Radionuclides as tracers for specific organ function.

Question 42

3 traits of an ideal radionuclide for nuclear medicine

Answer 42

Be short lived (i,e, short half lives). Emit gamma-rays with little more energy than required for detection (low patient dose). Why not a and b? Be non-toxic.

Question 43

what is the coincident event in PET scans?

Answer 43

when the detectors opposite from each other detect gamma rays/photon at the same time

Question 44

what do PET scans tell us and how are they used

Answer 44

cells that were the most active took up the most radionuclide substance and emitted more gamma rays and therefore appears brighter. tumours and abnormalities have blood vessel that take up more nutrients thus this technique is used to detect tumours