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Uni Of Bath 2023 > Medical Physics > Flashcards

Medical Physics Flashcards

(72 cards)

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1
Q

What is a nuclide?

A

An element with a specific number of neutrons and protons.

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2
Q

What is the purpose of an atomic number?

A

To specify which nuclide is which, e.g. different nuclides of iodine all have an atomic number of 53

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3
Q

What is an isotope?

A

An isotope is a nuclide with the same atomic number but different mass number

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4
Q

What is radioactive decay?

A

Radioactive decay sees a nucleus lose energy or particles to reach a more stable state

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5
Q

What is Alpha decay?

A

Nucleus ejects an alpha particle. Common in very heavy elements, alpha particles lose energy in multiple collisions (highly ionising)

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6
Q

What applications are there for alpha decay?

A

Therapeutic applications

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7
Q

What is beta decay?

A

A neutron within the nucleus is transformed into a proton and electron. Electron (beta minus particle) and antineutrino ejected from nucleus.

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8
Q

How is energy shared between the electron and antineutrino in beta decay?

A

The electron can have variable energy up to a max value defined by transition energy for decay process. Average energy is typically 1/3 max, with the energy being deposited over a relatively short distance.

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9
Q

What are beta particles deflected largely by?

A

Electrons and nuclei.

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10
Q

What are the applications of beta decay?

A

Non-imaging tests and therapy.

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11
Q

What is what state can beta decay sometimes leave a daughter nucleus?

A

Excited or metastable, decaying to a more stable state by emission of a gamma photon.

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12
Q

How long can an excited state exist?

A

From Picoseconds to Years depending on the nuclide.

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13
Q

What is the name for a long lived excited state?

A

Metastable or isomeric.

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14
Q

What is isomeric transition?

A

The decay of a nucleus from a metastable/isomeric state by rearranging the nucleus to a lower energy state with gamma photon emission or by internal conversion.

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15
Q

Why are metastable isotopes useful?

A

They can be chemically separated from their parent and are almost pure gamma emitters so they are useful in imaging.

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16
Q

What is the most important metastable radionuclide in nuclear medicine?

A

Technetium-99m.

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17
Q

What is electron capture?

A

Orbital electron is “captured” by nucleus and combines with proton to form neutron. Characteristic X-rays or Auger electrons emitted when orbital vacancy is filled. Often results in excited or metastable state.

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18
Q

What is positron decay?

A

Proton transformed into a neutron and positron. Positron loses energy in a number of interactions then annihilates with an electron. Mass converted to 2x 0.511MeV photons travelling in opposite directions.

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19
Q

What are the units of radioactivity relevant to nuclear medicine?

A

Bequerel (Bq), SI unit, means 1 disintegration per second. 1Bq is a very low activity, kBq and MBq usually used.

Curie (Ci), old unit of measurement. 1 Curie = 37MBq.

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20
Q

What types of radioactive decay are best suited to imaging?

A

Isomeric transition, Electron capture and Positron decay as they emit gamma photon and have the lowest radiation burden.

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21
Q

What types of radioactive decay are best suited to therapeutic applications?

A

Alpha and Beta decay because of the high dose deposition at short range.

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22
Q

What makes an isotope ideal for imaging?

A

-Pure gamma emitter
-Appropriate energy
-Half-life appropriate to study
-Good labelling chemistry
-Easy availability (low production costs)

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23
Q

What is the energy of Technetium-99m?

A

140keV

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24
Q

What is the half life of Technetium-99m?

A

6.02 hours

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25
How is Technetium-99m produced?
Using a Technetium-99m/Molybdenum-99 generator
26
How does Technetium-99m decay?
-89% gamma emission -11% conversion electrons and Auger electrons
27
What is the radioactive decay equation in terms of activity?
28
What is the decay constant?
29
What does the amount of a daughter isotope in a decay chain depend on?
-Initial activity of the daughter -Decay rate of the daughter -Amount of the parent
30
What is the Bateman equation?
Rate of change of daughter nuclei present = Rate of production of the daughter nuclei - Rate of decay of the daughter nuclei -Assuming each parent nucleus decays to one daughter nucleus -Rate of production of daughter equals decay rate of parent
31
How is activity defined?
Rate of change of parent atoms: A = dN/dt = -λN where N is the number of parent atoms at time t
32
What is Secular equilibrium?
-Parent half life is very long so decay rate is approx constant -Production of daughter approx constant -Assume daughter activity = 0 at t = 0 -Quantity of daughter builds up to constant equilibrium value
33
What is Transient equilibrium?
-Half life of parent = 10 to 50 times half life of daughter -Daughter builds up and then decays with parent -Assume daughter activity is zero at t = 0 to calculate ratio at equilibrium
34
What can the Bateman equations be used for?
-How much activity we will have at any time -Optimum time to elute the generator
35
What are the two main ways to generate radioisotopes?
-Charged particle accelerators such as Cyclotrons, Synchrotrons and Linear Accelerators (use charged particles, often lead to EC or positron decay) -Nuclear reactors (neutron interactions, often lead to beta decay)
36
What is the equation for the time to maximum in transient equilibrium (relevant to generator elution)?
37
Describe a Cyclotron
-Source of charged particles at centre (S) -Alternating voltage accelerates particles -High energy required to overcome Coulomb forces around target nucleus
38
What nuclides does a Cyclotron produce?
-Nuclide produced depends on target material and incident particle -E.g. Fluorine-18 from proton bombardment of Oxygen-18
39
What amounts of nuclides are produced by a Cyclotron and are they carrier free?
-Small amounts, so tends to be expensive - Carrier free normally, due to change in atomic number
40
How does a nuclear reactor work?
- U-238 fuel rod enriched with U-235 (<20% U-235) - U-235 naturally fissionable - 2-3 neutrons released during fission - Neutrons strike U-238 causing fission - More neutrons released - Chain reaction occurs
41
What 2 ways can reactors be used to produce radionuclides?
1. Place material in reactor to gain more neutrons - E.g Mo-98 to get Mo-99 - Not carrier free - Normally low specific activity 2. Separate fission products - E.g. "fission Molly" - Nuclides are chemically separated - High specific activity
42
Describe the line of stability for isotopes
43
Name 3 radionuclides commonly used for Imaging procedures and their properties
1. I-123 - Made in cyclotron, Decays by EC, 160keV photons, half life 13hrs 2. Tc-99m - Made in generator, Decays by IT, 140keV photons, half life 6 hours 3. Mo-99 - Made in reactor, Decays by beta decay, 780keV photons, half life 66 hours
44
Name 2 radionuclides commonly used for Non-Imaging procedures and their properties
1. Tc-99m - Made in generator, Decays by IT, 140keV photons, half life 6 hours 2. I-125 - Made in cyclotron, Decays by EC, 27 and 36 keV photons, half life 60 days
45
Name a radionuclide used in therapy procedures and its properties
I-131 - Made in Reactor, decays by beta decay, 3mm max tissue range, emits 606keV electrons and 364keV photons, half life 8 days
46
How does Mo-99 decay?
47
How is Mo-99 prepared?
As ammonium molybdate and absorbed onto the surface of the alumina column
48
Describe a Mo-99/Tc-99m generator
49
How is a generator eluted?
-Use 0.9% isotonic saline as eluting solvent -Ion exchange of Tc dioxide -10ml solution required for complete elution -75-85% available Tc-99m extracted -Eluate (saline) contains Sodium Pertechnetate and some Tc-99
50
What does Tc-99m elution look like?
51
What do radiopharmaceutical companies produce?
1. Mo-99/Tc-99m generators 2. Pharmaceutical kits to be labelled with Tc-99m 3. Ready to use non-Tc-99m radiopharmaceutical -Manufactured pharmaceutical and isotope produced in cyclotron on reactor
52
What do hospital radiopharmacies do?
1. Label the pharmaceutical kits with Tc-99m (may include blood labelling) 2. Dilute ready to use radiopharmaceuticals
53
What are the desirable properties of a radiopharmaceutical?
1. Suitable gamma emission (energy) 2. Suitable decay scheme (IT, EC or Positron) 3. Short "effective half life" 4. Good labelling chemistry 5. Easily available (how it is produced) 6. Good target to background ratio (Organ specific) 7.Good chemistry (Easy labelling, non-toxic, unaffected by other drugs) 8. Good quality - low particulates, sterile, stable
54
What are the 3 main detector types?
-Ionisation Chamber -Scintillation detector -Solid state detector (based on CZT tech)
55
What are examples of Ionisation chambers?
- G-M tube (contamination monitor) - Calibrator
56
What are examples of Scintillation detectors?
- Well counter -Contamination monitors -Gamma camera
57
What are examples of Solid state detectors?
- Gamma camera
58
What does an ionisation chamber look like?
-Radiation enters chamber volume and causes ionisation -High voltage separates electron and ion -Anode and cathode not necessarily flat plates
59
What does the Voltage response curve look like for an ionisation chamber?
60
What is a survey meter?
-Operate in ionisation region -Used to measure radiation levels for radiation protection -Displays a dose-rate on the front panel
61
What is a Geiger-Muller tube?
-Operates in Geiger-Muller region -Used for contamination monitoring -Can be used to detect different types of radiation
62
What is a dose calibrator?
-Operates in ionisation region -Doesn't measure dose or calibrate (misnomer) -Used to measure activity prior to injection
63
What does a dose calibrator look like?
64
What are the typical well dimensions of a dose calibrator?
- 370mm high x 65mm diameter - ~4π geometry
65
What is the measurement range of a dose calibrator?
- Activity up to 75 GBq -Resolution 0.001MBq - Energy range 25keV to 3MeV -Accuracy +/- 5%
66
What is the typical dose calibrator output for Tc-99m?
1.25 pA/MBq
67
Dose calibrators have a linear response to different photon energies? True or false?
False. They have a non-linear response
68
Does a dose calibrator measure more or less current from the same activity of Tc-99m than Cr-51?
It measures more current for Cr-51 than Tc-99m
69
What does one have to tell a dose calibrator before using it?
The specific radionuclide that is being measured - look up table of different factors
70
What daily tests are done on a calibrator?
Tests for consistency, background and voltage
71
What annual tests are done on a calibrator?
- Accuracy (compare to NPL) - Linearity (test over range of activities used - Precision - repeat measurements
72
What radionuclide is good for testing calibrators?
Cs-137 has 9.25MBq, 30 years half life. Therefore constant over short time scale

Uni Of Bath 2023 (3 decks)

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