17.18 Isotope diagnostics

Question 1

What is an isotope test?

Answer 1

determining the distribution of specific substance (in vivo or in vitro) by detecting radiation from a labelling radioisotope

Question 2

What are radiopharmaceuticals?

Answer 2

chemical agents or drugs having radioactivity. Can be used to label organic compounds

Question 3

What are the viewpoints for selection of an isotope for in vivo diagnostics (3)?

Answer 3

• gamma radiating (long effective range)
• short half-life (but not too shorter than exam time)
• moderate photon energy : higher energy means less absorption, but lower efficiency of detection)

Question 4

What are the viewpoints for selection of an isotope for in vitro diagnostics (2)?

Answer 4

• Longer half-life than for in vivo

- Beta or soft gama radiating

Question 5

What is a Tc generator used for?

Answer 5

To extract metastable Tc from a source of decaying Mb

Question 6

Half life and energy of 99mTc

Answer 6

• 6 hour half-life

- Gamma ray emission of 141eV

Question 7

What are the parts of the tc generator?

Answer 7

• saline solution
• Parent and daughter (99Mo + 99mTc)
• Aluminum : stops the 99Mo
• 99mTc eluted
• Lead shealding all around

Question 8

How does the Tc generator work?

Answer 8

Saline solution brings 99Mo and 99mTc down, but 99Mo stays stuck on the aluminum whereas 99mTC is less stuck (because of a negative charge)

Question 9

How often can we elute 99mb?

Answer 9

Every 24 hours, that’s when we get highest 99mTc activity

Question 10

When is 99Mo replaced and why?

Answer 10

Every week because at some point it doesn’t produce enough 99mTc to get the desired activity

Question 11

What is a gamma camera used for?

Answer 11

a device used to visualize the distribution of radiotracers injected in the body

Question 12

What are the parts of the gamma camera? (4)

Answer 12

• Collimator
• NaI crystal (scintillator)
• Photomultiplier tube
• Electronics

Question 13

What is the collimator made of?

Answer 13

hexagonal, parallel lead tubes. Lead absorbs non parallel gamma rays

Question 14

What is the minimum gamma photon energy for isotope diagnostics and why?

Answer 14

100keV or else absorbed by tissue

Question 15

What is the advantage of scintigraphy?

Answer 15

It is non invasive

Question 16

What is a typical isotope accumulation curve (IAC)?

Answer 16

activity vs time, represents time dependance of the activity in an organ

Question 17

How is the IAC obtained?

Answer 17

by measuring the isotope activity (decay) at different moments

Question 18

What are the 2 parts of the IAC curve?

Answer 18

• clearance (uptake rate of the organ)

- decay, elimination

Question 19

What is the physical half-life of an organ?

Answer 19

The time it takes to reduce the activity level of a source to 1/2 of its original value SOLELY due to radioactive decay (in vitro)

Question 20

What is the biological half-life of an organ?

Answer 20

The time it takes to reduce the amount of a substance in an organ to 1/2 of its original value, SOLELY due to biological processes

Question 21

What is the effective half-life of an organ?

Answer 21

Time it takes to reduce the radioactivity level of an organ or tissue to 1/2 of its original value, due to both the radioactive decay and normal elimination

Question 22

How is the biological half-life calculated?

Answer 22

1/Teff = 1/Tphys + 1/Tbiological

Question 23

What is SPECT?

Answer 23

single photon emission computed tomography

Question 24

How does SPECT work?

Answer 24

• Gamma emitting radioisotope in the body (often in blood)

- gamma camera turns around the body and detects emission of gamma photons, makes cross section

Question 25

What is PET?

Answer 25

positron emission tomography

Question 26

What type of isotope does PET use?

Answer 26

beta plus radiating isotope

Question 27

What are the parts of the detector ring of PET?

Answer 27

scintillation crystal and detector ring, no need for collimator (since emits only in 2 directions) so it has high sensitivity

Question 28

What is multimodal imaging?

Answer 28

combination of 2 or more imaging techniques in a single examination

Question 29

What are the advantages of SPECT/MRI?

Answer 29

MRI : better soft tissue contrast than CT? good resolution
SPECT : low resolution, but good sensitivity
combine the 2 : very accurate dynamic imaging

Question 30

What is relative depth dose?

Answer 30

ratio of an absorbed dose at a certain depth in the body to an absorbed dose at a reference point of the body (usually body surface)

Question 31

What is the applied dose?

Answer 31

The amount (dose) of substance that is in contact with parts of an organism through which it can be absorbed

Question 32

What is the most frequently used positron radiating radiopharmacon?

Answer 32

FDG (fluoro-deoxy-glucose) - behaves like glucose

Question 33

What is teletherapy?

Answer 33

using a source outside of the body to treat cancer.

Question 34

Which type of radiation can be used in teletherapy?

Answer 34

Highly active source with a long half-life, but given in small amount to not damage the healthy tissues

Question 35

What is a gamma knife used for?

Answer 35

used for treatment of intracranial tumors. Multiple beams that intersect at one point

Question 36

Which isotope is used for the gamma knife?

Answer 36

60Co (cobalt) : gamma radiating

Question 37

What is brachytherapy?

Answer 37

Type of therapy where radiation source is place permanently or temporarily in or near the tissue of concern. The source can also be placed in a body cavity

Question 38

What are the advantages of using brachytherapy compared to conventional radiation therapy?

Answer 38

Higher doses of radiation to more specific areas. It can also cause less side effects, while making treatment time shorter.