17.18 Isotope diagnostics Flashcards by Ines Crawford

What is an isotope test?

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

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What are radiopharmaceuticals?

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

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

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

Longer half-life than for in vivo

- Beta or soft gama radiating

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What is a Tc generator used for?

To extract metastable Tc from a source of decaying Mb

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Half life and energy of 99mTc

6 hour half-life

- Gamma ray emission of 141eV

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What are the parts of the tc generator?

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

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How does the Tc generator work?

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

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How often can we elute 99mb?

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

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When is 99Mo replaced and why?

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

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What is a gamma camera used for?

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

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

Collimator
NaI crystal (scintillator)
Photomultiplier tube
Electronics

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What is the collimator made of?

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

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

100keV or else absorbed by tissue

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What is the advantage of scintigraphy?

It is non invasive

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What is a typical isotope accumulation curve (IAC)?

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

How is the IAC obtained?

by measuring the isotope activity (decay) at different moments

What are the 2 parts of the IAC curve?

clearance (uptake rate of the organ)

- decay, elimination

What is the physical half-life of an organ?

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)

What is the biological half-life of an organ?

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

What is the effective half-life of an organ?

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

How is the biological half-life calculated?

1/Teff = 1/Tphys + 1/Tbiological

What is SPECT?

single photon emission computed tomography

How does SPECT work?

Gamma emitting radioisotope in the body (often in blood)

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

What is PET?

positron emission tomography

What type of isotope does PET use?

beta plus radiating isotope

What are the parts of the detector ring of PET?

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

What is multimodal imaging?

combination of 2 or more imaging techniques in a single examination

What are the advantages of SPECT/MRI?

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

What is relative depth dose?

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)

What is the applied dose?

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

What is the most frequently used positron radiating radiopharmacon?

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

What is teletherapy?

using a source outside of the body to treat cancer.

Which type of radiation can be used in teletherapy?

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

What is a gamma knife used for?

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

Which isotope is used for the gamma knife?

60Co (cobalt) : gamma radiating

What is brachytherapy?

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

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

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