Part 1: L6 Gamma Camera/SPECT Flashcards
1
Q
Overview of radiochemistry for SPECT:
A
- e.g. Parent nucleus of U-238 emits alpha particles -> excited daughter particles which emits gamma ray
- Alt: Beta and gamma decay modes - positron from unstable artificial isotopes
- SPECT requires gamma particles
2
Q
Radionuclides:
A
- Primarily showing physiological function -> localising
- Gamma-emitting radioisotopes which passes through tissue without being scattered, or causing much damage
- Can enter cells
- Generally administered in very low doses (ng)
3
Q
SPECT technology:
A
- Single photon emission computed tomography
- Gamma camera
- Radioactive event releases gamma particles -> collumate with lead fingers -> only photons directly upwards allowed -> Aluminium casing then scintillation crystal, photomultiply -> detect and trasnmit information to computer
- Collimator is also one of the sources of blurring within the image, attenuates most (>99%) of incident photons
- This limits the sensitivity
4
Q
Nucleotide Limitations in SPECT:
A
- Most gamma cameras are designed for specific energy windows (100-250 eV)
- Lower than this: too scattered
- Higher than this: difficult to collimate
- Half-life must be long enough to synthesis and allow accumulation in target tissue, while allowing clearance through non-target organs -> short as possible to limit radiation dose
- Desired half life is dependent on time required to localise to target tissue (heart/brain: shorter, tumour targeted radiolabelled monoclonal antibodies take longer)
5
Q
Typical sourcing of radionuclides:
A
- Generators considered ideal, preferably with low cost parent isotopes
- Daughter can be easily separated from the parent via either ion exchange chromatography or solvent extraction
- A few isotopes are produced in a nuclear reactor
- Several can be generated by an accelerator or cyclotron - expensive - and can only produce one isotope at a time
6
Q
Tc-99m qualities:
A
- 6hr half-life
- Generated in 99Mo/99mTc generator
- Decays via isomeric transition
7
Q
How are radionuclides administered?
A
- Intravenous injection in liquid or aggregate form (majority)
- Ingestion while combined with food
- Inhalation as a gas or aerosol (Xenon-133m/Krypton-81m)
- Some studies require the labelling of a patient’s own blood cells with a radionuclide leukocyte and red blood scintigraphy
8
Q
Iodine-123 in SPECT
A
- Half life of 13.13 hrs
- Decays via gamma particle
- Can be used for things like thyroid
9
Q
Xenon-133m in SPECT:
A
- Inhaled to assess pulmonary function and to image lungs
- Also often used to image blood flow
- Rapid clearance from brain
10
Q
Technecium-99m in SPECT:
A
- Most prevalent agent (85% of administered drugs)
- Can be gaseous
- Emits a 140 keV gamma-ray with 89% abundance and a 6hr half life
- There is no stable isotope (middle of transition metals)
- Versatile and simple chemistry (t.metal)
- Made in 99Mo/99mTc generator
11
Q
Thallium-201 in SPECT:
A
- Historically used to image heart
- Very toxic -> alopecia
- Cyclotron produced
- Half life 72.5hrs
- TI decays emitting Hg X-rays and photons
- Behaves similarly to potassium in body (Tl(I)) -> taken up well via sodium/potassium pump -> collects in myocardium then circulated out rapidly
- Able to image heart in real time -> Thallium stress test
- Has since been replaced by Sestimibi (Tc-99m based)
12
Q
99m-Tc generation:
A
- Molybdate (2- anion) decays to Pertechnetate in luminar column; single negative charge as Tc so able to wash off with 0.9% NaCl
- Obtained in evac vial with lead shielding
- Charge chromatography where Mo binds strongly and Tc elutes
13
Q
Tc essential:
A
- First class of 99mTc pharmaceuticals
- Tc is an essential structural part of the introduced drug
- Majority
14
Q
Tct-tagged
A
- Minority
- The physiologically active targeting moiety such as an antibody, peptide, hormone etc has been labelled with 99mTc by a bifunctional chelate
15
Q
Brain imaging with Technecium-99m
A
- Kits are available for the radiographer with essential components for ‘one pot’ solution
- Ceretec, neurolite etc -> Tc5+ formed by reduction with SnCl2, followed by complexation with the ligand in a saline solution
- Capable of penetrating the intact BBB due to their neutral and lipophilic character