2. Radioimaging Flashcards
What is SPECT? Adv and Disadv? Example of radionuclide
Single-photon emission computed tomography
- measures emission from gamma emitting radionuclide
- reactor produced, neutron rich
- ADV: cheap, low resolution, good tissue penetration (no damaging alpha or beta)
- DISADV: low sensitivity, low spatial resolution
E.g. Tc-99m can be used (gamma rays)
What is PET? Adv and disadv? Example of radionuclide
Positron emission tomography
- detects positron emitting radionuclide
- cyclotron produced, neutron deficient
- ADV: high resolution, high sensitivity, high tissue penetration
- DISADV: expensive, availability of nuclide hard to obtain
E.g. Y-86 to image tumour (cyclotron produced)
Why is Tc-99m ideal for imaging? Ligand design?
- emit gamma rays -> too weak to damage cells but energetic enough to be detected
- 6 h half-life -> ideal for preparation, administration and imaging
- ligand: need to bind tightly to radionuclide and complex rapidly (macrocyclic ligands react too slowly), use a variety of hard and soft donor groups
What is MRI and how does it work?
Magnetic resonance imaging
- apply magnetic field
- apply radio frequency and flips the mismatched spins
- radio frequency turned off and measure relaxation rate
Water in different tissue types have different relaxation rates.
Advantages and disadvantages of MRI
ADV: high spatial resolution, no harmful radiation produced
DISADV: expensive, low sensitivity - hard to tell difference between tissue types so need contrast agent
MRI contrasting agents and their ligand design
- Increase sensitivity by increasing relaxation rate. Agents have large number of unpaired electrons, low toxicity (chelate), targeting (high/low lipophilicity).
- Ligand design: Gadolinium is toxic, macrocyclic ligands used for stability and allow free coordination site on metal for water exchange.
Fluorescence microscopy (luminescence imaging).
- What are fluorophores used for?
- Example of metal used
- Fluorophores are fluorescent compounds used to label parts of the cell with different colours.
- E.g. Re, Ru, lanthanoid complexes normally contain cyclen macrocyclic ligand.
Advantages and disadvantages of fluorescence microscopy
ADV: cheap to perform, balance between sensitivity and resolution
DISADV: poor tissue penetration, can’t be used in vivo, many molecules in cell fluoresce and contribute to background
Ligand design in radiotherapy and radioimaging
- rapid binding to metal
- polydentate ligand (not macrocyclic - slow to react to metal)
- targeting ligand to specific tissue
Radioisotope for radiotherapy
- alpha and beta rays - damaging and destroy cells
- half-life in days
Radioisotope for radioimaging
- gamma rays, positron rays - high penetration
- half-life in low hours
