Ian Fallis

Question 1

Why Positron emission energies listed as maximum value

Answer 1

The energy of the beta+ particles is proportional to the mean free path. As there is various mean free paths there will be a spectrum of energy.

Question 2

How energy relates to the image resolution of the PET tracer

Answer 2

The higher the energy of the beta+ particle the larger the spot that shows up in the PET scan (worse resolution).
An ideal tracer will be the one with the lowest beta+ energy (best resolution).

Question 3

Why gamma rays are suitable for deep tissue

Answer 3

Gamma rays can penetrate deep tissue and are non ionising.

Can be collected for imaging where as alpha and beta can not.

Question 4

Why improvements in collimation and speed of PET detector instrumentation enhance PET image quality

Answer 4

Collimation: More precise determination of direction of trajectory. Only trajectory in line with collimator detected.

Speed of detector: Can accurately pinpoint the exact position of annihilation event along trajectory line

Question 5

Hypoxia selectivity

Answer 5

Hypoxia Selectivity = %uptake in hypoxic cells/%uptake in normoxic cells

CuATSM selectively images hypoxic tissue as enters all cells but only reduced (and therefore accumalated) in hypoxic cells.

Question 6

Functional Imaging agent

Answer 6

Imaging may be used to label molecules such as antibodies or to visualise metabolic function. Build up of imaging agent at required functional site.

Question 7

PET or SPECT has better spatial resolution?

Answer 7

PET has the better spatial resolution.
SPECT is very poor as it emits a single photon, rotating detector.
PET depends on mean free path. Low energy better resolution.

Question 8

How improvements of timing resolution of scintillation detectors results in an improvement of the spatial resolution in PET?

Answer 8

Will become much more accurate as it will be able to pinpoint the exact position of annihilation along the the trajectory line. At the moment only the trajectory line is known.

Question 9

Effective half life

Answer 9

Radionuclides decay with a characteristic t1/2 values(tr), but also have biological half lives (tb). They are eliminated from the body by processes such as perspiration and urination. [state equation].

Question 10

Why PET is usually combined with CT or MRI?

Answer 10

Poor spatial resolution of PET. Other techniques provide a better anatomical image.

Question 11

Typical limits of detection of SPECT and PET?

Answer 11

10^-14 mol (SPECT)

10^-15 mol (PET)

Question 12

Kinetic and thermodynamic criteria for a complexing agent for metal ion tracers?

Answer 12

• Must be kinetically robust (resistant to metal-ligand exchange i.e macrocyclic). This high stability prevents toxicity of a free metal ion. Avoid transchelation.
• Functionality (Must contain a site for tagging/conjugation/solubility/membrane permeability)

Question 13

Two factors that control the reducibility of Cu in complex

Answer 13

• Steric Control (Increase R group bulk > tetrahedral distortion > Cu (II) more stable > easier to reduce)
• Electronic control (Increase softness of donor > Cu (II) more stable > easier to reduce)

Question 14

Disadvantages of metals in SPECT/PET imaging

Answer 14

• Fancy facilities needed to make isotope
• Long lived species must be stable under physiological conditions for extended periods of time. Many chelate systems not stable for weeks/months
• Need to be stable long enough for transport, synthesis and in vivo applications

Question 15

Advantages of metals in SPECT/PET imaging

Answer 15

• range of half lives
• long times to synthesis probes
• aqueous chemistry
• bi-model imaging
• functional imaging possible

Question 16

Estimate the time period within which it would be possible to image a contrast agent?

Answer 16

Around 4-6 times the t1/2 value, but this improves with improvements in detector technology.

Question 17

Tc-ESSENTIAL and Tc-TAGGED

Answer 17

Tc-tagged: 99m Tc used in vivo applications to label biological molecules
Tc-essential: 99 Tc used in the design of radio-tracers. essential to achieve the size, shape, charge and
lipophilicity of the coordination complex.

Question 18

Why are hypoxic tumours difficult to treat with drugs and resistant to radiation treatment?

Answer 18

Drugs - Poor blood supply to deliver drugs to tumour

Radiation - requires oxygen for irreversible DNA damage but no oxygen present

Question 19

64Cu-ATSM

Answer 19

• Tracer enters all cells but only irreversible reduced in hypoxic cells
• acyclic ligand allows ligand conformation to rapidly change upon reduction of Cu(II) to Cu(I)
• Decomposition of Metal-ligand complex leads to accumulation of 64Cu in hypoxic tissue

Question 20

18F-FMISO

Answer 20

• Passively taken up by cells
• Only reduced in hypoxic tissues
• Once reduced too polar to leave the cell
• selectively accumulating the 18F-tracer
• disadvantage is the short half life of 18F

Question 21

18F-FDG (glucose)

Answer 21

• Mimics the action of glucose
• Glucose is too polar to diffuse through cell walls
• actively transported through cell wall of cells requiring glucose for energy
• 18F-FDG converted by enzyme to FDG-6-Phosphate
• This is too polar to leave cell and not a substrate for glycolysis, therefore accumulates in cell

Question 22

Hypoxia

Answer 22

Cancer cells grow faster than blood vessels can form.
Tumours starve themselves of oxygen but do not die
This low partial pressure of oxygen is called hypoxia