Tracer kinetics and basic compartmental analysis

Question 1

What are the main requirements of a tracer?

Answer 1

Must be both radio-logically distinguishable but biologically indistinguishable from the molecule it traces.

Question 2

Tracer requirements

Answer 2

• Follow a physiological / biochemical process
• Naturally occurring / analogues of natural substances
• Behave same as natural substance / in known, predictable way
• Mass of tracer must not alter process being studied, isotope is a different weight, i.e. 14C is heavier than 12C
  usually < 1% of natural compound
• High specific activity for imaging or assay
  without change in behaviour or process

Question 3

Describe Compartmental Analysis

Answer 3

A simplified model of how a tracer behaves in the human body

Question 4

What is a compartment? open? closed?

Answer 4

Compartment: A vol/space where the tracer rapidly become uniformly distributed

Open: tracer can escape to other compartments

Closed: tracer cannot escape

Question 5

What is the single compartment model? define concentration?

Answer 5

A closed system

Concentration, 𝐶= 𝑀/𝑉1 MBq / ml

Question 6

How can concentration be used to ind the volume of a 2nd compartment?

Answer 6

C1 . V1 = C2 (V1 + V2)

C1 . V1 = C2 .V1 + C2 .V2

𝑉2=𝑉1 (𝐶1/𝐶2 - 1)

Assuming V2&raquo_space; V1

𝑉2= (𝑉1 . 𝐶1)/𝐶2

Question 7

What percentage of blood volume is haematocrit (RBC) ?

Answer 7

50%

Question 8

What is the volume of distribution ?

Answer 8

Vd = (𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (𝐵𝑞))/(𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (𝐵𝑞/𝑚𝑙))

Question 9

How is loss from an open system determined?

Answer 9

Using the exponential rate factor k

dM = -κ.M
dt

𝑀=𝑀0 . 𝑒^(−κ𝑡)

Question 10

What is mean transit time?

Answer 10

τ = 1/ κ also called turnover time

Question 11

What is half time?

Answer 11

Time for half of the activity of an open system to depart

t½ = ln 2 / κ analog of half life

Question 12

Define total clearance

Answer 12

Biological clearance + Radiological clearance

𝑀=𝑀0 . 𝑒^(−(κ+λ)𝑡)

Effective half life:

1/𝑇𝑒 = 1/𝑇𝑏+ 1/𝑇𝑟

Question 13

Describe the flow of a two compartment model

Answer 13

(𝑑𝑀1 )/𝑑𝑡 = κ2 . M2 – κ1 . M1

i.e Flux out of 1st compartment plus flux back from 2nd compartment

Question 14

What are catenary and mamilliary systems?

Answer 14

Catenary: two compartment system - compartments are connected in series

Mamilliary: two compartment system - compartments are supplied by a central compartment (multi-expo)

Question 15

What a fundamental physiological variables?

Answer 15

• Distribution volume : ml : volume = Activity / conc
• Mean transit time : min
• Extraction :

= Blood flow : ml /min or Perfusion: ml /min per mg

= Clearance : ml / min

Question 16

What is transit time? and mean transit time ?

Answer 16

Time for a particle to travel between point A and B

Multiple particles following different pathways through the system

-> frequency distribution of transit times, h(t)

Mean Transit Time: Mean of all times taken over outflow curve,h(t)

𝞃= (∫ 𝑡 . ℎ(𝑡). 𝑑t)/(∫ℎ(𝑡). 𝑑t)