NMR mechanisms

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NMR mechanisms in gel dosimetry
L. John Schreiner, Ph.D., FCCPM Cancer Centre
of Southeastern Ontario and Queens University
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Start assuming some common knowledge
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FIRST QUESTION What is a gel dosimeter ?
89 bulk water
Small amount stuff that changes under irradiation
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What is a gel dosimeter ?
hn
89 bulk water
5 gelatin
Small amount stuff that changes under irradiation
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What is a gel dosimeter ?
hn
89 bulk water
5 gelatin
This small amount stuff changes the NMR
relaxation of adjacent water
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In general How does NMR relaxation happen ?
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How does NMR relaxation happen ?
consider the 89 bulk water
DIPOLE-DIPOLE Interaction
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Bulk water contd
Structure
Dynamics
Bo ? Bloc
Bloc Bloc (t)
SPIN-SPIN
SPIN-LATTICE
RELAXATION
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Magnetization
the spins macroscopic magnetization
Bo
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Bulk water contd
Bo ? Bloc
Structure
(in a liquid)
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Bulk water (ice) contd
Bo ? Bloc
Structure
(in a solid)
Effects T2
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Bulk water contd
Dynamics
Effects T1
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NMR Relaxation

• The dipolar power available to drive relaxation
  depends on the
• distance between the spins
• involved spins
• time scale of the motion

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Spin Lattice Relaxation

• time scale of the motion dynamics
• any particular time behaviour can be expressed as
  a sum of frequency components

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Spin Lattice Relaxation

• Molecular dynamics
• provide magnetic field fluctuations which have
  components at different frequencies

Area under curve is total power to cause
transitions
Power to drive transitions
frequency
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Spin Lattice Relaxation

• For T1 relaxation need transitions driven at
  Larmor frequency

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FAST MOTION
tc
Power to drive transitions
?o
frequency
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HALF FAST MOTION
tc
Power to drive transitions
?o
?o tc 1
frequency
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SLOW MOTION
tc
Power to drive transitions
?o
?o tc 1
frequency
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Spin Spin Relaxation

• For T2 relaxation also consider w 0

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FAST MOTION
tc
2
Power to drive transitions
frequency
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HALF FAST MOTION
tc
2
Power to drive transitions
frequency
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SLOW MOTION
tc
2
Power to drive transitions
frequency
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How does this go to relaxation times (BPP)
T1 T2 T1r (s)
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How does this go to relaxation times (BPP)
T1 T2 T1r (s)
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How does this go to relaxation times (BPP)
T1 T2 T1r (s)
(actually water freezes at b 3.6)
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What happens if water becomes restricted
T1 T2 T1r (ms)
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What is a Fricke gel dosimeter ?
hn
89 bulk water
Fe2
5 gelatin
Fe3
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Water in different environments not isolated
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Water in different environments not isolated
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Water in different environments not isolated
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Water in different environments not isolated
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In FAST EXCHANGE only one magnetization is
observed
l
i
Fract. water hydrating radn product
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In FAST EXCHANGE only one magnetization is
observed
NOTE in gel dosimeters T1 or T2 of bulk water
component is long (R1 R2 small) The
hydration water component DOMINATES the observed
relaxation
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Fricke gels
?S 658 ?I

• With iron ions we get paramagnetic enhancement
• Relaxivity change in R per unit concentration
  of paramagnetic species
• For Fe ions ionic radii and timescale (determined
  by electron spin relaxation rate) differ

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Ferrous and Ferric Relaxivity in water
Podgorsak and Schreiner (1992)
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Ferrous and Ferric Relaxivity in water
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What is a polymer gel dosimeter ?
hn
89 bulk water
momomers
5 gelatin
Large polymers
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What is a polymer gel dosimeter ?
hn
89 bulk water
John point this out!!!
momomers
5 gelatin
Large polymers
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What is happening to the dose dependent water now
T1 T2 T1r (ms)
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PAG without gel (a model system)
Babic and Schreiner. 06)
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But magnetization may exchange
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Baldock et al., 1999
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NMR of gel dosimeters (some points to take home)

• Dosimeters are mainly water
• Some of the water id perturbed by some species
  that is dependent of dose
• Bulk water is a poor relaxer (both T1 and T2 are
  long)
• Water hydrating the dose affected species if
  usually a far better relaxer

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NMR of gel dosimeters (some points to take home
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• So the overall relaxation mediated by the
  exchange of water between environments drives
  relaxation
• In Fricke gels T1 is a fine probe as R1
  relaxivity good
• In polymer gels better to probe T2 which is more
  dependent of structure

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