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A brief introduction to MR physics
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Hydrogen/Proton A spinning top
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With and without the presence of an external
magnetic field (B0) in the z direction
Randomly oriented hydrogen atoms (spins) in
absence of an external magnetic field
The protons aligns with the external field Low
field 0.47 T, Clinical fields B0 1.5 3.0 T
Together they create a macroscopic magnetic M0
vector parallel with B0 (and z)
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Radio Frequency (RF) coils in the xy-plane (B1)
A 900 pulse flips M0 into the xy-plane
M0 aligns with the z-axis
RF on
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An oscillating FID signal in the xy-plane
(readout plane) When the protons aligns with the
external field again they give up their energy to
their surroundings. This signal is registered by
a coil (like an antenna)
RF off The spins relax back to their
equilibrium state (along z)
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T1 relaxation The time it takes for M0 to
realign with B0 after the influence of an RF
pulse. Dependent of the exchange with the media.
T1 is short for fat and long for water
T2 relaxation The time it takes before the
spins (protons) reach equilibrium after
spin-spin exchange and thereafter dephasing in
the xy-plane T2 ? T1
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The MR signal

• Intrinsic parameters T1, T2, n(H)
• Instrument parameters TR, TE
• Pulse sequences vary TR and TE
• Contrast agents manipulate T1 and T2

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Contrast Agents (CA)

• Even though MR has high specificity, CA are often
  used to better visualize specific structures and
  pathology
• Indirect effect on hydrogen spins. The relaxation
  times (T1 and T2) are reduced, thus signal
  changes
• Either T1 or T2 CAs, where T1 CAs increase the
  signal and gives positive contrast, whereas T2
  CAs reduces the signal and gives negative
  contrast in the image

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Different CAs (exogenous)

• Paramagnetic
• Positive contrast n(H) increases, T1 reduces and
  T2 increases
• Much stronger T1 than T2 reducing effect, which
  in total gives an improved T1 contrast by the
  addition of paramagnetic agents
• T1-weighted images
• Some paramagnetic ions Gd3, Mn2

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• Electron spin
• Non-paired electrons

• A magnetic moment analogous to the magnetic
  moment of protons

• The magnetic moment of an electron 600 times
  stronger than the magnetic moment of protons, and
  will therefore have an influence on them

• Change the magnetic environment of the protons,
  where T1 and T2 will be reduced

1H
Paramagnetic ion
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Efficacy in a homogenous solution

• Relaxivity r1 and r2
• The measured effectiveness of an CA to reduce T1
  and T2
• Given in (s mM)-1
• A simple linear formula
• T1 -1with CA T1-1without CA r1 CA

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T1 relaxation shorter and shorter T1 (CA)
stronger and stronger signal in a T1 weighted
image where the CA is present

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Different types

• Free metal ions are toxic in large quantities,
  that is why they are bound to a chelate. This
  shielding influence the effectiveness of the
  CA, and safety is competing with efficacy
• Few side effects does not result in severe
  allergic reactions as with X-ray CAs
• Extracellular (infarction, pathology)
• Gd-DTPA (Magnevist)
• Liver CA (pathology)
• MnDPDP (Teslascan)
• Intravascular/E.C. (few protons lt 5 of all the
  tissue protons). Large complexes which are
  trapped in the blood pool
• Gd-DTPA-BMA (Omniscan)

Small molecules
Schering,Amersham Health/GE Healthcare
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Water Exchange Theory
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Intrinsic values
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A relaxivity in each homogenous water
compartment
Intracellular relaxivity r1ic
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Monoexponential decay Inversion Recovery pulse
sequence
Mz(t) M0 ( 1 - 2e-t/T1 )
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FAST EXCHANGE
T1
?-1
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The apparent biexponential T1s are sums of the
intrinsic relaxation rates and the water
exchange in each compartment
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1
1


T
T
?
1
ic
1
ic
ic
1
1
1


T
T
?
1
ec
1
ec
ec
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SLOW EXCHANGE Apparent values
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Biexponential signal
Mz pic ( 1 - 2e-t/T1ic ) pec ( 1 -
2e-t/T1ec ))
1/? (PA)/V
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Intermediate exchange
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2 SX equation for R1ic
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2 SX equation for R1ec
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2 SX equation for pic and pec
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Calculating relaxivities, lifetimes of water and
population fractions
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EKSTRASTOFF
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E.C. Contrast Agent
Intravascular
1H20ic
?p-1
1H20p
?ic-1

Plasma
1H20CAp
CAp
?m-1
Extracellular
Vessel wall
Interstitium
1H20ic
? ic-1
?m-1
Intracellular
CAec 1H20ec
1H20CAec
?ec-1
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Conclusions

• Multi-component analyses of T1 and T2 revealed
  two compartments with different chemical
  environments in the heart cells detectable with
  0.47 T MR

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Main findings

• Two compartment model more suitable for T1
  analysis
• Close correlation between tissue Mn content and
  T1relaxation
• Slow water exchange in excised cardiac tissue
• Remarkably high relaxivity of intracellular Mn2
  ions

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Physiology and Mn content
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