Tissue interactions MRI : part 3

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Tissue interactions MRI part 3

• Guy Marchal

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Contrast agents

• Endogeneous tracers
• Artificial tracers

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CA in MRI

• MR CA are not visible as such
• MR CA are only indirectly visible by their effect
  on proton relaxation
• T1
• T2
• Many different mechanisms influence relaxivity
• MR is 1000 times more sensitive to CA then XR,
  but 1000 times less than nuclear medicine

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CA in MRI

• Based on elements with a large number of
  unpaired electrons on the outer atomic orbits
• Lanthanide
• Gd 3 ( 7 unpaired electrons)
• Transition metals
• Mn 2 ( 5 unpaired electrons)
• Fe 5 ( 5 unpaired electrons)

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Molecular structure
Trade name OMNISCAN Generic name gadodiamide
injection Chemical name Gd-DTPA-BMA
(C16H28GdN5O9 xH2O)
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Types of MR contrast agents

• T1 relaxation agents
• T2 relaxation agents
• Paramagnetic
• Superparamagnetic

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Typical signal behaviour Paramagnetic agents
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Typical signal behaviour Paramagnetic agents

• Always combined T1 and T2 effect
• Increasing the injected dose does not necessarily
  mean a higher SI
• Dose optimization studies are a must in MRI

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Artificial tracers

• non specific
• specific

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Use of aspecific contrast media Gd chelates

• Dynamic multi-phase contrast- enhanced MRI
• arterial phase ( 25 sec )
• portal venous phase ( 70 sec )
• delayed phase ( 5-10 min )

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Vascular phases
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tumor of the pancreatic head compression /
invasion of the veins
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Stent iliaca communis /externa right
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Capillary phase
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Lungperfusion
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HCC
Courtesy B.Op de Beeck.VUB
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Multiple meningioma
PERFUSION !
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Diffusion
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Brain metastases
DIFFUSION
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Delayed enhancementAcute Inferoseptal Myocardial
Infarct
Strong T2-weighting TIR-TSE
Moderate T2-weighting
CE-MRI
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Artificial tracers

• non specific
• specific

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Specific contrast mediaHepatocyte - directed
agents

• Gd - BOPTA
• EOB -DTPA
• Mn DPDP
• Uptake modulated by degree of preserved
  specific metabolic activity

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Gadobenate diglumineGd-BOPTA
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Function
PV
BD
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BOPTA
0 sec
0 sec
30 min
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Serous cystadenoma pancreas
pre
T2 Haste
2DFLASH
post
Teslascan
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• The different hepatocyte - directed agents were
  all initially designed to do the same job
  improving the detection of focal liver lesions.
• The clinical trials however showed marked
  differences between these agents, particularly in
  terms of lesion characterization.

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HCC MnDPDP
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Effect of access on relaxation

• Compartments
• Intelligent tracers

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Effect of access on relaxation

• Compartments
• Intelligent tracers

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Molecular imaging using hyperpolarized 13C

• MRI provides unsurpassed soft tissue contrast,
  but the inherent low sensitivity of this modality
  has limited the clinical use to imaging of water
  protons.
• With hyperpolarization techniques, the signal
  from a given number of nuclear spins can be
  raised more than 100 000 times. The strong signal
  enhancement enables imaging of nuclei other than
  protons, e.g. 13C and 15N, and their molecular
  distribution in vivo can be visualized in a
  clinically relevant time window.

K Golman, PhD1, L E Olsson, PhD1, O Axelsson,
PhD1, S Månsson, PhD2, M Karlsson, PhD1 and J S
Petersson, PhD1 British Journal of Radiology
(2003) 76, 118-127
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Molecular imaging in vivo
Demonstrating gene transfection using MRI. In
this technique tumour cells were engineered to
overexpress the transferrin receptor (a cell
membrane receptor involved in regulating cellular
iron uptake). As a result the tumour
overexpressing transferrin receptors accumulated
iron in the form of MIONs (monocrystalline iron
oxide nanoparticles).
A S K Dzik-Jurasz, PhD, FRCS, FRCR British
Journal of Radiology (2003) 76, 98-109
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"smart" MRI contrast agents

• In vivo use of a gadolinium based smart contrast
  agent to demonstrate gene transfection.
• When the enzyme -galactosidase is expressed in
  engineered cells, the -galactopyranose ring
  protecting the Gd3 is cleaved allowing bulk
  water access to the paramagnetic gadolinium ion.

A S K Dzik-Jurasz, PhD, FRCS, FRCR British
Journal of Radiology (2003) 76, 98-109
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"smart" MRI contrast agents

• Example of a "smart" MRI probe. Ca2 causes a
  conformational change in the molecule such that
  access of bulk water to the Gd3 can be accessed
  or denied.

A S K Dzik-Jurasz, PhD, FRCS, FRCR British
Journal of Radiology (2003) 76, 98-109
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Molecular imaging using hyperpolarized 13C
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Molecular imaging using hyperpolarized 13C
Pictorial description of the orientation of the
nuclei at thermal equilibrium and in the
hyperpolarized state. In the figure, the
magnetic field (B0) is directed vertically
upwards.
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Molecular imaging using hyperpolarized 13C
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Molecular imaging using hyperpolarized 13C
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