MRI and MRS - from tissue to molecules Tools for translational research

Marc Dhenain UMR-CEA-CNRS-9199 – Fontenay aux Roses [email protected]

Master : Ingénierie de la santé et sciences du médicament Spécialité : Biologie et développement cellulaires http://marc.dhenain.free.fr/Teaching/Teaching.html

Overview

 Presentation of MRI  Clinical MRI  Small animal MRI  Cellular and molecular imaging

Presentation of MRI

1. Polarisation

2. Resonance

3. Relaxation

4. Localisation

To do MRI, you need…  A magnet  Radiofrequency probes  A gradient system

 Imaging sequences and a computer

Magnets and magnetic fields  Unit of measure of the magnetic fields  Gauss  Tesla : 1T = 10 000 G

 Earth magnetic field : 0,5 G  Used MRI  In clinic : 1 T : 20.000 times more intense than the

earth magnetic field  In research Up to 21 T

MRI and risks…

Description of a MR Image

Field of view (FOV) : 30 cm Matrix : 128

Resolution : 30/128 = 2.3 mm

H

Magnetic moment (or spin) 1. Polarisation = vector

H

Water protons

Random orientation of the spins otside the magnetic field

Magnet Bo

Outside the magnetic field

Spin alignment in the magnetic field

Bo

1. Polarisation

Use of a radiofrequency probe RF probe

Bo

2. Resonance

Tilting the spins RF probe

Bo

2. Resonance

Return to the equilibrium (relaxation)

Bo

2. Resonance

Emission of a radiofrequency wave

Origin of contrast in MRI ?

3. Relaxation

Origin of the contrast in MRI Observation of magnetization after some time...

 The intensity of the tilted magnetization 

evolves over time Different evolution in different tissues  Origin of the contrast between tissues

 Multiples causes of evolution  different types of contrasts

3. Relaxation

Longitudinal relaxation : T1 contrast Observation of the Mz part of the magnetization after some time...

Return to the equilibrium of the magnetization on Mz T1 = Time for a return to equilibrium at 63% 3. Relaxation

Longitudinal relaxation : T1 contrast

Source : http://www.simplyphysics.com/IntroToMRI.html 3. Relaxation

T1 contrast between different tissues

3. Relaxation

Transversal relaxation : T2 contrast Mxy part of the magnetization

T2 = Time to reach 37% of its initial value T2 relaxation depends on dephasing

3. Relaxation

Transversal relaxation : T2 contrast Mxy part of the magnetization

Source : http://www.simplyphysics.com/IntroToMRI.html 3. Relaxation

T2 contraste between tissues

3. Relaxation

3. Relaxation

Choice of the observed contrast The user can program sequences to select a particular contrast in the image.  Choice of the sequences  Spin echo sequence  Gradient echo sequence  Inversion recovery sequence  Diffusion weighted sequence …

 Choice of key parameters in the sequence (TR, TE…)

3. Relaxation

Example of different contrasts Spin echo images

T1

T2

FLAIR

Gradient echo image Sensitivity to local heterogeneity of magnetic fields

Artefact caused by dental filling

Functional MRI

Localization of the signal in images ?

4. Localisation

Signal localization in the images Role of the gradient system

 How can we measure magnetization in different location of space

  Create different magnetic fields in different location

 "Covering" of the space with different magnetic fields 4. Localisation

Nécessité des systèmes de gradients  Systèmes permettant de modifier le champ magnétique en différents points de l ’espace...

4. Localisation

Concl. : Some key parameters in an image

 Field of view ; Acquisition matrix (ex. 128x128)  Averaging  Imaging sequence (Gradient echo, Spin echo…)  Acquisition parameters – TR, TE, …

NMR Spectroscopy Identification of different chemical compounds on the basis of the analysis of the frequency of the samples

Spectrum of the brain of a normal person

Overview

 Presentation of MRI  Clinical MRI  Small animal MRI  Cellular and molecular imaging

Clinical MRI

 Anatomic MRI  Diffusion MRI  Functional MRI

Anatomic MRI T1-W

T2-W

T1-W

T2-W

Cardiac MRI

http://www.nums.nwu.edu/feinbergcardio/mri/ http://www.gemedicalsystems.com/rad/mri/products/cvi/cvi_apps_ventfunc.html

MR angiography

sténose http://www.gemedicalsystems.com/rad/mri/products/mri/mri_images.html

Imaging of vascular permeability Increased permeability of the blood vessels in the tumors Use of macromolecular contrast agents (ex. Chetated Gd-DTPA with polylysine or albumin)

Low grade tumor

High grade tumor

A-C : MRI ; B-D : Vascular permeability maps ; Shames DM, MRM, 1993 Source : http://www.synarc.com/Oncology/Oncology.htm

Diffusion MRI 

Early marker of tissue modifications



Detection of tissue orientation

Brownian movement of water r2 = 6 D t (Einstein equation) r = Travelled distance D = Diffusion coefficient = 2.2x10-3 mm2/sec for H20 (@ 25 °c) t = time

d = water movement during a time ‘ t ’

 Early changes during acute ischemia

Measure of diffusion anisotropy (DTI)

Diffusion anisotropy in oriented tissues

Visualisation of white matter

White matter dissection

Corpus callosum

Corona radiata Optical Tract

Zhukov and Barr, "Oriented Tensor Reconstruction: Tracing Neural Pathways from Diffusion Tensor MRI" ; IEEE Visualization 2002

Functional MRI Hemoglobin

- oxy-Hb = diamagnetic  No effect on MRI signal - deoxy-Hb = paramagnetic  Decrease MRI signal

Effect of the concentration of oxy-Hb on NMR signal

- oxy-Hb = diamagnetic  No effect on MRI signal - deoxy-Hb = paramagnetic  Decrease MRI signal Bandettini and Wong, Int J Imaging Systems and Technology, 6, 133, (1995)

BOLD signal Blood Oxygen Level Dependent signal

 neuronal activity   blood fsanguin   oxyhemoglobin   T2*   MRI signal

Basic state

‘normal’ MRI signal

Activated state

‘Increased’ MRI signal

Source: http://www.fmrib.ox.ac.uk/fmri_intro/physiology.html

Example of the first images

Kwong K K et al., PNAS, 1992

Functional MRI Pas d’activation neuronale

Hémoglobine désoxygénée

Activation neuronale

Flux sanguin Hémoglobine oxygénée

Stimulation visuelle Analyse statistique (signal versus stimulus)

Signal IRM ‘normal’

Signal IRM ‘augmenté’

Application in surgery

Imagerie fonctionnelle en l’absence de stimulus

Identification de réseaux neuronaux

Fox et al., Front Syst Neurosc 2010.

Réseaux neuronaux en mode repos

 Actifs chez un individu qui n’effectue aucune 

tâche précise = qd activité mentale non dirigée Inactivés quand tâche attentionnelle engagée

Réseau du mode par défaut  Fonction: Intégrer des informations sensorielles multimodales et affectives pour guider des comportements d'anticipation Informations sensorielles multimodales

Intégration de l’information

Manipulation flexible de l’information

Mémoires

 Quid de sa présence chez l’animal ?  Accès possible à l’état mental intérieur d’un animal

Clinical application of spectroscopy Ex of differentiel diagnostic of cerebral lesions

Anaplastic Glioma III

Abscess

T2*

 Cho  Ins

 NAA

T2*

 Lip 

other metabolites

http://www.mpibpc.gwdg.de/abteilungen/NMR/conferences/ismrm00_tumor.html

Overview

 Presentation of MRI  Clinical MRI  Small animal MRI  Cellular and molecular imaging

Small animal MRI Need for a better resolution

Source: R. E. Jacobs (Caltech)

Cerebral lesions in mouse lemurs

From fundamental research to therapies Understand mechanisms mechanism

Translational Translational Medicine --Animal Animal models models -- Biomarkers Biomarkers

Clinical Clinical research Research

Embryo imaging

Prevent from slicing the embryo

Post mortem imaging

http://www.mouseatlas.caltech.edu

Dhenain et al, Dev Biol. 232, 458-470, 2001

Cellular and molecular imaging

 Objective Visualise key events in biology

 Biological processes Angiogenesis, apoptosis, inflammation

 Cellular therapy  Stem cells

 Molecular imaging  Gene expression, Gene therapy…

 Cell function  Synaptic activity, pH changes…

Methods  Microscopic MRI  Visualisation of very small structures

 How can we detect the structures of interest  Natural contrast ?  Use of specific contrast agent  Sensitivity ?

Strategies to detect small structures Exemple of the detection of amyloid plaques Amyloid plaques : Lesions of Alzheimer's disease

Extra-cellular deposits of amyloid peptides

Size : 20 à 100 µm

Ultra-high resolution Detection of amyloid plaques by MRI 6 mois

9 mois

14 mois

20 mois

Petiet et al, Neurobiology of Aging, 2012

Ultra-high resolution Detection of amyloid plaques by MRI

Petiet et al, Neurobiology of Aging, 2012

Overview

 Presentation of MRI  Clinical MRI  Small animal MRI  Cellular and molecular imaging

Cell detection  Injection of contrast agent  Injection of cells labelled with a contrast agent  Optimisation of protocols to label the cells  Spontaneous internalization of contrast agents  Use of lipofection agents (FuGENE)  Magneto-dendrimers  Coupling to a translocator peptide (TAT)  Increased number of receptor to the cells (Transferrin recept.)

 Exemples of application…

Spontaneous internalisation of a contrast agent

T Lymphocyte + SPIO (Iron particle)

MRI (T2*-weighted) Dodd, Bioph J. 1999

0.25 106

1 106

2 106 Cell T/ml

Imaging of atherosclerose plaques

Plaque vulnerability is associated to their macrophages load

Macrophages detection (USPIO)

Labelling of the USPIO

Before USPIO

After USPIO Labelling of the macrophages (RAM11)

Tsuchiya K, Eur J Radiol, 2013

Optimisation of the labelling with a lipofection agent (FuGENE) Application to detect stem cell migration

USPIO + Fugene + Embryonic stem cell Rat model of stroke

+0d Lesion

+6d

+8d M Hoehn, PNAS, 2002

Internalisation of contrast agents Creation of cells overexpressing a receptor (gene reporter)

 Over-expression of transferrin receptor  Increased endocytosis of the contrast agent linked to transferrin Récepteur de la transferrine (ETR)

R. Weissleder et al., Nature Medicine, 2000

Application: Targetting of tumors Rat + Tumor (injected) overexpress Transferrine receptor (ETR) Injection of Tf-MION (endogen iron is not sufficient to detect the tumor)

Tumors ETR+

ETR-

T1w

T2w R. Weissleder et al., Nature Medicine, 2000

Detection of gene expression by MRI

Coupling of the transferrin receptor to other genes (gene reporting) Agent reporting gene expression

R. Weissleder et al., Nature Medicine, 2000

T. Ichikawa et al. Neoplasia, 2002

Visualisation of gene expression Exemple of a smart contrast agent (EgadMe)

Enzymatic cut -galactosidase (gene = Lac Z)

Invisible by MRI

Cleaved form Visible by MRI

A. Y. Louie et al. Nature Biotech, 2000

Xenopus Embryo EgadMe

GFP

EgadMe ARNm -gal GFP

-gal staining

MRI A. Y. Louie et al., Nature Biotech, 2000

From fundamental research to therapies Understand mechanisms mechanism

Translational Translational Medicine --Animal Animal models models -- Biomarkers Biomarkers

Clinical Clinical research Research

References (MRI theory)

 The basics of MRI  http://www.cis.rit.edu/htbooks/mri/  www.imaios.com/fr.

 Simply Physics http://www.simplyphysics.com/MAIN.HTM http://www.simplyphysics.com/MRIntro.html