Transcript An Introduction to Neuroimaging Paul Armitage SHEFC Brain Imaging Research Centre

An Introduction to Neuroimaging
Paul Armitage
SHEFC Brain Imaging Research Centre
Department of Clinical Neurosciences
The University of Edinburgh
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Overview
• Structural & functional imaging
• Different imaging modalities
• Advantages and disadvantages
• Magnetic resonance imaging
• Structural (T1, T2, PD)
• Blood flow
• Perfusion
• Diffusion
• Permeability
• Functional
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Imaging Modalities
Advantages and Disadvantages
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Acquiring Medical Images and Signals
• Imaging modalities
• X-ray computed axial tomography (CT)
• Ultrasound
• Single photon emission computed tomography (SPECT)
• Positron emission tomography (PET)
• Magnetic resonance imaging (MRI)
• Complementary techniques
• Electroencephalography (EEG)
• Magnetoencephalography (MEG)
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X-Ray Computed Axial Tomography
• X-ray based imaging modality.
• Same basic principals as conventional film based
photography (light  x-ray  EM wave).
• However, in CT projections are taken to reconstruct a series
of tomographic images.
Normal brain
Ischaemic Injury
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Haemorrhage
CT: Advantages and Disadvantages
• Advantages
• Fast.
• High resolution.
• Relatively cheap.
• Readily available.
• Disadvantages
• Uses ionising radiation.
• Limited for functional imaging applications.
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Ultrasound
• Sound waves are produced from a transducer.
• The waves are reflected back towards the probe at tissue
boundaries.
• Different tissues have different reflective properties giving
anatomical information.
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Ultrasound: Advantages and Disadvantages
• Advantages
• Fast (Real time).
• Very cheap.
• Portable.
• Disadvantages
• Low image quality.
• Diagnostic success can be highly operator dependent.
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Single Photon Emission Computed Tomography
• Uses labelled radioactive tracers.
• The tracers target a specific organ or ‘function’ and decay by emitting a
gamma-ray (photon).
• A gamma camera is then used to detect the emitted gamma-ray and
create an image.
CBF
Diagnosed with
clinical depression
Following recovery
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SPECT: Advantages and Disadvantages
• Advantages
• Good for targeted or functional imaging.
• Relatively cheap.
• Disadvantages
• Uses ionising radiation.
• Extremely low resolution.
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Positron Emission Tomography
• Uses positron emitting radioactive tracers.
• The emitted positron annihilates with an electron and emits 2 photons,
each travelling in opposite directions. These photons are detected.
• The tracers have very short half lives (minutes) and therefore require
and on-site cyclotron facility.
• Metabolic processes are studied using tracers such as 15O and 18FDeoxyglucose (FDG).
Brain Tumour (Astrocytoma)
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PET: Advantages and Disadvantages
• Advantages
• Excellent for functional imaging.
• Disadvantages
• Uses ionising radiation.
• Low resolution.
• Requires on-site cyclotron facility.
• Very expensive.
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Magnetic Resonance Imaging
• Uses intrinsic magnetic properties of water protons.
• Protons have a resonant frequency determined by the magnetic field [B0
= 1.5T gives 0 = 63 MHz (RF)].
• Applying RF radiation then excites the protons and their resonance
signal can be measured.
• The protons then relax with characteristic times (T1 & T2) which are
dependent upon the tissue type.
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MRI: Advantages and Disadvantages
• Advantages
• Fairly good resolution.
• No ionising radiation.
• Can perform structural and functional imaging.
• Disadvantages
• Noisy and ‘claustrophobic’.
• Expensive compared to CT.
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Magnetic Resonance
Imaging
Structural and Functional Imaging in a Single Scanner
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Structural Imaging
• Contrast is determined by imaging sequence used.
Proton Density
Weighted
T1-Weighted
T2-Weighted
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Clinical Examples
• The different imaging sequences can be used to highlight
different anatomical features or different pathologies.
T1
T2
PD
Tumour - easily seen on
T1, T2 and PD
Subtle lesions - clearly
seen on T2, difficult to
identify on T1
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Blood Flow Imaging (MRA)
• MR Angiography uses signal intensity to image arterial
structure either with or without a contrast agent.
• Blood velocity can be encoded in the phase of the MR
signals.
• This enables quantitative measurements of blood flow.
• Can be used to identify arterial diseases, stenoses and
aneurysms.
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Vessel Imaging (MRA)
Above: Transverse MRA Acquisition
Right: Maximum intensity projection
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Blood Flow Quantification
Phase contrast
MRA Image
Right: Flow
velocity vectors
Far Right: Wall
shear stress vectors
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MR Perfusion Imaging
• Measures blood flow in the capillary network.
• Perfusion is measured using MR by following a tracer
through the capillary network.
• The tracer can be either a contrast agent or magnetically
labelled protons.
• Gives measurements of:
• rCBV - regional cerebral blood volume.
• rCBF - regional cerebral blood flow.
• rMTT - regional mean transit time.
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Perfusion Images
• Perfusion Images Pre and Post thrombolytic therapy.
Pre tPA
18h
Post tPA
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Diffusion Imaging (DTI)
• Random Brownian motion of water molecules.
• Diffusion is restricted by cell membranes.
• Imaging diffusion gives information on white matter fibre
tracts, oedema etc.
• Applications in acute ischaemia, brain tumours etc.
Restricted
Diffusion
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Diffusion Maps in Stroke
T2
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DWI
ADC
3D Illustration of Diffusion
Sphere
Ellipsoid
Isotropic Diffusion
Anisotropic Diffusion
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Diffusion Ellipsoid Map of the
Brain
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White Matter Segmentation Using DTI Data
Fractional anisotropy
White matter segmentation
• Measuring diffusion in different directions enables white
matter fibres to be elucidated.
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Fibre maps
Diffusion Anisotropy Image
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Principal Fibre Direction Map
DTI Fibre Tracking in the Human Optic Radiations
• Uses DTI data to assess
fibre connectivity
• Image shows 1% most
likely tracts originating
from a given seed point
lateral to each LGN.
DTI tracking images appear courtesy
of Geoff Parker, University of Manchester
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DTI Based Fibre Tracking
• Human corticofugal /
corticospinal projections
originating from a seed
point in the middle of
the right cerebral
peduncle.
• The eventual goal of
fibre tracking is to
evaluate a probability or
likelihood for
connection between
given regions of the
brain.
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Permeability Imaging
•
•
•
•
Inject patient with contrast agent (e.g. Gd-DTPA).
Acquire a dynamic series of images over a period of  10 minutes.
Observe contrast agent uptake in different regions.
Rise and fall of curve can be related to cell permeability via a
pharmacokinetic model.
1.2
1
MALIGNANT
• Tumours – high and fast uptake.
• Normal tissue – slight uptake.
• Fat – no uptake.
SIGNAL ENHANCEMENT
0.8
BENIGN
0.6
0.4
NORMAL
0.2
FAT
0
0
1
2
3
4
5
6
7
8
TIME (mins)
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9
10
Example Pre / Post Treatment Images of the
Breast
Signal Enhancement
Pre-Chemo
Post-Chemo
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Permeability Maps
Functional Imaging (fMRI)
• Mismatch between blood delivery and oxygen uptake in
activated neurons.
• Haemoglobin and oxy-haemoglobin have slightly different
magnetic properties.
• Brain Oxygen Level Dependent (BOLD) contrast in MR
images.
• Cognitive and clinical research.
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Principle of Functional MRI
Brain images are
collected rapidly and
continually whilst the
subject carries out a
“cognitive paradigm”
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fMRI Procedure
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Hemifield Visual Stimulation
R
L
left
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R
L
right
fMRI - Motor Task
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Summary
• Medical imaging is now capable of giving a lot more than
just conventional structural information.
• Functional processes can be analysed:
• Blood Flow (in major vessels).
• Perfusion (in capillary network).
• Diffusion (water mobility).
• Permeability (cell interactions).
• fMRI (changes in blood oxygenation).
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Further Reading
• Functional Brain Imaging.
Orrison WW, Lewine JD, Sanders JA, Hartshorne MF.
Mosby 1995.
• Physics for Diagnostic Radiology.
Dendy PP, Heaton B.
IOP 1999.
• The Physics of Medical Imaging.
Webb S.
IOP 1988.
http://www.dcn.ed.ac.uk/studentnotes
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