Main Title Slide
Sensing Systems and Signal Processing
Dr Sidahmed Abayzeed
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Dr Kevin Webb
Magnetic Resonance Imaging
An introduction…
2003 for Sir at the University of Nottingham
MRI Overview
Basics of MR
MR spectroscopy – how it works
MR imaging as a diagnostic tool
Basics of functional MRI
– with thanks to , Sir Peter RI Centre!
Certain nuclei have
a magnetic moment
an angular momentum (spin)
Interaction with external
magnetic field
Magnetic field
Brain is composed of 73% water
Nuclear Magnetism
Consider a cubic cm of water molecules (no magnetic field applied)
→ magnetic moments are randomly oriented
From spins to magnetization
Magnetic field
Magnetic moments
align with magnetic field
From spins to magnetization
Advantages of MRI:
noninvasive (no ionizing radiation)
Magnetic Resonance Imaging
Radiofrequency
transmitter
Strong static
magnetic field
Keep strong static field on throughout the experiment
Experimental MR setup
Radiofrequency
transmitter
Strong static
magnetic field
Keep strong static field on throughout the experiment
Magnetization rotates with frequency ω
Magnetization
Experimental MR setup
Radiofrequency
transmitter
Alternating
magnetic field
Apply weak (1/10’000 Tesla) but alternating magnetic field
Perpendicular to main, static field
Effect on Magnetization only occurs if frequency of alternating field is equal to resonance frequency of nuclei
Magnetization
Experimental MR setup
Radiofrequency
transmitter
Radiofrequency
(receiver coil)
we can detect the precession and its frequency through the current induced in the receiver coil loop.
magnetic field
Experimental MR setup
Radiofrequency
Transmitter (off)
Radiofrequency
Relaxation =
return to initial conditions
Purcell & Bloch, Nobel prize in Physics 1954
Experimental MR setup
Signal has specific frequency
Signal decays (exponentially)
Measure signal
Echo time TE
Radiofrequency
transmission
Relaxation
Nuclei in different environments have different resonance frequencies
Frequency of signals
Lower frequency
Frequency of signals
Higher frequency
Frequency of signals
Two frequencies together
Frequency of signals
Simple MR experiment with a range of different frequencies
Frequency of signals
Transformation
, Nobel prize in Chemistry 1992
Protons in different molecules “see” different magnetic fields.
These different environments lead to slightly different resonance frequencies
Simple MR experiment
MR Spectroscopy – allows quantification
of metabolites in vivo
M. Stephenson, 7T human MRI,
Sir Peter R Centre, Nottingham
MR Spectrocopy
Data we get from MRS comes in the form of a spectrum. A spectrum is a plot in which the resonant frequency is given along the bottom and the signal strength is plotted at that frequency. Think of it like tuning in a radio.
The measured signal is proportional to the number of nuclei that are resonant at that frequency – able to calculate the concentrations of the chemicals in the brain.
MR Imaging
How can we get an image from the signal?
Water is the most abundant signal source in the body
Water bottles in different locations
MR Imaging
The frequency encodes the location.
Nobel prize in Medicine 2004
Locally varying magnetic field
(imposed gradient)
MR Imaging
Contrast in MR images
How can we distinguish
between two types of tissues
in same location?
Frequency is already
used for spatial encoding
and almost all signal is
from water …
MR Contrast
Magnetization
After radiofrequency transmission the signal decays exponentially depending on the type of tissue (T2 relaxation)
Measure signal
Echo time TE
Radiofrequency
Transmission (pulse)
The spins (magnetization) reverts to normal state (T1 relaxation)
MR Contrast
Timing of pulse experiment can be used to measure relaxation in different tissues
Echo time = 20 ms
Echo time = 80 ms
Relaxation
Hemoglobin provides neurons with oxygen necessary for energy production
Oxyhemoglobin contains bound oxygen
De-oxygenated hemoglobin shortens T2 relaxation times
MR Contrast
Experiment: Flashing light as experimental stimulus
MR Experiment example
neural activity blood flow oxyhemoglobin (overcompensation, i.e. relatively less deoxyhemoglobin)
lower field inhomogeneity
slower signal decay (T2) MR signal
Blood Oxygenation Level dependent (BOLD) signal
Increased neural activity
more energy required
MR Experiment example
Source: Kwong et al., 1992
Neural Activation measured by functional MRI
MR Experiment example
Activation in visual cortex
MR Experiment example
MRI Images take longer than one beat.
The heart moves, but not constantly
=> Image between the beats.
Imaging a moving target? Gated imaging
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