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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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