Hello everyone. With the appropriate choice of imaging sequence parameters, TE, TR and
TI, we have shown how spin-spin and longitudinal relaxation provide us with unique contrast
mechanisms that allow us to control the relative signal intensities to some extent and to optimise
the image contrast between tissues. There are a number of advanced MRI methods that
utilise other contrast mechanisms. One of the most important is diffusion weighted imaging.
In water, the molecules are free to move in random directions and undergo so-called Brownian
motion -- the translation of water molecules over large distances is known as diffusion.
In biological tissue, water diffusion rates are dependent on the viscoscity and temperature,
and diffusion may be restricted by interaction with cell membranes, proteins and other structures.
In tissues with co-linear structures such as muscle fibres or neurons in the brain,
the water molecules may have a preferred direction of diffusion. As we will see in a moment,
a magnetic field gradient will exaggerate the effect of diffusion on the MR signal of
water molecules. So what is the effect of diffusion on the MR signal in imaging?
Here we consider transverse magnetization during a spin--echo sequence, firstly for stationary
spins that are not diffusing. The spin vector in the rotating frame initially does not precess
but when a field gradient is applied, it will precess at a frequency given by the new value
of the field at that point. The 180 degree pulse then rotates the vector around the axis
of the pulse, and if an identical gradient is applied again then the vector will precess
through the same angle and align along the axis where it began forming a spin echo. This
echo forms because the precession imposed by the gradient is identical before and after
the 180 degree pulse. What happens then to spins that are moving. On the left we see
the stationary spins again for comparison. For the diffusing spins, we see that as the
spin moves through the field gradient, its rate of precession changes accordingly and
following the 180 pulse, the spin may not have experienced the same field strength as
before the pulse. Therefore, the spin vector will not be aligned along the same axis as
before. For a collection of spins, this results in a reduction in intensity of the spin echo
since all spins do not refocus. So the image intensity becomes weighted by the diffusion.
The rate of diffusion is defined by a rate constant known as the Apparent Diffusion Constant
and this ADC is a property of the particular tissue. Faster diffusion will result in loss
of signal whereas restricted diffusion which reduces the ADC will result in higher
signal. So diffusion weighted imaging can help detect regions of abnormal tissue where
the tissue structure is damaged and therefore the water diffusion is altered. This is important
for the detection of stroke and cancer where the cellular integrity is lost, water is taken
into the cells and so diffusion is significantly restricted resulting in an increase in signal
compared to healthy tissue. Often the relaxation characteristics of the tissue do not change
and so the effect of the damage not may be detected in T1 or T2 weighted images.
Here is a standard T2-weighted image of a stroke victim and it is difficult to identify any
regions of damage to the brain tissue. However, if a diffusion-weighted image is acquired
then the region of the brain damaged by the infarct shows a large increase in signal due
to restricted diffusion of water in that tissue and now the full extent of the tissue damage
can be measured. In diffusion, the water molecules may be restricted to moving predominantly
in a particular direction, perhaps along muscle fibres or neurons. We can measure the predominance
of water diffusion in a particular direction and therefore determine the direction of the
fibres. In this image, the predominant diffusion direction is indicated by a different colour
depending on whether the direction of motion is side-to-side, top-to-bottom or in-and-out
of the page. We can further use this information to visualise the fibre bundles
and the connection between parts of the brain. This is information that is not obtainable
by standard imaging methods and has become important in the understanding of brain function
and connectivity and may be an indicator of neuronal integrity in diseases such as Alzheimers
Disease and multiple sclerosis. As well as random diffusion, we can also measure the
flow of spins or the perfusion of agents introduced into the body which have an effect on the
MR signal. Special contrast agents that change the relaxation properties of water spins and
therefore change the image intensity in the vicinity of the agents can be introduced by
injection into the blood stream. MRI can observe the wash-in and wash-out of the bolus of contrast
agent delivered. These contrast agents allow detection of the tissue that is highly vascular
with enhanced blood flow, such as in tumours. Perfusion imaging is an important diagnostic
method for breast cancer for example. It is also possible to encode moving spins to measure
flow, for example, in blood. In MR angiography, the signal from moving spins in blood is enhanced
by saturating spins in a slab of tissue and observing the stronger signal of unsaturated
spins that flow into the slab. A maximum intensity projection of the image then produces a magnetic
resonance angiogram. So we have seen how we can enhance image contrast by a variety of
methods, by choosing appropriate imaging parameters such at TE and TR, encoding the diffusion
of water molecules in tissue to detect changes in tissue structure, using contrast agents
to enhance the signal of highly vascular tissue and also measure flowing spins to visualise
the entangled network of blood vessels. All these contrast mechanisms play an important
role in providing MRI with the versatility and range of application that make it a key
imaging modality for the detection of tissue damage and disease in modern medical diagnosis.