Relaxation The Key to Everything

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Relaxation(The Key to Everything!) 
and I dont mean lying on the couch!
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• What causes T1 relaxation?
• Why measure T1?
• How can it be measured?
• What are different kinds of T1?

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What causes T1 relaxation?
Relaxation is the transfer of energy from the
excited spins (protons) to the lattice
(spin-lattice relaxation).
Requires the presence of fluctuating fields at
the Larmor frequency to allow resonant transfer
of energy.
These fields are caused by molecular motion or
tumbling (translation, rotation or vibration)
in the environment in which the spin resides.
The local field experienced by a molecule
changes when the molecule reorients due to
motion.
T1 relaxation time is a measure of the time
taken for the magnetization to reach thermal
equilibrium along Z axis
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Hence T1 relaxation time is an indicator of the
structure and environment of the molecule.
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Long
T1 and T2 at short correlation times
T1
T1 or T2 relaxation time
T1 minimum
T2
Short
Fast motion Short tc
Slow motion Long tc
Correlation time
optimal frequency for T1 relaxation (MHz
frequencies)
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T1 is called spin-lattice or Longitudinal
relaxation
TR
M M0 (1-exp(-t/T1))
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Paramagnetic ions generate very large lattice
fields at the Larmor frequency thereby
considerably shortening the T1 relaxation. This
is the mechanism of the action of T1 shortening
due to contrast agents..
Because the Larmor frequency depends on the
strength of the external magnetic field, T1 is
field-strength dependent. Generally, T1 lengthens
with increasing field strength.
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Why measure T1?
To determine the repetition time (TR) to get
optimum contrast for the tissue of interest
T1 weighted contrast is a result of complex
inter-related physical changes. The signal
intensity is variable and interpretation can
be subjective. T1 relaxation provides a direct
and objective metric of changes in pathology of
tissue.
If T1 relaxation times are not taken into account
it can lead to erroneous interpretation of
signal behavior.
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Image Contrast and T1 Relaxation
TR500ms
TR250ms
TR750ms
TR1000ms
Shorter TRs maximize differences in T1
relaxation, generating tissue contrast Longer TRs
minimize differences in T1 relaxation, reducing
T1 tissue contrast
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How to measure T1
Accurate T1 relaxation time measurements can only
be obtained if the signal is allowed to recover
to full longitudinal magnetization
(i.e. TR 5T1)
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Inaccurate but reproducible?
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Two flip angle 3D SPGR method
The signal from a spoiled gradient imaging
sequence with flip angle (a) is given by the
following relation (assuming TE ltlt T2),
where
T1 maps were calculated by substituting the ratio
of steady state signal intensities taken at two
discrete flip angles, 40o and 8o
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T1 relaxation maps
This procedure generates high resolution, whole
brain quantitative T1 relaxation maps with good
signal to noise ratio.
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Reproducibility of T1 metrics derived from the
SPGR method
A high degree of reproducibility ( 1) is seen
in the metrics derived from relaxation maps
obtained with the 3D SPGR method. It incorporates
measurement, biological, inter and intra subject
variability.
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(a)
(b)
(c)
(d)
Figure Results of the T1 map calculation and
segmentation procedure from the same slice from a
normal control above and an MS patient below.
(a) T1 weighted image (b) T1 map. Segmentation
into (c) white matter and (d) gray matter.
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What are different kinds of T1?
As many as there are different structures!
-- free water, bound water --
white matter, gray matter -- chemical
compounds (eg. NAA, Creatine)
Important to have an knowledge of underlying T1
for the region of interest
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The behavior of T1 relaxation can follow
unanticipated trends between different molecular
species and in normal and disease conditions