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Biomedical imaging Macroscopy samenvatting UA
- Cours
- Biomedische beeldvorming
- Établissement
- Universiteit Antwerpen (UA)
Duidelijke samenvatting van het macroscopy deel van biomedical imaging. Toelichtingen met plaatjes!
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Biomedical Imaging Key points MacroscopyIntroduction:
Imaging techniques based on ‘waves’
o Imaging techniques are based on waves/light with different frequencies.
o Higher frequency smaller wavelength more ionizing (UV & X-rays & γ -radiation)
o Lower frequency larger wavelength less ionizing (infrared & visual light)
Processes we can visualize (anatomy vs molecules)
o To image small molecules we use more ionizing radiation which can be damaging.
o We will discuss techniques, they al use a different radiation from the spectrum:
o MRI radiowaves
o PET/SPECT γ -radiation
o CT X-rays
o Ultrasound soundwaves
o A perfect imaging technique would have:
o High molecular sensitivity (allows us to visualize small differences)
o Small spatial resolution (allows us to see deep into tissue)
o However, there is no technique that has both, it is a compromise between the
two: every technique will visualize different processes:
o We often combine techniques to visualize multiple processes in one image.
Chapter 1: MRI
MRI contrast mechanism
o MRI is an imaging technique where the patient is put into a big cylindric magnet. The
big magnet is constituted of different coils that are oriented in different directions
(x-,y-,z-axes). Within these coils there is a smaller cylindric radio-frequency coil that
can emit and detect radiofrequency waves. This radio-frequency coil can move along
the y-axes to scan the patient.
o MRI scans can be used for humans (1-3 Tesla), but they can also be used on animals
for pre-clinical studies. These scanners need a much more powerful magnet, since a
rodent is much smaller than a human (divide by a factor of 600/2400). The pre-
clinical scanners can have 7-9.4 Tesla. However, this is not enough, thus pre-clinical
scanners offer less resolution than the clinical ones.
o The resolution of a scanner is dependent on, the magnetic field strength and the
duration of the scan.
o MRI is based on the principle of NMR = nuclear magnetic resonance.
o NMR: We use the protons of an atom. Every proton has a charge and a spin.
Protons have a positive charge.
, The spin is the fact that a proton rotates around itself. We can have a
spin of ½ or 1 or 1½ etc.
Because the protons are a rotating charge, they are an electrical
current and they will create a magnetic field with a certain magnetic
momentum ( μ). Thus, every proton is a small magnet.
However, in a normal situation all these magnets are directed
randomly thus the netto-magnetic field is 0.
We will use the magnetic field of the atoms in NMR. Thus, we can only
use atoms with an unequal number of protons. If the atoms would
have an equal number of protons (12C) the spins of the protons would
cancel each other out.
The atom that is most aberrant in humans with an unequal number of
protons is 1H/hydrogen. (There is also 23Na & 13C etc. but they don’t
have a relevant biological abundance.)
o Every voxel in the MRI-image will be made with an NMR-signal to make a
visible picture. So, what do we do in NMR to obtain a signal?
1. Normal situation: the protons spin and have magnetic momentums that
are randomly oriented and cancelling each other out.
2. We apply an external magnetic field B0, the protons will now align with the
magnetic field. Either in the same direction, or in opposite direction.
a. Same direction = low energy state
b. Opposite direction = high energy state
c. The protons are not exactly aligned with B0. They ‘precess’ (the top
is spinning). The rate of precession can be exactly determined by
the Larmor precession equation: f =γ × B0
γ = angle at which spin is present (precession)
B0 = strength of external magnetic field
f = precession frequency
d. At 1 Tesla 42.58 MHz & At 2 Tesla 85.61 MHz
3. The protons will form a longitudinal magnetic field M z (the sum of all
their small μ.) This M z is pointed in the same direction as B0. Thus, we
cannot measure it because we can’t differentiate it from B0.
4. To be able to measure it, we apply a Radiofrequency pulse that is of the
same frequency as the precession of the protons.
a. This way, the protons will flip in such a way that 50% of the
photons is in high energy state and 50% of the protons is in low
energy state. Thus, the total longitudinal magnetization M z is
reduced to zero (the protons cancel each other out).
b. Another consequence of the radiofrequency pulse is that all the
protons will precess in the same phase. Thus, a transversal
magnetization M XY is formed.
5. We can detect the transverse magnetization with a coil/antenna. (the
precessing protons will create a current that we can measure)
a. With this signal we can measure the density of protons in the
tissue!
b. However, the proton density doesn’t differ that much between
tissues. There are other signal we can measure:
, 6. The proton will want to fall back to their low energy state (parallel with B0
). There are 2 ways in which the protons will ‘relax’ after they have been
excited with the RF-pulse:
a. The protons were in phase, but now the RF pulse is gone, they will
precess out of phase again. Because they are all positively charged,
they will repel each other. This leads to the loss of the transverse
magnetization M XY . (weak signal)
T2 – relaxation = spin spin relaxation
b. The protons will flip back to the low energy state, this will lead to
the gain of the longitudinal magnetization M z again They will emit
some heath to the surrounding tissue. (weak signal)
T1 – relaxation = spin lattice relaxation
Proton, T1, and T2 MRI
o To see contrast in the tissue we can use the T1 and T2 relaxations. They will each
provide different information depending on what you want to measure.
o We can apply the RF-pulses multiple times in a row to get more information from one
scan which will lead to a higher resolution of the image.
The time between 2 RF pulses = repetition time/TR
o To measure T2 or T1 we measure the transverse magnetization signal at different
times.
The time between the RF pulse and the EM-acquisition = echo time/TE
o T1 and T2-relaxation will differ between the tissues because of the structural variety in
tissue: in water the proton molecules are attached to form H2O. Here, the protons
move rather freely so they can stay in an excited state for a longer time. However, in
fat, the protons are attached to a chain of C-atoms. The protons move less freely and
will try to return to a lower energy state more rapidly.
o By choosing the TE and TR we can visualize the T2- or the T1-relaxation.
o In fatty tissue, the protons will have a faster T2- and the T1-relaxation.
o T2-relation we need a long TR and a long TE (minimizing T1 effects)
o T1-relaxation we need a short TR and a short TE (minimizing T2 effects)
o Short TR: the fatty protons will be recovered and the second RF pulse will
produce a large transverse magnetization M XY signal that we can quickly
measure with a short TE.
The water-protons will not be fully recovered when the second RF pulse is
applied, thus the RF pulse will push more low-energy water-atoms to the high
energy-state. This will result in a netto negative longitudinal magnetization
(pointing in the opposite direction of B0) It will give a low transverse
magnetization M XY that we can measure.
o Proton density image we need a long TR and a short TE (minimizing T1 and T2
effects.)
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