Let me welcome you to this lecture number 4. If you are just
joining us, may I advise that you read from MRI lecture 1 to be able to
sequentially follow the discussions for ease of understanding. We made a lot of
progress during our last discussion and we did introduce the concept of
resonance. But before we continue, let’s have a brief recap of what we
discussed so far.
MRI is made possible because of inherent physical ability of
hydrogen (abundant in the body) to exist as active MR nuclei. The magnetic moments
of these nuclei are able to align in either of high spin down or low spin up
energy level with the external magnetic field Bo when placed under
its influence and also exhibit precession in the axis of B0 with a precessional frequency unique to only hydrogen.
When a RF pulse with a frequency matching the precessional
frequency of hydrogen is applied, energy is transferred to the low energy spin
up magnetic moment and they move up to become high energy spin down magnetic moment. This does something very significant. The magnetic
moments or NMV are no longer aligned to the external magnetic field Bo
but flipped to an angle. This angle is called flip angle and depends on the
amplitude and duration of the RF pulse. If an RF pulse of 900 is
used, the NMV is flipped to a plane 900
to the external magnetic field and this is what is described as the
transverse plane. Before I continue, let me explain this. The external magnetic
field is on a plane generally accepted as the horizontal plane. A 900 RF
pulse flips the NMV 900 to lie perpendicular to the Bo
and this has been generally accepted as the transverse plane. Where RF pulses
below or above 900 are used, the reference transverse plane which is
900 and perpendicular to the Bo/ horizontal plane is
still applicable.
Another significant phenomenon that results from resonance
is that the magnetic moments are now in phase. This is termed phase coherence
and is a very important concept in MRI. Let me explain what a phase coherence
means. When the magnetic moments are placed in the external magnetic field Bo,
they precess along the precessional path of the external magnetic field Bo.
However, because the different magnetic moments possess different amounts of
energy, their speed of precession is
different and hence they are at different points (phase) along the precessional
path at any particular time. For example, assuming the magnetic moments are
cars with different engine power travelling on the highway. The more powerful cars
are ahead (at farther locations on the road) than the less powerful. That is to
say that at each time you check their location, they are in a different place
depending on their engine power and speed but along the same highway. What
resonance does is that magnetic moments aside been given more energy are
flipped and hence the less energy ones are in front of the precessional path
while the more powerful are behind. The cars are reversed and the faster cars
are now behind and have to drive up to the slower ones in front and a point is
reached where all the cars are at the same place at the same time. When this
point is reached, the entire magnetic moments are now precessing in sync and
can be found at the same point along the precessional path at a given time i.e.
they are in phase . Remember, this precession is no longer on the precessional
path of Bo having been flipped to the transverse plan. This phase
coherence is important because you can not get any MR image signal without it.
Let’s proceed forward. Resonance flips the NMV into the
transverse plane. These magnetic moments now precess at same Larmor frequency
but now on the transverse plane. Remember what we said about faradays law of
electromagnetic induction. Now we have magnetism and motion and to fulfil
faradays law, a charge is induced in the receiver coil placed in the transverse
plane. This charge has same frequency as the Larmor frequency and the magnitude
depends on the magnitude of the magnetization that has been flipped down to the transverse plane. The
charge induced in the receiver coil placed in the transverse plane is used in
image formation.
Let me summarize all what we have learnt so far. When the
body is placed in an external magnetic field, the magnetic moments of the
active MR nuclei align with the external magnetic field or Bo in the
horizontal plane. This cause another form of movement called precession along
the path of the external magnetic field. The alignment of the magnetic moments
in either high energy spin down or low energy spin up creates a quantity called
net magnetic vector NMV which is just the energy difference between the low
energy and high energy magnetic moments. Applying a radiofrequency pulse (RF)
with the same frequency as the Larmor frequency of hydrogen will cause the low
energy magnetic moments to acquire energy and move up to become high energy
spin up magnetic moments. This causes the NMV to be flipped to the transverse
plane (assuming the flip angle is 90o. The precession of the NMV
continues on the transverse plan but a phase coherence is established. A
receiver coil placed in the transverse plane will have an induced current with equal
frequency and amplitude as the frequency and amplitude of the flipped magnetization
on the transverse plane. In our next discussion, I am going to talk about how
fat and water are differentiated in MRI. This is called image weighting
