MR is a physical phenomenon fundamented in the mechanic and quantum properties of the atomic nucleus.
Matter is composed of atoms.
Elements that have atoms with odd number of electrons have a property known as magnetic moment or spin. This means that they have their own magnetic field. Fig. 1
If a tissue sample is exposed to a magnetic field,
its hydrogen atoms align with this external field with the property described as magnetic moment.
Alignment is a dynamic process,
in which there is a precession around the axis of the external magnetic field.
The frequency of this precession is exclusive for each element,
and depends on the intensity of the applied magnetic field.
This frequency is known as the gyromagnetic ratio .
Fig. 2
Resonance phenomenon
The way to interact with atoms in precession is through the application of a form of energy that has the same frequency as that of precession.
This form of energy is a radiofrequency pulse.
The interaction between the frequency of precession of the atoms subjected to the influence of an external magnetic field Fig. 3 Fig. 4 and a radiofrequency pulse,
produces a change in the orientation of the atoms,
which depends on the duration time of the radiofrequency pulse applied.
Alignment is a dynamic process,
similar to the movement of our planet.
It rotates on its own axis and precesses around the axis of the magnetic field,
at a specific frequency for each element and directly proportional to the intensity of the magnetic field Fig. 5 .
In the case of hydrogen,
42.5 MHz for each Tesla unit.
That is the frequency at which protons can be stimulated to obtain the resonance phenomenon.
The reason for doing this is that some transverse magnetization is always required.
Once this is achieved,
the RF pulse is interrupted,
and the atoms return to their original position - an artificial position,
created by the external magnetic field.
In the process of recovery towards the orientation prior to the radiofrequency impulse,
the atoms release the applied energy,
also in the form of radiofrequency pulses.
Through a complex process in which the described stimulus is repeated,
the signals emitted by the sample - radiofrequencies - are captured and processed to produce an image.
Basic vectorial theory:
The axis of orientation of atoms is represented as a vector.
The process of recovering the hydrogen atoms to their original precession position is divided into two main parts,
which correspond to the vector components of the orientation axis of the atoms examined.
The vertical component,
known as longitudinal,
depends on the interaction of the atoms with their environment and is related to thermal energy exchanges.
By following the behavior of this component over time,
it can be represented as an ascending exponential curve,
which is known as longitudinal relaxation time or T1.Fig. 6 .
The horizontal component,
known as transversal,
depends on the interaction of the atoms with each other.
Its behaviour in time is also exponential,
and corresponds to the transverse relaxation time or T2.
This curve is descending,
as corresponds to the vectorial component complementary to the longitudinal.
Both curves describe time constants,
and they reach two thirds of their final height when a time equivalent to 1/e has elapsed.
As it seems obvious,
the two curves happen simultaneously.
This explains why,
in some cases,
the reason why a tissue is "white" or "black" in a sequence "T1" is because of its "T2",
and vice versa Fig. 7 .
What is a sequence?
A sequence refers to one or more radiofrequency (RF) pulses,
which are applied in an orderly fashion for a certain time interval.
These processes are quite fast and are usually measured in milliseconds.
Sequence components
To describe the sequences,
a temporal diagram is used that demonstrates them as a series of events that occur in an orderly,
sequential or simultaneous manner.
The diagram can remotely resemble a physiological record,
such as an electrocardiogram,
which shows events that simulate waves and try to represent what happens each time the magnetized tissues are stimulated by radio waves Fig. 8 .
Image Formation (pulse RF sequences)
The sequential application of one or several RF pulses is the technique used to extract the information from the signals of the tissues.
There are several parameters that are relevant to acquire this information.
For the formation of the complete image of each section,
which in turn represents a matrix,
it is necessary to repeat the applied impulses several times.
In fact,
to complete each image - represented as a section - it is necessary to repeat the sequence of radiofrequency pulses as many times as rows has the final matrix Fig. 9 Fig. 10 .
Signals
Relaxation times are properties of tissues,
which we cannot modify.
By varying the technical parameters of the sequences,
we can observe preferentially one the two relaxation times,
but never completely separate them.
To facilitate the understanding of the terms "long time" and "short time",
in Fig. 11 we offer a guide,
only applicable for SE sequences.
Sequence terminology
In gradient echo (GE) sequences,
an additional parameter to be taken into account is the magnetization deflection angle or Flip Angle,
which can also be modified to obtain T1 or T2 weighting.
In fact,
in these GE sequences,
the flip angle is more important than the TR and the TE to determine the type of information acquired.
In general,
small deflection angles (<30o) produce predominantly T2 information,
and angles greater than 45o give T1 information.
TR and TE parameters are much shorter than those used in typical SE sequences Fig. 12 Fig. 13 .
Weighting basics: T1 and T2,
proton density,
susceptibility,
flow
In MRI,
there are at least five factors that determine the signal intensity of the examined tissues in a gray scale Fig. 14 .
All these factors are intrinsic to the examined tissue.
We use sequences that enhance these differences between tissues to obtain an image in which a gray scale is used to distinguish them.
Thus,
for example,
as mentioned earlier,
in a sequence that enhances the T1 information,
the liquid collections will have low signal,
while the same collections will have a very high signal in a sequence that shows more information about the T2 of the tissues Fig. 15 .
there are only TWO types of sequences: spin echo (SE) and gradient echo (GE)