Now that we know the appropriate background of the MRI,
lets play with sequences,
angles,
time,
pulses,
gradients and all the amazing tools that MRI offer to us.
Types of sequences: spin echo (SE) and gradient echo (GE)
If we want to classify sequences,
we do not divide them into «T1» and «T2» sequences,
but rather SE or GE sequences.
Spin echo formation
To facilitate the graphic representation of echo generation,
we present a scheme Fig. 16 of the artificial state of “rest” (created by the external magnetic field) as vectors (yellow) that revolve around the axis of the magnetic field (magenta).
The diagram is simplified Fig. 17 if this trend is represented as a single vector superimposed on the vector of the external magnetic field.
The excitation process Fig. 18 is illustrated with the application of a selective RF impulse. In the case of the spin echo sequence (SE),
the impulse is usually 90o.
This process is denominated magnetization deflection,
the chosen angle is usually 90o in SE sequences,
but it can be smaller or larger.
It is important to remember that it is really an energy change and not a real movement of the atoms.
As the vectors are getting out of phase in the transverse plane (due to their different relaxation speed,
which in turn depends on their molecular environment),
each vector in the transverse plane acquires a different position or angle respect to the magnetic field or “dephasing” Fig. 19 .
The idea is to refocus these vectors,
which is achieved stimulating the spins: a RF pulse forces them to return to the starting point at the same speed they had,
which is why every vector will meet at the same time at the starting point Fig. 20 .
This coincidence of spins is what generates the echo we are interested in: the spin echo,
Fig. 21 .
This process of refocusing the vectors or spins can be repeated as many times as desired,
as long as there is some transverse magnetization left Fig. 22 .
GRADIENT ECO (GE)
There is an additional way to acquire signals or echoes without the application of 180o pulses.
This is achieved by inverting the polarity of the local magnetic field to which the tissue is exposed Fig. 23 .
This technique is denominated Gradient Echo because it uses gradients to invert the polarity of the field.
Its name differentiates it from the previously described sequence,
where the echo is from the spin and not from the gradient.
GE sequence types
GE sequences can be coherent or incoherent Fig. 24 .
This description refers to the phenomenon known as the steady state,
which is reached thanks to the repetitive and rapid stimuli obtained by using small deflection angles of magnetization.
The result is an appearance of the liquid that is bright in all sequence types, independent of their T1 or T2 weighting. As it may be confusing to have sequences with T1 information but with white liquid, several ways of interfering with or destroying this "coherence" of the steady state have been found.
The application of "interference" pulses can "damage" or “spoil” the steady state,
and obtain a dark appearance of liquid instead of the high signal intensity that is seen on GE sequences in the steady state.
All GE “spoiled” sequences have many variants.
The classical non-spoiled GE sequence is also known as Gradient- Recalled Acquisition in the Steady State (GRASS).
The term “spoiling” refers to the destruction of or "interference" with the steady state,
as in the classical SPoiled GRASS or SPGR sequence.
There is also a faster variant (Fast SPGR) and one that acquires the information as a three-dimensional volume from which different reconstructions can be made (3D SPGR).
The” spoiled” sequences are also called incoherent.
For each TR / T1 ratio,
there will be an optimum angle that will give the largest transverse component with each RF pulse applied,
which in turn provides the largest signal.
This optimal angle is called the Ernst angle.
The sequences called echo planar imaging, are an ultra-fast variant of the 3D SPGR sequence,
in which a single repetition (TR) allows to fill all the information of a slice (instantaneous filling of all the rows of pixels in the time it takes to make one repetition).
This technique is used in advanced applications,
such as diffusion weighted images (DWI),
perfusion and functional MR imaging.
The counterpart of the incoherent GE sequences are the coherent sequences,
in which the steady state is not modified.
As mentioned above,
in these sequences,
liquid will always be bright,
regardless of whether the selected parameters favour the T1 or T2 information.
The "balanced" variant of coherent GE sequences includes the Balanced SSFP and the Constructive Interference Steady State (CISS).
In these,
the T2* effect is smaller,
and moreT2 information is obtained.
The contrast obtained is based on the relation T2 * / T1. This means that tissues with a T2 close to their T1 are those that will appear brighter.
What we owe to Fourier: turning frequencies into pixels (discovering k-space)
What we use sequences for: spatial location and image formation
According to the concept of gyromagnetic relationship,
the intensity of the magnetic field directly influences the frequency of precession.
Each of the sequences of RF pulses are designed to locate the signals in space.
Each signal is converted into a gray scale that reflects the interactions of tissues with the magnetic field.
K-space is also known as a frequency domain,
and is plotted as two perpendicular axes,
x and y.
One k-space plot corresponds to one slice.
If 20 slices are made,
each has its respective k-space or frequency domain plot Fig. 25 .
We owe Joseph Fourier a mathematical trick that allows us to convert one complex mathematical function into another,
the Fourier transform.
This function allows us to do a map of the existing frequencies in a complex RF wave,
which contains important information about the molecular content of tissues.
The k-space is a map of frequencies of the signals obtained from the tissues after being sequentially stimulated to obtain specific types of information about the relaxation times of tissues and other magnetic interactions of these same tissues.
There are several strategies that can enhance the contrast or the details of each image; they relate to the order in which the k-space is filled,
the so-called k-space trajectories.
The classic k-space filling is a Cartesian one,
in which this space is processed or filled one row at a time,
starting from the center,
and moving alternatively one row above and below the center one.
Fast imaging techniques include the filling of more than one row at a time,
non-Cartesian trajectories such as a spiral one commencing from the central point and spiraling to the periphery,
or over-sampling techniques that can emphasize the processing of specific information (e.g.,
contrast or spatial resolution).
Other time-saving strategies include undersampling techniques,
whereby partial filling of the k-space is attained.
This can be done by skipping lines or by processing only specific portions of the frequency domain plot.
Understanding the concepts of the information contained in this imaginary space and the ways to take advantage of such information is the clue to the better design of RF sequences and imaging protocols.