What is the MRI technology GRE


Measurement sequences for MR imaging

In order to obtain an MR image, a series of regularly successive RF excitation pulses is required, which are combined with magnetic field gradients in a precisely defined sequence.

S.pin-echo (SE) technology

The SE technology is based on the sequence of a 90 ° and a 180 ° pulse. In order to prevent the disruptive influence of external magnetic fields on the precessing spins (which leads to a decrease in the synchronization between the gyroscopic movements of the individual spins and therefore accelerates the transverse relaxation), a process takes place in the middle of the time interval between 90 ° excitation and data readout (time TE / 2 ) a 180 ° pulse is radiated. This pulse causes the gyroscopic movements to be synchronized and thus a maximally amplified signal occurs at the time of data readout (after the time TE) (Echo) (Fig. 25). Since the disruptive influence of the magnetic field inhomogeneities is eliminated with the 180 ° pulse, the transverse relaxation no longer runs with the time constant T2 *, but more slowly with the only tissue-specific constant T2. By appropriate selection of the sequence parameters TR and TE, images can be recorded in which the contrast is predominantly characterized by the PD or by the relaxation times T1 and T2 of the structures shown.

 


Fig. 25: Pulse-time diagram of the spin echo sequence

 


The advantages of the spin-echo technique are the good image quality and the low susceptibility artifacts (susceptibility: magnetic property of a substance), the low susceptibility to inhomogeneities and the possibility of a strong T2 weighting. The long measurement time proves to be a disadvantage with this technique.

Turbo spin echo (TSE) technology

The TSE technique is a variation of the conventional spin echo technique. With a normal spin-echo sequence, one echo is read out per excitation (90 ° pulse). With a turbo spin-echo sequence, several echoes are generated and received per excitation by additional 180 degree HF pulses. The successive echoes per excitation are referred to as the echo train (pulse train), their number as the turbo factor. Since several echoes are read out for each excitation, the number of echoes required for image construction is reached much earlier, and the total measurement time is shortened by the turbo factor. The advantages of the TSE are the greatly reduced measuring time, a higher resolution than the SE with the same measuring time and lower susceptibility artifacts than the SE (which can also be a disadvantage, especially when looking for small bleeds). The strong fat signals, even with long effective TE, the lower number of layers per TR and the reduced detail resolution, especially with short effective TE, are the disadvantages of this method.

Gradius echo (GRE) technology

The hallmark of all GRE sequences is that they do not use the additional 180 ° pulse. Instead, an artificial magnetic field inhomogeneity is created by applying a gradient magnetic field. The location-dependent Lamor frequency results in a dephasing of the nuclear spins and thus in a suppression of the MR signal. If the polarity of the magnetic field gradient is reversed after a certain time (gradient reversal), i.e. places that were previously exposed to a higher magnetic field are now in a correspondingly lower field, the artificially dephased spins can be re-phased, creating a so-called gradient echo. (Fig. 26)


In the case of gradient echo sequences, only the gradient generated by the gradient is reversed artificialMagnetic field inhomogeneity rephased. Magnetic field inhomogeneities (characterized by the T2 * relaxation time) are not compensated. In the case of gradient echo sequences, the level of the MR signal therefore depends on the T1 and T2 * relaxation times.

If one would like to select the times for TR to be shorter in order to shorten the measurement time, saturation effects will increasingly occur. These can be compensated for by a lower excitation angle (flip angle) of, for example, 30 °. This allows the longitudinal magnetization to relax more quickly to its initial state and thus reduce the measurement time. The shorter measuring time, due to the lower TR, TE and the smaller flip angle (fewer movement artifacts), combined with the possibility of a 3D measurement with the highest resolution, are among the advantages of the GE. The disadvantage is that there is no possibility of compensating for magnetic field inhomogeneities and thus no T2 but only a T2 * contrast is possible.

Basically, 2D and 3D measurements can be made. The difference is that with the 2D measurement, each slice is selectively excited and location-coded, while with the 3D measurement, the entire volume with all slices is excited at the same time.

The 3D measurement offers a number of advantages. It is possible to scan very thin slices without any gaps between them, so that even very small lesions can be imaged. In addition, images in any plane can be reconstructed from the data set obtained without loss of resolution. However, this requires a relatively long measurement time, which is why only sequences with a short measurement time are used for 3D measurements. 3D measurements (Fig. 27) are used when a good contrast with the highest resolution is required in all three spatial directions. MR angiography examinations are carried out using 3D technology. The volume of certain areas of the brain, such as the external liquor space, can also be used Can be realized using this technique.

 

Fig. 27:

3D reconstruction

 


E.cho Planar Imaging (EPI)

EPI is by far the fastest method in MR imaging. The classic EPI sequence uses a single excitation and then collects all data using gradient echo technology. An MR image can be created in less than 100 msec, which minimizes the susceptibility to movement artifacts. The frequency coding gradient is not constant with EPI, but oscillates. It generates a series of gradient echoes with constantly changing signs (Fig. 28).


Due to the very short measurement time, the EPI sequences are particularly suitable for recording physiological parameters in functional brain imaging.