Although the first systems of the MRI Scanners are characterized by relatively low static magnetic fields as well as time-consuming and multidisciplinary data recovery procedures,
nowadays the trend is the use of high-field magnetic resonators,
namely MRI Scanners with a static field greater than or equal to 3.0 Tesla.
Specifically MRI systems equal to 3 T mainly used in everyday clinical practice,
were called to replace the 1.5 T ones which are the majority of the operating magnetic resonators,
at the level of clinical practice.
The need for change: Better imaging,
quicker examinations,
new possibilities and perspectives.
However,
is this a panacea to increase the static magnetic field to achieve the above objectives? The answer to this question could be given by a thorough comparison of the 1.5T and 3.0T MRI systems,
which also describes the purpose of our work.
What is clear from the outset is the fact that the high field is not a solution to all the required MRIs.
High Field Magnetic Tomography,
compared to the lowest field,
obviously has very strong advantages which give medical imagery new perspectives and possibilities but at the same time they are characterized by disadvantages that should be taken into consideration every time while making our choices.
Concluding the advantages and the disadvantages of MRI Scanners with field ≥ 3.0 T with the corresponding ≤ 1.5 T,
we can note the following:
1.
Advantages (fig.1):
One of the strongest advantages of high field magnetic resonance imaging systems is the high ratio,
namely,
Signal to Noise Ratio (SNR).At 3.0T the SNR is twice as high as the 1.5T (Fig.2).High ratio means more signal and therefore better display or even display in shorter time or even images with better qualities such as thinner section thickness,
greater spatial resolution and image quality.
Respectively shorter time means greater comfort for the patient to co-operate seamlessly to avoid pseudo-motion.
Moreover,
this advantage creates the necessary conditions for the development of a plurality of otherwise specialized applications,
such as in vivo spectroscopy,
which in high field systems presents increased spectral analysis and distinction between the individual peaks of the metabolites recorded in the measured spectrum.
Other such applications are Functional Magnetic Resonance (MRI),
Diffusion Weighted Imaging and Diffusion Tensor Imaging techniques as well as Magnetic Resonance Angiography.
In particular,
in f MRI,
with BOLD (Blood Oxygen Level Dependent) techniques,
3.0 T systems have seen up to 40% increase in detected brain activation than in the 1.5T.
In terms of diffusion images,
increased signal leads to higher quality primary images,
increasing the sensitivity of the method and in cases of ischaemic strokes in their acute phase.
In DTI,
data can be retrieved at shorter times,
and with a much higher spatial resolution,
allowing for additional processing,
such as Fiber Tracking.
With regard to Magnetic Resonance Angiography and perfusion techniques in general,
high field systems have obvious advantages given the improved Contrast to Noise Ratio.
Moreover,
the prolongation of the T1 time,
which characterizes the 3.0T systems,
creates the conditions for better visualization of the vessels,
since we have a better suppression of the tissue signal and a prolonged time for filling the vessels,
which leads to the possibility of showing even smaller vascular.
Taking advantage of the prolongation of T1 time and high SNR,
on the other hand,
can be used for the best realization of TOF sequences but also for the fuller use of contrast media.
Also,
these phenomena are used catalytically for the application of ASL (arterial spin labelling) techniques (Fig.3).
In addition to the advantages of high field systems,
we could include increasing the magnetic sensitivity and the possibility of detecting the degradation of blood or structures containing calcium deposition,
exploiting the artifacts caused by them.
Sustained Weighted Imaging (SWI) sequences are used for this promotion (Fig.4).
Finally improved image quality due to a higher SNR also occurs for the small anatomical structures of a pediatric patient,
with the advantage that shorter scan times lead to a reduction in the total test time.
Relevant time and magnetic-time benefits for magnetic heart or embryonic imaging applications.
The stronger shift between fat and water is a powerful advantage of high field systems in liver imaging as well as in the musculoskeletal system.
2.
Disadvantages (Fig.
5):
In the drawbacks of high field magnetic resonance imaging systems,
we will initially mark the low contrast T1 gravity images.
In these systems,
ideal SE1 sequences cannot be produced,
given the prolongation of T1 time due to the intensity of the static magnetic field.
In the imaging of the musculoskeletal system,
the 10% to 30% prolongation of T1 time in the systems at 3.0 T,
compared to 1.5 T,
requires an increase in T1 to maintain a weighted T1 imaging.
In this case the suppression of spectral fat is particularly sensitive to field heterogeneities,
rendering this method inadequate for accurate T1 gravity imaging with fat suppression images.
Similarly,
in cases an important drawback of high-field systems is also the increased SAR,
which in the 3.0 T systems is four times higher than the corresponding 1.5T systems,
and the acoustic noise is multiple as far as the safety of the tests is concerned (Fig.6).
Metallic objects (surgical clips),
the artifacts are more intense.
Computational power requirements are also manifold.
In addition,
field heterogeneities are more common in MRI Scanners with filed ≥ 3.0T,
as well as susceptibility artifacts,
particularly in regions at the border of tissue change of different densities (eg air and fat,
etc.).
The reduction in the wavelength of the radio pads used in the high field magnetic resonance imaging systems,
combined with the size of the anatomical regions to be tested and the composition of the individual tissues,
can often result in the appearance of bright or dark areas,
ie artifacts that are connected with the dielectric phenomenon (Fig.
7).
In magnetic resonance imaging of the heart,
in the ≥3.0 T fields T-affinity is affected,
as well as in the imaging with the use of cine sequences for the cardiac muscle,
due to the heterogeneity of B1.
Finally,
there are plenty of materials used in a range of medical operations that are safe for 1.5T systems,
but not for systems ≥ 3.0 T.
In general,
both material and other equipment intended for ≥ 3.0T systems are more expensive and have a very limited availability,
compared with the equivalent equipment of the 1.5T systems.
From what we have outlined above,
which were analyzed in the main part of the paper,
it is clear that the use of high-field systems is not a one-way model in magnetic resonance imaging.
Clearly,
it excels in specialized applications,
however,
1.5T systems also have significant advantages in features useful in routine MRI scanning routines.