Proton MR spectroscopy of the brain at 3 T: an update
Eur Radiol
Proton MR spectroscopy of the brain at 3 T: an update
0 T. Schirmer GE Healthcare, Applied Science Laboratory , Munich , Germany
1 M. Tosetti Magnetic Resonance Laboratory, Scientific Institute “Stella Maris” , Pisa , Italy
2 F. Trojsi Department of Neurological Sciences, Second University of Naples , Naples , Italy
3 S. M. Lechner GE Global Research , Munich , Germany
Proton magnetic resonance spectroscopy (1H-MRS) provides specific metabolic information not otherwise observable by any other imaging method. 1H-MRS of the brain at 3 T is a new tool in the modern neuroradiological armamentarium whose main advantages, with respect to the well-established and technologically advanced 1.5-T 1H-MRS, include a higher signal-to-noise ratio, with a consequent increase in spatial and temporal resolutions, and better spectral resolution. These advantages allow the acquisition of higher quality and more easily quantifiable spectra in smaller voxels and/or in shorter times, and increase the sensitivity in metabolite detection. However, these advantages may be hampered by
Magnetic resonance spectroscopy; Brain; Diagnostic imaging
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intrinsic field-dependent technical
issues, such as decreased T2 signal,
chemical shift dispersion errors,
J-modulation anomalies, increased
magnetic susceptibility, eddy current
artifacts, challenges in designing and
obtaining appropriate radiofrequency
coils, magnetic field instability and
safety hazards. All these limitations
have been tackled by manufacturers
and researchers and have received
one or more solutions. Furthermore,
advanced 1H-MRS techniques,
such as specific spectral editing,
fast 1H-MRS imaging and diffusion
tensor 1H-MRS imaging, have been
successfully implemented at 3 T.
However, easier and more robust
implementations of these techniques
are still needed before they can
become more widely used and
undertake most of the clinical and
research 1H-MRS applications.
disorders. The list is very long and includes brain tumors,
degenerative diseases such as Alzheimer’s, Huntington’s
and Parkinson’s diseases, cerebrovascular diseases,
metabolic disorders such as adrenoleukodystrophy and
Canavan’s disease, epilepsy, multiple sclerosis and systemic
diseases such as hepatic and renal failure [1–3]. Rare
diseases also studied include creatine deficiency syndrome
[7], variant Creutzfeldt-Jakob Disease [8], pantothenate
kinase-associated neurodegeneration [9] and Rasmussen’s
encephalitis [10]. Most of these studies have been
performed using devices operating at 1.5 T, which has
been considered the standard field for years.
In the last decade, with the approval of the US Food and
Drug Administration for clinical use, MR systems at 3 T are
proliferating, particularly at research centers [11]. With
respect to the well-established MR technique at 1.5 T,
switching to a higher field brings several advantages, such
as an increased signal-to-noise ratio, with consequent
enhanced spatial and temporal resolutions, and better
spectral resolution, but also many limitations, such as
installation issues, higher acoustic noise, device
compatibility, system inhomogeneity, eddy current artifacts,
misregistration errors, J-modulation anomalies, magnetic
field instability and safety restrictions. Some technical
characteristics, such as changes in relaxation times,
chemical shift and susceptibility, can have both benefits
and disadvantages [11–15]. These limitations necessitating
changes in technical devices and acquisition strategies have
led to a debate about the usefulness of higher field strength
in clinical settings [14–20]. However, the new generation
of 3-T systems presents a number of fundamental technical
differences with respect to the first generation, which have
reduced the concerns about the limitations as well as the
benefits of 3-T over 1.5-T systems and increased the
penetration of 3-T scanners into the clinical setting [18–
20]. Furthermore, adapting the imaging procedures to
changes produced by the higher field allows obtaining
images with quality and/or acquisition speed superior to
1.5 T [21, 22]. A number of recent studies have evidenced
the advantages of 3 T over 1.5 T for both conventional MRI
and MR applications limited by insufficient sensitivity,
such as MR angiography, functional MRI and 1H-MRS
[19, 20, 23, 24]. This review focuses on brain 1H-MRS at
3 T, illustrating the advantages, the strategies to overcome the limitations and the advanced techniques.
Advantages and disadvantages of 1H-MRS at 3 T
Signal-to-noise ratio
The intensity of the MR signal is correlated linearly with
the strength of the static magnetic field. Thus, in theory, the
signal-to-noise ratio (SNR) would double when moving
from 1.5 T to 3 T, but in practice the improvement ranges
only from 20% to 50% [25–28]. In effect, the SNR depends
on several other variables, such as T1 and T2 relaxation
times, type of sequence, number of signal averages, size of
sample volume, radiofrequency (RF) ef (...truncated)