3 T MRI: Advances in brain imaging

doi:10.1016/j.ejrad.2008.02.045

European Journal of Radiology

Volume 67, Issue 3, September 2008, Pages 415-426

https://doi.org/10.1016/j.ejrad.2008.02.045 Get rights and content

Abstract

Since its approval by the FDA in 2000, brain MR imaging at 3.0 T has been increasingly used in clinical practice. Theoretically, the signal-to-noise ratio (SNR) of a 3 T MR scanner will be double that of a 1.5 T scanner. However, the relationship between the magnetic field used and the image obtained is very complex. Today, using a 3 T magnet in Neuroradiology has far more advantages than disadvantages, and the diagnostic potential of higher strength magnets for structural and vascular scans, diffusion and perfusion imaging, spectroscopy and cortical activation studies is improving. However, it is useful to have an awareness of how increasing field strength affects each of these techniques so that full advantage may be taken of them.

Introduction

Since MR imaging first came about, the strength of the magnetic field has been one of the variables which has sparked more interest. A great deal of effort has been invested in research into the magnetic field because of the difficulties of working with successively stronger fields. For a long time, the balance between the advantages and disadvantages of each system has meant that the most commonly used magnetic field has had a strength of around 1.5 T, although prototypes have been gradually developed to carry out research in stronger fields, particularly for Spectroscopy and fMRI. Recent technical advances meant that full body magnets of strengths up to 4 T were approved for clinical use in the year 2000. Even then, the first 3 T devices, which had large magnets and inefficient gradient systems (and, most importantly, pulse sequences and coils copied and adapted from those used in 1.5 T devices), offered relatively few advantages over 1.5 T scanners, which were far more optimised in both technical and economic terms. In theory, the signal-to-noise ratio (SNR) has a linear relationship with the magnetic field. So, in principle, the SNR of a 3 T MR scanner will be double that of a 1.5 T scanner. However, the actual relationship between the magnetic field used and the image obtained is very complex, as there are a series of other influencing factors, such as relaxation times, the body's dielectric properties, the effectiveness of radio frequency (RF) pulses and coil performance, which change when the field changes, causing a series of major problems at strengths of 3–4 T or more. Despite this, it takes far less time to obtain the same data using higher strength fields than it does using a 1.5 T scanner, and it is therefore possible to obtain images of a far greater spatial resolution. Today, using a 3 T magnet in Neuroradiology has far more advantages than disadvantages, and the diagnostic potential of higher strength magnets for structural and vascular scans, diffusion and perfusion imaging, spectroscopy and cortical activation studies (BOLD, blood oxygen level dependent) is improving. However, it is useful to have an awareness of how increasing field-strength affects each of these techniques so that full advantage may be taken of them. In addition to the higher cost, the need to have a good knowledge of how the technique works is one of the main drawbacks of using 3 T MR systems.

Section snippets

Consequences of increasing field strength

The main consequence of increasing field strength is the increase in the signal-to-noise ratio. In theory, the two variables are directly proportional to one another. The signal increase depends on the number of antiparallel spins and the voltage produced by each spin, which are both directly proportional to the field, while noise also increases in direct proportion to the field [1]. In theory, then, the relationship between the SNR and the magnetic field is linear, as indicated in the

3 T MRI and brain imaging

The main advantage of using 3 T MR in structural scans is its higher speed. This increases productivity, although obtaining the images is only one part of the MR process, so the final impact on the exploration time will still depend on the proportion of the process which is devoted to obtaining the data itself. In some situations, such as scans of uncooperative patients or children, speed is of the essence. In these cases, by using a combination of faster sequences which is less sensitive to

MR angiography

Vascular studies are one example where the advantages of increasing field strength become more apparent. In addition to the increased SNR, which makes it possible to increase spatial resolution, the TOF effect is increased (Fig. 5), making it possible to detect smaller vessels and distal vessels, and to reduce the likelihood of overestimating stenoses. The increased spatial resolution makes it slightly easier to delimit small aneurisms (Fig. 6) and see how they interact with vessels [17]. This

Perfusion

The most frequently used technique for cerebral perfusion scans is DSC (dynamic susceptibility contrast), which analyses the changes in magnetic susceptibility caused by passing a bolus of contrast material through the cerebral vascular system. Increasing the field causes a proportional increase in magnetic susceptibility, so with 3 T it is possible to work with half the dose of contrast materials, which increases the efficiency of the signal/time curve and limits any possible drawbacks of using

Diffusion

The contrast effects produced in diffusion scans are regarded as independent from the magnetic field because they result from a molecular movement which has no direct relationship with the magnetic field. Moreover, the use of a larger magnetic field results in increased distortion, particularly in areas around the base of the skull, as a result of the difference in magnetic susceptibility of the air and the bone compared with the cerebral parenchyma. In principle, therefore, increasing the

Activation (BOLD)

The BOLD technique has proved to be more useful when working with higher fields, as the combination of higher sensitivity to magnetic susceptibility with a higher SNR has particularly significant effects. Gati et al.[26] showed that the SNR increases in direct proportion to the field, and also made another important finding: the BOLD effect increases less in vessels larger than the voxel, while in vessels smaller than the voxel the increase in CNR is greater than the increase in a linear

Hydrogen spectroscopy (H-MRS)

Since MR first emerged, spectroscopy has been the technique which has always required the strongest possible magnetic field, as the method does not work with the water peak but with other peaks whose extent is several times smaller. It is therefore useful to increase the SNR as much as possible. In addition, another important requirement in H-MRS is the separation among different metabolites, which depends proportionately on the chemical shift. These two parameters are linearly dependent on the

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3 T MRI: Advances in brain imaging

Abstract

Introduction

Section snippets

Consequences of increasing field strength

3 T MRI and brain imaging

MR angiography

Perfusion

Diffusion

Activation (BOLD)

Hydrogen spectroscopy (H-MRS)

Acad. Radiol.

Neuroimag. Clin. N. Am.

The intrinsic signal-to-noise ratio in NMR imaging

Magn. Reson. Med.

Radiofrequency penetration and absorption in the human body: limitations to high-field whole-body nuclear magnetic resonance imaging

Med. Phys.

NMR relaxation times in the human brain at 3.0 T

J. Magn. Reson. Imag.

Time of-flight MR angiography: comparison of 3.0 T imaging and 1.5 T imaging—initial experience

Radiology

Perfusion imaging using arterial spin labeling

Top Magn. Reson. Imag.

Brain tumors: full- and half-dose contrast enhanced MR imaging at 3.0 T compared with1.5 T—initial experience

Radiology

B1 field homogeneity and SAR calculations for the birdcage coil

Phys. Med. Biol.

A hippocampal lesion detected by high-field 3 Tesla magnetic resonance imaging in a patient with temporal lobe epilepsy

Tohoku J. Exp. Med.

Evidence of neuronal injury outside the medial temporal lobe in temporal lobe epilepsy: N-acetylaspartate concentration reductions detected with multisection proton MR spectroscopic imaging—initial experience

Radiology

3 T phased array MRI improves the presurgical evaluation in focal epilepsies: a prospective study

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FLAIR imaging for multiple sclerosis: a comparative MR study at 1.5 and 3.0 Tesla

Eur. Radiol.

Does high-field MR imaging have an influence on the classification of patients with clinically isolated syndromes according to current diagnostic MR imaging criteria for multiple sclerosis?

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Magnetic resonance imaging contrast enhancement of brain tumors at 3 Tesla versus 1.5 Tesla

Invest. Radiol.

Tumors: full-and half-dose contrast-enhanced MR Imaging at 3.0 T compared with 1.5 T—initial experience

Radiology

Ultra high-field MRI at 8 Tesla of subacute hemorrhagic stroke

J. Comput. Assist. Tomogr.