Physikalische Grundlagen bei 3Tesla

Basic Physical Effects:

• T1 relaxation times are increased and converge. T2/T2* relaxation times are reduced.

• The Signal-to-noise ratio (SNR) is increased.

• The specific absorption rate (SAR) is increased.

• RF focusing effects lead to inhomogeneous RF fields.

• The static field (B0) is inhomogeneous near air/tissue interfaces.

Effects on Neuroimaging Techniques:

• General: The theoretical linear SNR increase is compromised by longer T1 and shorter T2/T2* values. Due to increased SAR, implants that are safe at 1.5 Tesla may constitute a risk at higher B0.

• Angiography: Time-of-flight (TOF) methods are based on magnetic labelling of blood spins. The increased tissue and blood T1 lead to better background suppression and a longer blood label lifetime.

• Perfusion: Arterial spin labelling (ASL) methods benefit from larger B0 in the same way as TOF angiography. The increased bolus lifetime allows for an increased number of slices. Contrast enhanced (CE) methods are based on the different magnetic susceptibility of intravascular contrast agents, leading to local field inhomogeneities and thus intensity variations in the vicinity of vessels. This effect is more pronounced at higher B0.

• Venography: Most methods are based on the acquisition of T2* weighted images that display signal losses in the vicinity of veins. This effect is enhanced at higher B0, leading to better contrasts.

• Diffusion Tensor Imaging (DTI): The increased SNR may be used to improve the spatial resolution or to use larger b-values (of prime importance for fibre tracking).

• Anatomical Brain Imaging: T1 weighted: Special sequences (like MDEFT) should be used to maintain satisfactory contrast. T2 weighted: Fast Spin Echo (FSE) techniques are problematic due to the high SAR. In general, a reduction of the flip angle is required.

• Functional Brain Imaging (fMRI): At higher B0, the increased SNR allows for enhanced spatial resolution. In addition, it becomes possible to base experiments on spin-echo EPI sequences that suppress the signal contribution from draining veins. Signal losses and image distortions due to B0 field inhomogeneities require special compensation methods.

Lernziele:

To understand the basic physical effects that affect MR imaging at static field strengths of 3 Tesla or more.

To understand how these effects influence some of the most common neuroimaging techniques.

Korrespondierender Autor: Deichmann R

Institute of Neurology, Wellcome Department of Imaging Neuroscience, 12 Queen Square, WC1N 3BG, London

E-Mail: [e-mail]