Artifacts in 3-T MRI: Physical background and reduction strategies
Introduction
Magnetic resonance imaging (MRI) systems working at a field-strength, B0, of 3 T have become more and more frequent in recent years. In an increasing number of radiological sites, 3-T MRI starts to play the same role for clinical imaging that was occupied by 1.5-T systems for about the last 10 years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The main motivation for the transition from 1.5 to 3-T MRI systems is the improved signal-to-noise ratio (SNR), which is approximately proportional to the field strength [11], B0; thus, under ideal conditions and with optimized acquisition techniques a doubled SNR can be expected at 3 T in comparison to 1.5 T. The growing availability of 3-T MRI systems is for instance demonstrated by the number of publications on 3-T MRI listed in the Medline database, which increased from around 30 in the year 2000 to more than 400 in 2006.
In the 1990s, 3-T MRI was available predominantly in a small number of specialized neuroradiological or neuropsychiatric research sites. Typical applications included functional MRI [12] or spectroscopy studies of the brain, which were performed with optimized protocols and under attendance of specialized engineers or physicists. Today, the availability of 3-T MRI systems has substantially broadened and these systems are applied for imaging of all anatomical areas including, e.g., musculoskeletal [7], [8], abdominal [4], [9], cardiac [5], angiographic [1], [2], and whole-body imaging [3], [6]. This development leads to an increasing diversity of applied pulse sequences and protocols, which are not always fully optimized for 3-T MRI. Consequently, the radiologist or technician working with such 3-T protocols will comparatively frequently be confronted with image artifacts related to 3-T MRI [13], [14].
The purpose of this review article is to present the most relevant artifacts that arise in 3-T MRI, to provide some physical background on the formation of artifacts, and to suggest strategies to reduce or avoid these artifacts. The discussed artifacts are classified and ordered according to the physical mechanism or property of the MRI system responsible for their occurrence: in the following sections, we distinguish artifacts caused by B0 inhomogeneity and susceptibility effects, B1 inhomogeneity and wavelength effects, chemical-shift effects, blood flow and magnetohydrodynamics, and artifacts related to SNR.
Section snippets
B0 inhomogeneity and susceptibility effects
An extremely homogeneous static magnetic field, B0, is required around the isocenter of the magnet for magnetic resonance imaging. The homogeneity of the static magnetic field influences the distribution of the Larmor frequencies of the protons and also the linearity of the magnetic field gradients required for spatial encoding. If the static magnetic field is disturbed, different effects of reduced B0 homogeneity can be observed: variations of the Larmor frequency within a single voxel result
Conclusions
Generally, a larger number of artifacts must still be expected in 3-T MRI than in routine 1.5-T imaging. These artifacts are caused either by physical limitations related to the higher field strength or by protocols transferred from 1.5-T MRI that are not yet fully optimized for the higher field strength. Most artifacts, however, can be mitigated or avoided by small modifications of the pulse sequences such as adapted receiver bandwidths or echo times, by new techniques such as parallel
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