As with other imaging modalities and especially with MR, the many variables that contribute to the complexity of image acquisition also make the imaging susceptible to artifacts. These can be caused by field or frequency shifts, aliasing or sampling error, instrument irregularities, and motion from the patient. Chemical shift is the most recognized example of artifact owing to frequency shift. As was discussed in earlier sections, the proton resonates at a spe-
cific frequency that is dependent on the strength of the external field and its local environment. It is the interactions with the surrounding local molecules that causes chemical shift, most noticably between that of water and fat. The hydrogen proton of the water molecule has a slightly different electron or magnetic shielding than those of the methylene group in fatty acids; therefore a small precessional frequency shift results. In a 1.5-T magnet this chemical shift of 3.5 ppm translates approximately into a separation of 224 Hz that is detected in the frequency-encoding or readout direction (42).
This chemical shift between water and fat manifests on imaging as spatial misregistration whereby as the imager is centered at water frequency, fat will be mapped at a slightly different site than its true spatial location, being most obvious at a water-fat interface such as the orbit and kidney. When the chemical shift misregistration is equal to or more than the size of a pixel, a dark or bright band will appear at the interface. The dark band on the lower frequency side of the interface is caused by shifting of fat proton to a lower frequency and away from the fat-water interface. On the higher frequency side of the interface, a bright band appears owing to the resulting overlapping water and lipid signals (Figs. 50 and 51). There are several ways to minimize this effect including changing the frequency-encoding direction, the field of view, or the bandwidth or employing fat suppression techniques. On the other hand, the chemical shift artifact itself can be exploited to clinical advantage to identify fat containing central nervous system lesions such as teratomas and dermoids (Fig. 52) and in differentiating fat from other high-signal T1 substances.
Aliasing or sampling error is best exemplified by truncation artifact/edge ringing and image foldover. The truncation phenomenon occurs when there is
a discontinuity in a function as it jumps abruptly from one value to another. This applies to a step function in a Fourier series: the function has a constant value until it reaches the point at which it jumps suddenly to another constant value. When the Fourier series consists of such a function represented by a finite number of terms, broad oscillations will occur about the point of abrupt change or truncation. The oscillations are squeezed closer as the number of terms increases. In other words, the step-like changes in signal intensities cannot be accurately portrayed by the limited bandwidth of the Fourier series. On imaging, this is most evident during spine imaging by alternating parallel light and dark lines or ripples conforming to the anatomical contour owing to the abrupt signal variations from sharp edges such as fluid/soft tissue/bony interface (43) (Fig. 53).
Image foldover or wraparound is caused by too small a field of view (FOV). To facilitate data reconstruction, the space outside the FOV is filled by identical copies of data from within. In other words, when the object is larger than the FOV, there will be spillover, appearing as wraparound (Fig. 54). To counteract this problem, one has to make sure either that the object to be imaged is completely included within the FOV or that the frequency- or phase-encoding data are oversampled, to create an image that is twice the size, with the portions of the image outside the selected FOV discarded. On gradient images, a "zebra"
pattern is created owing to interference between the aliased parts and the main image.
Artifacts caused by instrument or hardware malfunction include magnetic inhomogeneity with signal dropoff (from faulty shielding or foreign material on or within the patient), data clipping and spikes, broadband noise, and ghosting owing to temporal instability, just to mention a few. Inhomogeneity leads to
distortion of the image geometry that is particularly relevant in fat suppression imaging as this method depends on the frequency offset of fat and water within a homogeneous field. The inhomogeneous field broadens both the fat and water peaks, causing overlap and blurring.
Ghosting can be caused by any motion or suboptimal pulse sequence design of gradient profile owing to flow-related phase shifts. Periodic motion from the respiratory or cardiac cycle or arterial pulsatile flow can cause displacement and mismapping of spins along the phase encoding gradient. On imaging they appear as a series of "ghosts" conforming to the shape of the structure of motion along the phase-encoding axis (Fig. 55). On the other hand, this artifact can be exploited in the clinical setting to identify aneurysms: their periodic pulsations will give rise to ghosting, thus differentiating them from calcification or bony structures (Fig. 56). Various solutions to this issue have been proposed, including respiratory and cardiac gating, patient sedation, switching phase-and frequency-encoding direction, and fast scanning techniques. Flow artifact from CSF pulsation has previously been discussed.
Foreign material either inside or outside the patient can also contribute to signal and image distortion owing to the susceptibility phenomenon, whereby a material is partially magnetized when placed within a magnetic field. These include metallic objects, dental ware, cosmetics that contain ferromagnetic material, tattoos, bullets, or medical devices (Figs. 57-59). MR-compatible materials may torque or heat up during scanning, rendering them unsafe even if they do not cause significant distortion of the image. It is therefore imperative
that all patients be prescreened for potential foreign material within or on them before they are placed into the scanner. Gradient sequences are more sensitive to such artifacts than spin-echo sequences because of the lack of a 180° refocus-ing RF pulse. Again the susceptibility phenomenon can be exploited: a gradient sequence increases sensitivity to small foci of mineralization, cavernomas, and hemorrhage, because of their paramagnetic properties, which allows them to "bloom," appearing as an area of signal void with the extent of the area being exaggerated compared with the actual size of the abnormality (44) (Figs. 60-62).
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