Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

c:~...:L~"'FOii·"·"""~·~- ~; t---- --; ~t_- f.:::"' J;l ; Ci i i iFIGURE 15-38. A: Wraparound artifacts are caused by aliasing. Shown is a fixed sampling rateand net precessional frequencies occurring at position A and position B within the FOV thathave identical frequencies but different phase. If signal from position C is at twice the frequencyof B but insufficiently sampled, the same frequency and phase will be assigned to C as thatassigned to A. and therefore will appear at that location. B: A wraparound artifact example displacesanatomy from one side of the image (or outside of the FOV) to the other side.Nyquist sampling limit). Otherwise, the Fourier transform cannot distinguish frequenciesthat are present in the data above the Nyquist frequency limit, and insteadassigns a lower frequency value to them (Fig. 15-38A). Frequency signals will"wraparound" to the opposite side of the image, masquerading as low-frequency(aliased) signals (Fig. 15-38B).In the frequency encode direction a low-pass filter can be applied to theacquired time domain signal to eliminate frequencies beyond the Nyquist frequency.In the phase encode direction aliasing artifacts can be reduced by increasingthe number of phase encode steps (the trade-off is increased image time).Another approach is to move the region of anatomic interest to the center of theimaging volume to avoid the overlapping anatomy, which usually occurs at theperiphery of the FOY. An "antialiasing" saturation pulse just outside of the FOV isyet another method of eliminating high-frequency signals that would otherwise bealiased into the lower-frequency spectrum.Partial-volume artifacts arise from the finite size of the voxel over which the signalis averaged. This results in a loss of detail and spatial resolution. Reduction of partial-volumeartifacts is accomplished by using a smaller pixel size and/or a smallerslice thickness. With a smaller voxel, the SNR is reduced for a similar imaging time,resulting in a noisier signal with less low-contrast sensitivity. Of course, with agreater number of excitations (averages), the SNR can be maintained, at the cost oflonger imaging time.