If the receiving RF coil is sensitive to tissue signal arising from outside the desired FOV, this undesired signal may be incorrectly mapped to a location within the image, a phenomenon known as aliasing. This is a consequence of the acquired k-space frequencies not being sampled densely enough, whereby portions of the object outside of the desired FOV get mapped to an incorrect location inside the FOV. To prevent this effect in the frequency-encode direction, data oversampling is used and is performed automatically on the GE Signa scanner. In this method, the signal is sampled at a higher bandwidth than required, and a digital filter is applied to re-sample the data at the user-specified RBW. The internal high bandwidth sampling prevents this aliasing phenomenon in the frequency-encode direction, and filtering to provide the user-specified RBW improves the SNR.

A similar problem occurs in the phase-encoding direction, where the phases of signal-bearing tissues outside of the FOV in the y-direction are a replication of the phases that are encoded within the FOV. This signal will be mapped, or "wrapped" back into the image at incorrect locations, and is known as "phase-wrap" artifact. No Phase Wrap is a user-selectable parameter that maps this signal to its correct location outside the FOV, then discards any signal from outside the FOV before displaying the image. In the shoulder, for instance, No Phase Wrap should be used for coronal scan planes to eliminate signal originating from the chest medial to the shoulder. When using a shoulder coil, the signal drops off rapidly beyond the area of interest; however, there is still enough signal generated from outside the desired FOV to result in extremely distracting phase wrap artifact and the use of No Phase Wrap is therefore recommended.

No Phase Wrap works by filling k-space to the same extent, using twice as many phase-encoding steps (See Figure 1.7). (Because twice as many phase-encoding steps are required, the selected NEX must be two or greater when No Phase Wrap is selected).

Figure 1.7 No Phase Wrap works by acquiring twice as many phase-encode lines in the same extent of k-space. This results in a doubled FOV in the y-direction with the same spatial resolution. the two outer halves of the image which contained the unwanted signal are then discarded.

K-space is filled to the same outer extent, and the spatial resolution in the image space is therefore unchanged. However, using twice as many phase-encoding steps means that the FOV in the phase encode direction is doubled.* Following image reconstruction using an FFT, the two outer halves of image space are discarded. It should be noted that a sqrt(2) improvement in SNR occurs for a 2 NEX scan when the No Phase Wrap option is selected. Figure 1.8 shows an axial knee image with and without No Phase Wrap selected.

Figure 1.8 (Image reprinted with permission from: MRI Principles, by Donald G. Mitchell, W.B. Saunders Company, 1999).

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* K-space and image space have the following reciprocal relations: (1) Extent in k-space determines image spatial resoultion; (2) Resolution in k-space determines extent in image space (FOV).