Chemical Shift Effects

Historically, NMR has been used in isolated test tubes for measuring the frequency shift of nuclei in different chemical environments for far longer than for mapping the spatial distribution of those species. In MRI the nucleus of most interest is 1H, usually in the form of water. In human tissue the H20 peak dominates, but close examination reveals a multitude of lower intensity resonant lines in the 1H spectrum (i.e., in the data acquired without any spatial encoding field gradients). Second to water, the largest contribution is from the 1H nuclei in the lipid chains of the fatty tissues. This group of resonances is centered at a frequency displaced 3.4 ppm from that of water (e.g., at 1.5 tesla this corresponds to a 220-Hz shift). The signal from fat is sufficiently large that it can lead to

FIGURE 6 Diffusion weighted images collected in the human brain without (a) and with (b) navigator echo correction for microscopic bulk motions of the patient. A gradient b-value of 900° s/mm2 was used in the anterior-posterior direction. Note the substantial correction made by the navigator echo information.

FIGURE 6 Diffusion weighted images collected in the human brain without (a) and with (b) navigator echo correction for microscopic bulk motions of the patient. A gradient b-value of 900° s/mm2 was used in the anterior-posterior direction. Note the substantial correction made by the navigator echo information.

a low-intensity "ghosted" image of the fat distribution that is offset relative to the main water image.

5.1 Implications for Conventional MRI

The fat signal artifact that is generated may be understood quite easily using the same theory and approach as in Section 3.2. If we separate the signal into an on-resonance water spin density distribution, pw (x, y), and an off-resonance fatty spin density distribution, pf (x, y), the signal that is acquired is given by

S(kx, ky) = J J{pw(x, y) + pf (x, y) exp( - 2%ivt)}

In this equation v is the fat-water frequency separation and t is the time following spin excitation. Again it is clear that for a conventional image there is no phase evolution between adjacent points in the phase-encode dimension of k-space, since At = 0 for those points. In the readout direction, however, a fat image is expected that is shifted with respect to the main image by vNDW pixels. For a fat-water frequency shift corresponding to 1.5 tesla and a typical dwell time range of 25-40 ^s, the shift will be 1.5-2.5 pixels.

There are a variety of MRI methods for suppressing the fatty signal so that it never appears in the image. This is accomplished either by saturating the fat resonance with a long radio-frequency pulse directly at the fat frequency (fat saturation) or by selectively exciting the water resonant frequency and leaving the fat magnetization undetected along the longitudinal axis (selective excitation). Nevertheless, it is rare to get a complete suppression of the fat signal in the image even when these techniques are employed.

5.2 Implications for Ultrafast MRI

The effect of the fat signal in the case of ultrafast MRI pulse sequences is more significant than for conventional sequences. In particular, in echo planar imaging sequences the fat image can be significantly displaced from the main image. For a 64 x 64 pixel snapshot imaging sequence with a typical dwell time of 8 ^s, the shift of the fat image relative to the main water image will be 7 pixels at 1.5 tesla (i.e., 11% of the field of view). When interleaved k-space EPI methods [8,30] are used, the pixel shift will be proportionately reduced. Note also that for EPI the shift will be predominantly in the phase-encode direction rather than in the readout direction for the reasons discussed in Section 3.2. Because of the large shift in the fat image, it is important to use fat suppression techniques in EPI data whenever possible. Be aware that total suppression is difficult to achieve.

Spiral images [1,31] are also sensitive to the fat signal, but the artifact appears in a different way. This is because of the very different acquisition protocol for spiral scans, in which k-space is sampled in a spiral pattern. The result is a blurring and shifting of the fat image, which is off-resonance with respect to the scanner center frequency, superimposed on the main image, which is on-resonance. The extent of the blurring is given roughly by 2vTacq, where Tacq is the duration of the signal acquisition. For example, at 1.5 tesla with a 30 ms acquisition time the blurring is approximately 14 pixels in extent. Often this appears as a halo around regions of high fat content. Once again, fat suppression techniques should be employed when using spiral imaging sequences.

Examples of the fat ghost image for a conventional image of the leg collected at 1.5 tesla and an EPI spin-echo image of the brain collected at 3 tesla are shown in Fig. 7.

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