Effects of Motion

Bulk motion of the patient or motion of components of the image (e.g., blood flow) can lead to ghosting in the image. In MRI motion artifacts are most commonly seen in the abdomen (as respiratory related artifact) and thorax (as cardiac and respiratory related artifacts). The origin of these artifacts is easy to appreciate, since the phase of the signal for a particular line of fc-space is no longer solely dependent on the applied imaging field gradients but now also depends on the time-varying position or intensity of the sample and on the motion through the field gradients.

4.1 Pulsatile Flow Effects

The most common pulsatile flow artifact is caused by blood flowing perpendicular to the slice direction (i.e., through plane). Under conditions of partial spin saturation, in which the scan repeat time, TR, is too short to allow for full recovery of longitudinal magnetization of blood, the spin density term in areas of pulsatile flow will become time dependent. For periods when the flow is low, the blood spins will be heavily saturated by the slice selection. Conversely, when the flow is high, unsaturated blood spins will flow into the slice of interest, yielding a high signal in those areas. The modulation of the flow will have some frequency f Haacke and Patrick [16] have shown how the spin density distribution can be expanded into a Fourier series of time-dependent terms p(x, y, t') = S am(x, y) exp(2nimft'), where f is the fundamental frequency of the pulsatile flow (e.g., the cardiac R-R interval), and m e 0, +1, +2, etc. Their analysis shows that ghosted images of the pulsatile region appear superimposed on the main image. The ghosts are predominantly in the phase-encode direction and are displaced from the main image by Ay = mfTR x FOV. An example of an image with pulsatile flow artifact is shown in Fig. 4.

Even when long TR values are used, so that spin saturation effects are minimized, artifacts can still result from pulsatile flow. This is because of the phase shift induced in the flowing spins as they flow through the slice-select and readout field gradients. Again, ghosted images displaced by Ay = mf TR x FOV in the phase-encode direction result [39]. This form of image artifact can be dealt with at source by use of gradient moment nulling [12,17,38]. That technique uses a gradient rephasing scheme that zeroes the phase of moving spins at the time of the spin-echo or gradient-echo. An alternative strategy that can be used to suppress both the phase

FIGURE 4 Pulsatile flow artifact in an image of the knee. Note the periodic ghosting in the phase-encode dimension (vertical) of the popliteal artery.

shifts as spins move through the applied field gradients and intensity fluctuations as the spins attain a variable degree of magnetization recovery is to presaturate all spins proximal to the slice of interest. In this way, the signal from blood spins is suppressed before it enters the slice of interest and should contribute minimal artifact.

4.2 Respiratory Motion

A particularly profound form of motion artifact in MRI data is the phase-encode direction ghosting that results from motion of the anterior wall of the abdomen as the patient breathes. This superimposes a slowly varying modulation of the position term inside the double integral of Eq. 4. For motion in the x direction of amplitude Sx and frequency f this gives

S(kx, ky )=J J p{x, y) exp — 2rei|kx (x + Sx sin(2reff))

kyyI dxdy.

FIGURE 4 Pulsatile flow artifact in an image of the knee. Note the periodic ghosting in the phase-encode dimension (vertical) of the popliteal artery.

Haacke and Patrick [16] show how motions of this form (in x or y) also lead to ghosts in the phase-encode direction. Once again the ghosts appear at discrete positions Ay = mf TR x FOV away from the main image. The amplitude of the ghosts is proportional to the amplitude of the motion 8x. Solutions include breath hold, respiratory gating, or phase-encode ordering such that the modulation in fc-space becomes smooth and monotonic rather than oscillatory [4]. Signal averaging of the entire image will also reduce the relative intensity of the ghosts. An example of an image corrupted by respiratory artifact is shown in Fig. 5. The image shows an image of the lower abdomen collected from a 1.5-tesla scanner. Periodic bands from the bright fatty layers are evident in the image.

4.3 Cardiac Motion

The heart presents the most problematic organ to image in the body. In addition to gross nonlinear motion throughout the cardiac cycle, the heart also contains blood that is moving with high velocity and high pulsatility. In the absence of motion correction, any imaging plane containing the heart will be significantly degraded. Often the heart is blurred and ghosted.

The simplest solution is to gate the acquisition of the MRI scanner to the cardiac cycle. Indeed, by inserting a controlled delay following the cardiac trigger, any arbitrary point in the cardiac cycle can be imaged. For the highest quality, cardiac gating should be combined with respiratory gating or with breath hold. Another solution is to use very fast imaging methods so that the motion of the heart is "frozen" on the time scale of the acquisition. A hybrid combination of fast imaging methods and cardiac gating is often used as the optimum approach.

FIGURE 5 Respiratory artifact in an image of the lower abdomen.

gradient, 5 is its duration, and (r1 — r2) is the vector displacement of the patient. An example of ghosting from a diffusion imaging data set is shown in Fig. 6a. Gross phase encode ghosting is observed, rendering the image useless. Two solutions to this problem exist. One is to use ultrafast single shot imaging methods (e.g., snap shot EPI) since for these sequences the phase shift will be the same for all lines of fc-space and so will not Fourier transform to give an artifact. The other solution is to use navigator echoes [2,3,11,37] in which an extra MRI signal is collected before the phase-encode information has been applied. In the absence of bulk motions of the patient, the phase of the navigator information should be the same for each phase-encode step. Any differences may be ascribed to artifact and may be corrected for. Figure 6b shows the same data as Fig. 6a after the navigator correction information has been applied. A substantial correction can be realized.

FIGURE 5 Respiratory artifact in an image of the lower abdomen.

4.4 Bulk Motion

Random bulk motion, as the patient moves in the magnet or becomes uncomfortable, will also introduce phase-encode ghost artifacts in conventional MRI sequences or in interleaved hybrid MRI sequences. Since the motion is not periodic, the ghosts do not occur at discrete intervals and are much more random in appearance.

Diffusion imaging [25] is the technique most sensitive to random bulk motions of the patient. In this technique a large field gradient is applied to dephase the spins according to their position. A time A later a second field gradient is applied in the opposite direction. Stationary spins should be refocused by the second gradient pulse such that all the spins along the direction of the field gradient constructively add to give an echo. If there have been random diffusion processes along the field gradient direction, however, then some spins will find themselves in a different magnetic field for the second gradient pulse and will not be fully refocused. The amount of destructive interference can be related to the self-diffusion coefficient of water molecules in the tissue, and by repeating the experiment with a number of different gradient strengths the absolute value of the self-diffusion coefficient can be obtained.

The problem occurs if there is bulk motion of the patient between application of the dephasing and rephasing gradient pulses. Because small diffusion distances are being measured, this motion need not be large to produce a problem. The effect is to induce an artifactual phase in the fc-space line being collected equal to yGg ■ (r1 — r2)5, where Gg is the field

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