TwinTree Insert

17-03 Radiofrequency and Gradient Artifacts

17-03-01 Slice Profile Artifacts

adiofrequency directed at one slice might interfere with the ex­ci­ta­tion of a neigh­­bor­­ing slice. This undesired excitation is also known as cross ex­ci­ta­tion or cross­talk (Figure 17-06).

Figure 17-06:
(a) The ideal pulse shape is the square pulse which precisely defines the slice and ho­mo­ge­n­eous­ly ex­cites all spins within this slice.
(b) However, the actual pulse profile tends to be Gaussian. These pulses can overlap and deliver RF energy to neighboring slices.
(c) Increasing the interslice gap decreases the en­er­gy delivered to neighboring slices.

The effects of cross excitation are changes in image con­trast (cf. Chapter 6). Coun­ter­mea­su­res include an increase of interslice gaps or non-sequential excitation of ana­to­mi­cal slices.

The slice profile of an RF pulse can be distorted when repetition times not al­­low­­ing full recovery of the signal are used [⇒ Young 1987]. For any given profile, there is al­ways a transition zone where the value of the flip angle goes from zero up to the de­sir­ed value. In the case of incomplete recovery of the magnetization, the strong­est signal is produced at a flip angle which is less than 90°. The slice pro­fi­le will there­fore show maximum peaks on either side of a central dip.

This becomes particularly acute when very short TR values are used in con­­junc­­tion with a large flip angle (e.g., spoiled FLASH sequence with good T1 con­trast).

In such cases the desired contrast is not obtained since we will have a mixture of con­trasts due to the variation in the flip angle across the slice. This can be over­­come by using RF pulses which give very sharp transition zones or by using a 3D se­quen­ce.

17-03-02 Artifacts in Multiple Spin Echo Sequences

Multiple spin-echo (MSE) or SE-based sequences are widely used in MR imaging. Pro­b­­lems can arise with such sequences due to the fact that the refocusing pulses will not be perfect 180° pulses across the whole slice, and in the transition zone (from 0° to 180°) at the edge of the slice, a whole range of flip angles will be pre­sent.

This means that the refocusing pulses will not only form the desired spin echoes, but also will generate other signals which, if not suppressed, can degrade the ima­ges obtained from the second echo onwards [⇒ Graumann 1986].

There are a num­ber of solutions to this problem including the addition of spoiler gra­dients to the sequence (which increases the minimum echo times) or the use of phase-cycling to cancel out the unwanted components (which increases the overall scan time).

It is also possible to move the artifacts to the edge of the image by using suitable phase schemes for the RF pulses. This generally reduces their effect on the re­gion of interest.

17-03-03 Line Artifacts

A relatively common artifact is the presence of a high intensity line (sometimes looking like a zipper) at the center of the image, orientated in the phase en­cod­ing direction. This is usually caused by RF leaking from the transmitter to the re­cei­ver. Since the leakage is at the resonance frequency, it will appear at the center of each projection.

Slight variations in the amount of leakage in each projection cause the artifact to be smeared out across the field-of-view in the phase-encoding di­rec­tion. This prob­lem can be difficult to track down and eliminate completely, but can be re­mo­ved by col­lect­ing two averages in conjunction with phase alternation of the ex­ci­ta­tion pulses.

Line artifacts in the phase-encoding direction away from the center of the ima­ge usually result from RF interference at a well-defined frequency. For the most part they are caused by polluting RF, e.g., commercial radio or television stations. Com­mon­ly, the RF shielding supplied with commercial systems is ad­equa­te, but the door seals should be checked and cleaned periodically.

spaceholder redCentral point artifacts are white or black dots in the center of the MR image. They are only seen on older MR equipment.

spaceholder redA line artifact as a k-space artifact is shown in [Figure 17-18].