TwinTree Insert

06-05 Two-Dimensional Imaging

n an imaging experiment, definition and selection of a virtual (two-dimensional) slice through the ex­amined object or a patient are of great im­portance. They are de­ter­min­ed by cha­rac­te­ris­tics of the excitation pulse. One distinguishes between shaped and hard pulses (cf. Figure 01-08).

06-05-01 Slice Selection

We can express the gradient strength in ei­ther mT/m or in Hz/m. Since the pulse has a fixed bandwidth (provided that the pulse duration is held constant), raising the gradi­ent strength increases the number of Hz/m; this results in a decrease in slice thickness (Figure 06-15).

Figure 06-15:
Slice thickness: moving the gradient in the direction of the arrow increases the number of Hz/m, and thus gradient strength. It decreases slice thickness.
 For example, for a sinc pulse with a bandwidth of 2 kHz, increasing the slice gradient from 4 mT/m (1.7 kHz/cm) to 8 mT/m (3.4 kHz/cm) reduces the slice thick­ness from 11.8 mm to 5.9 mm.

Applying an RF pulse in the absence of any field gradients will excite the whole sam­ple. If a field gradient is applied at the same time as the pulse, the magnetic field, and therefore the resonance frequency, will change with position within the sam­ple. For an RF pulse at the resonance frequency ex­citation will oc­cur at the mag­net center where the gradient has no effect (cf. Figure 06-05).

Off-center, the nuclei cannot be excited by RF pulses at the Lar­mor fre­quen­cy. The distance (or slice thickness) over which the nuclei in the center resonate is de­ter­min­ed by the range of frequencies (bandwidth) contained in the excitation pulse and the strength of the field gradient. If the RF pulse contains only a well defined band of frequencies, then excitation will oc­cur for a well defined range of positions. This excitation corresponds to the selection of a slice in the sample.

spaceholder redThe length of the RF pulse, and thus also its bandwidth, is the se­cond factor in­­flu­enc­ing the slice thick­ness. The longer the pulse, the thin­ner the slice will be (Figure 06-16).

Figure 06-16:
(a) Long sinc pulses lead to thin slices, whereas (b) short sinc pulses increase slice thickness

The trade-off for thinner slices is the prolongation of the echo time (TE). Be­cause TE is measured from the center of the pulse, lon­ger pul­ses for thinner slices mean a lon­­ger initial TE, which, in turn, influ­ences imaging time, image artifacts, and con­trast.

Changing the frequency of the RF pulse corresponds to moving the position of the nuclei on resonance from the center of the sample. In this way we can move the slice to any desired location along the axis (Fig­ure 06-17). For a transverse slice, the slice gradient is applied along the z-axis; for a coronal slice, the slice gradient is applied along the y-axis; and for a sagittal slice, it is applied along the x-axis.

Figure 06-17:
Moving the slice position: at 1.0 T, the resonance fre­quency in the center of the sample corresponds to 42.57 MHz. Changing the pulse frequency by several kHz moves the slice off-center.

06-05-02 Slice Definition

In analytical NMR studies, the maximal RF power is applied for a time suf­fi­ci­ent­ly long to give the desired pulse angle (hard pulse). In more complicated ex­pe­ri­ments, it is nec­essary to adjust the pulse amplitude with time so as to give a bet­ter de­fin­ed fre­­quen­cy content (shaped pulse).

The pulse shape is used to give an ap­proximately rectangular slice profile for the slices in the imaging experiments (Gaussian and sinc pulses; see Figure 01-09) and can heavily influence image contrast in mag­­ne­tic re­so­nan­ce imag­ing.

The phase of the RF pulse is also deter­mined at this stage, with many MR ma­­chi­nes only allowing phases of 0°, 90°, 180° or 270° to be selected. The re­­sul­t­ing ex­ci­ta­tion pulses can be as short as 10 ms for non-selective hard pulses, and ty­pi­cal­ly a few milliseconds for the frequency-selec­tive shaped pulses used in mag­ne­tic reso­nance imaging with peak-to-peak ampli­tudes of up to several hun­dred volts.