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The repeated application of the A-scan is also suitable for producing cross-sectional images. In the case of this technique the emitting surface of the transducer moves steadily along a fan-shaped or a straight line. Instead of mechanical motion, using several crystals in the device is also feasible, in which case the crystal that emits the ultrasound is electronically controlled. In the case of straight-line motion, the colour-coded values of the A-scan, the time interval of which is $\Delta t$ , have to be placed in parallel columns. In the case of fan-shaped motion, the emitting surface turns with angle $\Delta \varphi$ during a time interval $\Delta t$ ; accordingly, the reflexivity values are plotted on the screen along turning segments, using colour codes. It is clear that in the direction perpendicular to the radius the lower the resolution will be, the higher the value of $\Delta \varphi$ is, and the farther the image segment is from the centre of the fan-shaped motion.

Figure 5.

The restriction $\Delta t \geqslant 2z_{max}/c$ needs to be considered in the case of this method as well. In practice it means that if we want to perform a scan the maximal depth of which is $z_{max}=20 cm$ , and if we want to produce the segment from n=200 lines, the time interval needed is

$\frac {2nz_{max}}{c}=\frac{2 \cdot 200 \cdot 0.2m}{1540 \frac{m}{sec}} \approx 51ms$ .

If we would like to achieve a higher resolution, the time of recording increases in direct proportion to the value of n.

Figure 7. Abdominal ultrasound image

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Medical Imaging

Site Language: English