Medical Imaging - Introduction to Biomedical Engineering - page 1069

This relationship shows that the location of the axial peak for a focusing aperture is now
moved in from the geometrical focal length. For example, if the natural focal length is
100 mm and the geometric focal length is 50 mm, the overall effective focal length is
33.3 mm. The shape of beams in the focal plane is the same as the far-field pattern of an
unfocused beam; consequently, Eqs. (16.44)-(16.45) can be applied with

.
So far, solid apertures have been described. Arrays can also be considered to be spatially
sampled apertures. To first order, the beams for both a solid and an adequately sampled
aperture are similar. The one-dimensional array actually has two types of focusing, as
shown in Figure 16.23. The azimuth or scan plane, here the

z ¼ F

x - z plane, is focused electroni-

cally, whereas the elevation or yz plane is focused by a fixed mechanical lens.

16.2.7 Ultrasound Imaging Systems
The formation of an image can be understood through the operation of an imaging sys-
tem. In Section 16.2.1, an imaging system was introduced as having a transducer that could
be scanned either mechanically or electronically to produce a pattern of sequential lines to
form an image. With reference to Figure 16.24, the management of an array imaging system
is accomplished by a computer or central processing unit (CPU). Once the scan depth and
mode are selected, transmit pulses, each repeating at the time interval for a line, are sent in
synchronism with a master timing clock like those in Figure 16.1. Each of these pulses initi-
ates a set of transmit pulses from the transmit beamformer that are sent to each element,
each one of which is delayed as necessary to form an electronic lens for focusing and steer-
ing the acoustic beam for the selected line direction in the azimuth plane.
Sound is scattered from tissue interfaces and inhomogeneities and is picked up as a
series of pulse echoes by the array acting as a receiver. Depth-dependent time gain compen-
sation (TGC) amplifiers can be controlled by the user to improve the image uniformity at
different depths. The receive beamformer applies electronic dynamic focusing (nearly

FIGURE 16.23 Focusing of a one-dimensional phased array in both the azimuth and elevation planes. Adapted
from [6].