of probe are used with ultrasound machines (see Figure 3.10):
linear arrays;
curved arrays (sometimes called "curvilinear" or "convex" arrays);
phased arrays.
Figure 3.10 (left to right) Typical linear, curved and phased array probes.
As we will see shortly, the word "array" refers to the fact that the transducer (the active element in the probe) is sliced into a large number of identical transducer "elements" to facilitate electronic focussing and scanning of the ultrasound beam.
Figures 3.11, 3.12 and 3.13 illustrate the scan patterns and image shape for these three probe types. Why are different types of probe used in different clinical applications? The answer lies in their relative strengths and limitations. These relate to considerations such as the physical size of the probe (often called its "footprint"), the available "acoustic window" into the patient (i.e. the area in the superficial tissues through which ultrasound can travel) and the shape and elasticity of the skin where the probe is to be placed.
In some areas (e.g. transthoracic echocardiography, where the ultrasound must travel between ribs and must avoid the lungs) the acoustic window is extremely limited. In other areas (e.g. late pregnancy scans) the acoustic window is large and therefore not such a concern.
Figure 3.11 (a) In a linear array the beam is stepped from left to right for successive transmit pulses (only the first three beam positions are shown).
Figure 3.11 (b) As the beam moves from one end of the probe to the other it sweeps out a rectangular field of view and so the image (black) is rectangular.
Figure 3.12 A curved array is similar to a linear array. However, due to the curvature of the probe the lines of sight are not parallel but instead sweep out a radial path. This type of probe therefore provides a significantly wider field of view than the linear array.
Figure 3.13 Phased array. The point of origin of the beam remains fixed. It is scanned by steering it in a series of different directions. The resulting image provides a good field of view at depth, but virtually no information about superficial tissues.
In summary:
Linear array: ideal for relatively flat surfaces such as the neck and limbs; however, the field of view is limited by the probe dimensions.
Curved array: widely useful, probably the standard probe in most clinical areas; the field of view diverges with depth, so it is not so limited by probe size or the acoustic window; the degree of divergence depends on the degree of curvature of the probe and so it can be tailored to suit a given clinical application.
Phased array: the dominant probe type in echocardiography; compact, easily manipulated and able to make use of a very small acoustic window; major limitation is the lack of detail close to the probe.
Two other types of probe are available for use in specific situations, namely invasive and 3D probes.
Invasive probes include transvaginal (see Figure 3.14), transrectal and transoesophageal. As with standard probes they use array technology to focus and scan the beam.
Figure 3.14 Transvaginal probe.
An advantage often claimed for ultrasound is that it is non-invasive, so why are invasive probes used? There are two reasons. First, some areas are virtually impossible to reach with ultrasound from the skin surface because of intervening gas and bone. An invasive probe can get very close to the region of interest (ovaries, prostate, aortic outflow tract etc) and so avoid the problems relating to overlying tissues.
Secondly, the fact that the probe is close to the tissues of interest means that a higher frequency can be used and so the image resolution is improved over what could be achieved with standard (non-invasive) scanning.
As discussed in chapter 12, 3D imaging places an extra requirement on the probe. The scan plane must be swept through the patient's tissues in order to acquire echo information from a three dimensional volume. Historically this has been achieved by moving the transducer, either manually or mechanically. More recently, however, it has become possible to achieve the same thing electronically using "matrix" transducers, which will be discussed more fully in chapter 4.
A, B and M mode
The technique used to produce the standard ultrasound image is referred to as "B-mode" (Brightness mode) imaging. Note that it produces a two-dimensional image of the patient's anatomy. Two other imaging modes are used in specific clinical areas, both of them one-dimensional.
"M-mode" (Motion mode) is widely use in echocardiography to provide detailed information regarding the movements of the heart walls and valves. To produce an M-mode display, the machine keeps the ultrasound beam in a fixed position and repeatedly transmits and receives along this beam. The display of the echoes is swept slowly from left to right on the screen over a period of several seconds.
Structures that are stationary relative to the probe (e.g. the chest wall) will be displayed at a constant depth and therefore as horizontal lines, while structures that move towards and away from the probe (e.g. the heart walls) will move up and down the screen and so the display will document their position as a function of time, as shown in Figure 3.15.
Figure 3.15 (Left) The beam is directed along the line of sight indicated by the broken line. (Right) The resulting M-mode display shows the depth of the tissue structures along this line of sight as a function of time over a period of several seconds.
The M-mode display thus provides information about the amount of movement of individual structures, the speed at which they move and their acceleration.
It also shows the relative position of structures and how that changes with time (e.g. the maximum and minimum diameters of a heart chamber, or the movement of two valve leaflets as the valve opens and closes). Often an ECG trace is also shown on the screen to provide a timing reference (as shown in Figure 3.16).
Figure 3.16 An M-mode trace showing the movement of the mitral valve leaflets.
The other one-dimensional imaging mode is the A-mode (Amplitude mode) display. Again the beam is kept in a fixed position and the machine transmits and receives along this line of sight. However, the display simply shows echo amplitude as a function of depth, as shown in Figure 3.17.
This
