As has been pointed out already, living human anatomy is essentially four-dimensional, with three time-variant spatial dimensions. Ultrasound, with its inherently rapid data acquisition capability, is exceptionally well placed to work in this way. Nevertheless, in order to achieve good spatio-temporal resolution, data will have to be acquired from a great many voxels (volume elements), and a choice is available as to how this can best be achieved for a particular type of application.
It will be useful at this point briefly to consider in more detail this issue of data acquisition rate. The conventional procedure for deriving a pulse-echo image does not make use of coded acoustic pulses or of receiving echoes along multiple lines of sight in parallel. As such, the method employed to distinguish echoes due to one pulse from those due to another, and avoid the echo position ambiguity that would otherwise ensue, is by means of recording a sequential series of image lines, allowing time for all echoes to return along one line before transmitting the next pulse. The time required to obtain a single such line, extending to depth d in the object, is:
where c is the speed of sound in the object material. The minimum time to obtain an w-line image is thus:
Hence the time to acquire an image of 100 lines (corresponding to rather poor sampling), covering a depth of 10cm in soft tissues (c ^ 1500ms_1), will be approximately 13ms. Practical considerations will increase this figure somewhat, particularly if, for example, the information to be displayed is not unprocessed echo magnitude and is not available from a single pulse: e.g. when some speckle reduction methods are employed, or when the information is associated with tissue motion or with the degree of propagation non-linearity (see later this chapter and Chapter 10). Nonetheless, it is feasible to obtain good B-scan image quality at frame rates of the order of 10-20 Hz, where human visual sensitivity to brightness modulation is low (see Chapter 8). As was suggested above, there are many schemes for altering the transmit and receive pulse and beam-forming process so as to increase this frame rate but it is an intriguing thought that so-called 'real-time ultrasound' using this simplest of image-forming methods is only made practicable by the apparently fortuitous set of relationships between speed of sound, the scale of human anatomy and the particular physiology of the human eye and brain!
If, as is commonly the case, acquired data are to be presented to a human observer, there will be a choice as to the format in which such presentation will be most appropriate for the investigative task on hand. Some of the earliest scanners simply displayed on an oscilloscope the amplitude of the incoming echo signals, in either r.f. or rectified ('video') form and usually following time-gain compensation (see below) against a time base triggered by the transmit pulse. This so-called 'A-scan' is now rarely, if ever, used apart perhaps from some restricted measurement applications in ophthalmology, where the anatomy lateral to the beam is expected to be reasonably invariant and predictable.
If multiple A-scans are recorded at high repetition rate and over a long time period, very rapid and small movements of tissue structures, which may have been imperceptible on older t\ = 2d/ c
or lower cost B-scan systems, may be evaluated. This feature has been exploited, particularly for cardiac application, in the M (movement)-scan, in which the time of arrival of the echo from a selected anatomical entity (e.g. a heart valve) is displayed against a slow time base (Figure 9.2). An important feature of this mode of data acquisition is that, even in systems of modest cost, it can be carried out at a repetition rate sufficiently high to follow the very fast movement of heart structures. Techniques such as parallel beam-forming that increase frame rate, and looped digital memory that makes inter-frame motion replay and comparison possible, have made the M-scan almost obsolete but at increased hardware cost.
The principle of the B-scan is well known because it is the basis of virtually all contemporary medical ultrasonic investigation. It can be seen, again, as a development of the A-scan, in which the video echo train is used to brightness-modulate a line in the image plane that is made to correspond instantaneously with the ultrasonic beam axis in the object plane. Thus a 'scan' of this axis across the object plane will build up a corresponding brightness-modulated image.
It is worth bearing in mind at this stage that this procedure, although of great practical value, is in a sense arbitrary and has evolved through considerations of convenience rather
Figure 9.2. An M-mode display (lower picture) showing systolic anterior motion of the mitral valve, where the valve can be seen (white arrow) to be hitting and maintaining contact with the septum. The black arrow in the upper, cross-sectional B-scan indicates the acoustic line of sight to which the M-mode pertains. This example has historical importance although, with modern high frame rate scanners, such a diagnosis might now be made from real-time B-mode imaging alone. Source: Images are courtesy of P. Nihoyannopoulos
Figure 9.2. An M-mode display (lower picture) showing systolic anterior motion of the mitral valve, where the valve can be seen (white arrow) to be hitting and maintaining contact with the septum. The black arrow in the upper, cross-sectional B-scan indicates the acoustic line of sight to which the M-mode pertains. This example has historical importance although, with modern high frame rate scanners, such a diagnosis might now be made from real-time B-mode imaging alone. Source: Images are courtesy of P. Nihoyannopoulos than from scientifically based strategy. It should become evident from later discussion that there may well be potential imaging parameters that, although at present less easy to implement in practice, could in some applications be more sensitive or reliable indicators of anatomy and pathology than a map of the modulus of echo amplitude, modified to an indeterminate extent by tissue attenuation.
The physical 'scanning' process referred to above - movement of the beam axis within a scanning frame - can be effected in two basic ways: by actual movement of a transducer having a fixed beam axis or by electronically controlled movement of a beam axis relative to an array-type transducer whose position is essentially fixed in space. The former approach has a long and creditable history but arrays are now in virtually universal use.
Array transducers in common use are broadly of two types, so-called 'linear' and 'phased' arrays, the terminology referring to the method by which scanning of the beam is achieved. The principles of these two arrangements are illustrated in Figure 9.3. In a linear array (Figure 9.3a), successive, spatially limited groups of array elements execute successive transmit-receive sequences, with the selected group being progressed, usually by one element width per image line, along the length of the array, thus acquiring a complete image frame by translating the point of origin of the beam from one end of the array to the other. As illustrated in Figure 9.3b, some degree of focusing in the array plane may be achieved by appropriately adjusting
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