Page 285 - Biomedical Engineering and Design Handbook Volume 2, Applications

INSTRUMENTATION DESIGN FOR ULTRASONIC IMAGING 263

9.5 SIGNAL PROCESSING AND SCAN CONVERSION

As has been noted earlier, the image data is acquired on a polar coordinate grid for sector scanners
(e.g., mechanically steered, curvilinear arrays, and phased-array systems) and in a rectangular grid for
linear arrays. It is clearly necessary to convert this image data into one of the standard TV raster for-
mats for easier viewing, recording, computer capture, and so on. This is performed in a module in
most systems referred to as the scan converter. The major function of the scan converter is that of
interpolation from, say, a polar data grid to that of the video pixel space. Given the data rates required,
it is a challenging task but one that most commercial systems appear to have well under control.
The early scan converters used several clever schemes to accomplish the required function. For
example, with sector scanners, certain systems would modulate the A/D converter sampling rate by
the look angle of the beam former in such a way that every sample would fall onto a raster line. Once
the acoustic frame was completed, and the data along each horizontal raster line was read out, simple
one-dimensional interpolation was performed (Park, 1984). The need for more sophisticated processing
brought about two-dimensional interpolation methods. Among the first in this area were Larsen et al.
(1980) whose approach involved the use of bilinear interpolation. With the Larsen et al. (1980)
approach, the sampling rate along each scan line was held at a uniform value that was high enough to
meet the sampling theorem requirements. With two axial samples along two adjacent acoustic lines,
the echo strength values at each of the pixels enclosed by the acoustic samples were determined. This
could be done by either (1) interpolating angularly new axial sample values along a synthetic acoustic
ray that traversed through the pixel in question and then performing an axial interpolation along the
synthetic ray or (2) by interpolating a new axial sample along both real acoustic rays and then per-
forming the angular interpolation. This basic approach has become the most widely used scan-conversion
method among the various manufacturers and seems to have stood the test of time.
More recently, researchers have continued to study the scan conversion problem with different
approaches. For example, Berkhoff et al. (1994) have evaluated “fast algorithms” for scan conversion,
that is, algorithms that might be executed by software as opposed to dedicated hardware. Given the
rapid trend to faster, more powerful, and cost-effective computers and their integration with ultrasound
systems, it is likely that more of the scan conversion function will be done in software. Berkhoff et al.
(1994) recommend two new algorithms that they compare with several conventional interpolators. With
the speed of computers improving at a steady pace, these approaches are increasingly attractive
(Chiang, 1997). In other words, with the cost of some of the hardware components such as A/D con-
verters coming down, oversampling the acoustic line data may permit the replacement of bilinear inter-
polation with simple linear interpolation. Oversampling by a factor of two along with linear interpolation
was found to be superior to bilinear interpolation under certain specific circumstances. It is clear there
is additional work in this area yet.

9.6 CURRENT TRENDS IN MEDICAL ULTRASOUND SCANNERS

In comparison to the other major imaging modalities such as CT, MRI, or PET, ultrasound is blessed
with a relatively simple image data transduction method. The arrays described earlier are all handheld;
this is in direct contrast with the other modalities that require room-sized gantries with rapidly rotating
64-slice or higher banks of detectors or superconductive magnetic field sources with rapidly changing
gradient fields. The remainder of an ultrasound scanner is largely digital signal processing. The activ-
ities of the microelectronics and personal computer industries have brought about major size, cost, and
power requirement reductions in the circuitry used by ultrasound scanners. This became apparent with
the rapid migration of key parts of the scanner from hardware to software, usually PC-based software.
Key among these were transmit beam formation, the scan converter function, and pled with the con-
tinuous reduction in feature size of semiconductors along the lines of Moore’s law, have enabled the
introduction of laptop-sized ultrasound scanners. This particular market segment is the fastest growing
for ultrasound scanners (Klein Biomedical Consultants). It is likely that this miniaturization trend will
continue even though Moore’s law is showing signs of slowing down.