Biomedical Engineering Reference
In-Depth Information
16.6.1 Ultrasound
Unlike other imaging modalities, ultrasound resolution and penetration depend on the
center frequency and type of transducer selected. The resolution is spatially variant and
depends on both the size of the active aperture and the center frequency (and bandwidth)
of the transducer and the selected transmit focal depth. A commonly used focal depth to
aperture ratio is five, so the half-power beam width is approximately two wavelengths at
the center frequency, and thus the transmitted lateral spatial resolution is about two
wavelengths. For typical frequencies in use ranging from 1 to 15 MHz, lateral resolution
ranges from 3 mm to 0.3 mm and is the smallest in the focal region and varies elsewhere
in a nonuniform way because of diffraction effects caused by apertures on the order of a
few to tens of wavelengths. For a short pulse, axial resolution is approximately two
wavelengths.
Another major factor in determining resolution is attenuation that limits penetration.
Attenuation increases with higher center frequencies and depth; therefore, penetration
decreases correspondingly, so fine resolution is difficult to achieve at deeper depths.
Ultrasound images are highly detailed and geometrically correct to first-order maps of
the mechanical structures of the body according to their “acoustic properties” such as dif-
ferences in characteristic impedance that depend on stiffness or elasticity and density.
The dynamic motion of organs such as the heart can be revealed by ultrasound operating
at up to hundreds of frames per second.
Diagnostic ultrasound is noninvasive. Ultrasound is also safe and does not have any
cumulative biological side effects. Two other strengths of ultrasound imaging are its rel-
atively low cost and portability. With the widespread availability of miniature portable
ultrasound systems for screening and imaging, these two factors will continue to
improve.
A high skill level is needed to obtain good images with ultrasound. This expertise
is necessary because of the number of access windows, differences in anatomy, the
many possible planes of view, and the experience required to find relevant planes
and targets of diagnostic significance and to optimize instrumentation. Furthermore, a
great deal of experience is required to recognize, interpret, and measure images for
diagnosis.
16.6.2 Computed Tomography Imaging
Computed tomography (CT) (also known as computed axial tomography, or CAT)
scanning involves x-rays and has been described in Chapter 15. As the x-rays pass
through the body, they are absorbed by tissue so an overall “mean attenuation” image
results along the ray path. Spatial resolution is not determined by wavelength but by focal
spot size of the x-ray tube and scatter from tissue; a typical resolution is about 1 mm.
Radioactive contrast agents can be ingested or injected to improve visualization of vessels.
Though exposures are short, x-rays are a form of ionizing radiation, so dosage effects can
be cumulative and extra precautions are needed for sensitive organs such as the eyes and
for pregnancies.
