Biomedical Engineering Reference
In-Depth Information
FIGURE 5.6
Geometric distortion resulting from ultrasound depth and refraction errors. In the upper
figures, outlines of a circular reflector of radius 50 mm at a depth (to center) of 100 mm
have been drawn. Superimposed on these are the new outlines due to propagation through a
uniform low velocity layer (left) and high velocity layer (right) using a linear array trans-
ducer; i.e., parallel scan lines. In the lower figures, the horizontal lines represent a plane
reflector of length 50 mm at a depth of 30 mm in a uniform medium. The curves represent
the appearance of this plane reflector when the medium has two layers of different velocities
separated by a horizontal interface when using a sector transducer. The ratio between the
velocity of the upper layer and that of the lower layer is 0.9 (left) and 1.1 (right). The
geometric distortion has been exaggerated for illustrative purposes—the maximum varia-
tion of the velocity of sound is only a few percent.
The tissue volume corresponding to an image pixel is determined by three
components of spatial resolution, each of which is dependent on depth and,
possibly, on the lateral position within the image. To a first approximation,
the spatial resolution is determined by the width of the ultrasound beam in
each of three directions: axial (along the beam), lateral (perpendicular to the
beam and in the plane of the image), and slice (perpendicular to the image
plane). Since the scan lines used to form an ultrasound image are not nec-
essarily parallel, these three components may not correspond to conven-
tional Cartesian axes. Each of these components can now be considered in
turn:
1.
Axial resolution is determined by the pulse width, which tends to
broaden with depth due to frequency-dependent attenuation.
However this is a small effect and in most cases the variation of
axial resolution with distance along the beam can be neglected.
