Biomedical Engineering Reference
In-Depth Information
which is useful for the treatment of a blood vessel, such as in percutaneous
transluminal coronary angioplasty (PTCA) [24].
We have been studying the ultrasound frontal imaging system [25]. This
imaging method requires one transmission of spherical ultrasound to recon-
struct a three-dimensional (3D) image. Therefore, this method enables us to
show a 3D frontal image with a very high frame rate, above 1000 fps.
In this section, imaging by a spherical ultrasound wave is described. This
imaging is an instantaneous imaging method [25] and is based on the synthetic
aperture method [26] and a spherical wave that is generated by laser-induced
breakdown (LIB).
2.3.1 An Instantaneous Imaging Method
In the conventional ultrasound imaging method, a pulsed ultrasound beam
scans the object electrically to reconstruct a two-dimensional (2D) image
[27]: this is called a “sequential imaging method”. The image has a frame
rate of about 10-100 fps, but it is dicult to increase the frame rate to about
1000 fps because ultrasound has a finite velocity in the human body, typically
1500 m/s. In the final decade of the last century, the sequential imaging me-
thod was often extended to 3D imaging [28]. However, the sequential imaging
method takes a much longer time to reconstruct a 3D image because of the
finite velocity of ultrasound. The sequential imaging method, especially 3D
imaging, is not suitable for use in a treatment that requires rapid imaging.
To overcome this diculty, we have proposed an imaging method that we
call an “instantaneous imaging method” [25], using the synthetic aperture
method and a spherical wave. In the experiment using this method, a 3D
object was imaged as follows:
1. A sound source, for example an ultrasound transmitter, a measurement
object, and several ultrasound receivers were positioned in water.
2. A limited measurement region was determined and divided into small
elements (voxels).
3. The ultrasound flight time from a sound source to a receiver by way of
an element was calculated for all receivers and elements.
4. Pulsed and spherical ultrasound were transmitted once.
5. The wave reflected from a measurement object was received at several
receivers.
6. The magnitudes of the echo signals from each element were summed for
all receivers.
7. The summed values were converted to gray levels in a 255-level scheme.
8. The 3D image of a measurement object was reconstructed.
As spherical ultrasound is transmitted, this imaging method enables us
to obtain a 3D image with a frontally wide view. Moreover, the 3D image
has a very high frame rate above 1000 fps because the imaging method needs
only one transmission of ultrasound. However, the following problems are
encountered:
