Biomedical Engineering Reference
In-Depth Information
increase the frame rate up to 15 fps or more to observe rapid movement of the
fetus in real time. However, an ultrasonic speed does have some limitations.
Ultrasound travels at a speed of 1540 m/s (mean) in soft tissue. This speed
limits the number of ultrasonic beams available per second. When a diagnostic
range is 15 cm, about 5000 ultrasonic beams per second are available at the
maximum. This means that only 18
×
18 pixels can be used for each 3D image
when the frame rate is 15 fps. Under these conditions, the resolution of the
image will be extremely low or the field of view will be extremely narrow if the
ultrasonic beam density is kept high enough. New technologies are needed to
realize real-time 3D ultrasound with high resolution and a wide field of view.
One of the promising technologies is parallel receiving processing, in which a
broad ultrasonic pulse is transmitted and ultrasonic echoes are received as if
many ultrasonic beams had come back from different directions. A 2D array
probe, in which transducers are arrayed two-dimensionally, can scan a 3D
space fast enough if parallel receiving technology is applied. Real-time 3D
ultrasound will be used not only for observation but also in treatment, such
as fetal surgery, as a noninvasive eye of the operator.
Tissue characterization by ultrasound will differentiate two kinds of tissue
and identify the border clearly. A 3D fetal surface image will be possible by
tissue characterization even when the fetus touches the uterine wall or the
placenta. Tissue characterization will realize automated fetal organ measure-
ment, which will be a new diagnostic method for some fetal conditions such
as hypoplastic lung, which is one of the most critical conditions for a newborn
baby and can be hardly diagnosed before birth. Even automated diagnosis of
fetal abnormalities is expected, by combining tissue characterization with a
computerized recognition technology.
Further development of 3D ultrasound will lead to many more fetuses in
the world being examined and treated. Three-dimensional ultrasound will be
used not only in obstetrics but in most fields of medicine, since the technolo-
gies being developed in 3D ultrasound for a fetus are applicable in all fields
of medicine.
2.3
Imaging by a Spherical Ultrasound Wave
Ultrasound imaging is widely used in various fields because it visualizes not
only the exterior structure of a measurement object but also the interior
structure that cannot be visualized with an optical measurement method,
such as the inside of a human body. Therefore, ultrasound imaging is an
essential technique for the medical field in diagnosis of human organs like
the heart, liver, and so on. Furthermore, intravascular ultrasound imaging
(IVUS) [23] has been developed to show the cross-section of a blood vessel
and reveal morphological information of a blood vessel, such as stenosis,
aneurysm, and so on in the clinical field of cardiovascular medicine. This
imaging method, however, cannot give the frontal image of a blood vessel,
