Biomedical Engineering Reference
In-Depth Information
bly be applicable as a diagnostic tool in mammography. In the future, this
technique will be applied to a reflection system, which is not limited by the
size of the target. The possibility of a reflection system can be considered, be-
cause the imaging mechanism in optical measurement with the assistance of
ultrasound assistance is explained by scattering theory. The theory indicates
that the main kind of scattering involved in biological tissue is forward light
scattering, and that backscattered light is only slightly involved. This shows
the possibility to reflection measurement, through the detection of the small
amount of backscattered light. The reflection system can be realized by repla-
cing some apparatus. The light source is replaced by a near-infrared light, to
reduce the light scattering effects. The photodetector is also exchanged for a
highly sensitive one, such as an avalanche photodiode and a photomultiplier.
Furthermore, a high voltage is applied to the ultrasound transducer produces
a high-power ultrasound field, and the pulse shape of the ultrasound wave is
optimized. In extending the application to biological tissue, a coupler is nee-
ded in which the acoustic impedance of the ultrasound transducer is matched
to that of biological tissue. The specification also demands a material that is
clear in the visible region so that light can be transferred eciently into the
tissue, and the power of the pulsed ultrasound and light should be increased,
while adhering to safety standards.
2.5
An Ultrasonic Drug Delivery System
Using Microcapsules
2.5.1
The Requirement for a Drug Delivery System
We have developed an ultrasonic drug delivery system (DDS) using air-filled
microcapsules. We have confirmed the physical possibility of the resonance
design for the drug carriers, depending on the shell structure of the micro-
capsule. In addition, we have developed a novel echographic imaging system
to locate microcapsules precisely. The path lines of microcapsules in blood
flow could be clearly visualized.
Cancer, which ranks as one of the principal causes of death, and arte-
riosclerotic thrombosis, are both local lesions. However, it is often inevitable
that drugs having that are strongly effective against such diseases will be
used, even though they have seriously adverse effects on the whole body. As
a result, there has been a great demand for a method that will release the
drug only to the diseased region.
To meet such a demand, various DDSs for realizing the most effective cure
by drugs while suppressing the side effects have been proposed with regard to
the desired absorption passage, degradation rate, metabolic rate, and other
factors. Nevertheless, none of the proposed methods have satisfied all of the
following important conditions that are necessary in clinical application:
