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矢印 High Frequency Ultrasound Imaging 矢印 Blood Flow Imaging 矢印 Parametric Imaging 矢印 Photoacoustic Imaging 矢印 Mobile Imaging
Resolution and Frequency of Ultrasound
Ultrasound is known as safe and portable imaging modality. In technical saying, ultrasound is not only “handy” but also high spatial and temporal imaging modality.
As the spatial resolution is anti-proportional to the frequency of ultrasound, higher frequency ultrasound enables higher resolution imaging.
Left figure shows the relationship between ultrasound frequency and resolution in clinical settings. Several MHz ultrasound is used in conventional echography and echocardiography, approximately 10 MHz ultrasound is used in carotid scan and ultrasound for surface organs such as thyroid, breast scan and muscle scans. 20-45 MHz ultrasound is used for intravascular ultrasound (IVUS).
Ultrasound microscope uses ultrasonic frequency higher than 100 MHz and it has achieved 15 micron resolution (equivalent to 40 times magnification in optical microscopy). Higher frequency such as GHz range ultrasound enables imaging of a single cell.

Principle of Ultrasound Microscope
Ultrasound is tightly focused to nearly wavelength by planar transducer with sapphire acoustic lens or concave transducer.
Transducer is two-dimensionally scanned over the specimen on slide glass or cultured cells on the dish by mechanical scanning. Staining of the tissue is not necessary. It takes 10 seconds for visualization only and quantitative measurement of sound speed, attenuation and acoustic impedance of the tissue is completed in 1 minute.

What Is Visualized by Ultrasound Microscope?
We have started application of ultrasound microscope in medicine and biology in 1985. Our first aim was quick and easy observation of the tissue during surgery. Secondly, our aim was changed to assess the origin of echo. Recently, application of the ultrasound microscope has covered elasticity and viscosity of the biological tissues from view point of acoustic properties as shown in the equations in the right column.
Biomechanical study is important in myocardium and atherosclerosis because pathophysiology of these organs are closely related to flow dynamics. Recently, our application also covers orthopedic surgery and dentistry and novel findings on tissue biomechanics are made.

Examples of Acoustic Microscopy
Left figures show optical microscopy and acoustic microscopy images of myocardial infarction (upper) and atherosclerosis (lower). Collagen fiber generated after myocardial infarction, had been believed to be an origin of the strong echo in myocardial infarction. In acoustic microscopy study, sound speed of collagen was high. However, the strong echo was generated from the large difference of acoustic impedance between collagen and surrounding tissues. The collagen fiber in dilated cardiomyopathy showed low sound speed and we confirmed the elasticity of collagen fiber changed according to the physiological state of the myocardium.
Histology of atherosclerosis is often characterized by the thickening of the intima and lipid deposit in the intima. Acoustic microscopy showed low sound speed tissue (lipid) was covered with high sound speed tissue (fibrosis of the intima). From these observations, we assessed how the plaque rupture occurred by finite element analysis of the elasticity information obtained with acoustic microscopy.

Ultrasound Impedance Microscope
We have already succeeded in acoustical “staining” by acoustic microscopy. However, thin-slicing of the tissue was still required and it took same time and procedures for preparation of the specimen of optical microscopy.
Ultrasound impedance microscope was developed by strong collaboration with Professor Naohiro HOZUMI at Toyohashi Institute of Technology. In conventional acoustic microscopy, ultrasound penetrated through thin-sliced tissue and reflected at the surface of the substrate (normally glass slide). In ultrasound impedance microscopy, ultrasound penetrates through the plastic plate and reflects at the interface between the plastic plate and the tissue. The interface usually becomes flat because the biological material is soft and it fits the surface of the plastic plate. The distribution of the specific acoustic impedance of the tissue is easily obtained by the mechanical scanning of the interface.
Right figure shows cross-section of rat cerebellum (collaboration with Dr. Sachiko YOSHIDA at Toyohashi Institute of Technology). Recently we have started an international collaboration how the folding of the brain is generated.

Skin Imaging
Three-dimensional (3D) ultrasound microscope was developed after our recent developments of ultrasound impedance microscope and ultrasound speed microscope. 3D ultrasound microscope can visualize tissue small vessels and har follicle of the dermis.
Since April 2010, our laboratory was also included in Smart Aging International Research Center at the Institute of Development, Aging and Cancer. We have stated integration researches on aging of the skin because brain, hair and skin are the three major interests in aging society. We have found that skin elasticity and sebum level are closely correlated with the size and number of sebaceous glands in the dermis.
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