New Noninvasive Diagnostic Method
Ultrasound elastography is a Japanese original technology that uses an ultrasound diagnostic device to visualize elasticity of tissues. In 2003, ultrasound elastography was developed and commercialized the first in the world by Tsuyoshi Shiina, a professor at Kyoto University, Ei Ueno, who was a professor at Tsukuba University and is now the director of Tsukuba International Breast Clinic, and engineers of Hitachi, Ltd. Ultrasound elastography was named Real-time Tissue Elastography (RTE). The technology has collected international attention as a new diagnostic method. RTE has been evolved in a dozen of years. RTE has attained clinical effectiveness in the mammal gland field. In recent years, RTE has been widely used for the upper abdominal area, thyroid, prostate, and so forth. Since RTE can qualitatively and noninvasively diagnose patients, it would revolve the diagnostic strategies.
Image: Hitachi's Noblus
Future Prospects and Current Situation of Real-time Tissue Elastography
Diffuse liver disease patients need to be evalu6ated whether they have hepatic fibrosis. Although they are regularly diagnosed by liver biopsy, it is invasive and may be subject to sampling errors. Diffuse liver disease patients may be diagnosed noninvasively with serum markers, elastography, or the like. Serum markers are susceptible to other than hepatic fibrosis. Likewise, some elastography devices are also susceptible to other than hepatic fibrosis.
Categories of Ultrasonic Elastography
Depending on "physical quantity ultrasound elastography measures," it is categorized as two types: strain imaging that measures relative strain and shear wave imaging that measures propagation velocities of share elastic waves.
It is known that cancer tissues of the mammal gland and prostate gland are harder (have higher Young's modulus) than their normal tissues. Strain imaging is highly desired because of high diagnostic yield. In shear wave imaging, the hardness of a tissue to be measured is calculated based on the relationship of hardness of living body = 3 x (propagation velocity)2.
When a tissue of the living body is pressured, hard tissues are more strained than soft tissues. RTE, which is one type of strain imaging, spatially differentiates relative changes of the tissue, calculates the strain amounts, and displays hard parts and soft parts in 256 blue levels and 256 red levels, respectively.
Measuring Liver with RTE
When liver is measured with RTE, the probe is placed between ribs of the patient so as to obtain images of the liver that is relatively strained by the beats of the heart. Although RTE of the liver can be measured without influence of ascites, there are artifacts such as large blood vessels, acoustic shadows, multiple reflection, and poor penetration.
Benefits of RTE
In strain imaging, there are several methods for measuring strains of tissues. The special correlation method (speckle tracking method) requires an enormous amount of computation. Likewise, the phase difference method (Doppler method) is subject to aliasing errors that occur if a large change that exceeds a half wavelength is measured.
On the other hand, the combined autocorrelation method used for RTE is a very fast, low error measurement method. In addition, this method does not require a large amount of computation. As a result, RTE can provide images in real time and does not require a large installation space.
Hitachi, Ltd. has marketed "Noblus" that is a portable ultrasonic RTE device. This device allows small-sized hospitals and clinics that do not afford high end devices to perform elastography for patients.