154th ASA Meeting, New Orleans, LA

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Modulated Ultrasound and Multifrequency Radiation Force

Matthew W. Urban, Urban.Matthew@mayo.edu
Mostafa Fatemi, and James F. Greenleaf
Department of Physiology and Biomedical Engineering,
Mayo Clinic College of Medicine,
Rochester, MN 55905

Popular Version of Paper 3pBB1,
Presented 1:15 p.m. Thursday afternoon, November 29, 2007,
154th ASA Meeting, New Orleans, LA

Ultrasound can provide a view into the womb to give the first glance of a new child or be used to assess how well the heart is working after a heart attack. An emerging application of ultrasound for medical imaging uses focused ultrasound to push tissues (through a phenomenon called radiation force) and measures the resulting deformation to assess the stiffness of these tissues. Images of these responses can be formed by pushing different parts of the tissue.

Previously methods have used ultrasound to push tissue or vibrate it at a known frequency. A new application of ultrasound would vibrate tissue at multiple frequencies for enhanced measurement of stiffness. This is important because the stiffening of tissue is an indicator of disease as occurs in cancer, cirrhosis, or atherosclerosis.  

Shaping the ultrasound signals used to push tissues or modulating these signals with different waveforms so that vibration occurs at multiple frequencies (or tones) simultaneously is advantageous because it increases the amount of information that can be obtained in a measurement or imaging scan. The extra information can assist in detecting cancer or measuring of tissue properties. This advance is analogous to shifting from playing a piano piece one note at a time to playing sets of chords and creating a richer piece of music.

This multifrequency radiation force can be tuned for specific applications by using different modulation or shaping waveforms. This allows flexibility to isolate and emphasize motion at specific frequencies and obtain more accurate or useful measurements in tissues.

This multifrequency force serves as virtual finger in tissue to non-invasively palpate the tissue. One application is an imaging technique called vibro-acoustography, which locally vibrates tissue in the kilohertz range and listens to its response with a nearby microphone. This has been employed in breast imaging, and by using multifrequency radiation force, multiple images can be obtained in a single scan. These multiple images obtained from the vibration response at different frequencies assists in detection of cancer or other diseased tissue.

Another area of application is using multifrequency radiation force and the resulting vibration to characterize materials and tissue in a technique called ultrasound vibrometry. In this study, a small steel sphere was embedded in a tissue mimicking material. Using the multifrequency force, the sphere was vibrated with frequencies from 100-2000 Hz and the motion was measured using a laser. Different modulation waveforms were utilized to change the multifrequency radiation force to produce different results with different weights. This motion was used to assess the stiffness of the tissue mimicking material with very good agreement to a previously established method.

Multifrequency radiation force provides a new tool to probe tissue in a very selective way. Different modulation waveforms provide flexibility for applications in vibro-acoustography and ultrasound vibrometry. The information gained using multifrequency radiation force is increased over current practice which can aid clinicians in detection and distinguish normal from diseased tissue.

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