Image features in medical vibro-acoustography: In vitro and in vivo results

Abstract

Vibro-acoustography is an imaging method based on audio-frequency harmonic vibrations induced in the object by the radiation force of focused ultrasound. The purpose of this study is to investigate features of vibro-acoustography images and manifestation of various tissue structures and calcifications in such images. Our motivation for this study is to pave the way for further in vitro and in vivo applications of vibro-acoustography. Here, vibro-acoustography images of excised prostate and in vivo breast are presented and compared with images obtained with other modalities. Resulting vibro-acoustography images obtained with a 3 MHz ultrasound transducer and at a vibration frequency of 50–60 kHz show soft tissue structures, tissue borders, and microcalcifications with high contrast, high resolution, and no speckle. It is concluded that vibro-acoustography offers features that may be valuable for diagnostic purposes.

Introduction

Ultrasonography is one of the most common medical imaging modalities. However, there are some limitations to this method. Namely, ultrasound images suffer from a speckle artifact. As a result, small objects, such as microcalcifications are hard to detect with this method. Speckle also can reduce the sensitivity of the imaging system in detection of masses in tissue. For these and other reasons, investigators have been seeking alternative non-invasive imaging methods that can offer higher quality images as well as new information about tissue.

Vibro-acoustography is an imaging method based on the radiation force of ultrasound [1], [2]. In this method, the image is formed from the acoustic response of the object to the oscillating radiation force of the amplitude modulated ultrasound. In vibro-acoustography, two intersecting continuous-wave ultrasound beams at slightly different frequencies of f₁ and f₂ = f₁+Δf, where Δf<<f₁, are used. The two ultrasound beams are focused, and they are aligned to intersect at their respective focal regions. At this intersection region, which is normally a small volume, the combined ultrasound field energy density is sinusoidally modulated at Δf, thus the field generates a highly localized oscillatory radiation force at the difference frequency when it interacts with the object. The harmonic force vibrates the object at Δf. The vibration results in a secondary acoustic field that propagates in the object. This acoustic field, which is at frequency Δf, is detected by an audio hydrophone. This signal is then filtered by a band pass filter centered at Δf to reject noise and any interfering signal. As the ultrasound beam is scanned across the object, the filtered hydrophone signal is recorded and its amplitude is mapped into an image.

To understand the interaction of two ultrasound fields at different frequencies and generation of a third acoustic field at the difference frequency, one needs to solve the nonlinear wave equation. The general theory of nonlinear wave propagation has been investigated extensively, for example see references [3], [4]. A detailed theoretical and simulation study of wave propagation and interaction of two ultrasound beams, as it applies to vibro-acoustography, is presented in another paper in this issue [5]. Therefore, we do not elaborate on the theoretical details of wave propagation in this paper, instead refer the reader to [5].

A comparison of conventional B-mode ultrasound imaging and vibro-acoustography is illustrated in Fig. 1. Ultrasound imaging is based on linear reflection or scattering of ultrasound. That is, the echo, which is used to construct the image, is at the frequency of the source ultrasound. In contrast, the secondary acoustic field that is used for making the vibro-acoustography image is at the difference frequency Δf, which is typically two order of magnitude smaller than the incident ultrasound frequency. This frequency conversion is the result of a nonlinear process that is central to vibro-acoustography methodology. The frequency conversion enriches the vibro-acoustography image with additional information not present in conventional ultrasound image.

A vibro-acoustography image contains two types of information: ultrasonic properties of the object, such as the scattering and power absorption characteristics, and the dynamic characteristics of the object at frequency Δf, which relates to tissue stiffness, boundary conditions, and coupling to the surrounding medium [6]. The former properties are those that are also present in conventional ultrasound imaging. The latter properties, which can be described in terms of object’s mechanical parameters at Δf, are not available from conventional ultrasound. Another feature of vibro-acoustography relates to image speckle. Speckle is the snowy pattern seen in conventional ultrasound images. Speckle results from random interference of the scattered ultrasound field. Speckle reduces the contrast of ultrasound images and often limits detection of small structures, such as microcalcifications in tissue. Because vibro-acoustography uses the secondary acoustic field, this modality is practically speckle-free, resulting in high contrast images that allow small structures to be visible. This feature makes vibro-acoustography particularly suitable for detection of breast microcalcifications.

Vibro-acoustography has been tested on various human tissues [6], [7], [8], [9], [10], [11], [12], [13], [14]. A comparative study of vibro-acoustography with other radiation force methods for tissue elasticity imaging is presented in [7]. The spatial resolution of vibro-acoustography is in the sub-millimeter range, making the technique suitable for high-resolution imaging [9], [11].

In this paper, we present some experimental results and discuss some features of vibro-acoustography images and their potential applications.