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Ultrasonics 44 (2006) e217–e220

Critical issues in breast imaging by vibro-acoustography

Azra Alizad a,*, Dana H. Whaley b, James F. Greenleaf a, Mostafa Fatemi a

a Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, 200, 1st St. SW Rochester, MN 55905, United Statesb Department of Radiology, Mayo Clinic College of Medicine, 200, 1st St. SW Rochester, MN 55905, United States

Available online 30 June 2006

Abstract

Clinically, there are two important issues in breast imaging: detection of microcalcifications and identification of mass lesions. X-raymammography is the main imaging method used for detection of microcalcification, and ultrasound imaging is normally used for detec-tion of mass lesions in breast. Both these methods have limitations that reduce their clinical usefulness. For this reasons, alternativebreast imaging modalities are being sought. vibro-acoustography is an imaging modality that has emerged in recent years. This methodis based on low-frequency harmonic vibrations induced in the object by the radiation force of ultrasound. This paper describes potentialapplications of vibro-acoustography for breast imaging and addresses the critical imaging issues such as detection of microcalcificationsand mass lesions in breast. Recently, we have developed a vibro-acoustography system for in vivo breast imaging and have tested it on anumber of volunteers. Resulting images show soft tissue structures and calcifications within breast with high contrast, high resolution,and no speckles. The results have been verified using X-ray mammography. The encouraging results from in vitro and in vivo experimentssuggest that further development of vibro-acoustography technology may lead to a new clinical tool that can be used to detect micro-calcifications as well as mass lesions in breast.� 2006 Elsevier B.V. All rights reserved.

Keywords: Vibro-acoustography; Breast imaging; Ultrasound; Mammography; Radiation force

1. Introduction

Pulse-echo ultrasound (ultrasonography) and X-raymammography are the common modalities used for breastimaging. Normally, ultrasonography is used to image softtissue imaging and detect lesions in breast. ray mammogra-phy is the only imaging modality clinically used fordetection of breast microcalcifications. The sensitivity ofmammography is greatly reduced in dense breasts. Theaccuracy of film screen mammography is also influencedby the experience of the radiologist, with experienced radi-ologists having the highest sensitivity in diagnosing breastcancer [1]. Furthermore, the ionizing nature of the X-raymammography limits its frequent use. Ultrasonographyapplication is hampered by the speckle patterns inherentto this imaging modality. Limitations in mammography

0041-624X/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultras.2006.06.021

* Corresponding author. Fax: +1 507 266 0361.E-mail address: aza@mayo.edu (A. Alizad).

and ultrasonography have prompted investigators toexplore alternative breast imaging techniques. Especially,non-invasive imaging methods that can show both the softtissue and microcalcifications are of particular interest.

Vibro-acoustography is a new imaging method based onthe radiation force of ultrasound [2,3]. This method can beparticularly useful for detecting hard inclusions in softmaterial. For example, vibro-acoustography has been usedto image calcifications in human arteries [4–6], microcalci-fications in breast tissue [7,8] and calcified arteries in breast[9]. A comparative study of vibro-acoustography withother radiation force methods for tissue elasticity imagingis presented in [10]. The transverse spatial resolution ofvibro-acoustography is in the sub-millimeter range, makingthe technique suitable for high-resolution imaging [6–8,10,11]. The depth resolution, representing the slice thick-ness, is in the sub-centimeter range.

Recently, we have developed a vibro-acoustographysystem for in vivo breast imaging and have tested it on anumber of volunteers. Here, we describe applications of

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vibro-acoustography in breast imaging, and present preli-minary in vivo breast vibro-acoustography results.

2. Method

Vibro-acoustography is based on vibro-acousticresponse of the object to a vibrating force [2,3,5,6,11].Radiation force is generated by a change in the spatial dis-tribution of the energy density of an incident acoustic field.This can happen when an acoustic field interacts with anobject. The energy density of the impinging sound maychange due to energy absorption, scattering, and reflection.Thus, a radiation force is exerted on the object. The mag-nitude of this force depends on a number of parameters,including the scattering and absorption properties of theobject. In a simple case of a plane wave reflected from aplanar object, the force is proportional to the power reflec-tion coefficient of the object. In vibro-acoustography, weuse two intersecting continuous wave (CW) focused ultra-sound beams of different frequencies. The two ultrasoundbeams are focused and they are aligned to intersect at theirfocal region. At this intersection region, which is normallya small volume, the combined ultrasound field energy den-sity is sinusoidally modulated, and hence, the field gener-ates a highly localized oscillatory radiation force wheninteracting with parts of the object in this region. Thus,the resulting radiation stress is confined to a small region,which acts as an oscillating point force placed remotelyinside the object.

The general principle of vibro-acoustography is illus-trated in Fig. 1. The confocal transducer produces twoco-focused beams that generate an oscillating radiationforce on the object at the difference frequency. The hydro-phone receives the resulting sound at the difference fre-quency. Image of the object is formed from the output ofthe hydrophone as the beam scans the object. The object,transducer, and hydrophone are placed in a water tank.

Amplitude and distribution of object motion is a func-tion of its mechanical parameter such as the mass density,elasticity, and viscosity, as well as the boundary conditions,such as coupling to and the loading effects of the surround-ing medium. The vibration motion results in a secondaryacoustic field (acoustic emission) that propagates in theobject. The acoustic emission which is at Df frequency is

ω1+ Δω

ω1

ConfocalTransducers

~

ConfocalTransducer

~

Fig. 1. Vibro-acoustography system setup

detected by an audio hydrophone. As the ultrasound beamis scanned across the object, the hydrophone signal isrecorded and its amplitude is mapped into an image.

A vibro-acoustography image depicts two types of infor-mation about the object: (1) ultrasonic properties of theobject, such as the scattering and power absorption charac-teristics; (2) the dynamic characteristics of the object at fre-quency Df, which also relates to the boundary conditionsand coupling to the surrounding medium [3]. The formerproperties are those that are also present in conventionalultrasound imaging. The latter properties, which arerelated to object stiffness, can be described in terms ofobject mechanical impedance at Df. Such information isnot available from conventional ultrasound. Another char-acteristic of vibro-acoustography relates to image speckle.Speckle is the snowy pattern seen in conventional ultra-sound images. Speckles result from random interferenceof the scattered ultrasound field. Speckles reduces thecontrasts of ultrasound images and often limits one to seesmall structures, such as breast microcalcifications intissue. Vibro-acoustography on the other hand uses theacoustic emission signal, which is at a low frequency. Theimage in this modality is practically speckle free, resultingin high contrast images that allow small structures to bevisible. This feature makes vibro-acoustography suitablefor detection of breast microcalcifications.

In vitro breast tissue imaging by vibro-acoustography:

Performance of vibro-acoustography in detection of breastmicrocalcifications and breast arterial calcifications hasbeen studied through a series of in vitro experiments con-ducted on human breast tissues samples in a water tank[7–9].

2.1. In vivo breast vibro-acoustography

We recently developed a vibro-acoustography systemfor in vivo breast imaging [12]. This system is integratedin a clinical stereotactic mammography machine (FischerImaging Inc., MammotestTM system). The combined sys-tem is designed in such a way that it enables us to producematching (from the same view angle) vibro-acoustographyand mammography images of human breast. Systemparameters are: transducer frequency = 3 MHz, resolution0.7 mm, scanning increments = 0.2 mm, ultrasound inten-

Object

Hydrophone

Image

ScanningMotionof Beam

Filter DetectorΔω

. Modified with permission from [12].

X-raytube

Patient bedBreast

X-ray detector

Slide Compression panelVA water tank

Ultrasoundtransducer

Hydrophone

Fig. 2. Combined vibro-acoustography-mammography system. Repro-duced with permission from [12].

Fig. 4. In vivo vibro-acoustography image from the breast of a volunteer.(Left) vibro-acoustography scan at 2.5 cm from the skin. (Right) anothervibro-acoustography scan at 3 cm depth. Reproduced with permissionfrom [12].

Fig. 5. (a) Mammogram. (b) vibro-acoustography image acquired atDf = 50 kHz with the beam focused at the depth of 4 cm from the skin.

A. Alizad et al. / Ultrasonics 44 (2006) e217–e220 e219

sity at the focal point = 700 mW/cm2 in compliance withthe FDA recommendation for in vivo ultrasound. Fig. 2shows combined mammography system. Patient lies inprone position on the examination bed with a breast hang-ing down through the hole. The breast is sandwichedbetween the back panel (X-ray detector), and a slidingcompression panel that keeps the breast slightly com-pressed and fixed for mammography and/or vibro-acous-tography scanning. The transducer is moved away duringmammography. Acoustic gel is applied to ensure properacoustic coupling.

3. Results

Vibro-acoustography images of a breast tissue samplewith calcified artery is shown in Figs. 3. The image of cal-cified artery of breast is clearly detected as seen in X-ray ofthis sample.

Fig. 4 demonstrates the in vivo vibro-acoustographyimage from the breast of a volunteer. These images covera 5 · 5 cm area taken at the depth of 2.5 cm (left image)and 3 cm (right image) from the skin. The calcification(diameter approximately 1 mm) is seen in the left imageas a bright spot in the top-left quadrant of the image onthe left. The presence of this calcification was proven bymammography. Tissue structure is visible especially inthe right image with remarkable contrast. The backgroundin the left image is darker because the image brightness isadjusted to show the calcification which happens to bemuch brighter than the soft tissue. These images wasacquired at frequency Df = 50 kHz. The scan time wasabout 7 minutes. This preliminary result demonstrates

Fig. 3. A breast tissue experiment: A=X-ray and B=vibro-acoustographyof the breast tissue.

one can produce high contrast in vivo images at ultrasoundintensities within the FDA guideline. These results alsodemonstrate that vibro-acoustography has enough resolu-tion and contrast to show both microcalcifications andthe soft tissue.

Fig. 5 demonstrates mammography and vibro-acoustog-raphy of a patient volunteer with a fibroadenoma in her leftbreast. The mammogram (a) showing a 5 · 5 cm area of thecoronal view of breast of a volunteer. The breast includes alarge calcification (bright spot) inside a fibroadenomaregion. The fibroadenoma, which is barely visible aroundthe calcification, is marked by a radiologist using thearrows. Vibro-acoustography image (b) acquired at Df =50 kHz with the beam focused at the depth of 4 cm fromthe skin. The fibroadenoma containing the calcification isseen as a dark region. The arrows are imported from (a)to the corresponding region on (b), verifying that the darkregion matches the radiologist reading of the fibroadenmain the mammogram.

4. Discussion

An ideal breast imaging device must be able to imageboth calcifications and soft tissue. It must also offer enoughresolution and sensitivity for detection of microcalcifica-tions. The experimental in vitro and in vivo studies havedemonstrated that vibro-acoustography has such capabili-ties. The spatial resolution can be improved by using trans-ducer with higher center frequency. However, one musttake into account the increase in tissue attenuation. The

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scanning speed may be improved by using array transduc-ers to steer the beam electronically.

Further investigation is needed to fully explore thepotentials of vibro-acoustography for in vivo breastimaging. A number of considerations must be taken intoaccount before implementing vibro-acoustography for clin-ical applications. For example, the coupling between thetransducer and the breast must be suitable for clinical prac-tice. Another consideration is the scanning time. This timemay be too long for routine clinical applications. The scan-ning time must be short enough to avoid excess patient dis-comfort during imaging. A clinical vibro-acoustographysystem may be implemented based on contact array trans-ducers. That is, instead of using a two-element confocaltransducer used in the present study, an ultrasound trans-ducer comprising of two two-dimensional arrays may beemployed to produce the two intersecting beams neededfor vibro-acoustography. The two beams from the arrayscan be focused at a common focal point and steered rapidlyacross a given plane within the breast. These methods arecurrently being studied [13].

5. Conclusions

Vibro-acoustography has potential to provide newerinformation in breast imaging.

Potential applications in vibro-acoustography breastimaging include microcalcifications, calcified arteries, anddetection of mass lesions. Future directions will includeelectronic beam forming with array transducers and tissuecharacterization.

Acknowledgements

The authors are grateful to the following individuals fortheir valuable work during the course of this study:Thomas M. Kinter for software support, Randall R. Kin-nick for laboratory support and scanning tissues, JoyceRahn for her help in scanning the patients, and ElaineC. Quarve for secretarial assistance. Supported by NIH

Grant EB-00535 and Grant BCTR0504550 from the SusanG. Komen Breast Cancer Foundation. Disclosure: Parts ofthe techniques used here are patented by MF and JFG.

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