ACOUSTIC RADIATION FORCE IMPULSE IMAGING REMOTE PALPATION OF THE MECHANICAL PROPERTIES OF TISSUE Kathryn Nightingale Mary Scott Soo Roger Nightingale Rex Bentley Deborah Stutz Mark Palmeri Jeremy Dahl and Gregg Trahey Duke University Durham NC 27708 kathy nightingale duke edu Abstract Acoustic Radiation Force Impulse ARFI imag ing is a new modality that utilizes brief high energy focused acoustic pulses to generate radiation force in tissue and conven tional diagnostic ultrasound methods to detect the resulting tissue displacements in order to image the mechanical properties of tis sue Tissue displacement magnitude is inversely related to local tissue stiffness and the temporal response of the tissue is related to its viscosity A high resolution and wide dynamic range ARFI image can be formed in 30 to 40 msec using modifi ed diagnos tic ultrasound scanners with maximum tissue displacements less than 15 microns and tissue heating less than 1 degree Celsius Tissue recovery to its original position typically occurs in 1 to 2 msec In vivo images of human breast abdomen and arter ies demonstrate good correlation between structures on matched B mode and ARFI images ARFI images are speckle free and demonstrate resolution similar to that of matched B mode im ages Differences in both peak displacement and recovery re sponse are observed for different tissues In general fatty tissues move farther and take longer to reach their maximum displace ment than fi brous or muscular tissues and fi brous tissues recover more slowly than fat or muscle In some cases malignant breast lesions appear larger in the ARFI images than they do on matched B mode images Results from ongoing human in vivo and ex vivo studies evaluating the correlation between ARFI images and tis sue pathology are presented The spatial and temporal patterns of radiation force induced target motion in phantoms and in vivo for varying transmit aperture characteristics transducer frequen cies and target characteristics are discussed Images of radiation force induced shear waves both in vivo and ex vivo are also pre sented Tissue structural boundaries are apparent in the propagat ing shear wave images ARFI imaging is currently implemented on both a modifi ed Siemens Elegra scanner with a 75L40 linear array transducer and a modifi ed Siemens Antares scanner using a VF13 5 linear array using radiation force application times rang ingfrom 03to1msec withreal timedataacquisitionandoffline post processing Differences in tissue response to radiation force suggest that ARFI imaging has the potential to provide clinicians with valuable insight into local differences in tissue mechanical properties that are not currently possible This work was supported by NIH grant R01 CA092183 01 and the Whitaker Foundation We thank Siemens Medical Systems Ultrasound Groupfortheirsystemsupport andAltairComputingfortheirFEMmod eling support 1 INTRODUCTION Acoustic Radiation Force Impulse ARFI imaging is a radiation force based imaging method that provides information about the local mechanical properties of tissue 13 Sugimoto et al was thefi rstgrouptoproposetheuseofradiationforcetoimagetissue mechanical properties 21 Since then several researchers have investigated the potential for using radiation force to image tissue properties 21 10 19 4 25 ARFI imaging uses short duration less than 1 msec acoustic radiation forces to generate localized displacements in tissue and these displacements are tracked us ing ultrasonic correlation based methods The tissue response to theseforcesismonitoredbothspatiallyandtemporally Displace ment magnitude is inversely related to local tissue stiffness and is typically on the order of ten microns Tissue recovery response is related to tissue visco elastic characteristics In this method a single transducer on a diagnostic ultrasound system is used to both apply localized radiation forces within tissue for short time periods and to track the resulting tissue displacements The pe riod and temporal profi le of the force application are determined by the transmitter pulse shape with the duration of the force ap plication limited to less than 1 millisecond The volume of tissue to which radiation force is applied is determined by the tempo ral average intensity fi eld of the transmitting transducer and the acoustic absorption of the propagating medium 1 1 Acoustic Radiation Force The spatial distribution of the radiation force fi eld is determined byboththetransmittedacousticparameters andtheacousticprop erties of the tissue In soft tissues where the majority of attenua tion results from absorption 17 2 the following equation can be used to determine radiation force magnitude 23 14 3 20 11 F Wabsorbed c 2 I c 1 where F dyn 1000 cm 3 or kg s 2cm 2 is acoustic radia tion force Wabsorbed W 100 cm 3 is the power absorbed by the medium at a given spatial location c m s 1 is the speed of sound in the medium m 1 is the absorption coeffi cient of the medium and I W cm 2