Official Proceedings - AIUM

American Institute of Ultrasound in Medicine Proceedings J Ultrasound Med 32(suppl):S1–S134, 2013Clinical Role and Potential of Ultrasound-Enhanced Thrombolysisin Peripheral Arterial DiseaseRichard Shlansky-Goldberg Radiology, University of Pennsylvania,Philadelphia, Pennsylvania USAThrombosis from peripheral arterial disease (PAD) due to underlyingatherosclerotic disease or thrombosis of a surgical bypass graftused to treat PAD can lead to limb loss. In addition, thrombosis of thedeep venous system (DVT) can lead to chronic leg swelling due to postphlebiticsyndrome. DVT can also lead to life-threatening pulmonary embolicdisease (PE). Catheter-directed pharmacologic thrombolysis is awell-established technique to treat these arterial, venous, and graft occlusions.The utility of this approach continues to be limited by several factors,including the cost of the lytic dose, the duration of treatment requiredfor effective clot lysis, costly intensive care monitoring, and the exposureof patients to the risk of intracranial hemorrhage and other life-threateningbleeding. Attempts to improve the efficacy of thrombolysis with differentcatheter configurations and mechanical devices have met withvaried success. The addition of ultrasound by itself or with pharmacologicagents has been demonstrated to primarily induce or enhance thrombolysis.To date, the clinical applicability of these approaches has been limited.We will explore the current clinical data to determine the success of theseapproaches to improve lysis in DVT, PAD, and PE. Although the use ofultrasound appears promising, the question still remains whether the currentiteration of techniques and devices will add enough efficiency to havea clinically significant impact on outcomes. We will evaluate what thresholdsneed to be crossed for ultrasound to dramatically improve on howthese diseases will be treated in the future.SPECIAL INTEREST SESSIONSWEDNESDAY, APRIL 10, 2013, 10:45 AM–12:30 PMAcoustic Radiation Force Impulse Imaging:Benefits and Challenges With Increasing AcousticOutput Beyond Diagnostic LevelsModerators: Kathy Nightingale, PhD, Thomas Szabo, PhDThe Historical Basis for the Food and Drug Administration’s MaximumExposure Level Guidance for Diagnostic UltrasoundGerald Harris US Food and Drug Administration, SilverSpring, Maryland USAUS Food and Drug Administration (FDA) regulations designatemost diagnostic imaging and Doppler ultrasound devices as class 2,which means that before a new device can be marketed in the UnitedStates, a “510(k)” (named for a section of the 1976 FDA Medical DeviceAmendments) premarket notification must be cleared by the FDA. In thisnotification, a device sponsor must demonstrate that the device is substantiallyequivalent (SE) in terms of safety and effectiveness to either adevice legally marketed before May 28, 1976, the date of enactment of theFDA Medical Device Amendments, or to a device that has been legallymarketed as a class 2 device since that date. To evaluate equivalent safety,the FDA has used several acoustic output quantities to compare maximumoutput levels, including the derated spatial-peak temporal-average intensityand the thermal index for thermal safety comparisons and the deratedspatial-peak pulse-average intensity and the mechanical index for nonthermalconsiderations. In this presentation, the origin and use of thesequantities in making SE determinations will be described. Also, their possibleshortcomings for evaluating the safety of applications that employlong-duration, high-intensity pulse bursts, such as acoustic radiation forceimpulse imaging, will be discussed.An Analysis of the Mechanical Index as a Means for Ensuring PatientSafety During Acoustic Radiation Force Impulse ImagingCharles Church,* Cecille Labuda National Center for PhysicalAcoustics, University of Mississippi, University, MississippiUSAThe mechanical index (MI) quantifies the likelihood that diagnosticultrasound will produce an adverse biological effect by a nonthermalmechanism. The current formulation of the MI is based on inertialcavitation thresholds in water and blood as calculated for pulse durationsS75of 1 period. However, tissue is not a liquid but a viscoelastic solid, andfurther, acoustic radiation force impulse imaging employs high-intensitypulses up to several hundred acoustic periods long. To quantify the importanceof these differences, thresholds for inertial cavitation were determinedin water, blood, and several representative tissues by performingnumerical computations similar to the analytical work underlying the MIfor pulse lengths of 1 to 1000 acoustic periods, equilibrium bubble radii(Ro) of 0.01 to 10.0 µm, a frequency range of 0.5 to 10 MHz, and 4 thresholdcriteria, including the criterion used for the MI (5000 K). Water andblood were modeled using the Gilmore equation, while tissues (smoothand skeletal muscle, kidney, liver, and skin) were modeled using a Keller-Miksis–like equation assuming a linear Voigt solid. It is shown that thelikelihood of an adverse biological effect due to cavitation is less in softtissues, and much less in muscle, than in blood. More importantly, the literaturesuggests that the experimental threshold for cavitation in tissue ismuch higher than predicted here, casting doubt on the value of this simpletheoretical approach in assessments of patient safety. By combiningtheoretical and experimental data, several options for transiently increasingoutput levels while ensuring patient safety become available.Investigation of the Use of Increased Acoustic Output Levels forAcoustic Radiation Force Impulse Imaging in the Research SettingMark Palmeri Biomedical Engineering, Duke University,Durham, North Carolina USAAcoustic radiation force impulse (ARFI) imaging has experiencedrapid development over the past decade, growing from a novel elasticityimaging method used in tissue-mimicking phantoms to clinicaltesting in a variety of target organs, including the liver, breast, prostate, vessels,and heart, to commercial implementation. While current commercialARFI imaging implementation operates within current US Food and DrugAdministration diagnostic ultrasound acoustic output guidelines, studies inthe research environment have not been similarly restricted. For example,pilot clinical ARFI imaging research studies at Duke University involve acustom method for characterizing acoustic intensity, tissue heating, transducerheating, and the mechanical index to support in vivo safety of usingincreased output during acoustic radiation force excitations. Given thestrong acoustic waveform nonlinearity that can occur when characterizinghigh pressures in water, acoustic radiation force pressure waveforms andintensity values are characterized using hydrophone measurementsthrough attenuating fluids similar to that of the target organ. Thermocou-