Focal ablation of prostate cancer: four roles for magnetic resonance imaging guidance

INTRODUCTION

There is currently a great deal of interest in the possible use of focal therapies for prostate cancer, since such treatments offer the prospect for control or cure of the primary disease with minimal side effects. Many forms of thermal therapy have been proposed for focal ablation of prostate cancer, including laser, high intensity ultrasound and cryotherapy. This review will demonstrate the important roles that magnetic resonance imaging (MRI) guidance can offer to such focal ablation, focusing on the use of high intensity ultrasonic applicators as an example of one promising technique.

MATERIALS AND METHODS

Transurethral and interstitial high intensity ultrasonic applicators, designed specifically for ablation of prostate tissue were tested extensively in vivo in a canine model. The roles of MRI in positioning the devices, monitoring prostate ablation, and depicting ablated tissue were assessed using appropriate MRI sequences.

RESULTS

MRI guidance provides a very effective tool for the positioning of ablative devices in the prostate, and thermal monitoring successfully predicted ablation of prostate tissue when a threshold of 52 ºC was achieved. Contrast enhanced MRI accurately depicted the distribution of ablated prostate tissue, which is resorbed at 30 days.

CONCLUSIONS

Guidance of thermal therapies for focal ablation of prostate cancer will likely prove critically dependent on MRI functioning in four separate roles. Our studies indicate that in three roles: device positioning; thermal monitoring of prostate ablation; and depiction of ablated prostate tissue, MR techniques are highly accurate and likely to be of great benefit in focal prostate cancer ablation. A fourth critical role, identification of cancer within the gland for targeting of thermal therapy, is more problematic at present, but will likely become practical with further technological advances.

Finite element static displacement optimization of 20-100 kHz flexural transducers for fully portable ultrasound applicator

This paper focuses on the development of a finite-element model and subsequent stationary analysis performed to optimize individual flexural piezoelectric elements for operation in the frequency range of 20-100kHz. These elements form the basic building blocks of a viable, un-tethered, and portable ultrasound applicator that can produce intensities on the order of 100mW/cm(2) spatial-peak temporal-peak (I(SPTP)) with minimum (on the order of 15V) excitation voltage. The ultrasound applicator can be constructed with different numbers of individual transducer elements and different geometries such that its footprint or active area is adjustable. The primary motivation behind this research was to develop a tether-free, battery operated, fully portable ultrasound applicator for therapeutic applications such as wound healing and non-invasive transdermal delivery of both naked and encapsulated drugs. It is shown that careful selection of the components determining applicator architecture allows the displacement amplitude to be maximized for a specific frequency of operation. The work described here used the finite-element analysis software COMSOL to identify the geometry and material properties that permit the applicator's design to be optimized. By minimizing the excitation voltage required to achieve the desired output (100mW/cm(2)I(SPTP)) the power source (rechargeable Li-Polymer batteries) size may be reduced permitting both the electronics and ultrasound applicator to fit in a wearable housing.

Therapeutic ultrasound and pain in degenerative diseases of musculoskeletal system

INTRODUCTION

Therapeutic ultrasound is a physical modality which is constantly expanding range of indications. Analgesic effect of ultrasound is still under discussion. Regardless the extensive application of pulsed ultrasound of low intensity, continuous ultrasound has a better analgesis effect, which is explained by its mechanism of action.

AIMS

The main research objective of this study was to determine the effect of continuous ultrasound to pain caused by degenerative diseases of the musculoskeletal system in the intensity and duration of ultrasound treatment. Other objectives are entailed determining the correlation between the degree of pain reduction with: location of pain, age, gender and body mass index (BMI).

MATERIAL AND METHODS

The study included 68 patients with chronic pain localized in the region of the spine or major joints of the extremities, depending on the localization of the degenerative changes. Patients are divided into two groups. The first group was treated with 10 applications of continuous ultrasound with frequency of 1 MHz, intensity 0.4 W/cm2 for 8 minutes, and the other group with 10 applications of ultrasound with frequency of 1 MHz, intensity 0.8 W/cm 2 for 4 minutes.

RESULTS AND DISCUSSION

Pain intensity was assessed before and after ultrasound therapy performed by subjective visual analogue scale (VAS) for pain, numbered from 0-10, where 0 is the rating for the state of no pain, and 10 grades for severe pain. The average VAS improvement in the first group was 3.97, and 4.74 in second one. The results of F (1.66) = 2.93, p = 0.09 analysis of variance showed no significance difference between the average improvement of two groups. Correlation between the degree of pain reduction showed significance only with BMI, or that higher BMI is associated with a lower degree of pain reduction. The results of this study showed that application of continuous ultrasound in patients with chronic pain, caused by degenerative changes in the musculoskeletal system, led to a significant reduction in pain. Different intensity and duration of ultrasound application showed no significant effect on the degree of pain reduction. Body mass index showed significant negative correlation with the degree of pain reduction, but age, gender and location of pain did not show significant correlation.

Understanding the structure and mechanism of formation of a new magnetic microbubble formulation

Magnetic nanoparticles and ultrasound contrast agents have both been used as vehicles for therapeutic delivery. More recently, magnetic microbubbles have been developed as a new theranostic agent which combines the advantages of the individual carriers and overcomes many of their limitations. In a previous study of gene delivery using magnetic microbubbles, it was found that a combination of magnetic liquid droplets and non-magnetic phospholipid microbubbles produced higher transfection rates than magnetic microbubbles. The reasons for this were not fully understood, however. The aim of this study was to investigate the hypothesis that conjugation between the droplets and the microbubbles occurred. A combination of optical and fluorescence microscopy and ultrasound imaging studies in a flow phantom were performed. No interaction between magnetic droplets and microbubbles was observed under optical microscopy but the results from the fluorescence and acoustic imaging indicated that magnetic droplets and microbubbles do indeed combine to form a new magnetically and acoustically responsive particle. Theoretical calculations indicate that the driving force of the interaction is the relative surface energy and thus thermodynamic stability of the microbubbles and the droplets. The new particles were resistant to centrifugation, of comparable echogenicity to conventional ultrasound contrast agents and could be retained by a magnetic field (0.2T) in a flow phantom at centre line velocities of ~6 cm s(-1) and shear rates of ~60 s( -1).

Optimization of un-tethered, low voltage, 20-100kHz flexural transducers for biomedical ultrasonics applications

This paper describes optimization of un-tethered, low voltage, 20-100kHz flexural transducers for biomedical ultrasonics applications. The goal of this work was to design a fully wearable, low weight (<100g), battery operated, piezoelectric ultrasound applicator providing maximum output pressure amplitude at the minimum excitation voltage. Such implementation of ultrasound applicators that can operate at the excitation voltages on the order of only 10-25V is needed in view of the emerging evidence that spatial-peak temporal-peak ultrasound intensity (I(SPTP)) on the order of 100mW/cm(2) delivered at frequencies below 100kHz can have beneficial therapeutic effects. The beneficial therapeutic applications include wound management of chronic ulcers and non-invasive transdermal delivery of insulin and liposome encapsulated drugs. The early prototypes of the 20 and 100kHz applicators were optimized using the maximum electrical power transfer theorem, which required a punctilious analysis of the complex impedance of the piezoelectric disks mounted in appropriately shaped metal housings. In the implementation tested, the optimized ultrasound transducer applicators were driven by portable, customized electronics, which controlled the excitation voltage amplitude and facilitated operation in continuous wave (CW) or pulsed mode with adjustable (10-90%) duty cycle. The driver unit was powered by remotely located rechargeable lithium (Li) polymer batteries. This was done to further minimize the weight of the applicator unit making it wearable. With DC voltage of approximately 15V the prototypes were capable of delivering pressure amplitudes of about 55kPa or 100mW/cm(2) (I(SPTP)). This level of acoustic output was chosen as it is considered safe and side effects free, even at prolonged exposure.

Prospective utility of therapeutic ultrasound in dentistry-Review with recent comprehensive update

BACKGROUND

The utility of ultrasound (US) for therapeutic purposes is still in its infancy. Therapeutic US (TUS) has been used widely in medical field for urological application, surgical intervention, bone healing, and osteointegration in cancer and healing of full thickness excised skin lesions, and within dentistry as a prediagnostic, diagnostic and therapeutic purpose. The purpose of the paper is to review and determine the efficacy of US as one of the treatment modalities for its role in maxillofacial region to reduce pain and promote soft tissue healing.

MATERIALS AND METHODS

A Medline search included of the international literature published between 1976 and 2011 and was restricted to English language articles, published work of past researchers including in vitro and in vivo studies, recent additions of textbooks on surgical and therapeutic applications of US and, current articles in conference papers and reports accessed from the internet using Google search engine on therapeutic ultrasound.

RESULTS

Very few article regarding effect of therapeutic of US for its use of insonation for treatment of patient with pain and soft tissue injury are available. This review article mainly emphasizes the therapeutic utility of US in dentistry for its effectiveness to decrease joint stiffness, reduce pain and muscle spasms and improve muscle mobility. In vivo studies have shown very little clinical effects.

CONCLUSIONS

Further research is warranted in this clinically important area to make the development of noninvasive, multifunctional ultrasound devices for repair, regeneration and other therapeutic utility a success.

HIFU for palliative treatment of pancreatic cancer

High intensity focused ultrasound (HIFU) is a novel non-invasive modality for ablation of various solid tumors including uterine fibroids, prostate cancer, hepatic, renal, breast and pancreatic tumors. HIFU therapy utilizes mechanical energy in the form of a powerful ultrasound wave that is focused inside the body to induce thermal and/or mechanical effects in tissue. Multiple preclinical and non-randomized clinical trials have been performed to evaluate the safety and efficacy of HIFU for palliative treatment of pancreatic tumors. Substantial tumor-related pain reduction was achieved in most cases after HIFU treatment, and no significant side-effects were observed. This review provides a description of different physical mechanisms underlying HIFU therapy, summarizes the clinical experience obtained to date in HIFU treatment of pancreatic tumors, and discusses the challenges, limitations and new approaches in this modality.

Can ultrasound enable efficient intracellular uptake of molecules? A retrospective literature review and analysis

Most applications of therapeutic ultrasound (US) for intracellular delivery of drugs, proteins, DNA/RNA and other compounds would benefit from efficient uptake of these molecules into large numbers of cells without killing cells in the process. In this study we tested the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro. A search of the literature for studies with quantitative data on uptake and viability yielded 26 published papers containing 898 experimental data points. Analysis of these studies showed that just 7.7% of the data points corresponded to relatively efficient uptake (>50% of cells exhibiting uptake). Closer examination of the data showed that use of Definity US contrast agent (as opposed to Optison) and elevated sonication temperature at 37°C (as opposed to room temperature) were associated with high uptake, which we further validated through independent experiments carried out in this study. Although these factors contributed to high uptake, almost all data with efficient uptake were from studies that had not accounted for lysed cells when determining cell viability. Based on retrospective analysis of the data, we showed that not accounting for lysed cells can dramatically increase the calculated uptake efficiency. We further argue that if all the data considered in this study were re-analyzed to account for lysed cells, there would be essentially no data with efficient uptake. We therefore conclude that the literature does not support the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro, which poses a challenge to future applications of US that require efficient intracellular delivery.

Therapeutic ultrasound as a potential male contraceptive: power, frequency and temperature required to deplete rat testes of meiotic cells and epididymides of sperm determined using a commercially available system

BACKGROUND

Studies published in the 1970s by Mostafa S. Fahim and colleagues showed that a short treatment with ultrasound caused the depletion of germ cells and infertility. The goal of the current study was to determine if a commercially available therapeutic ultrasound generator and transducer could be used as the basis for a male contraceptive.

METHODS

Sprague-Dawley rats were anesthetized and their testes were treated with 1 MHz or 3 MHz ultrasound while varying power, duration and temperature of treatment.

RESULTS

We found that 3 MHz ultrasound delivered with 2.2 Watt per square cm power for fifteen minutes was necessary to deplete spermatocytes and spermatids from the testis and that this treatment significantly reduced epididymal sperm reserves. 3 MHz ultrasound treatment reduced total epididymal sperm count 10-fold lower than the wet-heat control and decreased motile sperm counts 1,000-fold lower than wet-heat alone. The current treatment regimen provided nominally more energy to the treatment chamber than Fahim's originally reported conditions of 1 MHz ultrasound delivered at 1 Watt per square cm for ten minutes. However, the true spatial average intensity, effective radiating area and power output of the transducers used by Fahim were not reported, making a direct comparison impossible. We found that germ cell depletion was most uniform and effective when we rotated the therapeutic transducer to mitigate non-uniformity of the beam field. The lowest sperm count was achieved when the coupling medium (3% saline) was held at 37 degrees C and two consecutive 15-minute treatments of 3 MHz ultrasound at 2.2 Watt per square cm were separated by 2 days.

CONCLUSIONS

The non-invasive nature of ultrasound and its efficacy in reducing sperm count make therapeutic ultrasound a promising candidate for a male contraceptive. However, further studies must be conducted to confirm its efficacy in providing a contraceptive effect, to test the result of repeated use, to verify that the contraceptive effect is reversible and to demonstrate that there are no detrimental, long-term effects from using ultrasound as a method of male contraception.

Performance assessment of HIFU lesion detection by harmonic motion imaging for focused ultrasound (HMIFU): a 3-D finite-element-based framework with experimental validation

Harmonic motion imaging for focused ultrasound (HMIFU) is a novel high-intensity focused ultrasound (HIFU) therapy monitoring method with feasibilities demonstrated in vitro, ex vivo and in vivo. Its principle is based on amplitude-modulated (AM) - harmonic motion imaging (HMI), an oscillatory radiation force used for imaging the tissue mechanical response during thermal ablation. In this study, a theoretical framework of HMIFU is presented, comprising a customized nonlinear wave propagation model, a finite-element (FE) analysis module and an image-formation model. The objective of this study is to develop such a framework to (1) assess the fundamental performance of HMIFU in detecting HIFU lesions based on the change in tissue apparent elasticity, i.e., the increasing Young's modulus, and the HIFU lesion size with respect to the HIFU exposure time and (2) validate the simulation findings ex vivo. The same HMI and HMIFU parameters as in the experimental studies were used, i.e., 4.5-MHz HIFU frequency and 25 Hz AM frequency. For a lesion-to-background Young's modulus ratio of 3, 6 and 9, the FE and estimated HMI displacement ratios were equal to 1.83, 3.69 and 5.39 and 1.65, 3.19 and 4.59, respectively. In experiments, the HMI displacement followed a similar increasing trend of 1.19, 1.28 and 1.78 at 10-s, 20-s and 30-s HIFU exposure, respectively. In addition, moderate agreement in lesion size growth was found in both simulations (16.2, 73.1 and 334.7 mm(2)) and experiments (26.2, 94.2 and 206.2 mm(2)). Therefore, the feasibility of HMIFU for HIFU lesion detection based on the underlying tissue elasticity changes was verified through the developed theoretical framework, i.e., validation of the fundamental performance of the HMIFU system for lesion detection, localization and quantification, was demonstrated both theoretically and ex vivo.

Ultrasound contrast agents affect the angiogenic response

OBJECTIVES

The interaction of ultrasound contrast agents (UCAs) and ultrasound (US) provides a way to spatially and temporally target tissues. Recently, UCAs have been used therapeutically to induce localized angiogenesis. Ultrasound contrast agents, however, have been documented to induce negative bioeffects. To further understand the balance of risks and benefits of UCAs and to examine the mechanism of US-UCA-induced angiogenesis, this study explored the role of UCAs, in particular Definity (Lantheus Medical Imaging, Inc, North Billerica, MA), in producing an angiogenic response.

METHODS

The gracilis muscles of Sprague Dawley rats were exposed to 1-MHz US. The rats were euthanized the same day or allowed to recover for 3 or 6 days post exposure (DPE). Ultrasound peak rarefactional pressures (P(r)s) of 0.25, 0.83, 1.4, and 2.0 MPa were used while rats were infused with either saline or Definity. Assessments for angiogenesis included capillary density, inflammation, and vascular endothelial growth factor (VEGF), both acutely (0 DPE) and at 3 and 6 DPE.

RESULTS

The results of this study suggest that the angiogenic response is dependent on infusion media, P(r), and DPE. While capillary density did not reach significance, VEGF expression was significant for infusion media, P(r), and DPE with inflammation co-occurrence (P < .05).

CONCLUSIONS

These results suggest that the angiogenic response is elicited by a mechanical effect of US-UCA stimulation of VEGF that is potentially optimized when collapse occurs.

Mitigation of bone loss with ultrasound induced dynamic mechanical signals in an OVX induced rat model of osteopenia

This study tests the hypothesis that an ultrasound generated dynamic mechanical signal can attenuate bone loss in an estrogen deficient model of osteopenia. Eighty-four 16-week-old Sprague-Dawley rats were divided into six groups: baseline control, age-matched control, ovariectomy (OVX) control, OVX+5mW/cm(2) ultrasound (US), OVX+30mW/cm(2) US and OVX+100mW/cm(2) US. Low intensity pulsed ultrasound (LIPUS) was delivered transdermally at the L4/L5 vertebrae, using gel-coupled plane wave US transducers. The signal, characterized by 200μs pulses of 1.5MHz sine waves repeating at 1kHz with spatial-averaged temporal-averaged (SATA) intensities of 5, 30 or 100mW/cm(2), was applied 20 min/day, 5 days/week for 4 weeks. OVX treatment reduced bone volume fraction 40% and compromised microstructure at 4 weeks. LIPUS treatment, however, significantly increased BV/TV (+33%) compared to OVX controls for the 100mW/cm(2) treated group. SMI and Tb.N showed significant improvements compared with OVX for the 100mW/cm(2) treated group and Tb.Th was significantly improved in the 30 and 100mW/cm(2) treated groups. Improvements in bone's microstructural characteristics with 100mW/cm(2) US treatment translated into improved load bearing characteristics, including a significant 42% increase in apparent level elastic modulus compared to OVX controls. Significant improvement of trabecular mechanical strength was also observed in the treated animals, e.g., principal compressive stress (represent bone's ability to resist loads) was significantly higher compared to OVX controls. Histomorphometric analysis also showed that treatment with 100mW/cm(2) US resulted in a 76% improvement in MS/BS. In addition, measures of bone quantity and quality at the femoral metaphysis suggest that LIPUS is site specific. This study indicates that localized ultrasound treatment, delivered at specific intensities, has beneficial effects on intact bone and may represent a novel intervention for bone loss.

A temporal study of ultrasound contrast agent-induced changes in capillary density

OBJECTIVE

The ability of ultrasound (US) and ultrasound contrast agents (UCAs) to induce angiogenesis has been explored as a means of restoring blood flow to ischemic muscle. Because UCAs demonstrate an increasing percentage of collapse cavitation with increasing US pressure (Pr), this study sought to explore the effects of a US Pr that produces 100% collapse cavitation, determine the capillary density changes, and determine the time point of angiogenic rebound in a normal animal model.

METHODS

Using a 1-MHz focused transducer and a peak rarefactional US Pr of 3.8 MPa, rat gracilis muscles were exposed to US, and bioeffects were assessed. Capillary density, as a measure of angiogenesis, was examined. As an additional measure, inflammatory cells were quantified via a color threshold analysis to detect the presence of CD31 and CD34 as a percentage of the total section on stained slides. Six groups (0, 3, 6, 13, 20, and 27 days postexposure [DPE]; n = 3 each) and 5 cage controls were used to characterize the angiogenic response.

RESULTS

Ultrasound-UCA treatment caused the capillary density to decrease acutely (0 DPE) by 70% and inflammatory cells to increase by up to 250%. The angiogenic rebound was observed at 3 DPE but did not return to control levels by 27 DPE, suggesting an incomplete healing response.

CONCLUSIONS

Capillary destruction and inflammation played an important role in the angiogenic response induced by US-UCA. Exposure that causes 100% collapse cavitation causes capillary destruction from which normal rats are unable to recover and suggests a nontherapeutic effect.

Ultrasound-triggered release of recombinant tissue-type plasminogen activator from echogenic liposomes

Echogenic liposomes (ELIP) were developed as ultrasound-triggered targeted drug or gene delivery vehicles (Lanza et al. 1997; Huang et al. 2001). Recombinant tissue-type plasminogen activator (rt-PA), a thrombolytic, has been loaded into ELIP (Tiukinhoy-Laing et al. 2007). These vesicles have the potential to be used for ultrasound-enhanced thrombolysis in the treatment of acute ischemic stroke, myocardial infarction, deep vein thrombosis or pulmonary embolus. A clinical diagnostic ultrasound scanner (Philips HDI 5000; Philips Medical Systems, Bothell, WA, USA) equipped with a linear array transducer (L12-5) was employed for in vitro studies using rt-PA-loaded ELIP (T-ELIP). The goal of this study was to quantify ultrasound-triggered drug release from rt-PA-loaded echogenic liposomes. T-ELIP samples were exposed to 6.9-MHz B-mode pulses at a low pressure amplitude (600 kPa) to track the echogenicity over time under four experimental conditions: (1) flow alone to monitor gas diffusion from the T-ELIP, (2) pulsed 6.0-MHz color Doppler exposure above the acoustically driven threshold (0.8 MPa) to force gas out of the liposome gently, (3) pulsed 6.0-MHz color Doppler above the rapid fragmentation threshold (2.6 MPa) or (4) Triton X-100 to rupture the T-ELIP chemically as a positive control. Release of rt-PA for each ultrasound exposure protocol was assayed spectrophotometrically. T-ELIP were echogenic in the flow model (5 mL/min) for 30 min. The thrombolytic drug remained associated with the liposome when exposed to low-amplitude B-mode pulses over 60 min and was released when exposed to color Doppler pulses or Triton X-100. The rt-PA released from the liposomes had similar enzymatic activity as the free drug. These T-ELIP are robust and echogenic during continuous fundamental 6.9-MHz B-mode imaging at a low exposure output level (600 kPa). Furthermore, a therapeutic concentration of rt-PA can be released by fragmenting the T-ELIP with pulsed 6.0-MHz color Doppler ultrasound above the rapid fragmentation threshold (1.59 MPa). (E-mail: denise.smith@uc.edu).

Focused ultrasound effects on nerve action potential in vitro

Minimally invasive applications of thermal and mechanical energy to selective areas of the human anatomy have led to significant advances in treatment of and recovery from typical surgical interventions. Image-guided focused ultrasound allows energy to be deposited deep into the tissue, completely noninvasively. There has long been interest in using this focal energy delivery to block nerve conduction for pain control and local anesthesia. In this study, we have performed an in vitro study to further extend our knowledge of this potential clinical application. The sciatic nerves from the bullfrog (Rana catesbeiana) were subjected to focused ultrasound (at frequencies of 0.661 MHz and 1.986 MHz) and to heated Ringer's solution. The nerve action potential was shown to decrease in the experiments and correlated with temperature elevation measured in the nerve. The action potential recovered either completely, partially or not at all, depending on the parameters of the ultrasound exposure. The reduction of the baseline nerve temperature by circulating cooling fluid through the sonication chamber did not prevent the collapse of the nerve action potential; but higher power was required to induce the same endpoint as without cooling. These results indicate that a thermal mechanism of focused ultrasound can be used to block nerve conduction, either temporarily or permanently.

Size measurement of tissue debris particles generated from pulsed ultrasound cavitational therapy-histotripsy

Extensive mechanical tissue fractionation can be achieved using successive high intensity ultrasound pulses ("histotripsy"). Histotripsy has many potential medical applications where noninvasive tissue removal is desired (e.g., tumor ablation). There is a concern that debris generated by histotripsy-induced tissue fractionation might be an embolization hazard. The aim of this study is to measure the size distribution of these tissue debris particles. Histotripsy pulses were produced by a 513-element 1 MHz array transducer, an 18-element 750 kHz array transducer and a 788 kHz single element transducer. Peak negative pressures of 11 to 25 MPa, pulse durations of 3 to 50 cycles, pulse repetition frequencies of 100 Hz to 2 kHz were used. Tissue debris particles created by histotripsy were collected and measured with a particle sizing system. In the resulting samples, debris <6 microm in diameter constituted >99% of the total number of tissue particles. The largest particle generated by one of the parameter sets tested was 54 microm in diameter, which is smaller than the clinic filter size (100 microm) used to prevent embolization. The largest particles generated using other parameter sets were larger than 60 microm but the value could not be specified using our current setup. Exposures with shorter pulses produced lower percentages of large tissue debris (>30 microm) in comparison to longer pulses. These results suggest that the tissue debris particle size distribution is adjustable by altering acoustic parameters if necessary.

Ultrasound-enhanced thrombolysis using Definity as a cavitation nucleation agent

Ultrasound has been shown previously to act synergistically with a thrombolytic agent, such as recombinant tissue plasminogen activator (rt-PA) to accelerate thrombolysis. In this in vitro study, a commercial contrast agent, Definity, was used to promote and sustain the nucleation of cavitation during pulsed ultrasound exposure at 120 kHz. Ultraharmonic signals, broadband emissions and harmonics of the fundamental were measured acoustically by using a focused hydrophone as a passive cavitation detector and used to quantify the level of cavitation activity. Human whole blood clots suspended in human plasma were exposed to a combination of rt-PA, Definity and ultrasound at a range of ultrasound peak-to-peak pressure amplitudes, which were selected to expose clots to various degrees of cavitation activity. Thrombolytic efficacy was determined by measuring clot mass loss before and after the treatment and correlated with the degree of cavitation activity. The penetration depth of rt-PA and plasminogen was also evaluated in the presence of cavitating microbubbles using a dual-antibody fluorescence imaging technique. The largest mass loss (26.2%) was observed for clots treated with 120-kHz ultrasound (0.32-MPa peak-to-peak pressure amplitude), rt-PA and stable cavitation nucleated by Definity. A significant correlation was observed between mass loss and ultraharmonic signals (r = 0.85, p < 0.0001, n = 24). The largest mean penetration depth of rt-PA (222 microm) and plasminogen (241 microm) was observed in the presence of stable cavitation activity. Stable cavitation activity plays an important role in enhancement of thrombolysis and can be monitored to evaluate the efficacy of thrombolytic treatment.

Phonophoresis and the absorption of dexamethasone in the presence of an occlusive dressing

CONTEXT

Phonophoresis is purported to represent a method to apply topical medications through the skin to treat soft tissue injuries and inflammatory conditions. Few data are available to demonstrate the clinical effectiveness of the treatment.

OBJECTIVE

To determine the effect of ultrasound on the transcutaneous absorption of dexamethasone when occluded with a dressing.

DESIGN

Crossover design.

SETTING

University general clinical research center.

PATIENTS OR OTHER PARTICIPANTS

Ten healthy subjects (age = 29.2 +/- 8.8 years; height = 170.0 +/- 3.9 cm; mass = 67.5 +/- 18.4 kg).

INTERVENTION(S)

Two grams of 0.33% dexamethasone cream were applied to a 10-cm (2) area on the anterior forearm. The drug was applied to the skin and occluded with a dressing for 30 minutes before the ultrasound and sham ultrasound treatments. The treatments were applied over the drug and occlusive dressing. Ultrasound treatments were delivered at an intensity of 1.0 W/cm (2) (50% pulsed) at an output frequency of 3 MHz for 5 minutes and compared with sham ultrasound treatments that were delivered at an intensity of 0.0 W/cm (2) (50% pulsed) at an output frequency of 3 MHz for 5 minutes. All subjects received both the ultrasound and sham treatments, and the order in which subjects received the treatments was counterbalanced.

MAIN OUTCOME MEASURE(S)

Serum samples were drawn before treatment and immediately posttreatment and at 2, 4, 6, 8, and 10 hours posttreatment. Using high-performance liquid chromatography, we analyzed serum to determine dexamethasone concentrations.

RESULTS

A 2-way repeated-measures analysis of variance (condition x time) revealed a significant main effect for ultrasound treatment ( P = .047). The rate of appearance and the total concentration of dexamethasone in the serum were greater in subjects after phonophoresis than after sham ultrasound. The sham group had only trace amounts of dexamethasone in the serum, indicating that drug absorption was negligible without the ultrasound energy. The effect size of the phonophoresis condition fell within a 95% confidence interval after the baseline measurement.

CONCLUSIONS

We found that a phonophoretic effect occurred with dexamethasone when its application saturated the skin.

Variability in effective radiating area and output power of new ultrasound transducers at 3 MHz

CONTEXT

Spatial average intensity (SAI) is often used by clinicians to gauge therapeutic ultrasound dosage, yet SAI measures are not directly regulated by US Food and Drug Administration (FDA) standards. Current FDA guidelines permit a possible 50% to 150% minimum to maximum range of SAI values, potentially contributing to variability in clinical outcomes.

OBJECTIVE

To measure clinical values that describe ultrasound transducers and to determine the degree of intramanufacturer and intermanufacturer variability in effective radiating area, power, and SAI when the transducer is functioning at 3 MHz.

DESIGN

A descriptive and interferential approach was taken to this quasi-experimental design.

SETTING

Measurement laboratory.

PATIENTS OR OTHER PARTICIPANTS

Sixty-six 5-cm(2) ultrasound transducers were purchased from 6 different manufacturers.

INTERVENTION(S)

All transducers were calibrated and then assessed using standardized measurement techniques; SAI was normalized to account for variability in effective radiating area, resulting in an nSAI.

MAIN OUTCOME MEASURE(S)

Effective radiating area, power, and nSAI.

RESULTS

All manufacturers with the exception of Omnisound (P = .534) showed a difference between the reported and measured effective radiating area values (P < .001). All transducers were within FDA guidelines for power (+/-20%). Chattanooga (0.85 +/- 0.05 W/cm(2)) had a lower nSAI (P < .05) than all other manufacturers functioning at 3 MHz. Intramanufacturer variability in SAI ranged from 16% to 35%, and intermanufacturer variability ranged from 22% to 61%.

CONCLUSIONS

Clinicians should consider treatment values of each individual transducer, regardless of the manufacturer. In addition, clinicians should scrutinize the power calibration and recalibration record of the transducer and adjust clinical settings as needed for the desired level of heating. Our data may aid in explaining the reported heating differences among transducers from different manufacturers. Stricter FDA standards regarding effective radiating area and total power are needed, and standards regulating SAI should be established.

Investigation of intensity thresholds for ultrasound tissue erosion

Our previous studies have shown that short intense pulses delivered at certain pulse repetition frequencies (PRF) can achieve localized, clean erosion in soft tissue. In this paper, the intensity thresholds for ultrasound induced erosion and the effects of pulse intensity on erosion characterized by axial erosion rate, perforation area and volume erosion rate were investigated on in vitro porcine atrial wall tissue. Ultrasound pulses with a 3-cycle pulse duration and a 20-kHz PRF were delivered by a 788-kHz single element focused transducer. I(SPPA) values of 1000 to 9000 W/cm2 were tested. Results show the following: (1) the estimated intensity threshold for generating erosion was at I(SPPA) of 3220 W/cm2; (2) the axial erosion rate increased with higher intensity at I(SPPA) < or = 5000 W/cm2, while decreased with higher intensity at I(SPPA) > or = 5000 W/cm2; and (3) the perforation area and the volume erosion rate increased with higher intensity.