Application Of Ultrasound Contrast Agents Research Articles

Ultrasound mediated drug delivery—A new era using ultrasound contrast agents?

Ultrasound has been used therapeutically for decades. High energy ultrasound is used clinically in hyperthermia. Transdermal drug delivery is accomplished with ultrasound, perhaps more effectively at low frequencies because of more efficient cavitation. High intensity focused ultrasound (HIFU) is used to coagulate tissue for treatment of cancer and hemostasis. Recently, contrast agents have been developed for diagnostic ultrasound. These agents can also be used as exogenous cavitation nuclei for therapy. Compared to some of the other therapeutic ultrasound techniques, exogenous cavitation nuclei may enable use of lower power levels. Applications of ultrasound contrast agents as cavitation nuclei include sonothrombolysis and drug and gene delivery. Cavitation of contrast agents can be used to increase capillary permeability to deliver drugs. Co-administration of ultrasound contrast agents with transdermal ultrasound accomplishes sonothrombolysis. Acoustically active carriers (contrast agents) have been developed to carry drugs and genes; systemic administration produces local drug delivery within the field of insonation. Contrast agents may open a new era of therapeutic applications of ultrasound for drug delivery and sonothrombolysis.

Splanchnic vessels

Splanchnic vessels

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents.

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.

TRANSRECTAL ULTRASOUND OF THE PROSTATE: INNOVATIONS AND FUTURE APPLICATIONS

Purpose We present a critical evaluation of the use of ultrasound for prostate disease examination in urological practice, and provide perspectives on ultrasound applications that may become important for the future evaluation of prostate problems. Materials and Methods Based on an evaluation of clinical data in cases suspicious for prostatic malignancy, we addressed the apparent shortcomings of transrectal ultrasound for accurately diagnosing prostate cancer. Future applications presented in the literature were noted. Results Evaluating the ultrasound data in cases suspicious for malignancy indicated that imaging has little advantage over digital rectal examination for detecting malignant areas. The new applications of ultrasound that hold great promise for use in the urology clinic include the injection of contrast agents to obtain information on blood supply, temperature estimation for the noninvasive assessment of temperature distributions during heat treatment and a therapeutic application for local treatment of prostate cancer. Conclusions While differential diagnosis with ultrasound appears to result in disappointing sensitivity and specificity values, its use in volume measurement and biopsy guidance is unquestioned. The development of new applications may improve the clinical value of ultrasound in urological practice. The application of ultrasound contrast agents for the detection and clinical staging of prostate cancer is especially promising. Future research will indicate whether the promise evolves in clinical applications.

Clinical applications of ultrasound contrast agents

Within the last decade safe and practical ultrasound contrast agents have been introduced. Most of these are based on gasfilled microbubbles, which markedly enhance Doppler signals and, in some cases, also gray-scale images. The clinical improvements expected from ultrasound contrast is reviewed. Tissue-specific contrast agents constitute another area of potential clinical significance. One particular agent is taken up by the reticulo-endothelial system and produces so-called acoustic emission signals when imaged. An introduction to the unique clinical applications of acoustic emission is given. Harmonic imaging is a new contrast-specific imaging modality, which utilizes the nonlinear properties of some agents in an attempt to alleviate current limitations of ultrasound contrast studies. Examples of harmonic images are presented.