Ultrasound imaging

The transducers on most ultrasound imaging systems operate at a frequency between 1 and 20 MH2. The attenuation, of ultrasound by tissues is both frequency and tissue dependent. The attenuation coefficient, a, of a tissue is defined by equation 5 ... 

The resolution in an ultrasound image is, among other things, related to the duration of the ultrasound pulse, ie, the shorter the pulse the better the resolution. Imaging may not be performed when the pulse duration is longer than the time to receive an echo. The shorter the ultrasound pulse the more difficult it is to discern it from noise, and the poorer the SNR of the image. As the pulse duration is decreased, the power of the ultrasound pulse is typically increased to compensate for the poorer SNR. 

Special small ultrasound transducers, often referred to as endoscopic transducers, have been designed which can be inserted into blood vessels to examine blockages in arteries (43). These transducers operate at approximately 20 MHz and have a viewing distance of less than a centimeter. Such devices are capable of producing ultrasound images of the inside of arteries and veins. The quaUty of the ultrasound image is sufficient to determine the type of blockage. 

Why Use Ultrasound Imaging Modality and Ultrasound Contrast Agents ... 

The examinations are performed in real time and can visualize motion (30 frames per second or even more) respectively, needle biopsies and tissue ablation procedures are often performed under the guidance of ultrasound imaging. 

Ultrasound imaging is mostly two-dimensional. Real-time three-dimensional systems that give a field of view wider than a thin sHce at a time are still quite rare and expensive. Quasi-3D systems based on manual lateral translation of the probe are gradually becoming available as software upgrades for existing equipment. 

Fig. 6. Ultrasound imaging of a dilute microbubble dispersion in normal saline (right). Comparison with imaging of control normal saline (left). Imaging performed using a contrast pulse inversion scheme. Samples placed on top of an ultrasound tissue phantom (bottom)...

Fig. 8. Concentration dependence of ultrasound backscatter signal by plates coated with a layer of targeted microbubbles. Surface concentrations of microbubbles (as observed by bright-field optical microscopy, bottom) increase from left to right. Imaging performed using a fundamental frequency scheme. Samples placed on top of an ultrasound tissue phantom. Reprinted from Advanced Drug Delivery Reviews v. 37, A.L. Klibanov, Targeted delivery of gas-filled microspheres, contrast agents for ultrasound imaging, p. 145. Copyright, 1999, with permission from Elsevier Science...

Fig. 9. Ultrasound images of the liver before (left) and 10 min after Sonazoid administration (delayed phase, where ultrasound contrast is accumulated in the normal liver tissue, right). Copyright, 2001, L. Needleman, Thomas Jefferson University, Philadelphia, PA. Reprinted with permission from [66]...

The third chapter is dedicated to contrast agents for ultrasound imaging starting with the design, preparation and application of microbubbles. Additionally, the different presently available generations of contrast agents are... 

E.G. Schutt, D.H. Klein, R.M. Mattrey, J.G. Riess, Injectable microbubbles as contrast agents for diagnostic ultrasound imaging The key role of perfluorochem-icals, Angew. Chem. Int. Ed. 42 (2003) 3218-3235. 

A.L. Klibanov, Microbubble contrast agents Targeted ultrasound imaging and ultra-sound-assisted drug-delivery applications. Invest. Radiol. 41 (2005) 354-362. 

Dissolution of radiolucent, noncalcified gallstones when cholecystectomy is not recommended treatment of biliary cirrhosis PO 8-10 mg/kg/day in 2-3 divided doses. Treatment may require months. Obtain ultrasound image of gallbladder at 6-mo intervals for firsf year. If gallsfones have dissolved, confinue fherapy and repeat ultrasound within 1-3 mo. 

---