Ultrasound imaging focusing

First ferroelectric polymer - polyvinilidene fluoride (PVDF or PVF2) - was discovered in 1969. Extensive research has been focused on this substance and their copolymers withtrilluoroethylene (TrFE) since that time. Due to its resistivity to the harmful chemical substances is this polymer used in stractural coatings to prevent damage. Another excellent functional property is a veiy low value of the acoustic impedance, which allows for the better acoustic matching to water environment. Due to this property P(VDF/TrFE) copolymer is being applied mostly in hydrophones (Nalwa 1995) and ultrasound imaging transducers. PVDF polymer and its blends with TrFE are commercially available in the market. 

Figure 17.15 (a) Ultrasound image of a human cadaveric aorta obtained using a 2 mm focused polymer transducer (b) corresponding histological image. 

F. Kallel, R. J. Stafford, R. E. Price, R. Righetti, J. Ophir and J. D. Hazle, The feasibility of elastographic visualization of HIFU-induced thermal lesions in soft tissues. Image-guided high-intensity focused ultrasound, Ultrasound Med. Biol., 1999, 25, 641-647. 

N.J. McDannold, N.l. Vykhodtseva, K. Hynynen, Microbubble contrast agent with focused ultrasound to create brain lesion at low power levels MR imaging and histologic study in rabbits, Radiology 241 (2006) 95-106. 

Fig. 6.20 (a) Piezoceramic parts for a range of applications including accelerometers, underwater acoustics, pressure and liquid level sensors, medical diagnostics and therapeutics and NDT the ultrasound focusing bowls are for medical imaging and for producing high-intensity focused ultrasound. 

The delivery of macromolecules into the central nervous system (CNS) via the blood stream is seriously limited by the blood-brain barrier (BBB). Noninvasive, transient, and local image-guided blood-brain barrier disruption (BBBD) can be accomplished using focused ultrasound exposure with intravascular injection of pre-formed microbubbles. A detailed description of the method for MRI-guided focal BBBD in animals will be described in this chapter. The method may open a new era in CNS macromolecular drug delivery. 

Hynynen, K., Vykhodtseva, N.I., Chung, A., Sorrentino, V, Colucci, V., and Jolesz, F.A. (1997) Thermal effects of focused ultrasound on the brain determination with MR Imaging. Radiology 204,247-253. 

Fig. 17. A series of 2 s fast gradient refocused images (TR/TE/angle = 11/2.3/60) in the focal plane of a muscle specimen showing the displacement of a hot spot (dark area) by moving the focus of a 4 s ultrasound pulse, with a maximum acoustic intensity of 8100 Went" , in a vertical direction (a -d). (Reprinted from ref. 207 by permission of John Wiley Sons, Inc. Copyright 1993 John Wiley.)...

Fig. 20. Average spatial temperature profiles produced by ultrasound sonications given in tumours implanted in the skeletal muscle of rabbits, (left) Phase difference image showing a localized area of heating, (middle) corresponding temperature profile across the focus (right) profile across the focus with a plastic lens placed in front of the transducer. (Reprinted from ref. 273 by permission of Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc., Copyright 1998 John Wiley.)...

Nondestructive inspection (NDI) includes a set of techniques to enhance the ability to detect small and/or hidden malfunctions. One set of NDI techniques is those that enhance what is essentially still a visual inspection task, such as X ray, fluorescent particle, magnetic particle, or D Sight. They show cracks that are very small (fluorescent particle) or hidden within other structures (X ray). Apart from the steps necessary to ensure a good image, they have many of the human interface characteristics of visual inspection. The other set of NDI techniques is focused on specific malfunctions in specific locations, such as eddy current and ultrasound. For this reason, they are useful only for detection of malfunctions already predicted to exist. In practice, such NDI techniques are much more procedur-alized than visual inspection or NDI techiriques, which contain a human visual inspection component. 

Computed tomography (or CT) uses the variable absorption of x-rays by different tissues to visualize structures within the body. An x-ray source is needed and an image is formed by obtaining absorption data from many individual points as the x-ray is focused in different directions. A single CT image is the product of thousands of individual measurements made as the source encircles the body. Unlike ultrasound, the source and detector are not the same device. 

FIGURE 44.7 (a) Microscope image of the situation when the ultrasound is turned off. Both 3 pm polystyrene and 8 pm PMMA particles exit through the side outlets. (Reproduced from. Chemical Society Reviews, 36, 492-506, 2(X)7. With permission from The Royal Society of Chemistry.) (b) Microscope image of the situation when the ultrasound is turned on. The 8 pm particles are focused in the fundamental resonance pressure node and exit through the centre outlet while the 3 pm particles are gathered in the first harmonic pressure nodes and exit through the side outlets. The fundamental resonance and the first harmonic were typically active for 800 and 200 ps, respectively (total flow rate 90 pL/min). (Reproduced from Chemical Society Reviews, 36, 492-506, 2007. With permission from The Royal Society of Chemistry.)... 

C. Lorenzato, et al., MRI contrast variation of thermosensitive magnetoliposomes triggered by focused ultrasound a tool for image-guided local drug delivery. Contrast Media Mol. Imaging 8 (2)... 

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