Transducers 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 ... 

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. 

Ultrasound imaging is a non-invasive, portable and relatively inexpensive imaging modality, which is used extensively in the clinic. An ultrasound transducer (also called scanhead) sends short pulses of a high-frequency sound wave (1-10 MHz) into the body. At interfaces between two types of tissue, the wave will be refracted and part of the sound wave is reflected back due to Snells law. How much is reflected depends on the densities of the respective tissues, and thus the speed of the sound wave within the different tissues. In addition, parts of the sound wave are also backscattered from small structures at tissue boundaries or within the tissue. High-frequency sound waves propagate weU through soft tissue and fluids, but they are more or less stopped by air or bone. In clinical practice, this limitation is referred to as an acoustic window . The transducer not only sends the wave into the body but also receives part of the reflected and/or backscattered wave, also named echo . In clinical practice, ultrasound is used in a... 

Piezoelectric transducers are key components in medical ultrasound imaging and are used both as the acoustic source and the detector (pulse-echo teclmique). The uses for ultrasound are numerous and include examination of the fetus in the mother s womb as shown in Figure 31.22 and high-resolution imaging of intravascular structures. PZT is the ceramic of choice for this application mainly because it has a high k and is inexpensive compared to some of the other options such as polymer piezoelectrics. 

FIGURE 31.22 Medical ultrasound image using ceramic piezoelectric transducers. (Scale is in centimeters.)... 

Transverse ultrasound imaging is used to identify the vein between the two heads of the sternocleidomastoid muscle during the low anterior approach (Fig. 6.8). During needle advancement, the transducer must again be slightly moved or rocked in order to maintain visualization of the needle tip and vein. Once the tip is adjacent to the anterior vein wall, a short, sharp thrust is made with the small... 

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. 

Imaging of human tissues and nooinvasive assessment of their lesions is the most important diagnostic area of ultrasonics, and PVDF devices have also been exploited in m ical ultrasound imaging. Different transducer configurations have been adopted in the realization of the PVDF-based devices. 

Ultrasound images of the breast, obtained with P(VDF-T)rFE) transducers working at 75 MHz. have shown how the image quality is improved by the increased sensitivity due to the use of the copolymer, and demonstrated tint the capabilities of this kind of transducer can potentially allow imaging and diagnosis of very small lesions (even less than 5 mm in diameter) (45]. 

FIgura 8 Ultrasound image of the Ihyraid gland otneivcd with an annular PVDF array transducer. (Frao Ref. 39.)... 

M. Kobayashi, E. Nishihan. M. Matuyama, J. Morita, R. Omoto, M. Suzuki. T. Miya. and H. Ohigashi. High resolution ultrasound imaging with 73 MHz new potymer transducer, Ultrasamd Med. Bkd 8 (Suppl. 1) 99 (1982). 

Organic piezo-polymers such as polyvinylidene fluoride along with trifluoroethylene and their copolymers can be made ferroelectric by poling the crystalline regions of these polymers. Because the density of these materials is close to that of human tissue, they have foimd use as transducers for ultrasound imaging with no acoustic mismatch. 

The source and detector of ultrasound in an ultrasound medical imager is called a transducer. The transducer is a piezoelectric crystal which physically changes its dimensions when a potential is appHed across the crystal (38). The appHcation of a force to the piezoelectric crystal which changes its dimensions creates a voltage in the crystal. AppHcation of an oscillating potential to the crystal causes the dimensions of the crystal to oscillate and hence create a sound at the frequency of the oscillation. The appHcation of an oscillating force to the crystal creates an alternating potential in the crystal. 

Yang, M. et al. (1999) Real-time ultrasound process tomography for two-phase flow imaging using a reduced number of transducers. IEEE Trans. Ultrason. Ferroelectr. Freq. 

Medical Ultrasound Many of us have first seen our unborn children or grandchildren thanks to medical ultrasound technology. This application is somewhat related to sonar in that both use pulse echo methods to send and receive signals. However, in this short-range application, transducer arraj can be designed to form images. 

The field of view for the ultrasound equipment is rather small, typically, not more than 20-30 cm and just a few centimeters at higher spatial resolution. Normally, a cross-sectional image in the plane of the imaging probe is obtained. Respectively, the operator must know where to place the transducer to locate the area of an expected lesion, as compared with a whole-body CT, the wide flat view of planar X-ray or MRI scans. 

---