by Barnaby Meins

Created on: June 06, 2008

Ultrasound is the use of high-frequency sound waves (thus its name) to produce images of structures within the human body. Ultrasound imaging was first discovered in 1952 and used clinically in 1962. Non-ionizing sound waves in the typical frequency of 1 to 10 MHz above the range of sound audible to humans is used. Basically, echoes from discontinuities in tissue density or the speed of sound are registered. The imaging volume is about 20 cm and its resolution is high in the mm range. The more common ultrasound imaging applications are imaging soft tissue, blood flow based on Doppler shift and blood vessel conditions in vascular ultrasound.

The fundamental working principle of ultrasound imaging is as follow: A pulse is propagated and its reflection is received both by the transducer. The key assumption is that sound waves have an almost constant velocity of 1540 m/s in water and soft tissue. Size of reflected pulse detected gives acoustic impedance & brightness. The transducer is made of piezoelectric crystal creates sound waves aimed at a specific area of the body. Differences in tissue density reflect the sound waves and the echoes are recorded. Delay of reflected signal and amplitude determines the position of the tissue. There can be still images or a moving picture of the inside of the body

Generation and detection of ultrasound is by the piezoelectric effect. There is a conversion of electric energy into mechanical energy and vice versa in materials with intrinsic dipole moments in a scientific term known as structural anisotropy. An electric field of about 100 V causes re-orientation of dipoles, leading to deformation. Deformation, also causes shift of dipoles, which induces voltage. Examples of piezoelectric materials are crystalline (quartz), polycrystalline ceramic (PZT, lead zirconium titanate), Polymers-polyvinyldifluoride (PVDF).

Typical ultrasound scanning transducer contains an array of 200 piezoelectric crystals. Each is activated sequentially to scan beam over 2D field. The pulse rate is roughly 3000 per second and the frequency of each pulse is about 1- 15 MHz. Generally, high frequency means less penetration and higher spatial resolution. Linear array of crystals produce good images for superficial structures while curved array gives a wider field of view at depth. Generation and detection of ultrasound is by the piezoelectric effect. There is a conversion of electric energy into mechanical energy and vice versa in materials with intrinsic

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