Ultrasound Imaging

Introduction
Ultrasound, as currently practiced in medicine, is a real-time tomgraphic imaging modality. Not only does it produce real-time tomograms of the position of reflecting surfaces (internal organs and structures), but it can be used to produce real-time images of tissue and blood motion.

Theory and Instrumentation
Ultrasound denotes the use of acoustical (sound) waves at frequencies greater than 20 kHz. Generally, medical ultrasound is performed at frequencies in the range of 1 MHz. The technique is used to determine the location of surfaces within tissues by measuring the time interval between the production of an ultrasonic pulse and the detection of its echo resulting from the pulse reflected from those surfaces. By measuring the time interval between the transmitted and detected pulse, we can calculate he distance between the transmitter and the object. The ultrasound pulses are both produced and detected by a piezoelectric crystal. The crystal has the property of changing its physical dimensions in response to an electric field, and can produce an electric field if its physical shape is changed mechanically. Thus, ultrasonic compression waves (vibrations) are produced by applying an oscillating potential across the crystal. The reflected ultrasound imposes a distortion on the crystal, which in turn produces an oscillating voltage in the crystal. The same crystal is used for both transmission and reception.

Doppler Ultrasound
If a structure is stationary, the frequency of the reflected wave will be identical to that of the impinging wave. A moving structure will cause a back-scattered signal frequency shifted higher or lower depending on the structure's velocity toward or away from the sound generator (called a transducer).

For example, when an impinging sound pulse passes through a blood vessel, scattering and reflection occurs from the moving red cells. In this process, small amounts of sound energy are absorbed by each red cell, then re-radiated in all directions. If the cell is moving with respect to the source, the back scattered energy returning to the source will be shifted in frequency, with the magnitude and direction proportional to the velocity of the respective blood cell. Thus, if we use ultrasound to image the cross-sectional area of the blood vessel, the volume of blood flow can be calculated from the area of the vessel and the average velocities of the blood cells.

Clinical Applications
The major use of Doppler ultrasound is the study of the heart and human carotid artery disease wherein imaging and frequency shift are combined to produce images of artery and ventricle lumens. The frequency shift data is used to color the image, showing direction of flow (e.g. carotid arteries in red and veins in blue). Obstructions to blood flow are readily evaluated by this method using hand held scanning devices.
In addition to imaging heart valves and blood vessels, ultrasound is the most convenient and inexpensive method for medical evaluations such as fetal gender and gallbladder stones. Ultrasound imaging is also being used for monitoring therapy methods such as hyperthermia, cryosurgery, drug injections, and as a guide during biopses and catheter placements.

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© 2003 UC Regents. Last updated June 17, 2003 7:38 PM