بواسطة دكتور كمال سيد » الثلاثاء إبريل 23, 2019 9:42 pm
Ultrasound Medical Imaging—The Basic Process Briefly Stated
The basic ultrasound process goes as follows
:
The operator, usually a sonographer or radiologist, signals the generator which produces an electrical pulse and sends it to the transducer.1 The transducer, or probe, changes the electrical pulse into a sound pulse and sends it into the body.
1. An electric current can be produced in a continuous current form or a “pulsed” form where the current is on and then off in a periodic way.
The sound wave will travel through the first body tissue until it hits an interface, where two different tissues are contiguous. Because of this interface, some of the sound wave will be reflected back and some will continue to travel through the next tissue. The part that is reflected back, the echo, is picked up by the transducer and changed into an electric pulse.
The electric pulse is then sent to the computer/display. Depending on a) the time it takes an electrical pulse to make the round trip into the body and back and b) its intensity, a computer determines where on the display screen to make a dot and what shade of gray, from light to dark, it should be. The operator reads the information that appears on the screen.
Ultrasound medical imaging produces readable images of inner body structures and motion without surgical entry into the body or radiation. Along with conventional radiography (X-ray), nuclear magnetic resonance (NMR), and computed tomography (CT or CAT scan), ultrasound is one of modern medicines most powerful diagnostic tools.
The Basic Process in Greater Depth
Ultrasound is a sound wave. All waves have the following characteristics:
frequency, period, wavelength, propagation speed, amplitude and intensity.
Frequency, period, amplitude and intensity are determined by the sound source.
Propagation speed is determined by the medium, and wavelength is determined by both the source and medium.
The frequency of a wave tells how many cycles occur in a second. Frequency is measured in Hz and MHz. As stated above, the human ear can hear sound waves in the 20-20,000 Hz range and ultrasound refers to sound waves that are above 20,000 Hz. Diagnostic ultra-sound uses frequencies in the 1,000,000-5,000,000 Hz, or 1-5 MHz, range.
FREQUENCY/ AMPLITUDE
The frequency of a sound wave or pulse is important in image resolution (display) and depth of penetration. The higher the frequency the better the resolution but the less depth of penetration. The lower the frequency the greater the depth of penetration but the poorer the resolution.
Wavelength is the distance or space needed for one cycle to occur. In diagnostic ultrasound wavelength is measured in meters (m) and millimeters (mm). It is important in image resolution.
Sound waves must have a medium to pass through. The speed at which a sound wave travels through a medium is called the propagation speed or velocity. It is equal to the frequency times the wavelength. In ultrasound it is measured in meters per second (m/s) or millimeters per microsecond (mm/µ s). In general, the propagation speed of sound through gases is low, liquids higher and solids highest. The average propagation speed for sound in body tissue is 1540 m/s, or 1.54 mm/µs.
Carefully read the following table.
Velocity of Sound in Various Materials
Material Velocity (m/s)
air 331
fat 1450
water (50°C) 1540
human soft tissue 1540
brain 1541
liver 1549
kidney 1561
blood 1570
muscle 1585
lens of eye 1620
skull-bone 4080
brass 4490
aluminum 6400
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From Christensen, E.E.1
The period of a wave is the time it takes for one complete cycle to occur. It is the reciprocal of frequency. Period is measured in seconds(s) or microseconds (us).
Amplitude is the maximum height that occurs in a wave minus its normal value. It is measured in watts (W) and microwatts (µ W). Amplitude is important in determining the display and attenuation, the energy loss as sound travels through a tissue.
Intensity is a magnitude, such as energy or a force, divided by a unit of area, volume, etc. For sound it is the power, the amount of energy transferred measured in watts (W) divided by the area of the sound beam measured in square meters, (W/m2).
The sound beam will be discussed later. Amplitude and intensity describe the strength of a sound beam.
So far we have been considering terms that could be used to describe any wave—water, light, sound, radio,etc. Now we will consider ideas more specific to sound waves.
How is sound produced ?
An electric current, in a pulsed rather than continuous pattern, is sent to the transducer which converts the electrical energy into mechanical sound energy. The heart of the transducer is the piezoelectric (pi e zo i lek’ trik) crystal. This crystal, which is either natural or man made, has been processed to have the piezoelectric property of changing a mechanical sound to electrical current and vice versa
THE PROBE
Behind the crystal is backing material which dampens the sound pulse. In front of the crystal is an acoustical lens which helps to focus and cut down on the reflections of returning sound impulses.
Piezoelectric means pressure electricity. The piezoelectric crystal can be thought of as having many small particles called dipoles. Each dipole has a positive and negative charge.
Plating electrodes are placed on each side of the crystal.
A negative charge is induced on one side, a positive charge on the other.
This establishes an electric field between the plates, through the crystal. The dipoles are rearranged because of this= the positive charge of the dipole shifts slightly closer to the negative side and the negative charge of the dipole shifts slightly closer to the positive side. This realignment of the dipoles results in a small decrease in the thickness of the crystal. When the field is turned off, the dipoles return to their original position and the crystal expands.
PEIZOELECTRIC CRYSTAL
It is this contracting and expanding that produces the sound wave. When it expands it pushes the particles in its way and sends them crowding out in all directions. As the crystal becomes narrower there is a space created and the particles rush in to fill the space and return to their former positions. When the crystal pushes out causing a crowding of the particles, it is called compression.
Compression represents the maximum in a usual wave illustration or the crowded (dark) part of the line representation of a wave. The space caused by the contraction is called rarefaction; it is represented on the same illustration.
Now let’s go back. The generator, or pulser, sent an electric pulse to the transducer which changed it into a sound pulse.
It is important next that the sound pulse travel directly from the transducer into the body without any air pockets or bubbles.
Air acts as a sound barrier and would result in poorer resolution. To prevent this, a lubricant, such as mineral oil, is always placed on the skin. This provides a good connection between the transducer and the body.
