TMS
Didactic Muse
Page 17
Imaging Ultrasound Cont.
AXIAL RESOLUTION
Lambda (λ)
Lambda indicates
the wavelength of
any wave,
especially in
physics,
Resolution is the ability to differentiate between two or more objects in
space. Axial resolution in an ultrasound image is the resolution along the
ultrasound beam. The smaller the objects the more difficult to differentiate,
and if an equipment has poor resolution it may identify an object where indeed
there are two or three. Increasing the frequency of the probe also increase the axial resolution and a better quality of image is obtained. The parameter (λ) is defined as wave length, and is associated with the frequency and
represents the longitude of a entire cycle of the signal. The parameter (λ)
decrease with the frequency as is shown in (fig.19). As it was said, the
improvement of the image is given for the property to differentiate very small
objects of the size of (λ). If two ultrasound beams are emitted, one of 1 MHz and
another of 4.5 MHz, the (λ) of 1MHz is 1.5 mm., and the (λ) of 4.5 MHz is 0.3
mm. So with the frequency of 4.5 MHz, a better image is obtained because(λ)
is smaller. It is not possible to increase indefinitely the frequency because
increasing the frequency, also increase the attenuation of the signal. Then
for studies like gynecology and obstetrics where the beam has to penetrate
deep, there must be a compromise between an acceptable quality of image and
the penetration needed to reach the structure in study. This compromise leads
to the frequency choice of 3.5 MHz; commonly used and obtains a reasonable
good quality mage for medical diagnosis. In studies of the thyroid and
mammary glands, the beam does not need to penetrate deep, the frequency can be increased to 7.5 MHz.
electronics
engineering, and
mathematics.
TIME GAIN CONTROL (TGC)
The strength of the echoes received in the probe produce a very small pulse
of voltage and is not enough to be processed. These echoes must be amplified
to obtain adequate amplitude. The device named Time Gain Control
(TGC), Deep Gain Control (DGC), or Sensitive Time Control (STC) (the
abbreviation used is according to the manufacturer) is another kind of amplification. The amplitude of the echoes received by the probe
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