2 – 24 HS40 Service Manual
the output pulse duration in Pulsed Doppler. Increasing the Doppler sample volume, increases
the pulse length.
Probe selection affects intensity indirectly. Tissue attenuation changes with frequency. The
higher the probe operating frequency, the greater the attenuation of the ultrasonic energy.
Higher probe operating frequencies require higher output intensity to scan at an increased
depth. To scan deeper at the same output intensity, a lower probe frequency is required. Using
more gain and output beyond a point, without corresponding increases in image quality, can
mean that a lower frequency probe is needed.
■ Receiver Controls
Receiver controls are used by the operator to improve image quality. These controls have no
effect on output. Receiver controls only affect how the ultrasound echo is received. These
controls include gain, TGC, dynamic range, and image processing. The important thing to
remember, relative to output, is that receiver controls should be optimized before increasing
output. For example; before increasing output, optimize gain to improve image quality.
2.5.1.4. Additional Considerations
Ensure that scanning time is kept to a minimum, and ensure that only medically required
scanning is performed. Never compromise quality by rushing through an exam. A poor exam will
require a follow-up, which ultimately increases the scanning time. Diagnostic ultrasound is an
important tool in medicine, and, like any tool, should be used efficiently and effectively.
2.5.1.5. Output Display Features
The system output display com prises two basic indices: a mechanical index and a therm al index.
The thermal index consists of the following indices: soft tissue (TIs), bone (TIb) and cranial bone
(TIc). One of these three thermal indices will be displayed at all times. The thermal index to be
displayed is determined by the system preset or user choice, depending upon the application.
The mechanical index is continuously displayed over the range of 0.0 to 1.9, in increments of
0.1. The thermal index consists of the three indices, and only one of these is displayed all the
time. Each diagnostic probe application has a default selection. The TIb or TIs is continuously
displayed over its range of 0.0 to maximum output, based on the probe and application, in
increments of 0.1.
The default setting of the application-specific nature is also an important factor of index
selection. A default setting is a system control state which is preset by the manufacturer or the
operator. The system has default index settings for the probe application. The default settings
are applied automatically by the ultrasound system when the power is turned on, new patient
data is entered into the system database, or a change of application takes place.
The decision as to which of the three thermal indices to display should be based on the
following criteria:
Appropriate index for the application: TIs is used for imaging soft tissue, and TIb for a focus at
or near a bone. Some factors might create artificially high or low thermal index readings (e.g.
presence of fluid or bone, or the flow of blood). A highly attenuating tissue path, for example,
may cause the potential for local zone heating to be lower than the thermal index indicates.
The selection of scanned modes or unscanned modes of operation also affect the thermal index.
For scanned modes, heating tends to be near the surface; for unscanned modes, the potential
for heating tends to be deeper in the focal zone.
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