1 - 26 HS70A Service Manual
imaging mode, pulse repetition frequency, focus depth, pulse length, and probe selection.
The choice of imaging mode determines the nature of the ultrasound beam. 2D-mode is a scanning
mode, Doppler is a stationary or unscanned mode. A stationary ultrasound beam concentrates energy
on a single location. A moving or scanned ultrasound beam disperses the energy over a wide area and
the beam is only concentrated on a given area for a fraction of the time that is necessary in unscanned
mode.
Pulse repetition frequency or rate refers to the number of ultrasound bursts of energy over a specific
period of time. The higher the pulse repetition frequency, the more pulses of energy in a given period of
time. Several controls affect pulse repetition frequency: focal depth, display depth, sample volume depth,
color sensitivity, number of focal zones, and sector width controls.
The focus of the ultrasound beam affects the image resolution. To maintain or increase resolution at a
different focus requires a variation of output over the focal zone. This variation of output is a function of
system optimization. Different exams require different focal depths. Setting the focus to the proper depth
improves the resolution of the structure of interest.
Pulse length is the time during which the ultrasonic burst is turned on. The longer the pulse, the greater
the time-average intensity value. The greater the time-average intensity, the greater the likelihood of
temperature increase and cavitations. Pulse length, burst length or pulse duration is 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.
▐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-
To Next Page
Loading...