Philips EPIQ 7 User Manual page 53

Biological Safety Safety
The choice of imaging mode determines the nature of the ultrasound beam. 2D is a scanning
mode; Doppler is a stationary or unscanned mode. A stationary ultrasound beam concentrates
energy in a single location. A moving or scanned ultrasound beam disperses the energy over an
area and the beam is concentrated on the same area for a fraction of the time as that of an
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 period of time. Several controls affect pulse repetition frequency: focal depth, display depth,
sample volume depth, flow optimization, scale, number of focal zones, and sector-width
controls.
Focus of the ultrasound beam affects the image resolution. To maintain or increase resolution
at a different focus requires a variation in output over the focal zone. This variation of output is
a function of system optimization. Different exams require different focal depths. Setting the
focus at 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 cavitation. Pulse length, burst length, or pulse
duration is the output pulse duration in PW Doppler. Increasing the Doppler sample-volume
size increases the pulse length.
Transducer selection indirectly affects intensity. Tissue attenuation changes with frequency.
The higher the transducer operating frequency, the greater the attenuation of the ultrasonic
energy. A higher transducer operating frequency requires more output intensity to scan at a
deeper depth. To scan deeper at the same output intensity, a lower transducer frequency is
required. Using more gain and output beyond a point, without corresponding increases in
image quality, can mean that a lower frequency transducer 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 output is
increased. For example, before increasing output, optimize gain to improve image quality.
EPIQ 7 User Manual 4535 617 25341 53