TELEMED ClarUs User Guide, REV 4.0 2013.12.04
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ALARA (As Low As Reasonably Achievable):
a principle which is used to reduce
any unnecessary and potentially hazardous exposure to individuals by keeping doses
As Low As Reasonably Achievable.
As shown throughout this guideline, application of the ALARA principle to diagnostic
ultrasound differs from its common usage in diagnostic X-ray imaging where it is
assumed that there is no threshold exposure.
In the use of diagnostic ultrasound, there are three ranges of exposure, i.e.
combinations of Thermal or Mechanical Indices and dwell time that need to be
considered. At exposures that are clearly below the thresholds for health effects,
further reduction of exposure is not justified, whether it is via reductions in dwell time
or acoustic output. There can also be exposure that is or may be above thresholds
for health effects. In these cases, ALARA refers to using the lowest value of
potentially hazardous exposure, i.e. a combination of acoustic output and dwell time
needed to achieve the required diagnostic information.
Bone Thermal Index (TIB):
the Thermal Index for an exposure model in which the
ultrasound beam passes through soft tissue and a focal region is in the immediate
vicinity of bone.
Cranial Bone Thermal Index (TIC):
the Thermal Index for an exposure condition in
which the ultrasound beam passes through bone near the beam entrance into the
body.
derated:
a derated quantity is one which has been measured in water using standard
methods and then multiplied by a derating factor. This accounts for attenuation of the
ultrasound field by the tissue between the transducer and a particular location in the
body along the beam axis. The derating factor is 0.3 dB/cm-MHz in these guidelines.
derated spatial peak time average intensity:
the largest value in an ultrasound
beam of any derated time averaged intensity.
dwell time:
the amount of time that the transducer is actively transmitting ultrasound
while staying in any one place during part of an examination.
rarefactional pressure:
the amplitude of a negative instantaneous ultrasonic
pressure in an ultrasound beam
Soft Tissue Thermal Index (TIS):
the Thermal Index for an exposure model in which
the ultrasound beam heats primarily soft tissue.
spatial average, pulse average intensity at the face of the transducer:
the spatial
average, temporal average intensity at the face of the transducer divided by the duty
factor, where the duty factor is the product of the pulse duration and the pulse
repetition frequency.
spatial average, temporal average intensity at the face of the transducer:
the
time averaged intensity, averaged over the face of the transducer.
Thermal Index
(TI):
a quantity related to the potential for
ultrasonic heating
. It is
proportional to a calculated or estimated temperature rise for model exposure
