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4. Treatment methods > 4.5. Temperature monitoring
where:
= 1 °C, a constant to render the exponent dimensionless
C
T
T(t) = temperature (which may vary in time)
t = time
t' = time required to produce the bio effect at temperature T
R = thermal normalization constant, equal to 4,0 if T ≤ 43 °C
R = thermal normalization constant, equal to 2,0 if T > 43 °C
These parameters approximate human soft tissue.
Thermal dose is a measure of the biological effect of tissue heating. Thermal dose depends on the
temperature and the duration of the heating. Thermal dose is expressed in equivalent minutes
(EM) at 43 °C (109.4 °F). For example, a dose of 240 EM means that you would need to keep the
tissue at 43 °C for 240 minutes to cause similar damage.
Thermal dose gives an estimate of the expected tissue damage. Tissue receiving a dose of 30 EM
or less can be expected to avoid having any permanent damage. Tissue receiving a dose between
30 EM and 240 EM might become oedematic, or have some broken cell membranes, damage to
vasculature and other kinds of damage. This damage may or may not be reversible. A thermal
dose of 240 EM or more can be expected to cause irreversible coagulative necrosis in the tissue.
4.5.2. Thermal dose and temperature monitoring
Temperature maps graphically depict temperature rises in the tissue while thermal dose shows the
resulting tissue destruction. Both can be shown on the Sonalleve console as colored overlays on
top of the anatomical images. If a treatment cell cluster is selected, the cumulative thermal dose is
displayed for that cluster.
The Sonalleve MR-HIFU system records temperature in all the imaging planes. The system
automatically positions some stacks at the location of the expected maximum heating. Some
stacks are freely adjustable by the user, to be placed in locations where heating is not expected to
occur.
Temperatures below reference body temperature are not shown in the temperature monitoring
images: the color coding scale starts at 40 °C (104 °F). The colors in the temperature map are
more transparent outside the ultrasound beam path and the transparency increases with distance
from the beam.
There are two abrupt transitions in the color scale of the temperature maps to emphasize the
increasing physiological effect of heating:
Tab. 7: The temperature map color scale.
Color
Temperature
Blue
below 47 °C
(116.6 °F)
Yellow 47 °C to 56 °C
(116.6 °F to 132.8 °F)
Red
56 °C (132.8 °F) or
above
Instructions for Use
109745C2 / 02-2022
Physiological effect
A few minutes of exposure at this level is not likely to cause
necrosis.
Continued exposure at this level will cause necrosis within
seconds.
Necrosis is immediate.
50 (192)
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