Heat Drain Device For Ultrasound Imaging Probes
Heat Drain Device For Ultrasound Imaging Probes
Heat Drain Device For Ultrasound Imaging Probes
Abstract: Ultrasound imaging probes are widely The patent of the present invention is currently
used for several types of diagnosis applications. under submission.
Since the electroacoustic conversion efficiency
of a probe is never 100% and in most cases the Keywords: thermal analysis, ultrasound
duration of clinical examination can be quite transducer, piezoelectricity, thermally conductive
long, the surface temperature of the probe head, layer, FEM, COMSOL, PCM.
in contact with human body, must be kept under
control. International Safety Standard EN 60601- 1 Introduction
2-37 [1] sets an upper limit for the surface
temperature in still air of 50°C (43°C if Probes for diagnostic ultra-sonography
measured when coupled thermally and application are devices that generate a pressure
acoustically with a test object having thermal and field into the human body, according to an
acoustical properties mimicking those of an electrical signal [1]. The efficiency of energy
appropriate tissue). conversion, from applied electrical to stored
The temperature rise is caused mainly by joule mechanical one, is called (for a piezo-material)
heating effect inside the active piezo-transducer the electromechanical coupling factor k :
and could be very important under some
operating conditions (i.e. Continuous Wave I. =
Doppler (CW); Pulsed Wave (PW) Doppler;
Colour Flow Mapping (CFM)).
In the present work we focus our attention on a Since k is always lower than 1, a temperature rise
small footprint phased array probe. Temperature for the probe head occurs.
of the front face of the analysed probe must be The main cause of the temperature rise are the
reduced, because during CW and PW operating joule losses in the piezo-transducer with the
conditions it could exceed the limit imposed by dynamics of heating controlled by specific heat
International Safety Standard. We present here a and thermal conductivity values for the materials
detailed 3D FEM for the transducer , designed to used in the probe construction and the final
replicate the real operation of the device and to steady state temperature of the probe surface
be compared with measurement (electric and depending mainly on the internal loss of the
thermal) in order to optimize some key piezo-material and the electrical driving power.
parameters of modelled materials. Then we show As anticipated, the International Safety Standard
the way found to engage the thermal managing EN 60601-2-37 [1] sets an upper limit for the
requirement, consisting in a very thin layer of surface temperature in still air of 50°C for
super thermally conductive material that acts as whatever operating condition, in order to avoid
heat drain toward a heat sink (in this paper it’s patient discomfort. Such requirement should not
avoided to give more detail on the name and be a limitation for the optimum power output of
characteristics of this material for confidentiality the transducer,. Indeed, the power limitation of
reason). Also in this case a comparison between the transducer output leads to a reduction of
FEM and measurements will be presented. In maximum temperature reached on the top of the
particular, we will show how the transducer transducer lens, but also reduce transducer
performance, in terms of thermal response, performance in terms of imaging capability.
changes by varying the conductive layer This issue has been engaged by the introduction
thickness. In conclusion, a possible future work, of a very thin thermally conductive compound
consisting in a FEM for the transducer with an layer below the transducer silicone lens. In this
additional dissipation system based on Phase way heat is drained on the back of the transducer
Changing Materials (PCM), will be presented. where it can be dissipated by an adequate heat
sink. The present thermally conductive
The transducer’s basic performances can be Figure 6. Specialized plexiglass box and measurement set.
evaluated by measurement of the electrical
impedance; whose quality and reliability play an 3.2 Standard transducer characterization
important role in the comparison with simulation
results. For a complete transducer performance The FEM optimization on transducer material
analysis and optimization, we invite the reader to has been performed on each manufacturing stage
look at [3], [4]. of the transducer, following the step approach
Impedance measurement has been performed method presented in [4]. The comparison has
with Hewlett Packard 4195A Network Analyser. been made both in terms of electrical impedance,
As regard temperature measurements, the phased both in terms of temperature rise. Electrical
array transducer was powered with an Esaote impedance analysis allows to optimize PZT
scanner device that applies, at 5 MHz frequency, material parameters like elasticity matrix,
a driving voltage across the PZT poles. The coupling matrix, relative permittivity and also
voltage value changes with load changing, so it matching layers and backing elastic parameters
has been measured, at every step, in order to (Young modulus, Poisson coefficient).
replicate that on the FEM. The transducer has Temperature analysis allows to optimize
been left to free convection on the sides and basically all material losses.
bottom surfaces and a type k thermocouple, The first analysis step was the PZT plate with
applied on the top exposed surface (in the connection fingers soldered. The second step was
middle) has been used. The thermocouple head the PZT bound on the backing support. Then
The thermally conductive compound layer has It is possible to conclude that the thermally
been introduced between silicone acoustic lens conductive compound layer represents a concrete
and matching layers. The thermally conductive solution for heating issues of an ultrasound
compound layer has been designed to contact transducer. FEM and measurements match well
and transfer heat to the aluminium part of the and this allows to proceed to the next step of this
backing that could be used as a heat sink, thanks work.
to the very high thermal conductivity of the
compound material. This assumption is 5 Thermally conductive compound
confirmed by results of FEM simulation layer thickness influence on
compared with measurements (Figure 12). Here transducer thermal performance
it is visible that maximum temperature is about
44°C after 3000s, so that it is reduced of about After previous FEM validation and study, it is
12°C. In Figure 13 it is represented the 3D plot possible to carry out an analysis on thermally
of temperature and heat flux. It is possible to see conductive compound layer thickness
how heat flux is focused along thermally optimization. As mentioned before, thermally
conductive compound layer instead of scattered conductive compound layer thickness must be as
as in the standard case. low as possible in order to be “invisible” to
acoustic wave. An analysis of transducer thermal
performance has been carried out varying
thermally conductive compound layer thickness
from 0.05 mm down to 0.02 mm. In Figure 14 a
comparison of temperature vs time curve relative
to these two different analysis is represented. It is
possible to see that as thermally conductive
compound layer thickness is lowered, the final
transducer temperature rises. This means that
thermally conductive compound thickness layer
must be optimized to reach a trade-off between
thermal and acoustical performance.