Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-- ~1 3~A32
' :' .
ADJUSTABLE FOCUSING THERaPEUTIC APPARATUS WITH NO
SECONDARY FOCUSING
j This invention is chiefly related to apparatus for
therapy having variable focusing and no secondary
spurious focusing.
.: -
Preferably, the apparatus uses focalized ultrasounds. The
invention also entails the use of an electronic signal
which has an autocorrelation function of Dirac type for
the excitation of at least one ultrasonic transducer
element. The invention also relates to a process of
electronic focusing by at least one transducer element
which eliminates or minimizes secondary focusing by
2Q exciting the transducer elements with an electronic
signal generator which produces an electronic si~nal -
having a Dirac type autocorrelation function.
It is well-known that piezo-electric transducer~ produce
therapeutic focalized ultrasounds when powered by
electronic signals of sine type. The ultrasonic beam can
create lesions in the tissue which are l-imit~d--~o -t~e~- ~r , ~ .
focal volume of the transducer. This spatial limitation
is notably necessary for an efficient treatment in the
area of cancers therapy such as cancer of the prostate,
of the breast, of the brain etc... The beams can also be
used in the treatment of benign lesions such as benign
prostatic hypertrophy, benign breast lesions, nodules of
the thyroïd.
-~
Generally, these cancers have a predetermined volume so -~
that it is necessary to scan the focal point of the
transducer in order to treat the entire lesions.
-`
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Generally~ these cancers have a predetermined volume so
- that it is necessary to scan the focal point of the
transducer in order to treat the entire lesions.
This motion can be obtained by mechanical means, i.e.
translation stages which are expensive and bulky. However
it is also possible to use electronic means for the
scanning of the focal point. These means are less
cumbersome, less expensive, and eliminate the need of
mechanical motions of the transducer. These mechanical
motions can degrade the physical contact between the
transducer and the patient and are detrimental to the
proper transmission of the acoustic waves. In addition
these mechanical motions can cause unwanted motions of
the target area during the treatment.
When using electronically focused transducers, the
practitioner may lock the transducer head into position
against the patient's skin, control the ultrasonic waves
such that the deepest region of the target is treated,
then change the control of the transducer to focus at a
shorter distance to treat the most shallow region of the
target. This procedure may result in less burns of the
skin becau~e these originate from using low =aperture
transducers. The aperture being the ratio between the
diameter of the transducer and the focal distance, it isr- - ~~ ~ ~~ clear that the aperture is maximum when-the-tr~atment~
delivered near the patient skin, hence the risk of burns
is minimized.
These electronic stirring means of the focal point must
involve multi-element transducers. These elements can be
shaped as concentric rings so that the focal point can be
stirred along the axis of the transducer, i.e. in the
direction of the propagation of the waves. Multi-element
two dimensional arrays of transducers elements must be
used to stir the focal point in the three dimensions.
.~,-., ~.,
. . ~
~ 2~3~432
A physical motion of the focal point is obtained when
sine waves are applied to these elements, these sine
waves having a predetermined phase shift with respect to
- one another.
It is well-known to the one skilled in the art that for
large displacement of the focal point an ambiguity may
occur on the phase values which are applied to the
signals of each of 'che elements of the transducer
implying that secondary spurious focusing point appears
within the ultrasonic field which may induce necrosis of
the tissue in an unwanted area outside of the target thus
rendering the apparatus dangerous and unfit for clinical
use.
To solve these problems several solutions have been
proposed to control the ultrasonic field and obtain the
desired ultrasonic field without secondary focusing
points.
One example of known-methods includes the method of beam
synthesis which was described by ES EBBINI in the article
"Experimental evaluation of a prototype cylindrical
section ultrasound hyperthermia phased array applicator"
(in IEEE transactions on ultrasonics, ferroelectrics, and
frequency control, Vol. 38, n5, pp 510-520, September
1991). In this method, one calculate~-t~e~amplitud-e~and
the phase of the signal to be applied to each element of
the transducer in order to synthesize a predetermined
beam geometry . One example is given on figures 6,9,10,
and 11 of the article. EBBINI shows that with this method
it is possible to synthesize an ultrasonic field with one
or several focusing points.
EBBINI's method has major drawback. Each transducer does
not receive the maximum signal amplitude (see page 514).
So that the acoustic energy is not maximized. This is
undesirable when treating a tissue with an endocavitary
'. -:
~32~32
applicator which must be small so that the ratio between
- emitted power and transducer dimension must be maximum.
However this constraint is acceptable for a low power
hyperthermia treatment as foreseen in the article (see
page 514).
An another method for synthesizing ultrasonic beams while
limiting the number of spurious focal points is
described by UMEMURA in the article "The sector - vortex
, 10 phased array : acoustic field synthesis for hyperthermia"
also in IEEE transactions on ultrasonic, ferroelectrics
~ and frequency control, Vol. 36, n2, pp 249-257, March
i, 1989. This method is applicable to transducers divided in
several sectors and consists in applying a specific phase
distribution on each sector. Since it is necessary to
subdivide the rings into sectors, this method's results
in using a large number of transducers elements. To
obtain a correct beam shape the number of elements must
be high. Since the electronic circuits for the control of
each element is complex, the cost of such an apparatus is
very high and hardly compatible with industrial and
medical application where the cost of the therapy must be
kept low.
The goal of the present invention is to solve the
technical problem of electronic focusing of a therapeutic
transducer over large scannin~-~ dis~n~es- --while
eliminating spurious focal points and while using
transducers to the maximum power and while keeping a
spot-like focal area and at low cost such that the
apparatus can be widely used.
The present invention offers for the first time a
satisfactory solution to the technical problem as defined
above and carries other technical advantages which will
clearly appear to the one skill in the art from the
detailed description which follows.
2~32432
.
According to a first aspect, the present invention
introduces a process for the electronic focusing of at
least one transducer element for therapy which eliminates
or minimi~es secondary focusing; according to the
invention the transducer elements are excited by an
electronic signal generator delivering an electronic
signal of which the autocorrelation function is of DIRAC
type.
The definition of the autocorrelation function is as
follows.
- -
Given temporal signal X(t) it carries the energy :
~ :
Ex = [ X(t)]2 dt --
the intercorrelation function between 2 signals X(t) and -
Y(t) is defined by
r~ = x ( t) Y(t-~) dt ; ~
This equation translates the similarity of- the two
signals X(t) and Y(t) when they are shifted $n time by
the delay ~. If this function is always null, the two
~-signals are not correlated.-
In the same way, the autocorrelation function of a signal
is defined as :
rxx = X(t) X(t-l) dt
This function r~represents the similarity of function X
taken at time t with itself but taken at time t-~. The
less the similarity the more this function is closed to 0
but its maximum is always at t=0. In fact for any signal
~132432
, 6
"
X(t) the autocorrelation is maximum for t=O since G~(0)
is nothing else but the energy Ex of the signal.
"
One type of signal which is of interest corresponds to
the wide-band signals. A signal is wide-band when the
~ width of its autocorrelation function is narrow i.e. the
; autocorrelation function tends to a Dirac impulse ~(t).
i In the following we will call such a signal "signal with
Dira~-type autocorrelation function".
Known examples of Dirac-type autocorrelation function are
:'
- random signals of Gaussian or Poisonian-type ~ -
- signals which are modulated in frequency or phase
~':. ' '
Other examples of Dirac-type autocorrelation function
include : ~ -
20 - signals with "M" type sequence which are also known as ~ -~
"maximum length binary sequences" of the type described
by Jean-Yves CHAPELON in chapter 6 page 225-236, and more
especially page 230 in the book "Progress in medical
lmaging" edited by Dr Newhouse, Springer Ve~lag, New
York, 1988.
- "Golay" codes
- "Barker" codes.
Coded pseudo random signals may be used directly or may
modulate in phase or in frequency an electronic signal
whose carrier-frequency matches the nominal frequency of
the transducer.
It is better to use type "M" pseudo random coded signals.
These are described precisely in "Progress in medical
~i32432
.
imaging~ sriefly, these consist of sequences of binary
signals which are assembled by the pseudo random
repetition of impulses of an elementary duration. Each of
these sequences is repeated with a repetition period T
which is specific of a type "M" sequence.
A more precise description of type "M" sequence signal is
described in figure 8 ~
- the duration of the elementary pulse ~ 0.1 us < H <
100 ,us. Preferred value is about 1 ,us.
- repetition period T : 1 ~us < T < 10 s.
This pseudo random coded signals particularly of the type
"M" which is preferred for the application are easily
synthesized with electronic circuitry well-known by the
one skilled in the art.
Using these Dirac-type autocorrelation function type
electronic signals, it is possible to eliminate all
ambiguity in the definition of the time delays which
results in a single focus of ultrasonic waves hence
eliminating in a reliable way secondary focal zones which
25 were present in the earlier devices. -
- According to a second~-aspe~ e p~esent invention also
provides a therapeutic apparatus with electronic focusing
which includes at least an ultrasonic transducer, a
signal generator delivering an electronic signal, and a
control device such that the signal generator delivers a
Dirac-type autocorrelation function electronic signal.
The preferred embodiment of the signal or of the
35 apparatus results from the description of the process ~--
above and of the claims. ~
2~3~32
,~ 8
-
In one or the other of the aspects above, according to a
preferred embodiment mode, the signal generator delivers
¦ a Dirac-type autocorrelation function binary signal of
the sequence "M" type and particularly having a maximal
5 length.
Other characteristics will also appear to the one skilled
in the art from the description below as well as from the
claims which are also part of the description of the
10 invention.
3 ~ :
~ -:
The invention will now be described according to a
15 preferred embodiment which is which is given only as an
example and does not limit in any way the scope of the
invention. In the drawings :
- figure 1 represents the general schematics of
20 therapeutic apparatus for treating living tissues,
including a transducer array, the elementary transducers
being shaped as rings so as to produce an electronic
focusing along the axis of the transducer.
25 - figure 2 represents an ultrasonic pressure curve
(normalized to 1) versus the distance along the axis of
the transducer (ln millimE~s~ The transducer is the
one in figure 1 with a diameter of 100 mm and geometric
focus at 100 mm from the emitting surface. The curve in --
grey tone is obtained in a non-absorbing coupling medium
such as water. The curve in black is obtained in a medium
similar to living body tissues, i.e. having an acoustical -
absorption of 0.1 Neper/cm. The transducers are excited
with non de-phasing electronic sine signals according to
prior art.
' :
- figure 3 is a curve similar to figure 2, obtained with
the same transducer as figure 1 but the ring-elements are
. :::: ..
~13~432
- 9
powered by sine type electronic signals which are phased
such as to produce focal point 50 mm away from the
- emitting surface.
- figure 4 is a curve similar to figure 3, however the
focal point is 130 mm away from the transducer.
~ ~,
- figure 5 represents a curve similar to figure 2 but now
obtained with a sequence "M" pseudo random signal
according to the present invention. There is no time
delay between the transducers so that the focal point is
at its geometric locus, i.e. 100 mm from the surface.
3 - figure 6 represents a curve similar to figure 5 with ~-
15 the same "M" sequence pseudo random signal but with time
delays between the rings so that the electronic focusing
is 50 mm from the surface.
- figure 7 represents a curve similar to figures 5 and 6
20 with the same type of signals but with time delays
between the rings such that the focusing is 130 mm from
the surface.
- figure 8 represents a sequence "M" pseudo random binary
25 coded signal which are more precisely described above, in
which the amplitude is normalized to l and the time is in
microseconds. ~'~~ ~-~~~ -
.:
A therapeutic apparatus for treating living body tissues
according to the prior art is represented by general
reference number lO in figure 1.
This apparatus 10 includes a firing-head 20 which here is
shaped as a naturally focalizing spherical cup 22 itself
subdivided in an annular array of ring shaped
piezoelectric transducer elements such as 221 to 230.
Alternatively the cup could be subdivided as two- ~-
dimensional array (not represented here). This annular
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, . 10
... .
array or alternatively two-dimensional array is well-
known to the one skilled in the art so that no further
, description is necessary. One example of embodiment takes
the form of spherical cup 22 of diameter 100 mm and of
radius, i.e. focal length, 100 mm. Its frequency is about
r~ 1 MHz, its annular structure is made of 10 rings which
have the same surface area and are separated one from the
other by 0.1 mm spacing. Each transducer element 221 -
,' 230 is connected to an amplifier 300 which comprises
' 10 elementary amplifiers 301 - 310 and a time delay device
" 400 which comprises elementary time delays 401 - 410
themselves connected to a signal generator 50 which is
~ controlled by controller 60. In the figure, the generator
!1, 50 and the controller 60 are common to all elementary
amplifiers and delay lines. Controller 60 also supplies
supplying the time delay to delay lines 401 - 410
calculated such that the electronic focusing occurs to
~ the predetermined position on ths axis.
.~
With this design the device can generate an electronic
3 focusing at any point -on the axis of the transducer,
~ between two boundary points Fl and F2.
In case the transducer is made of two dimensional array
of elementary transducers a focal point can be obtained
J outside the main axis of the transducer. ~-
j ., ,,, . . .......................................... -: .
The operation of this apparatus will now be described in
reference to the curves in figure 2-7 : respectively
according to prior art (figures 2 -4) and according to
the invention (figure 5-8).
When generator 50 produces a typical sine wave electronic
signal of frequency about 1 MHz and when no delay has
been programmed into delay lines 401-410, one obtains
natural focusing at the center of the spherical cup which
is here 100 mm from the surface. Curve in figure 2 is
then obtained.
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:
When controller 60 programs the following delays into
delay lines 9201-410, and the same signal generator 50 is
used, one can move the focal point to 50 mm or to 130 mm
(other delays would also move the focal point along the
, axis) :
RING N FOCAL POINT 50 mm FOCAL POINT 130 mm
delays in ,usdelays in ,us
a 1 0
2 2 1.2 0.3
3 2.4 -0.6
15 4 3.5 -0.9
4.6 -1.2
6 5.7 -1.5
2 7 6.7 -1.8
8 7.8 -2
20 9 8.8 -2.3
3 lo 9.8 -2.7 ~ -
..
¦ When the delays above are used to obtain an electronic
focusing at 50 mm the curves on figure 3 are obtained. On
these curves lt is observed that a secondary focusing
appe~rs-~betwe~n 120 and 130 mm. However the secondary
focusing is much attenuated by the absorption of the
ultrasonic waves in the tissues (black curve) so that its
influence on the therapeutic treatment may be negligible.
If one does not take into account the absorption of the
tissues the secondary focusing has an amplitude higher
than the main focusing at 50 mm which demonstrates that
there is a risk of occurrence of a secondary focusing
which may impair safety.
However, delays are used to focus at 130 mm, the curves
of figure 4 are obtained where a secondary focusing
;,;, ! , , ' . ,, , ', . , . ' ' ' . ' ' ' ' ' .
- 213~432
12
appears around 50 mm. In fact, if one takes into account
the absorption of tissues (see black curve), the pressure
at the secondary focusing (50 mm) is higher than at the
primary focusing (130 mm). Therefore, it is impossible to
produce therapeutic treatments at 130 mm.
The discussion above shows that it is impossible to
obtain a large variation in focal length when using sine
wave electronic signals of the known type. The practical
range of electronic focusing is very limited which makes
the whole concept useless in a practical device. In
addition it would even be sometimes impossible to treat a
tissue at 130 mm as shown on figure 4.
Referring now to figures 5-7, which are produced with the
process and the apparatus according to the invention, the
major benefit of the invention is shown. The curves are
produced using a sequence "M" Dirac-type autocorrelation
function electronic signals such as the ones shown in
figure 8.
When comparing figure 5 to figure 2, it is seen that
Dirac-type autocorrelation function signal give the same
good results as sine type signal when the focusing occurs
at the geometrical focus F1 (here 100 mm from the
surface). Therefore, the results are similar whether
-u~ing the Dirac-type autocorrelation function signals or
the sine signals. This confirms that the former can be
used in a therapeutic application.
When one seeks to move the focal point well away from F1
by programming time delays as explained above, it is
observed that a single focal zone is obtained (figure 6)
whereas two focal zones are present with sine wave
according to the prior art (figure 3). In both figures
the time delays are according to the table above.
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13
The same phenomenon can be observed when comparing curve
in figure 7 (using pseudo random codes according to the
- invention) to figure 4 (using sine waves according prior
~- art), i.e. the invention makes possible a single focusing
zone.
.~
~- Therefore, the invention makes possible therapeutic
treatments of living tissues in a very reliable way. The
treatment is efficient in a wide-range of focal distances
- 10 when this was nearly impossible with the sine waves
signals according to the prior art because the latter
; would produce secondary focal zones outside of the main
focal zone.
It should be observed that all the elements of the
'~ embodiment described with reference to the figures form
'~' an integral portion of the invention and thus of the
present description. The invention also covers any
characteristic that appears to be novel over any of the
prior art.
