Note: Descriptions are shown in the official language in which they were submitted.
BACK~ROUND OF T~E INVENTION
Field of the Invention
. _ .
The invëntion relates to the field of ultrasonic scanners-
and,.in particular, to ultrasonic scanners for producing sector
scans in an o~ject to be scanned.
--1~
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1 i ~15~
11l 2. Prior Art
l ..~
2 Dynamic cross-sectional echography (DCE) is a com-
3 monly used technique for producing sequential two-dimensional
4 images of cross-sectional slices of the human anatomy by means
5~ of ultrasound radiation at a frame rate sufficiently high to
6 enable dynamic visualization of moving organs. Apparatus
7 utilizing DCE are generally called DCE scanners and transmit
81 and receive short, ultrasonic pulses in the form of narrow
9 beams or lines. The reflected signal strength as a func~ion
10 of time, which is converted to a position using a nominal
11 sound speed, is displayed on a cathode ray tube, or other
12 suitable device, in a manner somewhat analogous to radar or
13 sonar displays. While DCE can be used to produce images of
14 any object, it is frequently used for visualization of the
15 heart and main heart vessels.
16 Existing DCE scanners can be classified according
17 to the geometry of their field of view (linear or sector
18 scanning), according to the means used for scanning that
19 field of view (mechanical or electronic scanning), and
20 according to whether the transducer scans the patien~ or
21 object through an intervening water bath or by direct contact
22 with the surface of the object as, for example, the skin of
23 a patient using an appropriate contact gel or oil. Linear
24 scanners produce a scan of the anatomy consisting of a set
25 of nominally parallel scan lines, displaced with respect to
26 one another by a line spacing roughly comparable to the
27 effective width of each line, as determined primarily by the
28 transducers used in the apparatus. The cross-section imaged
29 by such scanners is theréfore approximately rectangular in
301 ///
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32 / 2.
1~2~
1 shape, its width being determined by the line _pccing and
21 total number of lines, while its depth is determined by the
3~ lesser of the useful penetration depth of the ultrasound
4 radiation into the body and the unambigous range of the device.
5! Linear scanners are generally used where there is a relatively
6~ extended region on the body surface from which access to the
7l parts of interest of the anatomy is possible, as in the abdo-
81 minal organs. Sector scanners produce a scan of the anatomy
9~ consisting of a fan of diverging lines spaced angularly from
one another but intersecting (nominally) at a point. The
11 angular spacing being even or uneven depending upon the
12 apparatus and roughly comparable to the effective angular
13 width of each line. The cross-section imaged by such scanners
14 i~ therefore approximately wedge or pie-shaped, i.e., it is
approximately a circular sector, the total angular width of
16 the sector~ or sector scan angle, being determined by the
17 angular line spacing and total number of lines, and the sector
18 radius being determined by the lesser of the useful penetra-
19 tion depth of the ultrasound radiation into the body and the
unabiguous range of the device. Sector scans are generally
21 used where the anatomical window or region on the body surface
22 from which access to the anatomical part of interest is
23 relatively small, as in the adult heart, the brain and the
eye in particular.
A large amount of work has gone into the develop-
26 ment of DCE sector scanners. Existing direct contact sector
27 scanners include both phased array and mechanical scanners.
29 In phased array scanners such as those exemplified in articles
by M~ G. Maginness et a~, "State-of-the-art in Two-dimensional
301 ///
32 /~'
l 3.
llZ3L5~}0
1l Ultrasonic Tranducer Array Technology", Medical Physics, Vol.
2 3, No. 5, Sept./Oct. 1976, Von Ramm et al, "Cardio-Vascular
3 Diagnosis in the Real Time Ultrasound Imaging", Acoustical
41 Holography, Vol. 6, 1975, and J. Kisslo et al, "Dynamic Cardiac
51 Imaging Using a Phased-Array Transducer System", published by
6~ Duke University, Durham, North Carolina, a large (16-60 element)
7 linear array of small transducers is used, with a variable time
8 (phase) delay inserted between elements of the array both in
9 the transmission and reception of the ultrasound signal, result-
ing in a transmitted beam and a receiving beam or sensitivity
11 pattern whose direction is determined by the magnitude of the
12 inter-element time delay. In sector scanning using phased
13 array scanners, such scanning is achieved without any mechanical
14 motion of the transducer array which remains in stationary con-
tact with, for example, the patient's skin. Such phased array
16 scanners have, however, several severe practical limitations.
17 One such limitation resides in the relative complexity of the
18 multi-element transducer array and especially of the transmit/
19 receive electronics necessary to achieve electronic beam
steering, resulting in a relatively high cost of phased array
21 scanners. In addition, the ultrasonic beam quality in phased
22 array scanners, in terms of lateral resolution and side lobe
23 levels and the possible occurance of grating lobes, is poor
24 compared to that of single transducer scanners, particularly
for beam direction angles greater than 30 degrees away from
26 the normal to the transducer, limiting its useful scanning
27 angles to about 60 degrees even though the beam might be
28 steered beyond that limit. Another significant limitation
29 of existing phased array scanners and all direct contact
33o ///
3Z ! 4.
llZ15C~0
11 scanners is that the scanned section is centered at the center
2~ of the transducer face, essentially at the skin or surface of
31~ the object and therefore outside of the patient or object, so
4 that in certain applications close-in structures are not well
resolved, while in other applications anatomical structures
6~ can limit the field of view of the scanner. This is particu-
7 larly the case in adult cardiac scanning, where the ult{asonic
8 access window to the heart is generally in the second to fifth
9 ~ intercostal spaces, just to the left of the sternum. In that
case, the ribs will tend to limit the scanner field of view,
11 particularly in obese adult patients where the ribs are close
12 to the patient's skin, so that ~he transducer window cannot
13 readily be pressed into the intercostal space. It would be
14 necessary, in order to avoid the rib interference problem, to
have the center of the sector scan replaced somewhat inside
16 the patient, in or near the space between the interfering ribs~
17 Limitations of the scanning sector angle to values significant-
18 ly below 90 degrees due to rib interference or beam steering
19 limitations or both, can prevent, in many cases, visualization
of the entire long dimension of the heart and can seriously
21 affect the diagnostic value of DCE in cardiac examinations as
22 well as in other examinations.
23 A further limitation of present phased array scanners
24 is that they can only be dynamically focused in range in one
25 ¦ lateral dimension, namely in the plane of scanning. Two-
26¦ dimensional focusing would require a two~dimensional matrix
27 ~ or array of phased transducer elements and is beyond the
2~1 present commercial state of the art.
29 1 -.
301 ///
31 //
3~ 5.
l~Z15UO
,1 Another class of sector scanners are mechanical in
2~ nature and can be divided into two classes, oscillating trans-
3~ ducer scanners and rotating transducer scanners. An oscillating
41 transducer scanner is exemplified by the scanner described by
J. Griffith et al, "A Sector Scanner for Real Time Two-Dimen-
6 sional Echocardiography", Circulation, Volume XLIX, June, 1974,
7 in which a single transducer is oscillated about an axis nomi-
8~ nally lying in the front plane and passing through the center
9 of the transducer with an appropriate angle sensor being used
to monitor the angular position of the transducer at any time.
11 Contact with the patient is maintained by the use of a gel, and
12 in operation the patient's tissues must conform to the movement
13 of the transducer which is essentially rigid. While the oscil-
14 lating transducer scanner described by Griffith is of the direct
contact variety, oscillating transducer scanners can also be of
16 the non direct or water bath variety as described by A. Ashberg,
17 "Ultrasonic Cinematography ~f the Living Heart", Ultrasonics,
18 April, 1967, in which the internal struc~ures of the human heart
19 have been investigated by using the ultrasound pulse-echo method
and an ultrasound optical mirror system immersed in a water tank
21 having one wall consisting of a thin rubber membrane pressed
22 against the chest wall of the patient through which ultrasonic
223 energy can easily penetrate. These mechanical sector scanners
also suffer from a number of limitations and drawbacks which
limit their use. Both of the above described mechanical sector
26 scanners s~ffer the same rib interference limitation as the
28 phased array scanners. In addition, the direct contact mechan-
29 ical sector scanners are limited in their useful scanning angle
by the problems of the ~oving contact and physical angulation
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31
~2 I ~
6.
1 of the tr~nsducer away from the skin, in most cases to values of
30 to 45 degrees. A limitation cor~mon to both of the mechanical
sector scanners described i5 that their angular rate of sweep is
not uniform, since the transducer or mirror system must reverse
direction at the end of each sweep in each direction, so that the
line density is greatest at the edges of the sector, where it is
usually least desirable, and is lowest at the center of the sector,
i.e., the cerlter of the region of interest. Concomitant with
this limitation is the fact that the alternate direction of sweep
means that an area at the end of a sweep is interrogated twice in
a very short interval, as the scan crosses it in opposite direc-
tions, and is not interrogated again until nearly the duration of
t~o frames. In addition~ only the mid-point of the scan is inter-
rogated at a constant frame rate. Another disadvantage of the
direct contact of the oscillating transducer scanner described by
Griffith arises from the physical transducer motion itself and
includes patient discomfort, vibration of the transducer in the
operator's hand, and mechanical ~ear of the tranducers moving
parts which are subjected to significant forces.
A further limitation of direct contact scanners, includ~
ing phased array scanners, arises from near field non-uniformities
in the so-called Fresnel zone of the transducer or array. As is
well known, the acousitc pressure field for an unfocused trans-
ducer exhibits large scale oscillations, including a series of
peaks and nulls, within a distance D=r /~ from the face of the
transducer, where r is the effectlve radius of the transducer, or
~rra~, ~n~ e ~ nc~ giv~ w~h~ a ~t~nc~
D, which is referred to herein as the "near" Fresnel zone. is
Z
characterized by particularly large fluctions in amplitude ~oth
3~ laterally and in range, target positions and strengths will be
--7--
1 falsely displayed as a sector scan is carried out in that region.
For typical transducer radium to wavelength ratio of 10, and
typical wavelengths of 0~7mm, the length D oE the near Fresna~
region, extends 3.5 centimeters in front of the transducer, and
this will frequently include portions of the body which are of
diagnostic interest.
Another type of mechanical sector scanner is the rotat-
ional scannlr described by Barber et al. "Duplex Scanner II: For
Simultaneous Imaging of Artery Tissuesand Flow", IEEE, 1974 Ultra-
sonics Symposium Proceedings, and by Daigle et al, "A DuplexScanning System for Pediatxic Cardiology", Proceedings 1st Meeting
of ~orld Federation for Ultrasound in Medicine and Biology~ 1976,
which uses a set of transducers mounted on a rotor coupled to the
patient through a water column which is separated from the skin
surface by a thin silastic or ru~ber membrane. ~hile the rotating
transducer water bath scanner described by Barber~ called the
"Duplex Echo-l)oppler Scanner" permits a stationary contact ~ith
the patient and provides a uniform beam spacing or line density
as well as uniform sampling, it is severely limited in its appli-
cation to adult cardiac scanning by the fact that the center oraxis of the sector .scan is removed or offset from the skin surface
by a distance equal to the sum of the rotating radius and the
length of the water column, resulting in a severe rih interference
problem, The device of Barber et al, is primarily intended for
use in pediatric cardiology where rib interference is not serious.
A further limitation of all present mechanical scanners
i5 that they cannot provide a s~multaneous M mode or Doppler scan
of any selected line of the scanned sector at
l~Z15(~0
1 rates adequate for measurements of heart valve and heart wall
2 ~ motions. While any line of the sector scan of a mechanical
3 ¦ scanner can be sampled at the frame rate of the sector scan
4 itself, typically 20 to 45 frames per second and displayed on
an M-mode type display, this rate is too slow since a minimum
6 of 300 frames per second is necessary in order to resolve
7 rapid motions, such as the motion of the mitral valve of the
8 heart, with existing M-mode single beam echocardlographic
9 probes operating at frame rates in excess of l000 frames per
second. In addition, even if such rates could be attained by
11 a mechanical scanner, the unambiguous range, or useful pene-
12 tration, corresponding to a frame rate of 300 or more frames
13 per second of the 80 to l00 lines typically forming a sector
14 scan would be less than two centimeters, and therefore totally
useless. One attempt to provide a Doppler scan in a mechanical
1~ scanner is shown in the rotational scanner described by Barber
17 in which an auxiliary transducer operated in the pulsed Doppler
18 mode is provided which permits obtaining information about
19 velocities of blood flow and movement of cardiac structures
essentially simultaneously (within less than a millisecond~
21 with the echoamplitude information. However, the Doppler scan
22 in the device described by Barber is not centered around the
23 same point as the echo scan, since the transducer is mounted
24 off to the side of the echo scanning head. Thus, the point of
entry of the Doppler beam and the corresponding interrogated
26 volume are different than the point of entry of the echo
27 sounding beam and its corresponding interrogated volume,
23 creating problems both of access and of interpretation as the
29 same line as one of the~sector lines is not simultaneously
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11~15~0
1 sampled.
2¦ The image producing capabilities of current ultra-
3 ~ sonic scanners are further limited by the existence of "echo"
4 ~ artifacts which degrade the quality of and complicate the
interpretation of the reflected signals from ~he object being
6~ visualized. Such echo artifacts are caused by ultrasound en-
7~ ergy being received by a detector which energy is not directly
8 reflected from the body or target under examination. In a
9 system utilizing mirrors and membranes, such as described by
~sberg, a portion of the echo artifacts are caused by partial
11 reflection of acoustic pulses along the path of the desired
12 or "target" echoes by the membranes and the mirrors before
13 they reach the body or target under examination. Another
14 portion of the echo artifacts are caused by partial reflection
of acoustic pulses from the membranes and mirrors not along
16 the return path of the target echoes but along other paths
17 resulting in reflections from numerous internal surfaces which
18 eventually inpinge upon the detector. A further portion of the
2 echo artifacts results from stray acoustic radiation that is
2 not intercepted by the membranes or mirrors but merely re-
22 flects around the scanner with some of it reaching the detector
2 and producing false echoes.
23 Accordingly, it is a general object of the present
4 invention to provide an improved ultrasonic sector scanner.
It is another object of the present invention to
26 provide a sector which has a sector scan center of focus which
27 can be located in front of the scanner so as to minimi~e inter-
28 f~rence problems.
31 /
10.
, ~ .
SUO
1, It is a further object of the present invention to
2 provide a sector scanner having a large effective sector scan
31¦ angle.
4~ It is another object of the present invention to
51 provide a sector scanner which has a stationary contact with
6~ the object being scanned and is free of vibration problems.
71~ It is still another object of the present invention
811 to provide a sector scanner which has uniform line density and
91 sampling rate at all angles, a high frame rate, and high
101~ quality radiating and receiving beam patterns.
11 It is a further object of the present invention to
12 provide a sector scanner which can provide a simultaneous
13 M-mode or pulsed Doppler scan of any selected line of the
14 sector scan at a high frame rate comparable to conventional
M-mode frame rates or pulsed Doppler systems.
16 It is another object of the present invention to
17 provide a sector scanner which is free of echo artifacts.
18 It is a further object of the present invention to
19 provide a sector scanner in which no part of the body of
diagnostic interest lies in the~Fresnel zone of large varia-
21 tions of acoustic intensity.
22
23 SUMMARY OF TEIE INVENTION
_ _ .
24 An ultrasonic scanner for producing a sector scan
in an object to be examined is provided in which one or more
26 ultrasonic transducers traverse an arcuate path with respect
27 to a reflector which is positioned to receive the ultrasonic
229 waves scanning the surface of the reflector from each of the
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11 .
l~Z1500
1 transducers and converge such waves at a point a preselected
2¦1 distance in front of the reflector. In general the ultrasonic
3¦ waves are converged at a point outside the scanner and inside
4~ the object to produce a sector scan in the object having its
51 center at the convergence point. In one embodiment of the
61 scanner, the reflector only partially reflects the ultrasonic
7 waves and an additional stationary transducer is provided which
8 is positioned to produce ultrasonic waves which radiate through
9 ¦ the reflector and coincide with one of the lines of the sector
10~ scan, thus permitting simultaneous M-mode or pulse Doppler echo
11 information to be obtained in perfect registration with the
12 sector scan lines~ Attenuation, absorbtion and anti-reflection
13 means are provided to suppress echo artifacts.
14 The novel features which are believed to be charac-
teristic of the invention, both as to its organization and its
16 method of operation, together with further objects and advan-
17 tages thereof, will be better understood from the following
18 description in connection with the accompanying drawings in
19 which a presently preferred embodiment of the invention is il-
lustrated by way of example. It is to be expressly understood,
21 however, that the drawings are for purposes of illustration and
22 description only and are not intended as a definition of the
23 limits of the invention.
24
25 BRIEF DESCRIPTION OF T~lE DRAWINGS
26 FIGURE 1 is a perspective view of a preferred
27 embodiment of the present invention illustrating the sector
8 scan produced within the object to be examined;
29 ,
32 "//
12.
llZi500
1~ FIGURE 2 is a cross-sectional view of the present
2 invention taken along the line 2-2 of FIGURE l;
3 FIGURE 3 is a cross-sectional view of the present
4 invention taken along the lines 3-3 of FIGURE 2;
FIGURE 4 is a perspective view of the present
6 invention illustrating the reflection and convergence of the
7 ultrasonic waves produced by the present invention;
8 FIGURE 5 and FIGURE 6 are cross-sectional views of
9 alternative embodiments of the face portion of the present
invention taken along the line 2-2 of FIGURE l; and
11 FIGURE 7 illustrates a partial reflector utilized
12l in the present invention.
~31
14 DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIGURE l, a preferred embodiment
16 of the present invention is illustrated. The ultrasonic scanner
17 lO is shown having a membrane 12 which is placed in contact with
18 the surface 14 of the object to be examined, such as the heart
19 region of the human body. The lower or face portion 16 of the
scanner lO houses the moving ultrasonic transducers and the
21 reflector while the upper portion 18 of the scanner lO houses
22 the electronics of the scanner lO. A motor 20 is provided on
23 top of the scanner lO to drive the transducers and a cable 22
24 provides the electrical power for the various elements of the
scanner lO. As is shown in FIGURE l, the ultrasonic waves 24
6 produced by the scanner lO converge at a point 26 outside of
27 the scanner lO, between the ribs 28 of the chest of a patient,
28 and then diverge to produce the ultrasonic waves 30 which per-
29 form a sector scan of t~e heart region. The ultrasonic waves
31 ///
32 / 13.
1~2~.S~I~
1~ 30 are reflected by the various portions of the heart region
2 and are received by the generating transducers and processed
3 in accordance with the pulse-echo method described in the prior
4 art literature referenced herein.
In FIGURES 2 and 3 a set of ultrasonic transducers
6 32a-e are shown affixed to ring 34 which is supported by shaft
7 36. The shaft 36 is supported by bearings 38 and 40 coupled
8 to the lower portion 16 and the upper portion 18, respectively,
9 of the scanner 10 and is driven by the motor 20 controlled
through leads 42. The ring 34 rotates within aperature 44
11 formed by the lower or face portion 16 and a dividing plate 46
12 and is immersed in a liquid 48 which is contained by seal 50
13 and membrane 12, plate 46 being opaque to ultrasonic waves and
14 having an aperature 52 therein to allow ultrasonic waves pro-
duced by the transducers 32a-e and reflected off the mirror 54
16 coupled to the plate 46 to pass through the aperature 52 in
17 the plate 46 and the membrane 12. As stated previously, part
18 of the energy of the ultrasonic waves is reflected by portions
19 of the heart region and returns along the same path to the
ultrasonic transducers 32a-e and is detected and displayed as
21 in conventional pulse echo instruments.
22 The transducers 32a-e are coupled to leads 56a-e
23 ¦ which pass through the ring 34 into the hollow center of the
24 shaft 36 and are coupled to slip rings 58, 60 mounted on disc
62 coupled to the shaft 36 and positioned above the transducers
26 32a-e. The slip rings 58, 60 are cut through at the midpoints
27 between the transducers 32a-e so that the brushes 64, 66 coupled
28¦ to leads 68 perform a commutating action upon rotation of the
s~;ft 36 and ehe disc 62 to ruccessively energize the trant-
32 / 14.
1 ducers 32a-e and to transmit information received by the trans-
ducers 32a-e to the processing e~uipment. An optical tachometer
70 coupled to leads 72 is affixed to the upper portion 18 and
provides an angular reference pulse train of about 500 pulses
per shaft revolution in addition to an index pulse once per
revolution which is used in conjunctlon with the pulse train to
indicate the angular position of the shaft 36, and hence the
position of the transducers 32a-e, at all times, and is also used
to servo control the speed of the motor with the pulse rate
applied to the transducers 32a-e to produce the ultrasonic wa~es.
As illustrated in FIGURES 3 and 4, the transducers
32a-e are located on the ring 34 at 72 degree intervals and are
sequentially energized in the vicin~ty of the reflector 54 to
produce a series of waves or pulses 74~ Due to the constrained
circular path of the transducers 32a-e, the ultrasonlc waves 74
are directed radially toward the center line of the shaft 36 and
scan across the surface of the reflector 54 which is mounted at
an angle of 45 degrees with respect to the axis of the shaft 36.
Because reflector 54 is located closer to the actuated transducer
32a-e than the radial distance between said transducer and the
axis o~ shaft 36, the ultrasonic waves 74 impinging on the surface
of the reflector 54 are reflected through the aperture 52, the
fluid 48 and the membrane 12, The emerging ultraso~ic waves 24
pass through the surface 14, converge at the point 26 and form a
sector scan comprised of waves 30 centered at the intersection
of the transducer axis with the shaft axis as projected by the
reflector 54 outside the scanner 10 ~hile sector scans can be
produced us;`ng only one transducer, higher frame rates are
achieved by the use of multiple transducers, as described above.
The plane of the sector is parallel to the axis of the shaft
. ?
~lS~ .
1 unless the shaft axis is tilted with respect to the surface of
2~l the object bein~ examined and the fluid 48 has a velocity of
3 propagation of sound therein different from the velocity of
4 propagation within the objectl in which case the actual angle
of the plane is defined by Snell's law, sin ~ / sin ~j = Cl/C~
6 where a, is the angle of incidence, ~ the angle of refraction,
7 Cj the velocity of propagation of sound in the incidence
8 medium and C~ the velocity of propagation of sound in the
9 incidence medium and Ct the velocity of propogation of
sound in the refracting medium.
11 The maximum usable sector angle is limited to those
12 angles for which the reflector 54 intercepts substantially all
13 of the ultrasonic beams. Since the distance between the shaft
14 axis and the mirror determines the distance of point 26 from
the scanner lO, and since it is desirable for certain applica-
16 tions, such as cardiography, to maintain the point 26, i.e.,
17 the effective or projected axis of rotation, within the object,
18 the reflector 54 must be placed sufficiently far from the
19 shaft axis to ensure that the point 26 is within the object.
For a hand held device of modest size, however, the maximum
21 scan sector angle is then limited by the mechanical interfer-
22 ence of the reflector 54 with the path of the transducers
23 32a-e as the reflector 54 moves farther from the shaft axis
24 toward the transducers. This limitation can be overcome by
using for the fluid 48 a fluid such as an emulsion of toluene
26 in 40% ethanol amine, 60% chloroform, which has a slower
27 ¦ velocity of propagation therein than in the object and thus
28 ¦ results, by Snell's law, in a larger scan sector angle in the
29 object. As is described more fully hereinafter, the fluid 48
should also have a specific impedance substantially equal to
31 //
~ 16.
1121SO(:~
1 the specific impedance of the object being scanned since
2l! multiple reflection artifacts result when a sufficiently large
3 echo results at the interface 14 due to an impedance mismatch.
4 It has been found that fluids with specific impedances of
5 ~ between 1.65 and 1.75 when used in conjunction with a thin
6~ latex membrane 12 yield acceptable results and can provide a
7~ sector angle of 90 degrees in a human object with a sector
8 angle of 70 degrees within the scanner 10. While the thin
9 I elastic membrane enables uneven body contours to be accommo-
dated and the scanner to be tilted to look behind interfering
12 structures, a rigid membrane can be used for industrial appli
catlons .
13 As shown in FIGURES 5 and 6, in order to minimize
14 echo artifacts on reflections from the membrane 12, a flexible
membrane may be used, as stated above, made of a material such
16 as latex, together with a window section cut out large enough
17 to accommodate the entire sector scan at the plane of the skin
18 or contact surface 14, with a very thin (eg. 25~thick) film
19 12' of a material such as polyethylene covering the window
section. Such a film 12' because it is much less than a wave-
21 length in thickness and has a characteristic impedance not
22 very different than that of the materials on either side of it,
23 will be essentially totally transparent to the ultrasonic
24 radiationO If it is desired to use a more rigid membrane 12,
made for example of polyethylene, quarter-wave anti-reflection
26 ~atching layers 80, 82 composed of low density polyethylene,
27 as shown in FIGURE 6, can be used on both surfaces of the
28 membrane 12 to match to the fluid 48 and to the skin or contact
Z9 surface 14.
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32 /
17.
11 In order to reduce the magnitude of those echoes
21l which result at least partly from reflection or scattering from
3 supporting structures within the scanner 10, the transducer ring
4 34, the portion support 55 of the reflector 54, the plate 46,
5~ and the inside surface of the face portion 16, can be made of
6l or covered with a layer 84 of an acoustically highly absorbing
7~ material, which has a characteristic impedance Z~ closely
8 matching the characteristic impedance ZL of the liquid 48,
9 and which may additionally be clad with a quarter-wave anti-
reflection matching layer 86, approximately .2mm ~hick and
11 with a characteristic impedance ZM ~ (Z~ ZL )
12 Since only echo artifacts (but no target echoes)
13 impinge on those internal support structures, the attenuation
14 of the materials may be chosen as high as i5 feasible without
any deleterious effects on the signal strength. A suitable
16 material choice in this case is low-density polyethylene,
17 preferably loaded with carbon, or carbon loaded natural rubber.
18 The material chosen for a matching layer will depend on the
19 liquid selected, and may be composed of a low density
polyethylene or an unloaded natural rubber to match a loaded
21 natural rubber wall to castor oil used as the fluid 48.
22 To further reduce echo artifacts, the fluid 48 can be
23 chosen to be a sound attenuating liquid, such as castor oil,
24 salt solutions such as solutions of hydrated manganese chloride
in water, or emulsions such as emulsions of toluene in water
26 which can be made highly attenuating, as described in an arti-
~7 cle by J. R. Allegra and S. A. Hawley, Journal Acoustical
28 Society of America, Vol. 151, 1972. While the provision of
29 a sound attenuating liq~id alone has been found quite effective,
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1~.
1 there are, however, limits on the values of attenuation coef-
2 ficient which are acceptable for the liquid 48, because the
3 target echoes must also travel in the liquid 48, and excessive
4 attenuation of those signals must be avoided to maintain a
51 useful signal-to-noise ratio and dynamic range based on signal-
6 to~noise ratio. For example, in order to meet the requirements
7 of having the entire scanned sector or field of view beyond the'
8 Fresnel zone ~ of the transducers 32, where d is the trans-
9 ducer diameter and ~ is the acoustic wave-length, and achieving
a large sector scan angle, within a scanner of minimum external
11 dimensions, the radius of a scanner used in cardiac scanning
12 should be between about 2.5 cm and 3.5 cm, and the corresponding
13 acoustic path length R within the scanner head will be between
14 2 and 3 cm, with a typical value of 2.5 cm. Pulses leading to
target echoes at a range R travel a distance 2R in the liquid
16 48 and a distance 2R~ = 2R-2R~ in the body or external medium
17 examined. Multiple echo artifacts appearing at range R travel
18 a distance 2R (or nearly 2R for echo artifacts arising from
19 subcutaneous layers) within the liquid 48. Thus, if the atten-
uation coefficient of the liquid 48 isC~ , measured in db/cm,
21 and the average attenuation coefficient of the examined tissue
22 isG~, then the target echoes from range R will suffer an
23 attenuation A=2R~ ~L+ 2Re~T on 2R~+ 2RC (~L -~r ) whereas
24 multiple echo artifacts will suffer an attenuation (due to the
liquid) of about 2R~. Clearly, the larger ~ compared toC~r ,
26 the greater the attenuation of the multiple echoes with respect
27 to the signal echoes, but also the greater the absolute atten-
28 uation of the signal echoes. l'he excess attenuation 2R~ c~,
29 of the signal means a dégradation of the system signal-to-noise
32 /
19 .
500
1 ratio, for a given transducer input power, efficiency, sensi-
2 tivity, and preamplifier noise figure. If attenuation in the
3 liquids 48 is used as the sole means of attenuating the basic
4 echo artifacts, and a liquid is chosen that will provide 40
db of attenuation for the multiple echoes at range 2R~(i.e.
6 the first spurious echoes from the membrane 12), then for a
7 typical value R =~2.5cm,~ = 4db/cm. The excess attenuation
8 of the target echoes will be 20db, providing a net reduction
9 of the echo artifacts of 20 db.
In conjunction with an attenuating liquid therefore
11 a layer of a highly absorbing or attenuating solid material 88
12 may be placed in the primary acoustic path, either on the
13 mirror 54 itself, or immediately adjacent to the transducer 32
14 or membrane 12. Since it is desirable to provide a scanner 10
which is as small in diameter as is practicable, a suitable
16 absorbing material such as carbon loaded rubber, with an at-
17 tenuation coefficient as hïgh as 25 db/cm can be utilized,
18 with a layer 2mm thick resulting in a two-way attenuation of
19 lOdb. Since the layer 88 should be located so as to intercept
(and therefore attenuate) all or nearly all artifactual echoes,
21 and since very close matching of the liquid to the attenuating
22 layer is required to avoid having their interface act as a new
23 source of echo artifacts, location on the transducer 32 is
24 preferred.
In addition to the use of an attenuating layer 88
26 or liquid 48, it has been found desirable to make the reflector
27 54 only partially reflecting and of highly acoustically absor-
28 ¦ bing material so that the portion of the ultrasonic energy
29 which is not reflected ~y the reflector will be absorbed and
3 ///
31 //
32 20.
11 ~9 ~ 0 0
1I not contribute to echo artifacts. The partial reflecivity of
2 the reflector 54 may be achieved by use of a reflector material
3~ with a suitable impedance mismatch with the liquid 48. Since,
4l however, this approach can lead to reflectivity which is de
pendent on the sector scan angle, and requires compensation of
6 the transducer receiver gain (or transmitted power, or both)
7l as a function of scan angle to lead to a uniform display
8 signal strength for a given target echo strength, a suitable
9 ~ partial reflector 54' may be employed, as is shown in FIGURE 7,
which is comprised of a non-reflecting or weakly reflecting
11 material 90, such as rubber, with narrow, closely spaced (less
12 than a half wave-length) strips 92 of a highly reflecting metal
13 such as tungsten. The use of such a partial reflector 54',
14 attenuating layers 84 and 88, an attenuating liquid 48, and
antireflection layers 82 and 86 has resulted in the reduction
16 of echo artifacts, compared to the target echoes and at a
17 range R~ in the examined body of about 3cm or greater, of
18 20db, sufficient for artifact-free or substantially artifact-
19 free operation of the scanner of the present invention in
medical diagnostic applications.
21 Since, in order to achieve the echo artifact sup-
22 pression as described above, the target echoes have also
23 been attenuated by approximately 20db, it is desirable to use
24 highly efficient transducers 32, and in particular transducers
with a relatively high Q-factor. This is, however, contrary
26 to current practice in which pulse excited low efficiency, low
27 (mechanical and electrical) Q transducers are used in order to
28 achieve extremely high range resolution. Since, however, the
29 very short wideband pulses used in the present art scanners
30 ///
32 /
21.
1 become stretched and distorted as they travel through tissues,
2 particularly muscle, because such tissues have a large fre-
3 quency dependent attenuation, the higher frequency components
4 are resultingly preferentially attenuated, with a resultant
effective stretching of the pulse. It has thus, been found
6 that the use of a higher Q transducer with a longer, narrower
7 bandwith pulse results in less distortion and pulse stretching,
8 with the effective lengths significantly greater than those
9 of the low Q, low efficiency transducers.
In FIGURE 6 an alternative embodiment of the present
11 invention is shown in which an auxiliary stationary transducer
12 94 is provided which is positioned in line with one of the
13 ultrasonic waves 24 and behind the partial reflector 54'. The
14 partial reflector 54' has a solid backing 96 with an attenua-
tion similar to the liquid 48 which is coupled to an additional
16 highly absorbing block 98 used to thoroughly dampen the energy
17 which passes through the partial reflector 54. The backing
18 96 may be composed, for example, of room temperature vul-
19 canized silicone rubber and the block 98 may be composed also
of rubber. The tr~nsducer 94 is coupled by bracket lO0 to
21 block 98 which i5 coupled to plate 46. Leads 102 are pro-
22 vided to energize transducer 94 and to transmit information
23 received by transducer 94 to the processing eguipment. The
24 ultrasonic waves produced by transducer 94 radiate through
the partial reflector 54' in coincidence with one of the lines
26 of the sector scan, thus permitting simultaneous M-mode or
27 pulse Doppler echo information to be obtained in perfect
28 registration with the sector scan lines.
29
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31 //
32 22.
50~
1I Having described the invention, it is obvious that
21 numerous modifications and departures may be made by those
3 skilled in the art; thus the invention is to be construed as
4 limited only to the spirit and scope of the appended claims.
What is claimed is:
17
22
2232
24
26
28
29 -
3~ :.
