Frequency controlled hybrid ultrasonic imaging arrays

This ultrasonic imaging apparatus has an array (44, 52 or 100) of transducer elements (44, 50 or 104) for transmitting ultrasonic signals having a first predetermined center frequency (fc1) into an object (12) to be analyzed through use of the transmitted signals reflected from within the object. A means (150, 116-1 through 116-X and 120-1 through 120-X) is connected to transmit the ultrasonic signals from the array (44, 52 or 104) in a stepped array mode. A means (150, 116-1 through 116-X and 120-1 through 120-X) is connected to transmit the ultrasonic signals from the array (44, 52 or 104) in an angle scanning mode. There is a means (150, 128-1 through 128-X) for focusing the transmitted signals at a desired depth within the object (12). The reflected signals sensed by the apparatus have a second center frequency (fc2) less than the first center frequency (fc1) as a result of signal attenuation by the object (12). There is a means connected to select between the stepped array transmission mode and the angle scanning transmission mode for operating the array elements based on the second center frequency (fc2). Varying the operating mode of the apparatus on this basis gives improved image resolution over a wider operating range than with prior art systems.

CROSS REFERENCE TO RELATED APPLICATION 
This application and a concurrently filed application, entitled "FREQUENCY 
VARIED ULTRASONIC IMAGING ARRAY" by David A. Wilson and James L. Buxton, 
are directed to related inventions. 
FIELD OF THE INVENTION 
This application is directed to improvements in ultrasonic imaging 
apparatus to produce a better image quality. More particularly, the 
invention relates to improved modes of operation for ultrasonic 
transducers utilized in such imaging apparatus. 
DESCRIPTION OF THE PRIOR ART 
The use of ultrasonic sound waves in apparatus for the analysis of solid 
objects is now a well-known and comparatively well developed art. In such 
apparatus, an array of ultrasonic transducer elements is used to transmit 
ultrasonic waves into an object, and echoes of the waves are used to 
define geometry and related characteristics of the object's interior. Such 
ultrasonic imaging apparatus has been found to be particularly useful in 
medical applications as a non-invasive diagnostic tool. The state of the 
art in such medical applications has been reviewed, for example, by 
Havlice and Taenzer, "Medical Ultrasonic Imaging", Proceedings of the 
IEEE, Volume 67, No. 4, April 1979, pages 620 to 641. 
As pointed out in the Havlice and Taenzer article, presently available 
electronically scanned medical ultrasonic imaging apparatus is of two 
principal types: linear stepped array and (linear) phased array. In the 
linear stepped array apparatus, each ultrasonic transducer element group 
in the array has a fixed beam direction, directly in front of the group. 
Successive groups of transducer elements are activated to define a 
rectilinear field of vision. In the (linear) phased array, all of the 
ultrasonic transducer elements in the array are activated simultaneously, 
but different length-of-delay lines are used to direct the ultrasonic 
waves in a sector scan and sometimes to focus the ultrasonic waves to a 
particular depth in the sector field of vision. 
It is well recognized that attenuation of higher frequency ultrasonic 
signals occurs as the signals penetrate more deeply into tissue. This 
causes a net frequency shift downward away from the average transmission 
frequency and therefore results in image degradation. 
It is also known to provide ultrasonic arrays that can be operated in more 
than one mode to provide a more complete image, as disclosed by Carpenter 
et al, "Multiple Mode Scanning Linear Array", Abstract, 4th World Congress 
on Ultrasonics in Medicine, 1979, page 2, and Ohmori et al, "A 
Multi-purpose Real Time Ultrasonic Imaging System with Dynamic 
Architecture", page 380 of the same publication, but those systems, as 
disclosed, do not change their operation based on frequency shifts due to 
signal attenuation. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an ultrasonic 
imaging apparatus in which operating mode of the apparatus is changed for 
lower frequencies resulting from attenuation by an object being analyzed 
of ultrasonic signals as sensed by the apparatus. 
It is still another object of the invention to provide a hybrid ultrasonic 
imaging apparatus which utilizes array stepping for near field imaging and 
sector scanning for deep field imaging. 
It is another object of the invention to provide a hybrid ultrasonic 
imaging apparatus in which a portion of an image obtained with the 
apparatus is generated in an array stepping mode and the remainder of the 
image is generated by an angle scanning mode. 
The attainment of these and related objects may be achieved through use of 
the novel ultrasonic imaging apparatus herein disclosed. Apparatus in 
accordance with this invention includes an array of transducer elements 
for transmitting ultrasonic signals having a first predetermined center 
frequency into an object to be analyzed through use of the transmitted 
signals reflected from the object. Within the object, the ultrasonic 
signals are reduced to a second center frequency, which is lower than the 
first center frequency as a result of round-trip attenuation by the 
object, as sensed by the apparatus after reflection by interfaces or 
discontinuities at a given depth. There will be a different second center 
frequency for echoes from each depth in the object. The apparatus has a 
means connected to transmit and receive the ultrasonic signals from and to 
the array in a stepped array mode. The apparatus also has a means 
connected to transmit and receive the ultrasonic signals from and to the 
array in an angle scanning mode. A means is connected to select between 
the stepped array transmitting and receiving means and the angle scanning 
transmitting and receiving means, based on the second center frequency at 
the depth of focus of the signals. Based on the degree of attenuation of 
the transmitted center frequency by the object, the selection means will 
choose the angle scanning array transmitting and receiving means when the 
apparatus is focused on a deeper field within the object, where a greater 
degree of attenuation takes place. As used herein, the term "angle 
scanning" means that the beam is directed at an angle using all or part of 
an array. Sector scanning is a special case of angle scanning in which all 
of the array is used for transmitting or receiving. 
The attainment of the foregoing and related objects, advantages and 
features of the invention should be more readily apparent to those skilled 
in the art, after review of the following more detailed description of the 
invention, taken together with the drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, more particularly to FIG. 1, there is shown a 
linear stepped array 10, which may be utilized with the invention. The 
array 10 is positioned to image a portion 12 of a person's body. The 
portion 12 is rectilinear in shape because successive groups of transducer 
elements 14 in the array 10, a portion of the groups being respectively 
indicated by brackets 16, 18 and 20, are activated and stepped from left 
to right in the array 10 as shown to define a rectilinear field of vision 
13 of the array 10. 
It should be noted that each group 16, 18 and 20 of transducer elements 14 
indicated in FIG. 1 is shown to contain three of the transducer elements 
14. As will be explained in more detail below, the number of elements 14 
in each group 16, 18 and 20 may be changed, depending on the center 
frequency of ultrasonic signals sensed at the transducer elements 14, due 
to attenuation by the body under investigation. In all cases the center 
frequency of the sensed ultrasonic signals will be less than the frequency 
of the ultrasonic signals as transmitted by the elements 14, due to the 
attenuation. The difference between the center frequency of the ultrasonic 
signals as sensed and the center frequency of the transmitted ultrasonic 
signals will depend on their depth of penetration into body 12 before 
reflection. Thus, as the transmitted ultrasonic signals are focused deeper 
into the body 12, a substantial decrease in the center frequency of the 
sensed ultrasonic signals occurs. 
FIG. 2 shows a sector scanned array 30 of ultrasonic transducers 32 of a 
known type. Through use of different electronically induced delays in the 
transmitted and received signals from and to transducers 22, the focal 
axis of signals is shifted to cause a scanning pattern from the array 30, 
producing a sector shaped field of view 34 of the body 12. As in the case 
of the FIG. 1 embodiment, there is a substantial decrease in the center 
frequency of the signals transmitted by the transducers 32 at the depth of 
focus of the signals, due to attenuation within the body 12. 
FIG. 3 shows an idealized representation of typical transmitted and 
received ultrasonic spectra 21 and 23 which may be obtained with 
transducer array 10. Since a proportionally greater attenuation of the 
transmitted spectrum 21 (0 cm into body 12) occurs at higher frequencies, 
its center frequency f.sub.c1 is greater than the center frequency 
f.sub.c2 of spectrum 23, which is reflected from 4 cm within body 12. 
There are certain mathematical relationships defining the nature of the 
attenuation useful for a more complete understanding of the present 
invention. For mathematical simplicity transducers 14 in FIG. 1 may be 
assumed to produce ultrasonic signals with a Gaussian frequency response 
as received after the reflection back at the transducers, which may be 
described by the following equation: 
EQU V(f)=Ae.sup.-.alpha.(f-f.sbsp.o.sup.).spsp.2 Equation (1) 
Where 
A=constant 
V(f)=round-trip voltage response 
f.sub.o =center frequency of received signal 
.alpha.=(1 n 4)/(.DELTA.f).sup.2 
.DELTA.f=-3 dB full bandwidth 
Equation 1 may be expressed in terms of a fractional bandwidth, B, which is 
equal to (.DELTA.f/f.sub.0), as follows: 
##EQU1## 
Attenuation of the signals produced by transmission through body tissue 12 
can be approximated by a loss term proportional to frequency in the 
following manner: 
EQU Loss (dB)=K.multidot.f(MHz).multidot.2.multidot.depth (cm) 
where K varies from 0.6 dB/cm/MHz to 1.2 dB/cm/MHz in typical soft tissue. 
This amplitude loss can be written exponentially as shown in the following 
equation: 
EQU A(f,d)=e.sup.-0.23.times.K(dB/cm/MHz).times.f(MHz).times.depth (cm) 
Equation (3) 
One of the advantages of the present invention is that no electrical 
filtering of the transmitted or received signals is required. Assuming no 
such filtering is done, equations 2 and 3 can be combined to get the round 
trip signal as a function of frequency and depth into the tissue. To find 
the sensed center frequency at each depth of focus, the resulting 
expression must be differentiated, e.g., 
##EQU2## 
In equation (4), f.sub.center is never less than 0. 
A basic distinction between the transducer elements 14 in the linear 
stepped array of FIG. 1 and the transducer elements 32 of the sector scan 
array 30 of FIG. 2 is the relationship between transducer element width 
and wavelength of the ultrasonic signals transmitted by the transducers in 
each case. For the linear array 10 of FIG. 1, the elements 14 are 
relatively wide. They typically have a width of about three times the 
wavelength of the transmitted signals. This is possible because the 
transducer elements 14 always have their focal axis normal to the plane of 
the elements, so that the severe grating lobes which would exist with such 
wide elements if the beam were steered are not a problem. In the case of 
the sector scanned array 30, the transducer elements 32 must be narrow 
with respect to the transmitted wavelengths. Typically, the elements 32 
have a width half the wavelength of the transmitted ultrasonic signals, so 
that no grating lobes are present in the response curves of the 
transducers 32. 
Due to design and operation based on the decrease of center frequency of 
the transmitted signals in the body 12 analyzed with the system of this 
invention, improved image definition with the linear stepped array 10 of 
FIG. 1 can be obtained, and a hybrid approach can be utilized, in which a 
transducer array is operated in the linear stepped array mode for near 
field observations, and in the sector scanning mode for deep field 
observations. The latter approach becomes possible because the lowered 
frequency for echoes from targets deep in the object makes the wavelength 
longer, and therefore the element spacing becomes smaller relative to the 
wavelength of the attenuated echo signal. When the signal from linear 
array transducer elements 14 has been attenuated sufficiently so that the 
element spacing is on the order of one wavelength or less, the signal beam 
can be scanned in the manner of a sector scan array, as shown in FIG. 2. 
The ability to scan, and the scan angle, are limited by the presence of 
grating lobes, which exist for transducer elements greater than half the 
wavelength of the sensed signal. For elements smaller than this, a scan 
angle of 90 degrees may be employed. 
For an array focused at angle .theta..sub.F, a grating lobe exists at angle 
##EQU3## 
where a is the center-to-center spacing of the elements, and .theta..sub.F 
&gt;0. This equation is exact for the cw case, and approximate for the pulsed 
case. To prevent grating lobes from occurring, the scan angle is 
restricted to 
##EQU4## 
For example, at 5 MHz frequency, the wavelength in water is 0.3 mm. For an 
array with elements spaced 0.2 mm apart, no grating lobes exist for scan 
angles up to +30.degree., but will exist if the beam is steered to larger 
angles. 
To determine the maximum scan angle as a function of depth, one must know 
the center frequency as a function of depth. This is controlled by a 
number of parameters, including: 
1. transducer response versus frequency (bandwidth, center frequency and 
bandshape) 
2. tissue attenuation versus frequency (may be a function of depth) 
3. frequency response of the receiving electronics (may be time-variable, 
and hence a function of depth). 
Equation 6 can be combined with Equation 4 to give the maximum angle to 
which the array can be scanned as follows: 
##EQU5## 
where V=acoustic velocity. 
Equation 7 has the following special cases: 
(1) When f.sub.center is very high: f.sub.center .gtoreq.(V/a). In this 
case .theta..sub.F =0, i.e., the array must be used in the "stepped array" 
mode. 
(2) When f.sub.center is very low: f.sub.center .ltoreq.(V/2a). In this 
case .theta..sub.F is allowed to be as much as 90.degree., i.e., the array 
can be used as a full sector scanner. 
(3) When (V/2a)=f.sub.center =(v)/a, the array can be scanned over some 
limited angle. 
FIGS. 4 and 5 show two different embodiments of hybrid transducer arrays in 
accordance with the invention. Lines 40 in FIG. 4 define focal axes for 
transducer elements 42 in the array 44. Centrally disposed lines 46 are 
perpendicular to the plane of the array 44 because the focal axes of the 
transducers 42 corresponding to these lines remain fixed at right angles 
to the plane of array 44. Remaining lines 48 to the left and right of 
lines 46 represent focal axes for elements 42 at the left and right edges 
of array 44 that are operated in an angle scanning mode to broaden the 
overall field of view represented by lines 40. In each case, the lines 48 
represent the maximum extent of scanning for each of their associated 
elements 42. The lines 48 begin at successively greater distances from the 
array 44 because the angled scan lines are activated only as the focus of 
array 44 is extended deeper into the field. 
In the embodiment of FIG. 5, elements 50 of the array 52 are operated in 
the stepped array mode when the array 52 is focused in the near field, as 
represented by lines 54. With focus into the deep field, elements 50 of 
the array 52 are then operated in the sector scan mode when attenuation of 
the transmitted ultrasonic waves is substantial enough to allow a 
substantial scanning angle, as represented by lines 56. As the selected 
observation depth increases, the scanning angle can be increased, as 
represented by lines 58. 
FIG. 6 shows one embodiment of the electronics necessary for implementing 
an ultrasonic imaging system incorporating the present invention. Array 
100 includes transducer elements 104 arranged in overlapping groups 106, 
108 and 110 as in FIG. 1. Each of the elements 104 of array 100 is 
connected by a line 114-1 to 114-x to a transmit and receive switch 116-1 
through 116-X. Each transmit and receive switch 116-1 through 116-X is 
connected by a line 118-1 through 118-X to transmit pulser circuits 120-1 
through 120-X. The transmit and receive switches 116-1 through 116-X are 
also connected by lines 122-1 through 122-X to preamplifiers 124-1 through 
124-X. The preamplifiers 124-1 through 124-X are in turn connected by 
lines 126-1 through 126-X to digital delay lines 128-1 through 128-X. 
Digital delay lines 128-1 through 128-X are connected by lines 130-1 
through 130-X to summing circuits 132. Summing circuits 132 are connected 
by line 134 to signal processing circuits 136. Signal processing circuits 
136 are connected by line 138 to scan converter circuits 140, which are in 
turn connected by line 142 to a video display 144. 
Central controller 150 is connected by lines 152-1 through 152-X to each of 
the transmit pulser circuits 120-1 through 120-X. Central controller 150 
is connected to each digital delay line 128-1 through 128-X by control 
lines 154-1 through 154-X. Control signal lines 152-1 through 152-X supply 
transmit-delay control signals to the transmit pulser circuits 120-1 
through 120-X. The control lines 154-1 through 154-X transmit receive 
delay control signals to delay lines 128-1 through 128-X. Central 
controller 150 is also connected by control line 156 to scan converter 
140. If controller 150 is implemented as a microprocessor, a suitable 
control program for implementing the control functions described herein is 
provided in a read only memory (ROM). 
The apparatus shown in FIG. 6 has the transmit pulser circuits 120-1 
through 120-X, preamplifiers 124-1 through 124-X and delay lines 128-1 
through 128-X implemented in fully parallel form. This construction allows 
the system of FIG. 6 to be operated in accordance with the present 
invention, with the above referenced Wilson et al "Frequency Varied 
Ultrasonic Imaging Array" related invention, the disclosure of which is 
incorporated herein by reference, or utilizing both inventions. In 
accordance with the present invention, the transducers 104 are operated in 
a stepped array mode for imaging in the near field and in an angle scan 
mode as shown in FIGS. 4 and 5 for imaging in the deep field. Selection of 
operating mode and scan angles when operating with a sector scan mode is 
made on the basis of sensed or calculated signal attenuation. 
In operation of a system as shown in FIGS. 1 and 4 to 6, assuming a 
transmitted frequency of 10 MHz, the ultrasonic signal would be attenuated 
as shown below in Table I, based on the mathematical relationships 
discussed above. 
TABLE I 
______________________________________ 
Attenuation .sup.f center 
.sup.f center 
Bandwidth 
factor at 2 cm at 4 cm 
(%) (dB/cm/MHz) (MHz) (MHz) 
______________________________________ 
40 0.8 7.9 5.8 
40 1.0 7.3 4.7 
40 1.2 6.8 3.6 
50 0.8 6.7 3.4 
50 1.0 5.9 1.7 
50 1.2 5.0 0.04 
______________________________________ 
In operation of a system as shown in FIG. 6, Table II below shows the 
sector scanning angles that may be achieved at various center-to-center 
spacings for the elements 104 of array 100 when scanning tissues with an 
attenuation of 1 dB/cm/MHz, a transmitted center frequency of 10 MHz, a 
bandwidth of 50%, and focusing depths into the tissue as indicated. Where 
a maximum scanning angle of zero is indicated, the array must be operated 
in the stepped mode, with increases in the maximum scan angle to 
90.degree. as depth imaging increases. 
TABLE II 
______________________________________ 
Depth (cm) 
Freq., MHz Max. Scan Angle, Degrees 
______________________________________ 
Center-to-Center Spacing = 1.15 mm 
0 10 0 
.5 8.9625 6.64747 
1 7.925 15.1727 
1.5 6.8875 26.866 
2 5.85 45.1863 
2.5 4.8125 89.9209 
3 2.775 89.9209 
3.5 3.775 89.9209 
4 1.7 89.9209 
Center-to-Center Spacing = .2 mm 
0 10 0 
.5 8.9695 0 
1 7.925 0 
1.5 6.8875 5.10201 
2 5.85 16.8827 
2.5 4.8125 33.9481 
3 3.775 80.6644 
3.5 2.7375 89.9209 
4 1.7 89.9209 
Center-to-Center Spacing = .25 mm 
0 10 0 
.5 8.9625 0 
1 7.925 0 
1.5 6.8875 0 
2 5.85 1.46929 
2.5 4.8125 14.2255 
3 3.775 86.1147 
3.5 2.7375 89.9209 
4 1.7 89.9209 
Center-to-Center Spacing = .3 mm 
0 10 0 
.5 8.9695 0 
1 7.925 0 
1.5 6.8875 0 
2 5.85 0 
2.5 4.8125 2.28287 
3 3.775 18.9855 
3.5 2.7375 55.7393 
4 1.7 89.9209 
______________________________________ 
The following listing is a program for calculating the center frequency of 
an ultrasonic spectrum at a given depth in tissue and can be used in the 
selection of appropriate depth for switching between a stepped array mode 
of operation, a combination of stepped array mode and angle scanning, or 
sector scanning for transmitting and/or receiving ultrasonic transducer 
arrays in accordance with the invention. The program runs on a Digital 
Equipment PDP 11/40 minicomputer. 
______________________________________ 
5 ! PEAK 
10 ! TISSUE ATTEN EFFECT ON CENTER FREQ 
30 INPUT "ATTEN (DB/CM/MHZ)",A 
40 ? "": ? "" 
50 ? "D","FR","RMAX" 
100 FOR D=0 TO 4 STEP .5 
103 RM=0 
105 FOR FR=.25 TO 15 STEP .25 
110 A1=.0493*(FR-9.75)*(FR-9.75) 
120 A2=.23*A*FR*D 
130 R=EXP(-A1-A2) 
140 IF R&gt;RM THEN RM=R:FM=FR 
150 NEXT FR 
155 ? "" 
160 ? D,FM,RM 
170 NEXT D 
______________________________________ 
The listing below is a program for a Hewlett-Packard HP-85 desk calculator 
which calculates and plots ultrasonic spectra at various depths in tissue, 
and is of further assistance in the selection of operating mode for 
transmitting and/or sensing transducers in accordance with the invention. 
______________________________________ 
1.0..0. GOSUB 1.0..0..0. ! INIT 
11.0. GOSUB 2.0..0..0.! INPUT 
12.0. GOSUB 4.0..0..0.! PLOT SET-UP 
13.0. GOSUB 3.0..0..0.! CALCULATE&PLOT 
14.0. GOSUB 5.0..0..0.! COPY 
16.0. GOTO 11.0. 
1.0..0..0. 
! INIT 
1.0.1.0. F.0.=1.0. ! MHz XDUCER CENTER F 
REQ 
1.0.2.0. F1=.0. ! MIN FREQ 
1.0.3.0. F2=15 ! MAX FREQ 
1.0.4.0. F3=.25 ! .DELTA.F 
1.0.5.0. B1=4.0. ! %BANDWIDTH 
1.0.6.0. B=B1*.01 ! FRAC BANDWIDTH 
1.0.7.0. K=.6 ! dB/cm/MHz 
1.0.8.0. Z1=6 ! DEPTH INTO TISSUE(cm) 
1.0.9.0. Z2=1 ! .DELTA.Z FOR PLOTTING 
11.0..0. CLEAR 
111.0. DISP "DATE" @ INPUT D9$ 
1999 RETURN 
2.0..0..0. 
! INPUT 
2.0..0.5 CLEAR 
2.0.1.0. DISP "F=";F.0.;"MHz XDUCER FR 
EQ" 
2.0.2.0. DISP "B=";B1;"%BANDWIDTH" 
2.0.3.0. DISP "K=";K;"dB/cm/MHz" 
2.0.4.0. DISP "ZMAX=";Z1;"cm DEEP" 
2.0.5.0. DISP @ DISP 
2.0.6.0. DISP "TYPE,VALUE";@ INPUT T 
$,V 
2.0.7.0. IF T$="F" THEN F.0.=V 
2.0.8.0. IF T$="B" THEN B1=V @ B=..0.1 
*B1 
2.0.9.0. IF T$="K" THEN K=V 
21.0..0. IF T$="Z" THEN Z1=V 
211.0. IF T$=".0." THEN RETURN 
212.0. GOTO 2.0..0.5 
2999 RETURN 
3.0..0..0. 
! CALCULATE 
3.0..0.5 FOR Z=.0. TO Z1 STEP Z2 
3.0..0.7 PENUP 
3.0..0.8 Q=.0. 
3.0.1.0. FOR F=F1 TO F2 STEP F3 
3.0.2.0. A=-K*F*2*Z 
3.0.3.0. T=-12*((F-F.0.)/(B*F.0.)) 2 
3.0.4.0. L=A+T 
3.0.5.0. IF L&gt;Y1 THEN PLOT F,L @ Q=1 
3.0.55 IF L&lt;Y1 AND Q&gt;.0. THEN LABEL 
VAL$(Z) @ PENUP @ Q=2 
3.0.6.0. NEXT F 
3.0.65 IF Q#2 THEN IMOVE .25,.0. @ L 
ABEL VAL$(Z) 
3.0.7.0. PENUP 
3.0.8.0. NEXT Z 
3999 RETURN 
4.0..0..0. 
! PLOT SET-UP 
4.0.1.0. CLEAR 
4.0.2.0. GCLEAR 
4.0.4.0. Y1=-8.0. ! YMIN 
4.0.5.0. Y2=.0. ! YMAX 
4.0.6.0. SCALE F1-1,F2+1,Y1-1.0.,Y2+1.0. 
4.0.7.0. XAXIS Y1,1,F1,F2 
4.0.8.0. XAXIS Y2,1,F1,F2 
4.0.85 XAXIS -4.0.,1,F1,F2 
4.0.9.0. YAXIS F1,1.0.,Y1,Y2 
41.0..0. YAXIS F2,1.0.,Y1,Y2 
411.0. YAXIS 5,1.0.,Y1,Y2 
4115 YAXIS 1.0.,1.0.,Y1,Y2 
412.0. FOR F=F1 TO F2 STEP 5 
413.0. MOVE .95*F,Y1-1.0. @ LABEL VA 
L$(F) 
414.0. NEXT F 
499.0. PENUP 
4999 RETURN 
5.0..0..0. 
! COPY 
5.0..0.5 PRINT "DATE IS ";D9$ @ PRINT 
5.0.1.0. PRINT "XDUCER FREQ=";F.0.;"MH 
z" 
5.0.2.0. PRINT "BANDWIDTH=";B1;"%" 
5.0.3.0. PRINT K;"dB/cm/MHz" 
5.0.4.0. PRINT "DEPTHS FROM .0. TO";Z1 
;"cm" 
5.0.5.0. PRINT @ PRINT 
5.0.6.0. GRAPH @ COPY 
5.0.7.0. PRINT @ PRINT @ PRINT @ PRI 
NT @ PRINT 
5999 RETURN 
______________________________________ 
The following listing is a program for calculating the maximum scan angle 
for angle scanned arrays or sector scanned arrays, based on decreases in 
center frequency of transmitted ultrasonic signals reflected from 
different depths within tissue being examined with the signals. It also 
runs on the PDP 11/40 minicomputer. 
______________________________________ 
100 V=1.5: ! MM/USEC 
110 INPUT "CENTER FREQUENCY(MHZ)";F0 
120 INPUT "% BANDWIDTH",BP:B=BP/100 
125 INPUT "MAX DEPTH, STEP SIZE (CM)";DM,DD 
127 INPUT "C-C SING RANGE: 
MIN,MAX,STEP (MM)";SL,SM,DS 
130 FOR A=.8 TO 1.2 STEP .2 
135 ? "LOSS=";A;"DB/CM-MHZ" 
140 FOR S=SL TO SM STEP DS 
145 ? "C-C SING=";S;"MM" 
150 K=0.083*A*B*B*F0*F0: ! T OF LOSS TERM 
160 ? "D(CM)","FREQ","MAX ANGLE": ? "" 
200 FOR D=0 TO DM STEP DD 
210 FC=F0-K*D: ! NEW XDUCER CENTER FREQ 
220 IF FCK=0 THEN FC=.0000001 
230 K1=(V/(FC*S))-1 
240 IF K1&gt;=1 THEN K1=.999999 
250 IF K1&lt;0 THEN K1=0 
260 TM=ATN(K1/SQR(1-K1*K1)): ! ARC SINE (K1) 
270 ? D,FC,TM*180/3.141592 
280 NEXT D 
290 ? "": ? "": ? "" 
300 NEXT S 
310 NEXT A 
READY 
______________________________________ 
It should now be apparent to those skilled in the art that an ultrasonic 
imaging apparatus capable of achieving the stated objects of the invention 
has been provided. The hybrid array approach of this invention allows the 
combination of a large field of view close to the array, obtained by use 
of the stepped array approach, and a very wide field deep in the image, 
obtained with a sector scanning or angle scanning approach. In this 
manner, the decrease in center frequency of a transmitted signal is 
utilized to permit sector or angle scanning with an array otherwise suited 
only for a stepped array mode of operation at higher frequencies near to 
the array. Thus, the different advantages obtainable with these three 
modes of operation can be obtained from a single apparatus. 
It should further be apparent to those skilled in the art that various 
changes in form and details of the invention as shown and described may be 
made. It is intended that such modifications can be included within the 
spirit and scope of the claims appended hereto.