Ultrasonic step scanning utilizing unequally spaced curvilinear transducer array

An electronically step scanned real time ultrasonic imaging system and method is disclosed. The system includes a transducer assembly having an unequally spaced array of elements, each with an axis of transmission along which it transmits its main ultrasonic energy when electrically stimulated. The elements are disposed in a curvilinear array, wherein their axes of transmission are approximately co-planar, but divergent in the common plane. In the preferred embodiment, the axes of transmission diverge from one another by unequal angles, such that the tangent functions of each of the angles varies in equal increments along the array. Scan conversion techniques are used for displaying purposes, operating in the Y .theta. and Y tan .theta. formats.

DESCRIPTION 
1. Technical Field 
This invention relates to the field of ultrasonic imaging equipment, and 
more particularly to an ultrasonic medical diagnostic system employing an 
improved transduc;er assembly and examination method. 
2. Background Art 
In recent years, the field of diagnostic ultrasound has seen the emergence 
of a so called "real time" ultrasonic B scanning examination system. The 
term "real time" means that the systems produce successive images at a 
rapid enough rate so that images are generated faster than the retention 
rate of the human eye so that moving objects appear in continuous motion. 
Thus, in real time operation, the course of the study can be influenced by 
the operator contemporaneously with the actual study, based on his 
observation of the rapidly produced image succession. This real time 
feature is considered an improvement over previous forms of ultrasonic 
examination, in which only a single image is developed slowly and 
gradually during the course of a study by moving a single transducer about 
the patient's skin. In addition to allowing the operator to influence the 
course of the study, real time systems allow for "stop action" images of 
moving body parts, and also for motion studies. 
Real time ultrasonic examination systems have mainly fallen into two 
general types, i.e., linear scanning and sector scanning. Electronic 
linear scanning systems utilize a transducer assembly including a large 
linear array of individual piezoelectric ultrasonic transducer elements. 
Imaging circuitry fires a succession of different groups of elements in 
accordance with a predetermined repeated sequence. This produces a 
succession of resultant ultrasonic beams propagated along respective 
parallel paths extending outwardly from the transducer assembly. The 
assembly is held stationary against the patient's body during image 
generation. 
This technique, in conjunction with known forms of imaging circuitry and 
display apparatus, produces from received ultrasonic echoes information 
defining a two dimensional rectangular image of the internal body 
structure of the patient taken in a common plane, or "slice" through part 
of the body near the transducer array. One coordinate of each point on the 
image plane is determined by the amount of time required for incident 
ultrasonic energy to be reflected back to the transducers from a tissue 
interface within the body. The other coordinate is determined by the 
location, along the transducer array, of the axis of the resultant 
ultrasonic beam which caused the reflected energy. 
By operating this system to repeatedly step the incident beam origin along 
the linear transducer array at, for example, thirty repetitions per 
second, the rapid sequence of ultrasonically produced image frames which 
result can show motion of a moving body part. Alternately, a single frame 
of image data can be held for display, in order to stop rapid motion of 
such a body part. 
The display area scanned by such linear step scanners is rectangular and 
suitable for presentation on a two dimensional display system, such as a 
CRT. The electronics required for such a system are relatively inexpensive 
and simple, since all the beams are parallel and stepped over uniform 
increments. Moreover, linear stepped scanning systems exhibit 
substantially uniform field of view throughout their display area. 
Linear systems, however, do have some disadvantages. For example, the 
transducer assembly must of necessity be rather long, and therefore clumsy 
to use, since the length of one side of the rectangular display equals the 
length of the transducer array. Since all the ultrasonic beams produced by 
the linear scanner are propagated along parallel lines, the linear scanner 
is not generally capable of imaging portions of the patient's body which 
are hidden behind other nearer portions, such as an organ which may, be 
located behind a rib. 
A known type of electrically stepped linear array ultrasonic system is 
described in the following publication, which is hereby expressly 
incorporated by reference: Havlice, J. F., et al, "Medical Ultrasonic 
Imaging: An Overview of Principles and Instrumentation", Proceedings of 
the IEEE, Vol. 67, No. Apr. 4, 1979, pp. 620-641. 
Another type of known real time electronic ultrasonic scanner is the 
electronic sector scanner. In such devices, a linear array of transducer 
elements is employed as in the case of linear step scanning. The length of 
the array, however, is considerably shorter than in the case of the step 
scanned linear device described above. 
In using the electronic sector scanner, the transducer assembly is held 
stationary near the portion of the patient's body to be examined. All 
elements are repeatedly fired in a single group. Phase delay circuitry is 
associated with imaging circuitry which is utilized to control ultrasonic 
beam emission and reception by the transducer elements. By proper phase 
delay of respective elements, the ultrasonic beam repeatedly produced by 
the transducer array is "steered" at different angles to the face of the 
transducer assembly. The angle of the ultrasonic beams produced by 
successive firings of all the elements of the transducer array is 
repeatedly scanned in increments from one side to another, such that the 
successive ultrasonic beams collectively sweep through the patient's body 
at different angles in a common plane. 
Several advantages over the linear stepped scanner are achieved by use of 
the electronic sector scanner. First, the transducer assembly is 
significantly more compact than in the case of the stepped scanner, and 
can thus be used at almost any location on the patient's body. Since the 
ultrasonic beams are directed into the subject at different angles, the 
electronic sector scanner can image portions of the body which might be 
hidden from view of the linear stepped scanner because of their location 
behind other more opaque portions of the body, such as bone. 
Electronic sector scanning, however, does have its own inherent 
disadvantages. One such disadvantage is that these scanners have a narrow 
field of view in regions of the body close to the transducer assembly. 
This is because the field of view of the sector scanner resembles a sector 
of a circle and, close to the transducer assembly, the excursion of the 
sweep of the ultrasonic beam is quite small. 
Another disadvantage of the electronic sector scanner is the relatively 
high cost of such units, due in large measure to the complexity of the 
electronics necessary to achieve the delay scheme employed to effect beam 
steering. While a typical linear transducer step scanner costs in the 
neighborhood of $15,000 to $30,000, the corresponding range of cost for 
electronic sector scanners is about $65,000 to $100,000 each. 
Mechanically steered real time linear and sector scanners, using 
oscillating or rotating single crystal transducers, have also been 
proposed. Such systems, however, suffer from relatively large physical 
size, and problems associated with reliability of the mechanical drive. 
They also usually require the transducer to be immersed in a fluid. 
Known proposals for electronic and mechanical sector scanners are described 
in the above referenced Havlice, et al, publication. 
Another system, a variant of ultrasonic step scanning, (Bushmann "New 
Equipment and Transducers for Ophthalmic Diagnosis", Ultrasonics, Vol. 3, 
pages 18 et seq, January-March, 1965,) has been proposed relating to 
ultrasonic examination of the eye. It is suggested to ultilize ten 
transducers arranged in an arc such that the ultrasonic beams emitted by 
each of the transducers mutually converge near the center of the eye ball. 
Pulsing circuitry is applied to separately fire each of the transducers in 
a sequence. 
A disadvantage of this type of examination stems from the fact that tissue 
interface points within the patient's body which generate ultrasonic 
echoes may be struck by primary incident energy from more than one 
transducer. Each such point could thereby lack uniqueness of location on 
the image display, causing blurring. 
This lack of unique location of multiply-struck interface points would be 
caused by inhomogeneity in the patient's body. Acoustic velocity differs 
among tissue types. If the time required for an ultrasonic echo from one 
transducer to return to that transducer from the subject point is 
different from the corresponding return time with respect to another 
transducer whose energy also strikes the point, the subject point will 
show up at slightly different spots on the display. 
It is an object of this invention to provide an economical ultrasonic 
scanning system having the flexibility, compactness and swept beam 
characteristics of an electronic sector scanner without the sector 
scanner's limited close up field of view and high price and which is 
susceptible of use with scan converters having simplified Y .theta. and Y 
tan .theta. memory formats, preserving the uniqueness of display location 
each imaged point, all for the cost of a simple linear. 
DISCLOSURE OF INVENTION 
The ultrasonic scanning system of this invention overcomes or reduces the 
disadvantages of the stepped linear scanner as well as those of the 
electronic and mechanical sector scanners, while combining advantages of 
both. 
A system embodying this invention includes an ultrasonic transducer element 
array, and imaging electronics coupled to actuate the transducer array for 
emitting incident ultrasonic energy and to convert received echoes to 
electrical signals. The system also includes appropriate display apparatus 
to convert the electrical signals to a visual image describing internal 
structure of the patient's body. 
The transducer array has a curvilinear arrangement of its elements. The 
transducer elements are disposed with their axes of primary transmission 
being divergent within a common plane. This feature enables the system to 
direct ultrasonic energy beams into a patient's body at different angles 
depending on which elements are fired. This facilitates the obtaining of 
ultrasonic echoes from body tissue interfaces located behind body parts 
which would obscure such interfaces if the ultrasonic beams were parallel. 
The divergent beams also provide a larger imaged area than would exist 
with a linear scanner employing the same length array. 
Another specific feature of this invention, involving the use of a 
curvilinear array of ultrasonic transducers, relates to the configuration 
of the array and to its particular adaptability to an especially efficient 
means of processing ultrasonically derived information into a visible 
image. 
In accordance with this specific feature, the ultrasonic transducers are 
distributed at unequal intervals along the curvilinear path. More 
specifically, the transducers are distributed at such intervals that the 
respective tangent values of each angle of ultrasonic beam divergence 
relative to ultrasonic axis of the other transducer, differ from one 
another by equal increments. Thus, where the axes of ultrasonic 
propagation of a series of ultrasonic transducers diverges from that of 
the center transducer by angles .theta..sub.1, .theta..sub.2 . . . 
.theta..sub.n, the angles .theta..sub.1, .theta..sub.2 . . . .theta..sub.n 
are chosen such that their respective tangents differ from one another by 
integral multiples of a constant. 
Such an unequally spaced array of ultrasonic transducers facilitates 
processing of ultrasonically derived information into a visual image by 
means of particularly efficient scan conversion. In such an embodiment, 
the scan converter has a memory with each address being dedicated to a 
particular Y and tan .theta.. In reproducing the image in a set of X,Y 
coordinates on a CRT monitor, the Y displacement of each event is read 
directly from the memory. The X displacement of the corresponding event is 
obtained by merely multiplying the Y displacement by the other value 
associated with the memory location from which the event data is sampled, 
namely Y-tan .theta..

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 illustrates in general form a system S incorporating the present 
invention. The system S directs ultrasonic energy into a subject, such as 
a patient's body, and in response to echoes produced by the incident 
energy, produces an image representing internal structure or condition of 
the body. 
The system S includes a curvilinear transducer assembly 10 for producing 
incident ultrasonic energy and for receiving echoes thereby caused. 
Imaging circuitry 12 actuates the transducer assembly to produce the 
incident energy, and receives electrical signals from the transducer 
produced in response to the echoes. Data from the imaging circuitry, in 
the form of these electrical signals, is directed to a display apparatus 
14 which produces the image. Display format adaptor circuitry 16 provides 
format generating signals to the display apparatus 14, defining an array 
of image lines constituting the image, in response to data and timing 
control signals received from the imaging circuitry 12. 
The transducer assembly 10 preferably includes 76 individual transducer 
elements, such as indicated by the reference character 18. Each transducer 
element comprises an individual piezoelectric ultrasonic transducer of 
known type, having a particular axis along which ultrasonic energy from 
the element is primarily directed. The ultrasonic transducer elements 18 
are arranged in a curvilinear disposition along a circular arc. The axes 
of transmission of the elements, such as indicated by the dotted lines 20 
in FIG. 1, diverge radially from the imaginary center of the circle 
defined by the arc along which the transducer elements are arranged. 
In this preferred embodiment, the radius of curvature of the arc along 
which the transducer elements are disposed is approximately 10 centimeters 
(cm). The arcuate length of the transducer element array is approximately 
5 centimeters. 
The imaging circuitry actuates the transducer elements to produce short 
bursts of ultrasonic energy, each burst having a frequency of 
approximately 3.5 megahertz (MHz.). The imaging circuitry actuates a 
sequence of groups of transducer elements 18 such that resultant 
ultrasonic beams transmitted from the transducer assembly 10 scan the 
subject body in a sequence of different angles relative to the transducer 
assembly. This mode of scanning is of known type, and is sometimes 
referred to as "real time stepped ultrasonic scanning". 
Echoes returning from tissue interfaces within the patient's body cause the 
transducer elements to produce electrical signals representing 
characteristics of those echoes. These electrical signals are received and 
processed by the imaging circuitry, which then directs them as data 
signals to the display apparatus 14, which may preferably comprise a 
cathode ray tube (CRT) display apparatus. 
Preferably, the imaging circuitry 12 actuates or "fires" successive groups 
of 16 transducer elements each. The imaging circuitry 12 fires each group 
of transducers in a phased delay fashion, such that incident ultrasonic 
energy produced by the transducer assembly 10 is focused at a distance of 
approximately 4 centimeters from the transducer assembly. Additionally, 
the receiving periods of the members of each group of transducers are 
delayed in varying amounts in order to focus the zone from which echoes 
are received most readily at a distance of approximately 6 centimeters 
from the transducer assembly. These focusing delay characteristics are 
described in more detail below. 
Display format adaptor circuitry 16 receives data and timing signals from 
the imaging circuitry 12, and produces format generating signals for 
causing the display apparatus 14 to produce a display comprising a number 
of divergent image lines collectively arranged in the form of radii of a 
truncated annulus. The arcuate length of the inner portion of the annulus 
display area (skin level) is approximately 5 centimeters, and the 
corresponding distance, or width, at the outer edge of the annulus 
(corresponding to the maximum range of about 20 cm.) is approximately 15 
centimeters. Where the interior edge of the truncated annulus is located 
at the patient's skin line, the range of system operations is 
approximately 20 centimeters into the body. The included angle of the 
truncated annulus is approximately 30 degrees. 
FIG. 2 illustrates in more detail an embodiment of an ultrasonic 
examination system incorporating the present invention. The imaging 
circuitry 12 includes timing and control circuitry 22 which sequences the 
operation of the remainder of the system S. The timing circuitry 22 
actuates pulser circuitry 24 to fire the appropriate groups of transducer 
elements 18. Electrical signals from the pulsers 24 are transmitted along 
respective parallel signal channels to actuate the transducer elements 18 
by way of delay control circuitry 26 and switching circuitry 28. 
The switching circuitry 28 is controlled by the timing circuitry 22 to 
close appropriate members of the switching circuitry in order to govern 
the sequence of actuation of the transducer elements 18. Likewise, the 
focusing delay circuitry 26 is controlled by the timing and control 
circuitry 22 to impose delays on the various channels into which the 
pulser produces the actuation signals. 
When echoes return to the respective transducer elements 18 which have been 
fired, the transducer elements convert the echoes to respective electrical 
signals. These received signals are transmitted back over each of the 
respective channels by way of the switching circuitry 28 and focus delay 
circuitry 26. 
The focusing delay circuitry is controlled in the receive mode by the 
timing and control circuitry 22 to impose receiving delays upon the 
received signals. These receiving delays focus the receiving zone of the 
transducer elements 18 in phase delay fashion to enhance sensitivity of 
the system to echoes generated in a particular reception zone relative to 
the transducer position. 
The received and delayed signals are passed through a summing circuit 30 
and directed to receiver circuitry 32. The receiver circuitry 32 transmits 
the summed received signals to the "Z", or intensity control, input of the 
display apparatus 14, which preferably is embodied by a cathode ray tube 
device. 
Delay modification circuitry 34, described in more detail below, is 
provided betwen the timing and control circuitry 22 and the delay focusing 
circuitry 26. The delay modification circuitry controls the delays 
interposed by the various delay elements in each channel, during both the 
transmit and receive modes, in order to impose the proper focusing delays 
on the various signals, taking into account the curvature of the 
transducer assembly 10. 
The pulser, receiver, and summing circuitry, as well as the focusing delay 
circuitry, switching circuitry and timing and control circuitry are 
exemplified for example in the analogous circuitry of an ultrasonic 
examination system, Model LS1000, sold by Picker Corporation, Northford, 
Conn. U.S.A. 
The display format adaptor circuitry 16 includes a pair of ramp generators 
36, 38 and ramp control circuitry 40. The outputs of the ramp generators 
36, 38 are coupled to the Y axis and X axis inputs, respectively, of the 
display apparatus 14. By proper adjustment by the ramp control circuit of 
the starting times, initial values, and slopes of the ramp signals 
produced by the ramp generators, an array of divergent radii having a 
common center can be generated on the screen of the display apparatus. As 
shown in FIG. 2, this array of lines provides a display in the format of a 
truncated annulus. Each of the divergent radii on the display corresponds 
in location to a respective one of the divergent ultrasonic beams 
generated in sequence by the transducer assembly 10. 
Thus, the system produces a display in the form of a truncated annulus 
whose interior edge represents the patient's skin surface, at the face of 
the transducer assembly, and whose outer edge represents the maximum range 
of the system field of view. The use of the curvilinear transducer 
assembly, with its corresponding truncated annular display, provides a 
much larger field of view than was previously obtainable by the use of a 
linear transducer assembly having the same length of that of the novel 
curvilinear transducer assembly. This larger field of view is obtainable 
without the aid of electronic delay circuitry for changing the incident 
angle of the produced ultrasonic energy. The larger field is likewise 
obtainable without the use of mechanical sector scanning techniques which 
can be expensive and cumbersome. 
In operation, the two ramp signals defining the slope of each line 
component of the display format are initiated in response to a signal 
appearing on the lead indicated "ramp start". The ramp start signal is 
produced by the timing and control circuitry 22, and is timed to be 
synchronized relative to the firing of the transducer elements by the 
pulser cirucitry 24. The ramp control circuitry 40 is controlled by a 
signal from the timing and control circuitry 22 appearing on the line 
"number" lead which identifies the particular radial line component of the 
image to be generated in response to information derived from the current 
firing of the pulser circuitry 24. 
Preferably, the transducers are fired in groups of 16, and the pulser and 
delay circuitry correspondingly define 16 electrical channels. The system 
is operated to produce real time images at approximately 30 frames per 
second. Each image preferably comprises 120 lines. A 120 line image can be 
obtained, if desired, from a 76 element transducer assembly by the 
employment of known fractional stepping techniques, such as described in 
the following publication which is hereby expressly incorporated by 
reference: Yoshikawa, Y. et. al., "Scanning Methods in Electro-Scanning 
Ultrasonic Diagnostic Equipment". 
As noted above, the incident ultrasonic energy produced in the transmit 
mode is focused by phase delay technique at 4 centimeters from the 
transducer array. The delay program for accomplishing this focusing, 
taking into account transducer array curvature, is defined in Table I: 
______________________________________ 
Transducer Delay 
Group Elements (Nanoseconds) 
______________________________________ 
1 and 16 0 
2 and 15 113 
3 and 14 210 
4 and 13 290 
5 and 12 355 
6 and 11 403 
7 and 10 437 
8 and 9 453 
______________________________________ 
Similarly, the reception focal zone is focused at approximately 6 
centimeters from the transducer array. The delay program for accomplishing 
this delay in the receive mode is defined by the following Table II: 
______________________________________ 
Transducer Delay 
Group Elements (Nanoseconds) 
______________________________________ 
1 and 16 347 
2 and 15 261 
3 and 14 187 
4 and 13 125 
5 and 12 75 
6 and 11 38 
7 and 10 13 
8 and 9 0 
______________________________________ 
FIGS. 3 and 4 illustrate in schematic form the circuitry embodying the ramp 
generators 36, 38 and the ramp control circuitry control 40. 
FIG. 3 shows the schematic diagram of a ramp generator circuit. The ramp 
generator circuit of FIG. 3 corresponds to either of the ramp generator 
circuits 36, 38, their circuitry being identical. For purposes of 
simplicity, only one such ramp generator circuit is illustrated in detail. 
The ramp generator circuit produces a ramp output voltage signal at a lead 
100 which is the output of operational amplifier 102. Control over the 
ramp characteristics is influenced by the RC circuit 104 coupled between 
input and output of the amplifier 102. Closure of a switch 106 in the 
circuit 104 initiates production of the ramp signal. The switch 106 is 
closed by way of a signal appearing on "ramp start" input 108, generated 
by timing control circuitry 22. 
Other signals, from the ramp control circuitry 40, govern aspects of the 
ramp signals generated at the lead 100. More specifically, a signal on a 
lead 112 defines the slope of the ramp signal generated. Another signal 
from the ramp control circuitry 40, appearing at a lead 114, governs the 
initial value of the ramp at its starting time. 
Signals on the leads 112, 114 are input to the operational amplifier 102 by 
way of a two-position switch 110. The condition of the signal on the lead 
108 controls the position of the switch 110. Prior to the initiation of 
the ramp signal output, the switch 110 is in its lower position, such that 
it defines the initial ramp signal value. Upon initiation of ramp signal 
production, the switch 110 is moved to its upper position, such that the 
ramp slope information input on the lead 112 is then applied to the 
operational amplifier 102, to control ramp slope. 
The ramp generators 36, 38, conjunctively define the X,Y location of each 
radial image display line generated on the display screen. The ramp 
generators govern both the starting and ending position of each display 
image line, and its slope or path on the screen. The ramp generators 
perform this function by application of the ramp signals to the X and Y 
deflection plates, respectively, of the display CRT. 
The slope of the actual display image line is a function of the ratio of 
the slopes of the respective ramp signals produced by the generators 36, 
38. Thus, the slope of the image line displayed is distinct from, but a 
function of, the slopes of the individual ramp signals produced by the 
generators 36, 38. 
The initial position of the trace of the image display line is determined 
by the initial values of the ramp signals produced by the two generators. 
Each initial position, in known fashion, provides the X,Y coordinate 
location of the beginning point of the corresponding image display line. 
FIG. 4 illustrates in schematic form a preferable embodiment of the ramp 
control circuitry 40. The ramp control circuitry produces four outputs, 
two outputs directed to each of the ramp generator circuits 36, 38. The 
ramp control circuit outputs to each ramp generator an analog signal 
indicating the initial value of the ramp to be generated and the slope of 
that ramp. These signals are produced in response to a digital signal from 
the timing and control circuitry 22 indicating by number the particular 
image display line which is to be generated by the next ramp signals 
produced. 
More specifically, signals appearing at the outputs 114, 112 indicate the 
initial value and slope, respectively, of the ramp signals to be produced 
by the ramp generator 38 for the image display line under consideration. 
Similarly, signals at the leads 114' and 112' define the analogous 
parameters for the Y axis ramp signal to be generated by the generator 
circuit 36. 
The outputs on leads 114, 114' 112, 112' are produced by the operational 
amplifiers 120, 122, 124 and 126 as indicated in FIG. 4. 
These operational amplifiers are fed input signals from the output of 
digital to analog converters, 130, 132, 134, 136, respectively. The inputs 
to the digital to analog converters are supplied as digital outputs from a 
series of six PROM (programmable read only memories) 140, 142, 144, 146, 
148, 150. The function of the PROMS circuits is to receive a digital input 
identifying the line number of the individual display line to be produced 
in response to the immediately subsequent action of the ramp control 
circuitry 40. In response to each line number input to the PROMS, each 
PROM produces a preprogrammed unique digital signal. 
The PROMS are programmed such that their digital signal outputs, as they 
are clocked by the "line number" digital signal, establish the proper 
initial conditions, ramp slopes and ramp timing to generate on the display 
the appropriate corresponding image line. 
It is believed that those of ordinary skill in the art relevant to the 
subject matter discussed here would be able, by the use of ordinary 
trigonometry to provide appropriate programming for the PROMS by analyzing 
the geometry of each desired image display line individually. However, for 
those not intimately familiar with this art, FIG. 5 is provided, 
illustrating the mathematical consideration involved in programming the 
PROM to generate appropriate initial conditions and slopes for each 
respective display line. In the embodiment described in FIG. 5, the 
display is configured as a truncated annulus having several individual 
display lines. The angle .theta. varies in increments equal to the total 
angular excursion of the display area divided by the number of lines. The 
equations for programming each output for the PROMS corresponding to each 
individual display image line, are set forth near the bottom of FIG. 5. 
The initial conditions and slopes for both X and Y are determinable by 
substituting for .theta. each individual angle of each display image line 
which is desired to be produced. The embodiment of the display format 
adaptor circuitry 16 described above comprises analog circuitry. As a 
matter of choice, however, those of ordinary skill in the art may embody 
the display format adaptor circuitry 16 in a digital form. 
More specifically, such an embodiment could suitably comprise a sector form 
digital scan coverter. A suggested embodiment for such a digital scan 
converter is illustrated in FIG. 6. The scan converter of FIG. 6 comprises 
an analog to digital converter 151, a random access memory 152, address 
counter circuitry 154, 156 and address counter control circuitry 158. 
In operation, the "Z" signal from the receiver circuitry, appearing upon a 
lead 160, is converted to digital form by the converter 151 and presented 
to the random access memory (RAM). The address counters and counter 
control circuitry determine the address in the RAM at which the incoming 
digitized Z signal is to be placed. The address counters 154, 156 are used 
to write the RAM in polar coordinates. The counters are operated by 
variable address clock rate signals from the counter control circuitry 
158. The counter control circuitry 158 operates in response to signals 
from the timing and control circuitry 22 appearing on the leads 164, 166. 
The signal on the lead 166 indicates the particular line of the composite 
image to, be currently displayed. The signal on the lead 166 is a 
synchronizing signal to synchronize the production of the displayed line 
relative to the firing of the transducers. 
Conversion to polar coordinates R, .theta. from X, Y coordinates is in 
accordance with the relation Y=R cos .theta.. This conversion is achieved 
in known form by controlling clocking rates, in each of the embodiments 
that are described below. 
When a digital representation of an image frame has been accumulated in the 
RAM by steering the incoming digitized Z axis signals among the 
appropriate RAM addresses, the RAM contents are read out in X,Y television 
format, and presented as inputs to a CRT video monitor display apparatus 
14. 
There are several ways in which ultrasonically derived data from the 
transducer array of this invention can be stored, processed and read out 
to form a visual display on a CRT monitor. 
One system uses a so-called "X,Y" memory format, wherein each pixel, or 
image portion, on the display has a corresponding memory location, 
expressed in X and Y coordinates. 
FIG. 7 illustrates a curvilinear array 200 of ultrasonic transducers for 
directing ultrasonic energy upward, as shown in this Figure, into a field 
of view denoted as 202. FIG. 7 illustrates two lines 204, 206 of 
ultrasonic propagation, and illustrates the manner in which data from 
those two lines are written into the memory and subsequently processed to 
form an image in a CRT screen. 
The line 204 emanates centered with the central one of the ultrasonic 
transducers of the array 200, and its angle of propagation is arbitrarily 
chosen as .theta.=0. 
A memory 210 is provided having an array of memory address locations which 
can be characterized graphically as a two dimensional pattern of dots 211. 
In the memory 210, each column of dots, are shown in FIG. 7, is dedicated 
to a particular value of the X coordinate of the image pixel. Each row of 
elements is dedicated to a particular value for the Y coordinate of the 
pixel. Thus, each memory address stores an image amplitude value for an 
image region about a particular X,Y location. 
Interposed between the curvilinear array 200 and the memory 210 is address 
calculator circuitry 208 whose function is described in more detail below. 
Since the X coordinate of each point on line 204=Y tan .theta., and 
.theta.=0 for line 204, the X coordinate of each point on line 204=0. It 
is thus quite simple to represent in the memory 210 each image pixel 
defined by the line 204, since X=0 for each point on the line. The line 
204 can be collectively represented by each of the memory addresses lying 
along the line 204 as defined in the portion of FIG. 7 describing the 
memory 210. 
Line 206, however, diverges from line 204 by an angle .theta..sub.1. Since 
not every point on the line 206 corresponds precisely to an address 
represented by one of the memory locations 211 of the memory 210, the scan 
converter hardware must choose which memory addresses are to be written 
into by information from the ultrasonic energy propagated along the line 
206, and which are to be left unwritten. This necessitates the use of a 
fairly complicated hardware system comprising the address calculator 
circuitry 208 to make these decisions and to avoid generation of digital 
artifacts in the displayed image. The address calculator circuitry, in 
responding to data derived from ultrasonic energy propagated along the 
line 206, must often write each data point into the memory address most 
closely approximating the actual location of the structure which caused 
the generation of the data. 
A description of this problem and its solution is provided by the 
publication Larsen, H., et al, "An Image Display Algorithm For Use In Real 
Time Sector Scanners With Digital Scan Converters", 1980 IEEE, Ultrasonics 
Symposium Proceedings, pp. 763-767, which publication is hereby expressly 
incorporated by reference. 
In the system as illustrated in FIG. 7, data thus stored in the memory 210 
can read out directly in X,Y format onto a CRT monitor to produce a visual 
display of an image corresponding to the information developed in response 
to ultrasonic energy emanating from the array 200. 
FIG. 8 illustrates another mode of scan conversion adaptable for use with 
the curvilinear array of this invention. FIG. 8 shows a curvilinear array 
220 of ultrasonic tranducers, three of which, for example, propagate 
ultrasonic energy into a field of view along lines 224, 226, 228. As in 
the case of the FIG. 7 embodiment, energy propagated along the line 224 is 
arbitrarily assigned an angle .theta.=0. Energy propagated along the line 
226 diverges from the energy of line 224 by an angle .theta..sub.1, while 
energy propagated along the line 228 diverges from that of the line 224 by 
an angle .theta..sub.2. 
The embodiment of FIG. 8 employs a memory 230 having a structure similar to 
that of the memory 210 in FIG. 7, but with a different format of 
geometrical correspondence between the memory address locations 232 and 
the geometry of the field of view 222. Instead of being formatted in 
rectangular coordinates, the memory 230 is formatted in Y,.theta. 
coordinates. In memory 230, each column of address locations is dedicated 
to a particular angle .theta., while each row of address locations is 
dedicated to a particular value of the coordinate Y. 
In FIG. 8, information from the memory 230 is read out through a 
calculation circuit 234 which subsequently transmits data to a CRT display 
236, which produces a visual display corresponding to the information 
developed by propagation and reflection of the ultrasonic energy. 
The system of FIG. 8 thus performs angle conversion between the memory and 
the display. This memory format is known as a "Y,.theta." format. In the 
Y,.theta. memory, there is a direct correspondence between the angle of 
divergence of the ultrasonic energy from each transducer element and 
memory location. 
Each angle .theta. to which each column of memory address locations 232 is 
dedicated corresponds to one of the angles .theta..sub.1, .theta..sub.2 . 
. . .theta..sub.n at which ultrasonic energy emanating from a particular 
ultrasonic element diverges from the angle .theta.=0. 
In the embodiment of FIG. 8, the required conversion of data to the display 
is performed as the data is read from the memory. 
In FIG. 8, each of the ultrasonic transducer elements is aimed at equally 
spaced angles .theta..sub.1, .theta..sub.2 . . . .theta..sub.n with 
respect to .theta.=0, which is the orientation of the central element. 
When reading data from the memory into the display for producing the 
image, the address calculator distinguishes a particular Y and .theta. 
value for the data from each memory location. In order to generate the 
image on the display in a sector scanning format, each point in the memory 
is sampled and displayed on the CRT screen in a pattern described by the 
following relations: The Y coordinate on the display screen is simply the 
value for Y associated with the particular address location being sampled. 
The X coordinate is equal to the product Y x tan .theta.. 
This value, Y tan .theta., is provided by the address calculating circuitry 
234. This circuitry is required to first calculate the tangent of the 
angle .theta. represented by the currently sampled memory location. The 
calculator circuitry then must produce a signal indicating the product of 
the tan .theta. times the Y value. 
This information is then applied to the display 236 to produce an 
indication of the X and Y coordinates of the image point represented by 
the Y, .theta. value of the particular currently sampled memory address 
location 232 of the memory 230. 
The coordinate conversion implemented by the embodiment illustrated in FIG. 
8 can be computed to a high degree of accuracy by proper digital hardware 
design. As mentioned above, only two mathematical operations need be 
performed, i.e., a multiply function and a tangent function. This 
Y,.theta. technique reduces expensive memory costs and provides images 
which are essentially free of digital artifacts. 
While the embodiments of FIGS. 6-9 are described in terms of only a single 
transducer element causing each ultrasonic energy line, this is done for 
simplicity and is not to be construed as limiting. Rather, each ultrasonic 
line can be a resultant line caused by phased or simultaneous firing of a 
different group of elements, as described above. Dynamic focusing can also 
be used. 
A third type of scan conversion technique is illustrated generally in FIG. 
9. This technique even further simplifies the required hardware for 
producing the visual image, while providing high quality displays. This 
method uses a Y,tan .theta. memory format. 
In the Y,.theta. memory system, as explained in connection with FIG. 8, 
data is acquired from the transducers at equal .theta. angle increments 
and stored in the memory under their correct .theta. coordinate. However, 
in the Y,tan .theta. system, data is acquired at unequal angles .theta., 
the angles .theta. having, however, equal tan .theta. increments. 
FIG. 9 illustrates a system incorporating the Y,tan .theta. memory format. 
A curvilinear array 240 of ultrasonic transducers directs ultrasonic 
energy into a field of view 242, such as along lines 244, 246, 248. As in 
the instance of the system described in connection with FIG. 8, the line 
244 of FIG. 9, being centrally located, is arbitrarily assigned an angle 
.theta.=0. Lines 246, 248, diverge from line 244 by angles .theta..sub.1 
and .theta..sub.2, respectively. 
An important aspect of this format is that the angles of divergence between 
adjacent ultrasonic propagation axes, such as .theta..sub.1, 
.theta..sub.2, are not equal. Rather, the angles .theta..sub.1, 
.theta..sub.2 etc., are chosen such that the tangents of each of the 
respective adjacent angles .theta. differ by a constant increment across 
the field of view 242. 
Data from the transducer array 240 is directed to a memory 250 having a 
plurality of memory locations graphically indicated by dots 252. Each of 
the columns of address locations in the memory 250 is dedicated to a 
particular value of tan .theta. corresponding to that tan .theta. value of 
one of the lines of ultrasonic propagation from the curvilinear array 240. 
Each of the rows of memory address locations is dedicated to a particular 
value of Y. 
A calculator 254 samples data from each of the memory locations and 
develops X,Y coordinates for input to the CRT display 256. It can be seen 
from the foregoing that the only function the calculation circuitry must 
perform is the multiplication of the Y value times the tan .theta. value 
associated with each sampled memory address. 
The Y coordinate of each displayed pixel is directly derived from the Y 
value to which the sampled address location is dedicated. To obtain the X 
value corresponding to that same location, the calculation circuitry need 
only multiply the Y value, already present in the memory, with the tan 
.theta. value, which is likewise already present. Thus, only a 
multiplication calculation must be made. 
A scan converter employing the Y,tan .theta. memory format is identified as 
a model 672, manufactured by Hughes Aircraft of Carlsbad, Calif. U.S.A. 
In the case of a sector format probe such as a mechanical sector scanner, 
having capability for propagating ultrasonic energy along only one axis at 
a time, the probe is directed, not to equal increments of angle .theta., 
but to increments of angle .theta. such that each function tan .theta. 
differs by equal increments from the tan .theta. of each of its adjacent 
angular positions. Under this format, the axes of ultrasonic transmission 
near the edges of the scan are spaced more closely in angle .theta. 
increments than they are near the center of the scan, i.e., where 
.theta.=0. 
Thus, the Y,tan .theta. memory format can minimize hardware costs, while at 
the same time providing high quality image displays. 
When using a Y,tan .theta. memory format with a convex curvilinear array 
such as described above, one must design that array to scan the ultrasonic 
energy at unequal .theta. intervals. A way to do this is to space the 
array elements unequally across the face of the transducer array. See FIG. 
10, reference character 260. The amount of such spacing varies across the 
array, depending upon the angle .theta.. The element spacing is designed 
such that the resultant ultrasonic axes correspond to the following 
mathematical relationships: 
##EQU1## 
Some examples of the ratio of spacing are as follows: 
______________________________________ 
.theta. 
Spacing 
______________________________________ 
0.degree. 
1.0 
.+-.7.5.degree. 
0.983 
.+-.15.degree. 
0.933 
.+-.30.degree. 
0.750 
______________________________________ 
For small angles .theta., the spacing changes very little across the array, 
as at 262. For larger angles, such as a 60.degree. total scan angle 
(+or-30.degree. ) such as at the elements referred to at 264 a more 
significant change occurs with elements closely spaced at the ends of the 
array. If the spacing changes significantly with one selected group of 
elements used to generate one ultrasonic resultant line, compensation may 
be required in the electronic focusing circuitry to provide a well focused 
beam. A typical array might comprise a 5 centimeter (cm) array line with 
15.degree. curvature and 80 elements, using 15 elements at a time to 
generate each resultant line. This provides an effective aperture of 0.94 
cm. When the scan is at the end of the array (7.5.degree. ) the spacing at 
this end will be 0.983, while at 15 elements inside of this point the 
spacing would be 0.993. Since each element is spaced from its neighbor by 
approximately one wavelength, the spacing error will be only in the order 
of one-tenth of a wavelength. This error can be easily accommodated. 
It should be kept in mind that the foregoing disclosure is intended to be 
illustrative, rather than exhaustive, of the invention. Those of ordinary 
skill in the pertinent art may be able to make additions, deletions, or 
modifications to the preferred embodiment described above without 
departing from the spirit or scope of the invention, as defined in the 
appended claims.