Ultrasound imaging system combining static B-scan and real-time sector scanning capability

A standard articulated B-scan arm provides positional and directional information for a transducer at its end, based on relative angular displacements of the separate arm segments. A real time scanner, preferably one having a fixed transducer and a fixed oscillating mirror, is freely attachable or detachable from the scan arm. The probe itself is operable either in a real time sector scan or in a static B-scan mode.

FIELD OF THE INVENTION 
This invention relates to ultrasound imaging systems, and more particularly 
to the classes of such systems which employ multiple segment, articulated 
arms for achieving positional information, and those which utilize 
self-contained scanning mechanisms for achieving a real-time scan. 
BACKGROUND OF THE INVENTION 
One well-known type of ultrasound imaging system, which is available 
commercially from numerous sources including the assignee hereof, is the 
static B-scan system. These typically involve a multiple segment (e.g. 
three) articulated arm, with a transducer attached at the end. The angular 
positions of the respective segments are monitored and encoded, typically 
by potentiometer means at the respective joints, whereby there is always 
available precise positional information concerning the transducer, and 
hence the portion of the patient being examined. Through mechanical 
manipulation of the transducer, images of high line density over a large 
field of view can be generated. Typically, these static images provide 
hard copy records of the highest image quality available in the ultrasound 
modality. Static scanning systems also involve disadvantages, however, 
arising partly from the limiting and cumbersome nature of the articulated 
arm. That is, the arm often hampers scanning in difficult apertures of the 
body, and complicates the initial surveying process to locate small 
lesions or to follow vascular structures. Additionally, the static nature 
of this display prevents the monitoring of organs in motion, due to 
respiration, heart action, or the like. 
Another class of commercially available imaging system is the real-time 
variety, typically a sector or a near rectangular segment of a sector. 
These systems typically utilize small hand-held probes, which allow the 
user to survey the anatomy, and to follow vascular structures and to 
observe organs in motion. These systems often involve disadvantages as 
well, however, chiefly due to the limited field of view, the low line 
density in the image, and the lack of positional information repeatably to 
coordinate the viewing of an image with some fixed datum position. These 
advantages aggregate when there is a need to obtain a high quality hard 
copy record of the examination, in that the image quality itself often is 
a limitation, and in any event there is little or no positional 
information available to determine the precise location from which the 
hard copy record was obtained. 
Recently, systems have been introduced which include dual functions, 
employing a real-time sector scanning aspect, as well as a static B-scan 
probe. One such system has been marketed since approximately November 1979 
by Rohe Scientific Corporation of Santa Ana, California under the trade 
designation Model 7000 Static B-Scanning System. The Rohe system employs 
two discrete and distinct probes, one for conducting a real-time sector 
scan and the other for a static B-scan. Preferred techniques for use 
entail utilization of the real-time sector scanning unit for surveying and 
coarse examination purposes, followed by use of the static B-scan for 
detailed evaluation and hard copy production with respect to any lesions 
or the like first located utilizing the hand-held sector probe. The Rohe 
system therefore entails most of the disadvantages attendant to 
utilization of separate sector scan and B-scan systems. First, utilization 
of the sector-scan image for hard copy is somewhat deficient with respect 
to image quality, and perhaps more seriously lacks positional information 
which indicates precisely the plane on which the image was observed. 
Secondly, assuming that the static B-scan aspect is to be utilized for 
detailed examination and recording of a hard copy, it may well be 
difficult to relocate a lesion first encountered utilizing the sector scan 
head; in any event, extensive readjustment of transducer dependent 
controls is required prior to shifting from one mode to the other. That 
is, it will be apparent that utilization of transducers of separate, 
distinct character for static B-scan and real-time sector scan 
applications, require different functional and signal processing 
constraints throughout the imaging system, with respect to timing, level 
selection, and the like. 
It is a primary object of the present invention to provide an ultrasound 
imaging system which combines the beneficial attributes of static B-scan 
and real-time sector scan systems, by combining those functions in a 
manner which substantially eliminates their respective disadvantages. 
It is a related object to provide a combined static B-scan and real-time 
sector scan system, utilizing a single transducer mechanism for both 
functions, whereby the user is free to shift operation back and forth 
between the respective modes, thereby vastly to increase the utility of 
the system. It is an associated object that such shifting back and forth 
may be accomplished free of the need for extensive readjustment or 
recalibration. 
It is yet another object of the principles of the present invention to 
provide a combined system whereby positional information is available even 
when images are being developed, and, as desired, recorded, utilizing the 
real-time scan mode. 
In a concurrently filed copending application of C. Hottinger entitled 
"ULTRASONIC IMAGING SYSTEM EMPLOYING REAL-TIME MECHANICAL SECTOR SCANNER", 
U.S. Ser. No. 178,482, which is assigned to the assignee hereof, there is 
disclosed and claimed a class of real-time sector scanners particularly 
useful in accordance with the principles of the present invention. The 
Hottinger system sets forth a form of real-time mechanical sector scanner 
wherein a positionally fixed, focusing transducer emits and receives 
ultrasound energy along an axis, and an oppositely facing sonic reflector 
or mirror is pivoted about a fulcrum on the axis, to reflect sonic energy 
between the transducer and the subject. Thus, beams between the reflection 
face and the subject lie in a different spatial plane than do beams 
between the transducer and the subject. In addition to setting forth the 
basic premise of such operation, the Hottinger application discloses 
respective embodiments wherein the mirror is located intermediate the 
transducer and the source of oscillatory motive power, and wherein the 
mirror is located "outboard" of the transducer relative to the source of 
oscillatory motive power. 
Another concurrently filed, copending application assigned to the assignee 
hereof, of J. Sorwick entitled "MECHANICAL SECTOR SCANNER HEAD AND POWER 
TRAIN", U.S. Ser. No. 178,488, sets forth a preferred design for sector 
scanning heads employing the rationale set forth in the Hottinger 
application. In accordance with the teachings of the Sorwick application, 
a preferred arrangement locates the transducer on the side of the 
oscillating mirror opposite the source of oscillatory power. A curved 
faced, disc-shaped transducer and an oppositely facing, circular angularly 
disposed mirror form the transmission and reception path of a mechanical 
sector scan imaging system. The transducer and mirror are mounted on a 
common axis, where the fulcrum for mirror movement also is located. A 
shaft upon which the mirror is affixed oscillates about the same axis, and 
through a belt drive mechanism, a spacially eccentric motor provides the 
oscillating motion to the mirror, in turn scanning ultrasound beams 
through the subject by virtue of mirror motion. Hence, the Sorwick 
application describes a configuration which is extremely compact and 
convenient to use for real-time scanning applications, allowing the user 
readily to follow patterns of vascularization, to image body portions 
which are in motion due to respiratory, cardiac, or the like movements, 
and to have access to difficult to reach portions of the body. 
It is an object of the present invention to utilize real-time sector scan 
apparatus and principles, as taught by the previously referenced Hottinger 
and Sorwick concurrently filed applications, in conjunction with static 
B-scan arms and systems, to achieve an efficient, mutually compatible 
combined system which meets the foregoing objects of the present 
invention. 
SUMMARY OF THE INVENTION 
The principles of the present invention are premised on beneficial 
utilization of a real time sector scanner, for example one such as 
described and claimed in the aforementioned Hottinger and Sorwick 
applications, in conjunction with a more traditional static B-scan 
segmented arm apparatus. 
In particular, the sector scan head is detachably mounted at the end of a 
multiple segment B-scan arm, and depending on whether the rotatable mirror 
is utilized in an oscillatory or stationary mode, either real time sector, 
or B-scan modalities may be employed while utilizing a single transducer 
means. Moreover, both the sector scan and the B-scan approaches may be 
respectively employed either with the scan head attached to the segmented 
arm (and hence precisely positionally located as well), or freely movable 
in the hand of the user (and hence not precisely positionally located). 
In a preferred embodiment, a real time sector scanning probe pursuant to 
the Sorwick application (i.e., with the transducer at an outboard end, and 
the motor and encoder in a "handle" which is eccentric from the 
transducer-mirror scan head portion), is freely attachable to and 
removable from a bracket located at the outermost extremity of a triple 
jointed B-scan arm. The mirror within the scan head is operable either in 
an oscillatory (i.e. scanning) mode, or a stationary (i.e. B-scan) mode. 
Positional information is derived from each joint of the scan arm, and 
additionally from the position of the mirror itself, whether in motion or 
fixed in a B-scan position. Pulse-echo information to and from the 
transducer thereby is spatially coordinated for assembly of a composite 
image. 
In a preferred mode of utilization of the present invention, the scanning 
probe is first detached from the B-scan arm, and thus hand-held and freely 
movable as the mirror therein oscillates back and forth through a 
predetermined sector, produces real time sector scan image for display. In 
this mode, lesions and areas of interest are easly located. Next, the 
probe is attached to the articulated B-scan arm while real time imaging 
again is performed; during this stage, positional information is derived 
from the joints of the articulated scan arm. This positional information 
advantageously is displayed along with the image, and, as desired, the 
superimposed positional information and real time sector image may be 
recorded. In such fashion, the lesions or other areas of interest are 
quite accurately and repeatably positionally fixed. Finally, automatic 
sweeping of the beam (i.e. the mirror) is halted, and the probe and 
articulated arm are utilized as a conventional B-scan apparatus, 
advantageously utilizing the identically same transducer, image scale and 
intensity factors, and the like parameters as were used during the real 
time procedure. A static image is thereby generated having wide field of 
view and high line density. 
It will be appreciated that the principles of the present invention 
facilitate ease and coordinated application of real time and static B-scan 
imaging techniques, substantially maintaining and adopting the superior 
aspects of each respective system, but for the most part avoiding their 
respective disadvantages. In particular, a single transducer system is 
used for both approaches, and the transition is made from sector scan to 
B-scan without probe changing or extensive readjustment of controls. 
Additionally, positional information and hence useful hard copy 
production, is available during the intermediate real time imaging 
sequences.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 1, there is shown symbolically a multisegmented 
B-scan arm of a type well known in the art. The arm of FIG. 1 includes 
three segments 104, 105 and 106, coupled to a fixed reference (not shown 
in detail) at a top joint 101, and coupled to one another at joints 102 
and 103. A bracket 107, shown symbolically in FIG. 1, holds a transducer, 
which in turn engages the patient in a pulse-echo exchange of sonic 
energy. The articulated arm of FIG. 1 has means (typically potentiometer 
circuits) located at each joint 101, 102, and 103 for determining the 
respective angles .theta.1, 174 2, and .theta.3. Voltages from the 
potentiometers thus correspond to the positions of arms 104, 105, and 106 
with respect to each other, thereby conclusively identifying the position 
and orientation of the outermost terminus 107, for example with respect to 
the positionally stationary joint 101. It will be appreciated from the 
principles of trigonometry that the location of terminus 107 relative to 
joint 101 may be resolved into an x and y component, the x component or 
position being a function of the lengths L1, L2 and L3 of the arms 104, 
105, and 106, respectively, and the cosines of the angles .theta..sub.1, 
(.theta..sub.1 +.theta..sub.2), and (.theta..sub.1 +.theta..sub.2 
+.theta..sub.3). Likewise, the y component or location of point 107 
relative to point 101 will be similarly obtained as a function of the 
sines of the various angles. Mathematically, the locations of the x and y 
positional components of point 107 components, are stated as follows: 
P(x)=L1 cos .theta.1+L2 cos (.theta.1+.theta.2)+L3 cos 
(.theta.1+.theta.2+.theta.3) 
P(y)=L1 sin .theta.1+L2 sin (.theta.1+.theta.2)+L3 sin 
(.theta.1+.theta.2+.theta.3) 
It will also be apparent that the slope or direction of an ultrasound beam 
from the transducer at point 107, may be determined based upon the angle 
(.theta.1+.theta.2+.theta.3), the x slope being a function of the cosine 
of that angle, and the y slope being a function of the sine of that angle. 
Referring next to FIGS. 2A and 2B, there is shown a preferred scan 
mechanism for use in accordance with the principles of the present 
invention. The scan head of FIGS. 2A and 2B embodies the generals precepts 
set forth in the concurrently filed Hottinger application, and the 
particular design configuration as set forth in the concurrently filed 
Sorwick application. In the figures, the transducer 302 is positionally 
fixed within a lower portion 313 of the head, which forms a fluid chamber 
314 and which carries therein a movable mirror 303. Mirror 303 is carried 
on a shaft 307, which is interconnected with a laterally displaced shaft 
308 by means of a drive belt 306. A motor 309 either oscillates shaft 308 
back and forth, or rotates it, as preferred, and correspondingly brings 
about similar movement of the mirror 303. For utilization in a static 
B-scan mode, the mirror 303 may also be "locked" in place by exerting 
similar control of the motor 309. In a preferred embodiment, the mirror 
303 is to be "locked" in a central position for generation of a single, 
directly downwardly extending ultrasound beam. 
In the real time imaging mode, as the mirror 303 is oscillated or 
"wobbled", the transducer 302 is fired at a frequency which is 
substantially faster than the mirror 303 oscillation rate, whereby the 
emission of a sonic pulse, and reception of an echo series, occurs as an 
event at substantially a single position for the mirror 303. In other 
words, the echo signal train returning the transducer 302, as well as the 
initial firing pulse which created it, occurs substantially 
instantaneously compared to the rate of motion of the mirror 303. As noted 
in FIG. 3A, the aggregate of these separate events, resulting from motion 
of the mirror 303 through a predetermined sector, is the assembly of a 
sector shaped image from the rotational plane in the body. 
Since the mirror 303 moves under power of the motor 309, it will be 
appreciated that the motion of the mirror 303 needs to be coordinated with 
the operation of the transducer 302, i.e., with the transmission of sonic 
energy into the body and receipt of echoes from the body. Accordingly, an 
encoder 310 is shown next adjacent the motor 309, which encoder 310 serves 
the function of encoding the angular position of motor 309, and in turn 
the angular position of mirror 303. Such positional information is 
important for production of a real time image display, by interrelating 
signals to and from the transducer, with respect to one another. 
It will be appreciated that numerous commercially available and well-known 
motor and encoder schemes will be suitable for utilization in accordance 
with the principles of the present invention. For example, the motor 309 
is properly embodied either as a continuous (e.g. three phase) motor, or 
as a stepping or incremental motor. Likewise, the encoder 310 is embodied 
by a number of alternative schemes, including a magnetic or Hall effect 
switch, or a continuous optical wheel type encoder which is suitable for 
direct mounting on a shaft from motor 309. 
It is to be noted that the physical configuration of the scan head of FIGS. 
2A and 2B is well-suited for utilization in accordance with the principles 
of the present invention. That is, the upper segment 325, enclosing the 
motor 309 and the encoder 310, is of a convenient size and shape to be 
grasped in one hand of the ultrasonographer, and useful for a free-hand 
sort of real time scanning. During such scanning, the lowermost face 315 
of the lower portion 313 of the scan head, is in physical contact with the 
body of the patient, and hence is the point of exit of sonic energy from 
the scan head, and also is the point of entry of echo signals from the 
body of the patient. Likewise, the intermediate portion 305, which houses 
the belt drive mechanism 306 between motor 309 and mirror 303 via shafts 
307 and 308, is quite a convenient portion for attachment to a B-scan arm. 
FIGS. 3A and 3B show a preferred form of bracket, and associated 
attachment scheme, whereby the sort of scan head shown in FIGS. 2A and 2B, 
may be utilized at the end point 107 of a segmented scan arm such as shown 
in FIG. 1. 
Prior to consideration of FIGS. 3A and 3B, however, it is appropriate to 
note that the principles of the present invention are not limited merely 
to utilization of a configuration shown in FIGS. 2A and 2B. Indeed, 
broader aspects of the principles of the present invention are not to be 
limited to the sort of real time scan heads set forth in the previously 
described Hottinger and Sorwick applications. Instead, it is contemplated 
that the principles of the present invention will be well-served by 
utilization of any real time imaging scan head which may be conveniently 
attached at the end of a scan arm, and which employ transducer schemes 
suitable both for scanning and static applications. Not the least of these 
are a number of linear and annular array transducer systems, some of which 
utilize mirrors and/or lenses to achieve the requisite scanning effect, 
and others of which utilize compound electronic pulsing techniques to 
achieve beam steering and shaping. 
Referring, then, to FIGS. 3A and 3B, there is shown a preferred embodiment 
of the principles of the present invention, wherein a mechanical sector 
scanner such as disclosed and claimed by Sorwick is mounted at the end of 
a multisegment B-scan arm. In particular, intermediate segment 105 is 
shown joined with terminating segment 106 at a joint 103 and 111, in 
conventional fashion. End portion 107 of segment 106 carries a 
cantilevered bracket 112, which at sleeve portion 113 engages the 
intermediate segment 305 of the mechanical sector scan head. It will be 
noted that, with cantilevered section 112 engaging intermediate section 
305, the lower, outermost section 313 of the scan head is located with 
ultrasonically transmissive window 315 immediately below and in 
substantial alignment with the end segment 106 of the B-scan arm. Hence, 
as shown in FIG. 3A in frontal view, and 3B in cross-section, as the scan 
head is operating in a real time sector scan mode, the field of imaging is 
as shown in phantom at 316. When the scan head has its mirror "locked" for 
more conventional B-scan operation, pulse transmission and echo receipt 
occurs on the center line 326 of the sector image field 316 and in direct 
alignment with segment 106. 
As is also noted from FIGS. 3A and 3B, upper section 325 of the scan head 
extends outwardly from the scan arm--bracket connection, and from the 
lower, scanning portion 313 and 315, thereby to furnish a convenient 
handle for manipulation of the scanning aspects and in turn of the B-scan 
arm. A cable 311 is connected into the system imaging electronics, for 
purposes of image production and display, with the ultrasound transmission 
and echo data, which also is coupled to the system electronics via cable 
311. Positional data from the respective joints of the segmented B-scan 
arm are conveyed separately to the imaging electronics, in conventional 
fashion, through the arm itself. Thus, the scan head 305, 313, and 325 may 
as desired be detached from the engaging portion 113 of bracket 112, and 
be utilized "free-hand" for conduct of scanning investigations. 
Referring next to FIGS. 4 and 5, there is set forth in block diagrammatic 
form a signal processing network for a diagnostic imaging system suitable 
for applications in the principles of the present invention. In 
particular, FIG. 4 shows an overall block diagram for processing signal 
bearing information as well as corresponding positional information, and 
FIG. 5 shows in somewhat greater detail aspects of the timing and control 
which relate to coordination of those functions, and especially with 
respect to the positional aspects thereof. 
In FIG. 4, the mechanical sector scanner is shown symbolically by means of 
a fixed transducer 501 facing a rotatable mirror 502. The mirror is 
located on a shaft drive from the motor 503, which receives energizing 
control from a servomechanism reference and feedback control 507. An 
encoder 504, shown being eccentric to the motor shaft 503, but in a 
preferred embodiment actually being mounted on that shaft, denotes at all 
times the position of the mirror by denoting the position of the motor 
shaft 503. The encoded positional information is coupled to signal 
conditioning circuitry 506, in order to provide suitably scaled 
information to the servo control 507. Hence, there is presented a closed 
servomechanism loop between control 507 and mirror 502 whereby the motor 
oscillation speed is maintained at a desired rate within predetermined 
tolerance limitations. It will also be clear that the servo control 507, 
when suitably energized, forces the motor 503 to assume an intermediate 
"rest" or "locked" position, thereby also affixing mirror 502 in a single 
position for the B-scan mode. 
Information with respect to the angular position of mirror 502 is coupled 
from the servo reference and feedback control 507 to a timing and control 
means. Additionally, the three reference angles from the segments of the 
scan arm are coupled to the timing and control means 508. As also shown in 
the drawing, control signals from the front panel controls 519 are also 
coupled to the timing control unit. In conventional fashion, the front 
panel controls, at the discretion of the user, select frame rate, 
interlace or noninterlace options, overall sector angle, and the like 
parameters which dictate the size, granularity, and overall presentation 
of the sector being imaged. Based on these parameters as selected by the 
user, the timing and control circuit 508 energizes pulser 509 to deliver 
electrical signals to fire the transducer 501. As previously described, 
the sonic energy pulses from transducer 501 are deflected by mirror 502 
into the subject. In turn, echoes from the subject are reflected from 
mirror 502 back to the transducer 501. The impinging of these echoes on 
transducer 501 is detected by a receiver 511. 
The receiver receives a "TGC" control signal from the timing and control 
module 508. TGC, or time gain compensation, is a standard form of 
correction, arising because the amplitude of the received pulses decrease 
exponentially as a function of the depth of tissue from which the echoes 
have come. Hence there is a need for compensation or equalization to 
increase the amplitude of echoes in a given train as a function of elapsed 
time, thereby accounting for the loss which actually occurs. 
The corrected signals from the receiver 511 are coupled to a logarithmic 
amplifier 512, and thence to an analog to digital converter 514. The 
amplifier 512 compresses the signal into a range which is appropriate for 
the gray scale being employed by the system. One method of display, known 
as the "A mode", involves direct coupling of these signals to the display 
513. Typically also, such display mode involves a simultaneous display of 
the TGC signal. 
The analog to digital converter 514 accomplishes suitable A to D 
conversion, typically utilizing a 5 or 6 bit code (depending upon the gray 
scale being employed), and couples these words preferably in a bit 
parallel fashion, to a memory 515. In essence, the digital image memory 
515 stores a composite image by appropriately locating the actual data 
from converter 514 in correspondence to the part of the body of the 
subject which is being displayed. Hence, the digital image memory 515 
receives coordination and control from the timing and control unit, 
whereby each word from converter 514 is conveyed to the proper location in 
the memory 515. As noted hereinbefore, the servo control loop and the 
segmented B-scan arm yield encoded information representing appropriate 
positional information. Therefore, this information may be utilized to 
place a digital word from A/D converter 514 in a position in 
correspondence direction and/or position of a beam directed from the 
mirror 502 into the subject. Likewise, the position and/or direction of 
each individual word along that beam will be a function of the timing of 
the received pulse at 511, with respect to its generation from the 
transducer 501. Such timing is conducted on an ongoing basis at module 
508. Hence, the digital image memory appropriately addresses and stores 
each word from the coder 504. 
The digital information in the memory 515, is coupled for display to and 
through a postprocessing unit 516, thence to a digital to analog converter 
517, and to a display 518. The postprocessing function at 516, under 
control of program selection controls, enables allocation and variation of 
gray scales in accordance with predetermined transfer curves, in a fashion 
known in the art. Such operation may utilize, as desired, a large variety 
of echo amplitude level versus display brightness level allocations, in 
order to enhance and/or suppress certain desired echoes, or in order to 
emphasize or de-emphasize particular aspects of the display. 
Referring next to FIG. 5, there is shown further detail with respect to 
some of the timing and control functions of the system of FIG. 4. 
Considering first the scan arm, angular positional information from the 
respective joints is coupled to gain and offset amplifier 521, thereby 
appropriately scaling the voltages representing the respective angles from 
the scan arm. Next, these three angular voltage quantities are coupled to 
an x-y converter, which utilizes the previously disclosed trigonometric 
relationships to convert the three angular quantities, .theta.1, .theta.2, 
and .theta.3, to x and y positional and directional information at the 
terminus of the multisegmented scan arm. Thus, four separate values, 
including x and y position, and x and y direction, are coupled to analog 
processing circuitry 523. In essence, the unit 523 is an analog 
multiplexer, which converts the four parallel quantities into a serial 
format, and as appropriate provides offsets to scale the quantities in 
accordance with optional "zoom" factors. Next, serialized analog data from 
the multiplexer 523 is coupled to an analog multiplier 524, where field of 
view scaling data is combined with the x and y slope and x and y position 
values. The data from the scaler 524, still in analog form, is coupled to 
a sample and hold unit 525, and thereupon are applied to an analog to 
digital converter 526 which reads each applied voltage from the sample and 
hold unit 525, and produces digital words representing the instantaneous x 
position, y position, x slope, and y slope values. These position values 
represent the position of the transducer face (i.e. the contact point with 
the body of the patient) when the scan head is coupled to the B-scan arm, 
as shown in FIGS. 3A and 3B. 
When the user selects a scan mode which utilizes positional information 
based on the connection of the scan head to the B-scan arm, as indicated 
by the function select block 519, the scan arm positional information is 
coupled from the converter 526 to appropriate accumulators 532. In turn, 
the information is utilized to guide pulse echo data into the digital 
image memory 515, as described hereinbefore. 
When real time scanning information is to be utilized, either alone or in 
conjunction with the scan arm positional information, it is necessary to 
know of the position of the mirror, either in place in a central location, 
or as it oscillates back and forth through a predetermined sector, and to 
coordinate this operation with the generation and receipt of sonic energy. 
Accordingly, the mirror angular position information is coupled from servo 
control 507 to one input of an angle comparator 529. The other input of 
angle comparator 529 receives data from a transmit angle reference memory 
528, which in turn is controlled as a function of the user controls via 
function selection block 519. 
The significance and operation of the reference memory 528 may be 
appreciated as follows. Sector scan timing for the embodiments disclosed 
differs from that for other modes in that transmissions are designed to 
occur when the mirror reaches certain angles, rather than at set time 
intervals. In turn, the spacing and location of these angles through a 
sector are variables to be chosen in accordance with the needs and desires 
of the user. Utilization of a transmit angle reference memory 528 allows 
for the enabling of sonic transmission and echo reception between 
transducer and patient in accordance with parameters established by the 
user via reference 528. 
Each time the angle comparator 529 denotes identity between the present 
mirror angle from the servo control 507, and a reference angle from 528, 
the timing and control generator 508 is notified to energize a 
transmit-receive sequence as described previously. It is noted that 
utilization of the scan head in simple B-scan mode will be promoted by 
utilization of a single, unvarying reference angle from memory 528. 
The reference angle from memory 528, together with actual positional 
information from the encoder, is processed at converter 530 to yield an 
x-y positional version of the angular information represented by the 
actual position of the mirror. This information in suitable digital form 
is coupled to a buffer or gate 531, which under the control of the 
function selection controls 519, is coupled to accumulators 532 and in 
turn to the digital image memory 515. 
In summary, it will be seen that the principles of the present invention 
advantageously utilize well-known precepts relating to real time sector 
scanning and to static B-scan imaging, and combine the two to yield 
substantially advantageous operation. Preferred and illustrative 
embodiments of the principles of the present invention have been 
presented. It will be understood, however, that those of ordinary skill in 
the art will devise numerous alternative embodiments which nevertheless 
fall within the spirit or the scope of the principles of the present 
invention.