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BACKGROUND OF THE INVENTION
This invention relates to scan conversion apparatus and a method for
converting data sensed in sector format to raster format for display. More
particularly, the invention relates to a scan converter and method for a
single-sector or multi-sector ultrasonic scanner for sampling and storing
the received echo data in a raster type geometry and processing the
read-out data for display on a cathode ray tube (television monitor).
Conventional analog scan converters employ delicate electron beam storage
tubes which are both expensive and difficult to maintain. Many previous
attempts to implement a digital scan converter have either been very
expensive or have introduced objectionable errors in the display resulting
in degraded image quality. The basic reason for the poor image quality of
such previous implementations is that the locations of the input data
samples have not corresponded to those of the output data in a manner that
permits a simple interpolation to obtain the correct output data. That is,
the physical locations of the input data are not related to those of the
output data of the scan converter in a simple way.
The single-sector scanner ultrasonic imager is a real time imaging system
having a linear transducer array as depicted in FIG. 1. To make a sector
scan the elemental transducers are excited in linear time sequence to
generate angulated acoustic beams at many different angles relative to the
normal to the array at the midpoint. Echoes returning from targets in the
direction of the transmitted acoustic beam arrive at the transducer
elements at different times necessitating relative delaying of the
received echo electrical signals by different amounts to focus the
received echoes, and the delayed echo signals are summed before being fed
to the scan converter. It is common in prior art single-sector scanners to
rotate the angulated transmitted beam by equal scan angle increments, and
in the scan converter to sample the focused echo signals at equal time
intervals so that the data samples are along arcs concentric to the origin
point. The cathode ray tube, on the other hand, is a rectangular grid type
display. The function of the scan converter is therefore relatively
complex and a picture of uneven quality often results, worsened by the
tendency of the eye to focus on uneven areas. A single-sector steered beam
cardiac scanner with a TV monitor display is described by Thurstone and
von Ramm in "A New Ultrasounc Imaging Technique Employing Two-Dimensional
Electronic Beam Steering", Acoustical Holography, Vol. 5, 1974, Plenum
Press, New York, pp. 249-259.
The present invention is applicable also to the multi-sector or "walking
beam" ultrasonic imaging system having a longer linear transducer array
for producing a set of sector scans with the points of the sequential
sector scans displaced longitudinally along the array. This real time
systrem capable of imaging randomly oriented targets and producing
improved images is disclosed and claimed in U.S. Pat. No. 4,159,462, H. A.
F. Rocha and C. E. Thomas, entitled "Ultrasonic Multi-Sector Scanner". A
scan converter operative to store and read out only the largest echo
amplitude at image points in overlapping areas of the sector scans is
disclosed and claimed in allowed copending application Ser. No. 825,529
filed on Aug. 18, 1977 by E. T. Lynk entitled "Peak Detecting Digital Scan
Converter" now U.S. Pat. No. 4,167,753. Both applications are assigned to
the same assignee as this invention.
SUMMARY OF THE INVENTION
In order to provide correct output echo amplitude data for display in
raster scan format, the input sector geometry is somewhat changed. The
scan angles of the acoustic scan lines are chosen so that they intersect a
lateral line at equal increments, i.e., the scan angles have equal tangent
increments. Along each of these lines the focused echo signal is sampled
and converted to digital format at a rate that varies with scan angle so
that corresponding samples are arranged in parallel rows or raster lines,
i.e., the sampling rate varies inversely with the cosine of the angle of
the scan line. Digitized echo data is written scan line by scan line into
a digital memory having a matrix of storage cell locations in rows and
columns, but is read out of memory raster line by raster line. In the
preferred embodiment, the data is written into adjacent columns of a
random access memory whereby data for a raster line is stored in a row of
memory, and then is read out row by row in sequence. To convert back to
sector geometry, the digital echo samples are read out into a shift
register or other buffer storage, and are clocked out of the shift
register at a variable rate dependent on the width of the sector at the
raster line being read out and delayed in time dependent on the location
of the edge of the sector from the side of the television screen or other
reference line. These data are passed through a digital-to-analog
converter and a low pass filter to produce the video output signal which
is fed to a cathode ray tube to control the electron beam intensity.
Memory is required only in the amount needed to store the input digital
echo data, as compared to prior art techniques in which there is a memory
location for every image pixel. For instance, a 32 K memory is sufficient
for a conventional TV raster of 400 lines with 300 picture elements each
or 120 K pixels. For improved efficiency and real time imaging, the memory
is operated in burst mode with alternate reading in and reading out, and
the memory is divided into segments to facilitate writing of echo data in
parallel into the segments and reading out in parallel into a like number
of output buffer shift registers. Some sampling points can be skipped to
avoid high rates of clocking out data in parallel from the shift
registers. These data will be "filled in" by the action of the low pass
filter at the output.
The digital scan conversion apparatus and method of converting ultrasound
echo signals for raster scan display are described with regard to real
time single-sector and multisector cardiology and laminography imaging
systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sketch illustrating operation of a singlesector steered beam
ultrasonic scanner;
FIG. 2 is an enlarged view of the acoustic scan lines of the sector scanner
on which data sample points, located on lateral raster lines designated by
letters, are shown as large dots;
FIG. 3 is a schematic plan view of a segment of the scan converter random
access memory showing the pattern of stored echo amplitude data;
FIG. 4 is a simplified block diagram of the electronics for processing echo
data read out of memory;
FIG. 5 is a system block diagram of the preferred embodiment of the
complete scan converter;
FIG. 6 illustrates the input sector geometry as in FIG. 2 with an addition
to facilitate explanation of the sequence of writing into and reading echo
amplitude data out of the four-segment random access memory in FIG. 5; and
FIG. 7 is a functional block diagram of a single-sector scanner imaging
system incorporating the scan converter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To facilitate conversion of ultrasound echo data derived by single sector
scanning for real time display on a conventional television monitor, the
format of the sector is somewhat changed. The scan angles are chosen so
that the acoustic scan lines intersect a lateral line at equal increments,
and along each of these scan lines a focused received echo signal is
sampled at a rate that varies with the scan angle. Corresponding data
samples are taken at the same value of the z-axis coordinate, whereby the
samples are arranged in a number of rows parallel to one another and the
linear transducer array. Successively sampled scan lines are buffered and
stored in a digital memory in adjacent columns of a row-column oriented
memory. Thus, memory is required only in the amount needed to store the
input data, and the only storage cell locations that are unused are those
intentionally skipped where the input data near the sector origin is too
dense to be processed at reasonable rates and viewed as individual pixels.
For readout, all data samples in a row and corresponding to a single value
of the z-axis coordinate are read into a shift register buffer, which is
then clocked out at a rate corresponding to the sector width at that value
of z. These data are fed to an analog-to-digital converter, whose output
is low pass filtered and presented to the display. Modifications of the
foregoing preferred embodiment of the scan converter will be discussed
subsequently.
The single-sector steered beam ultrasonic scanner in FIG. 1 has a linear
transducer array 10 comprised of equally spaced elementary transducers 11
which are energized by excitation pulses 12 in a linear time sequence to
form an ultrasound beam 13 and direct the beam in a preselected azimuth
direction to transmit a pulse of ultrasound. In order to steer the beam
electronically to an angle .theta. from the normal to the array
longitudinal axis, a time delay increment T.sub.i .apprch.(i-1)d sin
.theta./c, where c is the acoustic velocity, is added successively to each
ith element signal as one moves down the array from one end (i=1) to the
other (i=N) to exactly compensate for the propagation path time delay
differences that exist under plane wave (Fraunhofer) conditions. By
progressively changing the time delay between successive excitation
pulses, the angle .theta. at one side of the normal is changed by
increments. To form and steer the beam at the other side of the normal,
the timing of excitation pulses 12 is reversed so that the bottom
transducer in FIG. 1 is energized first and the top transducer is
energized last. The total sector scan angle is approximately 60.degree. to
90.degree.. Echoes returning from targets 14 in the direction of the
transmitted beam arrive at transducer elements 11 at different times
necessitating relative delaying of the received echo electrical signals by
different amounts so that all the signals from a given point target are
summed simultaneously by all elements of the array. The time delays of the
transducer element echo signals are the same as during the transmission
operation, to compensate for acoustic path propagation delay differences.
The linear transducer array is also known as a phased array. For further
information refer to "Electronic Scanning of Focused Arrays" by V. G.
Welsby, Journal of Sound Vibration (1968), Vol. 8, No. 3, pps. 390-394.
For real time imaging at a typical frame rate of 30 frames per second, the
system also requires a television monitor on which the total image is
built up line by line from the scan converter memory. From the foregoing
description, it is seen that an electronically controlled, steered
ultrasound beam is generated that is capable of oscillating or rotating
motion about the sector origin at the midpoint of the linear transducer
array. For each transmitted steered ultrasound beam, there is a
corresponding focused received echo electrical signal which is fed to the
digital scan converter and is data for the corresponding image line. A
single sector image depicting a tomographic slice of the insonified object
region is displayed in real time on the screen of the television monitor.
This is further explained in detail later with regard to FIG. 7. The
single-sector scanner has both industrial and medical applications, and is
especially advantageous in medical diagnostics for cardiology and
laminography. To image a portion of a heart, linear transducer array 10 is
manually held against the patient's chest wall while observing the image
on the cathode ray tube, and its position is changed until the desired
portion of the heart is imaged. A frame rate of at least 30 frames per
second is needed to prevent blurring of the image due to heart motion.
Assuming that a maximum image depth of 20 centimeters is required or a
round trip of 40 centimeters, and that the velocity of sound in tissue is
150,000 centimeters per second, the rate of generating steered acoustic
beams is limited to about 3,000 per second. For a good image there should
be between 200 and 300 scan lines on the television screen, and 300 lines
at 3,000 per second translates to a frame rate of 10 frames per second. To
obtain 30 frames per second, then, there should be three focused received
echo signals per transmitted acoustic beam. This is accomplished by
forming the transmitted acoustic beam using fewer transducer elements than
are used to receive the echoes. The transmitting beam lobe is three times
as wide, in the direction of the longitudinal axis of the transducer
array, as the "receiving beams" or focused echoes; that is, the "receiving
beams" are steered or focused within the lobe of the transmitting beam.
The result is that three lines of acoustic echo data are obtained on each
transmit-receive cycle. In general, there may be n echo signals per
transmitted beam, and the echo signals as well as the transmitted beam are
steered so that the angles have equal tangent increments.
FIG. 2 shows the format of the input sector geometry according to the
invention by which the physical locations of the input echo data to the
scan converter are related to those of the output data of the scan
converter in a simple way compatible with delay along a conventional
television raster. Acoustic scan lines are indicated by numbers 1-11, and
echo data samples along the scan lines are illlustrated as solid dots and
indicated by letters a-r. The scan angles .theta. on either side of the
normal through origin point 0 are chosen to have equal tangent increments,
and the acoustic scan lines intersect a lateral line perpendicular to the
normal at equal increments. Along each of these scan lines, the echo
amplitude signal is sampled at a rate which varies inversely with the
cosine of the angle of the scan line. With the x and z coordinates defined
as in FIG. 2, the echo signal is sampled at a rate that varies with the
scan angle so that corresponding samples are taken at the same value of
the z-axis coordinate. To emphasis that the echo data samples are arranged
in rows or raster lines parallel to one another and the linear transducer
array, data samples in three of the raster lines are circled. Within each
raster line the echo data samples are equally spaced, and in the z
direction the raster lines are also equally spaced. For small values of z
near the sector origin point 0, where the data samples come very close
together, it is permissible to skip over some of the samples provided the
actual sample rate stays above the Nyquist limit.
After being sampled and converted to digital form by an analog-to-digital
converter, the echo data samples are buffered and stored in a digital
memory having a matrix of storage cell locations in columns and rows. FIG.
3 shows a random access memory 15 preferably made with MOS (metal oxide
semiconductor) field effect transistors or bipolar transistors, and in
such memories storage cell locations are accessed for the writing in and
reading out of echo data by the coincidence of signals on X select lines
16 and Y select lines 17. Digitized echo data samples along successive
scan lines 1-11 are stored in adjacent columns of random access memory 15,
and a typical pattern of stored data is depicted by solid dots. The
sequence of accessing memory columns for the storage of echo data follows
the sequence of generating transmitted acoustic beams. For example, one
sequence is that transmitted beam 1 is produced and the time delays
progressively changed to rotate the transmitted beam in the clockwise
direction and produce beams 2-6, then transmitted beam 11 at the other
side of the normal is generated and the time delays progressively changed
to rotate the beam in a counterclockwise direction and produce beams 10-7.
The density of stored echo data along raster lines a-r increases with
distance from the sector origin point, and vacant storage cell locations
are intentionally skipped to keep readout of the output data from the scan
converter within reasonable rates. With this exception, the entire memory
is available for the storage of echo data, as contrasted to prior art scan
converter memories wherein data is stored in a sector pattern so that a
large percentage of storage cell locations are never used.
Whereas input echo data is written into the digital memory column by
column, stored data is read out of memory row by row in sequence. Further
processing of the read-out memory data, however, is required to convert
the rectangular grid memory format to sector image format. The
post-processing electronics is illustrated in simplified form in FIG. 4.
Assuming that readout from memory begins at raster line or row r, the data
samples are read into a buffer storage device such as an n-stage shift
register 20. Data is clocked out of shift register 20 to an output
digital-to-analog converter 21 at a variable rate corresponding to the
width of the sector at that raster line or value of z (see FIG. 2). In
addition to varying the shift register clocks, so that the sector geometry
is obtained, it is also necessary for the control circuitry 22 to delay
the start of the clock pulses by varying amounts on each raster line. As
readout from memory proceeds from raster line r toward raster line a, the
frequency of clock pulses increases and the time delay also increases,
which is dependent on the location of the edge of the sector at the row or
raster line being read out from a reference line such as the edge of the
television screen. The clock rate can reach a maximum and can be
approximately the same at raster lines near the sector origin, made
possible by skipping input sampling points. The stream of echo data is
presented to a low pass filter 22 before being fed out as a video signal
or Z control for varying the electron beam intensity of the cathode ray
tube. The purpose of filtering is to smooth out the step staircase
function at the DAC output. Echo data for a raster line is clocked out of
shift register 20 at TV rates, and in a conventional television monitor
the time for the electron beam to scan across a single line and retrace
itself is 63 microseconds.
The preferred embodiment in FIG. 5 of the complete digital scan converter
can now be explained. As was mentioned, for every transmitted acoustic
beam there are three focused received echo electrical signals from which
three image scan lines on the television screen can be derived. These
ultrasound echo signals are the outputs of the three summing amplifiers in
FIG. 7 which are supplied to the scan converter. Input analog-to-digital
converters 25a-25c sample the respective input amplitude signals under
control of a frequency synthesizer 26 at a rate which varies inversely
with the cosine of the angle of the scan line. It is permissible for
frequency synthesizer 26 to operate on the basis of multiplying a base
frequency by a rational fraction, because some frequency error can be
tolerated. This means that the data sampling points in FIG. 2 along the
several scan lines may not be exactly at the same z-coordinate. The
digitized data samples, having 8 bits each in the example being given, are
fed to a first-in-first-out input data buffer such shift registers
27a-27c. Data samples stored temporarily in the three shift registers are
clocked out successively by a buffer control 28 into a common memory input
bus 29.
The random access memory can be made from the Type 2102 Solid State Memory
Chip available commercially from a number of manufacturers including Intel
Corp. and National Semiconductor Corp. This is a 32 K.times.8-bit memory
divided into four 8 K.times.8 bit memory banks or segments 30A-30D, each
with a matrix of 45.times.180 storage cell locations. The cycle time for
this memory is 450 microseconds, and to achieve the object of real time
imaging four echo data samples, one per segment, are written into memory
simultaneously and four data samples are read out simultaneously. Improved
operation is attained by making one-half of the memory cycles available
for read in purposes and one-half for readout which occurs in apparently
simultaneous manner. If the sector is displayed during 30 microseconds out
of a possible 50 microseconds, there will be ample time for both functions
since each scanning line including retrace is 63 microseconds. Thus, a
burst mode in which writing occurs for 30 microseconds and reading occurs
for 30 microseconds is appropriate.
To simultaneously write four consecutively digitized echo data signals from
a scan line into the four memory segments 30A-30D, input bus 29 is
connected directly to segment 30A, through one unit delay element 31 to
segment 30B, through two such delay elements to segment 30C, and through
three of the delay elements to segment 30D. In each group of four data
samples, the first three samples are delayed by variable amounts and the
last has a zero delay so that the four samples can be written into the
accessed storage cell locations at the same time. During the available 30
microseconds for read in as many data samples as can be processed during
this time are entered into the four memory banks. Whenever data for a
complete scan line is emptied out of one of shift registers 27a-27c, data
is clocked out of the next input shift register in sequence. The sequence
of storage cell location addresses that controls where each sampling point
is stored, as well as the readout sequence, are stored in memory address
sequence read-only memories 32. ROMs 32, buffer control 28, and frequency
synthesizer 26 are controlled by a control and timing unit 33 which can be
part of the master digital controller in FIG. 7. The memory addresses read
out of ROMs 32 is 56-bit data, 14 bits for each memory segment.
Stored echo amplitude data is read out simultaneously from memory segments
30A-30D into four separate shift registers 34A-34D, one per memory
segment. These can be 64.times.8 bit shift registers. To clock out data
samples temporarily stored in the shift registers at a rate corresponding
to the width of the sector at the raster line being read out, an output
frequency synthesizer or variable clock 35 is provided for generating
clock pulses which are gated at the proper time to shift registers 34A-34D
by counter and gate circuitry 36. The clock rate at raster line r is, for
instance, 6 MHz and at line h is 12 MHz. The counters delay the start of
the clock pulses by varying amounts on each raster line, dependent on the
distance from the edge of the screen to the beginning of the raster line
being read out, and are operative to count down and open a gate at a
predetermined count to let the clock pulses through and then close the
gate at another point as the count continues. The counter is reset, ready
for the next cycle of operation. The parameters controlling the frequency
synthesizer and the counters and gates are loaded from output frequency
synthesizer and counter read-only memories 37 that in turn are controlled
by control and timing unit 33. Digital data samples read out in parallel
from the four shift registers are presented to a multiplexer 38, and at
the output the data samples are in serial groups of four containing one
sample from each shift register. After the data samples are passed through
digital-to-analog converter 39 and low pass filter 40, the resulting video
signal is fed to the cathode ray tube.
In order to realize the method of operation just discussed as to FIG. 5,
the pattern of storing echo data samples in the memory segments or banks
is shown in FIG. 6. Letters A-D adjacent the scan lines and the sampling
points, indicated by solid dots, designate that the data samples are
stored in memory segments 30A-30B. Looking at raster line j, it is seen
that the sequence across the row is A, B, C, D, A, etc. This is the proper
sequence for reading out stored data in parallel from memory segments
30A-30D. To get this arrangement of stored data, it is necessary to
precess the memory addressing of the memory segments from one scan line to
the next. Along scan line 1, data samples for raster lines A-D are stored
in the order D, C, B, A; along raster line 2 the order is A, D, C, B; and
so on, as directed by buffer control 28. Another aspect of alternately
reading into memory at 30 microseconds intervals, or one-half of the
memory cycle, is as follows. Less than one full frame of image pixels on
the television screen is changed at any one time, and therefore there is a
ripple effect from one frame to the next, resulting in improved picture
quality. Stored echo data is read out of a memory row at a time determined
by the memory address sequencing, and whatever data is there is read out
whether it be newly updated data or "old" data.
FIG. 7 is a system block diagram of the single sector scanner ultrasonic
imager incorporating the digital scan converter according to the
invention. The linear transducer array is illustrated with only three
transducer elements 43a-43c, but in practice the array has a larger number
of transducer elements. The three transmitting and receiving channels
44a-44c are each comprised by level and timing control circuitry 45 under
the control of master digital controller 46 for determining the level and
timing of a transmit pulse generated by transmit pulser 47 and applied to
one of the transducer elements. The receiving channel for processing the
received echo electrical signal is comprised of a preamplifier 48 having a
limiter to protect the sensitive preamplifier inputs from the high
transmitting voltage, and a compression amplifier 49 to reduce the larger
dynamic acoustic range to the smaller range a cathode ray tube display
device can handle. The amplified echo signal is next fed in parallel to
three digitally selected analog delay circuits 50, 50' and 50" having an
associated delay select switch matrix 51, 51' and 51" which, under the
control of digital controller 46, selects the delay element or elements to
focus the echo signal in the three delay channels. For each transmitted
acoustic beam, it is recalled, there are three different focused echo
signals within the lobe or angle of the transmitted beam. The other two
receiving channels are identical except for the values of the time delays
employed. Digital controller 46 can take various forms and can be a
hard-wired logic circuit, but is preferably a properly programmed
minicomputer or microcomputer. In operation, transducer excitation pulses
are generated by the three transmitting channels in time sequence to steer
the generated ultrasound beam and control the scan angle. The received
echo signals are time delayed by different preselected amounts in the
three receiving channels, and in the three delay channels within each of
the receiving channels. The delayed echo signals from the three analog
delay circuits 50, one per receiving channel, are fed to a summing
amplifier 52; and the delayed echo signals from the three delay circuits
50' are summed by summing amplifier 52', and those from delay circuits 50"
are summed by summing amplifier 52". The three focused ultrasound echo
signals at the summing amplifier outputs are now processed through digital
scan converter 53 to convert the sector scan format to raster scan format
as here described. The scan converter also controls sweep drivers 54 and
the generated X and Y deflection signals for cathode ray tube 55 on which
is displayed, in real time, the single sector image. The order of
generating transmitted acoustic beams is fixed and is preset by digital
controller 46, which supplies coordinating information to the scan
converter.
The components of the digital scan converter can be standard integrated
circuits or conventional circuitry as is presently known in the art. With
appropriate modifications that will be apparent, the scan conversion
apparatus and method of converting ultrasound echo signals are also
applicable to the multi-sector scanner ultrasonic imaging system described
in Rocha and Thomas application Ser. No. 825,528. This application may be
referred to for additional information on single-sector scanners and
specific transmitting and receiving channel circuitry that can be used to
practice the invention. Also, the digital echo output data can be
transferred to a storage medium like floppy disks for permanent storage
for display at a later time.
In summary, the method of scan conversion comprises generating input
digital echo data representing received echo amplitudes at sampling points
along scan lines angulated at angles on either side of the normal having
substantially equal tangent increments, the sampling points on each scan
line being taken at the rate which varies inversely with the cosine of the
scan angle whereby the sampling points are along lateral raster lines
perpendicular to the normal. Digital echo data is written scan line by
scan line into a memory having a matrix of storage cell locations in
columns and rows, but the stored data is read out of memory raster line by
raster line. The read-out digital echo data is then processed to generate
output data at a rate dependent upon the width of the sector at the raster
line and delayed in time dependent on the location of the edge of the
sector from a reference line. Memory is required only in the amount needed
to store the input data. In the preferred embodiment of FIG. 5, a 32
K.times.8 bit memory provides a display covering approximately 400 lines
of 300 picture elements each (120 K).
While the invention has been particularly shown and described with
reference to several preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and details may
be made therein without departing from the spirit and scope of the
invention.
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