- Email: [email protected]
Compound scanning with a phased array
ULTRASONIC IMAGING 4, 93-107
(1982)
COMPOUND SCANNING WITH A PHASED ARRAY David
P. Shattuck
Department
Compound scans have been produced in imaging, has a large acquisition of echoes the insonifying beam images. These gains and flexibility of a configuration of the scans are presented. Key words:
Abdominal focussing; ultrasound.
and Olaf
T. von Ramm
of Biomedical Engineering Duke University Durham, NC 27706
made with
a dynamically focussed phased array system intended for abdominal The scanner, The compounding improves the field of view. from specular targets by changing the orientation of and also reduces the speckle noise in grey scale are achieved while maintaining the high resolution computer controlled phased array sector scanner. The compound scanner is described, and
time.
imaging; arrays; compound electronic beam steering;
scanning; dynamic real time; speckle;
INTRODUCTION Real time ultrasound sector scanners have shown clinical utility and are finding widespread applications in abdominal, obstetrical, and cardiac diagnosis. These imaging systems combine relatively high frame rates with small hand held transducers which can be manipulated freely to obtain the anatomical views of interest. However, the sector format restricts the overall field of view and particularly limits the acquisition of targets which are close to the transducer. This restricted field of view is less a problem in cardiac imaging than in abdominal applications where targets of interest may be quite close to the skin surface. The small overall field of view only permits the simultaneous display of a small fraction of the abdominal tomographic anatomy thereby making the identification, location and determination of size of various organs more difficult. Additionally, these sector scan imaging systems suffer from reduced target acquisition due to the specular nature of many anatomical features. These systems produce "simple scans" in which targets are only insonified from one direction. A new real time compound imaging system with a field of view significantly larger than can be achieved with simple sector scanners has been developed for abdominal applications. The design, performance specifications and preliminary results are presented. This system is based on phased array principles to steer the ultrasound beam through the target volume and utilizes transmit focussing and dynamic receive focussing to obtain high resolution throughout the entire field of view. Unlike simple sector scanners this system produces compound images thereby increasing target acquisition and detectability. It is well known that spatial compounding improves the visualization of specular targets and consequently a more continuous delineation of anatomical structures is obtained. Indeed, the compounding process increases the apparent image resolution by reducing the speckle contrast. This speckle appears as 0161-7346/82/020093-15$02.00/O 93
Copyright 0 I982 by Academic Press, Inc. All rights of reproduction in an.v form reserved.
SHATTUCK AND VON RAMM
mottle or grain-like structure superimposed on the target information and can obscure the fine detail within an organ complex. Since these compound images are formed in real time, motion blurring typical in mechanical arm contact B-mode scanners is avoided.
METHODS The new compound imaging system was constructed as an adjunct to the Duke University phased array scanner [l]. In its present configuration, the compound scanner has 64 transmit and 16 receive channels. The entire system is under computer control and employs multiple interlaced transmit foci and dynamic focussing in receive. In traditional clinical cardiac or abdominal applications, linear arrays consisting of 32 elements operating in the 2 to 3 MHz range are employed. A typical clinical transducer operating at 2.5 MHz has a lateral dimension of 20 mm and measures 13 mm in the elevation dimension. The azimuthal resolution achievable with such transducers is about 2 mm at a range of 10 cm. The range resolution is 2 mm throughout the field of view, in water. The elevation resolution, that is, the thickness is approximately 5 mm at of the tomographic slice, a range of 10 cm. Sector scan images are produced at rates synchronous with standard television frame rates. In standard television, each image frame is comprised of two fields. Individual B-mode lines are interlaced in these two fields to produce acoustic frames consisting of 124 individual B-mode lines (with an 18 cm maximum range) at the rate of 30 per second. More B-mode lines may be written in an acoustic frame having a smaller maximum range. The transducer of the compound scanner described in this paper has 4 Each adjacent sections of 32 elements, or 128 total array elements. section can be used to produce an independent sector scan. In this way 4 sectors are written, and because of the large size of the sectors with respect to the transducer, these scans overlap. In overlapped areas the scans are compound. The area of overlap depends upon the range of the scan and the angular extent of each sector. Figure 1 illustrates the difference between normal simple sector scans and the new compound scans. Figure la shows a scale drawing of conventional simple sector scan, and figure lb shows the appearance of similarly scaled 4 sector compound scan. The figures are shown for a with a range of 15 cm, a sector angle of 76O, and an aperture of 2 cm 32 elements, or 8 cm for the entire 128 element array. The density of shading in figure lb indicates the locations of compounding, or overlap, of 2, 3, or 4 sectors. In these areas of overlap the line density will increase by a factor of 2, 3, or 4 respectively.
a a scan for the
The system organization is presented diagrammatically in figure 2. The POP 11/20 computer acts as a controller for the entire system. At the beginning of each scan frame, one of the four independent apertures within the large array is selected under software control. The computer then sends signals to trigger the firing of the transmitters. Received echo signals from scattering sites within the tissues are amplified, logarithmically processed, and then delayed by an amount appropriate for the depth and angle being interrogated. All transmit timing and receiver delays are controlled by the computer and the sequence of operation during the formation of each sector scan is identical to that required for a simple sector scan. The summed and detected signals are then displayed on This camera an HP 1311A Monitor which is viewed by a television camera.
94
COMPOUND SCANNING WITH PHASED ARRAY
(a) Fig.
(b)
1 The two drawings are to the same scale, and show (a) a simple The compound scan is sector scan, and (b) the compound scan. stippled to show the areas where 2, 3 or 4 scans overlap to form a compound image.
performs the scan conversion process, transforming the sector scan to a standard television format, which permits video tape recording of images for later playback and analysis. Once echoes from the maximum range have been received, the transmitters are reinitialized under computer control and another acoustic pulse is transmitted in a new azimuthal direction. Under computer control the displayed B-mode line orientation is modified to coincide with the direction of insonification. This process continues until all lines in an individual sector are written. This is limited to l/30 second, to coincide with video frame rates. when this is complete, a new aperture is selected, and another full sector scan is generated.
PDP
II/20
COMPUTFR
TRANSMITTER
MULTIPLEXER
MULTIPLEXER
Fig.
2
System
diagram
for
95
the
compound
scanner.
SHATTUCK AND VON RAMM
Since the sequence of independent apertures is under software control, the next aperture selected need not be adjacent to the first. Since this new sector has a different origin, the display origin is shifted by an appropriate amount under computer control. In this way the same target viewed by different apertures is kept in registration. In the ompound scanner the transducer and preamplifiers are contained in a hand held case. The preamplifier includes the multiplexing circuitry (4 to l), so that only 16 system receive and delay line circuits are required in the formation of a compound scan. This limits the production of scans to one sector at a time. Another major component of the compound scanner is the demultiplexing circuit for the transmitters. Pulses are sent by the computer to trigger the transmitters. Thus the computer determines the timing of the firing of the elements in the array. The demultiplexing circuit was designed so that each of the four sets of transmitter circuits in the compound scanner could be triggered by the computer signals that would normally be used to generate a single sector scan. These signals are sent to each of the sets of transmitters depending on the 2 bit 'sector number' signal, which is also sent to the multiplexing preamplifier. It is used to choose which aperture will be utilized to produce a sector scan at a given time. The sequence of scans is under the control of the operator, who can choose to iterate through each of the apertures, or make scans with any of the four in any combination. For example, if only the two center sectors were desired, only these would be utilized and the other sectors would not be generated. With the flexibility afforded by the computer control, scans with different ranges, sector angles, and resulting areas of overlap are easily obtained. The origin of each sector is determined by the size of the transducer that is used. This information is stored in the computer memory along with all of the transmit and receive timing data as part of the system initialization process. The compound transducer assembly has four 32 element apertures, each of which is 20 mm long, and 11 mm wide. It operates at approximately a 2 MHz center frequency. There are 64 receive channels in the transducer-preamplifier assembly which are multiplexed down to 16 channels after the preamplifier. Thus, 16 channels are used for each of the apertures in receive, using every other element of the 32 available array elements. Sixteen channels per aperture are used for transmitting, utilizing the 16 center elements of each aperture. The system produces scans at a rate of 30 sectors per second. As a result, if 4 sectors are being displayed in the compound scan, the entire image is written 7.5 times per second. The images are recorded on a video tape recorder. Since the sectors are produced synchronously with the video system, 4 video frames comprise a complete compound image. The B-mode lines in the compound scan can be written in a number of ways, due to the flexibility of the computer control. If, for example, it is decided that the areas of the scan where different sectors do not overlap are not needed, the lines in those areas can be removed and written in the overlap area to improve the line density. In this way a variety of scan formats is possible. RESULTS view
The compound scans provide over that of a simple scan.
a significant A compound
96
increase scan with
in the field of a 76O scan angle
COMPOUNDSCANNING WITH PHASED ARRAY
Fig.
3
Transducer
case and 128 element
transducer
for
compound
scanner.
would be 15.4 cm wide at a range of 6 cm, compared with a width of only 9.4 cm for a simple scan with the same angle. These gains in field of view are more significant near the transducer. Figure 3 shows a photograph of the transducer mounted on the the multiplexing preamplifiers. prototype transducer case, which contains The transducer face is 80.6 mm long and 11 mm wide. The case itself is The wires which enter approximately 16 cm long, 6 cm deep and 12 cm high. it include 64 transmit lines, 16 receive lines, two power supplies and 2 wires for the two bit 'sector number' signal. The results of a water tank experiment are shown in figure 4. A cylindrical object was scanned in a plane normal to the axis of the cylinder. This circular geometry best illustrates the advantages of compounding when imaging specular reflectors. The object being scanned is imaged only partially in each of the simple sector scans at the top of the figure. The side lobes and other image anomalies are effectively decreased in the compound scan, since they occur in different places in They are averaged out and appear less bright in the each sector scan. In addition, a more accurate image of the circular target compound image. is obtained in the compound scan than that obtained in any individual sector scan. The compound image is smoother and is a more complete circle. To bring out this improvement, plots of the grey scale around the circle, versus angle, are included in figure 5. A perfect image of the circular test object would yield an even grey scale level, above the Careful threshold level of 24, throughout the 360° of the circle. examination of these plots shows that the compound scan contains fewer subthreshold points than in any simple sector scan.
97
1
Fig.
Compound Scan
Sector
4
Comparison scan of closely scans.
2
of the four composite simple scans with the Note that the compound scan a circular target. smooth circle than any of approximates a full, This is due to the specular nature of the target.
Sector
compound more the simple
COMPOUND SCANNING WITH PHASED ARRAY
64~ 56 .
SECTOR
1
7
48. 40. 32 , 24
th 45
90
135
180 DEGREES
225
270
315
360
64 56 48 40 32 24
th 45
90
135
100 DEGREES
225
270
315
360
64 56 48 40 32 24
th 225
270
225
270
360
DEGREES 64~
SECTOR
4
56 . 48 .
45
90
135
180 DEGREES
315
360
64. COMPOUND
SCAN
th 45
Fig.
5
90
135
180 DEGREE?
225
,270
315
360
Plots of the grey scale level around the circular objects shown in figure 4. The plots show the grey scales as a function of angle, clockwise around the circle from the top. Note that the compound scan has a smoother graph, and remains above the threshold level (about 24) for more of the circle. Th = Threshold level above which the pixel is seen in the image.
99
SHATTUCK AND VON RAMM
E ”
100
COMPOUNDSCANNING WITH PHASED ARRAY
Figures 6 and 7 show some preliminary in vitro scans made with this compound scanner. All of these images are the averaged sum of four consecutive video frames required to record one complete scan from the Video images stored on video tape were digitized by a compound images. VIDCO 512 by 512 by 6 bit TV to TV converter, and this digitized information was then transmitted to and stored in a VAX 11/780 computer. In this computer, the digital information from four consecutive frames was added, then normalized, and the resultant data was transmitted back to the VIDCO for display on a standard TV monitor. This simulates the averaging in the eye that occurs in the real time situation. Images shown here were obtained by photographing this monitor. It is important to note that this As with is a real time scanner, and that these are just isolated frames. all real time images, a given still frame does not convey as much information as the real time presentation. Four frames were averaged for the single sectors as well. The averaging reduces the noise added to the images by the digitization process , and to equalize this variable all images were averaged to simplify comparisons. Figures 6 and 7 are abdominal scans of a normal 27 year old male subject. Figure 6 shows a liver bounded by the diaphragm (D), and shows a long axis view of the portal vein (PV). The compound scan in the right hand panel clearly has a larger field of view. The diaphragm is imaged more smoothly, with fewer bright spots and dark spots in the compound The salt and pepper like speckle pattern in the liver parenchymal view. The speckle pattern in the simple (LP) is smoother in the compound scan. scan has a higher contrast. This speckle noise reduction can be evaluated quantitatively. Once a digitized image is stored in the computer, selected areas can be masked off and image parameters calculated. The parameter we used was the signal to noise ratio proposed by Burckhardt [2], Signal
to Noise
Patio
= u/o
(1)
where n is the mean brightness and 0 is the standard deviation of the brightnesses in the masked off area. In the case where an image with no structures large compared to the size of the resolution cell are present, the signal to noise ratio will increase when the speckle noise contrast is reduced. It is important to the accuracy of these measurements that all images be made with the same gain settings throughout the entire process, from scanning to digitization. This procedure was followed for all these tests. Figure 7 shows each of the simple sector scans across the top of the figure and the corresponding compound scan of a different view of the same liver as shown in figure 6. Here, the scans are made with the subject motionless, in rapid succession, and with identical gain settings. A mask was applied to each scan, to isolate the liver tissue from the hepatic vein (HV) and the diaphragm (D). The boundaries of the mask are shown in the bottom panel of figure 8, which is the same image as the compound scan in figure 7. The non-stippled area, outlined in white, was the area of the standard deviation of the brightness, and interest. Mean brightness, the resulting signal to noise ratio was calculated for each sector and for As expected, the compound scan. The results are tabulated in table I. the signal to noise ratio for the compound scan was higher than in any individual sector scan. A more significant improvement was found when signal to noise ratios were calculated for scans of a tissue mimicking phantom made up of 7 micron graphite particles suspended in agar gelatin. This phantom was
101
Fig.
Compound Scan
7
O.,:“‘:
Liver scans, showing each of the sectors individually, and the compound scan. These are the images used for the calculation of in vivo signal to noise ratios shown in table I. Hv = hepatic vein, D = diaphragm.
Scale
COMPOUND SCANNING WITH PHASED ARRAY
Tissue Mimicking
Liver
Fig.
8
Phantom
Scan
Areas used for calculation of signal to noise ratios in table for the liver scan in figure 7, and for the scans of a tissue mimicking phantom. I
produced by Radiation Measurements type have been described by Madsen targets was chosen, as shown in the are also shown in table I. Again, the compound scan than in any single
Incorporated [3]. Phantoms of this et al. 141. An area free of large top panel of figure 8. The results the signal to noise ratio is higher sector scan.
DISCUSSION increase
The spatial in target
compounding acquisition,
in the Duke Scanner yields a significant and the scan format yields a large field
103
I
in
SHATTUCK AND VON RAMM
Table
I.
Signal
to noise A.
Sector 1 Sector 2 Sector 3 Sector 4 Compound Scan
Scans
a
17.8 19.7
3.7 5.4 3.5 3.3 3.6
19.4 17.8 20.1
Tissue
B.
Sector 1 Sector 2 Sector 3 Sector 4 Compound Scan
Liver
u
ratios
1! 20.3 23.7 22.0 19.1 20.1
mimicking 0 3,3 5.4 3.5 2.5 2.1
(Fig.
7)
-S/N 4.8 3.6 5.5 5.4 5.6
phantom S/N 6.2 4.4 6.3 7.6 9.6
of view. (See Appendix). Compound real time imaging has also been achieved by Carpenter et al. [5] in Australia and by Berson et al. [6] in France, although with a different scheme. Their scanners also utilized a linear array, but the image lines were not in a sector format. Instead they followed the format of a linear sequential scanner, with all lines normal to the transducer, and then additional lines were steered at a fixed angle to these lines in both directions. This approach has advantages and disadvantages with respect to the method described in this paper. The scanners of Berson and Carpenter will have a larger field of view adjacent to the transducer. In addition they are somewhat simpler and more straightforward to construct. The scanner described here has a more flexible scan format, which allows the image lines to be placed where they will be most beneficial. any object point imaged with Additionally, this scanner will be viewed with larger aperture displacements. The scanner described here will also have a larger overall field of view. The compound images are superior to those which can be made with a single sector on the same system. The sources of improvement include the increase in target acquisition from the compounding and the effective decrease in side lobes and grating lobes from the averaging of the different views. The signal to noise ratio parameter also increased in the compound scans, as compared to noncompound view of the same subject. We infer from this that the speckle noise contrast has been reduced. While it is possible that for a single system the speckle pattern may have diagnostic value, we believe that its system dependency and the degradation of resolution that results from speckle make the images a poorer representation of an organ with diffuse scatterers. From this and it is our hypothesis that more viewpoint speckle is noise, diagnostically useful images can be obtained by reducing this noise. The reduction in speckle contrast we found was less than predicted. The Burckhardt model would suggest that there would be a reduction of apertures speckle contrast by (N]1/2 , where N is the number of independent used in the compound scan [Z]. With 4 apertures, a resulting increase in signal to noise ratio by a factor of 2 would be expected. We suspect that the smaller increases shown in table I are due to the fact that the four apertures view any given target at different angles. Since the angular
104
COMPOUNDSCANNING WITH PHASED ARRAY
response of our system is not constant, the contributions from different apertures are not equal for a given image point in the scan. This can be seen in the reduced mean brightness in sectors in table I. Still, the signal to noise ratio is always improved, image itself is subjectively smoother in the compound case.
compound 1 and 4 and the
It might reasonably be asked-whether the same results could not be obtained by producing a single sector using the entire 128 element array as an aperture, instead of treating the aperture as four separate parts. It is true that some of the advantages we gain could also be obtained in this way, such as the improved imaging of specular reflectors. However, such an approach would require very long delay lines to produce a sector with an appreciable angular extent. Even if these fairly expensive longer delay lines were available, the field of view close to the transducer would remain small. In addition, smoothing that results from the summing of the views from different apertures would be lost with this alternative scheme. The performance of the scanner is somewhat limited since only half of the elements in each aperture are used in transmit and receive. This was due to the large cost of producing the circuitry needed to transmit and receive on every element. The image would be expected to improve if more elements were used, but this was not thought to be necessary for the testing of this prototype. Another limitation of this scanner is the low frame rate of the display. When operating with all four sectors the resulting rate of 7.5 frames per second results in an objectionable flicker. Although this flicker can be reduced in certain scan formats, it is clearly undesirable, and should be eliminated. This could be done by increasing the persistence of the display, refreshing the display with old data, or by increasing the data acquisition rate. Another problem arises in that the areas in which there are fewer than four sectors overlapping, the grey scale level will be correspondingly lower. However, in situations where the absolute grey scale is of importance, the flexibility of the system allows the operator to switch to a single sector at the touch of a switch, eliminating these grey scale anomalies.
CONCLUSIONS With this scanner we have been able to produce compound scans in real time. Using four overlapping sector scans, an increased near field of view was obtained. There was also a reduction in noise and target anomalies in the image due to the averaging of the different views. The scans were made with the high resolution of a phased array system, and with its resulting flexibility and ease of control. ACKNOWLEDGEMENTS This LM
07003,
work
was supported
5507-RR-07070-13,
in part GM 29680,
by USPHS and the and APR 7714694.
NSF under
HL
12715,
APPENDIX The dimensions given in table II. indicated dimensions
of the scan formats or data sets currently in use are The headings in the table tA,B,C,Rl correspond to the in figure 9. Here, 8 is the scan angle, the angular
105
SHATTUCK AND
Table
II.
Compound
A Wide Data Data Data
Scans Set 1 Set 2 Set 3
VON
RAMM
scan
dimensions
B
(cm)
C
Scan
Angle
13 15 18
7b0
24.5 28.2
10.0
18.0
13
12.4 16.1
20.5 24.2
15 18
76O 7b"
15.4 15.4
10.0 12.4
15.4
16.1
13.1
22.1
R
76O 7b"
Intermediate Data
Set
Scans 4
Data
Set
5
Data
Set
6
13.4 13.7
Compressed Data Data Data
Set Set Set
Scans 7 8 9
10.7 11.3 12.0
10.0 12.4 16.1
14.0 16.5 20.1
13 15 18
7b"
Tightest Data Data Data
Set Set Set
8.3 9.3 10.3
10.0 12.4 16.1
10.0 12.4
13 15
76" 76O
16.1
18
76O
extent width that center
10 11
12
of each of of the scan were written of the scan
9
76"
7b0 76O
Scans
the sectors. at its widest at the outer and can be C = 2[R
Fig.
0
Compound given in
scan table
geometry. II for
R is the range of the scan and point. This width will decrease edge of the scan are moved toward calculated from the formula: sin
(e/2)1
+ 2a
These different
letters scans.
106
C is the as lines the
(2)
cm
refer
to
the
dimensions
COMPOUND SCANNING WITH PHASED ARRAY
where a is a multiplicative factor equal to 3, 1, -1, and -3 for the four progressively tighter scans and where R is in cm. B is the width of the area where four sectors overlap. It is equal to C when a = -3. A is the width of the scan at a range of 6 cm. This width can be calculated from the formula: A = 2{
(R sin
(S/2)
-
2a)
(6/
[R cos
(O/2)1
1 } + 6
cm
(3)
where a is 0, 1, 2, and 3 for the four progressively tighter scans. again in cm. From the table it can be seen that a variety of formats available to fit the subject being imaged.
R is are
REFERENCES
[II
von F&nun, 0-T. array ultrasound 258-262
[21
and Thurstone, system, part
F-L., I.
C-B.,
Speckle
in ultrasound
Trans. Sonics Ultrasonics SU-2S, [31
Radiation Middleton,
[41
Madsen, E-L., Zagzebski, Tissue mimicking materials
Measurements, WI 53562.
391-394
[51 Carpenter, scanner,
[‘51 Berson, electrically
imaging
using
a phased
design, Circulation
63,
(1976).
Burckhardt,
5,
Cardiac system
l-6
B-mode
Incorporated, Model No. 412. J.A., for
IEEE
scans,
(1978).
P.0.
Banjavic, ultrasound
Box 44, R.A. and Jutila, R.E., phantoms, Me&car Physics
(1978).
D.A.,
Dadd, M-J.,
and Kossoff,
Ultrasound ?4ed. and Biot. M.,
Roncin, steered
A., and Pourcelot, beam, Ultrasonic
107
G.,
6, 279-284 L., Inmn~~ing
A multimode
real
time
(1980).
Compound 3,
scanning
303-308
(1981).
with
an
(1982)
COMPOUND SCANNING WITH A PHASED ARRAY David
P. Shattuck
Department
Compound scans have been produced in imaging, has a large acquisition of echoes the insonifying beam images. These gains and flexibility of a configuration of the scans are presented. Key words:
Abdominal focussing; ultrasound.
and Olaf
T. von Ramm
of Biomedical Engineering Duke University Durham, NC 27706
made with
a dynamically focussed phased array system intended for abdominal The scanner, The compounding improves the field of view. from specular targets by changing the orientation of and also reduces the speckle noise in grey scale are achieved while maintaining the high resolution computer controlled phased array sector scanner. The compound scanner is described, and
time.
imaging; arrays; compound electronic beam steering;
scanning; dynamic real time; speckle;
INTRODUCTION Real time ultrasound sector scanners have shown clinical utility and are finding widespread applications in abdominal, obstetrical, and cardiac diagnosis. These imaging systems combine relatively high frame rates with small hand held transducers which can be manipulated freely to obtain the anatomical views of interest. However, the sector format restricts the overall field of view and particularly limits the acquisition of targets which are close to the transducer. This restricted field of view is less a problem in cardiac imaging than in abdominal applications where targets of interest may be quite close to the skin surface. The small overall field of view only permits the simultaneous display of a small fraction of the abdominal tomographic anatomy thereby making the identification, location and determination of size of various organs more difficult. Additionally, these sector scan imaging systems suffer from reduced target acquisition due to the specular nature of many anatomical features. These systems produce "simple scans" in which targets are only insonified from one direction. A new real time compound imaging system with a field of view significantly larger than can be achieved with simple sector scanners has been developed for abdominal applications. The design, performance specifications and preliminary results are presented. This system is based on phased array principles to steer the ultrasound beam through the target volume and utilizes transmit focussing and dynamic receive focussing to obtain high resolution throughout the entire field of view. Unlike simple sector scanners this system produces compound images thereby increasing target acquisition and detectability. It is well known that spatial compounding improves the visualization of specular targets and consequently a more continuous delineation of anatomical structures is obtained. Indeed, the compounding process increases the apparent image resolution by reducing the speckle contrast. This speckle appears as 0161-7346/82/020093-15$02.00/O 93
Copyright 0 I982 by Academic Press, Inc. All rights of reproduction in an.v form reserved.
SHATTUCK AND VON RAMM
mottle or grain-like structure superimposed on the target information and can obscure the fine detail within an organ complex. Since these compound images are formed in real time, motion blurring typical in mechanical arm contact B-mode scanners is avoided.
METHODS The new compound imaging system was constructed as an adjunct to the Duke University phased array scanner [l]. In its present configuration, the compound scanner has 64 transmit and 16 receive channels. The entire system is under computer control and employs multiple interlaced transmit foci and dynamic focussing in receive. In traditional clinical cardiac or abdominal applications, linear arrays consisting of 32 elements operating in the 2 to 3 MHz range are employed. A typical clinical transducer operating at 2.5 MHz has a lateral dimension of 20 mm and measures 13 mm in the elevation dimension. The azimuthal resolution achievable with such transducers is about 2 mm at a range of 10 cm. The range resolution is 2 mm throughout the field of view, in water. The elevation resolution, that is, the thickness is approximately 5 mm at of the tomographic slice, a range of 10 cm. Sector scan images are produced at rates synchronous with standard television frame rates. In standard television, each image frame is comprised of two fields. Individual B-mode lines are interlaced in these two fields to produce acoustic frames consisting of 124 individual B-mode lines (with an 18 cm maximum range) at the rate of 30 per second. More B-mode lines may be written in an acoustic frame having a smaller maximum range. The transducer of the compound scanner described in this paper has 4 Each adjacent sections of 32 elements, or 128 total array elements. section can be used to produce an independent sector scan. In this way 4 sectors are written, and because of the large size of the sectors with respect to the transducer, these scans overlap. In overlapped areas the scans are compound. The area of overlap depends upon the range of the scan and the angular extent of each sector. Figure 1 illustrates the difference between normal simple sector scans and the new compound scans. Figure la shows a scale drawing of conventional simple sector scan, and figure lb shows the appearance of similarly scaled 4 sector compound scan. The figures are shown for a with a range of 15 cm, a sector angle of 76O, and an aperture of 2 cm 32 elements, or 8 cm for the entire 128 element array. The density of shading in figure lb indicates the locations of compounding, or overlap, of 2, 3, or 4 sectors. In these areas of overlap the line density will increase by a factor of 2, 3, or 4 respectively.
a a scan for the
The system organization is presented diagrammatically in figure 2. The POP 11/20 computer acts as a controller for the entire system. At the beginning of each scan frame, one of the four independent apertures within the large array is selected under software control. The computer then sends signals to trigger the firing of the transmitters. Received echo signals from scattering sites within the tissues are amplified, logarithmically processed, and then delayed by an amount appropriate for the depth and angle being interrogated. All transmit timing and receiver delays are controlled by the computer and the sequence of operation during the formation of each sector scan is identical to that required for a simple sector scan. The summed and detected signals are then displayed on This camera an HP 1311A Monitor which is viewed by a television camera.
94
COMPOUND SCANNING WITH PHASED ARRAY
(a) Fig.
(b)
1 The two drawings are to the same scale, and show (a) a simple The compound scan is sector scan, and (b) the compound scan. stippled to show the areas where 2, 3 or 4 scans overlap to form a compound image.
performs the scan conversion process, transforming the sector scan to a standard television format, which permits video tape recording of images for later playback and analysis. Once echoes from the maximum range have been received, the transmitters are reinitialized under computer control and another acoustic pulse is transmitted in a new azimuthal direction. Under computer control the displayed B-mode line orientation is modified to coincide with the direction of insonification. This process continues until all lines in an individual sector are written. This is limited to l/30 second, to coincide with video frame rates. when this is complete, a new aperture is selected, and another full sector scan is generated.
PDP
II/20
COMPUTFR
TRANSMITTER
MULTIPLEXER
MULTIPLEXER
Fig.
2
System
diagram
for
95
the
compound
scanner.
SHATTUCK AND VON RAMM
Since the sequence of independent apertures is under software control, the next aperture selected need not be adjacent to the first. Since this new sector has a different origin, the display origin is shifted by an appropriate amount under computer control. In this way the same target viewed by different apertures is kept in registration. In the ompound scanner the transducer and preamplifiers are contained in a hand held case. The preamplifier includes the multiplexing circuitry (4 to l), so that only 16 system receive and delay line circuits are required in the formation of a compound scan. This limits the production of scans to one sector at a time. Another major component of the compound scanner is the demultiplexing circuit for the transmitters. Pulses are sent by the computer to trigger the transmitters. Thus the computer determines the timing of the firing of the elements in the array. The demultiplexing circuit was designed so that each of the four sets of transmitter circuits in the compound scanner could be triggered by the computer signals that would normally be used to generate a single sector scan. These signals are sent to each of the sets of transmitters depending on the 2 bit 'sector number' signal, which is also sent to the multiplexing preamplifier. It is used to choose which aperture will be utilized to produce a sector scan at a given time. The sequence of scans is under the control of the operator, who can choose to iterate through each of the apertures, or make scans with any of the four in any combination. For example, if only the two center sectors were desired, only these would be utilized and the other sectors would not be generated. With the flexibility afforded by the computer control, scans with different ranges, sector angles, and resulting areas of overlap are easily obtained. The origin of each sector is determined by the size of the transducer that is used. This information is stored in the computer memory along with all of the transmit and receive timing data as part of the system initialization process. The compound transducer assembly has four 32 element apertures, each of which is 20 mm long, and 11 mm wide. It operates at approximately a 2 MHz center frequency. There are 64 receive channels in the transducer-preamplifier assembly which are multiplexed down to 16 channels after the preamplifier. Thus, 16 channels are used for each of the apertures in receive, using every other element of the 32 available array elements. Sixteen channels per aperture are used for transmitting, utilizing the 16 center elements of each aperture. The system produces scans at a rate of 30 sectors per second. As a result, if 4 sectors are being displayed in the compound scan, the entire image is written 7.5 times per second. The images are recorded on a video tape recorder. Since the sectors are produced synchronously with the video system, 4 video frames comprise a complete compound image. The B-mode lines in the compound scan can be written in a number of ways, due to the flexibility of the computer control. If, for example, it is decided that the areas of the scan where different sectors do not overlap are not needed, the lines in those areas can be removed and written in the overlap area to improve the line density. In this way a variety of scan formats is possible. RESULTS view
The compound scans provide over that of a simple scan.
a significant A compound
96
increase scan with
in the field of a 76O scan angle
COMPOUNDSCANNING WITH PHASED ARRAY
Fig.
3
Transducer
case and 128 element
transducer
for
compound
scanner.
would be 15.4 cm wide at a range of 6 cm, compared with a width of only 9.4 cm for a simple scan with the same angle. These gains in field of view are more significant near the transducer. Figure 3 shows a photograph of the transducer mounted on the the multiplexing preamplifiers. prototype transducer case, which contains The transducer face is 80.6 mm long and 11 mm wide. The case itself is The wires which enter approximately 16 cm long, 6 cm deep and 12 cm high. it include 64 transmit lines, 16 receive lines, two power supplies and 2 wires for the two bit 'sector number' signal. The results of a water tank experiment are shown in figure 4. A cylindrical object was scanned in a plane normal to the axis of the cylinder. This circular geometry best illustrates the advantages of compounding when imaging specular reflectors. The object being scanned is imaged only partially in each of the simple sector scans at the top of the figure. The side lobes and other image anomalies are effectively decreased in the compound scan, since they occur in different places in They are averaged out and appear less bright in the each sector scan. In addition, a more accurate image of the circular target compound image. is obtained in the compound scan than that obtained in any individual sector scan. The compound image is smoother and is a more complete circle. To bring out this improvement, plots of the grey scale around the circle, versus angle, are included in figure 5. A perfect image of the circular test object would yield an even grey scale level, above the Careful threshold level of 24, throughout the 360° of the circle. examination of these plots shows that the compound scan contains fewer subthreshold points than in any simple sector scan.
97
1
Fig.
Compound Scan
Sector
4
Comparison scan of closely scans.
2
of the four composite simple scans with the Note that the compound scan a circular target. smooth circle than any of approximates a full, This is due to the specular nature of the target.
Sector
compound more the simple
COMPOUND SCANNING WITH PHASED ARRAY
64~ 56 .
SECTOR
1
7
48. 40. 32 , 24
th 45
90
135
180 DEGREES
225
270
315
360
64 56 48 40 32 24
th 45
90
135
100 DEGREES
225
270
315
360
64 56 48 40 32 24
th 225
270
225
270
360
DEGREES 64~
SECTOR
4
56 . 48 .
45
90
135
180 DEGREES
315
360
64. COMPOUND
SCAN
th 45
Fig.
5
90
135
180 DEGREE?
225
,270
315
360
Plots of the grey scale level around the circular objects shown in figure 4. The plots show the grey scales as a function of angle, clockwise around the circle from the top. Note that the compound scan has a smoother graph, and remains above the threshold level (about 24) for more of the circle. Th = Threshold level above which the pixel is seen in the image.
99
SHATTUCK AND VON RAMM
E ”
100
COMPOUNDSCANNING WITH PHASED ARRAY
Figures 6 and 7 show some preliminary in vitro scans made with this compound scanner. All of these images are the averaged sum of four consecutive video frames required to record one complete scan from the Video images stored on video tape were digitized by a compound images. VIDCO 512 by 512 by 6 bit TV to TV converter, and this digitized information was then transmitted to and stored in a VAX 11/780 computer. In this computer, the digital information from four consecutive frames was added, then normalized, and the resultant data was transmitted back to the VIDCO for display on a standard TV monitor. This simulates the averaging in the eye that occurs in the real time situation. Images shown here were obtained by photographing this monitor. It is important to note that this As with is a real time scanner, and that these are just isolated frames. all real time images, a given still frame does not convey as much information as the real time presentation. Four frames were averaged for the single sectors as well. The averaging reduces the noise added to the images by the digitization process , and to equalize this variable all images were averaged to simplify comparisons. Figures 6 and 7 are abdominal scans of a normal 27 year old male subject. Figure 6 shows a liver bounded by the diaphragm (D), and shows a long axis view of the portal vein (PV). The compound scan in the right hand panel clearly has a larger field of view. The diaphragm is imaged more smoothly, with fewer bright spots and dark spots in the compound The salt and pepper like speckle pattern in the liver parenchymal view. The speckle pattern in the simple (LP) is smoother in the compound scan. scan has a higher contrast. This speckle noise reduction can be evaluated quantitatively. Once a digitized image is stored in the computer, selected areas can be masked off and image parameters calculated. The parameter we used was the signal to noise ratio proposed by Burckhardt [2], Signal
to Noise
Patio
= u/o
(1)
where n is the mean brightness and 0 is the standard deviation of the brightnesses in the masked off area. In the case where an image with no structures large compared to the size of the resolution cell are present, the signal to noise ratio will increase when the speckle noise contrast is reduced. It is important to the accuracy of these measurements that all images be made with the same gain settings throughout the entire process, from scanning to digitization. This procedure was followed for all these tests. Figure 7 shows each of the simple sector scans across the top of the figure and the corresponding compound scan of a different view of the same liver as shown in figure 6. Here, the scans are made with the subject motionless, in rapid succession, and with identical gain settings. A mask was applied to each scan, to isolate the liver tissue from the hepatic vein (HV) and the diaphragm (D). The boundaries of the mask are shown in the bottom panel of figure 8, which is the same image as the compound scan in figure 7. The non-stippled area, outlined in white, was the area of the standard deviation of the brightness, and interest. Mean brightness, the resulting signal to noise ratio was calculated for each sector and for As expected, the compound scan. The results are tabulated in table I. the signal to noise ratio for the compound scan was higher than in any individual sector scan. A more significant improvement was found when signal to noise ratios were calculated for scans of a tissue mimicking phantom made up of 7 micron graphite particles suspended in agar gelatin. This phantom was
101
Fig.
Compound Scan
7
O.,:“‘:
Liver scans, showing each of the sectors individually, and the compound scan. These are the images used for the calculation of in vivo signal to noise ratios shown in table I. Hv = hepatic vein, D = diaphragm.
Scale
COMPOUND SCANNING WITH PHASED ARRAY
Tissue Mimicking
Liver
Fig.
8
Phantom
Scan
Areas used for calculation of signal to noise ratios in table for the liver scan in figure 7, and for the scans of a tissue mimicking phantom. I
produced by Radiation Measurements type have been described by Madsen targets was chosen, as shown in the are also shown in table I. Again, the compound scan than in any single
Incorporated [3]. Phantoms of this et al. 141. An area free of large top panel of figure 8. The results the signal to noise ratio is higher sector scan.
DISCUSSION increase
The spatial in target
compounding acquisition,
in the Duke Scanner yields a significant and the scan format yields a large field
103
I
in
SHATTUCK AND VON RAMM
Table
I.
Signal
to noise A.
Sector 1 Sector 2 Sector 3 Sector 4 Compound Scan
Scans
a
17.8 19.7
3.7 5.4 3.5 3.3 3.6
19.4 17.8 20.1
Tissue
B.
Sector 1 Sector 2 Sector 3 Sector 4 Compound Scan
Liver
u
ratios
1! 20.3 23.7 22.0 19.1 20.1
mimicking 0 3,3 5.4 3.5 2.5 2.1
(Fig.
7)
-S/N 4.8 3.6 5.5 5.4 5.6
phantom S/N 6.2 4.4 6.3 7.6 9.6
of view. (See Appendix). Compound real time imaging has also been achieved by Carpenter et al. [5] in Australia and by Berson et al. [6] in France, although with a different scheme. Their scanners also utilized a linear array, but the image lines were not in a sector format. Instead they followed the format of a linear sequential scanner, with all lines normal to the transducer, and then additional lines were steered at a fixed angle to these lines in both directions. This approach has advantages and disadvantages with respect to the method described in this paper. The scanners of Berson and Carpenter will have a larger field of view adjacent to the transducer. In addition they are somewhat simpler and more straightforward to construct. The scanner described here has a more flexible scan format, which allows the image lines to be placed where they will be most beneficial. any object point imaged with Additionally, this scanner will be viewed with larger aperture displacements. The scanner described here will also have a larger overall field of view. The compound images are superior to those which can be made with a single sector on the same system. The sources of improvement include the increase in target acquisition from the compounding and the effective decrease in side lobes and grating lobes from the averaging of the different views. The signal to noise ratio parameter also increased in the compound scans, as compared to noncompound view of the same subject. We infer from this that the speckle noise contrast has been reduced. While it is possible that for a single system the speckle pattern may have diagnostic value, we believe that its system dependency and the degradation of resolution that results from speckle make the images a poorer representation of an organ with diffuse scatterers. From this and it is our hypothesis that more viewpoint speckle is noise, diagnostically useful images can be obtained by reducing this noise. The reduction in speckle contrast we found was less than predicted. The Burckhardt model would suggest that there would be a reduction of apertures speckle contrast by (N]1/2 , where N is the number of independent used in the compound scan [Z]. With 4 apertures, a resulting increase in signal to noise ratio by a factor of 2 would be expected. We suspect that the smaller increases shown in table I are due to the fact that the four apertures view any given target at different angles. Since the angular
104
COMPOUNDSCANNING WITH PHASED ARRAY
response of our system is not constant, the contributions from different apertures are not equal for a given image point in the scan. This can be seen in the reduced mean brightness in sectors in table I. Still, the signal to noise ratio is always improved, image itself is subjectively smoother in the compound case.
compound 1 and 4 and the
It might reasonably be asked-whether the same results could not be obtained by producing a single sector using the entire 128 element array as an aperture, instead of treating the aperture as four separate parts. It is true that some of the advantages we gain could also be obtained in this way, such as the improved imaging of specular reflectors. However, such an approach would require very long delay lines to produce a sector with an appreciable angular extent. Even if these fairly expensive longer delay lines were available, the field of view close to the transducer would remain small. In addition, smoothing that results from the summing of the views from different apertures would be lost with this alternative scheme. The performance of the scanner is somewhat limited since only half of the elements in each aperture are used in transmit and receive. This was due to the large cost of producing the circuitry needed to transmit and receive on every element. The image would be expected to improve if more elements were used, but this was not thought to be necessary for the testing of this prototype. Another limitation of this scanner is the low frame rate of the display. When operating with all four sectors the resulting rate of 7.5 frames per second results in an objectionable flicker. Although this flicker can be reduced in certain scan formats, it is clearly undesirable, and should be eliminated. This could be done by increasing the persistence of the display, refreshing the display with old data, or by increasing the data acquisition rate. Another problem arises in that the areas in which there are fewer than four sectors overlapping, the grey scale level will be correspondingly lower. However, in situations where the absolute grey scale is of importance, the flexibility of the system allows the operator to switch to a single sector at the touch of a switch, eliminating these grey scale anomalies.
CONCLUSIONS With this scanner we have been able to produce compound scans in real time. Using four overlapping sector scans, an increased near field of view was obtained. There was also a reduction in noise and target anomalies in the image due to the averaging of the different views. The scans were made with the high resolution of a phased array system, and with its resulting flexibility and ease of control. ACKNOWLEDGEMENTS This LM
07003,
work
was supported
5507-RR-07070-13,
in part GM 29680,
by USPHS and the and APR 7714694.
NSF under
HL
12715,
APPENDIX The dimensions given in table II. indicated dimensions
of the scan formats or data sets currently in use are The headings in the table tA,B,C,Rl correspond to the in figure 9. Here, 8 is the scan angle, the angular
105
SHATTUCK AND
Table
II.
Compound
A Wide Data Data Data
Scans Set 1 Set 2 Set 3
VON
RAMM
scan
dimensions
B
(cm)
C
Scan
Angle
13 15 18
7b0
24.5 28.2
10.0
18.0
13
12.4 16.1
20.5 24.2
15 18
76O 7b"
15.4 15.4
10.0 12.4
15.4
16.1
13.1
22.1
R
76O 7b"
Intermediate Data
Set
Scans 4
Data
Set
5
Data
Set
6
13.4 13.7
Compressed Data Data Data
Set Set Set
Scans 7 8 9
10.7 11.3 12.0
10.0 12.4 16.1
14.0 16.5 20.1
13 15 18
7b"
Tightest Data Data Data
Set Set Set
8.3 9.3 10.3
10.0 12.4 16.1
10.0 12.4
13 15
76" 76O
16.1
18
76O
extent width that center
10 11
12
of each of of the scan were written of the scan
9
76"
7b0 76O
Scans
the sectors. at its widest at the outer and can be C = 2[R
Fig.
0
Compound given in
scan table
geometry. II for
R is the range of the scan and point. This width will decrease edge of the scan are moved toward calculated from the formula: sin
(e/2)1
+ 2a
These different
letters scans.
106
C is the as lines the
(2)
cm
refer
to
the
dimensions
COMPOUND SCANNING WITH PHASED ARRAY
where a is a multiplicative factor equal to 3, 1, -1, and -3 for the four progressively tighter scans and where R is in cm. B is the width of the area where four sectors overlap. It is equal to C when a = -3. A is the width of the scan at a range of 6 cm. This width can be calculated from the formula: A = 2{
(R sin
(S/2)
-
2a)
(6/
[R cos
(O/2)1
1 } + 6
cm
(3)
where a is 0, 1, 2, and 3 for the four progressively tighter scans. again in cm. From the table it can be seen that a variety of formats available to fit the subject being imaged.
R is are
REFERENCES
[II
von F&nun, 0-T. array ultrasound 258-262
[21
and Thurstone, system, part
F-L., I.
C-B.,
Speckle
in ultrasound
Trans. Sonics Ultrasonics SU-2S, [31
Radiation Middleton,
[41
Madsen, E-L., Zagzebski, Tissue mimicking materials
Measurements, WI 53562.
391-394
[51 Carpenter, scanner,
[‘51 Berson, electrically
imaging
using
a phased
design, Circulation
63,
(1976).
Burckhardt,
5,
Cardiac system
l-6
B-mode
Incorporated, Model No. 412. J.A., for
IEEE
scans,
(1978).
P.0.
Banjavic, ultrasound
Box 44, R.A. and Jutila, R.E., phantoms, Me&car Physics
(1978).
D.A.,
Dadd, M-J.,
and Kossoff,
Ultrasound ?4ed. and Biot. M.,
Roncin, steered
A., and Pourcelot, beam, Ultrasonic
107
G.,
6, 279-284 L., Inmn~~ing
A multimode
real
time
(1980).
Compound 3,
scanning
303-308
(1981).
with
an