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doppler image presentation
COLOR DIGITAL ECHO/DOPPLER IMAGE PRESENTATION M. K. EYER, M. A. BRANDESTINI.D. J. PHILLIPS and D. W BAKER Center for Bioengineering, University of Washington, Seattle, WA 98195, U.S.A (First
received
28
March 1979;
infinalform
27 December
1979)
Abstract-Arterial occlusive disease in peripheral blood vessels can be evaluated with an ultrasound scanning system designed to generate real-time cross sectional tissue images and to assess the character of blood flow at any point using multi-gate Doppler methods. A new-generation echo/Doppler system. Duplex scanner TV, incorporates digital display technology to store both echo target location and flow velocity parameters for color composite imaging. Many display formats are possible. In B-mode format echographic tissue images are supplemented with flow information acquired at a specific point in time over 20-30 cardiac cycles by a slow spatial sweep of the Doppler beam in the region of interest. Image storage also makes possible a novel M/Q-mode presentation wherein anatomical interfaces and flow velocities are detected along a single line of sight and displayed as a function or time.
1. INTRODU~ION
Barber et al., 1974b; Daigle et al., 1976; Ramsey et al., 1976: and Phillips et al.. 1978b, is to combine the techniques of pulse echo and pulsed Doppler in one instrument. Each technique provides specific information, each supplementing the other in the combined instrument. To implement such an echo/Doppler scanning system, dedicated electronics are required to coordinate the data acquisition and storage and, in turn. to synthesize a display format capable of presenting multi-dimensional data on a real-time, or near real-time, basis. This paper describes the Duplex scanner IV, which utilizes a microprocessor and dedicated memory to display echo and Doppler information in a meaningful way.
The objective of arteriography is to define as accurately as possible the geometry of the interface artery wall and flowing blood. Because the acoustic reflectance of blood is at least 20 dB below that of an artery wall (Barber er al., 1974a), blood vessels can be readily imaged with ultrasonic pulsed echo. An echo image of a normal blood vessel is characterized by the vessel wall appearing as a bright target and the blood-filled cavity within showing as a uniformly dark or echo-free region. Echographic techniques alone are insufficient to pinpoint disease since non-calcific lesions appear as dark areas due to low acoustic reflectance. Pulsed Doppler techniques can be used to describe regions of flow (Baker, 1970), and have been used to map the flow channel of blood vessels (Hokanson er al., 1972). Although regions of flow are visualized with this method, the intimal vessel wall cannot be identified in the presence of plaque. Doppler systems provide an image of a flow channel, but there is no certainty that the flow channel detected is bounded solely by the vascular wall. Since assessment of atherosclerotic disease involves characterization of plaque formations, the combination of B-mode imaging (defining vascular walls and reflective plaques) with pulsed Doppler velocity detection capabilities might be used to accurately define soft, fatty plaque formations as well as those of a more reflective nature. The objective of the Duplex echo/Doppler scanning systems described by
2. DUPLEX SCANNER IV SYSTEM Figure 1 presents a simplified block diagram of the Duplex scanner TV system and shows the echo and Doppler sections of the unit
22
M. K. EYERet al.
r-4
Transmit/Receive Echo HPrZLg
1
1
I I I -I i I I Digital Field Store I--_-----_------
Duplex Scan Head \
I Doppler Transmit/Receive
1
Signal -----cProcessing 4
Audio Output
Fig. 1. Block diagram of the Duplex Scanner IV system.
semiconductor memory which independently record echo and Doppler processed data. The polar-to-rectangular conversion required for television compatibility is performed in the block labeled “Sector Address Generator”. Angular position information from the scan head is used in the conversion to compute a memory address where data is to be stored. Thurstone and Abbot (1977) have described a system for the Duke University phased array scanner, also incorporating digital scan conversion and echo field storage. A digital approach provides a convenient method to generate standard composite video signals, thus improvement in gray scale linearity and dynamic range over XYZ cathode ray tube displays, and the possibility of color presentation. Since the DFS produces a television compatibility is signal directly, videotape assured with minimal loss in image quality.
(1972); Curry and White (1978)]. The reason for the slow development of pulsed Doppler systems lies in the complex signal processing required and the presence of unwanted signals arising from specular reflections. Brandestini (1978) describes a multichannel pulsed Doppler unit incorporating discretetime and amplitude processing. The two major benefits offered by digital processing are: (1) amplitude quantization allows capacitors to be replaced by digital memory in sample and hold and filtering circuitry; and (2) time sampling allows a single arithmetic unit to service all range gates sequentially. Thus, a digital multiincreased channel system requires only memory capacity to increase range of detection. In addition, matching of gain, offset and filter responses among channels is assured with sequential digital processing. Incorporation of a digital multichannel Doppler (DMD) unit in scanner IV makes flow mapping practical. Using a single gate pulsed 2.1. Acquisition of blood jlow information Doppler, acquisition of a blood flow parameter One of the design goals in scanner IV was to in two dimensions could take as long as 5 min, be able to rapidly acquire and display blood assuming a single sample per cardiac cycle and flow parameters in two dimensions. While a 60” field of view. Use of multi-channel pulsed B-mode imaging is currently enjoying a wide Doppler can reduce this time to about 20 set, popularity, few imaging schemes using Doppler since the velocity data can be acquired and detection have been reported [Hokanson et al., stored along the entire depth of interest during 1972; 1977; Fish et al., 1978; Reid and Spencer each sample time. The reader is referred to
Color digitalecho/Doppler image presentation
Brandestini (1978) for a more complete description of this approach. 2.2. Use of color in image presentation The multichannel Doppler provides bidirectional velocity data so that simple brightness modulation is not sufficient to produce an unambiguous display. Color display can eliminate uncertainty and allow for the forward/ reverse flow differentiation. Others have reported schemes to color code thermograms (Anliker and Friedli, 1976) and M-mode echocardiograms (Flinn. 1976) in which 15 or more colors are chosen for their high contrast and used to indicate various ranges of temperature or echo amplitude. One cannot tell at a glance, however, which are “hotter areas” (brighter targets) without having memorized the color scheme being used. In the phase IV scanner, the primary factor calling for use of color is the requirement to display bi-directional flow information along with echo amplitude in a single image. Instead of a set of colors with high contrast, shades of one or two colors are used to indicate magnitude and type of data. Inspired by Chan and Pizer (1976), who described a “rational red” red scheme based on hot-body radiation spectra, shades of red and orange represent arterial flow in the Doppler image. Shades of blue and green show areas of flow in the opposite direction, and shades of gray differentiate tissue structure in the composite echo/flow images. 2.3. Di&laJl format The format used for television display of images and related data is diagrammed in Fig. 2. Surrounding the image field itself are color bars, a range scale, and four lines of alphanumeric characters reflecting examination data
23
and several of the programmable parameters chosen. The image field is organized as 160 x 192 by four bits deep, so that 30,000 picture elements. or pixels, are displayed, each having 1 of the 16 possible color values. Different color assignment rules are given to echo and flow datathe 16 flow values appear at the left; the set used for echo data is seen at the right. As indicated in Fig. 2, captions for the image field and various examination data, including patient identification, data, and scan head, are provided. The number following “R+ :” in the figure is the time in seconds since the last detected ECG trigger. Following this is the time selected by the operator to delay, following ECG trigger, before sampling flow data (in two-dimensional flow mapping mode). A frame counter for video tape indexing appears at lower right; above the frame counter, PRF rates for both Duplex and flow mapping modes are displayed. Above those, indication of the Doppler sensitivity appears: each step on the flow color scale represents a step in velocity (along the sound beam) by the amount shown. The centimeter scale at the top of the field indicates that the display window is 4 x 6 cm. Seven other choices for window size are available, ranging to a maximum of 13 x 21 cm. The numeral “2” in the left margin indicates that the vertical center of the displa,y field is 2 cm beneath the scan head boot. The arrow in the flow color scale indicates orientation of the Doppler transducer with respect to the arterial flow axis. 3. OPERATING
MODES
Four operating modes are incorporated into the current DFSDMD instrument: (1) Duplex scan mode, where real-time cross timeter Refererlce Scale
Forward Flow_ Color Scale Depth in cm. Transducer Orientation
Echo Color Sea-e
160 x 192
Reverse Flow Color Scale-
Display Field
’ c Doppler
Patient Identification Command EntryECG Timing-
R+:
Timing for Flow Recording
Fig. 2.
4.14
z 0.901.20
/UofW
HD:PVX2-5__@2778----Frame
Ultrasound
Sensitive.:!
,~B.BB--Duplex/Doppler-only
Seattlc_Sc,n
Counter
Head Number
Displayformat illustrating the display field, image parameters, and patient-related information.
PRF
24
M. K. EVERet al.
sectional imaging is combined with pulsed Doppler audio output at a single point indicated on the display. Desired scan head orientation is achieved with real-time, twodimensional imaging; the Doppler sample point is then positioned at various sites within the vessel to permit an overall audible assessment of flow characteristics. Figure 4 is an example of a two-dimensional image. The diagonal line and intensified point represent orientation of the Doppler ultrasound beam and location of the audible sample point. (2) B-Mode flow mapping, in which a 2D echo image is “frozen” in memory to form a tissue structure reference. Then, timed from the cardiac cycle, flow information is used to construct a two-dimensional “map” of the flow pattern in the blood vessel at a specified time in the cardiac cycle. The Doppler beam is swept slowly across the field of.view, probing a new region each cardiac cycle until a wide area has been covered (see .Fig. 7). (3) Composite echolpow M-mode, in which both echo and pulsed Doppler information are taken from the Doppler transducer and used to present a time-motion display. The resulting composite combines standard echo M-mode with time-motion flow profile to record flow activity and blood vessel/tissue movement throughout the cardiac cycle. Figures 5 and 6 provide examples of this format. Duplex scan mode can be used in conjunction to orient the Doppler beam at a desired location. (4) Single crystal M-mode, same as (3) above, except it is used with a separate echo transducer (Brandestini et al., 1979). Typically, Duplex scan mode is used to survey the anatomy and to place the sample volume at a desired location; then, either flow mapping or M-mode is selected by footswitch to record velocity information. 4. SYSTEM
HARDWARE
4.1. Field storage unit As shown in Fig. 1, the DFS consists of timing, sector generator, and memory circuits. Figure 3 presents a block diagram of circuits contained in the DFS; Eyer (1978) provides a more detailed hardware description than space permits here. Bus architecture and the two large banks of semiconductor memory are indicated along with the microprocessor and associated boards. The resident microcomputer input/output, program (CPU), associated
memory and working memory are commercially available printed circuit cards (Cromemco, Inc.) residing in a mainframe enclosure. A Zilog Z-80 microprocessor forms an intelligent base for the DFS. While speed considerations required many of the display and recording functions to be designed with hardwired logic, many others were put under control of the resident operating program. In the resulting system, operator interaction takes place through an alphanumeric keyboard, allowing the clinician to choose operating mode, depth of view desired, screen magnification factor, and other operations involving display format. Each image memory plane in the field storage unit allows for 30,000 picture cells (pixels) to be displayed in a 160 x 192 format. Memory plane depth is four bits, so that 16 possible shades of gray or color may be displayed at each point. The memory size chosen is a tradeoff among system cost, complexity and granularity in the constructed image. To generate a 4 x 6 cm tissue image with this format, 160 pixels must be written in approx. 52 psec (a rate of 300 nsec per pixel). This rate provides range resolution of 0.25 mm, and assures oversampling of the echo video signal for smooth image presentation. Direct memory access to fast semiconductor memory is necessary. Metal-oxide-semiconductor (MOS) memory boards having read/write cycle times of 250 nsec were used in a configuration of 32K words of 8 bits each. As each pixel is defined by a four-bit intensity, the memory modules were modified to allow either the upper or lower half of any byte to be written without affecting the adjacent pixel. Each memory bank is interfaced so that direct connection of address, data, and control busses can be made to any one of four channels. The write only channel connects image memory banks to the sector address generator for recording. A read only channel scans image memory line-by-line at television rates for readout. The CPU itself has access to the memory through a third channel, while a fourth can be used for future access by, for example, a floppy disk system. Memory interfacing in this arrangement means that one bank of memory can be read while the other is written, and a real-time display can be obtained. Coordinate information from the rotating
Rotor Angle
Echo Video
Doppler
*
1
Console
Data
Keyboard
16 Rit
Polar to Rectangular Conversion
CPU
4 Bit
iddress
m
I/O
Serial/ Parallel
I
I
I
I
8ank
I
Bank
in the DFS
Data
Read
Only\
II
2 x 32K x 4
Memory
Bi-directional
Iii-directional
8 Bit
of circuits contained
I
!aJrite Only
I
tlrite Only
I III
Address
Data
2 x 32K x 4
Nemory
Fig. 3. Block diagram
I
Data
16 Bit Address
16 Bit
8 Bit
Rl\M 16K x 8 CPU Working Storage
Future
ROE 1GK x 8 CPU Program Storage
I
Color TV 'rioniior
1 ’
else
26
M. K. EYER et al.
echo transducers is in the form of an angle and a range. The rotor angle is determined by an optical incremental shaft encoder in the scan head, which also provides speed and rotor position feedback to a phase-locked-loop. Servo control of motor speed then assures that each 60” sector will be covered by exactly 256 is emitted pulses. If a deeper penetration desired, the emission, and hence the frame rate, is correspondingly lowered. Range information is determined simply by using the knowledge of the average speed of sound in the body-the time at which an echo returns after transmit is directly related to depth. In order to store in memory the intensity value of an echo corresponding to a known range and angle, the X and Y axis memory coordinates are calculated. The polar-torectangular coordinate conversion is implemented in a straightforward way using binary multipliers and sine and cosine lookup tables stored in read-only memory. Images are read out in television format in a straightforward way : counters synchronized to television horizontal and vertical drive signals provide addresses which scan the image memory. System architecture allows both banks of memory to be read simultaneouslyreadout circuits form a composite of stored echo/flow images, and color scheme lookup tables make possible programmable color assignments for each field. A bank of 16 x 12 bit random-access memories (RAM) for each memory bank generates four-bit values for red, green and blue for each of the 16 possible values stored in each pixel. Address and data for these RAMS can be routed to the CPU so that color assignments can be made under program control. Resulting four-bit values for red, green and blue are converted to analog and fed to the RGB inputs of the color television monitor. The DFS also contains an RGB encoder circuit, modeled after a scheme described by Matic and Trottier (1977) which generates the color composite video signal used for videotape recording. Composite echo/flow readout, as mentioned, is accomplished by simultaneous readout of both memory banks-one representing the echo, the other the flow field. The method adopted at this time is to assume valid flow information will occur only in echo-free regions for composite display. The echo signal processing, including log-compression and
reject, is adjusted so that good contrast is obtained and regions of suspected blood flow appear black. The flow field takes the place of the echo field in these regions. For each pixel, then, flow velocity is displayed unless echo intensity is non-zero. Realizing that with the use of sensitive Doppler detection flow may occur in the presence of echo reflections, especially near vessel walls, other methods of superimposition are being considered. One possibility is to simply combine the two images in a 50:50 blending scheme. 4.2. Multigate
Doppler hardware
The version of the Doppler processor incorporated into scanner IV provides an axial resolution of one sample gate each millimeter of depth and operates at Doppler frequencies of either 3 or 5 MHz. Where the depth of interest is less than 12.8 cm, higher than the nominal PRF of 6 KHz can be used. The final stage of processing in the unit provides a time average of the instantaneous frequency from the Doppler sample points and holds it in digital memory for display readout. Details on the implementation of the processor can be found in Brandestini (1978). 4.3. System timing The Doppler and field store share a common 28.6 MHz master clock, from which all timing signals are synthesized. This frequency was chosen primarily for its TV timing compatibility-it is an integer multiple (8) of the television color subcarrier frequency, 3.58 MHz. In the pulsed Doppler timing chain, the master clock is divided by 20, producing a clock with 1.43 psec period, which represents the time interval between successive gates and translates to 1.04 mm spacing. (The Doppler data is thus oversampled by the DFS sampling clock.) Divisors of 10 or 6 generate Doppler carrier frequencies of 2.86 or 4.77 MHz, respectively. Another integer divider, 3N, generates the DFS sampling clock, where N varies between 3 and 10, depending upon the display window size chosen. Pulse repetition frequencies for echo and Doppler are synthesized in a similar way: the master clock is divided by 32 x N, where N is an eight bit number supplied by a CPU output port. Synchronous timing generation assures phase stability throughout the instrument.
Fig. 6. Composite
echo/Row
M-mode
vein with a venous vaive.
of a B-mode flow map of the common
carotid artery obtained
in the mid-neck
Increasing region.
maneuver.
example of venous valve study in Flig. 4.
example of the jugular vein and common carotid artery during a valsalva be from left to right with the entire horizontal axis covering 3.0 sec.
Fig. 7. Example
N-mode
echo/flow
Fig 4. Dupiex scan mode exampie oi a juguiar Fig. 5. Composite
time is defined
to
Color digital echo/Doppler 5. CLINICAL
EXAMPLES
Preliminary results Examples of the data display capabilities of the scanner IV system are presented in Figs 4-7. Figure 4 shows a two-dimensional image of a normal jugular vein obtained in long axis orientation just above the clavicle. The skin surface is seen anterior with the jugular vein located 2cm beneath and with diffuse reflections from muscle tissue intervening. Large peripheral blood vessels are generally not tortuous in this region permitting visualization of at least 6 cm of the jugular vein in a single B-mode image. As noted earlier, the anterior and posterior vessel walls are clearly defined as continuous bright targets, with the blood-filled lumen appearing as a dark, echo-free region. The orientation of the pulsed Doppler transducer is shown by the diagonal line with the intensified dot marking the position of the selected Doppler gate used for audio interpretation. The bright target located just to the right of the dot and along the posterior wall is a leaflet of a venous valve measuring approx. 1 cm in length. Although the function and competency of a venous valve located in this region is not known to the authors, its motion throughout the cardiac cycle is curious. The leaflet is noted to move within the lumen during the cardiac cycle. The familiar “click” heard in cardiac exams when a valve leaflet passes through the sample volume was clearly heard in this vascular exam with the audio sample gate positioned as shown by the intensified portion of the Doppler line. A composite echo/flow M-mode display was obtained during this exam and is illustrated in Fig. 5. Tissue interfaces are displayed in shades of gray with blood flow velocities along the transducer axis displayed in color. The detected velocity values correspond to the 6 cmjsec per color level sensitivity specified below the image. The horizontal sweep for the time motion display is fixed and covers nearly four complete heart cycles with increasing time defined to be to the right. The location of the audible pulsed Doppler gate is defined by the white line positioned 1 cm beneath the skin surface. The dynamic motion of a single valve leaflet is clearly seen as it moves within the lumen in synchrony with the cardiac cycle. Figure 6 illustrates a composite echo/flow M-mode display with the Doppler transducer orientation such that the jugular vein and com-
5.1.
image presentation
,mon carotid
29
artery are “in line”. The jugular vein is seen just beneath the skin surface with the common carotid artery posterior and in /close contact. Color coding relates the opposing directions of flow as well as the velocity magnitudes. With increasing time defined to be to the right, a Valsalva maneuver was performed. Ballooning of the vein with posterior movement of the common carotid artery is noted as well as the flow patterns detected during the maneuver. Three cardiac cycles are seen in this display. The second arterial pulse coincides with the Valsalva maneuver, producing a flow pattern appearing as a sustained high-velocity flow within the artery. Upon “release’“, a short velocity reversal is noted within the common carotid artery along with a simultaneous increase in venous flow returning blood to the heart. Also noticeable in this image is a considerable amount of apparent “flow” posterior to the carotid artery. This artifact is thought to be generated primarily by two sources. The first is system. noise generated by the preamplifiers, sampling, and multigate pulsed mixers, The second source Doppler processing. appears to be of physiologic origin and is due to reflections at vessel walls resulting in subsequent scattering within the flow channel. This scattering is perceived by the system to be flow located posterior to the vessel under study. The flow artifact in regions posterior to imaged vessels is evident in Figs 6 and 7. While little can be done to eliminate artifact of physiologic origin, the noise generated by system electronics can be reduced by future refinements. Figure 7 illustrates the combined B-mode flow mapping capabilities for a normal common carotid artery in the mid-neck region. While the scan head is held stationary, a twodimensional image is stored and the system changes from the 4.14 KHz PRF, Duplex mode, to a 10 KHz PRF Doppler-only mode. Although flow velocity data is continuously detected by the DMD, only flow data at a selected time during the heart cycle is, accepted for display. For this example, velocity data is accepted at 0.20 set after the R wave to yield a map of the systolic flow distribution. This point in time was chosen because important diagnostic patterns are thought to be,evident at systole or during early diastole. While holding the scan head stationary, the pulsed Doppler _ beam is moved about the tissue regron oi mter-
30
M. K. EVERet al.
During each cardiac cycle velocity information is stored to build up a two-dimensional flow velocity image occurring at the selected time of the cardiac cycle. Such an image can be constructed over 20-30 heart cycles and may be useful in detection of many types of lesions not readily found by either the B-mode or Doppler technique used independently. This method of data collection assumes that the vessel characteristics and spatial blood velocity parameters are essentially constant over successive cardiac cycles. This premise may not be valid in patients with cardiac arythmia or in patients with complex plaque formations that might move about within the vessel during the heart cycle. Our limited experience thus far with presumably normal arteries suggests, however, that the method of data collection and display can provide meaningful, twodimensional velocity information. An apparent loss in target continuity of the vessel walls can be noted in Fig. 7. This effect occurs when: the vessel is tortuous and does not lie exactly within the scan plane; or when the signal processing electronics is not optimally adjusted to detect and display the entire dynamic range of echo return. Signal processing techniques associated with temporal and spatial averaging may provide methods to more accurately delineate the continuous vessel walls in normal subjects. It should be mentioned that there is no correction made for Doppler angle with respect to the detection and display of velocity magnitudes in the composite two-dimensional image. The Doppler angle, formed by the transducer and velocity axes, could be accounted for, in software, by defining an appropriate velocity axis from the two-dimensional image. Then, for each Doppler line, appropriate scaling could be implemented according to the Doppler equation. This addition would add to the accuracy of the velocity data displayed. The Duplex scanner IV system is in an initial clinical evaluation stage. This type of instrumentation and display will, hopefully, permit presentation of important echo and Doppler data in a format for accurate assessment of peripheral vascular disease. est.
6. SUMMARY
AND CONCLUSION
Based upon clinical experience with a Duplex scanner employing two-dimensional imaging on a CRT monitor and a single gate
pulsed Doppler, a new instrument has been constructed with features designed to add a new measure of utility to the system. Digital storage of both echo and Doppler data in the new Duplex scanner IV, coupled with digital scan conversion and television video readout, produce higher quality images. Color is used to permit differentiation between types of data displayed-forward flow, reverse flow, and tissue structure. A multi-gate digital pulsed Doppler collects flow data in real time along the entire length of the sound beam. The system is under control of a microprocessor, resulting in greater display format flexibility and operator convenience. Initial clinical results are encouraging. Adjustments in B-mode video processing circuits (log compression and filtering) are more sensitive due to the gray scale sensitivity of the television monitor display. Two-dimensional images, however, show more detail than those obtainable on the XYZ monitor in scanner III. The requirement to hold the scan head in a fixed position while the Doppler arm is slowly moved to acquire two-dimensional flow data does not appear to be a problem in clinical studies. Wall motion artifacts, excessive noise, reverberant artifact, and marginal dynamic range still remain primary limiting factors to digital Doppler performance. “Smart” processing algorithms, including nonlinear and statistical principles, are being evaluated. The new scanner IV system provides a means for acquisition and flexibility in display of specific ultrasonic information. It is envisioned that this type of approach to echo/ Doppler imaging may become the preferred method for processing and display of multivariate acoustic data. Acknowledgements-We would like to acknowledge the technical assistance of E. Aaron Howard. Vernon Simmons, Robert Olson, George Mahler. Gordon Kirkendall and, in particular, John Ofstad. Much thanks goes also to Carolyn Phillips and Barbara Eyer for work in preparation of this manuscript. This work was supporte;d by NIH Grants NL-07293 and NL-20898. REFERENCES
Anliker, M. and Friedli, P. (1976) Evaluation of high resolution thermograms by on-line digital mapping and color coding. Appl. Radiol./Nucl. Med. 5, I 14-I 18. Baker, D. W. (1970) Pulsed ultrasonic Doppler blood-flow sensing. IEEE Trans. Sonics Uhsonics 17, 170-185. Barber, F. E., Baker, D. W., Nation, A. W. C., Strandness D. E., Jr. and Reid, J. M. (1974a) Ultrasonic duplex echo-Doppler scanner. IEEE Trans. Biomed. Engng. 21, 109-I 13.
Color digital echo/Doppler Barber. F. E., Baker. D. W.. Strandness, D. W.. Jr., Ofstad, J. M. and Mahler, Ci. D. (1974b) Duplex scanner II: for simultaneous imaging of artery tissues and flow. Lllrrasomcs Symp. Proc.. IEEE Cai. No. 74 CHO 896-ISU, 744-748. Brandestini. M. A. (1978) Topoflow-a digital full range Doppler velocity meter. IEEE Trans. Sonics Ultrasonics 25, 287-293.
Brandestini. M., Eyer, M. K. and Stevenson. J. G. (1979) M/Q Mode echocardiography-the synthesis of conventional echo with digital multigate Doppler. In Proc. 3rd Svmp. Echocardioloav (Edited bv Lancie. C. T.). 1. on. 267-212. Martinus Gyhoff. New York. Chan, F. H. and Pizer, S. M. (1976) An ultrasonogram display system using a natural color scale. J. of C/in. Ultrasound 4. 335-338. Curry, G. R. and White. D. N. (1978) Color coded ultrasome differential velocity arterial scanner (Echoflow). L’ltrasound Med. Biol. 4, 27-35.
Daigle. R. E.. Rubenstein, S. A. and Baker, D. W. (1976) A duplex scanning system for pediatric cardiology. Ulrrasound Med. 3B. 1205-1211. Eyer, M. K. (1978) A microprocessor based digital scan converter and color display system for ultrasonic image presentation. Masters thesis. Electrical Engineering. University of Washington. Seattle, Washington. Fish. P. J.. Wilson, I. M., Brown, T. I. H. and Barrett, M. (1978) Rapid vessel imaging and blood velocity measurement-an integrated system. Proc. ‘3rd Meeting of the Am. Instir. qf Ultrasound in Med. 1. 97. Flinn. G. S. (1976)Color encoded display of M-mode echocardiograms. J. C/in. Ufrrasound 4, 339-341.
image presentation
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Hokanson, D. E.. Mozersky, D. J., Sumner, D. S., McLeod. F. D.. Jr. and Strandness. D. E., Jr. (1972) Ultrasonic arteriography: a non-invasive method of arterial visualizationy Radiology 102, 435-436. Matic. G. and Trottier. L. (1977) MTX-256 Graohlc display and application notd. Publication of M.a&ix Electronics Systems, Montreal, Quebec. Phillips, D. J., Strandness, D. E.. Jr., Daigle. R. E. and Baker. D. W. (1976) An ultrasound duplex scanner for imaging peripheral vessels. Uhrasound in Med. 3B, (Edited by White. D. N and R. Braun). pp. 1349-1350. Plenum Press, New York. Phillips. D. J., Blackshear. W. M., Baker, 13. W. and Strandness. D. E.. Jr. (1978a) Ultrasound Duplex Scannine in Perioheral Vascular Disease. Radial. ;Yucl. Med. 8, &IO. 1 Phillips, D. J.. Blackshear. W. M.. Strandness. D. E.. Jr.. Powers, J. E.. Eyer M. K. and Baker, D. W. I 1978b) Use of Duplex scanner III in the assessment 01‘peripheral vascular disease. Proc. 23rd Meering of the Am. Instir. of Lilrrasound in Med., IO9 Ramsey, S. D.. Jr., Taenzer, J. C., Holzemer. J F.. Suarez. J. R. and Green, P. S. (1976) Composite B-scan/Doppler imaging of blood vessels in real-time. Uhrasound Med. 3B, (Edited by White, D. N. and Braun, R.), pp. 1347-1348. Plenum Press. New York. Reid, J. M. and Spencer, M. P. (1972) Ultrasomc Doppler technique for imaging blood vessels. Science 176. 1235-1236. Thurstone. F. L. and Abbott, J. G. (1977) Actual time scan conversion and image processing in a phased array ultrasound imaging system. Ultrasonics Svmp. Proc. IEEE Cat. No. 77CH 1264-ISU, 247-249.
received
28
March 1979;
infinalform
27 December
1979)
Abstract-Arterial occlusive disease in peripheral blood vessels can be evaluated with an ultrasound scanning system designed to generate real-time cross sectional tissue images and to assess the character of blood flow at any point using multi-gate Doppler methods. A new-generation echo/Doppler system. Duplex scanner TV, incorporates digital display technology to store both echo target location and flow velocity parameters for color composite imaging. Many display formats are possible. In B-mode format echographic tissue images are supplemented with flow information acquired at a specific point in time over 20-30 cardiac cycles by a slow spatial sweep of the Doppler beam in the region of interest. Image storage also makes possible a novel M/Q-mode presentation wherein anatomical interfaces and flow velocities are detected along a single line of sight and displayed as a function or time.
1. INTRODU~ION
Barber et al., 1974b; Daigle et al., 1976; Ramsey et al., 1976: and Phillips et al.. 1978b, is to combine the techniques of pulse echo and pulsed Doppler in one instrument. Each technique provides specific information, each supplementing the other in the combined instrument. To implement such an echo/Doppler scanning system, dedicated electronics are required to coordinate the data acquisition and storage and, in turn. to synthesize a display format capable of presenting multi-dimensional data on a real-time, or near real-time, basis. This paper describes the Duplex scanner IV, which utilizes a microprocessor and dedicated memory to display echo and Doppler information in a meaningful way.
The objective of arteriography is to define as accurately as possible the geometry of the interface artery wall and flowing blood. Because the acoustic reflectance of blood is at least 20 dB below that of an artery wall (Barber er al., 1974a), blood vessels can be readily imaged with ultrasonic pulsed echo. An echo image of a normal blood vessel is characterized by the vessel wall appearing as a bright target and the blood-filled cavity within showing as a uniformly dark or echo-free region. Echographic techniques alone are insufficient to pinpoint disease since non-calcific lesions appear as dark areas due to low acoustic reflectance. Pulsed Doppler techniques can be used to describe regions of flow (Baker, 1970), and have been used to map the flow channel of blood vessels (Hokanson er al., 1972). Although regions of flow are visualized with this method, the intimal vessel wall cannot be identified in the presence of plaque. Doppler systems provide an image of a flow channel, but there is no certainty that the flow channel detected is bounded solely by the vascular wall. Since assessment of atherosclerotic disease involves characterization of plaque formations, the combination of B-mode imaging (defining vascular walls and reflective plaques) with pulsed Doppler velocity detection capabilities might be used to accurately define soft, fatty plaque formations as well as those of a more reflective nature. The objective of the Duplex echo/Doppler scanning systems described by
2. DUPLEX SCANNER IV SYSTEM Figure 1 presents a simplified block diagram of the Duplex scanner TV system and shows the echo and Doppler sections of the unit
22
M. K. EYERet al.
r-4
Transmit/Receive Echo HPrZLg
1
1
I I I -I i I I Digital Field Store I--_-----_------
Duplex Scan Head \
I Doppler Transmit/Receive
1
Signal -----cProcessing 4
Audio Output
Fig. 1. Block diagram of the Duplex Scanner IV system.
semiconductor memory which independently record echo and Doppler processed data. The polar-to-rectangular conversion required for television compatibility is performed in the block labeled “Sector Address Generator”. Angular position information from the scan head is used in the conversion to compute a memory address where data is to be stored. Thurstone and Abbot (1977) have described a system for the Duke University phased array scanner, also incorporating digital scan conversion and echo field storage. A digital approach provides a convenient method to generate standard composite video signals, thus improvement in gray scale linearity and dynamic range over XYZ cathode ray tube displays, and the possibility of color presentation. Since the DFS produces a television compatibility is signal directly, videotape assured with minimal loss in image quality.
(1972); Curry and White (1978)]. The reason for the slow development of pulsed Doppler systems lies in the complex signal processing required and the presence of unwanted signals arising from specular reflections. Brandestini (1978) describes a multichannel pulsed Doppler unit incorporating discretetime and amplitude processing. The two major benefits offered by digital processing are: (1) amplitude quantization allows capacitors to be replaced by digital memory in sample and hold and filtering circuitry; and (2) time sampling allows a single arithmetic unit to service all range gates sequentially. Thus, a digital multiincreased channel system requires only memory capacity to increase range of detection. In addition, matching of gain, offset and filter responses among channels is assured with sequential digital processing. Incorporation of a digital multichannel Doppler (DMD) unit in scanner IV makes flow mapping practical. Using a single gate pulsed 2.1. Acquisition of blood jlow information Doppler, acquisition of a blood flow parameter One of the design goals in scanner IV was to in two dimensions could take as long as 5 min, be able to rapidly acquire and display blood assuming a single sample per cardiac cycle and flow parameters in two dimensions. While a 60” field of view. Use of multi-channel pulsed B-mode imaging is currently enjoying a wide Doppler can reduce this time to about 20 set, popularity, few imaging schemes using Doppler since the velocity data can be acquired and detection have been reported [Hokanson et al., stored along the entire depth of interest during 1972; 1977; Fish et al., 1978; Reid and Spencer each sample time. The reader is referred to
Color digitalecho/Doppler image presentation
Brandestini (1978) for a more complete description of this approach. 2.2. Use of color in image presentation The multichannel Doppler provides bidirectional velocity data so that simple brightness modulation is not sufficient to produce an unambiguous display. Color display can eliminate uncertainty and allow for the forward/ reverse flow differentiation. Others have reported schemes to color code thermograms (Anliker and Friedli, 1976) and M-mode echocardiograms (Flinn. 1976) in which 15 or more colors are chosen for their high contrast and used to indicate various ranges of temperature or echo amplitude. One cannot tell at a glance, however, which are “hotter areas” (brighter targets) without having memorized the color scheme being used. In the phase IV scanner, the primary factor calling for use of color is the requirement to display bi-directional flow information along with echo amplitude in a single image. Instead of a set of colors with high contrast, shades of one or two colors are used to indicate magnitude and type of data. Inspired by Chan and Pizer (1976), who described a “rational red” red scheme based on hot-body radiation spectra, shades of red and orange represent arterial flow in the Doppler image. Shades of blue and green show areas of flow in the opposite direction, and shades of gray differentiate tissue structure in the composite echo/flow images. 2.3. Di&laJl format The format used for television display of images and related data is diagrammed in Fig. 2. Surrounding the image field itself are color bars, a range scale, and four lines of alphanumeric characters reflecting examination data
23
and several of the programmable parameters chosen. The image field is organized as 160 x 192 by four bits deep, so that 30,000 picture elements. or pixels, are displayed, each having 1 of the 16 possible color values. Different color assignment rules are given to echo and flow datathe 16 flow values appear at the left; the set used for echo data is seen at the right. As indicated in Fig. 2, captions for the image field and various examination data, including patient identification, data, and scan head, are provided. The number following “R+ :” in the figure is the time in seconds since the last detected ECG trigger. Following this is the time selected by the operator to delay, following ECG trigger, before sampling flow data (in two-dimensional flow mapping mode). A frame counter for video tape indexing appears at lower right; above the frame counter, PRF rates for both Duplex and flow mapping modes are displayed. Above those, indication of the Doppler sensitivity appears: each step on the flow color scale represents a step in velocity (along the sound beam) by the amount shown. The centimeter scale at the top of the field indicates that the display window is 4 x 6 cm. Seven other choices for window size are available, ranging to a maximum of 13 x 21 cm. The numeral “2” in the left margin indicates that the vertical center of the displa,y field is 2 cm beneath the scan head boot. The arrow in the flow color scale indicates orientation of the Doppler transducer with respect to the arterial flow axis. 3. OPERATING
MODES
Four operating modes are incorporated into the current DFSDMD instrument: (1) Duplex scan mode, where real-time cross timeter Refererlce Scale
Forward Flow_ Color Scale Depth in cm. Transducer Orientation
Echo Color Sea-e
160 x 192
Reverse Flow Color Scale-
Display Field
’ c Doppler
Patient Identification Command EntryECG Timing-
R+:
Timing for Flow Recording
Fig. 2.
4.14
z 0.901.20
/UofW
HD:PVX2-5__@2778----Frame
Ultrasound
Sensitive.:!
,~B.BB--Duplex/Doppler-only
Seattlc_Sc,n
Counter
Head Number
Displayformat illustrating the display field, image parameters, and patient-related information.
PRF
24
M. K. EVERet al.
sectional imaging is combined with pulsed Doppler audio output at a single point indicated on the display. Desired scan head orientation is achieved with real-time, twodimensional imaging; the Doppler sample point is then positioned at various sites within the vessel to permit an overall audible assessment of flow characteristics. Figure 4 is an example of a two-dimensional image. The diagonal line and intensified point represent orientation of the Doppler ultrasound beam and location of the audible sample point. (2) B-Mode flow mapping, in which a 2D echo image is “frozen” in memory to form a tissue structure reference. Then, timed from the cardiac cycle, flow information is used to construct a two-dimensional “map” of the flow pattern in the blood vessel at a specified time in the cardiac cycle. The Doppler beam is swept slowly across the field of.view, probing a new region each cardiac cycle until a wide area has been covered (see .Fig. 7). (3) Composite echolpow M-mode, in which both echo and pulsed Doppler information are taken from the Doppler transducer and used to present a time-motion display. The resulting composite combines standard echo M-mode with time-motion flow profile to record flow activity and blood vessel/tissue movement throughout the cardiac cycle. Figures 5 and 6 provide examples of this format. Duplex scan mode can be used in conjunction to orient the Doppler beam at a desired location. (4) Single crystal M-mode, same as (3) above, except it is used with a separate echo transducer (Brandestini et al., 1979). Typically, Duplex scan mode is used to survey the anatomy and to place the sample volume at a desired location; then, either flow mapping or M-mode is selected by footswitch to record velocity information. 4. SYSTEM
HARDWARE
4.1. Field storage unit As shown in Fig. 1, the DFS consists of timing, sector generator, and memory circuits. Figure 3 presents a block diagram of circuits contained in the DFS; Eyer (1978) provides a more detailed hardware description than space permits here. Bus architecture and the two large banks of semiconductor memory are indicated along with the microprocessor and associated boards. The resident microcomputer input/output, program (CPU), associated
memory and working memory are commercially available printed circuit cards (Cromemco, Inc.) residing in a mainframe enclosure. A Zilog Z-80 microprocessor forms an intelligent base for the DFS. While speed considerations required many of the display and recording functions to be designed with hardwired logic, many others were put under control of the resident operating program. In the resulting system, operator interaction takes place through an alphanumeric keyboard, allowing the clinician to choose operating mode, depth of view desired, screen magnification factor, and other operations involving display format. Each image memory plane in the field storage unit allows for 30,000 picture cells (pixels) to be displayed in a 160 x 192 format. Memory plane depth is four bits, so that 16 possible shades of gray or color may be displayed at each point. The memory size chosen is a tradeoff among system cost, complexity and granularity in the constructed image. To generate a 4 x 6 cm tissue image with this format, 160 pixels must be written in approx. 52 psec (a rate of 300 nsec per pixel). This rate provides range resolution of 0.25 mm, and assures oversampling of the echo video signal for smooth image presentation. Direct memory access to fast semiconductor memory is necessary. Metal-oxide-semiconductor (MOS) memory boards having read/write cycle times of 250 nsec were used in a configuration of 32K words of 8 bits each. As each pixel is defined by a four-bit intensity, the memory modules were modified to allow either the upper or lower half of any byte to be written without affecting the adjacent pixel. Each memory bank is interfaced so that direct connection of address, data, and control busses can be made to any one of four channels. The write only channel connects image memory banks to the sector address generator for recording. A read only channel scans image memory line-by-line at television rates for readout. The CPU itself has access to the memory through a third channel, while a fourth can be used for future access by, for example, a floppy disk system. Memory interfacing in this arrangement means that one bank of memory can be read while the other is written, and a real-time display can be obtained. Coordinate information from the rotating
Rotor Angle
Echo Video
Doppler
*
1
Console
Data
Keyboard
16 Rit
Polar to Rectangular Conversion
CPU
4 Bit
iddress
m
I/O
Serial/ Parallel
I
I
I
I
8ank
I
Bank
in the DFS
Data
Read
Only\
II
2 x 32K x 4
Memory
Bi-directional
Iii-directional
8 Bit
of circuits contained
I
!aJrite Only
I
tlrite Only
I III
Address
Data
2 x 32K x 4
Nemory
Fig. 3. Block diagram
I
Data
16 Bit Address
16 Bit
8 Bit
Rl\M 16K x 8 CPU Working Storage
Future
ROE 1GK x 8 CPU Program Storage
I
Color TV 'rioniior
1 ’
else
26
M. K. EYER et al.
echo transducers is in the form of an angle and a range. The rotor angle is determined by an optical incremental shaft encoder in the scan head, which also provides speed and rotor position feedback to a phase-locked-loop. Servo control of motor speed then assures that each 60” sector will be covered by exactly 256 is emitted pulses. If a deeper penetration desired, the emission, and hence the frame rate, is correspondingly lowered. Range information is determined simply by using the knowledge of the average speed of sound in the body-the time at which an echo returns after transmit is directly related to depth. In order to store in memory the intensity value of an echo corresponding to a known range and angle, the X and Y axis memory coordinates are calculated. The polar-torectangular coordinate conversion is implemented in a straightforward way using binary multipliers and sine and cosine lookup tables stored in read-only memory. Images are read out in television format in a straightforward way : counters synchronized to television horizontal and vertical drive signals provide addresses which scan the image memory. System architecture allows both banks of memory to be read simultaneouslyreadout circuits form a composite of stored echo/flow images, and color scheme lookup tables make possible programmable color assignments for each field. A bank of 16 x 12 bit random-access memories (RAM) for each memory bank generates four-bit values for red, green and blue for each of the 16 possible values stored in each pixel. Address and data for these RAMS can be routed to the CPU so that color assignments can be made under program control. Resulting four-bit values for red, green and blue are converted to analog and fed to the RGB inputs of the color television monitor. The DFS also contains an RGB encoder circuit, modeled after a scheme described by Matic and Trottier (1977) which generates the color composite video signal used for videotape recording. Composite echo/flow readout, as mentioned, is accomplished by simultaneous readout of both memory banks-one representing the echo, the other the flow field. The method adopted at this time is to assume valid flow information will occur only in echo-free regions for composite display. The echo signal processing, including log-compression and
reject, is adjusted so that good contrast is obtained and regions of suspected blood flow appear black. The flow field takes the place of the echo field in these regions. For each pixel, then, flow velocity is displayed unless echo intensity is non-zero. Realizing that with the use of sensitive Doppler detection flow may occur in the presence of echo reflections, especially near vessel walls, other methods of superimposition are being considered. One possibility is to simply combine the two images in a 50:50 blending scheme. 4.2. Multigate
Doppler hardware
The version of the Doppler processor incorporated into scanner IV provides an axial resolution of one sample gate each millimeter of depth and operates at Doppler frequencies of either 3 or 5 MHz. Where the depth of interest is less than 12.8 cm, higher than the nominal PRF of 6 KHz can be used. The final stage of processing in the unit provides a time average of the instantaneous frequency from the Doppler sample points and holds it in digital memory for display readout. Details on the implementation of the processor can be found in Brandestini (1978). 4.3. System timing The Doppler and field store share a common 28.6 MHz master clock, from which all timing signals are synthesized. This frequency was chosen primarily for its TV timing compatibility-it is an integer multiple (8) of the television color subcarrier frequency, 3.58 MHz. In the pulsed Doppler timing chain, the master clock is divided by 20, producing a clock with 1.43 psec period, which represents the time interval between successive gates and translates to 1.04 mm spacing. (The Doppler data is thus oversampled by the DFS sampling clock.) Divisors of 10 or 6 generate Doppler carrier frequencies of 2.86 or 4.77 MHz, respectively. Another integer divider, 3N, generates the DFS sampling clock, where N varies between 3 and 10, depending upon the display window size chosen. Pulse repetition frequencies for echo and Doppler are synthesized in a similar way: the master clock is divided by 32 x N, where N is an eight bit number supplied by a CPU output port. Synchronous timing generation assures phase stability throughout the instrument.
Fig. 6. Composite
echo/Row
M-mode
vein with a venous vaive.
of a B-mode flow map of the common
carotid artery obtained
in the mid-neck
Increasing region.
maneuver.
example of venous valve study in Flig. 4.
example of the jugular vein and common carotid artery during a valsalva be from left to right with the entire horizontal axis covering 3.0 sec.
Fig. 7. Example
N-mode
echo/flow
Fig 4. Dupiex scan mode exampie oi a juguiar Fig. 5. Composite
time is defined
to
Color digital echo/Doppler 5. CLINICAL
EXAMPLES
Preliminary results Examples of the data display capabilities of the scanner IV system are presented in Figs 4-7. Figure 4 shows a two-dimensional image of a normal jugular vein obtained in long axis orientation just above the clavicle. The skin surface is seen anterior with the jugular vein located 2cm beneath and with diffuse reflections from muscle tissue intervening. Large peripheral blood vessels are generally not tortuous in this region permitting visualization of at least 6 cm of the jugular vein in a single B-mode image. As noted earlier, the anterior and posterior vessel walls are clearly defined as continuous bright targets, with the blood-filled lumen appearing as a dark, echo-free region. The orientation of the pulsed Doppler transducer is shown by the diagonal line with the intensified dot marking the position of the selected Doppler gate used for audio interpretation. The bright target located just to the right of the dot and along the posterior wall is a leaflet of a venous valve measuring approx. 1 cm in length. Although the function and competency of a venous valve located in this region is not known to the authors, its motion throughout the cardiac cycle is curious. The leaflet is noted to move within the lumen during the cardiac cycle. The familiar “click” heard in cardiac exams when a valve leaflet passes through the sample volume was clearly heard in this vascular exam with the audio sample gate positioned as shown by the intensified portion of the Doppler line. A composite echo/flow M-mode display was obtained during this exam and is illustrated in Fig. 5. Tissue interfaces are displayed in shades of gray with blood flow velocities along the transducer axis displayed in color. The detected velocity values correspond to the 6 cmjsec per color level sensitivity specified below the image. The horizontal sweep for the time motion display is fixed and covers nearly four complete heart cycles with increasing time defined to be to the right. The location of the audible pulsed Doppler gate is defined by the white line positioned 1 cm beneath the skin surface. The dynamic motion of a single valve leaflet is clearly seen as it moves within the lumen in synchrony with the cardiac cycle. Figure 6 illustrates a composite echo/flow M-mode display with the Doppler transducer orientation such that the jugular vein and com-
5.1.
image presentation
,mon carotid
29
artery are “in line”. The jugular vein is seen just beneath the skin surface with the common carotid artery posterior and in /close contact. Color coding relates the opposing directions of flow as well as the velocity magnitudes. With increasing time defined to be to the right, a Valsalva maneuver was performed. Ballooning of the vein with posterior movement of the common carotid artery is noted as well as the flow patterns detected during the maneuver. Three cardiac cycles are seen in this display. The second arterial pulse coincides with the Valsalva maneuver, producing a flow pattern appearing as a sustained high-velocity flow within the artery. Upon “release’“, a short velocity reversal is noted within the common carotid artery along with a simultaneous increase in venous flow returning blood to the heart. Also noticeable in this image is a considerable amount of apparent “flow” posterior to the carotid artery. This artifact is thought to be generated primarily by two sources. The first is system. noise generated by the preamplifiers, sampling, and multigate pulsed mixers, The second source Doppler processing. appears to be of physiologic origin and is due to reflections at vessel walls resulting in subsequent scattering within the flow channel. This scattering is perceived by the system to be flow located posterior to the vessel under study. The flow artifact in regions posterior to imaged vessels is evident in Figs 6 and 7. While little can be done to eliminate artifact of physiologic origin, the noise generated by system electronics can be reduced by future refinements. Figure 7 illustrates the combined B-mode flow mapping capabilities for a normal common carotid artery in the mid-neck region. While the scan head is held stationary, a twodimensional image is stored and the system changes from the 4.14 KHz PRF, Duplex mode, to a 10 KHz PRF Doppler-only mode. Although flow velocity data is continuously detected by the DMD, only flow data at a selected time during the heart cycle is, accepted for display. For this example, velocity data is accepted at 0.20 set after the R wave to yield a map of the systolic flow distribution. This point in time was chosen because important diagnostic patterns are thought to be,evident at systole or during early diastole. While holding the scan head stationary, the pulsed Doppler _ beam is moved about the tissue regron oi mter-
30
M. K. EVERet al.
During each cardiac cycle velocity information is stored to build up a two-dimensional flow velocity image occurring at the selected time of the cardiac cycle. Such an image can be constructed over 20-30 heart cycles and may be useful in detection of many types of lesions not readily found by either the B-mode or Doppler technique used independently. This method of data collection assumes that the vessel characteristics and spatial blood velocity parameters are essentially constant over successive cardiac cycles. This premise may not be valid in patients with cardiac arythmia or in patients with complex plaque formations that might move about within the vessel during the heart cycle. Our limited experience thus far with presumably normal arteries suggests, however, that the method of data collection and display can provide meaningful, twodimensional velocity information. An apparent loss in target continuity of the vessel walls can be noted in Fig. 7. This effect occurs when: the vessel is tortuous and does not lie exactly within the scan plane; or when the signal processing electronics is not optimally adjusted to detect and display the entire dynamic range of echo return. Signal processing techniques associated with temporal and spatial averaging may provide methods to more accurately delineate the continuous vessel walls in normal subjects. It should be mentioned that there is no correction made for Doppler angle with respect to the detection and display of velocity magnitudes in the composite two-dimensional image. The Doppler angle, formed by the transducer and velocity axes, could be accounted for, in software, by defining an appropriate velocity axis from the two-dimensional image. Then, for each Doppler line, appropriate scaling could be implemented according to the Doppler equation. This addition would add to the accuracy of the velocity data displayed. The Duplex scanner IV system is in an initial clinical evaluation stage. This type of instrumentation and display will, hopefully, permit presentation of important echo and Doppler data in a format for accurate assessment of peripheral vascular disease. est.
6. SUMMARY
AND CONCLUSION
Based upon clinical experience with a Duplex scanner employing two-dimensional imaging on a CRT monitor and a single gate
pulsed Doppler, a new instrument has been constructed with features designed to add a new measure of utility to the system. Digital storage of both echo and Doppler data in the new Duplex scanner IV, coupled with digital scan conversion and television video readout, produce higher quality images. Color is used to permit differentiation between types of data displayed-forward flow, reverse flow, and tissue structure. A multi-gate digital pulsed Doppler collects flow data in real time along the entire length of the sound beam. The system is under control of a microprocessor, resulting in greater display format flexibility and operator convenience. Initial clinical results are encouraging. Adjustments in B-mode video processing circuits (log compression and filtering) are more sensitive due to the gray scale sensitivity of the television monitor display. Two-dimensional images, however, show more detail than those obtainable on the XYZ monitor in scanner III. The requirement to hold the scan head in a fixed position while the Doppler arm is slowly moved to acquire two-dimensional flow data does not appear to be a problem in clinical studies. Wall motion artifacts, excessive noise, reverberant artifact, and marginal dynamic range still remain primary limiting factors to digital Doppler performance. “Smart” processing algorithms, including nonlinear and statistical principles, are being evaluated. The new scanner IV system provides a means for acquisition and flexibility in display of specific ultrasonic information. It is envisioned that this type of approach to echo/ Doppler imaging may become the preferred method for processing and display of multivariate acoustic data. Acknowledgements-We would like to acknowledge the technical assistance of E. Aaron Howard. Vernon Simmons, Robert Olson, George Mahler. Gordon Kirkendall and, in particular, John Ofstad. Much thanks goes also to Carolyn Phillips and Barbara Eyer for work in preparation of this manuscript. This work was supporte;d by NIH Grants NL-07293 and NL-20898. REFERENCES
Anliker, M. and Friedli, P. (1976) Evaluation of high resolution thermograms by on-line digital mapping and color coding. Appl. Radiol./Nucl. Med. 5, I 14-I 18. Baker, D. W. (1970) Pulsed ultrasonic Doppler blood-flow sensing. IEEE Trans. Sonics Uhsonics 17, 170-185. Barber, F. E., Baker, D. W., Nation, A. W. C., Strandness D. E., Jr. and Reid, J. M. (1974a) Ultrasonic duplex echo-Doppler scanner. IEEE Trans. Biomed. Engng. 21, 109-I 13.
Color digital echo/Doppler Barber. F. E., Baker. D. W.. Strandness, D. W.. Jr., Ofstad, J. M. and Mahler, Ci. D. (1974b) Duplex scanner II: for simultaneous imaging of artery tissues and flow. Lllrrasomcs Symp. Proc.. IEEE Cai. No. 74 CHO 896-ISU, 744-748. Brandestini. M. A. (1978) Topoflow-a digital full range Doppler velocity meter. IEEE Trans. Sonics Ultrasonics 25, 287-293.
Brandestini. M., Eyer, M. K. and Stevenson. J. G. (1979) M/Q Mode echocardiography-the synthesis of conventional echo with digital multigate Doppler. In Proc. 3rd Svmp. Echocardioloav (Edited bv Lancie. C. T.). 1. on. 267-212. Martinus Gyhoff. New York. Chan, F. H. and Pizer, S. M. (1976) An ultrasonogram display system using a natural color scale. J. of C/in. Ultrasound 4. 335-338. Curry, G. R. and White. D. N. (1978) Color coded ultrasome differential velocity arterial scanner (Echoflow). L’ltrasound Med. Biol. 4, 27-35.
Daigle. R. E.. Rubenstein, S. A. and Baker, D. W. (1976) A duplex scanning system for pediatric cardiology. Ulrrasound Med. 3B. 1205-1211. Eyer, M. K. (1978) A microprocessor based digital scan converter and color display system for ultrasonic image presentation. Masters thesis. Electrical Engineering. University of Washington. Seattle, Washington. Fish. P. J.. Wilson, I. M., Brown, T. I. H. and Barrett, M. (1978) Rapid vessel imaging and blood velocity measurement-an integrated system. Proc. ‘3rd Meeting of the Am. Instir. qf Ultrasound in Med. 1. 97. Flinn. G. S. (1976)Color encoded display of M-mode echocardiograms. J. C/in. Ufrrasound 4, 339-341.
image presentation
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Hokanson, D. E.. Mozersky, D. J., Sumner, D. S., McLeod. F. D.. Jr. and Strandness. D. E., Jr. (1972) Ultrasonic arteriography: a non-invasive method of arterial visualizationy Radiology 102, 435-436. Matic. G. and Trottier. L. (1977) MTX-256 Graohlc display and application notd. Publication of M.a&ix Electronics Systems, Montreal, Quebec. Phillips, D. J., Strandness, D. E.. Jr., Daigle. R. E. and Baker. D. W. (1976) An ultrasound duplex scanner for imaging peripheral vessels. Uhrasound in Med. 3B, (Edited by White. D. N and R. Braun). pp. 1349-1350. Plenum Press, New York. Phillips. D. J., Blackshear. W. M., Baker, 13. W. and Strandness. D. E.. Jr. (1978a) Ultrasound Duplex Scannine in Perioheral Vascular Disease. Radial. ;Yucl. Med. 8, &IO. 1 Phillips, D. J.. Blackshear. W. M.. Strandness. D. E.. Jr.. Powers, J. E.. Eyer M. K. and Baker, D. W. I 1978b) Use of Duplex scanner III in the assessment 01‘peripheral vascular disease. Proc. 23rd Meering of the Am. Instir. of Lilrrasound in Med., IO9 Ramsey, S. D.. Jr., Taenzer, J. C., Holzemer. J F.. Suarez. J. R. and Green, P. S. (1976) Composite B-scan/Doppler imaging of blood vessels in real-time. Uhrasound Med. 3B, (Edited by White, D. N. and Braun, R.), pp. 1347-1348. Plenum Press. New York. Reid, J. M. and Spencer, M. P. (1972) Ultrasomc Doppler technique for imaging blood vessels. Science 176. 1235-1236. Thurstone. F. L. and Abbott, J. G. (1977) Actual time scan conversion and image processing in a phased array ultrasound imaging system. Ultrasonics Svmp. Proc. IEEE Cat. No. 77CH 1264-ISU, 247-249.