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Two-dimensional diffraction scanning of normal and cancerous human hepatic tissue in vitro
Ultrasound in Med. & Biol. Vol. 9, No. 3, pp. 283--295,1983 Printed in Great Britain.
TWO-DIMENSIONAL AND CANCEROUS
0301-5629/83/030283--13503.~10 PergamonPress Ltd.
DIFFRACTION SCANNING OF NORMAL HUMAN HEPATIC TISSUE IN VITRO
D. NICHOLASand A. W. NICHOLAS Institute of Cancer Research/Royal Marsden Hospital, Sutton, Surrey, England (Received 16 February 1982; in .final form 8 July 1982)
A l ~ t r a e t n A technique and associated equipment are described which investigate the acoustic signals backscattered from small tissue volumes and interrogated over a solid angle of transmitter/receiver positions. This two-dimensional diffraction technique produces acoustic scatter maps termed, by us "interferograms", which are analysed to determine their usefulness in discriminating between normal, freshly excised liver tissue and cancerous tissue within the intact organ. The "interferograms" are collected at three different narrow-band frequencies and have been used in investigating 26 normal liver and 13 liver tumour samples. The preliminary results indicate that this novel technique has a 97% success in discriminating normal liver from turnout, compared to an 87% success achieved by two observers when scoring conventional sector B-scan images of the same tissue regions. Key words: Acoustics, Ultrasonics, Acoustic diffraction, B-scans, Liver tissue, Liver cancer, Tissue characterization, Acoustic scattering.
INTRODUCTION
originating from a preselected tissue volume and measures the backscattered intensity, for a specific acoustic frequency, as a function of viewing angle. Although the technique was empirically very useful, it was limited mechanically to performing a planar arc scan about the tissue volume of interest. This restricted the diffraction information to one dimension whilst the tissue structures responsible for the scattering can be expected to be three-dimensional (Nicholas and Hill 1975). In this paper an extended diffraction technique will be described which collects information over a two dimensional surface, thereby improving the interpretation of the ultrasonic interaction with human soft tissues. This is achieved by utilising backscattered signals interrogated over a solid angle of transducer positions. At the same time the technique is further extended by scanning at a variety of frequencies. The results presented will still be empirical but will emphasise the improved discrimination achieved by this novel technique, when compared with conventional sector B-scanning, in the investigation of cancerous, post-mortem liver tissue.
Conventional ultrasound scanning of human soft tissue utilises the time separation of backscattered echoes to construct a two-dimensional tomogram of the tissue section scanned. The echoes portrayed in such an image can be considered either as "specular", originating from large scale tissue structure such as organ boundaries and vessels, or "omnidirectional" due to structures whose dimensions are small in comparison with the investigating pulse-length. It is these latter structures which give rise to the 19w intensity parenchymal echoes (or "speckle") exhibited by conventional B-scan equipment. At present, the major diagnostic use of these parenchymal echoes is for the characterization of space-occupying lesions where a contrast difference exists between adjacent abnormal and normal regions while only a few subjective reports (e.g. Joseph et al. 1979; Dewbury and Clark 1979) attempt to utilise the spatial distributions of such echoes for diagnostic purposes. Although a few workers have discussed the nature of these parenchymal echoes (Burckhardt 1978; Bamber and Dickinson 1980) and some work has been done in quantifying their use as pathological discriminators (Nicholas et al. 1980) only limited attention has been paid to utilising the nature of this backscattered information for direct characterization of tissue structure and pathology. Previously, we reported an in vivo techique which was capable of characterizing pathological conditions in both the human liver (Nicholas 1979) and thyroid (Merton et al. 1982). This technique, termed diffraction scanning, selects, on a time basis, echoes
APPARATUS AND TECItNIQUE
If one models the acoustic structure of human tissues as a distribution of point scatterers with short range spatial regularity (Hill et al. 1978), then the physics of the scattering phenomenon is similar to that exhibited in other fields, such as X-ray crystallography. When an acoustic pulse is incident upon a tissue volume each local inhomogeneity will backscatter an acoustic wavelet of specific 283
28,1
D. NICHOLASand A. W. NICHOLAS
amplitude and phase. On arriving at the receiving transducer face the sum of the individual wavelets will produce an echo whose overall intensity is determined by the phase relationships between these wavelets and the incident beam direction. Altering the orientation of the interrogating beam will alter the phase relationship and thereby produce an interference or diffraction pattern dependent upon the degree of constructive or destructive interference. In order to evaluate more fully the backscattering from a complex three-dimensional tissue such as liver, it is necessary to construct a scanning head which permits movement of the transducer over two degrees of freedom. One could achieve the extra degree of freedom in a number of ways but we have chosen to adapt existing hardware. The apparatus illustrated in Fig. 1 (modified from an isocentric C-scanner; McCready and Hill 1971) permits the investigating transducer to move isocentrically over a spherical surface by spiralling in and out from a central position. The counterweight, shown in the photograph, moves in an opposing manner to balance the transducer. This mechanical arrangement extends our original isocentric sector scan to a two-dimensional isocentric scan with the point of pivot set within the focal zone of the transducer, 12 cm from its transmitting face. The choice of isocentre is determined by a consideration of various factors and can only represent an optimal compromise. Since the tissues to be scanned will be placed in a sound tank and coupled to the transducer via a water path it is important that as wide a range of tissue depth as is possible should be available for investigation. With a 12 cm isocentre up to 6 cm depth of tissue can be investigated before artefacts due to reverberations between the transducer and water/tissue interface occur. These are to be avoided if the scattering data are to be sensibly interpreted. Furthermore, this distance satisfies the requirement that backscattering measurements should be conducted in the far-field of the transducer employed (in our case a weakly focussed 2.5 MHz single element transducer). Near-field measurements will incorporate the diffractive features of the transducer aperture thereby masking the backscattering effects of the tissue. Employing a long transducer/isocentre distance, however, restricts the solid angle over which the information can be gathered unless the aperture of the spiral motion can be extended. Apart from simple size problems this latter option is undesir-
able for two reasons. Firstly, the scan duration will need to be extended in order that sufficient data samples (limited by the pulse repetition frequency) can be provided to form a continuous diffraction pattern and secondly, the circumferential speed of the transducer will be increased at the extremity of the spiral motion, thereby causing severe disturbance of the water to which it is coupled. Furthermore, the unconventional nature of the transducer motion has necessitated the incorporation of a variable pulse repetition frequency (PRF) so as to produce a uniform spread of echo information over the spiral scan. This is especially necessary for our measurements as the present mode of display is limited by a storage oscilloscope; a final record being kept on polaroid. Table 1 lists the salient features of the mechanical device used in the experiments reported here.
Pulse echo diffraction The basis of our diffraction technique has been described elsewhere (Nicholas and Hill 1975; Huggins and Phelps 1977) and only warrants brief mention here. The purpose of the technique is to collect echoe's originating from a specific tissue volume and to display their intensity as a function of interrogation aspect. To achieve this the received echo-trains (A-scans) are electrbnically time-gated to limit the signals to those originating from the small tissue volume (defined by the timegate duration and beam-width) situated at the isocentre of the scanning motion. The gated signals are then frequency filtered to allow investigation of the scattering at a specific frequency. The electronic filter has a bandwidth of approx. 20 kHz and is tunable over the frequency range 1-5 MHz. Table 1. Characteristics of 2-D diffraction scanning apparatus Transducer/isocentre
distance
Feasible depth into tissue path stand-off) Time-gate
duration
Time-gate
delay
(using water (6 cm 5 ~s
(for scanning
Maximum o{f-axis
12 cm
scanning
at 20°C)
angle
Maximum solid angle of scanning Angular
velocity
of scanning
No. of revolutions
in spiral
Max. pulse repetition
rate
(at periphery)
Min. pulse repetition
rate
(on axis)
Time
for complete
scan
155 ~s
17 ° 0.27 Sr 325 rpm 45 14OO s
-i
O s -1 8.3 S
Two-dimensional diffraction scanning
A
Fig. 1. Mechanics of scanning head facilitating two-dimensional isocentric scanning.
285
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D. NICHOLAS and A. W. NICHOLAS
(a)
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(b) Fig. 3. Rectilinear B-scan through a freshly excised human liver (a) and a photograph of the corresponding anatomical section (b). Marked "scan lines" indicate the depth within the tissue at which the isocentre of the spiral motion was set.
Two-dimensional diffraction scanning
Finally, the resulting signal is displayed as an intensity modulated point on a two-dimensional display at a position in the spiral raster corresponding to that of the scanning transducer. Several repeats of the scan at different frequencies (limited by the frequency range encompassed by our transducer operating in pulse-echo mode) provide three "dimensions" of acoustic information pertaining to the tissue volume under investigation, two in the angular backscattering domain and one in the frequency domain.
~
287
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base Fig. 2. Schematic arrangement of scanning motion and tissue within a water-filled sound tank. The circle at the point of isocentre delineates the tissue volume investigated.
EXPERIMENTAL PROCEDURE
The tissue specimens, whole human livers, were obtained at post-mortem immediately after excision and transferred to the laboratory while immersed in water. The specimens were then placed in physiological saline and left at a temperature of 7°C overnight. Simple experiments at our laboratory have indicated that this is the preferred method for degassing the tissues whilst restricting their deterioration. Previous measurements reported by Bamber et al. (1977) indicate that the basic acoustic properties of the tissue will not change appreciably within this period. Furthermore, we have conducted diffraction measurements on tissues obtained immediately after excision and on those stored in the manner described and have not noted any significant differences in the results. The problem with tissues scanned immediately after excision is the intrusion of air into the tissues which severely restricts the regions which can be usefully examined. In preparation for scanning, the organ is placed in a tank of degassed water at room temperature (20°C) and secured to a rubber mat, on the tank bottom, by steel pins. This allows selected regions of the specimen to be scanned and the geometry of the scan planes to be accurately noted. Figure 2 illustrates schematically the positioning of the scanning head over the tissue such that the isocentre lies within the region to be examined. The depth within the tissue at which the isocentre lies is determined by raising or lowering the organ within the sound tank. As the objective of our measurements is to extract diffraction information from both normal and cancerous tissue, it is advantageous that the organ should first be scanned, by conventional ultrasound techniques, to determine the position of regions of potential interest. Since our electronics utilises a pulse-echo mode of operation we have incorporated additional electronics capable of providing conventional sector B-scan information.
By locking the transducer in the central position and moving the whole scanning head horizontally, a rectilinear tomogram of a tissue cross-section can be produced. From such scans regions of interest are then selected for diffraction scanning. Furthermore, by carefully noting the scan plane chosen it is possible, at the conclusion of the experiment, to section the tissue and compare the sectional cut with the corresponding B-scan image. Not only does this permit the regions of the tissue corresponding to the sites from which the diffraction information was obtained to be excised for histological classification, but it also indicates the ability of conventional B-scanning to depict the differing anatomical structures. Figure 3 shows a rectilinear B-scan of 'fresh human liver obtained in the manner described and the corresponding anatomical section. Marked on the photographs are the "scan lines", the depths at which the isocentre of the spiral motion was set, and positions along these lines corresponding to regions considered (initially on the evidence of the B-scan appearance) to be respectively tumour (T) and normal liver tissue (N). This procedure allows us to place the site of the diffraction information to within 2-3 mm, thereby enabling us to accurately classify the pathological condition of that tissue region, retrospectively. RESULTSAND ANALYSIS In the preliminary trial reported here measurements were made at three separate frequencies, 2.0, 2.5 and 3.0MHz, these being adequately encompassed by the frequency bandwidth of the weakly focussed single element transducer used with our equipment. A timegate duration of 5/~s was used throughout these studies which, when considered with the acoustic beam-width at the point of isocentre (12cm from the transducer face), restricted the tissue volumes under investigation to approx. 110, 75 and 55 mm 3 at the
288
D. NICHOLAS alK! A. W. NICHOLAS
frequencies 2.0, 2.5 and 3.0 MHz respectively. A full description of the inter-relationship of timegates and acoustic beam profiles has been reported elsewhere (Nicholas et al. 1982) and will not be discussed further. The choice of these parameters is purely dictated by a desire to use the same transducer and tissue volumes as in the in vivo, one-dimensional measurements reported previously (Nicholas 1979; Merton et al. 1982) and does not imply that they are optimal. Figure 4 depicts typical two-dimensional diffraction patterns, termed interferograms, from a normal region of hepatic tissue and from a secondary liver neoplasm for the three frequencies previously mentioned. Simple visual consideration of these patterns indicates that the scans associated with the focal neoplasm all exhibit a finer structure than the corresponding patterns associated with normal tissue. This finding complements our original one-dimensional results pertaining to the difference between cancerous and normal liver tissue (Nicholas and Hill 1975). This is emphasised in Fig. 5 where a single line of information (line (a)) has been extracted from the 2-dimensional patterns resulting from the two different tissue types, and displayed as one-dimensional diffraction patterns. Comparison with typical patterns obtained from our previous equipment and reported elsewhere (Nicholas 1976) illustrates that our new approach not only mimics our original findings but adds a further dimension of information to improve the assessment of this scattering phenomenon. It must be noted that one-dimensional information could be chosen which was non-representative of the 2-dimensional patterns portrayed here. Thus many scans need to be performed with the one-dimensional technique in order to produce statistically acceptable results. In using the 2-dimensional version the amount of data is effectively squared and the angular diffraction information truly displayed. As mentioned previously, the somewhat bistable nature of the displayed information limits our extraction of quantitative data. To date, we have attempted two simple forms of parameterization using Polaroid records of the data; the ratio of dark to light areas within a scan (termed "contrast") and the number of discrete minima (dark regions) discernible within the image. Although the former has been accurately quantified by making graphical area measurements it will vary depending upon the intensity and contrast setting of the storage oscilloscope. In all these experiments these controls were set to produce a visually "best"
image, with good contrast and no "blurring" due to too high an intensity setting, for scans performed at 2.5 MHz and left unchanged for all measurements performed on a single organ. The sensitivity of the transducer at the three chosen frequencies was noted by examining the frequency spectrum of the pulse reflected from a plane Perspex? target oriented normal to the beam, 12cm from the transducer face. The overall gain of the receiver electronics was then present at each frequency to normalize for these sensitivity variations. Furthermore, the differing intensity of the backscattered echoes due to scanning at different depths within the tissue was corrected, to a first order approximation, by employing a conventional timegain control to account for attentuation of the sound waves within the tissue. The assessment of the number of discrete minima is somewhat subjective but, as all the assessments were performed as a "blind" study, the relative differences between the results for different tissue types should be observer independent. This parameter is chosen because it relates to our previous studies and is fairly insensitive to small changes in the gain settings of the equipment. Measurements were conducted on five excised livers with a total of 39 different tissue sites scanned. Retrospective histological examination of these sites indicated that 13 were regions of cancerous involvement (secondary liver metastases of mixed odgip,) whilst 26 were sites of normal liver parenchyma. Five other sites, originally scanned, were eliminated from this study as the uncertainty in placement (2-3 ram) encompassed tissue regions of differing pathology. This presented an ambiguous histological description of the region scanned although the interferograms were similar to those from other sites. Measurements for each site were conducted at the three frequencies previously mentioned. Unfortunately our criteria for recording the diffraction patterns, outlined previously, occasionally led to unacceptable images at the frequencies of 2.0 and 3.0 MHz in that some scans were too "dark", i.e. lacking in detail, to allow any realistic quantitation. These. problems were specific to our limited data display facilities and do not imply any criticism of the technique. As the measurements are made from tissue regions at differing depths into the tissue it is necessary to establish whether these diffraction features are dependent upon the depth of intervening tissue. Figure 6 represents a scatter diagram ?Trade name for polymethyimethacrylate.
Two-dimensional diffraction scanning
289
Fig. 4. Two-dimensional diffraction patterns, "interferograms", obtained from normal hepatic tissue and secondary liver neoplasm for three separate acoustic frequencies: (a), (c), (e) normal liver at 2.0, 2.5 and 3.0 MHz respectively, Co), (d), (f) liver neoplasm at 2.0, 2.5 and 3.0 MHz respectively.
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Two-dimensional diffraction scanning
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plotting the number of discrete minima against scanning depth into the tissue at 2.5 MHz, these being the most sensitive measurements. The graph illustrates that depth of intervening tissue has no significant influence on this parameter (correlation coefficient <0.0l). It has similarly been confirmed that all other parameters of the "interferograms" are uncorrelated with scanning depth into tissue. Figures 7-9 are scatter maps at 2.0, 2.5 and 3.0 MHz respectively plotting the "contrast" (ratio of "dark" to "white" area) vs the number of minima per st•tad°an of scanning angle. One can immediately conclude that the more useful parameter for discriminating between the normal and neoplastic tissue is the number of minima per steradian. Although Fig. 8 seems to indicate that
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measurements of this feature at 2.5 MHz can adequately separate the cancerous from normal liver tissue there.is no reason to suppose that this frequency should be a more accurate discriminator that the other two, apart from its being the frequency at which the data acquisition was optimised in respect of signal to noise ratio. The usefulness of scanning at different frequencies is 40 "Contrast"
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Fig. 10. Plot of "contrast" and number of minima per steradian as functions of frequency for normal liver parenchyma, • , and tumours (mixed origin), A. The tumour data are offset to allow separation of data.
292
D. NICHOLAS and A. W. NICHOLAS
emphasised by Fig. 10. Here the absolute values of "contrast" and "minima per steradian" are plotted as a function of frequency for both normal and cancerous tissue. The points represent the means of the data whilst the error bars depict the associated standard deviations. Whilst there are not enough discrete frequency intervals to permit a detailed discussion a few general observations can be made. Firstly, there appears to be an inverse relationship which cannot be determined in the turnout data between the two parameters representing normal liver tissue. Secondly, whilst the normal tissue shows a marked dependency upon frequency, the tumour data are independent of frequency. The significance of these findings is difficult to determine and is best related to theoretical modelling studies, which will be reported elsewhere; however, one may conclude that normal liver tissue possesses a pseudo-regular structural arrangement which will then produce frequency dependent scattering whilst the tumours, if composed of randomly distributed scatterers, will, for these frequency ranges and measurements, produce frequency independent scattering (Nicholas and Nicholas 1980). Further examination of Fig. 10 indicates that the difference between the features extracted from the normal and tumour data varies with frequency, i.e. "contrast" measurements are best separated at 3 MHz and the "minima" at 2.5 MHz. Since all the features reported here indicate some degree of discrimination they have all been used, with equal weighting, in a stepwise discriminant analysis (Cooley and Lohnes 1971). The resulting discriminant function has then been used to classify the individual scans into normal or tumour categories. In our analysis the classifier uses a minimum distance criterion in which the six measured features are weighted according to the discriminant function and a discriminant score for each case computed. These values are then compared with the average scores for each classification and the case assigned to the classification to which its score lies closest. In this trial the discriminant function is derived from the 22 cases where all six variables were recorded. The final computer classification uses all 39 cases and achieves a 97% success rate with only one focal neoplasm being incorrectly classified as normal. It must also be noted that a 90% success rate is achieved when the two "best" features (minima per steradian of scanning angle at 2.5 and 2.0 MHz) are employed. In this context "success" is defined in relationship to ability of the diffraction data, when sub-
jected to discriminant analysis, to separate (in multi-variate space) the two sets of tissue type ("normal" and "cancerous") which constitute this training set of 39 tissue specimens. A similar ability to separate the two histologies in subsequent blind trials is strongly implied but.not, of course, demonstrated. B-SCAN LIMITATIONS
The results of this limited study indicate the accuracy of the technique in differentiating neoplastic lesions and normal liver parenchyma. However, the major test of the worth of such a technique is in its comparison with conventional visual assessment of sector B-scans and with alternative approaches to quantitative tissue characterization. As mentioned previously, conventional rectilinear B-scans were performed on each excised liver to provide an initial selection of sites for diffraction scanning. Not only did this provide a location of potential regions of tumour and normal tissue but also indicated regions of vascularity and occasional air bubbles which if not avoided would result in uncharacteristic diffraction measurements. Although conventional sectional B-scans depicted many of the large lesions, comparison with the cut section (see Fig. 11) indicates the difficulty that can arise in accurately describing which regions of the scan are indicative of abnormality. This has long been a problem in diagnostic ultrasound in that, whilst accurate diagnosis of multiple metastatic involvement can be made, it is invariably difficult to determine which regions of the image pertain to normality or tumour. In this study the relevant B-scans were scored "blind" by two observers (i.e. without reference to clinical or histological information) and identification of individual metastases attempted. Diffraction information was then collected from sites that had been identified in this manner and both the visual and diffraction classifications assessed retrospectively with histological characterization of the cut sections. Tumour measurements have only been accepted for lesions of over 1.5cm dia. where the volume scanned could definitely be located within the tumour. Table 2 gives the results of this trial where the diffraction technique, computer classified, is compared with the ability of the observers to detect regions of normality and abnormality as defined by retrospective histology. The success rates should not be considered as indicative of poor B-scan information or inexperience on the part of the observers but reflect the extreme specificity of the diffraction technique employed.
Two-dimensional diffraction scanning
(a)
(b) Fig. 11. Conventional rectilinear B-scan through excised liver tissue (a) and the corresponding anatomical section (b), Visual selection of potential tumour sites outlined from the ultrasound image do not correspond completely with those seen in the cut section.
293
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D. NICHOLAS and A. W. NICHOLAS
Table 2. Comparison of the ability of two observers of rectilinear B-scan tomograms against computer analysis of "interferograms" in distinguishing between cancerous and normal regions of excised liver tissue B-scan V i s u a l i z a t i o n Observer A Observer B Correct
35
(90%)
33
(85%)
False +re
1
2
False
-re
2
2
Undecided
1
2
39 liver samples
Plffracticn Analysis 38
(97%)
1
her and Hill 1981) indicate a general decrease in average velocity of the order of 1.5%. Utilising all these variations allows us to predict, to a first order approximation, that the selection of a tissue volume corresponding to the desired point of isocentre can be achieved to within +-2.5 mm. Although the volume of tissue investigated is not identical throughout the scan our selection of relatively large homogeneous tissue regions has ensured that all the baskscattered information pertains to a specific tissue condition.
26 normal 13 turnouts
DISCUSSION
In summarising the achievements and potential of this new technique and its impact upon characterization of tissue pathology, the various problems and limitations of both the technique and apparatus should be considered. (1) Scanning mechanism The mechanical alignment of the scanning head is excellent and the position of the isocentre only deviates by a millimeter throughout the scan. A further constraint is for the transducer beam to be accurately aligned with the required scan direction. For the transducer employed in these experimerits concise beam plots indicate that at 12 cm distance from the transducer face the maximum beam intensity occurs within 0.5 mm of the axial alignment. (2) Tissue volume variations The time gate was electronically triggered to open 155 ~s after the initial transmission of the acoustic pulse. This centered the 5/zs gate at a depth of l~cm provided the pulse travelled through a medium supporting an average sound velocity of 1525ms -l. Since the measurements were conducted at a temperature of 20°C the corresponding velocities of degassed water, 1480 ms -I, and freshly excised normal liver tissue, 1570 ms -~ (Nicholas 1982) will ensure the correct placement of the time-gate and isocentre provided an equal depth of water and tissue are traversed. In the worst situation only a 2 cm depth of tissue was examined resulting in an average volume displacement of approx. 2 mm. Velocity variations within the tissue cannot be evaluated yet the success of compound B-scanning seems to suggest that such variations are unlikely to be more than a few percent. The few reports relating to the acoustic velocity measured in tumours (e.g. Barn-
CONCLUSION
In this preliminary /n vitro study we have described a two-dimensional diffraction scanning technique which yields information pertaining to the structural organisation of the small tissue volume examined. To date we have only reported on the empirical nature of our diffraction scans, termed "interferograms", and limited ourselves to classifying focal cancers and normal liver tissue. The 97% accuracy of our results in especially notable in the light of the rather crude quantification of the patterns and is also significant when compared with the ability of the authors to interpret the corresponding conventional rectilinear B-scans. The results not only confirm our previous one-dimensional findings (Nicholas and Hill 1975; Nicholas 1979) but provide an improved specificity and interpretation. The existing display system is currently being supplemented by incorporating an analogue scan conversion memory into the system to provide a full "grey-scale" presentation of the images. Furthermore digital acquisition of the scans will be provided to enable more sophisticated analysis and discrimination. Apart from the empirical success of this technique it should be emphasised that the nature of the information is in accordance with a detailed assessment of the structural characteristics of the tissue region under investigation (Nicholas and Nicholas 1980). It is intended that a subsequent paper should report on the ability of this technique to recognize the structures responsible for acoustic scattering. Although the technique, as outlined, is primarily devised for investigation of excised tissues, it is feasible to envisage clinical applications where specific regions of anatomy may be accessible for two-dimensional diffraction scanning. Problems of tissue motion will undoubtedly be encountered although it is hoped that these will be minimal if initial trials are conducted upon thyroid and breast
Two-dimensional diffraction scanning disorders. The full potential of this technique still to b e r e a l i s e d y e t t h e p r e s e n t s u c c e s s e s r e l a t i v e e a s e o f u s e w a r r a n t its c o n s i d e r a t i o n p o t e n t i a l c o m p l e m e n t to e x i s t i n g d i a g n o s t i c cedures.
has and as a pro-
REFERENCES
Bamber J. C. and Dickinson R. J. (1980) Ultrasonic B-scanning: a computer simulation. Phys. Med. Biol. 25, 463--479. Bamber J. C. and Hill C. R. (1981) Acoustic properties of normal and cancerous human liver--l. Dependence on pathological condition. Ultrasound Med. Biol. 7, 121-t33. Bamber J. C., Fry M. J., Hill C. R. and Dunn F. (1977) Ultrasonic attenuation and backscattering by mammalian organs as a function of time after excision. Ultrasound Med. Biol. 3, 15-20. Burckhardt C. B. (1978) Speckle in ultrasound B-mode scans. I E E E Trans. Sonics and Ultrasonics 25, 1-6. Cooley W. W. and Lohnes P. R. (t971) Multivariate Data Analysis. Wiley, New York. Dewbury K. C. and Clark B. (1979) The accuracy of ultrasound in the detection of cirrhosis of the liver. Br. J. Radiol. 52, 945-948. Hill C. Ro, Chivers R. C., Huggins R. W. and Nicholas D. (1978) Scattering of ultrasound by human tissue. In Ultrasound: Its Application in Medicine and Biology (Edited by F. J. Fry), pp. 441-493. Elsevier, Amsterdam. Huggins R. W. and Phelps J. V. (1976) Bragg diffraction scanner
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for ultrasonic tissue characterization in vivo. Ultrasound Med. Biol. 2, 271-277. Joseph A. E. A., Dewbury K. C. and McGuire P. G. (1979) Ultrasound in the detection of chronic liver disease. Br. J. Radiol. 52, 184-188. McCready V. R. and Hill C. R. (1971) A constant depth ultrasonic scanner. Br. J. Radiol. 44, 747-750. Merton J., Nicholas D., Hill C. R., Grover S., Queenan M. and Cosgrove D. O. (1982) Ultrasonic diffraction scanning of the thyroid. Ultrasound Med. Biol. 8, 145-153. Nicholas D. (1976) Ultrasonic Scattering and the Structure of Human Tissues. PhD Thesis, University of London. Nicholas D. (1979) Ultrasonic diffraction analysis in the investigation of liver disease. Br. J. Radiol. 52, 949-961. Nicholas D. (1982) Evaluation of backscattering coefficient for excised human tissues: results, interpretation and associated measurements. Ultrasound Med. Biol. 8, 17-28. Nicholas D. and Hill C. R. (1975) Tissue characterization by an acoustic Bragg scattering process. In Ultrasonics International 1975, pp. 269-272. IPC Science and Technology Press, London. Nicholas A. W. and Nicholas D. (1980) The analysis of threedimensional tissue structures using ultrasound diffraction patterns. Phys. Med. Biol. 25, 759. Nicholas D., Barrett A., Chu J. M. G., Cosgrove D. "O., Garbutt P., Green J., Pussell S. and Hill C. R. (1980) Computer analysis of grey-scale tomograms. In Acoustic Imaging (Edited by A. F. Metherell), Vol. 8, pp. 731-744. Plenum Press, New York.
TWO-DIMENSIONAL AND CANCEROUS
0301-5629/83/030283--13503.~10 PergamonPress Ltd.
DIFFRACTION SCANNING OF NORMAL HUMAN HEPATIC TISSUE IN VITRO
D. NICHOLASand A. W. NICHOLAS Institute of Cancer Research/Royal Marsden Hospital, Sutton, Surrey, England (Received 16 February 1982; in .final form 8 July 1982)
A l ~ t r a e t n A technique and associated equipment are described which investigate the acoustic signals backscattered from small tissue volumes and interrogated over a solid angle of transmitter/receiver positions. This two-dimensional diffraction technique produces acoustic scatter maps termed, by us "interferograms", which are analysed to determine their usefulness in discriminating between normal, freshly excised liver tissue and cancerous tissue within the intact organ. The "interferograms" are collected at three different narrow-band frequencies and have been used in investigating 26 normal liver and 13 liver tumour samples. The preliminary results indicate that this novel technique has a 97% success in discriminating normal liver from turnout, compared to an 87% success achieved by two observers when scoring conventional sector B-scan images of the same tissue regions. Key words: Acoustics, Ultrasonics, Acoustic diffraction, B-scans, Liver tissue, Liver cancer, Tissue characterization, Acoustic scattering.
INTRODUCTION
originating from a preselected tissue volume and measures the backscattered intensity, for a specific acoustic frequency, as a function of viewing angle. Although the technique was empirically very useful, it was limited mechanically to performing a planar arc scan about the tissue volume of interest. This restricted the diffraction information to one dimension whilst the tissue structures responsible for the scattering can be expected to be three-dimensional (Nicholas and Hill 1975). In this paper an extended diffraction technique will be described which collects information over a two dimensional surface, thereby improving the interpretation of the ultrasonic interaction with human soft tissues. This is achieved by utilising backscattered signals interrogated over a solid angle of transducer positions. At the same time the technique is further extended by scanning at a variety of frequencies. The results presented will still be empirical but will emphasise the improved discrimination achieved by this novel technique, when compared with conventional sector B-scanning, in the investigation of cancerous, post-mortem liver tissue.
Conventional ultrasound scanning of human soft tissue utilises the time separation of backscattered echoes to construct a two-dimensional tomogram of the tissue section scanned. The echoes portrayed in such an image can be considered either as "specular", originating from large scale tissue structure such as organ boundaries and vessels, or "omnidirectional" due to structures whose dimensions are small in comparison with the investigating pulse-length. It is these latter structures which give rise to the 19w intensity parenchymal echoes (or "speckle") exhibited by conventional B-scan equipment. At present, the major diagnostic use of these parenchymal echoes is for the characterization of space-occupying lesions where a contrast difference exists between adjacent abnormal and normal regions while only a few subjective reports (e.g. Joseph et al. 1979; Dewbury and Clark 1979) attempt to utilise the spatial distributions of such echoes for diagnostic purposes. Although a few workers have discussed the nature of these parenchymal echoes (Burckhardt 1978; Bamber and Dickinson 1980) and some work has been done in quantifying their use as pathological discriminators (Nicholas et al. 1980) only limited attention has been paid to utilising the nature of this backscattered information for direct characterization of tissue structure and pathology. Previously, we reported an in vivo techique which was capable of characterizing pathological conditions in both the human liver (Nicholas 1979) and thyroid (Merton et al. 1982). This technique, termed diffraction scanning, selects, on a time basis, echoes
APPARATUS AND TECItNIQUE
If one models the acoustic structure of human tissues as a distribution of point scatterers with short range spatial regularity (Hill et al. 1978), then the physics of the scattering phenomenon is similar to that exhibited in other fields, such as X-ray crystallography. When an acoustic pulse is incident upon a tissue volume each local inhomogeneity will backscatter an acoustic wavelet of specific 283
28,1
D. NICHOLASand A. W. NICHOLAS
amplitude and phase. On arriving at the receiving transducer face the sum of the individual wavelets will produce an echo whose overall intensity is determined by the phase relationships between these wavelets and the incident beam direction. Altering the orientation of the interrogating beam will alter the phase relationship and thereby produce an interference or diffraction pattern dependent upon the degree of constructive or destructive interference. In order to evaluate more fully the backscattering from a complex three-dimensional tissue such as liver, it is necessary to construct a scanning head which permits movement of the transducer over two degrees of freedom. One could achieve the extra degree of freedom in a number of ways but we have chosen to adapt existing hardware. The apparatus illustrated in Fig. 1 (modified from an isocentric C-scanner; McCready and Hill 1971) permits the investigating transducer to move isocentrically over a spherical surface by spiralling in and out from a central position. The counterweight, shown in the photograph, moves in an opposing manner to balance the transducer. This mechanical arrangement extends our original isocentric sector scan to a two-dimensional isocentric scan with the point of pivot set within the focal zone of the transducer, 12 cm from its transmitting face. The choice of isocentre is determined by a consideration of various factors and can only represent an optimal compromise. Since the tissues to be scanned will be placed in a sound tank and coupled to the transducer via a water path it is important that as wide a range of tissue depth as is possible should be available for investigation. With a 12 cm isocentre up to 6 cm depth of tissue can be investigated before artefacts due to reverberations between the transducer and water/tissue interface occur. These are to be avoided if the scattering data are to be sensibly interpreted. Furthermore, this distance satisfies the requirement that backscattering measurements should be conducted in the far-field of the transducer employed (in our case a weakly focussed 2.5 MHz single element transducer). Near-field measurements will incorporate the diffractive features of the transducer aperture thereby masking the backscattering effects of the tissue. Employing a long transducer/isocentre distance, however, restricts the solid angle over which the information can be gathered unless the aperture of the spiral motion can be extended. Apart from simple size problems this latter option is undesir-
able for two reasons. Firstly, the scan duration will need to be extended in order that sufficient data samples (limited by the pulse repetition frequency) can be provided to form a continuous diffraction pattern and secondly, the circumferential speed of the transducer will be increased at the extremity of the spiral motion, thereby causing severe disturbance of the water to which it is coupled. Furthermore, the unconventional nature of the transducer motion has necessitated the incorporation of a variable pulse repetition frequency (PRF) so as to produce a uniform spread of echo information over the spiral scan. This is especially necessary for our measurements as the present mode of display is limited by a storage oscilloscope; a final record being kept on polaroid. Table 1 lists the salient features of the mechanical device used in the experiments reported here.
Pulse echo diffraction The basis of our diffraction technique has been described elsewhere (Nicholas and Hill 1975; Huggins and Phelps 1977) and only warrants brief mention here. The purpose of the technique is to collect echoe's originating from a specific tissue volume and to display their intensity as a function of interrogation aspect. To achieve this the received echo-trains (A-scans) are electrbnically time-gated to limit the signals to those originating from the small tissue volume (defined by the timegate duration and beam-width) situated at the isocentre of the scanning motion. The gated signals are then frequency filtered to allow investigation of the scattering at a specific frequency. The electronic filter has a bandwidth of approx. 20 kHz and is tunable over the frequency range 1-5 MHz. Table 1. Characteristics of 2-D diffraction scanning apparatus Transducer/isocentre
distance
Feasible depth into tissue path stand-off) Time-gate
duration
Time-gate
delay
(using water (6 cm 5 ~s
(for scanning
Maximum o{f-axis
12 cm
scanning
at 20°C)
angle
Maximum solid angle of scanning Angular
velocity
of scanning
No. of revolutions
in spiral
Max. pulse repetition
rate
(at periphery)
Min. pulse repetition
rate
(on axis)
Time
for complete
scan
155 ~s
17 ° 0.27 Sr 325 rpm 45 14OO s
-i
O s -1 8.3 S
Two-dimensional diffraction scanning
A
Fig. 1. Mechanics of scanning head facilitating two-dimensional isocentric scanning.
285
286
D. NICHOLAS and A. W. NICHOLAS
(a)
~N IE
(b) Fig. 3. Rectilinear B-scan through a freshly excised human liver (a) and a photograph of the corresponding anatomical section (b). Marked "scan lines" indicate the depth within the tissue at which the isocentre of the spiral motion was set.
Two-dimensional diffraction scanning
Finally, the resulting signal is displayed as an intensity modulated point on a two-dimensional display at a position in the spiral raster corresponding to that of the scanning transducer. Several repeats of the scan at different frequencies (limited by the frequency range encompassed by our transducer operating in pulse-echo mode) provide three "dimensions" of acoustic information pertaining to the tissue volume under investigation, two in the angular backscattering domain and one in the frequency domain.
~
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base Fig. 2. Schematic arrangement of scanning motion and tissue within a water-filled sound tank. The circle at the point of isocentre delineates the tissue volume investigated.
EXPERIMENTAL PROCEDURE
The tissue specimens, whole human livers, were obtained at post-mortem immediately after excision and transferred to the laboratory while immersed in water. The specimens were then placed in physiological saline and left at a temperature of 7°C overnight. Simple experiments at our laboratory have indicated that this is the preferred method for degassing the tissues whilst restricting their deterioration. Previous measurements reported by Bamber et al. (1977) indicate that the basic acoustic properties of the tissue will not change appreciably within this period. Furthermore, we have conducted diffraction measurements on tissues obtained immediately after excision and on those stored in the manner described and have not noted any significant differences in the results. The problem with tissues scanned immediately after excision is the intrusion of air into the tissues which severely restricts the regions which can be usefully examined. In preparation for scanning, the organ is placed in a tank of degassed water at room temperature (20°C) and secured to a rubber mat, on the tank bottom, by steel pins. This allows selected regions of the specimen to be scanned and the geometry of the scan planes to be accurately noted. Figure 2 illustrates schematically the positioning of the scanning head over the tissue such that the isocentre lies within the region to be examined. The depth within the tissue at which the isocentre lies is determined by raising or lowering the organ within the sound tank. As the objective of our measurements is to extract diffraction information from both normal and cancerous tissue, it is advantageous that the organ should first be scanned, by conventional ultrasound techniques, to determine the position of regions of potential interest. Since our electronics utilises a pulse-echo mode of operation we have incorporated additional electronics capable of providing conventional sector B-scan information.
By locking the transducer in the central position and moving the whole scanning head horizontally, a rectilinear tomogram of a tissue cross-section can be produced. From such scans regions of interest are then selected for diffraction scanning. Furthermore, by carefully noting the scan plane chosen it is possible, at the conclusion of the experiment, to section the tissue and compare the sectional cut with the corresponding B-scan image. Not only does this permit the regions of the tissue corresponding to the sites from which the diffraction information was obtained to be excised for histological classification, but it also indicates the ability of conventional B-scanning to depict the differing anatomical structures. Figure 3 shows a rectilinear B-scan of 'fresh human liver obtained in the manner described and the corresponding anatomical section. Marked on the photographs are the "scan lines", the depths at which the isocentre of the spiral motion was set, and positions along these lines corresponding to regions considered (initially on the evidence of the B-scan appearance) to be respectively tumour (T) and normal liver tissue (N). This procedure allows us to place the site of the diffraction information to within 2-3 mm, thereby enabling us to accurately classify the pathological condition of that tissue region, retrospectively. RESULTSAND ANALYSIS In the preliminary trial reported here measurements were made at three separate frequencies, 2.0, 2.5 and 3.0MHz, these being adequately encompassed by the frequency bandwidth of the weakly focussed single element transducer used with our equipment. A timegate duration of 5/~s was used throughout these studies which, when considered with the acoustic beam-width at the point of isocentre (12cm from the transducer face), restricted the tissue volumes under investigation to approx. 110, 75 and 55 mm 3 at the
288
D. NICHOLAS alK! A. W. NICHOLAS
frequencies 2.0, 2.5 and 3.0 MHz respectively. A full description of the inter-relationship of timegates and acoustic beam profiles has been reported elsewhere (Nicholas et al. 1982) and will not be discussed further. The choice of these parameters is purely dictated by a desire to use the same transducer and tissue volumes as in the in vivo, one-dimensional measurements reported previously (Nicholas 1979; Merton et al. 1982) and does not imply that they are optimal. Figure 4 depicts typical two-dimensional diffraction patterns, termed interferograms, from a normal region of hepatic tissue and from a secondary liver neoplasm for the three frequencies previously mentioned. Simple visual consideration of these patterns indicates that the scans associated with the focal neoplasm all exhibit a finer structure than the corresponding patterns associated with normal tissue. This finding complements our original one-dimensional results pertaining to the difference between cancerous and normal liver tissue (Nicholas and Hill 1975). This is emphasised in Fig. 5 where a single line of information (line (a)) has been extracted from the 2-dimensional patterns resulting from the two different tissue types, and displayed as one-dimensional diffraction patterns. Comparison with typical patterns obtained from our previous equipment and reported elsewhere (Nicholas 1976) illustrates that our new approach not only mimics our original findings but adds a further dimension of information to improve the assessment of this scattering phenomenon. It must be noted that one-dimensional information could be chosen which was non-representative of the 2-dimensional patterns portrayed here. Thus many scans need to be performed with the one-dimensional technique in order to produce statistically acceptable results. In using the 2-dimensional version the amount of data is effectively squared and the angular diffraction information truly displayed. As mentioned previously, the somewhat bistable nature of the displayed information limits our extraction of quantitative data. To date, we have attempted two simple forms of parameterization using Polaroid records of the data; the ratio of dark to light areas within a scan (termed "contrast") and the number of discrete minima (dark regions) discernible within the image. Although the former has been accurately quantified by making graphical area measurements it will vary depending upon the intensity and contrast setting of the storage oscilloscope. In all these experiments these controls were set to produce a visually "best"
image, with good contrast and no "blurring" due to too high an intensity setting, for scans performed at 2.5 MHz and left unchanged for all measurements performed on a single organ. The sensitivity of the transducer at the three chosen frequencies was noted by examining the frequency spectrum of the pulse reflected from a plane Perspex? target oriented normal to the beam, 12cm from the transducer face. The overall gain of the receiver electronics was then present at each frequency to normalize for these sensitivity variations. Furthermore, the differing intensity of the backscattered echoes due to scanning at different depths within the tissue was corrected, to a first order approximation, by employing a conventional timegain control to account for attentuation of the sound waves within the tissue. The assessment of the number of discrete minima is somewhat subjective but, as all the assessments were performed as a "blind" study, the relative differences between the results for different tissue types should be observer independent. This parameter is chosen because it relates to our previous studies and is fairly insensitive to small changes in the gain settings of the equipment. Measurements were conducted on five excised livers with a total of 39 different tissue sites scanned. Retrospective histological examination of these sites indicated that 13 were regions of cancerous involvement (secondary liver metastases of mixed odgip,) whilst 26 were sites of normal liver parenchyma. Five other sites, originally scanned, were eliminated from this study as the uncertainty in placement (2-3 ram) encompassed tissue regions of differing pathology. This presented an ambiguous histological description of the region scanned although the interferograms were similar to those from other sites. Measurements for each site were conducted at the three frequencies previously mentioned. Unfortunately our criteria for recording the diffraction patterns, outlined previously, occasionally led to unacceptable images at the frequencies of 2.0 and 3.0 MHz in that some scans were too "dark", i.e. lacking in detail, to allow any realistic quantitation. These. problems were specific to our limited data display facilities and do not imply any criticism of the technique. As the measurements are made from tissue regions at differing depths into the tissue it is necessary to establish whether these diffraction features are dependent upon the depth of intervening tissue. Figure 6 represents a scatter diagram ?Trade name for polymethyimethacrylate.
Two-dimensional diffraction scanning
289
Fig. 4. Two-dimensional diffraction patterns, "interferograms", obtained from normal hepatic tissue and secondary liver neoplasm for three separate acoustic frequencies: (a), (c), (e) normal liver at 2.0, 2.5 and 3.0 MHz respectively, Co), (d), (f) liver neoplasm at 2.0, 2.5 and 3.0 MHz respectively.
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plotting the number of discrete minima against scanning depth into the tissue at 2.5 MHz, these being the most sensitive measurements. The graph illustrates that depth of intervening tissue has no significant influence on this parameter (correlation coefficient <0.0l). It has similarly been confirmed that all other parameters of the "interferograms" are uncorrelated with scanning depth into tissue. Figures 7-9 are scatter maps at 2.0, 2.5 and 3.0 MHz respectively plotting the "contrast" (ratio of "dark" to "white" area) vs the number of minima per st•tad°an of scanning angle. One can immediately conclude that the more useful parameter for discriminating between the normal and neoplastic tissue is the number of minima per steradian. Although Fig. 8 seems to indicate that
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measurements of this feature at 2.5 MHz can adequately separate the cancerous from normal liver tissue there.is no reason to suppose that this frequency should be a more accurate discriminator that the other two, apart from its being the frequency at which the data acquisition was optimised in respect of signal to noise ratio. The usefulness of scanning at different frequencies is 40 "Contrast"
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Fig. 10. Plot of "contrast" and number of minima per steradian as functions of frequency for normal liver parenchyma, • , and tumours (mixed origin), A. The tumour data are offset to allow separation of data.
292
D. NICHOLAS and A. W. NICHOLAS
emphasised by Fig. 10. Here the absolute values of "contrast" and "minima per steradian" are plotted as a function of frequency for both normal and cancerous tissue. The points represent the means of the data whilst the error bars depict the associated standard deviations. Whilst there are not enough discrete frequency intervals to permit a detailed discussion a few general observations can be made. Firstly, there appears to be an inverse relationship which cannot be determined in the turnout data between the two parameters representing normal liver tissue. Secondly, whilst the normal tissue shows a marked dependency upon frequency, the tumour data are independent of frequency. The significance of these findings is difficult to determine and is best related to theoretical modelling studies, which will be reported elsewhere; however, one may conclude that normal liver tissue possesses a pseudo-regular structural arrangement which will then produce frequency dependent scattering whilst the tumours, if composed of randomly distributed scatterers, will, for these frequency ranges and measurements, produce frequency independent scattering (Nicholas and Nicholas 1980). Further examination of Fig. 10 indicates that the difference between the features extracted from the normal and tumour data varies with frequency, i.e. "contrast" measurements are best separated at 3 MHz and the "minima" at 2.5 MHz. Since all the features reported here indicate some degree of discrimination they have all been used, with equal weighting, in a stepwise discriminant analysis (Cooley and Lohnes 1971). The resulting discriminant function has then been used to classify the individual scans into normal or tumour categories. In our analysis the classifier uses a minimum distance criterion in which the six measured features are weighted according to the discriminant function and a discriminant score for each case computed. These values are then compared with the average scores for each classification and the case assigned to the classification to which its score lies closest. In this trial the discriminant function is derived from the 22 cases where all six variables were recorded. The final computer classification uses all 39 cases and achieves a 97% success rate with only one focal neoplasm being incorrectly classified as normal. It must also be noted that a 90% success rate is achieved when the two "best" features (minima per steradian of scanning angle at 2.5 and 2.0 MHz) are employed. In this context "success" is defined in relationship to ability of the diffraction data, when sub-
jected to discriminant analysis, to separate (in multi-variate space) the two sets of tissue type ("normal" and "cancerous") which constitute this training set of 39 tissue specimens. A similar ability to separate the two histologies in subsequent blind trials is strongly implied but.not, of course, demonstrated. B-SCAN LIMITATIONS
The results of this limited study indicate the accuracy of the technique in differentiating neoplastic lesions and normal liver parenchyma. However, the major test of the worth of such a technique is in its comparison with conventional visual assessment of sector B-scans and with alternative approaches to quantitative tissue characterization. As mentioned previously, conventional rectilinear B-scans were performed on each excised liver to provide an initial selection of sites for diffraction scanning. Not only did this provide a location of potential regions of tumour and normal tissue but also indicated regions of vascularity and occasional air bubbles which if not avoided would result in uncharacteristic diffraction measurements. Although conventional sectional B-scans depicted many of the large lesions, comparison with the cut section (see Fig. 11) indicates the difficulty that can arise in accurately describing which regions of the scan are indicative of abnormality. This has long been a problem in diagnostic ultrasound in that, whilst accurate diagnosis of multiple metastatic involvement can be made, it is invariably difficult to determine which regions of the image pertain to normality or tumour. In this study the relevant B-scans were scored "blind" by two observers (i.e. without reference to clinical or histological information) and identification of individual metastases attempted. Diffraction information was then collected from sites that had been identified in this manner and both the visual and diffraction classifications assessed retrospectively with histological characterization of the cut sections. Tumour measurements have only been accepted for lesions of over 1.5cm dia. where the volume scanned could definitely be located within the tumour. Table 2 gives the results of this trial where the diffraction technique, computer classified, is compared with the ability of the observers to detect regions of normality and abnormality as defined by retrospective histology. The success rates should not be considered as indicative of poor B-scan information or inexperience on the part of the observers but reflect the extreme specificity of the diffraction technique employed.
Two-dimensional diffraction scanning
(a)
(b) Fig. 11. Conventional rectilinear B-scan through excised liver tissue (a) and the corresponding anatomical section (b), Visual selection of potential tumour sites outlined from the ultrasound image do not correspond completely with those seen in the cut section.
293
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D. NICHOLAS and A. W. NICHOLAS
Table 2. Comparison of the ability of two observers of rectilinear B-scan tomograms against computer analysis of "interferograms" in distinguishing between cancerous and normal regions of excised liver tissue B-scan V i s u a l i z a t i o n Observer A Observer B Correct
35
(90%)
33
(85%)
False +re
1
2
False
-re
2
2
Undecided
1
2
39 liver samples
Plffracticn Analysis 38
(97%)
1
her and Hill 1981) indicate a general decrease in average velocity of the order of 1.5%. Utilising all these variations allows us to predict, to a first order approximation, that the selection of a tissue volume corresponding to the desired point of isocentre can be achieved to within +-2.5 mm. Although the volume of tissue investigated is not identical throughout the scan our selection of relatively large homogeneous tissue regions has ensured that all the baskscattered information pertains to a specific tissue condition.
26 normal 13 turnouts
DISCUSSION
In summarising the achievements and potential of this new technique and its impact upon characterization of tissue pathology, the various problems and limitations of both the technique and apparatus should be considered. (1) Scanning mechanism The mechanical alignment of the scanning head is excellent and the position of the isocentre only deviates by a millimeter throughout the scan. A further constraint is for the transducer beam to be accurately aligned with the required scan direction. For the transducer employed in these experimerits concise beam plots indicate that at 12 cm distance from the transducer face the maximum beam intensity occurs within 0.5 mm of the axial alignment. (2) Tissue volume variations The time gate was electronically triggered to open 155 ~s after the initial transmission of the acoustic pulse. This centered the 5/zs gate at a depth of l~cm provided the pulse travelled through a medium supporting an average sound velocity of 1525ms -l. Since the measurements were conducted at a temperature of 20°C the corresponding velocities of degassed water, 1480 ms -I, and freshly excised normal liver tissue, 1570 ms -~ (Nicholas 1982) will ensure the correct placement of the time-gate and isocentre provided an equal depth of water and tissue are traversed. In the worst situation only a 2 cm depth of tissue was examined resulting in an average volume displacement of approx. 2 mm. Velocity variations within the tissue cannot be evaluated yet the success of compound B-scanning seems to suggest that such variations are unlikely to be more than a few percent. The few reports relating to the acoustic velocity measured in tumours (e.g. Barn-
CONCLUSION
In this preliminary /n vitro study we have described a two-dimensional diffraction scanning technique which yields information pertaining to the structural organisation of the small tissue volume examined. To date we have only reported on the empirical nature of our diffraction scans, termed "interferograms", and limited ourselves to classifying focal cancers and normal liver tissue. The 97% accuracy of our results in especially notable in the light of the rather crude quantification of the patterns and is also significant when compared with the ability of the authors to interpret the corresponding conventional rectilinear B-scans. The results not only confirm our previous one-dimensional findings (Nicholas and Hill 1975; Nicholas 1979) but provide an improved specificity and interpretation. The existing display system is currently being supplemented by incorporating an analogue scan conversion memory into the system to provide a full "grey-scale" presentation of the images. Furthermore digital acquisition of the scans will be provided to enable more sophisticated analysis and discrimination. Apart from the empirical success of this technique it should be emphasised that the nature of the information is in accordance with a detailed assessment of the structural characteristics of the tissue region under investigation (Nicholas and Nicholas 1980). It is intended that a subsequent paper should report on the ability of this technique to recognize the structures responsible for acoustic scattering. Although the technique, as outlined, is primarily devised for investigation of excised tissues, it is feasible to envisage clinical applications where specific regions of anatomy may be accessible for two-dimensional diffraction scanning. Problems of tissue motion will undoubtedly be encountered although it is hoped that these will be minimal if initial trials are conducted upon thyroid and breast
Two-dimensional diffraction scanning disorders. The full potential of this technique still to b e r e a l i s e d y e t t h e p r e s e n t s u c c e s s e s r e l a t i v e e a s e o f u s e w a r r a n t its c o n s i d e r a t i o n p o t e n t i a l c o m p l e m e n t to e x i s t i n g d i a g n o s t i c cedures.
has and as a pro-
REFERENCES
Bamber J. C. and Dickinson R. J. (1980) Ultrasonic B-scanning: a computer simulation. Phys. Med. Biol. 25, 463--479. Bamber J. C. and Hill C. R. (1981) Acoustic properties of normal and cancerous human liver--l. Dependence on pathological condition. Ultrasound Med. Biol. 7, 121-t33. Bamber J. C., Fry M. J., Hill C. R. and Dunn F. (1977) Ultrasonic attenuation and backscattering by mammalian organs as a function of time after excision. Ultrasound Med. Biol. 3, 15-20. Burckhardt C. B. (1978) Speckle in ultrasound B-mode scans. I E E E Trans. Sonics and Ultrasonics 25, 1-6. Cooley W. W. and Lohnes P. R. (t971) Multivariate Data Analysis. Wiley, New York. Dewbury K. C. and Clark B. (1979) The accuracy of ultrasound in the detection of cirrhosis of the liver. Br. J. Radiol. 52, 945-948. Hill C. Ro, Chivers R. C., Huggins R. W. and Nicholas D. (1978) Scattering of ultrasound by human tissue. In Ultrasound: Its Application in Medicine and Biology (Edited by F. J. Fry), pp. 441-493. Elsevier, Amsterdam. Huggins R. W. and Phelps J. V. (1976) Bragg diffraction scanner
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for ultrasonic tissue characterization in vivo. Ultrasound Med. Biol. 2, 271-277. Joseph A. E. A., Dewbury K. C. and McGuire P. G. (1979) Ultrasound in the detection of chronic liver disease. Br. J. Radiol. 52, 184-188. McCready V. R. and Hill C. R. (1971) A constant depth ultrasonic scanner. Br. J. Radiol. 44, 747-750. Merton J., Nicholas D., Hill C. R., Grover S., Queenan M. and Cosgrove D. O. (1982) Ultrasonic diffraction scanning of the thyroid. Ultrasound Med. Biol. 8, 145-153. Nicholas D. (1976) Ultrasonic Scattering and the Structure of Human Tissues. PhD Thesis, University of London. Nicholas D. (1979) Ultrasonic diffraction analysis in the investigation of liver disease. Br. J. Radiol. 52, 949-961. Nicholas D. (1982) Evaluation of backscattering coefficient for excised human tissues: results, interpretation and associated measurements. Ultrasound Med. Biol. 8, 17-28. Nicholas D. and Hill C. R. (1975) Tissue characterization by an acoustic Bragg scattering process. In Ultrasonics International 1975, pp. 269-272. IPC Science and Technology Press, London. Nicholas A. W. and Nicholas D. (1980) The analysis of threedimensional tissue structures using ultrasound diffraction patterns. Phys. Med. Biol. 25, 759. Nicholas D., Barrett A., Chu J. M. G., Cosgrove D. "O., Garbutt P., Green J., Pussell S. and Hill C. R. (1980) Computer analysis of grey-scale tomograms. In Acoustic Imaging (Edited by A. F. Metherell), Vol. 8, pp. 731-744. Plenum Press, New York.