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Midline echoencephalography with the Automatic Midline Computer — a clinical evaluation
Comput. Biol. Med.
Pergemon Press 1972. Vol. 2, pp. 273-284.
Printed in Great Britain.
Midline Echoencephalography with the Automatic Midline Computer-a Clinical Evaluation D. N. WHITE Department of Medicine (Neurology) Queens University, Kingston, Ontario, Canada (Received 21 March 1972) Abstract-1350 consecutive unselected cases had the position of their cerebral midline in the region of the pineal gland, measured by means of the Automatic Midline Computer. In 3 per cent of cases a satisfactory measurement could not be made. The majority of these patients were suffering from cerebral disease but not necessarily deforming cerebral disease. Of the 74 cases measured as having a displaced cerebral midline, the measurement was shown to be correct in 31 cases and considered, on clinical evidence, to be correct in a further 28. In 7 cases an erroneous measurement was almost certainly made, all in patients with cerebral disease. No false negative errors are known to have been made in this series.
IN NUMEROUS publications(1-5) we have emphasized that Midline Echoencephalography with the conventional video A-mode display only achieves acceptable accuracy by means of the clinical bias of the operator and interpreter. We do not consider such a test to be of clinical value if it depends upon the presence of a neurological specialist. Neurologists should be capable of giving far more valuable information about a patient in a few minutes than merely a statement of the position of some of the structures in the cerebral median plane. For this reason we have been interested in trying to develop more objective techniques.tG17) However the first successful objective technique was not developed by us but by Williams. Williams’ device, known as the Automatic Midline Computer,* achieves its objectivity by dispensing with the video display and measuring automatically the position of the highest amplitude echo in the region of the cerebral midline with respect to the position of the theoretical cerebral midline calculated by halving the distance from the scalp-air interface reflection from the opposite side of the head. Details of its timing and logic circuits have been described elsewhere.(5*7) The accuracy of the measurements it makes in an experimental situation where only far side and midline reflecting interfaces are present, is ihustrated in Fig. 1. In the clinical situation, where multiple reflecting interfaces are present within the head, we ensure that the measurements are randomized by taking the transducer off the head and reapplying it after each successful measurement. As a result the intensity distribution of the insonating energy is varied with every measurement.@) For this reason the maximal amplitude echo in the midgate often arises from interfaces which are not in the cerebral midline and therefore erroneous measurements of the position of the cerebral midline are made. However, if the insonating field is randomized with every measurement, * Diagnostic Electronics Corp., Box 500, Lexington, Mass. 02173. 273
D.N.
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WHITE
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60 40 20
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FIG. 1. A plot showing the accuracy of the Automatic Midline Computer under experimental conditions in a water bath in which only reflections from the “far side of the skull” and the “midline structures” were simulated. The “midline” target was moved at 1 mm intervals with respect to the calculated mid-position between the transducer and the “far side” reflector. At each position 20 measurements of its position were made by the Midline Computer. The Midline Computer only makes its measurements in increments of 2 mm from the geometrical midline. The measurements made are represented in percentages of the total 20 made at each position. Correct measurements are represented on the same vertical axis as the position of the midline target. Erroneous measurements made are represented as bars to either side of this axis. No measurements made by the computer were in error by more than 2 mm from the position of the “midline” target. It should be noted that since the Computer only makes its measurements in increments of 2 mm none of its measurements could be correct when the target was displaced an odd number of mm from the geometrical midline. It will be noted that the accuracy of the instrument is not affected by variations in the distance of the “farside” target from the transducer i.e. by the size of the head nor by the degree of displacement of the “midline” target from the geometrical midline.
the echoes from the interfaces in the cerebral midline are most often those of highest amplitude in the midgate. Thus more measurements will be made from reflections from the midline interfaces than from reflections from any other single structure even though the measurements in aggregate from all the non-midline interfaces will often exceed in number those from the midline interfaces. Figure 2 illustrates the fact that all the measurements made follow the usual bell-shaped distribution curve and that, even when 70 per cent of the individual measurements are made from interfaces which are not in the cerebral midline, the peak of the curve clearly allows an accurate measurement of the position of the cerebral midline structures to be made. For this reason we believe that our technique of removing the transducer after each measurement is an important safeguard. We also believe that our insistence that the number of the peak measurements must never be less than 25 also ensures better sampling of the measurements made with randomized intensity distributions of the insonating energy. In the clinical situation, the importance of this new technique also lies in its simplicity of operation and its objectivity. A printed set of instructions is adequate to enable persons unfamiliar with its operation to use it satisfactorily. The measurements it makes are in no way dependent upon the clinical skills of the operator. Thus it can readily be used by the nurses and residents in the Emergency department without the need to call an Echoencephalographer and the nurses and residents can have confidence that the measurements they make will be as accurate as those made by the Echoencephalographic technician or the neurologist.
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FIG. 2. Two histograms made on patients with radiologically confirmed undisplaced midline structures. The speckled bars indicate the number of measurements of the position of the cerebra1 midline with the transducer on the left side of the head; the cross hatched bars represent the numbers made with the transducer on the right side. They illustrate the extremes of scatter found in the measurements when the insonating fields are randomized by removing the transducer after each measurement is made. Presumably, even in normal persons, the midline structures give rise to an echo varying in degree by which its amplitude exceeds that of echoes from other intracranial interfaces. For this reason a significant and variable proportion of the measurements made will be erroneous and presumably arise from reflections from structures not in the cerebral midline. In the histogram on the left 30 per cent of all the measurements were erroneous while 70 per cent were erroneous in the histogram on the right. However, even where the histogram shows more scatter, the readings from both sides clearly peak at zero so that the absence of any midline shift can easily and correctly be inferred.
During the last eighteen months we have been using this technique routinely for all echoencephalograms made at the Kingston General Hospital. This report concerns 1350 consecutive, unselected cases examined in this way.
UNSATISFACTORY
EXAMINATIONS
In this series 45 histograms were read as Unsatisfactory which is just over 3 per cent. In examining this group of Unsatisfactory measurements, three main groups of histogram are identifiable comprising 41 cases in the entire group. Firstly there is a group in which the measurements show two peaks which do not overlap as normally they should. Each peak is displaced towards the side of the active transducer (Fig. 3). We believe that these histograms are due to mislocalization of the geometrical midline. If the transmitted pulse is of sufficient amplitude, a reverberation from the scalp-air interface to the outer table of the skull and back to the scalp-air interface may give rise to a signal of sufficient amplitude to trigger the discriminator (Fig. 4). It is even possible that a second reverberation might also be above the
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FIG. 3. A histogram made in a patient with an undisplaced cerebral midline demonstrating the far-side
reverberation error (see text). This histogram, and all subsequent histograms, are made with the same representation as in Fig. 2. It will be noted that almost all the measurements showing a shifted midline of 6 mm to the right were made with the transducer on the right side of the head while those showing a 2 mm shift to the left were made with the transducer on the left side. Such histograms are easily identified and can usually be corrected by reducing the power of the transmitted pulse.
discriminator threshold. Under these circumstances, since the geometrical midline is calculated by halving the distance of the most distal echo to trigger the distal timing
discriminator, the geometrical midline will be mislocalized away from the active transducer by the thickness of the scalp or, by twice the scalp thickness if the second reverberation exceeded the discrimination threshold. The scalp varies in thickness from 3 to 5 mm over the bony skull so that measurements of an undisplaced cerebral midline will be recorded as being shifted 3-10 mm towards the active transducer under such circumstances. The histograms produced are very easy to recognize because each separate peak is made from the readings of only the single ipsilateral transducer (Fig. 3). There were 13 such cases in
our series. As soon as we discovered the cause of such errors we adopted the practise of reducing the ampiitude of the generated pulse and consequently the twin peaks approximated towards each other. As a result the number of Unsatisfactory histograms made for this reason has sharply declined in the latter part of our series. It is true however that, by reducing the transmitted power, it is sometimes not possible to get the twin peaks to overlap. Too great a reduction results in no measurements being made. Under these circumstances, when the twin peaks are displayed more or less symmetrically 2 or even 4 mm to either side of the midline, we now feel sufficient confidence in understanding the cause for such an error that
FIG. 4. An A-mode display of the echoes from the far-side of the head demonstrating how the farside reverberation error is caused. The lower trace is the signal from the far-side discriminator which is triggered (upward deflection) when the amplitude of the far-side echoes exceeds a given threshold. A is the echo from the inner table of the skull. B may be from the outer table of the skull, C is from the scalp-air interface as can be demonstrated by modulating it by scalp movement. D is the first and E the second reverberation echoes from the scalpair interface to the outer table of the skull and back to the scalp-air interface. Note that D, the first reverberation echo was of sufficient amplitude to have triggered the discriminator; E was not. The diameter of the head will be measured to the last triggering of the discriminator (D) and in this case will be falsely lengthened (CD) by twice the scalp thickness (BC). This photograph was made by, and is reproduced by kind permission of Mr. James Williams.
CBM facing page 276
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such histogram as showing No Shift. So far this interpretation has not proved incorrect. The second group of Unsatisfactory histograms appear to result from prolongation of the reverberation echoes that normally occur between the active transducer and the skull beneath it. There were 9 such cases in the series. Such prolonged echoes, in certain cases, extend sufficiently far from the transducer as to be present at the opening of the midgate. Under these circumstances most of the measurements made will show a maximal shift of 16 mm towards the side of the active transducer. Again they are easy to recognize and, like the far-sided reverberation error, could never be mistaken for a true midline shift (Fig. 5). we will read
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5. A histogram demonstrating the near-side reverberation error in a patient with an undisplaced cerebra1 midline as was measured correctly on 18 occasions. This patient was seen on the video display to have reverberations between the proximal skull and the transducer extending along the time base almost as far as the position of the cerebral midline.
FIQ.
In a few cases this error occurs when the scalp is swollen as is often the case after head injuries when echoencephalography will be of the greatest value. Presumably the increased distance between the surface of the scalp and the outer table of the skull results in a given number of reverberations lasting for a longer time after the generation of the pulse, a sufficient time for such a high amplitude echo to enter the midgate and be measured as the echo of highest amplitude within the gate. In other cases, however, no such obvious cause is present. Examination of such cases with a conventional video display often shows that the reverberations normally seen from the near side of the skull are abnormally prolonged along the time base towards the midline. In these cases apparently the skull reverberates to a
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greater degree than usual for reasons we do not understand. Luckily this is not a common occurrence and was only present in 4 of our series. There is nothing that can be done to overcome this defect. When the defect results from swelling of the scalp, true midline measurements can often be made from the other side of the head and, if they comprise a clear peak, we have again been willing to make an interpretation from the measurements made from that one side alone. The last group of Unsatisfactory histograms are those in which the measurements are so scattered that no clearly defined peak can be seen from which a measurement can be made. Such histograms occur especially in cases of cerebral disease and presumably result from distortion, but not necessarily displacement, of the interfaces in the cerebral midline so that they no longer give rise to an echo of clearly higher amplitude than that from other intracerebral interfaces. There were 19 such cases in our series. 16 of these patients were suffering from intracranial disease. An example of this defect is shown in Fig. 6 where two
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FIG. 6. Two histograms made on a patient with a large, diffusely infiltrating cerebral tumour in the right temporal lobe. Angiograms showed there was no displacement of the cerebral midline structures. The very great scatter is apparent and makes the histograms unreadable. Presumably distortion of the interfaces in the cerebral midline has reduced the high amplitude of their echoes by means of which they are normally distinguished from the echoes from other intracerebral structures.
histograms were made from a patient with an infiltrating astrocytoma in the right temporal lobe which did not displace the cerebral midline. Thus only the 0 measurements are correct. Presumably the cerebral distortion reduced the amplitude of the midline echo so that it was no longer easy to distinguish from the other intracranial echoes. Indeed, of all the measurements made by the computer, 121 and 143 were in error in the two histograms and only 9 and 8 respectively were correct. An even more striking example is shown in Fig. 7 which shows 4 histograms made on the same patient successively during 72 hr after a head injury from a car accident. “A” was made within an hour of the accident and before cerebral oedema had occurred ; it clearly showed no midline shift. “B” was made 18 hr after the accident and was unreadable due to scatter presumably resulting from diffuse cerebral oedema. “C”, made 25 hr after the injury, was also felt to be Unsatisfactory due to scatter but now, because of our increased knowledge of the far-side reverberation error, described above, could probably have been improved by repeating with lower power when it might well have
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FIG. 7. Four histograms made on the same patient following a head injury. A, B, C and D were made 1, 18, 25 and 72 hr after the injury respectively. A & D clearly show there was no shift of the midline; there is little scatter in their measurements. B & C show sufficient scatter to prevent their interpretation. They were made during the phase of cerebral oedema and distortion which presumably reduced the amplitude of the echoes from the midline structures making their identification by the computer more prone to error.
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been readable as No Shift. Lastly “D”, made 72 hr after the accident when the oedema was subsiding, once again clearly showed no shift. It is this group of Unsatisfactory recordings that is responsible for the fact that the majority of patients in whom satisfactory histograms cannot be made are in fact suffering from cerebral disease. This is often due to cerebral oedema and is not necessarily associated with midline displacement if the oedema is diffuse. Some of the near-side reverberation errors are also most likely to be made, as has been mentioned, where the scalp is swollen after a head injury and therefore a number of these errors will also occur in persons with cerebral swelling or oedema. Indeed in our series, out of the toal number of 45 Unsatisfactory histograms, 30 or two-thirds were made in cases where we considered on clinical evidence that cerebral disease was present. Only 9 of these 30 patients were submitted to radiological contrast studies. Of these 9 patients the X-rays showed that the midline structures were displaced in 4 patients and undisplaced in 5. In the group of 16 patients whom we thought did not suffer from cerebral disease, X-ray studies were made in only 7 patients all of whom had undisplaced cerebral midlines. Thus the finding that the Midline Computer is unable to make a satisfactory measurement implies the possibility that cerebral disease, but not necessarily deforming cerebral disease, is the cause. In this way the test resembles Pneumoencephalography where failure to be able to make the examination often reinforces the neurologist in his suspicion that he is dealing with a case of cerebral disease and stimulates further investigation.
MEASUREMENTS
OF DISPLACED
CEREBRAL
MIDLINES
Seventy-four histograms were interpreted as showing Shifts in excess of 3 mm in our series. Previously when we attempted to validate the results of more conventional techniques, we were able to confine our study to those patients in whom subsequent X-ray contrast studies had demonstrated radiologically the position of the cerebral midline structures near the pineal gland.(g-ll) It was the radiological measurement we accepted as correct and with which we compared the echogram measurements. This was no longer possible in the present series because, as the confidence of my fellow clinicians began to grow in the technique, increasingly they accepted the echogram measurements and dispensed with radiological investigations. While it was very encouraging for us to see the technique being used in this way as we had always hoped it could be used, to speed and expedite thepatient’s treatment, it did deprive us of radiological confirmation of our measurements. For validation, therefore, we had to accept a second standard-that of the clinical picture and course of the patient’s disease. If the echogram showed a shift to one side and immediately afterwards the neurosurgeon removed a blood clot from the surface of the opposite cerebral hemisphere and the patient recovered, we considered that our echogram measurement had been correct. Similarly if the patient died and was found to have a space-occupying mass or clot on the side opposite to the shift we also considered the echogram had been correct. Of the total of 74 midline shifts, measured by echography, X-ray validation was only made in 35 cases. The echogram measurement coincided with the radiological measurement in 31. In 4 cases the echogram was shown to be in error. These 4 false positive errors were made in 3 cases of cerebral disease. Of the remaining 39 cases, the echogram measurement was considered to be correct in 28 on the basis of the clinical course of the patient’s disease. In 3 cases we were reasonably certain that the measured shift could not have been present and that a false positive error had
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been made. These 3 cases undoubtedly had cerebral disease but we believed it was not deforming. In 8 cases we were uncertain whether the shift measured was truly present. Again these were all cases in which cerebral disease was present but we were uncertain whether it was deforming. Thus it is apparent that even if all these 8 cases were errors, all 15 false positive errors occurred in patients with cerebral disease. That these errors also presumably resulted from distortion of the intracerebral interfaces, can be appreciated from Fig. 8. The figure shows histograms from two sepaiate patients with radiologically confirmed
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FIG. 8. Histograms from two patients with radiologically confirmed shifts of the cerebral midline of 10 mm to the left and 5 mm to the right. There is more scatter in the histogram from the patient with the larger displacement presumably resulting from the distorted interfaces in the cerebral midline returning an echo of lesser amplitude than in health and thus less easily identified by the computer.
displacement of the cerebral midline of 10 mm to the left and 5 mm to the right. The increase in the scatter of the measurements made from the case with the greater displacement can clearly be appreciated. Presumably the more severe is the disease and midline displacement, the more distorted are the interfaces in the cerebral midline. Thus the amplitude of their reflections is no longer clearly higher than those from other cerebral structures. The computer, therefore, makes an increasing number of false measurements and the scatter in the histograms increase. It will be noted however that even the histogram with the greatest scatter can clearly be interpreted as showing a shift of the midline structures of 8-10 mm to the left. The incidence of false positive errors would appear therefore to lie somewhere between 7-l 5 cases out of 1350 examinations in our series with from 59-67 correctly measured shifted midlines.
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It is interesting that in one of these cases of false positive error the same error was made independently by another interpreter with a conventional video technique. Another of these false positive errors occurred in a case which we do not understand. This was an 82-year-old woman admitted after being found unconscious with a right hemiparesis and aphasia and xanthrochromic C.S.F. She was considered to have had a left cerebral haemorrhage but no investigations were made and the patient improved sufficiently to be transferred to her local hospital. Five echograms were made, 2 showed No Shift and 3 showed a shift of 8 mm to the right. The operator found that the No Shift measurements could be obtained when the transducer was slightly posterior and superior to the conventional above-pinna position. The measurements of the shift, which were also seen on a video display, were found with the normal placement of the transducer. This is the only case that we have yet found where it appeared possible for the operator to bias the readings. We do not understand this case nor which of the echograms gave the correct reading. We wonder if there truly was an 8 mm shift and that the measurements showing No Shift and made with a slightly abnormal placement of the transducer were made from reflections from an undisplaced falx cerebri. It is however the false negative errors that are the most important and which will bring an examination into disrepute. It is forgivable falsely to measure a displaced cerebral midline and thereby instigate further investigation and especially if such errors only occur in persons with cerebral disease. It is unforgivable to examine a person with deforming cerebral disease and state that no deformity exists. Does the Midline Computer make such false negative errors? One thousand two hundred and thirty-one persons in our series were said to have no midline displacement. In 209 of these cases X-ray validation was made with contrast studies and the echogram was found to be correct in every instance. In the remaining 1022 cases we cannot be sure we were always correct. However, Kingston is a small medical centre and all cases of neurological disease admitted to the only neurological or neurosurgical centre in the region come to the attention of the author either directly or indirectly. Since this series started, no patient in this series on whom No Shift was measured has subsequently been brought to our attention with deforming cerebral disease.
CONCLUSIONS Our experience can therefore be summarized by the Table. If this series should prove to be representative and the Midline Computer technique, which can be operated by anyone with a minimum of instruction, makes no false negative errors; only makes false positive errors in cases of cerebral disease; and fails to provide a satisfactory histogram in only 3 per cent of examinations of which the majority of patients also have cerebral disease; then we believe the greatest advance has been made in midline echoencephalography since its discovery. For the first time the test can be used where and when the need for it is greatest; this is away from the neurological centres with their investigative facilities as well as by interns, nurses and technicians in emergencies and repeatedly by them, as needed. SUMMARY Conventional echoencephalographic measurements of the position of the cerebral midline by means of an A-mode video display, have been shown to depend for accuracy on the clinical skills of the echoencephalographer in biassing the measurements he makes.
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TABLE. SUMMARYOF RESULTS IN 1350 UNSELECTED PATIENTS
An automatic technique has been developed which is objective. Its measurements cannot be influenced by the operator. With this technique, while measurements are sometimes made from structures not in the median sagittal cerebral plane, the higher amplitude of the reflections from the midline structures ensures, in the majority of cases, that the greatest number of measurements will be made from their echoes provided the insonating field is randomized between each measurement. The measurements fail however when cerebral disease is present and the distortion of the midline interfaces results in their reflection no longer being of higher amplitude than those from other structures. Under these circumstances the measurements show a wide scatter and the position of the midline either cannot be estimated or it is measured falsely. Presumably the distortion of the cerebral interfaces may result in the reflections from structures not in the cerebral midline becoming of higher amplitude than those from the midline structures. Since these false positive errors only occur in cases of cerebral disease, but not necessarily displacing cerebral disease, the harm they cause is not great. To our knowledge we have made no false negative errors with this technique. Such errors are serious and destroy the value of a medical measurement. The results of 1350 consecutive unselected cases examined by this technique are described. If these results prove representative, then we believe that the automated technique is the greatest advance in echoencephalography since its discovery. At last this simply-made measurement can take its place with the other established neurological investigators to the benefit of patient and neurologist. REFERENCES I. D. N. WHITE, Accuracy of A-scan determination of midline echo, Diagnostic Ultrasound, pp. 142-147. Plenum Press (1966).
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2. D. N. WHITE, Limitations of echoencephalography, Ultrasonics 5,88-90 (1967). 3. D. N. WHITE,A. S. KRAUS,J. M. CLARKand J. K. CAMPBELL,Interpreter error in echoencephalography, Neurology 19,77X%4 (1969). 4. D. N. WHITE, Ultrasonic encephalography, Acta. Rudiol. Diag. 9, 671-674 (1969). 5. D. N. Wrrrr~, Ultrasonic Encephalography. Medical Ultrasonic Laboratory, Etherington Hall, Queen’s University, Kingston, Ont. (1970). 6. D. N. WHITE, A-scan echoencephalography at the cross-roads; Art or Science, Med. Biol. Engng 9, 289-297 (1971). 7. D. N. WHITEand A. C. HUDSON,The future of A-mode midline echoencephalography; the development of automated techniques, Neurology 21, 140-153 (1971). 8. D. N. WHITE, J. M. CLARK,D. A. W. WHITE, J. K. CAMPBELL,K. BAHULEYAN,A. S. KRAUSand R. A. BRINKER,The deformation of the ultrasonic field in passage across the living and cadaver head, hied. Biol. Engng 7, 607-618 (1969). 9. D. N. WHITE and J. B. BLANCHARD,A critical analysis of the amplitude-averaging A-scan technique, Neurology 16, 858-866 (1966). 10. D. N. WHITE, A critical analysis of the amplitude-averaged A-scan, Trans. Am New. Assoc. 91, 363% 365 (1966). Il. D. N. WHITE, Amplitude-averaged echoencephalography, Diagnostica Ultrasonica in Ophthalmologica. 93-100. Purkinje Univ. Press (1968).
Pergemon Press 1972. Vol. 2, pp. 273-284.
Printed in Great Britain.
Midline Echoencephalography with the Automatic Midline Computer-a Clinical Evaluation D. N. WHITE Department of Medicine (Neurology) Queens University, Kingston, Ontario, Canada (Received 21 March 1972) Abstract-1350 consecutive unselected cases had the position of their cerebral midline in the region of the pineal gland, measured by means of the Automatic Midline Computer. In 3 per cent of cases a satisfactory measurement could not be made. The majority of these patients were suffering from cerebral disease but not necessarily deforming cerebral disease. Of the 74 cases measured as having a displaced cerebral midline, the measurement was shown to be correct in 31 cases and considered, on clinical evidence, to be correct in a further 28. In 7 cases an erroneous measurement was almost certainly made, all in patients with cerebral disease. No false negative errors are known to have been made in this series.
IN NUMEROUS publications(1-5) we have emphasized that Midline Echoencephalography with the conventional video A-mode display only achieves acceptable accuracy by means of the clinical bias of the operator and interpreter. We do not consider such a test to be of clinical value if it depends upon the presence of a neurological specialist. Neurologists should be capable of giving far more valuable information about a patient in a few minutes than merely a statement of the position of some of the structures in the cerebral median plane. For this reason we have been interested in trying to develop more objective techniques.tG17) However the first successful objective technique was not developed by us but by Williams. Williams’ device, known as the Automatic Midline Computer,* achieves its objectivity by dispensing with the video display and measuring automatically the position of the highest amplitude echo in the region of the cerebral midline with respect to the position of the theoretical cerebral midline calculated by halving the distance from the scalp-air interface reflection from the opposite side of the head. Details of its timing and logic circuits have been described elsewhere.(5*7) The accuracy of the measurements it makes in an experimental situation where only far side and midline reflecting interfaces are present, is ihustrated in Fig. 1. In the clinical situation, where multiple reflecting interfaces are present within the head, we ensure that the measurements are randomized by taking the transducer off the head and reapplying it after each successful measurement. As a result the intensity distribution of the insonating energy is varied with every measurement.@) For this reason the maximal amplitude echo in the midgate often arises from interfaces which are not in the cerebral midline and therefore erroneous measurements of the position of the cerebral midline are made. However, if the insonating field is randomized with every measurement, * Diagnostic Electronics Corp., Box 500, Lexington, Mass. 02173. 273
D.N.
274
WHITE
loo 60
60 40 20
0) p
a@ 10: 60 60 .$ ::
t c Y x ,D I-
3 0 Lx too 60 60 40 20 0 Shift,
mm
FIG. 1. A plot showing the accuracy of the Automatic Midline Computer under experimental conditions in a water bath in which only reflections from the “far side of the skull” and the “midline structures” were simulated. The “midline” target was moved at 1 mm intervals with respect to the calculated mid-position between the transducer and the “far side” reflector. At each position 20 measurements of its position were made by the Midline Computer. The Midline Computer only makes its measurements in increments of 2 mm from the geometrical midline. The measurements made are represented in percentages of the total 20 made at each position. Correct measurements are represented on the same vertical axis as the position of the midline target. Erroneous measurements made are represented as bars to either side of this axis. No measurements made by the computer were in error by more than 2 mm from the position of the “midline” target. It should be noted that since the Computer only makes its measurements in increments of 2 mm none of its measurements could be correct when the target was displaced an odd number of mm from the geometrical midline. It will be noted that the accuracy of the instrument is not affected by variations in the distance of the “farside” target from the transducer i.e. by the size of the head nor by the degree of displacement of the “midline” target from the geometrical midline.
the echoes from the interfaces in the cerebral midline are most often those of highest amplitude in the midgate. Thus more measurements will be made from reflections from the midline interfaces than from reflections from any other single structure even though the measurements in aggregate from all the non-midline interfaces will often exceed in number those from the midline interfaces. Figure 2 illustrates the fact that all the measurements made follow the usual bell-shaped distribution curve and that, even when 70 per cent of the individual measurements are made from interfaces which are not in the cerebral midline, the peak of the curve clearly allows an accurate measurement of the position of the cerebral midline structures to be made. For this reason we believe that our technique of removing the transducer after each measurement is an important safeguard. We also believe that our insistence that the number of the peak measurements must never be less than 25 also ensures better sampling of the measurements made with randomized intensity distributions of the insonating energy. In the clinical situation, the importance of this new technique also lies in its simplicity of operation and its objectivity. A printed set of instructions is adequate to enable persons unfamiliar with its operation to use it satisfactorily. The measurements it makes are in no way dependent upon the clinical skills of the operator. Thus it can readily be used by the nurses and residents in the Emergency department without the need to call an Echoencephalographer and the nurses and residents can have confidence that the measurements they make will be as accurate as those made by the Echoencephalographic technician or the neurologist.
Midline echoencephalography
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FIG. 2. Two histograms made on patients with radiologically confirmed undisplaced midline structures. The speckled bars indicate the number of measurements of the position of the cerebra1 midline with the transducer on the left side of the head; the cross hatched bars represent the numbers made with the transducer on the right side. They illustrate the extremes of scatter found in the measurements when the insonating fields are randomized by removing the transducer after each measurement is made. Presumably, even in normal persons, the midline structures give rise to an echo varying in degree by which its amplitude exceeds that of echoes from other intracranial interfaces. For this reason a significant and variable proportion of the measurements made will be erroneous and presumably arise from reflections from structures not in the cerebral midline. In the histogram on the left 30 per cent of all the measurements were erroneous while 70 per cent were erroneous in the histogram on the right. However, even where the histogram shows more scatter, the readings from both sides clearly peak at zero so that the absence of any midline shift can easily and correctly be inferred.
During the last eighteen months we have been using this technique routinely for all echoencephalograms made at the Kingston General Hospital. This report concerns 1350 consecutive, unselected cases examined in this way.
UNSATISFACTORY
EXAMINATIONS
In this series 45 histograms were read as Unsatisfactory which is just over 3 per cent. In examining this group of Unsatisfactory measurements, three main groups of histogram are identifiable comprising 41 cases in the entire group. Firstly there is a group in which the measurements show two peaks which do not overlap as normally they should. Each peak is displaced towards the side of the active transducer (Fig. 3). We believe that these histograms are due to mislocalization of the geometrical midline. If the transmitted pulse is of sufficient amplitude, a reverberation from the scalp-air interface to the outer table of the skull and back to the scalp-air interface may give rise to a signal of sufficient amplitude to trigger the discriminator (Fig. 4). It is even possible that a second reverberation might also be above the
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FIG. 3. A histogram made in a patient with an undisplaced cerebral midline demonstrating the far-side
reverberation error (see text). This histogram, and all subsequent histograms, are made with the same representation as in Fig. 2. It will be noted that almost all the measurements showing a shifted midline of 6 mm to the right were made with the transducer on the right side of the head while those showing a 2 mm shift to the left were made with the transducer on the left side. Such histograms are easily identified and can usually be corrected by reducing the power of the transmitted pulse.
discriminator threshold. Under these circumstances, since the geometrical midline is calculated by halving the distance of the most distal echo to trigger the distal timing
discriminator, the geometrical midline will be mislocalized away from the active transducer by the thickness of the scalp or, by twice the scalp thickness if the second reverberation exceeded the discrimination threshold. The scalp varies in thickness from 3 to 5 mm over the bony skull so that measurements of an undisplaced cerebral midline will be recorded as being shifted 3-10 mm towards the active transducer under such circumstances. The histograms produced are very easy to recognize because each separate peak is made from the readings of only the single ipsilateral transducer (Fig. 3). There were 13 such cases in
our series. As soon as we discovered the cause of such errors we adopted the practise of reducing the ampiitude of the generated pulse and consequently the twin peaks approximated towards each other. As a result the number of Unsatisfactory histograms made for this reason has sharply declined in the latter part of our series. It is true however that, by reducing the transmitted power, it is sometimes not possible to get the twin peaks to overlap. Too great a reduction results in no measurements being made. Under these circumstances, when the twin peaks are displayed more or less symmetrically 2 or even 4 mm to either side of the midline, we now feel sufficient confidence in understanding the cause for such an error that
FIG. 4. An A-mode display of the echoes from the far-side of the head demonstrating how the farside reverberation error is caused. The lower trace is the signal from the far-side discriminator which is triggered (upward deflection) when the amplitude of the far-side echoes exceeds a given threshold. A is the echo from the inner table of the skull. B may be from the outer table of the skull, C is from the scalp-air interface as can be demonstrated by modulating it by scalp movement. D is the first and E the second reverberation echoes from the scalpair interface to the outer table of the skull and back to the scalp-air interface. Note that D, the first reverberation echo was of sufficient amplitude to have triggered the discriminator; E was not. The diameter of the head will be measured to the last triggering of the discriminator (D) and in this case will be falsely lengthened (CD) by twice the scalp thickness (BC). This photograph was made by, and is reproduced by kind permission of Mr. James Williams.
CBM facing page 276
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such histogram as showing No Shift. So far this interpretation has not proved incorrect. The second group of Unsatisfactory histograms appear to result from prolongation of the reverberation echoes that normally occur between the active transducer and the skull beneath it. There were 9 such cases in the series. Such prolonged echoes, in certain cases, extend sufficiently far from the transducer as to be present at the opening of the midgate. Under these circumstances most of the measurements made will show a maximal shift of 16 mm towards the side of the active transducer. Again they are easy to recognize and, like the far-sided reverberation error, could never be mistaken for a true midline shift (Fig. 5). we will read
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5. A histogram demonstrating the near-side reverberation error in a patient with an undisplaced cerebra1 midline as was measured correctly on 18 occasions. This patient was seen on the video display to have reverberations between the proximal skull and the transducer extending along the time base almost as far as the position of the cerebral midline.
FIQ.
In a few cases this error occurs when the scalp is swollen as is often the case after head injuries when echoencephalography will be of the greatest value. Presumably the increased distance between the surface of the scalp and the outer table of the skull results in a given number of reverberations lasting for a longer time after the generation of the pulse, a sufficient time for such a high amplitude echo to enter the midgate and be measured as the echo of highest amplitude within the gate. In other cases, however, no such obvious cause is present. Examination of such cases with a conventional video display often shows that the reverberations normally seen from the near side of the skull are abnormally prolonged along the time base towards the midline. In these cases apparently the skull reverberates to a
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greater degree than usual for reasons we do not understand. Luckily this is not a common occurrence and was only present in 4 of our series. There is nothing that can be done to overcome this defect. When the defect results from swelling of the scalp, true midline measurements can often be made from the other side of the head and, if they comprise a clear peak, we have again been willing to make an interpretation from the measurements made from that one side alone. The last group of Unsatisfactory histograms are those in which the measurements are so scattered that no clearly defined peak can be seen from which a measurement can be made. Such histograms occur especially in cases of cerebral disease and presumably result from distortion, but not necessarily displacement, of the interfaces in the cerebral midline so that they no longer give rise to an echo of clearly higher amplitude than that from other intracerebral interfaces. There were 19 such cases in our series. 16 of these patients were suffering from intracranial disease. An example of this defect is shown in Fig. 6 where two
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FIG. 6. Two histograms made on a patient with a large, diffusely infiltrating cerebral tumour in the right temporal lobe. Angiograms showed there was no displacement of the cerebral midline structures. The very great scatter is apparent and makes the histograms unreadable. Presumably distortion of the interfaces in the cerebral midline has reduced the high amplitude of their echoes by means of which they are normally distinguished from the echoes from other intracerebral structures.
histograms were made from a patient with an infiltrating astrocytoma in the right temporal lobe which did not displace the cerebral midline. Thus only the 0 measurements are correct. Presumably the cerebral distortion reduced the amplitude of the midline echo so that it was no longer easy to distinguish from the other intracranial echoes. Indeed, of all the measurements made by the computer, 121 and 143 were in error in the two histograms and only 9 and 8 respectively were correct. An even more striking example is shown in Fig. 7 which shows 4 histograms made on the same patient successively during 72 hr after a head injury from a car accident. “A” was made within an hour of the accident and before cerebral oedema had occurred ; it clearly showed no midline shift. “B” was made 18 hr after the accident and was unreadable due to scatter presumably resulting from diffuse cerebral oedema. “C”, made 25 hr after the injury, was also felt to be Unsatisfactory due to scatter but now, because of our increased knowledge of the far-side reverberation error, described above, could probably have been improved by repeating with lower power when it might well have
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FIG. 7. Four histograms made on the same patient following a head injury. A, B, C and D were made 1, 18, 25 and 72 hr after the injury respectively. A & D clearly show there was no shift of the midline; there is little scatter in their measurements. B & C show sufficient scatter to prevent their interpretation. They were made during the phase of cerebral oedema and distortion which presumably reduced the amplitude of the echoes from the midline structures making their identification by the computer more prone to error.
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been readable as No Shift. Lastly “D”, made 72 hr after the accident when the oedema was subsiding, once again clearly showed no shift. It is this group of Unsatisfactory recordings that is responsible for the fact that the majority of patients in whom satisfactory histograms cannot be made are in fact suffering from cerebral disease. This is often due to cerebral oedema and is not necessarily associated with midline displacement if the oedema is diffuse. Some of the near-side reverberation errors are also most likely to be made, as has been mentioned, where the scalp is swollen after a head injury and therefore a number of these errors will also occur in persons with cerebral swelling or oedema. Indeed in our series, out of the toal number of 45 Unsatisfactory histograms, 30 or two-thirds were made in cases where we considered on clinical evidence that cerebral disease was present. Only 9 of these 30 patients were submitted to radiological contrast studies. Of these 9 patients the X-rays showed that the midline structures were displaced in 4 patients and undisplaced in 5. In the group of 16 patients whom we thought did not suffer from cerebral disease, X-ray studies were made in only 7 patients all of whom had undisplaced cerebral midlines. Thus the finding that the Midline Computer is unable to make a satisfactory measurement implies the possibility that cerebral disease, but not necessarily deforming cerebral disease, is the cause. In this way the test resembles Pneumoencephalography where failure to be able to make the examination often reinforces the neurologist in his suspicion that he is dealing with a case of cerebral disease and stimulates further investigation.
MEASUREMENTS
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Seventy-four histograms were interpreted as showing Shifts in excess of 3 mm in our series. Previously when we attempted to validate the results of more conventional techniques, we were able to confine our study to those patients in whom subsequent X-ray contrast studies had demonstrated radiologically the position of the cerebral midline structures near the pineal gland.(g-ll) It was the radiological measurement we accepted as correct and with which we compared the echogram measurements. This was no longer possible in the present series because, as the confidence of my fellow clinicians began to grow in the technique, increasingly they accepted the echogram measurements and dispensed with radiological investigations. While it was very encouraging for us to see the technique being used in this way as we had always hoped it could be used, to speed and expedite thepatient’s treatment, it did deprive us of radiological confirmation of our measurements. For validation, therefore, we had to accept a second standard-that of the clinical picture and course of the patient’s disease. If the echogram showed a shift to one side and immediately afterwards the neurosurgeon removed a blood clot from the surface of the opposite cerebral hemisphere and the patient recovered, we considered that our echogram measurement had been correct. Similarly if the patient died and was found to have a space-occupying mass or clot on the side opposite to the shift we also considered the echogram had been correct. Of the total of 74 midline shifts, measured by echography, X-ray validation was only made in 35 cases. The echogram measurement coincided with the radiological measurement in 31. In 4 cases the echogram was shown to be in error. These 4 false positive errors were made in 3 cases of cerebral disease. Of the remaining 39 cases, the echogram measurement was considered to be correct in 28 on the basis of the clinical course of the patient’s disease. In 3 cases we were reasonably certain that the measured shift could not have been present and that a false positive error had
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been made. These 3 cases undoubtedly had cerebral disease but we believed it was not deforming. In 8 cases we were uncertain whether the shift measured was truly present. Again these were all cases in which cerebral disease was present but we were uncertain whether it was deforming. Thus it is apparent that even if all these 8 cases were errors, all 15 false positive errors occurred in patients with cerebral disease. That these errors also presumably resulted from distortion of the intracerebral interfaces, can be appreciated from Fig. 8. The figure shows histograms from two sepaiate patients with radiologically confirmed
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FIG. 8. Histograms from two patients with radiologically confirmed shifts of the cerebral midline of 10 mm to the left and 5 mm to the right. There is more scatter in the histogram from the patient with the larger displacement presumably resulting from the distorted interfaces in the cerebral midline returning an echo of lesser amplitude than in health and thus less easily identified by the computer.
displacement of the cerebral midline of 10 mm to the left and 5 mm to the right. The increase in the scatter of the measurements made from the case with the greater displacement can clearly be appreciated. Presumably the more severe is the disease and midline displacement, the more distorted are the interfaces in the cerebral midline. Thus the amplitude of their reflections is no longer clearly higher than those from other cerebral structures. The computer, therefore, makes an increasing number of false measurements and the scatter in the histograms increase. It will be noted however that even the histogram with the greatest scatter can clearly be interpreted as showing a shift of the midline structures of 8-10 mm to the left. The incidence of false positive errors would appear therefore to lie somewhere between 7-l 5 cases out of 1350 examinations in our series with from 59-67 correctly measured shifted midlines.
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It is interesting that in one of these cases of false positive error the same error was made independently by another interpreter with a conventional video technique. Another of these false positive errors occurred in a case which we do not understand. This was an 82-year-old woman admitted after being found unconscious with a right hemiparesis and aphasia and xanthrochromic C.S.F. She was considered to have had a left cerebral haemorrhage but no investigations were made and the patient improved sufficiently to be transferred to her local hospital. Five echograms were made, 2 showed No Shift and 3 showed a shift of 8 mm to the right. The operator found that the No Shift measurements could be obtained when the transducer was slightly posterior and superior to the conventional above-pinna position. The measurements of the shift, which were also seen on a video display, were found with the normal placement of the transducer. This is the only case that we have yet found where it appeared possible for the operator to bias the readings. We do not understand this case nor which of the echograms gave the correct reading. We wonder if there truly was an 8 mm shift and that the measurements showing No Shift and made with a slightly abnormal placement of the transducer were made from reflections from an undisplaced falx cerebri. It is however the false negative errors that are the most important and which will bring an examination into disrepute. It is forgivable falsely to measure a displaced cerebral midline and thereby instigate further investigation and especially if such errors only occur in persons with cerebral disease. It is unforgivable to examine a person with deforming cerebral disease and state that no deformity exists. Does the Midline Computer make such false negative errors? One thousand two hundred and thirty-one persons in our series were said to have no midline displacement. In 209 of these cases X-ray validation was made with contrast studies and the echogram was found to be correct in every instance. In the remaining 1022 cases we cannot be sure we were always correct. However, Kingston is a small medical centre and all cases of neurological disease admitted to the only neurological or neurosurgical centre in the region come to the attention of the author either directly or indirectly. Since this series started, no patient in this series on whom No Shift was measured has subsequently been brought to our attention with deforming cerebral disease.
CONCLUSIONS Our experience can therefore be summarized by the Table. If this series should prove to be representative and the Midline Computer technique, which can be operated by anyone with a minimum of instruction, makes no false negative errors; only makes false positive errors in cases of cerebral disease; and fails to provide a satisfactory histogram in only 3 per cent of examinations of which the majority of patients also have cerebral disease; then we believe the greatest advance has been made in midline echoencephalography since its discovery. For the first time the test can be used where and when the need for it is greatest; this is away from the neurological centres with their investigative facilities as well as by interns, nurses and technicians in emergencies and repeatedly by them, as needed. SUMMARY Conventional echoencephalographic measurements of the position of the cerebral midline by means of an A-mode video display, have been shown to depend for accuracy on the clinical skills of the echoencephalographer in biassing the measurements he makes.
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TABLE. SUMMARYOF RESULTS IN 1350 UNSELECTED PATIENTS
An automatic technique has been developed which is objective. Its measurements cannot be influenced by the operator. With this technique, while measurements are sometimes made from structures not in the median sagittal cerebral plane, the higher amplitude of the reflections from the midline structures ensures, in the majority of cases, that the greatest number of measurements will be made from their echoes provided the insonating field is randomized between each measurement. The measurements fail however when cerebral disease is present and the distortion of the midline interfaces results in their reflection no longer being of higher amplitude than those from other structures. Under these circumstances the measurements show a wide scatter and the position of the midline either cannot be estimated or it is measured falsely. Presumably the distortion of the cerebral interfaces may result in the reflections from structures not in the cerebral midline becoming of higher amplitude than those from the midline structures. Since these false positive errors only occur in cases of cerebral disease, but not necessarily displacing cerebral disease, the harm they cause is not great. To our knowledge we have made no false negative errors with this technique. Such errors are serious and destroy the value of a medical measurement. The results of 1350 consecutive unselected cases examined by this technique are described. If these results prove representative, then we believe that the automated technique is the greatest advance in echoencephalography since its discovery. At last this simply-made measurement can take its place with the other established neurological investigators to the benefit of patient and neurologist. REFERENCES I. D. N. WHITE, Accuracy of A-scan determination of midline echo, Diagnostic Ultrasound, pp. 142-147. Plenum Press (1966).
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2. D. N. WHITE, Limitations of echoencephalography, Ultrasonics 5,88-90 (1967). 3. D. N. WHITE,A. S. KRAUS,J. M. CLARKand J. K. CAMPBELL,Interpreter error in echoencephalography, Neurology 19,77X%4 (1969). 4. D. N. WHITE, Ultrasonic encephalography, Acta. Rudiol. Diag. 9, 671-674 (1969). 5. D. N. Wrrrr~, Ultrasonic Encephalography. Medical Ultrasonic Laboratory, Etherington Hall, Queen’s University, Kingston, Ont. (1970). 6. D. N. WHITE, A-scan echoencephalography at the cross-roads; Art or Science, Med. Biol. Engng 9, 289-297 (1971). 7. D. N. WHITEand A. C. HUDSON,The future of A-mode midline echoencephalography; the development of automated techniques, Neurology 21, 140-153 (1971). 8. D. N. WHITE, J. M. CLARK,D. A. W. WHITE, J. K. CAMPBELL,K. BAHULEYAN,A. S. KRAUSand R. A. BRINKER,The deformation of the ultrasonic field in passage across the living and cadaver head, hied. Biol. Engng 7, 607-618 (1969). 9. D. N. WHITE and J. B. BLANCHARD,A critical analysis of the amplitude-averaging A-scan technique, Neurology 16, 858-866 (1966). 10. D. N. WHITE, A critical analysis of the amplitude-averaged A-scan, Trans. Am New. Assoc. 91, 363% 365 (1966). Il. D. N. WHITE, Amplitude-averaged echoencephalography, Diagnostica Ultrasonica in Ophthalmologica. 93-100. Purkinje Univ. Press (1968).