|Year : 1966 | Volume
| Issue : 3 | Page : 105-116
Diagnostic ultrasonography in ophthalmology
Werner H Buschmann
GDR, Charity Eye Hospital, Humboldt University, Berlin
|Date of Web Publication
Werner H Buschmann
GDR, Charity Eye Hospital, Humboldt University, Berlin
Source of Support: None, Conflict of Interest: None
|How to cite this article:
Buschmann WH. Diagnostic ultrasonography in ophthalmology. Indian J Ophthalmol 1966;14:105-16
Ultrasound is the name for sound waves of high frequency beyond the range of the human ear. The principle of ultrasonic investigation in ophthalmology is to deliver a series of ultrasonic pulses to the eye by a `transducer probe' which are reflected like an echo by any solid or semisolid surface within the eye and are received by the same probe, rectified, amplified and indicated on an oscilloscope [Figure - 1],[Figure - 2]. This is called the pulse-echo technique. [Figure - 3] shows the whole apparatus with the oscillograph, whereas [Figure - 4] shows the different types of transducer probes.
To the transducer probe, which is placed on the surface of the eye, the electronic transmitter of the apparatus delivers an electric pulse of distinct quality (rhythm and intensity) which can be varied as required. This electric pulse is fed into the transducer probe [Figure - 4] by means of a flexible coaxial cable. The transducer consists of a piezoelectric barium titanate plate (or lead niobate, barium circonate) with damping block and coil. There the electric pulse is transformed into an ultrasonic pulse by the variations of the thickness of the plate due to its piezoelectric properties. The ultrasound pulse goes into the plastic material covering the plate as protecting layer and gives the first peak of the series of echoes in Fie. 1 and 2. Then the ultrasonic pulse enters the material over which the transducer is placed for investigation, e.g. the eyeball in our case.
The rhythm of the ultrasonic pulse (frequency 10-15 megacycles/ second = Mc/sec.) is much faster than that of the successive pulse repetition which is only 800 cycles per second. Thus the breaks between the pulses last about a thousand times longer than those of the ultrasonic pulse. This enables us to get sufficient time to receive the echoes from the sound reflecting surfaces and to have a good resolution in depth.
If there are interfaces of two or more media, the sound energy is partly reflected similar to the rules of light reflection and the further the reflecting surface is from the transducer probe, the later the echoes reach the transducer and will therefore be seen proportionately further removed from the initial pulse on the osciloscope [Figure - 1],[Figure - 2]. The amplitude of the echo blip on the screen corresponds within distinct limits to the echo intensity.
This is called A-system or time amplitude technique, which is the most simple technique for ultrasonic investigation and the most reliable for intraocular routine diagnosis. The more complicated B- and C- techniques will be considered later.
The first ultrasonic investigations in ophthalmology were made using ultrasonic flaw detectors (well-known in nondestructive material testing, especially for steel)-Oksala (1965), Nover (1963). Buschmann (1963). But soon we could clarify by experimental and clinical investigations that a special apparatus for ophthalmological use was absolutely necessary. The flaws which can be indicated in steel testing cause steel-gas interfaces reflecting about 99% of the ultrasound pulse energy, whereas the pathologic interfaces within the eye mostly are interfaces of waterlike fluid and soft tissues or tissue interfaces which reflect about 1 °% of the sound energy or even less. Thus the transmitter energy must be higher than in steel testing, and the receiver amplifier must have a better sensitivity, because the echoes are very weak. Devices not sufficient from this point of view cause extremely misleading diagnosis. Hitherto only one apparatus sufficiently specialized for ophthalmology is available-the type 7000 from Kretztechnic/Austria, [Figure - 3] developed in cooperation with us.* Furthermore, the pathologic alterations within the eye are often very small so that we had to improve the resolution power of the apparatus and transducers.
[Figure - 4] shows the transducers specially developed by us for ophthalmology. Minimized normal and flat stalked transducers (the sound beam of the latter runs normal to the probe handle) enables us to investigate the whole eye including the equatorial region with sufficient energy and frequency (10 to 15 Mc /sec). The so-called "Ultrasonolux"-transducer gives the facilities of simultaneous ophthalmoscopy through the transparent body of the transducer whilst the ultrasonic investigation is going on. The ophthalmologist is able to guide the sound beam exactly to distinct areas of small pathologic alterations, which saves much time and increases the accuracy of the diagnostic results. These developments are described in earlier papers-Buschmann 1963.). Just now we have succeeded in developing an extremely minimized suction cup transducer. The focused 9 Mc/sec transducer plate (diameter 2.5 mm) is built into a small corneal contact lens---Buschmann (1965). A small perforation of the contact lens is connected with a pump and enables us to rigidly fix this transducer at any place wanted on the anterior surface of the eye. The diagnostic advantages will he mentioned later.
The frequency of the transducers and the total performance resulting from the cooperation of transmitter, transducer, receiver, amplifier and oscilloscope must be measured exactly before successful clinical work with ultrasonic diagnoses of high safety and sufficient accuracy can be undertaken. Simple methods, easy to realise in every eye hospital under international standard conditions have been described by me for both of these measurements in earlier publications --- Buschmann (1963,). The total performance should be checked up with every transducer at different adjustments of the transmitter performance. For this purpose these adjustments must be done using a Beckman digital counting handle for the measurements as well as for later clinical work.
The total performance is measured in mm of height of a column of standardized paraffin oil which can be penetrated by the ultrasound pulse. Such measurement can thus be done by progressively increasing the height of the column of paraffin oil so that the echoes reflected from the opposite surface become critically unappreciable i.e. they reach the zero line on the oscillograph. The results are shown in a graph [Figure - 5]. After repairs to an instrument and with each new transducer, standardization should be repeated to maintain high diagnostic safety. In any case the instrument must be checked against a paraffin column at least once every three months se that any loss of performance due to ageing of valves may be rectified.
Ultrasound can go through a leucoma as well as through clear cornea, through sclera and through cataract lenses as well as through a detached retina and therefore this method enables us to get informations from regions behind light-opaque tissues. where optical instruments fail or give poor informations only. When I had been to South India (Madurai mainly) recently, I learnt that there ultrasonic investigation is needed still more urgently than in Europe due to the higher percentage of cases with cataract or leucoma and due to higher number of patients unable to co-operate in subjective classical methods (visual field e.g.). Moreover in their present situation Indian ophthalmologists are suffering still more than their European colleagues from a lack of time because of the high number of patients. which press them into adopting time saving methods. Diagnostic ultrasonography is really such a time-saving method if one is trained sufficiently, which takes time and intensive studies not only on the etechnical side but also in ultrasonic physics.
For most investigations the transducer is directly coupled to the anterior surface of the eye after giving anaesthetizing drops. One drop of a methylcellulose solution in water (Methocel) guarnatees best coupling, which is far better than with oil or pure water. The transducer must be coupled subsequently to some or all different points of the cornea and the anterior sclera for most diagnostic tasks. In every position the echogram on the screen is evaluated and a photo is taken. By recording the position of the transducer and the adjusted performance of each exposure, evaluation of the echo - --ram photos can be done later. To go on very fast, an automatic camera is used. Pressing one foot switch once moves the electromagnetic release, and when the switch springs back into off-position the film is moved and the camera is ready for the next photo. An electromagnetic counter (fixed in front of the screen) gives every photo a number as the film moves to the next position. Thus every second a photo could be taken. This is necessary, because a complete investigation of one eye needs about 30 photoes. Other diagnostic tasks (e.g. measurement of axial length or a foreign body position) need a few photoes only.
Measurements of distance:
These measurements are very simple. In clinical use it is herewith simple to differentiate a megalocornea from buphthalmos even in a cataract case, or to make an early diagnosis of a phthisis bulbi and to follow its further course. In aphakic children it is often difficult to do an exact retinoscopy and subjective refractometry may be impossible. By measurement of the corneal curvature (Javal ophthalmometer) and the axial length (ultrasonography) the correct glasses can be calculated. Sometimes, especially in cataract or leucoma cases it may be difficult to find out whether a protrusion is really existing (orbital tumor e.g.) or an enlargement of one eye (unilateral myopia, buphthalmos) is causing pseudo-protrusion. With exophthalmometry we measure the distance between the temporal martin of the orbits and the corneal vertex by optic means: an additional measurement of the eye length by ultrasound gives the correct position of the posterior pole and thus a more precise differential diagnosis can be obtained.
If single cells are completely dispersed in the whole vitreous, no pathologic echo will he registered ordinarily, but such cases are rare. Mostly there are clotted cells or more enlarged particles within a light opaque vitreous and those cause weak, but doubtless visible echoes. In retrolental fibroplasia and in clotted vitreous hemorrhages, somewhat more intensive echoes are indicated. For the former cases the typical position is in the anterior third of the vitreous, only in very bad cases the posterior parts are affected.
A luxated lens gives typical echoes and its position can be determined as well as when it is fixed or moving, regardless of a leucoma cornea or an occluded pupil.
Retinal and chorioideal detachment, retinoschisis, tumours, proliferative retinopathy, granulation tissue, Coats disease :
The diagnoses and differentiations of these conditions need special ultrasonic investigation in different directions and with different performance levels, using our specialised transducers. Most of our patients investigated with ultrasound belong to this group. Not only patients with light opaque refractive media are sent for clarification, whether there is a retinal detachment or an intraocular tumour but more often we have to investigate cases with clear media and prominent alterations in the fundus to clarify the nature of the lesions more accurately than is possible by optical means. A dark prominent lesion may be seen through an ophthalmoscope but it is still doubtful whether this is a pseudotumour, a subretinal hemorrhage or a melanoblastoma. A retinal detachment may be visible but it is not clear whether there is a pigmentless tumour behind this (perhaps with a fluid-filled space between the tumour surface and the detached retina) or not. We have already described a lot of cases belonging to this group in v hick the ultrasonic investigation has been very helpful, and in the same papers we mentioned the present limitations of this method. At present our experiences are based on 1000 patients investigated, about 800 belonging to this group. Progress in the technique of investigation and development of new special transducers further increase the value and accuracy of diagnostic ultrasonography.
Most difficult and important is the differentiation of tumour tissue and nonmalignant granulation tissue or clotted blood. In earlier times, using material testing apparatus, this was considered to be impossible with ultrasonography. Now, with the specialized apparatus and transducers and with the improvements of the investigation method (mainly measurement of performance height of penetrated oil column) we proved that diagnostic errors in this view happened in 8 of our tumour-suspicious cases only (other authors, not yet using measurements of performance, described recently 20% errors!). Further important points for improved ultrasonography are :
1. The scheme of typical echograms and their characteristic variations at different performance levels. [Figure - 6]
A foreign body and a retinal or choroidal detachment are both indicated by one single pathologic echo within the vitreous. Differentiation between a retinal detachment and a choroidal detachment (retina and choroid still in normal attachment to each other) is not yet possible through ultrasonic diagnosis; but a foreign body can be differentiated from them. Mostly it is small, and causes the pathologic echo to register only in a few directions, as against the larger area of a retinal detachment, which causes the pathologic echo in a larger number of neighbouring directions.
We have already shown how total performance can be standardized through a column of parallin oil penetration. Similarly by comparative performances levels, densities of tissues and objects can be ascertained by lowering or raising the intensity of the ultrasonic pulse to critical disappearance levels of the echoes. Thus the retinal and choroidal echoes being softer, disappear early on slight reduction of the intensity from the normal of 27 mm.* used for testing generally, to say 22 mm. In any case they disappear earlier than the echo from the posterior sclera, whereas an echo from an intraocular foreign body will still be obtained at say 12 mm. at which level the posterior scleral scho is not obtainable. Allowance for the attenuation effect from the lens should be made in testing at different performance levels it the ultrasonic pulse has to pass through it to reach a test surface. [Figure - 6]
Similarly, sometimes performance levels have to be increased to say 40 mm. paraffin oil height penetration level before we can obtain echoes from tumour tissue behind a tumour surface.
[Figure - 6] demonstrates that at medium performance levels (23-27 mm. oil column), differentiation between tumour and simple detachment is impossible. The echoes from the tumour tissue behind the tumour surface are so weak that they are below the sensitivity of the oscilloscope and thus they cannot be indicated on the screen. With appropriate performance levels they however become indicated and differential diagnosis is easily possible. (37-43 mm penetrable height of oil column).
On the other hand in high performance levels soft granulation tissue or clotted blood may appear in the echogram like tumour tissue. Here in most cases medium performance levels are necessary for differential diagnosis [Figure - 6]. But some cases with old granulation tissue, without spaces of waterlike fluid and consisting of cells and connective tissue mainly, as for example old pseudotumours of the macula and old Coats disease, cannot yet be differentiated from malignant tumours by ultrasound. Thus we never use the diagnosis "malignant tumour" in our ultrasonic findings but "solid tissue" only. That is all what ultrasound can decide at present--determination of the malignant nature is outside its scope. Nevertheless, solid tissue within the eye means mostly (but not always) malignant tumour tissue. Retinoblastoma, melanoblastoma and metastatic tumours are easy to find with ultrasonography, and if they project more than 0.8 mm, differentiation from simple retinal detachment is possible, but otherwise they cannot be differentiated one from the other.
The measurement of the thickness of the tumour tissue is more accurate through ultrasound than through optical means and is very important for control in conservative tumour therapy using light coagulation and modern radiotherapy. This is true again for the measurement of the distance between a detached retina and the choroid, especially to decide whether light coagulation or diathermic coagulation may be preferred. Attempted light coagulations in cases with too large distance cause shrinking of the detached retina only and prevent the success of later diathermic coagulation.
Retinoschisis as well as retinal detachment with underlying less prominent choroidal detachment are indicated by two pathologic echoes in the vitreous, separated from each other and from the sclera by echo free zero-lines. The importance for diagnosis and surgical treatment is doubtless considerable.
2. So far we have talked of a scheme where the transducer is placed on the cornea, but in localisation of tumours and other conditions like detachment the transducer will have to be shifted on to the sclera and another scheme will have to be required. The sound beam is directed normal to the posterior sclera to get a good choroidal echo which is important for localisation, depending upon the situation of the lesion.
[Figure - 7]a and b show such a localisation scheme which shows how the pathologic findings can be interpreted by investigating in the neighbouring directions.
Good echoes can be obtained only if the sound beam is directed normal or perpendicular to the surface and are indicated by "A" on the scheme. Surfaces situated tangentially to it will give tumour-suspicious echoes and are indicated by "x".
In this scheme a tumour, even a small one, always covers an area [Figure - 7]a and not only a line. Somewhat tumour-suspicious echoes can be received from the margin of a high circumscribed simple retinal detachment. In this scheme the echoes circumscribe an area in the case of simple detachment instead of covering an area, as in a tumour. This permits to make the correct difference diagnosis.
3. Already in the past attempts have been made to use the movements of the reflecting interfaces which occur after a small movement of the eye e.g. in a hemorrhage and in soft tissue with waterfilled spaces, which cause motions and variations of the amplitude of the pathologic echoes on the screen for differentiation from solid (tumour-) tissue, where such motions do not happen. But practically the value of this form of investigation was poor, because it was nearly impossible to prevent sufficiently small motions between the patient's eye and the transducer held in the doctor's hand--even when a fixation light was used. And such movements may cause echo variations in tumour cases as well as in cases with soft nonmalignant alterations.
The recently developed minimized suction-cup transducer [Figure - 4] can be fixed at any point of the anterior surface of the eye. Thus we fix it at some points from which we get tumoursuspicious echoes during the previous ultrasonic investigation with normal or flat-stalked transducers. With such a transducer in position the patient looks with the same or with the other eye to the light arrow projected on the ceiling. This arrow can be moved by the doctor. Small, fast movements gives best results. One must take care (using lid retractors etc.) that the transducer and its very flexible cable does not touch the lids. We could then demonstrate that a neoplastic tumour-echo gram remains completely unchanged inspite of repeated small movements of the eye, whilst the hitherto tumour-suspicious echograms of old, clotted hemorrhages showed considerable variations of the position and the amplitude of the pathologic echoes-during the movements as well as afterwards. The value of this method for accurate tumour diagnosis seems to be high, and the suction disc really prevents movements between the eve and the transducer. We still need experience with a large number of such cases.
Typical ecograms have been described previously, regarding different performance levels. Of course not only radio-opaque but also radio-lucid intraocular foreign bodies cause pathologic echoes, because their sound impedance is always different from the impedancy of the vitreous. Practically this is not so important. Plastic or glass slivers do not cause metallosis, and the possible infection may be treated sufficiently by antibiotics. On the other hand they are non-magnetic and even after correct ultrasonic localisation the operative removal may be very difficult and harmful and such attempts therefore are not justified in most cases. So, if surgical removal is not planned, ultrasonic localisation may be valueless. Ultrasonic diagnosis is more helpful in cases with radio-opaque metal slivers which must be removed due to the danger of siderosis etc. At present we prefer to do first X-ray localisation according to Comberg. The evaluation of the results in the case of X-Rays depends on the use of a scheme which shows the walls of an eye with average diameter and shape. But the eye tissues are not visible in the X-ray investigation and often it is doubtful, whether the sliver is still in the vitreous, within the scleral wall or already outside the eye within the orbit. The treatment depends on the position, because a sliver in the orbit generally should be left alone. Therefore additional ultrasonic investigation is very helpful. All diameters of the eye can be measured exactly and the echo from the foreign body enables us to determine its position and the distance from sclera or from the lens.
If the transducer is coupled to a steel block (plane, exactly parallel surfaces; thickness 10 mm), nearly 100% of the sound pulse energy is reflected at the backside (steel/air interface) and more than 90% of the reflected pulse energy is reflected again to the front (steel/ waterlike coupling fluid interface), only a small percentage goes back to the transducer. Thus the sound pulse is running back and forth within the steel block again and again, and from every backside-reflection an echo is indicated on the screen. We use this like a rule the multiple echoes of the steel block undergoes the same electronic and photographic magnification procedures as the echograms of the eye. To find the true distance between two eye echoes in millimeters we have to compare only their distance in the photo with the distance of the steel block echoes in the photo. [Figure - 8] shows such a "steel-calibration" as well as samples of echogram photoes of one eye with their counter numbers. For evaluation of the photoes it is better to arrange and fix them on cardboard.
Orbit : As against the eye which Dives normally only a few echoes but needs indication of very small pathologic alterations within it and is best investigated ultrasonically by the very sensitive A-System described (time-amplitude ultrasonography), for most investigations within the orbit as well as within most other parts of the human body we need much more complicated techniques of ultrasonography, the so-called B-system (compound scanning, time intensity method). As this method is being developed by us and still being in the experimental stage we defer our comments for the present.
Are there any dangers in doing an ultrasonic investigation?
The performance levels used in diagnostic ultrasonography are very low compared with the levels used in ultrasonic therapy when the average over a minute e.g. is considered; but even the performance level within one pulse does not exceed therapeutic levels. Thus theoretically the method seems to be without danger, and really neither we nor other ophthalmologists using this method have seen any harm in experimental or clinical work. The only harm we have seen had doubtless a simple mechanical origin: the first transducers used have not had a highly polished surface and sometimes a corneal erosion occured. Since we use now highly polished transducers only and methylcellulose solution always we have never seen this complication again.
The apparatus and the accessories specially developed for diagnostic ultrasonography in ophthalmology as well as the management of ultrasonic eye investigation are described. If one is once sufficiently trained it is easy to do diagnostic ultrasonography in intraoculary diseases accurately and fast. The ultrasound pulses go through sclera, leucoma cornea and cataract lenses and do not need clear optic media. They can give us even informations from the space between a detached retina or choroid and the sclera. For Indian ophthalmologists this method may be helpful still more than for Europeans due to the higher incidence of cataract and leucoma in India. Ultrasonography enables us to measure the axial length and all other diameters of the eye, the thickness of the lens, the distance between the sclera and the detached retina or any foreign body and the thickness of tumours. Furthermore diagnosis of retinal detachment, tumours, vitreous opacities and intraocular hemorrhages and their differentiation are possible with sufficient accuracy even in cases where optical instruments fail. These are the results of numerous experimental investigations and the clinical routine work in this method with 1000 patients.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8]