C-scan ultrasonography in orbital diagnosis

British Journal of Ophthalmology, 1977, 61, 735-740

C-scan ultrasonography in orbital diagnosisMARIE RESTORI AND JOHN E. WRIGHTFrom Moorfields Eye Hospital, London

SUMMARY A C-scan imaging facility has recently been added to the ultrasonic system in use atMoorfields Eye Hospital, London. The technique is explained and typical C-scans are presentedto demonstrate the normal orbital fat and optic nerve, together with selected pathological conditionsin the orbit. The C-scan facility permits imaging of the orbital contents in the coronal plane. Thiscoronal plane imaging, together with high resolution and sensitivity, makes this a useful technique fordemonstrating orbital lesions and it is hoped helps in the accurate measurement of the diameters ofthe optic nerve along its length. The problems associated with C-scanning are discussed.

The coronal plane of the orbital contents is inac-cessible to ultrasonic B-scan investigation becausethe surrounding orbital walls prove a barrier to thesound beam. This plane may, however, be imagedusing a C-scan technique. The C-scan also providesa means of measuring the diameter of the opticnerve, for which purpose it is potentially moreaccurate than both A-scan and B-scan.

This paper presents preliminary results of clinicaltrials of a C-scan facility which has recently beenadded to the Moorfields ultrasonic system (Aldridgeet al., 1974). This system is also capable of A-scan,mechanical B-scan (McLeod et al., 1977), andholographic imaging (Restori, 1977).

Methods

The coronal C-scan plane of the orbit, togetherwith the horizontal and saggital B-scan planes, areillustrated in Fig. 1. The method of scanning toobtain a C-scan is illustrated in Fig. 2.The eye is anaesthetised with oxybuprocaine HC1

drops and the eyelids are retracted with a Barraquerspeculum. A focused 10 MHz transducer (focallength 46 mm and focal spot diameter 0 8 mm), iscoupled to the eye by means of a bath (Purnell,1966) containing saline at 36°C. A fixation light isused to reduce eye movements to a minimum. Tocomplete the C-scan the transducer sweeps mechani-cally in a rectilinear fashion to cover a 4 cm squareaperture in which the eye is centralised. Each 4 cmsweep of the transducer takes approximately 140 msto complete. Either 80 or 160 transducer sweepsmay be employed to produce a C-scan takingAddress for reprints: Miss M. Restori, Ultrasound Depart-ment, Department of Clinical Ophthalmology, MoorfieldsEye Hospital, City Road, London ECIV

11 or 23 s respectively to complete. C-scans com-piled in 23 s contain more information than thosecompiled in 11 s and were the option chosen forall C-scans described in this paper. The transduceremits pulses of sound and, in the time intervalbetween emission of pulses, returning echoes arereceived by the same transducer. Only echoes fromwithin two pre-selected depths within the orbit arerecorded, these depths being selected by earlierreference to the B-scan display (Fig. 3). The focalplane of the transducer is arranged to lie at thecentre of these chosen depths. The resolution attain-able, therefore, is dependent on the focal spotdiameter of the transducer (0X8 mm for all C-scansdescribed in this paper). The echoes from the pre-selected depths correspond to those receivedwithin a period of time (time-gate) a certain timeafter the emission of a pulse from the transducer.If the time-gate is kept narrow, that is the pre-selected depths are close together, a thin section oftissue may be imaged. The time-gate is normallyset at 2 [Ls which, assuming a short examiningpulse of sound, is equivalent to 1-45 mm in thick-ness of orbital fat. The intensities of the recordedechoes are used to modulate the brightness of thespot of a cathode ray tube, which is driven insynchronism with the transducer. The C-scanimages the soft tissues within the time-gate in thecoronal plane and is recorded on Polaroid positive/negative film.

Results

To date 82 orbits have been examined using theC-scan in conjunction with the B-scan technique.C-scan examination was employed in all cases ofsuspected optic nerve lesions or for those lesions

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Marie Restori and John E. Wright

Sagittal tumour, possibly by psammoma bodies which were1B- scan plane demonstrated histologically. These echoes were

not demonstrated on the B-scan display. A lym-phoma wrapped around the optic nerve is illustrated

Horizontpl in the C-scan (Fig. 6). The optic nerve lies withinB-scon plane YJ 7

Transducer a 4 cm -

/ Coronal 4cm /4mmC-scan plane ' 1s

/ ll23seconds757-

Fig. 1 Planes of B-scan and C-scan examinations (Foca lengthof transducer

in which it was important to ascertain their positionrelative to the optic nerve. In addition, C-scaninvestigations were performed whenever there was /some doubt about the nature or extent of an orbital /lesion. C-scan /A normal C-scan section taken a few millimeters serltion

behind the globe is shown in Fig. 4. Individual /lobules of fat can be delineated and the optic /nerve is seen as an acoustically clear area. A variety /of pathological changes in the orbit are demon-strated in Figs 5 to 8. fThe C-scan in Fig. 5 displays a meningioma Tire gae

surrounding the optic nerve. The echoes arising 45mmfrom this mass represent sound scattered by the Fig. 2 C-scan imaging of the orbit

* Fig. 3 B-scan section showingpreselected depths (time-gate)in a normal orbit for C-scanexamination

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C-scan ultrasonography in orbital diagnosis

the clear area in the centre of the fat pad, and issurrounded by the acoustically homogenous lym-phoma. As neither the mass nor the optic nervegives rise to echoes the exact border of the nervecan only be surmised. The mass is seen to be lobu-lated. The attenuating properties of this mass, itshomogenous nature, and the lobulated shape

which was well demonstrated on C-scanning, ledto a correct preoperative ultrasonic diagnosis oflymphoma.The distorted fat pad of a dysthyroid patient is

shown in Fig. 7. The grossly enlarged superiorrectus muscle is seen as an acoustically clear areaabove the optic nerve. The full extent of the enlarge-

Fig. 4 A normal fat pad inC-scan section

Fig. 5 Optic nerve meningiomain C-scan section

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Fig. 6 C-scan section showinglobulated lymphoma around theoptic nerve

Fig. 7 Fat pad ofdysthyroidpatient with enlarged superiorrectus muscle seen directlyabove the optic nerve-C-scan

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C-scan ultrasoniography in orbital diagnosis

Fig. 8 Mass originating in thelacrimal gland shown in coronalC-scan plane

ment was determined by means of consecutiveC-scan sectioning of the orbit. In Fig. 8 a largelacrimal mass is illustrated and its position inrelation to the globe is demonstrated. The indenta-tion of the superotemporal equator of the globe bythe mass is clearly illustrated. Part of the mass givesrise to echoes, while other parts of the mass appearacoustically homogenous. This mass proved at sur-

gery to be a lacrimal carcinoma lying within a cyst.

Discussion

Several authors (McCready and Hill, 1971; North-eved et al., 1974; Suarez et al., 1975) have discussedthe possibility of employing a C-scan modality inmedical diagnosis, but the technique has not beenwidely used. C-scanning of the orbit has proved tobe particularly useful as it displays that plane in thefat pad which the surgeon progressively dissectsduring lateral orbitotomy. In addition, the positionof lesions in relation to the optic nerve can beclearly demonstrated. The C-scan technique usedin conjunction with the B-scan also permits threemutually perpendicular planes within the orbit tobe visualised, such that the full extent and shape ofa lesion can often be determined.

In the C-scan technique, the focal plane of thetransducer is arranged to coincide with the planeto be imaged so that certain advantages accrue.

Firstly the power of the sound beam is at a maxi-mum in the focal region, and thus the transducer isat its most sensitive here. This high sensitivityallows low amplitude echoes to be detected and,therefore, more information about the likelynature of a mass is received. Secondly, the soundbeam is narrowest in the focal region, and equal tothe focal spot diameter of the transducer. Conse-quently, since the resolution on the C-scan dependson the width of the examining sound beam in theimaged plane, resolution is optimised.

C-scan imaging employing strongly focusedtransducers, and hence having very small focalspot diameters, could be used as a means of measur-ing the diameter of the optic nerve with high resolu-tion. It is intended to employ such transducers in afuture study, correlating results with measurementstaken at surgery. The C-scan has advantages overboth B-scan and A-scan techniques for thesemeasurements. The B-scan is limited by the widthof the examining sound beam, and as transducerssuitable for B-scan are only weakly focused, theresolution attainable is not as great as that possibleemploying a C-scan technique. The A-scan measure-ments are limited by the difficulty in arranging thesound beam to strike the optic nerve perpendicu-larly, a prerequisite for accurate A-scan measure-ment. Thus the diameter of the optic nerve tendsto be overestimated.

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C-scans are degraded by gross eye movementsor by small involuntary movements (Coleman, 1971)occurring during construction of the image; forthis reason attempts are made to keep such move-ments to a minimum. C-scans taking 11 s ratherthan 23 s to complete may be the option chosen forpatients who find difficulty in holding the eye still.Because the orbital fat is more static than the eye,it is less susceptible to such degradation and thuslends itself to C-scan examination.

The authors would like to thank Terry Tarrant forall illustrations and Erica Banks for her secretarialassistance.

References

Aldridge, E. E., Clare, A. B., and Shephard, D. A. (1974).Scanned Ultrasonic Holography for Ophthalmic diagnosis.Ultrasonics, 12, 155-150.

Coleman, D. J. (1971). Measurement of choroidal pulsa-tions using M-scan ultrasound. American Journal ofOphthalmology, 71, 363-365.

McCready, V. R., and Hill, C. R. (1971). A constant depthultrasonic scanner. British Journal ofRadiology, 44, 747-750.

McLeod, D., Restori, M., and Wright, J. E. (1977). RapidB-scanning of the Vitreous. British Journal of Ophthal-mology, 61, 437-445.

Northeved, A., Holm, H. H., Kristensen, J. K., Pedersen,J. F., and Ramussen, S. N. (1974). An automatic ultra-sonic spherical section scanner. Ultrasound in Medicineand Biology, 1, 183-186.

Purnell, E. W. (1966). Ultrasound in Ophthalmic Diagnosis.In Diagnostic Ultrasound, p. 95. Edited by C. C. Crossman,J. H. Holmes, C. Joyner, and E. W. Purnell. Plenum Press:New York.

Restori, M. (1977). Ultrasonic Holography in OphthalmicDiagnosis. British Journal of Clinical Equipment (In press).

Suarez, J. R., Marich, K. W., Holzemer, J. F., Taenzer,J. F., and Green, P. S. (1975). Biomedical Imaging withthe SRI Camera. Proceedings of the 6th InternationalSymposium on Acoustical Holography. Edited by NewellBooth. Plenum Pres: New York.

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diagnosis.C-scan ultrasonography in orbital

M Restori and J E Wright

doi: 10.1136/bjo.61.12.7351977 61: 735-740 Br J Ophthalmol

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C-scan ultrasonography in orbital diagnosis

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