OPTHALMIC ULTRASONOGRAPHY BY DR TEJAS MANKESHWAR INTRODUCTION Ultrasonography

OPTHALMIC ULTRASONOGRAPHY BY DR. TEJAS MANKESHWAR

INTRODUCTION � � Ultrasonography of eye is a non invasive investigation of choice to study the posterior segment of eye in an opaque media. The other common indications are ocular tumours , ocular trauma and proptosis.

• B- scan (Brightness modulation scan) produces real time two dimensional image of particular plane of eye. Multiple images can be obtained by moving the probe in horizontal vertical and transverse direction. • Main advantage of B- scan is ability to study real time movement of lesions within the vitreous cavity. • Echo poor vitreous naturally acts as a contrast.

INSTRUMENTATION � � � Ophthalmic ultrasound should be ideally performed by a dedicated ophthalmic ultrasound scanner. A dedicated ophthalmic scanner includes: B- scan probe : - A 10 MHz small contact B- scan probe is ideal. Synchronised vector A scan : - Ophthalmic ultrasound scanners have A scan superimposed on B scan. The vector of A scan should be perpendicular to the structure whose reflectivity is being measured. Pre Programmed Time gain compensation : - It is necessary to obtain a uniform image of entire orbit as the echogenicity of vitreous (minimum gain) is different from retro orbital space (maximum gain).

OPTHALMIC USG PROBE

TECHNIQUE � � B-scan of the eye can be performed by placing the probe directly on the sclera in an open eye or on closed eyelid. Open eye technique is used as there is attenuation of sound by cornea. In open eye technique position of cornea helps in the better localisation of lesion in the eye. Open eye technique cannot be performed in patients with open corneal wounds and in children. The aim of B-scan is to obtain an image in two perpendicular planes to each other such that one can differentiate point lesion in vitreous from membrane lesions.

IMAGING OF POSTERIOR POLE

• There is a marker on the B-scan probe which represents the top of screen. • The first image is obtained with probe placed over cornea and marker over the temporal side.

The image shows optic disc in the centre of the screen with macula in the temporal side of disc

The next image is obtained by rotating the probe by 90* such that the marker faces superiorly. .

Scan passes through the optic disc and if it is angulated medially then a good view of optic nerve is obtained.

IMAGING OF TEMPORAL QUADRANT � � � Images are obtained by placing the probe on nasal side with eyeball turned temporally. B scan is obtained in two planes perpendicular to each other. In this optic disc is seen at the bottom of the disc with temporal retina superior to it. This is PA view of temporal retina. In the second image maker is placed superiorly to give supero-inferior view of temporal retina. These view help in localisation of the lesion.

In this optic disc is seen at the bottom of the disc with temporal retina superior to it. This is PA view of temporal retina.

NORMAL ANATOMY ON B-SCAN � � � � Normal anatomy is obtained by placing the probe on cornea such that the marker faces temporally. First structure to be visualised is the posterior capsule which is seen as a curvilinear echogenic structure. Vitreous cavity is circular in shape and is anechoic. Optic disc is small area at the anterior end of optic nerve. Macular region is temporal to the optic disc. Optic nerve is seen as an anechoic linear shadow in the retro-ocular space. Orbital fat is seen as a triangular echogenic structure. Extra ocular muscles are seen as linear hypo echoic structures at the periphery of the orbital fat.

LESION EVALUATION ON B- SCAN � When a lesion is detected in posterior segment on B-scan it is evaluated under following headings, -Location of the lesion -Echotexture of the lesion -Relation with the optic disc -Mobility of the lesion.

The most important role of B scan ultra sonography is to localise the lesion to eye or orbit by using various scan planes. This is called as topographic ultrasonography. The image shows elevated lesion in macula obtained by placing the probe over cornea with marker facing temporally.

ECHOTEXTURE OF LESIONS � � B scan helps in characterising the lesions on the basis of Echotexture. There are 3 types if lesions. - Dot like lesions - Membranous lesions. - Mass lesions.

Dot like lesions in the posterior segment mainly represent Vitreous floaters, haemorrhages or vitreous exudates.

Membranous lesions in posterior segment represent Vitreous membranes or retinal detachment.

Mass lesions in posterior segment may be choroidal or retinal tumours.

� B scan helps in establishing the relation of lesion to the optic disc. � � Retinal detachment attached to optic disc. Choroidal detachment not attached to optic disc.

� � B scan helps to study the mobility of the lesion in relation to eye ball. This is known as kinetic imaging. This is the most important part in the evaluation of posterior segment lesions. The two types of mobility are : -moves with eyeball. -Free after movements.

� � Retinal detachment and choroidal detachment move with the eyeball but movement stops as soon as the eyeball movement stops. Free movements are noted in lesions lying free in vitreous cavity. These lesions move with the movement of the eye but the movement continues even after eyeball movement stops. Typically complete after movements are noted in complete posterior vitreous detachment.

NORMAL ANATOMY ON A-SCAN � � � The normal anatomy of A scan is obtained when vector passes through the posterior capsule and the ocular coats near the optic disc. The first spike after the dead space corresponds to first structure visualised on B scan i. e. posterior capsule of lens. The flat scan posterior to posterior capsule corresponds to vitreous cavity. Few small spikes may be noted in the vitreous cavity. The first spike after the vitreous cavity corresponds to retina. The choroidal and scleral spikes are noted posterior to retinal spike.

LESION EVALUATION ON A-SCAN � � � � On A- scan lesions are evaluated on the basis of the reflectivity. The accurate reflectivity of lesion is obtained only when the vector A scan passes perpendicular to it. On A- scan the spike which reaches to the top of the scale is considered 100 percent reflective. All spikes are measured in terms of percentage of top spike. The various reflectivity's are: -High reflectivity(80 -100 percent) -Medium reflectivity(30 -80 percent) -Low reflectivity(0 -30 percent)

� High reflectivity on A – scan is seen in retinal detachment.

� � Medium reflectivity on A- scan is seen in posterior vitreous detachment. Organised vitreous haemorrhage and choroidal tumours also show medium reflectivity.

� � Low reflectivity is seen in vitreous floaters. It is also seen in early vitreous haemorrhage and vitreous exudates.

ANTERIOR SEGMENT EVALUATION � � � The anterior segment is not routinely visualised on B-scan. Posterior capsule is the only structure normally visualised. Ultrasound bio microscopy using 50 MHz probe is used for anterior segment evaluation. In this the probe is directly placed over cornea. Since the B-scan probe cannot be placed over cornea a water bath or gel pad is used as a standoff.

• A modified cup is used for anterior chamber sonography. • The cup fits in between the eyelids. • The cup is filled with normal saline. • The B- scan probe is placed within the saline.

ANATOMY OF ANTERIOR SEGMENT ON B-SCAN � � � � � The gross anatomy can be visualised using the water bath technique. Saline is seen as anechoic area anteriorly. Cornea is seen as curvilinear echogenic lesion. Posterior to cornea is the anechoic anterior chamber. The iris and anterior capsule are visualised as a dense echogenic membrane forming the posterior wall of anterior chamber. The gross image of angle is obtained between the iris and cornea. The normal lens is seen as anechoic or hypo echoic structure posterior to iris. The posterior capsule of lens is seen as a curvilinear echogenic lesion. The posterior chamber is not visualised using the water bath technique.

A- SCAN OF ANTERIOR SEGMENT • A- scan is only useful in the evaluation of ant chamber foreign body and ant chamber tumours. • It shows 3 spikes- first produced by cornea, second by iris and third by posterior capsule of lens.

ANTERIOR SEGMENT PATHOLOGIES

Above image shows echogenic mass lesion at limbus with high degree of irregular reflectivity on A scan suggestive of sq cell carcinoma of limbus.

On the left side image a small mass lesion is seen at uvea on B- scan. But on using water bath technique uniform echotexture of mass is noted. On A scan this shows classical cascading reflectivity characteristic of uveal melanoma.

A well defined anechoic cystic lesion is noted arising from iris without any echoes s/o iris cyst. An iris cyst wall shows high reflectivity whereas cyst cavity has low reflectivity.

Dome shaped elevation of iris suggestive of iris bombe resulting in complete obliteration of iris.

Echogenic foreign body in iris which is producing reverberation artifact in vitreous called as ringing phenomena.

Image shows nasal Subluxation of cataractous lens with posterior capsular rupture.

SOME VITREOUS PATHOLOGIES

• Vitreous floaters are produced due to liquefaction of vitreous. These are seen in old age or high myopia. • On B scan these are seen as few scattered low intensity echoes uniformly distributed in vitreous. On A scan they have low reflectivity (20 -25*/*)

• The deposition of calcium in vitreous cavity is called as asteroid hyalosis. On B scan it is seen as irregular echogenic dot like echoes scattered throughout vitreous with uniform clear band of vitreous between hylosis and retina. • On A scan it produces high irregular spikes.

Vitreous haemorrhage is seen as small dots and short lines on B scan and shows low reflectivity on A scan. It is difficult to distinguish from floaters an should be correlated clinically.

In diabetic retinopathy new blood vessels grow in the cortical gel leading to the formation of fibro vascular membrane, which appears as dense echogenic membrane over retina.

RETINAL PATHOLOGIES

Rhegmatogenous retinal detachment is seen as a uniform V shaped membrane attached to optic disc posteriorly and ora serrata anteriorly. It maintains its echotexture even at low gains. On dynamic B scan it moves with the eyeball and stops as soon as eyeball stops. On A scan it has 80 -100 percent reflectivity.

Retinoblastoma presents on B scan as an irregular dome shaped mass lesion within the vitreous cavity. The presence of calcification within the mass is a hallmark of retinobastoma. It has mixed echotexture due to irregular distribution of tumour cells and calcification.

On A- scan there is high irregular reflectivity due to multiple interfaces produced by irregular distribution of tumour cells.

• Dislocated lens is seen as an round or oval echogenic lesion within the vitreous. The mobility of the lesion depends upon the status of the vitreous. • The posterior capsular shadow is not visualised in case of dislocated lens.

Inflammation of optic disc is known as papillitis which on B scan presents as dome shaped elevation on optic disc which may have irregular outlines.

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