Ocular echography or ocular ultrasonography (Ophthalmic ultrasound) is an important modality of ophthalmic investigation.
Mechanism of ocular ultrasonogram
The ultrasonogram uses piezoelectric lead-zirconate-titanate crystals which generate ultrasonic (inaudible, more than 20kilohertz) sound waves from electricity. The ultrasonic wave goes and gets reflected from any echodense object. The piezoelectric crystal picks up the reflected sound and converts it to electric signals which give the image of the echogram.
Thus if the velocity of sound in the media is known, the distance of the object from the probe can be measured. The echodensity of the object determines the amount of reflected sound wave received by the probe and hence the intensity of whiteness in B scan and the amplitude in the A scan.
Speed of sound in various media
The speed of sound varies according to the media through which it passes. The distance or axial length is calculated from the speed and time (required by the sound to reach the lesion and come back from the lesion to be detected by the piezoelectric material). When the speed is slowed (eg silicone oil), the machine erroneously shows a long axial length.
|Medium||Speed in meter per second|
|Aqueous and Vitreous||1532|
|Intraocular and orbital soft tissue, blood||1550|
Frequency of the probe
Ophthalmic ultrasound probe usually uses 7.5, 8, 10 (MHz) megahertz or 12 MHz probes contrary to the 2.5MHz or 3.5MHz probes used for abdominal ultrasound. When the frequency of probe increases - the resolution of the image increases but the depth of penetration of the ultrasound decreases. Probe using 20MHz has been used for high resolution imaging of the retina including detection of cystoid macular edema, central serous retinopathy and evaluation of choroidal echoarchitecture. Ultrasound biomicroscopy (UBM) uses 35MHz and 50MHz probes. The 35MHz probe has a higher penetration and can evaluate the ciliary body and ciliary processes in detail.
Types of ocular ultrasound
Amplitude scan shows the amplitude of the echoes as vertical height from a baseline as the strength of the echoes. The horizontal distance between 2 echoes can be used to measure the distance between 2 structures as in measurement of ocular axial length using A-scan probe during ocular biometry before cataract surgery.
The brightness scan gives a 2 dimensional display showing the size and echotexture of a lesion.
Conventions during ocular ultrasound
The B scan probe has a marking. Conventionally the marking denotes the upper part of B scan. The orientation of B scan probe may be longitudinal, transverse or axial. In longitudinal scans, the longest diameter of the oval probe face is perpendicular to the limbus and the marking always faces towards the limbus. In transverse scans, the axis of the probe is parallel to the limbus. For upper and lower horizontal positions of the probe near the limbus, the marking faces nasally. In vertical and oblique probe positions, the marking is kept superior.
Indications of ocular ultrasound
Ocular ultrasound is indicated in
- For ocular biometry for calculation of the power the intraocular lens
- Cases with media haze in which the posterior segment cannot be evaluated clinically
- To determine the nature of ocular mass, optic disc lesion
- To differentiate retinal detachment (RD) of rhegmatogenous etiology from an exudative retinal detachment, and also to differentiate RD from retinoschisis
- To detect and localize intraocular foreign bodies
- To evaluate orbital lesions
- To evaluate Retina, choroid, sclera in various conditions including inflammatory diseases
Screening technique using B scan
In case of media haze, the systematic B scan examination includes 4 transverse scans (scanning superior, nasal, inferior, and temporal retina) and 2 axial scans (Horizontal and vertical).
Evaluation of an ocular ultrasonogram
The evaluation of ophthalmic ultrasonogram includes
- Location of the echo
- Size/shape of echo
- Aplitude of echo (the brightness of echo in B scan)
- Movements of the lesion with ocular movements
- Echotexture of the lesion (homogenous/heterogenous)
- Attenuation of sound beam due to reflection/scattering or absorption of sound energy- which leads to a decrease in the amplitude within the lesion. Structures with very strong sound attenuation like calcification/bone/metallic foreign body produce acoustic shadowing.
Ocular ultrasonogram (USG) in various ocular conditions
Ultrasonographic features of Retinal detachment versus posterior vitreous detachment
One of the most important use of ocular ultrasound is to rule out a retinal detachment (RD) which may require vitreoretinal surgical intervention. Differentiation of a posterior vitreous detachment (PVD) from RD is important. An old RD may also have retinal macrocysts at the macula or retinal periphery, which is detectable by the ultrasonography.
|Posterior vitreous detachment (PVD)|
|Location||Attached to the optic disc at both sides||Is not attached to the optic disc in a complete PVD. In incomplete PVD, one or both sides may be attached to optic disc margin.|
|Shape||Thick folded membrane. May be smooth.||Usually fine, smooth/folded membrane. May be thick especially in cases with neovascularization of disc/fibrovascular proliferation|
|Amplitude of Echo||100% compared to choroid-sclera.||Usually less than 100%|
|Persistence in low gain (around 40-50 decibel)||Persists||Disappears|
|Associated features||May have macrocysts (hemorrhagic or non-hemorrhagic), peripheral membrane also have 100% amplitude||Peripheral/anterior part of membrane usually has <100% spike, may be attached to the ocular coats (retina) at areas other than disc (at neovascularization of retina )|
|Doppler||Vascular||Usually avascular. The fibrovascular proliferations with attached vitreous membranes may have active vascularity.|
The retinal shortening and retinal to choroidal length ratio may denote an increased risk for recurrent retinal redetachment after vitreoretinal surgery. Ocular echography may detect the location of break and characteristic of break (eg. giant retinal tear). The amplitude of subretinal echoes may give a clue to its nature (clear fluid is anechoic, hemorrhage and turbid proteinaceous fluid show point echoes of mild to moderate amplitude).
[The funnel RD is attached to the disc and is not showing after-movements with ocular movements.]
In cases of bullous retinal detachments with 'shifting fluid' (in sitting position the RD is inferior bullous and superior retina seems attached; in supine position, there is a total retinal detachment), it may be necessary to rule out an exudative retinal detachment (ERD). ERD usually shows subretinal echoes, a choroidal mass may be detectable, and the ultrasound may also reveal the cause of ERD (eg, posterior scleritis, Vogt-Koyanagi-Harada syndrome).
A case of choroidal hemangioma with exudative RD. Note the limited after-movements.
An ocular USG also detects tractional retinal detachments which have concave or table-top appearance and limited after-movements. Most commonly, there is evidence of neovascularizations or fibrovascular proliferations.
A PVD is usually a thin wavy membrane which moves freely with ocular movements.
Cupping of the optic nerve head
The optic nerve head may show depression or bean pot like configuration in ultrasound, especially in blind eyes with absolute glaucoma. Ocular echography constitutes an important modality for detecting the cause of vision loss in severe visual impairment with opacity in the ocular media.
Excavation/ectasia/protrusion of ocular coats
The USG also detects elongated globe in pathological myopia with posterior staphyloma. The coloboma of optic nerve head or fundus can be documented.
Ultrasonographic features of Vitreous hemorrhage
Vitreous hemorrhage reveals multiple mobile point echoes in the vitreous cavity. They amplitude of such point echoes is usually mild to moderate. In cases of vitreous hemorrhage due to proliferative retinopathies (proliferative diabetic retinopathy, retinal vasculitis), it is also necessary to localize the area of neovascularization/fibrovascular proliferation which may guide during vitrectomy. The areas of neovascularization (NV) are denoted by areas of strong vitreoretinal adhesion, the PVD is attached to the NV in a 'v' shaped configuration. In vitreous hemorrhage related to PVD induced retinal tear, a PVD is obvious in the USG. Also, the anterior extent of the PVD may be seen attached to the anterior flap of retinal tear. In old patients with evidence of age related macular degeneration in the fellow eye, it is necessary to rule out a large choroidal neovascular membrane with subretinal bleed and breakthrough bleeding into the vitreous.
Note the after-movements of the PVD in this patient with subhyaloid hemorrhage and vitreous hemorrhage.
It is important to keep the gain in optimal setting (around 70decibels- varies with different machines), as high gains (eg 100 decibels) show many echoes in normal transparent vitreous and may give a false impression of vitreous hemorrhage.
Ultrasonographic features of Choroidal detachment (CD)
A CD is denoted by a round mound which is limited by the strong uveoscleral attachments (scleral spur, vortex veins, and optic nerve). It has limited mobility and an A scan passing perpendicularly through a CD shows 'M' (double peak) shape. The shape is typically of a smooth dome, usually not attached to the optic disc. The fluid in the suprachoroidal space may be anechoic (serous CD) or have mild to moderate amplitude point echoes (hemorrhagic CD). When CDs touch each other they are called kissing choroidals and such cases may be more prone to permanent adhesion and may guide the timing of surgery in suprachoroidal hemorrhage (SCH). USG also notes the clot lysis in SCH and guides timing of surgery and the location of SCH guides the area of drainage.
Asteroid hyalosis is denoted by moderate to high amplitude point spikes which are separated from the PVD by a clear space . In silicone oil the speed of sound is low (980 meter/second for 1000 centiStikes and 1040 meter/second for 5000 centiStokes ) and hence the measured axial length by echography is unusually long. Sound attenuation under silicone oil is high, making it unreliable for detection of retinal detachment under silicone oil. After removal of silicone oil, some emulsified droplet of silicone oil may remain in the vitreous cavity giving rise to mobile moderate to high amplitude point spikes in the vitreous cavity.
Retinoschisis is a thin immobile membrane with convex and smooth configuration usually on temporal retina of a hyperopic patient. The height of this round mound of typically does not decrease on indentation or pressure (versus retinal detachment the height of which reduces on indentation).
Ultrasonographic appearance of Choroidal melanoma
A choroidal melanoma shows a solid mass with homogenous appearance (moderate to low reflectivity) arising from the choroid with collar-button or mushroom appearance, acoustic hollow, and choroidal excavation. The A scan shows a typical reduction of amplitude with angle kappa which denotes the sound attenuation within the mass. The mass may be associated with retinal detachment, extrascleral extension, and uncommonly subretinal or suprachoroidal hemorrhage.
Ultrasonographic appearance of Retinoblastoma
Ocular echogram showing moderate to high amplitude echoes usually with acoustic shadow (calcification) in a child with leukocoria denotes a retinoblastoma unless proven otherwise.
Ultrasonographic features of Choroidal Osteoma
Choroidal osteoma shows a choroidal mass high amplitude and acoustic shadow.
Ultrasonography of Choroidal hemangioma
It is solid dome shaped mass of choroid with high amplitude (and thickening of the choroid) but no angle kappa (sound attenuation) unlike choroidal melanoma. The lesions are usually mildly elevated (height is less than width), unlike the melanoma which may have height more than width.
The causes of intraocular calcification extremely high amplitude lesion with acoustic shadow include
Inflammation and infection
Endophthalmitis is a purulent inflammation of intraocular fluids and adjacent structures usually due to infection. The USG shows mild to moderate amplitude point spikes in the vitreous which are very similar to vitreous hemorrhage. Clinical evidence of inflammation, hypopyon and vitritis with recent penetrating trauma or ocular surgery points towards endophthalmitis in such cases. It may be associated with retinal detachment, intraocular foreign body, choroidal detachment and others. The USG may also be used to look for the response to therapy and reduction in vitreous echoes.
It is denoted by thickening of the ocular coats, subtenon fluid, and clinical evidence of pain and limitation during ocular movements usually in cases with orbital cellulitis, penetrating ocular trauma with endophthalmitis.
The thickness of ocular coats is increased with fluid in subtenon space and around optic nerve (T sign). Inflammation of adjacent structures (eg, dacryoadenitis) may also cause a secondary posterior scleritis with subtenon fluid.
Cysticercus is denoted by a round cystic lesion with a hyperechoic dot like lesion (scolex). Rarely, retinal detachment may be associated with cysticercus which is demonstrated in USG. USG also picks up the real time movement of live cysticercus. Myocysticercus may also be detected and monitored for response to therapy.
In acute trauma with open globe injury, the ocular ultrasound may be avoided the as undue pressure on globe may lead to expulsion of intraocular contents. Trauma can lead to vitreous hemorrhage, retinal detachment, suprachoroidal hemorrhage, and others.
USG forms an indispensable part in the management of intraocular foreign body (IOFB). It detects and localizes the foreign body and the size of the foreign body can be measured. Metallic IOFB shows 100% amplitude with acoustic shadow.
Extreme anterior and inferior parts of the vitreous cavity need to be examined with USG along with detailed protocol for screening using B scan in every case of suspected IOFB. CT orbit with thin slices rules out multiple IOFBs/orbital foreign bodies and clearly delineates location/detection of IOFB especially if the foreign body is spanning across the vitreous cavity, ocular coats and retro-ocular space.
USG is helpful for evaluation of the posterior segment after ocular surgeries when media become hazy due to corneal edema, hyphema and other reasons. Other conditions in which USG plays a critical role includes endophthalmitis, retinal detachment, choroidal detachment, and rupture of the posterior ocular coats.
Lens matter drop (posterior dislocation of lens matter)
After posterior capsular rupture and drop of lens matter in the vitreous cavity, the USG locates the matter and gives an idea of the size of the lens matter. The lens matter appears as a lesion in the vitreous cavity (usually inferiorly) with moderate to high amplitude which moves on ocular movement.
Dropped intraocular lens (IOL)
IOL is identified by high amplitude spike which is associated with acoustic reverberations.
Most of the ocular ultrasound machines provide an orbital mode in which deep structures within the orbit can be evaluated like in patients with proptosis. Orbital abscess shows a cavitary lesion with mild to moderate amplitude point echoes. USG may be used to measure the muscle thickness. Due to pressure from the orbital lesion the globe may be compressed.
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