Normal Anatomy
Eye is fluid filled structure. It is situated in the anterior part of the orbit and embedded in the fat. The Tenon's capsule separates it from the orbital wall. The anterior segment forms 1/6th of eyeball and posterior segment forms 5/6th of eyeball. Normal axial length of eye is 22 mm.
Refractive Media
Cornea, lens, aqueous humor and vitreous gel form the refractive media. Aqueous humor is a saline solution, which fills the anterior segment. Lens is a transparent biconvex body, which rests over the vitreous gel. It is 3 to 4 mm in thickness. The vitreous is transparent gel, which fills the posterior segment. All of them are very good sound conducting media and stand out clearly on HRSG. The retrobulbar area is predominantly filled with retrobulbar fat and optic nerve is loosely embedded in it. It comes out as an echo poor strap in longitudinal axis. The retina is closely applied to the ocular coat and cannot be seen separately in normal eye. It is firmly adherent at ora serrata and optic nerve head.
Examination Technique
2-D Imaging
Short focus 7.5 mhz and 12 mhz real time small part sector transducers are pre-requisite for eye and orbital imaging.
FIGURE 1.1: Sagittal section of orbit. Sagittal section of the orbit shows eyeball, retrobulbar fat, optic nerve and ocular muscles.
The quality of images produced by general purpose Ultrasound with these probes are good and made eye scanning a practical proposition. Sector imaging is done with patient lying supine. The main method of examination is contact method. The probe is directly placed on closed eyelid with intervening coupling gel.
Complete visualization of the ocular structures is achieved with careful movement and orientation of transducers with eye fixed in primary position and in all directions of gaze (Straight head position).
FIGURE 1.2: Cross section of eye. Cross section of the eye shows lens, A.C. chamber, refractive media and posterior segment.
Color Flow Imaging and Doppler Study
Color Doppler study is highly useful in assessing vascular pathologies of eye and orbit. Ophthalmic artery and central artery of retina with short ciliary and posterior ciliary arteries supplying the choroids can be easily documented on color flow imaging. It is the useful method to examine intraocular tumors like melanomas, retinoblastoma and metastasis. It is an important investigation in retinopathies. Neo-vascularisation of tumor and infiltration in adjoining tissues can be better evaluated on color flow imaging.
FIGURE 1.3: Normal sono appearance of eye. HRSG shows normal sono appearance of eye showing pupil, lens- echogenic convex body sitting over vitreous body, iris and suspensory ligaments holding the lens.
FIGURE 1.4: Normal image of eye. HRSG shows normal image of eye showing lens, posterior chamber filled with anechoic vitreous gel.
FIGURE 1.5: Normal eye with retrobulbar space. HRSG shows normal eyeball with optic nerve in long axis as a hypoechoic strap traversing through echogenic retrobulbar fat.
FIGURE 1.6: Color Doppler study shows central artery of retina and vein traversing through optic nerve.
FIGURE 1.7: Spectral Doppler tracing shows flow in the central artery of retina. Short systolic and prolonged diastolic flow is seen in the central artery of retina typical of small arterial flow pattern.
FIGURE 1.8: Normal choroidal flow. Color Doppler flow study shows central artery of retina and its branches short and long posterior ciliary arteries supplying the choroid.