|ECR 2019 / C-3233|
|Ocular ultrasound: tips and tricks for general radiologists|
Ocular ultrasonography is one of the less frequently diagnostic procedures in the radiology departments day-to-day work. This might be secondary to the fact that ophthalmic pathologies are usually assessed by ophthalmoscopy performed by ophthalmologists.
Radiologists must know when the ocular ultrasound provides useful information beyond the ophthalmoscopy. Nevertheless, ocular ultrasonography is heavily operator dependent for accurate evaluation, which limits its overall utility. This is why it is prior to having in mind what information could be potentially assessed by different ultrasound modes and how to properly get and interpret it.
Ocular anatomy for radiologists
First of all, to understand properly the ocular ultrasound findings there are some anatomic and vascular structures that are mandatory for the radiologist to be reviewed and not get confused with.
Anterior segment: between the eyelid and the vitreous chamber. It is divided in anterior chamber between cornea and iris and posterior chamber between iris and the posterior capsule of the lens. Posterior chamber is not well visualized with sonography. Please, note that anterior segment and anterior chamber, likewise posterior segment and posterior chamber are not the same!
Posterior segment: it encompasses the vitreous chamber, crossed by the hyaloid conduct, and the posterior pole where there are three layers: sclera, choroid, and retina.
Vitreous chamber is a big space between the lens and the retina. It is composed of water, hyaluronic acid, and collagen fibrils. It is crossed by the hyaloid canal which is attached to the posterior capsule of the lens and to the optic nerve papilla.
The sclera is the most external layer and its function is support and protection.
The choroid is the vascular layer, from where there are vessels which cross to give vascular input to the retina. It is anteriorly fixed just anterior to the ora serrata and posteriorly fixed anterior to the optic disc.
The retina is the sensory layer extended from the optic nerve papilla posteriorly to the ora serrata anteriorly. The relevant vascular structures related to the retina are the central vessels of the retina and the aforementioned vessels coming from the choroid.
Vascularization of the eye is supported by the ophthalmic artery, branch of the internal carotid. The ophthalmic artery runs alongside the optic nerve and its main branches are the central retinal artery which irrigates the retinal internal layer and posterior ciliary arteries which irrigates choroid and the retinal external layer.
Ocular ultrasonography performance has two main roles in the evaluation of ophthalmic pathologies:
- Ocular conditions or circumstances which impede the appropriate evaluation of the ocular pathology by ophthalmoscopy such as blood in the anterior chamber or vitreous chamber, corneal or vitreous opacities, cataracts, post-surgical changes or oedema (Fig. 2 , Fig. 3, Fig. 4, Fig. 5)
- Ocular pathologies in which a thorough ocular ultrasonography, may add useful information in pursuit of reaching an accurate diagnosis. Specially useful in children with clinical leukocoria, foreign intraocular bodies or ocular trauma which leads to differential diagnosis of retinal, vitreous and choroid detachments as well as several posterior pole masses.
Previous to the ocular ultrasonography it is mandatory to assess that there is no ocular perforation, open wounds or severe inflammation which are the contraindications of ocular ultrasound performance. In this case, the elective imaging diagnostic method is computerized tomography.
According to the second point shown in the indications, it is recommendable to know the evaluations that an optimum ocular ultrasound performance must include:
Complete examination and measurements: using a high-frequency probe (7.5 - 15 MHz) through the closed eyelid with a great amount of sonographic gel looking forward to visualizing the whole ocular globe with the axial and longitudinal scan, including the optic nerve and its papilla. It is also critical to measure the diameters of the ocular globe as well as pathological findings.
Examination of the contralateral eye is always recommended to settle comparisons.
Gain modification: gain must be adjusted first to evaluate the anterior segment, then the posterior. The low gain setting will show the walls of the globe and the optic nerve. High gain enables the contents of the vitreous body to be studied. To modify the gain might show up some lesions that could not be noticed with the standard gain.
Vascularity: color Doppler imaging should depict the central retinal artery, the posterior ciliary arteries, and the retinal layer. When a lesion is found, to detect blood flow within the lesions will be helpful in differential diagnosis between diverse findings that seem similar in mode B examination. Low flow settings and a small gate should be chosen.
Convection: this term refers to the continuos slow movement secondary to convection currents. It is suitable to evaluate it when vitreous hemorrhage to be able to distinguish if there are membranes or detachments.
Mobility: the patient is asked to fix the sight and then move their eyes quickly away from their fixation point while the examiner continuously monitors the area of interest. Preparatory to the ultrasonography the patient should be instructed in the ocular movements that will be required.
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