9.1 Corneal Topography and Tomography
9.2 Confocal Microscopy
9.3 Optical Coherence Tomography - Macula
9.4 Optical Coherence Tomography Angiography (OCT-A)
9.5 Optical Coherence Tomography - Glaucoma
9.6 Optical Coherence Tomography – Anterior Segment
9.7 Fundus Autofluorescence Imaging
9.8 Fundus Angiography - Fluorescein
9.9 Fundus Angiography - Indocyanine Green
9.10 B-scan Ultrasonography & UBM
9.12 Automated Visual Fields
B (Brightness Display)-scan ultrasonography provides highly valuable dynamic information regarding the state of the intra-ocular contents. Its uses include but are not restricted to the diagnosis of:
The two main strengths of the B-scan ultrasound are the ability to examine all components of the globe and to penetrate through various media opacities. A B-scan ultrasound is made up of multiple A-scan ultrasounds rotating in a plane to form a two-dimensional picture. The strength of the image is dictated by the ultrasound probe (gain) as well as characteristics of the eye (reflectivity, absorption, scatter). The frequency (~10-20MHz for ophthalmic ultrasounds) dictates the resolution and depth (~3cm) of the image. Remember that B-scan ultrasonography is a dynamic process- the mobility of a retinal detachment cannot be reproduced in a print-out. Although print-outs are useful for documentation, the interpretation of scans by the examiner during the exam is crucial.
B-scan ultrasounds can be performed either through the eyelids or directly on the globe:
Axial scans are performed with the patient looking straight ahead (in primary). The probe is aimed directly through the pupil. Although commonly performed, it does not adequately view the retinal periphery. It is useful in scanning the macula- find the optic nerve then tilt the probe slightly temporal to this.
Be aware that cataractous lenses, intraocular lenses and intravitreal gas bubbles have high reflectivity, affecting the quality of images. Axial scans in these patients may produce significant distortion. Longitudinal and transverse scans may be more appropriate in these patients.
Axial scans pass through the pupil and lens of the eye. When tilted slightly nasal, the optic nerve will be seen. Document the scan by the type (e.g. axial) and the direction (in clock hours, in this case 12 because the marker is pointing superiorly) that the marker on the probe points to.
Transverse scans are performed with the patient looking in the direction of interest. The probe is placed on the opposite side of the globe at the scleral side of the limbus. The mark of the probe is directed parallel to the limbus. This way, images across the globe can be viewed. This can be further delineated by tilting the probe such that anterior (A), Equatorial (E) or Posterior (P) segments of the globe are scanned.
In a 9T scan, the probe is held on the nasal side of the right eye pointing towards the temporal (9 o’clock) meridian. The marker on the probe is directed parallel to the limbus and by convention is held pointing superiorly. This means that the top of the image is the superior (and in this case temporal) retina.
Longitudinal scans are performed with the patient looking in the direction of interest. The probe is placed on the opposite side of the globe at the scleral side of the limbus. The mark of the probe is directed towards the centre of the cornea (perpendicular to the limbus). This way, the ora serrata is shown at the top of the image and the posterior pole with optic nerve at the bottom. Longitudinal scans are useful in identifying very anterior lesions.
In a 9L scan, the probe is held on the nasal side of the right eye pointing towards the temporal (9 o’clock) meridian. The marker on the probe is directed perpendicular to the limbus. Longitudinal scans visualise from anteriorly (at the top of the image) to posteriorly (at the bottom of the image). The optic nerve is usually seen at the bottom of the image.
NB: This patient has abnormally thick sclera/choroid and fluid demonstrated posteriorly. This is called the T-sign and is seen in posterior scleritis.
When there is no view of the posterior segment (such as when there is a dense cataract or vitreous haemorrhage), perform 9 “screening” scans:
Simultaneous B-scan and A-scan of a choroidal melanoma
Note the echolucency on the B-scan, typical of a choroidal melanoma, as well as the inferior serous retinal detachment. The A-scan demonstrates a high initial spike and low internal reflectivity. The thickness of the tumour is measured between the “spikes” of the A-scan after placing the vector at the thickest point on the B-scan.
Ultrasound Biomicroscopy (UBM) has higher frequencies than conventional B-scan ultrasonography, allowing high resolution images of the anterior segment.
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