Ophthalmic ultrasonography Flashcards
What is the definition of ultrasound?
sound waves with frequency above the upper limit audible to humans (>20 kHz)
What is the range of frequency of ultrasound waves in ocular media?
8-10MHz
What is the range of frequency of ultrasound waves in orbital media?
4-5 MHz
What is the range of frequency of ultrasound waves in the anterior segment?
50-100 MHz
What is the overarching priniciple of ohow medical ultrasonography works?
sound waves are focused on the tissue of interest and the ‘echoes’ of reflected sound waves are measured
What structure exists in the ultrasonic probe that help it to function?
piezoelectric crystal for both sound wave generation and echo measurement
What is a piezoelectric material?
one that expands or contracts at high frequency when an electrical voltage is applied across it and produces an electrical current when sound waves are applied across it
How does a piezoelectric material work to produce an ultrasound image?
- once captured, the reflected signal is amplified; the ‘gain’ control can be used to adjust this amplification
- the resulting plot of amplitude vs time delay is known as an A-scan and describes the tissue structure at a specific point
How can A-scans be used to obtain accurate measurements of ocular structures?
by multiplying the time delay from any reflective interface by the speed of sound in the tissue
How are B scans genearted from A scans?
A scans can be repeated at multiple transverse locations and maped to a grey scale, producing 2D cross-sectional images (=B scans)
What frequency transducers is accepted by ultrasonography used for ocular utlrasonography?
8-10MHz
What is the usual a) axial resolution and b) transverse resolution of ocular ultrasonography?
- a) 150 microns
- b) 450 microns
What are 3 indications for A scan?
- measurement of axial length (biometry)
- measurement of AC depth or other intraocular distances
- measurement of intraocular mass thickness and characterisation of acoustic properties
What are 4 indications for B scan?
- identification of posterior segment pathology in the presence of media opacity preventing fundal view
- characterisation of intraocular masses
- location of intraocular FBs
- detection of calcification in retinoblastomas and optic disc drusen
What is an example of pathology in the posterior segment that B scan can be used to detect?
identifying retinal break/detachment obscured by vitreous haemorrhage
What are 3 stages to the method of preparing for ocular ultrasonography?
- Apply topical anaesthetic drops
- Apply coupling agent (e.g. methylcellulose) to tip of US probe or to closed eyelids
- Patient reclined/placed supine
What is an example of a coupling agent used for ocular ultrasonography?
methylcellulose
What is used for orientation of the ultrasound probe in ocular ultrasonography?
marker on the probe; when the marker is horizontal with the lids, the image displayed is in a horizontal plane; vertical placement (pointing to eyebrows) generates image in the vertical plane
In what position should the patient’s eye be for ocular ultrasonography?
primary position, then sequentially in all 4 quadrants, horizontally and vertically
What part of the retina is scanned if the probe is moved temporally while patient moves eye nasally?
nasal retina anterior to the equator
What is dynamic scanning in ocular sonography, and what is the benefit?
scanning during eye movements; can help differentiate between PVDs and retinal detachments
What are 3 limitations of A scan ultrasonography?
- is a 1D time-amplitude display
- need to correct for different media e.g. silicone oil in the eye causes speed of sound to vary
- artefactually low axial lengths in conditions e.g. asteroid hyalosis
How does the speed of sound compare in silicone oil vs vitreous media?
slower in oil vs vitreous media
What are 2 situations that can cause A-scan to result in artefactually low axial lengths?
- asteroid hyalosis
- inappropriate application
