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
What frequency ultrasonic transducer is used for orbital US?
lower-frequency - 3-5 MHz
What are 2 indications for orbital ultrasound?
- Assessment for orbital tumours
- Assessment of orbital disease (thyroid eye disease TED, measurement of muscles)
What is the basic principle of how corneal ultrasonic pachymetry works?
simple measurements of corneal thickness can be obtained using 20 MHz ultrasonic probe
What frequency ultrasonic probe is used for corneal ultrasonic pachymetry?
20MHz
What is the range of average central corneal thickness?
490-560 microns
What are 2 situations when corneal ultrasonic pachymetry is indicated?
- Prior to laser refractive surgery e.g. LASIK - assess risk of postoperative ectasia
- In suspected glaucoma, to estimate accuracy of applanation tonometry
How does corneal thickness influence applanation tonometry?
IOP may be overestimated in thick corneas
underestimated in thin corneas
What are the 3 stages to performing corneal ultrasonic pachymetry?
- topical anaesthetic applied
- no coupling agent required
- ultrasonic probe is held in direct contact with, and at 90 degrees to, the corneal surface
What frequency transducer is used for utlrasound biomicroscopy?
35-50 MHz - higher freqency
How does the depth penetration of ultrasound biomicroscopy (UBM) compare to that of other ultrasound forms?
much less depth penetration
Which structures is UBM suitable for imaging?
anterior segment structures
What are 6 indications for UBM?
- corneal biometry
- glaucoma
- unexplained hypotony
- anterior segment tumours
- assessment of crystalline lens and/or implant position, including phakic and piggyback implants
- assessment of anterior segment in corneal opacification
What are 4 things that can be seen with UBM in glaucoma?
- pupil block
- plateau iris
- malignant glaucoma
- pigment dispersion
What are 2 things that may be seen on UBM in unexplained hypotony?
- cyclitic membranes
- cyclodialysis clefts
What are 3 examples of anterior segment tumours that may be visualised on UBM?
- primary cyst
- secondary cyst
- iris and ciliary body melanomas
What are 2 types of implants that can be assessed with UBM?
phakic and piggyback implants
What are 2 examples of causes of corneal opacification when UBM can be used for anterior segment assessment?
- Peter’s anomaly
- sclerocornea
What are 3 stages to the method of UBM?
- topical anaestheic applied
- eyelids open with immersion bath (water or metyhlcellulose) used as coupling agent
- high frequency scans taken, radial and parallel to the limbus, at various predetermined positions
What is colour doppler imaging (CDI)?
US technique that combines conventional B-scan grey scale imaging wiht doppler-based assessment of blood flow
How is blood flow detected using CDI?
using the frequency shift of sound waves reflected from moving blood columns; colour is then added to represent the motion of blood through the vessels
How is colour applied in CDI?
it varies proportional to flow velocity and is colour-coded according to its direction to or from the probe
How can pulses Doppler measurements be recorded with CDI?
using the resulting image the operator can identify a vessel of interest and place the sampling window for pulsed Doppler measurements
What are 3 indications for CDI?
- assessment of blood flow in orbital vessels
- primary evaluation and follow up of orbital vascular lesions
- semi-quantitative assessment of perfusion in retinal and choroidal vascular disease
What are 4 types of vessels that can be imaged with CDI?
- central retinal artery
- posterior ciliary arteries
- ophthalmic artery
- central retinal vein
What are 3 examples of orbital vascular lesions that CDI can be used to evaluate?
- varices
- arteriovenous malformations (AVMs)
- carotid-cavernous sinus fistulae
What is an example of retinal/choroidal vascular disease that CDI can be used to assess perfusion in?
ocular ischaemic syndrome (OIS)
What are the 3 parameters that CDI describes blood flow in terms of?
- peak systolic velocity
- end-diastolic velocity
- resistance index
What information does CDI not provide?
absolute measurements of blood flow (no quantitative information on vessel diameter is obtained)
What size structures can be resolved by a 7.5MHz CDI probe?
0.2mm (200 microns) or larger; can als o measure Doppler shifts in smaller vessels (e.g. posterior ciliary arteries) (diameter 40 microns)
What is shown in the US B-scan?
posterior vitreous detachment
What is shown in the image?
US B-san and A san of retinal detachment - retina appears as highly reflective convex membrane, complete but irregular
What is shown in the image?
US (B scan, vector A-scan and standard A scan) of choroidal melanoma - dome-shaped membrane with low internal reflectivity on standar A scan
What does hte image show?
US (B scan) of buried drusen
What is shown in the image?
high frequency US of anterior segment in patient with phakomorphic glaucoma - anteriorly displaced iris with secondary narrowing of the angles
What is shown in the image?
high-frequency US of the anterior segemnt in a patient with ciliary body melanoma
