Clinical Techniques
Ultrasound
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Get accessPrinciples
- A piezoelectric crystal transducer produces high-frequency (8-100 MHz) sound waves
- Ultrasound waves have a frequency >20,000Hz
- Higher frequencies provide greater resolution but poorer tissue penetration
- Most ocular ultrasound is performed around 10 MHz
The sound waves travel through tissues and echos are generated from changes in the impedance of a tissue (therefore a homogenous tissue does not produce echoes)
The reflected echo signal is converted into an electrical signal and the amplitude is measured
A-scan
- Plots the intensity of the echo versus time delay: converted to distance
- 5 peaks in a normal A-scan
- Corneal surface
- Anterior lens
- Posterior lens
- ILM of retina
- Sclera
B-scan
- 2-dimensional images created from multiple A-scans.
Hot Topic
Malignant melanoma on ultrasound
- Mushroom, dome-shaped
- >2mm thickness
- High surface reflectivity
- Low internal reflectivity
- Choroidal excavation
Posterior segment tumours on ultrasound
- Choroidal haemangioma:
- Dome-shaped
- Acoustic solidity
High, regular internal reflectivity (due to tightly packed blood vessels)
- Choroidal melanoma:
- Smooth, dome-shaped
- Mushroom/collar button shape is pathognomonic
- Low to medium, regular internal reflectivity
- Retinoblastoma:
High, irregular internal reflectivity (due to focal calcification, necrosis and haemorrahge all within the tumour)
- Choroidal metastases:
- Irregular, lumpy
Medium-to-high internal reflectivity