What Is A-Scan Ultrasonography?
A-scan (amplitude scan) ultrasonography is a biometry technique that measures the axial length of the eye, which is the distance from the anterior corneal surface to the retina (specifically, the internal limiting membrane). This measurement is critical for calculating the correct power of an intraocular lens (IOL) before cataract surgery.
The instrument uses high-frequency sound waves (typically 10 MHz) that travel through the eye and reflect off tissue interfaces. By measuring the time between transmitted and reflected pulses and knowing the speed of sound through different ocular media, the instrument calculates the distance to each interface.
The A-Scan Display
The A-scan produces a one-dimensional display of spikes corresponding to tissue interfaces. In a normal scan, you should see five distinct spikes from front to back:
- Cornea (anterior and posterior surfaces)
- Anterior lens capsule
- Posterior lens capsule
- Retina/vitreoretinal interface
- Sclera/orbital fat interface
The spacing between these spikes allows the instrument to calculate individual segment lengths (anterior chamber depth, lens thickness, and vitreous length) as well as the total axial length.
Contact A-Scan
In the contact technique, the ultrasound probe tip is placed directly on the anesthetized corneal surface. The examiner holds the probe perpendicular to the cornea and gently touches it while watching the display for proper spike alignment.
Advantages
- Quicker to perform
- Simpler setup
- Less equipment required
Disadvantages
- Corneal compression: Pressing the probe against the cornea can indent it, shortening the measured axial length by 0.1-0.3 mm. Even a small error leads to a significant IOL power miscalculation.
- Operator-dependent accuracy
Immersion A-Scan
The immersion technique places the probe in a fluid-filled scleral shell (Hansen shell or Prager shell) positioned on the eye. A saline bath separates the probe from the cornea, eliminating any corneal compression.
Advantages
- No corneal compression: Provides more accurate axial length measurements
- More reproducible results
- Generally considered the preferred A-scan technique when optical biometry is not available
Disadvantages
- More time-consuming setup
- Patient must remain supine and still
- Fluid in the shell can create artifacts if bubbles are present
Normal Values
- Average adult axial length: 23.0-24.5 mm (mean approximately 23.5 mm)
- Short eye: Less than 22.0 mm (hyperopic, requires higher IOL power)
- Long eye: Greater than 25.0 mm (myopic, requires lower IOL power)
- Anterior chamber depth: Approximately 2.5-3.5 mm
- Lens thickness: Approximately 4.0-5.0 mm (increases with age and cataract formation)
Quality Control
A reliable A-scan reading requires:
- Proper probe alignment perpendicular to the visual axis
- Clear, tall, perpendicular spikes from all interfaces
- Consistent measurements (within 0.1-0.2 mm across multiple readings)
- The retinal spike should be the tallest spike in the display
- Compare both eyes; axial length difference between the two eyes should correlate with the refractive difference
Key Takeaways
- A-scan measures axial length using ultrasound, which is essential for IOL power calculation before cataract surgery
- Contact A-scan risks corneal compression, potentially shortening the measurement and causing a myopic outcome
- Immersion A-scan avoids corneal compression and is the more accurate ultrasound technique
- Normal adult axial length averages approximately 23.5 mm
- Consistent readings and proper spike alignment are essential quality control measures