NOBC 2025 SESSION B - ALL ABOUT A SCAN

1. Correct technique in performing an A-scan

• A. A-scan machine calibrated routinely

• B. Examiners seated within arm’s length from the machine right beside the patient

• C. Local anaesthetic drops are applied to patient’s eye

• D. The best position to hold the probe is at the wire

Answer explanation

D. False: The best position is actually to hold the probe at the body

2. In measuring A-scan

• A. Increase gain will display better resolution

• B. Gain setting is higher in aphakia compared to dense cataract

• C. Examiner should be able to reach the foot paddle by using the dominant foot

• D. If the gain setting is too low, the scleral spike may be mistaken for the retinal spike, leading to long reading

Answer explanation

A. False: Increase gain will increase spike and sensitivity, but it will display a lower resolution 

B. False: the gain setting is higher in dense cataracts compared to aphakia

3. 3. Validation guidelines for A-scan to be repeated by another operator

• A. Axial length >0.4 mm in the same eye

• B. Axial length < 21 mm or > 26 mm

• C. IOL power < 1.0 D between two eyes

• D. Axial length > 0.3 mm between two eyes

Answer explanation

A. False: axial length > 0.2 mm in the same eye, > 0.3 mm between the eyes

C. False: IOL power > 1.0 D between two eyes

4. 4. Regarding A-scan reading

• A. There are 4 clear echo spikes at maximum height

• B. The rise of echo must be perpendicular to the baseline

• C. Descending orbital fat spike must be present 

•  D. The A-scan reading is acceptable if the difference in the axial length between both eyes are > 0.5mm

Answer explanation

A. False: There are 5 (Contact) & 6 (Immersion) clear echo spikes followed by descending orbital fat

D. False: The discrepancies between the axial length of 2 eyes must be < 0.30mm

5. Regarding echo variability

• A. Significant difference between two tissues interface will produce higher amplitude

• B. The higher the angle of incidence, the higher the echo spike

• C. Misalignment of sound wave affect the quality of echo spikes

• D. Regularity of interface affects echo spike displayed

Answer explanation

B. False: Spike height is also impacted by the angle of the sound wave hitting the interface. When the transducer is held such that the angle of incidence is higher, some echoes will not return to the transducer. As a result, the spike height is lower.