A-Scan

✍️ frequently employed for two purposes

💧 calculate axial eye length by measuring distance between echo spikes

💧 obtain tissue diagnosis by measuring the amplitude and pattern of echo spikes

✍️ types of A-scans in ophthalmic practice

👉 Biometry A-scan

💧 measure axial eye length.

💧 utilizes linear amplification to display signals of highly reflective spikes of the familiar interfaces in the normal eye (the cornea, lens, and retina) which are required to measure the axial length.

👉 Vector A-scan

💧 often displayed simultaneously with B scan on the screen

💧 useful for reference and B-scan correlation

💧 limited for tissue differentiation, since its amplifcation is logarithmic as it follows the B-scan.

👉 Standardized A-scan

💧 specifically designed A-scan developed to combine dynamic range and sensitivity features of both linear and logarithmic amplifiers for optimum tissue differentiation

💧 can be calibrated by use of tissue model by the setting of ‘T-sensitivity’ to ensure the same appearance of similar tissues in different scanners.

🌕. Examination steps of A-scan

✍️ the instrument is set at ‘T-sensitivity’ and use tissue model for calibration

✍️ the patient is positioned with the head near the screen

✍️ Topical anesthetic drops are instilled into the eye

✍️ the probe is placed on the globe.

✍️ No coupling jelly is required, the tear film is more than enough for sound transmission.

✍️ in biometry ( measure axial length) an axial scan is obtained by placing the probe on the cornea with the patient gazing at the primary position

✍️ For diagnosis utilizing A-scan, the lens is better avoided and the probe is placed on the sclera near the limbus (posterior scan) with the patient gazing at the opposite meridian to that of the probe

✍️ Eight meridians are scanned by shifting and tilting the probe in a single, smooth arc movement in an anteroposterior direction from the limbus to the fornix( so the opposite globe wall is scanned posteroanteriorly

✍️ In all scans, perpendicularity of the sound beam incidence is maintained and verified by observing a smooth, steeply rising, tall retinal spike .

✍️ A suggested labelling system is to name the scans according to the clock hour and the anterior–posterior location of the beam on the globe wall for easy recall and diagnostic interpretation and correlation

👉Examples

💧in 12E , the scan beam is located at 12 o′clock equator and the probe is placed at 6 o′clock halfway between the limbus and the fornix.

💧 In 3P, the scan beam is at 3 o′clock posterior to equator and the probe is at 9 o′clock limbus.

💧 In 9A, the beam is at 9 o′clock anterior to equator and the probe is at 3 o′clock fornix.

🌕 Interpretation of diagnostic A-scans

✍️diagnostic A-scan mainly provides quantitative data and to a lesser degree kinetic data while B-scan mainly provides topographic and kinetic data .

✍️ the quantitative data obtained from A scan

👉 reflectivity

💧 the measurement of spike amplitude.

💧 can be an absolute value in dB or a percentage comparison between the initial spike (100% tall) and the spike in question.

💧 the sclera, for example, is the most highly reflective structure with a 95–100% tall spike.

💧 Retinal detachment, when scanned perpendicularly, normally produces a 90–100% tall spike.

💧 PVD usually displays a spike < 80% tall ( average 40 %).

💧scanning perpendicularly is mandatory in order to maximize the spike height.

💧 a low retinal spike is encountered if the retina is very folded

💧 a high spike is encountered in long-standing, thick inferior PVD, commonly seen in diabetic retinopathy with chronic VH

👉 internal structure

💧 describes the regularity of height and interval of a group of spikes representing a mass lesion.

💧 Regularly structured spikes tend to originate from a regular (small or large) histological unit, giving rise to homogeneous acoustic interfaces ( choroidal melanoma and choroidal hemangioma)

💧 irregularly structured spikes applies to irregularly structured lesions such as choroidal metastasis and disciform macular lesion.

👉 sound attenuation.

💧 the angle of decline in the height of spikes (the so-called angle kappa) along the beam path

💧 produced by factors such as sound absorption.

💧 Normally, choroidal melanoma produces strong sound attenuation, while choroidal hemangioma produces weak sound attenuation.

💧 reading the trace of A-scan requires a degree of experience akin to that required to read an ECG in cardiology .

💧 the skill of pattern recognition of A-scans is eventually gained with the experience of scanning a critical number of lesions.

🌕 Normal and abnormal A-scan results

✍️ Normal diagnostic A scan

👉 the A-scan trace is read from left to right.

👉 the first spike is that of the initial (100% tall) spike where the probe meets the globe

👉 followed by the flat, echo silent vitreous line

👉 followed by the retinal spike (100% tall)

👉 ending with a series of high reflective spikes, decreasing in amplitude from left to right, which represents the echoes from the orbital fat.

✍️ Abnormal diagnostic A scan

Globe lesions that produce abnormal A-scan can be summarized into three categories

👉 Membrane lesion

💧 Retinal detachment : a high amplitude and mobile spike( if recently) .

💧 choroidal detachment: a very high, thick (double peaked) and immobile spike

💧 PVD : a low amplitude and very mobile spike

👉 Mass lesion

1- All solid mass lesions demonstrate an immobile surface spike, and the appearance of the echoes between the surface spike and scleral spike is determined largely by the histological structure of the lesion

2- In a well-centered scan (showing high surface and scleral spikes), an accurate mea- surement of the height or thickness of the lesion is also obtained

Examples:

💧 Malignant melanoma

💧 metastasis

💧 hemangioma

💧 macular disciform

👉 Discreet opacities

1- vitreous line in these cases contains numerous small (low amplitude) and tightly packed spikes.

2- the number and height of spikes depend on the density and size of the units producing the echoes

3- asteroid hyalosis producing the highest amplitude and inflammatory cells the lowest

💧 VH
💧 asteroid hyaloid
💧 vitreous cells