Clinical Methods: A- and B-Scans

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Chapter 3 Clinical Methods

A- and B-Scans

Brandy C. Hayden, Arun D. Singh

Video material online

Introduction

Ophthalmic ultrasonography examination techniques are designed to evaluate all aspects of the globe in a methodical, reproducible manner.¹^–¹⁴ The specific type of examination performed is determined by the indication for examination. Contact B-scan and diagnostic A-scan are most commonly used to evaluate the posterior globe and orbit. Anterior ocular structures can be evaluated with a modified immersion B-scan examination, but are most commonly evaluated with ultrasound biomicroscopy (Chapter 4). This chapter describes the proper methods for performing A- and B-scans.

Basic positioning and patient preparation

Optimal positioning of the patient and the ultrasound display monitor significantly aids in the ease of obtaining and evaluating captured images. In most cases, scanning the eye with contact B-scan and diagnostic A-scan is most effective if performed with the patient in the reclined position. The ultrasonographer is positioned to the patient’s right or left side at the examiner’s discretion. The ultrasound display monitor and the patient’s head should be located parallel and in close proximity to each other to allow for simultaneous viewing of the ultrasound probe position on the eye and the display monitor (Figure 3.1).

Figure 3.1 Proper positioning of the patient in the reclined position with the ultrasonographer able to view both the probe on patient’s eye and the monitor simultaneously. Eye Cubed™ Ophthalmic Ultrasound System.

Reproduced with permission from Ellex Innovative Imaging, Minneapolis, MN, USA.

There are specific exceptions where examining the patient in the upright position is ideal. One common example is the examination of the posterior globe in a patient with a partial gas bubble filling the vitreous space. Such exceptions to the standard reclined patient position are noted in relevant chapters.

Once the patient, personnel and equipment are in the proper orientations, topical anesthetic drops are given to the indicated eye. A methylcellulose-based gel, used as coupling agent, is applied to the B-scan’s probe tip. The patient is instructed to open both eyes and gaze in the direction being imaged. When one of the patient’s eyes is closed, it narrows the contralateral eye and tightens the facial muscles, making the examination more irritating to the patient. When both eyes are open, the contralateral eye is relaxed and can be used to help fixation. The probe is placed directly on the eye. It is possible to obtain B-scan images through closed eyelids. However, it is not recommended in most cases for two reasons. First, the ultrasound waves are attenuated due to the soft eyelid tissue resulting in decreased echo differentiation. Second, it can be difficult to determine the exact position of the B-scan probe on the eye when the lids are closed. In some cases such as the examination of a patient with a ruptured corneal ulcer, or the examination of a small child, the probe should be placed on the eyelids. It is important to note in the examination report that the images were obtained through the eyelids in order to compare subsequent examinations.

B-scan probe

The B-scan probe is a two-dimensional echo display that is used to determine the topographic features of posterior segment pathology including location, shape and extent of lesions (Chapter 2) See Clip 3.1. B-scan probes have a marker along the side of the probe close to the probe tip that indicates the top of the B-scan ultrasound display (Figure 3.2A). The transducer inside the B-scan probe oscillates along the plane of the marker only, towards the marker and away from the marker. Therefore, the top of the B-scan display corresponds to the area indicated on the marker and the bottom of the display corresponds to the plane 180° away from the marker. The probe tip corresponds to the white line on the far left side of the B-scan display. The echoes to the right of this line correspond to the ocular structures opposite the probe tip. The further right the ocular structure, the further away is its echo (Figure 3.2B).

Figure 3.2 (A) B-scan probe. Note white line along side marking the top portion of the B-scan on the ultrasound display monitor. The probe tip corresponds to the white line on the far left side of the B-scan display (B, arrow). Echoes on the right represent the ocular structures. The further right the echo pattern, the further away is the ocular structure from the probe tip.

Reproduced with permission from Ellex Innovative Imaging, Minneapolis, MN, USA.

B-scan probe positions

Transcorneal scans

Axial scans

The axial scan is obtained by placing the B-scan probe tip directly over the cornea while the patient looks in primary gaze (Figure 3.3) See Clip 3.2. The resulting image shows the posterior segment of the globe where the marker tip is at the top of the B-scan display, the crystalline lens and the optic nerve in the center and the portion of the globe 180° degrees from the marker at the bottom of the screen. The axial scan is the easiest scan to interpret because the lens and the optic nerve are centered in the image. However, several issues make this scan less than ideal. There is significant sound attenuation and refraction as a result of going through the crystalline lens showing a diminished resolution in the B-scan image. In pseudophakic eyes, the intraocular lens causes intense sound reverberation echoes obstructing most views to the posterior segment. However, the axial scan can be very helpful in the evaluation of some specific disorders affecting the macula (Chapter 10), Tenon’s space (Chapter 12), and the optic nerve (Chapter 13).

Figure 3.3 Axial probe position. B-scan probe tip placed directly over the cornea (A). Corresponding ultrasonographed fundus (B, line). Resulting B-scan image showing the centered crystalline lens and optic nerve (C, L = lens, V = vitreous, R = retina, S = sclera, ON = optic nerve, O = orbital tissue).

Reproduced with permission from: Ophthalmic Ultrasonography: Theoretic and Practical Considerations. Hayden BC, Kelley L, Singh AD. Ultrasound Clin 2008; 3:179–183.

Axial scans are always obtained with the probe marker facing upward or horizontally.¹² The vertical axial scan is obtained with the marker in the upright position toward 12 o’clock and the horizontal axial scan is obtained with the marker nasally. For oblique axial scans the probe marker is facing toward the upper of the two meridians being examined.

Para-axial scans

Para-axial scans can be helpful in the evaluation of the peripapillary fundus. The para-axial scan images the fundus directly adjacent to the optic nerve. To obtain the scan, the probe tip is placed directly over the cornea as in the axial scan; however, the sound beam is shifted slightly to the peripapillary area of interest. The sound beam is directed through a portion of the crystalline lens in these scans and some sound attenuation, although not as marked as that occurring with the axial scan, occurs resulting in decreased resolution. Para-axial scans are integral in obtaining accurate dimensions of peripapillary mass lesions (Chapter 11).

Trans-scleral scans

Longitudinal and transverse scans are the two most commonly used scans in ophthalmic ultrasonography. These scans bypass the crystalline lens and thus result in better resolution of ocular structures than the axial scans. Additionally, patients are generally more cooperative when the cornea is not covered during the exam. Both types of trans-scleral scans are obtained by placing the probe tip on the conjunctiva 180° away from the area of interest. The patient’s gaze should be directed approximately 30° in the direction of the area to be examined. For example, if a lesion is located superiorly, the patient is instructed to look superiorly, and the probe tip is placed on the conjunctiva inferiorly near the limbus, angling upward. The sound beams will bypass the cornea and be directed superiorly to image the fundus lesion.

Unlike fundus photography, where the vasculature of the posterior fundus, the macula and the optic nerve can be used for anatomical reference, ophthalmic ultrasonography is limited to the use of the optic nerve and extraocular muscles. The macula, the true anatomic center of the posterior segment, is only evident on ultrasonographic examination when it is thickened. Therefore, in ophthalmic ultrasonography the optic disc is used as the reference center of the posterior segment.

Longitudinal scan

The longitudinal scan is obtained by placing the probe marker in the direction of the clock hour to be imaged (Figure 3.4) See Clip 3.3. The transducer located within the probe moves perpendicular to the limbus, sweeping along the radial plane of the fundus located opposite the probe tip. The resulting image shows the fundus along a specific clock hour. The fundus anterior to the equator is located at the top of the B-scan display, the fundus posterior to the equator is imaged centrally and the optic nerve is located at the bottom of the display. In this manner, the longitudinal scan shows the anterior to posterior extent of posterior segment pathology. The longitudinal scan does not usually require the examiner to actively rotate the probe. However, if the desired area to be examined is in the periphery, it can be helpful to place the probe tip closer to the fornix resulting in an image that demonstrates the peripheral fundus at the top of the display with the optic nerve at the far bottom of the display, or completely absent from the display. The longitudinal scans are labeled according to the clock hour imaged anterior to posterior. If the probe is placed at 9 o’clock with the marker towards the pupil, the transducer sweeps along the 3 o’clock plane (Figure 3.5). The resulting image is labeled as a longitudinal scan of 3 o’clock, or L3.

Figure 3.4 Longitudinal B-scan position. B-scan probe tip placed on the conjunctiva near the limbus with the marker pointed superiorly (A). Corresponding ultrasonographed fundus (B, line). Resulting B-scan image showing a radial plane with 12 o’clock in the peripheral fundus at the top of the display, 12 o’clock posterior to the equator in the middle and the optic nerve at the bottom (C, L = lens, V = vitreous, R = retina, S = sclera, ON = optic nerve, O = orbital tissue).

Reproduced with permission from: Ophthalmic Ultrasonography: Theoretic and Practical Considerations. Hayden BC, Kelley L, Singh AD. Ultrasound Clin 2008; 3:179–183.

Figure 3.5 Labeling of the longitudinal B-scan. B-scan probe tip placed on the conjunctiva near the limbus at 9 o’clock directing the sound beams towards 3 o’clock (A). Corresponding ultrasonographed fundus (B, line). Resulting B-scan image showing the longitudinal scan of 3 o’clock radial plane, labeled L3 (C). Note probe position for longitudinal scans on eye model for L3 (D), L12 (E), L9 (F), and L6 scans (G).

Longitudinal scan is the best orientation to evaluate membranes for insertion into the optic disc or adjacent to the optic disc (Chapter 10). It is also essential in the localization of small fundus abnormalities such as a retinal tear or a focal tractional retinal detachment as well as for the evaluation of the macula described later in this chapter.

Transverse scan

The transverse scan is obtained by placing the probe marker perpendicular to the clock hour to be imaged (Figure 3.6) See Clip 3.4. The transducer located within the probe moves parallel to the limbus, sweeping back and forth to the fundus located opposite the probe tip. The resulting image shows a circumferential section through many clock hours with the area of interest in the center of the displayed B-scan. For example, if the probe tip is placed on the limbus of the right eye at 6 o’clock with the marker facing towards 3 o’clock (perpendicular to 6 o’clock), the resulting image shows 3 o’clock at the top of the display, 6 o’clock in the middle and 9 o’clock at the bottom. The transverse scan is typically a dynamic exam. The probe is initially placed near the limbus to evaluate ocular pathology posterior to the equator. The probe is then slowly shifted more towards the fornix resulting in a scan near the equator. Continuing to move the probe away from the limbus results in transverse B-scan images that are anterior to the equator. The transverse scans are labeled according to the clock hour that is imaged in the middle of the scan. If the probe is placed in the right eye at 3 o’clock with the marker tip superiorly, the transducer sweeps from 12 o’clock to 6 o’clock with 9 o’clock in the center of the displayed image (Figure 3.7). The resulting image is labeled as transverse scan of 9 o’clock, or T9.

Figure 3.6 Transverse B-scan position. B-scan probe tip placed on the conjunctiva near the limbus with the marker pointed nasally (A). Corresponding ultrasonographed fundus (B, line). Resulting B-scan image showing 3 o’clock posterior to the equator at the top, 12 o’clock posterior to the equator in the middle and 9 o’clock posterior to the equator at the bottom (C, V-vitreous, R = retina, S = sclera, ON = optic nerve, O = orbital tissue).

Reproduced with permission from: Ophthalmic ultrasonography: theoretic and practical considerations. Hayden BC, Kelley L, Singh AD. Ultrasound Clin 2008; 3:179–183.

Figure 3.7 Labeling of the transverse B-scan. B-scan probe tip placed on the conjunctiva near the limbus at 3 o’clock with the probe marker facing superiorly (A). Corresponding ultrasonographed fundus (B, line). Resulting B-scan image showing the transverse scan of 9 o’clock lateral plane, labeled T9 (C). Note probe position for transverse scans on eye model for T9 (D), T6 (E), T3 (F), and T12 scans (G). Schematic drawing (H) and transverse section of the right eye (I) indicating labeling system. P, posterior; PE, posterior-equator; EP, posterior to equator; E, equator; EA, anterior to equator; O, ora serrata; CB, ciliary body; and AX, axial.

Modified with permission from: Bryne S, Green R. Ultrasound of the Eye and Orbit. 2nd ed. St. Louis: Mosby; 2002: 475.

The transverse scan shows the lateral extent of posterior segment pathology and is essential in the evaluation of retinal detachments (Chapter 10) and the circumferential extent of ocular masses (Chapter 11).

B-scan examination methods

A methodical approach to B-scan examination is essential to ensure the evaluation of all aspects of the globe. Depending upon the indication, “five scan screening” followed by detailed B-scan examination or only “five scan screening” may be performed.

Five scan screening

The most common indication for B-scan is the evaluation of the posterior segment in the presence of opaque media. The goal is to create a three-dimensional mental image of the globe from many two-dimensional B-scan images. By performing four transverse B-scans and one longitudinal B-scan, at both high and low to medium gains, the entire posterior segment can be well imaged (Figure 3.8) See Clip 3.5. Additional B-scan orientations may be necessary to further view and document specific areas of interest. However, performing these five basic B-scans properly will ensure that most significant posterior segment pathology will not be missed.

Figure 3.8 Five scan screening. Probe positions for four transverse B-scans of the right eye (T6 [A], T12 [B], T3 [C], T9 [D]) and one longitudinal B-scan of the macula (L9 [E]). Arrows indicate movement of the probe from limbus to the fornix.

The superior portion of the globe is examined first by placing the probe tip on the conjunctiva near the limbus at 6 o’clock with the marker nasally (Figure 3.9). The resulting B-scan image is a transverse scan of 12 o’clock posterior to the equator (T12, PE). The ultrasonographer then slowly shifts the probe inferiorly towards the fornix without changing the direction of the probe marker. The sound beams will shift from an examination of the fundus posterior to the equator to the fundus near the equator (T12, E) and finally to the fundus anterior to the equator (T12, AE). In this dynamic manner the entire superior portion of the eye is imaged. The inferior portion of the eye is imaged next by placing the probe tip on the conjunctiva near the limbus at 12 o’clock with the marker still nasally and again shifting the probe superiorly towards the fornix. The resulting B-scan image is a transverse scan of 6 o’clock posterior to the equator. Again, the ultrasonographer shifts the probe towards the fornix to scan the inferior portion of the globe. The nasal and temporal portions of the globe are likewise imaged with the probe tip placed across from the portion of the globe of interest and movement of the probe from the limbus towards the fornix. However, the probe marker is turned superiorly. The resulting B-scans are the transverse B-scan of 9 o’clock and the transverse B-scan of 3 o’clock.

Figure 3.9 Dynamic B-scan screening of the posterior segment. Transverse B-scan of 6 o’clock with the probe tip near the limbus showing the inferior portion of the globe posteriorly (T6 PE, Probe position [A], corresponding ultrasonographed fundus [B], and the B scan [C]). Movement of the probe midway between fornix and limbus assesses the inferior portion of the globe near the equator (T6 E, Probe position [D], corresponding fundus [E], and the B-scan [F]). Continued movement of the probe towards the fornix assesses the inferior portion of the globe anterior to the equator (T6 AE, probe position [G], corresponding fundus [H], and the B scan [I]).

In addition to these four transverse B-scans, a minimum of one longitudinal scan of the macula is recommended. In the right eye, this is a longitudinal scan at 9 o’clock. In the left eye, it is a longitudinal scan at 3 o’clock. The macula is not anatomically on the same horizontal plane as the optic disc, but approximately 5-degrees inferior to the horizontal axis. However, ultrasonographically 3 and 9 o’clock scans are considered to be macular scans.

Detailed B-scan examination

Five B-scan screening indicated above will provide the ultrasonographer with an overview of the entire posterior segment. A more detailed examination will ensure the detection of subtler posterior segment pathology. See Clip 3.6 Longitudinal scans of the superior, inferior and nasal planes will help in the delineation of membranes adherent near the disk, thickening of the peripapillary fundus and lesions of the peripheral fundus near the ora serrata. Additionally, axial vertical and axial horizontal scans aid in the evaluation of optic disc irregularities, the retrobulbar optic nerve and Tenon’s space. Adjusting the gain setting during all B-scan examinations is essential. Vitreous opacities are best detected at a high gain, the retina and choroid at a medium gain and the sclera and orbital tissue at a low gain setting (Table 3.1).

Table 3.1 B-scan: tissue specific gain settings.

Tissue	Decibel value (dB)	Gain setting
Vitreous	75–100	High
Retina/choroid	55–75	Medium
Sclera/orbit	35–55	Low

Diagnostic A-scan

Diagnostic A-scan is performed when a thorough B-scan examination reveals an intraocular or orbital lesion (Figure 3.10). The A-scan probe is placed on the conjunctiva 180° across from the lesion while the patient gazes in the direction of the lesion. For a lesion located at 12 o’clock posterior to the equator, the probe is placed near the limbus at 6 o’clock and shifted slightly upward until the sound beam is perpendicular to the lesion. The resulting A-scan shows a tall scleral spike at the far left of the display, a horizontal line at the baseline through the center of the display corresponding to the vitreous space, several closely spaced tall spikes representing the retina, choroid, and sclera, and at the right of the display, several spikes of variable height corresponding to fatty orbital tissue (Figure 3.11).

Figure 3.10 Diagnostic A-scan probe.

Reproduced with permission from Ellex Innovative Imaging, Minneapolis, MN, USA.

Figure 3.11 Diagnostic A-scan. A-scan probe tip placed near the limbus with a sound beam directed at 12 o’clock posterior to the equator (A). Resulting A-scan image shows a linear display with echo spikes representing each tissue interface (B, V = vitreous, R = retina, S = sclera, O = orbital tissue).

Differentiation of ocular lesions

Systematic and reproducible examination methods utilizing a combination of contact B-scan and diagnostic A-scan methods using topographic, quantitative, and kinetic ultrasonographic techniques are essential in accurately identifying ocular pathology (Table 3.2).

Table 3.2 Diagnostic features for assessing intraocular lesions.

Topographic	Quantitative	Kinetic
Location	Reflectivity	Mobility
Shape	Internal structure	Vascularity
Extent	Sound attenuation	Convection movement

Topographic ultrasonography

After a lesion is detected during a basic B-scan screening, topographic evaluation is initiated to determine location, shape and extent of the lesion. A transverse scan of the lesion should be performed first. See Clip 3.7 The B-scan probe tip is placed on the conjunctiva near the limbus 180° opposite the area of interest. For example, if the lesion is at 10 o’clock posterior to the equator, the probe tip is placed at 4 o’clock with the marker facing upward. The B-scan probe is then shifted peripherally towards the fornix to sweep through the lesion from posterior to anterior. The resulting B-scan displays the lateral extent of the lesion and the gross shape. When evaluating a solid intraocular mass lesion, the sweeping motion through the lesion is paramount in importance in order to determine the maximal height and lateral basal dimension of the lesion (Figure 3.12).^11,¹²

Figure 3.12 Topographic ultrasonography. Transverse B-scan of a solid intraocular mass at 8:30 posterior to the equator. Fundus photograph (A). Lines represent ultrasonographed fundus as shown in B, C, and D. The probe tip is initially placed at 2:30 near the limbus with the probe marker facing superiorly. Resulting B-scan image shows a dome-shaped elevated fundus lesion (B). Slight shifting of the probe anteriorly centers the sound beam on the thickest portion of the mass lesion (C). Continued shifting of the probe assesses the peripheral aspect of the mass lesion (D). (Arrows = mass lesion, arrowhead = cross-section of lateral rectus).

A longitudinal B-scan is performed next to evaluate the anterior to posterior topographic features of the lesion See Clip 3.8. For a lesion at 10 o’clock, the probe marker remains at 4 o’clock, but the marker is turned towards the clock hour of interest, perpendicular to the limbus. The resulting B-scan displays the anterior to posterior extent of the lesion and the gross shape. The probe is not shifted dramatically as required during the transverse scan. However, when delineating the height and anteroposterior basal dimension of a solid intraocular lesion, subtle shifts around the clock hour of interest will aid in the identification of the maximal dimensions.

Axial scans are not always necessary in the evaluation of topographic features; however they can be useful to establish the lesion’s location in relationship to the optic nerve. To image a lesion at 10 o’clock near the peripapillary area, the probe tip is placed directly over the cornea with the marker facing towards 10 o’clock. The resulting B-scan shows the plane 10 o’clock posterior to the equator, the optic nerve in the middle and 4 o’clock posterior to the equator. The height of a peripapillary fundus lesion is best imaged with the axial scan when the sound beams are directed perpendicular to the lesion.

The compilation of these various B-scan images allows the examiner to create a three-dimensional concept of the lesion’s location, shape and extent. These same B-scan techniques can also be used to determine the topographic features of a retinal detachment (Figure 3.13). Longitudinal B-scans will display a rope-like elevated membrane that inserts into the optic disc (Chapter 10).

Figure 3.13 Compilation of topographic B-scan images to differentiate membranes. Transverse B-scan showing a slightly rope-like, elevated membrane (arrows) (A). Corresponding longitudinal B-scan at the same clock hour showing a V-shaped elevated membrane inserting into the disk (B). Mental compilation of scans into a three-dimensional image differentiates membrane as an open funnel-shaped retinal detachment.

Quantitative ultrasonography

After the topographic features of a lesion are evaluated, quantitative ultrasonography utilizing diagnostic A-scan can determine reflectivity, internal structure and sound attenuation. Reflectivity can help delineate many different intraocular lesions such as opacities, foreign bodies, membranes, bands, and mass lesions. Reflectivity is graded by the height of the spike on A-scan (Table 3.3). The tissue sensitivity gain setting is used to compare the height of a lesion’s spikes compared with a zero baseline and a 100% spike of retina and all tissues equal or higher in density (Chapter 2). Reflectivity can only be accurately determined when the A-scan probe is calibrated for tissue sensitivity and the sound beam is directed perpendicular to the lesion being evaluated (Figure 3.14). Judging the reflectivity from contact B-scan signal intensity is only an estimate of reflectivity and should not be relied upon for diagnostic purposes. Contact B-scan displays are not standardized and vary with resolution, gray scale, and dynamic range.

Table 3.3 A-scan: grades of reflectivity.

Grade	A-scan spike height (%)
Low	0–33
Medium	34–66
High	67–100

Figure 3.14 Quantitative ultrasonography (reflectivity). Fundus photo showing choroidal melanoma with vitreous hemorrhage (A). Corresponding B-scan (B). Arrow represents the path of sound beam used to generate the A-scan. Diagnostic A-scan of the solid fundus mass shows high reflectivity of the retina (C, arrow) and low to medium internal reflectivity of the mass lesion (C, arrowheads).

After reflectivity is determined, intraocular mass lesions can be further differentiated by quantitative ultrasonography by determining the internal structure. The internal structure is correlated with the histologic composition of a mass lesion.¹⁵ A homogeneous cell architecture within a lesion results in little variation in the height and length of the spikes on A-scan, whereas a heterogeneous cell architecture results in marked variation (Chapter 11).

Sound attenuation or acoustic shadowing is the last component of quantitative ultrasonography. It refers to the diminished or extinguished echo pattern resulting from a strongly reflective or attenuating structure (Figure 3.15). Calcification of lesions, foreign bodies and bones are among the structures that cause sound attenuation. Sound attenuation is detected on both contact B-scan and diagnostic A-scan and is delineated by a marked, progressive decrease in the strength of the echoes within or behind a lesion. On B-scan, this results in reduction of the brightness of echoes or the complete absence of echoes termed as shadowing. On A-scan, progressive decrease in the height of the spikes can be depicted as an angular measurement (angle kappa).⁴ Angle kappa is proportional to the extent of sound attenuation, greater the attenuation of sound, greater the angle kappa (Figure 3.16).