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QUESTION

WHAT IS THE EASIEST WAY TO DO A GOOD B-SCAN?

Cathy DiBernardo, CDOS

Ophthalmic ultrasound has been a useful ancillary test in ophthalmology for over 40 years; however, depending on one’s exposure and training, the techniques used for screening the posterior cavity of the eye with B-scan are vastly different. There are a number of nuances to performing an ultrasound to ensure that you get the most useful information. This chapter summarizes some of the important facts in a stepwise manner in order to perform a good B-scan

First, having a room that is dedicated to ultrasound examinations is ideal. The machine should be positioned behind or a little to the left of the patient’s head, and the patient should be in a reclined position. The examiner should be seated (Figure 7-1). This orientation provides easier access to the buttons and knobs on the machine and when both the patient and examiner are comfortable, it is easier to obtain reliable images.

Second, after instilling anesthetic drops and explaining the examination to the patient, you are ready to begin. Keep in mind that the ultrasound waves do not travel through air, so you will need to use some form of a coupling agent to minimize air between the globe surface and probe. GenTeal Gel (Alcon), Systane Gel (Alcon), and methylcellulose work well. If you choose to use methylcellulose, be sure to rinse the eye when the exam is complete. Avoid using ultrasonic gel, as it is not made for contact with the globe.

Performing ultrasound on the globe as opposed to through the lids can often make a difference in the quality of information being obtained because every interface that the sound has to travel through (ie, lids) decreases the amount of energy that is getting into the eye. There are, however, instances when performing the examination directly on the eye is contraindicated, such as when there is suspicion of an open globe. Sometimes when patients may not be able to keep their eye open, it may be easier to perform the exam through the lids. It is important to then note that the examination was performed through the lids in the final report.

Third, the marker on the probe (white line) represents the upper portion of the echogram (Figure 7-2A) and changing the position of the marker changes the direction of the sound beam as it travels through the eye. The center of the sound beam provides the best detail of ocular structures. When the posterior segment is being evaluated, the probe is placed opposite the area to be examined and the patient should be looking in the direction where the sound beam is aimed. If the patient’s eyes are closed, it can be difficult for the patient and the examiner to know where the patient is looking. The eye can be examined in different planes to provide a 3D perspective using 2D images.

• Transverse scans provide cross-sectional views opposite of where the probe is positioned. The sound beam is aligned perpendicular to the globe wall and a thin section (2.0 mm in thickness) of tissue is examined along 6 clock hours (see Figure 7-2). To look at the superior or inferior fundus, the marker should be directed toward the patient’s nose. These are known as horizontal transverse scans. To look at the nasal or temporal fundus, the marker should be directed toward the 12 o’clock position. These are known as vertical transverse scans.
  
• When performing transverse scans, place the probe on the eye with the marker directed in the correct location and overlap the corneal limbus until you see the shadow of the optic nerve. Then, slowly shift the probe from the limbus into the fornix or toward the canthus, being sure to aim the sound into the periphery. Failing to move the probe limits the areas of the globe being examined. Failure to move the probe in this fashion often results in missed findings in the far periphery.
  
• The center of the image provides the best detail of intraocular structures. If, during your vertical and horizontal transverse scans, you see something that you want to evaluate better, but it is not in the center of the image, you can adjust the probe face and the marker to obtain an oblique transverse scan. For instance, during a horizontal transverse scan toward superior in the right eye, you see something more toward the bottom of the image (Figures 7-3A and 7-3B). Moving the probe face to the 4:30 location with the marker directed toward superonasal directs the center of the sound beam superotemporal and the area you were seeing in the previous scan is now in the center of the screen (Figures 7-3C and 7-3D). Shifting the probe from the limbus to the fornix is essential to evaluate the area from posterior to anteriorly.
  
• Longitudinal scans provide radial views of the intraocular areas opposite of where the probe face is positioned. The sound beam is aligned parallel to the globe wall and a thin section of tissue along a single meridian or clock hour is obtained (Figure 7-4). The probe face is placed opposite the area you want to examine and the marker on the probe should be placed near the corneal limbus, aimed at the clock hour you intend to evaluate. The patient should still be instructed to look in the area opposite the face of the probe. To visualize mobility during longitudinal scans, ask the patient to look straight and then back to the area you are scanning.
  
• For axial scans, instruct the patient to look straight up. In the obtained image, the lens and optic nerve should be in the center of the screen. Vertical scans (Figure 7-5) show the superior (above the nerve shadow) and inferior (below the nerve shadow) posterior pole. Horizontal scans show the nasal (above the nerve) and temporal (below the nerve) posterior pole. Oblique axial scans can be performed by directing the marker to one of the superior oblique clock hours.

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