Clinical Methods: A- and B-Scans

Chapter 3 Clinical Methods


A- and B-Scans



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Introduction


Ophthalmic ultrasonography examination techniques are designed to evaluate all aspects of the globe in a methodical, reproducible manner.114 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).



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)image 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).




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)image 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).



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.




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)image 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.




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.


Mar 5, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Clinical Methods: A- and B-Scans
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