BScan and Ascan in ophthalmology and eye field

BSCAN IS HELPFULL FOR ANY KIND OF RETINAL PROBLEM

1. B-Scan By: Saif ullah Email ID: [email protected] FIACLE, M.PHIL OPTM, MPH, BS OPTM Assistant Professor Optometry PIO, Al- Shifa Trust Eye Hospital

2. INTRODUCTION Ultrasound --- a vibrational form of energy, used in medical imaging. Formats for display of ultrasound signals --- include A-SCAN (A for Amplitude): B-SCAN (B for Brightness): C-SCAN (C for Coronal): D-SCAN (D for Deflection): M-SCAN (M for Motion): DOPPLER: The ultrasound display modalities used in Ophthalmological practice – A- and B-scans

3. INTRODUCTION A-scan: is a time-amplitude display. is one dimensional. echoes are displayed as vertical deflections from a base-line. the strength of the echo is indicated by the height of the spike. is a real-time display. B-scan: is a brightness intensity-modulated display. is two dimensional. echoes are displayed as dots. the strength of the echo is indicated by the brightness of the dots. is a real-time display.

4. BASIC PHYSICS SOUND – Audible frequency of vibrations (20 – 20,000 Hertz or cycles per second) ULTRASOUND – Sound frequency, exceeding the audible limit (> 20,000 Hertz). The higher the frequency of the ultrasound, the shorter the wavelength. The shorter the wavelength, the more shallow the tissue penetration. The shorter the wavelength, the more improved the image resolution. Ophthalmic ultrasound probes – 10 MHz – Shallow penetration and high resolution. Obstetrics ultrasound probes – 1 MHz – Deep penetration and low resolution. High-resolution ophthalmic B-scan probes (ultrasound biomicroscopy or UBM ) – 20-100 MHz – penetrate only about 5-10 mm into the eye – Incredibly detailed resolution of the anterior segment .

5. As the sound waves hit intraocular structures, they are reflected back to the probe and converted into an electric signal. The signal is subsequently reconstructed as an image on a monitor, which can be used to make a dynamic evaluation of the eye ( KINETIC SCANNING ) or, can be frozen and photographed to document pathology ( STATIC SCANNING ). BASIC PHYSICS

6. BASIC PHYSICS Normally the transmitting and receiving crystals are built into the same hand-held unit, which is a called an ULTRASONIC TRANSDUCER.

7. Velocity The velocity of the sound wave is dependent on the DENSITY OF THE MEDIUM through which the sound travels. Sound travels faster through solids than liquids. Velocity of sound through aqueous and vitreous -- 1532 m/sec & through the cornea and lens -- 1641 m/sec. Reflectivity When sound travels from one medium to another medium of different density, part of the sound is reflected from the interface between those media back into the probe. This is known as an ECHO The greater the density difference between the media, the stronger the echo, or the higher the reflectivity. BASIC PHYSICS

8. In A-scan ultrasonography , the echoes are represented as spikes arising from a base-line. The stronger the echo, the higher the spike. For example, the vitreous is less dense than the vitreous hyaloid , which, in turn, is much less dense than the retina. Therefore, the spike obtained as the sound strikes the interface of the vitreous and hyaloid is shorter than the spike obtained when the sound strikes the vitreous hyaloid -retinal interface. In B-scan ultrasonography, The echoes are represented as a multitude of dots that together form an image on the screen. The stronger the echo, the brighter the dot. For example, the dots that form the posterior vitreous hyaloid membrane are not as bright as the dots that form the retina. This is very useful in differentiating a PVD (a benign condition) from a more highly reflective RD (a blinding condition). BASIC PHYSICS

9. Angle of incidence The angle of incidence of the probe is critical for both A-scan and B-scan ultrasonography. When the probe is held perpendicular to the area of interest, more of the echo is reflected directly back into the probe tip and sent to the display screen. When held oblique to the area imaged, part of the echo is reflected away from the probe tip and less is sent to the display screen. The more oblique the probe is held from the area of interest, the weaker the returning echo and, thus, the more compromised the displayed image. BASIC PHYSICS

10. On A-scan, the greater the perpendicularity, the more steeply rising the spike is from baseline and the higher the spike. On B-scan, the greater the perpendicularity, the brighter the dots on the surface of the area of interest. Perpendicularity to the area of interest always should be maintained to achieve the strongest echo possible for that structure. The size and shape of the surface at each interface also affect that reflection. BASIC PHYSICS

11. Absorption Ultrasound is absorbed by every medium through which it passes. The more dense the medium, the greater the amount of absorption. The density of the solid lid structure results in absorption of part of the sound wave when B-scan is performed through the closed eye, thereby compromising the image of the posterior segment. Therefore, B-SCAN ALWAYS SHOULD BE PERFORMED ON THE OPEN EYE unless the patient is a small child or has an open wound. BASIC PHYSICS

12. Likewise, when performing an ultrasound through a dense cataract as opposed to the normal crystalline lens, more of the sound is absorbed by the dense cataractous lens and less is able to pass through to the next medium, resulting in weaker echoes and images on both A-scan and B-scan. For this reason, THE BEST IMAGES OF THE POSTERIOR SEGMENT ARE OBTAINED WHEN THE PROBE IS IN CONTACT WITH THE SCLERA RATHER THAN THE CORNEAL SURFACE , bypassing the crystalline lens or intraocular lens implant. When calcification of tissue is present, there is so much absorption and such a strong reflection of the echo back to the probe that there is no signal posterior to that medium. This is referred to as SHADOWING .

13. INTERACTION OF U/S IN TISSUES REFLECTION AND REFRACTION: B. SCATTERING: C. ABSORPTION:

14. B-SCAN ARTIFACTS Various acoustic artifacts may be encountered. If not recognized, may result in misinterpretation. REVERBERATIONS (MULTIPLE SIGNALS): SHADOWING: BAUM’S BUMPS: artifacts that appear as elevation of the fundus Caused by refraction of the sound beam as it sweeps thru the peripheral aspects of the lens, when an axial approach is used. Can be mis -interpreted as tumors, or they may give the impression of a posterior staphyloma To eliminate this artifact, probe should be re-positioned peripheral to the limbus to avoid the lens. COUPLING FLUID INSUFFICIENCY: ARTIFECTS OF SOUND BEAM INCIDENCE: artifect wo cheez hoti han jo procedure perform karny mai hindrance mushkil peash karti hab jo ap ki values ko galat kar daiti han

15. INSTRUMENTATION Ophthalmic ultrasound instruments use what is known as a PULSE-ECHO SYSTEM. PES consists of a series of emitted pulses of sound, each followed by a brief pause (microseconds) for the receiving of echoes and processing to the display screen. The amplification of the display can be altered by adjusting the GAIN, which is measured in decibels (dB). Adjusting the gain does not change the frequency or velocity of the sound wave but acts to change the sensitivity of the instrument’s display screen.

16. When the gain is high, weaker signals are displayed, such as vitreous opacities and posterior vitreous detachments. When the gain is low, the weaker signals disappear, and only the stronger echoes, such as the retina, remain on the screen. However, there is better resolution, or detail, of the area of interest when the gain is lowered. Typically, all examinations begin on highest gain so that no weak signals are missed; then, the gain is reduced as necessary for good resolution of the stronger signals.

17. PATIENT POSITIONING Reclining position on an adjustable chair Or Supine position on a couch

18. EXAMINATION TECHNIQUES As ultrasound waves do not travel thru air, some form of fluid coupling between the probe and the eye, is essential. CONTACT METHOD , HPMC or some other form of coupling material is being used. Contact method does not permit visualization of the anterior segment, as the first 5mm of the beam is poorly focused. IMMERSION METHOD , a water bath is placed over the eye and the probe is held in the water bath but not in contact with the globe. Immersion method permits more detailed examination of the anterior portions of the eye.

19. The probe face is usually oval in shape. There is a marker (usually a dot or line) on the side of the probe handle near its face. Knowing the orientation of the marker at all times is extremely important because it represents the upper portion of the echogram. The back-and-forth motion of the transducer occurs along the long portion of the oval; thus, the slice emitted occurs in the direction of the marker. In other words, if the area of interest is at the 3-o'clock position, the probe face is held on the globe at the 9-o'clock position with the marker aimed upward. The center of the probe is aiming at the 3-o'clock position, which appears in the center of right side of the echogram, the area of best resolution. INSTRUMENTATION

20. The top of the right side of the echogram represents the 12-o'clock position since that is the orientation of the marker, and the bottom of the echogram on the right represents the 6-o'clock position since that is the portion opposite the marker. Therefore, the slice of tissue on the right side of the display is from the 12 o'clock position to the 6 o'clock position, with the 3-o'clock position in the center. If the probe is held at the 9-o'clock position but rotated so the marker is now aimed inferiorly, the 3-o'clock position remains in the center of the display, but now the 6-o'clock position is at the top and the 12-o'clock position is at the bottom.

21. TYPE OF SCAN, BASED ON POSITION OF THE PROBE (I ) TRANS-OCULAR APPROACH: A. TRANSVERSE SCAN: B. LONGITUDINAL SCAN: C. AXIAL SCAN: (II) PARA-OCULAR APPROACH:

22. TRANS-OCULAR APPROACH TYPE OF SCAN, BASED ON POSITION OF THE PROBE Contd

23. TRANS-OCULAR APPROACH (A) Transverse Scans (Cross section ) TYPE OF SCAN, BASED ON POSITION OF THE PROBE Contd

24. TYPE OF SCAN, BASED ON POSITION OF THE PROBE TRANS-OCULAR APPROACH (B) Longitudinal scans Contd

25. TYPE OF SCAN, BASED ON POSITION OF THE PROBE TRANS-OCULAR APPROACH (C) Axial scans Contd

26. PARA-OCULAR APPROACH TYPE OF SCAN, BASED ON POSITION OF THE PROBE Contd

27. BASIC SCREENING TECHNIQUE The highest gain setting must be used to visualize any weak signals, such as vitreous opacities and posterior vitreous detachments, or to gauge the extent of vitreous hemorrhages. If any pathology such as retinal or choroidal detachments is found, then the gain may be reduced for better resolution of the stronger signals from these structures. It is vital that during basic screening the entire globe be examined, from the posterior pole out to the far periphery.

28. Using a limbus -to-fornix approach, each quadrant is evaluated carefully. The 4 major quadrants include the 12-o'clock, 3-o'clock, 6-o'clock, and 9-o'clock positions, each centered on the right side of the echogram in transverse approaches. Because approximately 6 clock hours are imaged at once, by examining each quadrant, the areas examined will overlap, thereby reassuring the examiner that the entire periphery of the globe is visualized. A photo or printed documentation of each of the 4 quadrants should be obtained. Document the posterior pole with a horizontal axial scan, which incorporates both the optic nerve and the macula in one echogram. If no additional pathology is detected, these 5 echograms complete the examination.

29. Localization of the macula The 4 methods of localizing and centering of the macula are as follows: horizontal,vertical,transverse,longitudinal Depending on the eye, one method may be preferable to another, or a combination of methods may be desired. THE HORIZONTAL METHOD involves first aligning a horizontal axial, ie , probe on the cornea with the marker nasally. In this particular scan, it is known that the macula lies just below the nerve. BASIC SCREENING TECHNIQUE

30. THE VERTICAL METHOD involves first aligning a vertical axial, ie , the probe on the cornea with the marker at the 12-o'clock position. Next, tilt the probe from slightly nasal to straight ahead. Just after the optic nerve shadow is lost, the probe should be aligned along the macula vertically. THE TRANSVERSE METHOD involves the patient fixating slightly temporally and placing the probe onto the nasal sclera with the marker at the 12-o'clock position. Using the optic nerve as the center of the imagined clock, the macula is at the 9-o'clock position at the posterior pole in the right eye and at the 3-o'clock position at the posterior pole in the left eye.

31. THE LONGITUDINAL METHOD involves directing the patient’s gaze slightly temporally, with the probe on the nasal sclera and the marker oriented toward the limbus . This is a horizontal scan of the macula, with the nerve at the bottom-right of the echogram and the macula just superior to the nerve. Because both the nerve and macula are imaged, the longitudinal method may be preferable if the patient has a cataract, because in this orientation the lens is bypassed, eliminating absorption of the sound from the crystalline lens. This method is also preferable when an intraocular lens implant is present, which causes both absorption and a chain of artifacts in the vitreous cavity with an even greater compromised echogram.

32. NORMAL OCULAR TOMOGRAPH On the extreme left are the echoes that represents the NOISE just in front of the probe. Behind it is an anechoic area of anterior chamber. Farther on right is the outline of normal crystalline lens . Anterior surface of which represents the iris diaphragm. In the middle of the screen, a large anechoic area represents the vitreous gel . On the right of vitreous cavity, a large echogenic area with its concave anterior surface and indented posterior outline, is the retro-bulbar fat.

33. Anterior chamber Lens outline Noise Iris diaphragm v i t r e o u s Retro-bulbar fat

34. There are two terms used, one is optic nerve shadow and the other is optic nerve head shadow . This optic nerve head shadow is produced by a phenomenon in which the beam of sound is bent from a curved surface and the structures behind this point don’t receive any sound, so they are not visible. Optic nerve head shadow Optic nerve shadow NORMAL OCULAR TOMOGRAPH

35. Optic nerve after exiting from the globe turns medially towards the optic canal. By this fact we can recognize that this part of the globe is medial to and this part is lateral to the optic disc. Macula has no peculiar echo-texture. It is only recognized by the fact that it is placed temporal to the point of exit of optic nerve. Lateral Medial Macula Optic disc Optic nerve NORMAL OCULAR TOMOGRAPH