Embed Size (px)
Transcript of Ophthalmic USG
Pabita DhungelB.Optometry3rd year ULTRASONOGRAPHY PRINCIPLE, METHODS AND INTERPRETATION
PRESENTATION LAYOUTIntroductionHistoryA- scanB- scanMethods for special purposesClinical picturesSummary
Clinical Procedure in OptometryOptometry: science, techniques and Clinical managementsOphthalmic ultrasonographyBorish Clinical Refraction
Introduction Ultrasound is an acoustic wave (above audible frequency) that consists of oscillations of particles within a medium, frequencies greater than 20kHz(20,000 oscillations/s)Diagnostic ophthalmic UltrasoundFrequency 8-10 MHz (Sandra Frazer et. Al) 8-25 MHz for Posterior segment & Orbit (Jagers Duane Oph.) 50 MHz for imaging Anterior segment (Jagers Duane Oph.)Produce short wavelength = 0.2 mmGood resolution of ocular structures Less Penetration
ContdExamination of larger structures e.g abdominal or obstetric ultrasound requires frequencies in the range of 1 to 5 MHz Such wavelengths produced by these lower frequencies enable these instruments to penetrate deeper into the body decreasing their resolution capability
ContdUltrasound is transmitted as a longitudinal wave so its speed is dependent upon the density of the medium it is passing through e.g air sound travels at 340m/s whereas in water its speed is much faster at approx 1480m/s.Fluid contact is essential betn the transducer and eye so normal saline is used in open eye or water soluble gel is used if reading taken through eyelids
Sound Wave Velocities Through Various Media
MediumVelocity (m/sec) Water1,480Aqueous/ Vitreous 1,532Silicon Lens1,486Crystalline Lens 1,641PMMA Lens2,718Silicon Oil986Soft Tissue 1,550Bone3,500
History In 1956, First time: Mundt and Hughes, American Oph.A-scan (Time Amplitude ) to demonstrate various ocular disease Oksala et. Al in FinlandUltrasound Basic Principle (Pulse-Echo Technique)Studied reflective properties of globeIn 1958, Baum and Greenwood Developed the first two-dimensional(immersion) (B-scan) ultrasound instrument for ophthalmology.In the early 1960s, Jansson and associates, in Sweden, used ultrasound to measure the distances between structures in the eye.
ContdIn the 1960s, Ossoinig, an Austrian ophthalmologistfirst emphasized the importance of standardizing instrumentation and technique.developed standardized A-scan.
In 1972, Coleman and associates madefirst commercially available immersion B -scan instrument Refined techniques for measuring Axial length, AC depth, Lens thickness
Bronson in 1974 made contact B scan machine.
Advantages of USGEasy to use.No ionizing radiationExcellent tissue differentiationCost effectivenessPrimary uses in ophthalmology:Posterior segment evaluation in Hazy media / Orbit Structural integrity of eye but no functional integrity Detection and differentiation of intraocular and orbital lesions Tissue thickness measurements.Location of Intra Ocular Foreign BodyOcular Biometry for IOL power calculations
Principle Average velocity in Eye = 1500 m/secTakes about 33 microseconds to travel & return back from the posterior part of eyePrinciple Pulse- Echo System Emission of multiple short pulses of ultrasound waves with brief interval to Detect, process and display the turning Echoes
Piezoelectric crystal (Quartz & Ceramic Crystal) Present in Probe tip Application of Pulse of Voltage electric energy Mechanical Vibration Rapid vibration Generate short pulse Ultrasound energy (Transducer) Longitudinal ultrasound wave propagate through medium (Eye) Echoes come back from different Acoustic Interfaces A pause of Several Milliseconds for receiving echoes Creates mechanical vibration as it strikes the probe tip and the piezoelectric crystals Produce electric energy
Transmitted to receiver and in a Cathode ray Tube to display Ultra sonogram
Terms PROBE Consists of piezoelectric transducer. A piezoelectric transducer consist of small ceramic plates which converts electricity into sound waves and sound waves into electric signals to be displayed on the screen.Damping material (metal powder with plastic or epoxy) Help to produce shorter Pulse Better for Axial resolutionAxial resolution The minimum distance between two interfaces (Echo Sources) along the direction of the sound beam
ContdShape of the Crystal: Planer crystal Produce relatively parallel sound beam (A- Scan) Acoustic lens Produce focused sound beam (B-scan)Improves lateral resolution Minimum separation between two interfaces perpendicular to the direction of sound beam
ContdRECEIVER (computer unit) Receives returning echoes
Produces electrical signal that undergoes complex processing Amplification, Compensation, Compression, Demodulation and Rejection
ContdRESOLUTION Ability to discern two interfaces close to each other. Higher the frequency of ultrasound Shorter the wavelength & better the resolution. Echoes- Reflected portion of the wave.
GAIN Relative units of Ultrasound intensity Expressed in Decibel (db)Adjust of gain doesn't change the amount of energy emitted by transducer but chance in intensity of the returning echoes for display Electric Amplification of the echo signals received by the transducer Higher the gain Greater the sensitivity of the Instrument in displaying weaker echoes (i.e Vitreous opacities) Lower the gain Weaker the depth of sound penetration only stronger echoes are displayed (i.e Retina / Sclera)
Gain contdStronger echoes are located in the centre of the returning sound wave Lowering gain effectively narrows the sound beam Improves both axial and lateral Resolution When the gain is Increased A-scan gets taller and B-scan echoes gets brighter conversely When the gain is turned down the echoes get shorter and dimmer.
Displaying the Ultrasound
A - scan A for amplitude provides one dimensional display of returning echoes in the form of vertical spikes of various heights and distances from the initial signalEchoes from the structures deeper within the eye take longer to return to the transducer for conversion back to electric signal, so appear further along the time baseline
ContdTwo fundamental data obtained are i) distance of echo source from the probe face - forms the basis of biometry ii) amplitude of echo signal (spike) which partly depends on the nature of reflecting interface - forms the basis of quantitative echography
Standardization of A scanCredited to Dr. Karl OssoinigUnique sound Amplification : S-shaped amplifier with flat upper and lower curves and a steep mid segment and a dynamic range of 36dBThis amplification enhances the difference between normal and abnormal signals
A-Mode DisplaySound velocity should be adjusted Time Dimension calculated according to the speed at which sound travels via a given medium Phakic Eye Average Sound Velocity : 1,550 m/sec
Average Velocity Adjustment for Eye Length measurementOcular MediaVelocity (m/sec)Aphakia (Aqueous/ Vitreous)1,532Phakia1,550Pseudophakia PMMA Implant Silicone Implant 1,532 + 0.2 mm or 1,5501,486Silicon oil986Note: Average Velocity : Average of Sound Velocities for the Aqueous + Vitreous + Lens
Examination stepsPatient is positioned with head near oscilloscopeTopical anaesthetic drops are placed in the eyeProbe is firmly placed on the globe without coupling jelly as tear acts as coupling agentEight meridians are scanned, postero-anteriorly, by shifting and tilting the probe in a single, smooth arc movement from limbus to fornix
ContdIn cases of traumatized or infected eyes, or soon after intraocular surgery, examination through the closed eyelids is saferAt the end of procedure, the eye is irrigated with sterile saline, and the probe tip is cleaned with alcohol wipe or other suitable disinfectant
Orientation and labelling of scanThe labelling of sections is determined by the projection of the beam and not the probe locationE.g a section labelled 12 equator (12E) is produced by placing the probe at 6 0clock and mid distance betn limbus and fornix, a section labelled 6 anterior (6 A) is produced by placing the probe at 12 0clock fornix
1) Axial section easier of the two approachesProbe is placed on cornea and directed axiallySuitable for measurement but not sensitive in detecting early macular thickening or in differentiation of its lesions because of strong sound attenuation by the lens
ContdPosterior sectionIn the RE this is 9P position and in the LE the 3P positionProduced by directing the patients gaze temporally and placing the probe at the nasal limbus and aiming it posteriorly thus avoiding the lensand achieving better resolution
Examination of fundus peripheryPatients gaze is directed maximally towards the meridian to be scanned and probe is placed at the opposite fornix the beam being aimed across the globe towards the opposite peripheryUseful for detecting peripheral retinal cysts/retino-schisis, choroidal detachments and ciliary body lesions
B - ScanB for brightness produces two dimensional slice of tissue images, composed of coalescing dots of varying degrees of brightness , depending on the reflectivity of the echo sourceProbe emits a focused sound beam at the frequency of 10 MHz , eye dedicated scanners produce a sound beam whose focal zone coincides with the posterior globe wall and anterior orbit
ContdMarker at the probe tip indicates the beam orientation and the top of the echogram as it displays on the screen