Ophthalmic Ultrasound

Ophthalmic UltrasoundEllison Bentley, DVM, Diplomate ACVOUniversity of Wisconsin-MadisonSchool of Veterinary Medicine

Principles of ultrasound• Standard: 10 MHz - 40 mm penetration• High resolution: 20 MHz - 10mm penetration• Ultrasound biomicroscopy - 50 MHz or higher- 7mm and less

• One dimensionaldisplay• Amplitude scan-thin,parallel sound beam;through one point ineye• Higher spike-greaterdifference betweeninterfacesA-scansProbe/cornea Anterior lens capsule Posterior lens capsule Vitreous A-scan of a dogRetina

• Brightness scanoscillatingA-scan,through cross sectionof tissue• Echos back are dotsforming image• Greater the differenceat the interface, brighterthe dotB-scans

Sound waves• Sound waves behave same as light raysrefracted,reflected, transmitted• Longitudinal wave-reflected toward sourcefrom tissue interface→echo• Refractive index = acoustic impedance

Acoustic impedance• Impedance = soundvelocity x density• Greater differencebetween two media,stronger reflectedsound wave• Ex-Aqueous and lenscapsule vs hypopyonand lens capsuleFrom Byrne, Green, 2002

Echo formation• Angle of incidence =angle of reflection• ‘Thou shalt beperpendicular’• Oblique-echoes divertfrom origin = weakerecho• Brighter echo =perpendicularFrom Byrne, Green, 2002

Echo formation• Irregular surfaceincreasedscattering– Smaller echo even ifperpendicular(wrinkled retina)• Small interfacepronouncedscatter• Small/irregularperpendicular↓importanceFrom Byrne, Green, 2002

Absorption• US energy-absorbed bytissue→low heat• ↑ Frequency = ↑absorption• ↑ Sound velocity/ ↑tissue thickness =greater absorption• Absorption=shadowing– FB, calcification, cataractIntraocular glass splinterFrom Byrne, Green, 2002

Refraction of sound waves• Occurs with differingsound velocitiesbetween media• Oblique soundwave→refraction• Highervelocity→lowervelocity = towardperpendicularFrom Byrne, Green, 2002

Refraction• Occurs inside eye• Refraction may help make some soundwaves strike objects more perpendicularly– Imaging of extraocular muscles

Instrumentation• Need:– Pulser (something to create sound waves)– Transducer (probe)– Receiver– Display screen

Instrumentation• Probe-piezoelectric element-convertsmechanical energy to electrical energy (andvisa versa)• Usually a crystal• Electric stimulation = mechanical vibration =sound wave

Instrumentation• Pauses between vibrations-receive returningsound waves (echoes)• Echo→Crystal vibration→Electricsignal→Receiver/display• Repeated > 1000 times/sec = ‘real time’display

Instrumentation• Dampening:– Back of crystal– Limits vibrations→shortening pulse– Improves axial resolution– Axial resolution=Minimum distance between 2interfaces along direction of sound wave whichcan be displayed

Instrumentation• Shorter pulses= better axialresolutionFrom Byrne, Green, 2002

Sound beam fields• Near field: Sound beam diameter ↓ with ↑distance from transducer• Far field: sound beam diameter ↑ with ↑distance from transducer• Greatest resolution in near field• Near field length-diameter and frequency ofcrystal– Larger diameter, higher frequency=longer nearfield

Crystal shape• Planar crystal-parallel beam• Concave crystal-focused sound beam• Focal distance = area of narrowest soundbeam• Focused beam-increased lateral and axialresolution of ultrasound

• Lateral resolution =minimumseparation between2 echo sourcesperpendicular todirection of soundwaveCrystal shapeFrom Byrne, Green, 2002

Signal processing• Echo produces weak radio frequency signal• Receiver processes signal:– Amplification– Compensation– Demodulation– Rejection

Signal processing• Amplification:– Processing of received sound waves-ability todisplay differences in echo strength– Measured in decibels-unit of ultrasound intensity

Signal processing-Amplification• Linear-limited range; displays minordifferences between echoes• Logarithmic-wider range but less sensitive• S-shaped-combination-increased range andsensitivity

Gain• Similar to volume control-increasesamplification of signal display• Decibels• Adjusting gain-no change in energy fromtransducer-only change in screen intensity• ↑ gain = ↑ sensitivity, ↓ resolution

Gain• Weaker signals (vitreal opacities) disappear• Strong signals (retina, sclera, masses) remainFrom Byrne, Green, 2002

Gain• ↓ gain = narrower sound beam (strongestechoes in central axis of returning soundwave)• ↓ gain = decreased penetration– Automatic time gain compensation-enhancesweak echoes from deeper tissues

A-scans• 1-D display, echoes are vertical spikes• Space between spikes dependent on time toreach interface and return• Distance:Distance = velocity x time• Height indicates strength of echo

Types of A-scans• Biometry-linear amplification, focusedtransducer, 10-15 MHz• Vector A-scan-simultaneous with B-scan;logarithmic amplification, focused transducer,10MHz• Standardized (diagnostic) A-scan-S-shapedamplification, non-focused 8 MHz transducer

Standardized A-scans• Tissue differentiation• Probe/instrumentcalibrated-TissueModel• TissueSensitivity=decibelsetting; unique to eachprobe/instrumentFrom Byrne, Green, 2002

Standardized A-scans• Images compared toknown patterns →nearly definitivediagnosis• Used most often forposterior disease, notwell described inveterinary medicineFrom Byrne, Green, 2002

• Oscillating beams -slice/sector of tissue– Each slice=6 clock hours• 2-D display• Multiple A-scans• Probe marker-directionof slice– Upper portion ofechogram=markerB-scans

B-scans• Angle of transducer-area of eye/orbitexamined• Rate of oscillation-10-60 slices/second• Range of gray scale-greater range, greatersensitivity to differences in echo intensity

Artifacts• Multiple signals-reverberation of soundwaves between interfaces– Multiple ‘fake’ echo signals produced– Must distinguish between true and artifactual

Artifacts• External multiple signals-reverberationsbetween probe tip, interface• Reflection of significant magnitude back toprobe– Then: True echo-strong enough to reflect offprobe surface– Back to original interface-resultant echo distal totrue echo (twice distance)

Artifacts• Surface of lens, IOL, airbubbles, sclera, bone• Distinguish by:– Decreasing strength– Equal distance– Greater movement ofmultiple signals withprobe pressure againsttissuesExternal multiple signals dueto insufficient contact betweenprobe and eyeFrom Byrne, Green, 2002

Artifacts• Multipleexternal signalsdue to IOL(luxated into AChere)IOL in AC

Artifacts• Internal multiple signals:– Reverberations within certain types of foreignbodies– Usually spherical FB, but can be flat (closelyspaced, parallel surfaces)– Closely placed signals emanating from FB echo

Artifacts• Energy from soundwave trapped in object• Bounces back/forth• Someescapes→returns toprobe• Chain of echoes ofdecreasing amplitudefrom objectFrom Byrne, Green, 2002

Artifacts• BBs, gunshotpellets, small airor gas bubble,slivers of glass• “Comet tail”• May help withidentification ofFBIntraocular BBFrom Byrne, Green, 2002

Artifacts• Shadowing:– Strong sound attenuation-absence or decrease ofechoes posterior to lesion– Shadowing also from refraction at edges ofsmooth curved surface• Globe, cystic lesion– Exam posterior to shadowing difficult

Artifacts-Shadowing• Usually caused bydense interface: bone,calcium, large FB• Aid to diagnosisFrom Byrne, Green, 2002

Artifacts• Enhancement:– Echoes ↑ amplitude posterior to weaklyattenuating structure/lesion• Orbital soft tissue/bone-enhanced through lowreflectivity vitreous cavity

Artifacts• Perpendicular soundbeam incidence– Strikes smooth, highlyreflective surface→brightfocal signal in center ofechogram• DDX-calcification, FB– Re-orient probeobliquely-shoulddisappear

Artifacts• Baum’s bumps– Appears elevations ofperipheral fundus– Refraction of soundbeam through peripherallens-axial positions– Probe positionperipheral to limbusFrom Byrne, Green, 2002

A-scan• Biometry• Awake, sedated, oranesthetized animals• Probe on cornea orscleral shell withwaterbath (immersiontechnique)www.eyetec.ne

A-scan• Axial probe placement• Equal spikes• Perpendicular spike (noslope)• Hard to get posteriorlens capsule in dogsProbe/cornea Posterior lens capsule Anterior lens capsule Vitreous

A-scan• Sound velocity: varies by study:– Canine lens:• 1710 m/s (Schiffer, 1982)• 1706 m/s (Gorig, 2006)-same study: 1535 m/s vitreous– Human lens-1641 m/s lens, 1532 m/s vitreous– Difference in IOL calculations:• Human values-43D IOL• Canine values-41.8 D IOL

A-scan• Used clinically for:– Disease-glaucoma– IOL calculations– Determination of cause of refractive errors• One study-Baptista 2006: Standardized A-scans for anterior uveal melanomas– Low to medium internal reflectivity, internalvascularity

B-scans• Intraocular exam with opaque media• Topical anesthetic, coupling medium, probedirectly on globe• Closed lids-Bell’s phenomenon• Left side of image-probe face• Upper part of image-probe marker• Best resolution-center of echogram

B-scan probe positions• Standardized positioning in human patients– Ensures complete examination– Standardized labeling– Facilitates communication between operators

B-scan probe positions• 3 main positions:– Transverse– Longitudinal– Axial• 2/3 bypass lensFrom Byrne, Green, 2002

B-scan probe positions• Humans directed tolook away from probe• More exposed sclera inhumansFrom Byrne, Green, 2002

B-scan probe positions• Optic nerve-reference point• Label according to what’sbeing examined, probeposition, and/or location ineye (anatomic, clock hour)• P=posterior pole• PE=posterior/equator• EP=equator/posterior• E=equator• EA=equator/anterior• O=ora serrata• CB=ciliary bodyFrom Byrne, Green, 2002

B-scan probe positions• Axial most common in veterinary medicine• Optic nerve-center of image• Sound attenuation, refraction through lens– Easiest to use in awake animals• Superior temporal position (through skin,dorsal to zygomatic arch)-best for orbitaldisease

B-scan probe positions• No standardizednomenclature in vetmed• Human standard:Probe-superior or nasal• 12-Vertical Axial• 3/9-Horizontal Axial• Clock hour at markerFrom Byrne, Green, 2002

• Anterior/posterior lenscapsule• Nucleus, cortexanechoic• Artifact from lenscapsuleB-scan lens

B-scan lens• Cataracts echogenic• Alterations in lensthickness• Luxated lens• Persistent hyaloid,PHPV, lenticonus

• Normal-anechoic• Turn down gain toDDX vitreal vs retinaldetachment• Floaters (vitrealdegeneration)• Asteroid hyalosismultiplebright whitedotsB-scan vitreous

Vitreal degeneration/asteroidhyalosis

Posterior vitreal detachment

B-scan-Vitreal hemorrhage• DDX-tumor,inflammatory exudate• Varies by location,amount, duration• Settles ventrally• Pseudomembranes• Hemorrhagedisappearswith ↓ gainHemorrhage for several weeks

B-scan-vitreal hemorrhage• Fresh hemorrhagelowerdensity, moves• Old hemorrhagedifficultdiagnosis• Monitor over time withconsistent probepositions/gain settingsFrom Byrne, Green, 2002

Vitrealhemorrhage

B-scan-retina• Retinal detachments-– Classic-’gull wing’– Different appearances:• Partial• Complete• Exudative• Disinsertional– Follow to optic nerve toprove retina– Turn down gain

B-scan-retinaPartial detachment inuveodermatologicsyndromeExudative detachmentin dog withblastomycosis

High gainLow gainAxialviewTransverseviewShih Tzu with hyphema

B-scan-choroid• Chorioretinitisthickeningof choroidand/or retina• Hyper or hypoechoic• Choroidal detachmentsharperangle atdetachment siteCourtesy D Ramsey

• Scleral rupture-look forhemorrhagic track• Scleritis: diffusethickening of posteriorsclera– Retinal/choroidalinvolvement common– Distension of sub-Tenon’s space=‘T-sign’B-scan-scleraOld scleral rupture

B-scan-scleritis

B-scan-orbit• Deformities-posterior globe• Diffuse, increased echogenicity retrobulbarspace• Discrete masses• Tumors-hyperechoic, deformation of globe,+/- discrete masses• Inflammatory-diffuse, hyper/hypoechoic– Fluid-abscess

B-scan-orbit10 year old sheepdog

B-scan-orbit

High resolution ultrasound• Ultrasound biomicroscopy (UBM) and highresolution ultrasound (HRUS)– Low power histology-like view• Resolution-20-80µm– 10MHz-300-400µm• Tissue penetration limited to 5-10mm

High resolution ultrasound• Clinical Uses: anteriorsegment evaluation, uvealtumors, cysts, scleral andcorneal disease, drainageangle, changes in thesestructures in disease• Research: effect of drugson anterior segmentstructures, pathophysiologyof anterior segmentdiseasesHRUS image of canine eye

UBM vs HRUSUBM• 50MHz-higherresolution• Smaller field of view• Less penetration• Positioning critical• Sedation/anesthesia• Higher costHRUS• 20MHz -35 MHz-lowerresolution• Larger field of view• Deeper penetration• Easier positioning• Topical anesthesia• Lower cost

UBM vs HRUS50MHz20MHz10MHzCourtesy C. Kendall, Innovative Imaging, Inc.

UBM vs HRUSUBM image High resolution ultrasound image

50MHz 50MHz 20MHz• Not all probes created equal

Technique• Topical anesthetic• Genteal Gel• Superior and temporal quadrants easiest– Ventral and nasal may require sedation

Probe position-longitudinalCan label by clock hour

Probe position-transverseNote ciliary processes

Cornea• Corneal thickness assessment• Sequestrums-deeply pigmented lesionshyperechoic• Deeper, faintly stained area may appearnormal via ultrasound• Use contralateral eye as control for depth

SequestrumHyperechoic sequestrum‘Normal’ cornea=0.27 Normal contralateral eyeCorneal thickness=0.58mm

Corneal tumors• Corneal tumors in horses– Sometimes difficult to assess depth• Limbal melanomas with corneal extension indogs

Hemangioma in a horse

Uvea• Useful to differentiate tumors versus cysts• Useful to evaluate tumor extension• Useful to determine which patients are goodcandidates for laser therapy for tumors;progression• Can use to determine location of ciliaryprocesses for cyclophotocoagulation

Uveal cysts2 yr FS Golden retriever Note bilobed uveal cysts

Uveal cysts14 year old Macaw referred for iris mass

Uveal cysts10 yr old DSH referred for ciliary body tumor

Uveal tumors4 yr MNmixed CNPre laser=0.80mm Immediatepost laser=1.17mm

Uveal tumors3 months post laser=0.58 mm15 months post laser=0.82 mm; re-lasered

Location of ciliary processes• 8 yr mixed breed withprevious lens luxation;sulcus IOL; developedsecondary glaucoma• Location of ciliary body:– 2.18 mm nasally– 2.08-3 mm temporal– 3.01 mm nasal

Lens• Useful to detectperipheral lensabnormalities9 yr FS mixed breed, diabeticLIU more severe OS; HRUSperipheral lens capsule rupture,closed cleft

Iridocorneal angle• Can examine ciliary cleft in greater detail• Can be useful in ‘suspicious’ cases, pre-opcataract cases (?)• 12 yr FS Cocker with goniodysgenesis;unilateral acutely red eye; improving by thetime owner presents dog• IOP 10 OD, 18 OS

Iridocorneal angleOD-note open ciliary cleftIOP 10 mm HgOS-note closed ciliary cleftIOP 18 mm Hg

Iridocorneal angle• Predictive value for onset of glaucoma?

Questions?