Explaining The Current Role Of High Frequency Ultrasound In Ophthalmic Diagnosis (Ophthalmic Ultrasound)

Abstract

Ultrasound has become as indispensable as indirect ophthalmoscopy or slit lamp in evaluation of the eye. It is an important adjuvant for the clinical assessment of a variety of ocular and orbital diseases. Advances in instrumentation, higher frequencies and more sensitivity and resolution have resulted in continuous improvement in image quality.Very high frequency ultrasound uses frequencies in the range of 35 to 100 MHz to show greater detail of the anterior segment. Penetration is limited for these higher frequencies to only a few millimeters and thus only the anterior vitreous behind the ciliary body and lens can be imaged. High frequency ultrasound in the range of 20 to 30 MHz has a penetration of about 10 mm and can be used for posterior pole evaluation of the retina and choroid.

Figure 1

A-scan and B-scan of an eye at 10 MHz with a sector scanner representative of a usual B-scan of an eye. The front of the eye is too close to the transducer to be imaged without a fluid standoff.

Figure 2

An A-scan can be taken from the B-scan or with a separate transducer. Best results with the A-scan require either “quantification” from a glass plate and a logarithmic amplifier, or with a tissue standard (“standardization”) in order to help identify tissue type, i.e. tumor type or hemorrhage. Routine use of the A-scan also is for axial length measurements to determine appropriate lens power for intraocular lens implants.

Figure 3

Posterior globe wall and orbit on B-scan show better definition with higher frequencies, but the usual 10MHz is best because of lens attenuation or other absorption due to pathology. a) 10 MHz B-scan of a normal eye using a sector scanner. b) 10 MHz B-scan of an eye using an arc scanner.

Figure 4

Vitreous hemorrhage in an eye precludes visual examination in order to determine other pathology such as underlying tumor or if a retinal detachment is present.

Figure 5

Retinal detachment shows a strongly reflective echo producing surface that can resemble blood along a detached vitreous (PVD) or indicating a choroidal detachment. Landmarks, such as the retina, always remaining attached at the optic nerve and the choroid showing a smooth convex surface due to pressure in the supra choroidal space exceeding that of the vitreous help to distinguish the pathology.

Figure 6

Intraocular foreign bodies are typically higher reflection due to their density and surface orientation. Ringing artifacts are a hallmark of glass, metal and even air bubbles or gas. In this scan, the foreign body (arrow) appears in the anterior lens with ringing artifacts, serving as a pointer to the foreign body.

Figure 7

High frequency (50 MHz) shows an anterior segment demonstrating a solid tumor of the ciliary body. Transillumination can be helpful, but this is an optically occult area of the eye.

Figure 8

a) B-scan of a melanoma, a solid tumor of the posterior pole. The A-scan is typical, demonstrating a rapidly decreasing amplitude due to tissue homogeneity. b) 20 MHz B-scan of a different melanoma, demonstrating improved resolution of the anatomic relationship of the tumor to the scleral wall.

Figure 9

a) Hemangiomas of the eye are filled with vascular channels and fluid spaces that give a uniformly high amplitude A-scan through the tumor. b) 20 MHz B-scan of a different hemangioma, demonstrating improved resolution. Metastatic carcinomas can also show uniformly high amplitude, but if only moderate amplitude compared to the vitreoretinal surface.

Figure 10

A representative low power histological section with color overlay (a,c). Areas in green denote tumor regions with the presence of arc and arc with branching features. Areas in red denote the presence of closed loop and network features. Companion spectral parameter images of acoustic concentration (b,d) given in db per mm³.

Figure 11

The ROC (Receiver Operating Characteristic) curve analysis for the linear discriminant analysis and support vector machine in terms of retrospective performance. The area under the curve, a measure of the performance of a classifier, was .866 for the linear discriminant analysis and .983 for the support vector machine.

Figure 12

B-scans at 20 MHz of small suspicious melanoma measuring less than 2 mm in thickness. Figure (a) shows a sold lesion with no evidence of subretinal or intraocular fluid, as seen in figure (b). These fluid spaces indicate a propensity for increased lethality.

Figure 13

Ultrasound arc scan of post-LASIK cornea. In this case, the image is displayed without geometric correction, allowing better appreciation on internal anatomy. Note keratome interface (L), Bowman’s membrane (B), which in this case shows several discontinuities consistent with breaks formed during surgery. Also shown in one line of radiofrequency echo data (RF) and its envelope, as determined from the deconvolved analytic signal of the RF data, which is accurate to 3 microns.

Figure 14

A complicated LASIK flap demonstrating breaks in Bowman’s membrane. Also shown are corneal thickness maps of reconstructed layers through multiple meridional planes.

Figure 15

An overall single B-scan of the anterior segment with the Artemis2 arc scanner permits angle to angle (a) and sulcus to sulcus (s) measurements in order to select the appropriate lens size. Lens sizing is a critical element for new lens designs.

Figure 16

The walls of the sclera adjacent to the equator of the lens are measurable in order to optimally place laser relaxing cuts or scleral implants in order to treat presbyopia surgically.

Figure 17

An IOL in proper position in the capsular bag.

Figure 18

An IOL with misalignment due to rupture of the supporting capsular bag and zonule.

Figure 19

An optic nerve outline showing separation of the nerve (N) and sheath (S) by inflammation in optic neuritis. This may be the only objective sign in these cases.

Figure 20

Swept scan of region of the retina and choroid to demonstrate choroidal blood flow. This technique can be used to quantify flow with different pharmacologic agents, such as sildenfil or niacin.

Figure 21

Wavelet imaging of the choroid in a human eye to demonstrate both thickness and microarchitecture. Comparison studies of normal and pathologic states, such as myopia or age-related macular degeneration can be sued to document variations in the in vivo eye.

Figure 22

20 MHz fundamental (left) and harmonic (right) image of the posterior segment of a normal human eye in the region of the optic nerve.