Comparison to other anterior segment imaging devices

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Roberta Patterson
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1 found helpful in a number of areas of ophthalmology. However, in the field of anterior segment tumors its usefulness is confined to a subset of tumors that are confined to the iris, hypo pigmented, and of limited thickness. Indirect signs such as iris contour and presence of some reflectivity may be helpful in presumption of tumor presence. UBM is more flexible and remains the best method of imaging and subsequently following the majority of anterior segment tumors (Charles et al., 2009). Figure 43: An irido-ciliary cyst (C). (Top) On OCT the iris shows smooth anterior bowing with no reflectivity below it. The cyst wall and underlying structures are not imaged. (Bottom) UBM image of the same irido-ciliary cyst. The cyst wall and underlying structures are imaged (Charles et al., 2009). -٨٢ -

Indirect signs such as iris contour and presence of some reflectivity may be helpful in presumption of tumor presence.

2 Figure 44: Hypopigmented iris tumor with ectropion uvea. (Top) Clinical photograph. (Middle) Anterior segment OCT image of the tumor (T). The tumor is completely penetrated and measurement is possible. The area of ectropion uveae shows shadowing (arrow). (Bottom) UBM image of this tumor(t) (Charles et al.,2009). -٨٣ -

The tumor is completely penetrated and measurement is possible.

3 Figure 45: Deeply pigmented iris tumor. (Top left) Clinical photograph. (Top right) Anterior segment OCT shows high reflectivity from the surface. Internal reflectivity indicates a solid tumor (T). The tumor is not completely penetrated and cannot be measured. (Bottom left) UBM shows complete penetration of the tumor (T) (Charles et al., 2009). -٨٤ -

Internal reflectivity indicates a solid tumor (T).

4 Figure 46: Large hypopigmented iris tumor: (Top left) Clinical photograph. (Top right) Anterior segment OCT shows some irregular reflectivity patterns in the superficial part of the tumor (T). The tumor is not completely penetrated. (Bottom left) UBM provides complete penetration of the tumor (T) (Charles et al., 2009). -٨٥ -

the superficial part of the tumor (T). The tumor is not completely penetrated.

5 Comparison of Anterior Segment OCT and A-scan Ultrasonography The disparities between AS-OCT and ultrasonic measurements in ocular tissues are different: AS-OCT tends to underestimate central corneal thickness (CCT),( Kim et al.,2008), but to overestimate the ACD in comparison to ultrasound (Nemeth et al.,2007). Comparison of OCT and A-scan for Detection of lens thickness Both AS-OCT and A-scan US, instruments were able to measure Lens thickness (LT). As a whole, the failure rate of AS-OCT was slightly higher than that of A-scan US. In young subjects with clear lenses, all LT measurements are successful, which suggests that AS-OCT is able to quantify the LT in eyes with clear lens and most of those with lens opacity. Zeng et al., also found that dense cortical lens opacity may compromise the visibility of the posterior capsule, but nuclear and posterior sub capsular won t affect the visibility. In a clear lens, the posterior capsule is not fully visible, but the posterior pole of the lens is clearly justified by the reflection (Zeng et al., 2009). AS-OCT imaging requires that the subject fixate on an internal target. One patient could not cooperate and fixate on the internal target. On the other hand, the depth of the scan was only 6 mm. Therefore, if an LT was more than 6 mm, the AS-OCT could not capture the posterior poles of the crystalline lens in a single image. In LT measurements with -٨٦ -

Comparison of OCT and A-scan for Detection of lens thickness Both AS-OCT and A-scan US, instruments were able to measure Lens thickness (LT).

6 A-scan US, we found that the cortical lens opacity also compromised the accuracy of measurements. It could be difficult to differentiate the peaks of lens posterior capsule and posterior cortical opacity (Zeng et al., 2009). In Zeng et al. study, the AS-OCT tended to give greater LT values (mean difference, mm in the elderly and mm in young subjects) than did the A-scan US, although both measurements correlated highly with each other and the difference was small. These two measures are designed based on different physical principles: A-scan ultrasound estimates the distances according to the speed of sound in ocular media and the ultrasonic echoes from the interfaces of ocular structure, whereas AS-OCT quantifies the distance based on a linear scan, using infrared light and the principles of low-coherence interferometry (Zeng et al.,2009). The difference in ACD may be explained by indentation of the ultrasound probe, but this should not be the case with LT measurements given that the indentation will not change the echoes of lens capsules. The disparities in the corneal and LT measurements are mainly attributable to the difference in physical principles of the measurements, optical refractive index, and ultrasonic velocity of the tissues and therefore the results in the cornea and lens are not directly comparable. Second, accommodation status in AS-OCT and A-scan US may also account for the differences on LT. AS-OCT is measured with minimal or no accommodation, whereas A-scan US inevitably introduces accommodation and therefore may yield greater LTs when subjects have to fixate on the blinking light on the probe (Lavanya et al.,2007). -٨٧ -

study, the AS-OCT tended to give greater LT values (mean difference, 0.135 mm in the elderly and 0.

7 However, the fact that the AS-OCT gives greater values than US may suggest that this difference could have been even greater. In fact, they observe that the difference (AS-OCT versus A-scan) becomes smaller in young subjects. This suggests that the accommodation makes the lens slightly thicker in A-scan US measurement. But given that the difference is very small, the influence of accommodation may be negligible in clinical evaluation (Zeng et al., 2009). In Zeng et al., study, the intraobserver and interobserver reproducibility of AS-OCT were better than those of A-scan US. It has been suggested that the accuracy of ACD measurements by A-scan US was affected by operator experience, differences in probe tip handling, off-axis measurement, and pupil diameter (Lavanya et al., 2007). In conclusion, Zeng et al. study demonstrated a good correlation between AS-OCT and A-scan US in LT measurements. AS-OCT tends to systematically give greater values, but the differences are small and clinically less important. AS-OCT is able to measure LT in most cases and may be subject to less measurement variation than US (Zeng et al.,2009). -٨٨ -

But given that the difference is very small, the influence of accommodation may be negligible in clinical evaluation (Zeng et al., 2009). In Zeng et al.

8 Figure 47: Lens thickness measurement by A-scan ultrasound. With A- scan ultrasound, it was difficult to differentiate the peaks of the lens capsule from cortical opacity. In this recording, the peaks of lens posterior capsule were not defined correctly by A-scan US (Zeng et al.,2009). -٨٩ -

9 Figure 48: Lens thickness measurement by AS-OCT. (1) Lens with grade 5 nuclear opacity. (2) AS-OCT could not penetrate dense cortical cataracts. (3) LT measurement with the manual calipers. (4) A clear lens (Zeng et al.,2009) -٩٠ -

10 Comparison of OCTand Pentacam for Assessment of Anterior Chamber Volume SL-OCT and Pentacam allow fast, non contact, and precise examination of the anterior segment of the eye. According to the current results, both devices revealed a highly repeatable measurement of anterior chamber volume. In addition, the results were found to be clinically interchangeable with a mean difference of 2.0 mm (Fig. 49) Figure 49: Mean values of each repeated anterior chamber volume Measurement (first, second, and third consecutive acquisitions) by SL- OCT and Pentacam are demonstrated (Asl et al., 2009). Wang and colleagues developed an image-processing software and evaluated the repeatability of anterior chamber volume measurements using Visante OCT (Wang et al., 2007). However, during daily practice -٩١ -

In addition, the results were found to be clinically interchangeable with a mean difference of 2.0 mm (Fig.

11 only anterior chamber depth, internal anterior chamber diameter, and anterior chamber angles can be measured manually using the measuring calipers and tools by Visante OCT. On the contrary, after the scleral spurs are marked by the clinician, the internal software of SL-OCT automatically calculates the anterior and posterior corneal radius, anterior chamber depth, anterior chamber volume, spur distance, pupil diameter, angle opening distance, and trabecular iris space area. There is no internal fixation target in SL-OCT and the strong light reflecting from the slit lamp may cause difficulty for the patients to keep their eyes open during acquisition, resulting in motion artifacts and miosis. Another limiting factor of SL-OCT acquisitions is the relatively long acquisition time for multiple images. The Scheimpflug technique requires complex reconstruction software since the distances observed on the images change depending on the position in the anterior segment (Baikoff et al., 2006). The ocular media quality affects the measurements obtained by Pentacam because of interfering signals reflected on iris. On the contrary, AS-OCT evaluation is less affected by the optical clarity of the anterior segment. The clinician is able to visualize the anterior chamber directly and measure ACD precisely by AS-OCT. The elimination of the subjectivity of the clinician by automatized acquisitions and data analysis is one of the most important advantages of Pentacam. However, the anterior chamber evaluation using SL-OCT depends on the examiner to some extent. The evaluation of AS-OCT data needs precaution, and the measuring calipers and points must be located accurately. Another shortcoming of AS-OCT devices is that the direct biometric evaluation is only accurate for strictly axial measurements. Therefore, off-axis -٩٢ -

anterior chamber volume, spur distance, pupil diameter, angle opening distance, and trabecular iris space area.

12 measurements should be excluded and such acquisitions should be repeated (Asl et al., 2009). Figure 50: Anterior chamber parameters and volume assessment using Pentacam (Asl et al., 2009). Figure 51: Anterior chamber parameters and volume assessment of the same eye in Figure 50 using SL-OCT (Asl et al., 2009). -٩٣ -

Figure 50: Anterior chamber parameters and volume assessment using Pentacam

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