Indian Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 1987  |  Volume : 35  |  Issue : 1  |  Page : 17--21

Computed tomographic method of axial length measurement of emmetropic Indian eye a new technique


Madhumati Misra, Santan Rath 
 Department of Neuro Surgery, S.C.B. Medical College, Cuttack -753 007, India

Correspondence Address:
Madhumati Misra
Department of Neuro Surgery, S.C.B. Medical College, Cuttack -753 007
India

Abstract

This is a study of 50 orbital CT Scans in emmetropic adults in «SQ»the neuro ocular plane. A technique is involved to measure the axial length of the eye using CT Scan. It was also tried in 20 new born infants. The measurements are compared with other techniques of axial length measurement.



How to cite this article:
Misra M, Rath S. Computed tomographic method of axial length measurement of emmetropic Indian eye a new technique.Indian J Ophthalmol 1987;35:17-21


How to cite this URL:
Misra M, Rath S. Computed tomographic method of axial length measurement of emmetropic Indian eye a new technique. Indian J Ophthalmol [serial online] 1987 [cited 2022 Dec 4 ];35:17-21
Available from: https://www.ijo.in/text.asp?1987/35/1/17/26323


Full Text

The axial length of the living human eye has been of concern since the beginning of. medicine. Various techniques have been so far described to establish the normal global length. These techniques include the indirect method (1909) [1], radiographic method [1] (1942), ultra sonic method [2] (1961), angiographic method (1962 [3],[4], and Photographic method (1976) [2],[5]. The axial length of the human eye as determined by the above methods averages between 22 - 27 mm. Dichiro [4] (1962) described the angiographic method of determining the actual length of the eye and obtained an average measurement of 27 mm. Gray et al [2](1977) compared the photographic method with the ultrasonic method and concluded that the photographic measurements were consistently larger than the ultrasonic by 3.5 mm. The Photographic method measured the radius of rotation of the corneal apex about the centre of rotation of the posterior scleral segment. The calculation assumed that the apex of the cornea lies on the circumference of the circle described by the posterior sclera and the centre of rotation of the globe is midway between the corneal apex and the posterior pole. These assumptions were later found to be wrong. Also, accommodation during ultrasonic and photographic procedures may also account for differences in axial length measurements, as measured by these two techniques [2]. It was concluded that a combination of various methods may be employed to determine the accuracy [1],[2].

Recently, the development of computed tomography (CT) has made invivo measurements of oculo-orbital structures possible and this has been taken advantage of in obtaining the axial global length [6],[7],[8],[9],[10],[11],[12],[13]. In order to eliminate any possible distortion of the images on the CT video screen, Crow et al (1982) first obtained photographic transparencies of the images. These transparencies were projected on to a screen, such that the projected image was of actual size. Axial length of the globe was obtained from the projected image [10].

We have however employed a new CT technique for obtaining the in vivo global length directly by utilising the computerises distance measurement technique [13]. This technique is more accurate, less expensive, takes little time and results. can be read at the end of the operation. To the best of our knowledge, this technique is first reported by us in the world literature and hence has been named as 'Misra and Math's technique.

 MATERIAL AND METHODS



50 orbital CT scans were taken strictly oriented in the neuro-ocular plane (N.P.0) i.e. a plane that passes through the transverse dimensions of the globe and shows the scleral rim, lens, papilla, optic nerve and horizontal recti on either side. None of these patients (all adults) had myopia, hypermetropia, glaucoma or any other abnormality that might affect the size of the globe. All CT images were made. using Hitachi 256 x 256 scanner and the axial global length was obtained by using the computerized distance measurement technique. The eye ball axial length was taken between two horizontal lines thrown along the corneal apex anteriorly and along the scleral rim posteriorly, (A in [Figure 1]. The distance between these two lines.

(a) give the computerized measurement of the axial length of the globe. The eye ball transverse diameter was similarly taken between two vertical lines (B in [Figure 1]. Ocular dimension were also obtained in 20 newborn infants scanned for suspected intracranial haemorrhage.

 RESULTS



Linear measurements and average values for the emmetropic adult subjects [Figure 1]

Adult i) Eye ball maximal length (E.M.L)

E.M.L. - 24.50 mm (SA - ± u.66)

ii) Eye ball maximal tansverse (E.M.T)

E.M.T. - 27.00 mm (SD - ± 0.60)

New born (i) Eye ball maximal length (E.M.L.)

E.M.L. - 16.00 mm (SD - ± 0.72)

 DISCUSSION



Estimation of the axial length of the globe is needed to establish axial emmetropic stages, glaumatous global enlargement, locating intraocular foreign bodies and intraocular tumours and for the diagnosis of shallow retinal detachment [1],[10]. The various techniques described so far to establish the axial length of the globe are depicted in [Table 1]. The CT technique is now accepted as the most accurate diagnostic procedure for obtaining invivo measurements [10],[12],[13]. Previous workers calculated the axial global length from the photographic transparencies obtained from the CT video screen in order to avoid possible image distortion. As we have used the computed distance measurement technique for obtaining global length, our data presents the actual length of the globe [13]. Hence we claim our method to be more accurate than the previously described CT technique [8],[10].

CT biometry is no doubt more convenient and accurate than the previously described techniques because of the following reasons.

i) CT is non-invasive, painless, takes few minutes of time and patient needs no hospitalisation. [6],[8],[10],[13]

ii) The accuracy figure can go upto 98% [11],[12]

iii) Some techniques (radiographic/photographic) (2) did not include choroido-scleral thickness (1.5 mm approx) and hence the axial length of the globe was under-estimated. CT measurement however includes this structure.

iv) Co-operation of the patient and gaze fixation are not required for obtaining CT biometry. Children and non-cooperative adults can be hence included in the study.

v) The previously estimated axial global length [Table 1] was affected by image magnification of the camera, corneal curvature and the degree of image separation on the corneal surface, the vergence of rays at each interface and the effect of accomodation. Such errors have been completely eliminated in CT technology.

vi) The previously employed techniques [Table 1]) were based on various wrong assumptions like the corneal apex lies on the circumference described by the posterior sclera and the globe rotates around its anatomical centre. Also, various values for the refractive indices of the media were assumed. Only CT gives the real anatomical length directly.

vii) As both orbits are simultaneously displayed, uniocular emmetropic states and subnormal anatomies can be detected.

In institutions in which CT scan is not available, our data may be utilised in establishing axial emmetropic states, glaucomatous global enlargement and in locating intraocular foreign bodies. The results, however will depend on the accuracy of observation.

References

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