ESTIMATION OF OCULAR AXIAL LENGTH USING MAGNETIC RESONANCE IMAGING TECHNIQUE AMONG ADULTS IN JOS METROPOLIS, NORTH-CENTRAL NIGERIA

CHAPTER ONE

INTRODUCTION: Background of the Study

The human eye is the sense organ with which we view the world. We use our eyes in almost every activity we perform whether reading, working, watching television, writing a letter, driving a car and in countless other ways (Lauterbur, 2003). Most people probably would agree that sight is the sense they value more than all the rest. The eye allows us to see and interpret the shapes, colours and dimensions of objects in the world by processing the light they reflect or emit. The eye is able to detect bright or dim light but it cannot sense objects when light is absent. The amount of light entering the eye is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. A mesh of blood vessels, the choroid, supplies the retina with oxygen and sugar. Lacrimal glands (left) secrete tears that wash foreign bodies out of the eye and keep the cornea from drying out. Blinking compresses and releases the lacrimal sac, creating a suction that pulls excess moisture from the eye’s surface.

Axial length of the eye is an important parameter in ophthalmology (Akduman et al, 2008). The establishment of this parameter helps in the classification of eye defects and normal eye (Montgomery , 1998). Radiology has been shown to play a major role in the establishment of this parameter (Nacke et al,2011). Studies have been carried out in the assessment of axial length using ultrasound and computed tomography (Leiva et al, 2008) and the results compared to findings in ophthalmology .

However, magnetic resonance imaging (MRI) is a diagnostic tool which has been under-utilized in the assessment of this parameter (axial length). Magnetic Resonance Imaging is one of the latest technological innovations in the health sector and a medical diagnostic technique that combines strong magnetic fields, radio waves and computer technology to create images of the body using the principles of nuclear magnetic resonance (Mansfield, 2003). It is a technology for producing detailed inner views of the body without the need for potentially harmful X-rays or invasive exploratory surgery (Lauterbur, 2003). A versatile, powerful and sensitive tool, MRI can generate thin section computerized images of any part of the body including the heart, arteries and veins from any angle and direction ( multiplanar imaging) in a relatively short period of time.

In current medical practice, MRI is preferred for diagnosing most soft tissue diseases because it gives excellent soft tissue contrast and superior anatomic detail (Abrikosov, 2000). It can distinguish soft tissue in both normal and diseased states. In using MRI, it is able to delineate the posterior aspects of the cornea and retina better than ultrasound and CT scan (Stone,2012). Since the lens of the eye is involved in axial length assessment, MRI is preferable to computed tomography ( CT ) because the former does not use ionizing radiation. No known risks have been recorded except for patients with cardiac pacemakers, patients who might have iron fillings next to their eyes (e.g. sheet metal workers), patients with inner ear transplants and patients with aneurysmal clips in their brains (Leggert et al, 1999). Although an MRI scan is relatively expensive, it may actually reduce costs to patients and hospitals by providing diagnostic evaluation to out patients and thereby frequently limiting more expensive hospitalization.

Axial length is defined as the distance from the back of the cornea to the back of the retina (Montogomery , 1998). Leiva et al (2008) defined axial length as the distance, through the visual axis from the posterior corneal surface to the posterior pole of the eye in axial view. The axial length of the eye at birth is approximately 17mm. It increases rapidly until age 10 and then increases slowly(Bradford,2012). It reaches approximately 24mm in adulthood. It is typically longer than 24mm in myopes and shorter than 24mm in hyperopes .The reason adduced was that myopes have longer eyeballs while hyperopes have shorter eyeball (Akduman, 2008).

According to Montogomery (1998), the average newborn’s eyeball is about 18mm in diameter from front to back (axial length). In an infant, the eye grows slightly to a length of approximately 19.5mm. The eye continues to grow, gradually to a length of about 24-25mm or about 1 inch in adulthood. A ping-pong ball is about one and half (1 ½) inch in diameter, which makes the average adult eyeball about two-third (2/3) the size of a ping-pong ball. The eyeball is set in a protective cone-shaped cavity in the skull called the “orbit” or “socket”. This bony orbit also enlarges as the eye grows. The orbits are symmetrical and of normal size with normal development of the orbital cones. The orbital walls show a normal configuration with smooth, sharp margins with no evidence of bone destruction, no circumscribed expansion of the bony or soft tissue components of the orbital wall (Moeller et al, 2000). Eye defects like myopia and hypertropia could be diagnosed by establishing a normogram for axial lengths of the eye.

1.2 Statement of Problem

There have been studies in Europe (Akduman, 2008; Nacke et al, 2009) which have assessed the axial lengths of the adult eye using magnetic resonance imaging. Due to possible racial differences in axial lengths of the eye, there is need to embark on similar study in our locality since no literature exists for any Nigerian adult population.

1.3 Objective of the Study

The purpose of this study is to evaluate the axial lengths of the eye in healthy Nigerian adult population using MRI.

The specific objectives of the study are:

1. To establish normogram of the axial lengths of the eye in a healthy Nigerian adult population using MRI.
2. To determine if there is any significant difference in the mean axial length measurement of the right and left eye.
3. To compare the findings in this study with what is found in the literature amongst Caucasians.
4. To compare mean axial lengths in different age groups.
5. To compare mean axial lengths in males and females.

Null hypothesis (Ho): There is no significant difference between the mean axial lengths of the right and left eye.

Alternative hypothesis (H1): There is a significant difference in the mean axial lengths of the right and left eye.

1.4 Significance of the Study

1. Magnetic resonance imaging evaluation of axial lengths of the eye will set a benchmark for normal value in an adult Nigerian population.
2. Magnetic resonance imaging evaluation of axial lengths of the eye will be a useful tool in diagnosing eye defects such as myopia and hypermetropia in radiology.
3. Magnetic resonance imaging evaluation of axial lengths of the eye will form a background for further ocular studies in Nigeria and Africa in general.

1.5. Scope of the Study

The study was limited to the residents of Jos, Plateau State ,Nigeria and was carried out at Radiology department, Jos University Teaching Hospital (JUTH), Jos, Nigeria.

1.6 Definition of Terms

1. Axial Length: This is the distance through the visual axis from the posterior corneal surface to the posterior aspect of the retina (Leiva et al,2008).
2. Emmetropes : These are individuals considered to have axial lengths within the normal range(23-26.5mm) (Bottomley, 2007).
3. Hypermetropia : This is the eye defect where the eyeball is shorter than the normal range (23.4-26.8mm),hence images fall behind the retina (Sobering et al, 2005).
4. Myopia : This is the eye defect where the eyeball is longer than the normal range, hence images fall in front of the retina ( Sobering et al, 2005).
5. Supine Position : Imaging position with the patient lying on his/her back as opposed to prone position (Gutteridge et al, 2006).
6. t-test : Statistical test used in medical research to compare the differences in the mean values of two groups of data. An appropriate level of significance must be used (Everitt, 2003).
7. T1-weighted : A sequence where signal contrast in the image is determined predominantly  by differences in T1 relaxation times. A short TE to minimize T2-weighting and a short TR is used (McAtamney, 2009).
8. T2-weighted : Image in which the signal contrast is determined predominantly by differences in T2 relaxation times. It uses a long TR to minimize T1-weighting and long TE (McAtamney, 2009).
9. Myopes : These are individuals with the eye defect myopia( Elster et al, 2000).
10. Hyperopes : These are individuals with the eye defect hypermetropia (Elster et al, 2000).