|Year : 1983 | Volume
| Issue : 5 | Page : 493-494
Computerised axial tomography in ophthalmology
CP Gupta, Netra Premraj
Government Ophthalmic, Hospital, Madras, India
C P Gupta
Government Ophthalmic Hospital, Madras-600 008
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Gupta C P, Premraj N. Computerised axial tomography in ophthalmology. Indian J Ophthalmol 1983;31:493-4
CAT is a new important radiographic scanning technique which is having a great impact in investigative Ophthalmology. Interpretation of X-rays depend upon images of large differential densities.
Hounsfield (1953) developed a method which employs Sodium Iodide crystals instead of photographic film for detection of emergent rays. With the use of normal X-ray beam, Sodium iodide crystal photomultiplier detector, Tomography and Computer he succeed in demonstrating tomographic configuration of the various soft tissue contents of orbit and cranium.
| Method|| |
Patient lies on adjustable couch and the head is inserted in a latex `cap' that projects into a plastic water containing box. Water is pumped into the box moulding the cap firmly onto the patient scalp excluding intervening air,
A slit X-ray beam of 26 min or 16 mm width scans the head and emergent photons are detected by 2 collimated sodium iodide crystals photomultiplier crystals that move parallel with the X-ray tube, During the movement 240 measurements of photo transmission are obtained from each position of the scan, The movement is repeated at 1° interval over an arc of 180° probing (240 x 180 = 43.200) absorption reading in 4z mins.
In routine examinations, collimator provides scans of tissue planes of 13 mm thickness, 8 mm thickness scans are also possible.
For orbital study the position of the head is adjusted to provide scan section parallel to Reid base line and the patient is so inserted that if possible the lowest slice scanned extends through the floor of the orbit. The orbit and contained structures are usually encompassed by four slices. Intra cranial contents are also surveyed in the orbital scans. In suspected vascular lesions or neoplasms it is customary to study the orbit with I.V. contrast media. The solution used is 300 ml 30 % Hypaque given I.V. for an adult.
By this means detection of small lesions are enhanced in proportion of their vascularisation and margins of larger lesion are well demarcated.
X-ray machine operates on an average K.V. range of 120 volte and milliampere value of 33. The maximum radiation does for a single scan is 0-5 rad.
Neoplasms that arise or extend into retrobulbar space were ideally shown by CAT scan. Masses which are greater than 5 mm are usually shown.
By measurable difference in tissue density we can differentiate between vascular tumours such as haemangioma, lymphangioma and haemangiopericytoma from lymphoma and rhabdomyosarcoma. They can be further differentiated by injecting Hypaque. It is not always possible to predict the histology of the tumour. It can differentiate between calcification and haemorrhage, optic nerve glioma is diagnosed by its 'banana shaped' enlargement of the optic nerve.
Meningioma enlarges the optic nerve along the entire length in a irregular fashion.
Foreign bodies like metallic fragments and glass are clearly distinguished because of their high absorption coefficient.
Graves disease is the commonest cause of bilateral/unilateral proptosis. It is difficult condition to diagnose by C.T. scanning. Majority have normal scans. Occasional cases may show muscle thickening as orbital apex masses more difficult in unilateral ex-ophthalmus where a thickening of a muscle may be seen which may be difficult to distinguish from a tumour mass. Most cases of psuedo tumour have normal scans.
CAT scanning is more accurate in demonstration of a orbital tumours than inflammatry lesions. Accuracy of CAT scanning for the evaluation of orbital space occupying lesion is thought to be 76 % (Dallow et al 1976).
A disadvantageous of CAT scanning so far has been that you could not study intra orbital and intra cranial lesions in multiple planes. This disadvantage is now being removed as some machines can produce from the same site a picture in the corneal plane and saggital plane.
In conclusion we may say that all cases of orbital lesions are not suitable for CAT scanning. If you suspect an orbital tumour or one extending from surrounding structures then CAT scan is best. If the lesion is thought to be inflammatry or endocrine diseases then ultra. sonography is more suitable. If the lesion is thought to be a venous anomaly than a venogram would be the best choice. Lastly though some intra ocular lesions may be seen in a CAT scan they are better investigated by ultra sonography.
[Figure - 1]