Introduction

Despite almost unrestricted access to the eye and orbit, additional diagnostic imaging is frequently required to diagnose a variety of ocular, orbital, and optic nerve conditions in veterinary medicine. Contrast radiography, ultrasonography, and computerized tomography have been reported as useful diagnostic aids in a variety of veterinary ophthalmic disorders. Since the eye is made of different soft tissues and liquids, there must be a good signal production by Magnetic resonance imaging (Bruce et al. 1993). Sometimes it is also difficult to detect inner parts of the eye because of cataract or keratitis. Various systemic diseases like fungal infections, lupus, and vitamin E deficiency can involve optical nerve which is not detectable by conventional ultrasonography or ophthalmoscopy (Nyland & Matton. 2002). In this study sagittal, transverse, and dorsal MRI planes of inter orbital structures such as cornea, anterior and posterior chambers, lens axis, iris, and post orbital structures such as optic nerve cats and the best plane for each individual structure are introduced. T1-weighted and T2- weighted signal intensities were also compared together. All the orbital structures of the cats were investigated for their hypointensity, hyperintensity, homogeneity, and heterogeneity. The sizes of the orbital structures were compared in left and right eyes.

Materials and Methods

A total of 6 females Domestic short hair cats weighing 3.2±0.4 kg were normal in clinical and paraclinical examinations were selected. Magnetic resonance imaging data were collected using GEMSOW (Philips) at a magnetic field strength of 1.5 tesla. Dorsal, sagittal, and transverse planes images were obtained from left and right eyes. A 13 cm diameter RF coil was used. All cases underwent general anesthesia by Ketamine (20mg/kg) and Diazepam (0.02 mg/kg) and positioned in ventral recumbency with their head inside the RF coil The field of view (FOV) was selected as smaller as it was possible to reduce Wrap-around artifacts and it was not to reduce SNR (Signal Noise Ratio). Dorsal plane images were obtained using T2-weighted (TE = 97 msec, TR = 42000 msec, matrix = 256 x 192, and FOV = 17.17) to measure the optical nerve (Figure 2). Saggital plan images were obtained using T2-wightes (TE = 88.3 msec TR = 4000 msec, matrix = 320 x 256 and FOV = 16.16) and T1-Wighted (TE = 36.8 msec, TR = 1500 msec, matrix = 256 x 224 and F0V = 16.16) to measure the orbital structure (Figure 1, 2).

Results

Intraocular structures of the cats visible on T1-weighted and T2-weighted images include cornea, anterior chamber, posterior chamber, lens, iris, sclera, and chiasm. Cornea was well detected in T1-weighted and the iris in T2-weighted. The image signal intensity of the aqueous and vitreous humor were low on T1- weighted and high on T2-weighted images when compared to brain tissue. Sclera has low signal intensity on T1- weighted and T2-weighted images. Aqueous, vitreous humor, and the lens were homogenous on T1-weighted and T2-weighted images and the iris was heterogeneous. Optic nerve was moderate high signal on T1-weighted in compare with vitreous humor and it was moderate low signal and more visible on T2-weighted images in compare with T1-weighted images and chiasma optic was seen better in T2-weighted in dorsal plan (Figure 1, 2). Measurements of the visible structures on T1- weighted and T2- weighted images did not show any significant difference between the left and right eyes (P > 0.05) (Table 1). Optical nerve size was measured in axial plane and anterior chamber, lens, vitreous humor, and orbital axis on sagittal plane (Figure 1, 2).

Table 1.Mean Measurement (mm) and SD internal structure and optic nerve of the right and left eyes in feline by MRI.

Discussion

This study confirms that MRI is useful in investigating of orbital structures. There is a potent high contrast from grate amount of fat surrounding the anatomical structures of cat's eyes. This fat accumulation is a good tool to differentiate soft tissues (muscles and optical nerve) from hard tissues (bones). Magnetic resonance imaging can demonstrate most of pathologic processes which destroyed bony structures (e.g., tumors). Magnetic resonance imaging is considered an excellent imaging modality for a variety of human ophthalmic conditions, because it provides high soft tissue contrast, multiplanar imaging capabilities, precise anatomical detail, flexible image contrast, and tissue pathology-specific images. Magnetic resonance imaging studies of the human eye, orbit, and optic nerves have shown not only the extent of pathological conditions, but also specific tumor images based on anatomical location and signal intensity. For example melanomas appear bright (high signal intensity) on T1-weighted and dark (low signal intensity) on T2-weighted images (Felix et al. 1985). Sarcomas, however, are characterized by low signal images on T1-weighted and high signal intensity on T2-weighted images (Peyster et al. 1985). Volume and distribution of melanomas are also possible by this technique to help preoperational planning, precluded successful surgical removal, and response to radiation. (Garcia et al. 2005) could measure the optical nerve diameter by MRI and three-dimensional ultrasonography in human. Whereas, there are a few reports in veterinary practice using MRI in animal eye examinations. For instance, different orbital structures were investigated in rabbit (Cecker et al. 1991) and cow evaluated in T1 and T2 Weighted (William et al. 1990). Orbital melanoma and meningioma were once reported in cats by using MRI and they also used Gd-DTPA to assess the eye in proton density-weighted scanning (Bruce et al. 1993). In this study the measurement of the internal structural of the eye and optic nerve were the same as another references (Evans & Christensen 2002). In conclusion, our results demonstrate the great potential that MRI offers the veterinarian for imaging ocular, orbital, and optic nerve lesions in the cats. Its lack of ionizing radiation and its direct multiplanar, multislice imaging capabilities are specific advantages in regard to the ocular structures. Its disadvantages include its limited availability to the veterinarian, and the requirement for general anesthesia to limit motion artifacts in the image. Metal objects in the patient's body are also a contraindication in the use of MRI.

Figure 1. T2-weighted magnetic resonance coronal images through the brain, eyes and orbit of a normal cat (TE = 97 msec, TR = 42000 msec). Normal structures are identified using the following abbreviations: ac = anterior chamber; l = lens; on = optic nerve; oc = optic chiasma; vh = vitreous humor. In this plan was measured the optic nerve.

Figure 2. T2-weighted magnetic resonance saggital images through the brain, right eye and orbit of a normal cat (TE = 88.3msec,TR = 4000msec). Normal structures are identified using the following abbreviations: i = iris; l = lens; ac = anterior chamber; on = optic nerve. In this plan anterior chamber, lens and vitreous humor were measured. Iris was well detected in this plan.

References

1.  Bruce HG, Wendy AS, Rheal AT, Michael D. 1993.Magnetic resonance imaging of the canine and feline eye. Can Vet J., 34:418-422

2.  Cecker TL, Karino K, Kador PF, Balaben RS. 1991 .Magnetic resonance imaging of the rabbit eye. Invest Ophthalmol Vis SCI., 32:31093113

3.  Evans HE, Christensen GC. (2002). Miler's Anatomy of the dog. 2nd Edition. W. B. Saunders. 1073-1076.

4.  Felix R, Scharner W, Laniado M. 1985.Brain tumours: MR imaging with gadolinium--DTPA. Radiology., 156:681-688

5.  Garcia JP, Garcia PM, Rosen RB, Finger PT. 2005. Optic nerve measurements by 3D ultrasound-based coronal C-Scan imaging. Ophthalmic Surgery, Lasers & Imaging., 36: 142-146

6.  Nyland TG, Matton JS. 2002. Small animal diagnostic ultrasound. 2nd Edition. W. B. Saunders. 306-322

7.  Peyster RG, Augsburg JJ, Shields JA, Herchey BL, Eagle Rehashing ME. 1985. Intraocular tumours: Evaluation with MR imaging. Radiology., 156:669-674

8.  William TR, Perry BC, Koenig JL. 1990. Magnetic resonance imaging of bovine ocular tissue. Ophthalmic Res., 22:89-94