الفهرس 
INTRODUCTION
OCULAR LESIONSOnly 14 pages are availabe for public view
 |  | from | 110 |  |  |
| | | from | 110 | | |
Abstract
Orbital space-occupying lesions represent a broad spectrum of benign and malignant diseases, many of which are difficult to differentiate on the basis of clinical or radiologic criteria. Diffusion-weighted (DW) imaging is a promising magnetic resonance (MR) imaging technique that may help distinguish malignant lesions from benign mimics.
DW imaging is predominantly used to identify acute infarct, which has characteristic high signal intensity on DW images, with associated low signal intensity on apparent diffusion coefficient (ADC) maps. Increasing emphasis on quantitative measurement of ADC has led to an increasing role in the characterization of solid masses, with at least one prospectively validated algorithm that incorporates DW imaging in the characterization of indeterminate brain masses. Despite technical limitations related to the commonly used echo-planar imaging pulse sequences, DW imaging has also been increasingly used to help characterize lesions of the head and neck. Parallel imaging has helped decrease echo-planar imaging–related artifacts in the head and neck, likely by decreasing effective echo time. Some of the larger studies have proposed specific threshold ADC values that may differentiate benign from malignant lesions.
The aim of this study is to evaluate the role of diffusion weighted MRI in the adult non osseous orbital lesions.
This study was conducted on 20 patients presenting with proptosis or visual impairment, collected during 6 months duration, starting from November 2010.
All patients were subjected to full history taking, thorough clinical and ophthalmological examination. Conventional and diffusion weighted MRI examinations of the orbit were performed for all patients included in this study on the 1.5 T MRI scanner (Siemens).
Diffusion weighted MRI of the orbit was processed. Final diagnosis was reached either by surgical findings & histopathological examination or therapeutic response or a consensus of clinical and imaging modalities or therapeutic response.
The studied cases included 5 males and 15 females (with their age ranged from 19 to 61 years and a mean age of 42 years). The patients were categorized on pathological basis into two main groups:
-Group A: 15 patients with benign orbital lesions; including:
• 7 inflammatory lesions (orbital pseudotumor, orbital abscess, orbital phlegmon and optic neuritis).
• 5 benign neoplastic lesions (orbital meningiomas and optic pathway gliomas).
• 2 tumor like lesions (cholesterol granuloma and venolymphatic malformation).
• One miscellaneous lesion (thyroid orbitoptahy).
-Group B: 5 patients with malignant orbital tumors; including
• 4 lymphoma patients.
• One choroidal malignant melanoma.
Significantly low ADC values were seen in malignant lesions compared with benign lesions and between lymphoma and pseudotumor. With a threshold value of 0.9 × 10−3 mm2/sec, ADC was 100% sensitive, 80% specific, and 85% accurate for differentiating malignant from benign lesions. DW imaging was highly sensitive and specific in differentiating lymphoma from pseudotumor. A threshold ADC of 0.9×10−3 mm2/sec was both 100% sensitive and 100% specific for predicting orbital lymphoma.
Diffusion weighted MRI added valuable information as follows:
1) Malignant orbital masses demonstrate significantly and visually appreciable lower apparent diffusion coefficients (ADCs) than benign orbital masses.
2) An ADC of less than 0.9 × 10−3 mm2/sec represents the optimal threshold value for predicting malignancy.
3) Diffusion-weighted (DW) imaging is 100% sensitive, up to 80% specific, and up to 85% accurate for predicting malignancy.
4) Orbital lymphoma can be differentiated from pseudotumor with extremely high accuracy by using a threshold ADC of 0.9 × 10−3 mm2/sec.