ORBITAL CT SCAN

1. ORBITAL CT SCAN Interpretation of CT imaging of the eye and orbit A systematic approach Prof. Ahmad Mostafa 2014

2. Introduction • A simple question anyone might ask at this stage is: • Why should an ophthalmologist learn to interpret CT scans? • Why not just read the radiologist's report and proceed?" 2014

3. Computed tomography (CT) is an important imaging tool in the evaluation of most orbital and some ocular lesions. • This technique allows us to diagnose the location, extent and configuration of the lesion and its effect on adjacent structures. • It also allows us to comment on the possible tissue mass composition. • In addition, knowing the precise location of a lesion facilitates the planning of an appropriate surgical approach to minimise morbidity. • A general ophthalmologist should have the ability to review a CT scan by himself, especially if orbital diseases and ocular oncology are areas of his special interest. 2014

4. The CT machineEvolution and Principle • On their way through tissues, X-rays are attenuated due to absorption of energy. • This "attenuation" is determined by the atomic number of the major tissue constituent. • Different tissues provide different degrees of X-ray attenuation, and it is this property that forms the basis of all imaging techniques. 2014

5. Plain radiography involves X-rays that pass through the patient, and create an image directly on a photographic film. • A three-dimensional structure is thus depicted on a two dimensional plane, Moreover, its sensitivity to small differences in the attenuation is low, i.e., its contrast resolution is poor. • In traditional tomography, the X-ray tube and the film is moved simultaneously in such a way that only a thin plane through the patient is imaged sharply, while structures located in other planes become blurred. 2014

6. Thus, the CT differs fundamentally from the plain radiography methods. • The X-ray tube of the CT machine emits a thin collimated fan-shaped beam of X-rays that are attenuated as they pass through the tissues, and are detected by an array of special detectors [Figure - 1]a. • Within these detectors, X-ray photons generate electrical signals, which are converted into images. 2014

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10. High density areas are depicted as white whereas low density areas appear black. • The CT images contain information from thin slices of tissue only, and are thus devoid of superimposition. • The result contrast resolution is far superior to projection X-ray techniques. 2014

11. Recent technological advances have greatly enhanced the applications of CT scan. • Today, it can be used for imaging any part of the body, and its role in the diagnosis of ocular and orbital disorders is well established. • The widespread use of CT is accompanied by an excessive technical and radiological expressions. • An intimate knowledge of this, however, is not necessary to interpret a CT scan, just as it is not necessary to understand the complexities of a personal computer to benefit from it. 2014

12. Requesting a CT Scan(Indications) • The list of indications for CT scanning of the orbit is exhaustive. • However, The common indications for CT scanning of the orbit are given in [Table - 1]. 2014

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14. Technical parameters • Several technical parameters are printed beside each CT image (Fig. 2 a). 2014

15. Though their location and number varies across machines, some common and important ones include: • Slice thickness • Scan time • Hounsfield number • Window width • Window level 2014

16. Slice thickness represents the thickness of tissue scanned at one time, and is mentioned in millimeters. 2014

17. Scan time represents the time taken (in seconds) to image each tissue slice. • The ideal scan time is less than 1-2 seconds per image. • A longer scan time (few seconds) can lead to motion artifacts (especially in children and uncooperative patients) and the findings need to be interpreted in that condition. 2014

18. Hounsfield units (HU) represent a scale of radiation attenuation values of tissues. • The number assigned is called the Hounsfield number. • This number can range from -1000 to +1000 HU or above, and a higher number represents greater attenuation of X-rays, indicating higher tissue density. • The Hounsfield numbers of various ocular and periorbital tissues are shown in (Table 2). 2014

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20. This can aid in the differential diagnosis. • For example, a dermoid cyst (Fig. 6 a) will have a Hounsfield number below zero due to its fat content, whereas a haematoma, will have a number of +70 to + 80 HU. 2014

21. Window width (WW) refers to the span of CT numbers on the Hounsfield scale that are selected to display the given image. • It can vary from a few CT numbers to the entire range available on the system. • Since the Hounsfield scale usually ranges from -1000 to +1000 HU or above, the maximum WW can be approximately 2000. • Thus at a WW of 2000, air will be black and bone will be white. • The rest of the tissues will be depicted in shades of gray between these two extremes of the spectrum. • A wider WW thus depicts a large number of tissues, and bone details can be better appreciated. 2014

22. Window level (WL) refers to the midpoint of the selected span of CT numbers, or in other words, represents a point midway between totally black and totally white. • For example a WW of 100 with a WL of +50 (Fig. 2a) displays all tissues with Hounsfield value ranging from zero to +100 HU. • Values above +100 HU will be white, those below zero will be black, and those between the two will have all shades of gray. This window setting is ideal for soft tissue evaluation. 2014

23. On the other hand, a WW of 2000 with a WL of +200 (Fig.2b) displays all tissues with Hounsfield value ranging from -800 HU to +1200 HU. This setting is ideal for evaluation of bone. 2014

24. Major Considerations • CT scan is most informative when the ophthalmologist seeks active participation of the radiologist in the diagnostic work-up. • The clinical information supplied by the referring ophthalmologist is used by the radiologist both in the selection of appropriate techniques for imaging, and in deriving the most specific conclusion. • Failure to communicate the patient's clinical data and the need for special scans to the radiologist is the most common cause for falsely negative CT scans. 2014

25. The following prerequisites should be considered and if necessary, discussed with the radiologist before an ocular or orbital CT scan is requested. (a) Slice thickness (b) Imaging plane (c) Tissue windows (d) Contrast enhancement (e) Modification of CT procedure (f) Simultaneous brain CT 2014

26. (a) Slice thickness • Spatial resolution of a CT depends on slice thickness. • The thinner the slice, the higher the resolution. • The slice thickness can varies from 1-10 mm. • Thin slices are good for spatial resolution, but require higher radiation dose, a greater number of slices, and eventually longer examination time. • The choice of slice thickness therefore is a balance of these factors. • Usually, 2mm cuts are optimal for the eye and orbit. • In special situations (like evaluation of the orbitalapex), thinner slices of 1mm can be more informative. 2014

27. (b) Imaging plane Routine CT scan of the orbit involves axial as well as coronal views [Figure - 1]b. 2014

28. However, reformatted sagittal views along the axis of the inferior rectus muscle can have special application in evaluation of orbital floor blow-out fractures and should be preferred in such cases. [Figure - 1]c 2014

29. (c) Tissue windows ► Tissues around the orbit form a spectrum of composition and density, ranging from air (within the para-nasal sinuses) to the bony orbit. • Tissue window refers to the selection of a small range from this variable spectrum to decipher the finer details of the tissue of interest. • Each tissue window has a specific windowwidth and window level. • Thus we havebone window, soft tissue window, brain window and so on. 2014

30. A thorough evaluation of any tissue is possible only when it is scanned under appropriate window settings: • Soft-tissue window is best for evaluating orbital soft tissue lesions, whereas fractures and bony details are better seen with bone window settings. • Though tissue windows are selected by the radiologist, it is important for the ophthalmologist to be familiar with the concept 2014

31. (d) Contrast enhancement • Contrast study involves imaging the area of interest after intravenous injection of a radiological contrast medium[Figure - 3]a and [Figure - 3]b. • Fortunately, orbital fat provides intrinsic background contrast, and most orbital pathologies can be easily visualised without infusion of a contrast medium. • However, in certain situations, a contrast medium is essential. • A contrast-enhancing lesion is one which becomes bright or more intense after contrast medium infusion. 2014

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33. Evaluation of optic chiasma, perisellar region and extra-orbital extensions of orbital tumors is best possible with contrast enhancement. • Contrast enhancement also helps to define vascular and cystic lesions as well as optic nerve lesions, particularly meningioma and glioma. • In short, the use of contrast enhancement is not routinely necessary for orbital pathologies unless they have intra-cranial extension, and its use is best left to the discretion of the radiologist. 2014

34. (e) Modification of CT procedure • Certain cases may require special modifications during the scanning procedure to aid diagnosis. • For example, in a suspected case of orbital venous varix, in addition to the routine cuts, it is important to request for special scans (with contrast) while the patient performs a Valsalva maneuver. 2014

35. (f) Simultaneous brain CT • In certain situations it is mandatory to request CT brain along with that of the orbit. • These include: 1. suspected neurocysticercosis with orbital involvement 2. head injury with orbital trauma 3. optic nervemeningiomas, 4. bilateral heritable retinoblastomas (to rule out pinealoblastoma), and 5. suspected perisellar lesions. 2014

36. Components of a CT Scan Plate 2014

37. For a complete and systematic evaluation of any CT scan, it is mandatory to familiarize oneself with the CT plates. • Unlike a plain radiograph, a CT scan usually provides three to four plates, each carrying multiple sequentially arranged images, with and without contrast. • Since we need to view sagittal and coronal images together, it is ideal to have a large X-ray viewer board which can simultaneously display at least two plates. 2014

38. The various components of a CT plate can be categorized under the following headings: • Patient data - This includes the name, age, sex of the patient as well as the date of the CT scan. - It is always necessary to confirm that you are looking at the correct CT of the specific patient, done at the specific time before you begin to interpret. 2014

39. (b) Type of CT scan - Note whether the plates provided are plain CT scans or contrast enhanced. - Though the image brightness and the Hounsfield value enables us to identify the contrast images, it will be printed next to each image whether the scan is plain or contrast enhanced. 2014

40. (c) Laterality- Though the eye depicted on the right side of the image usually depicts the right eye, it is important to note that these conventions are not universal. - Therefore, the best way to confirm laterality is to look for the "R" or "L" mark which represents right or left respectively  2014

41. (d) Axial scan orientation - Axial scans are traditionally made to pass through a plane parallel to the Reid's baseline or the orbito-meatal line [Figure - 1]b. 2014

42. - To orient yourself to the axial scans, always begin with the lateral scout view depicted as a plain radiogram that shows the exact location of planes chosen for axial slices. - It is usually displayed as the first image on the plate, preceding the serial axial images. - A simple way to identify the level of the axial slice is to note that as we move from inferior to superior, the prominence of the nose flattens out anteriorly, and increasingly more brain parenchyma appears posteriorly. - Slices that depict the lens represent the mid-level axial plane. [Figure - 4]. 2014

43. Therefore, according to the level in the orbital cavity, we will have: • A. Inferior axial orbital CT scan • B. Mid-axial orbital CT scan • C. Superior axial orbital CT scan 2014

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45. (e) Coronal scan orientation • The coronal scans should ideally pass through a vertical plane perpendicular to that of the axial scans, but are usually angled slightly obliquely [Figure:1b]. • This is done to avoid artifacts due to dental fillings if present, and does not significantly affect the anatomic relationships. 2014

46. Evaluation of the coronal scans too, should begin with the lateral scout view. • Coronal images are arranged to progress from anterior plane to posterior plane within the orbit. • One must remember, that because of the oblique direction of coronal scans, the first few anterior images do not show the orbital floor. 2014

47. If a given image depicts the globe, it is an anterior coronal section [Figure - 5]a. • The image with maximum globe diameter roughly represents the equator of the eyeball. • A posterior coronal section is devoid of the globe image, and demonstrates the optic nerve and extra ocular muscles [Figure - 5]b. • The cross-sectional size of the orbital cavity reduces as we move to the posterior [Figure - 5]. 2014

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49. Systematic Evaluation of Ocular and Orbital Structures on CT 2014

50. CT evaluation is most convenient and informative if performed on the CT machine screen itself. But this is not practical. One almost always has to read the hard copy plates sent by the radiologist. 2014