Ординатура / Офтальмология / Английские материалы / Ophthalmology Investigation and Examination Techniques_James, Benjamin_2006

Calcium in retinoblastoma

a

Tumour (retinoblastoma)

b

Orbital Computed Tomography

Fig. 14.5 (a) Axial computed tomography (CT) scan showing coarse calcification in an enlarged left globe due to retinoblastoma. This is well shown (as opposed to magnetic resonance, in Fig. 14.5b) but

CT does not allow an assessment of extension into the optic nerve or through the choroid. Wholebrain imaging is needed to look for metastases and careful evaluation of the opposite eye to looking for bilateral disease is essential. (b) Axial T1w magnetic resonance scan of the same child. Calcium is poorly seen but the soft-tissue tumour and its extent are more clearly delineated. Fat suppression technique failed in this small child (see later).

The high intrinsic contrast between bone, muscles, orbital fat and air produces excellent visualisation of orbital structures. CT is excellent for the detection of calcification, which aids in diagnoses such as retinoblastoma (Fig. 14.5) and optic nerve sheath

meningioma (Fig. 14.6). CT optimally demonstrates bony structures, including erosion (Figs 14.7 and 14.8), scalloping and bone defects, as in the case of a dermoid cyst (Box 14.5). It is the technique of choice for imaging in preseptal/septal cellulitis.

Box 14.4

Imaging features of an orbital lesion

■Contour and surface: smooth, round, irregular, poorly defined

■Surrounding structures: bony changes, mass effect on globe

■Internal character of lesion: calcium, cystic, vascularity

■Extent of abnormality

Box 14.5

Advantages of computed tomography

■High contrast between bone, muscles and orbital fat produces excellent visualisation of orbital structures

■Quick, readily available

■High spatial resolution: excellent anatomy

■Excellent for bone and calcification

■Less patient cooperation needed

■Multiplanar capability in modern scanners

But:

■ Uses ionising radiation

Fig. 14.7 Axial computed tomography image showing an aggressive soft-tissue mass in the left orbit in a young patient with rapidly progressive proptosis. The mass extends through the lateral orbital wall, which is eroded and destroyed, into the infratemporal fossa. Diagnosis: sarcoma.

b

See text

Contrast is essential. Evaluation of the paranasal sinuses to identify the possible source of infection and provide anatomical detail for the ear, nose and throat surgeons if surgery is necessary is possible within the same investigation (Fig. 14.9). It is the investigation of choice for the evaluation of orbital bony trauma (Fig. 14.10) and in the assessment of foreign bodies within the orbit.

Advantages and disadvantages

Irradiation of orbital structures is a potential disadvantage, although the radiation dose to the lens is low. Other limitations are artefact from dental amalgam and relatively low sensitivity for detecting intracranial extension of disease. Detail of the globe and optic nerve may be less good than that provided by MR scanning (Fig. 14.11).

See text

Fig. 14.8 (a) A young child with proptosis. Axial computed tomography image shows a soft-tissue mass in the upper outer quadrant of the left orbit, with thinning and erosion of bone. An aggressive lesion is likely. (b) Axial image through brain/skull vault of the same child shows further soft-tissue lesions in the occipital region and right temporal fossa. Multiple lesions are likely in metastases and in histiocytosis, which was the diagnosis here. It is always important to look for other lesions.

Inferior rectus muscle

Fracture

Fig. 14.10 Coronal computed tomography reformat showing a fracture of the orbital floor with downward displacement of the inferior rectus; fluid in the maxillary antrum.

Optic nerve glioma

Fig. 14.11 Computed tomography showing optic nerve glioma extending through a widened optic foramen into the intracranial compartment.

ORBITAL MAGNETIC RESONANCE IMAGING

Introduction to magnetic resonance imaging

In MR scanning the patient is placed within a magnetic field and a radiofrequency coil is used to transmit a radio signal to the body part being imaged. This radiofrequency pulse causes a change in the steady-state proton magnetisation of the tissue and results in a transient small radio signal which is detected by the receiver coil in the magnet. That radio signal undergoes spatial encoding and is subsequently converted into an image by a computer using complex mathematical formulae.

The signal on MR depends on the proton density. Proton density is the concentration of protons in the tissue in the form of water and macromolecules such as fat and protein. The T1 (longitudinal) and T2 (transverse) relaxation times define the way that the excited protons revert to their original state following the radiofrequency pulse.

The most common imaging sequences are T1and T2-weighted (w) sequences. Signal intensities relate to specific tissue characteristics.

In general, T2w sequences show fluid as a high or hyperintense signal and this will appear bright or ‘white’ on the images. T2w sequences are sensitive to changes in water content, and thus pathology, but are not very specific. T1w sequences show fluid as a low or hypointense signal, thus it appears dark or black on the images. T1w sequences are excellent at demonstrating anatomical detail and can also be used after administration of intravenous contrast to show enhancement of structures and pathology.

By utilising different radiofrequency pulses it is possible to produce images that are T1or T2weighted. Modifications to these pulses can be used to alter the signal return of certain tissues, e.g. fat to change the final image. Fat suppression techniques where the normal bright signal returned from fat is suppressed are useful in orbital imaging to allow improved visualisation of the other orbital structures and disease processes (Fig. 14.12). Fat suppression removes the high signal fat component from the image. This can be achieved in several ways, including short T1 inversion recovery (STIR) and spectral presaturation with inversion recovery (SPIR).

On T2w scans the fluid within the globe (vitreous) is typically high signal or ‘white’, the lens appears dark, the fat is bright but less so than fluid and the muscles are dark. On T1w scans the vitreous appears dark, the orbital fat is very bright and the muscles are dark (see Fig. 14.21).

Technique

Orbital MR can be obtained on any diagnostic fieldstrength magnet (0.2–3.0 T) but higher field magnets are preferred as the image acquisition time is shorter and resolution better. The standard head coil (or newer multichannel coils) is used routinely, with surface coils added if needed.

Images should be obtained in axial, coronal and sagittal planes using thin sections (2–4 mm) acquired on a small field of view with a high-resolution matrix.

Fig. 14.12 (a) Demonstration of a fat suppression technique on axial and sagittal magnetic resonance

images resulting in better anatomical

Fat

detail in the lower pictures. (b) The fat suppression technique, now

showing the lesion of the right optic Lesion nerve with greater clarity in the

lower pictures.

Suppressed

fat

b

Axial images should be aligned along the course of the optic nerve, i.e. –10º to the orbitomeatal base line.

Any number of pulse sequences can be used and examinations are often directed by the radiologist’s preference and experience. T1w and T2w techniques with and without fat suppression are generally recommended.

The use of fat suppression or STIR techniques may reveal pathology that might be missed if the bright orbital fat signal is not suppressed. Most pathological lesions have a long T1 and T2w relaxation time and thus will appear ‘bright or of increased signal intensity’ against the dark suppressed orbital fat (Figs 14.12 and 14.13).

Contrast agents are generally used to fully evaluate orbital masses and optic nerve/nerve sheath lesions (Fig. 14.14).

Surface coils can be used to provide higher resolution and detail, particularly of the globe. Images obtained with a surface coil provide better spatial resolution but, because of signal drop-out, apical lesions and intracranial extension may not be as well shown. If surface coils are used the ideal diameter is between 6 and 12 cm to allow both eyes to be imaged; the head should be tilted at 45º to the unaffected eye.

a

Mass

Fig. 14.13 (a) Computed tomography in a child with proptosis shows an ill-defined homogeneous mass in the upper orbit. The nature of the lesion is unclear. (b) Magnetic resonance using T1w fatsuppressed technique shows the mass to be hyperintense or bright – a haemorrhage. There was a history of trauma and the haematoma subsequently resolved.

Orbital Magnetic Resonance Imaging

Advantages and disadvantages

The advent of stronger gradients (these determine the magnetic field), faster pulse sequences and surface coils have overcome the earlier limitations of MR in orbital imaging – namely the time taken to obtain the information. MR provides optimal soft-tissue contrast and allows excellent visualisation of the globe, optic nerve and any intracranial extension of disease (Box 14.6). Further advantages include the lack of radiation. There are currently no known biological side-effects to MR. The ease of multiplanar imaging (that is, the ability to view or obtain images in any plane without moving the patient, as in multiplanar CT) and the ability to detect abnormal flow in vessels are additional advantages. The use of specific pulse sequences such as fat suppression and inversion recovery techniques enable abnormalities to be demonstrated with great clarity. The use of a paramagnetic contrast agent may be helpful, particularly in assessing the extension of disease into adjacent structures.

The limitations of MR relate, as with CT, to patient movement and cooperation – most sequences still take several minutes to acquire. This may require the use of anaesthesia in children. Motion artefact significantly degrades image quality. Patients should close their eyes during the examination and keep

Bright mass due to blood products

b

Fig. 14.14 Contrast-enhanced axial T1w magnetic resonance scan in a child with sarcoma, showing extension into the intracranial cavity.

Box 14.6

Advantages of magnetic resonance

■No radiation

■Multiplanar imaging

■Better ocular and soft-tissue detail; ideal for evaluating optic nerve and extraocular disease, e.g. melanoma extension

■Can see abnormal flow in blood vessels

■Visualisation of anterior optic pathway

But:

■Longer exam times; movement and patient cooperation

■Must be aware of safety issues and contraindications

the eyes still. Eye make-up, including conventional mascara and tattooed eyeliner, can lead to artefacts with distortion of contours. MR is absolutely contraindicated in patients with possible intraocular foreign body as ferromagnetic foreign bodies are induced to move during the scan and could cause severe injury and death (Fig. 14.15). Additional artefacts may be caused by problems with the scanning process itself, such as inhomogeneous fat suppression. It is important to be aware of the possible appearances caused by such artefacts.

Fig. 14.15 Axial T2w magnetic resonance scan showing susceptibility artefact resulting from a metallic foreign body in the soft tissues over the face. This was not disclosed by the patient.

In the axial plane there is often volume averaging of the optic nerve sheath complex and thus correlation with coronal images is essential if lesions are not to be missed.

Contrast studies of the orbit

1.Carotid angiography

2.Orbital phlebography

These have largely been replaced by modern crosssectional imaging, although carotid angiography is still used in cases of caroticocavernous dural fistula. Intracranial aneurysms may present with orbital pain or proptosis and thus the diagnosis of a caroticoophthalmic artery aneurysm or cavernous sinus aneurysm may be made by angiography initially, although diagnostic angiography is now often replaced by CT angiography (CTA).

For full evaluation of a dural fistula, selective injection of both internal and external carotid

arteries and the vertebral arteries may be required (Fig. 14.16).

Interpretation of orbital imaging

Imaging must be supplementary to clinical history and examination. It should not be interpreted in isolation. The best-quality study possible should be obtained. This will be determined by the availability

of scanners and the condition of the patient. It is better to obtain a good-quality CT study than a suboptimal MR examination. The techniques are complementary and both may be necessary. MRI potentially has advantages in allowing some physiological assessment to be made, e.g. in thyroid eye disease where ‘activity’ may be evaluated depending on the signal return from the muscles on T2w and STIR sequences (Figs 14.17 and 14.18).

Fig. 14.17 (a) Axial computed tomography showing bilateral proptosis and enlarged extraocular muscles in thyroid eye disease. Note good bony detail at optic foramen. (b) From the same patient, showing the inferior recti. (c) Coronal reformat showing bilateral muscle enlargement.

Fig. 14.18 (a) Axial T2w; (b) coronal T2w; (c) T1w fat-suppressed magnetic resonance scans in a patient with acute thyroid eye disease. There is bilateral proptosis, streaky orbital fat and enlarged muscles. The medial recti contain hyperintense (bright) signal suggestive of acute/active disease.