MRI of the head

MRI imaging depends on the rearrangement of hydrogen atom nuclei (positively charged protons) in tissues under the influence of a short electromagnetic pulse. After the pulse, the nuclei return to their normal position, emitting part of the absorbed energy, and sensitive receivers capture this electromagnetic echo. Unlike CT, the patient is not exposed to ionizing radiation during MRI. The tissues being examined become a source of electromagnetic radiation, characterized by a certain intensity and time parameters. The signals processed by the computer are displayed as a tomographic projection, which can be: axial, coronal, sagittal.

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Relaxation time

T1- and T2-weighted tomography are two methods of measuring the relaxation time of excited protons after the external magnetic field is switched off. The body's tissues have different relaxation times, and this is the basis for distinguishing T1- or T2-weighted tomograms (i.e. with better visualization in a specific image). In practice, both methods are used.

T1-weighted images better depict normal anatomy.

• Low-intensity (dark) structures including water and vitreous body.
• High-intensity (light) structures including adipose tissue and contrast agents.

T2-weighted tomograms are preferred for displaying pathological changes in tissues.

• Low-intensity structures including adipose tissue and contrast agents.
• High intensity structures including vitreous body and water,

Bone tissue and calcifications are invisible on MRI.

Contrast enhancement

1. Gadolinium is a substance that becomes magnetic in an electromagnetic field. The drug, administered intravenously, remains in the bloodstream unless the blood-brain barrier is breached. Such properties are useful for detecting tumors and inflammatory lesions that appear light on T1-weighted tomograms. It is best to perform MRI of the head before and after gadolinium administration. Specially designed receiving coils can be used to improve spatial resolution of the image. Gadolinium is less dangerous than iodine-containing substances: side effects are rare and usually relatively harmless (e.g. nausea, urticaria, and headache).
2. Fat suppression is used to image the orbit, where bright fat signal on conventional T1-weighted images often obscures other orbital contents. Fat suppression eliminates this bright signal, allowing better visualization of normal structures (optic nerve and extraocular muscles) as well as tumors, inflammatory lesions, and vascular changes. The combination of gadolinium and fat suppression helps highlight areas of abnormal signal enhancement that may otherwise remain undetected. However, fat suppression can introduce artifacts and should be used in conjunction with, and not in place of, conventional imaging.

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Limitations of using MRI of the head

• It does not visualize bone tissue (it looks black on the image), which is not a significant drawback.
• Does not detect fresh hemorrhages and is therefore not suitable for patients with acute intracranial hemorrhage,
• Should not be administered to patients with paramagnetic objects (eg, pacemakers, intraocular foreign bodies).
• The patient must remain still during the MRI.
• Difficult to perform in patients with claustrophobia.

Neuro-ophthalmological indications for MRI of the head

MRI of the head is the imaging modality of choice for lesions of the intracranial pathways. It is important to provide the radiologist with an accurate medical history and to focus on diagnostically significant areas to obtain appropriate images.

1. The optic nerve is best visualized with contrast-enhanced fat suppression on axial and coronal scans that should include both the optic nerve and brain. MRI of the head can detect intraorbital optic nerve lesions (eg, gliomas) and intracranial extension of orbital tumors. In patients with retrobulbar neuritis, MRI can detect plaques in the periventricular white matter and corpus callosum. MRI does not visualize calcium salts and is therefore not useful for detecting fractures or bone loss.
2. Pituitary tumors are best visualized with contrast enhancement. Coronal views optimally show the contents of the sella turcica, while axial views show adjacent structures such as the carotid arteries and cavernous sinuses.
3. Intracranial aneurysms can be visualized with head MRI, although intra-arterial angiography may be required.

Magnetic resonance angiography

Magnetic resonance angiography is a noninvasive imaging technique for the intracranial, extracranial carotid, and vertebrobasilar circulation to detect abnormalities such as stenosis, occlusion, arteriovenous malformations, and aneurysms. However, MRA is not as reliable as intra-arterial angiography in detecting aneurysms less than 5 mm in diameter. Consequently, angiography remains the gold standard for diagnosing and determining the surgical indication for small aneurysms that may be the cause of oculomotor nerve injury or subarachnoid hemorrhage. Although MRA shows an aneurysm, standard angiography is preferred for detecting undetected aneurysms.

CT scan of the head

The tomograph uses narrow beams of X-rays to obtain information about tissue density, from which a computer constructs detailed tomographic projections. These can be coronal or axial, but not sagittal. Vascular lesions are better visualized with iodine-containing contrast agents.

Indications

CT is easier and faster to perform than MRI, but CT exposes the patient to ionizing radiation.

• The main advantage over MRI of the head is the detection of bone lesions such as fractures and erosions and details of the skull structure, so CT is useful for evaluating patients with orbital trauma and helps to detect fractures, foreign bodies and blood, entrapment of extraocular muscles and emphysema.
• CT reveals intraocular calcification (optic disc drusen and retinoblastoma).
• CT is preferred for acute intracerebral or subarachnoid hemorrhage, which may not be detected by MRI in the first hours.

CT is superior to fat-suppressed MRI in detecting extraocular muscle enlargement in endocrine ophthalmopathy.

CT of the head is used in cases where MRI of the head is contraindicated (for example, in patients with metallic foreign bodies).

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