MRI of the head

Visualization with MRI depends on the rearrangement of the nuclei of hydrogen atoms (protons charged positively) in tissues under the action of a short electromagnetic pulse. After the pulse, the nuclei return to their normal position, radiating some of the absorbed energy, and sensitive receivers catch this electromagnetic echo. Unlike CT, the patient is not exposed to ionizing radiation during MRI. The tissues under investigation become a source of electromagnetic radiation characterized by a certain intensity and time parameters. Signals processed by the computer are displayed in the form of a tomographic projection, which can be: axial, coronal, sagittal.

[1], [2], [3]

Relaxation time

T1 and T2-weighted tomographs are two methods for measuring the relaxation time of excited protons after the external magnetic field is turned off. The tissue of the body has a different relaxation time, and on this basis T1 or T2-weighted tomograms are distinguished (ie, with better visualization on a specific image). In practice, both methods are used.

T1-weighted tomograms better display normal anatomy.

• Low-intensity (dark) structures, including water and vitreous.
• Strongly intensive (light) structures, including adipose tissue and contrasting substances.

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

• Low-intensity structures, including adipose tissue and contrast agents.
• Strongly intensive structures, including the vitreous and water,

Bone tissue and calcifications on MRI are invisible.

Contrast enhancement

1. Gadolinium is a substance that acquires magnetic properties in an electromagnetic field. The drug, administered intravenously, remains in the vascular bed, if the blood-brain barrier is not violated. Such properties are useful for detecting tumors and inflammatory foci that appear light on T1-weighted tomograms. It is best to perform the MRI of the head before and after the introduction of gadolinium. To improve the spatial resolution of the image, you can use the receiving coils of a special design. Gadolinium is less dangerous than iodine-containing substances: side effects are rare and usually relatively harmless (eg, nausea, urticaria and headache).
2. Suppression of the signal from the adipose tissue is used to visualize the orbit, where the bright signal of adipose tissue on conventional T1-weighted tomograms often hides other contents of the orbit. The suppression of the signal from the adipose tissue removes this bright signal, which promotes a better display of both normal structures (optic nerve and extraocular muscles), and tumors, inflammatory lesions and vascular changes. The combination of gadolinium administration and suppression of the signal from the adipose tissue helps to identify regions of anomalous signal amplification that may remain undetected. But suppression of the signal from fatty tissue can lead to the appearance of artifacts and should be used in combination, and not instead of the usual visualization.

[4], [5], [6], [7], [8]

Limitations of the use of MRI of the head

• Do not visualize the bone tissue (in the picture it looks black), which is not a significant drawback.
• Does not reveal fresh hemorrhages and, therefore, is not suitable for patients with acute intracranial hemorrhage,
• Do not administer to patients with paramagnetic objects (eg, pacemakers, intraocular foreign bodies).
• The patient should be immobile during the MRI.
• It is difficult to perform in patients with claustrophobia.

Neuroophthalmic indications for MRI of the head

MRI of the head is the method of choice for lesions of intracranial pathways. In order to obtain suitable images, it is important to provide the radiologist with an accurate medical history and focus on diagnostically relevant areas.

1. The optic nerve is best visualized with contrast enhancement with suppression of the signal from fat tissue on axial and coronal tomograms, which should include both the optic nerve and the brain. MRI of the head can detect lesions of the intraorbital part of the optic nerve (for example, gliomas) and intracranial spread of orbital tumors. In patients with retrobulbar neuritis, MRI can detect plaques in periventricular white matter and a callosized body. MRI does not visualize calcium salts, so the method is useless for detecting bone fractures and fractures.
2. Tumors of the pituitary gland are best visualized with contrast enhancement. Coronal projections optimally show the contents of the Turkish saddle, whereas axial projections show contiguous structures such as carotid arteries and cavernous sinuses.
3. Intracranial aneurysms can be visualized using the MRI of the head, although intra-arterial angiography may be required.

Magnetic resonance angiography

Magnetic resonance angiography is a non-invasive method for visualizing intracranial, extracranial carotid and vertebrobasilar circulation to detect abnormalities such as stenosis, occlusion, arteriovenous developmental defects and aneurysms. However, if an aneurysm is detected less than 5 mm in diameter, the MRA is not as reliable as intra-arterial angiography. " Consequently, angiography remains the "gold standard" of diagnosis and determination of indications for surgical intervention in small aneurysms, which may be the cause of oculomotor nerve injury or subarachnoid hemorrhage. Although MRA shows an aneurysm, standard angiography is preferred for detecting undetected aneurysms.

Computer tomography of the head

The tomograph uses narrow beams of X-rays to obtain information about the tissue density, over which the computer builds detailed tomographic projections. They 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 the patient under CT is exposed to ionizing radiation.

• The main advantage of MRI of the head is the detection of bone lesions, such as fractures and erosion, and the details of the structure of the skull, therefore CT is useful for examining patients with trauma to the orbit and helps detect fractures, foreign bodies and blood, infringement of extraocular muscles and emphysema.
• CT scan reveals intraocular calcification (druses of the optic nerve disk and retinoblastoma).
• CT is preferable for acute intracerebral or subarachnoid hemorrhage, which can not be detected on an MRI in the first hours.

CT scan is superior to MRI with suppression of the signal from adipose tissue in revealing the increase in extraocular muscles with endocrine ophthalmopathy.

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

[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]