MRI

LECTURER DR. DALAL AHMED

RADIOLOGY DEPARTMENT / SURGERY

Objective of the lecture�the student will be able to

• Interpret principle of MRI
• Enumerate types of MRI
• Enumerate indication and contraindication
• Define mri sequences
• Define different pathologies diagnosed by mri

• MRI (an abbreviation of magnetic resonance imaging) is an imaging modality that uses non-ionizing radiation to create useful diagnostic images.
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Types of MRI

Open mri

Any other advantage ??

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• MRI is the preferred investigation in intracranial sepsis, tumours, inflammatory diseases, epilepsy and congenital malformations

The disadvantages of MRI compared with CT include the

• limited visualization of calcification and lack of bone detail.
• scan time is much longer, during which the patient has to lie still, and monitoring seriously ill patients within the scanner can be difficult. If intubated, the life support and monitoring equipment must be MRI compatible.
• Any intracranial ferrous metal present such as aneurysm clips or cochlear implants are absolute contraindications

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• Any metal object is contra indication to mri
• Like ……….

Principle of MRI

Mri physics

• MRI is based on the magnetization properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined. This alignment (or magnetization) is next perturbed or disrupted by introduction of an external Radio Frequency (RF) energy. The nuclei return to their resting alignment through various relaxation processes and in so doing emit RF energy. 

Sequences of MRI

• The routine sequences used for MRI vary but usually include T1- and T2-weighted sequences in orthogonal planes .
• A FLAIR (fluid attenuated inversion recovery) sequence is useful as the pulse sequence nulls the bright signal from CSF, making brain pathology (usually T2 bright) more visible.
• Magnetic resonance can also recognize flowing blood within larger arteries and veins, which can be examined without the need for contrast medium to create intracranial angiograms .

• The diffusion-weighted imaging (DWI) sequence looks at the normally random movement of water molecules in brain tissue and is extremely useful, particularly when the patient is suspected of having an acute stroke.
• Within areas of cytotoxic oedema such as an infarct, the movement of water molecules in the extracellular space is limited by the swollen, dying cells and this produces bright signal, whereas the movement of water within vasogenic oedema is not limited (free) and does not produce any signal
• change. The DWI scan becomes positive within minutes of an acute stroke whereas CT abnormalities can take hours and will miss very small infarcts. Restricted diffusion within a cystic mass is also relatively specific for pus within a pyogenic abscess

Normal brain MRI. The images are axial sections at the level of the lateral ventricles. (RT) T2-weighted image. (LT) FLAIR image.

�How to know it is TI weighted image or FLAIR?�

• One of the method is By looking to grey and white matter as white matter is higher than grey matter on T1 and lower than grey matter on T2 ,so if we saw white matter lower than grey matter it mean it is T2 and fluid is suppressed resulting in FLAIR

DWI in stroke. (RT) An unenhanced CT image showing low attenuation in the right frontal lobe and basal ganglia (arrow). (LT) Diffusion-weighted MRI showing bright signal indicating the extent of restricted diffusion due to an acute infarct; it is more extensive anteriorly (arrow)

Brain Abscess

• An abscess can be caused by bacterial, tuberculous, fungal or parasitic organisms.
• Necrosis and pus formation occur in the centre of the abscess, which appears as low density on CT or fluid on MRI.
• The wall of the abscess enhances with intravenous contrast and may be surrounded by oedema, giving an appearance known as ‘ring enhancement’ .
• The pus within the centre of a pyogenic abscess will typically demonstrate restricted diffusion.

Cerebral abscess. (RT) Post contrast CT scan showing a right frontal ring-enhancing lesion with surrounding vasogenic oedema (arrow). (LT) DWI showing the contents of an abscess (arrow) with restricted diffusion in keeping with pus.

Glioma

• Glioma is a non-specific term used to describe a group of tumours that arise from glial cells,
• which normally support the brain neurons, such as astrocytes (astrocytoma is the commonest tumour type). They range from low grade to high grade

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• . For accurate assessment of gliomas, both pre and post contrast scans should be performed as enhancement can be associated with a higher grade of malignanacy.

• The MRI features are similar to those of a mass, often with adjacent signal change.
• ,in general, the tumour is lower in signal intensity than the normal brain on the T1-weighted images and higher in signal intensity on the T2-weighted images .

Brain metastasis

• On MRI, they usually show contrast enhancement and are often surrounded by substantial oedema (Fig. 15.15). Metastases are typically multiple but a solitary metastasis can be indistinguishable from a primary intracerebral brain tumour with either ct or mri technique.