MRI

Magnetic resonance imaging has made the revolution in the diagnostic imaging of the neurological patients in both human and veterinary medicine. It has become more available for dogs and cats in the last decade only. In contrary to normal x-ray or computer tomography, the magnetic resonance imaging is not using any radiation which would be a health hazard for the patient or the personnel.

The magnetic resonance imaging equipment of Aisti.

Magnetic resonance imaging (MRI) allow the clinician to have good quality images and in contrary to x-ray to examine intracranial and spinal structures without superimposition of other tissues. MRI may be considered of crucial importance in the diagnosis of intracranial and spinal diseases.

The head of the German Shepherd is examined.

MRI makes visible such diseases as brain tumors, developmental abnormalities, trauma, vascular diseases, abscesses and granulomas of the brain. MRI is also of paramount importance in clarifying conditions of the spine, spinal cord and nerve roots that have not been diagnostically solved with more traditional techniques, including contrast radiology.

The physical method of magnetic resonance imaging is rather complicated. Shortly, the magnetic qualities of the particles are used for generation of the image. Every particle in the magnetic field, but mainly protons is oriented along the forces of the magnetic field. If we place a patient in to the strong magnetic field all its protons in the body will get oriented in the same direction. If the radio signal is send to these protons, they absorb energy and their orientation in the magnetic field is changed. If the radio signal is switched of, the protons are giving out the absorbed energy in the form of radio signal and they return back to the initial orientation. The emitted radio signal is measured and has intensity according to the proton density in the tissue. Different tissues have various concentrations of the protons and therefore can be imaged in different shades of grey. This is an extreme simplification of the principle of the magnetic resonance imaging, but is enough in order to understand the method.

The presence of different concentrations of free protons in different tissues allows perfect visualization of the fluid (liquor), tissue rich in fat (white matter), cell rich tissue (grey matter), tissue with low content of water (bone) and therefore leads to image with enough contrast to visualize all the mentioned structures. In this way the anatomy, the symmetry and the size of the nervous system can be studied.

  The MRI examination is performed so that the images in all 3 planes are generated. Firstly so called localiser or scout image in all 3 planes is performed. This initial image is helping to plan the further sequences.

The lines in the image above are showing from where the images in the next sequences will be taken. Two major types of images are recognized: T1 weighted and the T2 weighted images. The biggest difference between the T1 and T2 images is the appearance of liquor: in T1 liquor is black and in T2 liquor is white.  This can be seen well in the images below:

   The difference in the color of cerebrospinal fluid in the T1 and the T2 images can be even better visualised in patient with hydrocephalus. Remember that the fluid is black in T1 and white in T2. Note how the ventricle system containing liquor (brain fluid) is expanded in comparison to the  normal images shown before.

T1 images are the preferred images to evaluate the anatomy of the central nervous system. The T2 images are sensitive in visualization of the presence of the free protons in the tissue (fluid) what is frequent if bleeding/infarction, brain edema, inflammation and neoplasia respectively:

The big advantage of MRI is in possibility to perform the images in all three planes in various sequences sagital, transversal and dorsal. This enables the veterinary neuroradiologist to get the three dimensional appearance of the brain and lesion. The images below  are sagital T1 (left) and sagital T2 (right) images. Sagital images gives the best view about the anatomy of the brain in the midline. The size of cerebellum and forebrain as well as anatomy of the brainstem can be judged well in sagital images.

The image below is dorsal. Dorsal images are taken like from above. Both sides of the brain can be compared well in dorsal images and this helps to detect asymmetries and other changes. Also position of the eyes and the space between the eye and the brain can be well evaluated.

The method of contrast examination in MRI is different to the conventional used in x-rays or CT. The mostly used contrast substance, Gadolinium, is a paramagnetic substance which is able to pass through the damaged blood brain barrier. It stays in the area with the CNS disease and enhances the magnetic resonance characteristics of the tissue near to it so that the tissue is seen with the much stronger signal in the MRI imaging. This makes the areas with damaged blood brain barrier look white and enables visualization of the neoplasias, infections, inflammations, traumas and vascular events.

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