För veterinärer
 

Magnetröntgen

MRI

Magnetisk resonans tomografi har revolutionerat diagnostiken av patienter med neurologiska besvär inom både human och veterinärmedicinen. Tekniken har blivit mer tillgänglig för hundar och katter under det senaste årtiondet. Till skillnad från vanlig röntgenundersökning eller datatomografi, används ingen strålning vid magnetisk resonans tomografi och utgör därför ingen hälsorisk för patienten eller personalen.  

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Den magnetiska resonans tomografi utrustning som används på Aisti.

 

Magnetisk resonans tomografi (MRT) ger klinikern bilder av god kvalitet och till skillnad mot röntgenundersökningar kan strukturer i hjärnan och ryggmärgen undersökas utan att bilden påverkas av att vävnader läggs på varandra. MRI är en ytterst värdefull teknik för att kunna ställa diagnoser vid sjukdomar i hjärnan och ryggmärgen.  

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Huvudet på en schäfer undersöks.

MRI visualiserar ett mångfald av sjukdomar, bl.a. hjärntumörer, utvecklingsdtörrningar, trauma, vaskulära sjukdomar, abscesser och granulom I hjärnan. MRI är av yttersta vikt när det gäller att ställa diagnoser i ryggmärgen och nervrötter som inte har kunnat diagnostiseras med hjälp av mer traditionella undersökningstekniker, t.ex. kontraströntgen.

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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.

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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.

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  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.

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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:

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   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.

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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:

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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.

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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.

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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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Ta kontakt
Virtatie 9
Myyrmäki
01600 Vantaa
Tel: 010-4232142
Fax: 09-4540409
E-mail: info@aisti.info,
förnamn.efternamn@aisti.info
Internet: www.aisti.info