Diagnosis of acoustic neuroma: MRT

Proof of an acoustic neuroma through an MRT

Whilst hearing and balance tests prove functional disorders and thereby permit conclusions on the (possible) existence of an acoustic neuroma, the Magnetic Resonance Tomography (MRT) shows the anatomic relationships in a selected part of the body.

The Magnetic Resonance Tomography (MRT), previously also called a nuclear spin tomography, has an outstanding importance for tumour diagnoses as an imaging method. The possibility to generate very high contrast and overlap-free images of soft tissue is particularly valuable. Its specialty, in contrast to a CT, lies in the fact that sectional images cannot only be represented horizontally, but rather in every and any level of space. For example, this makes it possible to have a virtual view of the area being examined from an angle which an observer cannot normally view the area in question. Therefore, the MRT not only provides the best data for diagnoses, but also immediate information for later treatment, both for planning radiotherapy and operations.

Another advantage is that no x-ray or other ionising rays are applied. Sophisticated: the hydrogen atoms of a human body are used themselves to produce the image: the core of the hydrogen atoms aligns itself in a magnetic field, like a compass needle. After the introduction of radio waves, they record the energy and are deflected. After switching off the radio waves, they rotate back to their original state and emit the energy in the form of resonance waves. These signals are caught by coil-shaped antenna and converted by high-performance computers into images.

The resulting images have outstanding spatial resolutions, they are high-contrast and display even the smallest changes in soft tissue. The documentation is especially successful with coronary examination films. Coronary section images are tomographies that are produced from the crown towards the feet and parallel to the face. Through prior administration of contrast agents (Gadolinium) this effect can be further emphasised. Nowadays, by using an MRT an acoustic neuroma with an average diameter of only a few millimetres can be detected and displayed. With the so-called CISS sequence, you can even display the individual cranial nerves in the ear canal and cerebellopontine angle, as well as clearly detecting the spread of the acoustic neuroma.

To take the pictures, the patient lies down on a bed, which is slid into a so-called "tube" about one meter in length. The periodic, extremely loud beating of the crashing magnetic fields is not altogether pleasant. However, wearing earplugs makes it more manageable.

Recently , so-called open nuclear spin tomographies (High Field Open) have been used increasingly. With these, the patient lies freely and not in a narrow tunnel. These machines are particularly suited to those patients who suffer from claustrophobia in closed spaces.

No genuine acoustic neuroma diagnosis: CT

A Computer Tomography uses x-rays. While with classic x-rays a virtual screening of the body in one direction is taken leading to a summary picture with overlays, a Computer Tomography produces layer images at right-angles to the direction of the radiography, free from overlays. Therefore, this also creates a three-dimensional impression of the body part being examined. The patient also lies on a bed, while the x-ray machine silently rotates around his/her body axis.

With CT images the body is x-rayed in layers as the x-ray machine rotates round the body axis. This ensures that it is able to take overlay-free, cross-sectional images, in contrast to conventional x-ray images. Iodine can be used as a contrast agent to increase the contrast of the images.

Although a Computer Tomography doesn't show an acoustic neuroma directly, it does, however, show whether the bony ear canal has been widened, which would indicate the existence of an acoustic neuroma. CTs have more importance for operation planning, because they display the bone structures - the posterior fossa and the bony ear canal - very well.

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