The advantages of Magnetic Resonance Tomography (MRT) are even more obvious. It demonstrates very small differences of contrast between different tissues, and this without any x-ray exposure. In contrast to CT, MRT uses information of rotating hydrogen nuclei (protons) which are exitated (and responding) by a high frequency radio wave pulse. This is carried out in in the presence of a very strong magnetic field. The concentration of those nuclei and their position within a molecule influences the signal encoded in the responding electro-magnetic waves which are received from the excitated tissues inside the magnet and computed to images. The researchers who first described the the basis of magnetic resonance were also honoured by the Nobel prize (in 1949).
MR “cuts” of the human body can be acquired in any direction and allow imaging of alteration of tissue structure and function with very low risk for the patient. An exception are patients with cardiac pace makers or metal implants or splinters containing iron. The patient is positioned on a table and, as in CT, pushed into the bore of the strong magnet which unfortunately is smaller than the CT gantry, for which reason claustrophobic reactions can occur. In this case, the patient can indicate his wish to interrupt the examination which usually lasts 15 to 30 minutes. During the application of additional (gradient) magnetic fields, a rather loud knocking noise is heard. Otherwise, the examination does not evoke any sensations in the patient.
In neuroradiology, MRT is usually applied for examinations of the brain, face, eyes, ears and of the spine and spinal cord as well as the supplying arteries and veins. Due to the different types of contrast which can be calculated, it allows more sophisticated differentiation of tissue alterations than CT. Acute infarctions and arterial occlusions can be diagnosed by measurement of tissue diffusion and perfusion and MR angiography within ten minutes in a “one-stop shop” procedure. Tiny tumors, degenerative brain disease (Alzheimer), cerebral inflammations, structural lesions in epileptic fits, demyelinations in multiple sclerosis, and posttraumatic lesions can be diagnosed and differentiated. Vascular malformations as arterio-venous angiomas and fistulas (AVMs, AVFs) and aneurysms can be imaged reliably from a diameter of 3 millimeters. By measurement and postprocessing of 3D data sets in special workstations, minimal-invasive operations can be planned and the intracerebral target can be localised exactly during operation by neuronavigation.
In addition to structural imaging, MR can deliver spectroscopic data of tissue biochemistry with information about tumour proliferation or degeneration under radio- and chemotherapy and about pathological molecular tissue components in metabolic disease. It can also image cerebral activation by memory, emotions, speech, vision, hearing, touch, and movements, which also can be used for operation planning in order to avoid damage to the so-called eloquent regions of the brain. By diffusion-weighted imaging, not only acute infarctions can be seen, but also fiber tracts within the white matter by a special postprocessing called “fiber tracking”, another tool for planning minimal-invasive neurosurgical interventions. Perfusion imaging can help to differentiate tumor recurrences from radiation necrosis and to demonstrate “misery” perfusion in case of narrowing (stenosis) of the carotid or vertebral artery. Finally, flow of cerebro-spinal fluid (CSF) can be imaged to demostrate narrowings or occlusions of the ventricular outlets or disturbed CSF absorption in ventricular dilatation (hydrocephalus).