MRI Scan
MR imaging uses a powerful magnetic field up to 8,000 times stronger than that of the earth. This strong magnetic field is combined with radio frequency pulses and specialised computer software to produce detailed pictures of the brain. 
MRI does not use ionizing radiation (xrays). Before an MRI scan you will have a dye (contrast material) injected into a vein, usually in your arm. The most commonly used contrast dye is Gadolinium. This contrast material concentrates more in tumour tissue than healthy brain cells and hence highlights where the tumour is most active.
The MRI machine is a tunnel-shaped piece of equipment; the patient lies on a table that slides inside the tunnel of the MRI scanner. Inside, a magnetic field surrounds the patient's head whilst radio energy is beamed to the area. The computer workstation that processes the imaging information is located in a separate room from the scanner, but you will be able to communicate with the staff through headphones and a microphone throughout the process. In most MRI units, the magnetic field is produced by passing an electric current through wire coils. Other coils located in the machine send and receive radio waves, producing signals that are detected by the coils.
Your body is composed of atoms, with 95% of your body being made up of water or hydrogen atoms. Usually the hydrogen atoms within the body spin at random. The strong magnetic field of an MRI scanner causes the axes of spinning protons, which are the nuclei of hydrogen atoms, to realign and spin all in the same direction. When the beam stops, the atoms relax and return to their original position. During relaxation, the atoms give off signals in differing amounts and at different time intervals. These signals are recorded and fed into a computer which assembles a picture. Because different atoms have their own characteristic radio signals, the computer can distinguish between healthy and tumour tissue.
Because the MRI does not register bone which can obstruct CT images, this technique provides clearer pictures of brain tumours and is more commonly used for studying their progress than CT scans. MRI scanning can also produce a wider variety of image angles. The MRI can detect Edema (excess fluid on the brain) but has difficulty distinguishing edema from tumour; the same applies to calcification which is present in some brain tumours.
MRI imaging takes longer than a CT scan, and is very noisy; you will usually be offered headphones which can either play music or simply muffle the noise levels. MRI exams usually include multiple runs (sequences), some of which may last several minutes. The entire scanning procedure usually lasts approximately 45-90 minutes, depending upon the level of information required.
Patients with cardiac monitors, pacemakers, or surgical clips cannot be given an MRI because of the magnetic fields. If you have claustrophobia (fear of enclosed spaces) or anxiety, you may want to ask your medical team for a prescription for a mild sedative; hypnosis, EFT, CBT or other self-help techniques can also be helpful if used before the scan to reduce anxiety levels (see our complementary therapies section for more information).
MR spectroscopy
MR spectroscopy provides information about non-water molecules present in brain and tumour cells, and may also be performed during the MRI scan. The molecules being assessed include choline, creatine, glucose, glutamine, beta-glutamine, n-acetylaspartate, inositol and lactate. This extra sequence of radio signals adds approximately 15 minutes to the overall time.
Diffusion MRI
Diffusion MRI is a technique for the evaluation of the mobility of water molecules and can give important information about the structure and grade of brain tumours, as well as the status of surrounding brain tissue.
Perfusion MRI
Perfusion MRI is a technique to evaluate the rate of blood flow into brain tissue, both healthy and tumourous. If a low-grade tumour starts to become more active and potentially change to a higher grade (“transform”), it pulls new capillary blood vessels towards it in order to create its own blood supply and hence use the blood to bring the nutrients it needs to grow more quickly; this process is known as angiogenesis. These new blood vessels are “leaky” when they are in the early stages of development, and hence areas of enhancement where the contrast material has leaked into brain tissue surrounding the tumour may indicate that the tumour is becoming more active. This type of information is crucial in the management of low-grade gliomas as signs of transformation will mean that treatment needs to be considered in order to halt the changes and return the tumour to a “low-grade” stage.
FUNCTIONAL MRI SCANS
Functional magnetic resonance imaging (fMRI) uses MR imaging to measure the tiny metabolic changes that take place in areas of the brain when they are used for “functions” such as movement, speech, sensation and thought processes. In an fMRI examination, you will perform a particular task or series of tasks during the imaging process (for example, finger tapping), which will cause increased metabolic activity in the area of the brain responsible for the task. This metabolic activity includes chemical changes, expanding blood vessels, increased blood flow and the delivery of extra oxygen. This functional data is then superimposed onto standard MRI images, enabling the brain to be “mapped”. A common use of Functional MRI scans is to enable surgery or radiation therapy to be carefully planned, to ensure that vital areas of the brain are avoided.
Interpretation of Scans
A Neuroradiologist interprets the computer images produced by CT, MRI and PET scans. The pictures help establish a tentative diagnosis of tumour type and grade, indicate changes in your tumour through comparison with previous scans, or give more detailed information if a more accurate form of scanning is undertaken than on previous occasions. However, only examination of an actual sample of tumour tissue gained through a biopsy or surgical resection can provide an exact diagnosis of tumour type and grade.
Sources:
ABTA’s Primer for Brain Tumours downloaded 17.5.09 from http://neurosurgery.mgh.harvard.edu/abta/primer.htm
Colorado Springs Imaging, downloaded 18.5.09 from http://www.coloradospringsimaging.com/index.php
PatientInfo UK, downloaded 18.5.09 from http://www.patient.co.uk/showdoc/27000361/
Radiology Consultants of North Dallas, downloaded 18.5.09 from http://www.rcnd.com/AboutUs.html
Radiology Info, downloaded 17.5.09 from http://www.radiologyinfo.org/en/info.cfm?PG=headct
Image source: Wikimedia Commons
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