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Information on MRI Technique

 

Doctors often recommend that children with giant nevi undergo MRI exams to look for nevus spots in the brain. Probably nearly all of us have nevus spots in the brain because that area is where the skin cells originally come from. So whether they are seen or not, they are probably still there.... Many can fade away with age. MRI exams are not cost-effective for most of us. No one who self-pays for medical care in our group has yet decided to drop a grand or two on an MRI (average cost $1,000-$2,000) because it just isn't worth the waste of money if a person doesn't have symptoms. Most nevus adults without symptoms have never had one. Of course, there is always a rare person here or there who had some other correctible problem thankfully found on a routine MRI, but that still doesn't change the fact that they are not cost-effective in general. If the doctor doesn't know what else to do for a nevus kid (and isn't able to handle the stress of standing by idle and doing nothing) and his or her parents don't mind the trauma of their kid going through it or for the rare nevus person who is actually symptomatic (headaches, vomiting, seizures, etc) and needs an MRI, then here is some information on appropriate MRI technique:

The MRI needs to be done properly in order to have the most chance of seeing any melanin in the brain. As nevus people age, the melanin usually fades both on the skin and in the brain. MRI's of older nevus people may be normal for that reason. An MRI is done in a tube which acts like a big magnet. The tube has tanks of liquid helium and nitrogen to keep the magnet from overheating. MRI's make loud, grinding noises, so earplugs are sometimes helpful. The noise comes from all the electronic circuits in the wall of the tube. These circuits change the the magnetic field so that different images can be taken. MRI's work by making use of quantum physics. Cells are made up of atoms. The nucleus or center of an atom contains protons and neutrons. They are considered either paired or unpaired, depending on whether they have an even or odd number of electrons surrounding them. Unpaired electrons are slightly off balance in their spin and momentum. In a strong magnetic field like the MRI scanner, the nuclei of hydrogen, H, atoms--single protons, all spinning randomly, all slightly off balance--line up like compass needles. So the water molecules, H2O, in the human body are temporarily made into small magnets. If the protons are hit with a short, precisely tuned burst of radio waves (radio frequency-RF-pulses), they get excited and will momentarily flip around. Then when they fall back to normal, they send an echo radio signal back. The strength of this signal reflects the amount of hydrogen in human tissue.

The terms T1 and T2 refer to relaxation times, the time it takes for the excited nuclei to fall back to normal after being excited by the different radio waves. Each type of body tissue has its own T1 and T2 time. Data from 3 planes must be obtained to create an MR image. The imaging techniques for the radio frequency pulses are called pulse sequences. They include spin-echo (SE), inversion recovery (IR), saturation recovery (SR), gradient echo (GE) and rapid acquisition relaxation enhanced (RARE) sequences, and FLAIR sequences. By changing the scan repetition time (TR, the time between RF pulses) and the echo time (TE, the time between the RF pulse and the recording of the MR signal), it is possible to change the tissue image. MRI's are generally safe, as far as is known, but one is still zapping one's brain with radio waves and magnetic fields. Whether that has any long-term negative side affects is unknown.

T1-weighted images (short TR and TE) are used to evaluate fat-containing body tissues. Fat will appear bright or white on these films and water will be darker. T2-weighted images (long TR and TE) blot out fat and are used to see tissues with a high water contact, such as tumors and cysts. Water in these images will be white or bright. H2O is bright on T2-weighted images.

How strong are the magnets used for MRI? Magnetic field strength is measured in units of tesla(T) and gauss(G), named after Mssrs. Tesla and Gauss of course. 1 T = 10,000 G. A simple refrigerator magnet is 50 G. The earth's magnetic field is 0.5 G. A midstrength MRI is 0.5 T = 5,000 G = 100 times the strength of a refrigerator magnet. A high-strength MRI can be 200 or more times greater than a refrigerator magnet.

The areas of the brain where our nevus spots usually are found are small and hard to see. They can be easily missed on an MRI. SE T1-weighted images of 4 mm or less and small or no interslice gap should be obtained in 3 planes, the coronal, sagittal, and axial planes. The best sequences are 2D SE images using a TR of 500-600 ms and TE of 10-15 seconds. 3D GE sequences with a partition size of 2-3 mm can also be obtained in the 3 planes. These are better with TR of 35 ms, TE of 7 ms, and flip angle (theta) of 35 degrees.

How can one tell the difference between benign nevus spots and melanoma? The noncancerous nevus spots do not have water around them and they do not become brighter if a dye or contrast agent is given. Cancers with melanin do have water and become brighter. To determine this, T2-weighted (H2O is bright on T2) images are done. To look for water in children less than 1 year old, SE T2-weighted images using TR of 3000 ms and TE of 120 ms are used. In older children, TR of 2500 ms and TE of 70 ms is used. FLAIR images can be substituted for SE T2 in nevus kids older than 2. To see if the spots become brighter or not, a dye such as gadolinium is given thru an IV in the child's vein. Allergic reactions are possible from the dye, so it is avoided when it is not really needed.

Where do we nevus kids usually have spots located on the brain? The most common locations are the anterior temporal lobes near the amygdalae, the brain stem and especially the pons, and the cerebellum. The lesions will look white on T1 images. In infants, the lesions will be darker compared to normal white matter of the brain on T2-weighted images. So now you know more than you ever wanted about MRI!


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This page last updated on January 6, 2005.   Copyright © 2005  Nevus Network  The Nevus Network logo and content of this site is the sole property of the Nevus Network.  Any use of this material without the express written consent of the Nevus Network is strictly prohibited.