The neuroimaging of interest here is brain and spinal cord. The CT Scan intro’d in mid 1970s preceded introduction of the MRI by several years. Essentially, both give the same picture, which is millimeters thick slices of body tissue. With CTs, the slices are mostly transverse (horizontal, side to side) because technical issues limit longitudinal slices. But MRI allows equally easy longitudinal (sagittal) slices and this is an advantage of MRI, especially for the spinal cord, which may then be viewed along its length to show continuous, continuous lesions like tumors and multiple sclerosis.The most important advantage of MRI is no damaging radiation. MRI has its limitations: the high magnetic pull makes it impossible with patients who have pacemakers or paramagnetic prostheses; and the relative isolation of the patient (Now being improved by open MRI) makes it hard on children, especially infants, and for older persons with agoraphobia or for persons with mental illnesses; and the greater affect of movement on the MRI image sometimes makes CT superior for moving part images like the heart valves.
Today in 2021 all modern hospitals are equipped with CT and MRI imagers, which to a large extent may duplicate each one's function. In general, an MRI scan does everything a CT scan does and does it a little better with no damaging radiation. But the decision to use CT or MRI on a patient for neuroimaging often rests on economic reasons - the necessity to justify having an expensive CT machine. However, there are specific indications for CT over MRI in emergency neuroimaging, mostly in acute brain hemorrhage where the speed and ease of doing an initial CT makes it first choice. But this is a changing picture as technical improvements may make MRI best in all cases.
For the reader who may be a patient subjected to these decisions, the general approach if a CT is ordered is to ask your physician "Why can't I have an MRI instead of CT?"But I do not advise arguing with your doctor. Simply asking the question will at least cause your doctor to carefully reconsider reasons why your CT is being ordered.
Now we shall focus on findings that come up when normal patients without symptoms get a screening MRI. Note that both MRI and CT scans may be done with and without chemical dye. The dye emphasizes internal anatomic structures so it makes the scans more accurate. But it also carries risk of kidney damage that, rarely, has resulted in the need for kidney dialysis for life. So a suggestion to use dye with neuroimaging should be met with the question: "Why is dye necessary?".
With MRI there are also a number of imaging techniques that may use not involve dye or risks but which are useful for bringing out aspects of the imaging and these are not necessary to describe.
The simplest indication to get an MRI neuroimaging is where you, a normal person with no symptoms, have a reason to worry about your brain or spinal cord function or the possible presence of an early curable tumor or a serious nervous disease like multiple sclerosis. A very frequent worry is the older person's about getting Alzheimer's Disease.
One important consideration of the person before getting an MRI is to think over whether one is prepared to act on a positive result and also whether the positive result might cause only worry but not result in any gain for your health and long life. For example, if you are 80 years old and worry that you might get Alzheimer's Disease, an MRI of the brain that shows early changes that might be from Alzheimer's will be of no value to you because preventive measures should be being carried out by all 80-year-olds, and the finding of some suggestive abnormality may destroy life's happiness for no good treatable reason. On the other hand at age 45 you want to know if you show early changes because you will have time to do preventive measures.
All in all, it is a sensible part of a healthy longevity program to submit one's self to an MRI brain- and spinal cord scan by age 60s. The following data from a Rotterdam study will give an idea of what simple MRI brain scan without dye may discover and how it could prolong your healthy life or merely cause anxiety. Here follows the findings in 2000 consecutive simple MRI brain scans of normal persons between ages 45 and 100 with no important symptoms of disease. Again keep in mind these are persons who normally would not have requested an MRI brain scan; they got it only because they volunteered as part of a study of normals. The criteria for entry into the study was that the volunteer has no significant symptoms or worries about brain disease. The MRI scan was a standardized high-resolution structural brain MRI (1.5 Tesla; important that it is the "enclosed" MRI not the "open" MRI which has a lower Tesla and lower accuracy) without dye. All the readings were done by 2 experts and samples checked by 2 other experts. This therefore is an excellent estimate of what one should expect when thousands of patients without symptoms undergo screening MRI of the brain.
Results were that 7.2% (1 in 13 scans) showed old brain infarcts (damage to brain from obstructed artery branch; the usual cause of brain stroke but sometimes without symptom), 1.8% (1 in 60) showed small brain aneurysms (weakening of a brain artery branch that may be the cause of strokes from hemorrhage), and 1.6% (1 in 67) showed small benign brain tumors (meningioma). Additionally, vascular changes in the brain white matter were found increasingly with older age. This last may be an early warning of Dementia and the larger the lesion the more serious the warning. Interestingly, in the youngest group, ages 45 to 59, about 35% of the scans showed potentially pathological white matter brain changes but, among the total of such white matter changes, this youngest age group showed the smallest volume of change (between 1 to 5 ml, a small size) and the progressively higher 10-year age groups showed progressively larger lesions. This suggests that white matter lesions may be a good MRI marker for taking preventive action against later Dementia, Alzheimer's or other type.
Another observation of this study was the sensitivity. Aneurysms as small as 2 mm diameter could be detected by simple MRI. This compares to a lower limit of 10 mm for the most sensitive CT with dye.
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