The MRI has been known to cause minor long-term shrinking of the male genitalia. The amount it will shrink it is hardly anything to be worried about, however; as little as 1/10 of an inch for a single MRI. In the worst cases, it's never been more than 1/2 of an inch from a single MRI.
The potential side effects of magnetic and electric fields on human health remain a source of debate. In particular, the possible effects on an unborn baby are not well known.
No. A magnetic resonance imaging machine does not cause dehydration. However, the medical reason for having the MRI may indeed be causing dehydration. Dehydration is not listed as a side effect on the web site: http://mri-side-effects.net/ Some of the side effects listed are the same symptoms of dehydration, but are not an indication that the patient has lost water. The contrast dye that is injected for some MRI scans may have its own issues though: http://www.levinsimeskaisergornick.com/gadolinium.html
MRI scanners have practically no adverse effects. MRI or magnetic resonance imaging works by noting difference of various images created by varying response of different body tissues.
MRI contrast agents do not usually cause side effects. Rarely, they can cause Allergies (mostly skin rash), nausea. When used in large quantities, they sometimes cause renal failure, but usually not on healthy kidneys.
spinal injuries
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I have never heard of frequent urination as being a side effect of an MRI. It is likely just coincidental.
It is best to wait for the results of the MRI - they will then tell you what they have found.
an MRI
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If the MRI technologist positions you with your leg going into the magnet (the MRI machine) then your head would be out of it. Call the facility where you are scheduled to have the MRI and ask to speak with the MRI technologist and they can fully explain to you exactly how you will be positioned.
Almost none. There are some surgical procedures a patient may have had done, prior to having an MRI ordered, that will not allow them to have an MRI study. A pace maker would be a good example. The magnetic field can erase or reprogram the pacemaker, and depending on the material used move the pace maker or the wires in the heart. Most modern surgical implants are safe in MRI machines, but it is safest if you make sure to let the technologist know you have had surgery in the past. I have read some studies (forgive me I don't remember references) when I was doing MRI that stated that some people could have arm numbness when left inside a strong MRI machine for extended periods of time. This seems to effect men more so than women. I never experienced this nor met a patient who has. The magnets are large and the biggest problem I have seen with the use of them is patients becoming claustrophobic. The strength of the magnet is measured in teslas from 1 tesla to 3 tesla. Usually over 3 is not considered safe for humans and is only used in research.