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What is Magnetic
Resonance Imaging?


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MRI Info ...
 
What is Magnetic Resonance Imaging ?
 Magnetic resonance imaging – MRI – is a scanning technique uses strong magnetic fields to form clear, detailed images of the internal tissue of the body , that can be studied by physicians . MR images come from signals originating from protons in the human body . Almost all of the signals come from one or two sources : water protons or from the protons in fat molecules . In a strong magnetic field, these protons act like little magnets, with a tendency to line up with the main magnetic field . When these protons are excited by radio – frequency energy, give off energy in the form of radio waves of the same frequency, which is usually received by the same coil that transmitted the original pulse . The signal intensity from different tissues is dependent on different tissue parameters, like T1 , T2 and proton density in a particular bit of tissue . These parameters are inherent to the tissue. Different tissues are different in one or more of these parameters and these differences are the basis for tissue contrast in MRI . Using appropriate pulse sequences we can generate images that bring out these differences . Finally, very sophisticated electronic hardware and computer soft ware are used to produce images that can be studied by physicaus . 

When might MRI be needed ?
  
Your physician may request an MRI study for a number of reasons. MRI is often used to obtain specific diagnostic information not already provided by other imaging technologies such as ultrasound, computed tomography, and nuclear medicine . MRI has commonly been used as an important diagnostic tool in medicine . MRI can be considered as the best neuroimaging modality . It has also been very useful in imaging of joints, musculoskeletal structures, pelvic and abdominal cavities, mediastinum , heart and breast . Thanks to different developing hardware and software, MRI is able to delineate vascular, biliary and urinary tract systems, effectivly and noninvasively .

What else is MRI used for ?
 
In addition to providing anatomical pictures, research is showing that MRI potentially has other uses . For instance with MRI spectroscopy, it is now possible to measure the amounts of various chemicals within the brain, which gives information about the health and viability of brain cells . This helps to determine how and where in the brain these are affected in different pathologies .

How should I prepare for the procedure ?
 
You may eat or drink as usual unless the MRI examination requires the injection or ingestion of a contrast agent for image enhancement, if sedation or anesthesia is necessary, or if there are other special care considerations . In these situations, specific dietary restrictions and other instruction are provided by your physician in advance of the appointment . It is very important to adhere to these instructions to insure your safety while undergoing the MRI examination . Otherwise , the procedure will be rescheduled to a time and date that allows for the appropriate preparation . It is also important to notify the MRI staff of any active illness, allergy or previous drug reaction that may prevent the patient from undergoing the examination safely .

What will happen during the procedure ?
 
Upon arriving at the MRI suit your appointment and registration are confirmed by the MRI scheduling coordinator .You will be asked to fill out the MRI screening questionnaire . The MRI nurse then asks a few questions and does a clinical screening examination . The patient must remove all metal objects ( e.g. jewelry ) and electronic devices ( e.g. watch ) and change into hospital attire or wear clothing from home that is comfortable and free of metal ( zippers , buttons , etc ) . The nurse then prepares you for the MRI procedure, which may include oral or intravenous sedation by the radiologist, or anesthesia by an anesthesiologist . The MRI Technologist positions and secures you comfortably on a narrow bed within the large, doughnut – shaped magnet and adjusts the radiowave coil . The technologist then performs the MRI examination with the radiologist and is in contact with the patient at all times . During the MRI procedure, the patient will hear several series of repetitive pulsing noises . It is particularly important for the patient to remain still during these noises since the MRI machine is obtaining images at those times . If the first pictures are motion – free, this will help shorten the total exam time and prevent delays . An entire MRI examination may take from twenty minutes to one and a half hours depending on the type of information required by the radiologist and your physician . All patient undergoing sedation or anesthesia receive continuous vital monitoring and support .

Will I feel anything ?
 
If an injection is required, you may feel minor discomfort from the needle . MRI itself is a painless procedure, in that the patient is not touched . Rather loud pulsations are routinely heard but can be reduced with ear plugs or earphones provided by the MRI staff . Occasionally, claustrophobia is a problem . If there is concern about this possibility, the MRI staff should be contacted in advance of the appointment for information and assistance . MRI, like other imaging technologies in radiology, is FDA approved for its safety and effectiveness . MRI has been used to image patients since 1981 and no adverse biological effects have been reported from exposure to the magnetic fields or radiowaves .

What will I experience during a scan and how long does the scan take ?  
 
You will be asked to lie on a table and this table will slowly move to the center of the magnetic field . The scanner is a long tube opened at both ends . The part of the body being scanned must be in the middle of this tube . Some patients find that they can be somewhat claustrophobic during the scan . However, the Philips scanner has a very short bore and claustrophobia is not usually an issue . The scanner generates loud knocking noises during the scan so the staff will provide earphones during the procedure . You can listen to your favorite radio station or CD . The scan times are very short and every consideration will be taken to make you comfortable during the procedure . The procedure usually lasts 15 – 30 minutes and you must lie very still . Some patients like to take a light sedative or pain medication before they arrive for the test .

What happens after the images are obtained ?
 
After the images are obtained, you are free to go and resume your normal activity .

When will I get the results ?
 
The exam will be reviewed by your radiologist shortly after the completion of your exam . Some procedures require post processing and can take some time to complete . So a report is not alway readily available . Then, the images and report will be sent to your referring physician within 24 hours of the procedure . A complimentary CD-ROM or x-ray film copies of the exam are available to you or your doctor .

Are there any risks involved ?
 
MRI is a painless imaging procedure that has the advantage of not using any x – ray radiation . There are no known side effects of the scan . However, since the MRI scanner is a large magnet, it is very important to notify the technologist of metal implants and it is necessary to remove all metal objects such as keys and watches before entering the scan room . A screening form must be completed before starting a scan . In some cases, the exam cannot be performed due to certain implants . The most common contraindication for scanning are :

•  Cardiac pacemaker
•  Implanted defibrillator
•  Brain aneurysm clips
•  Metal in the eyes
•  Implanted devices , such as insulin pumps
•  Certain types of ear implants
•  Other contraindications

include the hardware and support devices accompanying some patients . Some ventilators and monitors are MR compatible , whereas others are not . Credit cards, wrist watches, beepers , and other metal objects should be removed before the patient or staff enter the scan room . The technologist will give you a more complete list and can answer any questions you may have before the scan . The MRI exam may require an injection of a contrast material . The most common contrast is called gadolinium . This basically does the same thing for MRI as iodine does for CT scans . The complication rate for this contrast is very low .

Contrast agents for Magnetic Resonance Imaging :
 
The alteration of signal intensity in diseased tissue forms the basis for MR imaging in diagnostic radiology . Tissue signal intensity observed in MR images is the result of a complex interaction of numerous factors , which can be classified as those that reflect intrinsic properties of biologic tissue , e.g. , T1 and T2 relaxation times and proton density , and those that are equipment related , e.g. , field strength or pulse sequences . However, due to a wide biologic variation, relaxation times of normal and abnormal tissues overlap . This limits the ability of plain MRI to detect and characterize abnormal tissues . By using MR contrast agents, you can alter tissue relaxation times in favor of better characterization of abnormal tissues . One of the commonly used extracellular contrast agents is GD-DTPA . Due to the high hydrophilicity of the Gd chelates and their low molecular weight, they rapidly diffuse into the interstitial space after IV injection and a short intravascular phase . The protein binding is negligible . The elimination of the unmetabolized Gd complexes from the body occurs via renal excretion and with a plasma half-life of about 90 min . The compounds are completely eliminated after a maximum of 24 hr if the glomerular filtration is not diminished . The half-life is prolonged in patients with impaired renal function, but, this does not change the safety profile. It can be assumed that this class of contrast agents is by far the safest compared with other contrast agents . An overall incidence of adverse reactions is 1-2% . This incidence is about two or three times higher in patients with a history of allergies or in patients with asthma . The most frequent adverse reactions are as follows : Nausea / vomiting , local warmth / pain , headache , paresthesia , dizziness , urticaria , focal convulsion ; all being less than 0.5% . The most relevant adverse reaction which may occurs after IV injection of Gd compounds is an anaphylactoid reaction that also occurs with other contrast agents . The incidence of anaphylactoid reactions is about six times lower than with nonionic X-Ray contrast agents . As far as it is known, there is no relationship between adverse reactions and doses of up to at least 0.3 µmol/kg BW . Also, at no time and in no patient was there an effect on renal function that was attributed to the Gd injection in the standard doses . Nowadays, different kinds of contrast agents are available . There are also other contrast agents which are more tissue specific, particularly the liver or lymph node – specific contrast agents .

Pregnant patients :
 
As yet, there are no known biological effects of MRI on fetuses . However, there are a number of mechanisms that could potentially cause adverse effects as a result of the interaction of electromagnetic fields with developing fetuses . Cells undergoing division, which occurs during the first trimester of pregnancy , are more susceptible to these effects . It has been suggested that any examination of pregnant patients should be delayed until the first trimester and then a written consent form should be signed before the examination, by the patient . Pregnant patients or those who suspect they are pregnant should be identified before undergoing MRI, in order to assess the relative risks versus benefits of the examination . Gadolinium enhancement is at present best avoided when examining a pregnant patient .
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