MRI; Types, Indications, Patients Safety, Procedure

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MRI (Magnetic resonance imaging) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, electric field gradients, and radio waves to generate images of the organs in the body. MRI does not involve X-rays and the use of ionizing radiation, which distinguishes it from CT or CAT scans. Magnetic resonance imaging is a medical application of nuclear magnetic resonance (NMR). NMR can also be used for imaging in other NMR applications such as NMR spectroscopy.

Types of MRI

Indications/Uses of MRI

MR imaging of the body is performed to evaluate

Physicians use an MR examination to help diagnose or monitor treatment for conditions such as

Contra-Indications of MRI

The following are some items that might be contraindicated

  • Foreign bodies from trauma, mechanical heart valves, surgical implants, plates, screws, staples and clips, and prosthetics that contain metal
  • Pacemakers, cochlear implants, drug infusion ports, insulin pumps, deep-brain stimulators, and other electrical devices
  • Metal tooth implants and fillings
  • Accessories such as keys, glasses, piercings, jewelry, hairpins, pagers, watches, wallets, identification badges, and pens
  • Oxygen tanks, carts, chairs, IV poles, and other medical equipment
  • Implanted electric and electronic devices are a strict contraindication to the magnetic resonance imaging, and in particular:
  • heart pacemakers (especially older types)
  • insulin pumps
  • implanted hearing aids
  • neurostimulators
  • intracranial metal clips
  • metallic bodies in the eye

Benefits of MRI

  • MRI is a noninvasive imaging technique that does not involve exposure to ionizing radiation.
  • MR images of the soft-tissue structures of the body—such as the heart, liver and many other organs— is more likely in some instances to identify and accurately characterize diseases than other imaging methods. This detail makes MRI an invaluable tool in early diagnosis and evaluation of many focal lesions and tumors.
  • MRI has proven valuable in diagnosing a broad range of conditions, including cancer, heart and vascular disease, and muscular and bone abnormalities.
  • MRI enables the discovery of abnormalities that might be obscured by bone with other imaging methods.
  • MRI allows physicians to assess the biliary system noninvasively and without contrast injection.
  • The contrast material used in MRI exams is less likely to produce an allergic reaction than the iodine-based contrast materials used for conventional x-rays and CT scanning.
  • MRI provides a noninvasive alternative to x-ray, angiography and CT for diagnosing problems of the heart and blood vessels.

Risks Factors of MRI

  • The MRI examination poses almost no risk to the average patient when appropriate safety guidelines are followed.
  • If sedation is used, there are risks of excessive sedation. However, the technologist or nurse will monitor your vital signs to minimize this risk.
  • Although the strong magnetic field is not harmful in itself, implanted medical devices that contain metal may malfunction or cause problems during an MRI exam.
  • Nephrogenic systemic fibrosis is currently a recognized, but rare, complication of MRI believed to be caused by the injection of high doses of gadolinium-based contrast material in patients with very poor kidney function. Careful assessment of kidney function before considering a contrast injection minimizes the risk of this very rare complication.
  • There is a very slight risk of an allergic reaction if contrast material is injected. Such reactions are usually mild and easily controlled by medication. If you experience allergic symptoms, a radiologist or other physician will be available for immediate assistance.
  • Manufacturers of intravenous contrast indicate mothers should not breastfeed their babies for 24-48 hours after contrast medium is given. However, both the American College of Radiology (ACR) and the European Society of Urogenital Radiology note that the available data suggest that it is safe to continue breastfeeding after receiving intravenous contrast.

Patient safety tips prior to the procedure of MRI

EAT/DRINK

You may eat, drink and take medications as usual.

CLOTHING

You must completely change into a patient gown and lock up all personal belongings. A locker will be provided for you to use. Please remove all piercings and leave all jewelry and valuables at home.

What to expect

Imaging takes place inside of a large tube-like structure, open on both ends. You must lie perfectly still for quality images. Due to the loud noise of the MRI machine, earplugs are required and will be provided.

Allergy

If you have had an allergic reaction to contrast that required medical treatment, contact your ordering physician to obtain the recommended prescription. You will likely take this by mouth 24, 12 and two hours prior to examination.

Anti Anxiety Medication

If you require anti-anxiety medication due to claustrophobia, contact your ordering physician for a prescription. Please note that you will need some else to drive you home.

Strong Magnetic Invironment

If you have metal within your body that was not disclosed prior to your appointment, your study may be delayed, rescheduled or cancelled upon your arrival until further information can be obtained.

  • Pacemaker
  • Pregnancy
  • Claustrophobia
  • History of kidney problems
  • Skin tattoos
  • Neurostimulators (TENS-unit)
  • Implanted drug infusion device (i.e., insulin pump)
  • Exposure of metal fragments to your eye
  • Artificial heart valves
  • Aneurysm clips
  • Cochlear implants
  • Metallic implants and prosthesis
  • Vascular stent or stent graft
  • History as a metal worker
  • Shrapnel or bullet wounds
  • Dorsal column stimulators
  • Allergy to iodine, or gadolinium
  • History of diabetes
  • Other conditions you believe to be relevant
  • jewelry, watches, credit cards and hearing aids, all of which can be damaged
  • pins, hairpins, metal zippers and similar metallic items, which can distort MRI images
  • removable dental work
  • pens, pocket knives and eyeglasses
  • body piercings

Other Tips of MRI

  • Please leave your valuables at home, including jewelry, to prevent it from being lost or stolen, for they have to be removed prior to entering the scan room.
  • Please let us know if you need interpreting services, this can be arranged for you.
  • Please bring a list of your current medications.
  • If you have claustrophobia, your doctor may prescribe an oral medication for you to take with you for your MRI appointment.
  • cochlear (ear) implant
  • some types of clips used for brain aneurysms
  • some types of metal coils placed within blood vessels
  • nearly all cardiac defibrillators and pacemakers

What happens during an MRI ?

www.rxharun.com/Block diagram of an MRI imaging system.

MRI may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor’s practices.

Generally, MRI follows this process

  • You will be asked to remove any clothing, jewelry, eyeglasses, hearing aids, hairpins, removable dental work, or other objects that may interfere with the procedure.
  • If you are asked to remove clothing, you will be given a gown to wear.
  • If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast dye.
  • You will lie on a scan table that slides into a large circular opening of the scanning machine. Pillows and straps may be used to prevent movement during the procedure.
  • The technologist will be in another room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. Speakers inside the scanner will enable the technologist to communicate with and hear you. You will have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist will be watching you at all times and will be in constant communication.
  • You will be given earplugs or a headset to wear to help block out the noise from the scanner. Some headsets may provide music for you to listen to.
  • During the scanning process, a clicking noise will sound as the magnetic field is created and pulses of radio waves are sent from the scanner.
  • It will be important for you to remain very still during the examination, as any movement could cause distortion and affect the quality of the scan.
  • At intervals, you may be instructed to hold your breath, or to not breathe, for a few seconds, depending on the body part being examined. You will then be told when you can breathe. You should not have to hold your breath for longer than a few seconds.
  • If contrast dye is used for your procedure, you may feel some effects when the dye is injected into the IV line. These effects include a flushing sensation or a feeling of coldness, a salty or metallic taste in the mouth, a brief headache, itching, or nausea and/or vomiting. These effects usually last for a few moments.
  • You should notify the technologist if you feel any breathing difficulties, sweating, numbness, or heart palpitations.
  • Once the scan is complete, the table will slide out of the scanner and you will be assisted off the table.
  • If an IV line was inserted for contrast administration, the line will be removed.

While the MRI procedure itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure such as surgery.

How does the procedure work?

Unlike conventional x-ray examinations and computed tomography (CT) scans, MRI does not utilize ionizing radiation. Instead, radiofrequency pulses re-align hydrogen atoms that naturally exist within the body while you are in the scanner without causing any chemical changes in the tissues. As the hydrogen atoms return to their usual alignment, they emit different amounts of energy that vary according to the type of body tissue from which they come. The MR scanner captures this energy and creates a picture of the tissues scanned based on this information.

The magnetic field is produced by passing an electric current through wire coils in most MRI units. Other coils, located in the machine and in some cases, placed around the part of the body being imaged, send and receive radio waves, producing signals that are detected by the coils. The electric current does not come in contact with the patient.

A computer then processes the signals and generates a series of images, each of which shows a thin slice of the body. The images can then be studied from different angles by the interpreting radiologist.

Frequently, the differentiation of abnormal (diseased) tissue from normal tissues is better with MRI than with other imaging modalities such as x-ray, CT and ultrasound.

Report that are found in imaging

The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows:

SignalT1-weightedT2-weighted
High
  • Fat
  • Subacute hemorrhage
  • Melanin
  • Protein-rich fluid
  • Slowly flowing blood
  • Paramagnetic substances, such as gadolinium, manganese, copper
  • Cortical pseudolaminar necrosis
  • More water content, as in edema, tumor, infarction, inflammation and infection
  • Extracellularly located methemoglobin in subacute hemorrhage
Inter- mediateGray matter darker than white matterWhite matter darker than grey matter
Low
  • Bone
  • Urine
  • CSF
  • Air
  • More water content, as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage
  • Low proton density as in calcification
  • Bone
  • Air
  • Fat
  • Low proton density, as in calcification and fibrosis
  • Paramagnetic material, such as deoxyhemoglobin, intracelullar methemoglobin, iron, ferritin, hemosiderin, melanin
  • Protein-rich fluid

References

mri

 

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