|
MRI uses magnetic fields and radio waves to produce images of thin slices of tissues (tomographic images). Normally, protons within tissues spin to produce tiny magnetic fields that are randomly aligned. When surrounded by the strong magnetic field of an MRI device, the magnetic axes align along that field. A radiofrequency pulse is then applied, causing the axes of all protons to momentarily align against the field in a high-energy state. After the pulse, some protons relax and resume their baseline alignment within the magnetic field of the MRI device. The magnitude and rate of energy release that occurs as the protons resume this alignment (T1 relaxation) and as they wobble (precess) during the process (T2 relaxation) are recorded as spatially localized signal intensities by a coil (antenna). Computer algorithms analyze these signals and produce anatomic images.
The relative signal intensity (brightness) of tissues in an MRI image is determined by factors such as the radiofrequency pulse and gradient waveforms used to obtain the image, intrinsic T1 and T2 tissue characteristics, and tissue proton density.
By controlling the radiofrequency pulse and gradient waveforms, computer programs produce specific pulse sequences that determine how an image is obtained (weighted) and how various tissues appear. Images can be T1-weighted, T2-weighted, or proton density–weighted. For example, fat appears bright (high signal intensity) on T1-weighted images and relatively dark (low signal intensity) on T2-weighted images; water and fluids appear relatively dark on T1-weighted images and bright on T2-weighted images. T1-weighted images optimally show normal soft-tissue anatomy and fat (eg, to confirm a fat-containing mass). T2-weighted images optimally show fluid and abnormalities (eg, tumors, inflammation, trauma). In practice, T1- and T2-weighted images provide complementary information, so both are important for characterizing abnormalities.
Uses
MRI is preferred to CT when soft-tissue contrast resolution must be highly detailed (eg, to evaluate intracranial or spinal cord abnormalities, inflammation, trauma, suspected musculoskeletal tumors, internal joint derangement). MRI is also useful for evaluating the following:
MRI can also be substituted for CT with contrast in patients with a high risk of contrast reactions.
Contrast:
With MRI, contrast agents may be used to highlight vascular structures (for MRA) and to help characterize inflammation and tumors. The most commonly used agents are gadolinium derivatives, which have magnetic properties that affect proton relaxation times. MRI of intra-articular structures may include injection of a gadolinium derivative into a joint.
Variations
Diffusion (diffusion-weighted) MRI:
Signal intensities are related to diffusion of water molecules in tissue. This type of MRI can be used to detect early cerebral ischemia and infarction and to differentiate intracranial cysts from solid masses.
Echo planar imaging:
This ultrafast technique (images obtained in > 1 sec) is used for diffusion, perfusion, and functional imaging of the brain and heart. Its potential advantages include showing brain and heart activity and reducing motion artifacts. However, its use is limited because it requires special technical hardware and it is susceptible to other artifacts.
Functional MRI:
Functional MRI is used to assess brain activity by location. In the most common type, the brain is scanned at low resolution very frequently (eg, every 2 to 3 sec). The change in oxygenated Hb can be discerned and used to estimate metabolic activity. Mechanisms of various neural mechanisms can be studied in research settings.
Gradient echo imaging:
Gradient echo is a pulse sequence that can be used for fast imaging of moving blood and CSF (eg, in MRA). Because this technique is fast, it can reduce motion artifacts (eg, blurring) during imaging that requires patients to hold their breath (eg, during imaging of cardiac and abdominal structures).
Magnetic resonance spectroscopy (MRS):
MRS combines the information obtained by MRI (mainly based on water and fat content of tissues) with that of nuclear magnetic resonance, or NMR; NMR provides information about tissue metabolites. Such information can help differentiate certain abnormalities (eg, certain types of tumors).
Perfusion MRI:
Perfusion MRI is a method of assessing relative cerebral blood flow. It can be used to detect an area of ischemia during imaging for stroke.
Disadvantages
MRI is relatively expensive and may not be available or available immediately.
Magnetic field:
MRI is relatively contraindicated in patients with implanted materials that can be affected by powerful magnetic fields. These materials include ferromagnetic metal (containing iron), magnetically activated or electronically controlled medical devices (eg, pacemakers, implantable cardioverter defibrillators, cochlear implants), and nonferromagnetic metal electronically conductive wires or materials (eg, pacemaker wires, certain pulmonary artery catheters). Ferromagnetic material may be moved by the strong magnetic field and injure a nearby organ; movement is more likely if the material has been in place < 6 wk (before scar tissue forms). Ferromagnetic material can also cause imaging artifacts. Magnetically activated medical devices may malfunction when exposed to magnetic fields. Magnetic fields may induce current in conductive materials; this current may produce enough heat to burn tissues. Whether a specific device is compatible with MRI depends on the type of device, its components, and its manufacturer (see the MRI safety web site). Also, MRI machines with different magnetic field strengths have different effects on materials, so safety in one machine does not ensure safety in another.
The MRI magnetic field is very strong and always on. Thus, a ferromagnetic object (eg, an O2 tank, a metal pole) at the entrance of the scanning room may be pulled into the magnet bore at high velocity and injure anyone in its path. The only way to separate the object from the magnet may be to turn off the magnetic field.
Claustrophobia:
The imaging tube of an MRI machine is a tight, enclosed space that can trigger claustrophobia even in patients without preexisting phobias or anxiety. Also, some obese patients do not fit on the table or within the machine. Premedication with an anxiolytic (eg, alprazolam or lorazepam 1 to 2 mg po) 15 to 30 min before scanning is effective for most anxious patients. MRI scanners with an open side can be used. Its images may be inferior to those of enclosed scanners depending on the field strength of the magnet, but they are usually sufficient for making a diagnosis. Patients should be warned that the MRI machine makes loud, banging noises.
Contrast reactions:
Gadolinium derivatives, if used, can cause headache, nausea, and pain, as well as sensation of cold at the injection site. However, serious contrast reactions are rare and much less common than with iodinated contrast agents. However, in patients with impaired renal function, nephrogenic systemic fibrosis is a risk. Nephrogenic systemic fibrosis is a rare but life-threatening disorder that involves the skin and probably internal organs, resulting in severe disability or death. For patients with impaired renal function, the following is recommended:
Last full review/revision July 2008 by Jon A. Jacobson, MD
Content last modified February 2012
| 
