Diffusion-weighted imaging (DWI) is a technique that measures the free diffusion of water molecules within tissue. Preferential movement of water molecules along a particular direction, for example, parallel to white matter tracts, is referred to as anisotropy (i.e., nonisotropic movement). Many abnormal processes can produce anisotropy as well. In acute ischemic stroke, failure of the sodium-potassium ATPase pump leads to cellular swelling and reduced intercellular space, thus limiting the free movement of water and producing hyperintensity on DWI. This imaging technique reveals the abnormalities of ischemic stroke earlier than standard T1or T2-weighted MRI, or CT. Pus-filled abscesses and hypercellular tumors can also show DWI hyperintensity, reflecting the limitation of free diffusion of water in these lesions.
True restricted diffusion, appearing hyperintense on the DWI sequence in acute infarction, is hypointense on a related sequence termed apparent diffusion coefficient (ADC). If the hyperintense DWI signal is also hyperintense on ADC, then diffusion is termed facilitated rather than restricted. This phenomenon is seen when the free movement of water within a tissue becomes increasingly isotropic, as with vasogenic edema. Therefore, the interpretation of DWI signal hyperintensity must be gauged in the context of the ADC signal in the same region.
The administration of gadolinium, a paramagnetic agent that accelerates the process of proton relaxation during the T1 sequence of MRI, permits even sharper definition and highlights regions surrounding many types of lesions where the blood–brain barrier has been disrupted in the brain, spinal cord, or nerve roots.
Limitations and Safety of MRI
The degree of cooperation in holding still that is required to perform MRI limits its use in young children and in the cognitively impaired. Some form of sedation may be required in these individuals and most hospitals have services to safely accomplish conscious sedation for this purpose. Studying a patient who requires a ventilator is also difficult but manageable by using either manual ventilation or nonferromagnetic ventilators.
The main dangers in the use of MRI are torque, dislodgement or heating of metal clips on blood vessels, of dental devices and other ferromagnetic objects, and of small metal fragments in the orbit, the last of these often acquired unnoticed by operators of machine tools. For this reason it is wise, in appropriate patients, to obtain plain radiographs of the orbits so as to detect metal in these regions. Corneal metal fragments can be removed by an ophthalmic surgeon if an MRI is necessary. The presence of a cardiac pacemaker, defibrillator, or implanted stimulator in the brain or spinal cord is an absolute contraindication to the use of MRI as the magnetic field induces unwanted currents in the device and the wires exiting from it. However, many new implantable medical devices have been developed that are unaffected by and do not distort the magnetic field. Most of the newer, weakly ferromagnetic prosthetic heart valves, joint prostheses, some cochleae implants, intravascular access ports, aneurysm clips, and ventricular shunts and adjustable valves do not represent an untoward risk for magnetic imaging although shunt valves may require resetting after MRI. An extensive list of devices that have been tested for their ferromagnetic susceptibility and their safety in the MRI machine can be found at www.mrisafety.com. MRI entails some risk in these situations unless there is direct knowledge of the type of material contained in the device. It should be noted that devices or materials that are deemed safe for 1.0 or 1.5 Tesla scanners may not be compatible with higher magnetic field strength scanners.
Because of the development of cataracts in the fetuses of animals exposed to MRI, there has been hesitation in performing MRI in pregnant patients, especially in the first trimester. However, current data indicate that imaging may be performed provided that the study is medically indicated. In a study of 1,000 pregnant MRI technicians who entered the magnetic field frequently (the magnet remains on between procedures), no adverse effects on the fetus could be discerned (Kanal et al).