special subjects
Magnetic Resonance Imaging
Variations of 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 To detect white matter disease of the brain To differentiate abscess from a cystic tumor To stage various tumors such as non–small cell lung cancer This ultrafast technique (images obtained in<1 second) 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 is more sensitive to various artifacts than conventional 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 seconds). The change in oxygenated hemoglobin can be discerned and used to estimate metabolic activity of different parts of the brain. Researchers sometimes do functional MRI while subjects do different cognitive tasks (eg, solve a math equation); the metabolically active parts of the brain are presumed to be the structures most involved in that particular task. Correlating brain function and anatomy this way is called brain mapping. Functional MRI can be used in both research and clinical settings. It is especially helpful clinically in mapping of motor or language cortices (ie, cortical areas that when removed result in deficits in sensory processing, motor function, or language processing) in patients with intracranial abnormalities such as tumors and arteriovenous malformations for whom surgery is being planned. It is also used increasingly to plan epilepsy surgery. Gradient echo is a pulse sequence that can be used for fast imaging of moving blood and cerebrospinal fluid (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, pulmonary, and abdominal structures). MRS combines the information obtained by MRI (mainly based on water and fat content of tissues) with that of nuclear magnetic resonance (NMR). NMR provides information about tissue metabolites and biochemical abnormalities; this information can help differentiate certain types of tumors and other abnormalities. Magnetic resonance enterography has become popular, especially for follow-up imaging of children with known inflammatory conditions of the small bowel. Because magnetic resonance enterography does not require ionizing radiation, it has an advantage over CT enterography. Perfusion MRI is a method of assessing relative cerebral blood flow. It can be used to detect Areas of ischemia during imaging for stroke Areas of increased vascularity that can indicate tumors This information can help direct biopsy. PET-MRI combines functional Positron Emission Tomography (PET) with whole-body MRI. T1-weighted and short T1 inversion recovery (STIR) sequences are frequently used. This method is new and is available in only a few major medical centers. 