The basic process of MRI involves excitation of hydrogen nuclei (i.e., protons) with radio frequency energy. After sending electromagnetic energy into the patient, protons are excited to a higher-energy state. The radio frequency wave is then turned off, and protons "relax" to their baseline state. As the protons relax, they emit a radio frequency signal that can be picked up by an antenna, amplified, and used to form an image.
Although hydrogen nuclei are typically used in MRI, other atoms, such as phosphorus (particularly important for cardiac energetics and infarction) can also be used for MRI. Hydrogen is particularly useful for biologic tissues: it is extremely abundant (for example, in water and fat) and is extremely sensitive to the MRI process. In addition, with pathologic processes, the water content of tissues often changes. For example, myocardial infarctions have a higher water content than normal tissues. Therefore, pathologic processes can be readily detected by imaging changes in water content of tissues.
To summarize, the basic MR process involves the following:
• The patient is placed in very high field strength magnet
• The magnetic field causes alignment of protons in the patient
• A radio wave is sent into the patient, disturbing the alignment
• The radio wave is turned off
• The patient emits a radio frequency signal
• The signal is picked up by an antenna and used to form an image
The MR signal is a function of multiple tissue parameters, including T1, T2, proton density, and the rate and type (oxygenated or deoxygenated) of flowing blood. T1 and T2 parameters are numbers that characterize the response of biologic tissue to an applied magnetic field. T1-weighted images are generally those that help define the anatomy that is being viewed (Fig..57-11). T2 images are those that better define the pathology of the tissue. Proton-density images may also be generated, in which the image intensity is proportional to the number of protons present. Finally, images can be generated that are sensitive to flowing blood ( bright blood images or MR angiograms). It is the unique property and strength of MR imaging that four different physical factors (T1, T2, proton density, and blood flow) may be used to derive a variety of imaging results; this is very different from CT scanning or echocardiography, in which only one factor (x-ray attenuation and acoustic attenuation, respectively) is used to generate contrast in images. Although these multiple factors add to the complexity of understanding MR, it also enhances the power of the technique for detecting subtle changes in pathologic tissues relative to normal tissues.
FIG. 57-11. Paracardiac mass demonstrated by MRI. Axial images of the chest demonstrating a mass posterior to the left ventricle. A. T1-weighted image, showing thickening of the posterior wall of the left ventricle. B. T2-weighted image, showing bright increase signal in the mass. Biopsy revealed hemangiopericytoma. (Reprinted from Bluemke and Boxerman,19 with permission.)
Two parameters are particularly helpful in determining MR image contrast: time to repetition (TR) describes the time between radio frequency pulses, and time to echo (TE) describes the time after each radio frequency pulse when the MR scanner "listens" for signal from the patient. By changing the TR and Te values, different types of image contrast are obtained (Tab.leii57.-1). For more detailed information regarding MR physics, the reader is referred to standard textbooks on the subject, such as that by Berning and Steensgaard-Hansen.11
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