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              Magnetic Resonance in Medicine                                                              977

                The total time required to complete a scan is M times
              T R , so that T 2 -weighted images generally take longer to
              acquire. Often it is desired to enhance the SNR by repeat-
              ing the entire sequence one or more times and averaging
              the results of corresponding cycles. If there are n repeti-
              tions of the basic sequence, the total scan time increases to
                                          √
              nMT R  and the SNR is increased by  n. The total time to
              complete an individual scan usually ranges from about 1
              to 15 min. Because it is usually necessary to make more
              than one series of images, the patient is normally in the
              magnet from 15 to 90 min to complete a diagnostic study.


              IV.  CLINICAL APPLICATIONS OF
                 MAGNETIC RESONANCE IMAGING

              A.  Proton Imaging                                FIGURE 10  Sagittal head image. The image represents a 3-mm
                                                                thick slice near the midline of a normal volunteer’s head. Like all
              Once clinical MR scanners became available certain ad-  the images shown here this was taken at 1.5 T. (Courtesy of GE
              vantages  and  disadvantages  of  their  use  became  estab-  Medical Systems.)
              lished. Among the chief advantages of MR are the ability
              to image almost any region of the body, the very high con-
                                                                available on CT scanners. The contrast in these images
              trast available between soft tissue structures, the ability to
                                                                is said to be T 1  weighted—that is, the scan repetition time
              vary the plane of imaging at will, the ability to vary the tis-
                                                                T R  was short enough that not all tissues had time to mag-
              sue appearance by varying the scan parameters, the lack of
                                                                netize completely. Thus, tissues with larger values of T 1 ,
              the need for any invasive step such as the injection of con-
                                                                such as the cerebrospinal fluid, appear dark on these scans.
              trast agents, and the absence of any ionizing radiation (i.e.,
                                                                Good contrast is seen between gray and white matter of
              X rays). The relative disadvantages include the cost, time
                                                                the brain. MR is completely free of any interference from
              requirements, and the inability to detect certain materials.
                                                                bone.  This  makes  imaging  of  the  cerebellum,  near  the
              The cost of the examination varies with the time required
                                                                base of the brain at the back of the head and surrounded
              tocompleteit,thenumberofseparateimagesrequired,and
                                                                by rather dense bone, more effective with MR than with
              other factors. A large portion of the scanner cost is associ-
                                                                X-ray methods such as CT.
              ated with the magnets, which require expensive materials
              such as large amount of superconducting wire. Each im-
              age acquisition requires a time ranging from less than one
              second to as much as 20 min. During this time it is nec-
              essary for the patient to remain still to avoid blurring the
              image.CTscanscanbetakenmuchmorequicklythanthis.
              Certain materials (calcium, in particular) that are readily
              seen in X-ray studies such as CT do not give a NMR sig-
              nal, and therefore appear only as voids on MR images.
              This is a drawback especially in the diagnosis of certain
              tumors. One consequence of this balance of advantages
              and disadvantages is that CT and MR have become com-
              plementary imaging modalities with neither one showing
              signs of displacing the other.
                The advantages of MR scanning have been especially
              pronounced in imaging of the brain and spinal cord. At
              the present time a large fraction, perhaps around 70%,
              of  all  MR  studies  are  done  for  central  nervous  system
              indications.
                                                                FIGURE 11  Coronal head image. The subject was in the same
                Figures 10 and 11, both done on normal volunteers,
                                                                position as in Fig. 10 but by interchanging the gradients used for
              illustrate several advantages of MRI for brain imaging.  selective excitation an image of a slice at right angles to that shown
              Neither  the  sagittal  nor  coronal  section  are  routinely  in Fig. 10. (Courtesy of GE Medical Systems.)
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