Quantum Physics

962 Chapter 29 Nuclear Physicsassociated with the absorption. In this example, compartment D would be verydark and compartment A would be very light.The steps outlined previously are representative of how a CAT scanner producesimages of the human body. A thin slice of the body is subdivided into perhaps10 000 compartments, rather than 4 as in our simple example. The functionof the CAT scanner is to determine the relative absorption in each of these 10 000compartments and to display a picture of its calculations in various shades of gray.Note that “CAT” stands for computed axial tomography. The term axial is usedbecause the slice of the body to be analyzed corresponds to a plane perpendicularto the head-to-toe axis. Tomos is the Greek word for slice and graph is the Greekword for picture. In a typical diagnosis, the patient is placed in the position shownin Figure 29.14 and a narrow beam of x-rays is sent through the plane of interest.The emerging x-rays are detected and measured by photomultiplier tubes behindthe patient. The x-ray tube is then rotated a few degrees, and the intensity isrecorded again. An extensive amount of information is obtained by rotating thebeam through 180° at intervals of about 1° per measurement, resulting in a set ofnumbers assigned to each of the 10 000 “compartments” in the slice. These numbersare then converted by the computer to a photograph in various shades of grayfor the segment of the body that is under observation.A brain scan of a patient can now be made in about 2 s, and a full-body scanrequires about 6 s. The final result is a picture containing much greater quantitativeinformation and clarity than a conventional x-ray photograph. Because CATscanners use x-rays, which are an ionizing form of radiation, the technique presentsa modest health risk to the patient being diagnosed.APPLICATIONMagnetic ResonanceImaging (MRI)Magnetic Resonance Imaging (MRI)At the heart of magnetic resonance imaging (MRI) is the fact that when a nucleushaving a magnetic moment is placed in an external magnetic field, its momentprecesses about the magnetic field with a frequency that is proportional to thefield. For example, a proton, with a spin of 1/2, can occupy one of two energystates when placed in an external magnetic field. The lower energy state correspondsto the case in which the spin is aligned with the field, whereas the higherenergy state corresponds to the case in which the spin is opposite the field. Transitionsbetween these two states can be observed with a technique known as nuclearmagnetic resonance. A DC magnetic field is applied to align the magnetic moments,and a second, weak oscillating magnetic field is applied perpendicular tothe DC field. When the frequency of the oscillating field is adjusted to match theprecessional frequency of the magnetic moments, the nuclei will “flip” betweenX-raydetectorsX-rayPatient(a)Figure 29.14 (a) CAT scanner detector assembly. (b) Photograph of a patient undergoing a CATscan in a hospital.Jay Freis/The Image Bank(b)