Chapter 1: An Introduction to Anatomy and Physiology

FOCUS Sectional Anatomy and Clinical Technology

Radiological procedures include (1) scanning techniques that involve the use of beams of radiation, such as X-rays, to create a photographic or computer-generated image of internal structures and (2) methods that involve the administration of radioactive materials.
Physicians who specialize in the performance of these procedures and the analysis of the resulting images are called radiologists. Radiological procedures can provide detailed information about internal systems. Figures 1-12 through Figure 1-14 compare the views provided by several techniques radiologists and other clinicians use. These figures include X-rays, CT scans, MRI scans, ultrasound images, spiral-CT scans, and DSA images. These and several other clinical technologies, such as those that produce PET (positron emission tomography) scans and DSR (dynamic spatial reconstruction) images and more-specialized MRI procedures, are described in the Applications Manual; many examples are included as figures later in this text. Diagnostic Procedures

Whenever you see anatomical diagrams or clinical procedures that present cross-sectional views of the body, remember that the sections are oriented as though the observer is standing at the feet of the subject and looking toward the head.

FIGURE 1-12 X-Rays (a) X-rays of the skull, taken from the left side. X-rays are a form of high-energy radiation that can penetrate living tissues. In the most familiar procedure, a beam of X-rays travels through the body and strikes a photographic plate. Not all of the projected X-rays arrive at the film; some are absorbed or deflected as they pass through the body. The resistance to X-ray penetration is called radiodensity. In the body, radiodensity increases in the following sequence: air, fat, liver, blood, muscle, bone. The result is an image with radiodense tissues, such as bone, appearing in white, and less-dense tissues in shades of gray to black. (b) A barium-contrast X-ray of the upper digestive tract. Such an X-ray is produced by introducing a relatively radiodense material into the body to provide sharp outlines and contrast and to check the distribution of fluids or the movements of internal organs. In this instance, the patient swallowed a solution of barium. Barium is very dense, and the contours of the stomach and intestinal linings are clearly outlined against the white of the barium solution.

FIGURE 1-13 Common Scanning Techniques (a) The relative position and orientation of the scans shown in parts (b)–(d). (b) A color-enhanced CT scan of the abdomen. Computerized tomography (CT), formerly called computerized axial tomography (CAT), uses computers to reconstruct sectional views. A single X-ray source rotates around the body, and the X-ray beam strikes a sensor monitored by the computer. The source completes one revolution around the body every few seconds; it then moves a short distance and repeats the process. The result is usually displayed as a sectional view in black and white, but it can be colorized for visual effect. CT scans show three-dimensional relationships and soft-tissue structure more clearly than do standard X-rays. (c) A color-enhanced MRI scan of the abdomen. Magnetic resonance imaging (MRI) surrounds part or all of the body with a magnetic field about 3000 times as strong as that of Earth. This field affects protons within atomic nuclei throughout the body. The protons line up along the magnetic lines of force like compass needles in Earth's magnetic field. When struck by a radio wave of a certain frequency, a proton will absorb energy. When the wave pulse ends, that energy is released and the source of the radiation is detected. Each element differs in terms of the radio frequency required to affect its protons. (d) An ultrasound scan of the abdomen. In ultrasound procedures, a small transmitter contacting the skin broadcasts a brief, narrow burst of high-frequency sound and then picks up the echoes. The sound waves are reflected by internal structures. An echogram, or ultrasound picture, can be assembled from the pattern of echoes. These images lack the clarity of other procedures, but no adverse effects have been reported, and fetal development can be monitored without a significant risk of birth defects. Special methods of transmission and processing permit analysis of the beating heart, without the complications that can accompany dye injections. Note the differences in detail among this image, the CT scan, and the MRI image.

FIGURE 1-14 Special Scanning Methods. (a) A spiral-CT scan of the chest. Such an image is created by special processing of CT data to permit rapid three-dimensional visualization of internal organs. Spiral-CT scans are becoming increasingly important in clinical settings. (b) Digital subtraction angiography (DSA) is used to monitor blood flow through specific organs, such as the brain, heart, lungs, or kidneys. X-rays are taken before and after radiopaque dye is administered, and a computer "subtracts" details common to both images. The result is a high-contrast image showing the distribution of the dye.