Introduction to Diagnostic Imaging

X-rays (roentgen rays):

• are produced by creating a large potential difference between a filament (cathode) and a tungsten target (anode) in a vacuum.

• at ~125,000 volts (125 kV), the filament reaches a high temp.

• electrons boil off cathode and are accelerated as a beam which strikes the tungsten anode producing X-rays
• X-rays penetrate people or objects to variable degrees, depending on thickness or density, to reaching the film or detector placed on the side opposite the beam source.

• Use ionizing radiation (rays) of very short wavelength and very high energy, produced and directed as a beam in a particular direction.

• produce images as a result of their systematic attenuation – their selective removal from the beam by absorption and scatter (reflection) – as they pass through structures of various thicknesses and variable densities.
• ➢ Air causes very little attenuation – nearly all the X-rays passing through it reach the film (a plastic sheet coated w/ an emulsion of silver bromide and a little silver iodide sensitive to light and radiation, producing a chemical change causing the emulsion to turn black) or beam detector

• ➢ Bone--being much denser than air—causes nearly all the x-rays to be absorbed or reflected (scattered), so that very few reach the film/detector, leaving the emulsion/screen white.

• NOTE: “Density” is used 2 ways by radiologists discussing radiographs: physical density (just referred to – as in air vs. bone) and radiographic density, which refers to the degree of “whiteness” on the film. The effect on film is paradoxical: structures of high physical density produce less photographic density (the film is less exposed) and vice versa. The difference in radiographic densities (range from nearly pure black through a wide range of grays to nearly pure white) is referred to as radiocontrast. Structures which produce more blackening on the film (i.e., structures which cause less attenuation of the x-rays, such as air) are radiolucent; those which produce less blackening (ie, which cause more attenuation of the beam) are radiodense or radiopaque.

Radiographs are “shadow-grams”! Therefore the same principles apply:

• They are two dimensional; because of the absence of the 3rd dimension (depth):

• structures nearer the source and farther from the surface (wall or film tray) onto which the shadow is cast produces a larger but less sharp shadow than structures farther from the source and nearer to the surface onto which the shadow is cast. Thus patients are positioned with the area under study closest to the film to reduce distortion and maximize sharpness of features.

• Each radiograph presents a composite view of the tissues penetrated by the beam; thus structures overlap (are projected on top of) each other. Because of this overlapping and the absence of depth, more than one view is usually necessary to detect and localize an abnormality accurately. Whenever you are baffled (except during a gross anatomy exam), call for an additional view!

More terms and definitions:

Projection: refers to the direction in which the beam penetrates the body

• PA (posteroanterior) projection: source (X-ray tube) is posterior, film anterior; beam traverses the body posterior to anterior. This is the projection usually used for chest films if the patient is capable of standing, since it puts the heart closest to the film, making it sharper, less magnified.

• AP (anteroposterior) projection: source is anterior, film is posterior; beam traverses anterior aspect first, posterior aspect last. This is the projection used for patients that must remain supine, or for vertebrae studies, since it places vertebrae closest to film.

• View: refers to the direction from which the body or body part is viewed. By convention, both PA and AP projections are examined in an AP view, as if you were facing the patient. The view is thus often opposite the projection (normal chest studies are AP views of PA projections).

• Plain film: conventional radiology without use of enhancing contrast media; depends on the natural contrast between air, soft tissue (water), fat and bone to define structures and abnormalities (eg, chest films, skeletal studies).

• Contrast studies: Due to an absence of inherent contrast difference from surrounding tissue, contrast agents (media) are employed to enhance the contrast. This technique is applied most often to hollow, tubular structures or systems – the vast majority of contrast studies are of the G.I. tract (including the hepatobiliary tract), urinary tract and blood vessels. Most common is barium sulfate administered by mouth (swallow, antegrade) or rectum (enema, retrograde), which may be enhanced by the use of gas (“air contrast”).

Angiography is the non-specific term for any contrast study of the vascular system, and includes arteriography, venography (the venous phase of an arteriogram), cardiography, and lymphangiography (iodinated oil is injected into the lymph vessels on the dorsum of the hand or foot, and x-rays are taken of lymph nodes after a delay).


Myelography involves the injection of radiopaque dye into the subarachnoid space to examine for cord or nerve root compression.


Computerized tomography (CT scanning):

• uses X-rays to produce planar images that resemble (mostly transverse) anatomical sections. The x-ray tube and detectors (electronic x-ray sensors or receivers that replace the film tray of conventional radiography) move in an arc or circle around the body, assigning each volume element or unit (voxel) an absorption value or attenuation number (expressed in Hounsfield units). The attenuation number of each voxel in the mosaic slice is converted to a pixel of a particular gray-scale value on a television monitor screen. The picture produced is equivalent to an x-ray of an anatomical slice of the living patient: air is black, bone is white, soft tissues as a variety of gray tones. Radiologists initially referred to the transverse sections produced as “transaxial sections” since they intersect the axis of body and limbs at right angles; unfortunately, this has been shortened to “axial” sections, which is actually a contradictory term since they are perpendicular to the axes. By convention, transverse sections are viewed inferiorly, as if you were standing at the foot of the bed of a supine patient, looking toward their head

• Three-dimensional CT: 3-D CT images are created through computerized “stacking” of contiguous CT slices. No additional scanning is required. With suitable software, the 3-D model can be rotated, sliced and “dissected”

Magnetic resonance imaging (MRI):

• A noninvasive technique which does not use ionizing radiation; it has no known health hazard. Patients are placed in the bore of a powerful magnet which aligns the body’s free protons (hydrogen atoms in fat and water molecules) with the magnetic field, like a compass needle in the earth’s magnetic field. Radiowaves of a particular radiofrequency (RF) are passed through the body in a particular sequence of very short pulses, the energy of which excites the protons, and they flip out of alignment. The hydrogen atoms eventually flip back to become realigned (“relax”); as they do, they emit the radiofrequency wave they absorbed. The distribution of the emitted radiofrequency waves is mapped by computer to produce images on a monitor. A variety of techniques or algorithms (spin-echo sequences) can be applied to enhance the visualization of difference tissues and disease processes which involve altering the pattern in which the radiopulses are administered (repetition time or TR) and the signals are returned (echo time or TE). Thus MRI is in essence a display of where the body’s fat and water is and is not located. Thus various body tissues emit characteristic MR signals: well-hydrated tissues, such as fat and brain, emit strong signals (have high signal strength) and appear white; relatively dehydrated tissues (eg, compact bone) emit little or no signal and appear black; moderately hydrated tissues, such as muscle, appear in a variety of gray tones. Usually rapidly-moving blood appears black because the it has moved out of the section being imaged by the time it emits its RF signal, being replaced with blood which was not subjected to the exciting signal.

• One of the great advantages of MR over CT imaging is that MR is able to produce primary images in almost any plane. Further, greater differentiation of soft-tissue structures, such as between the gray and white matter of the CNS, is possible with MR. MR is also able to demonstrate blood vessels without the use of contrast media.

Ultrasonography (US):

• Like MR, US does not utilize ionizing radiation, and most US procedures are non-invasive. US uses the same principles as sonar: recording the attenuation of pulses of high frequency sound (ultrasonic) waves as they are reflected, slowed, or freely transmitted during their attempt to pass through the body, demonstrating especially the reflection which occurs at the interfaces between the organs, tissues and substances of the body. Among its advantages are its low cost, the relatively compact, portable nature of the machine—allowing its use in the office and in the operating room -- and especially its ability to demonstrate the body’s structure in “real time”, i.e., in motion. Through the Doppler effect, it also yields information about the flow and velocity of blood. Thus it has become especially useful in cardiology. Small transducers have been developed which enable trans-esophageal, trans-vaginal and trans-rectal US for “up close” examination of surrounding structures. Because of its non-ionizing, non-invasive nature, it is especially suited for obstetrical study of the gravid uterus, placenta and developing fetus