INTRODUCTION TO PLAIN FILM
RADIOGRAPHY AND FLUOROSCOPY
1
Linda K. Petrovich, MD, and
E. Scott Pretorius, MD
PLAIN FILM RADIOGRAPHY
1. How do diagnostic x-rays differ from other kinds of electromagnetic radiation?
X-rays have higher frequencies and shorter wavelengths than visible light, microwaves, and radio waves. Diagnostic
x-rays typically have energies between 20 keV and 150 keV. Gamma rays, which are emitted by many agents used in
nuclear medicine, have even higher frequencies and shorter wavelengths than x-rays.
2. What are the components of an x-ray tube?
An x-ray tube contains a negatively charged cathode and a positively charged anode. The cathode contains a filament
(usually made of coiled tungsten wire) that is the source of electrons. The electrons are accelerated toward the anode.
The anode contains the target (most commonly made of tungsten), where x-rays are produced.
3. How are diagnostic x-rays produced?
In an x-ray tube, x-rays are produced when an energetic electron passes close to an atomic nucleus in the anode of
the x-ray tube. The attractive force of the positively charged nucleus causes the electron to change direction and lose
energy. The energy difference between the initial energy of the electron and the energy of the electron after it changes
direction is released as an x-ray photon. This process is called bremsstrahlung, which means braking radiation in
German.
4. What happens to most of the energy entering the x-ray tube?
Most (99%) of the electrical energy entering the tube is converted to heat, and tube heating is often the limiting
factor in how much the x-ray tube can be used. About 1% of the electrical energy entering the tube is converted to
x-rays.
5. What are the key parameters that can be manipulated on an x-ray generator?
The voltage across the x-ray tube (measured in kilovolts [kV]), the current flowing through the x-ray tube (measured
in milliamperes [mA]), and the exposure time (measured in milliseconds [ms]) can be manipulated. Current and
exposure time can be combined and expressed in milliampere-seconds (mAs).
6. What happens if kV is increased?
X-ray penetration increases, exposure increases (darker film), and contrast decreases. The maximal energy of the x-rays
produced is increased. Film contrast primarily depends on kV.
7. What happens if mAs is increased?
Increasing mAs means increased film exposure (more x-rays produced), which darkens the film. The maximal energy
of the x-rays produced is not changed.
8. How are plain film radiographs generated?
The patient is placed between an x-ray tube and a film cassette. The x-ray tube produces x-rays that pass through the
human body and are attenuated by interaction with body tissues. The film cassette contains film that is adjacent to
fluorescent screens. When the x-rays reach the film cassette, a photochemical interaction occurs between the x-rays
and the screen coated with fluorescent particles. The x-rays activate the fluorescent particles to emit light rays. The light
rays expose the photographic film, and an image is produced.
9. What are collimators, grids, and screens? Where are they?
From the x-ray tube, x-rays pass through a collimator, which consists of lead sheets (Fig. 1-1). Collimators narrow the x-ray
beam. X-rays travel through air to the patient and then through a grid, which consists of narrow lead strips separated by
plastic. The grid allows passage of x-rays that have passed directly through the patient (and contain useful information)
and stops scattered x-rays, which would contribute to noise and decreased contrast. After passing through the grid, x-rays
9
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10 INTRODUCTION TO PLAIN FILM RADIOGRAPHY AND FLUOROSCOPY
penetrate the film cassette, which in standard dual-
screen systems includes an intensifying screen, the film,
X-ray tube
and a second intensifying screen. The screens absorb
the x-ray photons and emit visible light. The light exposes Filter
the film.
Collimator
10. What is the inverse square law?
The intensity of the x-ray beam decreases with the
square of the distance from the x-ray tube. In other
words, if one doubles the distance between oneself
and the x-ray tube, x-ray exposure decreases by a
factor of 4. This concept is important in determining
x-ray exposure and absorbed dose.
11. What are the advantages and
disadvantages of a small focal spot?
The focal spot is the source of x-rays in the tube.
Smaller focal spots produce sharper images, but
larger focal spots can tolerate greater amounts of
heat. Small focal spots are used for mammography
Grid
(which has few exposures, but very sharp images are
required). Large focal spots are used in fluoroscopy
Screen-film
(for continuous exposure, but lesser resolution).
cassette
Regular diagnostic x-rays use focal spot sizes
between the sizes used for mammography and Figure 1-1. Apparatus for film screen radiography.
fluoroscopy.
Key Points: Radiographic Exposures
1. Increasing voltage (kV) increases the maximal energy of the x-rays produced, decreases the film contrast, and
increases exposure, making the film darker.
2. Increasing mAs increases the amount of x-rays produced, does not change the maximal energy of the x-rays
produced, and increases exposure, making the film darker.
3. The focal spot is the source of x-rays in the tube.
4. A small focal spot produces sharper images and is used in mammography.
5. A large focal spot can tolerate more heat and is used in fluoroscopy.
12. What are the five basic densities seen on an x-ray? How do they appear?
The five basic densities are air, fat, soft tissue, bone, and metal. Air attenuates very little of the x-ray beam, allowing
nearly the full force of the beam to darken the film, so air appears black. Bone and metals attenuate a large proportion
of the x-ray beam, allowing very little radiation through to blacken the film. Bone and metallic objects appear white on
radiographs. Fat and soft tissues attenuate intermediate amounts of the x-ray beam. They appear somewhere between
white and black as a spectrum of shades of gray.
13. How does computed radiography differ from film screen radiography?
In computed radiography, a phosphor plate is used to capture x-ray exposure information. Electrons are trapped in the
phosphor layer by the x-ray exposure, and the plate is read by a device that uses lasers to compel the electrons to
emit light. This pattern is electronically stored as a digital image. The ability of the observer to manipulate the brightness
and contrast of the digital image is a great advantage of this technique.
14. How do mammography film screen combinations differ from combinations used for
chest and abdominal examinations?
Mammography film screen combinations are slower than combinations used for chest and abdominal imaging. This
means that mammography film screen combinations are thinner and have higher spatial resolution.
15. What is the difference between a posteroanterior (PA) and an anteroposterior (AP) film?
The names of these views describe the path of the x-ray beam through the patient. In a standing PA chest radiograph,
the beam enters the patient from the back, and the image is acquired in front of the patient. For acquiring portable chest
radiographs, a cassette is placed behind the patient, and the x-ray beam is transmitted from the front, creating an AP
film. A similar naming convention is followed for all radiographic and mammographic images.
INTRODUCTION TO IMAGING MODALITIES 11
MAMMOGRAPHY
16. How does mammographic technique (kV and mA) selection differ from that selected
for chest and abdominal examinations?
Mammography uses lower kV (for higher image contrast) and higher mA (for shorter exposure times) compared with the
technique for chest and abdominal examinations.
17. What are the two standard views obtained in mammography?
The two standard views are medio-lateral oblique (MLO) and craniocaudal (CC). The direction of the x-ray beam is
defined by the name of the view. In CC views, the x-ray beam enters the cranial portion of the breast, traverses the
breast, and exits on the caudal side of the breast onto the film. By convention, metallic markers indicating view type are
placed closest to the axilla laterally on the CC view and superiorly on the MLO view.
18. Can any other views be obtained?
Yes. If an abnormality is seen on only one of the previously described views, or if it is seen on both views but needs
further characterization, or if it is unclear whether an apparent mass is real or just superimposed tissue, additional
views are often obtained. If a lesion is seen on only one of the two standard views (MLO, CC), a 90-degree medio-
lateral (ML) view may be helpful to localize the lesion. Rolled views may be performed in any projection, and tissue can
be rolled in any direction desired. This is done to position the area of a potential lesion away from adjacent tissue that
may obscure it.
19. How are spot compression and magnification views used in diagnostic
mammography?
Spot compression views are often obtained to determine whether a density presses out meaning that on a
compression view it is found to have represented a superimposition of normal breast tissue rather than an actual mass.
Magnification views are obtained to visualize calcifications better or to characterize a mass better.
20. How are women with breast implants imaged with mammography?
Four views of each breast are done:
" Two CC views one with the implant in the field of view and one with the implant displaced as much out of the field
as possible.
" Two MLO views one with the implant in the field of view and one with the implant displaced as much out of the field
as possible.
21. What are the two main tissue types in the breast? How do they appear on a
mammogram?
The two main tissues are fibroglandular tissue, which appears bright, and fat, which appears darker.
22. Why are single screens (rather than dual screens) usually used in mammography?
Dual-screen systems are more efficient in detecting x-rays and are used for most purposes. The two screens produce
more scatter than a single screen, however, and decrease image sharpness. Because sharpness is so important in
mammography to characterize calcifications, single-screen systems are used.
FLUOROSCOPY
23. What is fluoroscopy? How are fluoroscopic images obtained?
Fluoroscopy uses continuously emitted x-rays and allows real-time visualization of anatomic structures. An x-ray tube
located beneath the table emits a continuous x-ray beam that passes through the patient and falls onto a continuously
fluorescing screen and image intensifier located above the patient (Fig. 1-2). The fluorescing screen emits a faint light.
The emitted light is amplified electronically by an image intensifier, and the image is displayed on a television screen.
When an image of interest is identified, a radiograph may be obtained by placing a film between the patient and image
intensifier and exposing the film with a pulse of radiation.
24. Why are image intensifiers used?
Image intensifiers substantially decrease the amount of radiation needed to produce clinically useful images.
25. How do fluoroscopic conventions differ from radiographic conventions?
On fluoroscopy, things that are white on x-rays (e.g., bones or oral contrast agent) are presented as dark. Things that are
dark on x-ray (e.g., air) are presented as white on fluoroscopy.
12 INTRODUCTION TO PLAIN FILM RADIOGRAPHY AND FLUOROSCOPY
26. Discuss the contrast agents used
in fluoroscopic studies of the upper
and lower gastrointestinal (GI) tract.
Barium sulfate is the standard contrast agent for
routine fluoroscopic contrast studies of the upper
and lower GI tract. Thin, more fluid suspensions
are used for single-contrast studies. Thick, more
viscous suspensions are used for double-contrast
(air and oral contrast agent) studies. If perforation
of the GI tract is suspected, a water-soluble,
iodinated contrast agent such as diatrizoate
(Gastrografin) is used instead of barium because
of the high mortality rate from barium peritonitis.
Water-soluble agents are quickly resorbed through
the peritoneal surface. Aspiration of water-soluble
agents may result in chemical pneumonitis and
should not be used in patients with an increased
risk of aspiration.
Figure 1-2. Typical C-arm unit for fluoroscopically guided procedures.
27. Why is it important to obtain a scout film before administering a contrast agent in a
fluoroscopic study?
Objects that appear white on a radiograph include contrast agents and calcium. If a white object is seen after
contrast administration, it may be either the contrast agent or calcium. The way to differentiate is to see whether it was
present on the scout (precontrast) image. If it was present, it is calcium; if it was not present, it is the contrast agent.
Before administration of a barium enema examination, the scout film may help determine the adequacy of the bowel
preparation before the procedure. This scout film helps avoid nondiagnostic examinations because of retained stool.
28. Name four types of fluoroscopic studies and a possible clinical indication for each.
" Esophagogram: dysphagia (to rule out stricture or mass)
" Upper GI study: abdominal pain (to rule out gastric or duodenal ulcer disease)
" Small bowel follow-through: diarrhea or constipation (to rule out Crohn disease or other small bowel pathologic
conditions)
" Barium enema: rectal bleeding (to rule out a polyp or mass)
29. What is digital subtraction angiography?
Digital subtraction angiography is a technique in which a precontrast image is electronically subtracted from an image
obtained after intravascular contrast injection. This technique results in a greater contrast-to-background image
because background structures such as bone and soft tissues have been removed.
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