Diagnostic Imaging Flashcards
Describe the distance:inverse square law of radiology.
If the distance from the primary source is doubled, the intensity will decrease by a factor of four
What is the kVp?
- The potential difference between the cathode and the anode of the x-ray machine
- A higher potential difference leads to a faster acceleration of electrons, increases the number of x-ray photons produced, and increases the penetrating power of the photons
- Low values provide image contrast
What is mA
- Miliamperes
- Increasing the mA increase the number of electrons being produced at the filament
What is the focal spot?
The focal spot is the region of the target struck by electrons and is the site of x-ray production. The smaller the focal spot,the better the detail on the radiograph. A practical example of this principle is the sharpness of a shadow cast by a large versus a small light source. A shadow of an object cast by a small light source will be sharper than the shadow cast by a large light source. Therefore, radiographs produced using a small focal spot will have more detail than radiographs produced with a large focal spot. Angling the anode is one way to make the focal spot appear smaller than it really is, creating an effective focal spot, while at the same time maintaining a larger area being struck by electrons to facilitate heat distribution
Some x-ray machines allow the operator to select the size of the filament from which the electrons emanate. Use of the smaller filament will lead to a smaller effective focal spot. This is illustrated in the right side of Figure 1-14, where the width of the electron path from the filament has been reduced by 50% by using a smaller filament. As a result, the size of the effective focal spot would also be reduced. Reducing focal spot size is useful theoretically when enhanced image detail is necessary, as for radiographing a small bird or reptile, but most of the time only the large focal spot (large filament) is used. A disadvantage of using the small filament is that lower mA values must be used to prevent the filament from overheating and burning out.
What is mAs?
mAs is the product of mA and time in seconds. A variety of combinations of milliamperage and time can be used to produce the same mAs value and thus the same number of x-rays
What is X-ray transmission?
X-ray passing through matter without interaction
What is the photoelectric effect?
- Ionization occuring as a photoelectron is removed from its shell in the patient.
- An x-ray from the x-ray tube ejects an electron, usually from an inner shell (K shell) of a tissue atom. The incoming x-ray is absorbed completely in the atom. The ejected electron, the photoelectron, has sufficient energy to create additional ionizations in tissue.
- The photoelectric effect occurring in the patient should not be confused with collisional x-ray production in the x-ray tube. A characteristic x-ray is produced in both instances because an electron is ejected from an atom in each processand the void created by the ejected electron is filled by another electron leading to characteristic x-ray production. However, the photoelectric effect a photon ejects an electron from an atom in tissue, while in collisional x-ray production it is a high-energy electron that ejects the electron from an atom in the target of the x-ray tube. When the vacancy filled by the electron ejection is filled, a characteristic x-ray is emitted in both processes. In the case of the photoelectric effect this has has very low energy because of the low binding energy inner shell electrons in tissue versus collisional x-ray production where the characteristic x-ray is of much higher energy because of the much higher inner shell binding energy elections in tungsten, the target material in an x-ray tube.
What is the Compton effect?
Incoming photon ejects free outer shell electron from the tissue atom and the photon is scattered. The scattered photon has lower energy but may produce more ionizations, fog the film and is a radiation safety hazard
How should the use of the x-ray grid be compensated for?
Requires increased exposure by a factor of 2 to 3 to compensate for the portion of the primary x-ray beam absorbed by the grid
Describe the layers of the x-ray film.
- Protective coating
- silver halide emulsion
- Upon radiographing, the electron in the silver halide (silver bromide) is released by light energy which forms a silver atom at the sensitivity speck with a trapped electron to create the latent image. A developer will provide electrons to exposed crystals and allow the radiographic image to be seen. The resulting black colour on the film is the exposed silver crystals
- Polymer base
Describe computed radiography. What are the advantages and disadvantages of this radiographic method?
- Filmless, no chemistry, no darkroom required
- Image is captured in cassettes containing phosphor storage layer
- Laser beam in developer stimulates phosphor crystal to released stored light energy
- Cassette is put into laser film reader
- Computer generates digital image in DICOM format
- Image is sent and stored in server with PACS
- Advantages
- Time efficient, robust, provides good quality image
- Lower radiation does can be used
- Cost effective
- Disadvantages
- Laser reader is sensitive to dust
- Maintenance of moving parts
- Manual labour still required
Describe direct digital radiography.
X-ray photon strikes a selenium plate which creates an encoded charge that is transmitted to the computer for image reconstruction.
Describe indirect digital radiography.
X-ray photons strike a flat panel detector that creates visible light. Light photons stimulate a photocathode which is encoded as electrical energy and transmitted to the computer for image reconstruction
Describe the levels of radiopacities.
- Gas
- Fat
- Soft tissue or fluid
- Mineral
- Metallic
What is border effacement?
Two structures of the same radiopacity which are in contact will not be able to have their margins identified (their borders are effaced).
What are the Rotgen signs?
- Size
- Shape
- Opacity
- Decrease opacity of bone is indicative of bone destruction
- Location
- Number
What is the Focal spot-film distance (FFD)?
- The distance from the x-ray tube to the film
- As the FFD increases, film blackness decreases because the intensity of x-rays in the x-ray beam (x-rays/unit area) decreases
What is the Object-focus distance (OFD)?
The further away the to be imaged object is from the film, the more magnification there is, which is undesirable. Therefore we place patients as close to the cassette as possible.
What are the groups of contrast medium?
- Barium sulfate
- Iodinated compounds
Describe the use of Barium Sulfate as a contrast medium. What are the cautions for its use?
- Applications
- Oral for upper gastrointestinal study
- Rectal for barium enema study
- Outlines wall and lumen of GI tract
- Coats mucosa and provides mucosal detail
- Biologically inert, not hypertonic, metabolized or absorbed
- Provides a muco-protective coating
- Cautions for use
- Can cause granuloma or adhesions in peritoneal cavity
- Can be aspirated
- Dangerous if infiltrates a lesion in the GI tract- likely to cause a neoplasia
