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117 Cards in this Set
- Front
- Back
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What are X-rays?
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A form of electromagnetic radiation (EM)
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What makes x-rays different from other forms of EM radiation?
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Their wavelength!
** X-rays carry much more energy than visible light or radio waves |
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What is electromagnetic radiation?
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Alternating electric and magnetic fields that propagate space
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Ionizing radiation
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They can knowck electrons out of their shells around atoms and break chemical bonds
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How are x-rays produced?
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When high energy electrons bombard a substance with a high atomic number (certain metals), that energy is converted to (released as) x-rays
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Bremsstrahlung radiation (breaking radiation)
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* When a beam of high energy electrons hits a metal target, these electrons feel the pull of positively charged protons in the target nuclei and rapidly decelerate to change course
* As they decelerate, the energy they carry is released * 99% of this energy is released as heatm and 1% is released as x-rays |
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Why do x-rays produce a "polychromatic" beam?
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because electrons will decelerate to dofferent degrees based on how close to a nucleus they travel, they will give off different amounts of their energies and produce x-rays of different energies
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Most x-rays are formed by wghat process?
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Bremsstrahlung (breaking) radiation
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How does an x-ray machine turn electricity into a beam of x-rays?
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Every x-ray machine has basically the same components which:
1. Produce a cloud of electrons 2. Accelerate those electrons with extremely high charge to give them high energy 3. Bombard a metal target with that high energy electrom stream to create x-rays |
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X-ray tube
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A glass tube with all internal air evacuated to keep those high energy electrons from colliding with air molecules instead of the metal target
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7 things found in an x-ray tube
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1. Cathode (the electron source)
2. Anode (target,focal spot) 3. Focusing cup 4. Tube housing 5. Filter 6. Collimator 7. Control panel |
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What is the cathode found in x-ray tube?
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** The electron source
* A small coil filament of tungsten metal which has milliampere current (mA) of electricity pass through it. * The filament becomes very hot and electrons are "boiled off" and surround the filament as an electron cloud (strong neg. charge) |
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What determines the number of x-ray photons created?
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* The number of electrons boiled off the cathode
* Higher mA = more electrons = more x-rays |
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What is the anode found in x-ray tube?
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* Target, focal spot
* A disc of tungsten metal which develops a very high positive charge when a high volatage (kV) is suddenly applied across the tube * When electrons strike the anode, x-rays are produced |
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Focusing cup
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A negatively charged cup that surrounds the cathode filament to keep the electron cloud tightly packed and to prevent the electron beam from fanning out
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Characteristic radiation
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* When an electron in a high energy beam strikes an inner shell orbital electron from the metal target, the orbital electron is knocked out of its shell
* One of the electrons from a higher shell will replace the lost electron and gives up its extra energy as an x-ray * The energy of the x-ray released is exactly equal to the energy difference between two shells, which is a cha"racteristic" amount for a specific metal |
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Difference between x-rays and gamma rays
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* X-rays are made outside of the atom nucleus
* Gamma rays are emitted from a decaying nucleus |
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What is the tube housing found in x-ray tube?
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Glass envelop and loter lead housing to shield everything but the window from x-rays
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What is the filter found in an x-ray tube?
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removes low energy x-rays from the beam
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What is the collimator found in an x-ray tube?
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An adjustable lead shutter device which restricts the size and shapre of the x-ray beam
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What is the control panel found in an x-ray tube?
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allows crucial adjustment of the exposure parameters
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What determines the number of x-rays in exposure?
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mAs
* The product of mA times the seconds * Adjusting either the mA or seconds will adjust the number of x-rays in the exposure |
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Why should you select a technique with the higest mA and lowest seconds possible?
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To reduce the chance of movement
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kVp control
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* Kilovoltage peak
* voltage with which the electrons are accelerated toward the anode |
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What determines the maximum energy of x-rays in the exposure?
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kVp
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What are 3 possible fates of a single x-ray photon when it enters a patient's body?
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1. Complete transmission
2. Complete absorption 3. Scatter |
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Complete transmission
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the x-ray photon passes right through the patient in a straight line and results in a black speck on the film
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Complete Absorption
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the x-ray photon strike an electron the patient's body and all of the x-ray energy is consumed in the interaction.
* This results in NO black speck on the film |
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Scatter
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The x-ray photon strikes an outer shell electron in the patient's body and is diverted from its original path
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What is the enemy of radiography?
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Scatter- it reduces radiographic contrast
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3 factors that affect the production of scatter
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1. Size of the collimated field
2. kVp 3. Thickness of the body part |
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How does the size of the collimated field affect the production of scatter?
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* Larger exposure fields create more scatter radiation
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How does the kVp affect the production of scatter?
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Higher kVP creates more scatter
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How does the thickness of a body part affect the production of scatter?
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Thicker body parts cause more scatter
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What is a technique chart?
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A table with predetermined x-ray machine settings that enables the radiographer to select the correct machine setting based on the thickness of the animal.
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What is a radiographic filter?
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A sheet of metal placed against the window of x-ray tube
* Usually built into the machine |
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What is the purpose of a radiographic filter?
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To the low energy x-ray photons before they hit the patient
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What is a grid?
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A plate composed of hundred of alternating thin lead strips with aluminum or fiber interspaces focused at the x-ray beam anode
* Grid is placed directly in front of the cassette (can be built into machine) |
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What is the purpose of a grid?
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To increase contrast of the film by reducing the amount of scatter radiation that hits the film
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How does a grid increase contrast of the film by reducing the amount of scatter radiation that hits the film?
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because the thin lead septa are focused to the anode, they allow primary x-ray photons traveling alonf their path pass.
* Scattered photons, origination in the patient, travel in other directions and many will be absorbed as they hit the lead septa obliquely |
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When should a grid be used?
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Anytime a bodypart over 10cm is radiographed
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Does the use of a grid affect technique settings?
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YES!
* SInce some of the primary photons are also absorbed by the grid, fewer photons are available to make the image.....so more are needed by increasing the mAs |
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grid ratio
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Height of the lead strips divided by the distance between them
Common ratios: 5:1, 8:1, and 12:1 |
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Bucky Factor
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Amount the mAs must be increased to account for the grid
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Common grid ratios and their bucky facors
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* 5:1, bf of 2
* 8:1, bf od 3 * 12:1, bf of 4 |
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What is a cassette?
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A rigid, light tight container that holds the x-ray film for exposure
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What are intensifying screens?
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Thin sheets lining the inside of a cassette which emit visible light (blue or green) when struck with x-ray radiation
* The film is sandwiched between these screens |
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Why do we use intensifying screens?
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To decrease the amount of radiation the patient is exposed to
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What is the downside to using an intensifyinf screen?
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Since light photons diffuse a small distance from their creation point in the screen, the radiographic detain is slightly reduced
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Is radiographic film ever used without an intensifying screen?
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Yes- when very fine detail is required
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What is radiographic film?
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A sheet of plastic with a coating of photographic emulsion on both sides.
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What is the key ingredient in the photographic emulsion found on radiographic film?
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silver halide
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What is the SPEED of radiographic film?
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The ability of a film to make an image from a certain number of x-ray photons
* The greater the ability to make an image with a giver number of protons, the greater the speed |
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What is the result of increased speed of radiographic film?
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Decreased detail
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"film fog"
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* Exposure of film to extraneous light and x-rays will cause "fog," which is an increase in background film density (blackness)
* Film fog reduces contrast, which makes the image less diagnostic |
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6 sources of film fog
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1. Light leaks in the dark room
2. Improper safelight intensity, distance of filter color 3. Cassettes placed to close to x-ray beam 4. High heat and humidity 5. Pressure 6. Old developer solution |
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What 4 things should be permanently imprinted on all radiographs?
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1. Hospital name
2. Date 3. Owner's name 4. Animal ID * Radiographs are legal documents |
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What does optical density mean?
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How black the film is
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What controls optical density?
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The number of photons that hit the film
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What 4 things control the number of photons that hit the film and ultimately the optical density?
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1. mAs
2. Focal Distance 3. kVp 4. Subject density |
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What is the MAJOR determinant of film blackness?
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mAs
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How to correct film blackness by adjusting the mAs?
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* To increase film blackness: increase mAs
* To decrease film blackness: decrease the mAs * These changes are usually done on the order of 100% |
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Why does changing the distance of the x-ray tube from the cassette have a direct film on film blackness?
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Because of the diverging x-ray beam, moving the tube closer to the cassette will result in more photons hitting the film
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What determines the enrgy of the x-rays?
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kVp
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Corrections in kVp to affect film blackness tend to be what?
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10-15%
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What is the heel effect?
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The phenomenon in which x-ray beam is of lower intensity on the side of the beam towars the anode
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Why does the heel effect happen?
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x-rays are absorbed as they pass through the anode metal, so there are fewer photons available to make the film black on the anode end of the x-ray beam
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What does the heel effect look like?
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the film will be slightly lighter on one end than the other
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Is the heel effect apparent on all films?
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No, it is most apparent when a low level exposure is made on a long, thin body part
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What is contrast?
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how much black and white
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What is latitude?
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how many shades of gray
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What is the appearance of a film with high contrast/low latitude?
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Black and white with very few shade of gray
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What is the appearance of a film with low contrast/high latitude?
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Many shades of gray with little black and white
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What controls contrast?
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kVp
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Guideline for high contrast studies
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Low kVp and high mAs
* Ex: orthopedic films |
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Guidelines for low contrast studies
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High kVp and low mAs
* Ex: thoracic films |
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5 basic radiographic opacities
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1. Air
2. Fat 3. Soft Tissue 4. Mineral (bone) 5. Metal |
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3 things that determine radiopacity
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1. Atomic #
2. Physical density 3. Thickness of tissue |
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Summation
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When 2 structures overlay the same path of the x-ray beam and do not touch each other, their radiopacities will combine while preserving visibility of their individual margins
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Silhouette Sign
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This sign is in direct contact to summation. When two structures of the same radiopacity are in contact, theur margins cannot be distinguished
* AKA Border Effacement |
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Distortion
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The artifical change in shape of an image due to one aspect of the object being a different distance from the cassette than another aspect
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3 general causes of a dark film
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1. overexposure
2. over development 3. excessive fog |
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2 general causes of a light film
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1. Underexposure
2. Underdevelopment |
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5 causes of loss of film contrast
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1. Fog
2. Lack of grid when needed 3. Film out of date 4. Improper film storage 5. Old developer |
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5 causes of loss of detail
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1. Patient, tube, or cassette motion
2. Increased object-film distance 3. Beam not perpendicular to cassette 4. Poor film-screen contact 5. Fast screens with large crystal size |
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5 causes of black artificats
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1. developer splashed on film
2. scratches 3. bending of film 4. static electricity 5. exposure of edge of film to light |
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5 causes of white artifact
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1. fixer or water splash
2. foreign object 3. not developed properly 4. emulsion scratches 5. poor film-screen contact |
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4 stages of film development
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1. Development
2. Fixed 3. Rinsed 4. Dried |
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4 stages of film development
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1. Development
2. Fixed 3. Rinsed 4. Dried |
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What is the purpose of film developer solution?
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To reduce all silver atoms within the exposed silver halide crystals to metallic silcer atoms (which are black)
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What 3 factors affect the amount of development?
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1. Time the film spends in the developer
2. Temp of the developer solution 3. Concentration of the developer solution |
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How does temp of developer solution affect the time the time the film needs to remain in the solution?
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Higher temp = less time needed
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Puropse of fixer solution?
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To stop the development process and remove excess development
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An x-ray is a form of ionizing radiation, which has a low but very real potential for causing what 2 things?
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Malignant cancer and Germ Cell mutation in patients and radiographers
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ALARA stands for?
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As Low As Reasonably Possible
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The ALARA principle
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A diagnostic radiographic study should always be obtained while keeping exposure to x-ray radiation as low as possible
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3 variables which can be controlled to reduce radiation exposure
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1. Time
2. Distance 3.Shielding |
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the mathematical unit used in radiation safety for describing radiation exposure to an individual
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The Rem (old unit) or the Sievert (SV, new unit)
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The maximum acceptable radiation per year to a non-occupationally exposed person for purposes other than personal medical diagnosis or therapy, regardless of age or pregnancy?
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0.1 Rem (1 mSv)
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An occupationally exposed pregnant woman can make x-ray exposures, provided she dose not receive an exposure in excess of what?
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A monthly limit of 50 mRem (0.5 mSv) or an amount equal to 500 mRem ( 5mSv) over the duration of her pregnancy
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The annual max permissible dose of radiation from occupational exposure?
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* 5 Rem (50mSv) for whole body
* 50 Rem (500mSv) for extrmities * 15 Rem (150 mSv) for lens and eye |
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3 methods that each individual should use to minimize their own radiation
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1. Increase distance between individual and the radiation souce
2. Reducing the duration of the radiation exposure 3. Using protective barriers |
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What type of lead approns must be worn?
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At least 0.25 mm lead equivalent
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7 Advantages of digital radiography
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1. Prompt image viewing
2. Improve patient throughput 3. Ability to perform image processing 4. Ability to perform digital measurements 5. Automatic filing 6. Automatic image backup 7. Enhanced ability to perform remote consultation |
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3 Disvantages of digital radiography
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1. Expense of equipment
2. Technical expertise requirements 3. Potential for increased radiation exposure |
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3 Types of digital detectors
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1. Computed Radiography (CR)
2. Direct Digital Radiography (DR, flat panel) 3. Charge Coupled Device (CCD) |
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Computed Radiography (CR)
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X-rays interact with a photostimulatable phosphor (PSP) detector
* The detector is read-out by a machine in a step analogous to film development = disadvatange because it prolongs time between exposure and image viewing |
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Direct Digital Radiograhphy (DR, flat panel)
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* X-rays interact with excitable molecules in the detector elements of the flat panel plate
* This produces an electrical current that is converted directly into a digital image |
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Charged Couple Device (CCD)
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* X-rays interact with fluorscent detectors that give off light which is focused through a lens and then captured by the CCD chip - can result in "veiling glare"
* Image can be viewed seconds after exposure |
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Ultrasound definition
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acoustic energy with a frequenct above human hearing
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Piezielectric effect
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electricity interacts with a crystal producing ultrasound
* The ultrasound wave moves through the tissues and is partially reflected back to the crystal * The ultrasound interacts once more with the crystal producing an electrical current that is used to construct an image based on the magnitude and timing of the electrical signals |
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Advantages of ultrasound
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1. Evaluate fluid and soft tissue structures
2. Evaluate blood flow 3. Real-time movement 4. Guide needle biopsies 5. Widely available 6. Relatively inexpensive 7. Portable 8. Generally doesnt require anesthesia |
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Major disadvantage of ultrasound
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Its accuracy is highly dependent of the competency of the ultrasonographer
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Computed Tomography (CT)
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An x-ray based imaging modalit in which a computer creates a slice based image based on numerous x-ray projections
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Bone appearance on CT vs. MRI
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CT: white
MRI: Black |
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What things in CT best for evaluating?
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* Bones
* Lungs * Nasal Cavity |
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Disadvantages of CT
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* Anesthesia needed
* Limited availability * Small size limit * Excess radiation * Expensive |
