Quiz 1 Flashcards
properties of x-rays
a type of electromagnetic radiation, product of electron interactions, travel at the speed of light, invisible, cannot be felt, can penetrate matter, can ionize atoms
energy characteristics
composes all electromagnetic (EM) radiations, amount is inversely related to wavelength, unit is electron volt (eV), EM > 15 eV (low) can produce ionization energy in living cells
most diagnostic x-rays
in the kilovolt range (keV)
what is an ion?
an electron ejected out of its shell
ionization of DNA can result in?
increased rate of mutation, rate of abortion/fetal abnormalities, susceptibility to disease, risk of cancer, risk of cataracts
radiation safety: ALARA
as low as reasonably achievable
roentgen
a unit of exposure
rad/gray
a unit of absorbed dose
rem/sievert
a unit of dose equivalent
1 rem = ?
1,000 mrem
what is the radiation dose limit set by the NRC?
T/F: lead protective gear protects you from the primary beam
false: lead protective gear only protects against weaker scatter radiation
how are x-rays produced?
high speed electrons are hurled at a metal target
what are the main components needed in an x-ray machine?
cathode/filament, anode/target, focal spot, generators
cathode/filament
electric current is passsed through to generate an electron cloud that is measured in milliamperes (mA)
the filament is made of which material?
tungsten
anode/target
x-rays are produced here via a voltage potential (Vp) that accelerates the electron cloud towards itself
what are the electrical charges of the cathode/anode?
negative at the cathode/positive at the anode
the anode is made of which material?
tungsten
what are bremsstrahlung x-rays?
slowing or ‘breaking’ of the electrons as they pass the atom
what is so important about kVp and mA?
to penetrate an object in order to detect the x-ray on the opposite side of the object, adequate energy and sufficient quantities are needed
what is kVp?
quality of the x-ray
what happens when kVp is increased?
the energy of the x-ray photon increases and the number of x-rays (due to more interactions within the anode)
what are mA and s?
quantity of the x-ray (the product is mAs); the current that passes through the filament
what subsequently happens if the time of x-ray exposure is increased?
the number of x-rays produced is also increased
five types of radiation effects?
photoelectric effect, compton effect, coherent scattering, pair production, photodisintegration
what is the photoelectric effect?
occurs when the x-ray photon (energy) is totally absorbed and no scatter radiation is produced in which characteristic x-rays form (responsible for patient’s absorbed dose)
which effect is the key for the generation of the radiographic image?
the photoelectric effect; the probability is proportional to Z^3
in regards to the photoelectric effect, which will have better absorption of x-rays; fat or bone?
bone; following the Z^3 rule: fat - mostly carbon (C12) bone - mostly calcium (C20) 12^3 = 1,728 20^3 = 8,000
in regards to the photoelectric effect, what happens if the energy is too high?
the effect is reduced, resulting in a loss of contrast with the image
what is compton scattering?
it’s similar to the photoelectric effect but the incoming photon energy is not completely absorbed and therefore is responsible for scatter radiation (an orbital electron is ejected and an energy reduced photon continues on)
does compton scattering contribute to the radiographic image?
no, but it does contribute to film fogging and exposure to the patient and surrounding participants
how likely is it that compton scattering will occur?
the probability is independent of Z, but is proportional to physical and electron density of the object
how is a radiograph produced?
when x-ray beams pass through a target (patient) and expose photographic film
what physically makes the radiographic image?
exposed photographic emulsion; the x-ray film is similar to photographic film, the emulsion contains silver halide crystals that precipitate as elemental silver (black) when exposed to x-rays, unexposed emulsion is washed away
what factors contribute to the ‘blackness’ of a film?
increased kVp, increased mAs, decreased focal spot-film distance, heel effect
how do you increase (double) film blackness?
double the mAs or increase kVp by 15%
how do you decrease (half) film blackness?
half the mAs or decrease kVp by 15%
in regards to focal spot-film distance, what is the inverse square law?
I1/I2 = (d2)^2/(d1)^2
how does changing focal spot-film distance change mAs?
mAs is proportional to I
a specific focal spot-film distance is chosen to accomplish what?
avoids high mAs and preserves radiographic detail
what is the standard focal spot-film distance?
40 to 60 inches
what is the heel effect?
due to a portion of the x-ray beam being absorbed by the anode that results in an x-ray beam that is less intense on the anode side than the cathode side
in regards to the heel effect, how should the patient be positioned under the x-ray beam?
always place thicker body parts under the cathode side to give a uniform exposure across the radiograph
what factors affect image detail?
motion, film speed, focal spot size, focal spot-film distance, object film distance, intensifying screens, grids
what does motion look like on a radiograph?
reduced image sharpness
how do you correct a motion artifact?
decrease exposure time, use sedation or general anesthesia, use a regular (non-detail) screen/film combination, reduce grid ratio, reduce focal spot-film distance
what is the focal spot?
a function of the actual size of the cathode and the angulation of the anode surface in which the large filament of the two (the other being the small one) is able to produce a larger number of x-rays (decreases sharpness of the image)
what is the object film distance?
the distance from the object (patient) to the film; as it increases, magnification occurs and image detail is reduced
what is the purpose of the intensifying screen?
they convert x-rays into visible light through phosphorescence since the film emulsion is insensitive to x-rays compared to visible light
what material are intensifying screens made of?
rare earth screens are more efficient at producing visible light
what is a grid?
a plate containing rows of lead strips that is placed between the patient and the cassette (film and screens) in order to reduce scatter radiation (more rows = more effective but must compensate 2-3x mAs)
what is the purpose of a bucky?
a device that moves the grid during exposure (since a stationary grid results in lines where the lead strips are)
when are grids necessary?
scatter radiation increases as patient size increases, but not always necessary; 10 cm use grid
what is contrast?
the opacity difference between adjacent areas on a radiograph
what is the scale of contrast?
the number of density gradiations between the lightest radiograph shadow and the darkest radiograph shadow
what is meant by an image having a lot of contrast?
it has a short scale of contrast; there are only a few shades of grey between the lightest and darkest areas of the radiograph
what is meant by an image having a low contrast?
it has a long scale of contrast; there are many shades of grey between the lightest and darkest areas of the radiograph
how do you achieve a short scale of contrast?
high mAs and low kVp
when would you want a short scale of contrast on a radiograph?
when taking abdominal radiographs
how do you achieve a long scale of contrast (also referred to as latitude)?
low mAs and high kVp
when would you want a long scale of contrast on a radiograph?
when taking thoracic radiographs
what factors affect contrast?
subject contrast, film contrast, fog/scatter
what composes subject contrast?
thickness, density, atomic number differences, x-ray beam energy
what composes film contrast?
inherent property of the film
in regards to contrast, what composes fog and scatter?
the use of grids, avoid high temperature/visible light/pressure, expiration dates
how can films be processed?
automatic processing, manual processing (developing (deposits), fixing (removes), washing)
what are some examples of imaging modalities?
radiography, ultrasonography, nuclear scintigraphy, computed tomography, magnetic resonance imaging
what is nuclear scintigraphy?
the patient is injected with a substance that becomes the source of radiation that is picked up by a dectector (most commonly done to see portosystemic shunts or feline hyperthyroidism)
what are roentgen (imaging) signs?
a visual clue or signal that is produced by a pathologic change of tissue and may be detected in a medical image
what characteristics do roentgen signs hold?
size, shape, margin, opacity, number, location
what substances can create different opacities on an image?
gas, fat, soft tissue/fluid, mineral, metal
what errors can occur with test interpretations?
false positive and false negative diagnoses
what does an error mean for the patient?
inappropriate therapy, euthanasia, untreated disease
what is the mach phenomenon?
mach lines are false white or black lines that appear at sharp boundaries (they are optical illusions that enhance boundaries but can mimic lesions)
how does the contrast background effect result?
the eye is a poor judgement of absolute brightness
how can vision be explained?
what the eye sees (reality)
how can perception be explained?
what the brain sees (how we interpret it); previous experiences influence it and visual signals are compared to memory in which the most likely explanation is ‘seen’
what is a contour?
perceived when there is change in brightness or color between adjacent objects, can occur without changes (results from mental completion of partial lines to form something expected
what are the 5 opacities seen on radiographs?
gas, fat, soft tissue/fluid, bone (mineral), metal
what determines the opacity?
molecular structure, atomic elements, thickness, density, manifested as ‘shades of grey’ on a film or computer monitor
what is the summation sign?
an area of increased radiopacity (‘whiteness’) occurring when two overlapping objects are in the path of the x-ray beam and are not in contact with one another
what is the silhouette sign?
margins of two objects cannot be distinguished because of similar opacity and margins are in contact
how should radiographs be oriented when viewing them?
head to the left, tail to the right, dorsum on top, ventrum on bottom, right side of the patient on the left, left side of the patient on the right (no convention for medial/lateral)
how are radiographs named?
according to the direction of the primary x-ray beam as it penetrates the area of a body, from point of entrance to point of exit
what about oblique or special views?
should be named in the same method used in naming the standard views, the angle is measured and then added after the first directional designation
what is tomography?
a type of imaging that depicts a slice of the body free of superimposition by overlying structures (includes US, CT, MRI)
what is a pixel (picture element)?
a tiny two dimensional square that represents the average number of a voxel of tissue
what is a voxel (volume element)?
a three dimensional block of tissue that is represented on an image matrix by the pixel (determined by the product of the pixel size and the thickness of the scan slice)
what does a voxel on an MRI represent?
the signal intensity that depends on the respective transverse magnetization
what does a voxel on a CT represent?
the hounsfield unit that depends on the linear attenuation coefficient of tissue
when is MRI used?
for soft tissue evaluation, requires anesthesia
when is CT used?
for bone and lung evaluation (gold standard for thoracic metastasis), requires sedation or anesthesia
how is a CT image generated?
x-rays are attenuated as they rotate around the patient and the computer reconstructs the image using voxels to create the hounsfield unit (HU) to be converted to a grey scale final image
what kind of contrast does a wide (1,000) window achieve?
long scale of contrast/low contrast, shows all subtle shades of grey
what kind of contrast does a narrow (250) window achieve?
short scale of contrast/high contrast, helps accentuate subtle differences
how are lesions described using CT?
hypodense/hypoattenuating - black
hyperdense/hyperattenuating - white
which diagnostic imaging technique is the gold standard for checking for metastasis, especially in the thorax?
CT
what is the normal view of a CT scan?
can only image in one transverse plane, other planes are reconstructed
what are the advantages of CT compared to conventional x-rays?
no superimposition of overlying structures, multiple imaging planes (via reconstruction), superior soft tissue differentiation (fluid vs. soft tissue), manipulation of grey scale, improved lesion detection, guided biopsy possible
what are the advantages of CT compared to MRI?
normally easier to interpret, faster and cheaper, guided biopsy possible
what are the disadvantages of CT?
general anesthesia or sedation needed, radiation exposure, limited referral centers, less soft tissue (compared to MRI)
name some examples when CT might be used?
surgery planning, radiation therapy planning, excellent for diagnosis/ treatment of thoracic metastasis checks, thoracic disease, nasal/oral cavity disease, elbow dysplasia, brain disease (MRI), spinal disease (MRI)
what does MRI stand for?
magnetic resonance imaging; strong magnet (0.5-3.0 Tesla)
how does an MRI machine work?
the patient is placed in a strong magnetic field in which protons of hydrogen atoms align with, a pulse is applied to tip over the protons and a signal is released once they relax (measured and converted to a grey scale)
how are lesions described using MRI?
most lesions result in increased tissue water, the level of the signal is described in terms of intensity
hypointense - black
hyperintense - white
what are the T1 characteristics?
(hypointense) bone, fluids –> grey matter –> white matter –> fat (hyperintense)
what are the T2 characteristics?
(hypointense) bone –> white matter –> grey matter –> fat –> CSF (hyperintense)
what are the advantages of MRI?
best low contrast resolution, no ionizing radiation used, direct multiplanar imaging can be obtained
what are the disadvantages of MRI?
based on complex physics, cost, time consuming, focal areas should only be imaged, no ferromagnetic metals can be used, anesthesia and monitoring needed
name some examples when MRI might be used?
neuroimaging (brain/spinal/nerve disease), musculoskeletal disease, tumor staging
when is T1 MRI used?
best for anatomical overview, fat is bright/free fluid and edema are dark, contrast studies using gadolinium IV which is hyperintense
when is T2 MRI used?
for diagnosis of pathology, fat is intermediate grey, free fluid and edema are bright
what is STIR?
inversion recovery images with short TI, fat is selectively suppressed (void signal as black), fat is black/free fluid and edema are white (distinguishes fat from fluids/edema which makes lesions more obvious)
what are FLAIR sequences?
fluid attenuated inversion recovery, pure fluids are selectively suppressed (void signal as black), fat is grey/white/free fluid is black and edema is white (distinguishes free fluid from edema)
