DI Flashcards
the amount of current is corrected by
mA
the potential difference between the cathode and anode is adjusted with
- the kilovoltage peak KVp)
- increasing the KVp increses the potential difference between the filament and the target and the electrons are accelerated to higher velocities and have more velocity when striking the target.
discuss the effect of the size of the focal point on the quality of the image
small focal spot>>shapper/more detailed image
discuss the effect of the angle of the focal spot on the quality of the image
the smaller the angle,the smaller the effective focal spot
the type of xray interaction where all photons are completely absorbed
- photoelectric effect (PE)
- no scattered radiations is produced
explain the basis for the image production with photoelectric effect (PE)
- probability of the PE is directly proportional to the third power of the atomic # (Z3)–this is the basis for image production
- because it amplifies the differential absoption of x-rays between tissues ( otherwise there wouldbe no image contrast and all tissues would be of similiar radiographic opacity.)
- Pe is inversely proportional to the 3rd power of the photon energy (1/E3)
which xray interaction with the patient creates more scattering and exposure to the personnel
- Compton scattering
- most important film fogging (deteriorating of film quality)
explain how the campton scattering cause x-ray scattering
- incoming xray photon ejects an electron from an outer shell and is deviated and loses some of its energy
- the ejected electron may cause further ionization or fog the film.
- cs is independent of the atomic # (Z)
- CS can increase with increasing photon energy
- reduces image contrast where it predominates
which image generation is independent of the atomic number
campton scattering
the blackening of the film in analog radiography depends on
- the amount of xrays hitting the film
- the energy of the x-ray, thus the KVp since the high energy x-ray are more likely to penetrate the patient and strike the film
- it is also related to the focal film distance(FFD).increaseing the FFD, reduces the intensity of the xray beam and vice versa
what are the factors that increases scatter
- mainly the campton scatter
- increases with the size of patient
- size of exposed area
what are ways to prevent scatter
- expose as much as needed
- use a grid for any structure > 10cm
what is the grid ratio
height of lead stripes to width of the spaces between them
factor of increase in mAs that is needed to compensate for the absorbed radiation
grid factor
the type of artefact which occurs when increasing the distance between the object and film/cassette
magnification
what is the advantages of magnification
- improves resolution of small objects
- enlarged images easy ro interprete
- air gap eliminates some scatter
what are the disadvantages of magnification
- penumbra (blurring at the edges of a a structure on a radiograph) increase
increased FFD needs more output
- exacebate motion blur
unequal magnification of various parts of the patient due to differences in distance to the source
- distortion
- since our patients are 3 dimension,there will always be some distortion
- consistent radiographic positioning help recognising/ minimising distortion
how are the shades of grey in high contrast
few shades of grey
how are shades of grey in low contrast radiograph
- many shades of grey (wide lattitude)
image contrast depends on
- thickness
- physical density
- atomic #
- x ray beam energy
explain how the indirect flat panel detector system works
- light created by an x-ray in an intensifying screen as in analog radiography is an intermidiate step in image formation
- the screen is in contact with an array of pgotodioets
- the photodiotes convert the emited light into an elecreical signal
- readout electronics convert the electronic signal into a file
discuss how the direct flat panel detector system works
- the x ray strikes a photoconductor array made from selenium that absorb xray efficiently –>liberated electrons from the se forms a charge which is read and converted into an electronic file by the readout electronics.
discuss how the charged couple system(CCD) works
- a light senstitive CCD chip with an intensifying screen to capture the light emited by the screen when hit by xrays
- not commonly used
roentgen signs
- use to describe any structure visible on the radiograph
- Location
- –Number
- – Size
- – Shape
- – Margination
- – Opacity
define frequency
number of times a wave is repeated per second
define wavelength and write down the equation
- waveleng is the distance travelled by a wavesound in one sec.
- wavelength=/velocity /frequency
explain the relationship of wavelength and frequency at a constant velocity
- at a constant velocity(this is the case as sound travel though soft tissue), frequency is inversly related to wavelength
- thus high frequencies implies short wavelenghth vice versa
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define amplitude
amplitude refers to the strenght/intensity of the soundwave and decreased with increasing tissue depth.
discuss the impedence of a tissue
the aucostic impedance (z) of a tissue is the product of its physical density (p) and sound velocity (V) within.
Z=p*v
the difference in impedence between tissue determines how much sound is reflected and how much is transmitted further into the adjacent tissue
the amplitude of the echo that returns is proportional to the difference in impedence between tissue
a black, no signal/echo ultrasound image
anechoic
dark grey ultrasound image
hypochoic image
medium grey ultrasound image
echogenic
white ultrasound image
hyperechoic image
define Acoustic.impedance (Z)of a tissue
is the product of tissues physical density (ρ) and sound velocity (ν) within
- Z= ρxν
- The difference of impedance between tissues determines how much sound is reflected and how much is transmited further into the adjacent ti.ssue
- the differnce in impedencebetween tissues determines how much sound is reflected and howmuch is transmitted further into the adjucent tissue
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