13. Radiographic image quality and contrast media Flashcards
what does absorbed radiation create on the image in terms of appearance and brightness
increased brightness and white areas
what does transmitted radiation create on the image in terms of appearance and brightness
decreased brightness and black areas
what are the 3 factors in terms of anatomy visibility
brightness
contrast
signal to noise ratio
what are the 3 factors in terms of how accurately the image represents the anatomy (sharpness)
sharpness/accuracy of structural lines
spatial resolution
sharpness
distortion
how is image brightness evaluated
2 things
by the amount of luminance/light emission of the display monitor
the amount of density or overall blackness after processing
what is image brightness
amount of luminance/light emission from an image or display
what does brightness depend on
2 things
absorption and transmission of x-ray through tissue
what is the exposure intensity
amount and energy of x-rays reaching an area of the IR
the exposure intensity must be high enough to__ and low enough to __
high enough to overcome quantum noise
low enough to minimise patient dose
what causes noise in terms of primary exposure factors
mAs too low = grainy images
what is image contrast
differences in brightness/density levels to differentiate between anatomic tissues
the range of brightness levels/contrast is a result of what
the tissues differential absorption of the x-ray photons
what is subject contrast
what 2 things is it the result of
different densities in the patient
result of absorption characteristics of the anatomic tissue radiographed and the quality of the x-ray beam
what is the difference between the gray scale and scale of contrast
gray scale = number of different shades of grey that can be stored and displayed in a digital image
scale of contrast = range of densities visible on film
higher contrast results from what
great differences in radiation absorption between tissues that vary greatly in composition
what is contrast resolution
ability of IR to distinguish between tissues with similar subject contrast
what is spatial resolution
how much system is able to give info about detail by distinguishing between different structures even though the structures might have similar appearance to each other
what does spatial resolution refer to
the smallest object that can be detected in a digital image
what is sharpness/spatial resolution
accuracy of representing anatomical structural boundaries
what 2 things affects the sharpness/spatial resolution of an image
focal spot size
magnification
what is distortion
what 2 features of the anatomic part is involved
misrepresentation of either the size or shape of the anatomic part
what are the 3 things that can affect distortion
CR alignment or the x-ray tube
CR alignment of the part
CR alignment of IR
what are the 2 types of shape distortion that can occur
foreshortening
elongation
what is shape distortion
change in the imaged proportions of an object
what 3 things is shape distortion affected by
orientation and alignment of object, IR and central ray
how is the size and number of pixels in the matrix related to the spatial resolution
larger number of pixels = greater spatial resolution
smaller pixel size = capture smaller entities and higher resolution
anatomy must be aligned how to avoid shape distortion
in terms of the IR and x-ray beam
anatomy must be parallel to the IR plate and perpendicular to the x-ray beam
what is size distortion and what does it affect
magnification
affects accuracy of representing anatomical structural boundaries
what 3 things is size distortion due to
longer OID
shorter SID
larger focal spot size
what should be done to the SID and OID to get rid of sharpness
shorter OID
extend/larger SID
what is motion unsharpness
decrease in sharpness resulting from voluntary or involuntary patient motion
what is scatter
unwanted exposure to the image receptor resulting in fog
what is scatter the result of
compton interactions
what does scatter and fog do to the image contrast
decreases contrast
what do you reduce scatter via the primary exposure factors
decrease kV
what is quantum noise
what is it dependent on
visible as brightness or density fluctuations and is photon dependent
too few x-ray photons results in increased quantum noise
what is contrast media
group of chemical agents developed to aid the characterization of pathology by improving the contrast resolution of an imaging modality
what are the 3 types of contrast administration
swallowed/orally
enemia/rectally
intravenously/intra-arterially
what is a barium enema
what structures does it visualise
into large colon to see ascending, transverse and descending colon
what are positive contrast in terms of what it does to the attenuation
increases attenuation
what are 2 examples of positive contrast
barium, iodine
what may need to be done when using positive contrast
may need to adjust exposure factors
what are negative contrast in terms of what it does to the attenuation
decreases attenuation
what is an example of negative contrast
air
do exposure factors need to be adjusted when using negative contrast
no
why can contrast agents be seen on images
due to differences in tissue density (increased x-ray absorption based on higher density)
