5. Making a Radiographic Image: Projection Geometry & Characteristics of Diagnostic Radiograph Localization
most radiolucent
air
radiolucent structures
air space foramen, canal, suture, fossa, pdl
radiolucent
absorbs little xray - most photons pass thru air space, foramen, canal, suture, fossa, pdl space
radiopaque
absorbs most xray radiation - few photons pass thru cortical bone, lamina dura, dentin, enamel, metal restorations
foreshortening
beam over angulation (improper vertical angulation) receptor not parallel to long axis of tooth
if film tilted and not parallel to object then
elongation
density of anatomy and density
increase density of anatomy will decrease density - lighter image
to decrease optical density/lighter image in terms of beam intensity/quantity
increase filtration increase collimation increase distance (inverse square law)
collimation and density
increase in collimation will decrease density - lighter image
concentration processing and density
increase in concentration will increase density - darker image
distance and density
increase in distance will decrease density - lighter image
exposure time and density
increase in exposure time will increase density - darker image
filtration and density
increase in filtration will decrease density - lighter image
kVp and density
increase in kVp will increase density - darker image increasing kVp will increase quantity and increase energy so more photons will pass through and will be darker
mA and density
increase in mA will increase density - darker image
temp film and density
increase in temp will increase density - darker image
time processing film and density
increase in time will increase density - darker image
to increase optical density/darker image in terms of beam energy/quality
increase kVp
to increase optical density/darker image in terms of beam intensity/quantity
increase mA increase exposure time
by decreasing SOD
increase magnification and decrease sharpness
if you decrease ORD
increase sharpness and decrease magnification
to decrease optical density/lighter image in terms of anatomy being imaged
increase thickness increase density of anatomy (g/cm3) increase atomic number
thickness and density
increase thickness will decrease density - lighter image
to increase optical density/darker image in terms of processing (for film)
increase time increase temp increase concentration
clinical applications of short ORD
place anatomy of interest closest to receptor intraoral imaging - challenge in paralleing technique cephalometric radiography - standardizes ORD = standardized magnification
atomic number and density
increase atomic number will decrease density - lighter image
factors affecting density
1. anatomy being imaged 2. beam energy/quality 3. beam intensity/quantity 4. film speed 5. processing (for film only) 6. artifacts
TUSDM intra oral radiography collimated extention
16 in
right angle radiograph to lateral cephalometric radiograph
PA cephalometric radiograph
projection geometry principle
SOD should be as long as possible
tube shift technique
Utilizes two radiographs with altered horizontal or vertical angulations SLOB rule - Same Lingual Opposite Buccal
xrays should originate from
a small focal spot
characteristics of diagnostic radiograph
anatomical accuracy detail radiographic density contrast
artifacts and density
artifacts will block some xrays so less density and lighter film
bone v muscles attenuation of xrays
bone attenuates more xrays than muscle of same thickeness
radioopaque structures
cortical bone lamina dura dentin enamel metal restorations
who determines focal spot size
decided by industry standard, so we dont have control over it
film processing (improper time, temp, chemical concentration) = ___ contrast
decrease
increase in filtration = ____ contrast
decrease (increase in mean energy)
when you move object closer to film/sensor
decrease magnification and increase sharpness
increase in kVp = ___ contrast
decrease, longer gray scale
scatter radiation (compton or coherent) = ___ contrast
decrease, produces fog and overall darkening
contrast ____ with increasing kVp (photon energy)
decreases
parallelism between the object and receptor
decreases distortion improves image sharpness
overall darkness of image
density
anatomic accuracy
depends on projection geometry focal spot size as small as possible xray source to object distance as long as possible object to receptor distance as short as possible detector plane parallelism xray beam axis perpendicularity
radiographic contrast
difference in densities between the light and dark regions in radiograph (aka range) result of attenuation/absorption of xray photons
at TUSDM do we use film or digital sensor
digital sensor since 2005
film speed and density
faster film speed will increase density - darker image
if tooth is tilted and not parallel to film then
foreshortening
change in angulation of beam results in
foreshortening of image
long axis of object not being parallel to receptor causes
foreshortening of image elongation of image
fast film speed = ___ optical density
greater
lateral cephalometric view = ___ contrast
high
what type of contrast do we want to detect caries
high
increase in density = ___ contrast
increase contrast within the range of optimal densities if image is too dark or too light, contrast of anatomic structures is diminished
short scale contrast
high contrast large or abrupt differences among optical densities low kVp
resolution
how well can you differentiate 2 very small objects close to each other
uses for localization
impacted or supernumerary tooth root canal foreign bodies fractures study for pathoses
moving object closer to receptor
increases sharpness decreases magnification of image
factors affecting radiographic contrast
inherent anatomical contrast (subject contrast) - thickness, density, atomic # beam energy/quality beam quantity density film processing scatter radiation
tube shift technique is limited to ___ radiographs
intraoral
SOD as long as possible achieved by
intraoral radiography - collimated extensions: 8in, 12in or 16in cephalmetric radiography - source object distance 5ft
xray tube with small focal spot size
intraoral, panoramic, CBCT: 0.4-0.7mm
magnification and sharpness have ____ relationship
inverse
kVp and contrast relationship
inverse low kVp = high contrast
biggest factor that controls contrast
kVp
right angle radiograph to PA cephalometric radiograph
lateral cephalometric radiograph
paralleling technique
less distortion - ideal
short object to receptor distance
less magnification sharper images
SOD should be as ___ as possible
long
long scale contrast
low contrast a small gradual change among different opical densities high kVp
moving source closer to object results in
magnification of image decreases sharpness of image
right angle radiograph to periapical
maxillary cross sectional occlusal
most radiopaque
metal restorations (amalgam)
bisecting angle technique
more distortion usefull in pts with severe gag reflex, small mouth.. etc
OFD
object to film/receptor distance
right angle technique
obtain 2 radiographs at 90 degrees to each other of area of interest ie periapical and occlusal radiographs ie lateral cephalometric and postero-anterior view of skull
radiographic density
overall degree of darkening a radiograph
long axis of an object should be ___ to the receptor
parallel
right angle radiograph to max cross sectional occlusal
periapical
central ray should be ___ to receptor
perpendicular
anatomical accuracy depends on
projection geometry
xray beam perpendicualrity to long axis
reduces geometric distortion
radiographic techniques in localization
right angle tech tube shift tech - SLOB rule, BO rule special studies
smaller focal spots produce
sharper image and higher spatial resolution
detail
sharpness and resolution focal spot size SOD OFD movement (tube or pt) film/receptor speed density
ORD should be as ___ as possible
short
radiographs are
superimpositions
elongation
teeth look stretched if object not parallel to film
SLOB rule
tube shift technique Same Lingual Opposite Buccal images of objects that are superimposed can be separated by changing the angle of projection object moves in the same direction as the xray tubehead - lingual object object moves in the opposite direction as the xray tubehead - buccal object
buccal object rule
tube shift technique when 2 different radiographs are made of a pair of objects, the image of the buccal object moves, relative to the same image of the lingual object, in the same direction that the xray beam is directed
how to gather 3D radiologic information
use 3D imaging modality CBCT techniques
if you decrease size of focal spot...
will result in sharper images and increased resolution
long source to object distance
x rays less divergent sharper images less magnification
SOD
xray source to object distance
reality of shadow casting
xrays originate from an area and not a point source xrays diverge from the point source 3D object is projectted on the 2D receptor resulting in unequal magnification and inaccuracy
principles of shadow casting
xrays should originate from a small focal spot SOD should be as long as possible ORD should be as short as possible long axis of an object should be parallel to the receptor central ray should be perpendicular to receptor