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