oral radiology lab exam 1

density

overall degree of darkening of exposed processed radiograph

radiolucent

dark area from more exposure to film or sensor and passing through less dense structure

radiopaque

lighter area in radiographic image from less exposure to film or sensor and more dense structure

less dense tissue

radiolucent (dark) and allows more x-ray to pass through

more dense tissue

radiopaque (light) and absorbs more x-ray

factors affecting density

control, influence and number of x-ray photons that interact with image receptor

dependent on quantity of x-ray photons that interact

density

control factor of density

major factor

milliaperage-seconds (mAs)

output of step down transformer

mAs

mAs

limit/varying amount of current from AC power source and uses low voltage to heat x-ray tube filament

increases heat -> increase electrons -> increase photons -> increase density

mAs

x-ray been intensity is proportional to

mAs

i(1)/i(2) = mas(1)/mas(2)

influence factor of density (minor effect)

kilovoltage (kVp)

collimation

focal spot to film sensor difference

filtration

patient factor

exposure time

contrast

kVp

generates high potential differences and supply high voltage step-up

what happens to density when kVp increases

density increases

for density, an increase in kVp causes

increase in energy -> high potential difference -> more photons and increases density (darker image)

What happens to the x-ray intensity and energy as the voltage increases from zero to its peak value?

X-ray, intensity and energy increase, slowly at first and then rapidly as peak voltage is obtained

higher density

light image, radiopaque

low density

dark image, radiolucent

collimation

The restriction of the size and shape of the x-ray beam in order to reduce patient exposure. (

decreases in density due to reduction of fog: removal of scattered photons

collimation

focal spot to film sensor distance (inverse square law)

intensity is inversely proportional to the square distance

I(1)/I(2) = (D2)^2 / (D1)^2

FFD (focal spot to film sensor distance)

short FFD

has high intensity and increases in density

increase in FFD

decreased intensity

decreases density

decrease FFD

increases intensity

increases density

filtration on density

removal of low energy photons (decreases density to make darker image)

patient factors for density

thicker tissue -> more bean attenuation (absorbtion) lighter

dense tissue -> more dense = more attenuation -> lighter image

Subject thickness

The thicker the subject, the more the beam is attenuated ( more photons absorbed), and the lighter the resultant image

Subject density/mass

The greater the density of a structure within the subject, the greater the attenuation (more photons absorbed) lighter image

exposure time

interval of time during which x-rays are produced

increase in exposure time

increases quantity of photons

increases density (darker image)

contrast (quality)

differences in density between areas of image

high contrast (short gray scare)

has higher visible difference (less gray area) good for detecting carries and bone detail

low contrast (long gray scale)

many shades of gray and is good for looking at soft tissue and perio evals

for contrast, an increase in kVP causes

increase energy -> increase penetration-> decrease beam attenuation (absorption)-> decrease contrast(more grey)

a decrease in contrast equals,

increase in power penetration (less difference between structures because all photons are absorbed/long grey scale)

low kVp (40)

small gray scale

higher contrast

high kVp (100)

long gray scale

lower contrast

increases contrast due to reduction of fog (less scatter) less gray scale

collimator

filtration cause on contrast

increases energy by removing low energy = decrease contrast

patient factors for contrast

thicker- less contrast (more variability/ more scattering)

thinner- more contrast (more uniform)

density : high atomic # = more contrast/more absorption

increase density and no effect on contrast

mAs

increase density and decrease contrast

kVp

What are the two patient factors of contrast

Subject thickness and density

Effective atomic number

Subject contrast ___________ with decreasing atomic number

decreases

decrease density and no effect on contrast

FFD (focal spot to film distance)

decrease density and increase contrast

collimation

image sharpness

how well edges and boundaries are defined

spatial resolution

ability to distinguish small, close objects

penumbra

zone of un-sharpness caused by finite size of focal spot

focal spot size

The smaller the focal spot, the sharper the image appears (less penumbra)

smaller angle of focal spot

increases sharpness (20degrees)

focal spot on object distance

longer FsOD = more sharp (done by using long opened ended cylinder)

Object to film distance

OFiD - want to minimize to reduce magnification and unsharpness to make image more sharp

kVP and MAs on sharpness

high kVp - increases energy -> increase scatter = less sharp

high mAs- increase time -> reduce noise & more sharp

more motion in an image

decreases sharpness

digital sensor vs. film

has higher sharpness

long source to object distance

decreases magnification to increase sharpness

short object to film distance

decreases magnification increase if sharpness

magnification factor

image size / object size = image distance/object distance

elongation

x-ray beam perpendicular to object not receptor and image appears longer

foreshortening

beam perpendicular to film/sensor not object and image is shorter

bisecting able technique

film placed close to teeth and beam directed perpendicular to imaginary bisector

(causes more distortion)

parallel technique

film is parallel to tooth and beam is perpendicular to both (preferred method)

ideal image technique

use small effective focal spot

increase focal distance between Fs and object

minimize object film distance

position sensor parallel to long axis of object

orient central ray perpendicular to object sensor

electrons

located in energy levels or shells around the nucleus (K,L,M) or (1,2,3)

k shell electrons

strongest attraction to the nucleus and highest bonding energy (closest to the nucleus)

electrostatic force

if binding energy is stronger than electrostatic force there will be more electrons

radiation

transimission of energy through space or matter

particulate radiation

atomic nuclei or subatomic particles that have both mass, energy and may be + or - or neutral in charge and move at high velocity

electromagnetic radiation

movement of energy through space as a combination of electric and magnetic fields. has no mass or charge and moves at speed of light

types of electromagnetic radiation

non-ionizing ( microwave)

ionizing (sufficient energy to remove an orbital from atomic x-ray)

wave theory

radiation propagated in form of a wave consisting of electrical and magnetic fields or originated in a plane at a right angel to one another and oscillating perpendicular to the direction of motion

short wavelength

higher frequency and higher energy

longer wavelength

lower frequency and lower energy

quantum theory

radiation depicted as small bundles of energy called photons that travel at the speed of light and contain specific amount of energy (E) (keV)

smallest bundle of energy that radiation is depicted as

photons

E = h x v = h x c/frequency

energy = (Planck's constant h) x (frequency)

parts of the xray tube

cathode

anode

focal spot

glass envelope

aluminum filter and collimator

cathode (-)

the tungsten filament and source of electrons

molybdenum focusing cup

part of cathode that is a concave reflector that focuses on electrons

anode (+)

the tungsten target which has high atomic numbers, high melting point, high thermal conductivity, and low vapor pressure

made of copper and is a good thermal conductor to dissipate heat from the tungsten target

anode

focal spot of x-ray tube

area on the target (anode) to which the focal cup directs the electrons from the filament

place to convert the kinetic energy of the electron from the filament into xray photons which generate the x-ray

focal spot to tube

if focal spot of xray tube decreases

sharpness increases and heat generated per unit of target area increases

target placed at angle (20 degrees) to the electron beam

focal spot of xray tube

glass envelope

provides a vacuum environment to withstand high heat

xray tube window

allows maximum emission of xray and minimum absorption of the glass in the glass envelope

aluminum filter and collimator

reduces unnecessary exposure and limits/shapes beam size

protective housing of xray machine

lead-lined metal casing that provides mechanical support and prevents excessive radiation exposure and shock

power supply

creates high and low voltage circuit with transformer

low voltage from the power supply (mA)

heats up the xray tube filament (cathode) and makes electrons available

provides low voltage to heat up x-ray tube filament

mA

generates high potential difference between anode and cathode to move electrons

the power supply (kVp)

direct current/constant potential

produces higher frequency

alternating current

most common and is in 60 cycles

self-rectification

converting an alternating current voltage to direct current voltage

mA control

output of stepdown transformer (connected to the cathode)