X ray unit and films Revision Questions Flashcards
What does an xray unit consist of?
X rays tube head Collimator Positioning arm Control Panel Circiutary
Filament - cathode
Made of tungsten and negatively charged
Goes through step down transformer - low voltage and high current
z = 74, mp = 3210 degrees
Target - anode
Positively charged and made of tungsten
Converts electrons to xray photons
effective area = 0.7mm^2
Target surround
copper and effective heat conductor
z=24, mp = 1080 degrees
Shielding
lead surround to absorb xrays trying to get out of tube head
Z = 82
Ensure does rate in vinicity >7.5uSvh^-1
Evacuate glass envelope
Evacuated glass and vacuum prevents risk of interaction of electrons with air atoms prior to meeting target
Filtration
Alluminium allows high energy electron through but not low ones Z = 13 1.5mm <=70kv 2.5mm >70kv
Collimator
lead
Controls the shape of the xrays beam
max beam = 60mm
Spacer cone
a beam indicating device
Controls the distance between the focus to skin
100mm<60kv
200mm>60kv
What does rectangular collimation do?
Controls the shape, size of the xray beam
Dose reduction, improved quality, fewer rejects
Compoenents of an intra oral film?
Radiographic film
Protective paper (black) - protects the radiographic film from light from light exposure, damage fingers and saliva
Lead foil - absorbs any excess xray photons - furthers away from tube - lets xrays get to the patient
Outer wrapper
What is the structure of the radiogrpahic film?
Transparent plastic base - supports the emulsion
Adhesive - attaches the emulsion to the plastic base
Emulsion - silver bromide in gelatin
Protective coating of clear gelation - shields the emulsion from mechanical damage
What does film holder packet consist of?
Connecting rod
bite block
image receptor support
beam aiming deveice with rectangular devices
Where is the xray dot located
convex faces tube head
Placed occulsally so does not superimpose over tooth
PA - dot bottom of film holder
BW - top of holder
What is a latent image?
A hidden image that becomes visible once beam is processed
Steps of fim processing
Developing - converts sentitised crystals to black metallic silver patches
Washing - removes resisdual developer solution
Fixing - removes non-sensitised crystals and hardens emulsion
Washing - removes reisdual fixer solution
Drying - removes water so film is ready to be handled
Advantages of self developing film
No dark room or processing facilities required
Faster time to complete
Disadvatnages of self devleoping films
No lead foil Easily bent Expensive Poor quality image detoeriiates more rapidly over time Difficult in positoning holders
Advantages for digital xrays
no need for chemical processing easy back up of images images can be manipulated no processing faults easy storage and archiving images can be integrated with patient notes easy transfer of images low radiaiton dose
Disadvatanges of xrays
worse resolution
risk of data loss
hard copy print outs tend to have decreased image quality
image enhancment can increase misleading images
requires a diagnositic level computer monitor for viwing images
Brown/green stains
inadqueate fixing time
Non sensititsed crystals are left behind
crazed pattern
film dired too quickly over a strong heat source
Presence of crystals
insufficient washing after fixing
Black lines
film bent
black spots
film splashed with fixer before developing
Fading image
inadequate fixing time
faint image
developer solution too weak, too cold temp
left in for too short a period
Why can cone cutting occur
incorrect assembly of receptor holder
incorrect allingment between xray tube and recptor holder
incorrect orientation of the rectangualr collimation
What is film speed
amount of xray exposure required to produce an adequate image
What affects film speed
number and size of hallide crystals
larger crystals, faster film but pooer image quality
Proclined teeth - angulation of tube head
increases
Retroclined teeth - angulation of tube head
decreases
Ideal projection geometry
image recpetor and object in contact and parallel
parallel beam of xrays
xray beams perpendicular to object and image recptor
Paralleling techinque
Film parallel to long axis of tooth, xray beam perpendicular to long axis of tooth
Image size same as actual tooth
Problem - divergent xrays - long distance between image recpetor and tooth so potential for magnification - use long axis x-ray focus to skin distance to reduce magnification @20cm
Bisecting angle technique
angulation of long axis of tooth determined, angle bisected at right angles with the xray beam
Still uses long xray focus to skin distance @20cm
What are head positions for maxiallry occlusal
Mid sagittal plane perpendicular to the floor
Occlusal plane horizontal
Ala tragus line horizontal and parallel to floor (runs from ala of nose to upper border of tragus of ear)
Anterior teeth horizontally wider
patient too far back in machine
canine behind the guidance marker
Posterior teeth horizontally wider
patient rotated in machine
Anterior teeth, out of focus
Patient postioned too far back in machine
Molars larger on one side
Head rotated to one side
Patient not biting symmetrically between incisors on bite peg
Vertical/horizontal distortion
patient moved
<90 degrees
elongate tooth
> 90 degrees (steep)
Shortens tooth
