Principles of Radiology 1 Flashcards
How are radiographic images obtained?
They are obtained by X-rays travelling through the patient’s tissues which are then captured on the image receptor.
Different tissues have different attentuation gradients causing X-rays to travel through the tissues and show up differently on the phosphor plates.
What kind of image is displayed by X-rays that have hit the phosphor plates?
It will result in a black image whereas X-rays that have been attentuated will cause a white image.
What are the types of interactions that can result from X-rays touching tissue?
It can either be transmitted 100%
100% absorption can occur.
It can be scattered forwards and backwards with partial absorption
What is the attenuation coefficient?
The fraction of the X-rays removed from a beam per unit thickness of the attenuating medium.
Density of the material matters.
What is rayleigh scattering?
Happens when the energy of the incoming photon is considerably less than the binding energy of orbiting electrons. The photon is then deflected but does not lose energy.
No excitation or ionization of orbiting electron takes place and thus can be ignored for dental/medical diagnostic imaging.
What is photoelectric absorption?
Incoming photon has enough energy (equal or more than the orbiting electron’s binding energy) to eject an orbiting electron from its orbit.
The photon’s energy is completely absorbed and the electron is ejected.
The ejected electron is called a photoelectron and the vacancy is filled up by electrons from higher orbits cascading down emitting characteristic radiation (very low infrared radiation which results in heat production)
What equation describes the amount of absorption that takes place?
1/E^3
Z^3
What kind of action does the photoelectric effect have on radiographic images?
It contributes to absorption of X-rays but not scatter
What is compton scattering?
Incoming photos has energy considerably larger than binding energy of orbiting electron.
Electron considered a free electron as it is ejected from its orbit while incoming photon is scattered.
During this interaction the incoming photon’s energy is not conserved but the total energy of the interaction is unchanged.
What is pair production?
It is the formation of 2 charged particals (negatron and positron from a single high energy photon)
THIS IS IRRELEVANT TO DIAGNOSTIC IMAGING BECAUSE IT ONLY OCCURS AT ENERGIES ABOVE 1.02 MeV
What affects a tissue’s attenuation coefficient?
Thickness
Density
Atomic number or component atoms
What is the total attenuation of an X-ray based on?
Photoelectric effect + Compton scattering = Total attenuation
How much does bone attentuate a diagnostic X-ray?
A given thickness of bone will attenuate X-rays approximately 12 times the level of an equal thickness of soft tissue
What kind of scattering occurs more in bone?
Photoelectric effect will occur more in bone while compton scattering occurs more in soft tissues.
How can scattered radiation affect the image?
The scattered radiation may hit the image detector and cause loss of contrast (fogging the image)
How can scattered radiation be minimized?
Using a collimator to direct the X-ray beam only to the target tissue.
Scattered radiation increases when the volume of tissue that is irradiated is larger.
What happens to X-ray image if kV is increased?
There is less differentiation in absorption between bone and soft tissue which results in less contrast in the image.
There will also be more scattered radiation that can hit or reach the image detector which also results in less contrast.
Higher kV allows for shorter exposure time which means a lower absorbed radiation dose for the patient.
How does the X-ray machine create X-rays?
A vacuum glass tube is used to position cathode and the anode. From the cathode end a current is applied which creates a cloud of electrons. Electrons are attracted to the positively charged anode and when electrons hit the anode the energy is converted into head and x-rays.
What are the anode and cathode made of?
Tungsten
How is the rate of movement of electrons in the X-ray tube from cathode to anode?
A vacuum tube is used which decreases resistance between them
A DC current is used
What are the real and apparent focus and how do they differ from each other?
Real focus is the point on the anode that the electrons hit and where the X-rays are generated.
The apparent focus is where the x-rays go afterwards and generate the image when they hit the receptor.
How are x-rays filtered and why is this needed?
An aluminum disk is placed behind the opening in the lead casing of the x-ray machine to filter out the weaker X-rays. This is important to reduce radiation dose as it will be absorbed by patient tissues either way for no reason.
On the outside of the machine the spacer cone/tube is used to regulate where the x-rays exit the machine.
What is the result of having a good filtering of the X-rays?
The lower energy X-rays are filtered out resulting in overall higher X-ray energy output and less absorption of x-rays by patient tissues.
How far should the anode and the surface being irradiated be?
20 cm minimum at all times.
It is usually 20 - 40 cm
What kind of focus is the anode theoreticall?
The anode is theoretically considered to be a point focus. However, in reality it is about 0.4mm x 0.4mm
What happens to focus in anode?
Over time of using the x-ray machine the focus will enlarge and instead of being a point focus it becomes a line focus making the image less sharp.
What results from unsharpening of the image?
A penumbra forms at the edges of the images.
This occurs with time at such a slow rate that the dentist is hardly aware it is happening.
What are the most basic intraoral radiographs?
Periapical radiograph
Bitewing radiograph
Occlusal radiograph
Oblique occlusal radiograph
What does intraoral radiography refer to?
Images taken where the image RECEPTOR is inside the oral cavity.
How can distortion of the image be minimized?
Positioning the image receptor as close and parallel to the teeth as possible.
Parallel xrays should be aimed perpendicular at both the teeth and at the image receptor.
Why are spacer cones different in length between different machines?
Different machines have different lengths due to positioning of the anodes within the machine.
Why is a cone shaped PID not acceptable?
Due to the lack of collimation
What happens to image if the radiation source is too close to the object?
The object will be magnified substantially.
What are the problems with the 2D nature of radiographic images?
There is overlap of structures
Image is foreshortened or elongated if the correct vertical projection angle is not respected
Distortion of the image in width if the correct horizontal projection angle is not respected.
How do analogue images work?
Incoming photons sensitize the silver halide crystals in the film emulsion and convert them into black metallic silver
Unsensitized silver halide crysstals in the film emulsion are removed during the fixation step of processing revealing transparent parts of the film.
Finally, the film needs to be washed and dried before it can be used for diagnostic purposes.
THIS IS TIME CONSUMING
What are the disadvantages to using analogue imaging?
Chemicals (affected too much by temperature and concentrations)
Space to store chemicals and films
Need for a dark room
Duplication isn’t simple
Sharing images is not simple
Lead foils and chemicals are toxic so safe disposal is needed
No 3D imaging possible
Retakes are necessary if film was over- or under- exposed
Technician needs to know more about exposure time, kV and mA as well as about developing processes.
What are the limitations of using solid state sensors?
Sensor is much larger and some patients will not be able to handle them. They are not available in different sizes.
Actual size of the sensor is significantly smaller than the size of the captor of the image.
Over exposure is possible and results in blooming.
No double images possible
Barrier envelopes required to avoid cross contamination
What are the limitations of using phosphor plates?
Latent image is captured that is sensitive to white light
Red helium neon laser scan is needed
Blue fluorescent light will be emitted which is proportional to the number of X-ray photons that hit the PSPP
What are the advantages of using phosphor plates?
Re-usable
Double images are possible
It is re-usable
Barrier envelopes are required to prevent cross contamination.
What happens inside the scanner that scans the phosphor plates?
Red helium neon laser scans the phosphor plate
Plate emits blue fluorescent light
Light is caught by a photomultiplier tube
Analogue digital converter displays the image on the computer monitor
A bright white light array of LEDs erases the plates
Plate is dropped down at the bottom of the scanner
What can PSPPs do that CCDs and CMOS cannot?
They can form double images
Bending is possible
Can fit more comfortably into patients’ mouth due to being much more thin and small based on size.
What are the advantages to using CCDs and CMOS over PSPPs?
Scanner is not required
They don’t get damaged very easily
They have better resolution
How does exposure time affect PSPP and Solid State sensors differently?
The longer the exposure time the more the optical density in both. However this occurs more quickly with Solid State sensors resulting in a phenomenon known as “blooming”
What is latitude?
The difference in minimal and maximal exposure time to which the receptor can be exposed.
Minimal exposure time corresponds to the highest grey value that will still be detectable.
Below minimal exposure time the image receptor shows a white image (noo exposure detected). Above the maximal exposure time, the image receptor will show a pitch black image.
How does latitude differ between PSPPs and Solid State sensors?
PSPPs have a wider latitude than solid state sensors.
Solid state sensors have a more narrow latitude compared to PSPP.
Solid state sensors are more sensitive to overexposure to light.
How should X-rays be taken? (work flow)
Clean your hands
Take plastic wrapped plates required for your patient and place on clean paper towel
Place the black plastic box in a dimmed area with lid open
Glove hands
Expose wrapped plates and collect them on dry paper towel with black side facing up (no confusion possible with already exposed plates avoiding double images)
If more plates needed ask someone to provide them to you
Dry plastic barriers with paper towel
In dimmed area where you placed the black plastic box open each plastic barrier by tearing the V shaped indentation in the wrap while holding the barrier with the transparent side facing up
Let the plates drop in the box without touching the plates with gloved fingers
Shredded plastic barriers go into general waste
Take gloves off and close lid of the box
Wash hands
Proceed to scan-X scanner
Login to titanium and subsequently into romexis to scan the plates
Load plates one by one making sure the blue surface faces the scanner and avoid plates being exposed to light
Close lid of box while waiting for scanner to process slides
Collect plates at bottom of the scanner
With clean hands re-wrap plates in appropriate plastic barriers and place in appropriate box for the next user
Issue: I am not sure if I have used the phosphor
plate already (messy organisation on
your bench top)
Action?
Take another phosphor plate for the
exposure and scan all the plates you
have.
Issue: I dropped an exposed wrapped plate
onto the floor.
Action?
Avoid stepping onto it and proceed with the next exposures if you have any. Once you are done, pick up the plate from the floor with your gloved hands and open the wrap as like under “normal” conditions. Open the other barriers as well as under normal conditions and proceed as usual (see work flow slides)
Issue:I dropped an exposed plate on the floor
while opening the plastic barriers
Action?
Take off your gloves and pick up the plate as quickly as possible and make sure you place it in the black box with the phosphor layer facing down. Clean hands and don new gloves and proceed with the other plates.
Issue: The scanner is being used and there is
a queue
Action?
Keep the black box with your plates in
closed and wait in line
Issue: The scanner is not working
Action?
Keep the black box with your plates in
closed and ask for IT help if necessary
to solve the scanner issue
Issue: I loaded two plates too close to another
(did not wait for green light)
Action?
The images will be overlapping and the
scanner will consider the images of the 2
plates as one image. After assessing the
diagnostic quality, discuss with
supervising faculty if a retake of one or
both is required.
Issue: I have taken my radiographs and just
see that the settings of the X-ray
machine were wrong
Scan the plates as usual and assess the
image quality and diagnostic yield before
you consider retaking the images.
Issue: The patient “may” have moved during
the exposure
Scan the plate first to see if the image is
diagnostic. Then decide if a retake is
required.
Issue: I have scanned the images under the
wrong patient
Inform your supervisor in the clinic and
proceed to the radiology clinic and
explain what happened. The radiography
staff will help you move the images.
Issue: I have a bad quality image
Action
NEVER ERASE images. Records
should be kept of all exposures, whether
good or bad
Issue: The black transport box was dropped
on the floor and all the plates are on
the floor now
Action?
Put the plates as quickly as possible back in the black box. Try to avoid scratching the plates while picking them up from the floor. Use 2 fingers to lift them from the floor instead of shuffling them over the floor
What is a pixel?
A three dimensional cuboid or voxel of the patient’s tissues
Depth of the voxel depends on the thickness of the part of the body that is being imaged
How many grey shades are used on the typical grey scale?
256 (8 bit 2^8)
What determines the amount of information, the size of the file, and resolution of the digital image?
Number and size of the pixels
Number of shades of grey
Resolution = lp/mm (typical 7 to 25 lp/mm)
What are the digital system limitations that affect the type of image generated on x-rays?
The actual number of meaningful attenuation levels that can be captured is limited due to system’s inherent “noise”
Conventional computer monitors can only display 8 bit depth images
Operating system windows can only show 242 shades.
Human eye can only see about 60 shades of grey under ideal viewing circumstances (in a dental office that decreases to less than 30 shades)
What is the spatial resolution of intraoral films?
20 lp/mm (solid state sensors)
7 lp/mm (PSPP systems)
What causes detector resolution to vary?
Electronic noise
Diffusion of photons in the scintillation coating or phosphor layer
Potential imperfect optical coupling between scintillator, fiber optic screen and photodetector
Thickness of the phosphor layer and diameter of the laser beam
Scan modus of the PSPP system
Size of the focal sspot of the X-ray machine
Source-to-object distance
Object-to-image distance
How can image be manipuled on PC?
Alteration in contrast
Alteration in brightness
Sharpening / smoothening
Inversion
Embossing
Magnification
Automated measurement
Pseudocolourisation
What should be ensured about the monitor being used for diagnosis?
Avoid reflection on the monitor
Avoid monitor facing light
Make sure monitor is clean
Test calibration regularly as monitor performance may change
Sit in front of monitor
Laptops have a lower display intensity
Most of the recent displats will do for 2 dimensional X-ray diagnosis in dentistry
There are displays witha digital video interface which eliminates the digital to analog conversion (direct digital information displaed), which decreases the electronic noise of the monitor and eliminates signal noise.
What are the features of a medical grade monitor?
High resolution (Mega pixel screens)
Grey scale calibration
Not used for chart notes – exclusive diagnosis
display
Placed in a room with minimal ambient light
Calibration repeated regularly
Controlled voltage/signal fluctuations (high
quality electronics)
DICOM part 14 GSDF standards
