Principles of Radiology 1

Question 1

How are radiographic images obtained?

Answer 1

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.

Question 2

What kind of image is displayed by X-rays that have hit the phosphor plates?

Answer 2

It will result in a black image whereas X-rays that have been attentuated will cause a white image.

Question 3

What are the types of interactions that can result from X-rays touching tissue?

Answer 3

It can either be transmitted 100%

100% absorption can occur.

It can be scattered forwards and backwards with partial absorption

Question 4

What is the attenuation coefficient?

Answer 4

The fraction of the X-rays removed from a beam per unit thickness of the attenuating medium.

Density of the material matters.

Question 5

What is rayleigh scattering?

Answer 5

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.

Question 6

What is photoelectric absorption?

Answer 6

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)

Question 7

What equation describes the amount of absorption that takes place?

Answer 7

1/E^3

Z^3

Question 8

What kind of action does the photoelectric effect have on radiographic images?

Answer 8

It contributes to absorption of X-rays but not scatter

Question 9

What is compton scattering?

Answer 9

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.

Question 10

What is pair production?

Answer 10

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

Question 11

What affects a tissue’s attenuation coefficient?

Answer 11

Thickness

Density

Atomic number or component atoms

Question 12

What is the total attenuation of an X-ray based on?

Answer 12

Photoelectric effect + Compton scattering = Total attenuation

Question 13

How much does bone attentuate a diagnostic X-ray?

Answer 13

A given thickness of bone will attenuate X-rays approximately 12 times the level of an equal thickness of soft tissue

Question 14

What kind of scattering occurs more in bone?

Answer 14

Photoelectric effect will occur more in bone while compton scattering occurs more in soft tissues.

Question 15

How can scattered radiation affect the image?

Answer 15

The scattered radiation may hit the image detector and cause loss of contrast (fogging the image)

Question 16

How can scattered radiation be minimized?

Answer 16

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.

Question 17

What happens to X-ray image if kV is increased?

Answer 17

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.

Question 18

How does the X-ray machine create X-rays?

Answer 18

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.

Question 19

What are the anode and cathode made of?

Answer 19

Tungsten

Question 20

How is the rate of movement of electrons in the X-ray tube from cathode to anode?

Answer 20

A vacuum tube is used which decreases resistance between them

A DC current is used

Question 21

What are the real and apparent focus and how do they differ from each other?

Answer 21

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.

Question 22

How are x-rays filtered and why is this needed?

Answer 22

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.

Question 23

What is the result of having a good filtering of the X-rays?

Answer 23

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.

Question 24

How far should the anode and the surface being irradiated be?

Answer 24

20 cm minimum at all times.

It is usually 20 - 40 cm

Question 25

What kind of focus is the anode theoreticall?

Answer 25

The anode is theoretically considered to be a point focus. However, in reality it is about 0.4mm x 0.4mm

Question 26

What happens to focus in anode?

Answer 26

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.

Question 27

What results from unsharpening of the image?

Answer 27

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.

Question 28

What are the most basic intraoral radiographs?

Answer 28

Periapical radiograph Bitewing radiograph Occlusal radiograph Oblique occlusal radiograph

Question 29

What does intraoral radiography refer to?

Answer 29

Images taken where the image RECEPTOR is inside the oral cavity.

Question 30

How can distortion of the image be minimized?

Answer 30

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.

Question 31

Why are spacer cones different in length between different machines?

Answer 31

Different machines have different lengths due to positioning of the anodes within the machine.

Question 32

Why is a cone shaped PID not acceptable?

Answer 32

Due to the lack of collimation

Question 33

What happens to image if the radiation source is too close to the object?

Answer 33

The object will be magnified substantially.

Question 34

What are the problems with the 2D nature of radiographic images?

Answer 34

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.

Question 35

How do analogue images work?

Answer 35

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

Question 36

What are the disadvantages to using analogue imaging?

Answer 36

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.

Question 37

What are the limitations of using solid state sensors?

Answer 37

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

Question 38

What are the limitations of using phosphor plates?

Answer 38

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

Question 39

What are the advantages of using phosphor plates?

Answer 39

Re-usable Double images are possible It is re-usable Barrier envelopes are required to prevent cross contamination.

Question 40

What happens inside the scanner that scans the phosphor plates?

Answer 40

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

Question 41

What can PSPPs do that CCDs and CMOS cannot?

Answer 41

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.

Question 42

What are the advantages to using CCDs and CMOS over PSPPs?

Answer 42

Scanner is not required They don't get damaged very easily They have better resolution

Question 43

How does exposure time affect PSPP and Solid State sensors differently?

Answer 43

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"

Question 44

What is latitude?

Answer 44

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.

Question 45

How does latitude differ between PSPPs and Solid State sensors?

Answer 45

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.

Question 46

How should X-rays be taken? (work flow)

Answer 46

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

Question 47

Issue: I am not sure if I have used the phosphor plate already (messy organisation on your bench top) Action?

Answer 47

Take another phosphor plate for the exposure and scan all the plates you have.

Question 48

Issue: I dropped an exposed wrapped plate onto the floor. Action?

Answer 48

``` 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) ```

Question 49

Issue:I dropped an exposed plate on the floor while opening the plastic barriers Action?

Answer 49

``` 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. ```

Question 50

Issue: The scanner is being used and there is a queue Action?

Answer 50

Keep the black box with your plates in | closed and wait in line

Question 51

Issue: The scanner is not working Action?

Answer 51

Keep the black box with your plates in closed and ask for IT help if necessary to solve the scanner issue

Question 52

Issue: I loaded two plates too close to another (did not wait for green light) Action?

Answer 52

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.

Question 53

Issue: I have taken my radiographs and just see that the settings of the X-ray machine were wrong

Answer 53

Scan the plates as usual and assess the image quality and diagnostic yield before you consider retaking the images.

Question 54

Issue: The patient “may” have moved during | the exposure

Answer 54

Scan the plate first to see if the image is diagnostic. Then decide if a retake is required.

Question 55

Issue: I have scanned the images under the | wrong patient

Answer 55

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.

Question 56

Issue: I have a bad quality image Action

Answer 56

NEVER ERASE images. Records should be kept of all exposures, whether good or bad

Question 57

Issue: The black transport box was dropped on the floor and all the plates are on the floor now Action?

Answer 57

``` 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 ```

Question 58

What is a pixel?

Answer 58

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

Question 59

How many grey shades are used on the typical grey scale?

Answer 59

256 (8 bit 2^8)

Question 60

What determines the amount of information, the size of the file, and resolution of the digital image?

Answer 60

Number and size of the pixels Number of shades of grey Resolution = lp/mm (typical 7 to 25 lp/mm)

Question 61

What are the digital system limitations that affect the type of image generated on x-rays?

Answer 61

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)

Question 62

What is the spatial resolution of intraoral films?

Answer 62

20 lp/mm (solid state sensors) 7 lp/mm (PSPP systems)

Question 63

What causes detector resolution to vary?

Answer 63

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

Question 64

How can image be manipuled on PC?

Answer 64

Alteration in contrast Alteration in brightness Sharpening / smoothening Inversion Embossing Magnification Automated measurement Pseudocolourisation

Question 65

What should be ensured about the monitor being used for diagnosis?

Answer 65

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.

Question 66

What are the features of a medical grade monitor?

Answer 66

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