Principles of imaging 1 Flashcards
What does the current do to x-ray image?
mAMPs determine the number of X-rays
How do the cathode and anode interact on X-ray tubes?
Between anode and cathode there is a constant voltage applied which enables electrons to fly faster towards the anode
What are the anode and cathode of an X-ray made of?
Tungsten
How are X-rays directed in a safe and appropriate way?
The anode is positioned under an angle facing towards a small opening in the lead casing. this allows only certain X-rays to exit the machine
How long should the distance between the tube and the patient be and why?
Cannot be shorter than 20cm. To avoid overradiating the patient.
Why are some manufactured X-rays with longer spacer cones and others with shorter ones?
Some machines have smaller and others longer spacer cones. Depending on the position of the X-ray tube.
What are the functions of the spacer cone?
Indicates where X-rays exit the machine to aim at patient
Keeps distance between patient and X-ray source
What is the job of the aluminum disk in an X-ray?
Aluminum disk is placed behind the opening in the lead casing. To filter out the weaker X-rays as these X-rays would be absorbed by patient but are useless.
How is the position of the anode indicated on the machine?
A small dot on the outer casing of the machine indicates where the anode is in the device. The distance between the anode and the spacer cone is usually between 20 and 40cm (never <20cm)
What are the dimensions of the anode?
Anode is theoretically a point focus but reality is its dimensions are 0.4 x 0.4 mm.
Initially the anode is flat but over time it gets bent and becomes less clear.
This causes the focus to enlarge and instead of being the theoretically point focus, it turns into a line focus.
What current is ideal for imaging?
7mAMPs ideal for imaging and this can go down to 4 without issue
How are parallel, focused X-rays produced?
Image receptor should be placed as close as possible to the teeth and as parallel as possible to the axis of the teeth.
To achieve parallel X - rays a collimator is positioned in front of the aluminum filter. This collimator is either circular (cylinder shape of PID) or rectangular (beam shape of PID).
Why is geometry important?
If the X - rays are not parallel, but diverging, the image
will be distorted severely and be very inaccurate
.
What are the types of solid state sensors?
CCD Sensor same as the phone camera sensor. It converts X-rays into light energy that can be detected by this device.
CMOS-APS
What digital image detectors are used in ohcwa and how do they work?
Photo-Stimulable Phosphor Storage Plates. They don’t caputre light, they capture energy from the X-rays.
When were Solid State Sensors made?
1987 by a company called trophy
What are the disadvantages to using Solid State Sensors?
Not available in any size
Actual size of sensor is different to size of captor
Easy to use, however, fast image acquisition time
Overexposure is possible and results in blooming
What are the important limitations to using phosphor plates?
Latent image is captured – sensitive to white light (erases the latent image)
Can be scratched
Barrier envelopes required to prevent cross contamination
What are the advantages to using phosphor plates
A photomultiplier tube catches the light and via an analogue digital converter the image is displayed on a computer screen
Very forgiving with regard to exposure time
Double images are possible
Re - usable (1000s x)
How do solid state sensors compare to phosphor plates?
Solid state sensors give a better image overall
CCDs are bulky
Solid state sensors also don’t require a scanner
Phosphor plates damage easily and are bendable
Which of the 2 types of imaging has better latitude of exposure time under optical densities between 0.3 and 2?
Phosphor plate has the widest latitude of exposure time under optical densities between 0.3 and 2
What does a pixel represent?
a 3D cuboid or voxel of a patient’s tissues.
Depth of the pixel depends on the thickness of the part of the body being imaged.
Each pixel measures the total x-ray absorption throughout the whole of each voxel
Why is it impossible to have perfect images using X-rays?
The system has a lot of inherent “noise” from scattering etc
The computer systems it is diplayed on are limited by 8 bit processing
How is image manipulated on romexis?
Alteration in contrast (difference between black and white)
Alteration in brightness (degree of blackening)
Sharpening / smoothening (uses filters)
Inversion (positive image)
Embossing (pseudo 3D)
Magnification (limited as pixilation occurs with too much magnification)
Automated measurement (calibration required, but not in CBCT)
Pseudo colourisation (arbitrary)
What are the features of the diagnostic monitor being used to view X-ray images?
Screen should never face a window
Sit in front of the monitor (less important today than in the past)
Laptops have a lower display intensity
Most of the recent displays will do for 2 dimensional X-ray diagnosis
Avoid reflection on the monitor (matt displays are best)
Avoid monitor facing light (window, artificial light source)
Make sure the monitor is clean
Test calibration regularly as monitor performance may
change
