X-ray Detection & Aspects of Image Formation Flashcards
creating a digital image
photons are selectively attenuated depending on the anatomy of a patient.
This creates a beam whose intensity varies as a function of spatial location
We refer to this as the latent image
what is a latent image
Latent - (of a quality or state) existing but not yet developed or manifest; hidden or concealed.
what is a digital XR image receptor responsible for?
A digital X-ray image receptor (or detector) is responsible for detecting the X-ray photons incident upon it and forming a digital image from the latent image
The image is divided into pixels, and each pixel has a value proportional to the X-ray dose incident upon it
A simple image receptor
Energy from X-ray photons incident on a pixel are converted to electrical charge in the pixel electronics
A number of intermediate stages may be involved
This charge is passed via an amplifier into an analogue-to-digital convertor
The digital (number) value produced is proportional to the energy absorbed by the pixel
In practice a square matrix of pixel detector elements is used to produce digital values that are stored in a computer
Typical detector pixel dimensions are 0.1 – 0.2 mm square
creating a digital image- pixels
stored values can be displayed on a monitor
Each pixel element of the image has an associated X and Y position in a matrix
The value of each pixel (i.e. the digital number) defines the shade of grey which is displayed
creating a digital image- size of the displayed image
the size of the displayed image is not generally the same as the size of the detected image
once stored in a computer the image is merely recorded as numbers in a matrix (sometimes called an array)
these numbers can be represented in many different ways before displaying the final image (magnified/reduced)
creating a digital image- grey level values
Note that the grey level values (displayed pixel values) of the displayed image are not generally the same as the pixel values initially recorded for the detected image
these numbers can be manipulated in many different ways before displaying them – for example by adding or subtracting a number to all the values to make the image brighter or darker
grey level range (pixel depth)
Each pixel must take on a discrete value. Use of 1 bit (binary digit) to “code” for the pixel value means that the pixel can take on one of two discrete values (1 or 0), which would correspond to a pixel that is either black (0) or white (1). If two bits are used to code for a pixel, then four discrete values are possible (i.e., 00, 01, 10, and 11).
In general, if n bits are used to code for one pixel, the number of discrete values is 2n. 8 bits (equal to one Byte) can code for 256 discrete values (shades of gray); adding an extra bit will double the number of discrete values (i.e., 9 bits codes for 512 shades of gray), whereas subtracting one bit halves the number of shades of gray (i.e., 7 bits allows 128 shades of gray).
energy band concepts of conductors
in a conductor. there are no band gaps between the valence and conduction bands.
In some metals the conduction and valence bands partially overlap. This means that electrons can move freely between the valence band and the conduction band
energy band concepts of insulators
An insulator is a material that does not allow charge or heat to pass through it easily. has a large gap between the valence band and the conduction band.
The valence band is full as no electrons can move up to the conduction band. As a result, the conduction band is empty.
energy band concepts of semiconductors
-In a semiconductor, the gap between the valence band and conduction band is smaller. At room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band. This allows some conduction to take place.
An increase in temperature increases the conductivity of a semiconductor because more electrons will have enough energy to move into the conduction band.
energy band concepts of lumicescence
energy band concepts of photo stimulated luminescence
computed tomography principles
-Uses an imaging plate to store a latent X-ray image
-The imaging plate is later scanned by a laser beam which causes the latent image to be released as light
-This light is detected and converted to electrical charge using a photomultiplier tube
-The charge is readout, amplified and converted to a digital image using an analogue-to-digital (ADC) convertor
phosphor plate
-Transparent protective layer
-phosphor crystals with doping (activating element).
-Light reflective layer
-Conduction layer to eliminate static
-Support for strength
-Light shielding layer
-Backing layer
what do flat panel (FP) detectors consist of?
-XR photons
-conversion layer
-amplifiers and ADC’s
-a-Si TFT active matric
-Timer and line driver electronics
indirect detectors:
-CsI scintillator – converts X-ray photons to light
-CsI scintillator – converts X-ray photons to light
-Readout electronics
direct detectors:
-a-Se photoconductor converts X-ray photons to electron-hole pairs
-Readout electronics
Each absorbed X-ray photon in the a-Se layer creates about 1000 charge carriers.
The charge reaching the active matrix is stored on capacitors
Read out then proceeds in much the same was as with indirect detectors
Scintillator
-The CsI scintillator used on indirect detectors is often around 500 µm thick.
-This gives an absorption of the incoming X-ray beam of about 80%.
-Each absorbed X-ray photon in the scintillator causes ~3000 light photons to be emitted
FP imaging chain
-Flat panel systems have a rather more complex imaging chain than CR.
-Gain and offset corrections correct for differences in amplifier response, CsI variations, etc.
-Defect pixel correction is essential for adequate images.
-Image enhancement is also performed prior to display.
FD image corrections
Required for all flat panel systems and comprises:
-Offset correction
-Gain correction
-Defect pixel correction
Requires calibration, which is performed by service engineers
what is geometric magnification:
the ratio of the actual size of the sample to the size of the X-ray image projected on the X-ray camera.
what are the assumptions of geometric magnification
-Thin object
-Object parallel to image plane
-Object at centre of image field
-Central ray of X-ray source -normal to object and image plane
-Point X-ray source
What are some key points about geometric magnification
-Magnification results in size distortion of an object
-Magnification is reduced by:
-Decreasing distance between object and detector
-Increasing the distance between the focus and the detector
geometric magnification practical cases
Real objects:
-Not thin
-Not necessarily at centre of image field
-Central ray of X-ray source
Not necessarily normal to object or image plane
-Practical X-ray tubes have finite focal spot sizes
key points:
-Magnification results in shape distortion of real objects
-Shape distortion is reduced by careful arrangement of the object with respect to the image plane and the X-ray beam
what are different types of XR beam collimation devices
-cones
-iris diaphragm
-plate diaphragm
Xr beam centring devices
Light beam diaphragm
Varay lamps
(e.g. some skull units)
Laser beams
(e.g. Computed Tomography)
