Digital Imaging Chapters 20,21,22

Carlton and Adler

Charged Couple Device (CCD)

a photodetector device that is capable of converting visible light into an electric charge and storing it in a sequential pattern

• The electronic signal produced by the CCD is sent to the computer for image processing

Flat Panel Thin Film Transistors (TFTs)

used by amorphous silicon (indirect conversion) and amorphous selenium (direct conversion) flat plate detectors for electronic signal readout

• TFTs collect the electric charges produced by either the selenium or silicon as an array or matrix of pixel-sized detector elements (DEL)

Fill Factor

refers to the sensing area of a detector element, such as the pixel or DEL area of a detector 

• if a detector has a fill factor of 80% then the other 20% would be covered by electronics

• The fill factor has a direct relationship with spatial resolution and contrast resolution 

• detectors with HIGH fill factor will yield HIGHER spatial and contrast resolution 

Direct Conversion (Digital system) 

1-step process, converts X-ray photons into an electronic signal 

• uses a photoconductor to convert X-rays into an electronic signal

• Direct DR systems use a thin-film transistor array with amorphous selenium (a-Se)

Indirect conversion (digital system)

2-step process, converting x-ray photons into light and then converting light into an electronic signal

• uses a scintillator to convert X-rays into light and uses a photodetector to convert light into an electronic signal

• indirect DR systems include either a charged-couple device (CCD) or amorphous silicon with a thin-film transistor (TFT) array

CR Technical Factors Selection

• technique charts should be used to avoid repeats and minimize exposure

• kVp does NOT control image contrast, but DOES control subject contrast

• not recommended to use over 80 kVp for non-grid images bc higher kVp produces scatter that will decrease image contrast significantly more with digital imaging

• digital imaging has a wide dynamic range (wider window for optimal exposure)

Factors that cause histogram errors

improper collimation

incorrect part selection

presence of unexpected materials

• causes the system to rescale the image incorrectly

• can cause an incorrect exposure indicator number

Exposure field recognition 

used to recognize the clinically useful areas on the imaging plate, so it is the only data that will be manipulated, and it eliminates the signals from outside of the collimated field 

• important to have two recognized sides of collimation on the imaging plate to prevent errors 

pre processing

includes all processing of raw data performed to correct flaws in image acquisition

operations

• exposure field recognition

• histogram analysis

• grayscale analysis

post processing

includes manipulation of data that has been pre processed

operations

• edge enhancement

• smoothing

• stitching

• cropping/masking

• windowing

analog to digital conversion (ADC) 

process where the light emitted from a photostimulable phosphor plate (PSP) is converted into an electrical signal by a photomultiplier tube (PMT) and then transformed into discrete digital values by an analog-to-digital converter 

pixel bit depth

The number of bits that represent an analog signal will determine the number of gray levels

• determines the amount of color information per pixel, indicating how many colors an image can display

pixel pitch

the physical distance between pixels and is measured from center to center

• in CR it is dependent on the sampling frequency or nyquist frequency which is how the laser scans the plate

• in DR it is determined by the detector element (DEL) size

• the smaller the pixel pitch, the higher the spatial resolution (indirect relationship)

Ghosting artifact 

may be seen on the next exposed image if a PSP imaging plate is grossly overexposed 

CR Reader Process 

1) the cassette is fed into the CR reader

2) the PSP plate is slid out of the cassette into the reader

3) the plate is scanned line by line (in a raster pattern) by a helium-neon laser beam

4) the laser beam causes phosphors to emit the stored latent image in the form of light photons, which are converted to an electrical signal and then turned into a unique digital value 

5) the plate is erased by exposure or flooding of an intense light to release any remaining trapped electrons 

PSP latent image

• Radiation causes fluorescence of
  the imaging plate, but some of the
  energy of the beam is also stored in
  the plate.

• Some of the electrons are excited by
  absorbed energy and are trapped in
  the crystal structure of the
  phosphor.

• The latent image will lose about 25%
  of its energy in 8 hours, so it is
  important to process the cassette
  shortly after exposure. (the longer an
  image is stored the more energy the
  CR cassette loses)

PSP imaging plate layers

1) front protective layer- protects the PSP from mechanical damage during handling

2) phosphor layer- composed of europium-activated barium fluorohalide

3) reflective layer- reflects emitted light photons towards the photomultiplier tube during scanning

4) electroconductive layer- prevents static build up which could cause significant artifacts on the image

5) polyester base layer

6) light shielding layer- prevents extraneous light erasing the plate before it is scanned

7) back protective layer

PSP plate characteristics

A PSP imaging plate is a rigid sheet
with several layers that are designed
to recored and enhance
transmission of the image from a
beam of ionizing radiation.
• Most common phosphors for CR
and barium fluorohalide Bromides
and Iodides with Europium
activators (traps and release the
electrons information in a usable
form)