Radiographic Image Acquisition Flashcards
What is the basic concept of computed radiography (CR)
Cassettes are used that have a phosphor screen.
- When the x-rays hit they form a latent image in the phosphor.
- The cassette is then placed into a reader with a laser shone on to it which releases the stored photons, collects the signal, and digitises it to be displayed on a display screen.
Describe the imaging process of CR
- Latent image formation - cassette made of amorphous barium fluoro bromide/halide crystals (phosphor) doped by (covered with) europium ( BaFBr:Eu )/ (BaFX:Eu) - disrupts structural integrity of the crystals and creates F centres (fluoride atoms - relatively positive charge) - valence band (lower energy state with a lot of electrons available a) and conduction band (theoretical energy energy that we would need to apply through the system in order for electrons to reach the band)
As X-rays come in to system electrons release from valence band to conduction band - oxidation loss of electron - Eu 2+ to Eu 3+ - this electron goes to fluoride which gains an electron (reduced) - metastable fluoride atom
End product: Eu 3+ and stable uncharged fluoride atom - forms latent image
- summary: X-ray photons are absorbed into phosphor that give rise to high energy photoelectron which ionizes several atoms releasing several electrons. The electrons become temporarily trapped at specific sites throughout the phosphor - this produces the latent image
- Laser simulated emission - processing of image as it is brought to the CR reader - stored electron in fluoride atom - has energy and has created memory within the system.
Casette placed in a machine with rollers:
Red laser light is shone onto the cassette in wavelength of 700 nm - has enough energy to release electron from F into conduction band which can then drop down into the valence band which reduces Eu3+ to Eu2+ (reverses process). Higher energy to lower energy state cases electromagnetic radiation release (in blue visible light spectrum in 400-450 nm range)
Overall X-ray energy has been turned into light energy after delayed process - phosphorescence. Small amount of fluorescence occurs once X-rays hit cassette - but doesn’t contribute to production of image.
Red laser light is shone onto a mirror which rotates - which moves light along x axis of cassette - back and forth as mirror rotates - scan along axis - but sequentially read out x axis
The red laser light causes blue light to be released upwards which are channeled along the fiber optic channels towards a photomultiplier. Red light that goes along with it is removed by a filter before the photomultiplier.
Intensity if blue light proportional to intensity of X-rays - more X-rays (more F centres filled) hitting machine means more blue light
- summary: During readout the image plate is scanned with a red laser beam - causing trapped electrons to relax back to ground state - release their stored energy as light photons - blue spectrum. These are collected by optical fibers to a OM tube which produces electrical current
- electrons relax back to ground state if left long enough - causes image decay.
- Resetting cassette - Bright white light shone onto cassette clears it completely (releases all electrons from F Centres) - so casette can be used for another radiograph
- Post-processing of image - Photomultiplier tube sends blue light to analogue to digital converter (ADC) - converted into pixel values - digital signal - 1s and 0s so we can assign a value to the pixel on computer screen
Because we have created a digital image, our dynamic range is more than screen film. Specific pixel values for each exposure on cassette - can change exposure (steepness of graph), contrast and manipulate pixel values vs screen film drawback
Describe the process of screen film radiography
- Incident X-ray enters casette
- X-ray photon converted to light (fluorescence)
- Light cause reduction of Ag+ to stable Ag
- Film removed from casette
- Film placed in aqueous solution
- Reducing agent with the solution causes the reduced (stable) silver atoms to catalyse further reduction in nearby Ag
- Film then placed in an oxidizing fixer to dissolve the inactivated silver halide
Explain the factors that affect image quality in computed radiography
- Exposure index (speed) - a measure of the amount of exposure on the image receptor - even though can digitally alter - an underexposed or overexposed image can introduce noise and reduce contrast
- Latitude (dynamic range) - dynamic range very high and dose response is linear - CR produces good contrast over a much wider range if exposures
- Spatial resolution - described by modulation transfer function (MTF) - ratio of output:input modulation
As spatial frequency increases MTF decreases
Detective quantum efficiency (DQE) = SNR2 out/ SNR2 in
Higher DQE more efficiently the detector can record information. DQE 0.25 implies detector can only exploit 1/4 incident X-ray photons - standard vs 0.12 for high resolution CR IP
- improved by :
- smaller diameter of readout laser beam
- smaller pixels
- smaller size of phosphor crystals
- thinner phosphor layer
- no light reflection/absorption backing layer - uses more of photons for image formation even though produces scatter
Discuss the impact of image processing on image quality (factors that affect quality)
- Contrast / subject vs image
- Resolution - spatial
- Noise
- Unsharpness - geometric, image receptor, movement, edge
- Magnification
- Distortion
- Artifacts
Compare indirect DR CCD to TFT
Indirect :
Charged coupled device (CCD chip) - converts light into a digital signal (same chip used in video cameras) - first layer scintillation layer that allow light to be funelled through Cseium iodide crystals. Then coupling layer allows light photons to be funneled to chip by fiber optic channels or lenses that focuses the light down to be converted into electrons.
Drop off occurs as electron energy is less than initial X-ray energy (secondary quantum sink) - due to coupling required and large surface area of where X-ray enters compared to chip
- Dexels correspond to X-ray intensity hitting scintillating layer. Higher intensity more electrons released in a dexel.
As light strikes surface of chip (silicone layer) electrons are released - voltage gates between dexels can be changed to allow dexels to be released into an amplifier so that signal can be digitized into a pixel value
More electrons available darker it appears within that pixel
Each dexel read out row by row to create radiograph
Thin Film Transistor (TFT) Array (flat panel)
Xray photons transverse the scintillator caesium iodide layer that contact a photodiode layer (made of hydrogenated amorphous silicon - semiconductor that converts light into electrons) - allow passage of electrons one way - converts light photons into electrons which are then released into the TFT itself
(All 3 layers tightly packed to not allow light to spread out)
Each detector element (del) in a TFT has 3 components - electronic (capacitor - ability to store charge) , TFT switch to release charge from capacitor), photosensitive component where electrons from photodiode are channel towards capacitor for storage
- electron released from photodiode proportional to number or intensity of xrays hitting caesium iodide layer. Charge stored in capacitor also proportional to number of electrons that have been released
When TFTs exposed to X-rays charge build up in capacitor. When we want to read out the signal we sequentially switch the TFT switch so the charge can be released from the capacitor - so can read out row by row in TFT array - the electronic signal then goes through an amplifier and we get a pixel value - grey scale value for each one of the dels in the detector
Electrons don’t hit photosensitive component on del and won’t contribute to the image itself
Only electrons hit photosensitive layer (blue in image) will be stored in capacitor
Explain how a direct DR flat panel detector works
X-ray energy is converted directly into an electronic signal
Shares common detector element - del as indirect TFT array
In direct- an amorphous selenium layer is used (semi-conductor) - has ability to convert x-ray energy into an electronic signal
Charge differential created across layer by creating a Positive anode region created at top of layer where x-rays strike detector and negative cathode bottom that interacts with TFT array
As X-rays strike the layer ion pairs are created. Movement of electrons creates electrons flowing in the opposite directions (electron holds/pairs flow to negative end and electrons flow to positive end)
The electron holds can be measured as current on TFT array - specific spatial resolution - as where the electron pair is created in the semiconductor is where it contacts the TFT array
Drawback - not good for high energy X-rays - best at low energies - ideal for mammogram - need good spatial resolution and contrast - due to compressed breast tissue were able to use lower energy X-rays
Electrical current goes directly to sensitive layer of del unlike in indirect
Then dels are read out as charge stored in capacitor and after X-ray is exposed we can sequentially close the TFT switches and reach out charge that has been stored
Charge then amplified and sent to computer where each del represents a pixel , amount of current stored in each capacitor correlates to grey scale value on image
What is base fog
film base plus fog density - reference value
the density of an unexposed, but processed area of film. It’s the density of the film base plus the density of the fog; fog being the silver that has received no exposure to light, yet gains a little density during development
What is Dmax
Dmax is an abbreviation for maximum density and Dmin for minimum density. The abbreviations are used both in describing the characteristics of an image or an imaging device such as a scanner.
Compare display monitors used in radiography
Cathode ray tube (CRT)
Visible image generated by scanning a phos- phor screen with a focused beam of electrons all contained within an evacuated glass tube.
Flat panel displays
Most display monitors are based on liquid crystal technology. Application of the appro- priate voltage distribution to an active matrix modulates light polarisation on a pixel-by-pix- el basis varying the light emission that com- prises the image seen on the screen. It pro- duces a higher contrast image with greater resolution and less power usage.
Hardcopy
On occasions it is necessary to print a hard- copy image. A hardcopy image is recorded using a laser printer onto a film with silver crystals to create a latent image. This is con- verted into a visible image by applying heat to the film. This ‘dry’ film processing eliminates the need for traditional chemical processing.
What is (DEXA or DXA) - dual energy x-ray absorptiometry
A procedure that measures the amount of calcium and other minerals in a bone by passing x-rays with two different energy levels through the bone.
A dual energy x-ray absorptiometry shows the strength and thickness of a bone and is usually done in the lower spine, hip, lower arm, wrist, fingers, and heel.
Describe the concept of dual energy radiography
The concept by which a patient is exposed to X-rays with two different energy levels. This enables the selective imaging of separate materials from each other eg. Bone vs soft tissue
The energy dependent differences of each material can be used to eliminate one tissue or the other
