Fluoroscopic Image Acquisition

2. Image Intensifier

An image intensifier (II) is a vacuum tube that amplifies low input x-ray doses by converting x-rays to light, such that the output image is 10⁵ times brighter than the input image.

2.1. Key Components

1. Vacuum housing
   • consists of lead (absorb scatter radiation), mu-metal (shields electron optics from external magnetic fields) and Al.
   • vacuums allow for the acceleration of electrons across open space.
2. Input screen
   • vacuum window: 1mm Al, curved to withstand the force of atmospheric pressure. The low atomic number of Al (Z = 13) allows x-ray photons to enter with minimal attenuations, whilst also providing mechanical rigidity for maintaining the vacuum.
   • support layer: 0.5mm Al, curved for accurate electron focusing, thin with low absorbency/scatter and supports input phosphor and photocathode layer
   • input phosphor: Cesium iodide (CsI), absorbs and converts incident x-rays into visible light, thick enough (0.1 – 0.4 mm) to absorb the majority of incident x-rays but not degrade spatial resolution
     • The long needle-like CsI crystals channel visible light toward the photocathode with minimal lateral spreading
     • The curved shape of the input screen leads to the pin-cushion distortion
   • photocathode: thin (20 nm) of Sb and alkali metals, good spectral matching to CsI light output, converts light into electrons with a conversion efficiency of 10-20%
3. Electron optics
   • acceleration: electrons are accelerated by a 25 – 35 kV electric field between the anode and cathode, gaining kinetic energy and resulting in electronic gain
     • Electronic gain or flux gain is defined as the number of photons generated at the output phosphor for every photon generate at the input phosphor
   • focusing: three focusing electrodes shape the electric field onto the output layer, where energetic electrons strike the output phosphor and cause visible light to be emitted
4. Anode
5. Output phosphor
   • thin (4 – 8 μm) ZnCdS:Ag (Zinc cadmium sulfide doped with silver) layer, emits green light when struck by an electron
   • The image is much smaller at the output phosphor than it is at the input phosphor. This leads to amplification since the energy incident on a large area is concentrated on a very small area. The minification gain is the ratio of the area of the input phosphor to that of the output phosphor
6. Output window
   • thick 14 mm clear glass reduces scatter of light (halation)
   • halation degrades the contrast of the output image. Anti-halation techniques typically include the use of smoked glass, special optical coatings, very thick glass or a fibre-optic bundle
   • contributes to veiling glare (reduces image contrast)

2.2. Principle of Operation

1. Input phosphor (CsI) absorbs and converts incident x-ray photons to a large number of light photons.
2. A photocathode (Sb and alkali metals) then absorbs and converts the light photons to photoelectrons.
3. Electrons are accelerated across a vacuum by the anode (25 – 35 kVp) and focused onto the output phosphor (ZnCdS:Ag) by the electron optic system.
4. The output phosphor absorbs and converts accelerated electrons to many light photons.
5. Photons are then captured by a TV camera and displayed on a monitor

2.3 Image Intensifier Characteristics

• Brightness gain = electronic gain x minification gain (typically 2,500 – 7,000)
  • Electronic gain: amplification of photon number due to acceleration of electrons (50 photons are produced in the output phosphor for ever 1 photon emitted from the input phosphor)
  • Minification gain: concentration of photons from a large input screen to a small output screen
    • Brightness gain therefore decreases with decreasing field size (due to less minification gain)
• Vignetting: reduction in brightness at periphery of image
  • reduced exposure rate along periphery of input screen
  • longer focal distance from curved input to flat output phosphor (less brightness gain)
  • less scattered light from surround regions of output phosphor 
• Veiling glare: addition of background brightness causing reduction in image contrast (i.e. dark regions in the image appear lighter because of surrounding light areas)
  • light scattered and reflected within glass output window
  • light scattered back through the anode and hitting the photocathode producing electrons with no spatial information
  • x-ray the pas through the input screen and interact with output screen
  • extend of veiling glare can be measure with contrast ratio
• Contrast ratio: the ability of the II to convey the contrast of an object

2.4 Distortion

• Pincushion: Stretching of the physical dimensions of the periphery of the image (should not be significant) due to electronic focusing from the curved input surface, to the flat output surface (the path lengths to the periphery of the output phosphor are longer, images of linear structural lines will appear curved). Inherent to the design of the image intensifier.

• S distortion – spatial warping of the image in an S shape due to stray magnetic fields. Electrons move along designated lines of flux.

3. Flat Panel Detectors

Flat panel detectors (FPD) are composed of thin film transistor arrays of individual detector elements called dexels. Each detector element has a capacitor which accumulates the signal as an electric charge and a transitor which serves as a switch.

Scintillator – material which exhibits luminescence when excited by ionising radiation.

3.1 Indirect detection systems

• Incident x-rays are converted to light in a phosphor layer
• Each dexel has a transistor, capacitor, in addition to a photodiode (converting the x-ray induced light from the phosphor into a corresponding charge) made usually of amorphous silicon.

Thin flat-panel transistors

Composed of three main layers;

1. Top scintillation layer – an incident photon is absorbed by a scintillating material (usually small rods of cesium iodide) and re-emitted into a pulse of light photons. The light produced travels isotropically, which decreases spatial resolution as the convert light photons are of greater size than the incident photons. The effect is that this create a small degree of blurring.Light produced is proportional to incident x-ray intensity.

   x-ray photon → visible light photon

2. Middle photodiode layer (comprised of amorphous silicons) – converts incoming photons from scintillation layer into electric chargers. The electrons are then transferred to to the thin-film transistor layer.

   visible light photon → electron

3. Bottom TFT array – (comprised of a matrix of small detector elements called DELs). Each del is comprised of a capture element or pixel detector, a storage capacitor and a thin-film transistor or switch that acts to open and close the release of electric charge leaving each del, thus producing the digital image.

   electron → visible light photon

   • Spatial resolution: determined by fill factor – the percentage of the active pixel area within each del. i.e. the higher the fill factor, the greater percentage of space within the DEL occupied by the active pixel area, the higher the spatial resolution.

Charged Coupled Devices

CCD systems differ from the TFT systems in that there is no photocathode or TFT layer. In CCD systems, the scintillation layer is optically coupled to each CCD sensor chip by either lenses or fibre optics. The CCD system are able to convert light photons into electrical signals, which are then processed by a computer.

Each light photon (from the intensifier output) absorbed in the silicon substrate of the CCD gives rise to an electron–hole pair. The quantity of electronic charge that accumulates at each pixel is directly proportional to the intensity of the incident light and the frame integration time

The technological benefits of CCD sensors include:

• Small, inexpensive and compact with low power consumption
• Self-scanning image readout (no large electromagnetic deflection coils required)
• Negligible lag (less temporal unsharpness)
• Resilience against burned-in signals at high-intensity lights
• Geometrical precision and spatial uniformity
• Excellent thermal, electrical and magnetic stability
• Excellent serviceability and long life-time
• Compatibility with digital X-ray imaging modalities

Spatial resolution can be improved by increasing the number of pixels in the array.

3.2 Direct detection systems

1. Semiconductor layer between electrodes – comprised of amorphous selenium based semi-conductor situated between two electrodes. The interaction between incident x-ray photons and the a-Se causes the release of electrons.

   x-ray photon → electron-hole pairs

2. TFT layer

• This systems works by applying a high-voltage charge to the top surface nanoseconds before the exposure is made. The interaction between the incident X-ray photons and the higher voltage charges causes the selenium atoms to release electrons. The electrons are then collected and processed by the TFT layer

Qauntum Detection Efficiency

QDE is the fraction of incident X-ray photos that interact with the detector

QDE can be increased by increasing the thickness of phosphor (CsI) however this degrades the spatial resolution

Where is the best place to image using an image intensifier?

Resolution, brightness and contrast are greatest in the centre

4. Comparison of II and FPD

5. Automatic Brightness Control

Automatic brightness control ABC (also known as Automatic Exposure Rate Control), automatically adjusts the exposure factors to maintain a constant image SNR. It achieves this through use of a sensor which measures output and a feedback loop to the generator to adjust kV, mA, pulse width (for pulses systems), filtration (modern systems) or lens aperture (old II systems)

6. Modes of Operation

6.1 Continuous mode

• Produces a continuous x-ray beam (0.5 – 6 mA)
• Video camera displays the image at 30 FPS (i.e. each frame = 33 ms)
• Usually used to assist with positioning or guidance of wires for example.

6.2. Pulsed mode

• The x-ray generator produces a series of short x-ray pulses (30 pulses/s)
• Each pulse can be 3 – 10 ms (i.e. pulse width), mA is usually increased to match exposure rate of continuous fluoroscopy
• Better temporal resolution (3 – 10ms) than continuous fluoroscopy (33 ms), hence less motion blur artifact

6.3 Variable frame rate pulsed mode

• Allows pulsed fluoroscopy at selectable frame rates (e.g. 30, 15, 7.5, 3.75 FPS)
• Lower frames rates reduces patient dose and is used when a lower temporal resolution is acceptable for a portion of the procedure (e.g. guidance of a catheter to it’s target vessel)

6.4 High dose/Boost mode

• Maximum patient entrance skin dose rate must be less than 150mGy/min

6.5 Acquisition mode

• High dose (~ 20x compared to continuous mode), high quality images acquired in rapid succession for analysis
• Can be used for single-shot or continuous mode (30 frames/s)

7. Magnification modes

Depending on the region of interest, smaller fields of view (FOV) can be selected to magnify the anatomy and increase spatial resolution. This is achieved by 

• A collimator adjusts the x-ray beam to a small FOV, resulting in a smaller central region of the input phosphor being irradiated
• Magnification is then achieved electronically with electronic focusing of the electron beam using the focusing electrodes of the optic system. The focal spot of the beam is brought closer to the input screen.

7.1. Effects on image quality

• Magnification reduces brightness on the output screen (as less signal from the input screen is available)
• Magnification however improves spatial resolution as the output image size remains constant.

7.2. Effects on dose

As exposure factors are increased to compensate for the reduction of brightness, this increases the patient’s skin dose. However, because the surface area is reduced, the effective dose would be less in comparison with a full FOV without collimation

Quality Assurance

Contrast resolution is tested with a low-contrast test object, such as a Leeds test
object, comprising a flat disc 6 mm thick and 200–300 mm in diameter containing
circular inserts of high-atomic-number material that produce varying levels of contrast.