Fluoroscopy

Fluoroscopy is the use of real time x-ray imaging. It either utilises image intensifiers, which have been in use since the late 1950's, or the newer flat panel detector which is similar to the digital radiography used in projection radiology.

Definitions

Fluoroscopy

  • Real time imaging viewed on a display monitor in the clinical room
  • Matrix is smaller (512 x 512 pixels) and 8 bits of grey scale only needed as temporal, not spatial, resolution is prioritised
  • Can acquire continuous (cine) or pulsed fluorographic images 

Fluorography

  • Images usually formed and viewed after the x-ray exposure is complete
  • Better quality images acquired than in fluoroscopy but at higher doses
  • Matrix is typically 1024 x 1024 pixels with each pixel representing 10 bits of grey scale information

'Greyscale' digital x-ray imaging modes

  • e.g. fluoroscopy, digital cardiac and digital spot imaging in which the image is similar to a plain film radiograph or inverted.

Subtractive digital imaging

  • e.g. angiography. Base image taken which is then subtracted from the image with contrast to result in an image showing the contrast only.

 

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Image intensifier (II)

The IITV system is characterised by the ability to convert the input light into a much higher output (gain or intensification). The x-ray II tube is a vacuum glass or ceramic envelope surrounded by a metal housing (to shield from external light sources and magnetic fields).

The degree to which an image is intensified (or amplified) is given by the brightness gain (Gbrightness) which is the ratio of the brightness of the output screen compared to the input screen.

Gbrightness = Gminification x Gflux
 
where:
Gminification = minification gain
Gflux = flux gain

 

In turn, the minification gain describes how much the brightness has increased due to the demaginification of the image in the II tube.

Gminification = (Dinput / Doutput)2
 
where:
Dinput = diameter of the input screen
Doutput = diameter of the output screen

 

In measuring the ability of the II tube to amplify the signal we are unable to measure the brightness of the input screen which limits the use of the Gbrightness. We now use the image intensifier conversion factor (Gx).

Gx = L / X'
 
where:
L = luminance of the II output (units = candelas m-2)
X' = II entrance dose rate (units = μGy s-1)

 

Factors that affect the brightness gain of an II tube:

  • Magnification -
    • The demagnification (i.e. minimising) electron-optical factor. More demagnification = higher gain
    • Zoom field: Gx falls in proportion to the reduction in the area of the input field
  • Flux -
    • Higher voltage difference applied across II tube = more flux gain

Some numbers:

  • Gminification ~ 100
  • Gflux ~ 100
  • Gx ~ 10-30

Configuration of equipment

  • Standard R and F:
    • Under table x-ray tube with an overcouch II system
    • Table can be tilted and rotated
    • Radiologist stands beside patient to operate system
  • Remote control R and F:
    • X-ray tube mounted over the bed and II system underneath
    • Higher scattered dose so radiologist has to stand behind screen to operate system via remote control
  • Fixed C-arm:
    • II x-ray tube and II system attached to a C-structure that allows it to be rotated around and moved up and down patient as required
    • Can be ceiling or floor mounted
  • Mobile C-arm:
    • System that can be transported on wheels to where it is needed e.g. theatres

Image intensifier system (IITV)

 

II system

 

Internal construction of an II x-ray tube:

  1. Input screen
  2. Electron-optics
  3. Output stage

 

1) Input screen

Input screen of II tube

 

1. II input window

  • Convex metal shield that covers the input face of the II
  • Usually made of aluminium or titanium foil (low Z metal) to allow x-ray beam to enter with minimum attenuation
  • Provides protection for sensitive input components of the tube and maintains the vacuum

2. Input phosphor

  • Layer of sodium activated caesium iodide (CsI:Na) for good x-ray absorption efficiency (70-90%)
  • Channelled into tiny needle-like crystals (5µm in diameter) with fibreoptic-like characteristics
  • Deposited on a thin aluminium substrate
  • CsI:Na usually 400-500µm thick
  • Each x-ray photon produces ~3000 light photons in the blue spectrum

3. Photocathode

  • Fluorescent emission from phosphor then absorbed in a light-activated photocathode comprising a very thin layer of antimony caesium (SbCs3) alloy that has a spectral sensitivity well matched to the blue light emission of CsI:Na
  • Absorption of the fluorescent light photons releases a pattern of electrons in the body of the II tube
  • Approximately 200 electrons released per absorbed x-ray photon

 

2) II electron optics

The input screen is maintained at a negative voltage with respect to the anode (output screen) with a potential difference of 25 kV. This means the electrons produced are accelerated across the II tube and carefully focused on the output screen. The output screen is 1/10 the diameter of the input screen and, therefore, a minified and inverted image is produced.

Electron Focusing

Focusing electrodes are metal rings within the tube that are held at positive voltages with respect to the photocathode. This constrains the electrons in the tube to travel along paths that lead them directly to the output screen, such that the pattern of electron intensities falling on the screen are an exact (but minified) replica of the pattern intensities on the input screen.

Magnification

 

Magnification in fluoroscopy

 

Magnification is achieved electronically with electronic focusing of the electron beam. If a smaller area of the input screen is sampled the image is still shown on the same area of output screen (the output image size remains constant). This results in a magnified image. Because less signal is used, the image is less bright and, therefore, a higher dose is needed. However, as the image magnified, the resolution is better.

Magnification causes:

  • Less bright image and increased dose required
  • Better resolution

 

3) II Output Stage

Output Screen

Thin layer of silver-activated zinc cadmium sulphide (ZnCdS:Ag) crystals deposited on the inner surface of the output window that convert the electrons into light photons. The output image is intensified significantly by the acceleration of the electrons and the minification of the image that occurs in the II tube. The screen is normally 25-35 mm in diameter and a few micrometres thick.

This inner surface of the output screen is coated in a very thin layer of aluminium that:

  • Forms part of the anode structure
  • High speed electrons travel through the aluminium layer
  • The layer is opaque preventing the light emitted by the phosphor from back-illuminating the photocathode and degrading II performance. The light is reflected back towards the output increasing the gain of the II tube.

Output Window

This is an optically transparent glass block port through which the intensified light image exits the II tube. Scatter of light, or halation, in the output window can seriously degrade the contrast of the II output image. Minimising halation can be done with:

  • Smoked glass
  • Special optical coatings
  • Very thick glass
  • Fibre-optic bundle

 

Summary

  1. X-ray photons enter tube through aluminium or titanium window
  2. Hit input phosphor layer of sodium activated caesium iodide and release light photons
  3. Light photons detected by photocathode that then release electrons into the tube
  4. Electrons accelerated and focused onto the output screen (silver-activated zinc cadmium sulphide crystals) as a minified and inverted image
  5. Light photons released that then leave through the output window 

Display of image

The image from the output screen is displayed on a monitor using a TV imaging system.

 

II TV camera

These are no longer used in clinical practice.

Electronic TV camera tube

A TV camera tube uses an electron beam that scans across, line by line. The scanning direction is determined by focusing and deflection coils arranged around the outside of the tube.

 

II TV camera

 

Determining resolution:

  • Vertical resolution is determined by the number of scan lines
  • Horizontal resolution is determined by the bandwidth of the system (higher bandwidth = lower resolution)

 

Charged coupled device (CCD sensors)

Solid state CCD sensors are superseding the electronic TV camera tube. Each pixel in the CCD has an associated electrode. A positive bias voltage is applied to the electrode that forms a "potential well" in the region of the silicon substrate.

  1. The light photons (from the II output) are absorbed into the silicon substrate of the CCD (light-sensitive array)
  2. Each light photon gives rise to an electron-hole pair
    • The positive "hole" drains away
    • The negative electrons accumulate in a potential well
  3. These charge packages are then transferred to the light shielded storage array
  4. The data is then read from the storage section line-by-line
    • The quantity of electronic charge which accumulates at each pixel is directly proportional to the intensity of the incident light

 

Charged coupled device

 

Benefits

  • Small, inexpensive, compact, low-power consumption
  • Self-scanning image readout
  • Negligible lag (important for video fluoroscopy)
  • Excellent thermal, electrical and magnetic stability
  • Excellent serviceability and long life-time
  • Compatibility with digital x-ray imaging modalities

 

Image quality

Automatic brightness control (ABC)

Aka automatic dose rate control. The purpose of the ABC is to maintain constant viewing condition independent of examination. This is done by mA and kV regulation. The need to alter the mA or kV is determined by either electronically sampling the video signal or by measuring the II light output with a photo-sensor.

The allowable dose allowed by the ABC is determined by the mode used:

  • Minimum patient dose rate mode
  • Standard patient dose rate mode
  • High patient dose rate mode (high image quality)

N.B. ABC is used in fluoroscopy, automatic exposure control (AEC) is used in fluorography

Digital image processing

To improve the appearance of the image on the screen there are several algorithms that can be applied to the digital image.

Greyscale processing

  • Greyscale range compression
  • Used to suppress or highlight intensities and improve contrast balance of image. Achieved by using either an analogue (video) circuit or via a look-up-table (LUT)
  • Contrast and brightness adjustment

Spatial filtering

  • Similar to edge enhancement in projection radiography
  • Improves displayed spatial resolution
  • Best used for high contrast image e.g. barium GI studies

Temporal filtering

  • This is used to decrease the level of noise
  • The current frame is averaged with a set of the preceding frames. This creates a digitally generated lag to smooth the noise fluctuations. Also, the higher signal created by combining several frames results in a smaller proportion of noise
  • This is best used for structures that arec quasi-static 

Flat panel detector

Flat panel detectors utilise the same technology as digital radiography in that there is a flat panel of detectors that provide a direct electronic readout instead of requiring the conversion of analogue to digital as is seen in the IITV. Similar to digital radiography dynamic FP detectors can be direct or indirect. However, they are more commonly indirect with a CsI:Tl x-ray scintillator layer which is superimposed onto an a-Si high resolution active matrix.

Benefits

  • Smaller equipment
  • Video signal emerges in digital form, reducing electronic noise
  • Square or rectangular field (unlike circular field in IITV) = better coverage in the corners
  • Better temporal resolution with matrix size of 2048 x 2048 pixels
  • Greyscale of 12 or 14 bits per pixel
  • Produces better quality images than IITV
  • Fewer artefacts such as geometrical distortion, vignetting or contrast loss
  • Detective quantum efficiency 10-20% better than IITV so can afford to reduce patient dose
  • Zoom option available (but doesn't increase spatial resolution as it does in IITV)

Digital subtraction angiography

A common procedure performed using fluoroscopy is a digital subtraction angiography (DSA). In this procedure the contrast outline structure is highlighted by removing the background anatomical structures from the images. This is done in four stages:

  1. Acquire mask image (IM) to record anatomical background
  2. Contrast injected. Series of images acquired which show arrival and run-off of contrast (contrast medium enhanced image, IC)
  3. Image frames subtracted via digital processor. Any structures that are common to set 1 and set 2 are subtracted (i.e. all background anatomy but not the contrast filled structures)
  4. Amplify contrast signal to boost displayed contrast of the vessels

Artefacts

The artefact most unique to DSA is misregistration

  • The movement of a structure by even 1 mm can cause misregistration.
  • These are corrected by the computer with:
    • Pixel shifting (contrast and mask images spatially offset prior to subtraction to compensate for movement) and
    • Remasking (re-mask to an image later in the run-off phase instead of the initial mask)

Dose

Dose to patient

The dose to the patient is better represented by the skin dose rate i.e. the dose per unit of time.

Maximum entrance skin dose rate limit 100 mGy per minute

IITV patient entrance skin dose

3 to 10 mGy per minute

10 to 30 mGy per minute for larger patient

Digital fluoroscopy 10 - 40 nGy per frame at II entrance
Digital cardiac image acquisition Dose per frame 10x higher than fluoroscopy
Digital spot imaging (fluorography) Dose 100x higher than fluoroscopy
Subtractive digital fluorography (e.g. angiography) Dose 1000x higher than fluoroscopy

 

Minimising patient dose

  • Setup of equipment
    • Tight collimation of x-ray beam
    • Appropriate x-ray beam spectral filter to minimise patient skin dose rate
    • Increase distance between patient and x-ray source
    • Minimise gap between patient and II entrance
    • Remove anti-scatter grid if possible
  • Imaging procedure
    • Avoid constantly imaging at same projection angle
    • Minimise x-ray beam on time
    • ABC mode with lowest dose rate possible for diagnostic images
    • Pulsed fluoroscopy with minimum acceptable pulse rate if possible
    • Avoid use of II zoom
  • Digital processes
    • Last-image-hold
    • Road mapping - digital fluorographic image acquired during contrast injection phase. This image is then subtracted from subsequent fluoroscopy images in real time to highlight the contrast-injected structure and remove the background anatomy
  • Fluoroscopy
    • Maximise concentration of contrast medium in vessel of interest, e.g. intra-arterial rather than intra-venous, to increase signal and therefore enable lower dose

Dose to staff

  • Stray radiation:
    • Leakage of from tube housing should be less than 1 mGy per hour at 1 metre from the focus
    • Scatter of x-rays from patient is the most significant contribution to staff dose. ~0.1% of patient dose at 1m distance
    • Secondary scatter of x-rays from structures in the room

Minimising staff dose

  • Use of lead aprons and other radiation shields e.g. gloves, glasses and thyroid protection
  • Lead-rubber drapes and movable lead glass shields
  • Maintain maximum possible distance from patient
  • Monitor individual staff doses

 


Σ  Summary

Definitions:

  • Fluoroscopy: Real-time imaging viewed on display monitor in clinical room. Higher temporal but lower spatial resolution than fluorography
  • Fluorography: Image displayed after x-ray exposure
  • Fluorography and fluoroscopy imaged using image intensifier system (IITV) or, more recently, digital flat panel detectors (FP detector)

IITV system

Measuring intensification

  • Calculations
    • Brightness gain = minification gain x flux gain
    • Minification gain = (Dinput / Doutput)2 (where D is diameter of input and output screen respectively)
    • Image intensifier conversion factor (Gx) = L / X' (where L = luminance of II ouput, X' = II entrance dose rate)
  • Factors that affect brightness gain
    • More minimisation = higher gain
    • Gx falls in proportion to the reduction in the area of the input field in zoom setting
    • Higher voltage applied across II tube = more flux gain

II x-ray tube

  • Input screen
    • II input window: aluminium or titanium foil to allow x-rays to enter tube and maintain vacuum
  • Input phosphor
    • Layer of CsI:Na for good x-ray absorption efficiency. Each x-ray photon produces ~3000 light photons in blue spectrum
  • Photocathode
    • Fluorescent emission from phosphor absorbed by light-activated photocathode made of SbCs3 which then releases electrons into body of II tube

II electron optics

  • Input screen at negative charge compared to output screen to direct electron towards output screen.
  • Electron focusing: positively charged electrodes along tube direct electron path to create exact but minified and inverted image on input screen
  • Magnification: achieved via electronically focusing electron beam. Magnified images use less signal and so need a higher dose but improve the resolution

II output

  • Output screen made of thin layer of ZnCdS:Ag that convert electrons into light photos that then leave through the output window

Display of image

  • II TV camera
    • Electronic TV camera uses electron beam that scans across photoconductive target to create flow of electrons, the rate of which corresponds to the amount of light photons striking that area
  • Charged coupled device (CCD sensors)
    • Now more commonly used
    • Each pixel has an associated electrode. The accumulation of charge is directly proportional to the intensity of the incident light
    • Flat panel detector
      • Utilises same technology as digital radiography
      • Most commonly indirect dynamic FP detector with CsI:Tl x-ray scintillator layer superimposed onto a-Si high resolution active matrix

Image quality

  • Automatic brightness control (ABC, fluoroscopy) (c.f. automatic exposure control, AEC, fluorography)
    • Alters kV and mA to ensure stable quality of images. This, in turn, alters the patient dose
    • Done by measuring II light output with a photo-sensor or electronically sampling video signal
  • Digital processing
    • Grey scale processing with greyscale range compression: suppress or highlight intensities and improve contrast balance. Uses analogue (video) circuit or via a look-up-table (LUT)
    • Spatial filtering: similar to edge enhancement in projection radiography. Improves displayed spatial resolution
    • Temporal filtering: decreases level of dose by summing current image with previous frames, averaging out signal and resulting in smaller proportion of noise

Dose

  • Patient dose
    • Measured in skin dose rates
  • Staff dose
    • Greatest contribution to staff dose is from scatter, amounts to ~0.1% of patient dose at a distance of 1m

Next chapter: CT imaging


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