Fluoroscopy

Fluoroscopy provides real-time x-ray 'videos' of anatomy. The images are of low spatial resolution but high temporal resolution. In fluoroscopy, a high frame rate of up to 30 frames per second is applied, meaning nearly 1000 images are taken over a period of minutes. Thus, if the same dose was delivered as in conventional radiography, the patient would be severely over irradiated. In fluoroscopy, the dose is reduced to 1/1000th of conventional radiography, resulting in poor spatial resolution. Because of this low dose, either an image intensifier or flat panel detector is used.

Image Intensifier

As such low doses are delivered per frame, the detected output signal from the patient is very small and produces an almost black image. An image intensifier aims to amplify the signal from the patient in order to produce an image of adequate brightness.

An image intensifier consists of an external vacuum, input layer, electron optics system and an output phosphor. After the image intensifier exists a lens assembly, solid-state charged coupled device and video monitor. The external vacuum functions to keep air out, which would impede electron trajectory.

The input layer consists of an input phosphor, the most outer layer of the input phosphor, and the photocathode. The input phosphor converts transmitted x-rays into light, which is then converted into electrons by the photocathode. The input phosphor consists of caesium iodide crystals arranged in to columns which guide light onto the photocathode. The columns are approximately 400 micrometers long and 5 micrometers in diameter. For a 60keV photon, approximately 3000 light photons are created.

The photocathode consists of antimony and alkali metals, and converts light produced by the input phosphor into electrons. For a 60keV photon, approximately 400 electrons are produced.

The electron optics system consists of an anode, cathode and 3 intermediate electrodes (G1, G2 and G3). The system functions to accelerate electrons and focus these electrons onto the output phosphor. A large electric field is shaped by the intermediate electrodes, up to 30,000kV. Whilst moving through the II and being accelerated, the electrons gain kinetic energy termed electronic gain.

Electrons incident on the output phosphor are converted into light, which is a significantly amplified version of the incident signal. It consists of zinc cadmium sulphide, which has a green emission. The output phosphor is very thin, only 4-8micrometers, and thus produces images of high spatial resolution. In general, one electron results in the emission 1000 light photons at the output phosphor.

When the light is emitted at the output phosphor, some is reflected back by the glass window, termed veiling glare. This reflection results in reduced image contrast, and is corrected by placing a black pigment on the sides of the glass window which absorbs any internally reflected light.

NB: The input phosphor is significantly larger (15-35cm) than the output phosphor (2.5cm), meaning there is significant light amplification.

Electronic Gain

When electrons are accelerated from the photocathode to the output phosphor, they gain kinetic energy. This energy is termed electronic gain and normally has a value of 50.

Minification Gain

Minification gain is a result of the difference in size of the input and output phosphors. As the input phosphor is significantly larger than the output phosphor, electrons emitted by the photocathode must be focused onto the small area of the output phosphor. This results in light amplification at the output phosphor, termed minification gain. Minification gain can be calculated by dividing the input phosphor area by the output phosphor area. It normally has a value of 100.

Total Brightness Gain

Total brightness gain is the product of electronic and minification gain, and has a value between 2500 and 7000.

f Number

The f number gives information on the size of the adjustable aperture of the lens assembly, thereby detailing the amount of light let through.

• A high f number indicates a small aperture has been used and the electric signal is small. More light is required to create an image of suitable brightness and thus patient dose is higher.
• A low f number indicates a large aperture has been used and the electric signal is large. Less light is required to create an image of suitable brightness and thus patient dose is less, but significant light scattering occurs and the image is noisy.

Distortion of an Image Intensifier

There are three types of distortion; pincushion distortion, S distortion and vignetting. 

Pincushion distortion occurs due to the curved surface of the input screen and the flat surface of the output phosphor. Although the curved surface of the input screen is required for electron focusing, projecting images onto the output phosphor results in distortion. A way to correct this is by placing the anatomy of interest in the central area of the image intensifier.

S distortion occurs when there is spatial wrapping of the image into an S shape. This is a result of a stray magnetic field and the Earth's magnetic field affecting the trajectory of the electrons inside the image intensifier.

Vignetting occurs because of the increased concentration of electrons at the centre of the output phosphor compared to the periphery, causing the centre of the image to appear brighter.

Flat Panel Detector

Fluoroscopy using an image intensifier is 'conventional', however digital fluoroscopy involves a flat panel detector. Flat panel detectors are solid-state radiation detectors which are highly sensitive to low output signals and replace the need for the image intensifier, lens assembly and camera system. Flat panel detectors record real time fluoroscopic images and use either direct or indirect conversion into an electrical signal.

In indirect conversion, incident x-rays are converted into light by a photocathode, which is then converted to charge by a photodiode. This charge is read out by a TFT. In direct conversion, incident x-rays are converted directly to charge by a selenium semiconductor. The charge is again read out by a TFT.

The benefits of digital fluoroscopy are:

• More compact
• High sensitivity to low output signals
• Generally require less patient dose
• Allow for post-processing
• Enable direct acquisition of data in the digital format
• Produce less distortion than image intensifiers
• Higher dynamic range
• High quality dynamic and static image capture

Binning

In digital fluoroscopy, dexel size is generally larger than in conventional radiography. This is because, unlike in radiography, we do not require images of incredible spatial resolution. Binning is a process in which large amounts of data can be compressed into small amounts, thereby quickening the transfer rate and allowing for real-time display of the images. Adjacent dexels can be binned together to form one dexel, improving the transfer rate and reducing the effect of noise on the image.

Frame Averaging

Noise is a significant issue in fluoroscopy as a result of the low mAs delivered to the patient. This can be reduced by averaging adjacent frames, however causing a decrease in temporal resolution. Aggressive use of frame averaging reduces image noise and patient dose but potentially introduces image lag and a loss of the required temporal resolution.

Last Frame Hold

Last frame hold is a mechanism used to display the last image of the sequence even without delivering excess radiation to the patient. In last frame hold, the technician will remove their foot from the exposure pedal and the last image will be displayed on the tv or computer, allowing for inspection or diagnosis without unnecessarily exposing the patient.

Automatic Exposure Rate Control

For fluoroscopy where high frame rates are used, manual adjustment to dose is impractical. Thus, an automatic system is used which employs a feedback loop between the x-ray generator and x-ray intensity measured by the detector. AERC through the feedback system makes any changes necessary which are detected over time by changes in x-ray intensity transmitted through the patient, i.e. increase mAs for thicker patient regions (e.g. abdomen) but decrease mAs for thinner patient regions (e.g. hand).

In fluoroscopy, AERC may change kVp, mAs or both.

• An increase in kVp will increase dose but reduce contrast resolution, as the relative amount of Compton scattering to photoelectric absorption increases.
• An increase in mAs will increase dose but preserve contrast resolution.

Pulsed and Continuous Fluoroscopy

There are two modes of operation in fluoroscopy, pulsed and continuous. In continuous fluoroscopy, the simplest, analogue mode, a continuous beam is applied of 0.5-6mAs. The camera displays the images at 30 frames per second, with each image being displayed for approximately 33ms. As this is a very high dose procedure, continuous fluoroscopy is restricted to the mini c-arm.

In pulsed fluoroscopy, a pulsed x-ray beam is delivered to the patient. The pulsed mode is achieved by switching the generator off an on, or introducing a grid control tube which is an additional electrode between the anode and cathode of the x-ray tube which repels electrons from the anode. The exposure time in pulsed fluoroscopy is smaller, between 3 and 10ms, which is particularly useful when imaging either a moving patient or a moving anatomical site, e.g. blood vessel. By using pulsed fluoroscopy, motion blurring is reduced.

Dose rate in pulsed fluoroscopy is proportional to pulse rate, which is varied depending on the clinical task. In all cases, the lowest possible pulse rate is used to reduce rate. A high pulse rate may be used for fast movement or to avoid blurring, and a low pulse rate may be used for slow movement.

Magnification Mode

Magnification mode is a technique possible in image intensifiers. A higher voltage is applied across the image intensifier to focus the electrons released by the photocathode onto a smaller area of the output phosphor. With increasing magnification factor, a smaller area of the input phosphor is seen in the final image.

In magnification mode, x-ray collimators adjust the x-ray beam to the required field size. 

NB: As magnification increases, minification and overall brightness gain decrease. Because of this decrease, automatic exposure rate control increases patient dose to compensate. Thus, the lowest possible magnification and collimation must be used to reduce patient dose.