An x-ray image intensifier (XRII) is an image intensifier that converts x-rays into visible light at higher intensity than mere fluorescent screens do. Such intensifiers are used in x-ray imaging systems (such as fluoroscopes ) to allow low-intensity x-rays to be converted to a conveniently bright visible light output. The device contains a low absorbency/scatter input window, typically aluminum, input fluorescent screen, photocathode, electron optics, output fluorescent screen and output window. These parts are all mounted in a high vacuum environment within glass or more recently, metal/ceramic. By its intensifying effect, It allows the viewer to more easily see the structure of the object being imaged than fluorescent screens alone, whose images are dim. The X-ray II requires lower absorbed doses due to more efficient conversion of x-ray quanta to visible light. This device was originally introduced in 1948.
Contents
Operation
The overall function of an image intensifier is to convert incident x-ray photons to light photons of sufficient intensity to provide a viewable image. This occurs in several stages. The first is conversion of x-ray photons to light photons by the input phosphor. CsI:Na is typically used due to its high conversion efficiency thanks to high atomic number and mass attenuation coefficient. The light photons are then converted to electrons by a photocathode. A potential difference (25-35 kilovolts) created between the anode and photocathode then accelerates these photoelectrons while electron lenses focus the beam down to the size of the output window. The output window is typically made of silver-activated zinc-cadmium sulfide and converts incident electrons back to visible light photons. At the input and output phosphors the number of photons is multiplied by several thousands, so that overall there is a large brightness gain. This gain makes image intensifiers highly sensitive to x-rays such that relatively low doses can be used for fluoroscopic procedures.
History
X-ray image intensifiers became available in the early 1950s and were viewed through a microscope.
Viewing of the output was via mirrors and optical systems until the adaption of television systems in the 1960s. Additionally, the output was able to be captured on systems with a 100mm cut film camera using pulsed outputs from an x-ray tube similar to a normal radiographic exposure; the difference being the II rather than a film screen cassette provided the image for the film to record.
The input screens range from 15–57 cm, with the 23 cm, 33 cm and 40 cm being among the most common. Within each image intensifier, the actual field size can be changed using the voltages applied to the internal electron optics to achieve magnification and reduced viewing size. For example, the 23 cm commonly used in cardiac applications can be set to a format of 23, 17, and 13 cm. Because the output screen remains fixed in size, the output appears to "magnify" the input image. High-speed digitalisation with analogue video signal came about in the mid-1970s, with pulsed fluoroscopy developed in the mid-1980s harnessing low dose rapid switching x-ray tubes. In the late 1990s image intensifiers began being replaced with flat panel detectors (FPDs) on fluoroscopy machines giving competition to the image intensifiers.
Clinical applications
"C-arm" mobile fluoroscopy machines are often colloquially referred to as image intensifiers (or IIs), however strictly speaking the image intensifier is only one part of the machine (namely the detector).
Fluoroscopy, using an x-ray machine with an image intensifier, has applications in many areas of medicine. Fluoroscopy allows live images to be viewed so that image-guided surgery is feasible. Common uses include orthopedics, gastroenterology and cardiology. Less common applications can include dentistry.
Image intensifier configurations
A system containing an image intensifier may be used either as a fixed piece of equipment in a dedicated screening room or as mobile equipment for use in an operating theatre.
Permanent/Fixed Fluoroscopic Systems
There are two main configurations of permanently installed fluoroscopic systems. One class commonly utilizes a radiolucent patient examination table with an under-table mounted tube and an imaging system mounted over the table, while the other is commonly referred to as a C-arm system used where greater flexibility in the examination process is needed such as neuro or cardiac imaging.
Modern imaging systems on both configurations are limited in capability only by the desired features the users will want. All frame rates, storage (local or PACS), image capture devices etc. are now far lower in cost than before, software configurable and based on COTS components for all but the camera/II or flat panel devices.
The non-C-arm based systems are used in most X-ray departments as 'screening rooms'. The types of investigations for which this machine can be used for is vast, including:
The C-arm systems are commonly used for studies requiring the maximum positional flexibility such as:
General configuration and range of movements
A mobile fluoroscopy unit (or "c-arm") generally consists of two units, the X-ray generator and image detector (II) on a portable imaging system (C-arm) and a separate workstation unit used to store and manipulate the images. The imaging system unit can perform a variety of movements that allow for use in a range of surgical procedures such as cardiology, orthopedics and urology. This unit provides the appropriate structure to mount an image intensifier and an X-ray tube with a beam limiting device positioned directly opposite from and aligned centrally to each other.
The C-arm is capable of many movements:
The X-ray generator, dose control system and collimator controls are usually housed in the chassis on which the C-arm is mounted. All of the control systems are closed loop systems which are directed by the master controller initial program settings. The master controller generally is found in the work station. User controls on the C-arm allow the operator to modify the operation of the system while in use. I.e. format size, slot collimator position, dose rate etc.
The imaging system must be compact and lightweight to allow easy positioning with adequate space to work around and a wide range of motion while remaining inflexible enough so as to avoid misalignment due to flexion caused by the mass of the X-ray tube or Image system assemblies.
Image intensifier size and features
X-ray systems may be fitted with a range of different types of image intensifiers; typically 16 cm or 22 cm.
Typical specifications for a 16 cm intensifier are:
Typical specifications for a 22 cm intensifier are:
Flat panel detectors
Flat Detectors are an alternative to Image Intensifiers. The advantages of this technology include: lower patient dose and increased image quality because the X-rays are always pulsed, and no deterioration of the image quality over time. Despite FPD being at a higher cost than II/TV systems, the noteworthy changes in the physical size and accessibility for the patients is worth it, especially when dealing with paediatric patients.
Feature Comparison table of II/TV and FPD Systems
Feature comparison of II/TV and FPD Systems
3D imaging
Some imaging systems using either image intensifiers or flat panel detectors are capable of taking images in multiple planes that can be used to reconstruct a 3D volume of the patient anatomy. This capability is typically used for surgical navigation. It can also be helpful for surgeons who want to check the placement of implanted devices in the patient, such as spinal screws. See: