X-ray image intensifier

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.[1]

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. Further advancements include FPD being produced in the late 1990s giving competition to the image intensifiers. (Seibert, 2006).[2]

 

Clinical applications

Modern imaging systems will use the image intensifier as the source of images supplied to a storage system.

It will be used either as a fixed piece of equipment in a dedicated screening room or as mobile equipment for use in an operating theatre.

Fixed X-ray image intensifier

Components

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:

Mobile Fluoroscopic System AKA "portable C-arm"

C-arm of a mobile X-ray image intensifier

General configuration and range of movements

A mobile image intensifier generally consists of two units, the X-ray generator and image system on a portable imaging system (C-arm) and the 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 variety 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.

Workstation unit

Much of the operation of the machine is from the workstation unit. This has the following features:

Types of X-ray tube

Two types of X-ray tube may be fitted, fixed anode or rotating anode. Typical features of fixed anode tubes include:

1.8 by 1.8 mm focal spot size for radiographic applications.

Typical features of rotating anode tubes include:

The housing also has a heat storage limitation, typically 1200-1250kHU

[3][4]

Electronic capabilities

The images can be manipulated in many ways on the computer screen. Examples of this are:

Generator and range of exposures

Modern systems use a digital high frequency generator with typically 20,000 cycles per second. The range of kVp settings may be from 40kV to 120kV. The tube current is typically 0.1mA to 6mA for fluoroscopy examinations. For radiographic mode the mA is fixed at about 20mA to 60mA. mAs values vary from 0.16 to 160 for radiographic application. The electronic timer varies from 0.1sec to 4.0sec for radiographic exposures.

Image intensifiers, size and features

They 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 - image intensifier replacement

Flat Detectors are an alternative to Image Intensifiers. They are currently offered on imaging systems manufactured by Liebel-Flarsheim (Direct Digital Imaging System), OrthoScan,[5] Ziehm Imaging, Siemens, GE, Medtronic (The "O-arm" System), FujiFilm Japan and Philips Healthcare The Flat Detector (FD) will replace the Image Intensifier. 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. (Seibert, 2006).[2]

Feature Comparison table of II/TV and FPD Systems

Feature Digital Flat Panel Conventional II/TV
Dynamic range Wide, about 5,000:1 Limited by TV, about500:1
Geometric distortion None Pin-cushion and ‘S-distortion
Detector size (bulk) Thin profile Bulky, significant with large FOV
Image area FOV 41 x 41cm 40cm diameter (25% less area)
Image quality Better at high dose Better at low dose

Table 1: Feature comarison of II/TV and FPD Systems, From Pediatric Radiology. Vol36(s2).p185. By Seibert, J. A., (2006)[2]

Type of TV camera and coupling to II

Older machines may have a vidicon type pickup tube, with direct fiber-optic coupling to the image intensifier. Modern machines may have a CCD camera.

Radiation safety features

Special features

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:

Potential safety issues

Failure of the X-ray beam collimation may lead to primary beam X-ray exposure outside of the selected image intensifier input area. This would result in image degradation. Light generated outside the area of the image intensifier input at magnification causes additional loss of contrast of the image with increased noise. Additionally, unnecessary additional dose to the patient would result. If the C-arm or fittings are damaged, the X-ray tube and intensifier may become misaligned resulting in image degradation or loss, as well as presenting a potential injury to staff and patient if the structural integrity of the C-arm or mounted components are compromised.

Technical capabilities

Image intensifiers are usually set up for two purposes. For either plain fluoroscopy or digital subtraction angiography (DSA). All image intensifiers are set up with software capable of adjusting settings to suit different user requirements, depending on the procedure and body area being imaged. In simple fluoroscopy for example, imaging of the throat would not require the same amount of exposure as that of the abdomen. And on DSA capable models, preset programs are available which enables the user to decide a rate of how many images or frames per second are acquired.

Future developments

References

  1. ↑ Krestel, Erich (1990). Imaging Systems for Medical Diagnostics. Berlin and Munich: Siemens Aktiengesellschaft. pp. 318–327. ISBN 3-8009-1564-2.
  2. 1 2 3 4 Seibert, J. A. "Flat panel detectors: how much better are they?". Journal of pedeatric radiology Vol36(s2). doi:10.1007/500247-006-0208-0.
  3. ↑ OEC-DIASONICS, series 9400, date: 12/1991
  4. ↑ Product Brochure, Philips Medical Systems
  5. ↑ http://orthoscan.com/mini-c-arms/fd-pulse/flat-detector-advantages-2
  6. ↑ walterrobinson.com > Last Image Hold Feature Retrieved on April 3, 2010
  7. ↑ Karch, J., Krejci, F., Barti, B., Dudak, J., Kuba, J., Kvacek, J., & Zemlicka, J. "High contrast X-ray micro-tomography of low attenuation samples using a large area hybrid semiconductor pixel detector array of 10 x 5 Timepix chips". . Journal of Instrumentation. Vol 11.
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