What are Image Intensifiers?

The brightness on the X-ray fluorescent screen is very weak, and it is not suitable to use a television camera to directly capture images. There are three ways to solve this difficulty. The first is to increase the brightness of the fluorescent screen, but this must increase the X-ray dose. The second one uses an enhanced fluorescent screen and a high-sensitivity camera tube, but the quality of the obtained image is not ideal. The third and most reasonable way is to use an image intensifier to convert the X-ray image into a visible light image and increase its brightness thousands of times before taking a picture. According to the type of input light, image intensifiers can be divided into X-ray image intensifiers , infrared image intensifiers, and visible light image intensifiers . The image intensifier is composed of a reinforcing tube, a tube container, a power source, an optical system, and a support (support) part. Its main part is an image enhancement tube, which has an input screen (receiving X-ray radiation to generate electron flow) and an output screen (receiving electron bombardment to emit light), so that the former enhances the image with thousands of times brightness on the output screen. The reinforced tube is a vacuum tube made of glass. For the purpose of protection, a tube container is needed. This tube container also plays a role of shielding X-rays and shielding the external electromagnetic field and protecting the human body from high voltage damage. In addition, the booster also has a set of power sources, including a high-voltage power supply for the booster tube, a focusing power supply, and a power source for driving the deaeration ion pump in the booster tube. The image enhancement tube is the heart component of the image intensifier.

In early fluoroscopy techniques, X-rays emitted by a patient hit the phosphor screen directly. The visible light emitted by each area on the screen is related to the rate of energy deposited by the incident X-rays. A radiologist observes visible light images on a phosphor screen at a distance of 10 or 15 inches. Behind the screen is a thin lead glass plate to protect the radiologist from X-ray radiation that passes through the screen.

Using this fluoroscopy technique, radiologists observe a dim image with low detail visibility. In order to view such images, they had to prolong their time in the dark to make their eyes "dark fit". In the 1940s, radiologists realized that the low visibility of details during fluoroscopy was related to the dim images of early fluoroscopy. They emphasized the need for brighter fluorescent images and encouraged the development of image intensifiers. Image intensifiers increase the brightness of fluorescent images, and observers can use bright vision (cone cells) instead of scotopic vision (rod cells) required in earlier fluoroscopy. Because the image is bright, fluoroscopy using an image intensifier does not require dark adaptation. Although image intensifiers increase the cost and complexity of fluoroscopy systems, the use of non-image-enhanced fluoroscopy is obsolete ^[1]

X-ray image intensifiers "enhance" or increase the brightness of the image through two processes: (1) shrinking, emitting a given number of visible light photons from a smaller area; (2) flux gain, high-voltage accelerated When electrons hit the phosphor screen, more visible light photons are produced. The "night vision" device used in the military also uses this principle to observe targets in low light conditions.

X-ray photons hit a fluorescent screen (input screen) with a diameter ranging from 4 inches to more than 16 inches and a slight convex surface. The fluorescent emulsion of the input screen is a thin layer of cesium iodide (Csl). Earlier image intensifier input screens were composed of zinc cadmium sulfide (ZnS: CdS). The main advantage of CsI over ZnS: CdS: Ag is that it can increase the absorption of X-rays. This is due to the presence of a higher Z (atomic number) component in the CsI fluorescent substance and the accumulation of CsI molecules in the fluorescent particles Higher density.

For each X-ray photon absorbed, 2000 ~ 3000 visible light photons can be emitted on the input screen. These photons are not directly observed, but fall on a photocathode containing antimony (Sb) elements, such as cesium antimony oxide (Sb-CsO). Visible light photons released in a direction opposite to the photocathode are reflected to the photocathode by a mirror-like aluminum support on the outer surface of the input screen. If the spectral sensitivity of the photocathode matches the wavelength of visible light emitted by the phosphor screen, the photocathode can emit 15-20 electrons for every 100 visible photons received. The number of electrons released by any area on the photocathode depends on the number of visible photons incident on that area. The potential difference between the photocathode and the anode of the image intensifier tube is 25 to 35 kilovolts. This voltage accelerates the electrons. The electrons pass through a large hole in the anode and hit a small fluorescent screen (output screen) mounted on a flat glass holder. The emulsion on the output screen is similar to that on the input screen, except that the fluorescent particles are much smaller. Most output screens are 0.5 inches to 1 inch in diameter. A booster with a small output screen can be used for TV fluoroscopy because the input screen diameter of a TV camera is also small. To prevent light from entering outside the booster, the output screen is coated with a layer of metal (usually aluminum). The metal layer can also remove electrons accumulated by the output screen.

A cylindrical electrode placed between the photocathode and the anode focuses the electrons emitted from the photocathode on the output screen. Three focusing electrodes are usually used. The glass package is contained in a high-permeability alloy (iron-containing alloy) housing. The housing attenuates the magnetic fields generated outside the booster and prevents these magnetic fields from distorting the movement of electrons inside the booster. The strong magnetic field around the image intensifier may still distort the motion of the electrons, so the image on the output screen is distorted. In addition, the strong magnetic field around the image intensifier can magnetize the high-permeability alloy casing and focusing electrode, and cause permanent distortion of the fluorescent image. Therefore, the image intensifier should not be placed near a permanent or transient strong magnetic field (such as near a magnetic resonance imaging system).

With X-ray image intensifiers, there are four different information carriers that pass patient information to the radiologist, and X-ray beams pass information from the patient to the input screen of the image intensifier. On the input screen, the information changes from X-rays to visible photons. When visible photons are absorbed by the photocathode, the information becomes an electron beam directed at the output screen of the enhancer; the information is transmitted from the output screen to the observer's retina in the form of a visible light image.

The brightness of the image on the output screen of the image intensifier can be compared with the brightness of the image on a standard non-image enhanced phosphor screen. When the image intensifier and the fluorescent screen receive the same radiation exposure, the brightness ratio of the two images is called the brightness gain of the image intensifier .

Brightness gain = brightness of the output screen on the image intensifier / brightness on the standard screen .

The image intensifier has a gain range of 1000-6000, depending on the particular image intensifier used and the phosphor screen to which it is being compared. The brightness gain is caused by two independent processes that occur inside the enhancer, which are image reduction and flux gain, respectively.

The input screen of the image intensifier absorbs the light image formed by X-rays and reproduces it on the output screen in a reduced form. Because the output screen is much smaller than the input screen, the number of visible photons per unit area of the output screen is larger than that on the input screen. The increase in image brightness caused by image reduction is called reduction gain

,

It is equal to the ratio of the area of the input screen to the output screen.

= Area of input screen / area of output screen = [(input screen diameter) / 2] ² / [(output screen diameter) / 2] ² = (input screen diameter) ² / (output screen diameter) ² .

For example, an image intensifier with an input screen diameter of 9 inches and an output screen diameter of 1 inch has a reduction gain of 81:

The electrons generated by the photocathode are accelerated when they are directed to the output screen, which will also increase the brightness of the image. When these electrons hit the output screen, the number of photons released varies with the energy of the electrons. The brightness gain caused by the acceleration of electrons is called the flux gain of the image intensifier

. Typical image intensifier

At least 50.

The total brightness gain of the image enhancer is

versus

Product of:

For example, an image intensifier with a reduction gain of 81 and a flux gain of 50 has a brightness gain of 4050:

Two image intensifiers can be compared by describing the conversion factor of each image intensifier. Conversion factor

Is the quotient of the brightness of the output screen of the image intensifier and the exposure of the input screen:

= Brightness of the output screen (canddela / m ² ) / exposure rate of the input screen (mR / sec) .

The conversion factor of an image intensifier depends on the radiated energy and should be measured using X-rays generated by a full-wave rectifier or constant-voltage X-ray generator operating at around 85KVp. Most image intensifiers have a conversion factor of 50 ~ 100 (candela) / (mR · m2).

The resolution of an image intensifier is limited by the resolution of the input and output phosphor screen and the ability of the focusing electrode to hold the image when the image is passed from the input screen to the output screen. For an image intensifier with an input screen of CsI, its average resolution is 4 line pairs / mm. The reasons for the reduced resolution that originated from the outside of the image intensifier include: scattered radiation in the X-ray beam received by the input screen; image sharpness caused by patient motion and the limited size of the focal spot. In addition, the quality of fluoroscopy images is also affected by statistical fluctuations in the number of X-rays hitting the input screen.

The resolution, brightness, and contrast of the image produced by the image intensifier are the largest at the center of the image and gradually decrease toward the periphery. The brightness reduction of the image along the peripheral direction usually does not exceed 25%. The decrease in brightness and image quality along the edges of a fluoroscopy image is called vignetting. Halo is a reflection of a reduction in the exposure rate along the periphery of the input screen and a decrease in the accuracy with which electrons from the periphery of the photocathode hit the output screen. In addition, the center of the output phosphor receives some scattered light from the area around the output phosphor, while the periphery receives only scattered light from the center. Therefore, the absence of light from areas outside the output screen is also the cause of vignetting.

The straight lines on the target are usually shown as outward-facing curves in the fluorescence image. This effect is called

The input screen diameter of the image intensifier ranges from 4 inches to 16 inches. Enhancers with small input screens are more flexible and cheaper to operate. Small image intensifiers can slightly improve resolution because electrons from the photocathode strike the output screen with greater accuracy. However, the range of the patient's body that the input screen can surround is limited. Larger intensifiers are more expensive and inflexible, but they can provide a larger field of view and image magnification opportunities.

The diameter of the input screen of the image intensifier should be larger than the diameter of the area to be studied on the patient's body ^[1] .