Principles of Fluoroscopic Image Intensification and Television Systems

Workbook and Laboratory Manual

Robert J. Parelli

• Editore: CRC Press
• Anno: 2020
• ISBN:
  9781000154535
• Edizione: 1
• Argomento: Droit
• Categoria: Science médico-légale

This unique workbook can be used as a stand-alone text or supplemental text for any course designed to enhance the work of radiologic technology students. It will also serve the needs of graduate radiographers as well as the physician in learning specific areas of the Fluoroscopic Image Intensifier such as:

Chapter 1

Image Intensifier System

Objectives

Upon completion of Chapter 1, the reader will be able to:

1. Describe the basic parts of the image intensifier tube: input phosphor and photocathode, electrostatic lens, accelerating anode, and output phosphor.

2. Explain how to use a dual field image intensifier tube for magnification procedures.

3. Calculate the amount of patient exposure in the magnified mode.

4. Define conversion factor and brightness gain.

5. Calculate the minification ratio of the image intensifier tube.

6. Explain how image quality of the image intensifier tube is affected by scintillation, resolution, contrast, and distortion.

7. Describe the automatic brightness stabilizer system.

8. Recall the types of brightness sensing devices.

9. Identify the types of automatic brightness sensing circuits that are used with the x-ray generator.

Conventional fluoroscopy, without image intensification, has two serious limitations: it produces a statistically inferior image and one too dim for photopic (daylight) vision. In the early 1950s, the x-ray image intensifier was developed, which has revolutionized fluoroscopy. Its image is bright enough for scotopic vision and small enough to be conveniently coupled to cine, television, or spot-film cameras.

Image Intensifier Tube Design

The image intensifier tube is an evacuated glass envelope, a vacuum tube, which contains four basic parts (Figure 1.1):

1. Input phosphor and photocathode.

2. Electrostatic focusing lens.

3. Accelerating anode.

4. Output phosphor.

Input Phosphor and Photocathode

First-generation image intensifier tubes had silver-activated zinc cadmium sulfide crystals in the input screen. Second-generation intensifiers have cesium iodide (Csl) input screens. The fluorescent material is deposited on a thin substrate of the glass envelope. It serves the purpose of converting incident x-ray beam photons into a light image. The fluorescent screen and the photocathode are separated by a thin transparent layer only a fraction of a millimeter. Very little of the image is lost as the image is transferred from the screen to the photocathode.

Figure 1.1: Cross-section of an image intensifier tube.

The photocathode is a photo-emissive metal (usually a combination of antimony [Sb] and cesium [Cs] compounds). The photocathode receives the light emitted from the input screen and emits electrons in proportion to the intensity of the light it receives. One can appreciate the necessity of the input screen and photocathode being in very close proximity. Otherwise the image detail transmitted from one to the other would be significantly degraded. The photocathode also serves as the cathode of the image intensifier tube. It is kept at ground potential. By establishing a voltage or difference in electrical potential between the cathode and the accelerating anode, the electrons can be made to transverse the tube toward the output screen.

Electrostatic Focusing Lens

The focusing of electrons is accomplished by a series of rings called the electrostatic lenses, which are located inside the tube envelope and concentric with the tube axis. Applying a positive electrical potential difference to the electrostatic lens will cause the electrons emitted from the photocathode to be focused into an extremely fine beam. Each point on the input screen is focused to a specific point on the opposite side of the output screen. Electron focusing inverts the image on the output phosphor. The image on the output phosphor is reduced in size, which is one of the principal reasons why it is brighter.

Accelerating Anode

Located at the neck of the image intensifier tube, the accelerating anode draws electrons from the photocathode and accelerates them toward the output screen. The anode of a 6-in. image intensifier tube has a positive potential difference (voltage) of 25,000 volts (25 kVp), so it accelerates electrons to a tremendous velocity.

Output Phosphor

The output fluorescent screen of the 6-in. image intensifier tube is made from cesium iodide. The crystal size and layer thickness are reduced to maintain resolution in the minified image. Since the electrons are greatly accelerated, they emit more light photons from the output screen than were originally present in the input screen. The number of light photons is increased approximately 50 times. A thin layer of aluminum is plated onto the fluorescent screen to prevent light from moving retrograde through the tube and activating the photocathode. The aluminum layer is very thin, and high-energy photoelectrons easily pass through it en route to the output screen. This layer also serves as a ground to remove spent electrons from the image tube. If they were not removed, they would accumulate on the output phosphor and build up a negative charge.

The output screen is optically coupled to a viewing system by subjective lenses. The image is viewed either directly through a series of lenses and mirrors or indirectly through a closed circuit television system.

Dual Field Image Intensifier Tubes

Field size on the output phosphor is changed by applying a simple electronic principle: the higher the voltage on the electrostatic focusing lens, the more the electron beam is focused. Figure 1.2 shows this principle applied to a dual field image intensifier.

Figure 1.2: Dual field image intensifier.

In the 9-in. mode, the electrostatic focusing voltage is decreased. The electrons focus to a point, or cross, close to the output phosphor, and the image is actually smaller than the phosphor. In the 6-in. mode, the electrostatic focusing voltage is increased, and the electrons focus farther away from the output phosphor. After the electrons cross, they diverge so the image on the output screen is larger than the 9-in. mode. The physical size of the input and output screens is the same in both modes; the only thing that changes is the size of the output image. Therefore, the 6- and 9-in. modes have different minification gains. Exposure factors are automatically increased when the unit is used in the magnified mode to compensate for the decreased brightness from minification. The ratio of patient exposure is calculated using this formula:

$\frac{{\left(\text{Normal}\text{mode}\text{size}\right)}^2}{{(\text{Magnified mode size})}^2}=\text{Increase in patient exposure}$

Example: When operating the image intensifier in the magnified 6-in. mode from the normal 9-in. mode, the patient will receive how many times more exposure?

Answer:

$\frac{{\left(9\right)}^2}{{(\text{6})}^2}=2.25\text{times more exposure}$

It is important to note that the normal mode is always used to view larger anatomic areas with less patient exposure. When size is unimportant, the magnified mode is used for better resultant image quality. The mA is automatically increased when the unit is used in the 6-in. magnified mode to compensate for the decreased brightness. Therefore, patient dose is increased.

Conversion Factor and Gain

When x-ray image intensifier tubes were first introduced, some means was needed to express their increased intensity. Because of the past familiarity with conventional fluoroscopic screens, it became commonplace to compare the image intensifier tube to the screen. The brightness gain, or “intensification gain” as it was called, was then expressed as the luminance of the output screen compared to the luminance of a standard Patterson fluoroscopic screen with the same incident radiation. The brightness gain is the ratio of the two illuminations:

$\text{Brightness gain}=\frac{\text{Intensifier luminance}}{\text{Patterson B-2 luminance}}$

The first image tubes that gained widespread acceptance had gains on the order of 3,000 or possibly a little higher. As improvements in design and manufacturing came along, this gain grad...