Fluoroscopy

Fluoroscopy is an imaging technique in which a live xray image of the patient's internal structure can be taken and viewed. Image Intensifier Fluoroscopy replaced Direct vision fluoroscopy due to the many limitations of Direct vision fluoroscopy. Image Intensifier (II) fluoroscopy is a modern system, consisting of an Image Intensifier and closed circuit TV system. Hence, it is also called IITV.

Fluoroscopy image should have maximum image detail, which requires more image brightness. Image intensifier has higher image brightness, which makes this image detail more visible. Image brightness depends on anatomy, mA and kVp. Hence image brightness can be changed in fluoroscopy by controlling mA and kVp.

A Fluoroscopy examination lasts for several minutes, which als

o increases the radiation dose of the patient. The exposure rate is kept as low as 200 mR / Min to reduce the radiation dose of the patient. Fluoroscopy systems have low current output that ranges from 1 to 5 mA and produces 30 images per second. Whereas the current output in radiographic exposure ranges from 100 to 200 mA which is much higher than the fluoroscopic current output. In Fluoroscopy, few X-ray photons are taken to create a single fluoroscopic image. Therefore, comparing the number of X-ray photons, the fluoroscopic image is inferior to the radiographic image.

A special feature of a fluoroscopy machine is the Deadman switch. Continuous xray beam is achieved by continuously pressing either the hand switch or the foot pedal. The exposure is terminated by releasing the pressure applied on the pedal or switch.

Image intensifier tube converts exit radiation from the patient into visible radiation. The image intensifier consists of an evacuated glass envelope. The surface of the patient side of this vacuum bottle is called aluminum. It is made of 1 millimeter aluminum.

This surface is made curved to reduce the resistance of air pressure. Due to the evacuated glass envelope, its size is limited and its diameter range is 23 to 57 centimeters. Glass envelope is kept in a metal container to prevent rough handling and damage.

Image intensifier has four parts -

1. Input screen
2. Focusing electrodes
3. Anode
4. Output phosphor

Input screen

It is inside the aluminum window. It is made up of three layers.

II Input Window

This is a convex substrate layer, which is generally made of aluminum or Titanium-like metal with low atomic number. Its thickness is about 0.5mm. This allows the x-ray beam to come into the image intensifier tube with minimal attenuation. In addition, it provides protection to the sensitive input components of the tube and maintains the vacuum in the tube.

Input Phosphor has a thickness of 200 to 400 microns. Sodium activated caesium iodide is used as input phosphor. Its xray absorption efficiency is 70 to 90%. Caesium iodide crystal has a special property. Its crystals, whose diameter is 5 micrometers, are very tightly packed as a vertical needle, and send the light towards the output screen in a forward direction, like a fiber optic, allowing the lateral of the light. The spread is also less. Caesium iodide crystal produces about 3000 light photons of the blue light spectrum from an x-ray photon.

Photocathode

Light photons resulting from photocathode input phosphor fall on the photocathode. The photocathode is made of a very thin layer of Antimony caesium (SbCs3). The photocathode input is coated on the inside surface of the phosphor. It converts a visible light into an electron. Antimony caesium (SbCs3) absorbs blue light. That is, it has spectral matching to the blue light emitted from Caesium iodide. Photocathode emits 200 electrons per X-ray photon.

The input screen is placed at a higher negative potential than the anode, so that electrons move faster towards the anode. The input screen has a diameter of 150 to 400 millimeters depending on the clinical application.

Focusing Electrodes

Input screen is placed at negative potential relative to anode. There are D1, D2 and D3 electrodes between the input screen and the anode. These are usually metal rings placed at a positive voltage relative to the photocathode. These electrodes accelerate the electron and focus the output screen. The length of the image intensifier tube is about 50 centimeters and there is a voltage difference of 25000 volts between the photocathode and the anode.

Electrons come to the anode carrying images of input phosphors with high velocity and energy. Therefore, the electrons receive energy and form a minified, inverted image on the output phosphor. Due to the curved nature of image intensifier, electron foxing is promoted, but this produces pincushion distortion.

Anode

It is a circular plate between which there is a hole. Through this hole, electrons reach the output phase. It is made by coating 2 mm of aluminum on the output phosphor inside surface. It is electric conductive and its potential difference is 25kV more than the input screen. It works for receiving accelerated ekectrons.

Output screen

Silver dopped zinc cadmium sulfide crystal (ZnCdS: Ag) is used as output phosphor. It is dopped on the inside surface of the output window. Its diameter is 25 to 35 mm and phosphor thickness is 4-8 mm. It absorbs electrons and emits large amounts of green light. In this green light, the video camera is the most sensitive. Each electron produces about 1000 light photons.