Fluoroscopy: Imaging Technique

Overview of the Fluoroscopy Machine

• Fluoroscopy is a medical imaging technique that uses X-rays to create a real-time video of the inside of the body.
• It is also referred to as "fluoro".
• The image intensifier is a complex device that receives the X-ray beam, converts it into light, and increases light intensity for better viewing.

History

• Thomas Edison invented the fluoroscope in 1896.
• It had a zinc-cadmium sulfide screen.
• Fluoroscopy has been a valuable tool in medical imaging.
• Image intensifiers became routine in the 1960s.

Overview

• Diagnostic fluoroscopy is a daily procedure performed in radiology suites across the country.
• This modality visualizes anatomy using radiation in real-time.
• Patient doses can be significant, increasing the risk of adverse reactions.

Interventional Fluoroscopy

• Interventional fluoroscopy guides minimally invasive medical procedures.
• It's used to evaluate dynamic biological processes, such as the movement of a body part, an instrument, or a contrast agent.

How it Works

• An X-ray beam passes through the body part being examined.
• X-ray images are transmitted to a monitor.
• Healthcare providers can see the body part in motion.

What it's Used For

• Healthcare providers use fluoroscopy to monitor and diagnose conditions.
• It guides treatments like implants and injections.
• It guides orthopedic surgery.
• It allows the evaluation of specific body areas like bones, muscles, joints, organs, and blood vessels.

Imaging Chain Components

• X-ray source: Emits an X-ray beam that passes through the patient's body.
• Flat panel detector (FPD) / Image-intensifier tube: A receptor that converts the X-ray signal into digital images.
• Monitor: Displays digital images as a continuous X-ray image.

Imaging Chain of Interventional Fluoroscopy

• The chain involves an X-ray source, a flat panel detector/image intensifier tube, and a monitor.
• The chain produces real-time X-ray images that guide medical procedures.
• Key components include: High Voltage Generator, X-Ray Tube, X-Ray Image Intensifier, and Video Camera/Monitor.

Image-Intensifier Tube

• It receives the image-forming X-ray beam and converts it into a high-intensity visible-light image.
• Tube components are contained within a glass or metal envelope that provides structural support and maintains the vacuum.

Input Phosphor

• X-rays that exit the patient and are incident on the image-intensifier tube are transmitted through the glass envelope and interact with the input phosphor.
• Energy is converted into visible light.

Input Phosphor (X-Rays to Light)

• The input phosphor converts X-rays into light.
• Cesium Iodide (CsI) crystals are commonly used, arranged in a dense needle-like structure (approximately 300 μm).
• It prevents lateral light spread.

Photocathode

• The photocathode is the next element in the image intensifier tube.
• It is bonded directly to the input phosphor.
• It is a thin metal layer, often composed of cesium and antimony compounds, responding to the input phosphor stimulation.
• Photocathode emits electrons through photoemission when illuminated by visible light from the input phosphor.

Photocathode converts Light to Electrons

• Photocathode converts light photons into electrons through the photoelectric effect.
• This occurs when photons strike a thin bi- or multi-alkali photocathode.
• Electrons are released and repelled from the photocathode.

Photoemission

• Photoemission is electron emission resulting from visible light stimulation.
• Numerous light photons are required to cause the emission of a single electron.
• The number of emitted electrons is directly proportional to the light intensity.

Electron Optics

• Proper electron travel is maintained.
• Electrons generated in the active area of the input are focused onto the fixed-size output screen.
• Electrostatic focusing lenses are responsible for reducing the electron pattern from the large cathode end of the tube to the small output phosphor.

Flux Gain

• Flux gain is the ratio of the number of output light photons to the number of input X-ray photons at the phosphor.

Minification and Brightness Gain

• Minification gain is the increased illumination of the image due to light photon multiplication at the output phosphor.
• Brightness gain is the ability of the image intensifier to increase illumination; it's the product of minification and flux gain.

Intensification

• Intensification has two mechanisms:
• Electronic (flux) gain - electrons gain kinetic energy from acceleration (~50 typically).
• Minification gain - reducing the large X-ray image at the input phosphor to a smaller diameter at the output phosphor (e.g., 40 cm to 2.5 cm).

Output Phosphor

• Accelerated electrons interact and produce visible light.
• Output phosphor converts electrons to light via focused electron beams onto a thin powder phosphor.

Optical System

• The optical system couples the image intensifier to the video camera.
  • It includes a collimating lens to shape divergent light from the output phosphor,
  • and an aperture to limit the light reaching the camera.

Flat-Panel Detector

• Replaces the image intensifier tube, optical system, and cameras.
• It directly converts X-rays into a digital form, improving image quality.
• Carbon fiber covers reduce X-ray attenuation and increase DQE (Detective Quantum Efficiency).

FPD (Indirect/Direct Systems)

• Indirect systems use a phosphor layer that converts X-rays to light, and a photodiode converts this light into a charge.
• Direct systems directly produce a charge from X-ray absorption in a semiconductor material.

Video Camera

• Video camera captures the XRII output image and converts it to an analogue electrical signal in a recognized video format (e.g., NTSC/PAL/SECAM).
• Older cameras used photoconductive targets scanned by electron beams. Modern cameras use Charge-Coupled Devices (CCDs).

Interventional Procedures and X-Ray Tube Positioning

• X-ray tubes are positioned beneath the patient for interventional procedures to minimize scattered radiation reaching medical staff. The majority of scattered radiation originates from the patient's entrance side.