What Do You Know About Ct Tube? - Principles

Efficiently converting electrical energy into X-rays, achieved by electron flow impacting the anode target surface, is essentially a balance between energy conversion and thermal management.

 

Basic Principles

 

Efficiently converting electrical energy into X-rays, achieved by electron flow impacting the anode target surface, is essentially a balance between energy conversion and thermal management.

 

(1) Electron emission (cathodic action)

 

● Filament heating

 

The cathode filament (tungsten wire) is heated to about 2200 ° C by electricity, and thermal electron emission occurs (electrons detach from the metal surface).

 

Dual filament design (some high-end CT): Switching between large focus (high current, fast scanning) and small focus (low current, high-resolution scanning).

 

● Electronic Focusing

 

The cathode focusing cup (metal cup-shaped structure) focuses the electron beam through negative voltage to prevent electron divergence and ensure precise bombardment of the anode target surface.

 

(2) Electron Acceleration (High Voltage Electric Field)

 

High voltage generator: A strong electric field (40-150 kV) is formed between the cathode (negative high voltage, such as -140 kV) and the anode (grounded or positive high voltage).

 

Electrons are accelerated to near the speed of light (energy up to 140 keV) in an electric field, and the kinetic energy formula is:

 

 

(e: Electronic charge, V： Tube voltage)

 

(3) Electron impact on anode target (X-ray generation)

 

Anode rotation: The anode target disk (tungsten or molybdenum rhenium alloy) rotates at high speed (10000-15000 rpm) to disperse the heat generated by electron bombardment and avoid local melting.

 

For every 1000 rpm increase in anode speed, the local temperature on the target surface can be reduced by about 15%, which is also the reason why modern CT tubes commonly use high-speed bearings.

 

X-Ray Generation Mechanism:

 

● Bremsstrahlung: High speed electrons are deflected by the Coulomb force of the target atomic nucleus, and their kinetic energy is converted into continuous spectrum X-rays (accounting for 85% of the total X-ray energy). Wide energy range (from 0 to peak tube voltage, such as 0-140 keV).

 

● Characteristic Radiation: High speed electrons hit the inner layer electrons of the target atom, and the outer layer electrons fill the vacancies, releasing specific energy X-rays (such as tungsten's K α line: 59.3 keV). Generate a single energy peak for spectral CT imaging.

 

● Energy conversion efficiency: Only about 1% of the kinetic energy of electrons is converted into X-rays, while the remaining 99% is converted into thermal energy (requiring an efficient heat dissipation system).

 

X-ray Output and Collimation

 

● Window material: A beryllium window (low atomic number, less absorption of X-rays) is installed at the bottom of the tube, allowing X-rays to be directed and output to the patient.

 

● Collimator: Adjust the shape and thickness of the X-ray beam, and control the scanning layer thickness (such as 1 mm or 5 mm).

 

 

Working Mode And Tube Load

 

(1) Continuous scanning mode (such as spiral CT) with continuous anode rotation: The electron beam continuously bombards the target surface, requiring the tube to have high thermal capacity and fast heat dissipation capability. Accumulation of heat load: Long term scanning may cause the anode temperature to exceed the safe threshold (real-time temperature monitoring is required).

 

(2) Pulse scanning mode (such as sequential scanning) intermittent exposure: the electron beam starts and stops as needed, reducing heat accumulation and extending the lifespan of the tube.

 

Clinical application: low-dose pediatric scanning or low-frequency examination.

 

Difficulties In Thermal Management

 

Anode heat capacity depletion: If the scanning protocol heat load exceeds the rated value of the tube, the system will force a pause ("Cool Down" prompt).

 

Thermal technology upgrade:

 

Liquid metal bearings (GE): reduce frictional heat generation and improve heat dissipation efficiency. Graphite based composite target (Toshiba): improves thermal conductivity and prolongs target surface life.

 

CT tube is like a 'miniature nuclear reactor', with 1% of energy used for imaging and 99% of energy requiring instantaneous' suppression '(heat dissipation)

Electron beam control technology

 

● Flying focus technology (Siemens): The electron beam quickly switches the impact point on the target surface, reducing the thermal load on a single focus while increasing the sampling rate (reducing spiral artifacts).

 

● Dynamic zoom (Philips): automatically adjusts the focus size based on the scanned area, balancing resolution and heat dissipation requirements.