Mechanisms of X-RAY tube used in ct .pdf

X ray tube in CT

1. XRAY TUBE IN A CT
Discovery of XRays –William Roentgen 1895
Creation of Xray Tube –William D Coolidge 1913
Parts of the tube : Vacuum tube with a cathode and anode with 30-150 kV
power source.
1)Glass envelope :
Highly evacuated tube with cathode ( negative electrode ) and anode ( positive
electrode ). Thin glass/beryllium window is present as tube envelope to
decrease attenuation of the Xray beam
2)Cathode:
Made up of tungsten due to high melting point
Emits electrons by thermionicemission
Electrons are targeted at anode at the focal spot. Diagnostic tubeshavedual
focusspots.
3)Anode :
Made up of tungsten –due to high atomic number ( Z = 74 ) and meltingpoint(
3370degC). Rhenium added to preventcracking.
The heat generated conducted outside by thick copper anode, oil reservoir,
cooling water lines, rotating anodes and air fan.
Someanodesarehoodedbycopperwhichabsorbsstray electrons.
Mechanism of flowof current :
Current through tungsten filament(heated up ) →Electron cloud formation
→Strikes a target ( Bremsstrahlung and Characteristic XRays produced )→Max
electrons lost in form of heat →Xray photons released as beam/spectrum out
beryllium window.
A CT scanner contains –Gantry + Computer + Operating console
Gantry houses the heart of the CAT scanner + Xray tube + Xray detectors
Generations of Xray tubes :
1st gen CT Xray tube –Pencil beam Xray , Single detector , 1-180 deg rotation,
translates across, very slow, 1-10 mm thickness, minutes to take one slice
2nd gen CT Xray tube–Fan beam Xray , Multiple detectors , 30 deg rotation,
translates across, slow, 1-10 mm thickness, 20s to take one slice, led to high
radiation doses due to poor beam restriction
3rd gen CT Xray tube –Fan beam Xray , Multiple detectors ( 500-1000 ) , 1-360
deg rotation ( Rotate –Rotate geometry ), no translation, 0.5s to take one slice,
bow tie shaped filters to suit body size and shape , overcome beam hardening
4th gen CT Xray tube –Wide fan beam Xray , Multiple detectors ( 500-1000 ) , 1-
360 deg rotation ( Rotate –Fixed geometry ), 10s to take one slice, poor
radiographic geometry with large geometric magnification
Developments and Future :
Till 1980s, X-ray generators were merely high-voltage, AC to DC. Later high-speed
switching emerged for more accurate control of the X-ray unit, higher quality
results, and reduced X-ray exposures.
X-ray tubes are subjected to far higher thermal loads in CT. Recentadvances
comprise of increase inanode heat preventing the need for cooling delays. Heat
dissipation rate has improved with thick graphite backing of target disks, anode
diameters of 200 mm or more, improved high temperature rotor bearings, and
metal housings with ceramic insulators. The mass has reduced to withstand
increasing gantry rotational rates required by faster scan times. The working life
of tubes ranges from 10,000 to 40,000 hours, compared with the 1,000 hours
typical of conventional CT tubes.
With increased awareness about the radiation dose encountered during CT
scanning, developments are under way to develop real-time exposure control to
reduce radiation dose without loss of image quality.
-Dr. Rijushree Saha JR1 Rad Onco