Cath: X-ray

(Extract from Morton Kern’s The Cardiac Catheterization Handbook. 3rd edition 1999)

X-ray tube: converts electrical energy into X-ray beam. Electron beams from a heated cathode are accelerated towards the rotating disc of anode. The electrons become X-rays upon contact with anode. But, only 0.2-0.6% of electrical energy provided is eventually converted into X-ray. This process generates extreme heat.

Three factors influence the X-ray image:
1) Electrical current (mA): Number of photons (electrical particles) generated per second. Increasing milliamperage increases quality of image. But, heat generated is the limiting factor. Of course, this increases radiation exposure to patient and increases scatter – leading to increase in exposure to lab personnel.

2) Kilovoltage level (kV): The wavelength of the X-ray beam. Higher the kV, shorter is the wavelength. Less tissue penetration. So, for obese patients, high kV is needed to get adequate penetration. But, high kV leads to poor resolution of the image. Higher kV also increases scatter and therefore, higher exposure to lab personnel.

High kV - short wavelength, uniform and better penetration. But, low contrast in image.
Low kV - provides better contrast in image.

High kV allows to keep mA low, but the trade-off is low-contrast in the image. A balance has to be struck. kV has more effect on image contrast than mA.
3) Exposure Time (s): Pulse width

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