X-ray

X-rays (or Röntgen radiation) (TV: Smith and Jones) were a type of electromagnetic radiation. They were used in medical examinations because they penetrated soft tissue, allowing a non-invasive exam of the internal anatomy; they could also be used to examine the interiors of sealed objects using the same method. High levels of X-ray radiation could be harmful to many forms of life. When focused into a coherent beam or laser, X-rays could be used as an offensive weapon.

Instances of medical X-ray use
In the 1970s, X-rays were used on the Third Doctor at Ashbridge Cottage Hospital. They revealed his binary vascular system and astounded Dr Henderson. (TV: Spearhead from Space)

On 31 December 1999, Dr Grace Holloway was similarly astounded at the Seventh Doctor's X-ray results. (TV: Doctor Who)

On Wengrol, Yend biologists saw on their X-ray screens that the First Doctor (whom they had mistaken for a shapeshifter) didn't change shape, giving them hope that they might have found a cure for their mutation. (PROSE: The Sons of the Crab)

Instances of X-rays as non-medical imaging
The Mantis leader, in his attempts to learn the nature of the Doctor's TARDIS, had it subjected to X-rays, to find it was dimensionally transcendental and that the Third Doctor was inside it. (COMIC: Size Control)

Dr Carter took X-rays of Eldrad's fossilised hand which, unlike normal fossils, didn't show any tissue differentiation (since it belonged to a silicon-based life form). (TV: The Hand of Fear)

Instances of harmful X-ray use
The Third Doctor cured Liz Shaw and Captain Harvey of their possession by a sentient virus by using an X-ray machine on them. (PROSE: Invaders Invisible)

In 2003, the Seventh Doctor used a singularity which generated X-ray pulses to sterilise Agent Yellow, saving humanity at the cost of killing millions. (PROSE: Eternity Weeps)

In 2008, the Tenth Doctor destroyed one of Florence Finnegan's Slabs by using his sonic screwdriver to increase the radiation of Royal Hope Hospital's X-ray machine by 5000%. (TV: Smith and Jones)

X-ray-based weapons
In the 21st century, Space Station W3 was armed with an X-ray laser. (TV: The Wheel in Space)

On the Moon's surface in 2070, the Cybermen employed a cannon-sized X-ray laser. (TV: The Moonbase)

The Cyber-gun found on Telos by the Parry expedition was a hand-held X-ray laser. (TV: The Tomb of the Cybermen)

Natural occurrence
Vega was known for its x-ray storms, which the Captain used as a curse: "By all the x-ray storms of Vega!" (TV: The Pirate Planet)