A vacuum is a volume of space that is substantively empty of matter so that gaseous pressure is much less than standard atmospheric pressure.
A vacuum is used in the x-ray tube because otherwise the electrons would be stopped by air or any other gas. Unlike in closed (or sealed) x-ray tubes, in which the vacuum is permanent, open tube systems have to be outfitted with a vacuum pump for the purpose of creating vacuum during tube warm-up.

A vector is a quantity characterized by a magnitude (in mathematics a number, in physics a number times a unit) and a direction (and a point of application), often represented graphically by an arrow. The length of the line segment represents the magnitude, and its orientation in space represents its direction. Vector quantities can be added to or subtracted from one another. Used in diagnostic imaging to describe forces, e.g. magnetic moment, spin, magnetization etc.

A venography (also called phlebography) is the imaging of veins. To perform a venography, a contrast agent is injected into a vein during a fluoroscopy, to show the details of venous structure and abnormalities as for example blood clots and occlusions. The examination of the veins is called venogram.
Other methods include contrast enhanced MR venography and venous ultrasound techniques.

See also Angiogram, CT Angiography, Digital Subtraction Angiography and Computed Tomography.

Virtual colonoscopy provides a less invasive option to conventional polyp detection in the large intestine (colon and rectum). A virtual colonoscopy is a synthesis of a computed tomography (CT) scan, digital processing and virtual reality computer software.
A virtual colonoscopy is less invasive and more comfortable for patients than either conventional colonoscopy or a barium enema. No sedation is required and the examination takes less than 30 minutes.
A CT colonography offers a new option for a total colon evaluation and cancer detection and has the potential to be used for screening. A problem is the amount of information captured in a CT exam. Reviewing these images can be time-consuming and challenging.

See also Computed Tomography Enterography.

Viscosity is measured in centipoise (cP). The viscosity of water at 20° C is approximately 1 cP. Viscosity depends on temperature, size and shape of the dissolved particle.
The viscosity of contrast agents is an important factor in diagnostic imaging procedures like angiography and other studies where the injection rate can be of critical importance. Warming the contrast agent to body temperature may achieve higher flow rates. Contrast media are approximately twice as viscous at 20° C as at body temperature.