(N) The SI unit of force.
Definition: 1 newton will accelerate a mass of 1 kilogram at the rate of 1 meter per second.
The relationship between force (F), mass (m), and acceleration (a) is expressed by the formula: F = ma.
The newton is named for Isaac Newton (1642-1727), the British mathematician, physicist, and natural philosopher.

Noise is an undesirable background interference or disturbance that affects image quality. The image noise is commonly characterized by the standard deviation of CT numbers in the image of a uniform object (phantom - generally water) relative to the difference in CT numbers between water and air.
Noise is an important limiting factor of CT image quality and is characterized by a grainy appearance, sometimes described as a salt and pepper pattern. Noise as a result of low radiation dose requires a higher tube current (mA) in combination with narrow slices to maintain image noise at an acceptable level.

Nonionic contrast agents are a group of contrast media that do not dissociate into charged particles. Many of the side effects of contrast agents are due to the injection of a solution with high osmolality compared to blood. Nonionic contrast agents have a low osmolality and tend to have less toxicity. However, as with all iodinated contrast agents, serious or fatal reactions have been associated with their use.
Due to their lower neurotoxicity, nonionic contrast agents can also be used for myelography. The only FDA approved agent, iodixanol, is iso-osmolar to blood, which makes it preferable for examinations of the central nervous system and cardiovascular studies. Because of its higher costs relative to other nonionic agents, the use is sometimes reserved for patients with high risk of nephrotoxicity.

See also Nonionic Dimer, Nonionic Monomer and CM Ratio.

A nonionic dimer consists of 2 joined nonionic monomers. Nonionic dimers are used as x-ray and CT contrast media.
Nonionic dimers have the lowest osmolarity. These contrast materials contain 6 iodine atoms for every 1 particle in solution (CM ratio=6). Modern CT techniques require fast bolus injections and thus low-viscosity, high-concentration, well-tolerated contrast agents. Nonionic dimers have the highest viscosity, which limits the clinical usefulness. Examples of nonionic dimers are iotrolan (Isovist®) and iodixanol (Visipaque).

See also Contrast Agents.

Nonionic monomers are used as x-ray contrast agents. To create a nonionic monomer, the tri-iodinated benzene ring is made water soluble by the addition of hydrophilic hydroxyl groups to organic side chains (CM ratio=3). Nonionic monomers have an intermediate osmolarity, intermediate viscosity and elevated hydrophilicity with three atoms of iodine per molecule. Lacking a carboxyl group, nonionic monomers do not ionize in solution. A nonionic monomer is potentially less chemotoxic than an ionic monomer.
Common nonionic monomers are iohexol (Omnipaque), iopamidol (Isovue®), ioversol (Optiray®), and iopromide (Ultravist®). Nonionic monomers are contrast agents with a wide range of indications due to their nonionic nature and lower osmolalities.