The aim of medical imaging is to create images of the body, or of particular areas of it in order to reveal, diagnose or examine disease in a non-invasive way. As a field, it comprises a number of different procedures including:

• conventional X-ray
• fluoroscopy, e.g. angiography
• computed tomography (CT)
• positron emission tomography (PET)
• single photon emission computed tomography (SPECT)
• magnetic resonance imaging (MRI)
• ultrasound
• techniques using laser and other light sources

Some, e.g. X-rays,fluoroscopy, CT, PET and SPECT, rely on the use of ionizing radiation or X-rays while others such as MRI, light and ultrasound do not. All, however, require the means to be able to distinguish between different structures in the body.

Nanotechnology can be applied across all of these types of imaging to improve the contrast between healthy and diseased or damaged tissues by virtue of the fact that the nanoscale materials used are in a similar size range to some of the biological structures they are targeting. Some novel imaging applications include:

• the use of iron-containing molecules in the range of 2 nm (molecular nanomagnets) in magnetic resonance imaging (MRI)
• the use of gold nanorods in combination with a laser light sources
• the use of fluorescent carbon nanotubes

A relatively new approach is to combine imaging with therapy, so called "theranostics" and here nanotechnology has great advantages. In detecting and treating early stage cancer, for example, a material needs to be at a scale where it can readily enter a cell where it can be imaged, to ensure that it has reached the desired site, and then activated into a therapeutic state, e.g. by the application of radiation or a magnetic field.