Edinburgh Preclinical Imaging

The modalities explained

The 4 imaging modalities in our facility each have their own advantages and disadvantages.

Magnetic Resonance Imaging

The subject is placed within a powerful magnetic field which results in the realignment of hydrogen atoms in the body, when the hydrogen atoms return to their normal alignment, they emit energy. The energy emitted will differ according to the tissue, structure or cell type in which they reside. This released energy is detected by coils around the subject and converted by complex algorithms into an image.


An ultrasound probe emits sound waves with a frequency above 20000Hz, beyond the range of human hearing. The ultrasonic waves are scattered and reflected from organs and structures within the body. The returning echoes are received by the transducer and  transformed in real time into an image.

The delay in receiving the echo is a measure of the depth from which the echo has been reflected or scattered. A comparison of the frequency of the emitted and received ultrasound wave also gives information, using the Doppler effect,  on the speed and direction of movement of the moving scatterer.

Positron emission tomography–computed tomography

In order to obtain a positron emission tomography (PET) image, a radiotracer (a compound of interest labelled with a radionuclide) is administered into a living subject. Immediately after administration, the radiotracer distributes in the body, reaching the target site, where it is retained and then eliminated. As the radionuclide attached to the compound of interest decays, gamma photons are emitted and the PET cameras can be used to measure those photons, in order to generate images showing radiotracer biodistribution inside the subject body. The computed tomography (CT) image generated by using X-rays allows for the acquisition of high resolution images of the subject's anatomy and together with the PET image helps identify areas of radiotracer uptake inside the living subject.

Optical in vivo imaging

Two main methods: 1: Bioluminescence – an enzyme catalysed reaction resulting in the emission of light or 2: Fluorescence - the excitation of a reporter by exposure to light resulting in the emission of light upon return to its resting state. The choice of method will depend on the question, the anatomical location and the brightness of your reporter. The tissue of the animal absorbs much of the green and yellow spectrum so is suitable only for near surface or very bright imaging, by contrast the near infrared spectrum is not absorbed so heavily thus can be quantitative in even deep tissue imaging.