In order to understand the general concept of an MRI, it is crucial to understand the basic construction of atoms. All materials in the environment, including the human body, are made up of atoms. Atoms are tiny particles that consist of an outer shell containing electrons and a nucleus that contains protons and neutrons. About two thirds of the human body consist of water molecules which are composed of oxygen and hydrogen atoms. The nucleus of those hydrogen atoms play a crucial role for MRI.

In certain cases, such as for hydrogen, the nucleus of an atom has a non-zero intrinsic angular momentum (also called nuclear spin). Such an intrinsic "rotation" is a quantum-mechanic property. Since protons have a positive charge, in a classical sense they can be considered as moving charges that possess a magnetic moment and therefore generate a measurable magnetic field. Hence, the hydrogen atoms behave like "mini-magnets".

When putting the patient into the static magnetic field of the MR scanner, those "mini-magnets” in the patient align with the magnetic field. The alignment along and against the field direction generates a net magnetisation that rotates with the so-called Larmor frequency that is dependent on the strength of the static magnetic field. When a radiofrequency wave with the equal frequency is sent into the tissue, due to the resonance phenomenon the aligned spins start to flip their energy state, thus changing the direction of the net magnetization. In this "excited" state, a MR signal can be recorded by receiver coils that are placed around the subject. Since the excited spins tend to return to their equilibrium state after the radiofrequency pulse has stopped, this signal will decay with a tissue characteristic relaxation time. The origin of the individual MR signal fractions is inscribed by additional pulsed magnetic field gradients.

A reconstruction computer processes the received signals and converts them into grey-scale images. The image contrast depends on the number of spins in the sampled volume, the present tissue types (hence the relaxation of the signal) and the used pulse sequence type (i.e. the timing of radiofrequency and magnetic gradient pulses) and its parametrisation. Usually a number of pulse sequences are applied throughout an MR scan session to acquire images with manifold types of contrast, depending on the underlying focus of interest.

MRI is not limited to the generation of anatomical images. Hence, also information about chemical structure, metabolism or functional contrasts, such as molecular mobility or activity tracking, are accessible.

fMRI is a sub-category of MRI (f = functional). It obtains neural activities and tries to derive the active sections of the brain. fMRI enables the documentation of brain activities during different tasks such as talking, thinking or movement, which can then be matched to active sections of the brain.

The whole brain is therefore consecutively scanned, while the subject is exposed to different types of stimuli or has to solve tasks. Blood flow increases in the active sections of the brain, with oxygenated blood flowing in and a subsequent decrease of the blood oxygen level. fMRI measures this change of blood flow. Oxygenated and deoxygenated blood has different magnetic properties and therefore yields a change of contrast in the active brain sections on an image. This is called BOLD effect.

What is the BOLD effect?

BOLD is short for Blood Oxygen Level dependant. Red blood pigment (haemoglobin) transports oxygen to the brain where it is predominately consumed by the active brain cells. In general, more oxygen-rich blood than needed gets delivered to the active regions. Hence, one can find a local enrichment of oxygenated blood, where normally oxygen-poor blood predominates.

Oxygen-poor blood is paramagnetic, oxygen-rich blood is diamagnetic. Thus, the magnetic properties (i.e. T2* relaxation times) in the respective domains change over time, which can be observed in the acquired fMRI signal.