MR Physics

MRI uses the magnetic moment of non-zero spin nuclei, such as hydrogen, to generate a detectable signal. The spin of a nucleus can be seen as a compass needle. Normally the orientation of spins of the nuclei within the human body are not aligned. In order to create an image, MRI uses a constant magnetic field (B0), along which all the spins align.

A second magnetic field, which oscillates at radio frequencies (RF pulse), is then applied to the aligned spins. The RF pulse excites the spins, which leads them to rotate away from their orientation. This will not happen for any frequency, but is directly related to the so-called Larmor frequency (the resonance frequency), which is proportional to the magnetic field strength. Subsequently the spins start precessing about B0. Both rotation and precession start to decay to bring the spins back to their original position, during which they lose energy that can be detected by the MRI scanner. The spins' rotation and precession decays exponentially with tissue specific time constants T1 and T2 respectively. In newborns these time constants are in the order of 2.5s (T1) and 0.2s (T2).

There are two key parameters that can be tuned, in order to highlight different tissues in the image. One is called the repetition time (TR), which is related to time constant T1. It is the time that passes between the application of the first excitation pulse (RF pulse) and the next. The time related to the time constant T2 is called echo time and represents the time between the application of the excitation pulse and the peak of the response signal (detected signal).

So far, however, we have no information as to where the signal that we are recording is exactly coming from in the body. By applying a third magnetic field, the so-called gradient, we can gain spatial information. Gradients are magnetic fields which change in intensity across the MRI scanner. This additional magnetic field strength, in combination with Larmor frequency mentioned above, allows then for tissues at separate positions within the body to experience a different resonance frequency.

Gradients are not only useful to select individual slices (approximately 2-D planes) in the brain to be imaged, but also to investigate different aspects, such as the diffusion of water molecules within the brain (see diffusion MRI). Commonly, MRI images are stacks of slices rather than the entire brain volume, which necessitates that the patient inside the MRI scanner does not move. Research into motion correction and MRI methods that can image entire volumes at once is therefore of particular importance.

A useful concept when talking about MRI acquisition strategies or sequences is k-space. It is based on the mathematical principle of Fourier transformation of the gradients and the spatial components x and y of the image and can be seen as a temporary image space. In k-space, measurements around the centre (low values of kx and ky) correspond to gradual changes in the intensities, i.e. constant or slowly varying areas of intensity. High values on the other hand corresponds to edges in the image. Imaging sequences are often described as paths (trajectories) in k-space, as it allows a relatively simple representation of the elements used in the sequence.

The time it takes to acquire MR images can be improved by modifying these paths. One example is called echo planar imaging (EPI), which acquires all of k-space (both slow and rapidly varying areas of the pictures) in one sweep, rather than line by line. Another method to speed up the imaging sequence is by either not acquiring all of k-space and estimating the missing parts, or using parallel imaging, which basically uses many cameras at once in a divide and conquer approach to take the image (similar to panorama pictures made from individual images stitched together).