ABSTRACT
Magnetic resonance imaging (MRI) is probably the most flexible of the medical imaging modalities. Images in MRI can be acquired using many different acquisition techniques, known as imaging pulse sequences, and a whole range of images with differing contrast properties can be achieved. A tissue can be made to give a signal higher or lower than another tissue, or even to give no signal at all. All these possibilities are a direct consequence of the physical principles that underpin the technique, which are fundamentally different from those of other medical imaging methods. You may already be familiar with x-ray imaging, in which images are formed by detecting the x-rays after they have been attenuated by tissue. X-rays are electromagnetic radiation with a high frequency. Electromagnetic radiation is involved in MRI too, but in a very different way. In MRI the radiation has a much lower frequency, and is not attenuated by tissue and then detected. Instead, the radiation is one of the components of the process of nuclear magnetic resonance, which also requires an interaction between atomic nuclei and a strong magnetic field. For MRI, the nuclei concerned are hydrogen nuclei, which are abundant in the human body. Measuring the signals that the hydrogen nuclei emit gives information about the tissue in which they are situated, and much of the practice of MRI is concerned with ensuring that signals from hydrogen nuclei in an environment of interest can be clearly distinguished. To engage with all the different ways in which an MR image can be acquired requires an understanding of the basic principles and concepts. Such an understanding is hugely valuable and provides a solid foundation because the same principles form the basis of every imaging pulse sequence or new development in the field.
