So, in typical MRI machines, the electromagnet coils are made super conducting by keeping the coils immersed in a very cold liquid. The hydrogen nucleus ‘A’ is in a stronger magnetic field and therefore has an higher resonant frequency. Just as an example, let us say nucleus ‘A’ has a resonant frequency of 70 MHz (MHz = mega hertz = millions of cycles per second). I have chosen this number just as an example. The actual values found in MRI scanners will be different.

Now you are ready to understand how the MRI machine selects various areas to study. Let us imagine that it wants to image the foot area (i.e. it needs the foot area hydrogen nuclei (B) to respond). It does this by sending RF energy (from the pink RF coil) at the resonant frequency of the nuclei of interest, in this case 60 MHz. Each hydrogen atom consists of a nucleus with one electron going around it. The MRI machine doesn’t “see” these electrons either. FLAIR is also similar to T2, however, the CSF signal is nullified. This is particularly useful for evaluating structures in the central nervous system (CNS), including the periventricular areas, sulci, and gyri. For example, FLAIR can be used to identify plaques in multiple sclerosis, subtle oedema after a stroke, and pathology in other conditions whereby CSF may interfere with interpretation (Figure 4). 1 Figure 4. Multiple sclerotic plaques in periventricular regions and corpus collosum 7 Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) Now let us return to discussing the MRI machine. You will recall that we said that MRI machines can “see” hydrogen nuclei contained in water molecules. If you decide to have a sedative during the scan, you'll need to arrange for a friend or family member to drive you home afterwards, as you won't be able to drive for 24 hours.If you have been next to or inside an MRI machine (as a patient !), you would know that it can be noisy. However, if you have only been inside the MRI control room, you may not appreciate how loud it is. Furthermore, the MRI machine produces a large variety of different sounds. To summarise, regarding water molecules, the MRI machine does not see the oxygen atoms or the electrons of the hydrogen atoms. So you need to be cautious of the magnet at all times, even when the machine appears “quiet and inactive “ and doesn’t have a patient in it. Quenching The hydrogen nuclei best absorb energy given in a very specific frequency called the ‘resonant frequency’. Energy given in frequencies that are below or above the resonant frequency are not absorbed by the nuclei. MRI is an elegant technology, and its clinical application is better appreciated by having at least a basic understanding of the basics of its underlying physics. The acquisition of an MR image can be simplified into three processes based on its very name: magnetic (the inherent electromagnetic activity of atomic nuclei), resonance (the behavior of these nuclei in an external magnetic field and the method of manipulating them), and imaging (manipulation of these nuclei resulting in an observable signal to give every point in the sampled tissue a unique signal and location).[ 1] The near-ubiquitous nature of hydrogen atoms in biological molecules makes it ideal for clinical MR imaging.[ 1, 2]

A single scan may take from a few seconds to 3 or 4 minutes. You may be asked to hold your breath during short scans. What I am trying to say is that quantum physics is weird and I recommend, for your sanity, that you don’t try to understand it too deeply. Unfortunately, MRI machines deal with very tiny particles (i.e. hydrogen nuclei) and therefore they follow the rules of quantum physics. It is difficult to ‘imagine’ quantum physics, because often there is nothing in our everyday life that behaves like it. If you can’t physically remove the cylinder using a normal force, your only option might be is to put the magnet off by quenching it. Now that the magnetic field is gone, you can easily remove the steel cylinder. Potential issues during magnet quenching:Once the initial current is given, because the wire is super conducting and has no resistance, it simply continues to flow round and round “forever”. No further current supply is required ! This free flow of electrical current means that the coil keeps producing a magnetic field forever. This is because it's very important to stay still during the scan, which babies and young children are often unable to do when they're awake. During the scan

As you have seen before, the gradient coils make the main magnetic field have a gradient : stronger at one end and weaker at the other end. This means that, from the head of the patient to the foot of the patient, the hydrogen nuclei are exposed to different strengths of the magnetic field. At the head of the patient, for an example, the hydrogen nuclei (blue) are exposed to an high magnetic field, and therefore have an high resonant frequency. On the other hand, at the foot of the patient, the hydrogen nuclei (green) are exposed to a low magnetic field and therefore have a lower resonant frequency. In addition to enabling the use of thinner wires, super conductivity has another very interesting benefit. Normally, an electromagnet needs a constant current applied to it to generate a magnetic field. The MRI machine has “receiver coils ” (blue coil shown below) that receive the energy waves sent out by the nuclei. Having given up their energy, the nuclei change their spin direction and return to the low energy state that they were in before. Depending on the size of the area being scanned and how many images are taken, the whole procedure will take 15 to 90 minutes.After the scan, you can resume normal activities immediately. But if you have had a sedative, a friend or relative will need to take you home and stay with you for the first 24 hours.