MRI Made Easy

Product Information

The MRI machine uses certain properties of these hydrogen nuclei to compose an image for you to see. The concepts it uses to do this can be a little complex, but if we approach it in a step by step manner, we will eventually understand the basics. Dr Mahmoud Rashed. Multiple sclerotic plaques in periventricular regions and corpus callosum. Licence: [ CC BY-SA].

Bitar, R. et al. MR pulse sequences: What every radiologist wants to know but is afraid to ask. Published in 2006. Available from: [LINK] Depending on the size of the area being scanned and how many images are taken, the whole procedure will take 15 to 90 minutes. It's also possible for contrast agent to cause tissue and organ damage in people with severe kidney disease. The gradient coils create a gradient by producing a small magnetic field within the main magnetic field. The two magnetic fields interact and result in the magnetic gradient.As the MRI machine scans different areas of the body, it changes the gradients as necessary. The changes of the gradients are made by rapidly changing the magnetic fields produced in the gradient coils. The gradient coils have a tough job to do. It is quite “difficult” for them to create their modifying magnetic fields in the presence of the extremely strong main magnetic field. When the gradient coils produce magnetic fields to alter the main magnetic field, due to the huge magnetic forces involved, they move slightly. Now let us imagine that the MRI machine wants to image part of the head of the patient i.e. the area where nucleus ‘A’ resides. To do this, the MRI machine changes the emitted RF energy frequency to now match the resonance frequency of nucleus A. Therefore, this time, hydrogen nucleus A absorbs the energy as it matches its resonance frequency, whereas nucleus B at the foot end ignores this energy as it does not match its resonance frequency. An MRI scan is usually carried out as an outpatient procedure. This means you won't need to stay in hospital overnight. A magnetic resonance imaging (MRI) scan is a painless procedure that lasts 15 to 90 minutes, depending on the size of the area being scanned and the number of images being taken. Before the scan

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. 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. Extensive research has been carried out into whether the magnetic fields and radio waves used during MRI scans could pose a risk to the human body. The MRI machine can get in the way of anaesthesia, and the reverse is also true, where provision of anaesthesia can mess up MRI scans. However, for the sake of producing good images and keeping the patient safe, the couple needs to work together, taking care not to upset each other. Three years later, in 1978, Hugh Clow and Ian R. Young, who were working at a British company called EMI, achieved another groundbreaking milestone in medical imaging. They successfully created the first transverse Nuclear Magnetic Resonance (NMR) image of a human head. This achievement opened up new possibilities for non-invasive imaging of the brain and other internal structures, providing valuable insights for medical diagnosis and research.Let us magnify, for clarity, one hydrogen nucleus from the head end (A) and one from the foot end (B). As explained, nucleus A is in a stronger magnetic field and therefore has an high resonant frequency (e.g. 70 MHz). Nucleus B is in a weaker field and therefore has a lower resonant frequency (e.g. 60 MHz). Lesions that appear hyperintense on DWI are said to show “diffusion restriction,” which is always a pathological finding.[ 11] The mechanism for restricted diffusion of water may be varied; however, for e.g. in abscesses, the motion of water molecules is limited by the viscosity of the fluid and dense inflammatory infiltrate. In ischemia or severe demyelinating optic neuritis, cytotoxic edema results in cellular swelling and decreased intercellular spaces with cellular swelling and resultant diffusion restriction. In highly cellular tumors, such as lymphomas, reduced interstitial spaces result in similar diffusion restriction. An additional calculated map called the apparent diffusion coefficient (ADC) map, which is similar to an inverted color image, is also provided. True diffusion restriction will appear hypointense on ADC maps and hyperintense on DWI. Advanced applications based on diffusion include quantitative analysis of the degree of diffusion restriction, which allows differentiation between benign and malignant lesions.[ 12] 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. You have seen that the receiver coil receives the energy released by the hydrogen nuclei that are moving from the high energy state to a low energy state. There are various types of receiver coils. It is often best to use a receiver coil that can be kept as near as possible to the area being imaged. For an example, for imaging structures in the head, one can use a “head coil”. 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]