MRI Made Easy

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To further make the diagrams clearer, I will be showing a close up of only a few of the hydrogen nuclei. You might also be interested in our OSCE Flashcard Collection which contains over 2000 flashcards that cover clinical examination, procedures, communication skills and data interpretation.

As mentioned before, the MRI machine has an extremely strong magnet, shown as the green coil below. This magnet produces a field that is equally strong everywhere. The MRI machine does something similar to detect the hydrogen nuclei. It first “irritates “ the hydrogen nuclei and then from their “responses”, detects their presence. How the MRI machine does this is somewhat more complicated than shouting to detect three grumpy men, but don’t worry, I will explain it to you. To understand how the MRI machine selects one region at a time, we need to first understand the concept of ‘resonant frequency’. Resonant frequency:To make the diagrams clearer, I will no longer draw the patient. Instead, I will only show the hydrogen nuclei that are inside him. 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

The MRI scanner is operated by a radiographer, who is trained in carrying out imaging investigations. You should not use the emergency quench button for every type of emergency. Quenching results in the loss of Helium which is extremely expensive to replace. Furthermore, the process of quenching can damage the magnet which can take a long time to repair, making the MRI unavailable for other patients. in clinical practice, to identify what is being explored, optimize the examination and interpret it in a relevant way; When magnetic field is turned on, the axes align with the more powerful external magnetic field with some aligning parallel (low energy state) to the field and some aligning anti-parallel (high energy state) to magnetic field, cancelling each other. The larger the external magnetic field, the greater the difference in energy levels and the larger the excess number aligned with the field. 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.

Bitar, R. et al. MR pulse sequences: What every radiologist wants to know but is afraid to ask. Published in 2006. Available from: [LINK] However, with super conductivity, since there is no resistance, any electric current given to the coil will simply keep flowing ‘forever’. This property can be made use of for the magnet not to need a continuous current supply. When the magnet is first installed, the engineers give it an electric current. Dr Hidayatullah Hamidi. Normal brain MR shows differences between T1 and T2 images. Licence: [CC BY-SA]. The MRI machine has a very powerful computer that controls various aspects of the scanning process. The computer controls the gradient coil to create an appropriate magnetic gradient. It then controls the frequency of the emitted RF energy according to which part of the body it wants to look at. It then collects the return signals that arrive at the ‘listening coil’. It repeats this process till the required anatomy of the body has been scanned.

For the hydrogen nuclei to get aligned, the magnetic field needs to be extremely strong. The units of magnetism commonly used are Tesla and Gauss. A typical MRI magnet has a strength of 1.5 Tesla. This is about thirty thousand times stronger than Earth’s magnetic field. An MRI scan is usually carried out as an outpatient procedure. This means you won't need to stay in hospital overnight.

After the scan

Anatomy, as with all scans, is key. MRIs produce a very clear view of structures, therefore strong anatomical knowledge is particularly helpful. Generally, MRI is used less commonly than plain films and CT scans. They are often reserved for superior viewing of soft tissues. MRI is particularly helpful in patients with suspected neurological or musculoskeletal pathology, however, they can be used in many other specialities too. It takes slightly longer to acquire MR images and they are more expensive. MRI is contraindicated in patients who have ferromagnetic metal implants or foreign bodies. 1 Consideration should be given to patients who are claustrophobic as well.