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MRI Made Easy

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The gradients that I have shown in our discussions have been simplified greatly. In reality there are many gradient coils and as the MRI machine scans different parts of the body, the coils work together to create very complex gradient fields. Before I proceed, let me tell you that this an extremely rare occurrence. So please do not worry excessively about it happening to you either as a member of the staff or even as a patient. Therefore when the MRI sends the radio frequency (RF) energy wave, it does this at the resonant frequency of the hydrogen nuclei. Anyone who stays with you will be asked if they have a pacemaker or any other metal objects in their body.

DWI is an imaging modality that combines T2 images with the diffusion of water. With DWI scans, ischaemia can be visualised within minutes of it occurring (Figure 5). This is because DWI has a high sensitivity for water diffusion, thereby detecting the physiological changes that happen immediately after a stroke. Figure 5. Recent right-sided ischaemic stroke 8T2 relaxation (dephasing): loss of transverse magnetization; T2 time refers to time where only 37% of original transverse magnetization is present

However, quantum physics complicates things slightly. According to its rules, the hydrogen nuclei don’t absorb just ‘any’ energy. They can be quite fussy about ‘what’ energy they accept. 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. Hydrogen nuclei have a quantum physics property called “spin “. Now you probably imagine hydrogen nuclei “spin” to look something like this. Dr Hidayatullah Hamidi. Normal brain MR shows differences between T1 and T2 images. Licence: [CC BY-SA]. The MRI machine then looks at the section shown by the green arrow and the process goes on. Combining information from these different sections enables it to eventually construct an image for you to see. Now the question is, how does the MRI machine make only hydrogen nuclei in the area of interest respond while keeping all the other hydrogen nuclei quiet ?The “ magnet stop “ button is a button that you certainly don’t want to press by mistake. The button is connected to the venting system. At the centre of each hydrogen atom is an even smaller particle called a proton. Protons are like tiny magnets and are very sensitive to magnetic fields. 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. 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

Your MRI scan needs to be studied by a radiologist (a doctor trained in interpreting scans and X-rays) and possibly discussed with other specialists.In the example shown below, as we go from the patients head towards his feet, the main magnetic field becomes weaker. We would now say that there is a ‘magnetic gradient” along the patient. The coils that modify the main magnetic field are called “gradient coils”. How these magnetic field gradients are created is fascinating but I will explain that later. Once the RF coil stops transmitting energy, the listening coil (green) listens for a return signal of energy. Since it was only the hydrogen nuclei at the foot end (B) that absorbed energy, it is only these nuclei that now release energy. The other nuclei, e.g. ‘A’, do not respond, since they did not absorb energy. In this way, the MRI machine is able to get information specifically from the lower limb area of the patient. The MRI machine applies a current to this energy producing coil for a short period. During this period, the coil produces energy in the form of a rapidly changing magnetic field (pink waves in diagram below). The frequency (i.e. how often it changes in one second) of this changing field falls within the frequency range commonly used in radio broadcasts. Therefore this energy is often called “radio frequency” energy (RF energy) and the coil is often called an radio frequency coil ( RF coil). Unlike CT, MRI consists not of one scan but a series of “sequences.” Each MRI sequence is designed to manipulate the magnetization and achieve amplification or suppression of a particular tissue characteristic. Hence, it is said to be “weighted” to the specific tissue characteristic. The appearance of a lesion, specifically its brightness (generally referred to as signal intensity), may vary on each sequence depending on the particular amplified or suppressed features of the tissue. In contradistinction, in CT, the lesion has a fixed appearance or “density” measured in Hounsfield units. A lesion can thus be described as iso-, hypo-, or hyperintense relative to recognizable normal reference structures, including fluid (CSF, vitreous, etc.) or soft tissue structures like extraocular muscles in that particular sequence. Because the RF energy of frequency 60 MHz is below the resonant frequency of the hydrogen nucleus at the head end of the patient (A), it simply passes this nucleus without getting absorbed. On the other hand, the 60 MHz RF energy exactly matches the resonant frequency of the hydrogen nucleus at the foot of the patient (B). This nucleus therefore absorbs the energy and changes its spin direction to be become an high energy nucleus.

Learn why each image type is used – this will enable you to know what you are looking for (e.g. for MR brain it’s useful to look at T2, then FLAIR, then DWI/ADC, as this will help distinguish between most differentials). Gadolinium enhances vasculature (i.e. arteries) or pathologically-vascular tissues (e.g. intracranial metastases, meningiomas). This process involves injecting 5-15ml of contrast intravenously, with images taken shortly thereafter. Gadolinium appears bright in signal, allowing for detection of detailed abnormalities (e.g. intracranial pathologies). Typical intracranial abscesses have a “ring-enhancement” pattern, while metastases enhance homogeneously. Meningiomas will have a homogenous enhancement after the contrast, but will also have a “dural tail,” meaning the lesion appears continuous with the dura (Figure 2). 4 Figure 2. Meningioma is shown more clearly by gadolinium contrast with a dural tail 5 Instead, the MRI machine needs to scan the body in sections. It needs to record signals from hydrogen nuclei in one area before moving onto the next. For an example, let us imagine, for the purpose of discussion, that we are doing a scan of the head. Imagine that it is currently interested in scanning the section of the head shown by the blue arrow. It needs a way to make the hydrogen nuclei of interest (blue ones in image) to respond by taking and returning energy. At the same time, it needs the other hydrogen nuclei not to respond (i.e. all the red dots). 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. 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

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To make the diagrams clearer, I will no longer draw the patient. Instead, I will only show the hydrogen nuclei that are inside him.

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