Inside an MRI scanner

 When doctors need the highest quality images possible they turn to MRI scanners, but how do they work?

Doctors often plan treatments  based on imaging. X-rays,  ultrasound and CT scans  provide useful pictures, but  when the highest quality images are  needed, they turn to MRI scanners. While  CT scanners use x-rays and therefore  expose the patient to radiation, magnetic  resonance imaging (MRI) uses powerful  magnets and is virtually risk free. 

 MRI scans are obtained for many  medical conditions, although since they  are expensive and complicated to  interpret, they certainly aren’t as easy as  taking a chest x-ray. Examples for which  they are used include planning surgery  for rectal cancers, assessing bones for  infection (osteomyelitis), looking at the  bile ducts in detail for trapped gallstones,  assessing ligamental damage in the knee  joints and assessing the spinal cord for  infections, tumours or trapped nerves.

  Physicists and engineers use and  manipulate the basic laws of physics to  develop these incredible scanners for  doctors to use. MRI scans provide such  details because they work at a submolecular  level; they work on the protons  within hydrogen atoms. By changing  the position of these protons using  magnetic fi elds, extremely detailed  pictures of the different types of particles  are obtained. Since these pictures rely  on the tiny movements of these tiny  particles, you need to lie very still during  the scan.  

Planning from the detail

The detail provided by MRI  scanners enables doctors of all  specialties to plan their  treatment. When footballers  damage their knees, an MRI  scan will tell if the ligaments  are ruptured. Knee surgeons  can then reconstruct the  damage, often via keyhole  incisions (arthroscopically).  MRI scans are used to  characterise a variety of  tumours, such as those of the  rectum (the lowest part of the  colon) and within the brain.  MRI gives enough detail to  determine the size and stage of  the tumour. This helps  specialist surgeons plan  whether the tumour is  resectable, and also how to  perform the operation.

  MRI’s key lies in its ability to  differentiate soft tissues – it  can even tell the difference  between infected and normal  tissues. Infections within  bones are best identifi ed using  MRI, and then surgeons can  plan whether to treat with  antibiotics, an operation, or, if  the infection is spread too far,  an amputation.  

Slice by slice images

Specially wound coils, known as gradient  coils, allow for the detailed depth imaging  which creates the slice-by-slice pictures.  While the main superconducting magnet  creates a very stable magnetic fi eld, these  gradient coils create variable magnetic fi elds  during the scan. These fi elds mean that the  magnetic strength within the patient can be  altered in specifi c areas. Since the protons  realign at different rates in different tissue  types, the relationship between the strength  of the fi eld and the frequency of the emitted  photons is different for various tissues.  Detecting these differences allows for very  detailed images.

 Powerful computers outside the main  machine then reconstitute all of this data to  produce slice-by-slice imaging. Depending on  what’s being scanned, 3D reconstructions can  then be created, such as for brain tumours.  

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