1. How can magnetic resonance be used for imaging?
    (click here for a pictorial explanation)

Answer
An imaging technique that uses the magnetic properties of the hydrogen atoms to produce images. The nucleus of the hydrogen is a spinning charged proton with magnetic properties. 
Two magnetic fields are used in MRI. The first being a strong static magnetic field which cause the hydrogen atoms in the bodies to align in a direction parallel to the field. A second magnetic field (radio-frequency RP pulse) is then applied at right angle to the first field causing the hydrogen atoms to change their alignments. When the RP pulse is turned off the hydrogen atoms return to their alignment with the static magnetic field. In so doing, they release the extra energy as magnetic signal which can be detected by coil placed around the patient.
The magnetic signals depends on many factors amongst which are: density of the hydrogen, the environment of the hydrogen atoms (in fat or in water), flow (CSF or blood) and relaxation time (T1 and T2). T1 (longitudinal relaxation) depends on the time the hydrogen atoms take to return tot he axis of the magnetic field where as T2 (transverse relaxation time) measure the interaction of hydrogens with other tissues. 

2. What are the differences between T1 and T2 weighted images?
    (Click here for the differences between T1 and T2 images, the details included 
       here is for completeness of the answers) 

Answer
There are two main forms of MRI, known as T1 weighted and T2 weighted.  The two image types are also known as T1 contrast and T2 contrast. As these names imply, the reason that more than one image type is required is that some tissues are contrasted better, and thus are easier to see, with one technique rather than the other. T1 contrast shows better anatomical detail and better distinction between solid and cystic structures. T2 contrast shows pathological changes better.

T1 Weighted Imaging

A 90-degree pulse is used to displace the protons. The protons are rotated around into the X-Y plane where they move in phase (in time with each other). After the RF pulse is switched off the protons come out of phase (dephase). The decaying signal sent out as the protons return to the Z-plane is measured over a time known as TR. The faster protons realign the stronger the signal that is emitted. Early measurement of this signal is required to differentiate between various tissues. 
To maximize T1 contrast a short TR sampling time must be used.
In T1 imaging you are measuring how the protons realign relative to the external magnetic field.

T2 Weighted Imaging

A 90-degree pulse is used, as in T1 weighting. However, as the protons dephase a 180-degree pulse is fired which puts them back in phase. After this second pulse is switched off the protons again dephase. In a T2 image we are looking to emphasize tissues which remain in phase for a long time and therefore maintain a strong signal. This can be achieved by delaying the sampling time after the 180-degree pulse (a time known as TE). 
To maximize T2 contrast a long TE sampling time must be used.
In T2 imaging you are measuring how the protons realign relative to each other

3. What are the advantages of MRI scans?

Answer
Non-ionizing radiation.  Because MRI does not ionize tissue it is considered amongst the safest of radiological techniques. There are no known physiological side effects of being exposed to a magnetic field. 
High soft-tissue contrast MRI images provided very detailed information about soft tissues. They can differentiate between normal and abnormal tissues and may show damage missed on CT 
Visualization of areas deep within bony structures MRI is thus invaluable for the diagnosis and treatment of brainstem tumours. 
Shows vasculature without contrast.
Good for angiography.  MRI is excellent for imaging blood flow. Functional MRI maps changes in blood flow in the brain during specific tasks. This provides valuable information about how the brain works. 

4. What are the main disadvantages of MRI scans?

Answer
High cost of equipment.
Claustrophobia. Up to 10% of patients experience claustrophobia during an MRI scan and 1% of scans have to be aborted because of it. 
Long imaging time. A complete image may take up to30 mins and movement at the wrong time can cause artefacts in the image. However, new techniques have reduced imaging time. 
Strong magnetic field. MRI imaging is unsuitable for many patients with metal implants (e.g. artificial joints) and is especially dangerous for patients with pacemakers, neural stimulators, 
or cochleal implants. Loose metal objects must be removed before coming near to the scanner otherwise they may be attracted so strongly by the magnet that they fly through the air like tiny missiles! 
Unable to image calcium  Because MRI detects water rather than molecular density, calcium is not well visualized. This means that tissue calcification, a feature of a number of disease processes, can not be detected. Bone is also less obvious than on a CT scan. 
Acoustic noise  Switching on and off of the gradient coils causes repeated loud bangs. Noise levels may reach 95 dB for much of the scan. Ear plugs are advisable to reduce the risk of temporary or even permanent hearing loss. 

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