Magnetic Resonance Imaging (MRI)

MRI is based on the magnetic properties of hydrogen which is one of the constituents of cerebral tissue and other tissues. The hydrogen atom has a single proton which possesses a magnetic moment or behaves like a small magnet called a spin. The spins of hydrogen nuclei have a random orientation in any region and the orientation of all the spins is such that the resultant magnetic moment is zero.

MRI uses a strong magnetic field and radio waves to obtain images of many types of soft tissues. When the radio signal is beamed through the body, hydrogen in the water molecules of the body absorbs the energy. The excited spins enter into resonance and align in a direction different from that of the field of the magnet. This is called magnetic resonance. When the radio wave pulse subsides, the spins return to the previous direction of the magnetic field. The molecules then produce small radio signals by re-emitting the energy absorbed. The signal is picked up by the computer and the density of body tissues at each point is calculated. The magnitude of the signal depends upon the richness of hydrogen in the tissue. For fat, muscles, water or air the signals are of different wavelengths. They also differ from grey matter and white matter. MRI is used to obtain images of the brain.

Blood Oxygen Level Dependent Response

Any synaptic activity consumes oxygen which is provided by an increase in the flow of blood to that region. This increase is by vasodilation of the surrounding arterioles. Regions in which the blood flow increases and regions where it does not increase, can be mapped using MRI.

Blood haemoglobin contains iron. Oxyhaemoglobin in blood is diamagnetic while deoxyhaemoglobin is paramagnetic. When the subject is placed in a magnetic field each molecule of deoxyhaemoglobin causes a disturbance in the homogeneity of the magnetic field, resulting in a reduction in the MRI signal. When there is a cerebral activity, there is an increase in the signal. Deoxygenated haemoglobin is an endogenous magnetic tracer.

MRI is one of the best diagnostic tools for imaging soft tissues of the brain, blood vessels of the neck, for examining the heart, aorta, lumbar spine, upper abdomen, liver, kidney, spleen, pancreas, pelvis, hips, male and female reproductive systems, and the bladder. It is also used to examine shoulder joints, wrists, ankles and feet.

MRI provides excellent contrasts details between different tissues. It has become a powerful tool for diagnosing many forms of cancer, for diagnosing and treatment of heart disease, heart attack and stroke.

MRI Procedure

The MRI examination can last from 10 minutes to 1 hour, depending upon the tissue being examined. The patient is placed on a special table inside a large, cylindrical-shaped magnet. The cylindrical magnet has a large aperture of 11″ to 26″radius with a length of 5′3″ to 8′6″.

Many patients, when placed inside the magnet, become claustrophobic. A new open system has been developed to overcome claustrophobic. The open system has a C-shaped magnet.

The open system has a magnetic field strength of 0.2 to 0.3 tesla; the closed system has field strength of 1.5 tesla, which is thirty thousand times stronger than the magnetic field of the earth.

During examination, a radio signal is turned on and off, intermittently. There are 2-6 sequences lasting 2-15 minutes. The radio waves are absorbed by the body and the echoes are continuously measured by the scanner.

The figure above shows the schematic functional diagram of the MRI unit. The gradient coil is made up of loops of wire embedded in a hard plastic tube. During the scanning process, current pulses pass through the gradient coil every few milliseconds. When these pulses pass, the coil vibrates inside the plastic casing, creating a knocking sound which may be irritable to some patients.

Magnets

For magnetic field strengths of less than 0.3 tesla, electromagnets with coil windings are used. If permanent magnets are used, the weight of the magnets is very large and the entire equipment becomes very heavy. For low field strength nowadays, permanent magnets of small size are used. However, the most commonly used magnets are superconducting magnets, which are electromagnets with coils bathed in liquid helium at -452°C. Since the coils are insulated, the patient does not feel so cold. Superconducting magnets are extremely expensive and can create fields of 2.0 tesla, allowing higher quality imaging.

In MRI, images are displayed in three X, Y, Z planes or axial, coronal and sagittal planes respectively as shown in the diagram below:

Oblique views can also be obtained from MRI. Many diseases, like brain tumours, are more apparent in MRI scans than in CT scans. Even without a contrast medium, MRI creates detailed images of blood vessels, however nowadays there is a tendency to use a contrast medium called Gadolinium for imaging blood vessels.

The Benefits, Risks and Limitations of MRI

Benefits

1. Soft tissue structures of the body such as heart, lungs, liver and other organs are clearer as compared to the images obtained by other methods.
2. There is no exposure of the body to radiation as in X-rays.
3. MRI can help to detect abnormalities that might be obscured by bones in other imaging techniques.
4. MRI helps to evaluate the structure as well as the function of many organs.
5. The detailed MRI images help in the early diagnosis and evaluation of tumours.
6. For diagnosis of heart and cardiovascular problems, MRI is a good alternative to X-ray angiography, since it is non-invasive.
7. The contrast medium Gadolinium used in MRI is less likely to cause allergic reaction as compared to iodine-based materials used for conventional X-rays and CT scans.

The Risks

1. Undetected metal implants will be affected by the strong magnetic field, and maybe pulled out causing damage to tissues.
2. MRI should be avoided in the first 12 weeks of pregnancy.

Limitations

1. MRI cannot always distinguish between tumour tissue and oedema fluid.
2. It does not detect calcium present in a tumour.
3. For images of the bones, X-rays are better than MRI and sometimes CT is preferred for patients with severe bleeding.
4. Patients with metal implants cannot undergo MRI tests. MRI cannot be used in early pregnancy.
5. Cost of MRI is more than that of CT scanning.

You can also read: X-ray imaging