**10. Magnetic Resonance Imaging**

It is a imaging modality which uses magnetic fields and radiofrequency energy to stimulate hydrogen nuclei which emits radiofrequency waves that are used to construct images. Over the 1990's the field has evolved from a procedure which takes a long time to produce a series of static images to one in which real time 3-D visualization is possible. MRI uses magnetic field of strength 0.5 Tesla to 3 Tesla (1 Tesla is 10,000 Gauss; earth magnetic field is 0.5 G). MRI uses fields which are 5000 to 60000 more powerful than the earth's magnetic field.

*Synchronisation or respiratory motion compensation* is required as the heart is a moving object and 'gating' or ' synchronization' is required to return to the same point in the cardiac cycle in order to freeze the cardiac motion. Pulse oximetry, ECG signal or MRI navigator echoes can be used for gating. Advances in *gradient coil and parallel acquisition* methods can obviate the need for synchronization.

Two main modalities of MRI are:

**a.** Spin Echo – uses a radiofrequency pulse that tilts the hydrogen protons by 90 degrees followed by a second 180 degree pulse, which are used to generate images. Produces images in which the flowing 'blood is black'. This provides static anatomic information, with excellent blood myocardium contrast. Used for assessing cardiac tumours, pericar‐ dial disease and thoracic masses. Takes a relatively longer time

**b.** Gradient Echo - sequences uses radiofrequency pulses that are less than 90 degrees, that are faster than spin Echo images and used to produce images in which the flowing 'blood appears white'. 'Steady State free precession' MR sequences allow real time MR fluoro‐ scopy, with shorter imaging times. Used to assess ventricular function and flow of blood in the cardiac pathways and for identifying stenotic or regurgitant jets.

Myocardial tagging – uses 'spatial modulation of magnetization' so that protons in selected volume are incapable of producing a signal. This produces stripes across the image, tagging the myocardium. As the heart moves the tags are followed and this allows calculation of myocardial strain

Velocity encoded Cine MRI- can be used to measure blood flow velocity and quantify blood flow rate.- can measure the regurgitation volume and can even calculate the shear stress exerted by the blood on the vessel wall.

Contrast enhanced MRI – Uses gadolinium chelate which produces bright blood signal used for clear delineation of spatial relationship and for imaging of baffles and outflow tracts. Hyper enhancement of myocardial regions observed 10-15 minutes after administration of gadolini‐ um contrast is indicative of scar tissue and irreversible myocardial injury.

MRI in patients younger than 5-6 years would require sedation, Surgical clips, sternotomy wires, coils stents and occluding devices are MRI safe once the surrounding fibrous tissue grows over these implants and makes them immobile. Cardiac pacemakers, presence of intracranial, intra ocular or intracochlear implant are considered contraindication to MRI.

Cardiovascular MRI is fast becoming a tool which can provide us with anatomic and functional information not provided by echo or cardiac catheterization. [17]
