Q.Why is MRI considered a harmless diagnostic technique?

Several imaging and monitoring techniques let clinicians look inside the body or track an organ's activity without surgery.

Radiographs (X-rays) are obtained by passing a controlled beam of X-rays — a high-energy form of electromagnetic radiation — through the body onto a photographic film or digital detector on the far side. Dense tissue such as bone absorbs more of the beam and so appears light/white on the resulting image, while soft tissue absorbs less and appears darker, producing a single two-dimensional picture useful for viewing bones, detecting fractures, and examining the chest.

A tomogram is a single cross-sectional image of the body produced by tomography (as in a CT/CAT scan): X-ray images are taken from many different angles around the body and combined by computer into a detailed slice-by-slice picture, unlike a plain radiograph, which flattens the whole thickness of the body into one two-dimensional projection.

MRI (Magnetic Resonance Imaging) uses a strong magnetic field and radio waves, not ionising radiation, to build detailed cross-sectional images of soft tissue. In the procedure, the patient lies inside the scanner's large magnet, which aligns the hydrogen nuclei present throughout the body's water and fat molecules; brief pulses of radio-frequency energy are then applied, which knock these nuclei briefly out of alignment; as the nuclei realign afterward, they emit detectable radio signals, which the scanner's computer processes into detailed images. MRI is considered a harmless diagnostic technique specifically because it uses no ionising radiation at all — unlike X-ray or CT imaging, the magnetic fields and radio waves involved do not damage tissue or DNA.

The ECG (electrocardiogram) records the heart's electrical activity over time as a characteristic trace of waves and intervals: the P wave (depolarisation, i.e. electrical activation, of the atria), the QRS complex (depolarisation of the ventricles), and the T wave (repolarisation, or electrical recovery, of the ventricles). Between these waves, clinically important intervals are measured, including the P-R interval (the time from the start of atrial activation to the start of ventricular activation, reflecting how long the signal takes to pass through the conducting tissue between the atria and the ventricles) and the Q-T interval (spanning ventricular depolarisation together with its subsequent repolarisation). A prolonged P-R interval indicates a delay in the electrical signal passing from the atria to the ventricles along the conducting pathway — a finding associated with a conduction disturbance often referred to as heart block. …