Final answer:
The term 'echo' in MRI refers to the reemitted radio waves from the nuclei after they are excited by the machine's magnetic field. These echoes provide the necessary information to create detailed images of the body's internal structures, and different sequences of MRI make use of the varying relaxation times of protons for enhanced imaging contrast.
Step-by-step explanation:
The term echo in MRI refers to the reemitted radio waves by the nuclei after they absorb radiowaves and flip during magnetic resonance imaging. When the external magnetic field is turned off, the protons return to the equilibrium state, releasing energy in the form of echoes that are captured to form MRI images. The intensity of these echoes is proportional to the local concentration of nuclei, providing information about the tissue type and its environment. The contrast in the images can be enhanced using different MRI sequences, such as T1, T2, or proton density scans, which rely on different relaxation times of the protons after the field is removed.
MRI provides high-resolution images for medical diagnosis, using the echoes of radiofrequency waves to create detailed images of the body's internal structures. Unlike X-rays, MRI does not use ionizing radiation, making it a safer option for certain types of tissue. However, it is less effective in imaging certain tissues, such as bone, and is more expensive compared to X-rays. Despite these disadvantages, MRI's ability to create detailed images without the hazards of ionizing radiation makes it a valuable tool in medical imaging.