MRI is based on the magnetization properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined This alignment (or magnetization) is next perturbed or disrupted by introduction of an external Radio Frequency (RF) energy The nuclei return to their resting alignment through various relaxation processes and in so doing emit RF energy. After a certain period following the initial RF, the emitted signals are measured. Fourier transformation is used to convert the frequency information contained in the signal from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of gray in a matrix arrangement of pixels By varying the sequence of RF pulses applied & collected, different types of images are created. Repetition Time (TR) is the amount of time between successive pulse sequences applied to the same slice. Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal
The MRI scan "sees" the spine by using a large magnet that stimulates (excites) the hydrogen atoms in the vertebrae (bony building blocks of the spine), spinal sac (contains the spinal cord, nerves and spinal fluid), supporting muscles and ligaments Because the human body is mostly comprised of water (which is 2 parts hydrogen and I part oxygen, or H,O), an accurate picture of the spinal anatomy can be attained
The most common MRI sequences are T1-weighted and T2-weighted scans. TI- weighted images are produced by using short TE and TR times. The contrast and brightness of the image are predominately determined by T1 properties of tissue Conversely, T2-weighted images are produced by using longer TE and TR times. In these images, the contrast and brightness are predominately determined by the T2 properties of tissue.
