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What is Contrast mechanisms in MR imaging (MRI) ?
Contrast mechanisms in MR imaging are established on tissue specific parameters, utilized with the appropriate imaging technique, sometimes in conjunction with preparation of the magnetization or application of a contrast agent.
MR images have contrast if there are zones of high signal (hyperintensity - white in the image) and areas of low signal (hypo intensity - black in the image). Some regions have an intermediate signal (shades of gray in between white and black). The NMV is separated into discrete vectors of the tissues such as fat, cerebrospinal fluid (CSF), and muscle.
A tissue has a high signal if it has a huge transverse component of coherent magnetization at time TE. If there is a big component of coherent transverse magnetization, the amplitude of signal received by the coil is large, resulting in a hyperintense zone on the image. A tissue go back to a low signal if it has a compact or no transverse component of coherent magnetization at time TE. If there is a small or no component of transverse coherent magnetization, the amplitude of signal received by the coil is tiny, resulting in a hypointense zone on the image.
Tissue-specific parameters:
The primary sources of inherent tissue contrast in MRI are threefold: the proton density (PD), the longitudinal relaxation time T1, and the transverse relaxation time T2. Exposed to an outermost field, a macroscopic magnetization builds up within soft tissue, since the parallel alignment of the spins with the magnetic field corresponds to a lower energy state of the proton. The larger the macroscopic magnetization, the stronger the induced signal of the tilted magnetization and the brighter the pixel intensity displayed on the monitor.
T1 and T2 relaxation depend on two factors:
T1 relaxation
If the molecular tumbling rate matches the Larmor frequency of hydrogen. If there is a good match between the rate of molecular tumbling and the Larmor frequency of magnetic moments of hydrogen, energy exchange between hydrogen nuclei and the molecular lattice is efficient. When there is a bad match, energy exchange is not as efficient. This is important in both T1 recovery and T2 decay processes.
T2 relaxation
If the molecules are closely packed together. In tissues where molecules are closely spaced, there is more efficient interaction between the magnetic fields of neighboring hydrogen nuclei. The reverse is true when molecules are spaced apart. This is especially important in T2 decay processes, which rely on the efficiency of interactions between the magnetic fields of neighboring hydrogen nuclei (spin-spin relaxation).