Spin-spin relaxation time
Encyclopedia
Spin–spin relaxation is the mechanism by which Mxy, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value of zero, in nuclear magnetic resonance
(NMR) and magnetic resonance imaging
(MRI). It is characterized by the spin–spin relaxation time, known as T2, a time constant characterizing the signal decay. It is named in contrast to T1, the spin-lattice relaxation time
.
It is the time it takes for the magnetic resonance signal to reach 37% (1/e) of its initial value after its generation by tipping the longitudinal magnetization towards the magnetic transverse plane. Hence the relation:
T2 relaxation generally proceeds more rapidly than T1 recovery, and different samples and different biological tissues have different T2. For example, fluids have the longest T2s (in the order of seconds for Protons), and water based tissues are in the 40-200 ms range, while fat based tissues are in the 10-100 ms range. Amorphous solids have T2s in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.
decay experiment can be used to measure the dephasing time, as shown in the animation below. The size of the echo is recorded for different spacings of the two pulses. This reveals the decoherence which is not refocused by the
pulse. In simple cases, an exponential decay is measured which is described by the T2 time.
, T2 weighted images can be obtained by setting long repetition time(TR) ( >1500 ms) and echo time(TE) ( > 75 ms) values in conventional spin echo
sequences, while in gradient echo sequences they can be obtained by using flip angles of less than 40° instead of 90° while setting TE values to above 30 ms.
T2 weighted images in MRI are often thought of as "pathology scans" because collections of abnormal fluid are bright against the darker normal tissue. In the MRI of prostate cancer
, cancerous regions of the peripheral zone of the gland often appear as hypo-intense regions of the gland in T2 weighted MR images.
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR) and magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI). It is characterized by the spin–spin relaxation time, known as T2, a time constant characterizing the signal decay. It is named in contrast to T1, the spin-lattice relaxation time
Spin-lattice relaxation time
Spin–lattice relaxation is the mechanism by which the z component of the magnetization vector comes into thermodynamic equilibrium with its surroundings in nuclear magnetic resonance and magnetic resonance imaging. It is characterized by the spin–lattice relaxation time, a time constant known as T1...
.
It is the time it takes for the magnetic resonance signal to reach 37% (1/e) of its initial value after its generation by tipping the longitudinal magnetization towards the magnetic transverse plane. Hence the relation:
T2 relaxation generally proceeds more rapidly than T1 recovery, and different samples and different biological tissues have different T2. For example, fluids have the longest T2s (in the order of seconds for Protons), and water based tissues are in the 40-200 ms range, while fat based tissues are in the 10-100 ms range. Amorphous solids have T2s in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.
Measuring spin–spin relaxation
A Hahn echoSpin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....
decay experiment can be used to measure the dephasing time, as shown in the animation below. The size of the echo is recorded for different spacings of the two pulses. This reveals the decoherence which is not refocused by the
pulse. In simple cases, an exponential decay is measured which is described by the T2 time.T2 weighted images
In MRIMagnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
, T2 weighted images can be obtained by setting long repetition time(TR) ( >1500 ms) and echo time(TE) ( > 75 ms) values in conventional spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....
sequences, while in gradient echo sequences they can be obtained by using flip angles of less than 40° instead of 90° while setting TE values to above 30 ms.
T2 weighted images in MRI are often thought of as "pathology scans" because collections of abnormal fluid are bright against the darker normal tissue. In the MRI of prostate cancer
Prostate cancer
Prostate cancer is a form of cancer that develops in the prostate, a gland in the male reproductive system. Most prostate cancers are slow growing; however, there are cases of aggressive prostate cancers. The cancer cells may metastasize from the prostate to other parts of the body, particularly...
, cancerous regions of the peripheral zone of the gland often appear as hypo-intense regions of the gland in T2 weighted MR images.