T2*-weighted imaging
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''T''2*-weighted imaging is an
MRI sequence An MRI sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance. A multiparametric MRI is a combination of two or more sequences, and/or includin ...
to quantify observable or effective ''T''2 (T2* or "T2-star"). In this sequence, hemorrhages and
hemosiderin Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderin Hemosiderin or haemosiderin is an iron-storage complex that is composed of partially digested ferritin and lysosomes. The breakdown of heme gives rise ...
deposits become hypointense.


Physics

''T''2*-weighted imaging is built from the basic physics of magnetic resonance imaging where there is spin–spin relaxation, that is, the transverse component of the
magnetization In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Di ...
vector exponentially decays towards its equilibrium value. It is characterized by the ''spin–spin relaxation time'', known as 2. In an idealized system, all nuclei in a given chemical environment, in a magnetic field, relax with the same frequency. However, in real systems, there are minor differences in chemical environment which can lead to a distribution of resonance frequencies around the ideal. Over time, this distribution can lead to a dispersion of the tight distribution of magnetic spin vectors, and loss of signal ( Free Induction Decay). In fact, for most magnetic resonance experiments, this "relaxation" dominates. This results in dephasing. However, decoherence because of magnetic field inhomogeneity is not a true "relaxation" process; it is not random, but dependent on the location of the molecule in the magnet. For molecules that aren't moving, the deviation from ideal relaxation is consistent over time, and the signal can be recovered by performing a spin echo experiment. The corresponding transverse relaxation time constant is thus ''T''2*, which is usually much smaller than ''T''2. The relation between them is: : \frac=\frac+\frac = \frac+\gamma \Delta B_0 where ''γ'' represents
gyromagnetic ratio In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol , gamma. Its SI u ...
, and Δ''B''0 the difference in strength of the locally varying field. Unlike ''T''2, ''T''2* is influenced by magnetic field gradient irregularities. The ''T''2* relaxation time is always shorter than the ''T''2 relaxation time and is typically milliseconds for water samples in imaging magnets. ''T''2*-weighted imaging can be created as a postexcitation refocused
gradient echo Gradient echo is a magnetic resonance imaging (MRI) sequence that has wide variety of applications, from magnetic resonance angiography to perfusion MRI and diffusion MRI. Rapid imaging acquisition allows it to be applied to 2D and 3D MRI imagin ...
(GRE) sequence with small flip angle. The sequence of gradient echo ''T''2*-weighted imaging (GRE T2*WI) requires a high uniformity of the magnetic field.


Clinical applications

''T''2*-weighted sequences are used to detect deoxygenated hemoglobin, methemoglobin, or
hemosiderin Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderin Hemosiderin or haemosiderin is an iron-storage complex that is composed of partially digested ferritin and lysosomes. The breakdown of heme gives rise ...
in lesions and tissues. Diseases with such patterns include intracranial hemorrhage,
arteriovenous malformation Arteriovenous malformation is an abnormal connection between arteries and veins, bypassing the capillary system. This vascular anomaly is widely known because of its occurrence in the central nervous system (usually cerebral AVM), but can appea ...
,
cavernoma Cavernous hemangioma, also called cavernous angioma, venous malformation, or cavernoma, is a type of venous malformation due to endothelial dysmorphogenesis from a lesion which is present at birth. A cavernoma in the brain is called a cerebral ca ...
, hemorrhage in a tumor, punctate hemorrhages in diffuse axonal injury,
superficial siderosis Superficial hemosiderosis of the central nervous system is a disease of the brain resulting from chronic iron deposition in neuronal tissues associated with cerebrospinal fluid. This occurs via the deposition of hemosiderin in neuronal tissue, and ...
, thrombosed aneurysm, phleboliths in vascular lesions, and some forms of calcification. ''T''2*-weighted GRE sequences can detect microhemorrhages as seen in most vestibular schwannomas, thereby differentiating them from
meningioma Meningioma, also known as meningeal tumor, is typically a slow-growing tumor that forms from the meninges, the membranous layers surrounding the brain and spinal cord. Symptoms depend on the location and occur as a result of the tumor pressing o ...
s. The ''T''2*-weighted GRE sequence can detect a "middle cerebral artery susceptibility sign", which is a dark linear filling defect that is wider than the corresponding artery on the contralateral side. This sign is 83% sensitive and 100% specific for thrombotic occlusion of the internal carotid artery. It can detect hemosiderin deposition in joints as seen in arthropathy by
hemophilia Haemophilia, or hemophilia (), is a mostly inherited genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. This results in people bleeding for a longer time after an injury, easy bruising, ...
, as well as pigmented villonodular synovitis . ''T''2*-weighted sequences are very useful for evaluation of articular cartilages and
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
s because a relatively long ''T''2* makes the articular cartilage becomes more hyperintense, while bone becomes hypointense. ''T''2*-weighted sequences can be used with MRI contrast, mainly ferucarbotran or superparamagnetic iron oxide (SPIO), to depict liver lesions.


See also

*
MRI sequence An MRI sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance. A multiparametric MRI is a combination of two or more sequences, and/or includin ...


References

{{reflist Magnetic resonance imaging