MET Full Form

Much of the excitement around Mass Emission Tomography’s rapidly growing uses stems from its clear identification as a unique imaging modality with demonstrated therapeutic utility in treating cancer patients. In addition, unlike other imaging modalities, Mass emission tomography (MET) quantitatively provides a scintigraphic depiction of tumour physiology and architecture, giving it a distinct edge over other cross-sectional imaging methods like computed tomography (CT). The primary properties used to allow tumour imaging with MET, or any other tracer technology are variations in physiological and metabolic characteristics of tumor and normal tissues.

The phenotype of tumour cell surface antigen vs normal tissues is one of these distinctions. In general, tumours grow faster than normal tissues, resulting in more significant usage of DNA precursors like thymidine and higher protein synthesis rates. Tumor cells have been seen transporting and incorporating numerous kinds of amino acids and rates of anaerobic and aerobic glycolysis.

MET Full Form

MET Full Form is Mass emission tomography. The glucose consumption rate in tumour cells is much higher than in normal cells, and this metabolic property is used in MET imaging with FDG as the primary workhorse. MET systems, as well as cyclotrons that produce positron-emitting radiopharmaceuticals, have been continually improved. At the same time, MET (CT) systems allow for fusion pictures and exact attenuation correction. The self-shielded cyclotrons were created to provide specialised techniques for producing many MET radiopharmaceuticals in-house. MET imaging is used to diagnose lung, colorectal, breast, lymphoma, head and neck, bone, ovarian, and gastrointestinal malignancies. MET has been identified as a promising diagnostic technique for predicting biological and physiological changes at the molecular level and may have future implications in Stem Cell research.

Basic Principles

A positron is an electron’s antiparticle with the same mass and charge. After emission, the positron has some kinetic energy, which is dissipated by repeated collisions with electrons in the surrounding tissues. As a result, the positron loses all or practically all of its power, which causes it to combine with an electron. This eventually creates positronium, a short-lived compound. Because positronium is short-lived, it eventually annihilates, transforming all of its mass into energy and producing two 511 keV photons each. This assures energy and momentum conservation. The ability to identify and localise positron emitters using a revolutionary approach termed coincidence detection is based on the unique property of simultaneous emission of two destroyed photons. 

Scintillation detectors, such as bismuth germinate (BGO) or Lutetium Oxyorthosilicate (LSO), and photomultiplier tubes are positioned opposite the positron emitter source. Separate amplifiers and energy discriminating circuits are then used to process the signals. As a result of this procedure, a coincidence event is detected, which pinpoints an annihilation event somewhere along the line connecting the two detectors. In a conventional MET scanner, the patient is surrounded by hundreds of such detector banks in the form of a ring. As a result, it can be concluded that MET scanning entails the detection of millions of coincidence occurrences and offers information on the concentration and geographical position of positron emitters within the patient.

Image Formation

A coincidence line is produced by each set of parallel and opposite detectors, which is unique in terms of position and direction. A vast number of these coincidence lines make up the data set, which may be used to rebuild a cross-sectional picture. The data for coincidence occurrences are kept as a two-dimensional matrix. The horizontal direction indicates the offset from the centre of the field of vision (CFOV), and the vertical direction shows the projection angle. The ‘Sinogram’ is a set of projection data in terms of a two-dimensional matrix that may be used to rebuild a picture.

On the other hand, Sinogram data must be adjusted for tissue variations and detector non-uniformities. Due to geometrical variance, changes in energy discrimination, and detector gains, the detection effectiveness of various detector components in a MET system is likely to vary. To avoid the appearance of artefacts, such variances must be equalised. Furthermore, compensation owing to intra-tissue absorption of one or both destroyed photons is taken into account via attenuation correction. After appropriate modifications, the Sinogram depicts all the coincidence occurrences along a given coincidence line. The picture is then reconstructed using filtered back projection or an interactive approach based on the sinogram data.

Conclusion

Mass emission tomography (MET) has ushered in a new age of tumour physiology and anatomical visualisation. The inherent challenges of intra-patient absorption and attenuation correction have been addressed because of technological advancements in which MET systems are integrated with CT to provide channel fusion pictures and approximate attenuation correction. This is to underline that CT-measured attenuation correction factors are only scaled up for 511 keV, and MET-CT can only correct segmental attenuation. Therefore, generating attenuation data for transmission scan utilising 68-Ge source rather than integrated CT is preferable to accomplish exact attenuation correction. Dedicated and self-shielded cyclotrons have been built to offer many radiopharmaceuticals for therapeutic and research uses.