Positron imaging

Positron Imaging. Creating images of distributions of positron emitters requires a somewhat different type of apparatus. Positron cameras use many of the same technologies as do cameras for other isotopes, but there is a broader array of methods and physical arrangements. AH of these systems take advantage of the physical characteristics of positrons. 

M. Lonneux, I. Borbath, M. Berliere, C. Kirkove, S. Pauwels, The place of whole-body PET FDG for the diagnosis of distant recurrence of breast cancer, Clin. Positron. Imaging 3(2) (2000) 45-49. 

A subsequent investigation (6 ) studied the resolution and sensitivity of Mn-52m as a quantitative measurement of the size and location of myocardial ischemia. Comparison between microsphere distributions and Mn-52m images taken at 1.5 cm levels revealed that an ischemic area down to the size of 2.5 cm with 50% of the normal myocardial perfusion can be seen in the positron images. In addition, a relative change of 10% or more of the normal perfusion in the ischemic area can also be observed. Large clinical studies with the Fe-52m/Mn-52m generator have recently been initiated at the National Heart and Lung and Institute, National Institutes of Health, Bethesda, Maryland ... 

Applications of positron imaging, particularly PEPT, have been extensive and very diverse. In general, they vary from quick diagnostic studies for industrial users to fundamental work on multi-phase systems, such as "granular gases" (see, e.g., Wildman et al., 2005). Here, we concentrate on work aimed at industrial understanding. 

As in other imaging fields, developments in positron imaging occur in a symbiotic way new capabilities enable new applications and new demands drive enhancement in capability. In capability development, there is a continuing demand to make tracers which are... 

Although PEPT is likely to remain the more appropriate choice for most industrially oriented positron-imaging work, PET can be useful in some circumstances, particularly for following slow processes such as diffusion and some classes of diffusion/reaction problems, as well as multi-phase pipeline flow. 

Since fatty acids are the primary substrate for aerobic metabolism within the heart, they are also of interest as potentially useful tracers of myocardial perfusion (12-18). This class of compounds labeled with the positron emitter is being used with specially designed positron imaging (ECT) systems to quantitatively measure the regional distribution of myocardial blood flow in three dimensions (12,13,14). Carbon-11 labeled palmitic acid is also being evaluated as a tracer for studies of myocardial metabolism. Likewise, the o-iodofatty acids, which are structural analogs of the physiologic substrates, have been shown to be taken up by myocardial tissue in proportion to blood flow (15). Iodine-123 labeled 16-iodo-9-hexadecenoic acid (18,17) and 17-iodo-heptadeca-noic acid (18) have been used in conjunction with conventional scintillation cameras for in vivo studies in humans. In all cases with this class of compounds, turnover of the label within the tissue is related to some aspect of metabolism (17,18). 

Table 9.2. Some imponant radio isotopes in positron imaging and some examples of their applieation [5]...

Grigsby PW, Dehdashti F, Siegel BA (1999) FDG-PET evaluation of carcinoma of the cervix. Clin Positron Imaging 2 105-109... 

Liu RS. Clinical application of [C-ll]acetate in oncology. Clin Positron Imaging 2000 3 185. 

Larson SM, Grunbaum Z, Rasey JS. Positron imaging feasibility studies selective tumor concentration of H-thymidine, H-uridine, and C-2-deoxyglucose. Radiology 1980 134 771-773. 

The camera actually images the annihilation events, not the radioactive decay events directiy. Thus imaging of high energy positron emitters can have a limiting resolution owing to the range of the positron. 

Because few scatter events are recorded, attenuation compensation is relatively easier for PET using an external positron emitting source. As a result, the technology for quantitative determinations of radioactivity distributions is significantly more advanced in PET imaging. Technology development for SPECT, however, is improving this parameter. 

Brain imaging technique that allows visualization of the brain, in order to understand which brain regions are involved in specific functions. Its functioning is based on the measurement of the regional cerebral blood flow which increases when a specific brain region is activated. Its use is similar to that of positron emission tomography (PET). 

Positron emission tomography (PET) is an imaging technique that relies on the emission of positrons from radionucleotides tagged to an injectable compound of interest. Each positron emitted by the radioisotope collides with an electron to emit two photons at 180° from each other. The photons are detected and the data processed so that the source of the photons can be identified and an image generated showing the anatomical localization of the compound of interest. 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

Considerable interest has been focused on the efficient and rapid synthesis of 2-deoxy-2-[ F]fluoro-D-gIucose, a popular imaging agent for positron-emission tomography (see Section III, 1). However, introduction of a fluorine atom at C-2 by nucleophilic displacement is generally not easy on account of the weak nucleophilic character of the fluoride ion. One possible... 

A PET scan requires a substance called a tracer. A suitable tracer must accumulate in the target organ, and it must be modified to contain unstable radioactive atoms that emit positrons. Glucose is used for brain imaging, because the brain processes glucose as the fuel for mental and neural activities. A common tracer for PET brain scans is glucose modified to contain radioactive fluorine atoms. Our molecular inset shows a simplified model of this modified glucose molecule. 

C22-0094. Two isotopes used in positron-emission imaging are C and O. On which side of the belt of stability are these nuclides located Write the nuclear reactions for their disintegrations. 

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