PET scanners

In preparation for a PET scan, a patient is injected with a dose of the tracer, which quickly accumulates in the brain. The patient is placed inside the PET scanner, and the instrument detects the gamma rays emitted by the tracer. The result is an image showing the distribution of glucose in the brain, which indicates where brain activity is greatest. 

In the first of these studies, the researchers identified the areas of the brain that are associated with normal sadness. They asked volunteer subjects to think about some very sad personal experiences - and about some emotionally neutral experiences - while their brains were being imaged in a PET scanner. When thinking about the sad experiences, the volunteers reported feeling intense sadness, and many of them became tearful. The PET scans showed the changes in brain activity that accompanied these sad feelings. They demonstrated increased blood flow in the limbic system - a part of the brain that is involved in the control of emotion - and decreased blood flow in parts of the brain that are involved in the control of attention. 

At present, most PET scanners can acquire in both a two-dimensional as well as a three-dimensional mode, whereas SPECT cameras measure in a three-dimensional mode. The physical property of the dual-positron gamma-rays emission lends itself to mathematical reconstruction algorithms to produce three-dimensional images in which the calculations are much closer to exact theoretical ones than those of SPECT. This is, in part, due to the two-photon as opposed to single-photon approach. PET can now achieve resolutions, for example in animal-dedicated scanners, in the order of 1 or 2 mm. The resolution is inherently limited theoretically only by the mean free path or distance in which the positron travels before it annihilates with an electron, e.g. those in biological water 2-8 mm. SPECT, although achieving millimeter resolution with the appropriate instrumentation, cannot quite achieve these levels. 

TABLE 58-S Performance characteristics for the current commercial clinical PET scanners... 

The main feature of the short-lived positron-emitting isotope F is the relative long half-life of 110 min. The ultrashort-lived isotopes C, N, and O have half-lives of 2-20 min, and they must subsequently be produced in the vicinity of the PET scanner(s). Radiopharmaceuticals labeled with F, on the contrary, can be shipped to distant satellite PET centers that are notequippedwith a cyclotron. Several commercial companies are today producing 2-[ F]fluoro-2-deoxy-D-glucose... 

What can sleep neurophysiology and the new neuropsychology tell us about lucid dreaming, and how can we use that information to increase our access to that state And what, beyond entertainment, can we learn about the brain-mind from a scientific exploration of lucid dreaming The last question has one ready answer by placing experienced lucid dreamers in a PET scanner (or preferably an fMRI), we could test the hypothesis that, when lucid, dreamers increase the blood flow to their dorsolateral prefrontal cortices as that cortex reactivates to a level consistent with wake state executive function. 

A redundant medical PET scanner, a CTI ECAT931/08, was acquired. This comprises 128 detector blocks (Figure 2a), each consisting of four photomultiplier tubes viewing a 30 mm thick crystal of bismuth germanate scintillator approximately 49 x 56 mm2 in area, which is cut into an array of 8 x 4 elements (each approximately 5.6 x 12.9 mm2,... 

New materials with higher sensitivity or resolution have been designed A 3-dimensional PET scanner using gadolinium oxyorthosilicate (GSO) crystals (Surti). 

Carbon-11 has a very short half-life (just 20.4min) but the chance to substitute a carbon atom in any biological molecule by a positron-emitting nC is a very interesting possibility. This has led to a substantial development of nC-labeled tracers. The short half-life conditions everything and only PET centers equipped with a cyclotron can have a chnical program with nC tracers. The production of the radiopharmaceutical must in these cases be performed just before the imaging study and is usually not started until the patient is already on the PET scanner. 

Positron emitting nuclides have very short half lives, on the order of minutes to tw o hours. This makes operation of a cyclotron and a radiochemistry laboratory essential to the use of PET scanners. is the longest radionuclide with a half-life of 1.87h, making a central production facility within a city feasible for radiopharmaceuticals employing this nuclide. Most clinical PET facilities have on-site cyclotrons and radiopharmaceutical laboratories to allow the use of short-lived isotopes in clinical studies. 

Geworski L, Knoop BO, de Wit M, Ivancevic V, Bares R, Munz DL. Multicenter comparison of calibration and cross-calibration of PET scanners. J Nucl Med 2002 43(5) 635-639. 

A recently introduced detector, yttrium-activated lutetium oxyorthosilicate (LYSO), has the physical properties similar to LSO and has been used in PET scanners by a commercial vendor. 

In PET scanners with block detectors, each detector pair is not connected by coincidence circuitry for practical difficulty, so the detectors are grouped together into banks or sets, which are then connected in coincidence opposite to each other. Essentially each detector in a set is connected by a coincidence circuit with a time window to a set of opposite detectors (both in plane and... 

Figure 2.5. The transverse field of view determined by the acceptance angles of individual detectors in a PET scanner. Each detector is connected in coincidence with as many as half the total number of detectors in a ring and the data for each detector are acquired in a fan beam projection. All possible fan beam acquisitions are made for all detectors, which define the FOV as shown in the figure. (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved).

Because of the increased diagnostic accuracy offered by PET/CT, the worldwide market share of PET/CT is increasing exponentially, whereas that of the stand-alone PET scanners has dwindled to almost zero. 

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