PET Data Acquisition

In a full ring system, the data are collected simultaneously by all detector pairs, whereas in partial ring systems, the detector assembly is rotated around the patient in angular increments to collect the data. In acquiring the coincidence events, three steps are followed  

Basics of PET Imaging Physics, Chemistry, and Regulations, 

The location of the detector pair in the detector ring is determined for each coincident event. 

The pulse height of the photon detected is checked if it is within the pulse energy window set for 511 keV. 

Finally, the position of the LOR is determined in terms of polar coordinates to store the event in the computer memory. 

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Figure 3.3. PET data acquisition in the form of a sinogram. Each LOR data (a) is plotted in (r, ) coordinates. Data for all r and values are plotted to yield sinogram indicated by the shaded area (b). (Only a part is shown.) (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved)...

The 2D PET data acquisition is accomplished in a PET scanner by the use of (a) parallel-hole collimator, (b) lead shield, (c) tungsten septa, or... 

After the completion of the PET data acquisition for the last bed position, the patient is released. 

Bacharach SL, Bax JJ, Case J, Delete D, Cordial KA, Martin WH et al. PET myocardial glucose metabolism and perfusion imaging Part 1-Guidelines for data acquisition and patient preparation. J Nucl Cardiol 2003 10 543-556... 

Dedicated PET systems are usually based on lull ring detector systems with an axial field of view exceeding 15 cm, and can be operated in septa extended (2D mode) or septa retracted mode (3D) for patient examinations. Some systems only provide 3D-data acquisition modes. Most systems allow the simultaneous acquisition of 36 transversal slices and more with a theoretical slice thickness of 2-5 mm. Transmission scans for a total of 10 min are obtained prior to the radionuclide application, for the attenuation correction of the acquired emission tomographic images. All PET images are attenuation corrected and iteratively reconstructed. 

PET/CT systems are gaining major attention in the last few years. Due to the sequential acquisition of CT and PET data, morphological and functional information can be easily correlated and analysed. The CT data can also be used for attenuation correction of the PET images, thus decreasing the total examination time of the patient. 

The last step in data acquisition is the storage of the data in the computer. Unlike conventional planar imaging where individual events are stored in an (X, Y) matrix, the coincidence events in PET systems are stored in the form of a sinogram. Consider an annihilation event occurring at the position in Fig. 3.3a. The coincidence event is detected along the LOR indicated by... 

Correction for scattered events is essential, particularly for 3D acquisition of PET data, because of the loss of contrast and resolution in the image. A review by Zaidi and Koral (2004) provides an extensive account of scatter events in patient imaging and methods to correct them. Several methods have been applied for scatter correction, which are briefly described below. 

Bailey DL (2003). Data acquisition and performance characterization in PET. In Valk PE, Bailey DL, Townsend DW, Maisey MN (eds) Positron emission tomography. Springer, New York... 

Fahey FH (2002). Data acquisition in PET imaging. J Nucl Med Technol 30 39... 

Note that the sensitivity of a PET scanner increases as the square of the detector efficiency, which depends on the scintillation decay time and stopping power of the detector. This is why LSO, LYSO and GSO detectors are preferred to Nal(Tl) or BGO detectors (see Table 2.1). In 2D acquisitions, system sensitivity is compromised because of the use of septa between detector rings, whereas these septa are retracted or absent in 3D acquisition, and hence the sensitivity is increased by a factor of 4-8. However, in 3D mode, random and scatter coincidences increase significantly, the scatter fraction being 30—40% compared to 15-20% in 2D mode. The overall sensitivities of PET scanners for a small-volume source of activity are about 0.2-0.5% for 2D acquisition and about 2-10% for 3D acquisition, compared to 0.01-0.03% for SPECT studies (Cherry et al, 2003). The greater sensitivity of the PET scanner results from the absence of collimators in data acquisition. 

Fig. 3. Block diagram of data acquisition and analysis system. In our laboratories two systems based on the above have been built. One is a Durrum-Gibson stopped-flow spectrophotometer coupled through a Datalab DL901 transient recorder to a Commadore PET 32K minicomputer (used for data in Fig. 2). Another system consists of a Canterbury SF-3A stopped-flow instrument interfaced, via a Datalab DL901 transient recorder to an Exidy Sorcerer minicomputer. Software is available.

Because of the complex behavior of the tracers employed in metabolic studies with PET, rather sophisticated mathematical models and data acquisition schemes have been developed to extract metabolic rates from the data provided by the quantitative PET images and samples of arterial blood specific activity. These are illustrated in Figure 2. 

Figure 2, A general approach to data acquisition and analysis in the measurement of metabolism by PET is shown (41).

Philip ABAC forte dual head gamma camera installed with Pegasys software for data acquisition, image reconstruction, display, analysis was used. A modified/designed PET/SPECT cylindrical phantom was scanned by using Tc-99m radionuclide. The tank was filled with water. Cold and hot regions insert was placed in the phantom tank. The insert comprises on various sized pairs (29.9, 22.4, 17.9, 14.3, 11.4, 9.7, 7.3, 5.9 and 4.7 mm diameter) of cold and hot regions at different locations... 

F. Fahey, "Data acquisition in PET imaging," Journal of nuclear medicine technology, vol. 30, p. 39,2002. 

A feature of data acquisition that is becoming of greater interest in PET is that of list mode. In this, the events are not stored over preselected time frames in... 

There are only four installed human brain PET-MR imagers in the world at this moment. The PET insert is inside the magnet covered by the gradient coil. Data acquisition can be done separately or simultaneously by using two separate computers. The platform of the PET insert is the same as in the latest version of Siemens Inveon animal PET system. The MR imager is Siemens 3T TRIO. 

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