Background: The positron emission tomography (PET) technology has undergone continuous innovation in recent years. New-technology digital PETs are silicon photomultiplier (SiPM) PET systems with digital readouts, which contribute to improved image resolution. This study aimed to compare the image quality of sub-centimeter lesions of NEMA PET phantom images obtained under identical imaging conditions (identical lesion volumes, identical activity and identical scanning time) using dPET, analog PET-1 and analog PET-2 acquired in the clinic. Materials and Methods: For image analysis, a standard NEMA IEC body phantom was used. In the present study, the lesion detection performance of all PETs was evaluated in two categories, sub- and over-centimeter size. The imaging durations of this study were 1, 2, 3, and 5 minutes, while the injection doses were 2.33 and 5.33 kBq/ml for the 1/4 and 1/8 background-to-lesion ratios, respectively. For a quantitative assessment of image quality, a circular ROI with activity concentrations (ACmean) and the mean recovery coefficient were calculated for each lesion via the ACmean. Results: Our study revealed approximately 15% greater RCmean values for dPET with SiPM technology compared to the analog PET-2 with PMT technology. However, analog PET-1 exhibited a significant lack of performance, especially when compared to analog PET-2 and dPET. Conclusion: Although dPET, the first generation of dPETs analyzed in the present study, yields relatively better RCmean values than analog PETs, it is not able to entirely eliminate the unfavorable impacts of PVE for sub-centimeter lesions.

In this paper, we review novel techniques in the emerging field of spatiotemporal 4D PET imaging. We will discuss existing limitations in conventional dynamic PET imaging which involves independent reconstruction of dynamic PET datasets. Various approaches that seek to attempt some or all of these limitations are reviewed in this work, including techniques that utilize iterative temporal smoothing, advanced temporal basis functions, principal components transformation of the dynamic data, wavelet-based techniques as well as direct kinetic parameter estimation methods. Extension of 4D PET to 5D PET in which the additional dimension of (respiratory or cardiac) gating is considered has also been discussed.

PET (Positron Emission Tomography) is a powerful imaging technique through which quantitative information on the distribution of positron emitter labeled radiopharmaceuticals (PET radiopharmaceuticals) in the body can be realized. Positrons (b+) are positively charged beta particles. They are emitted when the atom is proton enriched. A positron has only a transient existence. After losing all of its kinetic energy, it interacts with an electron and is annihilated. Both the mass of positron and electron are converted to energy during annihilation and two 511 KeV photons are emitted at a 180° angle to each other. The PET is based on the coincidence detection of the two aforementioned photons. Coincidence detection is a powerful method enhancing sensitivity and dynamic-imaging capabilities of PET. PET camera systems contain a ring of detectors that encircles the patient. The data collected over many angles around the body axis of the patient is used to reconstruct the image of the activity distribution in slice or tomographic form.

Radiation therapy has a major role in treatment of malignant tumor not only as an adjuvant treatment for surgery but also as a primary modality in treatment of inoperable tumors or as an alternative to surgery in order to preserve organ function and/or avoiding surgical complications.Treatment planning has evolved from simple conventional planning to conformal techniques such as Three-Dimentional conformal radiation therapy and intensity Modulated Radiation Therapy (IMRT). These techniques which are based on anatomical information from CT scan or MRI allow us to confirm radiation dose distribution to the target volume with a minimum possible dose to normal tissues. A major step in planning process is delineating tumor bearing tissues (target volume). The most important limitation is that the extent of target volume is not always discernible based on CT scan or MRI information.Positron emission tomography (PET) usingradioactive fluorine-18 labelled FDG as a tracer enables biological imaging of tumors and highlights its proliferating areas. This imaging technique which has an important role in diagnosis, staging and restaging of tumors has been introduced in radiation planning to facilitate tumor delineation. By using the same table and positioning for a PET / CT scan it is possible to fuse both images and combine the physiologic information from PET and superior images of anatomy and localization from CT scan. Using PET / CT scan in radiotherapy planning has the following potential advantages: 1- Differentiating between tumor and normal tissue. Take for an example; in lung cancer a distal atelectasis could be mixed up with a central tumor in CT scan.2- Including the tumor spread such as metastatic lymph nodes into the target volume which could be missed in CT information.3- Assessing the tumor response after terminating the planned treatment and prescribing additional boost dose to the remaining functional area.4- In some organs such as lung which lesions move considerably during respiration and radiotherapy, acquiring gated 4-D PET scan and hybrid PET / CT make it possible to assess the tumor motion precisely and allowing tighten tumor margins.In non-small cell lung cancer using FDG-PET has resulted in safe decrease in radiotherapy volume in a considerable number of patients and allowed dose escalation in tumor. In esophageal and Head and neck carcinoma it is possible to detect unrecognized lymph node metastasis and to delineate the tumor volume more precisely. In Hodgkin’s lymphoma, PET may be essential in involved field radiotherapy after chemotherapy to safely decrease the treatment volume while avoiding geographic miss. In cervical carcinoma PET can help a clinician to decide whether to encompass the para-aortic lymph nodes in treatment volume or not. PET has also an emerging role in radiotherapy of primary brain tumors.Multiple studies suggest an increasingly important role for PET in radiotherapy planning. However, using PET should follow strict standardized protocols. Further investigations are needed to reveal its exact role in radiation planning for different malignancies and to define safe recommendations.