Physical characteristics of the ECAT EXACT3D positron tomograph.

The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. A single time frame of data without any compression is very large (> 300 Mbyte) and two approaches are employed to overcome this difficulty: (a) adjacent sinograms can be summed automatically into different combinations and (b) listmode (event-by-event) acquisition has been instituted, which is both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional side shielding. The mean spatial resolution is 4.8 +/- 0.2 mm FWHM (transaxial, 1 cm off-axis) and 5.6 +/- 0.5 mm (axial, on-axis). Its absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively. The rate of random events in blood flow studies in the brain and body, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding.

Mitomycin C-treated or irradiated concanavalin A-activated T cells augment the activation of cytotoxic T cells in vivo.

The activation of cytotoxic T lymphocytes (CTL) in vivo after immunization of normal or cyclophosphamide-treated mice with allogeneic cells was strongly augmented by the administration of mitomycin C-treated or irradiated concanavalin A-activated spleen cells (Con A-spl). This effect of the Con A-spl was abrogated by treatment with Anti-Thy 1 antibody plus complement, and was therefore presumably mediated by activated "helper" T cells. (The term "helper" cell is only operationally defined in this context and indicates that the augmenting irradiation resistant T cells are obviously not CTL precursor cells). These observations indicated (i) that even the cytotoxic response against allogeneic stimulator cells suffers in vivo from insufficient "helper" T cell activity, and (ii) that the injection of Con A-spl may serve as a simple procedure to apply this "helper" activity in vivo. This procedure was at least as effective as the repeated injection of interleukin 2 (IL-2)-containing cell supernatants with up to four 30-unit doses of IL-2 per mouse. IL-2-containing cell supernatants were found to mediate similar effects only if injected into the footpads but not intravenously. This was in line with the reported observation that IL-2 has an extremely short half-life in vivo. The injection of Con A-spl was also found to augment the proliferative response in the regional lymph nodes.

Capacity of different cell types to stimulate cytotoxic T lymphocyte precursor cells in the presence of interleukin 2.

Plastic-adherent cells enriched for dendritic cells (AC) were found to be among the most potent stimulator cells for the activation of cytotoxic T lymphocytes (CTL) in vitro in the presence of interleukin 2 (IL 2) and a constant second set of allogeneic stimulator cells. Concanavalin A-activated nylon wool-nonadherent spleen cells ( CNWT ), concanavalin A-activated unfractionated spleen cells ( Cspl ), and some variants of the ESb T lymphoma line were equally effective as stimulator cells, however, and provoked a substantial cytotoxic response at concentrations of 10(4) cells per culture or less. In contrast, nonactivated nylon wool-nonadherent spleen cells ( NWT ) or unfractionated spleen cells (Spl) and cells of the P815 mastocytoma, the Meth A fibrosarcoma, and the T cell lymphomas Ly 5178 Eb and ESb did not stimulate cytotoxic responses at these cell concentrations. The strong stimulatory potential of the Cspl preparation was reduced by treatment with anti-Thy-1 antibody plus complement, whereas the stimulatory activity of the AC preparation was resistant to this treatment. All cell types tested expressed class I major histocompatibility antigens. Nonactivated NWT cells, in contrast to the CNWT preparation, showed no detectable staining with anti-I-E or anti-I-A antibodies and also a slightly weaker staining with class I antisera. Experiments with the tumor cell lines revealed, however, that there was no strict correlation between stimulatory potential and density of class I alloantigens or the expression of I-E determinants. Experiments on primary cytotoxic responses in vivo gave similar results. Experiments in cultures with a single set of stimulator cells and I region-compatible responder cells indicated that AC and Cspl or CNWT also have a markedly stronger capacity than NWT to induce IL 2-dependent DNA synthesis.

Antigenically activated helper T cells are required as stimulator cells for optimal activation of cytotoxic T lymphocytes under conditions of limiting helper factors.

This paper deals with the question of how antigenically activated helper cells interact with cytotoxic T-lymphocyte (CTL) precursor cells in an environment where helper factor is limiting. Experiments in culture systems with limiting concentrations of helper factor indicated that the (optimal) activation of CTL required antigenically activated helper T cells as stimulator cells. These experiments were performed partly in macrocultures which contained prostaglandin E2 (PGE2) and partly in microcultures with small numbers of responder cells and without additional helper factors. The results showed that a strong activation of CTL against TNP-haptenated syngeneic or semiallogeneic cells occurred only if the cultures contained TNP-haptenated stimulator cells from euthymic but not athymic donors and if the haptenated stimulator cells were exposed to allogeneic determinants. Moreover, combinations of F1-hybrid stimulator cells and parental responder cells generated no substantial cytotoxic responses against determinants of the other parent, unless the cultures were supplemented with a source of I-region determinants which were foreign to the semiallogeneic stimulator cells. Strong responses against haptenated syngeneic or semiallogeneic stimulator cells were obtained, however, when helper factors were added to the cultures. It was concluded that our cultures with limiting concentrations of helper factors required a close proximity between helper T cells and CTL precursor cells; and this proximity was obviously provided by the receptors of the CTL precursor cells with no detectable contribution from the helper-T-cell receptors. Allogeneically activated helper T cells in the responder cell population or in a second irradiated spleen cell population which did not bear the target antigens delivered no substantial helper effect to the CTL precursor cells under test.