In this work we present a measurement setup for the determination of scintillation pulse shapes of fast scintillators. It is based on a time-correlated single photon counting approach that utilizes the correlation between 511 keV annihilation photons to produce start and stop signals in two separate crystals. The measurement is potentially cost-effective and simple to set up while maintaining an excellent system timing resolution of 125 ps. As a proof-of-concept the scintillation photon arrival time histograms were recorded for two well-known, fast scintillators: LYSO:Ce and LaBr 3 :5%Ce. The scintillation pulse shapes were modeled as a linear combination of exponentially distributed charge transfer and photon emission processes. Correcting for the system timing resolution, the exponential time constants were extracted from the recorded histograms. A decay time of 43 ns and a rise time of 72 ps were determined for LYSO:Ce thus demonstrating the capability of the system to accurately measure very fast rise times. In the case of LaBr 3 :5%Ce two processes were observed to contribute to the rising edge of the scintillation pulse. The faster component (270 ps) contributes with 72% to the rising edge of the scintillation pulse while the second, slower component (2.0 ns) contributes with 27%. The decay of the LaBr 3 :5%Ce scintillation pulse was measured to be 15.4 ns with a small contribution (2%) of a component with a larger time constant (130 ns).

Simultaneous acquisition of nuclear and fluoroscopic projections could be of benefit for image-guided radionuclide administration. A gamma camera positioned behind an x-ray flat panel detector can accomplish such simultaneous acquisition, but the gamma camera performance suffers from the intense x-ray dose. A regular NaI(Tl)-based camera has nominal performance up to 0.02 nGy dose per pulse, whereas 10 nGy dose is expected for our foreseen applications. We evaluated the performance of CeBr3- and CZT-based detectors and investigated a cost-effective improvement of a regular NaI(Tl)-based camera by the introduction of a high-pass filter and shorting circuit. A CeBr3-based detector was exposed to 5 mGy x-ray dose and the resulting light emission was measured over time to quantify the crystal afterglow, allowing comparison with a previously measured NaI(Tl)-based detector. The NaI(Tl)-, CeBr3- and CZT-based detectors were exposed to x-ray pulse sequences with dose from 0.06 to 60 nGy, while being irradiated with a gamma source. The mean gamma energy and energy resolution in between the x-ray pulses were measured as a reference of the detector performance. The afterglow signal after 3 ms was 14.1% for the NaI(Tl)-based detector, whereas for the CeBr3-based detector it was only 0.1%. The limits for a proper functioning detectors are 0.32 nGy for the NaI(Tl)-based detector with high-pass filter and shorting circuit and 18.94 nGy for the one with CeBr3. No energy degradation was observed for the CZT module in the studied dose range. The performance of regular NaI(Tl)-based gamma cameras deteriorates when exposed to high x-ray doses. CeBr3 and CZT are much better suited for introduction into a dual-layer detector but have high associated costs. Addition of a high-pass filter and shorting circuit into the PMT of a NaI(Tl)-based detector is a cost-effective solution that works well for low dose levels.

In SPECT/CT systems x-ray and γ-ray imaging is performed sequentially. Simultaneous acquisition may have advantages, for instance in interventional settings. However, this may expose a gamma camera to relatively high x-ray doses and deteriorate its functioning. We studied the NaI(Tl) response to x-ray pulses with a photodiode, PMT and gamma camera, respectively. First, we exposed a NaI(Tl)-photodiode assembly to x-ray pulses to investigate potential crystal afterglow. Next, we exposed a NaI(Tl)-PMT assembly to 10 ms LED pulses (mimicking x-ray pulses) and measured the response to flashing LED probe-pulses (mimicking γ-pulses). We then exposed the assembly to x-ray pulses, with detector entrance doses of up to 9 nGy/pulse, and analysed the response for γ-pulse variations. Finally, we studied the response of a Siemens Diacam gamma camera to γ-rays while exposed to x-ray pulses. X-ray exposure of the crystal, read out with a photodiode, revealed 15% afterglow fraction after 3 ms. The NaI(Tl)-PMT assembly showed disturbances up to 10 ms after 10 ms LED exposure. After x-ray exposure however, responses showed elevated baselines, with 60 ms decay-time. Both for x-ray and LED exposure and after baseline subtraction, probe-pulse analysis revealed disturbed pulse height measurements shortly after exposure. X-ray exposure of the Diacam corroborated the elementary experiments. Up to 50 ms after an x-ray pulse, no events are registered, followed by apparent energy elevations up to 100 ms after exposure. Limiting the dose to 0.02 nGy/pulse prevents detrimental effects. Conventional gamma cameras exhibit substantial dead-time and mis-registration of photon energies up to 100 ms after intense x-ray pulses. This is due PMT limitations and due to afterglow in the crystal. Using PMTs with modified circuitry, we show that deteriorative afterglow effects can be reduced without noticeable effects on the PMT performance, up to x-ray pulse doses of 1 nGy.

Content codes:Purpose: To develop and evaluate a dual-layer detector capable of acquiring intrinsically registered real-time fluoroscopic and nuclear images in the interventional radiology suite. Materials and Methods:The dual-layer detector consists of an x-ray flat panel detector placed in front of a g camera with cone beam collimator focused at the x-ray focal spot. This design relies on the x-ray detector absorbing the majority of the x-rays while it is more transparent to the higher energy g photons. A prototype was built and dynamic phantom images were acquired. In addition, spatial resolution and system sensitivity (evaluated as counts detected within the energy window per second per megabecquerel) were measured with the prototype. Monte Carlo simulations for an improved system with varying flat panel compositions were performed to assess potential spatial resolution and system sensitivity. Results:Experiments with the dual-layer detector prototype showed that spatial resolution of the nuclear images was unaffected by the addition of the flat panel (full width at half maximum, 13.6 mm at 15 cm from the collimator surface). However, addition of the flat panel lowered system sensitivity by 45%-60% because of the nonoptimized transmission of the flat panel. Simulations showed that an attenuation of 27%-35% of the g rays in the flat panel could be achieved by decreasing the crystal thickness and housing attenuation of the flat panel.