Photostimulable phosphors

Giovanni De Crescenzo, Kuo Yan, Kristina Watt

A photostimulable phosphor (PSP) is a type of phosphor that can store information generated by X-rays. When a photostimulable phosphor is excited by X-rays, electrons and holes are generated. Several cascaded processes are possible:

1. Some of the electrons and holes can recombine at luminescent centres (number [1] in Figure 1). Their energy excites the luminescent centres [2]. When the excited luminescent centres return to their ground states, they give off 'prompt' visible light [3].
2. Some of the electrons and holes are captured in meta-stable traps [4]. It is possible to release these trapped electrons and holes using a stimulating laser [5]. Once the electrons are released, they have some probability (not necessarily the same) of returning to the traps once again [4] or recombining at luminescent centres [1]. When the laser is operated at continuous mode, all trapped electrons and holes can be pumped out and converted to 'stimulated' visible light [3].

Figure 1: Energy levels for processes in a PSP screen.

A computed radiography (CR) system uses the stimulated light emitted from the photostimulable phosphor. A red laser with adequate power and the correct wavelength scans the surface of the phosphor screen. The stimulated light emerging from the screen surface can be collected by a photomultiplier tube and converted to a digital signal for diagnostic purposes.

In our project, it is the prompt light that is investigated. This light carries the same amount of information as does the stimulated light. This method reduces the system complexity and still allows the meaningful physical processes occurring in the screen to be revealed. For instance, our measurements of the modulation transfer function (MTF) by coupling the screen to a CMOS detector with a fibre optic plate shows that two photostimulable phosphor screens produced by the same manufacturer with different design parameters resemble each other. This indicates that their design parameters have been intentionally optimized by the manufacturer for practical purposes (Figure 2). Similar results from these two screens were also found in our pulse-height spectra study.

Figure 2: Presampling MTF scaled to unitless dimensions to remove the effect of thickness.

The screen physics could also be analyzed by two powerful tools: transport theory and Monte Carlo simulation. The models built on the theory and simulation can shed light on our experimental measurements and also the computed radiography system performance. We believe that, ultimately, all the fundamental imaging processes and qualities of the photostimulable phosphor screen and the computed radiography system possibly can be understood with the description of a handful of design parameters.

Computed radiography topics at Fujifilm: http://www.fujimed.com/

References

1. Watt KN, Yan K, DeCrescenzo G, Rowlands JA.The physics of computed radiography: measurements of pulse height spectra of photostimulable phosphor screens using prompt luminescence. Med Phys. 32(12):3589-98, 2005.
2. Rowlands JA. The physics of computed radiography. Phys Med Biol. 7;47(23):R123-66, 2002.