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Characterization of a Miniature X-ray Tube with the Medipix2 Detector

01.02.2011

The use of a miniature X-ray device for intra- or post-operative irradiation has increasingly established itself as an alternative to the radioactive sources used today in brachytherapy, the so-called "seeds". An advantage is that the local irradiation is electrically regulated and is limited in time. Thus, no radioactive sources must remain permanently in the body, as is usually the case in brachytherapy with LDR (low dose rate) seeds. The dose applied by means of a miniature X-ray facility strongly decreases with distance so that the tumour tissue situated directly at the applicator surface receives a high dose, whereas the healthy tissue situated further away is spared.

The X-ray spectrum of the Xoft company’s miniature X-ray facility "Axxent HDR X-ray S700" was investigated within the framework of a doctoral thesis in cooperation with the University of Erlangen-Nuremberg. This facility is operated at tube voltages of up to 50 kV and currents of up to 300 µA. At a distance of 1 cm, this device achieves an air kerma rate comparable to HDR (high dose rate) seeds which – as a matter of routine – are used in brachytherapy.

The hybrid photon-counting pixel detector Medipix2 with 256 x 256 pixels and a pixel size of 55 µm was used as a detector. A 300 µm thick silicon layer serves as the sensor material. The Medipix2 has an adjustable lower energy threshold which makes it possible to count only such energy contributions in the sensor which are above the threshold. By gradually increasing the energy threshold, the spectrum can be sampled. To be able to draw conclusions as to the incoming spectrum, the measured spectrum is unfolded with the simulated detector answer. Thus – due to its pixellation – this detector is suitable both for imaging and for measuring the spectrum.

Figure 1 : Medipix2 detector with USB readout (left) and the examined miniature X-ray source of the Xoft company (right).

Figure 2 : Representation of the focal spot at 10 µA (left) and at a tube current of 300 µA (right).

The two-dimensional distribution of the focal spot as a function of the tube current was measured by means of the pinhole camera principle. The measurements showed that the position of the focal spot changes at low tube currents, but remains unchanged at high currents. In the case of this particular tube, the focal point shifted by up to 160 µm, which represents approx. 5 % of the total diameter of the tube (of 2.8 mm) and is therefore not negligible. The shift has an impact on the spectrometry on which the (basic) dosimetry is based. The latter uses a Ge detector and its counting rate is limited, so that the measurement must usually be carried out at a low photon flux and, thus, at a low tube current. The results show that comparability with clinical practice (full load) is then no longer guaranteed.

Furthermore, the emitted spectrum of the tube was measured using the Medipix2 at a distance of 60 cm for different emission angles. Contrary to the use of a germanium detector, the incident photons are, in this case, distributed over 65536 pixels, so that the tube could be operated at a high tube current. The tube’s intensity was highest at an emission angle of 90°. With increasing deviation from this angle, the intensity decreases down to 60 %. The shape of the emitted spectra also varies according to the emission angle.