Table 7.3: Results of XRF measurements without and with HOPG produced with the PMMA/mouse phantom (phantom 2) filled with various iodine concentrations. The values for the SNRs are given according to table 7.1.
here, no additional collimators are used, which considerably filter the XRF spectrum.
In contrast, the flux here is large enough so that counting statistics are not impaired.
Figure 7.6 (right) shows the results for the measured photon flux emitted by the two phantoms depending on the detection angles. Error bars were not included, since the errors are situated within the symbols for the photon counts. For phantom 3, the number of background counts increases with increasing detection angle (backward scatter), which is in accordance with the findings of [31]. In contrast, the results for phantom 2 reveal that a broad range of angular detection configurations is suitable for an optimal XRF signal yield.
This result shows that a 90° angular detector configuration, as implemented in pre-vious measurements, is suitable (lowest amount of Compton scatter) for all further investigations with the initial spectrum derived in the previous section. Moreover, the findings show that angular detector positions between 80° and 110° do not significantly increase the amount of scatter for phantom 2. This implies that detectors with larger sensitive areas could be implemented without deteriorating the quality of the results.
In contrast, the XRF signal yield would be improved.
7.2 Measurements of various iodine concentrations
With the derived initial spectrum (UA = 80 kV, I = 37.5 mA, 1.0 mm Al filter) var-ious XRF measurement were performed using the PMMA/mouse phantom (phantom 2) and the simple Eppendorf test tube (phantom 1). Each phantom underwent a mea-surement sequence with an experimental setup omitting the HOPG crystal and a setup where the crystal was included. In total four measurement series were performed. The first subsection deals with the PMMA/mouse phantom (phantom 2) and the second subsection will summarise the results implementing the simple Eppendorf test tube (phantom 1). The resulting XRF spectra form the basis for the feasibility tests of the Monte Carlo code presented in part III, Monte Carlo Simulation GEANT4, of this work.
7.2. Measurements of various iodine concentrations 69
Figure 7.7: XRF spectra emitted by the PMMA/mouse phantom without (left) and with (right) HOPG crystal. The HOPG isolates the energy region around the Kα signal of iodine and suppresses other energy regions.
7.2.1 PMMA/mouse phantom (phantom 2)
For the XRF measurements the PMMA/mouse phantom was filled with iodine con-centrations of 0.6 mg/ml, 1.2 mg/ml, 2.3 mg/ml and 5.0 mg/ml and one measurement was performed without iodine. Figure 7.7 shows an XRF spectrum performed with phantom 2 using an iodine concentration of 5.0 mg/ml (top) and 2.3 mg/ml (bottom), left without the HOPG and right including the crystal. The results nicely confirm the 50 % reduction of counts due to the HOPG crystal and the suppression of other energy bins. The energy range of the second order 004-plane reflection in the energy area around 57.2 keV shows a minimal increase of photon counts, but is nevertheless very well reduced.
Table 7.3 summarises the results of the XRF spectra for each iodine concentration, displaying the SNR for both experimental setups including and omitting the HOPG crystal. Figure 7.8 visualises these findings. As expected, the SNR shows a linear increase with the iodine concentration. A contrast agent concentration of 2.3 mg/ml or lower yields an SNR that does not fulfil the Rose criterion. Only the SNR derived with a 5.0 mg/ml iodine concentration lies clearly above this threshold. Recordings
70 7.2. Measurements of various iodine concentrations
Figure 7.8: SNRs of recorded XRF spectra (with/without HOPG crystal) emitted from the PMMA/mouse phantom, applying various iodine concentrations between 0.6 mg/ml and 5.0 mg/ml.
with a considerably increased measuring time and dose would reduce the statistical fluctuations and improve the values for the SNRs.
7.2.2 Eppendorf phantom (phantom 1)
This subsection deals with XRF measurements recorded using the Eppendorf test tube phantom. Due to its lack of an absorbing PMMA layer, it is expected that even lower iodine concentrations than the ones used for phantom 2 will fulfil the Rose cri-terion. Measurements were performed without iodine and with contrast agent concen-trations of 0.15 mg/ml, 0.3 mg/ml, 0.6 mg/ml, 1.2 mg/ml, 2.3 mg/ml and 5.0 mg/ml.
Figure 7.9 shows the resulting XRF spectra recorded with iodine concentrations of 5.0 mg/ml (top) and 2.3 mg/ml (bottom), left without the HOPG and right including the crystal. The results clearly show the influence of phantom size: The absolute values of the XRF signals and resulting SNRs are notably larger than for the PMMA/mouse phantom. Also the number of background counts is considerably reduced compared to the spectra performed with the PMMA/mouse phantom (compare figure 7.7). Even the signal of the Kβ emission line is discriminable for the spectra without HOPG implementation and the 5.0 mg/ml spectrum including the crystal.
7.2. Measurements of various iodine concentrations 71
Figure 7.9: XRF spectra emitted by the Eppendorf test tube without (left) and with (right) the HOPG crystal. The amount of background photons is considerably reduced compared to the spectra emitted by the PMMA/mouse phantom. The Kβ signature is clearly distinguishable from the background for an implementation without HOPG crystal, but is reduced in the HOPG results. In the results using an iodine concentration of 5.0 mg/ml there is an artefact due to escape events at ≈5 keV.
w/o HOPG w/ HOPG
I [mg/ml] [kV] SNR SNR
0.15 3.37 ±1.03 (±30.6 %) 1.55 ±0.83 (±53.5 %) 0.3 1.59 ±0.61 (±38.4 %) 2.67 ±1.17 (±43.8 %) 0.6 6.03 ±1.56 (±25.9 %) 5.26 ±1.69 (±32.1 %) 1.2 8.84 ±2.07 (±23.4 %) 8.21 ±2.21 (±26.9 %) 2.3 13.95 ±2.94 (±21.1 %) 12.13 ±2.70 (±22.3 %) 5.0 24.66 ±4.63 (±18.8 %) 19.56 ±3.88 (±19.8 %)
Table 7.4: Results of XRF measurements without and with HOPG produced with the Ep-pendorf test tube phantom filled with various iodine concentrations. The values for the SNRs are given according to table 7.1.
72 7.2. Measurements of various iodine concentrations
Figure 7.10: SNR of recorded spectra emitted from the Eppendorf test tube phantom, ap-plying various iodine concentrations between 0.15 mg/ml and 5.0 mg/ml.
The signature at about 5 keV for the 5.0 mg/ml measurements is an artefact due to escape events which could not entirely be removed by the stripping algorithm. Fig-ure 7.10 shows the linear increase with iodine concentration of the SNRs for both experimental setups (the values are summarised in table 7.4). Even an iodine concen-tration of 0.6 mg/ml leads to an SNR of 6.03±1.56 (without HOPG) and 5.26±1.69 (with HOPG), thus fulfilling the Rose criterion. The measurements with lower iodine concentrations, i.e. with 0.15 mg/ml and 0.3 mg/ml, do not show the linear increase when measured without HOPG. It is possible that the iodine concentration in this configuration is too low, in order to be discriminable from the scatter background.