Influence of gain setting

Tables 21, 22 and 23 show the results when changing the gain settings on the three different transducers respectively. The median was calculated. In all three transducers, the gain setting did not give any significant difference in the mean of the output curve, with p-values of .393 for the linear probe and .853 for both the curved and TV probes (see Figures 31 and 32).

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Figure 31: ASQ window with High Gain of 100 % Figure 32: ASQ window with low Gain of 85 %

Median p-value Significance?

High gain 112

0.393 No

Low gain 110

Table 21:Summary of the influence of the gain setting on the mean of output curve using linear Probe.

Median p-value Significance?

High gain 101

0.853 No

Low gain 101

Table 22:Summary of the influence of the gain setting on the mean of output curve using curve Probe.

Median p-value Significance?

High gain 102

0.853 No

Low gain 102.5

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3.3.3.6. Influence of transducer frequency

The influence of frequency on the output curve is summarised in Tables 24, 25 and 26. The mean and median of the ASQ output curve were calculated for all frequencies. For the linear probe, the 8.4 MHz frequency was significantly different when compared to the 7.2 MHz frequency as well as when compared to the 6.2 MHz frequency, with both p-values of < .001. However, there was no significant difference seen when comparing the p-values of 7.2 with 6.2 MHz, with a p-value of .739. For the curved probe, four different frequencies were tested against each other. In all frequencies, no significance difference was seen when compared to each other, with p-values ranging from .165 to .796 using the Mann-Whitney U test.

When testing the TV probe, no significant differences were seen between the frequencies 8 and 7 MHz and between frequencies 7 and 6 MHz, with p-values .280 and .089 respectively. The only significant difference shown was between 8 and 6 MHz, with a p-value of .003.

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Frequency

(MHz)

Mean Median P-value Significance?

8.4 71.6 72 (8.4 with 7.2) <.001 Yes 7.2 103.6 104 (7.2 with 6.2) .739 No 6.2 103.3 103.5 (8.4 with 6.2) <.001 Yes

Table 24: Influence of frequency in linear transducer.

Frequency

(MHz)

Mean Median P-value Significance?

6 96.4 97 (6 with 5) .436 No 5 95.6 95.5 (5 with 4) .739 4 95 95 (4 with 3) .796 3 93.8 95 (6 with 3) .353 (6 with 4) .165

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Frequency

(MHz)

Mean Median P-value Significance?

8 104.5 105 (8 with 7.2) .280 No 7.2 103.1 104.5 (7.2 with 6) .089 No 6 101.1 101 (8 with 6) .003 Yes

Table 26: influence of frequency in TV transducer.

3.3.4. Discussion

The success of applying the ASQ technique is subject, among other factors, to the reliability of the extracted features. Five factors associated with the reliability were considered in this section: (i) ROI size, (ii) ROI depth, (iii) focus, (iv) gain setting and (v) transducer frequency.

The results for the influence of ROI size on the mean of the output curve show that the size of the ROI does not affect the occurrence curve mean. This can be clearly seen in both the TV transducer and the curved transducer. Yet, in the linear transducer, the variation is inversely proportional to the size of the ROI. In other words, the larger the ROI size, the more reliable the mean of the output curve, with less variation, and the smaller the ROI, the more variation can be seen in the mean. This indicates that the linear transducer is ROI-size dependent. This could be explained by the fact that a smaller number of samples are used in the smaller ROI, which gives more variation. In other words, a small ROI would have an insufficient

121 numbers of pixels to reliably compute the ASQ mean output. As different lesions may have different sizes, a minimum size that ensures that the variation in ASQ parameters in each ROI is captured needs to be established.

It is important to note that a relatively large ROI size should be used for any particular study using the linear probe to avoid misleading results due to the difference in the ROI size. This transducer, however, was not used in the study of images of ovarian masses.

These results do not apply to the TV or the curved transducer. It has been shown that the ROI size does not affect the mean of the output curve. Moreover, the mean of the output curve will have minimal variation when using different ROI sizes that will not affect its reliability.

ROI depth is another determinant of the mean of the output curve. The ultrasound beam causes a depth dependence of the B-mode image texture, as described by Oosterveld et al. (1991). This can be explained by the fact that the intensity progressively decreases as the beam passes though tissues as a result of scattering, refraction and absorption phenomena (Lagalla and Midiri, 1998).

The attenuation on the ultrasound signal as it advances through the tissue is one of the underlying factors that affect the B-mode image texture. According to Morris (1988), the correction of this variation in the raw image is not straightforward and might not even be possible. To overcome this problem, some studies have confined the ROI

122 position to a fixed depth to avoid depth dependency. For instance, (Bader et al., 2000) stated in their study that all tumours were located at depths between 2 cm and 3 cm.

Since the position of the lesion may vary, it is crucial to understand the influence of the ROI depth on the mean of the output curve. We have evaluated the effect of ROI depth on the mean of the output curve on three different transducer types. In both the TV and the curved transducers, the depth did not significantly affect the mean of the output curve, while in the linear transducer, a significant difference was seen when using different depths for the ROI.

The third factor that may affect the ASQ output curve is the position of the focus. Generally, the ultrasound beam spreads out, or undergoes divergence, as it moves away from the transducer (International Society of Radiology: (Tole, 2005)): therefore, it is possible to focus the ultrasound beam to cause narrowing of the beam and thus improve (lateral) resolution. The beam can be narrowed at a predetermined distance from the transducer. The point at which the beam is at its narrowest is the focal point or focal zone and is the point of greatest intensity and best lateral resolution (George, 2006).

The fact that the focus position does affect the mean of the output curve in the TV probe led us to decide to keep the focus position at the level of the drawn ROI to acquire the greatest intensity and the best lateral resolution to ensure reliable results.

The fourth factor that may influence the mean of the output curve in the ASQ software is the gain setting. The results of testing the effect of high gain (100 %) and

123 low gain (85 %) showed that all three probes are independent of the gain setting. This means that using either high or low gain when acquiring the raw data on the ultrasound machine will not affect the mean of the output curve in the ASQ software. This could be explained by the fact that the ASQ software uses the raw data, which is initially not affected by the gain setting.

Transducer frequency results showed that the mean of the output curve is not affected by the frequency chosen to capture the ultrasound images except when choosing 8.4 or 8 MHz in the linear and the TV probe respectively. These higher frequencies seem to affect the mean of the output curve in the ASQ.

Based on the findings from this chapter, three factors do not influence the mean of the output curve in the ASQ software using the TV probe: these are ROI size, ROI depth and the gain setting. Nevertheless, the focal position does have a significant effect on the output curve. Moreover, transducer frequency does affect the mean of the output curve only if a high frequency is chosen (8 MHz); otherwise lower frequencies do not influence the mean of the output curve. These findings permit us to use the ASQ software on ovarian masses with confidence that these factors will not affect the reliability of the reading, while taking caution when positioning the focus on the image before storing the raw data as well as using low transducer frequencies.