In this thesis four different studies were performed: Comparison of mean ADC values of benign and malign lesions, repeatability of ADC measurements, correlation between SNR and ADC values, and effect of lesion size to ADC values. The results of these studies are introduced next.
5.1 Comparison of mean ADC values
ADC values of lesions were measured as described in Chapter 4.3.1. Malign suspect were excluded from statistical analysis due to small sample size. ADC values of lesions were normalized by dividing the ADC value of lesions with ADC values of reference tissue. The distribution of original ADC values of 48 malign and 8 benign lesions is shown in Figure 12 and normalized ADC values in Figure 13. Also box plot is included.
Figure 12. Measured ADC values of malign and benign lesions. Box plot shows the median of the sample (middle line inside the box) and 25 to 75percentiles of data. The whiskers represent the minimum and maximum of the sample, i.e. range of values.
Figure 13. Normalized ADC values of malign and benign lesions. Box plot shows the median of the sample (middle line inside the box) and 25 to 75 percentiles of data. The whiskers represent the minimum and maximum of the sample, i.e. range of values.
It can be seen in Figure 12 and 13 that ADC values of malign and benign lesion overlap.
The normalization makes the box plot of malign sample narrower. The benign sample distribution does not change much in normalization. The statistical information of the data is also gathered in to Table 5.
Table 5. Statistical information of the ADC measurement.
Type of lesion
Number of lesions
ADC values Normalized ADC values Range
(x10-3 mm2/s)
Mean
(x10-3 mm2/s)
Range
(x10-3 mm2/s)
Mean
(x10-3 mm2/s)
Benign 8 0.68 – 1.38 1.03 ± 0.29 0.54 - 1.23 0.80 ± 0.23 Malign 48 0.130 – 1.43 0.76 ± 0.23 0.18 - 1.05 0.63 ± 0.20 Normal distribution of the ADC values was tested with Kolmogorov-Smirnov separate-ly for benign and malign samples. Both normalized and not normalized data were con-sidered as normally distributed, with significance level 0.05 test (Table 6). The statisti-cal significance of difference between malign and benign means was tested with Inde-pendent-Samples’ t test. The malign and benign lesions ADC value means were
cantly different for both normalized (0.03) and not normalized cases (0.01) with signifi-cance level of 0.05. These results are presented in Table 6.
Table 6. Results for testing significance of benign and malign mean difference.
Statistical test
Significance
Not normalized Normalized Kolmogorov-Smirnov
test
Benign 0.08 0.20
Malign 0.20 0.20
Independent-Samples
T test 0.01 0.03
5.2 Repeatability of ADC measurement
ADC measurements were repeated for 16 randomly chosen malign lesions. The meas-urement results for real ADC values are shown in Figure 14 and for normalized ADC values in Figure 15. Old measurement results from corresponding 16 lesions are plotted beside for comparison.
Figure 14. ADC values from repeated measurement on left and for comparison the cor-responding ADC values from original measurement on right. Box plot shows the medi-an of the sample (middle line inside the box) medi-and 25 to 75 percentiles of data. The whiskers represent the minimum and maximum of the sample, i.e. range of values.
Figure 15. Normalized ADC values from repeated measurement on left and for compar-ison the corresponding ADC values from original measurement on right. Box plot shows the median of the sample (middle line inside the box) and 25 to 75 percentiles of data. The whiskers represent the minimum and maximum of the sample, i.e. range of values.
From Figures 14 and 15 can be seen that the medians lie at same levels both in original and repeated measurements and distributions and ranges of samples are similar. Nor-malization makes the box plot wider i.e. ADC values spread around median a little. The statistical information of these measurements is collected into Table 7.
Table 7. Statistical information of the repeatability measurement
Measurement
ADC values Normalized ADC values Range
(x 10-3 mm2/s)
Mean
(x10-3 mm2/s)
Range
(x10-3 mm2/s)
Mean
(x10-3 mm2/s)
Original 0.23 – 1.43 0.77 0.18 - 1.04 0.63 Repeated 0.30 – 1.36 0.76 0.26 - 1.00 0.60
The normal distribution of old and new ADC values of lesions was tested with Kolmogorov-Smirnov test. Both old and repeated samples (normalized and not normal-ized) were considered normally distributed (Table 8) with significance level 0.05. The reliability of the repeated measurements was tested with ICC values and the reliability
test was performed with absolute agreement. ICC value for not normalized data was found 0.99 and for normalized 0.94 (Table 8). Thus repeatability was excellent.
Table 8. Results for statistical reliability test.
Statistical test
Reliability test 0.99 0.94
5.3 Correlation between SNR and ADC values
The mean SNR value was determined with equation (17) from 19 b=0 s/mm2 DW im-ages. The obtained SNR value was 40.6 ± 15.6.
SNR values were compared with ADC values of 19 random lesions (the original ADC values, not repeated measurements). The true ADC values were used, i.e. not normalized values. Comparison was performed with correlation calculation in SPSS software. First the normality of ADC value distribution was tested with Kolmogorov-Smirnov test and both variables were considered as normally distributed with signifi-cance level 0.05 (Table 9).
Table 9. Normal distribution test for ADC and SNR values
Kolmogorov-Smirnov Test ADC SNR
Significance 0.08 0.20
Next the actual correlation was tested with a correlation matrix, shown in Table 10. The correlation coefficient between ADC and SNR was positive (correlation coefficient 0.49) and statistically significant (significance level 0.05). The not normalized and nor-malized ADC values are also plotted against SNR values in Figures 16 and 17, respec-tively.
Table 10. Correlation matrix for ADC and SNR values
Correlation matrix, N=19 ADC SNR
ADC Pearson Correlation
Figure 16. Graphical presentation of correlation between SNR and not normalized ADC values. Line shows least square fit for correlation.
Figure 17. Graphical presentation of correlation between SNR and normalized ADC values. Line shows least square fit for correlation.
From Figure 16 and 17 can be seen that there is a slight trend that ADC values increase while SNR values increase. However, like the correlation coefficient (0.49) shows, the correlation is not very strong, because it is far away from unity.
5.4 Effect of lesion size to ADC values
Effect of lesion size to ADC values was tested with the repeated measurements. The ICC values were calculated for different lesion size groups: small, medium, and large. It was seen that ICC increased with increasing lesion size (Table 11.)
Table 11. ICC results for three different size groups of lesions.
Size group Size range (mm2) N was analysed with correlation matrix. The normal distribution of data was already tested in part 5.1. The correlation was tested for both not normalized and normalized ADC values, shown in correlation matrix (Table 12).
Table 12. Correlation matrix for area, normalized and not normalized ADC values.
Correlation matrix, N=48 Area ADC, not normalized
Lesion size and ADC values show poor correlation for both normalized (0.15) and not normalized ADC values (0.22). Moreover, results are not statistically significant (>0.05). Consequently, according to this study, no correlation can be seen between le-sions size and ADC values. In Figures 18 and 19 the not normalized and normalized ADC values are plotted against lesion size.
Figure 18. Graphical presentation of correlation between lesion size and not normalized ADC values. Line shows least square fit for correlation.
Figure 19. Graphical presentation of correlation between lesion size and normalized ADC values. Line shows least square fit for correlation.
Figures 18 and 19 support the result of correlation matrix. There is not any correlation between ADC and lesion size.