Visualisation of Entire Image Volumes

Figure 6.4 depicts axial slices of the scoring volume of the two benign cases B1 and B2 and the two malignant cases M4 and M6. The kinetic signals spof voxels marked by the breast mask were

evaluated with the LSM using the raw -features. The outcome yp reflecting the confidence that

the signal belongs to the class of malignant (yp1), normal (yp2) and benign (yp3) was mapped to

the red, green and blue component of the pseudo-colour cp, respectively. All four lesions are easy

to locate in terms of their size and pseudo-colours assigned to the lesion voxels. The lesion masses of B1 and B2, which are both attributed as benign according to the histological examination, are displayed by voxels with shadings of blue to purple indicating high confidence values for the benign and slightly increased values for the malignant component of the LSM output. The malignant lesion of case M4 exposes a ring of intense red and purple voxels surrounding a green core which may consist of necrotic tissue with low vascularity. The average signal of the whole-tumour ROI exhibits a delayed uptake and weak washout between the fourth and fifth postcontrast image suggesting a benign disorder of the tissue. In this situation, the pseudo-colours provide valuable architectural information about the lesion mass and allow for localising subregions which are malignant at high probability. The lesion of case M6 is a bifocal lesion consisting of a benign (intense blue) and a malignant (intense red) compartment. The visualisations of the lesions of all twelve cases will be investigated in more detail at the end of this section. The major part of the breasts is homogenously depicted with intense green voxels indicating breast tissue which is healthy at high probability.

Selective Augmentation of Conventional DCE-MR Images

An alternative visualisation of the data can be obtained by superimposing pseudo-colours only onto those voxels whose signals are rated as suspicious with a probability above a certain threshold. The depiction of the entire DCE-MRI volume with pseudo-colours facilitates the localisation and examination of suspicious masses due to the high visual contrast between the lesion voxels depicted with shadings of blue and red and the surrounding normal tissue homogenously coloured green. In contrast, a limited augmentation of conventional DCE-MR images with pseudo-colours enables

B1 B2

M4 M6

Figure 6.4:Subregion of an axial slice of the LSM based scoring volume for the benign cases B1 and B2 (top row) and the malignant cases M4 and M6 (bottom row). The locations of lesions are designated by white arrows. In all four images, the major part of the breast consisting of normal tissues such as fat or glands is depicted intense green. The benign lesions are presented as intense blue or purple masses, whereas the carcinoma of the malignant cases are predominately displayed with intense red and purple voxels. Next to the lesion masses, the pseudo-colouring suggests additional benign masses for case B2 (right breast, below the lesion) and M4 (left breast, centre).

the observer to interpret the spatial distribution of the pseudo-colours in regions of suspicious tissue in the context of the structure of the surrounding tissue.

For an automatic selection of regions which are to be presented in pseudo-colours, the le- sion detection setup as proposed in the previous chapter can be applied. The voxel value in the confidence volume computed for the patient under investigation reflects the probability P (Suspicious|xp) = P (¬Normal|xp) that the associated temporal kinetic signal was caused by

suspicious tissue. A comparable confidence value can be obtained from the outcome y of the LSM and MSVM, respectively. The second component yp2 = P (Normal|xp) reflects the proba-

bility that the temporal kinetic signal associated with voxel p is caused by normal tissue. Hence, a probability value P (¬Normal|x) = 1 − P (Normal|xp) = 1 − yp2 above a certain threshold

indicates suspicious temporal kinetic signals.

Examples for selectively augmented conventional DCE-MR images can be observed in figure 6.5. The figure depicts the same axial slices as figure 6.4, but pseudo-colours are only superimposed onto voxels p with P (¬Normal|xp) > 0.5. Thereby, tissue regions with an increased probability

of being malignant or benign are presented in pseudo-colours, whereas regions depicting glands, fat or muscle tissue exposing unsuspicious kinetic signals are displayed with the gray values of the precontrast image. Through this, suspicious regions are augmented by information about the temporal data component which therefore be correlated with the anatomical structures of the surrounding tissue.

M4 M6

B1 B2

Figure 6.5: Subregion of an axial slice of the LSM based scoring volume for the benign cases B1 and B2 (top row) and malignant cases M4 and M6 (bottom row). The figure depicts the same slices as figure 6.4, but the pseudo-colouring is only superimposed onto voxels with an increased probability of suspicious signal characteristics according to P (¬Normalxp) > 0.5. The remaining voxels reflect the intensity value of the precontrast image.

Table 6.2:Az (first and second column)and Az(SE>0.9) (third and fourth column) for the different combi- nations of classification algorithms and feature types.

Raw Allratios Raw Allratios LSM 0.9912 0.9825 0.9667 0.9112 MSVM-L 0.9905 0.9865 0.9658 0.9689 MSVM-G 0.9899 0.9880 0.9535 0.9499

of suspicious subregions, a ROC analysis is performed. Similar to the ROC analysis in the previous chapter, the indices area-under-the-ROC-curve (Az) and partial-area-under-the-ROC-curve above a sensitivity of 0.9 (Az(SE>0.9)) are calculated. The ROC curve for each combination of classifier

(LSM/MSVM-L/MSVM-G) and feature (raw /allratio) is calculated by pooling of the validation data [Swets and Pickett, 1982]: The data of the twelve average scoring volumes are merged (pooled ) and a single ROC curve is computed using the manual lesion segmentation as ground truth. Subsequently, the indices Az and Az(SE>0.9) are determined for each ROC curve. The

results are listed in table 6.2. Classifiers trained with the raw -features outperform classifiers trained with allratio-features. Only in the case of the MSVM-L, a slightly higher Az(SE>0.9) is

yielded for the allratio-features.

Spuriously Highlighted Tissue Regions

For the cases B2 and M4, the visualisation points to additional regions of suspicious tissue which were not marked by the radiologist during the manual segmentation of the lesion. For instance in

C D A B (c) (d) (a) (c) (d) (b) (a) (b)

Figure 6.6: Pseudo-colouring superimposed onto a subregion of the precontrast image of the left breast of case M4. The image exhibits several regions of tissue for which the pseudo-colouring indicates suspicious signal characteristics (P (¬Normal|xp) > 0.5), but which are false positives according to the expert label. Though, exemplary examination of the kinetic signals of such false positive voxels as can be observed on the plots affirms that the kinetic signals exhibit characteristics which are indicative for benign and malignant tissue.

figure 6.6, the pseudo-colouring superimposed onto the precontrast image of the left breast of case M4 highlights several regions by pseudo-colours indicating malignant or benign tissue. According to the expert label, the lesion of case M4 is located in the right breast. Hence pseudo-coloured

regions in the left breast have to be treated as spuriously highlighted tissue masses. Though, examination of the temporal kinetic signals of such spuriously highlighted regions reveals that, from the viewpoint of assessment of temporal kinetic signals of single voxels, the outcome of the pixel-mapping appears reasonable. The four plots in figure 6.6 visualise temporal kinetic patterns as measured inside clusters of spuriously highlighted voxels. Additionally, a pie chart illustrates the distribution of the ’probability mass’ onto the three RGB components of the pseudo-colour. The pseudo-colour itself is reflected by a filled circle to the right of the pie chart. Plot (a) exposes a kinetic signal with an uptake in the early postcontrast period followed by a steady wash-out and, therewith, exposes a typical course of a malignant kinetic signal. Correspondingly, the LSM classifies the signal as malignant with a probability which is twice the probability of a benign classification. Since the region is located inside the thorax, it can be easily distinguished from potential lesion masses. The signal shown in plot (b) is classified with high confidence as malignant. The signal exhibits a delayed but strong uptake followed by a weak wash-out in the very late postcontrast period. A signal which steadily increases over the entire period of time can be observed in plot (c). The signal is classified by the LSM as being benign leading to an intense blue pseudo-colour. The last plot depicts a steadily increasing signal with a fast uptake during the first 220sec, followed by a period of slower signal enhancement. The assessment by the LSM indicates that the signal is more likely to be caused by benign tissue than by malignant tissue.