on the tissue until reaching the desired ranges specified on the monitor. The results of palpation inside the MIS training box also confirm that incorporating force feedback in either form with tactile feedback can result in successful tumor localization.
4.4
Experiments
To quantitatively assess the effectiveness of force feedback in robot-assisted tac- tile sensing, three scenarios were chosen for further considerations including visu- ally presented tactile feedback only (VPTF), tactile feedback with visual force feed- back (VPFF+VPTF), and tactile feedback with directly reflected force feedback (DRFF+VPTF). The main objective of this study was to evaluate quantitatively how force feedback could enhance the performance of tactile sensing for MIS tumor localization and determine which way of presenting force readings could be more effective in terms of accuracy and ease of use.
4.4.1
Experimental Conditions
Eight subjects were chosen for our study: one medical professional with previous experience with haptics, three subjects with some experience with haptics and four subjects with no experience with haptic feedback in teleoperated environments. These subjects were asked to palpate four ex vivo livers each (thirty two ex vivo livers were prepared in total). The livers had the possibility of containing from zero to two tumors (eight livers with no tumor, eight with one small tumor, eight livers with one large tumor, and eight livers with two tumors, one small and one large, but they were given to the subjects randomly). Presenting a liver with no embedded tumor allows the statistical results of specificity and negative predicted value to be determined. The subjects received some visual clues on a monitor from a camera overlooking the tissue but it was not possible to discern the location of the lump in the tissue from the camera image. Figs. 4.6(a) and (b) show the tissues with the location of the tumors embedded. Each subject palpated each of the livers three times for three different scenarios. Before the experiment, the subjects were trained to use the master-slave teleoperation system for tumor localization and they were allowed practice trials until they became comfortable with use of the setup for palpation. In this experiment, the livers were placed on the table and the subjects were asked to use
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the master to palpate the tissue using the TSI through the slave in left-right direction in one dimension (Fig. 4.5(a)).
4.4.2
Experimental Procedure
In the first trial, the only information provided to the subjects was the color contour map produced by the PPS software and the subjects were asked to find the tumor by looking at the pressure distribution profile when they palpated the tissue using the TSI. In the second trial, force was also reflected to the subjects’ hands. The subjects were asked to decide about the location of the tumor based on the pressure map and the force reflected on their hands. In the third trial, force feedback was presented to the subjects on a screen including the palpation force and the lateral forces they applied during palpation. The subjects were told to keep the lateral forces in the range of ±0.5N and the palpation force within the range of 4N-5N and then decide about the location of tumors based on what they observed on the pressure maps.
4.4.3
Performance Assessment Criteria
Various criteria were employed to evaluate the performance of the subjects for tumor localization in different methods. Statistical measures are the most common metrics used to asses the effectiveness of a diagnostic test: accuracy which represents the proportion of the tests that were successful in identifying the presence or absence of a tumor; sensitivity which shows the proportion of the samples with tumors present and tested positive; specificity which indicates the proportion of the samples with no tumors and which tested negative; Positive Predictive Values (PPV) which denotes the proportion of the samples that tested positive and had a tumor; and Negative Predictive Values (NPV) which denotes the proportion of the samples that tested negative and had no tumor.
The average and maximum force applied on the tissue were also recorded for each trial to see how much damage occurred during tumor localization in different trials. These forces were measured by the environment force sensor.
The task completion time is the other criterion which is defined as the time required to locate the tumors in the tissue for each trial. The recorded time begins once the probe has touched the surface of the tissue and ended when the subject stops palpating the tissue.
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Table 4.1: Accuracy measures of tumor localization using different methods Method Accuracy Sensitivity Specificity PPV NPV
VPTF 55% 81% 6% 62% 14%
VPTF+DRFF 74% 84% 45% 82% 50%
VPTF+VPFF 83% 88% 63% 90% 56%
Table 4.2: Forces applied and task completion time for the various tests Method Favg (N) Fmax (N) Tct (s)
VPTF 7.10±2.64 15.09±4.37 131±51
VPTF+DRFF 2.80±0.84 7.81±2.20 119±40
VPTF+VPFF 3.21±0.45 6.43±0.78 160±56
4.4.4
Results and Discussion
Table 4.1 shows the statistical measures obtained from tumor localization in different methods. These statistical measures are also displayed in the bar graph in Fig. 4.14(a). Fig. 4.14(b) shows the accuracy results for the cases that the tissue contained only small or large tumors. Table 4.2 shows these forces along with the average task completion time for each method. The scaled results are also presented in Fig. 4.14(c) (force scale=5, time scale=0.4).
Among these three methods, the results show the best performance for the VPTF+VPFF method: 50% increase in tumor localization accuracy,8% increase in sensitivity, huge improvement in specificity (close to 10 times), 46% increase in PPV and more than
280%improvement in NPV when compared with the results obtained from the method using tactile feedback only. These improvements were achieved while the average and maximum force applied to the tissue were decreased by more than 55% and 57%, respectively when compared to those of using tactile information alone. However, the task completion time for this method, on average, was 22% longer than that for the tactile feedback method which was due to the time the subjects needed to keep the forces in the desired ranges. The following success rates were achieved for the case of
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(a) Statistical measures.
(b) Accuracy measure for the small and large tumors.
(c) Force applied on the tissue and the task completion time.
Figure 4.14: A bar graph comparison among the methods using tactile feedback only, force-tactile feedback fusion with visual presentation, and force-tactile feedback fusion with direct reflection.
using directly reflected force feedback incorporated with tactile feedback compared to the case using tactile feedback alone: 35%improvement in accuracy,4% improvement in sensitivity, more than670% improvement in specificity, 32% improvement in PPV and 250% improvement in NPV while decreasing the average and maximum force applied to the tissue 61%, and 48%, respectively. Using force feedback reflected to the participants’ hands not only significantly decreased the amount of force applied
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on the tissue during palpation but also reduced the task completion time by9%. Fig. 4.14(b) shows the performance of each method in tumor localization when there is only one small or one large tumor embedded in the tissue. The results show that the VPTF+VPFF method is more accurate for large tumors which indicates that the exploration force applied on the tissue containing one small tumor might not be enough to detect the tumor in all cases. The worst performance was also observed for the VPTF method for small tumors which was caused by numerous false posi- tives detected during experiments because of too much exploration force applied on the tissue. Compared to the VPTF method for small tumors, both fusion methods showed better performance because of limiting the force applied on the tissue which resulted in significant decrease in the false positives. After finishing the experiments, each participant was asked to choose the method he/she was more comfortable with. Novice participants preferred to have visual feedback from the interaction forces since they could then better know how much force they were applying on the tissue, but they found it difficult to distinguish between forces when they felt them in 3DOF at the same time on their hands. Those who had some experience with haptics pre- ferred direct force reflection on their hands; however, they pointed out that when they palpated a thicker part of the liver they felt higher force and they could not decide if there was any tumor in that region (they were asked to locate tumors based on either the pressure map or the force reflected to their hands). In the visual presenta- tion of force feedback, they had this opportunity to push the TSI on the tissue until reaching the desired forces regardless of the thickness of the liver palpated and they decided only based on the pressure map. The main reason that the VPTF+DRFF method showed some success for tumor localization is because of preventing excessive forces on the tissue and thereby reducing the false positives that resulted in accuracy enhancement. However, the VPTF+VPFF method was much more successful in con- trolling the forces applied on the tissue and showed better achievements in accuracy measures. In conclusion, visual force feedback incorporated with tactile feedback was chosen as the method with the best performance for tumor localization in robotics- assisted master-slave soft-tissue palpation.