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2.2 Phantoms (Feasibility Study)

We studied and tested two types of phantom.

Agar Based Phantom

We arranged a prostate phantom composed of a prostate capsule, a prostatic hyperplasia, and surrounding tissue combining agar, distilled water, Psyllium and chlorhexindine 0.2%. The chemical composition of sections differed on the percentages of the cited ingredients. Table 1 shows the optimal compositions obtained after experimental trials under US examination.

Fig. 4. Screenshot of the navigation system visualizations: at the top left corner the endoscopic visualization; at the bottom the lateral 3D view of the prostate and the surgical tool

Table 1. Composition of agar-based phantom

Phantom

Agar (g)

Psyllium (g)

Distilled water (g)

Chlorhexidine (g)

Prostate

0.6

2.6

95.6

0.2

Adenoma

1.5

0.7

97.6

0.2

Surrounding tissue

15

7

976

2

To prepare the prostatic capsule we added 0.6 g of agar and 2.6 g of Phylum to

95.6 g of distilled water and then we stirred until the ingredients were completely dissolved. We boiled the mixture. After two minutes cooking we added 0.2 of chlorhexidine, we poured it in a realistic mold and we refrigerated it for 30 minutes [10-11].

We prepared the adenoma and the surrounding tissue using the same concentration. We combined 15 g of agar, 7 g of psyllium and 976 g of distilled water. We added 488 ml of cold water to the agar and we stirred to dissolve the agar. We stirred 7 g of psyllium with 488 ml of cold water. After boiling the agar solution for two minutes we mixed it with psyllium solution adding 2 g of chlorhexidine and stirring the mixture to make it homogeneous. Figure 5 shows the phantom acquired by an US linear transducer: the boundaries are evident and it is possible to distinguish the prostate, the urethra and the adenoma that occluded the urethral channel. The materials chosen had same acoustic impedance in respect of human soft tissue. On the other hand the stiffness of adenoma and prostate were too different oversimplifying the enucleating performance.

Fig. 5. Ultrasound scan of the phantom in aqueous environment: the prostate capsule, the adenoma and urethral channel are well distinguished from the surrounding environment

Wilkin and Hamm like Phantom

We investigated the optimal way to preserve the acoustic impedance of the phantom and to employ materials that simulate the density and the stiffness ratio of the real structures. We exploited the method described by Wilkin and Hamm to develop a cheap prostate phantom making use of cornstarch, beets, corned beef and gelatin [12]. Figure 6 shows the phantom acquired by using the trans-vaginal V9-4v volume curved array transducer. The inner adenoma appeared high-echogenic: in real patients the inner gland usually results hypo-echoic compared to the peripheral zone of the prostate and the central zone is often discernible. Despite the different US features, our phantom allowed to well distinguish the zones and thanks to its comparable densities it simulated the operative conditions to enucleate the prostate.

Fig. 6. Ultrasound scan of phantom obtained by using Wilkin and Hamm recipe: the conic shape adenoma is clearly visible in respect of the surrounding prostate

 
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