Results

et al. 2007c): the stent geometry was reconstructed from caliper and optic microscope measurements; the manufacturing company supplied material data.

Contact algorithms were set up in the FE code (ABAQUS/Standard, Simulia, USA), and the PPVI stent model was inflated inside the RVOT model. The stresses induced by the stent on the arterial wall and the stress distribution on the device itself were analysed to verify safe anchoring and stent mechanical performance.

Using FE modelling, a ‘stent-in-stent’ technique (Fig.11a) was investigated as a possible solution to common occurrence of stent fractures (Schievano et al.2007c), which remains the major technical issue of the current PPVI device, occurring in approximately 20% of the patients (Nordmeyer et al.2007).

To extend the indication of the percutaneous approach to the whole range of possible RVOT dimensions, innovative solutions that enable downsizing of the vessel diameter to the available valves, have been designed and tested, such as stent-graft geometries with a narrowing in the central portion that hosts the valve, and wider terminal parts that anchor the device to the RVOT wall (Fig.11b) (Capelli et al.2010). These stents are made of nitinol, a shape memory alloy with important biocompatibility, fracture resistance and MR compatibility properties and the pecu- liar characteristic of recovering very large deformation. In this study, patient- specific computational analyses have been applied to investigate the suitability of new device designs, using imaging data from 62 patients who had undergone surgical pulmonary valve replacement.

patients underwent PPVI (Fig.12): none of them had Type I morphology, which was felt unsuitable because of the high risk of proximal device dislodgement.

From a morphological perspective, we could have attempted PPVI in all types of RVOT except Type I. In fact, we performed PPVI only in 11 out of 42 patients with suitable morphology, as other factors influence patient selection for this procedure, such as the dimension of the implantation site, which cannot be greater than 22 mm.

Diameter measurements clearly show that most of the analysed RVOTs were larger than 22 mm. In addition, suitable implantation sites must be rigid conduits which can guarantee protection from fracture, while the dynamics that developed in these patients late after surgical repair of congenital heart disease presented large deformations in all directions, as shown by the 4DCT analysis (Fig.13).

The development of larger devices with a new design that permits downsizing of dilated RVOTs to the size of biologically available valves will significantly increase the number of patients that could benefit from a percutaneous approach in the future.

50 1/2

Other

13 11/83

TOTAL

27 3/11

Type V

21 3/14

Type IV

67 2/3

Type III

16 2/12

Type II

0 0/41

Type I

% PPVI

Type I 50%

Type II 14%

Type III 4%

Type IV 17%

Type V 13%

Other 2%

Fig. 12 Distribution of patients in the five morphological classes and number of PPVIs performed in the studied population

40 50 70 ₁

2 3 4 5 6 7 8 9 10 11 12 Systole Patient

60 50 40 30 20 10 0 40

30 20 10 0 30

20

a b c

Diameter [mm] 10

0

0 50 0

Cardiac cycle [%] Cardiac cycle [%] Cardiac cycle [%]

50

100 100 0 50 100

Fig. 13 RVOT diameter changes from 4DCT in three different sections along the pulmonary outflow tract (A¼sub-valve,B¼supra-valve,C¼pre-bifurcation) of 12 patients

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2.2.2 Rapid Prototyping

The RP results are shown in Fig.14. The cardiologists made the correct decision in 50% and 66% of the cases respectively, using conventional MR assessment, which improved for both to 75% with the availability of the 3D RP models. In the patients where PPVI was attempted but failed, the decisions were 50% correct with the MR images alone, but 100% correct using the models for both observers.

Physical RP models can help the selection of patients for PPVI more accurately than conventional MR because they provide a complete appreciation of the 3D RVOT anatomy and enable easy measurement of RVOT size (Fig.15a). The models also enabled trial implantation of the PPVI device (Fig. 15b) to test the correct Fig. 14 3D RP models of 12 patients’ RVOT

Fig. 15 Calliper measurements (a) andin-vitrotrial insertion of the current PPVI device (b) and of a new prototype (c,d) in different patients’ RP models

positioning and anchoring into the RVOT before the procedure was actually performed for borderline patients. This guaranteed a more accurate selection process for PPVI and enhanced patient safety. Furthermore, the RP models were used to test prototypes of new possible devices for PPVI (Fig.15c, d) (Schievano et al.2010).

2.2.3 Finite Element Analysis

The FE analysis of the current stent identified the areas at potential risk for fracture as seen in clinical practice (Fig.16a). The stent-in-stent technique appeared to be an effective solution to increase the device mechanical performance and reduce the rate of fracture. However, the design of this stent would still not be suitable to treat patients who have large RVOTs. Purposely designed nitinol stent grafts could be the optimal choice for the next generation PPVI device. FE analysis can aid the optimisation of these devices (Fig.16b) before prototypes are manufactured and tested in RP models.

FE analyses allowed the evaluation of the stresses not only in the stent but also those induced in the RVOT wall by the deployment of the device. These stresses should be not too elevated to avoid wall damage, but also high enough to hold the stent in the correct position, thus avoiding valve dislodgment.

Deployment and anchoring of a potential new PPVI stent-graft with three differ- ent dimensions were tested in patient specific implantation sites using FE analyses (Fig.17). By varying the dimensions of the device, the number of patients that could potentially benefit from PPVI can be increased to 70% (Capelli et al.2010).

Fig. 16 Von Mises stress distribution in the current PPVI stent after FE inflation in a patient’s implantation site (a) and an example of an optimised, nitinol ring for a new PPVI stent (b) – maximum principal strain contour when the ring is crimped inside the catheter

Fig. 17 Implantation of a new stent graft in selected patients’ outflow tracts

Biomedical Imaging and Computational Modeling in Cardiovascular Disease. . . 189