Design of heart phantoms for ultrasound imaging of ventricular septal defects

Effect of Injection Parameters on the MRI and Dielectric Properties of Condensation-Cured Silicone

Phantoms with tissue-mimicking properties play a crucial role in the calibration of medical imaging modalities, including Magnetic Resonance Imaging (MRI). Among these phantoms, silicone-based ones are widely used due to their long-term stability in MR imaging. Most of these phantoms are manufactured using traditional pour-mold techniques which often result in the production of air bubbles that can damage the phantom. This research investigates the feasibility of utilizing extrusion techniques to fabricate silicone phantoms and explores the effects of extrusion parameters including plunger speed and nozzle diameter on void content, T1 and T2 relaxation times, and dielectric properties. A custom double-syringe silicone extrusion apparatus was developed to prepare the silicone samples. The void content, relaxometry, and dielectric properties of extruded samples were measured and compared with traditional poured samples. The results show that extrusion parameters can affect the void content of the silicone samples. The presence of voids in the samples resulted in lower T1 values, indicating an inverse relationship between void content and relaxometry. This study demonstrates the potential of extrusion techniques for manufacturing silicone phantoms with reduced air bubble formation and provides valuable insights into the relationship between extrusion parameters and phantom properties.

Navigation guidance for ventricular septal defect closure in heart phantoms

PURPOSE

Transesophageal echocardiography (TEE) is the preferred imaging modality in a hybrid procedure used to close ventricular septal defects (VSDs). However, the limited field of view of TEE hinders the maneuvering of surgical instruments inside the beating heart. This study evaluates the accuracy of a method that aims to support navigation guidance in the hybrid procedure.

METHODS

A cardiologist maneuvered a needle to puncture the patient's heart and to access a VSD, guided by information displayed in a virtual environment. The information displayed included a model of the patient's heart and a virtual needle that reproduced the position and orientation of the real needle in real time. The physical and the virtual worlds were calibrated with a landmark registration and an iterative closest point algorithms, using an electromagnetic measurement system (EMS). For experiments, we developed a setup that included heart phantoms representing the patient's heart.

RESULTS

Experimental results from two pediatric cases studied suggested that the information provided for guidance was accurate enough when the landmark registration algorithm was fed with coordinates of seven points clearly identified on the surfaces of the physical and virtual hearts. Indeed, with a registration error of 2.28 mm RMS, it was possible to successfully access two VSDs (6.2 mm and 6.3 mm in diameter) in all the attempts with a needle (5 attempts) and a guidewire (7 attempts).

CONCLUSION

We found that information provided in a virtual environment facilitates guidance in the hybrid procedure for VSD closure. A clear identification of anatomical details in the heart surfaces is key to the accuracy of the procedure.

Characterization of blood-mimicking fluids for echocardiography imaging of ventricular septal defects

PURPOSE

Ventricular septal defects (VSD) are congenital heart malformations and, in severe cases, they require complex interventions under echocardiography guidance. Heart phantoms can be helpful to train and to understand the complex hemodynamics of VSD. The goal of this study was to characterize the best blood mimicking fluids in such heart phantoms for modelling the hemodynamics of VSD patients using echocardiography.

METHODS

Four fluid compositions were considered. Distilled water was used as a baseline, while the other three fluids were developed based on physical properties of human blood, such as the viscosity and the refractive index. Three bi-ventricular heart phantoms of three different pediatric patients with complex VSD were designed from preoperative CT imaging. Custom molds were printed in 3-D and the anatomical structure was casted in polyvinyl alcohol cryogel. The VSD in each heart phantom were observed using echocardiography and color Doppler imaging was used for the hemodynamic study.

RESULTS

Heart phantoms with blood mimicking fluids of 30% glycerol and 27% glycerol, 10% sodium iodide were found to be anatomically realistic under echocardiography imaging. Hemodynamic parameters such as the pressure gradient and the volume of the shunt were characterized using color Doppler imaging.

CONCLUSION

Proper composition of blood mimicking fluids are important for improving the realism in echocardiographic heart phantoms and they contribute to better understand the complex hemodynamic of VSD under echocardiography.