The Development of a Novel Multi-modality Heart Phantom for Cardiac Applications in Radiotherapy

Abstract

Purpose & Introduction: For radiotherapy in the thoracic region and radioablation of non-cancerous conditions such as ventricular tachycardia (VT), geometric and dosimetric accuracy near the beating heart is critical. For this project, a flexible multi-modality heart phantom was designed which incorporates cardio-respiratory motion and novel scintillation detectors to facilitate high-precision radiotherapy in the thoracic region. Eight essential design goals were developed according to physicist user needs for the project: (1) the CT number of the heart material should be close to myocardium; (2) the system must be MR-compatible; (3) the phantom must be durable, withstanding many usage cycles; (4) the design must be water-compatible; (5) the heart model must move according to both cardiac and respiratory influences; (6) the heart model must be anthropomorphic and include at least 12 heart substructures; (7) the prototype design should be compatible with current clinical equipment; (8) the prototype design should allow for dosimetry at several locations throughout the model. Material specifications were developed from the design goals and weighted using a 1-5 scale of importance, to inform a decision matrix for evaluating five candidate materials. The weights were multiplied by individual scores (1-5) evaluated from a literature review and divided by a maximum score to obtain an overall score out of 5. Each material that is capable of being 3Dprinted (3D-P) scored highly in the preliminary evaluation, where ease of fabrication and material durability drove the scoring rubric. The high scorers from this decision matrix guided the investigation toward further characterization of 3D-P materials. Materials & Methods: Five sample materials (Agilus30, Durometer 60, EPU 40, FPU50, SIL30) were printed (5mmx50mm) and obtained from Midwest Prototyping. A second cardiac model was prototyped using Elastic50A on a FormLabs Form 3BL printer. Imaging experiments were performed using a CT-simulator, T1-weighted (T1W) and T2-weighted (T2W) sequences on a 1.5T MR-simulator, and T2W and true fast and steady precession (TrueFISP) sequences on a 0.35T MR-Linac. Three Elastic50A sample cylinders (2cm height, 2.5cm diameter) were obtained to perform nondestructive compression testing on a Material Testing System (MTS). Eight compression cycles were performed at a strain rate of 0.2 mm/s. Twelve heart substructures were contoured following a published cardiac atlas on a contrast-enhanced CT with coronary angiography (CTCA) scan. The contours were verified by a cardiovascular radiologist and exported as an STL file using MIM Maestro. The heart model was simplified using the shrink-wrap feature in ANSYS Workbench to generate a mesh with 0.5cm element sizing. The boundary conditions for the test were chosen according to the maximum safe driving force of the MR-safe motor to match the expected design dynamics. The superior-most faces were held fixed while a force with magnitude 20N was applied upwards for a duration of 0.5s at the inferior-most faces. A novel anthropomorphic phantom module has been simulated using 3D engineering design software. Multi-point scintillator dosimeter devices considered for the final fabrication of this module were validated in a low-field MR-linac using a similar motion phantom. Several gating parameters were tested to evaluate the efficacy of the novel dosimeters in future motion experiments. Results: The average CT numbers were Agilus30 (15 ± 22 HU), Durometer 60 (-37 ± 28 HU), EPU 40 (-57 ± 39 HU), FPU50 (51 ± 34 HU), SIL30 (20 ± 30 HU), and Elastic50A (38 ± 22 HU), with Elastic50A most comparable to the average CT number of the human heart (39 ± 23 HU). All materials displayed a difference of signal to water in standard MRI sequences, allowing for image contrast against the water background. An Elastic50A 3D model was printed and imaged on all modalities, with the model’s substructures appearing in each imaging modality. The experimentally-determined average compressive Young’s modulus of Elastic50A is 3.87 ± 0.14 MPa. Using the compressive modulus, the Elastic50A heart phantom will be capable of deformable movement up to 0.44 ± 0.04 cm (k=2) at the apex, which is approximately equal to the centroid movement of the LV reported in the literature. The maximum simulated equivalent stress and strain were found to be 289 ± 26 kPa (k=2) and 8.3 ± 0.7% (k=2) respectively, at the outer wall of the superior pulmonary trunk. These values are less than the ultimate tensile strength and elongation at failure. Therefore, the material is not projected to fail under a compressive deformable load. The simulated design has been retrofitted to a commercially available MR-safe ceramic motor for rigid motion. The programmable motor software allows for linear and rotational motion in decoupled respiratory and cardiac waveforms. A modified hollow body oval and cylindrical heart module has been designed to contain the heart for rigid movement. The design for the heart module has several openings to allow for the insertion of cavities that may contain a variety of objects for medical device inserts or dosimeters for determining radiation dose. Multi-point scintillation detectors have shown promise for reporting gating efficiency via the unique real-time readout of these dosimeters. No apparent image artifacts were observed when using the novel detectors, indicating acceptable use for future motion-gating experiments. Conclusions & Future Work: The initial prototyping has been performed for a novel 3D-printed heart model for use in radiation therapy applications. Elastic50A was determined to be the optimal material of choice due to its similarities to heart tissue in CT and contrast to water in MRI. A novel multi-modality anthropomorphic heart phantom has been designed for integration with validated scintillation dosimeters for cardiac applications in radiotherapy. Future work includes the fabrication of the phantom module and experimental testing. The flexible nature of the material allows for deformable motion to be considered for future design iterations.