| Author | RA Crescenti, JC Bamber, AA Oberai, PE Barbone, JP Richter, C Rivas, NL Bush and S Webb |
|---|---|
| Title | Quantitative ultrasonic elastography for gel dosimetry |
| Year | 2010 |
| Journal | Ultrasound in Medicine & Biology |
| Volume | 36 (2) |
| Pages | 268-275 |
| Abstract | Advanced radiation techniques such as intensity modulated radiotherapy (IMRT) for complex geometries in which targets are close to organs at risk have been introduced in radiation therapy, creating a need for procedures that allow easy three-dimensional measurement of dose for verification purposes. Polymer gels that change their material properties when irradiated have been suggested for such use and, mostly, the change in their magnetic properties has been investigated with magnetic resonance imaging (MRI). Previously we have shown that the mechanical stiffness, i.e. Young's modulus, of these gels also changes with dose. This finding prompted us to assess whether we can image a radiation-induced stiffness distribution with quantitative ultrasound elastography and whether the stiffness distribution is correlated with the dose distribution. A methacrylic-acid-based gel was loaded with scatterers to create ultrasound echoic signal. It was irradiated to create a rod-like region of increased stiffness with a 10 × 10 mm2 cross-section. The gel block was compressed in a frame that restricted the movement of the gel to planes orthogonal to the long axis of the irradiated region, and ultrasonic echo data were acquired in the central plane during compression. This simplified irradiation pattern and experimental setup were designed to approximate plane strain conditions and was chosen for proof of concept. The movement of the gel was tracked from ultrasound images of a different compressional state using cross-correlation, enabling a displacement map to be created. The shear modulus was reconstructed using an inverse algorithm. The role of the magnitude of the regularization parameter in the inverse problem and the boundary conditions in influencing the spatial distribution of stiffness, and thus final dose contrast, was investigated through parametric studies. These parameters were adjusted using prior knowledge about the stiffness in parts of the material, e.g. the background was not irradiated and therefore its stiffness was homogenous. It was observed that a suitable choice for these reconstruction parameters was essential for a quantitative application of stiffness measurement such as dosimetry. The dose contrast and distribution found with the optimal parameters were close to those obtained with MRI. Initial results reported in this paper are encouraging, and indicate that with ongoing refinement of ultrasound elastography techniques and accompanying inverse algorithms this approach could play an important role in gel dosimetry. |