A CT based correction method for speed of sound aberration for ultrasound based image guided radiotherapy
SOS versus density linear relationship for human soft tissues at 37 °C: the data were taken from: Mast T.D., “Empirical relationship between acoustic parameters in human soft tissues” (Ref. 19).
(a) The corrected US scan is created sequentially adding along each column a number of voxels corresponding to the value of SOS variation in the physically matching SOS variation map voxel. (b) The final matrix is obtained by resampling the corrected US matrix to the original US scan resolution and assigning to each voxel sequentially the value averaged every 100 voxels of the corrected US matrix (100 is the rescaling factor chosen in order to lose as little information as possible considering that expected SOS variations are in the order of a few percents).
(a) Setup for the measurements with the PMMA phantom. (b) Setup for the measurements with the ClarityTM Calibration phantom.
CT and US scans superposition with Implicit RegistrationTM in the ClarityTM system: the bottom of the container filled with a 20% saline solution is indicated with arrows according to the US scan (a) and to the CT scan (b). Its depth is measured as 76.9 mm in the US scan, while it is at 80 mm in reality as can also be seen from the CT scan.
Comparisons between the positions of the bottom surface of the PMMA phantom in the CT scans and in the US scans, before and after the application of the SOS aberration correction algorithm. The lines are indicated by arrows in order to distinguish them from other image artifacts.
Comparisons between the US scans of the ClarityTM Calibration phantom as seen through three liquids with different SOSs: 1450 m/s for water, 1480 m/s for sunflower oil, and 1600 m/s for 20% saline solution. In the second line the images were split in two halves to show visually the difference between the positions of the top rods in the different cases. In the third line the same comparison is shown after the SOS aberration correction was applied. The rods result at a much more similar depth after the application of the SOS correction. The horizontal white lines, instead, are image artifacts which have an erratic behavior and do not represent any physical object, so they are not supposed to align after the correction has been applied.
Clinical cases: a breast seroma and a prostate. The US scan and the correspondent CT scan transformed in SOS variation map are shown on the top line. A superposition image of original and corrected US scans, with different contrast values in order to distinguish them, as it is shown in the inferior left part of the image. The prostate case was scanned with the curved probe so the algorithm is strictly applicable only in the central part of the figure (indicated). On the other parts, lines of view should be extracted from the RF data and the algorithm needs to be adapted to follow them, to have a reliable correction. In the inferior right part, instead, a map of the cumulative shifts is plotted, calculated as the total amount of shift applied to the tissue in that specific voxel. Shifts of up to 1.5 and 3.0 mm are evident in the breast and prostate cases, respectively.
SOS values for different human soft tissues at 37 °C: the data were taken from: Mast T.D., “Empirical relationship between acoustic parameters in human soft tissues” (Ref. 19).
The values of SOS and density at 20 °C for the three liquids used to obtain the SOS versus density relationship to test the algorithm applied to the PMMA container.
Quantitative comparison between the position of reference structures in phantoms before and after SOS aberration correction with errors in distances: in the PMMA phantom the reference structure is the bottom of the phantom and is at 8 cm depth; in the QA phantom, the reference structures are the most shallow rods and their centers are at 3 cm depth with respect to the surface of the phantom but at 6 cm depth with respect to the position of the probe at the surface of the liquids container on top of the phantom.
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