| Structure Index | Tissue Type | Sound Speed (m/s) | Conductivity (mS/m) |
| 1 | Healthy Lung | 1440 | 109.0 |
| 2 | Pleural Effusion | 1500 | 275.1 |
| 3 | Heart | 1560 | 222.1 |
| 4 | Spine | 1640 | 22.9 |
| 5 | Adipose | 1532 | 367.1 |
| 6 | Background | 1540 | 0 |
In previous work by Rueter et al., low frequency (10–750 kHz) ultrasound was shown to penetrate the lung, motivating its use as a nonionizing tomographic technique for pulmonary monitoring. Here, we present a method for regularized full waveform inversion for low-frequency tomographic ultrasound data. A novel structural similarity index metric (SSIM)-based regularization term is introduced that compares the structural correlation of the current sound speed iterate to a prior reconstruction computed by electrical impedance tomography (EIT). Full waveform reconstructions of sound speeds from numerically simulated low frequency ultrasound data with 0.1% additive Gaussian noise are computed, and the results are compared using Tikhonov regularization, total variation regularization, and both terms combined with the SSIM-EIT regularization term. The EIT reconstruction was computed from simulated voltage data on the same phantom using one step of a Newton-Raphson method with a high-pass Gaussian filter as a regularizer. Reconstructions including the SSIM-EIT regularization term converged fastest, and an adaptive regularization parameter provided the most accurate reconstructions. The method is then iterated by computing an EIT reconstruction using the USCT result as a prior, and a subsequent USCT reconstruction is computed using the SSIM-EIT regularization term with the updated EIT reconstruction.
| Citation: |
Figure 5. (A)–(D): USCT sound speed reconstructions for four different regularization terms. The reconstruction with the optimal choice of regularization parameter is plotted. (E)–(F): Comparison of the error term $ E(c) $ and $ L^2 $ error for the best performing reconstructions using the four different regularization terms
Table 1. USCT and EIT parameters in the numerical lung phantom
| Structure Index | Tissue Type | Sound Speed (m/s) | Conductivity (mS/m) |
| 1 | Healthy Lung | 1440 | 109.0 |
| 2 | Pleural Effusion | 1500 | 275.1 |
| 3 | Heart | 1560 | 222.1 |
| 4 | Spine | 1640 | 22.9 |
| 5 | Adipose | 1532 | 367.1 |
| 6 | Background | 1540 | 0 |
Table 2. Normal distribution parameters of the conductivity region variations
| Organ | Mean (S/m) | Std. (S/m) |
| Lungs | 0.0 | 0.240 |
| Heart | 0.0 | 0.192 |
| Domain | 0.0 | 0.140 |
Table 3. Threshold segmentation
| Organ | Threshold | Region |
| Lungs | $ \text{Ref. Image}< {1490} $ | 2 |
| Heart | $ {1580} < \text{Ref. Image} < {1610} $ | 3 |
| Domain | - | 1 |
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The transducer locations surrounding the lung phantom (left) and the filtered, transmitted ultrasonic signal sent through each transducer in succession (right)
EIT phantom and conductivity reconstruction. Note that the conductivity values are negative because these represent difference images
Left: The Tikhonov+SSIM-EIT USCT image used to construct the prior for the OSGN EIT reconstruction. Center: The threshold segmentation with assigned conductivity values in mS/cm for the construction of the prior. Right: OSGN EIT reconstruction using equation (13)
Left: The TV+SSIM-EIT USCT image used to construct the prior for the OSGN EIT reconstruction. Center: The threshold segmentation with assigned conductivity values in mS/cm for the construction of the prior. Right: OSGN EIT reconstruction using equation (13)
(A)–(D): USCT sound speed reconstructions for four different regularization terms. The reconstruction with the optimal choice of regularization parameter is plotted. (E)–(F): Comparison of the error term
(A)–(B): USCT sound speed reconstructions using a second iteration of the Tikhonov+SSIM-EIT prior at iteration 14 and TV+SSIM-EIT prior at iteration 5 of Algorithm 1 with an adaptive regularization term. (C)–(D): error measures for the same reconstructions