## 5 Conclusions
### Symology  
\(\mathbf{r}=(x,y,z)\) and \(\mathbf{k}=(k_{x},k_{y},k_{z})\)--the radius vector of the point in space and the wave-vector with their components. \(t\)--the time. \(f\) and \(\omega\)--the temporal and angular frequency. \(p\), \(p(\mathbf{r},t)\)--the acoustic pressure field. \(p_{0}\), \(p_{0}(\mathbf{r})\)--the initial pressure distribution. \(c\)--the speed of sound in an infinite homogeneous medium, or a homogeneous background speed of sound in an inhomogeneous medium. \(\widetilde{c}\), \(\widetilde{c}(\mathbf{r})\)--the speed of sound in an inhomogeneous medium. \(\kappa\), \(\rho\), \(T\)--the compressibility, density and temperature of an infinite homogeneous medium, or the corresponding homogeneous background values for an inhomogeneous medium. \(\widetilde{\kappa}\), \(\widetilde{\kappa}(\mathbf{r})\) and \(\widetilde{\rho}\), \(\widetilde{\rho}(\mathbf{r})\)--the compressibility and density of an inhomogeneous medium. \(\gamma_{\kappa}\), \(\gamma_{\kappa}(\mathbf{r})\) and \(\gamma_{\rho}\), \(\gamma_{\rho}(\mathbf{r})\)--the relative deviations of compressibility and density. \(\delta(t)\)--the Dirac delta function. \(\Gamma\), \(\Gamma(\mathbf{r})\)--the Gruneisen parameter. \(H\), \(H(\mathbf{r})\)--the absorbed light energy density. \(\beta\)--the volume thermal expansion coefficient. \(c_{\mathbf{r}}\)--the specific heat capacity at constant pressure. \(\mathbf{r}_{m}=(x_{m},z_{m})\)--the radius vector of the \(m\)-th sensor. \(r_{i}\) and \(\mathbf{r}_{ij}=\left(x_{i},z_{j}\right)\)--the radius vector of the point of the image. \(N\)--the number of sensors. \(t_{0}\)--such large time instant that the acoustic pressure in the medium has decayed to zero. \(V\)--the volume of the investigated object. \(p_{tr}\), \(p_{tr}(\mathbf{r},t)\)--the time-reversed pressure field. \(p_{0}^{\prime}\), \(p_{0}^{\prime}(\mathbf{r})\)--the reconstructed initial pressure distribution. \(P_{0}(\mathbf{k})\) and \(P_{0}^{\prime}(\mathbf{k})\)--the spatial spectrum of the actual (\(p_{0}(\mathbf{r})\)) and reconstructed (\(p_{0}^{\prime}(\mathbf{r})\)) initial pressure distributions. \(A\left(k_{x},k_{y},t\right)\)--the spatial spectrum of the pressure field at \(z=0\). \(c_{0}\), \(\rho_{0}\), \(Z_{0}\) and \(c_{1}\), \(\rho_{1}\), \(Z_{1}\)--the speed of sound, density and acoustic impedance of the immersion liquid and the sample. \(S_{0}\) and \(dS_{0}\)--the detection surface and its element. \(d\Omega_{0}\)--the solid angle of the element of the detection surface. \(\Omega_{0}\)--the solid angle of the whole detection surface. \(\mathbf{p}\) and \(\mathbf{P}_{0}\)--the vector representations of the recorded pressure signals and the initial pressure distribution. \(\mathcal{A}\)--the model (system) matrix. \(\mathcal{R}\)--the regularization term. \(\mathcal{T}\)--the travel time of the acoustic pulse. \(\mathcal{T}_{ijm}\), \(\mathcal{T}_{ij}^{\prime\prime\prime}\) and \(\mathcal{T}_{ijm}^{\prime\prime\prime}\)--the total, forward and backward travel times of the LU pulse from the PA generator to the image point \(r_{ij}\) and to the sensor at point \(\mathbf{r}_{m}\). \(\epsilon\), \(\epsilon\left(\mathbf{r}_{ij}\right)\) and \(\epsilon^{\prime}\left(\mathbf{r}_{ij}\right)\)--the effective strength of an acoustic scatterer. \(\mathcal{D}\left(\mathbf{r}_{m},\mathbf{r}_{ij}\right)\)--the directivity factor used for streak artifact reduction in LU imaging. \(PSF\left(\mathbf{r}|\mathbf{r}_{0}\right)\)--the value of the point spread function at point \(\mathbf{r}\) when the point-like PA source is located at \(\mathbf{r}_{0}\). \(R\) and \(F\)--the radii of curvature of the sensor array in the image plane and perpendicular to it. \(D\)--the full width at half-maximum of the sensitivity map along \(z\)-axis. \(Lat\)--the lateral resolution. \(D_{F}\), \(Lat_{F}\) and \(D_{\infty}\), \(Lat_{\infty}\)--the values of \(D\) and \(Lat\) for the special cases of \(R=F\) and \(R\to\infty\). \(h\), \(h(t)\)--the sensor's temporal response.