Due to the symmetry of the sensor, the system matrix depends only on $|m\u2212n|$. According to the discrete representation of the source distribution $w\varphi $, the meaning of $Wj,l,n$ is actually the strength of a uniform, spherical voxel located at position $(sl,zn)$ in the object rotated by $\varphi j$, which is integrated over an infinite line in direction of the cylindrical detector axis. The elements of the system matrix are thus given by Display Formula
$ak,l,|m\u2212n|=cst\u222b0tpsph(sl,|m\u2212n|\Delta z,t\u2032)t2\u2212t\u20322dt\u2032|t=k\Delta t,$(5)
where $psph(sl,|m\u2212n|\Delta z,t\u2032)$ is the response of the detector to a spherical source at position $(sl,|m\u2212n|\Delta z)$, given by Display Formula$psph(sl,|m\u2212n|\Delta z,t)=\u222bC(s\u2032,z\u2032)h(b,dl,|m\u2212n|,t)dCwith\u2009\u2009dl,|m\u2212n|=(sl\u2212s\u2032)2+(|m\u2212n|\Delta z\u2212z\u2032)2.$(6)