Systems and methods for digital detection of an analog tomographic signal are described. The methods include receiving an amplitude-modulated analog signal containing tomographic information, the analog signal having a modulation frequency, f.sub..lamda.i; and converting the analog signal into a digital signal at a sampling frequency, f.sub.s, to produce a number of samples, K. The digital signal is multiplied by an in-phase reference signal to obtain an in-phase signal component, the in-phase reference signal having the frequency, f.lamda.i; and the digital signal is multiplied by a quadrature reference signal to obtain a quadrature signal component, the quadrature reference signal having the frequency, f.sub..lamda.i. The in-phase signal component and the quadrature signal component are passed through the K-point averaging filter. A signal amplitude is computed based on the filtered in-phase signal component and the filtered quadrature signal component, the signal amplitude being representative of the tomographic information.