Systems and methods for digital detection of a tomographic signal

1. A digital detection method for an analog tomographic signal, the method comprising:

receiving an amplitude-modulated analog signal containing tomographic information, the analog signal having a modulation frequency, f λi ;

converting the analog signal into a digital signal at a sampling frequency, f s to produce a number of samples, K;

multiplying the digital signal by an in-phase reference signal to obtain an in-phase signal component, the in-phase reference signal having the frequency, f λi ;

multiplying the digital signal by a quadrature reference signal to obtain a quadrature signal component, the quadrature reference signal having the frequency, f λi ;

passing the in-phase signal component through a K-point averaging filter;

passing the quadrature signal component through the K-point averaging filter;

computing a signal amplitude based on the filtered in-phase signal component and the filtered quadrature signal component, the signal amplitude being representative of the tomographic information; and

outputting the demodulated signal amplitude.

2. The method of claim 1 , wherein the signal amplitude is computed based on a square-root of a sum of the in-phase signal component squared and the quadrature signal component squared.

3. The method of claim 1 , further comprising computing a signal phase shift based on the filtered in-phase signal component and the filtered quadrature signal component, the signal phase shift being representative of the tomographic information.

4. The method of claim 1 , wherein the in-phase reference sequence is K points in length.

5. The method of claim 1 , wherein the quadrature reference sequence is K points in length.

6. The method of claim 1 , wherein K is between about 150 and about 160.

7. The method of claim 1 , wherein the sampling frequency, f s is between about 75 kHz and about 80 kHz.

8. The method of claim 1 , wherein the modulation frequency, f λi , is between about 3 kHz and about 9 kHz.

9. The method of claim 1 , wherein the modulation frequency, f λi , is given by m i *(f s /K), where m i is an integer between 1 and K/2.

10. The method of claim 1 , wherein a settling time of the averaging filter is given by K/f s .

11. The method of claim 1 , further comprising transmitting the demodulated signal amplitude to a host computer for storage.

12. The method of claim 1 , wherein the tomographic information comprises diffuse optical tomographic information.

13. The method of claim 1 , wherein the amplitude-modulated analog signal is an optical signal obtained by directing a light source at an object and detecting attenuated light exiting the object.

14. The method of claim 13 , wherein detecting the attenuated light exiting the object comprises using a photo-detector to convert light energy into an electronic representation.

15. The method of claim 13 , wherein the object is one or more of a tissue, an organ, and a limb.

16. The method of claim 1 , further comprising amplifying the analog signal prior to converting the analog signal into the digital signal.

17. The method of claim 1 , further comprising passing the analog signal through a low-pass anti-aliasing filter prior to converting the analog signal into the digital signal.

18. A digital detection system for processing an analog tomographic signal, the system comprising:

a transducer configured to receive an amplitude-modulated analog signal containing tomographic information, the analog signal having a modulation frequency, f λi ;

an analog-to-digital converter configured to convert the analog signal into a digital signal at a sampling frequency, f s to produce a number of samples, K; and

an instrument comprising a digital signal processor configured to:

multiply the digital signal by an in-phase reference signal to obtain an in-phase signal component, the in-phase reference signal having the frequency, f λi ;

multiply the digital signal by a quadrature reference signal to obtain a quadrature signal component, the quadrature reference signal having the frequency, f λi ;

pass the in-phase signal component through a K-point averaging filter;

pass the quadrature signal component through the K-point averaging filter;

compute a signal amplitude based on the filtered in-phase signal component and the filtered quadrature signal component, the signal amplitude being representative of the tomographic information; and

output the demodulated signal amplitude.

19. The system of claim 18 , wherein the digital signal processor computes a signal phase shift based on the filtered in-phase signal component and the filtered quadrature signal component, the signal phase shift being representative of the tomographic information.

20. The system of claim 18 , wherein K is between about 150 and about 160.

21. The system of claim 18 , wherein the sampling frequency, f s is between about 75 kHz and about 80 kHz.

22. The system of claim 18 , wherein the modulation frequency, f λi , is between about 3 kHz and about 9 kHz.

23. The system of claim 18 , wherein the modulation frequency, f λi , is given by m i *(f s /K), where m i is an integer between 1 and K/2.

24. The system of claim 18 , wherein the instrument transmits the demodulated signal amplitude to a host computer for display and storage.

25. The system of claim 18 , further comprising a low-pass anti-aliasing filter configured to filter the analog signal prior to converting the analog signal into the digital signal.

26. The system of claim 18 , further comprising a trans-impedance amplifier configured to amplify the analog signal prior to converting the analog signal into the digital signal.

27. The system of claim 18 , further comprising a programmable gain amplifier configured to amplify the analog signal prior to converting the analog signal into the digital signal.