To calculate the Signal-to-Noise Ratio (SNR) in a single image using MATLAB, you can follow these steps:

1. Read in the image using the imread function:

img = imread('image_file.jpg');

2. Convert the image to double precision:

img = im2double(img);

3. Calculate the mean signal value in the image using the mean2 function:

signal_mean = mean2(img);

4. Calculate the standard deviation of the signal in the image using the std2 function:

signal_std = std2(img);

5. Calculate the mean noise value in the image by subtracting the mean signal value from the overall mean value:

noise_mean = mean2(img) - signal_mean;

6. Calculate the SNR using the formula:

SNR = 20 * log10(signal_mean / noise_mean);

This formula is in units of decibels (dB), which is a logarithmic measure of the ratio of the signal power to the noise power.

Here's the complete code:

img = imread('image_file.jpg');
img = im2double(img);

signal_mean = mean2(img);
signal_std = std2(img);
noise_mean = mean2(img) - signal_mean;

SNR = 20 * log10(signal_mean / noise_mean);

Note that the actual SNR value will depend on the specific image and the imaging conditions used to capture it.