Yes, it is possible for a dog to go into heat if she has been spayed. This is only true if the dog had her tubes tied versus having a whole ovariohysterectomy, which removes all of the female reproductive organs.
A female dog is traditionally spayed by a licensed veterinarian when she is between the ages of 2 and 6 months. The average age for a spay is 4 months. It is common to remove both the uterus and the ovaries during this operation. Spaying a female dog helps to prevent unwanted pregnancies. Spaying is also useful for preventing ovarian and breast cancer.