It usually indicates a false positive. Attenuation is the decrease in intensity of the signal and it varies through tissues of varying densities such as the diaphragmatic tissue, chest wall, spine and breasts.
This results in an attenuation artifact whose extent varies with location of soft tissue, overall patient body size, and depth of target organ (heart). So attenuation artifact leads to a loss of diagnostic accuracy as artifacts may be confused with true perfusion abnormalities, resulting in an increase in false positives.
Hope this helps and if you have any furtheer questions you are welcome to respond. Thanks for the question, take care.