An automatic procedure to convert ECGs from graph paper to digital signals

Today most commercial equipment already provides ECG signals in a digital form and allow their registration in a local archive. However, the recovery of past ECGs from graph paper is still needed since ECGs are a valuable source of information regarding the clinical history of a patient and, consequently, it is very useful to have all the ECG recordings of the same patient in the same digital archive for subsequent analyses and comparisons. In this paper we propose a new method to convert ECGs from graph paper to digital signals. First a digital image of the ECG on graph paper is obtained with a flatbed scanner (600 dpi) and then a line detection procedure is used to detect and locate the ECG trace. The first order absolute moment (FOAM) is used as a mathematical operator to highlight the ECG trace with a ridge and the ECG signal is subsequently located at the top of the ridge with a multi-resolution approach.