The paper describes comparative and verification measurements of three developed prototypes of sensors based on the photoplethysmography (PPG) principle for non-invasive acquisition of vital information about the cardiovascular system of an examined person lying inside the scanning area of the running magnetic resonance scanner. All three prototypes are assembled from non-ferromagnetic components and all their parts are shielded. The performed experiments verify practical functionality of all developed sensors for real-time continual pickup of PPG signal in the weak magnetic field up to 0.2 $T$ with the inherent radiofrequency and electromagnetic disturbance. Comparative measurements by commercial pulse oximeter and blood pressure monitor (BPM) devices show good stability and proper accuracy of subsequently determined heart rate values. The supplementary mapping of pressure effect of the inflated BPM cuff certifies necessity of placement of the pressure cuff of the BPM device on the opposite arm than the tested PPG sensor.