A new approach to physiologic triggering in medical imaging using multiple heart sounds alone

A new method for physiological synchronization of medical image acquisition using both the first and second heart sound has been developed. Heart sounds gating (HSG) circuitry has been developed which identifies, individually, both the first (S1) and second (S2) heart sounds from their timing relationship alone, and provides two synchronization points during the cardiac cycle. Identification of first and second heart sounds from their timing relationship alone and application to medical imaging has, heretofore, not been performed in radiology or nuclear medicine. The heart sounds are obtained as conditioned analog signals from a piezoelectric transducer microphone placed on the patient's chest. The timing relationships between the S1 to S2 pulses and the S2 to S1 pulses are determined using a logic scheme capable of distinguishing the S1 and S2 pulses from the heart sounds themselves, using their timing relationships, and the assumption that initially the S1-S2 interval will be shorter than the S2-S1 interval. Digital logic circuitry is utilized to continually track the timing intervals and extend the S1/S2 identification to heart rates up to 200 beats per minute (where the S1-S2 interval is not shorter than the S2-S1 interval). Clinically, first heart sound gating may be performed to assess the systolic ejection portion of the cardiac cycle, with S2 gating utilized for reproduction of the diastolic filling portion of the cycle. One application of HSG used for physiologic synchronization is in multigated blood pool (MGBP) imaging in nuclear medicine. Heart sounds gating has been applied to twenty patients who underwent analysis of ventricular function in Nuclear Medicine, and compared to conventional ECG gated MGBP. Left ventricular ejection fractions calculated from MGBP studies using a S1 and a S2 heart sound trigger correlated well with conventional ECG gated acquisitions in patients adequately gated by HSG and ECG. Heart sounds gating provided superior definition of the diastolic filling phase of the cardiac cycle by qualitative assessment of the left ventricular volume time-activity curves. Heart sounds physiological synchronization has potential to be used in other imaging modalities, such as magnetic resonance imaging, where the ECG is distorted due to the electromagnetic environment within the imager.