Abdominal Wall Vibration Analysis for Evaluation of Biomechanical Properties and Physiological Diagnosis of Internal Abdomen

The human abdominal cavity is an interesting part of the human body, containing a lot of important organs which, due to relative soft boundaries, are quite accessible for examination. Conditions like mechanical bowel obstruction, obstruction of the inferior vena cava, abnormalities of the viscera and existence of abdominal aortic aneurysms can be detected using physical examination. This technique involves ‘feeling’ for abnormal mechanical behavior, unusual masses or pulsation sources, and is basically a manual assessment of mechanical properties and excitation sources. Changes in mechanical properties change the feel, but also changes the dynamical properties of the abdominal system. As such, it can be expected that changes in mechanical properties or changes in excitation sources should also cause an alteration in system dynamics, and should be visible as a change in vibration pattern of the abdomen. This provides an quantifiable parameter which can be measured and monitored. Previous research showed that it was possible to use a Laser Doppler Vibrometer to detect vibration patterns on the skin surface above the carotid artery and the thoracic cage, and it is assumed that this approach can also be applied in a scan of the abdominal surface. A measurement setup is designed to measure the cardiac induced vibration of the abdomen. An ECG monitor registers heart activity and is used to synchronize the measurement with the QRS complex of the heart. An commercial Laser Doppler Vibrometer is used to measure the vibration on a grid consisting out of several dozen of reflective stickers placed on the abdomen surface. By using the QRS complex as a trigger, a sequential scan passes along the defined gridpoints and measures their velocity over a sample period of a few seconds. Afterwards the sequential data is used to generate an approximate parallel scan of the surface. By including a Finometer into the setup it is possible to monitor the brachial blood pressure waveform and to define frequency response functions between the input blood pressure and output skin vibrations. The measurements reveal that there is indeed a repeatable vibration pattern present at the abdomen, which synchronizes with heart rate. Although the velocity of this vibration pattern is affected by respiration, its effect can be reduced by applying averaging over multiple scans, making it is possible to obtain a clear pattern which synchronizes with the cardiac cycle and is most likely generated by pulsation in the abdominal aorta and possibly by other arteries. It is also observed that the vibration pattern differs in shape depending on location on the abdominal surface. Gridpoints located above the abdominal aorta show a clear cardiac pulse, even appearing to show the wave velocity of the pressure pulse, while gridpoints positioned further away show an increased complex signal and a significant reduction of vibration amplitude. Several different data processing techniques are tested. It is found that the vibration patterns show quite some detail, and its is necessary to discriminate between measurement location. Frequency response functions between bloodpressure waveform and skin vibrations are defined and appear to show the presence of dynamical behavior. Accompanying coherence functions show quite high values of coherence up to 10 Hz, but this tends to vary between measurements. This suggests that, although maybe not on all occasions, abdomen vibration is primarily caused by blood pressure waveform.