P. Schneider, B. Bosinski, A. Trimper, K.W. Oh
University at Buffalo's Sensors & MicroActuators Learning Lab (SMALL),
Keywords: acoustic impedance, subdermal imaging, test phantom, ultrasonic imaging, ultrasonic material characterization, vascular compliance
Summary:This paper reports the creation of an ultrasonically equivalent test phantom arm. The phantom arm replicates the key physiological features of a human arm in terms of an acoustic and mechanical standpoint. As shown in Figure 1., the physiological feature set includes; arteries, blood cells, bone, muscle, fat, fingerprints, skin, and veins. Mechanical aspects include heart pumping mechanics, vascular compression and distention, and elasticity of tissue layers. Designed with the purpose of being used to evaluate, analyze, and tune the performance of an ultrasonic blood pressure wearable healthcare sensor, this test phantom allows for a number of physiological functions to be measured. These functions are blood pressure, heart rate, blood flow, subdermal tissue and vascular imaging, and vascular compliance features. Test phantoms can offer controllable parameters for measurements and simulate real world conditions in a controlled testing environment. It also offers known constant parameters like arterial cross sectional area, skin layer thickness, heart rate, and internal fluid pressure. The phantom was broken down into two major components: the physiological and anatomical replication of the human arm and the material properties challenge to mimic the acoustical and mechanical aspects of an arm. Using an ultrasonic oil test bath, two polyvinylidene fluoride (PVDF) thin film ultrasonics transducers, an Olympus 5072 pulser-receiver, and a Tektronix MDO 3024 oscilloscope, a variety of materials were characterized acoustically using techniques as discussed in . The key acoustics aspects measured were longitudinal velocity, density, acoustic impedance and signal attenuation, the results of which can be seen in Table 2. The materials were then matched to known biological parts of the human arm based upon their acoustic properties (Table 1). Once fabricated, the phantom was imaged using a TeleMed SmartUS portable ultrasound machine along with single element fixed/plane focused transducers of varying frequencies of 2.5MHz, 5MHz, 10MHz and 20MHz. The results of the ultrasound imaging can be seen in Figure 2b, 2c, 2d where the artery was measured in terms of heart rate, vascular distention and compression, and blood flow rate, respectively. The ability to accurately measure these parameters (heart rate, pulse wave velocity, arterial cross sectional area, and arterial compression/distention) within the test phantom yields the derivation of vascular compliance and ultimately blood pressure . Validating the realism of the skin, fat, muscle later of human tissue, an A-Scan was performed using a 10MHz fixed focused transducer and the reflections were measured and analyzed as shown in Figure 3. This showed that the three layers of tissue with three different thicknesses could be imaged, identical to what would be seen in a human arm. The thicknesses acoustically measured were directly related to their physical thicknesses. Knowing the propagation time and compressional velocity of the material, we calculated the theoretical and experimental thickness of each layer, the results of which showed concurrence with one another.