Ergonomic Flexible Drivers for Hepatic MR Elastography

Introduction: Normal liver tissue is soft when assessed by palpation, whereas cirrhotic livers are typically very hard [1]. Liver stiffness is a very sensitive and specific biomarker for staging liver fibrosis in chronic liver disease patients, which has been proved by a number of elastographic techniques such as ultrasound elasticity imaging and Magnetic Resonance Elastography (MRE) [2-6]. Emerging data further suggest that increased liver stiffness occurs even before collagen deposition takes place[7], implying that tissue stiffness change is a prerequisite for injury responses such as stellate cell activation [8], therefore presymptomatic diagnosis of fibrosis could be possible. At our institution, an average of 2-3 hepatic MRE scans are prescribed every day for evaluating liver fibrosis in patients. This allows for a systematic approach to determine which patients would benefit most from liver biopsy referral [8]. To measure liver stiffness, MRE uses a mechanical transducer called a driver to send low-frequency mechanical waves into the liver from the body wall. The resulting propagation of the waves within the liver is imaged by an MRE sequence. Based on different assumptions about the tissue properties, MRE inversion algorithms are applied to the wave images to estimate liver stiffness [9]. Electromechanical drivers [5, 6] and pneumatic drivers [4] are widely used for in vivo human hepatic MRE. Mechanical drivers are usually small and rigid, therefore the human-driver mechanical coupling is not optimal since human bodies are soft, large and contoured; they have the potential of causing discomfort to patients. Our pneumatic driver system [4] uses the air in a flexible PVC tube to transmit acoustic energy from an active driver to a passive driver, which is coupled to the anterior chest wall close to the liver via an elastic strap around the patient. The current hepatic passive driver is 19 cm in diameter, has a rigid structure and therefore has the similar limitations, especially for female patients. Our goals were 1) to design an ergonomic flexible passive driver to improve human-driver mechanical coupling and patient comfort; and 2) to compare the flexible driver with the rigid driver on volunteers and patients.