Non-Invasive Blood Pressure Device for Use in fMRI Imaging Applications February 18, 2005 Jose Alvarado Benjamin Huh Sanjeet Rangarajan Dr. André Diedrich Dr. John Gore Dr. Richard Shiavi StudentsAdvisors

Background Functional Magnetic-Resonance Imaging (fMRI) has recently allowed novel insights into the function of individual brain sites. Patients with baroreflex failure have extremely labile blood pressure due to loss of buffering function of blood pressure control. Higher centers of the brain stem and cortical structures may have potentiating effects on changes in autonomic outflow. Measuring blood pressure continuously during fMRI procedures could provide numerous benefits to the study of autonomic disorders.

The Penaz Measurement Technique If an externally applied pressure is equal to arterial pressure at all times, the arterial walls are unloaded and the photoplethysmogram will be constant. The Finometer uses two methods to determine the set point, which represents the unloaded blood volume.

The Penaz Measurement Technique This set point is used in a servo-loop, so as the measured photoplethysmogram varies from the set point a servo-valve is driven to increase and decrease the cuff pressure to maintain the set point. A fast servo-valve allows cuff pressure to equal arterial pressure throughout each cycle. The cuff pressure is measured with a transducer and the resulting signal is displayed as the arterial pressure.

Existing Schematic

Set Point Criteria Two methods Servo start-up adjustment method Steps cuff pressure upward and interprets magnitude and shape of photoplethysmogram at each step. When cuff pressure is between systolic and diastolic pressures the pulsations of the photoplethysmogram become maximal. Servo self-adjustment method Provides a fine-tuning of the set point and corrects for slowly changing physiologic conditions in the finger. Occurs periodically throughout measuring time as often as every ten beats, and as infrequent as 70 beats.

The Problem Current commercial devices are able to continuously measure blood pressure but not in the presence of magnetic fields. The electrical sensor system for the finger cuff as well as the pneumatic pump interfere with the highly sensitive fMRI magnet leading to distortion of the MRI images. Also, some components of the cuff are made up of ferromagnetic materials are dangerous when placed in close proximity to an fMRI machine.

The Problem (Setup)

Market Analysis Use for only in research and hospital settings. Large market potential because of applications in other MRI and fMRI studies. An optical continuous non-invasive blood pressure measuring device could be used in conjunction with electromagnetic trackers which are used in image guided surgery. Profit will come from selling the design of the fMRI compatible finger cuff to pre-existing companies that manufacture the cuffs. Perhaps too specific of an item to be able to market the device independently and for this reason it could be a better to sell the design to a current company.

Our Solution Retrofit finger cuff blood pressure devices to use optical transmission techniques instead of electrical transmission techniques. Replace electrical cables and sensors with optical components: Fit cuff with an optical cable terminator. Develop an fiber-optic interface compatible with existing commercially available electrical systems. Design fMRI compatible shielding. Extend length between cuff and electrical components without losing pneumatic function. Keep all electrical components that may cause potential interference in the observation room.

Hydraulic System

Estimated Design Cost ItemQuantityCost per ItemTotal USD Fiber Optic Cable15 feet$1/foot$15.00 LEDs4$5-$10$40.00 Air Piping15 feet$0.50/foot$7.50 Photodetector1$25$25.00 Rubber Insulation2 square feet$3/foot 2 $6.00 Inflation Bag for Cuff1$5$5.00 Hydraulic SuppliesVaries$10 Total Cost: $108.50

Completed Tasks Tested pneumatic extension with Finometer and found a maximal extension length of 6 ft using an air-only system. Dissected finger cuff. Visited 3T fMRI scanning facility and measured required extension length (13 feet, 3 inches). Met with Dr. Jansen to discuss strategy for fiber optic transmission. Procured parts for hydraulic extension system.

Current Work We have been researching the use of fiber optics to be able to transmit the blood pressure signal with the aid of Dr. Duco Jansen. Pneumatic extensions from the Finometer have failed when extended past 6 ft due to extensive dampening in the lines. We have decided to pursue a hydraulic method of pressure transduction between the pump and cuff.

Future Work Build fiber optic linkage Purchase fiber optic components after verifying the ones that are needed through work in Dr. Jansen’s optics lab. Test hydraulic system for accuracy and precision upon completion of prototype. Test entire system during 3T fMRI procedures.

Questions Questions?