Non-Invasive Blood Pressure Device for Use in fMRI Imaging Applications

February 18, 2005

Jose AlvaradoBenjamin Huh

Sanjeet Rangarajan

Dr. André DiedrichDr. John Gore

Dr. Richard Shiavi

Students Advisors

Baroreflex Failure

Efferent autonomic activity controlling blood pressure is determined at the level of medullary brainstem nuclei.The input information is integrated in these brainstem nuclei, where it serves to determine the balance of efferent parasympathetic and sympathetic activity.

An essential portion of the input information comes from the baroreflexes, whose function is to maintain arterial pressure within a narrow range.

The baroreflex as a system can potentially be damaged at any site (e.g. aortic arch, carotid arteries). Failure of the baroreflex at any point produces a characteristic clinical syndrome of volatility of blood pressure and heart rate

The incidence rate of this disorder is extremely small.

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

Item Quantity Cost per Item Total USD

Fiber Optic Cable 15 feet $1/foot $15.00

LEDs 4 $5-$10 $40.00

Air Piping 15 feet $0.50/foot $7.50

Photodetector 1 $25 $25.00

Rubber Insulation 2 square feet $3/foot2 $6.00

Inflation Bag for Cuff 1 $5 $5.00

Hydraulic Supplies Varies $10 $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.

Finished and refined hydraulic system twice, ready for final testing.

Current Work

Complete and test fiber optic transmission system.

Construct mounts for the hydraulic system that will keep the system at a consistent height to eliminate hydrostatic pressure.

Test the whole system in the fMRI lab for accuracy.

Finish writing final paper and construct the poster presentation.

Future Work

Test the whole system in the fMRI lab for accuracy.

Finish writing final paper.

Construct the poster presentation.

Questions

Questions?