Design of a respiratory indicating and triggering unit
for MRI in small animals

team members: Amit Padaki, Jacqueline Papapietro, Ryan Stokes

sponsor: Dr. Andrew Webb, Professor of Bioengineering, Penn State University

Executive Summary

Respiratory motion blurs MRI images of abdominal organs such as the liver, kidneys, and stomach. When a human has images taken in an MRI, the patient is asked to hold his or her breath while the image slices are being taken in order to reduce movement caused by breathing. However, this breath-hold technique is not applicable for animal subjects. In order to obtain clear images, it is necessary to acquire each image slice at the same point in the breathing cycle of the animal.

The objective of this project is to develop a non-invasive respiratory sensor for small animals that triggers an MRI to take an image slice at the same point in each respiratory cycle.

Design Criteria

•  MRI compatible

•  Rapid sensor response (<10 ms delay)

•  Biocompatible adhesives and materials

•  Output a 5 V trigger signal

•  Less than $1000 total cost

Design Overview

The design utilizes a simple circuit consisting of a resistor, LED, and a physical switch. The switch consists of one lead placed on the animal, with other lead positioned directly above on a custom-made support beam. The beam is made out of acrylic and screws into holes on the end of the animal tray used in the MRI system. The device is versatile, with vertical and horizontal sliders for precise calibration. When the animal inhales, the lead connect and complete the circuit, producing a 5 V signal. The final design contains the LED that is lit when the circuit is complete.

   
left: Acrylic support arm and stationary lead; right: elastic belt with copper tape for animal-lead interface

       
left: custom-made attachment to tray; right: completed project box

Alternative Design

Piezo film produces a voltage when pressure is applied to it. Here, it was used to detect the respiration cycle of the animal. The signal was put through an amplifier circuit and a reference voltage circuit. The output signal is 5 V when the amplified voltage from the PVDF film is above a certain threshold value, and 0 V when it is below this threshold. The adjustable reference voltage circuit allows the threshold value to be calibrated to each animal.

The sample output of the circuit (shown below) consists of the amplified voltage waveform on the top and the final output on the bottom. It can be seen that there is only a 5 V signal output when the waveform reaches a certain threshold value. When the voltage is below this threshold, the output remains at 0 V.

   
left: PVDF piezo film; right: sample output

above: circuit diagram

Budget