The Electrical Impedance Imaging Project at Rensselaer has developed a series of non-invasive medical imaging devices, Adaptive Current Tomographs (ACT), which create images of a patient based upon the naturally varying conductivity of the body. The ACT instruments use a technique known as impedance imaging. The generated image is a picture of how well electric currents can travel through the subject. The word "impede" means "to hold back" ; impedance is a technical term for how much electric current a substance (such as human tissue) holds back at a given time.
For example, the heart is full of blood. Blood conducts electricity well, so the heart has a low impedance. The lungs are filled with air. Air does not conduct electricity well, so the lungs have relatively high impedance. Figure 1 below shows the cross-section of a human chest. As the viewer, you are facing the patient, looking down from above. Blue indicates low impedance, red indicates high impedance.
Notice the dark blue spot in the lower center - that is the heart. The two red oval shaped objects are the lungs.
The previous image is a single snapshot of a series, one frame of many. The third ACT instrument, ACT 3, produces images at 20 frames per second. Below is a QuickTime video of ACT 3 monitoring a patient (actually, a patient grad student) as he breathes.
The right hand circle displays the relative changes in impedance, while the left displays the total impedance. Watching the right circle, you will see two regions change from red to blue and vice versa. These are the lungs, which become blue as the amount of air in them increases and red as the patient exhales.
If you are running a Quicktime Enabled browser, such as Netscape 3.0, it should eventually show up above. Otherwise, you can download the movie here (1.9 Mb) and get QuickTime here.
The Impedance Imaging Laboratory is a leader in the development of this technology and its application to clinical problems. This team has built hardware and software needed to test these systems, explore their ultimate limits, and test them in several clinical applications.
Before the ACT systems could be tested on live graduate students (a cheap but finite resource), many test apparatus were built. The test tanks are circular and saline-filled, with electrodes at their periphery. Figure A1 is a photograph of a two-dimensional tank, 30 cm in diameter, with thirty-two 2.54 cm square stainless steel electrodes on its inside surface. There are two lung-shaped structures and a heart-shaped structure made of agar immersed in saline. The lower right figure is a color resistivity image reconstructed from data obtained from this test phantom, while the lower left sketch shows the impedance values of the agar structures.
To test the system in three dimensions, a cylindrical tank 30 cm in diameter and 72 cm high was constructed, with a row of thirty-two 2.54 cm square stainless steel electrodes on its inside surface. A three-dimensional agar casting of lungs was made from sequential MRI images of the chest of a normal subject. These lungs were suspended in the tank with their center at different levels with respect to the electrode plane. Final images were made by subtracting the image of the homogeneous tank with no lung present from the images with the lungs in place.
Beyond the technical challenges of fabricating the ACT series hardware, the team maintains a strong theoretical base. The laboratory pioneered Isaacson's discovery of an adaptive algorithm to determine the best current patterns to apply to a patient. This work has spawned a number of US Patents.
This interdisciplinary research and development effort may result in improved impedance imaging for a variety of disciplines: materials testing, manufacturing processes, chemical processing, and many biomedical and clinical applications.
For Questions and comments, please send an email to newelj at rpi dot edu.