Pulmonary edema -- an abnormal accumulation of water in the lungs -- may someday be routinely monitored at a patient's bedside with a new device being developed at LBL. Called a lung densitometer, the device is designed to safely, quickly, and accurately determine fluid concentrations in the lungs through the detection of scattered gamma radiation.
No clinical instrument exists today for accurately monitoring pulmonary edema, a serious disorder that often results from congestive heart failure. Doctors must rely on chest x-rays which, among other shortcomings, cannot detect small changes in lung fluid concentrations. Also, whereas the treatment of pulmonary edema depends upon detection at the onset, conventional chest x-rays are not safe to be used for frequent monitoring.
Dr. Daniel Simon, a staff member of the Cardiovascular Research Institute at UC San Francisco, spent years attempting to apply gamma rays to the problem before coming to Billy Loo and Fred Goulding (now retired) of LBL's Engineering Division. His concept of the medical needs led them to develop an inexpensive clinical lung densitometer that would be accurate and noninvasive, give real-time results, and be easily portable for bedside use in hospitals or in outpatient clinics.
"Ideally, we wanted something that would be as convenient as measuring a person's blood pressure," says Loo. "If our instrument lives up to its promise it should be very useful, because as modern medicine helps to increase our life span, a significantly larger percentage of hospital patients are suffering from pulmonary edema as a consequence of heart disease."
Loo and Goulding based their technique on Compton scattering -- the scattering of gamma ray photons after collisions with electrons -- because it reveals the absolute density of a sample, which is a good measuring stick of the sample's water content. All previous attempts at using Compton scattering to measure lung density were based on total scattered counts and were thwarted by the chest wall surrounding the lung. Chest walls -- a mixture of bone and dense tissue -- vary so much in thickness and composition that determining their contribution to the total scattering count is extremely difficult.
The LBL lung densitometer avoids the complications of the chest wall by sending a pencil-sized collimated beam of gamma rays through the chest and detecting the number of gamma rays scattered at different points along the path of the beam.
"This yields a spectrum of scattered count rate per unit of distance along the incident beam," says Loo. "The slope of the counting curve, corresponding to a limited region in the lung, is proportional to density."
By using the slope instead of the magnitude of their counting curve, Loo and Goulding do not have to worry about the chest wall.
"Even though the wall will attenuate the overall scattering count, the slope of our spectrum remains the same," says Loo. "Furthermore, unlike scattering or transmission measurements that are based on absolute counts, we do not have to keep close track of the source decay."
The lung densitometer starts with a gamma ray source capable of generating a monoenergetic beam with an energy between 100 and 200 keV (thousand electron volts) -- the range in which Compton scattering is the dominant interaction in tissue. At the recommendation of Eddie Brown, with LBL's Isotopes Project Group, Loo and Goulding have been using a tellurium isotope, Te-122, which is encapsulated in quartz and activated to Te-123 by thermal neutrons in a high flux reactor. The source is placed inside a tungsten holder and closely coupled with a high- resolution germanium detector that was designed and fabricated at LBL. This compact configuration increases counting efficiency and minimizes the effects of multiple scattering, which means exposure times and the amount of radiation needed are substantially reduced.
"It takes us about one minute to make our measurements and the radiation risk is less than a thousandth that of a chest x-ray," says Loo. Also, unlike chest x-rays, which must be processed and then interpreted by a radiologist, the lung densitometer's data is instantly analyzed by computer and immediately available for diagnostic use.
Loo and Simon have been testing the densitometer using commercial "anthropomorphic phantoms" -- plastic models that closely mimic human tissue in the absorption and transmission of radiation -- into which they stuff foam "lungs" of known densities. Through these tests, they have been able to correlate the slopes of their measurements to specific gravity, which is the ratio of lung density to the density of water.
"We know that the specific gravity of a normal lung is between 0.2 and 0.3," says Loo. "But we need to determine the norm and variations that we can expect from a group of healthy volunteers."
To date, the beam has been directed through the lower portion of the right lung in order to avoid the heart and any other organ that might interfere with the scattering signal. One of the next steps, Loo says, will be to determine if this is indeed the best approach. Other optimal operating conditions must also be decided before the device is tested on healthy human volunteers in order to establish a reference baseline. A collaboration with the University of California at San Francisco is already in the planning stages for using the lung densitometer on actual patients once such a baseline has been established.
Loo expects the necessary refinements and testing on human subjects to be completed during the next twelve months. He says he thinks a prototype could be ready for testing in a hospital within a year after that. Companies interested in collaborating on the production of such a prototype are currently being sought by the Technology Transfer Office. LBL holds the patent rights to this invention.