See Also: Brain Imaging and the Law


The Misapplication of Brain Scans in Toxic Tort Litigation

by Brickford Brown and Samuel Tarry


In recent years, two exciting and relatively new diagnostic medical procedures, PET and SPECT, have been propelled from the hospital into the courtroom, most typically by plaintiffs. As a result, many toxic and environmental tort defendants have faced the daunting task of explaining, challenging, and refuting the dynamic color images of plaintiffs’ brains provided by these computer-aided scans. Although untested by appropriate peer review in the realm of toxic torts, both PET and its less-expensive neuroimaging cousin, SPECT, do provide an indirect glimpse into the functioning of the human brain. The compelling visual effects virtually ensure a future proliferation of PET and SPECT scans proffered as evidence of brain damage.

However, the diagnostic value of these neuroimaging technologies is, at best, uncertain. Increasingly, lawyers and judges in toxic tort litigation will have to wrestle with this controversy. Recent experience suggests that the outcomes of these evidentiary battles may determine, more than any other aspect of trial, a defendant’s judicial fate. Until adequate testing and studies have been conducted, PET and SPECT should not be allowed as proof of toxic brain injuries.

The Procedures

PET—Positron Emission Tomography—measures metabolism. This is accomplished by injecting into the patient a radioisotope tracer which "tags" glucose in the bloodstream. The PET scanner detects gamma rays from the positron-emitting radioisotopes and generates an image of the variations in metabolism across different sections of the brain. Although this process obviously yields a snapshot of brain function, there is some debate as to what "function" is being measured. As Jennifer Kulynych has noted in "Brain, Mind and Criminal Behavior: Neuroimages as Scientific Evidence," 36 Jurimetrics 235 (1996), to rely on PET images as a measure of the brain’s cognitive activity requires one to assume that changing metabolic rates actually represent the degree to which different brain structures participate in thinking and reasoning.

The PET scanning process involves a needle, a patient, and a radiology technician. A typical protocol starts with the technician explaining the process to the patient and, if possible, to an accompanying family member. After discerning the patient’s medical history, the technician will have the patient lie down on a table. Often, this is done in a dimly lit room in an effort to achieve a relaxed state. The technician then inserts an IV and leaves the patient alone for ten to twenty minutes. When the technician returns, he or she injects the patient with the radioisotope. After another short wait, the IV is removed and the patient is asked to return for scanning within a short time period which is dependent upon the half-life of the particular radioisotope used.

The actual scanning looks and feels very similar to the more commonly performed types of x-rays. The patient lies on a table which slides into the middle of a circular scanner. With the patient’s head kept in still position, the PET scanner collects multiple images of the patient’s brain from different angles. The computer then processes the information collected and produces color-coded images of metabolism throughout the brain.

In 1996, there were 73 PET scanning sites in the United States, located mainly at university medical centers in the East and Midwest. Costs averaged approximately $1,000 per PET study. Because of the spectacular nature of PET images, toxic tort plaintiffs appear more and more willing to foot these bills in the hope of a larger reward. A review of recent litigation demonstrates that plaintiffs often solicit PET scanning after obtaining normal or "negative" results from more standard examinations such as Magnetic Resource Imaging (MRI). As will be discussed later, the ability of PET scans to aid in the diagnosis of chemically-induced brain disorders is unproven and unsettled in the medical community.

SPECT—Single Photon Emission Computerized Tomography—provides a cheaper alternative to PET scanning while achieving a similarly impressive visual effect. Because the spatial resolution of SPECT technology is inferior to PET, the resulting images are less exact, with the colors tending to blur together. Technically, SPECT measures cerebral metabolism only indirectly. It is viewed by clinicians as a cost effective tool for gauging regional cerebral "perfusion" or blood flow. Although SPECT technology was developed over twenty-five years ago, its use was limited until recently due to the lack of effective tracers. By the end of 1996, SPECT scanners far outnumbered PETs.

PET/SPECT in Toxic Tort Litigation

The most striking example of the persuasive power of brain neuroimaging evidence is Guilbeau v. W. W. Henry Co., 85 F.3d 1149 (5th Cir. 1996). In Guilbeau, a Louisiana jury returned a $2.9 million verdict to the plaintiff and his wife as damages for chronic toxic encephalopathy (dementia). The plaintiff’s condition, which was hotly disputed, allegedly resulted from a short-term exposure to carpet adhesive containing either sodium pentachlorophenate or pentachlorophenol. Following an $850,000 remittitur of the wife’s loss of consortium award by the trial court, and a subsequent appeal, the United States Court of Appeals for the Fifth Circuit reversed the verdict and rendered judgment for the defendant, holding that no rational juror could find the defendant’s carpet adhesive to be defective under Louisiana’s strict product liability law.

Plaintiff Guilbeau, a disabled mobile home salesman, allegedly suffered from repeated dizziness, headaches, weakness, loss of sleep, arm and wrist pain, fatigue, nervousness, itching, sweating, shaking, and hallucinations. Mr. Guilbeau’s psychologist, Dr. Lisa Morrow, described him as having a psychiatric disorder resulting from toxic exposure. According to the plaintiff’s wife, he was sickened by odors of all variety of substances, including shampoos and new clothing. Mrs. Guilbeau also described temper tantrums, and explained the necessity of a sign placed on the front door of their home which instructed visitors not to enter if they were wearing hair spray, colognes, powder, make-up, or new clothes. The sign thanked visitors for their efforts to help Mr. Guilbeau avoid "severe seizures." Id. At 1156 fn. 21. As the Fifth Circuit noted "amazingly," Mr. Guilbeau was fortunate enough to escape adverse effects both from the fluid in his cigarette lighter and from the tobacco in the two packs of cigarettes which he smoked daily. Id.

At trial, the plaintiff offered the testimony of Dr. Thomas Callender, a board certified internist. Dr. Callender testified as an expert in internal medicine, neurotoxicology, and occupational medicine. He was allowed to opine that Mr. Guilbeau suffered from severe chronic toxic encephalopathy caused by a two-week exposure to the adhesive. Dr. Callender based his opinion on the plaintiff’s reported exposure history, his reported hypersensitivity to smells, and a 1990 SPECT scan of his brain which showed decreased blood flow in the left frontal lobe, the left thalamus, and parts of the right basal ganglia. Plaintiff’s neurologist, Dr. Harper, concurred with Callender’s diagnosis in spite of the fact that his own neurological examination of Guilbeau rendered no abnormal physical findings aside from the plaintiff’s complaints of odor sensitivity. Like Dr. Callender, Dr. Harper’s opinion was based primarily on the SPECT scan, which showed abnormal areas of decreased circulation in parts of Guilbeau’s brain.

Rather than launch a pre-trial assault on the SPECT evidence through a Daubert hearing, the defendant chose to refute the SPECT scan through cross-examination and the testimony of its expert, Dr. Berger. On cross-examination, Dr. Callender was asked about his 1991 published article concerning the helpfulness of SPECT scans in diagnosing toxic encephalopathy. Coincidentally, Dr. Morrow, the psychologist, contributed to the article, which included a case study of Mr. Guilbeau. Dr. Callender admitted that the defendant’s adhesive did not contain many of the chemicals to which the article reported the plaintiff had been exposed. The article, "Three Dimensional Brain Metabolic Imaging in Patients with Toxic Encephalopathy," 60 Environmental Research, 295-319 (1993), subsequently has been cited by other experts attempting to support the proposition that SPECT scans showing decreased perfusion in the temporal and frontal lobes are consistent with toxic encephalopathy. Dr. Harper admitted on cross-examination that the SPECT scan was the "basic supporting" evidence of his theory of toxic encephalopathy along with the plaintiff’s history. Dr. Harper agreed that a SPECT scan is a measure of "blood flow" rather than a test of "function" and admitted that a SPECT could not establish the cause, date or duration of the decrease in perfusion. Dr. Callender, however, who was not admitted either as a neurologist or a radiologist, testified that he could infer actual brain damage on the basis of a SPECT scan. Original Brief of Defendant/Appellant W.W. Henry Co., at 19.

The defendant also pointed out that the 1990 SPECT scan, which was taken four years after the alleged exposure, showed decreased blood flow primarily in the left hemisphere. Other evidence demonstrated that the plaintiff had a fat-obstructed left carotid artery. An MRI of Mr. Guilbeau’s brain revealed no structural damage or abnormalities. Additional rebuttal evidence showed that a second SPECT scan, taken in 1993, demonstrated normal blood flow in the left temporal lobe, left thalamus, and right basal ganglia. Dr. Harper, who was unaware of the second SPECT until the time of his cross-examination, conceded that the remaining area of mildly abnormal perfusion in the left frontal area "would not explain most of the complaints that we had talked about." Id. at 20. The plaintiff’s psychologist, Dr. Morrow, further conceded that the article which she co-authored with Dr. Callender was, to her knowledge, the first case report of SPECT scan analysis of a patient reporting chemical exposure.

The jury was unmoved by defendant W.W. Henry Co.’s efforts to explain or minimize the SPECT evidence. Indeed, courtroom observers recall the color enlargements of Guilbeau’s brain SPECT as nothing short of spectacular. According to one courtroom viewer, the pictures were the centerpiece of Guilbeau’s case and "appeared to show that something was clearly wrong with the [plaintiff’s] brain."

On appeal, Henry asserted that Guilbeau had failed to present sufficient scientific evidence either of causation or actual brain damage. Part of this argument included an assault on the reliability of SPECT technology as a diagnostic tool for toxic encephalopathy. More specifically, Henry challenged the admissibility of Guilbeau’s experts’ conclusions in that they were based on medically unfounded inferences about the SPECT results. Henry rightly demonstrated the lack of medical studies on the issue and correctly pointed out that even the World Health Organization, which plaintiffs often cite as the oracle of toxic encephalopathy knowledge, has concluded that tests for cerebral blood flow measurements among workers exposed to toxic solvents are of poor diagnostic value. In his reply brief, Guilbeau did little more than reiterate that SPECT scans "are well recognized diagnostic tools," reference a TIME magazine article containing a SPECT picture of a patient with attention deficit disorder, and reassert the Callender article as valid support for his position. Original Brief of Appellees/Cross Appellants, at 17.

Because the case was reversed based upon the insufficiency of plaintiff’s proof of product defect under prior Louisiana law, there is little indication of how the SPECT argument was received by the Fifth Circuit. However, the written opinions suggest that even those on the bench may have a tendency to get caught up in SPECT’s state-of-the-art technology and overlook the void of medical and scientific support for its use in diagnosing toxic brain injuries. In his dissenting opinion, sitting Circuit Judge Reynado G. Garza flatly states his belief that Guilbeau carried his burden. He writes: "the SPECT scan showed areas of decreased brain function, which was consistent with toxic encephalopathy." 85 F.3d at 1174.

Judge Garza is not alone in his tacit acceptance of SPECT/PET evidence in toxic tort cases. In Hose v. Chicago Northwestern Transportation Co., 70 F.3d 968 (8th Cir. 1995), the court upheld the limited use of PET scan evidence as an appropriate means of eliminating alternative theories of injury advanced by the defense. The obvious problem with this type of holding is that it allows evidence of a differential diagnosis suggestive of causation. Simply put, a jury cannot be faulted for attributing liability to a defendant when PET scan evidence purports to discount all other potential causes of the plaintiff’s injuries. The prejudice is compounded when, as in Hose, a radiologist is allowed to testify that a PET scan is "consistent" with the plaintiff’s alleged diagnosis and the suggested causation. Id. at 973.

In Hose, the plaintiff sued the employer railroad pursuant to the Federal Employers Liability Act, alleging that he contracted manganese encephalopathy as a result of work place exposures to manganese fumes and dust between 1976 and 1991. Prior to litigation, Hose’s neurologist, Dr. Jan Golnick, scheduled a PET scan for March 4, 1992. The scan was interpreted by Dr. Naresh Gupta, a physician board certified in nuclear medicine. Over the objection of defense counsel, the court allowed the plaintiff to read into evidence Dr. Gupta’s deposition testimony concerning his interpretation of the PET scan. Id.

Dr. Gupta’s testimony described the plaintiff’s PET scan and explained why the images did not match the expected patterns for Alzheimer’s, alcoholism, and stroke. Dr. Gupta then testified that the plaintiff’s scan showed a pattern consistent with dementia and opined that the plaintiff’s "overall picture is very consistent with manganese encephalopathy and toxicity as the cause of this dementia." Appellant’s Brief, at 32. The court admitted Dr. Gupta’s statement even though his other testimony made it clear that the scientific literature had not adequately described how a manganese toxicity would appear on PET. As defense counsel pointed out, Hose’s PET scan showed a normal basal ganglia, the area most often affected by manganese poisoning. An apparent abnormality in the basal ganglia region detected in an April 1991 MRI had diminished by the time a second MRI was taken in October of that year.

Ultimately, the jury believed Dr. Gupta’s PET evidence and returned a $1.33 million verdict for Mr. Hose. On appeal, the Eighth Circuit concluded that the district court did not abuse its discretion by allowing Dr. Gupta to testify about the PET scan. 70 F.3d at 973. The court seemed impressed by the fact that Hose’s treating physician ordered the PET scan prior to the initiation of litigation, giving it the appearance of being more diagnostic than calculated to create evidence for court. The written opinion also included a reminder that challenges to the scientific reliability of expert testimony should be addressed prior to trial. Id. at 973 fn. 3. Ultimately, the court viewed Dr. Gupta’s body of testimony as relevant because it was limited to showing consistency with, not diagnostic proof of, manganese encephalopathy. Id. at 973.

Regardless of whether Dr. Gupta told the jury that the PET scan showed manganese encephalopathy or was "consistent" with it, his message was understood. The jury heard, and saw, evidence of what misleadingly appeared to be a thoroughly performed differential diagnosis, aided by reliable, high-tech equipment, which seemingly proved brain damage caused solely by manganese toxicity.

When given the time to review the literature and educate themselves about the limited application of PET and SPECT, courts are more apt to see through the transparent shield of scientific reliability. In Summers v. Missouri Pacific Railroad System, 897 F.Supp. 533 (E.D. Okla. 1995), the court examined SPECT technology in the scope of a pre-trial Daubert hearing to exclude expert testimony concerning multiple chemical sensitivity syndrome (MCS). Mr. Summers, along with co-plaintiff Potts, sued his employer for MCS pursuant to the FELA. The plaintiffs claimed to have been virtually disabled by exposure to diesel fumes while riding in the second locomotive of a four locomotive train on a two-hour trip taken June 5, 1993. Their alleged symptoms included headaches, memory loss, concentration difficulties, irritability, and fatigue.

Prior to trial, Missouri Pacific filed a motion to strike the testimony of plaintiffs' expert, Dr. Alfred Johnson, a specialist in "clinical ecology." Dr. Johnson diagnosed "chemical sensitivity" based upon a physical examination, the plaintiffs' work history, and the results of a Booth test, another procedure of questionable diagnostic value. Dr. Johnson later ordered a SPECT scan which was interpreted as demonstrating a mismatch between blood flow and function. Summers’s radiologist, Dr. Theodore Simon, described these "abnormalities" as "typical of that seen in patients with neurotoxic exposure." Dr. Frederick Bonte Dep., 18. As an instructional tool, Missouri Pacific submitted, in its motion to exclude, the deposition transcript of Dr. Frederick Bonte, a physician board certified in radiology and nuclear medicine. Dr. Bonte criticized the methodology employed by Dr. Simon in interpreting the SPECT scan, refuted his opinions, and discussed the lack of reliable scientific and medical data to support the use of SPECT technology to diagnose neurotoxic exposure. Id. at 24-35.

At the hearing, Magistrate Judge James H. Payne found himself confronted with misleading statements about the nature of PET/SPECT technology. For example, Summers’s attorney represented to the court that the SPECT scan provided objective evidence of causation. "I mean, this is an objective test. Yes, you can see the effects of what this material is." June 12, 1995 Hearing Tr., at 51. Fortunately, the Summers court reviewed the necessary materials from which to learn about the application of PET/SPECT and to make a proper ruling. The court ultimately granted Missouri Pacific’s motion to exclude plaintiff’s expert testimony based on the lack of reliable, scientific supporting evidence. In reaching its decision, the court rejected the SPECT test as an acceptable means of diagnosing chemical sensitivity.

The record reveals that both of these tests [SPECT and Booth] have been the subject of much criticism by the scientific community as not having met acceptable scientific levels of methodology and criteria, and are not designed to test for the recognized medical condition of chemical sensitivity.

897 F.Supp at 540.

In 1991, the federal district court in Montana granted summary judgment to Burlington Northern Railroad in a 27-plaintiff consolidated FELA action alleging injuries from chemicals. Eggar v. Burlington Northern Railroad Co., 1991 Westlaw 315487. The plaintiffs' evidence included PET images. Burlington Northern’s motion, heard pretrial, involved the admissibility of plaintiffs' expert testimony. Because the court granted the motion based on broader aspects of reliability, it did not reach the issue of whether PET scans offered appropriate diagnostic value in the cases before the court. Id. at p. 4 fn. 9.

Similar situations have arisen in other areas of toxic tort litigation. In 1991, Emma Jean Hando submitted a PET scan of her brain to an administrative law judge in an effort to establish disability pursuant to the Social Security Act, 42 U.S.C. 401-433. Ms. Hando's treating physician had concluded that the PET scan was compatible with a diagnosis of chemical sensitivity. The administrative law judge and the appeals counsel rejected Hando's claim; the federal district court in Wyoming and the Tenth Circuit Court of Appeals affirmed. Hando v. Shalala, 1993 Westlaw 423213, 13 F.3d 405 (10th Cir. 1993).

The Future of PET/SPECT

Although multiple chemical sensitivity syndrome has not been widely recognized by the medical profession, certain physicians are actively working to prove the disease and are employing neuroimaging as a means to do so. Among others, Dr. Gunnar Heuser has been working to establish before-and-after SPECT comparisons to demonstrate that MCS patients show less blood perfusion in the temporal, frontal, and parietal lobes following the inhalation of an offending substance. In his September 19, 1996 testimony before Congress, Dr. William E. Baumzweiger described Dr. Arthur Vento's SPECT scan research on Gulf War veterans. Dr. Baumzweiger stated that Dr. Vento has found persistent, if subtle, abnormalities in the orbitofrontal, right hemisphere, and paralimbic timbic cortices which he believes to be consistent with a brainstem-limbic system encephalitis. "The Gulf War Disorders: Hearings Before the House Committee on Human Resources and Government Oversight," September 1996 (statement of William E. Baumzweiger, M.D.). PET and SPECT scans also have been utilized by certain physicians investigating silicone toxicity in breast implant patients. A recent case report on fifteen implanted women purports to demonstrate abnormal SPECT scans showing decreased perfusion in the temporal lobes bilaterally indicating impaired memory. According to the abstract, repeated scans after the removal of the implants showed significant improvement in most patients. Shoaib, Abnormal SPECT Scans and Impaired Memory in Patients with Adjuvant Breast Disease (unpublished).

The increasing use of PET and SPECT scans by the medical community, coupled with an awareness of their evidentiary potential by lawyers, suggests that these scans will be used in greater numbers as evidence of toxic brain damage. The United States Supreme Court's decision in Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), with its perceived relaxation of standards for the admission of scientific testimony, has encouraged the use of PET and SPECT scans to substantiate claims of neurological injury due to toxic exposures. As discussed above, the courts, in certain instances, are permitting the limited introduction of this evidence despite the admonitions of the medical community that these methods, except in limited circumstances, have not reached the level of sophistication and reliability necessary to diagnose neurological or cognitive deficits.

It is well recognized that PET and SPECT scans can be used in the management of patients with stroke, epilepsy, brain tumors, and Alzheimers disease. Scan abnormalities have been recognized in patients with an array of conditions such as depression, schizophrenia, and cerebral vascular disease. While these techniques can be used to identify abnormalities of the brain, there is not, at this time, adequate evidence to support the confident use of PET and SPECT scans to establish causal relationships.

At best, the use of these techniques to establish cause-and-effect in the area of toxic torts is premature. Before such evidence is permitted, there must be accepted scientific evaluation to establish supportable patterns of abnormalities associated with specific conditions and their causes. This requires reliable, controlled studies of patterns and conditions to establish a database of consistent information to substantiate the relationships. Currently, sufficient data simply does not exist to support the findings and conclusions offered by plaintiffs' experts on the issues of causation and cognitive deficit.

In considering the admissibility of these neuroimaging techniques, it is necessary to determine whether the methods are technically adequate, whether a clear distinction can be made between "normal" and "abnormal," whether sufficient reliability and validity have been established, and whether science has demonstrated that the intended application can be effective. The PET/SPECT expert should be able to demonstrate to the court that the method has been adequately tested, based on substantial peer-reviewed publications. He or she should also be able to testify to the potential rate of error for the method as well as the existence and maintenance of standards for controlling its use for clinical applications. This is not possible in the context of the toxic tort case because the data on which the plaintiffs' experts rely is without sufficient scientific foundation.

Presently, the scans are so variable and subject to so many dependent factors that it is impossible for a witness to testify with intellectual honesty that the data currently available permits him or her to determine the cause of any abnormalities seen on the scans.

First, the equipment comes in different configurations: camera based and non-camera based. Of the non-camera based equipment there can be single or multi-head systems, each potentially giving slightly different resolutions. Generally speaking, the equipment is either high resolution or low resolution. This makes a difference in the quality of the scan and any subsequent interpretation. As an example, low resolution scanners can produce artifacts in an image which can influence the interpretation.

Similarly, the type of equipment employed must be appropriate for the type of abnormality being investigated. Low resolution equipment, which older scanners tend to be, may be appropriate for identifying a large stroke, but may not be appropriate for locating a temporal lobe seizure where high resolution is necessary.

Next, the quality of a specific scan is dependent on many factors, including: the equipment used, the filters used, the tracers used, the time between the tracer injection and the scan, the duration of the scan, the mental state of the patient during the scan, movement of the patient during the scan, audio and visual stimulation of the patient during the scan, the alignment of the scanner to the patient, variations in the processing and display of the scan, the method of reconstruction and analysis, and the overall quality control of the scan.

Additionally, the typical methods of interpreting scans are subjective and can vary from one interpreter to another, based on experience, expertise, and/or clinical bias. Subjective differences in interpretation can occur even between institutions, based on differences in equipment, institutional experience, and protocols.

Finally, and perhaps most importantly, findings are influenced by a lack of standardized definitions of "normal" and "abnormal," the lack of standardized quantitative analysis of the scan, the lack of published standards for disease pattern identification, and the lack of a recognized protocol for producing the scans to identify specific diseases. The most basic parameters for scan interpretation, the terms "normal" and "abnormal," do not have common and universal definitions. These "standards" must, however, be developed in order to build the kind of information storehouse which is necessary to support the diagnostic inferences currently being made by certain experts.

It is difficult, though, if not impossible, to reach agreement when the current data by which these terms would be defined has been collected from instruments of varying resolution without standardized scanning, reconstruction and interpretation protocols. This lack of appropriate controls ensures that there is inadequate underlying data on which the inferences drawn or reached by plaintiffs' experts can be supported.

The attempts at authentication of PET and SPECT scans also can be fertile ground for objection at trial by defense counsel. Generally, no single witness is expert in all aspects of producing and reading a brain scan. Neuroimaging is multifarious and oftentimes requires one image study to be reviewed by physicists, radiologists, psychiatrists, psychologists, neurologists, and biochemists. When, however, the image is offered as evidence at trial, the issue of diagnostic inference is usually addressed by only one witness, a neurologist or an internist, who has only limited knowledge of the technical aspects of producing an image, aspects which affect the image's appearance.


It is accepted that new medical diagnostic techniques eventually make their way into the courtroom. It is necessary, thought, for courts to adhere to procedural safeguards when scrutinizing testimony about these techniques. Danger lies in the fact that PET and SPECT scans have an alluring power. They appear as living photographs of the structure and function of the brain and over-simplify complicated data about the brain. These "photographs" of the brain suggest a portrait of scientific objectivity where none exists. In reality, PET and SPECT scans are more akin to charts and line graphs than to photographs. The admission of the accompanying testimony interpreting "objective" scans and linking blood flow or metabolism with a substance-induced injury is an insupportable and dangerous move by the courts.

The courts, therefore, must define the appropriate use of neuroimaging evidence. Legal procedure demands that courts rigidly apply the appropriate evidentiary tests to determine the admissibility of PET and SPECT studies. If the traditional standards for the admission of scientific evidence are used, there is no doubt that this evidence will be excluded. Until valid medical and scientific research establishes that PET and SPECT scans can demonstrate neurological and cognitive deficits, along with their causes, judges should keep such testimony out of the courtroom. The propriety of expert findings and conclusions of cognitive deficiencies resulting from exposure to toxic substances, based on PET or SPECT studies, is impossible to substantiate based on the current level of scientific and clinical knowledge.

Reprinted with permission from For the Defense, Vol. 39, No. 3, March 1997 Defense Research Institute, Inc.