Patent Publication Number: US-2023148942-A1

Title: Detection of concussion using cranial accelerometry

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 14/788,683, filed Jun. 30, 2015 which claims the benefit of U.S. Provisional Application No. 62/019,280, filed Jun. 30, 2014. The application is also a continuation-in-part of U.S. patent application Ser. No. 14/565,337, filed Dec. 9, 2014, now U.S. Pat. No. 10,092,195, issued Oct. 9, 2018 which is a continuation-in-part of U.S. patent application Ser. No. 11/894,052, filed Aug. 17, 2007, now U.S. Pat. No. 8,905,932, issued Dec. 9, 2014 which claims the benefit of U.S. Provisional Application No. 60/838,624, filed Aug. 17, 2006. The aforementioned applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns noninvasive detection of anomalies of the brain, and in particular the invention is concerned with detection of concussion in a patient. The equipment and the method of the invention investigate skull motion produced by pulsatile cerebral blood flow, as measured by cranial accelerometry using one or more accelerometers attached to a patient&#39;s head. 
     Sports impacts are only one source of concussion. 18% of concussions are pediatric, 0-4 years of age. Another important segment is persistent TBI that accounts for up to 15% of all TBI. Persistent TBI does not resolve within 90 days and in some cases never resolves. Separating persistent TBI from PTSD is important to direct therapy for the symptoms. 
     Concussion has undergone a reevaluation in recent years with only loss of conciseness being the diagnosis of a concussion, to a list of symptoms providing the diagnosis to neurophysiological measures of concussion. So older data on the number of concussions sustained in a contact sport may be under reported by an order of magnitude or more. Approximately 300,000 sports concussions are reported annually but most authorities believe this is vastly under reported due to lack of awareness, misunderstanding of the definition of concussion and more recent realization that even what was considered minor head impacts previously are in fact concussions. Football in particular has received much media attention because of the frequency of concussions and other forms of traumatic brain injury (TBI). 
     A concussion is “a complex pathophysiologic process affecting the brain, induced by traumatic biomechanical forces.” It is caused either by a direct blow to the head or by a blow to the body with forces transmitted to the head. A concussion results in neurological dysfunction that is usually transient and resolves spontaneously. The majority of concussions do not result in loss of consciousness; however, they result in clinical and cognitive symptoms that commonly resolve in a sequential course. Importantly, no abnormalities are seen acutely on standard neuroimaging (computed tomography and magnetic resonance imaging (MRI)). Standard neuroimaging may be useful to evaluate for skull fractures, extra-axial bleeding, and contusions; however, these are unlikely to be present after concussions, given the likely microstructural nature of this injury. Functional magnetic resonance imaging (fMRI), which measures changes in blood flow during brain activation, shows potential in the evaluation of concussion. However, fMRI is expensive, time-consuming, not applicable when metal such as orthodontic braces are present and is not easily transportable or widely available. Multimodal MRI studies, notably including diffusion tensor imaging to detect axonal pathology and susceptibility weighted imaging to detect micro-hemorrhage, have been proposed as relevant imaging modalities. Dysfunction in cerebrovascular blood flow and auto-regulation, as demonstrated by fMRI and multimodal MRI, may one day be considered an objective definitive diagnostic measure of concussion. 
     Given the lack of a validated definition, or of proven physiological diagnostic testing or imaging criteria, concussions in clinical practice are not diagnosed or followed to resolution by objective findings or imaging, but rather by the constellation of neurocognitive signs and symptoms that commonly accompany mild TBI. The list of these signs and symptoms is long, and many of them are included in the symptom score of Sport Concussion Assessment Tool 2 (SCAT2), and now SCAT3. Other tests used to identify and follow concussions include Standardized Assessment of Concussions, Balance Error Scoring System, and Immediate Post-Concussion Assessment and Cognitive Testing. 
     Because less-than-optimal recovery or repeated concussion potentially leads to worse clinical outcome, a technology to objectively detect concussion is needed. In TBI, cerebral autoregulation can be impaired and persons can develop brain edema. This perhaps affects other physical properties of brain tissue and adjacent structures. A new physiological phenomenon developed pursuant to the invention uses highly sensitive accelerometers to detect movement of the human skull during a normal cardiac cycle. Because pulsatility of cerebral blood flow is transmitted through tissue to the skull surface, we sought to measure this phenomenon using highly sensitive cranial accelerometers in resting subjects suspected of having suffered a concussion, and therefore perhaps by measuring cerebral autoregulation dysfunction or subtle amounts of brain edema. 
     SUMMARY OF THE INVENTION 
     With the method and system of the invention, brain concussion is detected in a human patient by detecting and measuring natural motion of the patient&#39;s head due to blood flow in the brain and resultant movement of tissue in the brain. 
     At least one accelerometer (and preferably at least two) is attached to the patient&#39;s head such that the accelerometer measures the acceleration of the head where attached. Acceleration of the skull is detected and measured using the accelerometer, and these measurements are sent to a computer. The data include frequency and intensity of vibrations of the skull at the location of the accelerometer, preferably in a frequency range of about 1 to 20 Hz. 
     The data from the accelerometer are analyzed and investigated. If the data show an increase in frequency content of skull motion in selected frequency ranges above about the fourth harmonic of the patient&#39;s heartbeat, as compared with data corresponding to non-concussion, this is taken to indicate probable concussion. 
     The acceleration of the skull can be measured with apparatus other than actual accelerometers. Laser-based interferometers, including fiber-based interferometers can be used to measure movement at selected parts on the skull, and from these measurements acceleration intensity and frequency can be determined. 
     Importantly, the system and method of the invention do not require baseline data for the particular patient being tested. A graph of the data, either in the time domain or in the frequency domain, will reveal, in a patient having concussion, a particular pattern that will not be seen in a graph of data from a non-concussed patient. The pattern indicating concussion can be observed visually by a trained physician or technician, or the data can be analyzed using an algorithm operated by a computer. 
     The algorithm can be in several forms, but primarily it will detect, as noted above, an increase in energy of skull motion in frequency ranges above about the fourth harmonic of the patient&#39;s heartbeat. In a mathematical algorithm these data need to be “normalized”, and in one form of algorithm the averaged energy of, for example, the fifth and sixth harmonics of the heartbeat is compared against (i.e. divided by) a maximum value of one or more or an average of several of the lower harmonics, below the fourth harmonic. This ratio, which is often called herein R.sub.1, is compared against a selected threshold. Such a threshold, in one embodiment of the invention, is set at 1.0, but different thresholds can be selected, based on desired levels of sensitivity and specificity. The higher the threshold for R.sub.1 is set, the lower the sensitivity but the higher the specificity, and vice versa. 
     The waveform is obtained by averaging data over a number of heartbeats, which can be, for example, about 45 heartbeats, or a range of about 40 to 60 heartbeats. 
     Preferably another ratio factor is also included in the described algorithm, which can be called R.sub.2. R.sub.2 represents “normalized” data from the eighth and ninth harmonic peaks. The average value of those peaks is also divided by a maximum or average of one or more of the lower harmonics, the same denominator used for R.sub.1. In one embodiment of the algorithm the R.sub.2 threshold is set at 0.66. Thus, concussion is indicated if R.sub.1.gtoreq.1.0 and R.sub.2.gtoreq.0.66. Concussion is contra-indicated in the case where both R.sub.1 and R.sub.2 are below the set thresholds. 
     An important aspect of the invention is the ability to detect concussion early. Typically peak data exceeding thresholds, in the procedure described above, are not exhibited for a few days after a concussive event, e.g. about day 4. The factors R.sub.1 and R.sub.2 may not cross the thresholds until day 3 or day 4, or even longer in some cases. It is known through cognitive concussion testing that some delay occurs in concussion manifesting itself (although maximum indications in cognitive testing tend to occur earlier than those from the subject algorithm). However, the system of the invention can detect the rise of these factors toward concussion even in the first day or two after an event of trauma to the head. This is accomplished by observing the velocity of change in the patient&#39;s R.sub.1 and R.sub.2 values in the period leading up to the concussion symptoms exhibiting themselves, referred to herein as the period of “developing” concussion, typically about four days. With the system of the invention, the data can be observed as showing clear and definitive movement toward crossing the thresholds for R.sub.1 and R.sub.2, in a patient with concussion. Thus, days before the data analysis shows positive for concussion, the velocity of movement in that direction will indicate concussion. There previously existed no other reliable means for detecting concussion in this “developing” period. 
     After R.sub.1 and/or R.sub.2 reach peak values, i.e. in the case of R.sub.1 the maximization of the frequency-domain peaks at about the fifth and sixth harmonics of the patient&#39;s heartbeat, a period of usually several weeks of declining values ensues, called the “recovery” period. 
     Although the data comparison step to determine concussion can be accomplished by a trained physician reviewing a graph, it is preferably performed by an algorithm operated on a computer, which can be the same computer to which the raw data is fed. 
     Importantly, the data corresponding to non-concussion, to which the accelerometer-derived data is compared, preferably are data observed from a plurality of normal persons without concussion; baseline data for the particular patient being examined is not required. In contrast, with cognitive testing pre-season baseline data for football players, for example, is required. 
     Another important aspect of the invention is the use of a Campbell diagram as an indicator of concussion, or as a confirmation. The responses of a concussion patient&#39;s brain to vascular pulsing are frequency dependent, and the Campbell diagram was developed for frequency dependent functions, such as turbine design in jet engines. Vibration response in a turbine tends to be different at different revolution speeds. Since concussed brain responses are also frequency dependent, i.e. heartbeat rate dependent, a typical waterfall diagram of the gathered data on a patient will usually not produce sharp lines—a patient&#39;s heartbeat rate can vary with time. However, the data can be plotted to produce sharper lines, as eigenfrequency lines, if heart rate is represented on the vertical axis and frequency on the horizontal axis. The eigenfrequency of a concussion patient typically are essentially radial lines emanating and fanning out from the theoretical point 0,0. In particular if the “hot” color bands (red, orange, indicating high intensity) follow such radial, fanning lines, this indicates the harmonics of the system are changing in frequency with the driving function. That is to say, as the speed decreases, then the harmonics as an ensemble decrease in a pattern of the fan going down to zero. If, on the other hand, the structure is not responding to the driving force, i.e. to the frequency, but is simply a structural response, then the bands will be vertical. Therefore, one can use these bands to detect whether this is a structural change. It appears that in the normal brain (without concussion), the bands tend to be vertical; changing the heart rate does not shift the peaks of the harmonics. With concussion, changing the heart rate does change the position of the harmonics such that they follow the eigenfrequency lines down to zero. This is another method of detecting concussion and potentially detecting it much earlier than the R.sub.1 or R.sub.2 or velocity can do. The Campbell diagram provides an efficient reference that can be used as a primary determination for concussion indication (or not), or which can be used as a check against the conclusion reached via another algorithm such as that discussed above. The harmonic peak locations can be compared to the eigenfrequency lines by a correlation function to determine how well the structure responds to the varying heart rate. 
     The method of the invention also encompasses additional techniques to optimize the identification of peaks on the plot of accelerometer data in the frequency domain, using algorithms. As discussed above, one preferred algorithm developed pursuant to the invention has been to look at the values at the fifth and sixth harmonics as a basis for R.sub.1, and also at the eighth and ninth harmonics as a basis for R.sub.2. When an algorithm is applied based on the harmonics, the peaks will seldom fall precisely at the harmonic frequencies being investigated. In spite of this, the algorithm functions very well, with high sensitivity and specificity. Improvement can be made, however, by more closely identifying the actual frequency locations of the peaks that tend to occur around the fifth and sixth harmonics and also around the eighth and ninth harmonics. This can be done visually on a computer monitor, and it could also be done using a pattern recognition program, similar to those used for facial pattern recognition. Techniques are also available to identify these peaks accurately using an algorithm that does not use the visual appearance of a time domain or frequency domain chart. As one example, the algorithm can include a maximizing or optimizing feature by which, after the data are analyzed at the fifth and sixth harmonics and the eighth and ninth harmonics (as well as at lower harmonics as discussed above), the frequency under examination can be shifted up or down for each of the fifth, sixth, eighth and ninth harmonics to find the peak value within a selected frequency band of the nominal harmonic. By this method the actual peaks of the four approximate harmonics that are used to calculate R.sub.1 and R.sub.2 can be pinpointed, for more accurate analysis. 
     By the system and method of the invention, concussion can be determined with a high degree of sensitivity and also specificity, without any baseline data for a patient, and it can be determined at a very early stage, by observing results at repeated intervals (daily or more frequently) immediately following a potential concussive event. In addition, the period of recovery from concussion can be monitored accurately, and the point of and the time of actual recovery can be confirmed. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are frontal and side views schematically indicating accelerometer sensors located on a patient&#39;s head according to the invention. 
         FIG.  2 A  is a block diagram showing components of the system and method of the invention. 
         FIG.  2 B  is a flow chart showing an embodiment of the process of the invention. 
         FIG.  3    is a chart indicating Phase 1 testing of subjects using the system of the invention. 
         FIG.  4    is a chart indicating Phase 2 testing of subjects. 
         FIG.  5    is a frequency plot of a particular subject, showing a pattern indicating non-concussion. 
         FIGS.  6 A to  6 D  are FFT plots of four different subjects, all indicating concussion. 
         FIG.  7    is a chart of the R.sub.1 factor of the algorithm of the invention against time. 
         FIGS.  8 A- 8 C  are charts plotting R1 and R2 factors for several different populations of recordings using the system of the invention. 
         FIG.  9    shows an example of a Campbell diagram, as an indication of concussion. 
         FIGS.  10  and  11    show examples of time-domain waveforms for typical non-concussed and concussed subjects. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention are explained below in terms of system components and tests that have been performed. Initial discussion is in regard to testing performed to identify a specific pattern indication of concussion using cranial accelerometry (phase 1), to test the identified pattern against blinded data to verify its efficacy in detecting concussion (phase 2). 
     In all testing, subjects had accelerometry measurements and concurrent two-lead electrocardiograms. In players with a concussion, multiple sequential measurements were obtained. Sport Concussion Assessment Tool 2 was used to assist clinical determination of concussion. 
     As explained in greater detail below, phase 1 was the process whereby accelerometry data indicative of a concussion pattern were determined, and phase 2 was evaluation of these findings against a blinded set of accelerometry data. 
     The following explanation pertains to methods used to acquire data for both phases 1 and 2. 
     Accelerometry Measurements/Equipment and Methodology 
     This investigational device comprises a headset with accelerometer  10 , as indicated in  FIGS.  1 A and  1 B , as well as a data collector unit  12  with amplifier  14  and digitizer  16 , a detachable coaxial cable  18 , and laptop computer  20 , all shown schematically in  FIG.  2 A . The system collects and stores data derived by sensing small motion produced by pulsatile cerebral blood flow and its impact on the skull. This is accomplished by six highly sensitive accelerometers  10  that are positioned within an adjustable headset (not shown) to contact the scalp bitemporally, bifrontally, occipitally, and at the skull vertex. Transduced accelerometer data are digitized with the digital signal processor  12  and sampled with the laptop computer  20 . An omnidirectional sound pressure level (SPL) sensor (not shown) is positioned on the top of the headset for the purpose of ambient noise eradication. Correlated noise patterns between the SPL sensor and accelerometer data may be removed during processing. A heart rate sensor (not shown) is also provided using a PPG (photoplethysmograph). Note that concussion can be detected using only a single temporally-placed sensor, although two opposed temporal sensors are preferred. 
       FIG.  2 B  is a simplified flow chart outlining the method of the invention. The block  25  indicates detection and recording of acceleration at one or more selected points on the head of a subject. As noted above, this can be done using accelerometer sensors  10  placed against the head, but other methods and equipment for detecting acceleration could also be used. For example, a laser interferometer could be used to monitor extremely small motions of the skull, the second derivative of position being acceleration. In any event, this data is digitized and, in a preferred embodiment, a fast Fourier transform (FFT) is performed on the data, as indicated at  26 . A waveform is produced, per the block  28 , as a plot of energy or intensity versus frequency. This waveform is compared to a reference waveform (block  30 ), to determine whether concussion is indicated. The dependent blocks  32 ,  34  and  36  indicate that this comparison can be done visually, by algorithm, or, as explained below, via a Campbell diagram. If comparison is done visually, the FFT waveform will be shown on a computer monitor and can be compared to a typical non-concussion waveform or to a typical concussion waveform, or both. A mathematical algorithm developed according to the invention is explained below, as is the Campbell diagram. The block  38  indicates the system provides a conclusion as to whether concussion is indicated, and the probability of accuracy can accompany this conclusion. 
     Sport Concussion Assessment Tool 
     Sport Concussion Assessment Tool 2 (SCAT2) was documented and accompanied all accelerometry recordings except those obtained postseason. Sport Concussion Assessment Tool 2 is a standardized tool introduced at the 2008 third Consensus Conference on Concussion in Sport held in Zurich. All scores from sections of the SCAT2 tool contribute to a possible total of 100 points. Scoring fewer points in any section lowers the aggregate score. For instance, baseline scores for high school athletes averaged 88.3+6.8. In our study, declines in SCAT2 scores were used to assist in the clinical determination of whether a head impact resulted in a concussion. SCAT2 or SCAT3 or similar neurocognitive testing can be used for other indications of concussion such as accidents. 
     Subjects and Sample Size 
     Candidates for this observational study included all football players (grades 9-12) at a northern California high school during the 2011 football season. 
     Visual inspection of data from concussed and non-concussed subjects revealed that in the non-concussed subjects, cranial motion had a series of peaks that typically declined (diminished in amplitude) with increased frequency.  FIG.  5    shows the frequency components plot obtained from the cranial accelerometry recording of a typical non-concussed subject. It is notable that the fifth and sixth harmonic peaks have lower amplitude than do the first to third harmonic peaks. This was a consistent finding in subjects without concussion. 
     In the subjects with concussion, several additional peaks, representing higher frequencies, appeared. These new peaks arose at frequencies including and between 5 and 12 times the heart rate frequency. Three high-frequency ranges were used in algorithm development to differentiate between concussed and non-concussed subjects.  FIGS.  6 A- 6 D  show the frequency component plots of four concussed subjects. Note that the fifth and sixth harmonic peaks have higher amplitude than do the first to third harmonic peaks. This increase in higher frequency harmonic content continues in paired bands at about 5 to 6 times the heart rate, 8 to 9 times the heart rate and 11 to 12 times the heart rate. This increase in higher frequency harmonic content can also be observed in the time-domain waveforms as shown in  FIGS.  10  and  11   , described below. 
     Several algorithms are described to quantify the differences between concussed and non-concussed subjects. Other algorithms are also possible. One such algorithm defines up to three factors (R.sub.1, R.sub.2 and R.sub.3) based on the relative height of the peaks, with the higher frequencies compared with the lower. The lower three harmonic values (first, second and third) define the denominator for these three variables. R.sub.1 is defined as the average of harmonics 5 and 6 amplitude divided by the maximum of the lower three harmonic amplitudes. R.sub.2 is the average of harmonics 8 and 9 amplitude divided by the maximum of the lower three harmonic amplitudes. R.sub.3 is the average of harmonics 11 and 12 amplitude divided by the maximum of the lower three harmonic amplitudes. Higher values of R.sub.1 through R.sub.3 indicate higher energy within the higher frequencies of skull motion. R.sub.1 and R.sub.2 can be used to create a Boolean value (true or false). This Boolean value is true if R.sub.1&gt;1.0 AND R.sub.2&gt;0.66; a Boolean value of “true” defines concussion. R.sub.1 and R.sub.2 (or either of them) can also be used as a continuous variable that follows the clinical time course of concussion. Likewise R.sub.3 can used to discriminate between concussed and non-concussed subjects by its increase over non-concussed subjects&#39; values and by comparing it to R.sub.1 and R.sub.2. In this case the values of R.sub.1, R.sub.2 and R.sub.3 should diminish with increasing harmonic number. 
     The sensitivity and specificity of the described algorithm in detecting concussion from a set of subjects playing American football are shown in Tables 1 and 2. 
     TABLE-US-00001 TABLE 1 Sensitivity in Detecting Concussion Clinically Concussed Subjects*C (True+) NC (False−) Sensitivity, % CI, % 13 10 3 76.9 46-94*Thirteen subjects were confirmed by SCAT2 and clinical assessment to have suffered a concussion. The concussion algorithm pattern was seen in 10 of these subjects and was not seen in 3 of these subjects. C, concussion; NC, no concussion; CI, confidence interval (P=0.95). 
     TABLE-US-00002 TABLE 2 Specificity in Detecting Concussion No. Recordings C (False+) NC (True-) Specificity, % CI, % Baseline and postseason 18 5 13 72.2 46-89 recordings*Baseline recordings.dagger. 15 0 15 100.0 74-100 Baseline recording.dagger-dbl. 58 7 51 87.9 76-94 Total recordings 91 12 79 87.0 78-93*Baseline and postseason recordings from subjects (n=9) with no reported or suspected concussion for the duration of study. .dagger.Baseline recordings from subjects (n=15) who, at some time after baseline recording, reported a possible concussion and had multiple recordings related to that event. Thirteen of these 15 subjects were confirmed to have a concussion by SCAT2 and clinical assessment (Table 1). .dagger-dbl.Baseline recordings from subjects with no reported or suspected concussion at the time of recording. C, concussion; NC, no concussion; CI, confidence interval (P=0.95). 
     To gather insight into the time course of the presence of the concussion pattern, we plotted factor R.sub.1 over time in subjects who were able to provide multiple recordings after concussion.  FIG.  7    is an example of R.sub.1 plotted against time for one subject with a concussion. The first cranial accelerometry recording was the day following the suspected concussive event (SCE). R.sub.1 generally varied smoothly and followed the clinical course of concussion. However, in subjects with multiple recordings, the concussion pattern returned to a normal pattern at a time beyond self-reporting of symptoms as measured by neurocognitive testing (catalog and assessment of symptoms). This is notable in that concussion patients tend to be cleared by neurocognitive to return to sports activities too early. Analysis indicates that the time course of this pathophysiological change begins immediately after concussion and outlasts the duration of clinical concussion before it eventually resolves. Using the method described above to detect concussion leads to delays of 0 to 7 days from injury to detection. Another factor, R.sub.3, the rate of change of R.sub.1 or R.sub.2 with time (R.sub.1 or R.sub.2 velocity) may be used to predict a concussion within hours of an injury. A given individual, when measured repeatedly, has a cluster of R.sub.1 and R.sub.2 which may be similar to that as shown in  FIG.  8 A , at lower left. When a recording provides an R.sub.1 and R.sub.2 that differs by an amount greater than the average variation in typical subjects, then that change provides a measure of concussion detection. The more recordings and factors obtained, the more confident the diagnosis of concussion. These factors could be obtained from continuous recordings to further improve the detection speed. 
     Likewise a visual examination of the time domain or frequency domain plots of the average brain pulse or a visual examination of both domains of the average brain pulse provides a rapid method of detecting concussion, as explained below with reference to  FIGS.  10  and  11   . These changes can be further displayed in another visual algorithm using color intensity waterfall (CIWF) plots. In this type of plot, the overlap FFT of the subject recordings are plotted in a horizontal line with frequency on the horizontal line axis. The intensity of each frequency bin of the FFT is color-coded; a typical color-coding uses blue for the lowest value and red as the highest value following the color spectrum for values in between. Each subsequent recording FFT is plotted below the previous recording so that the vertical axis is time. The convention is that time increases toward the plot origin. With this type of plot, shifting harmonic content is observed as increasing color intensity over time in the 5 to 13 harmonic frequencies. Another related algorithm to detect concussion is a variation of CIWF, the Campbell diagram. The Campbell diagram was developed to detect damaging harmonic resonances in rotating machinery such as jet turbine engines. In human subjects the heart rate varies with respiration, providing a natural variation to the brain pulse, so that the Campbell diagram is based on heart rate. Each of the individual heart rate brain pulse recordings is sorted. The CIWF plot is modified so that the vertical axis is no longer time but heart rate with the highest heart rate FFT line plots farther from the plot origin and the lowest heart rate FFT line plots closer to the plot origin; see  FIG.  9   . Lines, called eigenfrequency lines, are drawn from each harmonic on the highest frequency FFT line plot, with each eigenfrequency line connecting to the waterfall plot origin. If the changes in the brain pulse are due to a concussion, the subsequently lower heart rate frequency FFT line plots will have lower harmonic peak locations, aligning with each corresponding eigenfrequency line, and higher heart rate FFT plots will have higher harmonic peak locations. Correlation of the harmonic peak location to the eigenfrequency lines, fanning out essentially from the plot&#39;s origin, provides a factor for concussion detection. This fanning out Campbell diagram seems not to occur without concussion. 
     The precise pathophysiology of concussion remains undefined, but likely is related in some way to injury that includes blood flow abnormalities, fluid accumulation, and/or structural changes. A shift of harmonic energy toward higher frequencies (or an increase at the higher frequencies) is likely caused by an increase in brain resonance from the force induced by pulsatile blood flow entering the skull. This may be caused by a stiffer brain and could be accounted for by brain edema or perhaps changes in cellular structure. However, brain edema has not been demonstrated in persons with clinical concussion, so this explanation is by no means certain. It is possible that disruption of cerebral autoregulation might produce phase changes in brain resonance and result in higher harmonics of the cardiac-induced forcing function into the closed skull space. 
     It was not part of our study to determine which subjects continued to suffer from concussion at the end of the season as determined by SCAT2 test results. We did not obtain SCAT2 test results from subjects at the end of the season, so cannot conclude who did or did not continue to register a concussion by that test method. Throughout the study, we used clinical judgment to determine the presence or absence of concussion. It is a recognized limitation of concussion diagnosis that clinical judgment and psychometric testing are subject to variability. However, this clinical judgment could not have been biased by knowledge of accelerometry data because these data were kept blinded until they had been fully collected and analyzed. 
     The observation that in certain subjects the concussion pattern returned to the non-concussion pattern after reporting of symptoms resolution but then returned to the concussion pattern after exercising suggests that return to activity is perhaps premature if it is not based on an objective physiological determination. 
       FIGS.  8 A,  8 B and  8 C  are population type charts showing results of tests on a number of subjects, plotted on a graph of R.sub.1 and R.sub.2, with dotted lines indicating thresholds of R.sub.1 (vertical dotted line) at 1.0 and a threshold for R.sub.2 (horizontal dotted line) at 0.66, according to one preferred embodiment of an algorithm. When both R.sub.2&lt;1.0 and R.sub.2&lt;0.66, the conclusion is non-concussion (lower left of chart). Also, if either the R.sub.1 or R.sub.2 is below threshold, the conclusion is non-concussion (a few in  FIG.  8 B  have only one threshold met). If both these factors equal or exceed the threshold, i.e. the R.sub.1, R.sub.2 plot is in the upper right of the chart, concussion is indicated according to this algorithm. All three charts indicate specificity of the algorithm of the invention. 
     In  FIG.  8 A  twenty-two subjects were evaluated. These were athletes in non-contact sports, aged 14 to 18. The recording sequence was two recordings every week for four weeks, for a total of 176 recordings. Of these, 159 resulted in valid recordings (due to the inability of the PPG sensor to function properly in bright sunlight). As shown on  FIG.  8 A , all tests come squarely in the lower left non-concussion region of the chart. Since there were zero false positives, the specificity of the test and algorithm was 100%, as to each of the 22 subjects and as to each of the 159 valid recordings. Since all subjects reported no possibly concussive events in recent months before testing or during the test, it appears probable that none of these tests represent a false negative. 
       FIG.  8 B  is another plot of R.sub.1 and R.sub.2 for 1205 recordings taken over approximately two years, from eight subjects, all of whom had no reason to believe any concussion event had occurred. No recording indicated concussion, for a specificity of 100%. As the chart shows, a few recordings were outside the lower left quadrant, with R.sub.1.gtoreq.1.0 or R.sub.2&gt;0.66, but none exceeded both thresholds. Those are considered not concussed. 
       FIG.  8 C  is a similar chart but for 1784 recordings, including the 1205 reported in  FIG.  8 B . The tested population included persons known to have dementia, pediatric DKA (diabetic ketone acidosis), stroke, vasospasm and 91 impact sports subjects who could possibly have had concussions, but without diagnosis. As the chart of  FIG.  8 C  shows, 36 of the total population of 1784 recordings indicated concussion, with both R.sub.1 and R.sub.2 exceeding thresholds. Since none of the persons tested was known to have concussion, all must be assumed as non-concussed. Still, this produces a specificity of 98%, which is very high. 
     Note that many of the subjects reported in the testing and chart of  FIG.  8 C  had other structural brain issues, including dementia, stroke, ischemic stroke, DKA and vasospasm. 42 had dementia and 86 had DKA, and none of these were recorded as concussed. 25 had stroke and 296 had vasospasm; only one stroke patient was recorded as concussed, and only 13 vasospasm subjects were shown as concussed. 91 of the subjects were involved in impact sports, and 12 were shown as having concussion. It is possible (but unknown) that these were true positives, but for purposes of this specificity analysis they are assumed as false positives. 
       FIGS.  10  and  11    show plots of accelerometer readings taken on a subject with only two opposing accelerometer sensors, at left and right temporal lobes. These are plots of acceleration events during a single heartbeat (at a heartbeat rate of 1 Hz), against time (i.e. in the time domain).  FIG.  10    is from a subject without concussion;  FIG.  11    is a plot from the same subject but after a concussion event.  FIG.  10    indicates non-concussion, while  FIG.  11    indicates concussion. The opposed positive and negative values on the charts represent data from the two opposed temporal accelerometers. Both charts have data averaged together over 45 heartbeats. From  FIG.  10    it is seen that during the systole portion of a heartbeat, which is up to about 0.3 on the time scale, only two large positive peaks appear (one from the left sensor and one from the right sensor). However, the graph of  FIG.  11    is strikingly different, with four prominent positive peaks showing during the systole portion of the heartbeat. This is a typical differentiating pattern to indicate non-concussion or concussion in a graph of acceleration in the time domain. Other differences occur in the remainder of the heartbeat, but the systole portion is the most prominent. As an alternative, analysis can be done by counting zero crossings with up to 8 or 10 being typical of non-concussed and a higher number being typical of concussed. 
     The described methodology identifies and utilizes a novel, unique pattern of cranial accelerometry that correlates with human concussion. It is influenced by movements of the brain within the skull that occur during cardiac-induced blood flow pulsations. 
     The terms “intensity of frequency”, “frequency content”, and “amplitude of signal data” are used in the above description and in the claims. These terms refer to how much of each of a series of particular frequencies occurs in a waveform analyzed by the algorithm, through a band of frequencies (e.g. 1-28 Hz). In the frequency domain they refer to the amplitude, i.e. height of signal data in the plot of amplitude (which can be called energy or power) versus frequency. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.