Compressive sampling of swallowing accelerometry signals using time-frequency dictionaries based on modulated discrete prolate spheroidal sequences
© Sejdić et al; licensee Springer. 2012
Received: 30 November 2011
Accepted: 4 May 2012
Published: 4 May 2012
Monitoring physiological functions such as swallowing often generates large volumes of samples to be stored and processed, which can introduce computational constraints especially if remote monitoring is desired. In this article, we propose a compressive sensing (CS) algorithm to alleviate some of these issues while acquiring dual-axis swallowing accelerometry signals. The proposed CS approach uses a time-frequency dictionary where the members are modulated discrete prolate spheroidal sequences (MDPSS). These waveforms are obtained by modulation and variation of discrete prolate spheroidal sequences (DPSS) in order to reflect the time-varying nature of swallowing acclerometry signals. While the modulated bases permit one to represent the signal behavior accurately, the matching pursuit algorithm is adopted to iteratively decompose the signals into an expansion of the dictionary bases. To test the accuracy of the proposed scheme, we carried out several numerical experiments with synthetic test signals and dual-axis swallowing accelerometry signals. In both cases, the proposed CS approach based on the MDPSS yields more accurate representations than the CS approach based on DPSS. Specifically, we show that dual-axis swallowing accelerometry signals can be accurately reconstructed even when the sampling rate is reduced to half of the Nyquist rate. The results clearly indicate that the MDPSS are suitable bases for swallowing accelerometry signals.
Continuous monitoring of physiological functions such as swallowing can pose severe constraints on data acquisition and processing systems. Even when sampling physiological signals at low rates (e.g., 250 Hz), we end up with close to a million of samples in the first hour of monitoring. Similar computational burdens are ever-present in telemedicine, and in recent years we have witnessed numerous efforts to deal with this problem. One such effort is to compress the acquired signals immediately upon sampling using various schema (e.g. ). The other is to rethink the way we acquire the data, and a number of recent publications have begun looking at this approach (e.g., [2–5]).
The idea of compressive sensing (CS) has gained considerable attention in recent years. The main idea behind CS is to diminish the number of steps involved when acquiring data by combining sampling and compression into a single step [3, 4]. Specifically, CS enables one to acquire the data at sub-Nyquist rates, and recover it accurately from such sparse samples .
In this article, we propose an approach for CS of swallowing accelerometry signals based on a time-frequency dictionary. In particular, the members of the dictionary are recently proposed modulated discrete spheroidal sequences (MDPSS) [6, 7]. The bases within the time-frequency dictionary are obtained by modulation and variation of the bandwidth of discrete prolate spheroidal sequences (DPSS) to reflect the vaying time-frequency nature of many biomedical signals, including the swallowing acclerometry signals considered in this article. Using the proposed approach, we carry out a numerical analysis of synthetic test signals and real swallowing accelerometry signals. The numerical analysis using the synthetic test signals showed that the CS approach based on MDPSS was more accurate than the CS approach based on DPSS (e.g., [7, 8]). Additionally, the analysis of swallowing accelerometry signals showed that we can obtain 90% cross-correlation between the reconstructed signals and the actual signals using only 50% percent of samples. This has been observed for three different types of swallowing tasks.
The article is organized as follows: Section 2 describes swallowing accelerometry and outlines the advantages of this approach for detecting swallowing difficulties. In Section 3, we describe the proposed approach for CS using the time-frequency based dictionary consisting of MDPSS bases. Section 4 reports the data analysis steps that we carried out to obtain the reported results, which are presented in Section 5 along with the discussion of the same results. The conclusions are drawn in Section 6.
2 Swallowing accelerometry
Swallowing (deglutition) is a complex process of transporting food or liquid from the mouth to the stomach consisting of four phases: oral preparatory, oral, pharyngeal, and esophageal . Dysphagic patients (i.e., patients suffering from swallowing difficulty) usually deviate from the well-defined pattern of healthy swallowing. Dysphagia frequently develops in stroke patients, head injured patients, and patients with others with paralyzing neurological diseases . Patients with dysphagia are prone to choking and aspiration (the entry of material into the airway below the true vocal folds) . Aspiration and dysphagia may lead to serious health sequelae including malnutrition and dehydration [11, 12], degradation in psychosocial well-being [13, 14], aspiration pneumonia , and even death .
The videofluoroscopic swallowing study (VFSS) is used widely in today's dysphagia management and it represent the gold standard for assessment [9, 17]. However, VFSS requires expensive X-ray equipment as well as expertise from speech-language pathologists and radiologists. Hence, only a limited number of institutions can offer VFSS and the procedure has been associated with long waiting lists [18, 19]. In addition, day-to-day monitoring of dysphagia is crucial due to the fact that the severity of dysphagia can fluctuate over time and VFSS is not suitable for such day-to-day monitoring.
Cervical auscultation is a promising non-invasive tool for the assessment of swallowing disorders  involving the examination of swallowing signals acquired via a stethoscope or other acoustic and/or vibration sensors during deglutition . Swallowing accelerometry is one such approach and employs an accelerometer as a sensor during cervical auscultation. Swallowing accelerometry has been used to detect aspiration in several studies, which have described a shared pattern among healthy swallow signals, and verified that this pattern is either absent, delayed or aberrant in dysphagic swallow signals [22–34].
However, these previous studies used single-axis accelerometers and exclusively monitored vibrations propagated in the anterior-posterior direction at the cervical region. Proper hyolaryngeal movement with precise timing during bolus transit is vital for airway protection in swallowing . Since the motion of the hyolaryngeal structure during swallowing occurs in both anterior-posterior (A-P) and superior-inferior (S-I) directions, the employment of dual-axis accelerometry seems well motivated. Since correlation has been reported between the extent of laryngeal elevation and the magnitude of the A-P swallowing accelerometry signal , it is hypothesized that vibrations in the S-I axis also capture useful information about laryngeal elevation. From a physiological stand point, the S-I axis appears to be as worthy of investigation as the A-P axis because the maximum excursion of the the hyolaryngeal structure during swallowing is of similar magnitude in both the anterior and superior directions [36, 37]. Recent contributions have indeed confirmed that dual-axis accelerometers yield more information and enhance analysis capabilities [38–43].
Sample signals used in this article were collected from 408 participants (ages 18-65) over a 3 month period from a public science centre in Toronto, Ontario, Canada. All participants provided written consent and had no documented swallowing disorders. The research ethics boards of the Toronto Rehabilitation Institute and Holland Bloorview Kids Rehabilitation Hospital (both located in Toronto, Ontario, Canada) approved the study protocol.
To collect data from participants, we used a dual-axis accelerometer (ADXL322, Analog Devices), which was attached to the participant's neck (anterior to the cricoid cartilage) using double-sided tape. The axes of acceleration were aligned to the anterior-posterior and superior-inferior directions. Data were band-pass filtered in hardware with a pass band of 0.1-3,000 Hz and sampled at 10 kHz using a custom LabVIEW program running on a laptop computer. With the accelerometer attached, each participant was cued to perform five saliva swallows (i.e., dry swallows), five water swallows by cup with their chin perpendicular to the floor (i.e., wet swallows) and five water swallows in the chin-tucked position. The entire data collection session lasted 15 min per participant.
3 Proposed scheme
where T s is the sampling period and Ωmax represents the maximum frequency present in the signal. In other words, the Shannon sampling theorem states that in order to ensure accurate representation and reconstruction of a signal with Ωmax, we should sample it at least at 2Ωmax samples per second (i.e., the Nyquist rate). However, many recent publications have challenged this approach for a number of reasons (e.g., [44, 45]). First, by using the Shannon sampling theorem we rely on bases of infinite support, while we generally reconstruct signal samples in the finite domain . Second, large bandwidth values can severely constraint sampling architectures . Third, even when we consider signals with a relatively low band-width values such as swallowing accelerometry signals, continuous monitoring of swallowing function can produce large number of redundant samples, which severely constraints our processing efforts.
where η is the expected noise of measurements, ||x||0 counts the number of nonzero entries of x and || • ||2 is the Euclidian norm. Unfortunately, the above minimization is not suitable for many applications as it is NP-hard . To avoid the computational burden, approaches like thresholding, (orthogonal) matching pursuit and basis pursuits have been proposed . In this article, we will focus on the matching pursuit .
Given the CS framework, the immediate question is how to define the sensing matrix Φ, that is the bases used in the recovery of the signal. Most commonly used sensing matrices are random matrices with independent identically distributed (i.i.d.) entries formed by sampling either a Gaussian distribution or a symmetric Bernoulli distribution . Previous publications have shown that these matrices can recover the signal with high probability . However, when dealing with biomedical signals, we would like to "precisely" recover the signals (i.e., with a very small error). Therefore, we propose to use a time-frequency dictionary (also known as frames ) based on modulated discrete prolate spheroidal sequences (MDPSS).
3.1 Time-frequency dictionaries based on MDPSS
where n = 0, ± 1, ± 2, . . . and k = 0, 1, . . . , N − 1.
where ω m = 2πf m is a modulating frequency. It is easy to see that MDPSS are also doubly orthogonal, obey the same Equation (4) and are bandlimited to the frequency band [−W + ω m : W + ω m ].
However, in practical applications, exact frequency band is known only with a certain degree of accuracy and usually evolves in time. Therefore, only some relatively wide frequency band is expected to be known. In such situations, an approach based on one-band-fits-all may not produce a sparse and accurate approximation of the signal. In order to resolve this problem it was suggested to use a band of bases with different widths to account for time-varying bandwidths . However, such representation once again ignores the fact that the actual signal bandwidth could be much less then 2W dictated by the bandwidth of the DPSS. In order to provide further robustness to the estimation problem we suggest to use of a time-frequency dictionary containing bases which reflect various bandwidth scenarios.
To construct this time-frequency dictionary, it is assumed that an estimate of the maximum frequency is available. The first few bases in the dictionary are the actual traditional DPSS with bandwidth W. Additional bases could be constructed by partitioning the band [−ω; ω] into K subbands with the boundaries of each subband given by [ω k ; ωk+1], where 0 ≤ k ≤ K − 1, ωk+1> ω k , and ω0 = −ω, ωK-1= ω. Hence, each set of MDPSS has a bandwidth equal to ωk+1− ω k and a modulation frequency equal to ω m = 0.5(ω k + ωk+1).
3.2 Matching pursuit and MDPSS-based frames
As mentioned at the beginning of Section 3, the CS approaches can be NP-hard, which are not practically viable. Fortunately, efficient algorithms, known generically as matching pursuit [47, 49], can be used to avoid some of the computational burden associated with the CS. The main feature of the algorithm is that when stopped after a few steps, it yields an approximation using only a few basis functions . The matching pursuit decomposes any signal into a linear expansion of waveforms that are selected from a redundant dictionary of functions . It is a general, greedy, sparse function approximation scheme with the squared error loss, which iteratively adds new functions (i.e. basis functions) to the linear expansion. In comparison to a basis pursuit it significantly reduces the computational complexity, since the basis pursuit minimizes a global cost function over all bases present in the dictionary . If the dictionary is orthogonal the method works perfectly. Also, to achieve compact representation of the signal, it is necessary that the atoms are representative of the signal behaviour and that the appropriate atoms from the dictionary are chosen.
where . The process continues till the norm of the residual R(k)(m) does not exceed required margin of error ε > 0: ||R(k)(m)|| ≤ ε.
An alternative stopping rule can mandate that the number of bases, , needed for signal approximation should satisfy . In previous contributions (e.g., ), is set equal to ⌈2NW ⌉ + 1 to compare the performance of the MDPSS-based frames with DPSS.
where are L bases from the dictionary with the strongest contributions.
3.3 Estimation of sampling times
A thorough description of the procedure can be found in [2, Appendices 1 and 2].
4 Data analysis
4.1 Synthetic test signals
where 0 ≤ n < N, T s = 1/256, N = 256, A i is uniformly drawn from random values in 0 and f i ~ N(30, 102). ζ(n) represents white Gaussian noise and σ is its standard deviation.
where A† denotes the pseudo-inverse of a matrix; U(n, k) is the matrix containing K bases (i.e., DPSS) and each sequence is of length N; m denotes the time instances at which the samples are available.
In the second experiment, we vary the number of available samples from 50 samples to 200 samples in increments of 10 samples in order to understand how the number of samples affects the overall accuracy of the proposed scheme. The samples are uniformly distributed, and the normalized half-bandwidth is set to 0.30. The lower boundary of 50 samples denotes a very aggressive scheme, as it represents approximately 20% of the original samples. On the other hand, the upper boundary of 200 samples represents a very lenient scheme for compressive sampling since it represents approximately 78% of the original samples. Additionally, we use the following four SNR values: 5, 15, 25 and 35 dB. The accuracy of the proposed CS-approach is examined using a 7- and 15-band MDPSS based dictionaries against the CS-approach based on DPSS. The accuracy metric is the MSE value defined by Equation (30) and 1,000 realizations are used to obtain its values.
The third experiment examines the effects of non-uniform sampling times on the overall performance of the CS-based schemes. In particular, we use 100 non-uniform samples and the SNR values were incremented by 1 dB from 0 to 30 dB. Also, the normalized half-bandwidth is varied in 0.025 increments from 0.30 to 0.375. The accuracy of the proposed approach based on MDPSS is compared against the CS-approach based on DPSS. Specifically, we use 7- and 15-band MDPSS-based time-frequency dictionaries. The accuracy metric is again the MSE value defined by Equation (30). 1,000 realizations are used again to obtain the MSE values, and for each realization new 100 time positions are achieved.
4.2 Swallowing accelerometry signals
Using the proposed scheme, we analyze how accurately we can recover dual-axis swallowing accelerometry signals from sparse samples. Specifically, we assume two different scenarios: only 30% of the original samples are available and only 50% of the original samples are available. In both cases, we examine whether the uniform or non-uniform sub-Nyquist rates have significant effects on the overall effectiveness of the proposed scheme. In this numerical experiment, we use a 10-band MDPSS based dictionary with the normalized half-bandwidth equal to 0.15. To evaluate the effectiveness of the proposed approach when considering dual-axis swallowing accelerometry signals, we adopted performance metrics used in other biomedical applications (e.g., [5, 55, 56]). Those metrics are:
Cross-correlation (CC): CC is used to evaluate the similarity between the original and the reconstructed signal, and is defined as:(32)
where x(n) is the original signal and represents a reconstructed signal. In addition, µ x and denote the mean values of x(n) and , respectively.
Percent root difference (PRD): PRD measures distortion in reconstructed biomedical signals, and is defined as:(33)
Root mean square error (RMSE): RMSE also measures distortion and is often beneficial to minimize this metric when finding the optimal approximation of the signal. RMSE is defined as:(34)
Maximum error (MAXERR): MAXERR is used to understand the local distortions in the reconstructed signal, and it particularly denotes the largest error between the samples of the original signal and the reconstructed signal. The metric is defined as:(35)
In order to establish statistical significance of our results, a non-parametric inferential statistical method known as the Mann-Whitney test was used , which assesses whether observed samples are drawn from a single population (i.e., the null hypothesis). For multi-group testing, the extension of the Mann-Whitney test known as the Kruskal-Wallis was used . A 5% significance was used.
5 Results and discussion
In this section, we present the results of numerical experiments and discuss those results. First, we will discuss the results based on the synthetic test signals. In the second part, we will discuss the results of numerical experiments considering the application of the proposed approach to dual-axis swallowing accelerometry signals.
5.1 Synthetic test signals
5.2 CS of swallowing accelerometry signals
Performance of the proposed method for recovery of dual-axis swallowing accleremetry signals when considering 30% of samples and a uniform sampling scheme
96.6 ± 4.30
96.8 ± 4.28
92.8 ± 9.13
93.3 ± 8.85
90.5 ± 11.1
97.4 ± 5.54
23.2 ± 12.3
21.8 ± 13.2
33.5 ± 19.6
31.7 ± 20.2
37.8 ± 23.4
17.1 ± 15.6
0.04 ± 0.03
0.06 ± 0.04
0.05 ± 0.04
0.10 ± 0.08
0.12 ± 0.08
0.11 ± 0.08
0.34 ± 0.40
0.67 ± 0.69
0.56 ± 0.57
1.15 ± 1.06
1.51 ± 1.24
1.36 ± 1.19
Performance of the proposed method for recovery of dual-axis swallowing accleremetry signals when considering 30% of samples and a non-uniform sampling scheme
89.5 ± 7.17
92.5 ± 6.60
84.5 ± 11.3
87.8 ± 11.3
84.3 ± 13.7
94.4 ± 7.35
43.9 ± 14.9
36.5 ± 15.7
53.3 ± 20.2
46.2 ± 22.5
52.4 ± 26.1
30.0 ± 17.4
0.07 ± 0.04
0.10 ± 0.06
0.09 ± 0.04
0.15 ± 0.09
0.17 ± 0.11
0.23 ± 0.13
0.55 ± 0.53
0.88 ± 0.73
0.72 ± 0.62
1.35 ± 1.18
1.93 ± 1.60
2.38 ± 1.96
Performance of the proposed method for recovery of dual-axis swallowing accleremetry signals when considering 50% of samples and a uniform sampling scheme
98.1 ± 2.53
98.1 ± 2.83
95.8 ± 5.99
95.9 ± 5.69
94.1 ± 7.70
98.5 ± 3.66
17.3 ± 8.87
16.4 ± 10.0
24.7 ± 14.1
23.6 ± 14.8
28.3 ± 17.6
12.6 ± 11.5
0.03 ± 0.02
0.04 ± 0.03
0.04 ± 0.03
0.08 ± 0.06
0.09 ± 0.06
0.08 ± 0.06
0.26 ± 0.29
0.51 ± 0.52
0.41 ± 0.42
0.87 ± 0.77
1.12 ± 0.85
1.02 ± 0.87
Performance of the proposed method for recovery of dual-axis swallowing accleremetry signals when considering 50% of samples and a non-uniform sampling scheme
95.8 ± 4.44
96.4 ± 4.23
92.2 ± 8.77
93.2 ± 8.30
90.4 ± 10.6
97.1 ± 5.23
26.4 ± 11.6
23.8 ± 12.4
35.4 ± 17.4
32.1 ± 18.2
38.4 ± 21.6
19.7 ± 14.1
0.04 ± 0.03
0.07 ± 0.04
0.06 ± 0.04
0.11 ± 0.07
0.12 ± 0.08
0.14 ± 0.09
0.38 ± 0.37
0.69 ± 0.64
0.55 ± 0.54
1.08 ± 0.93
1.53 ± 1.22
1.69 ± 1.42
Several observations are in order. First, we achieved very high agreement between the reconstructed data and the original signals with uniformly spread out samples. Statistically higher results were achieved with 50% of samples than with 30% of samples when considering the CCs results (p << 0.01), which resulted in statistically lower errors with 50% of samples when considering the three error metrics (p << 0.01).
Second, statistically worse results have been obtained when using non-uniform (random) sampling times (p << 0.01) in comparison to uniform sampling for both 30% of samples and 50% of samples. This result is expected, as it becomes more challenging to recover the signal accurately with non-uniform samples. Additionally, it is difficult to recover swallowing vibrations accurately, given that these vibrations are short-duration transients. Unless the non-uniform samples capture the behavior of these short-duration transients, a larger recovery error is achieved. However, with 50% of samples, we still obtain very high agreement between the recovered data and the original signals. As a matter of fact, the results obtained with 50% of samples with non-uniform sampling are comparable to the results obtained with 30% of samples when using uniform sampling.
Third, amongst the considered swallowing tasks, dry swallows tend to be recovered most accurately, followed by the wet swallows and lastly by the wet chin down swallows. From a physiological point of view, this is expected since during the dry swallowing manoeuver only small amounts of liquid (i.e., saliva) are swallowed. It is also expected that wet chin down swallows will be more difficult to recover due to the complex maneuvering required during these swallows, which may introduce signal components otherwise not present during the dry and/or wet swallowing tasks.
Therefore, based on the presented results, we can state with high confidence that CS based on the time-frequency dictionary containing MDPSS is suitable scheme for dual-axis swallowing acceleromtry signals. Particularly accurate results have been obtained when we use 50% of samples. We expect that further improvements can be achieved by optimizing the parameters of the recovery process with respect to the considered error metrics.
In this article, a CS algorithm for accurate reconstruction of dual-axis swallowing accelerome-try signals from sparse samples was proposed. The proposed algorithm uses a time-frequency dictionary based on MDPSS. The modulating of DPSS was performed in order to account for the time-varying nature of the dual-axis swallowing accelerometry signals. The proposed CS algorithm was tested using both synthetic test signals and swallowing accelerometry signals. In both cases, we achieved very accurate representations with MDPSS, which makes these bases suitable for CS approaches of swallowing accelerometry signals. Specifically, we showed that even when the dual-axis swallowing accelerometry signals were subsampled at by 50% below the Nyquist rate, we still achieved very accurate representations of these signals.
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