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Proceedings ArticleDOI

Adaptive spatial compounding for improving ultrasound images of the epidural space on human subjects

06 Mar 2008-Vol. 6920, Iss: 37, pp 157-168
TL;DR: The development of an updated compounding algorithm and results from a clinical study show a significant improvement in quality when median-based compounding with warping is used to align the set of beam-steered images and combine them.
Abstract: Administering epidural anesthesia can be a difficult procedure, especially for inexperienced physicians. The use of ultrasound imaging can help by showing the location of the key surrounding structures: the ligamentum flavum and the lamina of the vertebrae. The anatomical depiction of the interface between ligamentum flavum and epidural space is currently limited by speckle and anisotropic reflection. Previous work on phantoms showed that adaptive spatial compounding with non-rigid registration can improve the depiction of these features. This paper describes the development of an updated compounding algorithm and results from a clinical study. Average-based compounding may obscure anisotropic reflectors that only appear at certain beam angles, so a new median-based compounding technique is developed. In order to reduce the computational cost of the registration process, a linear prediction algorithm is used to reduce the search space for registration. The algorithms are tested on 20 human subjects. Comparisons are made among the reference image plus combinations of different compounding methods, warping and linear prediction. The gradient of the bone surfaces, the Laplacian of the ligamentum flavum, and the SNR and CNR are used to quantitatively assess the visibility of the features in the processed images. The results show a significant improvement in quality when median-based compounding with warping is used to align the set of beam-steered images and combine them. The improvement of the features makes detection of the epidural space easier.

Summary (4 min read)

1.1 Epidural anesthesia in obstetrics

  • Anesthesia is injected into the epidural space as shown on Fig. 1 and a nerve block for the lower body is then provided.
  • The catheter is then inserted through the epidural needle shaft into the epidural space.
  • The ultrasound group achieved a success rate of 84% in the first 10 attempts whereas the control group had a success rate of 60%.
  • Which is around 7mm and much smaller than the epidural depth of adults (20-90mm), it shows the potential benefit of using ultrasound to visualize the lumbar region during an epidural needle insertion procedure.

1.2 Ultrasound imaging of lumbar region

  • Ultrasound is noninvasive, harmless at low power, portable, accurate and cost effective.
  • Ultrasound of the lumbar region shows an image filled with speckle and artifacts which can impede detection of important features such as the ligamentum flavum and other hard to detect structures.
  • The received echo is based on ultrasound reflections from large-scale (relative to wavelength) structures, such as bone (i.e. specular reflection) and reflections from small-scale structures, such as cells (i.e. random scattering).
  • If the specular reflection is strong enough, such as a bone, it casts shadows in the beam direction and structures underneath are obstructed.
  • Terms of Use: http://spiedl.org/terms with respect to the reflectors.

1.3 Image processing techniques

  • Many post-processing methods employ filters to reduce speckle.
  • Examples are the diffusion filter 10 , the adaptive weighted median filter11 , homogeneous region growing mean filter, and aggressive region growing filter 12 .
  • Frequency compounding captures several images at the same location at different transmission frequencies, decorrelating the speckle patterns among images.
  • Spatial compounding is now popular among commercial manufacturers and will be presented in the next section.

2.1 Spatial compounding

  • Spatial compounding uses beam steering19 20 21 , which captures several frames by sending the ultrasound pulses at different angles of incidence (see Fig. 2(b) for an illustration of the principle).
  • The application of spatial compounding to these images reduced speckle noise and improved the boundary continuity.
  • Spatial compounding also has other benefits, such as the possibility of enhancing structures that are only visible at certain beam angles.
  • Certain weak but important features, such as a biopsy needle 9 24 only appear at certain Proc. of SPIE Vol.
  • The structure of main interest in this work is the epidural space, which is immediately under the ligamentum flavum and initial experience suggests it is only clear at certain beam angles.

2.2 Registration techniques

  • Conventional spatial compounding still suffers from blurring due to misalignment of features.
  • The speed of sound varies by as much as 14% in soft tissue25 and the resulting distortion (including refraction) causes the apparent positions of structures to be slightly different under different angles of incidence.
  • Re-alignment of the features using an additional block-matching non-rigid registration was previously proposed by their group to properly align the structures of each image 13 .
  • The result was a sharper ultrasound image.
  • Building on those results, the warping/compounding method is extended here to improve visibility of the ligamentum flavum in vivo, and therefore the epidural space.

2.2.1 Similarity measures

  • Registration performance is highly dependent on the similarity measure used as a cost function for finding the best alignment.
  • A good similarity measure will yield a single strong peak upon best alignment.
  • Previous literature used several methods such as sum of absolute differences (SAD), mean squared error (MSE) 27 , normalized covariance (NCOV), normalized crosscorrelation (NCC), entropy of the difference image and mutual information28 .
  • Mutual information29 30 is a very popular similarity measure for registration of multimodality images but is too easily affected by artifacts.
  • Since the two images to be registered are quite similar, the means are assumed to be small enough so that the NCC and NCOV will yield similar results.

2.2.2 Interpolation and mapping

  • The beam-steered images are divided into blocks and each block registered to the reference image.
  • Once the individual warping vectors have been found for each block, each pixel is assigned a warping vector by smooth interpolation.
  • Many interpolation techniques are well known and have been compared on operations such as resizing and rotation31 32 33 .
  • Popular interpolation techniques are placed in order of performance as follows: nearest neighbour, linear, cubic and cubic B-spline.
  • Inverse mapping does not encounter the problem of holes and overlapping like in forward mapping as each pixel has only one associated value.

2.3 Linear prediction techniques

  • In order to further reduce computational cost, a coarse to fine or multi-resolution approach is often used where lower resolution blocks are registered and then higher resolution blocks are registered using a smaller search region 13 .
  • The top of the image is often noisy with poor resolution so it is not suitable as the basis for finding the initial warping vectors.
  • It is not guaranteed to be the location with strong feature content and may contain shadows.
  • The Canny edge detector is used to detect areas containing edges, and then the block with the highest count is assumed to be the best starting candidate.

2.4 Median-based compounding

  • Since the features of interest do not appear on all frames, taking the average may not highlight weak anisotropic reflectors.
  • Accordingly, any edges are weighted more than homogeneous regions.
  • Many gradient calculation methods can be used.
  • This parameter should be set according to each type of image since speckle scale depends on probe characteristics such as frequency and depth setting.

2.5 Finding the right parameters

  • The first choice is the number of beam-steered images and the number of degrees between each image.
  • Too small an angle between each image and the speckle noise pattern will be highly correlated; too large and there will be few images within the range of angles that provide good image quality.

2.5.1 Warping parameters

  • The second parameter to choose is the size of the blocks.
  • The blocks must be large enough so that a block contains significant anatomical features, therefore making the registration more accurate, and small enough to produce a different warping vector for each block as each block is associated with a different refraction error.
  • A small search region size means that the best registration may not be found.
  • Quantitative measures are computed on the regions of interest in each image and the parameters giving the best results are chosen.
  • Table 1 shows the maximum Laplacian at the ligamentum flavum as the measure of interest.

3.1 Experimental setting

  • This study was approved by the Ethical Review Boards of the University of British Columbia and British Columbia Children’s and Women’s Health Centre and written informed consent was obtained from all subjects.
  • Subjects who had contraindictions to neuraxial anesthesia or who could not communicate in English were excluded.
  • Each subject was scanned in the sitting position with L3/L4 or L2/L3 interspaces identified using surface landmarks and confirmed by ultrasound.
  • Pre-scan converted B-mode images showing the ligamentum flavum and laminas were captured over the range of beam steering angles.
  • The sonographer finds the best image and then the beam-steered frames are acquired.

3.2 Qualitative evaluation

  • The authors now compare the images before and after compounding with different compounding methods.
  • Simple compounding (Fig. 4(b) and 5(b)) averages out most speckle, however, the ligamentum flavum and the bone boundaries are blurred.
  • Using warping (Fig. 4(c) and 5(c)) sharpens the compounded image and the ligamentum flavum is seen as a doublet again.
  • Fig. 5 shows a case where the doublet is a very faint structure, compounding indeed loses the depiction of the doublet by blurring.

3.3 Quantitative measures

  • There are two structures of interest in this clinical application: the ligamentum flavum and the lamina which is the bone seen in the images.
  • Ideally, the authors would see a set of sharp lines, therefore the Laplacian of the leading line is taken as the quantitative measure.
  • The quantitative measures are calculated on all 20 sets of images and results are compiled in Table 2.
  • The features are difficult to discern due to speckle, as shown in Figs. 4(a) and 5(a).
  • Here the compounding methods show a large improvement.

3.4 Computational cost

  • The different methods presented above have various individual computational costs which are summarized in Table 4.
  • Spatial compounding alone adds very little extra cost.
  • Terms of Use: http://spiedl.org/terms frame rate down to two frames per second which makes it impractical for real-time implementation.

4.1 Summary

  • One usually relies on the lamina which is a stronger reflector to predict where the ligamentum flavum is.
  • The images are heavily affected by speckle noise.
  • Using warping makes edges clearer and thus makes the skin-to-epidural depth easier to measure.
  • The choice of parameters will not only affect the intelligibility of the compounded image but also affects the computational cost.
  • The median-based compounding yields sharper edges and certain details on surrounding tissues can be resolved but at a very high cost.

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Citations
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Journal ArticleDOI
TL;DR: Paramedian ultrasound can be used to estimate the midline depth to the epidural space, but the surrogate measures are not sufficiently correlated with the Depth of the Epidural space to recommend them as a replacement for the actual depth.
Abstract: BACKGROUND: Ultrasound is receiving growing interest for improving the guidance of needle insertion in epidural anesthesia. Defining a paramedian ultrasound scanning technique would be helpful for correctly identifying the vertebral level. Finding surrogate measures of the depth of the epidural space may also improve the ease of scanning. METHODS: We examined 20 parturients with pre-epidural ultrasound in the paramedian plane, and the predicted depth was compared with the actual midline depth. The actual depth was also compared with subject biometrics, depth of transverse process, and thickness of lumbar fat. RESULTS: The scanning technique allowed the depth of the epidural space to be measured in all subjects. The depth measured in ultrasound was strongly correlated to the actual depth (R 2 = 0.8 and 95% limits of agreement of -14.8 to 5.2 mm), unlike patient biometrics (R 2 < 0.25), the depth of the neighboring transverse processes (R 2 = 0.35 and 95% limits of agreement of -13.8 to 19.1 mm), or the thickness of overlying fat (R 2 = 0.66). The duration of the ultrasound scan was 10 min at the beginning of the trial and 3 min for the last subjects. CONCLUSIONS: Paramedian ultrasound can be used to estimate the midline depth to the epidural space. The surrogate measures are not sufficiently correlated with the depth to the epidural space to recommend them as a replacement for the actual depth to the epidural space measurement.

56 citations


Cites background from "Adaptive spatial compounding for im..."

  • ...The possible correlation with fat thickness was initially thought to be helpful with automatic image processing of the features to emphasize the epidural space.(13,14) This is being investigated further because speed of sound variations can cause image distortion through compression and refraction....

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Proceedings ArticleDOI
23 Jun 2008
TL;DR: This paper investigates reconstruction of the acoustic impedance from ultrasound images for the first time by combining multiple images to improve the estimation and uses phase information to determine regions of high reflection from an ultrasound image.
Abstract: Reflection of sound waves, due to acoustic impedance mismatch at the interface of two media, is the principal physical property which allows visualization with ultrasound. In this paper, we investigate reconstruction of the acoustic impedance from ultrasound images for the first time. Similar to spatial compounding, we combine multiple images to improve the estimation. We use phase information to determine regions of high reflection from an ultrasound image. We model the physical imaging process with an emphasis on the reflection of sound waves. The model is used in computing the acoustic impedance (up to a scale) from areas of high reflectivity. The acoustic impedance image can either be directly visualized or be used in simulation of ultrasound images from an arbitrary point of view. The experiments performed on in-vitro and in-vivo data show promising results.

13 citations

Journal ArticleDOI
TL;DR: A significant improvement in quality is shown when using warping with adaptive median-based compounding, and linear prediction is used to find the warping vectors and decrease computational cost.

11 citations

Patent
08 Dec 2014
TL;DR: In this paper, a pixel processor for beamforming with respect to a pixel from among the pixels, and for assessing the amount of local information content of respective ones of the images.
Abstract: An image compounding apparatus acquires, via ultrasound, pixel-based images (126-130) of a region of interest for, by compounding, forming a composite image of the region. The image includes composite pixels (191) that spatially correspond respectively to pixels of the images. Further included is a pixel processor for beamforming with respect to a pixel from among the pixels, and for assessing, with respect to the composite pixel and from the data acquired (146), amounts of local information content of respective ones of the images. The processor determines, based on the assessment, weights for respective application, in the forming, to the pixels, of the images, that spatially correspond to the composite pixel. In some embodiments, the assessing commences operating on the data no later than upon the beamforming. In some embodiments, brightness values are assigned to the spatially corresponding pixels; and, in spatial correspondence, the maximum and the mean values are determined. They are then utilized in weighting the compounding.

2 citations

References
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Journal ArticleDOI
TL;DR: TE material made with agar is environmentally stable and can be manufactured to exhibit the important speed of sound, 1,540 m/s.
Abstract: Two limitations on the animal-hide gelatin and graphite powder tissue equivalent (TE) materials are that they cannot be produced consistently with speeds of sound less than 1,570 m/s at room temperature (22 degrees C) and that irreparable damage can result if the materials are raised to temperatures above 32.5 degrees C. An acceptable substitute polysaccharide gel (agar) has a high melting point (78 degrees C) and can be made to exhibit speeds of sound over the range 1,498 m/s to over 1,600 m/s at 22 degrees C. Thus TE material made with agar is environmentally stable and can be manufactured to exhibit the important speed of sound, 1,540 m/s.

218 citations

Journal ArticleDOI
TL;DR: A good level of success in the ultrasound-determined insertion point, and very good agreement between UD and ND suggests that the proposed ultrasound single-screen method, using the transverse approach, can be a reliable guide to facilitate labor epidural insertion.
Abstract: BACKGROUND: Ultrasound imaging of the spine has recently been proposed to facilitate identification of the epidural space. In this study, we assessed the accuracy and precision of the transverse approach, using a “single-screen” method, to facilitate labor epidurals. METHODS: We enrolled 61 patients requesting labor epidurals. Ultrasound imaging (transverse approach, 2–5 MHz curved array probe) identified the midline, the intervertebral space, and the distance from the skin to the epidural space (ultrasound depth/UD). During the epidural puncture, we recorded the success of the insertion point, and measured the distance to the epidural space to the nearest half-centimeter of the marked Tuohy needle (needle depth/ND). We calculated the agreement between UD and ND by the concordance correlation coefficient and Bland–Altman analysis with 95% limits of agreement. RESULTS: The average maternal age was 33 4.6 yr, body mass index 29.7 4.8, UD 4.66 0.68 cm, and ND 4.65 0.72 cm. The success of the insertion point was 91.8%, with no need to redirect the needle in 73.8% of the patients. The concordance correlation coefficient between UD and ND was 0.881 (95% CI 0.820–0.942). The 95% limits of agreement were 0.666 to 0.687 cm. CONCLUSIONS: We found a good level of success in the ultrasound-determined insertion point, and very good agreement between UD and ND. This suggests that our proposed ultrasound single-screen method, using the transverse approach, can be a reliable guide to facilitate labor epidural insertion. (Anesth Analg 2007;104:1188‐92)

215 citations

Journal ArticleDOI
TL;DR: The speckle reduction is analyzed numerically and the results are found to be in excellent agreement with existing theory, and an investigation of detectability of low-contrast lesions shows significant improvements compared to conventional imaging.

201 citations

Journal ArticleDOI
T. Grau1, Erika Bartusseck1, R. Conradi1, Eike Martin1, Johann Motsch1 
TL;DR: Using ultrasound imaging for teaching epidural anesthesia in obstetrics the authors found a higher rate of success during the first 60 attempts compared to conventional teaching, which shows the possible value of ultrasound Imaging for teaching and learning obstetric regional anesthesia.
Abstract: Epidural anesthesia may be difficult in pregnancy. We intended to evaluate the teaching possibilities of ultrasonography as a diagnostic approach to the epidural region. Two groups of residents performed their first 60 obstetric epidurals under supervision. One proceeded in the conventional way using the loss of resistance technique (control group = CG). The other group proceeded in the same way but was supported by prepuncture ultrasound imaging, giving them information about the optimal puncture point, depth and angle (ultrasound group = UG). Success was defined as adequate epidural anesthesia requiring a maximum of three attempts, reaching a visual analogue scale score of less than I, while neither changing the anesthesia technique, nor starting at another vertebral level. In addition, intervention by the supervisor was defined as failure. In the CG we observed asuccess rate of 60% ± 16% after the first ten attempts followed by a nearly continuous rise of the learning curve. Within the next 50 epidurals the rate of success increased to 84%. In the UG the rate of success started at 86% ± 15%. Wthin 50 epidural insertions it rose up to a level of 94%. The difference between the two groups remained significant (P < 0.001). Using ultrasound imaging for teaching epidural anesthesia in obstetrics we found a higher rate of success during the first 60 attempts compared to conventional teaching. We believe this shows the possible value of ultrasound imaging for teaching and learning obstetric regional anesthesia.

201 citations


"Adaptive spatial compounding for im..." refers background in this paper

  • ...Moreover, it is suggested that the use of ultrasound images to get apriori estimates of a suitable puncture point and needle insertion angle greatly improves the learning curve of inexperienced doctors and failures happen less frequently[4]....

    [...]

  • ...A study shows two groups of residents performed epidural anesthesia, one with the help of ultrasound (ultrasound group) and the other without (control group)[4]....

    [...]

Journal ArticleDOI
T. Grau1, R. W. Leipold1, S. Fatehi1, Eike Martin1, Johann Motsch1 
TL;DR: Real-time ultrasonic scanning of the lumbar spine is an easy procedure that provides an accurate reading of the location of the needle tip and facilitates the performance of combined spinal-epidural anaesthesia.
Abstract: SummaryBackground and objective: The quality of combined spinal–epidural anaesthesia mainly depends on accurate identification of the epidural space. The real-time ultrasound control of the procedure for puncture was therefore evaluated.Methods: Thirty parturients scheduled for Caesarean section were randomized to three equal groups. Ten control patients received conventional combined spinal–epidural anaesthesia. Ten of the remaining patients received ultrasonic scans by an offline scan technique, and 10 received online imaging of the lumbar region during epidural puncture. The epidural space was identified and needle advancement was surveyed through the interspinal and flaval ligaments. The number of attempts to advance the needle to achieve a successful puncture was measured and compared, as well as the number of vertebral interspaces punctured before successful entry into the epidural space.Results: There was no difference between patient characteristics in the three groups. The visualization of the epidural structures and of the needle manipulations was very effective. In the ultrasound group, the reduction in the number of attempts at puncture was significant (P < 0.036). The number of interspaces necessary for puncture was reduced (P < 0.036) in the ultrasound online group compared with controls. The number of spinal needle manipulations was significantly reduced (P < 0.036).Conclusions: Real-time ultrasonic scanning of the lumbar spine is an easy procedure. It provides an accurate reading of the location of the needle tip and facilitates the performance of combined spinal–epidural anaesthesia.

176 citations


"Adaptive spatial compounding for im..." refers background in this paper

  • ...A failed epidural procedure in which the needle goes beyond the epidural space and perforates the dura mater may cause the patient to experience headaches and, in more severe cases, paralysis or death[2]....

    [...]