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.

Ultrasound has been proposed for aiding epidural needle insertion, but challenges remain in differentiating spinal structures due to noise, artifacts, and inexperience by anesthesiologists in ultrasound interpretation. Moreover, the anesthesiologist needs to measure relevant distances while preserving sterile conditions; therefore, interaction with the ultrasound controls must be minimal. Automated measurement is needed. Beam-steered ultrasound images are captured and spatial compounding is used to improve image quality. Phase symmetry is used to enhance bone (lamina) and ligamentum flavum (LF) ridges. A lamina template is matched to this ridge map using Pearson's cross-correlation, and the most likely lamina positions are found. Then, the lamina is traversed using a LF template with the Pearson's cross-correlation, and the location of the LF is obtained. Tests are performed on 39 sets of compounded ultrasound images in the L2-3 and L3-4 levels of the spine in the paramedian plane. The proposed algorithm can detect the laminas in 38 of the 39 images, and the LF in 34 of the 39 images. In successful detections, the automatic detections versus manual segmentation has an rms error of 0.64 mm and average error 0.04 mm, versus independent sonographer-measured depth has a root-mean-squared error of 3.7 mm and average error 2.5 mm, and versus the actual needle insertion depth has a root-mean-squared of 5.1 mm and average error -2.8 mm. The computational time is 4.3 s on a typical personal computer. The accuracy, reliability, and speed suggest this method may be valuable for helping guide epidurals in conjunction with the traditional loss-of-resistance method.

Epidural anesthesia is the most common form of anesthesia in obstetrics. The loss-of-resistance to saline injection is used to confirm when the needle tip enters the epidural space. This procedure is highly dependent on skill and expertise, so it is useful to quantify the tissue resistance during insertion. Sensors are used to measure the force and displacement of the plunger of the syringe and the pressure at the needle tip. A model is also developed to estimate the pressure from the force and displacement. Tests are first performed on porcine tissue to compare the continuous-pressure and intermittent-pressure versions of the technique and to compare the paramedian and midline needle approaches. The accuracy of the pressure model is 12% of peak pressure for the continuous technique and 20% for the intermittent technique. Significant differences in injection flow rate were also found for the muscle, interspinous ligament, and ligamentum flavum encountered in the two approaches. A small clinical study on human subjects was performed and again significant differences were found in flow rate for different tissues. These quantitative results improve the understanding of small differences in feel that have been previously known qualitatively and may help in the development of simulators.

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Purpose In conventional practice of epidural needle placement, determining the interspinous level and choosing the puncture site are based on palpation of anatomical landmarks, which can be difficult with some subjects. Thereafter, the correct passage of the needle towards the epidural space is a blind ''feel as you go'' method. An aim-and-insert single-operator ultrasound-guided epidural needle placement is described and demonstrated. Method Nineteen subjects undergoing elective Cesarean delivery consented to undergo both a pre-puncture ultrasound scan and real-time paramedian ultrasound-guidance for needle insertion. Following were the study objectives: to measure the success of a combined spinal-epidural needle insertion under real-time guidance, to compare the locations of the chosen interspinous levels as determined by both ultrasound and palpation, to measure the change in depth of the epidural space from the skin surface as pressure is applied to the ultrasound transducer, and to investigate the geometric limitations of using a fixed needle guide. Results One subject did not participate in the study because pre-puncture ultrasound examination showed unrecognizable bony landmarks. In 18 of 19 subjects, the epidural needle entered the epidural space successfully, as defined by a loss-of-resistance. In two subjects, entry into the epidural space was not achieved despite ultrasound guidance. Eighteen of the 19 interspinous spaces that were identified using palpation were consistent with those determined by ultrasound. The transducer pressure changed the depth of the epidural space by 2.8 mm. The measurements of the insertion lengths corresponded with the geometrical model of the needle guide, but the needle required a larger insertion angle than would be needed without the guide. Conclusion This small study demonstrates the feasibility of the ultrasound-guidance technique. Areas for further development are identified for both ultrasound software and physical design. RésuméObjectif Dans le positionnement traditionnel de l'aiguille pe´ridurale, l'identification du niveau intere´pineux et le choix du site de ponction se fondent sur la palpation de points de repe`re anatomiques, ce qui peut s'ave´rer difficile chez certains patients. Par la suite, le passage appropried e l'aiguille vers l'espace pe´ridural est re´alise´par une me´thode en aveugle sur la base de sensation subjective pendant l'insertion. Nous de´crivons et de´montrons ici le positionnement d'une aiguille pe´ridurale e´choguide´e par un ope´rateur unique en 'ciblant et inse´rant'. Méthode Dix-neuf patientes subissant un accouchement par ce´sarienne non urgent ont consenti a`subir un Electronic supplementary material The online version of this article (doi:10.1007/s12630-009-9252-1) contains supplementary material, which is available to authorized users.