**4. Pressure measurement for realistic epidural simulation**

A sterile wireless measurement device was developed to record the resultant pressure of the saline inside the syringe during an epidural needle insertion. This measurement device is used to enable data collection to quantify the pressure during the epidural procedure. Quantifying the pressure will enable accurate configuration of an epidural simulator.

Our novel pressure measurement device has wireless functionality and by using entirely sterile components allows in-vivo trials to be conducted with patients. A wireless data transmitter is utilized to minimize the equipment and disruption in the hospital room (Figure 2).

**Figure 2.** Remotely monitored wireless epidural pressure measurement system.

The design aims to minimise changes to the standard epidural set up. A small sterile threeway tap (BD ConnectaTM) is connected between the Tuohy needle and syringe (Figure 3). The tap is connected to the pressure transducer via a one metre length of saline-filled sterile manometer tubing. The transducer's electrical plug is connected by a short electrical cable to our wireless transmitter. At the remote site, a wireless receiver is connected via Universal Serial Bus (USB) to the computer.

The UTAH Medical Deltran disposable transducer is used for the pressure measurement sensor. These transducers are commonly used in hospitals to monitor systemic blood pressure and central venous pressure. Transducers produce a small electrical signal based on the pressure of the liquid inside the manometer tubing. Disposable transducers are designed to have accuracy of +/- 3% with the average output of 100.03 +/- 0.55 mm Hg and the worst cases being 98.53 and 101.36 when 100 mm Hg was applied [10].

Biomedical Engineering in Epidural Anaesthesia Research 393

human spinal research and can be a representative anatomical model for the human spine and tissues [12]. The porcine tissue specimen was a double loin saddle cut. The cadaver was obtained from a livestock farm within 24 hours of slaughter without being frozen or modified in any way to avoid desiccation and deterioration of the spinal tissues which would affect the pressure measurements. The pig was a standard hybrid Large White cross Saddleback. The specimen contained the entire back in one piece, with the whole spine, and all tissue layers from external skin, through to the thoracic cavity. The porcine tissue was mounted vertically against a wooden support to mimic sitting position, resting upon, but

Epidural insertions were performed by two experienced anaesthetists. The epidural space was located using a Portex 16-gauge Tuohy needle (Smiths Medical International Ltd, Kent, UK) at L2/3 or L3/4 intervertebral levels using a midline approach. Subsequently a number of different vertebral levels ranging from T12-L5 were targeted. The porcine spine was palpated to locate anatomical landmarks prior to insertion. The Tuohy needle with its introducer stylet penetrates the skin as is standard procedure. The recordings of pressure were then started and continuously recorded throughout needle insertion until after the loss

The majority of insertions located the epidural space during the first attempt. Data from hitting bone was also recorded to analyse the effect on pressure. In some cases, the number of attempts to find the space was greater than three so those recordings were abandoned. The maximum pressure during ligamentum flavum was 500 mmHg. The highest pressures

The results demonstrated that during needle insertion the saline pressure started low and gradually built up, although the increase was not entirely steady due to the various tissues encountered. A similar pressure trend was found; a depression occurred on insertion 2 during 3-6 seconds and insertion 3 during 12-15 seconds (Figure 6, circular area). This may have been caused by the interspinous ligament and the pressure required to traverse this was 350 mmHg on insertion 2 and 470 mmHg on insertion 3. The final peak pressure was 500 mmHg which was caused by the ligamentum flavum (Figure 6, rectangular area). It was

not attached onto, a platform beneath (Figure 5).

**Figure 5.** Porcine cadaver set up for Tuohy needle insertions

were obtained when the Tuohy needle hit bone.

of resistance had been experienced.

**Figure 3.** Wireless Device for recording measured pressure of saline during insertion.

The computer can process pressure data, display a real-time graph on screen and simultaneously record the data to a file. When the anaesthetist presses on the syringe plunger, the saline inside the syringe is pressurised and the device quantifies this pressure. The computer runs our custom built software (Figure 4) which monitors pressure data as it arrives [11]. The software displays the live data on screen in the form of a real-time graph, can save graphs as images to file and writes data to a text file. The data files can be used for further analysis using statistical software. Before each insertion, the graph and start-time are reset and a new data file is created. Pressure can be converted into various units. In the current implementation the pressure is measured in mmHg or kPa and also a provision is given to determine force on the plunger in Newtons. This directly provides actual pressure measurement of saline inside the needle as applied to a continuum. To test this device a pilot trial was conducted on a porcine cadaver.

**Figure 4.** Screen print of the software to monitor and record pressure of saline during insertion

#### **5. Trial on porcine cadaver**

A trial using a section of a porcine cadaver was conducted to test the pressure measurement device during epidural insertions. The pig is claimed to be the closest animal model for human spinal research and can be a representative anatomical model for the human spine and tissues [12]. The porcine tissue specimen was a double loin saddle cut. The cadaver was obtained from a livestock farm within 24 hours of slaughter without being frozen or modified in any way to avoid desiccation and deterioration of the spinal tissues which would affect the pressure measurements. The pig was a standard hybrid Large White cross Saddleback. The specimen contained the entire back in one piece, with the whole spine, and all tissue layers from external skin, through to the thoracic cavity. The porcine tissue was mounted vertically against a wooden support to mimic sitting position, resting upon, but not attached onto, a platform beneath (Figure 5).

Epidural insertions were performed by two experienced anaesthetists. The epidural space was located using a Portex 16-gauge Tuohy needle (Smiths Medical International Ltd, Kent, UK) at L2/3 or L3/4 intervertebral levels using a midline approach. Subsequently a number of different vertebral levels ranging from T12-L5 were targeted. The porcine spine was palpated to locate anatomical landmarks prior to insertion. The Tuohy needle with its introducer stylet penetrates the skin as is standard procedure. The recordings of pressure were then started and continuously recorded throughout needle insertion until after the loss of resistance had been experienced.

**Figure 5.** Porcine cadaver set up for Tuohy needle insertions

392 Practical Applications in Biomedical Engineering

**Figure 3.** Wireless Device for recording measured pressure of saline during insertion.

pilot trial was conducted on a porcine cadaver.

**5. Trial on porcine cadaver** 

The computer can process pressure data, display a real-time graph on screen and simultaneously record the data to a file. When the anaesthetist presses on the syringe plunger, the saline inside the syringe is pressurised and the device quantifies this pressure. The computer runs our custom built software (Figure 4) which monitors pressure data as it arrives [11]. The software displays the live data on screen in the form of a real-time graph, can save graphs as images to file and writes data to a text file. The data files can be used for further analysis using statistical software. Before each insertion, the graph and start-time are reset and a new data file is created. Pressure can be converted into various units. In the current implementation the pressure is measured in mmHg or kPa and also a provision is given to determine force on the plunger in Newtons. This directly provides actual pressure measurement of saline inside the needle as applied to a continuum. To test this device a

**Figure 4.** Screen print of the software to monitor and record pressure of saline during insertion

A trial using a section of a porcine cadaver was conducted to test the pressure measurement device during epidural insertions. The pig is claimed to be the closest animal model for The majority of insertions located the epidural space during the first attempt. Data from hitting bone was also recorded to analyse the effect on pressure. In some cases, the number of attempts to find the space was greater than three so those recordings were abandoned. The maximum pressure during ligamentum flavum was 500 mmHg. The highest pressures were obtained when the Tuohy needle hit bone.

The results demonstrated that during needle insertion the saline pressure started low and gradually built up, although the increase was not entirely steady due to the various tissues encountered. A similar pressure trend was found; a depression occurred on insertion 2 during 3-6 seconds and insertion 3 during 12-15 seconds (Figure 6, circular area). This may have been caused by the interspinous ligament and the pressure required to traverse this was 350 mmHg on insertion 2 and 470 mmHg on insertion 3. The final peak pressure was 500 mmHg which was caused by the ligamentum flavum (Figure 6, rectangular area). It was

also noted that after the final drop of pressure there was often a 'step' before the bottom pressure was reached (square area). One explanation is that the initial pressure is the effect of opening up the epidural space which is a potential space and also saline pushing the dura away.

Biomedical Engineering in Epidural Anaesthesia Research 395

information about the depth of ligaments. We have developed image processing algorithms

During the epidural insertion procedure, the needle is slowly advanced through layers of tissue into the epidural space which is on average somewhere between 40-80mm deep. It is possible to record the depth of the needle tip by viewing the 10mm markings printed on the metallic needle; however, it is important to measure the needle depth precisely so that the needle travel can be guided with available measurements from techniques such as ultrasound scanning or magnetic resonance imaging for precise needle placement in the actual procedure. We have developed a novel image processing technique which aims to measure insertion depth of an epidural Tuohy needle in real-time. The implemented technique uses a single wireless camera to transmit depth data remotely to a host computer. Combining length and pressure data enables more accurate interpretation of the data in that the various changes in pressure can be linked to the actual depth at which the changes

The 16 gauge Portex Tuohy needle of 80mm length (Figure 7) is the most common epidural needle used in hospitals. The needle has grey and silver markings on the metallic shaft at 10mm intervals which are used by the software as a reference length. The blue handle is the plastic part at the base which is held by the operator and connected to a LOR syringe. This is

The actual technique of length and size measurement by digital image processing is well established, however, in this specific circumstance, image processing is much more complex and challenging due to many reasons; (i) the needle is a thin, narrow object, (ii) the needle is composed of reflective stainless steel, (iii) the needle is circular in cross-section causing colour changes around the shaft of the needle, (iv) wireless camera introduces transmission noises, (v) as the needle is tilted it reflects in different directions, (vi) the needle will not be the only object in the foreground due to the operator's hands and patient's back, (vii) lighting conditions vary from room to room. We have overcome these problems by advanced analysing techniques focusing on a small area of the dynamic environment.

The actual technique involves placing a wireless camera in the procedure room, one metre away from the needle insertion, which will transmit data to a remotely located computer.

to measure the needle depth by a wireless camera during insertion [13].

**Figure 7.** Properties of the 80mm Tuohy needle used for image processing

occurred.

used for colour detection.

**Figure 6.** Pressure recordings durings two successful insertions to the epidural space.

The opinion of the trial anaesthetists was that porcine tissue did feel like a close approximation to human tissue and the shape of the graphs were similar to graphs previously reported from human insertions [8]. In most cases the resulting pressure-time graphs clearly show a drop when the loss of resistance occurred as the needle entered the epidural space (Figure 6). The maximum pressure peak during successful insertions ranged from 470 to 500 mmHg (62.7 - 66.7 kPa) caused by ligamentum flavum. After this the needle tip enters the epidural space causing a sudden loss of pressure back to the starting pressure. The shapes of each graph in successive trials were similar but also different to reflect individual variations.

The results of this pilot trial demonstrate that the wireless pressure measuring system is accurate and responsive in the porcine model. Such measurements from patients could be used to create realistic epidural simulators.

#### **6. Image processing for non-contact needle depth measurement**

The reason why needle depth is important is that it relates the depths at which each resultant pressure occurred during the epidural procedure. This can also provide information about the depth of ligaments. We have developed image processing algorithms to measure the needle depth by a wireless camera during insertion [13].

394 Practical Applications in Biomedical Engineering

away.

individual variations.

used to create realistic epidural simulators.

also noted that after the final drop of pressure there was often a 'step' before the bottom pressure was reached (square area). One explanation is that the initial pressure is the effect of opening up the epidural space which is a potential space and also saline pushing the dura

**Figure 6.** Pressure recordings durings two successful insertions to the epidural space.

The opinion of the trial anaesthetists was that porcine tissue did feel like a close approximation to human tissue and the shape of the graphs were similar to graphs previously reported from human insertions [8]. In most cases the resulting pressure-time graphs clearly show a drop when the loss of resistance occurred as the needle entered the epidural space (Figure 6). The maximum pressure peak during successful insertions ranged from 470 to 500 mmHg (62.7 - 66.7 kPa) caused by ligamentum flavum. After this the needle tip enters the epidural space causing a sudden loss of pressure back to the starting pressure. The shapes of each graph in successive trials were similar but also different to reflect

The results of this pilot trial demonstrate that the wireless pressure measuring system is accurate and responsive in the porcine model. Such measurements from patients could be

The reason why needle depth is important is that it relates the depths at which each resultant pressure occurred during the epidural procedure. This can also provide

**6. Image processing for non-contact needle depth measurement** 

During the epidural insertion procedure, the needle is slowly advanced through layers of tissue into the epidural space which is on average somewhere between 40-80mm deep. It is possible to record the depth of the needle tip by viewing the 10mm markings printed on the metallic needle; however, it is important to measure the needle depth precisely so that the needle travel can be guided with available measurements from techniques such as ultrasound scanning or magnetic resonance imaging for precise needle placement in the actual procedure. We have developed a novel image processing technique which aims to measure insertion depth of an epidural Tuohy needle in real-time. The implemented technique uses a single wireless camera to transmit depth data remotely to a host computer. Combining length and pressure data enables more accurate interpretation of the data in that the various changes in pressure can be linked to the actual depth at which the changes occurred.

The 16 gauge Portex Tuohy needle of 80mm length (Figure 7) is the most common epidural needle used in hospitals. The needle has grey and silver markings on the metallic shaft at 10mm intervals which are used by the software as a reference length. The blue handle is the plastic part at the base which is held by the operator and connected to a LOR syringe. This is used for colour detection.

**Figure 7.** Properties of the 80mm Tuohy needle used for image processing

The actual technique of length and size measurement by digital image processing is well established, however, in this specific circumstance, image processing is much more complex and challenging due to many reasons; (i) the needle is a thin, narrow object, (ii) the needle is composed of reflective stainless steel, (iii) the needle is circular in cross-section causing colour changes around the shaft of the needle, (iv) wireless camera introduces transmission noises, (v) as the needle is tilted it reflects in different directions, (vi) the needle will not be the only object in the foreground due to the operator's hands and patient's back, (vii) lighting conditions vary from room to room. We have overcome these problems by advanced analysing techniques focusing on a small area of the dynamic environment.

The actual technique involves placing a wireless camera in the procedure room, one metre away from the needle insertion, which will transmit data to a remotely located computer. The camera transmits a 640x480 pixel image in full colour over a 20MHz wireless link. The computer contains the image processing algorithm to detect the visible needle in the image and measure its length. The first step in the algorithm is to automatically calibrate the background model. For ten seconds, with no objects in the foreground, the colour values (HSV) for each pixel are analyzed. Maximum and minimum values are stored in an array and used later as a background model. HSV values from each frame are compared to the background model. Foreground objects are identified by HSV values outside of the expected range. The pixels from foreground objects are scanned for HSV values which match the blue handle. The centre point of the blue handle is found by taking an average and is stored for object tracking in subsequent frames, and is assumed to be approximately at the level of the needle shaft. The rightmost edge is stored and assumed to be the start point of the metal shaft. The blue handle is removed from further processing. The algorithm scans horizontally from the position of the blue handle to find the metal needle shaft by matching HSV values. The leftmost and rightmost pixels in the metal shaft are identified. These are stored for tracking in subsequent frames. At this point a strip of image remains over the needle. For each column, an average HSV value is taken. This average is used to create four separate histograms for H, S, V and the total along the length of the metallic shaft. The histograms identify sudden changes in colour, caused by the boundaries between 10mm markings (Figure 8). Histograms make the markings more detectable under reflective conditions. The number of visible 10mm markings is counted. The number of pixels in each division is counted to find how many pixels equate to 10mm. If the final marking is only partially visible the length is calculated by comparing it to a full division.

Biomedical Engineering in Epidural Anaesthesia Research 397

when the needle was 500mm from the camera. The graph shows two erroneous readings at about 4 and 8 seconds, which was due to camera noise and in these frames the needle shaft was not detected properly, but all other frames were successfully measured and verified by the actual measurement. The total insertion took about fifteen seconds with 10 frames per second. The failure rate was 3 frames out of 150 which gave an overall 97.8% reliability during this insertion [13]. Errors like this could potentially be removed by ignoring sudden jumps in the data. The graph currently displays length but this can be converted from length to needle depth by simply subtracting the value from 80 mm, which is the total length of the

The distance between needle and camera can be varied because length is measured using the 10mm markings as a reference length. At distances over 150cm the reliability dropped but this could be improved with a higher resolution camera. The needle can be tilted up or down to +/-30 without any adverse effect to measurements. Tilting towards or away from the camera does not affect measurement as long as the divisions are clearly visible because division length differentiates between length reductions caused by tilt and caused by insertion. Failures occurred on some frames, due to blur in the image, or at certain angles where silver and grey areas became merged. The background model successfully removed

In order to simulate the whole epidural procedure a realistic user interface must be provided together with the flexibility of 3D visualization and haptic interaction. The 3D models for the epidural simulator were generated with an object modelling software. Each vertebra is an individual wireframe model, constructed from 514 vertices. The vertices are positioned and then wrapped by a texture. Shadows and light sources are applied through OpenGL interfaces. The spine in the simulator contains 26 separate objects for the thoracic, cervical and lumbar spinal vertebrae, sacrum and coccyx. Layers of tissue, fat, muscle and

the majority of background, even with cluttered multi-colour backgrounds.

**Figure 9.** Software showing plot of needle length during insertion

**7. 3D spine modelling for epidural training** 

needle.

**Figure 8.** Output of the algorithm to measure needle length with histograms

The image processing algorithm was tested during insertions. The needle was successfully detected and measured accurately in most frames. The developed software was used to draw a graph of the length in real time and write the length data into a data file. Figure 9 shows the graph during an insertion in which the needle was slowly advanced and then rapidly withdrawn. We found that length measurement was accurate to within +/-3mm, when the needle was 500mm from the camera. The graph shows two erroneous readings at about 4 and 8 seconds, which was due to camera noise and in these frames the needle shaft was not detected properly, but all other frames were successfully measured and verified by the actual measurement. The total insertion took about fifteen seconds with 10 frames per second. The failure rate was 3 frames out of 150 which gave an overall 97.8% reliability during this insertion [13]. Errors like this could potentially be removed by ignoring sudden jumps in the data. The graph currently displays length but this can be converted from length to needle depth by simply subtracting the value from 80 mm, which is the total length of the needle.

**Figure 9.** Software showing plot of needle length during insertion

396 Practical Applications in Biomedical Engineering

The camera transmits a 640x480 pixel image in full colour over a 20MHz wireless link. The computer contains the image processing algorithm to detect the visible needle in the image and measure its length. The first step in the algorithm is to automatically calibrate the background model. For ten seconds, with no objects in the foreground, the colour values (HSV) for each pixel are analyzed. Maximum and minimum values are stored in an array and used later as a background model. HSV values from each frame are compared to the background model. Foreground objects are identified by HSV values outside of the expected range. The pixels from foreground objects are scanned for HSV values which match the blue handle. The centre point of the blue handle is found by taking an average and is stored for object tracking in subsequent frames, and is assumed to be approximately at the level of the needle shaft. The rightmost edge is stored and assumed to be the start point of the metal shaft. The blue handle is removed from further processing. The algorithm scans horizontally from the position of the blue handle to find the metal needle shaft by matching HSV values. The leftmost and rightmost pixels in the metal shaft are identified. These are stored for tracking in subsequent frames. At this point a strip of image remains over the needle. For each column, an average HSV value is taken. This average is used to create four separate histograms for H, S, V and the total along the length of the metallic shaft. The histograms identify sudden changes in colour, caused by the boundaries between 10mm markings (Figure 8). Histograms make the markings more detectable under reflective conditions. The number of visible 10mm markings is counted. The number of pixels in each division is counted to find how many pixels equate to 10mm. If the final marking is only partially

visible the length is calculated by comparing it to a full division.

**Figure 8.** Output of the algorithm to measure needle length with histograms

The image processing algorithm was tested during insertions. The needle was successfully detected and measured accurately in most frames. The developed software was used to draw a graph of the length in real time and write the length data into a data file. Figure 9 shows the graph during an insertion in which the needle was slowly advanced and then rapidly withdrawn. We found that length measurement was accurate to within +/-3mm, The distance between needle and camera can be varied because length is measured using the 10mm markings as a reference length. At distances over 150cm the reliability dropped but this could be improved with a higher resolution camera. The needle can be tilted up or down to +/-30 without any adverse effect to measurements. Tilting towards or away from the camera does not affect measurement as long as the divisions are clearly visible because division length differentiates between length reductions caused by tilt and caused by insertion. Failures occurred on some frames, due to blur in the image, or at certain angles where silver and grey areas became merged. The background model successfully removed the majority of background, even with cluttered multi-colour backgrounds.
