Synthetic Methods for Ultrasound-Guided Epidural Insertion Simulation1 (2014)

This work presents developed methods for simulating ultrasound of a patient's lumbar spine of various body mass index (BMI), for ultrasound guided needle insertion. In 2008 National Institute for Health and Clinical Excellence (NICE) introduced new interventional procedure guidance for using ultrasound during epidural needle insertions. There are six main benefits that ultrasound guidance offers the epidural procedure (i) identifying the midline (longitudinal view) (ii) identifying the sacrum (iii) finding the level L3/L4 interspace (articular processes) (iv) identifying the ligamentum flavum (v) identify angle of insertion (vi) measuring the depth of the epidural space.Currently there are no simulators allowing ultrasound guided epidural practice. Ultrasound is a desirable method to measure epidural space depth and tissue layer thickness because compared to alternatives like X-ray, magnetic resonance imaging (MRI) or fluoroscopy because it is radiation free, nonhazardous, real-time, relatively cheap, can view any angle, and can target small remote tissue areas.This work developed computer-based synthetic ultrasound methods to enable needle insertion training. Training with 3D spine models improves ultrasound performance of the spine in vivo [1]. Spare ultrasound machines are not always available for trainees to practice. Ultrasound can be generated on the graphics processing unit, such as by using ray-casting [2]. Anaesthesia simulators with incorporated simulated ultrasound will strengthen skill of trainees [3].A clinical trial was arranged to recruit twenty obstetric patients at Poole Hospital NHS Foundation Trust. We took ultrasound scans from patients prior to their epidural. After the epidural MRI scans were taken within 72 h to measure ligament thicknesses and depths. Patient information sheets and recruitment forms were completed prior.Novel aspects of this work include simulated ultrasound for patients of various BMI. In simulated scans on obese or various sized patients the simulated needle depths vary according to tissue layer thickness. Anaesthetists find it difficult in overweight and obese patients to correctly place epidural needle due to difficult palpation.The objectives for obtaining MRI and ultrasound were to take high accuracy measurements describing how ligament thicknesses and epidural space depth change in various BMI patients. Obtained images were also incorporated into an MRI and ultrasound simulator for epidural. Experienced MRI radiographers identified tissue boundaries for measurement.The ultrasound measurements from patients of various BMI depict how ligament thicknesses correlate to patient BMI or to the pressures that we recorded during needle insertion which allows the simulator to model BMI variation (Fig. 1). It would not otherwise have been possible to measure tissue thicknesses in vivo. Since the images were taken during or directly after pregnancy, obstetric patients may have higher proportions of body fat than the average patient population.The MRI images were able to show depth and thickness of ligaments and fat which are pierced by the epidural needle such as ligamentum flavum, epidural space, interspinous and supraspinous ligaments, subcutaneous fat layers, and skin thickness as are labeled in Fig. 2 showing MRI from obstetric patient. The simulated patient BMI can be varied based on anthropometric measurements of the patient's parameters.For simulating ultrasound a modeled 3D ultrasound probe was created. A panel that simulates the ultrasound view is on the screen during the ultrasound guided epidural procedure (Fig. 3). Algorithms were developed which use image processing to model a synthetic ultrasound image based on MRI data, and Fig. 4 shows resulting display after processing and image filters. Synthetic ultrasound needs to incorporate many characteristics to be realistic such as speckle, range distortion, refraction, and acoustic shadowing. These real-time artifacts are essential for the interpretation of ultrasound.Alternative methods to display simulated ultrasound were developed by recording ultrasound video from patients in vivo. The probe was moved at a constant rate to known locations on the back to enable tracking position over time. The videos were then processed to extract individual frames and stored as images with reference to the location on the back. During simulation these images are loaded corresponding to the location of the simulated probe so that actual ultrasound images are displayed in real time (Fig. 5).The ultrasound training system is based around the NICE guidelines which are built into the system as learning objectives and scoring outcomes show accuracy of the trainee's ultrasound measurements. The measurement simulation allows the user to measure lengths between two points on the display to identify depth from skin surface to epidural space and the thickness of each tissue layer (Fig. 5). When measurements are made, scoring can be given for accuracy of marker positioning for ultrasound measurements.The resulting synthetic ultrasound is incorporated into the epidural simulator for ultrasound guided needle insertion using a modeled ultrasound probe. The probe can be moved to any location on the vertebrae at any angle in either transverse or longitudinal alignment. The generated synthetic ultrasound images update in real time from volumetric data.Ultrasound provides real time internal views of lumbar back anatomy and additionally allows the epiduralist to measure directly the depth from skin to epidural space, since it is vital for epidural to monitor the depth of the needle so that it does not proceed too deeply and cause injuries.Ultrasound is becoming increasing common for guided needle insertion. NICE guidelines considerably increase the chances of successfully identifying the bone landmarks such as spinous processes and the identification of correct spinal levels which are commonly used such levels as L2/L3 or L3/L4. There are currently limited methods available for trainees to become competent without practicing on real patients and it is envisaged that a computer based simulator would result in enhanced epidural training and reduced risk.Our MRI scans were T1-weighted which highlight fat, proteins or white matter as light grey, whereas water or body fluid such as contrast-sensitivity function or blood is dark. By comparing the 20 patient MRIs, ligaments on the obese show brighter, indicating adipose tissue collections and possibly lower density. In lower BMI patients, ligaments may appear darker since the ligaments were low fat, high density. Previous work into locating tissue density from MRI includes tissue density of breasts can be found from MRI scan [4]. CT scanning can also find tissue density since hardest tissue such as bone shows brightest. The volumetric MRI tissue density distribution data can then be used as a basis for generating haptic feedback from the imaged anatomical tissues.