Ultrasound simulator and recording system
A free-hand recording system as part of an ultrasound simulation system (Ultrasound simulator Reslice, registered trademark by Schallware GmbH, Berlin, Germany, http://www.schallware.de)  was coupled to a high-end ultrasound device (Vivid-i, registered trademark by GE Healthcare Ultrasound, Solingen, Germany) with a 12 MHz linear probe (12L). The free-hand recording system consists of a PC with Ubuntu software platform (Linux-based public license, non-commercial software, version 7.10, http://www.ubuntu.com), an electromagnetic tracking system, a mannequin and a probe dummy with localization transmitter, and also consists of a PC with recording software (Schallware Acquisition 2007, Fa. Schallware, Berlin, Germany), and an additional electromagnetic tracking system. The ultrasound device was also coupled with a VGA2USB-LR converter (Epiphan Company, Springfield, NJ, USA) to get a lossless video-real time compression with high frame rates from up to 80 frames per second (fps). To save position of the probe of the ultrasound device once, the probe had to be connected and calibrated with a localization transmitter. Calibration of the probe had only to be done once before the beginning of the trial by the manufacturer of the ultrasound simulator. Frame rates while recording 3D-UV were always above 30 fps and could reach 80 fps depending on the number of focus points. 3D-UV regularly were recorded as transverse parallel planes.
Acquisition of 3D-UV and quality assessment
Study population, recruitment of models
With informed consent, four healthy models (n = 2 females, age 26 and 24 years; and 2 males, age 17 and 24 years) served to obtain normal sonoanatomy. Models had no co-morbidity and no pathologic finding, and the ultrasound exam of respective body areas was recorded. The staff of the university hospital were recruited by announcements. Inclusion criteria were: age above 18 years and a body mass index (BMI) of less than 30; exclusion criteria were: age below 18 years and BMI over 30.
Setting, location and team
Data were collected at the Hospital of the University of Frankfurt am Main. For recordings within a standardized environment, a room of the emergency department was equipped with the complete ultrasound simulator system and regularly maintained by two staff technicians. One medical doctor recorded 3D-UV in a team with the technician on a model. The medical doctor is an expert practitioner on basic and intermediate UGRA level blocks . The staff technician was educated by the manufacturer to perform ultrasound 3D-UV recordings. Recordings were viewed later and selected by eye-balling, whenever a target anatomical site was clearly identified. Those 3D-UV were collected and stored in a case library on the ultrasound simulation system and combined to 3D-multivolumes (3D-MV).
On every model, respective regions of the right body area were scanned. Recordings were also obtained as 2D-B-Mode images or clips by the ultrasound device. After recording and storing, 3D-UV were positioned with an electromagnetic tracking system into a soft foam mannequin to the corresponding anatomical sites originally obtained on the model. Using developer software, single B-Mode 3D-UV were combined into 3D-MV. The advantage of 3D-MV is that one single case of the ultrasound simulation can combine multiple 3D-UV and allows more examinations and workflow without changing or interrupting by the operator. This system allows operators to examine any position in similar way as using an ultrasound device and can be performed near-real close to the examination of a model or patient. Image adaptations can be made related to depth, contrast and gain. For documentation, a freeze function and storing of screenshots are available which can be evaluated by trainers.
Single recordings of 3D-UV were continuous transversal views along the body axis (“horizontal sweep”). Regions included carotid trigonum for jugular vein, carotid artery and soft tissues or bone, interscalene region to record brachial plexus, peripheral nerves distal to the axilla (median, ulnar, radial and musculocutaneous), inguinal region for femoral vessels and femoral nerve and popliteal crease for distal sciatic nerve and branches (popliteal and tibial branch).
Evaluation of reslices and statistical methods
Neither a reference method for UGRA was described nor does a reference ultrasound machine or setting serve as a gold standard. High-resolution ultrasound technology using high frequency ultrasound (>10 MHz) is considered to be somewhat a major prerequisite. Therefore, no blinding for the test results of 3D-UV was included.
Original SAX and long axis (LAX) B-Mode planes were compared with the reconstructed planes (=reslices) by the expert practitioner who recorded the 3D-UV. Image quality was assessed by eye-balling always during repositioning. This was done qualitatively for descriptive statistic purposes only. Reslices were compared qualitatively to original recordings of 2D clips or 2D images stored on the ultrasound device. Additionally, quality of SAX view was assessed by obtaining an image of a recorded 3D-UV target structure and rotation of 90°, resulting in a longitudinal slice (LAX) of the reconstructed 3D-UV. These LAX 3D-UV reslices were also compared to original LAX images of the model. Comparison resulted only in selecting or discarding of the recordings. A further visual estimation test for correlation analysis between original images and 3D-UV was not planned. Test reproducibility was controlled by repeated recording of an anatomical site up to ten times. A quantitative test reproducibility was not planned. From these repeated recordings, 3D-UV were chosen for further analysis.
This study and project plan was granted by the local review board of the University of Frankfurt am Main.