Application of robotic transcranial Doppler for extended duration recording in moderate/severe traumatic brain injury: first experiences

The probes/robotic drive

The Delica EMS 9D robotic TCD system is a portable TCD system allowing for bilateral simultaneous MCA insonation. The standard probes available with the system are 1–2 MHz Doppler ultrasound probes, each attached to a separate robotic drive. The entire drive/probe construct is encased within a tough plastic shell and supported using a head-band type frame (Fig. 1A). An option exists for the probes are also surrounded by a small rubber ring around its periphery, which is designed to hold ultrasound gel for longer, allowing for preserved signal quality (See Additional file 1). Figure 1 displays various pictures of the Delica EMS 9D system.

The head-band frame is a composite of plastic and Velcro straps with fabric (Fig. 1C and 1D). The diameter of the head-band may be adjusted using either the Velcro straps, or the ratcheting wheel located on the front of the head-band (Fig. 1C). Ideal location of the band is just above the orbital margins. Near the temporal windows, the head-band frame contains plastic inverted “U-shaped” pieces following the course of the superior temporal line, meant for mounting the robotic drive/TCD probe construct (Fig. 1B). This is accomplished using wing-nut fasteners. The entire head-band is padded with exchangeable Neoprene inserts, for comfort (Fig. 1A).

The monitor—gross interface

The Delica EMS 9D monitor is designed for portability, with a carrying handle situated at the top. The device itself is a Windows based machine, allowing for the installation of various other Windows based software packages directly onto the machine. As mentioned above, the device supports bilateral 1–2 MHz Doppler probes. However, it also has the ability to record signals with a single 4, 8, or 16 MHz probe. The device also supports a touch screen method of interaction with the software. Various ports are present on either side of the monitor, including: two external VGA monitor ports, four USB 2.0 ports, two USB 3.0 ports, one HMDI port, one serial RS232 COM port, and one ethernet port.

The Delica TCD software

The Delica EMS 9D comes with its own specially designed software for recording TCD signal. This provides both an easy to use interface with the robotic drive system, and continuously updating CBFV waveforms and M-mode signals. Furthermore, the left side of the screen provides both simultaneous left and right CBFV waveforms at various depths of insonation, allowing for selection of the optimal depth for recording. Various other more complex functions are available within the software, such as microemboli detection, however, we will not focus on these, given the goal was to assess the basic ability to record in critically ill TBI patients. Figure 2 Displays the software interface.

Finally, the robotic drive system carries four main functions: scan, search, direction, and track. Scan is an automated algorithm which moves the TCD probe position in a square grid pattern, insonating at each spot, assessing for MCA CBFV signal intensity and providing a color code for its findings (black = poor/no signal found, red = good; with colors ranging from blue, to green, to yellow, to orange, to red). It must be acknowledged that the amount the probe can change in position is limited, so large position corrections still required manual manipulation. After completion of the grid, the proprietary automated algorithm then chooses the best position. The search function provides an automated circular search pattern around the initial starting point, changing both probe position and insonation angle. As with the scan function, it insonates each spot, finding the optimal signal intensity for a final position. The directional function alters the TCD probe angle, using an automated algorithm, with the goal of finding the optimal insonation angle. It should be noted, at any point, the operator can manually change the probe position using the direction touch pad on the screen.

Finally, the track function is enabled after the user selects the optimal position of insonation. This function is designed to automatically detect any deterioration in signal quality/intensity, and then automatically adjust the TCD probe (both via position and angle), using a proprietary algorithm, to restore optimal signal. Figure 3, demonstrates TCD CBFV signal recorded over a 4-h period, displaying continuous uninterrupted acquisition of maximum envelope flow velocity signal which retained good quality without the need of any manual adjustments over the whole duration of recording.

Advantages

With above described set-up, we were able to record at least 4 h of continuous, non-interrupted, good quality TCD signals. The TCD probe held ultrasound gel for 4 h without any drying issues or need for re-application. Both the recording length and gel integrity duration are far longer than most standard TCD systems can provide. The optional rubber ring around the margins of the TCD probe appeared to also hold the gel in situ for longer. Though we must acknowledge, this rubber ring was only utilized in 2 of the 10 patients and appeared to be cumbersome at times to insonation. Thus, further testing with this added feature is required prior to definitive conclusion on its benefit being made.

Aside from gel integrity and duration of recording, we were able to capture bilateral MCA flow velocities in the presence of all other multi-modal monitoring devices described, confirming its applicability in the multi-monitoring moderate/severe TBI patients. ICM+ software ran seamlessly off of the Delica monitor, allowing for simultaneous digital signal acquisition from all multi-modal monitors.

The head-band appeared to be comfortable, with the padding protecting surgical wounds from injury, to rarely moving once secured. Once in situ, the head-band, with attached robotic drive/TCD probes tolerated patient turning in each patient, without loss of CBFV waveform or intensity. Furthermore, 2 of the 10 patients underwent portable chest x-ray while recording, with no change in signal intensity throughout this procedure. Finally, 1 of the 10 patients underwent a bedside chest-tube insertion without change in signal quality.

The robotic drive system worked well, with the search and directional functions aiding with set-up and re-acquisition of signal if it needed to be optimized. The algorithms for these two functions appeared to work well, finding the optimal angle and position of insonation. This, in the author’s opinion, was a massive “time-saver”, preventing the need to loosen the head-band or drives and manually adjust the TCD probe. Especially important in patients with numerous monitors in situ, where repeated loosening and manipulation could potentially lead to disruption of one or more of these other monitors. This advantage cannot be overstated, as the presence of automated robotic adjustment of the probe both during the initial set-up phase and during episodes of frame shift is invaluable. With this step forward in technology, the ability to obtain extended duration, mostly uninterrupted, CBFV recordings is achievable. This allows for continuous non-invasive assessment of various aspects of cerebrovascular physiology using this technology. This includes continuous measures of CBFV, pulsatility index, and cerebrovascular reactivity (in the presence of finger-cuff based continuous non-invasive blood pressure), to name a few. As robotic TCD device technology continues to improve, we are likely to see further widespread application across various pathologies, obtaining longer and higher quality recordings. This is a step towards more non-invasive means of continuous cerebral monitoring.

Finally, all plastic elements of the device were easily cleaned, allowing for rapid equipment turn-over between patients, an important aspect for those wishing to employ the device for multiple short duration recordings within a single day.

Disadvantages

The advantages of this system far outweigh any disadvantages listed within this section. However, the system suffered from a few important initial issues/limitations, which we will highlight. Other minor annoyances with the device can be seen in the table within Additional file 2.

The main issues stemmed from initial poor real-time functioning of certain aspects of the robotic drive system. As we mentioned above, the search and direction functions appear to work beautifully. However, the two other functions (scan and track), within the initial version of the system, were less useful. The scan function, within all patients recorded, failed to select automatically the appropriate optimal position for insonation. This occurred during repeated attempts. As a result, we elected to manually position the probe using the CBFV waveform and M-mode, selecting an appropriate starting position and depth for insonation. We then applied the search and direction functions, which provided improvement and overall optimization of the signal acquired. This issue was raised with the manufacturer, whom responded promptly with a software update, leading to a remedy to the scan function. This was subsequently trialed on additional patients, confirming its functionality and fix to the previous issues.

As well, with the initial software version, the track function rarely appeared to work. This was a disappointment to us, as the hope with this function is that the drive would automatically correct for any shift in the probe during recording. There were two issues we found with this function in the initial software version. First, despite being enable, it was rarely triggered, even in the rare case when the headframe/probe shifted and lost signal. Second, when the track function was triggered, and we could audibly identify that the robotic drive was attempting to move the TCD probe, however, it never appeared to accurately correct the probe for signal loss. Both issues were likely a result of inefficiencies in the embedded algorithm for the track function. As with the scan function, our concerns were raised with the manufacturer and resolved with a subsequent, most recent version of the software. Trialing this updated software version on additional patients, the track function was correctly engaged during shift of the probe, and corrected the probe position/angle, optimizing the CBFV signal.

In addition, the overall available TCD CBFV signal recording frequency should be commented on. Currently the signal recording frequency is at 100 Hz. This is adequate for basic waveform analysis techniques. However, if one were to use this signal for more complex heart rate variability analysis, it could be argued that higher recording frequencies of 200 Hz or higher would be ideal. Thus, this might be considered a limitation of the current signal recording. However, pressure signals (as well as TCD-based CFBV) do not necessarily require as high of frequencies, and their content extends to 10–20 pulse harmonics at best. Therefore, a sampling frequency of 100 Hz may be entirely sufficient for recording those signals and this is what the majority of bedside and NCCU monitors currently offer.

Finally, we observed that in one patient with scalp soft tissue bruising, the ratcheting head-band triggered a sustained intra-cranial pressure elevation above 20 mmHg lasting 5 min, with an initial baseline ICP of 10–15 mmHg. This was promptly resolved by re-adjusting the head-band. In this one patient, however, the sedation was relatively light, and thus the ICP elevation was likely secondary to pain experienced during the head-band tightening. Nevertheless, this is an important potential complication to highlight.

Interaction with manufacturer

An important aspect of new technologies is the responsiveness of the manufacturer to queries regarding their devices. We were fortunate enough to have this device on trial/loan for the purpose of this evaluation. As eluded above, the concerns outlined within this document were relayed to the manufacturer. They were extremely prompt in response to these raised issues. The main concerns raised above were well received and we were provided with a timely updated software version, leading to resolution of these encountered issues. Delica appears to be vigilant in providing a clinically useful TCD tool, with desires to continually improve their device, rapidly including the concerns of their clients in the process.