Conceptualization, M.B., P.J.E.E., B.D., S.Y. and F.V.; methodology, M.B., P.J.E.E. and S.Y.; software: M.B. and P.J.E.E.; validation: M.B., P.J.E.E., S.Y. and B.D.; data curation, P.J.E.E.; writing—original draft preparation: M.B. and P.J.E.E.; writing—review and editing: M.B., P.J.E.E., M.J.C. and D.S.; visualization: M.B. and P.J.E.E.; supervision: M.J.C. and D.S.; funding acquisition, D.S. All authors have read and agreed to the published version of the manuscript.

Figure 1. System design, showing (a) the setup of the ultrasound, cube tracker and phantom; and (b) System in use: Navigating the tracked US probe to the holographic standard plane while wearing the HoloLens 2.

Figure 1. System design, showing (a) the setup of the ultrasound, cube tracker and phantom; and (b) System in use: Navigating the tracked US probe to the holographic standard plane while wearing the HoloLens 2.

Figure 2. Unity scene showing the various components of the holographic setup: (a) complete holographic menu for experts; (b) reduced holographic menu for the trainee; (c) navigation to head standard plane including the navigation instruction card, navigation arrows, position and rotation offset between probe and target plane and elapsed time for the navigation task; and (d) closeup of the head ultrasound plane including the four pink guidance arrows pointing at the plane’s corners.

Figure 2. Unity scene showing the various components of the holographic setup: (a) complete holographic menu for experts; (b) reduced holographic menu for the trainee; (c) navigation to head standard plane including the navigation instruction card, navigation arrows, position and rotation offset between probe and target plane and elapsed time for the navigation task; and (d) closeup of the head ultrasound plane including the four pink guidance arrows pointing at the plane’s corners.

Figure 3. Illustration of the CAL-Tutor’s user work flow phases, shown from the HoloLens 2 perspective: (a) The initial manual registration of the baby model (by the expert); (b) the manual placement of the holographic standard planes at their respective baby locations (by the expert); and (c) trainee navigation to the standard planes.

Figure 3. Illustration of the CAL-Tutor’s user work flow phases, shown from the HoloLens 2 perspective: (a) The initial manual registration of the baby model (by the expert); (b) the manual placement of the holographic standard planes at their respective baby locations (by the expert); and (c) trainee navigation to the standard planes.

Figure 4. Hologram alignment options: two options of a standard manual plane definition: (a) placement of new standard planes via the US probe; (b,c) adjustment of already existing standard planes whose coordinates have been loaded via a .csv file—(b) Unity concept and (c) HoloLens 2 view.

Figure 4. Hologram alignment options: two options of a standard manual plane definition: (a) placement of new standard planes via the US probe; (b,c) adjustment of already existing standard planes whose coordinates have been loaded via a .csv file—(b) Unity concept and (c) HoloLens 2 view.

Figure 5. NASA-TLX workload assessment result, represented as a box and whisker chart, grouped into the two experimental conditions A = with MR guidance and B = without MR guidance.

Figure 5. NASA-TLX workload assessment result, represented as a box and whisker chart, grouped into the two experimental conditions A = with MR guidance and B = without MR guidance.

Figure 6. Product assessment result, represented by the comparison of scale means: the chart shows the scale means and corresponding 5% confidence intervals.

Figure 6. Product assessment result, represented by the comparison of scale means: the chart shows the scale means and corresponding 5% confidence intervals.

Figure 7. Visual attention profiles of study participants during standard plane navigation: Amount of time (in %) spent looking at specific game objects during navigation to the three standard planes, namely (a) head, (b) abdomen; and (c) femur.

Figure 7. Visual attention profiles of study participants during standard plane navigation: Amount of time (in %) spent looking at specific game objects during navigation to the three standard planes, namely (a) head, (b) abdomen; and (c) femur.

Table 1. Data recorded by the CAL-Tutor application.

Table 1. Data recorded by the CAL-Tutor application.

ARToolkitHoloLens 2Voluson US scannerProbePositionx, y, zEyeGaze: Game object hit Positionx, y, zUS videoProbeRotationx, y, zEyeGaze: name of game object user is looking at HandPalmPositionx, y, zNDI Aurora HandWristPositionx, y, zProbePositionx, y, z HeadPositionx, y, zProbeRotationx, y, z HeadRotationx, y, z External cameraExternal camera video of the overall scene

Table 2. The two experimental conditions of the user study.

Table 2. The two experimental conditions of the user study.

Experimental ConditionCondition 1 (Baseline): Probe navigation without mixed reality assistance The participant has to wear the HoloLens 2 device during standard plane navigation since user data will be recorded. Despite the fact that the user has to wear the HoloLens 2, no holographic information is being displayed.Condition 2 (MR guidance): Probe navigation with mixed reality assistance The user is asked to perform the standard plane navigation with holographic guidance which includes the instruction card, the guidance arrows, directional indicator, elapsed time and numerical offset between the probe’s US plane and the target standard plane, as described in Section 3.1.2.

Table 3. NASA-TLX questionnaire results for the six study participants (engineering students) in two conditions: With MR guidance (A) and without MR guidance (B).

Table 3. NASA-TLX questionnaire results for the six study participants (engineering students) in two conditions: With MR guidance (A) and without MR guidance (B).

User NumberConditionMentalPhysicalTemporalPerformanceEffortFrustrationMean1A403050508020451B706080509580732A4065551010232B759040256030533A453525754530433B652550556050514A252530352020264B603550656545536A954055407075636B35253025204029WorkloadValue Workload ComponentWith MR guidanceWithout MR guidanceLow0–9 Mental5362Medium10–29 Physical3946Somewhat high30–49 Temporal3346High50–79 Performance4442Very high80–100 Effort5063 Frustration3143

Table 4. Individual product assessment result of all six study participants in two conditions: with MR guidance (A) and Without MR guidance (B).

Table 4. Individual product assessment result of all six study participants in two conditions: with MR guidance (A) and Without MR guidance (B).

ScaleConditionMeanSTDNConfidenceConfidenceIntervalAttractivenessA2.000.8560.681.322.68B0.470.6860.54−0.071.02PerspicuityA1.790.8660.691.112.48B−0.421.3761.09−1.510.68EfficiencyA1.880.5960.471.412.34B0.290.9560.76−0.471.06DependabilityA1.710.8760.701.012.41B0.501.0060.80−0.301.30StimulationA2.130.6860.551.582.67B0.960.8960.710.251.67NoveltyA2.210.2560.202.012.41B−0.132.0861.66−1.791.54