## 3 Results
### Cell Imaging  
Based on these findings, we applied rMM-OCT to cell imaging. For this, we magnetically labeled mesenchymal stem cells (MSCs) with the fluorescent magnetic particles, as mentioned in Section 3.1, and embedded them in agar scaffolds resembling the elastic properties of soft tissue, as described in [6]. Concerning cellular particle load and its effect on cell function, recent literature indicates that MSCs incubated with 200-nm magnetic particles coated with dextran-sulfate take up \(\sim\)10 pg of iron oxide per cell, and show a cell viability of -70% after 24 h [12]. After preparation, we placed the samples on the object stage of our multimodal imaging setup containing the permanent magnet and kept them under resting conditions for 30 min in order to provoke the formation and alignment of magnetic chains inside the cells. After that, we analyzed single cells using rMM-OCT in combination with light microscopy and laser speckle reflectometry. In this context, we applied bright-field transmission and fluorescence microscopy in order to identify single cells inside the sample volume and evaluated cell labeling using the fluorescent properties of the magnetic particles. Additionally, we applied laser speckle  
Figure 4: Magnetic particle behavior in the switchable magnetic field. (**a**) Bright-field transmission microscopy of magnetic nanoparticles dispersed in glycerin (0.5 mg/mL) in a freshly prepared sample; (**b**) after 30 min exposure to the permanent magnetic field with the solenoid switched off; and (**c**) shortly after with the solenoid switched on (30 V). The dominant direction of the magnetic field is indicated by the symbols in the upper left corner of the images (\(\uparrow-\)transverse, \(\odot-\)longitudinal). The indicated coordinate system \((x,y)\) is identical to Figure 2a. As illustrated by the magnified views (red rectangles), the particles in (**b**) have formed long chains that are aligned parallel to the image plane; when the solenoid is switched on in (**c**), these chains rotate and align perpendicular to the image plane. This rotation effect is reversible across many on/off cycles of the solenoid (see Scheme 1 and Video S1) (color online). reflectometry as a 2D reference imaging method to evaluate the sample motion. This time, we operated the solenoid in pulsed mode (pulse duration 80 ms; repetition rate 1 Hz) in order to induce repetitive magnetomotion. According to our previous experiments, we set the capacitor charging voltage to 30 V, expecting the magnitude of the pulsed magnetic field to be similar to the values measured in DC mode (Figure 3a-c). Finally, we retrieved motion contrast from both OCT and laser speckle (LS) images (\(f_{\text{OCT}}=f_{\text{LS}}=25\) Hz) by calculating the temporal variance of the OCT signal amplitude and the laser speckle intensity over an interval of five B-scans and five video frames, respectively. Exemplary imaging results for five labeled and two unlabeled control cells are shown in Figure 5. It can be seen that labeled cells are characterized by an intense red fluorescence signal in the microscopy channel originating from the fluorescent magnetic particles, as well as significant color-coded motion signals in the OCT and laser speckle channels resulting from magnetomotion. In comparison, unlabeled cells show no fluorescence and no motion contrast. Hypothetically, the pulse length of 80 ms is not long enough to allow the sub-micron particles to fully respond to the magnetic field. However, we choose this pulse duration to make the results comparable to previous work [6]. In this context, we measured the magnetomotive signal of five different cells imaged with rMM-OCT and compared the results to our previous pulsed MM-OCT method. For measurement, we selected a region-of-interest (ROI) representing the cell, and calculated the normalized magnetomotive signal as the quotient of the ROI's mean variance and the average amplitude of the OCT signal across the magnetic pulse.