Page 58 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis
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                                Chapter 3
the Netherlands) and Prussian blue staining (Sigma-Aldrich, Zwijndrecht, The Netherlands) was performed. The slides were covered with a cover slip and imaged with an Axiovert S100 microscope (Zeiss, Oberkochen, Germany). In order to eliminate extracellular presence of SPIO, samples for cytospin slides were taken from labeled cell stocks. After visual inspection with microscopy, stocks were further processed (washed) if extracellular SPIOs were detected or otherwise approved for sample preparation (Fig. 1).
MR imaging and post-processing
Imaging was performed on a GE Signa 3.0 T whole-body clinical scanner (General Electric Healthcare, Milwaukee, USA). For radio-frequency (RF) excitation, the built-in whole body coil was used, and signal detection was performed with receive-only custom-made single channel 5 cm surface loop coil. R2 Relaxometry was carried out with a single-slice (coronal plane) two- dimensional (2D) spin-echo (SE) sequence at various echo times (12 echoes, echo time TE = 10–210 ms, repetition time TR = 1500 ms). The image matrix was 256 x 256, at a field-of-view (FOV) of 40 x 40 mm, with a slice thickness of 0.7 mm (resolution 0.16 x 0.16 x 0.7 mm).
For R2* relaxometry, a three-dimensional (3D) RF-spoiled gradient recalled echo (SPGR) sequence was used at the same resolution as the R2 measurements (0.16 x 0.16 x 0.7 mm), with eight echoes TE = 4–25 ms, TR = 33 ms and a flip angle of 13o. Coronal (horizontal plane) images were used for evaluation.
The TE values for both measurements were chosen to sample the expected exponential decay curves for different SPIO concentrations. Signal-to-noise ratios were taken into account to provide a good fit. Imaging resolution was chosen to match in vivo protocols, where R2 and R2* measurements were also matched in order to achieve a voxel based comparison with the in vitro results.
Mono-exponential curve fitting was carried out on T2- and T2*-weighted images with MATLAB (Levenberg–Marquardt algorithm, the MathWorks, Natick, MA, USA). The voxel-based and selected-region-of-interest based curve fitting was performed. Figure 2(b) illustrates the quality of data and fit results for spin echo (R2) measurements, and Fig. 2(d) for gradient echo (R2*) measurements. A three-parameter exponential function was used:
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