Purpose: To investigate the usefulness of magnetic particle imaging (MPI) for predicting the therapeutic effect of magnetic hyperthermia (MH). Materials and Methods: First, we performed phantom experiments to investigate the relationship between the MPI value and the temperature rise of magnetic nanoparticles (MNPs) under an alternating magnetic field (AMF). The MPI value was defined as the pixel value of the transverse image reconstructed from the third-harmonic signals. Samples filled with various iron concentrations of MNPs (Resovist ®) were prepared and were imaged using our MPI scanner. These samples were also heated using the AMF, and the specific loss power (SLP) and volume-specific loss power (vSLP) were calculated from the initial slope of the time-dependent temperature rise. Second, we performed animal experiments using tumor-bearing mice, which were divided into untreated (n = 10) and treated groups (n = 20). The tumors in the treated group were injected with Resovist ® at an iron concentration of 250 mM (n = 10) or 500 mM (n = 10), and received MH for 20 min, during which the temperatures in the tumor and rectum were measured. The relative tumor volume growth (RTVG) was calculated from (V 15 − V 0)/V 0 , where V 0 and V 15 represented the tumor volume on day 0 and day 15 after MH, respectively. Results: In phantom experiments, the MPI value had significant correlations with the iron concentration of MNPs (r = 0.997), temperature rise (r = 0.981), and vSLP (r = 0.961). In animal experiments, the MPI value had significant correlations with the temperature rise in the tumor (r = 0.731) and RTVG (r = −0.687). Conclusion: Our preliminary results suggest that MPI is useful for predicting the therapeutic effect of MH.

Purpose: To investigate the feasibility of applying magnetic particle imaging (MPI) to pulmonary imaging using nebulized magnetic nanoparticles (MNPs) and to quantify the mucociliary clearance in the lung, using small animal experiments. Materials and Methods: Intrapulmonary administration of MNPs was performed in seven-week-old male ICR (Institute of Cancer Research) mice (n = 8) using a nebulized microsprayer connected to a high-pressure syringe containing 50 μL of MNPs (500 mM Resovist®). We imaged the lungs using our MPI scanner 2.5 hours, 1 day, 3 days, and 7 days after the intrapulmonary administration of MNPs. The average MPI value was calculated by drawing a region of interest (ROI) on the lungs by taking the threshold value for extracting the contour as 20% of the maximum MPI value within the ROI. The MPI value was defined as the pixel value of the transverse image reconstructed from the third-harmonic signals. Mice were sacrificed immediately after the last MPI and X-ray CT studies on day 7, and 5 lobes of the lung in each mouse were extracted to confirm the accumulation of iron using Berlin blue staining. Results: We could visualize the distribution of MNPs in the lungs as positive contrast using MPI with use of nebulized MNPs. The presence of iron in the lung was confirmed by Berlin blue staining. The average MPI value decreased with time and tended to saturate. The clearance rate was calculated to be 0.505 day −1 from the time course of the average MPI value in the lungs. Conclusion: Our preliminary results suggest that MPI can be applied to pulmonary imaging by nebulizing MNPs and can be useful for quantifying the mucociliary clearance in the lung.

Purpose: Magnetic hyperthermia treatment (MHT) is a strategy for cancer therapy using the temperature rise of magnetic nanoparticles (MNPs) under an alternating magnetic field (AMF). Recently, a new imaging method called magnetic particle imaging (MPI) has been introduced. MPI allows imaging of the spatial distribution of MNPs. The purpose of this study was to investigate the feasibility of visualizing and quantifying the intratumoral distribution and temporal change of MNPs and predicting the therapeutic effect of MHT using MPI. Materials and Methods: Colon-26 cells (1 × 10 6 cells) were implanted into the backs of eight-week-old male BALB/c mice. When the tumor volume reached approximately 100 mm 3 , mice were divided into untreated (n = 10) and treated groups (n = 27). The tumors in the treated group were directly injected with MNPs (Resovist ®) with iron concentrations of 500 mM (A, n = 9), 400 mM (B, n = 8), and 250 mM (C, n = 10), respectively, and MHT was performed using an AMF with a frequency of 600 kHz and a peak amplitude of 3.5 kA/m. The mice in the treated group were scanned using our MPI scanner immediately before, immediately after, 7 days, and 14 days after MHT. We drew a region of interest (ROI) on the tumor in the MPI image and calculated the average, maximum, and total MPI values and the number of pixels by taking the threshold value for extracting the contour as 40% of the maximum MPI value (pixel value) within the ROI. These parameters in the untreated group were taken as zero. We also measured the relative tumor volume growth (RTVG) defined by (V−V0)/V0, where V 0 and V are the tumor volumes immediately before and after MHT, respectively. Results:

Magnetic particle imaging is a novel method of imaging the spatial distribution of magnetic nanoparticles. When considering the practical application of magnetic particle imaging, it is important to correct the inhomogeneous sensitivity of the receiving coil together with the feedthrough interference. In this study, we developed a simple and practical method for these corrections in which projection data are multiplied by correction factors obtained by fitting projection data acquired in a blank scan to a sixth-degree polynomial. Phantom experiments suggest that our method can be simply and easily implemented to realize the above corrections.

Purpose: Magnetic targeting refers to the attachment of therapeutic agents to magnetizable particles such as magnetic nanoparticles (MNPs) and then applying magnetic fields to concentrate them to the targeted region such as solid tumors. The purpose of this study was to investigate the usefulness of magnetic particle imaging (MPI) for monitoring the effect of magnetic targeting using tumor-bearing mice. Materials and Methods: Colon-26 cells (1 × 10 6 cells) were implanted into the backs of eight-week-old male BALB/c mice. When the tumor volume reached approximately 100 mm 3 , the mice were divided into treated (n = 8) and untreated groups (n = 8). The tumors in the treated group were directly injected with MNPs (Resovist ® , 250 mM) and a neodymium magnet was attached to the tumor surface, whereas the magnet was not attached to the tumor in the untreated group. The mice were imaged using our MPI scanner and the average and maximum MPI values were obtained by drawing a region of interest (ROI) on the tumor, with the threshold value for extracting the contour of the tumor being taken as 40% of the maximum MPI value in the ROI. The relative tumor volume growth (RTVG) was calculated from (V − V0)/V0, where V0 and V represented the tumor volume immediately before and after the injection of MNPs, respectively. Results: The average and maximum MPI values in the treated group were significantly higher than those in the untreated group 3 days after the injection of MNPs, suggesting the effectiveness of magnetic targeting. There were no significant differences in RTVG between the two groups. Conclusion: Our preliminary results suggest that MPI is useful for monitoring the effect of magnetic targeting.