Long-term in vivo imaging of cells is crucial for the understanding of cellular fate in biological processes in cancer research, immunology or in cell-based therapies such as beta cell transplantation in type I diabetes or stem cell therapy. Traditionally, cell labelling with the desired contrast agent occurs ex vivo via spontaneous endocytosis, which is a variable and slow process that requires optimization for each particular label-cell type combination. Following endocytic uptake, the contrast agents mostly remain entrapped in the endolysosomal compartment, which leads to signal instability, cytotoxicity and asymmetric inheritance of the labels upon cell division. Here, we demonstrate that these disadvantages can be circumvented by delivering contrast agents directly into the cytoplasm via vapour nanobubble photoporation. Compared to classic endocytic uptake, photoporation resulted in 50 and 3 times higher loading of fluorescent dextrans and quantum dots, respectively, with improved signal stability and reduced cytotoxicity. Most interestingly, cytosolic delivery by photoporation prevented asymmetric inheritance of labels by daughter cells over subsequent cell generations. Instead, unequal inheritance of endocytosed labels resulted in a dramatic increase in polydispersity of the amount of labels per cell with each cell division, hindering accurate quantification of cell numbers in vivo over time. The combined benefits of cell labelling by photoporation resulted in a marked improvement in long-term cell visibility in vivo where an insulin producing cell line (INS-1E cell line) labelled with fluorescent dextrans could be tracked for up to two months in Swiss Nude mice compared to two weeks for cells labelled by endocytosis.
Purpose To investigate the effects of frequency drift on chemical exchange saturation transfer (CEST) imaging at 3T, and to propose a new sequence for correcting artifacts attributed to B0 drift in real time. Theory and Methods A frequency‐stabilized CEST (FS‐CEST) imaging sequence was proposed by adding a frequency stabilization module to the conventional non‐frequency‐stabilized CEST (NFS‐CEST) sequence, which consisted of a small tip angle radiofrequency excitation pulse and readout of three non‐phase‐encoded k‐space lines. Experiments were performed on an egg white phantom and 26 human subjects on a heavy‐duty clinical scanner, in order to compare the difference of FS‐CEST and NFS‐CEST sequences for generating the z‐spectrum, magnetization transfer ratio asymmetry (MTRasym) spectrum, and amide proton transfer weighted (APTw) image. Results The B0 drift in CEST imaging, if not corrected, would cause APTw images and MTRasym spectra from both the phantom and volunteers to be either significantly higher or lower than the true values, depending on the status of the scanner. The FS‐CEST sequence generated substantially more stable MTRasym spectra and APTw images than the conventional NFS‐CEST sequence. Quantitatively, the compartmental‐average APTw signals (mean ± standard deviation) from frontal white matter regions of all 26 human subjects were –0.32% ± 2.32% for the NFS‐CEST sequence and –0.14% ± 0.37% for the FS‐CEST sequence. Conclusions The proposed FS‐CEST sequence provides an effective approach for B0 drift correction without additional scan time and should be adopted on heavy‐duty MRI scanners.
Transplantation of pancreatic islets is a possible treatment option for patients suffering from Type I diabetes. In vivo imaging of transplanted islets is important for assessment of the transplantation site and islet distribution. Thanks to its high specificity, the absence of intrinsic background signal in tissue and its potential for quantification, F MRI is a promising technique for monitoring the fate of transplanted islets in vivo. In order to overcome the inherent low sensitivity of F MRI, leading to long acquisition times with low signal-to-noise ratio (SNR), compressed sensing (CS) techniques are a valuable option. We have validated and compared different CS algorithms for acceleration of F MRI acquisition in a low SNR regime using pancreatic islets labeled with perfluorocarbons both in vitro and in vivo. Using offline simulation on both in vitro and in vivo low SNR fully sampled F MRI datasets of labeled islets, we have shown that CS is effective in reducing the image acquisition time by a factor of three to four without seriously affecting SNR, regardless of the particular algorithms used in this study, with the exception of CoSaMP. Using CS, signals can be detected that might have been missed by conventional F MRI. Among different algorithms (SPARSEMRI, OMMP, IRWL1, Two-level and CoSAMP), the two-level l method has shown the best performance if computational time is taken into account. We have demonstrated in this study that different existing CS algorithms can be used effectively for low SNR F MRI. An up to fourfold gain in SNR/scan time could be used either to reduce the scan time, which is beneficial for clinical and translational applications, or to increase the number of averages, to potentially detect otherwise undetected signal when compared with conventional F MRI acquisitions. Potential applications in the field of cell therapy have been demonstrated.
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