ABSTRACT
As the ‹eld of stem cells and its applications for tissue regeneration in a wide variety of diseases continues to grow rapidly, there is increasing interest in noninvasive imaging techniques both to evaluate the therapeutic response to cell-based therapies and to track cells after their delivery [1,2]. Ultrasound imaging is one technique that has become a widely utilized modality within medical imaging and clinical cardiology, with important applications in experimental research, due in large part to its many advantages: (a) noninvasive imaging, (b) lack of ionizing radiation, (c) real-time imaging of anatomical structures, (d) visualization and quanti‹cation of ¨ow through vascular structures via Doppler techniques, (e) excellent temporal resolution (30-120 Hz with clinical imaging systems and up to 1,000 Hz with small-animal imaging systems), (f) very good spatial resolution (<1 mm with clinical imaging systems and all the way down to 30 µm with ultrahigh-frequency probes using small-animal imaging systems), and (g) portability. Ultrasound, however, is hampered by limited access to certain anatomic structures (e.g., the lungs, bones, and intracerebral structures); susceptibility to imaging artifacts [3]; a limited depth of penetration (especially with higher-frequency probes); and lack of suf‹cient contrast resolution in grayscale imaging.
