High‐field arterial spin labeling (ASL) perfusion MRI is appealing because it provides not only increased signal‐to‐noise ratio (SNR), but also advantages in terms of labeling due to the increased relaxation time T₁ of labeled blood. In the present study, we provide a theoretical framework for the dependence of the ASL signal on the static field strength, followed by experimental validation in which a multislice pulsed ASL (PASL) technique was carried out at 4T and compared with PASL and continuous ASL (CASL) techniques at 1.5T, both in the resting state and during motor activation. The resting‐state data showed an SNR ratio of 2.3:1.4:1 in the gray matter and a contrast‐to‐noise ratio (CNR) of 2.7:1.1:1 between the gray and white matter for the difference perfusion images acquired using 4T PASL, 1.5T CASL, and 1.5T PASL, respectively. However, the functional data acquired using 4T PASL did not show significantly improved sensitivity to motor cortex activation compared with the 1.5T functional data, with reduced fractional perfusion signal change and increased intersubject variability. Possible reasons for these experimental results, including susceptibility effects and physiological noise, are discussed. Magn Reson Med 48:242–254, 2002.

Purpose: Autophagy consists of lysosome-dependent degradation of cytoplasmic contents sequestered by autophagic vesicles (AV). The role of autophagy in determining tumor aggressiveness and response to therapy in melanoma was investigated in this study. Experimental Design: Autophagy was measured in tumor biopsies obtained from metastatic melanoma patients enrolled on a phase II trial of temozolomide and sorafenib and correlated to clinical outcome. These results were compared with autophagy measurements in aggressive and indolent melanoma cells grown in two- and three-dimensional (3D) culture and as xenograft tumors. The effects of autophagy inhibition with either hydroxychloroquine or inducible shRNA (short hairpin RNA) against the autophagy gene ATG5 were assessed in three-dimensional spheroids. Results: Patients whose tumors had a high autophagic index were less likely to respond to treatment and had a shorter survival compared with those with a low autophagic index. Differences in autophagy were less evident in aggressive and indolent melanoma cells grown in monolayer culture. In contrast, autophagy was increased in aggressive compared with indolent melanoma xenograft tumors. This difference was recapitulated when aggressive and indolent melanoma cells were grown as spheroids. Autophagy inhibition with either hydroxychloroquine or inducible shRNA against ATG5 resulted in cell death in aggressive melanoma spheroids, and significantly augmented temozolomide-induced cell death. Conclusions: Autophagy is a potential prognostic factor and therapeutic target in melanoma. Three dimensional culture mimics the tumor microenvironment better than monolayer culture and is an appropriate model for studying therapeutic combinations involving autophagy modulators. Autophagy inhibition should be tested clinically in patients with melanoma. Clin Cancer Res; 17(10); 3478–89. ©2011 AACR.

Magnetic susceptibility, as a physical property of materials, plays important roles in many physical, chemical, engineering, and medical applications. Its quantification becomes of significant interest when MRI becomes a commonly used technique in biomedical applications. A general method is presented here for quantifying arbitrary magnetic susceptibility distributions in a localized region on the basis of first principles of magnetic induction field distributions in space. A proof of the concept was demonstrated by computer simulations. The study establishes the methodological basis for quantitative magnetic susceptibility imaging with MR. Magn Reson Med 51:1077–1082, 2004.

Predicting tumor metastatic potential remains a challenge in cancer research and clinical practice. Our goal was to identify novel biomarkers for differentiating human breast tumors with different metastatic potentials by imaging the in vivo mitochondrial redox states of tumor tissues. The more metastatic (aggressive) MDA-MB-231 and less metastatic (indolent) MCF-7 human breast cancer mouse xenografts were imaged with the low-temperature redox scanner to obtain multi-slice fluorescence images of reduced nicotinamide adenine dinucleotide (NADH) and oxidized flavoproteins (Fp). The nominal concentrations of NADH and Fp in tissue were measured using reference standards and used to calculate the Fp redox ratio, $Fp∕(NADH+Fp)$. We observed significant core-rim differences, with the core being more oxidized than the rim in all aggressive tumors but not in the indolent tumors. These results are consistent with our previous observations on human melanoma mouse xenografts, indicating that mitochondrial redox imaging potentially provides sensitive markers for distinguishing aggressive from indolent breast tumor xenografts. Mitochondrial redox imaging can be clinically implemented utilizing cryogenic biopsy specimens and is useful for drug development and for clinical diagnosis of breast cancer.

Noninvasive or minimally invasive prediction of tumor metastatic potential would facilitate individualized cancer management. Studies were performed on a panel of human melanoma xenografts that spanned the full range of metastatic potential measured by an in vivo lung colony assay and an in vitro membrane invasion culture system. Three imaging methods potentially transferable to the clinic [dynamic contrast-enhanced (DCE) MRI, T 1 _ρ -MRI, and low-temperature fluorescence imaging (measurable on biopsy specimens)] distinguished between relatively less metastatic and more metastatic human melanoma xenografts in nude mice. DCE-MRI, analyzed with the shutter-speed relaxometric algorithm and using an arterial input function simultaneously measured in the left ventricle of the mouse heart, yielded a blood transfer rate constant, K _trans , that measures vascular perfusion/permeability. K _trans was significantly higher in the core of the least metastatic melanoma (A375P) than in the core of the most metastatic melanoma (C8161). C8161 melanoma had more blood vascular structures but fewer functional blood vessels than A375P melanoma. The A375P melanoma exhibited mean T 1 _ρ values that were significantly higher than those of C8161 melanoma. Measurements of T ₁ and T ₂ relaxation times did not differ significantly between these 2 melanomas. The mitochondrial redox ratio, Fp/(Fp + NADH), where Fp and NADH are the fluorescences of oxidized flavoproteins and reduced pyridine nucleotides, respectively, varied linearly with the in vitro invasive potential of the 5 melanoma cell lines (A375P, A375M, A375P10, A375P5, and C8161). This study shows that a harsh microenvironment may promote melanoma metastasis and provides potential biomarkers of metastatic potential.

Mitochondrial redox states provide important information about energy-linked biological processes and signaling events in tissues for various disease phenotypes including cancer. The redox scanning method developed at the Chance laboratory about 30 years ago has allowed 3D high-resolution (~50 × 50 × 10 μm³) imaging of mitochondrial redox state in tissue on the basis of the fluorescence of NADH (reduced nicotinamide adenine dinucleotide) and Fp (oxidized flavoproteins including flavin adenine dinucleotide, i.e., FAD). In this review, we illustrate its basic principles, recent technical developments, and biomedical applications to cancer diagnostic and therapeutic studies in small animal models. Recently developed calibration procedures for the redox imaging using reference standards allow quantification of nominal NADH and Fp concentrations, and the concentration-based redox ratios, e.g., Fp/(Fp+NADH) and NADH/(Fp+NADH) in tissues. This calibration facilitates the comparison of redox imaging results acquired for different metabolic states at different times and/or with different instrumental settings. A redox imager using a CCD detector has been developed to acquire 3D images faster and with a higher in-plane resolution down to 10 μm. Ex vivo imaging and in vivo imaging of tissue mitochondrial redox status have been demonstrated with the CCD imager. Applications of tissue redox imaging in small animal cancer models include metabolic imaging of glioma and myc-induced mouse mammary tumors, predicting the metastatic potentials of human melanoma and breast cancer mouse xenografts, differentiating precancerous and normal tissues, and monitoring the tumor treatment response to photodynamic therapy. Possible future directions for the development of redox imaging are also discussed.