n Z. Li

Comparison of quantitative perfusion imaging using arterial spin labeling at 1.5 and 4.0 Tesla

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.

Arterial spin labeling perfusion fMRI with very low task frequency

Functional magnetic resonance imaging (fMRI) has become the most widely used modality for visualizing regional brain activation in response to sensorimotor or cognitive tasks. While the majority of fMRI studies have used blood oxygenation level‐dependent (BOLD) contrast as a marker for neural activation, baseline drift effects result in poor sensitivity for detecting slow variations in neural activity. By contrast, drift effects are minimized in arterial spin labeling (ASL) perfusion contrast, primarily as a result of successive pairwise subtraction between images acquired with and without labeling. Recent data suggest that ASL contrast shows stable noise characteristics over the entire frequency spectrum, which makes it suitable for studying low‐frequency events in brain function. The present study investigates the relative sensitivities of ASL and BOLD contrast in detecting changes in motor cortex activation over a spectrum of frequencies of experimental design, where the alternating period between the resting state and activation is varied from 30 s up to 24 hr. The results demonstrate that 1) ASL contrast can detect differences in motor cortex activation over periods of minutes, hours, and even days; 2) the functional sensitivity of ASL contrast becomes superior to that of BOLD contrast when the alternating period between the resting state and activation is greater than a few minutes; and 3) task activation measured by ASL tends to have less intersubject variability than BOLD contrast. The improved sensitivity of the ASL contrast for low task frequency and longitudinal studies, along with its superior power in group analysis, is expected to extend the range of experimental designs that can be studied using fMRI. Magn Reson Med 49:796–802, 2003.

Measurements of Tumor Cell Autophagy Predict Invasiveness, Resistance to Chemotherapy, and Survival in Melanoma

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.

Quantifying arbitrary magnetic susceptibility distributions with MR

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.

Quantitative magnetic resonance and optical imaging biomarkers of melanoma metastatic potential

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, T1ρ-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, Ktrans, that measures vascular perfusion/permeability. Ktrans 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 T1ρ values that were significantly higher than those of C8161 melanoma. Measurements of T1 and T2 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.

Quantitative mitochondrial redox imaging of breast cancer metastatic potential

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.

Magnetic susceptibility quantification for arbitrarily shaped objects in inhomogeneous fields

Magnetic susceptibility measurement has wide‐ranging applications in MR technical development and medical applications. A general susceptibility quantitation method for objects of arbitrary shapes in inhomogeneous magnetic fields is presented in this study. Based on the mean value properties of magnetic fields, the polarizing magnetic field at the location of interest inside an object can be exactly obtained in situ from the field values on a spherical surface enclosing the object. With numerical computation of the self‐demagnetizing field and correction of contact shifts, magnetic susceptibilities were quantitatively measured for CuSO4 phantoms based on their MR gradient echo phase maps. Magn Reson Med 46:907–916, 2001.

Reduced susceptibility effects in perfusion fMRI with single-shot spin-echo EPI acquisitions at 1.5 tesla

Arterial spin labeling (ASL) perfusion contrast is not based on susceptibility effects and can therefore be used to study brain function in regions of high static inhomogeneity. As a proof of concept, single-shot spin-echo echo-planar imaging (EPI) acquisition was carried out with a multislice continuous ASL (CASL) method at 1.5T. A bilateral finger tapping paradigm was used in the presence of an exogenously induced susceptibility artifact over left motor cortex. The spin-echo CASL technique was compared with a regular gradient-echo EPI sequence with the same slice thickness, as well as other imaging methods using thin slices and spin-echo acquisitions. The results demonstrate improved functional sensitivity and efficiency of the spin-echo CASL approach as compared with gradient-echo EPI techniques, and a trend of improved sensitivity as compared with spin-echo EPI approach in the brain regions affected by the susceptibility artifact. ASL images, either with or without subtraction of the control, provide a robust alternative to blood oxygenation level dependant (BOLD) methods for activation imaging in regions of high static field inhomogeneity.

Monitoring response to chemotherapy of non‐Hodgkin's lymphoma xenografts by T2‐weighted and diffusion‐weighted MRI

An effective method for in vivo detection of early therapeutic response of patients with non‐Hodgkins lymphoma would enable personalized clinical management of cancer therapy and facilitate the design of optimal treatment regimens. This study evaluates the feasibility of T2‐weighted MRI (T2WI) and diffusion‐weighted MRI (DWI) for in vivo detection of response of human diffuse large B‐cell lymphoma xenografts in severe combined immunodeficient mice to chemotherapy. Each cycle of combination chemotherapy with cyclophosphamide, hydroxydoxorubicin, Oncovin, prednisone, and bryostatin 1 (CHOPB) was administered to tumor‐carrying mice weekly for up to four cycles. T2WI and DWI were performed before the initiation of CHOPB and after each cycle of CHOPB. In order to corroborate the MRI results, histological analyses were carried out on control tumors and treated tumors after completion of all MRI studies. DWI revealed a significant (P < 0.03) increase in the mean apparent diffusion coefficient in CHOPB‐treated tumors as early as 1 week after initiation of CHOPB. However, a significant (P < 0.03) decrease in mean T2 was observed only after two cycles of CHOPB. Both MRI methods produced high‐resolution (0.1 × 0.1 × 1.0 mm3) maps of regional therapeutic response in the treated tumors based on local apparent diffusion coefficient and T2. Only a specific region of the tumors (in 3 of the 5 tumors) corresponding to about one third of the tumor volume exhibited a response‐associate increase in ADC and decrease in T2. An adjacent region exhibited an increase in T2 and no change in ADC. The rest of the tumor was indistinguishable from sham‐treated controls by MRI criteria. The therapeutic response of the treated tumors detected by MRI was accompanied by changes in tumor cell density, proliferation and apoptosis revealed by histological studies performed upon completion of the longitudinal study. The mechanism producing the regional response of the tumor remains to be elucidated. Copyright

MITOCHONDRIAL REDOX IMAGING FOR CANCER DIAGNOSTIC AND THERAPEUTIC STUDIES.

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.