A prepolarized MRI (PMRI) scanner was used to image near metal implants in agar gel phantoms and in in vivo human wrists. Comparison images were made on 1.5-and 0.5-T conventional whole-body systems. The PMRI experiments were performed in a smaller bore system tailored to extremity imaging with a prepolarization magnetic field of 0.4 T and a readout magnetic field of 27-54 mT (1.1-2.2 MHz). Scan parameters were chosen with equal readout gradient strength over a given field of view and matrix size to allow unbiased evaluation of the benefits of lower readout frequency. Results exhibit substantial reduction in metal susceptibility artifacts under PMRI versus conventional scanners. A new artifact quantification technique is also presented, and phantom results confirm that susceptibility artifacts improve as expected with decreasing readout magnetic field using PMRI. This proof-of-concept study demonstrates that prepolarized techniques have the potential to provide diagnostic cross-sectional images for postoperative evaluation of patients with metal implants. Magn Reson Med 56:177-186, 2006.

An optical coherence microscope (OCM) has been designed and constructed to acquire 3-dimensional images of highly scattering biological tissue. Volume-rendering software is used to enhance 3-D visualization of the data sets. Lateral resolution of the OCM is 5 µm (FWHM), and the depth resolution is 10 µm (FWHM) in tissue. The design trade-offs for a 3-D OCM are discussed, and the fundamental photon noise limitation is measured and compared with theory. A rotating 3-D image of a frog embryo is presented to illustrate the capabilities of the instrument.

Imaging of tumor microvasculature has become an important tool for studying angiogenesis and monitoring antiangiogenic therapies. Ultrasmall paramagnetic iron oxide contrast agents for indirect imaging of vasculature offer a method for quantitative measurements of vascular biomarkers such as vessel size index, blood volume, and vessel density. Here, this technique is validated with direct comparisons to ex vivo micro-CT angiography and histologic vessel measurements, showing significant correlations between in vivo vascular MRI measurements and ex vivo structural vessel measurements. The sensitivity of the MRI vascular parameters is also demonstrated, in combination with a multispectral analysis technique for segmenting tumor tissue to restrict the analysis to viable tumor tissue and exclude regions of necrosis. It is shown that this viable tumor segmentation increases sensitivity for detection of significant effects on blood volume and vessel density by two antiangiogenic therapeutics (anti-VEGF and anti-neuropilin-1) on an HM7 colorectal tumor model. Anti-VEGF reduced blood volume by 36 6 3% (P < 0.0001) and vessel density by 52 6 3% (P < 0.0001) at 48 h posttreatment; the effects of anti-neuropilin-1 were roughly half as strong with a reduction in blood volume of 18 6 6% (P < 0.05) and a reduction in vessel density of 33 6 5% (P < 0.05) at 48 h posttreatment.

Prepolarized MRI uses pulsed magnetic fields to produce MR images by polarizing the sample at one field strength (ϳ0.5 T) before imaging at a much lower field (ϳ50 mT). Contrast reflecting the T 1 of the sample at an intermediate field strength is achieved by polarizing the sample and then allowing the magnetization to decay at a chosen "evolution" field before imaging. For tissues whose T 1 varies with field strength (T 1 dispersion), the difference between two images collected with different evolution fields yields an image with contrast reflecting the slope of the T 1 dispersion curve between those fields. Tissues with high protein content, such as muscle, exhibit rapid changes in their T 1 dispersion curves at 49 and 65 mT due to cross-relaxation with nitrogen nuclei in protein backbones. Tissues without protein, such as fat, have fairly constant T 1 over this range; subtracting images with two different evolution fields eliminates signal from flat T 1 dispersion species. The ability to manipulate image contrast is one of MRI's strongest advantages over other imaging modalities. The search for new contrast mechanisms and better visualization of various tissues and pathologies drives a substantial segment of MRI research. Cooperative research efforts between the world of medicine and that of physics and engineering have sought and found an ever-increasing number of endogenous tissue parameters that can produce useful contrast for specific medical applications, including T 1 relaxation, T 2 relaxation, diffusion, perfusion, blood oxygenation, and others. T 1 dispersion contrast is a new contrast mechanism that uses the variation in T 1 with magnetic field strength to probe the behavior of macromolecules in tissue, similar to T 1 imaging (1) or magnetization transfer contrast (MTC) (2,3).The values of T 1 and T 2 for a given tissue depend on many factors; some are intrinsic, such as the molecular makeup of the tissue, and some are extrinsic, relating to the local cellular environment of the tissue. In particular, the T 1 of many tissues depends strongly on the local magnetic field strength. T 1 values usually get longer as the field strength increases, but the rate of change in T 1 , known as T 1 dispersion (4), varies from tissue to tissue. T 1 dispersion is therefore a potential contrast mechanism that may yield different information than standard T 1 contrast; rather than comparing the relative values of T 1 at a particular field strength, T 1 dispersion contrast would reveal how those values of T 1 change as the field increases.To access T 1 dispersion as a contrast mechanism, we must be able to change the field strength while the image is being acquired. This requires a pulsed-field MRI scanner using a technique such as prepolarized MRI (PMRI) (5-7) or field-cycling MRI (8,9). If we use a pulsed-field scanner to collect two images with T 1 -weighted contrast at different field strengths (normalized to account for the different equilibrium magnetizations) and then subtract those two images, the resulting im...