We demonstrate a technique for detecting magnetically labeled Listeria monocytogenes and for measuring the binding rate between antibody-linked magnetic particles and bacteria. This sensitive assay quantifies specific bacteria in a sample without the need to immobilize them or wash away unbound magnetic particles. In the measurement, we add 50-nm-diameter superparamagnetic magnetite particles, coated with antibodies, to an aqueous sample containing L. monocytogenes. We apply a pulsed magnetic field to align the magnetic dipole moments and use a hightransition temperature superconducting quantum interference device, an extremely sensitive detector of magnetic flux, to measure the magnetic relaxation signal when the field is turned off. Unbound particles randomize direction by Brownian rotation too quickly to be detected. In contrast, particles bound to L. monocytogenes are effectively immobilized and relax in about 1 s by rotation of the internal dipole moment. This Né el relaxation process is detected by the superconducting quantum interference device. The measurements indicate a detection limit of (5.6 ؎ 1.1) ؋ 10 6 L. monocytogenes in our sample volume of 20 l. If the sample volume were reduced to 1 nl, we estimate that the detection limit could be improved to 230 ؎ 40 L. monocytogenes cells. Timeresolved measurements yield the binding rate between the particles and bacteria.A ntibodies are widely used as biological probes to identify specific microorganisms or molecules (1, 2). The antibodies are linked to a label and introduced into the sample, where they bind to the targets of interest and provide a means of detection. Common labels include enzymes, fluorescent dyes, radioisotopes, or magnetic particles. This general technique has various applications. In an immunoassay, the goal is to detect and quantify specific targets. Tagged antibodies can also be used to separate target antigens selectively or to measure the affinity between antibody and antigen. In this article, we present a sensitive method for detecting magnetically labeled bacteria by using a superconducting quantum interference device (SQUID), a highly sensitive detector of magnetic flux. This assay can be used to monitor bacteria in a liquid sample and to determine the rate of binding between antibody-linked particles and bacteria.Magnetic particles have several advantages as labels. They are stable and nontoxic and can be manipulated with a magnetic field, making it possible to separate target antigens magnetically (3). Methods have been developed to detect small numbers of such particles by using Hall probes (4), giant magnetoresistance arrays (5), atomic force microscopy (6), force-amplified biological sensors (7), and SQUIDs (8-10).Weitschies, Kötitz, and colleagues pioneered the use of SQUIDs for magnetic immunoassays (8,(11)(12)(13)(14)(15)(16). They developed a magnetic relaxation immunoassay in which magnetic particles bound to targets are distinguished from unbound particles by their different relaxation times. By using a low-criticalte...

T 1 -weighted contrast MRI with prepolarization was detected with a superconducting quantum interference device (SQUID). A spin evolution period in a variable field between prepolarization and detection enabled the measurement of T 1 in fields between 1.7 T and 300 mT; T 1 dispersion curves of agarose gel samples over five decades in frequency were obtained. SQUID detection at 5.6 kHz drastically reduces the field homogeneity requirements compared to conventional field-cycling methods using Faraday coil detection. This allows T 1 dispersion measurements to be easily combined with MRI, so that T 1 in a wide range of fields can be used for tissue contrast. Images of gel phantoms with T 1 -weighted contrast at four different fields between 10 T and 300 mT demonstrated dramatic contrast enhancement in low fields. A modified inversion recovery technique further enhanced the contrast by selectively suppressing the signal contribution for a specific value of the low-field

In magnetic resonance imaging (MRI) performed at fields of 1 T and above, the presence of a metal insert can distort the image because of susceptibility differences within the sample and modification of the radiofrequency fields by screening currents.Furthermore, it is not feasible to perform nuclear magnetic resonance (NMR) spectroscopy or acquire a magnetic resonance image if the sample is enclosed in a metal container. Both problems can be overcome by substantially lowering the NMR frequency. Using a microtesla imaging system operating at 2.8 kHz, with a superconducting quantum interference device (SQUID) as the signal detector, we have obtained distortion-free images of a phantom containing a titanium bar and threedimensional images of an object enclosed in an aluminum can; in both cases high-field images are inaccessible.

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.

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We describe the development and utilization of a new imaging technology for plant biology, optical coherence microscopy (OCM), which allows true in vivo visualization of plants and plant cells. This novel technology allows the direct, in situ (e.g. plants in soil), three-dimensional visualization of cells and events in shoot tissues without causing damage. With OCM we can image cells or groups of cells that are up to 1 mm deep in living tissues, resolving structures less than 5 m in size, with a typical collection time of 5 to 6 min. OCM measures the inherent light-scattering properties of biological tissues and cells. These optical properties vary and provide endogenous developmental markers. Singly scattered photons from small (e.g. 5 ϫ 5 ϫ 10 m) volume elements (voxels) are collected, assembled, and quantitatively false-colored to form a threedimensional image. These images can be cropped or sliced in any plane. Adjusting the colors and opacities assigned to voxels allows us to enhance different features within the tissues and cells. We show that light-scattering properties are the greatest in regions of the Arabidopsis shoot undergoing developmental processes. In large cells, high light scattering is produced from nuclei, intermediate light scatter is produced from cytoplasm, and little if any light scattering originates from the vacuole and cell wall. OCM allows the rapid, repetitive, non-destructive collection of quantitative data about inherent properties of cells, so it provides a means of continuously monitoring plants and plant cells during development and in response to exogenous stimuli.Studies in plant physiology and development characteristically follow changes in space and time that occur as part of normal plant activity or in response to exogenous stimuli. Typical studies require the destruction and analysis of a plant or a tissue sample, followed by the collection and analysis of a second distinct plant or sample. Thus, biological responses or changes are inferred by comparing different plants or samples. Such approaches have been used for centuries and have produced a great deal of knowledge. However, when scientists are able to nondestructively follow biological changes, important concepts and insights have emerged. For example, critical genes involved in programmed cell death were found in Caenorhabditis elegans partially because the developing nematode is nearly transparent, allowing the fate of each cell to be followed in vivo by light microscopy (Gilbert, 1998). Similarly, an elegant fate map for Arabidopsis roots was constructed because the relatively transparent roots allow changes in individual plants to be followed continuously (Dolan et al., 1993). This study led to new discoveries such as the presence of downward communication between mature root cells and the root apical meristem and short-range control of differentiation signals (van den Berg et al., 1997a(van den Berg et al., , 1997b.Except for the relatively transparent Arabidopsis root, plants provide a challenge for in vivo an...

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The proton T 1 was measured at 132 mT in ex vivo prostate tissue specimens from radical prostatectomies of 35 patients with prostate cancer. Each patient provided two specimens. The NMR and MRI measurements involved proton repolarization, a field of typically 150 mT and detection of the 5.6-kHz signal with a superconducting quantum interference device. Values of T 1 varied from 41 to 86 ms. Subsequently, the percentages of tissue types were determined histologically. The theoretical image contrast is quantified for each case by d 5 [1 -T 1 (more cancer)/T 1 (less cancer)]. A linear fit of d versus difference in percentage cancer yields T 1 (100% cancer)/T 1 (0% cancer) 5 0.70 6 0.05 with correlation coefficient R 2 5 0.30. Two-dimensional T 1 maps for four specimens demonstrate variation within a single specimen. These results suggest that MR images with T 1 contrast established at ultra-low fields may discriminate prostate cancer from normal prostate tissue in vivo without a contrast agent. Magn Reson Med 67:1138-1145,