| nuclear magnetic resonance imaging | A special imaging technique used to image internal stuctures of the body, particularly the soft tissues. An MRI image is often superior to a normal X-ray image. It uses the influence of a large magnet to polarize hydrogen atoms in the tissues and then monitors the summation of the spinning energies within living cells. Images are very clear and are particularly good for soft tissue, brain and spinal cord, joints and abdomen. These scans may be used for detecting some cancers or for following their progress. Acronym: MRI (11 Nov 1997) |
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| nuclear magnetic resonance tomography | A special imaging technique used to image internal stuctures of the body, particularly the soft tissues. An MRI image is often superior to a normal X-ray image. It uses the influence of a large magnet to polarize hydrogen atoms in the tissues and then monitors the summation of the spinning energies within living cells. Images are very clear and are particularly good for soft tissue, brain and spinal cord, joints and abdomen. These scans may be used for detecting some cancers or for following their progress. Acronym: MRI (11 Nov 1997) |
| toroidal magnetic cusps | A hybrid confinement scheme operating at high beta. A region of closed toroidal magnetic flux with high-beta plasma is separated by a narrow sheath from the surrounding field, which contains externally produced poloidal components arranged in a toroidal line-cusp configuration. Plasma migrating to the outer sheath is temporarily mirror-confined before being removed in a divertor system. (09 Oct 1997) |
| lens, magnetic | <microscopy> Circular electromagnets capable of projecting a precise circular magnetic field in a specified region. The field acts like an optical lens, having the same attributes (focal length angle of divergence etc.) and errors (spherical aberration, chromatic aberration, astigmatism etc.). They are used to focus and steer electrons in an Electron Microscope. (05 Aug 1998) |
| auditory field | The space included within the limits of hearing of a definite sound, as of a tuning fork. (05 Mar 2000) |
| bright field illumination | <microscopy> The method of lighting the specimen with a solid cone of rays. Transmitted bright field illumination is performed by a substage condenser. Reflected bright field illumination is performed by a vertical illuminator. Compare: dark field illumination (05 Aug 1998) |
| bright field imaging | <microscopy> An imaging mode in a transmission electron microscopy that uses only unscattered Electrons to form the image. Contrast in such an image is due entirely to mass-thickness variations in amorphous samples, and may include diffraction contrast in crystalline samples. (05 Aug 1998) |
| bright field microscopy | <technique> Optical microscopy, in which absorption to a great extent and diffraction to a minor extent give rise to the image, as opposed to phase contrast or interference methods of microscopy. (18 Nov 1997) |
| Broca's field | The posterior part of the inferior frontal gyrus of the left or dominant hemisphere, corresponding approximately to Brodmann's area 44; Broca identified this region as an essential component of the motor mechanisms governing articulated speech. Synonym: Broca's area, Broca's field, motor speech centre. (05 Mar 2000) |
| cardioid dark field condenser | <microscopy> A condenser designed with two reflecting surfaces, the first, a spherical surface which reflects the rays to a second, cardioid (heart-shaped) surface. The virtue in such an arrangement is that, if the cardioid surface is of true figure, the lens is both achromatic and aplanatic. It has a limiting numerical aperture of about 1.0. Thus objectives of a greater numerical aperture cannot be used successfully with it. A true cardioid figure is the trace of a point on the circumference of a circle rolling around an equal, fixed circle. (05 Aug 1998) |
| paraboloid dark field condenser | <microscopy> A lens of parabolic shape. The vertex end is ground back so that its focus can be brought into coincidence with the specimen on the slide. A central stop is provided to block the central rays. It is used chiefly for medium- power work. (05 Aug 1998) |
| receptive field | That part of the retina whose photoreceptors (rods and cones) pertain to a single optic nerve fibre. The response of a neuron to stimulation of its receptive field depends on the type of neuron and the part of the field that is illuminated; an "on-centre" neuron is stimulated by light falling at the centre of its receptive field and inhibited by light falling at the periphery; an "off-centre" neuron reacts in exactly the opposite fashion; that is, it is inhibited by light falling at the centre of its receptive field. In either case, the net response depends on a complex switching action in the retina. When an entire receptive field is equally illuminated, the response of receptors at the centre of the field predominates. (05 Mar 2000) |
| visual field | The area simultaneously visible to one eye without movement; often measured by means of a bowl perimeter located 330 mm from the eye. (05 Mar 2000) |
| visual field test | <ophthalmology> A test which measures the extent of visual field loss. This test may be performed by a number of methods including confrontation, tangent screen exam and automated perimetry. Diseases that affect visual field include stroke, diabetes, hypertension, multiple sclerosis, glaucoma, hyperthyroidism, Jacob-Creutzfeldt disease and optic glioma. (27 Sep 1997) |
| reversed-field pinch | <radiobiology> A toroidal magnetic confinement scheme which could constitute an alternative to the Tokamak for building a fusion reactor. It is characterised by a magnetic field mostly generated by the plasma itself, with toroidal and poloidal components of comparable intensities, in contrast with the Tokamak where most of the field is toroidal and externally applied. The name of the configuration is given by the fact that the toroidal component of the magnetic field changes sign in the outer region of the plasma. The main attractivness of the Reversed Field Pinch is that, according to presently established scalings, it could reach ignition without the need of auxiliary heating. (09 Oct 1997) |