Another term for detector, by analogy to broadcast radio receivers. Other terms used for devices of this type include modulators, phase sensitive detector and mixers.

(DWI) Magnetic resonance imaging is sensitive to diffusion, because the diffusion of water molecules along a field gradient reduces the MR signal. In areas of lower diffusion the signal loss is less intense and the display from this areas is brighter. The use of a bipolar gradient pulse and suitable pulse sequences permits the acquisition of diffusion weighted images (images in which areas of rapid proton diffusion can be distinguished from areas with slow diffusion).
Based on echo planar imaging, multislice DWI is today a standard for imaging brain infarction. With enhanced gradients, the whole brain can be scanned within seconds. The degree of diffusion weighting correlates with the strength of the diffusion gradients, characterized by the b-value, which is a function of the gradient related parameters: strength, duration, and the period between diffusion gradients.
Certain illnesses show restrictions of diffusion, for example demyelinization and cytotoxic edema. Areas of cerebral infarction have decreased apparent diffusion, which results in increased signal intensity on diffusion weighted MRI scans. DWI has been demonstrated to be more sensitive for the early detection of stroke than standard pulse sequences and is closely related to temperature mapping.
DWIBS is a new diffusion weighted imaging technique for the whole body that produces PET-like images. The DWIBS sequence has been developed with the aim to detect lymph nodes and to differentiate normal and hyperplastic from metastatic lymph nodes. This may be possible caused by alterations in microcirculation and water diffusivity within cancer metastases in lymph nodes.

See also Diffusion Weighted Sequence, Perfusion Imaging, ADC Map, Apparent Diffusion Coefficient, and Diffusion Tensor Imaging.

Diffusion weighted imaging can be performed similar to the phase contrast angiography sequence. The gradients must be increased in amplitude to depict the much slower motions of molecular diffusion in the body.
While a T1 weighted MRI pulse sequence is diffusion sensitive, a quantitative diffusion pulse sequence was introduced by Steijskal and Tanner. Its characteristic features are two strong symmetrical gradient lobes placed on either side of the 180° refocusing pulse in a spin echo sequence. These symmetrical gradient lobes have the sole purpose of enhancing dephasing of spins, thereby accelerating intravoxel incoherent motion (IVIM) signal loss.
Dephasing is proportional to the square of the time (diffusion time) during which the gradients are switched on and the strength of the applied gradient field. Therefore, the use of high field gradient systems with faster and more sensitive sequences, make diffusion weighting more feasible.
Areas in which the protons diffuse rapidly (swollen cells in early stroke, less restriction to diffusion) will show an increased signal when the echo is measured relative to areas in which diffusion is restricted. For increased accuracy of diffusion measurement and image enhancement, useful motion correction techniques such as navigator echo and other methods should be used. In addition to this, applying the b-value calculated by the strength and duration of motion probing gradients with a high rate of accuracy is very important.

See also Apparent Diffusion Coefficient, ADC Map, Lattice Index Map.

From ONI Medical Systems, Inc.;
MSK-Extreme™MRI system is a dedicated high field extremity imaging device, designed to provide orthopedic surgeons and other physicians with detailed diagnostic images of the foot, ankle, knee, hand, wrist and elbow, all with the clinical confidence and advantages derived from high field, whole body MRI units. The light weight (less than 650 kg) of the OrthOne System performs rapid patient studies, is easy to operate, has a patient friendly open environment and can be installed in a practice office or hospital, all at a cost similar to a low field extremity machine.
New features include a more powerful operating system that offers increased scan speed as well as a 160-mm knee coil with higher signal to noise ratio, and the option of a CD burner.

The schematic figures of a pulse sequence timing diagram illustrate the steps of basic hardware activity that are incorporated into a pulse sequence. Time during sequence execution is indicated along the horizontal axes. Each line belongs to a different hardware component. One line is needed for the radio frequency transmitter and also one for each gradient (G_s = slice selection gradient x, G_f = phase encoding gradient y, G_f = frequency encoding gradient z, also called readout gradient).
In picture 1, a timing diagram for a 2D pulse sequence is shown.
Slice selection and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. A single phase encoding component is present each time the sequence is executed.
Additional lines are added for ADC (Analog to Digital Converter) and sampling. A gradient pulse is shown as a deviation above or below the horizontal line. Simultaneous component activities such as the RF pulse and slice selection gradient are indicated as a non-zero deviation from both lines at the same horizontal position. Simple deviations from zero show constant amplitude gradient pulse. Gradient amplitudes that change during the measurement, e.g. phase encoding are represented as hatched regions.

The second picture shows a timing diagram for a 3D pulse sequence.
Volume excitation and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. Two phase encoding components are present, one in the phase encoding direction and the other in slice selection direction (irrespectively incremented in amplitude) in each time the sequence is executed. A description of the comparison of hardware activity between different pulse sequences.