Normal Findings

The TOF method uses the longitudinal magnetization, such that the blood flowing into the selected volume during the RF excitation period is fully relaxed, whereas the magnetization from the stationary tissue is saturated, and hence the signal from the blood is brighter compared with that of the stationary tissue. This phenomenon was first described in a nonimaging context by Singer.y After RF excitation, the longitudinal magnetization recovers exponentially with time constraint T1 and is typically much longer than with T2 weighting. In a volume of stationary tissue and flowing blood, during every TR interval, the

magnetization recovers incompletely throughout the volume, acquiring T1 weighting that suppresses particularly the signals with longer T1. This is referred to as saturation. The degree of T1 weighting depends on the solution of the Bloch equation, which is a function of TR, T1, and the excitation flip angle sigma. Usually, a TR of 30 to 100 msec and flip angles of 10 to 60 degrees are used. These imaging parameters can be optimized for maximum suppression of the stationary tissues and better flow visualization.

In TOF imaging, faster flow through a vessel lumen running perpendicularly through the slice partially or completely exits the slice in the interval between the 90- and 180-degree excitation pulses.y Consequently, the 180-degree pulse fails to invert the nuclei and the signal is diminished. However, with slow flow, washout of the spins between the 90-degree and the 180-degree pulses is minimal. The previously excited outgoing slice is replaced during the TR interval by the inflow of fully magnetized spins from outside the volume, giving a brighter signal than the one acquired from stationary tissue. This phenomenon is generally referred to as entry phenomenon or flow-related enhancement. Furthermore, the heavy T1 weighting associated with these short TR sequences suppresses stationary tissue, an important contributor to the contrast between the stationary and the flowing signal. The TOF method primarily uses this flow-related enhancement phenomena to image the blood vessels.

Two major techniques exist in TOF MRA: two-dimensional TOF (2D TOF) and three-dimensional TOF (3D TOF) imaging techniques. Two-dimensional TOF is advantageous because of its sensitivity to slow flow rates, minimal saturation effects for normal flow velocities, and short acquisition times (5 to 7 minutes). Although widely used, the technique has a few limitations. The appearance of the moving blood varies when the imaging plane is not perpendicular to the slice select direction. Owing to the inplane flow, the blood vessels that run parallel to the imaging plane experience multiple RF pulses and eventually become saturated. Hence, the portion of the vessel traversing the imaging plane may exhibit some reduction in signal intensity and appear artifactually narrowed or even discontinued. Preliminary work with 2D TOF and magnetization transfer (MT) in the carotid artery bifurcation has shown an improved signal-to-noise ratio (SNR) and greater sharpness of vessel margins.y Contrary to the 2D TOF technique, the 3D TOF MRA technique offers a wide number of advantages over its 2D counterpart. The 3D volume acquisition techniques offer superior SNR and high spatial resolution and are sensitive to fast and intermediate flow. Also, the other major advantage of this technique is that the acquisition can be made in thin slices, thereby reducing voxel size and hence decreasing intravoxel dephasing of the moving blood. The use of surface coil along with small imaging voxels has been shown to significantly improve SNR and vessel conspicuity in normal and diseased carotid arteries. y Three-dimensional TOF is relatively insensitive to slow flow and is effective only for relatively small 3D volumes, because blood is saturated by multiple RF pulses while traversing the slab. Recently, MRA using 3D TOF with MT has been widely used in routine clinical imaging of the circle of Willis owing to their excellent stationary tissue suppression capabilities. y Preliminary work with 3D TOF acquired with dual 20- and 60-degree flip angles demonstrates improved SNR and vessel definition. y

In contrast to TOF, the phase-contrast angiography technique uses the flow-induced phase variations of the MRA signal caused by the motion of the blood. These phase shifts induced by the flow can be selectively modulated using bipolar flow-encoding gradients, and images of flow can be generated. With this strategy, two sets of data are acquired under identical conditions, except that the polarity of the bipolar flow encoding gradients is alternated during data acquisition. This modulates the velocity-induced phase shift for spins having macroscopic motion (flowing blood) and has no effect on stationary spins. MRA projections are created by subtraction of one data set from another, remove the signal from stationary tissues, and retain the signal from the flow. Moreover, the phase shift of the image can also be used to obtain a phase map of the image. This phase map is usually displayed in gray scale and indicates the regions where flow has occurred.

The two most popular methods that are currently used image the slab volume either with sequential time-averaged 2D slices y or with simultaneous acquisition of 3D volume slabsy and are generally referred to as 2D or 3D phase- contrast angiography. The 2D phase-contrast angiograms, like 2D TOF, are obtained as sequential individual slices. In 3D phase-contrast MRA the data are collected in three dimensions. This has several advantages over the 2D phase-contrast technique, such as isotropic voxels, reduction in phase variations, and flexibility in cross-sectional presentation of the feature of interest. However, the overall imaging time is much higher as compared with the 2D technique. In the past few years, cine phase-contrast MRA, a variant of 2D phase-contrast MRA in which cardiac gating is used to generate images at different points within the cardiac cycle, has become widely used. This method has been used to analyze temporal flow of blood in carotid arteries in the necky and flow of CSF in the brain. y

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Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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