The spin-warp technique

Spin-warp imaging denotes the most common form of Fourier imaging with data acquisition in Cartesian k-space coordinates [Edel]. Instead of a variable evolution time t for phase encoding, the gradient amplitude is stepped during t in order to halt phase evolution from spin interactions other than with the applied space-encoding gradient. 

During the detection time the signal is recorded with fiequency encoding of the spatial information (cf. Section 2.3.4). 

The phase acquired during the evolution period t in the presence of the applied gradient Gy (cf. Fig. 6.1.1(b)) is given by 

The acquisition time T of the signal is then determined by the number noMi of discrete frequency components corresponding to the number of pixels in this dimension, 

In medical imaging, T is of the order of 5 ms and is kept constant. For a different field of view, Xmax the gradient strength Gmax is adjusted rather than the acquisition time. 

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The limitation of the resolution by signal decay in the absence of a gradient applies only for frequency encoding of the space information, that is for the space dimension which is acquired directly (cf. Section 2.3.4). For indirect detection of the space information by phase encoding with the spin-warp technique, the spatial resolution is unrestricted in theory. It is determined just by the range of gradient values scanned in the experiment. Apodization techniques can be used for giving the voxel the desired shape (cf Section 6.2.4) [Marl]. 

The intensity of the signal acquired by the spin-warp technique depends on the echo time tE and on the repetition time t of successive scans. The echo time introduces T2 weights to the pixels of the image, resulting in relaxation-time contrast in cases where T2 is a function of space coordinates x, y, and z- Within the validity of the Bloch equations, the weight of the echo at point r = (x, y, z) is given by... 

Furthermore, the magnetic field intensity in the Y direction and application time are systematically varied and repeated in the same manner. By determining the 2D Fourier transform on every point on the XY coordinates, and performing inverse Fourier transformations, the 2D image is obtained as a cross section on the Z axis. The spin warp technique [112], which is often seen in newer commercial instruments, maintains ty as the constant, systematically varies Gy and applies phase encoding. 

The former technique was that known as filtered back-projection, which, as originally applied, was a direct MR analogue of the original translate-rotate CT-X-ray scanner developed by Hounsfield. The first version of the other strategy, involving Fourier transformation in two or more directions, was proposed by Ernst and his colleagues in a form that ultimately proved much less useful than the spin warp method that has become the basis of the vast majority of clinical MRI. 

Spin-warp technique Method of using gradient fields to encode position-dependent information into NMR signals, then using Eourier transform mathematics to decode the signal and produce an image. 

Normally, the first step in textile conservation is the examination of the piece to determine the fibers from which it was made and the technique by which it was created. One determines the spin of the yarns, whether Z or S, and the number of wefts and warps per inch. Next, one tests for colorfastness first with distilled water, then with glycerin and water, and finally with a neutral detergent and water. If the colors run with any of these solutions, obviously the piece cannot be washed, and one tests with dry-cleaning solvents and other chemicals. 

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