Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 18

A clinical scanner has many different pulse sequences available, broadly designated as either gradient echo (GRE) or spin echo (SE). In GRE (e.g. Figure 1.7) each signal collected arises from a low flip angle RF pulse, typically less than 40°. T₁‐weighted images are generated using so‐called spoiled gradient echo. Rewound GRE uses slightly higher flip angles (>40°), producing bright‐fluid images, popular in cardiac MRI. GRE images are shown in Figure 1.9.

Figure 1.9 Gradient echo images: Gradient echo (GRE) abdomen; Rapid Acquired Gradient Echo (RAGE); 3‐D Gradient Echo (3D GRE); Steady State Free Precession (SSFP) heart; BOLD‐EPI with brain activation map overlay. Source: Flinders Medical Centre, Adelaide, Australia; Charing Cross Hospital, London, UK. Reproduced with permission.

The SE sequence was initially developed for its ability to recover signal loss arising from B₀ inhomogeneities. These occur across the field‐of‐view (FOV) as dephasing, or “fanning out”, of transverse components of the magnetization (Figure 1.10) following a 90° pulse. By applying a 180° pulse orthogonal to the 90° pulse (and also to B₀), the fan of magnetization vectors is twisted around in such a way that those with a phase lag advance in phase and vice‐versa. At time TE, equal to twice the interval between the 90° and 180° pulses, the magnetization rephases, giving a strong echo whose magnitude depends upon the tissue T₂.

Figure 1.10 Spin echo formation: following a 90° pulse aligned with the x’ axis, magnetization in the x’y’ plane dephases; a 180° pulse aligned with the y‘‐axis inverts the phase of the magnetization to form a spin echo at time TE. The prime (‘) indicates a frame of reference rotating at the Larmor frequency.

Spin echo can be further enhanced by using multiple 180° pulses to form a series or train of echoes, each of which can have different phase‐encoding applied. In Turbo or Fast Spin Echo (TSE/FSE) the overall scan time is reduced by the echo train length (ETL) or Turbo‐factor (TF). Typical ETL/TFs are in the range 3–20, although single shot acquisitions with 128–256 echoes are also possible.

The addition of a preparation 180° pulse prior to the 90° inverts the magnetization to lie along the ‐z axis. As each tissue recovers towards +M₀, there is a time at which its magnetization passes through zero. An image formed at this point, will not contain signal from that tissue. Short TI Inversion Recovery (STIR) removes fat from the image, whilst FLuid Attenuated Inversion Recovery (FLAIR) removes the cerebrospinal fluid (CSF). Typical SE images are shown in Figure 1.11.

Figure 1.11 Spin echo images: (a) Spin echo (SE) T₂‐weighted brain; single shot Turbo (fast) Spin Echo (TSE) MR cholangio‐pancreatogram; Fluid Attenuated Inversion Recovery (FLAIR) brain; Short TI Inversion Recovery (STIR) knee; Diffusion EPI (Echo Planar Imaging) showing white matter directional anatomy. Source: Flinders Medical Centre, Adelaide, Australia. Reproduced with permission.