multiple slices at different locations (Figure 1.6).
Figure 1.6 Multiple slice imaging. Changing the frequency of each RF pulse whilst a gradient Gss is applied selects a different slice position.
In‐plane localization
The localization of the MR signal within a slice is usually achieved by two processes: phase‐encoding (PE) and frequency‐encoding (FE), each using gradient pulses along orthogonal directions. These pulses encode the MR signal in terms of spatial frequencies. Image acquisition requires multiple repetitions of the basic block of a pulse sequence using a different amplitude of PE pulse each time (Figure 1.7). TR is the time interval between successive repetitions. Image reconstruction is achieved by the mathematical operation of a two‐dimensional (2D) Fourier transform.
Figure 1.7 Simple 2D pulse gradient echo (GRE) sequence showing pulse amplitudes and timings of the components.
The application of a second set of PE gradients in conjunction with the selection of a thicker slab of tissue makes three‐dimensional (3D) imaging possible (Figure 1.8).
Figure 1.8 3D imaging sequence with a second phase‐encode gradient in the slab direction.
Pulse sequences
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.
Parallel imaging
In parallel imaging a multi‐element RF receive array coil is used to provide additional spatial information, and to reduce the number of lines of signal required to form an image. Parallel imaging reduces the number of TR periods of an acquisition by an amount known as the reduction factor R, SENSE factor or iPAT factor. The use of parallel imaging reduces the patient’s overall RF exposure.
Overview of MRI applications
Since its adoption in the late 1980s the scope of MRI’s clinical applications has grown, and continues to grow. Brain, spine, and musculoskeletal imaging were the first major applications.
MRI’s ability to differentiate between grey and white matter in the brain led to its deployment in neuroradiology, particularly for white
