Synopsis

This talk will cover the physical principles of acquiring data rapidly and introduce commonly-used fast pulse sequences. Gradient-recalled echo and balanced SSFP pulse sequences reduce scan time by reducing the repetition time. Spin-echo-train pulse sequences reduce scan time by acquiring a series of k-space lines interspersed between refocusing pulses. A third way to reduce scan time is to collect more than a single line of k-space during a single readout, as in echo-planar imaging and spiral scanning. The audience will learn the fundamentals and applications of these fast pulse sequences.

Overview of Presentation

A spin echo pulse sequence with rectilinear (Cartesian) k-space sampling is a robust method for acquiring T1-weighted or T2-weighted images, and this sequence played an important role in the early days of MRI. However, the scan time with such a sequence can be many minutes, so alternative pulse sequences have been developed to sample data more quickly. This presentation will give an overview of sampling data more quickly using three general approaches.

The first approach is to shorten the sequence repetition time (TR). Gradient-recalled echo (GRE) sequences can enable the TR to be reduced substantially relative to a spin echo sequence. For spoiled GRE, this reduction in TR necessitates a smaller flip angle, with associated reductions in signal-to-noise ratio (SNR) and image contrast. It is possible to increase image contrast using contrast agents or preparation pulses. 3D MP-RAGE is an example of a GRE method that uses preparation pulses to enhance T1-weighted contrast, and it can rapidly collect a 3D volume. A sequence with an even shorter TR and larger flip angles is balanced steady-state free precession (bSSFP). This method keeps the magnetization in the transverse plane over multiple TR’s, and thus can have higher SNR for blood and similar tissues.

A second way to shorten scan time is to use a series of 180-degree refocusing pulses after a single 90-degree excitation pulse, sampling a different k-space line after each of the refocusing pulses. This approach, known as a RARE or spin-echo train sequence, is a standard way to collect T2-weighted images. There also spin-echo train sequences that extend the length of the echo train by modulating the refocusing flip angles. These methods can generate T1-weighted or T2-weighted contrast and can collect 3D images very rapidly.

A third way to sample data more quickly is to collect more than one line of k-space during each readout. Echo planar imaging (EPI) is a classic method for collecting data rapidly by sampling rectilinear k-space in a raster fashion. EPI is widely used for single-shot imaging, especially for functional and diffusion MRI. It is also possible to sample k-space in a non-rectilinear fashion, which is known as non-Cartesian scanning. Radial spokes and interleaved spirals are two such non-Cartesian k-space trajectories. This presentation will cover various properties of non-Cartesian sampling, including high scan efficiency and robustness to patient motion, as well as how they can be incorporated into the various pulse sequences described above.

Acquiring data more quickly is a key step in rapid imaging. Scan time can be reduced further by collecting a subset of the data and estimating the missing data in image reconstruction. Methods to do this using parallel imaging and sparse reconstruction will be covered in other talks in this session.

Acknowledgements

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References

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