MR Toolbox

Martin Krssák¹

¹Medical University Vienna, Austria

Synopsis

This lecture will introduce and describe most of the “Tools” we are using as building blocks of MR imaging and spectroscopy sequences. It will start with radiofrequency (RF) excitation, use of the phase cycling and building up of spin echo. Principles of slice selection, use of the gradients for signal encoding and spoiling will also be described. A concept of the extended phase graph (EPG) theory, which is a tool for depicting and understanding the magnetization response of a broad variety of MR sequences will also be introduced.

RF excitation

Radio frequency (RF) excitation is a resonance process where external RF field interacts with the magnetization of the sample. Angular tilting of the magnetization (flip angle) is dependent on the RF power applied whereas its frequency-selectivity is dependent on the envelope function of the RF field – the shape of the RF pulse.

RF train

A sequence of different RF pulses applied in the MR sequence for the creation of signal. The role of RF pulses varies from excitation, through refocusing of desired signal down to suppressing of unwanted magnetization.

Spin-echo

Signal appearing after the simple 90°– tau – 180°– tau MR sequence due to the refocusing of disappeared magnetization in the transversal plane.

Phase graphs

Phase graphs of the magnetization evolution during the time course of MR sequence. Phase graphs focus on echo generation as well as on classification and use a Fourier based magnetization description in terms of “configurations states”.

Gradient

A well-defined, time restricted, mostly linearly changing magnetic field applied for the purpose of dephasing (spoiling) or rephrasing the magnetization, slice selection, frequency or phase encoding.

RF spoiling (phase cycling)

Suppression of unwanted signals can be achieved by changing pulse phases with respect to the reference phase of the RF field or the phase of the transverse magnetization. By alternating phases of the excitation pulses, followed by either addition or subtraction of the measured signals, one can amplify the echo of interest with respect to other components of the signal that overlap with the echo.

Slice selection

Slice selection is a concomitant application of a frequency selective RF pulse and a gradient of the magnetic field resulting in the RF excitation of pre-defined slab within the sample (slice). The position of the selected slice depends on the central frequency of the RF-pulse and the thickness of the slice depends on the combination of the gradient amplitude and the frequency width of RF-pulse.

Acknowledgements

No acknowledgement found.

References

Bernstein M.A., King K.F., Zhou X.J. – Handbook of MRI Pulse Sequences. Elsevier Academic Press, 2004

Hanson L. - Bloch Simulator for education in MRI and NMR, http://www.drcmr.dk/bloch

Weigel M. - Software for Extended Phase Graph Simulation, http://epg.matthias-weigel.net/

Elster A.D. - Questions and Answers in Magnetic Resonance Imaging, http://mriquestions.com/index.html

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)