Café com Física



17 de junho de 2015
Sala Celeste

Michael Garwood
University of Minnesota, USA

Frequency- and Gradient-Modulated MRI: Imaging with Extreme Field Inhomogeneity

MRI has evolved into an incredibly powerful tool due to impressive innovations over the past 3-4 decades. However, the general approach used to obtain MR images has not deviated significantly from Lauterbur’s original MRI experiment (1). That is, in state-of-the-art MRI, the pulse sequence used to generate spatially-encoded signals for imaging requires two basic elements: a sufficiently short RF pulse to excite all resonance frequencies of interest simultaneously, followed by a finite time during which magnetization is allowed to evolve in the presence of a field gradient to encode their spatial locations. This universal approach to MR imaging is optimal only under stringent experimental conditions. Specifically, frequency variation due to field inhomogeneity and magnetic susceptibility must be limited to a small fraction of the Larmor frequency, γB0. Although recent advances have improved the ability of Fourier-based imaging to tolerate B0 inhomogeneity, these improvements fall short of meeting the technical requirements for human imaging using small and portable magnets. For example, MRI of the human brain cannot be performed with the subject’s body from the shoulder down placed outside the magnet bore. No MRI technology has ever been conceived to accomplish this; that is, until possibly recently. Namely, spatiotemporal encoding (2-7) has the capability to go far beyond standard Fourier imaging in terms of tolerating B0 inhomogeneity, allowing B0 to vary by a significant fraction of the nominal field.

Improved tolerance to B0 inhomogeneity with spatiotemporal-encoded MRI has already been demonstrated by us and others. However, these improvements were limited because B0 compensation was restricted to one or possibly two spatial directions simultaneously. Now, due to the recent invention of 3D frequency-swept excitation pulses in STEREO (7), in combination with parallel transmit “beam-steering”, multi-coil shimming, and simultaneous transmit and receive electronics, it should be possible to fully compensate extreme B0 inhomogeneity and thus accomplish MR imaging of brain using a small and portable head-only magnet. STEREO, which stands for steering resonance over the object (7), produces non- Fourier based images by moving a localized resonant region along a curved trajectory in space such that the excitation itself is spatially selective and temporally sequential. In STEREO, the gradients are modulated to produce a “spatial rapid passage”. By producing a sweeping 3D-selective excitation in time, temporal adjustments to the pulse can be made to compensate for the spatial variation of B0 and B1 as the “resonant region” traverses through space. Simultaneous RF transmission and signal detection preserves ultrashort-T2* signals. This method of achieving spatial isolation is particularly novel in MR imaging and opens opportunities for numerous novel applications that could not be considered with standard Fourier-based techniques.