Strategies for inner volume 3D fast spin echo magnetic resonance imaging using nonselective refocusing radio frequency pulsesa)

Abstract

Fast Spin Echo (FSE) trains elicited by non-selective "hard" refocusing radio frequency (RF) pulses have been proposed as a means to enable application of FSE methods for high resolution 3D magnetic resonance imaging (MRI). Hard-pulse FSE (HPFSE) trains offer short (3-4 ms) echo spacings, but are unfortunately limited to imaging the entire sample within the coil sensitivity thus requiring lengthy imaging times, consequently limiting clinical application. In this work we formulate and analyze two general purpose combinations of 3D HPFSE with Inner Volume (IV) MR imaging to circumvent this limitation. The first method employs a 2D selective RF excitation followed by the HPFSE train, and focuses on required properties of the spatial excitation profile with respect to limiting RF pulse duration in the 5-6 ms range. The second method employs two orthogonally selective 1D RF excitations (a 90 x°-180 y° pair) to generate an echo from magnetization within the volume defined by their intersection. Subsequent echoes are formed via the HPFSE train, placing the focus of the method on (a) avoiding spurious echoes that may arise from transverse magnetization located outside the slab intersection when it is unavoidably affected by the non-selective refocusing pulses, and (b) avoiding signal losses due to the necessarily different spacing (in time) of the RF pulse applications. The performance of each method is experimentally measured using Carr-PurcellMeiboom-Gill (CPMG) multi-echo imaging, enabling examination of the magnetization evolution throughout the echo train. The methods as implemented achieve 95% to 97% outer volume signal suppression, and higher suppression appears to be well within reach, by further refinement of the selective RF excitations. Example images of the human brain and spine are presented with each technique. We conclude that the SNR effciency of volume imaging in conjunction with the short echo spacing afforded by hard pulse trains enable high resolution 3D HPFSE MRI of a small fieldof-view (FOV) with minimal aliasing artifact.

On the correlation between T2 and tissue diffusion coefficients in exercised muscle: quantitative measurements at 3T within the tibialis anterior

“…A series of axial T 2 -weighted images were acquired with a T 2 -weighted fast spin-echo (FSE) sequence in order to identify a single 4 mm thick axial slice through the TA muscle for quantitative T 2 and D measurements. A Carr-PurcellMeiboom-Gill (CPMG) imaging sequence [12,[24][25][26] with a 10 ms echo spacing, 16 echoes, 1 s repetition time (TR), 128 × 64 image matrix (frequency by phase encode), with a 22 × 22 cm 2 field-of-view (FOV), 4 mm slice thickness, nominal voxel volumes of 0.02 ml and a scan time of 64 s, including four "dummy" scans, was used to obtain data for image based T 2 measurement. The CPMG sequence utilized a slice selective 90 x excitation pulse, non-selective 180 y pulses sandwiched by crusher gradients of constant amplitude, and phase encode rewinding applied between the refocusing pulses [12,25].…”

Basis function cross‐correlations for Robust k‐space sample density compensation, with application to the design of radiofrequency excitations

“…This is often of paramount importance, since multidimensional RF excitations are often employed precisely in order to reduce the imaging FOV (14,15,19). Conversely, a significant portion of the increased error observed in the experiments stemmed from a broadening of the transition region of the profile, likely due to the finite resonance linewidth and differences between the designed and actual k-space trajectory traced.…”

“…Multidimensional RF excitation (1,10,11) has many applications, such as inner volume imaging, navigator acquisition, and accounting for field inhomogeneities (10,(12)(13)(14)(15)(16)(17)(18)(19). Conversely, non-Cartesian sampling (20) has applications in fast imaging (21).…”

Reduction of B1 sensitivity in selective single‐slab 3d turbo spin echo imaging with very long echo trains

“…1 (D 1 ϭ D 2 , D 1 ӷ D 3 ), while the remainder of the train uses a shorter echo spacing (ESP 2 ) as permitted by the short, nonspatially selective refocusing RF pulses. The unequal echo-spacing strategy has been also used for other applications such as 3D inner-volume turbo/fast SE imaging (15).…”