Investigations of the origin of phase differences seen with ultrashort TE imaging of short T2 meniscal tissue

Abstract

Ultrashort echo time MRI requires specialized pulse sequences with nominal echo times as low as a few microseconds to detect signals from the short T2 tissues frequently encountered in the musculoskeletal system. Usually, magnitude images are reconstructed and these often show low tissue contrast. Ultrashort echo time phase images of the meniscus show surprisingly high contrast despite their very short echo time. In this article, we investigated the source of this contrast using the Bloch equations, simulations, phantom experiments, and tissue studies. Phase evolution was shown to occur in ultrashort echo time sequences during the finite radiofrequency pulse and readout periods, and previously unrecognized susceptibility differences between fiber groups were observed in the meniscus. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.

Tissue Electrical Property Mapping From Zero Echo-Time Magnetic Resonance Imaging

“…Image encoding starts immediately, leading to a nominal echo time TE = 0; consequently, the acquired images have little sensitivity to chemical shift and B 0 inhomogeneities. Note that the three sources of off-resonance phase accumulation in UTE imaging [30], i.e., finite excitation pulse width, ramp sampling, and readout interval, are minimized or eliminated through use of a short hard pulse, lack of ramp sampling (Fig. (3)), and high-bandwidth readout.…”

“…The effective off-resonance dephasing time estimated from the theories of ref. [30] was on the order of 20 µs.…”

Ultrashort Echo Time T1ρ Is Sensitive to Enzymatic Degeneration of Human Menisci

“…The syringe containing the specimen was placed inside a 2 cm diameter transmit-receive solenoid coil (ProbeTek, San Diego, CA) 18 , near the iso-center of the scanner, with the meniscal slice positioned anatomically as it would be expected if it were located inside of a patient. All sequences (conventional and quantitative) were performed in the sagittal plane, with a single slice acquisition, 1 mm slice thickness and a field of view of 4 cm.…”

Signal and contrast effects due to T2 decay during k-space readout of UTE (ultrashort TE) sequences

“…Dec( k )S( k ) and I( r ) of this “box image” model are shown in figures 1B–C, demonstrating increasing decay in Dec( k )S( k ) and I( r ) with decreasing T2 values. To permit detailed theoretical analysis, we also evaluated an additional “box k-space” model (similar approach to references [9,10]), where the source spin density s( r ) is chosen to be a sinc function, with the corresponding S( k ) to be a box function of limits k L = ±1/(2 L ) in k-space. The box function S( k ) in k-space of this “box k-space” model is much more tractable to detailed analysis, while yielding qualitative and quantitatively similar results to the “box image” model (see below, and also references [9, 10]).…”

“…To permit detailed theoretical analysis, we also evaluated an additional “box k-space” model (similar approach to references [9,10]), where the source spin density s( r ) is chosen to be a sinc function, with the corresponding S( k ) to be a box function of limits k L = ±1/(2 L ) in k-space. The box function S( k ) in k-space of this “box k-space” model is much more tractable to detailed analysis, while yielding qualitative and quantitatively similar results to the “box image” model (see below, and also references [9, 10]). In figures 1D–E, we demonstrate that decreasing T2 values also leads to increasing decay in Dec( k )S( k ) and I( r ) for the “box k-space” model.…”