HYFI: Hybrid filling of the dead‐time gap for faster zero echo time imaging

Abstract

The aim of this work was to improve the SNR efficiency of zero echo time (ZTE) MRI pulse sequences for faster imaging of short-T 2 components at large dead-time gaps. ZTE MRI with hybrid filling (HYFI) is a strategy for retrieving inner k-space data missed during the dead-time gaps arising from radio-frequency excitation and switching in ZTE imaging. It performs hybrid filling of the inner k-space with a small single-point-imaging core surrounded by a stack of shells acquired on radial readouts in an onion-like fashion. The exposition of this concept is followed by translation into guidelines for parameter choice and implementation details. The imaging properties and performance of HYFI are studied in simulations as well as phantom, in vitro and in vivo imaging, with an emphasis on comparison with the pointwise encoding time reduction with radial acquisition (PETRA) technique. Simulations predict higher SNR efficiency for HYFI compared with PETRA at preserved image quality, with the advantage increasing with the size of the k-space gap. These results are confirmed by imaging experiments with gap sizes of 25 to 50 Nyquist dwells, in which scan times for similar image quality could be reduced by 25% to 60%. The HYFI technique provides both high SNR efficiency and image quality, thus outperforming previously known ZTE-based pulse sequences, in particular for large k-space gaps. Promising applications include direct imaging of ultrashort-T 2 components, such as the myelin bilayer or collagen, T 2 mapping of ultrafast relaxing signals, and ZTE imaging with reduced chemical shift artifacts.

Mapping the myelin bilayer with short‐T2 MRI: Methods validation and reference data for healthy human brain

“…With the maximum gradient strength of 221 mT/m employed at the shortest TE, the nominal isotropic resolution was limited to 1.56 mm. The second protocol utilized zero‐TE imaging with hybrid filling of the dead‐time gap (HYFI) 32 to acquire a high‐resolution (0.39 mm isotropic) ultrashort‐T 2 reference image. Further details on the protocols can be found in Table 1 .…”

“… Note : The first protocol uses an SPI 31 sequence with spherical k‐space support to acquire a multi‐TE image series. The second protocol uses a HYFI 32 sequence to acquire a high‐resolution ultrashort‐T 2 reference image. Pulse power was adjusted for maximum steady‐state signal, calibrated in a D 2 O sample to specifically optimize non‐aqueous signals.…”

Single point imaging with radial acquisition and compressed sensing

“…However, FID-based projection techniques have been proposed that offer ultra-short acquisition delays: UTE imaging, 7 back-projection low angle shot (BLAST), 8 rotating ultra-fast imaging sequence (RUFIS), 9,10 zero TE (ZTE), 11 point-wise encoding time reduction with radial acquisition (PETRA), 12,13 water-and fat-suppressed proton projection MRI (WASPI), 14 and HYFI (hybrid filling of the dead-time gap). 15 A more extreme version of 3D radial projection imaging has been implemented by simultaneous RF excitation and signal acquisition, such as in sweep imaging with Fourier transform (SWIFT). 16,17 In UTE, radial k-space data are acquired directly after a non-selective RF pulse.…”

Sub-millisecond 2D MRI of the vocal fold oscillation using single-point imaging with rapid encoding

“…The long acquisition times of SPI can be shortened by applying the excitation pulse already in the presence of ramped up encoding gradients [ 39 ]. SPI is also applied in hybrid zero echo time sequences to acquire data close to k-space center [ 40 , 41 ].…”