Exploring the bandwidth limits of ZTE imaging: Spatial response, out‐of‐band signals, and noise propagation

Abstract

The tightest bandwidth limits in ZTE arise from background signal and radiofrequency (RF) switching transients. Significant advances in ZTE performance will be afforded by faster transmit-receive (T/R) switching with negligible transients and RF coils free of background signal.

A virtually 1H‐free birdcage coil for zero echo time MRI without background signal

“…4a). In agreement with previous findings (13), noise appeared unchanged up to gap 3.0 and showed some amplification and correlation for gap 3.8. Imaging of the empty coil (with RF excitation) did not lead to perceivable signal with a gap size up to 3.0.…”

“…Even with a gap of 3.8 dwells, images were obtained nearly free of background artifacts, which is the basis for enabling ZTE imaging at very high bandwidths. These results are comparable or even superior to what has been reported recently with 19F ZTE imaging (13), where avoiding background signal is considerably easier. Further efforts to eliminate 1H content will face diminishing returns as switching transients take over as the dominant source of out-of-band signal.…”

“…This situation is aggravated when imaging short‐T2 tissues at the required large bandwidths, where the finite initial RF dead time [i.e., the time between the magnetic center of the RF pulse and the first usable sample after digital filtering ] leads to a loss of early, low‐frequency data. This introduces a considerable spherical gap in central k‐space of ZTE data, which leads to strong amplification of out‐of‐band signal (Fig. ).…”

Long‐T2‐suppressed zero echo time imaging with weighted echo subtraction and gradient error correction

“…This led to total dead times below 8 μs corresponding to k-space gaps below 2 Nyquist dwells, which is sufficiently small for the AR technique. 36 To adjust the timing between RF excitation and reception, the position of the RF pulse with respect to the data was calibrated by recording it in a separate acquisition with the same experimental setup. For RF transmission and reception, a custom-built protonfree quadrature coil was used.…”

Analysis of ZTE MRI application to sandstone and carbonate

“…Additionally, gradient switching can be very taxing on the instrument hardware and cause excessive heating. To avoid gradient switching, ZTE uses nonselective radio frequency (RF) excitation while the gradients are already acting 11,12 . The strength of the gradient gradually increases with each pulse over time, so that spatial encoding is still possible, but the encoding happens with zero delay.…”