MRI physics: The physics underlying image acquisition in the Zero-Echo-Time sequence

In contrast to conventional MRI modalities, the ZTE sequence prioritizes imaging of tissues with ultrashort T2 relaxation times, such as bone tissues. Therefore, CT-like images can be obtained from these tissues, which are seen as signal-void areas in conventional MRI (Figure 1).

In the ZTE sequence, readout gradients are ramped before the radiofrequency pulse to ensure immediate data acquisition after stimulation. A readout period is started immediately after the radiofrequency (RF) pulse, allowing for imaging of the ultrashort-T2 structures.  There is a near-zero-time interval between the end of the RF excitation and the start of data acquisition. Gradients are used continuously and reoriented gradually in the x,y, and z directions between repetitions. Due to the dead time (Δt) following the radiofrequency pulse, a spherical gap forms at the center of the k-space, resulting in missed data points (Figure 2). Accordingly, data are sampled along center-out 3D radial trajectories in k-space using purely frequency encoding (Figure 3). This trajectory is crucial for effectively capturing signals with rapid signal decay from tissues. Short RF pulses with high peak power are used to achieve the desired flip angle for excitation. Rapid switching between transmit (for RF excitation) and receive (for signal reception) modes is essential for preserving the signal-to-noise ratio (SNR) during data acquisition.

Scan time focuses on data acquisition, with minimal dead time during the RF pulse and a single gradient step. This feature allows for rapid isotropic scanning with high resistance to artifacts caused by susceptibility and patient motion. This sequence reduces mechanical vibration and allows for an almost silent technique.

MRI protocols: Protocols for using the Zero Echo Time sequence in 1.5 Tesla and 3 Tesla MRI

The ZTE sequence can be applied with 3T and 1.5T field strength and anywhere in the body imaging. Protocol values may vary depending on the location of the body where the scan is performed, but some value ranges can be determined. The sequence type of ZTE is ultrashort echo-time 3D isotropic, repetition time is 1–2 msec, and echo time (TE) is 8 µsec. The number of signals acquired at 3 T and 1.5 T are 3-6 and 5-8, respectively. The flip angle is chosen as 1° or 2°. Bandwidth is adjustable as 50 kHz at 1.5 T and 31.25 or 83.33 kHz at 3 T.  Depending on the location of the imaging area field of view (160x160 mm–420x420 mm), section thickness 0.6–1.4 mm, and section numbers (<640) are between certain values. As a result of using the described protocols, the acquisition time is 2-5 minutes. The parameters and values used in our institution are summarized in Figure 4.

Clinical feasibility: Case-based evaluation of the ZTE sequence in whole-body imaging

Due to its remarkable success in imaging bone structures, arthropathies, fractures, bone pathologies, and calcifications, the ZTE sequence has started to be utilized in many centers, including our institution (Figure 5-12).

This technique is not commonly used but has been employed in studies at certain centers for specific purposes. It has been utilized to image kidney stones in children, to enhance magnetic resonance angiography (MRA) by reducing artifacts in patients with stents and coils, and in thoracic MRI for imaging patients with cystic fibrosis or lung nodules.