Variable ultrasound trigger delay for improved magnetic resonance acoustic radiation force imaging

Abstract

Magnetic resonance acoustic radiation force imaging (MR-ARFI) allows the quantification of microscopic displacements induced by ultrasound pulses, which are proportional to the local acoustic intensity. This study describes a new method to acquire MR-ARFI maps, which reduces the measurement noise in the quantification of displacement as well as improving its robustness in the presence of motion. Two MR-ARFI sequences were compared in this study. The first sequence 'variable MSG' involves switching the polarity of the motion sensitive gradient (MSG) between odd and even image frames. The second sequence named 'static MSG' involves a variable ultrasound trigger delay to sonicate during the first or second MSG for odd and even image frames, respectively. As previously published, the data acquired with a variable MSG required the use of reference data acquired prior to any sonication to process displacement maps. In contrary, data acquired with a static MSG were converted to displacement maps without using reference data acquired prior to the sonication. Displacement maps acquired with both sequences were compared by performing sonications for three different conditions: in a polyacrylamide phantom, in the leg muscle of a freely breathing pig and in the leg muscle of pig under apnea. The comparison of images acquired at even image frames and odd image frames indicates that the sequence with a static MSG provides a significantly better steady state (p < 0.001 based on a Student's t-test) than the images acquired with a variable MSG. In addition no reference data prior to sonication were required to process displacement maps for data acquired with a static MSG. The absence of reference data prior to sonication provided a 41% reduction of the spatial distribution of noise (p < 0.001 based on a Student's t-test) and reduced the sensitivity to motion for displacements acquired with a static MSG. No significant differences were expected and observed for thermal maps acquired with a variable MSG and a static MSG. The use of a static MSG with a variable ultrasound trigger delay improves the ARFI displacement map quality without additional acquisition time and remains compatible with the simultaneous acquisition of MR thermal maps.

Real‐time monitoring of tissue displacement and temperature changes during MR‐guided high intensity focused ultrasound

“…sequence with respiratory gating at 3T, the spatial resolution in this study was considered sufficient to monitor temperature and displacement changes during HIFU sonication. Displacement amplitudes found ex vivo on pig muscle ($40 mm) are in agreement with those from Mougenot et al (17). Auboiroux et al (18) reported from in vivo pig liver, a lower displacement (less than 15 mm) during a transcostal sonication, whereby a significant portion of acoustic power was absorbed by the rib cage.…”

“…Such a method may also be implemented with a single‐shot EPI. Although voxel size reported in Mougenot et al and Auboiroux et al was smaller in the liver using a segmented EPI acquisition sequence with respiratory gating at 3T, the spatial resolution in this study was considered sufficient to monitor temperature and displacement changes during HIFU sonication. Displacement amplitudes found ex vivo on pig muscle (∼40 µm) are in agreement with those from Mougenot et al .…”

“…Such a method may also be implemented with a single-shot EPI. Although voxel size reported in Mougenot et al (17) and Auboiroux et al (18) was smaller in the liver using a segmented EPI acquisition Orange squares represent artifacts in temperature and displacement estimates resulting from the data processing (see Eqs. [5] and [6]), when ultrasound is turned on and off, respectively.…”

Simultaneous MR thermometry and acoustic radiation force imaging using interleaved acquisition

“…Multiple acquisition schemes have been proposed for acquiring MR‐ARFI measurements, including spin echo sequences, which are inherently insensitive to temperature change; and gradient recalled echo (GRE) pulse sequences, which are sensitive to temperature changes. Although the majority of ARFI techniques have been demonstrated in phantoms and ex vivo tissues, in vivo studies recently have been published as well. Depending on the application, MR‐ARFI scan time has been reduced using keyhole or parallel imaging techniques when multiple MR radiofrequency (RF) coils are available .…”

“…The correlation between changes in tissue mechanical properties and thermal dose has been investigated using both ultrasound (US) and MR imaging . For MRI assessment of tissue mechanical properties, this typically has been accomplished using a standalone MR‐ARFI sequence, but methods for acquiring simultaneous MR‐ARFI and PRF thermometry have been proposed . These combined MR‐ARFI/PRF sequences are valuable because they can improve the safety of using MR‐ARFI for tissue stiffness investigation, focal spot localization, and phase aberration correction by simultaneously measuring temperature rise to ensure no thermal dose is delivered by the ARFI pulses.…”