Magnetic Resonance Imaging monitoring of histotripsy effects in agar phantom

Abstract

Background: While the technical aspects of histotripsy have seen significant advancements, research on Magnetic Resonance Imaging (MRI) monitoring remains limited.

Purpose: This preliminary study explored the use of conventional T2-Weighted (T2-W) Turbo Spin Echo (TSE) imaging to monitor histotripsy lesions in a pure agar gel, as a potential research tool for MRI-guided histotripsy (MRgHt).

Methods: Histotripsy experiments were conducted in a 3T MRI scanner using a 2% weight per volume agar solution in water. Pulsed Focused Ultrasound (FUS) was applied at a duty factor of 2%, with a pulse repetition period of 1s, delivering up to 2000 pulses. T2-W TSE imaging was employed for intra-procedural monitoring with 10s updates and post-sonication assessment, with the first prioritizing rapid acquisition and the second focusing on resolution. Supplementary, MR thermometry tracked temperature variations throughout the procedure.

Results: Dynamic T2-W TSE imaging provided insights into the progress of phantom disruption. Mild thermal effects served as early signs of phantom response, as mechanical effects required more time to manifest. Typical histotripsy lesion characteristics were observed, including a hyperintense core with an elongated shape and a thin hypointense border. The developed temperatures remained well below hyperthermia levels, reaching a maximum of 34°C.

Conclusions: The presented methodology, utilizing a pure agar gel within a simple, versatile setup alongside conventional T2-W TSE imaging, holds promise for advancing MRgHt research.

Keywords: MRI; T2‐W; agar gel; histotripsy; lesion monitoring.

FIGURE 1

(a) Experimental setup for phantom histotripsy experiments in the MRI setting, and (b) Axial T2‐W TSE image of the setup, showing the location from the transducer to the focal point within the phantom. MRI, Magnetic Resonance Imaging.

FIGURE 2

Axial T2‐W TSE images showing the first 10 scans acquired following amplifier activation (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm², PRP = 1s, DF = 2%, 2000 pulses), alongside the pre‐sonication reference image. DF, duty factor; PRP, pulse repetition period; SPPA, spatial peak pulse average intensity; T2‐W TSE, T2‐weighted turbo echo spin.

FIGURE 3

Selected axial T2‐W TSE images from a series of scans, spaced at regular intervals throughout the sonication session (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm², PRP = 1 s, DF = 2%, 2000 pulses). DF, duty factor; PRP, pulse repetition period; SPPA, spatial peak pulse average intensity; T2‐W TSE, T2‐weighted turbo echo spin.

FIGURE 4

(a) CNR measured from T2‐W TSE images of the agar phantom during histotripsy sonication. (b) Focal temperature changes during and shortly after sonication (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm², PRP = 1s, DF = 2%, 2000 pulses), c) Color‐coded thermal map produced upon completion of sonication, and (c) Corresponding thermal dose map. DF, duty factor; PRP, pulse repetition period; SPPA, spatial peak pulse average intensity; T2‐W TSE, T2‐weighted turbo echo spin.

FIGURE 5

(a) High‐resolution T2‐W TSE axial image showing the inflicted histotripsy lesion (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm², PRP = 1s, DF = 2%, 2000 pulses) following resolution of thermal effects, (b) Coronal slices at two different cross‐sections (1 and 2) of the lesion, and (c) Photo of the actual lesion after phantom cross‐sectioning at FD. DF, duty factor; FD, focal depth; PRP, pulse repetition period; SPPA, spatial peak pulse average intensity; T2‐W TSE, T2‐weighted turbo echo spin.

FIGURE 6

(a) High‐resolution T2‐W TSE axial image showing histotripsy lesions created with different protocols: [1] pulse numbers (N) = 2000 and PRP = 1, [2] N = 1000 and PRP = 1, [3] N = 2000 and PRP = 0.5 (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm² and DF = 2%), (b) Corresponding coronal slice revealing lesion diameter, with the cross‐section location indicated by the red dotted line in the axial view, (c) Contrast‐to‐noise ratio between lesion and background, and lesion area measured in axial plane. (d) T2‐W TSE images acquired at 5‐, 10‐, 15‐, and 30 min post‐sonication (${\mathrm{I}}_{\mathrm{SPPA}}$ = 29.4 kW/cm² and DF = 2%, PRP = 1) showing lesions formed by histotripsy at pulse counts of [4] N = 1000, [5] N = 750, [6] N = 500, and [7] N = 250, compared to a thermal lesion (200 W, 60 s) indicated by the red arrow. DF, duty factor; PRP, pulse repetition period; SPPA, spatial peak pulse average intensity; T2‐W TSE, T2‐weighted turbo echo spin.