Image-Guided High-Intensity Focused Ultrasound, A Novel Application for Interventional Nuclear Medicine?

Abstract

Image-guided high-intensity focused ultrasound (HIFU) has been increasingly used in medicine over the past few decades, and several systems for such have become commercially available. HIFU has passed regulatory approval around the world for the ablation of various solid tumors, the treatment of neurologic diseases, and the palliative management of bone metastases. The mechanical and thermal effects of focused ultrasound provide a possibility for histotripsy, supportive radiation therapy, and targeted drug delivery. The integration of imaging modalities into HIFU systems allows for precise temperature monitoring and accurate treatment planning, increasing the safety and efficiency of treatment. Preclinical and clinical results have demonstrated the potential of image-guided HIFU to reduce adverse effects and increase the quality of life postoperatively. Interventional nuclear image-guided HIFU is an attractive noninvasive option for the future.

Keywords: PET/MR image guidance; hyperthermia; image-guided HIFU; targeted drug delivery; tumor ablation.

FIGURE 1.

Schematic representation of HIFU ablation. (A) High level of sound energy in focal spot leads to heating of tissue to temperatures above 55°C, resulting in coagulation with subsequent necrosis in tissue. Sensitive structures in near and far fields can be spared. (B) Ablation of large-volume tumors is realized by scanning focal zone through entire tumor using phased-array transducer via beam forming. (C) Histotripsy destroys tissue by induction of cavitation events, generating lesions in target region.

FIGURE 2.

MRgHIFU for uterine fibroid treatment. (A) Schematic drawing of MRgHIFU treatment of uterine fibroids with Sonalleve or ExAblate 2000. Diagnostic T2- and T1-weighted contrast-enhanced (gadolinium) images are shown of patients with hyper- and dysmenorrhea and Funaki type 1 fibroid before (B, white arrow) and after (C, black circle) MRgHIFU. Fibroid volume is 184 mL; treatment duration is up to 4 h. w = weighted.

FIGURE 3.

Clinically approved HIFU systems. (A) MRgHIFU system for neurologic application (ExAblate Neuro). Patient table includes FUS transducer mounted on mechanical positioning unit with MRI-compatible head frame that can be docked to MRI scanner. (B) EchoPulse USgHIFU system for treatment of thyroid nodules and breast fibroadenomas. (C) Transurethral TUSLA-Pro MRgHIFU ablation system for prostate cancer. (D) JC USgHIFU system for solid tumor ablation of all types of soft tissue. (Images courtesy of Insightec, Theraclion, Profound Medical, and Chongqing Haifu.)

FIGURE 4.

Integration of Innomotion robotic arm (InnoMedic GmbH) into Biograph mMR PET/MRI scanner (Siemens Healthineers). (A) Robotic arm was modified to fit into PET/MRI system holding ultrasound imaging probe. (B) MRI-compatible FUS system Diphas (Fraunhofer IBMT) can be integrated into PET/MRI scanner. (C) Three-dimensionally printed adaptor and fixation parts were manufactured to hold ultrasound transducer. (D) Attenuation map of robotic arm visualized by 2 perpendicular maximum-intensity projections of PET annihilation photons (in 1/cm) with linear attenuation coefficient of 511 keV, obtained by stand-alone PET scanner equipped with ⁶⁸Ge-/⁶⁸Ga rod sources.