An experimental model to investigate the targeting accuracy of MR-guided focused ultrasound ablation in liver

Abstract

BackgroundMagnetic Resonance-guided High Intensity Focused Ultrasound (MRgHIFU) is a hybrid technology that aims to offer non-invasive thermal ablation of targeted tumors or other pathological tissues. Acoustic aberrations and non-linear wave propagating effects may shift the focal point significantly away from the prescribed (or, theoretical) position. It is therefore mandatory to evaluate the spatial accuracy of ablation for a given HIFU protocol and/or device. We describe here a method for producing a user-defined ballistic target as an absolute reference marker for MRgHIFU ablations.MethodsThe investigated method is based on trapping a mixture of MR contrast agent and histology stain using radiofrequency (RF) ablation causing cell death and coagulation. A dedicated RF-electrode was used for the marker fixation as follows: a RF coagulation (4 W, 15 seconds) and injection of the mixture followed by a second RF coagulation. As a result, the contrast agent/stain is encapsulated in the intercellular space. Ultrasonography imaging was performed during the procedure, while high resolution T1w 3D VIBE MR acquisition was used right after to identify the position of the ballistic marker and hence the target tissue. For some cases, after the marker fixation procedure, HIFU volumetric ablations were produced by a phased-array HIFU platform. First ex vivo experiments were followed by in vivo investigation on four rabbits in thigh muscle and six pigs in liver, with follow-up at Day 7.ResultsAt the end of the procedure, no ultrasound indication of the marker’s presence could be observed, while it was clearly visible under MR and could be conveniently used to prescribe the HIFU ablation, centered on the so-created target. The marker was identified at Day 7 after treatment, immediately after animal sacrifice, after 3 weeks of post-mortem formalin fixation and during histology analysis. Its size ranged between 2.5 and 4 mm.ConclusionsExperimental validation of this new ballistic marker method was performed for liver MRgHIFU ablation, free of any side effects (e.g. no edema around the marker, no infection, no bleeding). The study suggests that the absolute reference marker had ultrasound conspicuity below the detection threshold, was irreversible, MR-compatible and MR-detectable, while also being a well-established histology staining technique.

Muscle Dysmorphic Disorder Inventory (MDDI): Validation of a German version with a focus on gender

“…The samples mainly consisted of male bodybuilders, weightlifters or college students. Prevalence rates of MD ranged from over 50% in studies assessing highly selective groups like competitive bodybuilders to about 6–7% in primarily male college students [ 18 – 20 ]. However, it is important to note that most studies used screening instruments (self-report) and not a clinical interview.…”

Prediction of HIFU Propagation in a Dispersive Medium via Khokhlov–Zabolotskaya–Kuznetsov Model Combined with a Fractional Order Derivative

“…Usually, the shift was measured to be around 1-3 mm for 1-MHz HIFU transducers [24], 4-5 mm for~2.2-MHz excitation [51], and an empirical formula was also used to calculate the focus shift [22]. Although different possible mechanisms have been proposed and a series of correlation studies have been carried out [21][22][23][24][25][26][27], detailed theoretical proof that could quantitatively explain the inherent mechanisms of the observed focal shifts is still lacking. The proposed model modification here clarifies how the acoustic dispersion played a role in the complicated physics of this problem.…”

“…Several techniques such as ultrasound B-Scan and Magnetic Resonance Imaging (MRI) have been combined with HIFU to achieve real-time monitoring of focal areas [17][18][19][20]. With these methods, the actual focal profiles were usually found to deviate from those predicted through theoretical models [21,22]. As was indicated by Petrusca et al, the shift of the focal point away from the prescribed position caused by acoustic aberrations and non-linear wave propagating effects make it mandatory to evaluate the spatial accuracy of HIFU ablation [21].…”

“…With these methods, the actual focal profiles were usually found to deviate from those predicted through theoretical models [21,22]. As was indicated by Petrusca et al, the shift of the focal point away from the prescribed position caused by acoustic aberrations and non-linear wave propagating effects make it mandatory to evaluate the spatial accuracy of HIFU ablation [21]. Li et al observed a focal shift of 1-2 mm and ascribed it to the layered distribution of tissues.…”

“…Li et al observed a focal shift of 1-2 mm and ascribed it to the layered distribution of tissues. Several different mechanisms might contribute to these deviations, such as the thermos-lensing effect [23,24], bubble formation [25][26][27], acoustic radiation force [27] and the nonlinear nature of acoustic waves [21,22]. Connor et al pointed out that the positioning error of the focal spots could be mainly related to the thermos-lensing effect and nonlinear propagation of ultrasonic waves [23].…”