J Korean Neurosurg Soc.  2019 Nov;62(6):712-722. 10.3340/jkns.2018.0226.

Factors Related to Successful Energy Transmission of Focused Ultrasound through a Skull: A Study in Human Cadavers and Its Comparison with Clinical Experiences

Affiliations
  • 1Department of Neurosurgery, Brain Research Institute, Yonsei University College of Medicine, Seoul, Korea. JCHANG@yuhs.ac
  • 2InSightec Ltd., Tirat Carmel, Israel.

Abstract


OBJECTIVE
Although magnetic resonance guided focused ultrasound (MRgFUS) has been used as minimally invasive and effective neurosurgical treatment, it exhibits some limitations, mainly related to acoustic properties of the skull barrier. This study was undertaken to identify skull characteristics that contribute to optimal ultrasonic energy transmission for MRgFUS procedures.
METHODS
For ex vivo skull experiments, various acoustic fields were measured under different conditions, using five non-embalmed cadaver skulls. For clinical skull analyses, brain computed tomography data of 46 patients who underwent MRgFUS ablations (18 unilateral thalamotomy, nine unilateral pallidotomy, and 19 bilateral capsulotomy) were retrospectively reviewed. Patients' skull factors and sonication parameters were comparatively analyzed with respect to the cadaveric skulls.
RESULTS
Skull experiments identified three important factors related skull penetration of ultrasound, including skull density ratio (SDR), skull volume, and incidence angle of the acoustic rays against the skull surface. In clinical results, SDR and skull volume correlated with maximal temperature (Tmax) and energy requirement to achieve Tmax (p<0.05). In addition, considering the incidence angle determined by brain target location, less energy was required to reach Tmax in the central, rather than lateral targets particularly when compared between thalamotomy and capsulotomy (p<0.05).
CONCLUSION
This study reconfirmed previously identified skull factors, including SDR and skull volume, for successful MRgFUS; it identified an additional factor, incidence angle of acoustic rays against the skull surface. To guarantee successful transcranial MRgFUS treatment without suffering these various skull issues, further technical improvements are required.

Keyword

High-intensity focused ultrasound ablation; Skull; Acoustics

MeSH Terms

Acoustics
Brain
Cadaver*
High-Intensity Focused Ultrasound Ablation
Humans*
Incidence
Pallidotomy
Retrospective Studies
Skull*
Sonication
Ultrasonics
Ultrasonography*

Figure

  • Fig. 1. Skull mounting to the placement frame. Each hole indicates a specific location inside the skull. D4, for example, is the center and A3 is a target, 3 cm right and 1 cm anterior from the center (A). Therefore, D4 indicates that the skull is located in the center of the transducer, while A3 indicates that it is in a more lateral location (B).

  • Fig. 2. Reference drawing showing how to measure the SDR on brain computed tomography (A) and how to average SDR on all elements of Exablate 4000 (InSightec, Tirat Carmel, Israel) (B). CT : computed tomography, SDR : skull density ratio.

  • Fig. 3. Positive correlation between SDR and relative intensity of ultrasonic energy through the skull. These correlations were similarly identified in conditions with either mid frequency (A) or low frequency (B). In the clinical results, there was a positive correlation between SDR and temperature rise per unit energy, revealing greater transmitted energy (C). SDR : skull density ratio, dT/dMaxE : temperature rise per unit energy.

  • Fig. 4. Linear regression of skull thickness, skull volume, and Tmax (A and B). Skull volume is also correlated with maximal energy delivery to reach maximal temperature (C). Tmax : maximal temperature, Edelivery/Tmax : energy delivered to reach maximal temperature.

  • Fig. 5. Two-dimensional scan showing appearance of acoustic rays when focusing on a specific target location. A center location (D4) revealed more centralized acoustic rays than a laterally located target with a high incidence angle. However, acoustic beam fields demonstrated a similarly centralized shape during optimal correction of acoustic rays by hydrophone.

  • Fig. 6. Cumulative number of sonication elements according to incidence angle. When setting the incidence angle <25 as the standard cut-off value, a laterally positioned target acquires relatively fewer elements that could be usable for energy transmission. Similar patterns are confirmed in the skull experiments (A) and clinical data (B). ET : essential tremor, OCD : obsessive compulsive disorder, PD : Parkinson's disease, MDD : major depressive disorder.

  • Fig. 7. Mean value of transmission amplitude according to the incidence angle of acoustic rays. When dividing the three groups by SDR value, energy transmission through the skull generally exhibited less effective as the incidence angle become larger (A-C). Interestingly, cases with SDR ≥0.6 showed slightly improved amplitude transmission at incidence angles >25 degrees (C), indicating that high SDRs are less affected by the influence of the incidence angle. Moreover, low frequency ultrasound demonstrated much better transmission than mid frequency in all SDR conditions. SDR : skull density ratio.


Reference

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