MR-Guided Percutaneous Radiofrequency Interstitial Thermal Ablation (RF-ITA) of the Tongue Base: An Alternative Minimally Invasive Approach to Obstructive Sleep Apnea (OSA) Treatment

Purpose:

To (1) test the feasibility and safety of a percutaneous MR guided technique for RF-ITA of the tongue base, and to (2) correlate MR imaging appearance of induced thermal lesions to the histopathological findings in acute and chronic animal models.

Materials and Methods:

On a 0.2T open MR system (Magnetom open, Siemens, Germany), and under IV anesthesia, a 1cm tip, 17G RF electrode (Radionics, MA) was inserted percutaneously into the tongues of 10 pigs under real time MR guidance using a FLASH sequence. The electrode was advanced aiming at a midline plane between the genioglossus/geniohyoid muscle complexes avoiding lingual arteries and was stopped short of tongue mucosa. RFA then followed for 10 minutes at 90°C using a 200W generator (Radionics, MA). Post ablation scans were performed on a 1.5T scanner (Sonata, Siemens, Germany) and consisted of T2, STIR, CE T1, and CISS. 5 animals were immediately sacrificed (acute model) while the remaining 5 were followed up for one month (chronic model) before sacrifice. MR-compatible fiducial coils were inserted into the tongue musculature under MR guidance prior to survival ablations. Tongues were harvested for histopathology.

Results:

Acute Model

Successful MR guided electrode positioning was achieved in all procedures without intra- or immediate post-procedure complications. The high vascular conspicuity and tissue contrast allowed safe electrode navigation without vascular or mucosal injury. Thermal lesions displayed well-defined dark signal on all sequences with bright surrounding rims on T2, STIR, and CE T1 scans. At gross pathology, an acute lesion appeared as a pale necrotic area surrounded by thin dark hyperemic rim.

Chronic Model

Successful use of MRI to guide electrode insertion was again achieved in all experiments. The procedure was well tolerated by all animals. Thermal lesions displayed the same signal characteristics observed in acute models, yet with smaller central dark zones on day 30. Mean lesion volume decreased from 3.0 ± 0.4 ml on day 0 to 0.4 ± 0.4 ml on day 30. No evidence of infection or other delayed complications was noticed. Pathology of chronic lesions demonstrated peripheral invasion by fibrous tissue.

Conclusion:

We introduce a novel technique of percutaneous RF-ITA of the tongue base under direct MR fluoroscopic guidance as an alternative to the currently practiced blind transoral approach used for treatment of OSA. Our investigation demonstrates the feasibility and safety of the procedure and illustrates the imaging / pathologic phenomena associated with the creation and evolution of thermal lesions in the tongue base.

Figure 1: (A) Diagramatic representation of the new percutaneous approach to tongue base ablation showing the RF-electrode (13) inserted percutaneously through the chin and advanced cranially in a strict midline trajectory to reach the tongue base without puncturing the tongue mucosa and without risking any of the vital structures at the floor of the mouth.

(1) mucous membrane of tongue; (2) tongue muscules; (3) genioglossus muscle; (4) geniohyoid muscle; (5) hyoglossus muscle; (6) sublingual gland; (7) lingual vessels& nerve; (8) Mylohyoid muscle; (9) Body of mandible; (10) Submandibular gland; (11) Platysma muscle; (12) Skin.

(B) Selected coronal scan from a series of FISP images (TR/TE/FA/NSA: 17.8/8.1/90º/3, temporal resolution: 22 sec/3 frames) acquired during MR fluoroscopic guidance of the RF electrode into the base of the tongue. MR guidance enhances procedure safety by offering high vascular conspicuity thereby allowing excellent identification of the lingual arteries (straight arrows) and by providing high enough soft tissue contrast to distinguish the surface mucosa (arrowheads) from the intrinsic muscles of the tongue, so that the RF electrode (curved arrow) can be stopped short of the mucosa and the induced thermal lesion can be planned to lay entirely with the muscular portion of the tongue base.

Figure 2: Chronic thermal lesion – Follow-up MRI appearance and histopathological correlation.

(A-C) Sagittal TSE STIR images (TR/TE/ETL/NSA: 5300/ 35/ 7/ 1) acquired on a high field MR scanner (1.5 T) immediately (A), 2 weeks (B), and one month (C) after percutaneous RF ablation of the tongue base. Note the rapid rate of thermal lesion shrinkage from 3.7 ml on the immediate post-ablation scan (A) to 1 ml at 2weeks (B) and ending up as a thin band of enhancing scar tissue on the one-month scan (arrows, A-C).

(D-F) Corresponding gross pathological (D), H&E stained (X ) (E) and Trichrome stained (X ) (F) histological specimens demonstrating the total replacement of the area of necrosis by a grayish dense fibrous tissue (arrowheads, D) that lacks inflammatory cells, denoting a healed scar (arrowheads, E) and stains blue on the Trichrome stain (arrowheads, F). Note the inward traction of the yet intact surface mucosa by the contracting scar tissue (curved arrows, D-F). This effect illustrates the basic theory behind using thermal energy to induce tongue base volume shrinkage as a treatment of OSA syndrome.