Chronic back pain is often a diagnostic and management challenge due to multiple potential pain sources with both biomechanical and inflammatory mechanisms. Both the facet joints (FJ) and sacroiliac (SI) joints account for a large fraction of back pain. Interventional pain procedures such as RF ablation provide target specificity but lack long-term efficacy and are associated with procedural risks. MR-guided focused ultrasound is a completely non-invasive modality approved for various applications such as uterine fibroids, and painful bone metastases, but these systems designed for body applications lack the image quality necessary to precisely target small structures in the spine.
Results from swine MRgFUS ablation. (A) Coronal oblique 3D segmented EPI image with real-time MR temperature monitoring shows focal ablation of medial branch (MB) nerve at L2 level (yellow arrow). (B) Temperature rise as a function of time with a peak temperature rise of ~33 °C above body temperature. (C) Swine positioned in oblique supine position above the FUS transducer. White volume overlay on (C) T2w MRI and (D) T2 map (black arrow) shows lethal thermal dose volume along MB nerve course.
Histology results showed successful ablation in the targeted area. (a) Sagittal cut through center of spine showing vertebral levels. (b) Axial cut through S1 showing the left SI joint (white arrow), spinal canal (dotted arrow), and ablated area (black arrow). (c) Axial cut through S1/S2 after fixation in formalin. (d) Margin of ablated area is outlined as blue dotted line. The ablated area near the joint shows fibrosis and hypertrophic cartilage with a sharp transition between normal (N) and ablated zone (AZ) in muscle.
MRgFUS allows for non-invasive and precise ablation of targets in the spine and has been used as an effective and safe thermal ablation technique for outpatient management of facet and SI joint related low back pain outside the United States. In addition to ablative procedures, FUS has the potential to treat pain by neuromodulation of the spinal pain generators without surrounding tissue damage.
FUS performed under non-ionizing MR-guidance offers many benefits including better treatment visualization, excellent soft tissue characterization, three-dimensional imaging, and real-time treatment feedback and monitoring. These factors allow for accurate and precise targeting while avoiding injury to adjacent structures in an anatomic landscape of closely apposed critical structures.
We are currently designing prototypes of spine-specific FUS systems for the cervical and lumbar spine. This work is funded by the NIH HEAL Initiative and BACPAC.
Related Publications
Krug R, Do L, Rieke V, Wilson MW, Saeed M. Evaluation of MRI protocols for the assessment of lumbar facet joints after MR-guided focused ultrasound treatment. J Ther Ultrasound. 2016;4:14. doi: 10.1186/s40349-016-0057-8. eCollection 2016. PubMed PMID: 27054038; PubMed Central PMCID: PMC4822243.