Evaluation of thermometric monitoring for intradiscal laser ablation in an open 1.0 T MR scanner

MR-guided lumbar facet radiofrequency denervation for treatment of patients with chronic low back pain in an open 1.0 Tesla MRI system

Background

To evaluate the feasibility, safety and efficacy of magnetic resonance imaging (MRI)-guided lumbar facet joint radiofrequency denervation (FRD) in patients with chronic low back pain.

Methods

The study consisted of two parts. First, a preclinical analysis using an ex vivo animal model was performed to define optimal technical parameters for ablation. Then, 17 patients with chronic lumbar facet joint pain syndrome were prospectively included and underwent MRI-guided FRD in an open 1.0-Tesla MRI. We analyzed technical feasibility and complications as well as clinical outcome in terms of subjective pain assessed on a numerical visual analogue scale (VAS) before and after 1 week/6 months after FRD. Clinical assessment was complemented by measurement of paravertebral muscle volume and fat content before the intervention and at 6-month follow-up.

Results

All interventions were technically successful without major complications. Initial VAS scores (median: 8, IQR: 1, range: 6-9, CI: 7.14-8.04) decreased significantly both after one week (median: 4, IQR: 5, range: 0-7, CI: 1.9-4.69, P=0.003) and after 6 months (median: 1, IQR: 6, range: 0-7, CI: 1.06-4.23, P<0.001). Mean multifidus muscle volume increased significantly in the patient population (from 366.8±130.8 cm³ before to 435.4±146.7 cm³ after FRD, P=0.031).

Conclusions

This proof of principle study shows MRI-guided FRD in an open 1.0-Tesla MRI system to be a potential therapy option for patients with chronic low back pain.

Improved MR thermometry for laser interstitial thermotherapy

OBJECTIVES

To develop, test and evaluate improved 2D and 3D protocols for proton resonance frequency shift magnetic resonance temperature imaging (MRTI) of laser interstitial thermal therapy (LITT). The objective was to develop improved MRTI protocols in terms of temperature measurement precision and volume coverage compared to the 2D MRTI protocol currently used with a commercially available LITT system.

METHODS

Four different 2D protocols and four different 3D protocols were investigated. The 2D protocols used multi-echo readouts to prolong the total MR sampling time and hence the MRTI precision, without prolonging the total acquisition time. The 3D protocols provided volumetric thermometry by acquiring a slab of 12 contiguous slices in the same acquisition time as the 2D protocols. The study only considered readily available pulse sequences (Cartesian 2D and 3D gradient recalled echo and echo planar imaging [EPI]) and methods (partial Fourier and parallel imaging) to ensure wide availability and rapid clinical implementation across vendors and field strengths. In vivo volunteer studies were performed to investigate and compare MRTI precision and image quality. Phantom experiments with LITT heating were performed to investigate and compare MRTI precision and accuracy. Different coil setups were used in the in vivo studies to assess precision differences between using local (such as flex and head coils) and non-local (i.e., body coil) receive coils. Studies were performed at both 1.5 T and 3 T.

RESULTS

The improved 2D protocols provide up to a factor of two improvement in the MRTI precision in the same acquisition time, compared to the currently used clinical protocol. The 3D echo planar imaging protocols provide comparable precision as the currently used 2D clinical protocol, but over a substantially larger field of view, without increasing the acquisition time. As expected, local receive coils perform substantially better than the body coil, and 3 T provides better MRTI accuracy and precision than 1.5 T. 3D data can be zero-filled interpolated in all three dimensions (as opposed to just two dimensions for 2D data), reducing partial volume effects and measuring higher maximum temperature rises.

CONCLUSIONS

With the presented protocols substantially improved MRTI precision (for 2D imaging) or greatly improved field of view coverage (for 3D imaging) can be achieved in the same acquisition time as the currently used protocol. Only widely available pulse sequences and acquisition methods were investigated, which should ensure quick translation to the clinic. Lasers Surg. Med. 51:286-300, 2019. © 2019 Wiley Periodicals, Inc.

Tailored interactive sequences for continuous MR-image-guided freehand biopsies of different organs in an open system at 1.0 tesla (T) - Initial experience

OBJECTIVES

To assess the feasibility, image quality, and accuracy of freehand biopsies of liver, bone, muscle, vertebral disc, soft tissue, and other lesions using balanced steady-state free precession (SSFP, balanced fast field echo: bFFE), spoiled and nonspoiled gradient echo (FFE), and turbo spin echo (TSE) sequences for interactive continuous navigation in an open magnetic resonance imaging (MRI) system at 1.0 tesla (T).

METHODS

Twenty-six MR-guided biopsies (five liver, five bone, four muscle, four vertebral disc, one lung, one kidney, one suprarenal gland, and five soft or other tissue) were performed in 23 patients in a 1.0-T open magnetic resonance (MR) scanner (Panorama HFO, Philips Healthcare, Best, the Netherlands). A total of 42 samples were obtained. Depending on lesion size and location, 14-18-gauge MR-compatible biopsy sets with a length of 100 or 200 mm (Somatex Medical, Teltow, Germany), 14-18-gauge MR-compatible semiautomatic biopsy guns with a length of 100 or 150 mm (Invivo, Schwerin, Germany), or 11-gauge MR-compatible bone marrow biopsy needles with a length of 100 mm (Somatex Medical, Teltow, Germany) were employed.

RESULTS

All lesions were visible with continuous interactive imaging. Our initial results indicate that bFFE is particularly suitable for fast-moving organs (pulmonary, paracardial); moving organs are targeted better with T1-weighted (T1W) TSE, T1W FFE (liver) or T2-weighted (T2W) TSE (complicated cysts, adrenal glands), and static organs are successfully approached with proton density (PD) (spine) or T1W TSE (peripheral bones, musculoskeletal system). No adverse events related to the use of MRI were obtained. No complications occurred according to the Society of Interventional Radiology (SIR) clinical practice guidelines.

CONCLUSION

Applying tailored interactive dynamic imaging sequences for continuous navigation to liver, bone, muscle, vertebral disc, soft tissue, and other lesions can improve the feasibility, image quality, and interventional accuracy of freehand MR-guided biopsies and may hence reduce the risk of complications.

MR guidance and thermometry of percutaneous laser disc decompression in open MRI: an ex vivo study

PURPOSE

To assess the feasibility of guidance and thermometry by open 1.0 T magnetic resonance (MR) imaging during percutaneous laser disc decompression (PLDD).

METHODS

A fluoroscopic proton-density-weighted turbo spin echo sequence was used for positioning a laser fiber and a reference thermosensor within the targeted spinal disc. In 30 lumbar discs from human donors, nonspoiled gradient-echo (GRE) sequences with different echo times (TE) were compared to monitor thermal laser effects (Nd:YAG laser, 1,064 nm). Temperature distribution was visualized in real time on the basis of T1-weighted images and the proton resonance frequency (PRF) technique. Image quality, temperature accuracy, and correlation with macroscopic lesion sizes were analyzed. Image quality was confirmed in healthy volunteers.

RESULTS

MR-guided placement of the laser fiber in the center of the targeted disk was precise. Best overall PLDD results-considering image quality (contrast-to-noise ratio), temperature accuracy (R (2) = 0.96), and correlation between the macroscopic and MR lesions (R (2) = 0.63)-were achieved with TE at 7 ms. The same TE value also gave the best image quality with healthy volunteers.

CONCLUSION

Instrument guidance and PRF-based thermometry of PLDD in the lumbar spine are feasible and accurate. Open 1.0 T MR imaging with fast spin-echo and GRE sequence designs may render laser discectomies more effective and controllable.

Mathematical models of laser-induced tissue thermal damage

Laser sources are under increasing study for in vivo tumour ablation. Photo-thermal ablation in tissues varies tremendously in governing physical phenomena, depending on wavelength, owing to wide variation in the optical properties of tissues, specifically the dominant chromophore and degree and type of scattering. Once converted into local tissue heating, however, the governing thermodynamic principles remain the same. Observed irreversible thermal alterations range from substantial structural disruption due to steam evolution in high temperature short-term activations to low temperature rise, longer-term initiation of the complex protein cascades that result in apoptosis and/or necroptosis. The usual mathematical model in hyperthermia studies, the thermal isoeffect dose, arising from the relative reaction rate formulation, is not an effective description of the higher temperature effects because multiple processes occur in parallel. The Arrhenius formulation based on the theory of absolute reaction rates is much more useful and descriptive in laser heating since the multiple thermodynamically independent processes may be studied separately.

Intradiscal temperature monitoring using double gradient-echo pulse sequences at 1.0T

PURPOSE

To validate an unspoiled gradient-recalled echo pulse sequence with dual echo acquisition as a means to increase temperature sensitivity while monitoring intradiscal laser ablation therapy.

MATERIALS AND METHODS

Phantom experiments as well as in vitro thermal ablation simulations were performed in an open 1.0T magnetic resonance (MR) scanner. Three methods of noninvasive MR-thermometry based on the signal void decrease caused by T1-relaxation time increase (T1), the temperature-dependent proton resonance frequency (PRF) shift, and a combination of both methods with complex differences (CD) were compared. Temperature accuracy and reliability of temperature distribution were the main assessment criteria.

RESULTS

The optimum temperature sensitivity was found using CD in phantom experiments. During in vitro experiments the PRF showed the smallest margin of error (T1: +/-1.64 degrees C, PRF: +/-1.23 degrees C, CD: +/-1.29 degrees C) and the best qualitative evaluation of temperature.

CONCLUSION

Intradiscal temperature monitoring with an unspoiled dual-echo sequence is most accurate with PRF-thermometry in combination with the long echo time. Magnitude images with an initial short echo time permit high image detail of the heat-induced lesion.