Magnetic Resonance Imaging Characteristics of Six Radiofrequency Electrodes in a Phantom Study

Artefact and ablation performance of an MR-conditional high-power microwave system in bovine livers: an ex vivo study

Background

We evaluated a magnetic resonance (MR)-conditional high-power microwave ablation system.

Methods

An exvivo 1.5-T evaluation was conducted by varying the sequence (T1-weighted volume interpolated breath-hold examination, T1w-VIBE; T1-weighted fast low-angle shot, T1w-FLASH; T2-weighted turbo spin-echo, T2w-TSE), applicator angulation to B₀ (A-to-B₀), slice orientation, and encoding direction. Tip location error (TLE) and artefact diameters were measured, and influence of imaging parameters was assessed with analysis of variance and post hoc testing. Twenty-four exvivo ablations were conducted in three bovine livers at 80 W and 120 W. Ablation durations were 5, 10, and 15 min. Ablation zones were compared for short-axis diameter (SAD), volume, and sphericity index (SI) with unpaired t test.

Results

The artefact pattern was similar for all sequences. The shaft artefact (4.4 ± 2.9 mm, mean ± standard deviation) was dependent on the sequence (p = 0.012) and the A-to-B₀ (p < 0.001); the largest shaft diameter was measured with T1w-FLASH (6.3 ± 3.4 mm) and with perpendicular A-to-B₀ (6.7 ± 2.4 mm). The tip artefact (1.6 ± 0.7 mm) was dependent on A-to-B₀ (p = 0.001); TLE was -2.6 ± 1.0 mm. Ablation results at the maximum setting (15 min, 120 W) were SAD = 42.0 ± 1.41 mm; volume = 56.78 ± 3.08 cm³, SI = 0.68 ± 0.05. In all ablations, SI ranged 0.68–0.75 with the smallest SI at 15 min and 120 W (p = 0.048).

Conclusion

The system produced sufficiently large ablation zones and the artefact was appropriate for MR-guided interventions.

In vitro artifact assessment of an MR-compatible, microwave antenna device for percutaneous tumor ablation with fluoroscopic MRI-sequences

OBJECTIVE

To evaluate artifact configuration and diameters of a magnetic resonance (MR) compatible microwave (MW) applicator using near-realtime MR-fluoroscopic sequences for percutaneous tumor ablation procedures.

MATERIAL AND METHODS

Two MW applicators (14 G and 16 G) were tested in an ex-vivo phantom at 1.5 T with two 3 D fluoroscopic sequences: T1-weighted spoiled Gradient Echo (GRE) and T1/T2-weighted Steady State Free Precession (SSFP) sequence. Applicator orientation to main magnetic field (B₀), slice orientation and phase encoding direction (PED) were systematically varied. The influence of these variables was assessed with ANOVA and post-hoc testing.

RESULTS

The artifact was homogenous along the whole length of both antennas with all tested parameters. The tip artifact diameter of the 16 G antenna measured 6.9 ± 1.0 mm, the shaft artifact diameter 8.6 ± 1.2 mm and the Tip Location Error (TLE) was 1.5 ± 1.2 mm.The tip artifact diameter of the 14 G antenna measured 7.7 ± 1.2 mm, the shaft artifact diameter 9.6 ± 1.5 mm and TLE was 1.6 ± 1.2 mm. Orientation to B₀ had no statistically significant influence on tip artifact diameters (16 G: p = .55; 14 G: p = .07) or TLE (16 G: p = .93; 14 G: p = .26). GRE sequences slightly overestimated the antenna length with TLE(16 G) = 2.6 ± 0.5 mm and TLE(14 G) = 2.7 ± 0.7 mm.

CONCLUSIONS

The MR-compatible MW applicator's artifact seems adequate with an acceptable TLE for safe applicator positioning during near-realtime fluoroscopic MR-guidance.

MR-guided microwave ablation in hepatic tumours: initial results in clinical routine

OBJECTIVES

Evaluation of the technical success, patient safety and technical effectiveness of magnetic resonance (MR)-guided microwave ablation of hepatic malignancies.

METHODS

Institutional review board approval and informed patient consent were obtained. Fifteen patients (59.8 years ± 9.5) with 18 hepatic malignancies (7 hepatocellular carcinomas, 11 metastases) underwent MR-guided microwave ablation using a 1.5-T MR system. Mean tumour size was 15.4 mm ± 7.7 (7-37 mm). Technical success and ablation zone diameters were assessed by post-ablative MR imaging. Technique effectiveness was assessed after 1 month. Complications were classified according to the Common Terminology Criteria for Adverse Events (CTCAE). Mean follow-up was 5.8 months ± 2.6 (1-10 months).

RESULTS

Technical success and technique effectiveness were achieved in all lesions. Lesions were treated using 2.5 ± 1.2 applicator positions. Mean energy and ablation duration per tumour were 37.6 kJ ± 21.7 (9-87 kJ) and 24.7 min ± 11.1 (7-49 min), respectively. Coagulation zone short- and long-axis diameters were 31.5 mm ± 10.5 (16-65 mm) and 52.7 mm ± 15.4 (27-94 mm), respectively. Two CTCAE-2-complications occurred (pneumothorax, pleural effusion). Seven patients developed new tumour manifestations in the untreated liver. Local tumour progression was not observed.

CONCLUSIONS

Microwave ablation is feasible under near real-time MR guidance and provides effective treatment of hepatic malignancies in one session.

KEY POINTS

• Planning, applicator placement and therapy monitoring are possible without using contrast enhancement • Energy transmission from the generator to the scanner room is safely possible • MR-guided microwave ablation provides effective treatment of hepatic malignancies in one session • Therapy monitoring is possible without applicator retraction from the ablation site.

Technical Note: MR-visualization of interventional devices using transient field alterations and balanced steady-state free precession imaging

PURPOSE

In interventional magnetic resonance imaging, instruments can be equipped with conducting wires for visualization by current application. The potential of sequence triggered application of transient direct currents in balanced steady-state free precession (bSSFP) imaging is demonstrated.

METHODS

A conductor and a modified catheter were examined in water phantoms and in an ex vivo porcine liver. The current was switched by a trigger pulse in the bSSFP sequence in an interval between radiofrequency pulse and signal acquisition. Magnitude and phase images were recorded. Regions with transient field alterations were evaluated by a postprocessing algorithm. A phase mask was computed and overlaid with the magnitude image.

RESULTS

Transient field alterations caused continuous phase shifts, which were separated by the postprocessing algorithm from phase jumps due to persistent field alterations. The overlaid images revealed the position of the conductor. The modified catheter generated visible phase offset in all orientations toward the static magnetic field and could be unambiguously localized in the ex vivo porcine liver.

CONCLUSIONS

The application of a sequence triggered, direct current in combination with phase imaging allows conspicuous localization of interventional devices with a bSSFP sequence.

Image-based monitoring of magnetic resonance-guided thermoablative therapies for liver tumors

Minimally invasive treatment options for liver tumor therapy have been increasingly used during the last decade because their benefit has been proven for primary and inoperable secondary liver tumors. Among these, radiofrequency ablation has gained widespread consideration. Optimal image-guidance offers precise anatomical information, helps to position interventional devices, and allows for differentiation between already-treated and remaining tumor tissue. Patient safety and complete ablation of the entire tumor are the overriding objectives of tumor ablation. These may be achieved most elegantly with magnetic resonance (MR)-guided therapy, where monitoring can be performed based on precise soft-tissue imaging and additional components, such as diffusion-weighted imaging and temperature mapping. New MR scanner types and newly developed sequence techniques have enabled MR-guided intervention to move beyond the experimental phase. This article reviews the current role of MR imaging in guiding radiofrequency ablation. Signal characteristics of primary and secondary liver tumors are identified, and signal alteration during therapy is described. Diffusion-weighted imaging (DWI) and temperature mapping as special components of MR therapy monitoring are introduced. Practical information concerning coils, sequence selection, and parameters, as well as sequence gating, is given. In addition, sources of artifacts are identified and techniques to decrease them are introduced, and the characteristic signs of residual tumor in T1-, T2-, and DWI are described. We hope to enable the reader to choose MR sequences that allow optimal therapy monitoring depending on the initial signal characteristics of the tumor as well as its size and location in the liver.

MR-guided radiofrequency ablation of hepatocellular carcinoma: long-term effectiveness

PURPOSE

To evaluate long-term effectiveness of magnetic resonance (MR)-guided radiofrequency (RF) ablation of hepatocellular carcinoma (HCC).

MATERIALS AND METHODS

This prospective study was approved by the institutional review board. In 20 patients, 28 HCCs (mean diameter, 28.0 mm; range, 6-58 mm) were treated with 25 sessions of MR-guided RF ablation. Previous chemoembolization had been performed in nine HCCs with diameters greater than 3 cm. The entire RF ablation procedures were carried out on a 0.2-T open MR system. Placement of MR-compatible internally cooled electrodes was performed under MR fluoroscopic imaging with fast gradient-echo sequences. Therapeutic assessment was based on dynamic MR-imaging (1.5 T) at a mean follow-up of 24.2 months (range, 6-52 mo).

RESULTS

MR-guided RF ablation was technically successful in all 25 sessions (100%), as assessed at the end of each session. T2-weighted sequences were accurate to monitor the ablation zone and supported guidance of overlapping ablations if necessary. Technique effectiveness, defined as complete ablation confirmed at MR imaging 4 months after RF ablation, was achieved in 27 of 28 HCCs (96.4%). To achieve complete ablation, 25 of 27 tumors (92.6%) were treated in a single session and two tumors were treated twice. In one tumor initially defined as having been treated with technically effective RF ablation, local tumor progression was detected more than 4 months after ablation. Consequently, the available follow-up indicated complete ablation in 26 of 28 HCCs (92.9%). There was one major complication (4.0%) and one minor complication (4.0%).

CONCLUSIONS

On a long-term basis, MR-guided RF ablation is an effective therapy option in the treatment of HCC.

Tumour ablation: technical aspects

Image-guided percutaneous radiofrequency ablation (RFA) is a minimally invasive, relatively low-risk procedure for tumour treatment. Local recurrence and survival rates depend on the rate of complete ablation of the entire tumour including a sufficient margin of surrounding healthy tissue. Currently a variety of different RFA devices are available. The interventionalist must be able to predict the configuration and extent of the resulting ablation necrosis. Accurate planning and execution of RFA according to the size and geometry of the tumour is essential. In order to minimize complications, individualized treatment strategies may be necessary for tumours close to vital structures. This review examines the state-of-the art of different device technologies, approaches, and treatment strategies for percutaneous RFA of liver tumours.

Feasibility of real-time MRI with a novel carbon catheter for interventional electrophysiology

BACKGROUND

Cardiac MRI offers 3D real-time imaging with unsurpassed soft tissue contrast without x-ray exposure. To minimize safety concerns and imaging artifacts in MR-guided interventional electrophysiology (EP), we aimed at developing a setup including catheters for ablation therapy based on carbon technology.

METHODS AND RESULTS

The setup, including a steerable carbon catheter, was tested for safety, image distortion, and feasibility of diagnostic EP studies and radiofrequency ablation at 1.5 T. MRI was performed in 3 different 1.5-T whole-body scanners using various receive coils and pulse sequences. To assess unintentional heating of the catheters by radiofrequency pulses of the MR scanner in vitro, a fluoroptic thermometry system was used to record heating at the catheter tip. Programmed stimulation and ablation therapy was performed in 8 pigs. There was no significant heating of the carbon catheters while using short, repetitive radiofrequency pulses from the MR system. Because there was no image distortion when using the carbon catheters, exact targeting of the lesion sites was possible. Both atrial and ventricular radiofrequency ablation procedures including atrioventricular node modulation were performed successfully in the scanner. Potential complications such as pericardial effusion after intentional perforation of the right ventricular free wall during ablation could be monitored in real time as well.

CONCLUSIONS

We describe a newly developed EP technology for interventional electrophysiology based on carbon catheters. The feasibility of this approach was demonstrated by safety testing and performing EP studies and ablation therapy with carbon catheters in the MRI environment.

Validation of fast MR thermometry at 1.5 T with gradient-echo echo planar imaging sequences: phantom and clinical feasibility studies

The purpose of this work was to validate in phantom studies and demonstrate the clinical feasibility of MR proton resonance frequency thermometry at 1.5 T with segmented gradient-echo echo planar imaging (GRE-EPI) sequences during liver tumour radiofrequency (RF) ablation. Classical GRE acquisitions and segmented GRE-EPI acquisitions were performed at 1.5 T during simultaneous RF heating with an MR-compatible RF electrode placed in an agar gel phantom. Temperature increments were calculated and compared with four optical temperature probe measurements using Bland- Altman analysis. In a preliminary clinical feasibility study, the rapid GRE-EPI sequence (echo train length = 13) was used for MR temperature monitoring of RF ablation of liver tumours in three patient procedures. For phantom experiments, the Bland-Altman mean of differences between MR and optical probe temperature measurements was <0.4 degrees C, and the 95% limits of agreement value was <1.4 degrees C. For the in vivo studies, respiratory-triggered GRE-EPI acquisitions yielded a temperature accuracy of 1.3 +/- 0.4 degrees C (acquisition time = 0.6 s/image, spatial coverage of three slices/respiratory cycle). MR proton resonance frequency thermometry at 1.5 T yields precise and accurate measurements of temperature increment with both classical GRE and rapid GRE-EPI sequences. Rapid GRE-EPI sequences minimize intra-scan motion effects and can be used for MR thermometry during RF ablation in moving organs.

Magnetic resonance guidance for radiofrequency ablation of liver tumors

Image-guided thermal ablation of liver tumors is a minimally invasive treatment option. Techniques used for thermal ablation are radiofrequency (RF) ablation, laser interstitial thermotherapy (LITT), microwave (MW) ablation, high-intensity focused ultrasound (HIFU), and cryoablation. Among these techniques RF ablation attained widespread consideration. Image guidance should ensure a precise ablation therapy leading to a complete coagulation of tumor tissue without injury to critical structures. Therefore, the modality of image guidance has an important impact on the safety and efficacy of percutaneous RF ablation. The current literature regarding percutaneous RF ablation mainly describes the use of computed tomography (CT) and ultrasonography (US) guidance. In addition, interventional MR systems offer the possibility to utilize the advantages of MR imaging such as excellent soft-tissue contrast, multiplanar and interactive capabilities, and sensitivity to thermal effects during the entire RF ablation procedure. Monitoring of thermally induced coagulation by MR imaging is supportive to control the ablation procedure. MR imaging can be advantageously used to guide overlapping ablation if necessary as well as to define the endpoint of RF ablation after complete coverage of the target tissue is verified. Furthermore, monitoring of thermal effects is essential in order to prevent unintended thermal damage from critical structures surrounding the target region. Therefore, MR-guided RF ablation offers the possibility for a safe and effective therapy option in the treatment of primary and secondary hepatic malignancies. The article summarizes the role of MR guidance for RF ablation of liver tumors.

New horizons in MR-controlled and monitored radiofrequency ablation of liver tumours

There is a sustained interest in using magnetic resonance (MR) thermometry to monitor the radiofrequency ablation of liver tumours as a means of visualizing the progress of the thermal coagulation and deciding the optimal end-point. Despite numerous technical challenges, important progress has been made and demonstrated in animal studies. In addition to MR thermometry, MR can now be used for the guidance of the tumour targeting with ‘fluoroscopic’ rapid image acquisition, and it can provide several contrast mechanisms for post-procedural assessment of the extent of the thermal coagulation zone. Challenges of in vivo simultaneous MR thermometry implementation and the current limitations of the thermal dose model for the estimation of the extent of the thermal coagulation zone are discussed. MR imaging could enhance the success of RF ablation of liver tumours due to its potential to provide accurate targeting, monitoring, and post-procedural evaluation.

MR thermometry for monitoring tumor ablation

Local thermal therapies are increasingly used in the clinic for tissue ablation. During energy deposition, the actual tissue temperature is difficult to estimate since physiological processes may modify local heat conduction and energy absorption. Blood flow may increase during temperature increase and thus change heat conduction. In order to improve the therapeutic efficiency and the safety of the intervention, mapping of temperature and thermal dose appear to offer the best strategy to optimize such interventions and to provide therapy endpoints. MRI can be used to monitor local temperature changes during thermal therapies. On-line availability of dynamic temperature mapping allows prediction of tissue death during the intervention based on semi-empirical thermal dose calculations. Much progress has been made recently in MR thermometry research, and some applications are appearing in the clinic. In this paper, the principles of MRI temperature mapping are described with special emphasis on methods employing the temperature dependency of the water proton resonance frequency. Then, the prospects and requirements for widespread applications of MR thermometry in the clinic are evaluated.

Development of a cone-shape phantom for multi-voxel MR spectroscopy

The purpose of this study was to develop a cone-shape phantom for multi-voxel magnetic resonance spectroscopy (MRS) and to evaluate MR spectra using the cone-shape phantom we developed in this study. A cone-shape MRS phantom was developed with a combination of cone-shape vials. The cylindrical main body was made of acrylic resin and the cone-shape vials were fabricated from poly-ethylene cones. Each cone of the phantom was filled with various metabolite materials. 1.5T GE and 3T Philips systems were used for the single voxel spectroscopy (SVS) as well as for the multi-voxel spectroscopy (MVS). Identification and quantification of the metabolite materials in the cone-shape phantom were done by the SAGE post-program. The MR images and spectra of the cone-shape phantom were obtained from the assigned slice position. The high order shimming control provided enhanced resolution in the SVS and MVS. The area and amplitude were proportional to the metabolite volume in the voxel. The present study demonstrated that the cone-shape phantom was useful for the metabolite quantification. Thus, we propose that the cone-shape phantom can be used for the evaluation of quality control of the MR spectra obtained from SVS and MVS.

MR-guided radiofrequency ablation in a 0.2-T open MR system: Technical success and technique effectiveness in 100 liver tumors

PURPOSE

To evaluate the feasibility and technique effectiveness of magnetic resonance (MR)-guided radiofrequency (RF) ablation of hepatic malignancies.

MATERIALS AND METHODS

In 64 patients, 100 primary (N = 19) or secondary (N = 81) liver tumors (mean diameter = 24.7 mm; range = 4-60 mm) were treated with 87 sessions of MR-guided RF ablation. The entire ablation procedure was carried out at an 0.2-T open MR system by using MR-compatible internally cooled electrodes. T2-weighted turbo spin echo sequences (TR/TE = 3500 msec/110 msec) were used to monitor thermally induced coagulation. Technique effectiveness was assessed four months after the last RF ablation by dynamic MR imaging at 1.5-T.

RESULTS

MR-guided RF ablation procedures were technical successful in 85 of 87 (97.7%) assessed at the end of each session. Complete coagulation was intended in 99 of 100 tumors. Technique effectiveness was observed in 92 of 99 (92.9%) of these tumors. To achieve complete coagulation 82 of 92 (89.1%) tumors required a single session. T2-weighted sequences were accurate to monitor the extent of coagulation and were supportive to guide overlapping ablation. There were two of 87 (2.3%) major and seven of 87 (8.0%) minor complications.

CONCLUSION

MR-guided RF ablation is a safe and effective therapy in the treatment of hepatic malignancies. MR imaging offers an accurate monitoring of thermally-induced coagulation, thus enabling complete tumor coagulation in a single session.

MRI-based localization of electrophysiological recording sites within the cerebral cortex at single-voxel accuracy

The localization of microelectrode recording sites in the layers of primate cerebral cortex permits the analysis of relationships between recorded neuronal activities and underlying anatomical connections. We present a magnetic resonance imaging method for precise in vivo localization of cortical recording sites. In this method, the susceptibility-induced effect thickens the appearance of the microelectrode and enhances the detectability of the microelectrode tip, which usually occupies less than a few percent of the volume of an image voxel. In a phantom study, the optimized susceptibility-induced effect allowed tip detection with single-voxel accuracy (in-plane resolution, 50 mum). We applied this method to recording microelectrodes inserted into the brains of macaque monkeys, and localized the microelectrode tip at an in-plane resolution of 150 mum within the cortex of 2-3 mm in thickness. Subsequent histological analyses validated the single-voxel accuracy of the in vivo tip localization. This method opens up a way to investigate information flow during cognitive processes in the brain.

Magnetic resonance imaging for hepatic radiofrequency ablation

Image-guided radiofrequency (RF) ablation is a minimally invasive therapy option in the treatment of primary and secondary hepatic malignancies. Magnetic resonance (MR) imaging offers an accurate pre-interventional imaging having important impact on patient selection and planning of the ablation procedure. Peri-interventional imaging is used for targeting, monitoring, and controlling of the ablation procedure. Due to a high soft-tissue contrast offering delineation of tumor tissue and the surrounding anatomy, coupled with multiplanar capabilities, MR imaging is an advantageous targeting technique compared with ultrasonography (US) or computed tomography (CT). MR imaging is sensitive to thermal effects enabling a monitoring of ablation therapy subsequently being supportive to control the ablation procedure. Therefore, MR imaging can fulfil the conditions for overlapping ablations by enabling a precise repositioning of the MR compatible RF applicator if required. Thus, the probability of achieving complete coagulation in larger tumors within a single therapy session is potentially increased. A monitoring of thermal effects is moreover essential in order to prevent unintended tissue damage from critical structures in the surrounding of the target tissue. Post-interventional imaging is performed to assess treatment response after RF ablation and has prognostic impact, as an early detection of treatment failure, e.g. residual tumor tissue, enables immediate therapy. Nevertheless, differential diagnostic difficulties arise from benign periablational enhancement which may cover tumor tissue. Hence, further evaluation and improvement in the assessment of treatment response is essential.

Magnetic resonance-guided percutaneous radiofrequency ablation of renal cell carcinomas: a pilot clinical study

OBJECTIVE

The objective of this study was to assess the feasibility and efficacy of magnetic resonance imaging-(MRI) guided percutaneous radiofrequency (RF) ablation of renal cell carcinomas (RCC).

SUBJECTS AND METHODS

Twelve patients with RCC (63 to 82 years old) were treated with RF ablation in an interventional 0.2-Tesla open MR unit. Tumor sizes varied from 1.6 cm to 3.9 cm in maximum diameter (tumor volumes 1.9 cm3 to 28.7 cm3). RF procedures were entirely performed in the MR suite. For positioning of the MR-compatible RF-electrode, near real-time MR fluoroscopy by means of rapid gradient echo sequences (acquisition time approximately 2 seconds) was used. Monitoring of ablation was obtained by intermittent imaging with T1- and T2-weighted spin echo sequences.

RESULTS

Accurate placement of the RF electrodes was possible in all cases using near real-time MR fluoroscopy. Eleven of 12 patients were successfully treated within 1 single session; 1 patient had to be retreated for tumor relapse at 13 months follow up. Mean number of electrode repositionings under MR guidance during 1 session was 1.7; ablation time ranged between 12 and 28 minutes. Mean duration of 1 treatment session was 5 hours. Coagulation volumes ranged from 7.3 cm3 up to 30.2 cm3. All patients now appear to be disease-free with a mean follow up of 10.3 months (range, 3-23 months).

CONCLUSION

MRI-guided RF ablation of RCC in an interventional MR unit is safe and feasible. Fast MR imaging is a convenient method for rapid positioning of MR-compatible RF electrodes. MR monitoring of ablation procedure with T2-weighted imaging allows for immediate assessment of coagulation extent.

Real-time monitoring of radiofrequency ablation of rabbit liver by respiratory-gated quantitative temperature MRI

PURPOSE

To evaluate the feasibility and precision of magnetic resonance imaging (MRI) thermometry for monitoring radiofrequency (RF) liver ablation in vivo and predicting the size of the ablation zone.

MATERIALS AND METHODS

At 1.5T, respiratory-triggered real-time MR temperature mapping (the proton resonance frequency (PRF) method) was used to monitor RF ablation in rabbit liver (N = 6) under free breathing. The size of the ablation zones, as assessed by histological analyses, was compared with that predicted from MR thermal dose (TD) maps or derived from conventional T1-weighted (T1w), T2-weighted (T2w), and T1w gadolinium (Gd)-enhanced (T1w-Gd) images acquired immediately after the ablation, and on days 4 and 8 postprocedure.

RESULTS

MR temperature uncertainty remained under 1-2 degrees C even during RF deposition. The TD maps were shown to be more predictive and precise than the other MR images, with an average predictive precision for the final ablation zone size of about 1 mm as compared to the histologically proven lesion on day 8.

CONCLUSION

Quantitative temperature MRI during RF ablation is feasible and offered a precise indication of the ablation zone size in this preclinical study based on the lethal dose threshold.