Feasibility, efficacy, and safety of percutaneous MR-guided ablation of small (≤12 mm) hepatic malignancies.

BACKGROUND: Percutaneous tumor ablation is commonly performed using computed tomography (CT) or ultrasound (US) guidance, although reliable visualization of the target tumor may be challenging. MRI guidance provides more reliable visualization of target tumors and allows for real-time imaging and multiplanar capabilities, making it the modality of choice, in particular if lesions are small. PURPOSE: To investigate the feasibility, technical success, and safety of percutaneous MR-guided ablation (RFA n = 27 / MWA n = 16) of small (≤12 mm) hepatic malignancies. STUDY TYPE: Retrospective case study. POPULATION: In all, 45 patients (age: 61.1 ± 11.8) with hepatic malignancies and a lesion diameter of ≤12 mm scheduled for percutaneous MR-guided tumor ablation based on a tumor board decision were included. FIELD STRENGTH: A 1.5T MR system was used for planning, targeting, and monitoring. ASSESSMENT: Feasibility assessment included the detection of the target tumor, tumor delineation during MR-fluoroscopy guided targeting, and the number of attempts needed for precise applicator placement. Technical success was defined as successful performance of the procedure including a safety margin of 5 mm. Safety evaluation was based on procedure-related complications. STATISTICAL TEST: Frequency. RESULTS: Tumor ablation (mean diameter 9.0 ± 2.1 mm) was successfully completed in 43/45 patients. Planning imaging was conducted without a contrast agent in 79% (n = 37). In 64% (n = 30), the target tumors were visible with MR-fluoroscopy. In six patients (13%), planning imaging revealed new, unexpected small lesions, which were either treated in the same session (n = 4) or changed therapy management (n = 2) due to diffuse tumor progress. Postprocedural imaging revealed a technical success of 100% (43/43), with no major complications. During follow-up, no local tumor progression was observed (mean follow-up 24.7 ± 14.0 months) although 28% (12) patients developed new hepatic lesions distant to the ablation zone. No major complications were observed. DATA CONCLUSION: MR-guided ablation is a feasible approach for an effective and safe treatment of small hepatic malignancies. LEVEL OF EVIDENCE: 4 Technical Efficacy: Stage 4 J. Magn. Reson. Imaging 2019;49:374-381.

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.

Diagnostic accuracy of different magnetic resonance imaging sequences for detecting local tumor progression after radiofrequency ablation of hepatic malignancies.

OBJECTIVE: To evaluate the diagnostic accuracy of the individual sequences of a clinical routine liver MRI protocol for the detection of local tumour progression after radiofrequency (RF) ablation of hepatic malignancies. MATERIAL AND METHODS: A cohort of 93 patients treated for 140 primary and secondary hepatic malignancies with RF ablation was assembled for this retrospective study. The cohort contained 31 cases of local tumour progression, which occurred 8.3±6.2months (range: 4.0-28.2 months) after treatment. All patients underwent clinical routine follow-up MRI at 1.5T including following sequences: unenhanced T1-weighted fast low angle shot (FLASH-2D), T2-weighted turbo-spin-echo sequence, contrast-enhanced (CE) T1-weighted volume-interpolated breath-hold examination (VIBE), diffusion-weighted imaging (DWI). Follow-up was 32.7±22.5months (range: 4.0-138.3 months). Two readers independently evaluated the individual sequences separately for signs of local tumour progression. Diagnostic confidence was rated on a 4-point scale. Inter-reader agreement was assessed with Cohens kappa. Long-term follow-up and histological specimen served as standard of reference. RESULTS: Both readers reached the highest sensitivity for detection of local tumour progression with unenhanced T1-FLASH 2D (88.2% and 94.1%, respectively) and the highest specificity with CE T1-VIBE (96.2% and 97.2%, respectively). Highest inter-reader agreement was reached with T1-FLASH-2D (kappa=0.83). Typical pitfalls for false-positive diagnoses were focal cholestasis and vasculature adjacent to the ablation zone. Diagnostic confidence was highest with CE T1-VIBE for reader 1 and DWI for reader 2. CONCLUSION: Unenhanced T1-FLASH-2D is an essential sequence for follow-up imaging after tumour ablation with a high sensitivity for detection of local progression and a high inter-reader agreement.

Preclinical evaluation of an MR-compatible microwave ablation system and comparison with a standard microwave ablation system in an ex vivo bovine liver model.

OBJECTIVE: Evaluation of a newly developed MR-compatible microwave ablation system with focus on ablation performance and comparison with a corresponding standard microwave ablation system. MATERIALS AND METHODS: A total of 52 ablations were performed with a non-cooled microwave ablation system in an ex vivo bovine liver model using the following settings: [A] 16G-standard antenna, 2 cm active tip, 2.4 m cable; [B] MR-compatible 16G-antenna, 2 cm active tip, 2.4 m cable; [C] MR-compatible 16G-antenna, 2 cm active tip, extended 6 m cable; and [D] MR-compatible 16G-antenna, 4 cm active tip, extended 6 m cable. Ablation durations were 3, 5 and 10 min, and additionally 15 min for [D]. Ablations zones were measured for short-axis diameter (SA) and long-axis diameter (LA). Settings [A]-[C] were compared regarding SA, volume (V) and generator energy output (E) with analysis of variance and Tukey-Kramer post hoc test. Ablation performance of the MR-compatible settings [C] and [D] were compared regarding SA, V, E and sphericity index (SA/LA) with unpaired t-test. p < 0.05 was considered as statistically significant. RESULTS: No significant differences were found between [A], [B] and [C] regarding SA and V (10 min; SA[A] = 25.8 ± 2.4 mm, SA[B] = 25.3 ± 1.9 mm, SA[C] = 25.0 ± 2.0 mm, p = 0.88; V[A] = 17.8 ± 4.4 cm³, V[B] = 16.6 ± 3.0 cm³, V[C] = 17.8 ± 2.7 cm³, p = 0.85); however, the highest energy output was measured for setting [C] (10 min; [A]: 9.9 ± 0.5 kJ, [B]: 10.1 ± 0.5 kJ, [C]: 13.1 ± 0.3 kJ, p < 0.001). SA, V and E were significantly larger with setting [D] than [C] with 10 min ablations (SA[C] = 25.0 ± 2.0 mm, SA[D] = 34.0 ± 2.9 mm, p = 0.003; V[C] = 17.8 ± 2.7 cm³, V[D] = 39.4 ± 7.5 cm³, p = 0.007; E[C] = 13.1 ± 0.3 kJ, E[D] = 16.7 ± 0.8 kJ, p = 0.002) without significant difference in sphericity index (SA/LA[C] = 0.46 ± 0.02, SA/LA[D] = 0.52 ± 0.04, p = 0.08). CONCLUSION: The tested MR-compatible system can be used without loss of ablation performance compared to the standard system.

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.

Comparison of different population-averaged arterial-input-functions in dynamic contrast-enhanced MRI of the prostate: Effects on pharmacokinetic parameters and their diagnostic performance.

PURPOSE: To assess the effect of different population-averaged arterial-input-functions (pAIF) on pharmacokinetic parameters from dynamic contrast-enhanced MRI (DCE-MRI) and their diagnostic accuracy regarding the detection of potentially malignant prostate lesions. MATERIALS AND METHODS: 66 male patients (age 65.4±10.8y) with suspected prostate cancer underwent multiparametric MRI of the prostate including T2-w, DWI-w and DCE-MRI sequences at a 3T MRI scanner. All detected lesions were categorized based on ACR PI-RADS version 2 and divided into 2 groups (A: PI-RADS ≤3, n=32; B: PI-RADS >3, n=34). In each DCE-MRI dataset, pharmacokinetic parameters (Ktrans, Kep and ve) and goodness of fit (chi(2)) were generated using the Tofts model with 3 different pAIFs (fast, intermediate, slow) as provided by a commercially available postprocessing software. Pharmacokinetic parameters, their diagnostic accuracies and model fits were compared for the 3 pAIFs. RESULTS: Ktrans, Kep and ve differed significantly among the 3 pAIFs (all p<.001). Ktrans and Kep were significantly higher in group B compared to group A (all p<.001). For chi(2), lowest results (representing highest goodness of fit) were found for intermediate pAIF (chi(2) 0.073). ROC analyses revealed comparable diagnostic accuracies for the different pAIFs, which were high for Ktrans and Kep and low for ve. CONCLUSION: Choosing various pAIF types causes a high variability in pharmacokinetic parameter estimates. Therefore, it is of great importance to consider this as potential artifact and thus keep AIF type selection constant in DCE-MRI studies.

In vitro artefact assessment of a new MR-compatible microwave antenna and a standard MR-compatible radiofrequency ablation electrode for tumour ablation.

OBJECTIVE: To evaluate and compare artefact configuration and diameters in a magnetic resonance (MR)-compatible prototype microwave (MW) applicator and a standard MR-compatible radiofrequency (RF) applicator for MR-guided tumour ablation. METHODS: Both applicators were tested in a phantom study at 1.5T with three sequences: T1-weighted three-dimensional volume interpolated breath-hold examination (VIBE), T1-weighted fast low angle shot (FLASH), T2-weighted turbo spin echo (TSE). Applicator orientation to main magnetic field (B0) and slice orientation were varied. Needle tip location error (TLE) was assessed, and artefact diameters were calculated. Influence of imaging parameters on artefacts was assessed with analysis of variance (ANOVA) and post hoc testing. RESULTS: MW applicator: the shaft artefact diameter measured 2.3 +/- 0.8 mm. Tip artefact diameter and length measured 2.2 ± 0.8 mm and 2.4 ± 1.3 mm, respectively. A prominent oval artefact (diameter: 16.5 +/- 1.8 mm, length: 19.1 +/- 2.5 mm) appeared close to the tip. TLE: - .3 +/- 0.6 mm. RF applicator: shaft and tip diameter measured 8.9 +/- 4.7 mm and 9.0 +/- .0 mm, respectively. TLE: -0.1 +/- 0.8 mm. Minimal artefacts were measured with RF applicator orientation parallel to B0 (P < 0.0001), whereas no such influence was found for MW applicator. For both applicators, significantly large artefacts were measured with T1 FLASH (P = 0.03). CONCLUSION: The MW applicator's artefact is satisfactory and seems useable for MR-guided ablation procedures. KEY POINTS: MW applicator's artefact appearance is independent of angulation to main magnetic field. MW applicator's prominent distal artefact may increase visibility under MR-guidance. RF and MW applicator's artefacts are precise concerning tip depiction. Largest artefact diameters are measured with T1-weighted fast low angle shot sequence.