MR thermometry for monitoring tumor ablation

Magnetic Resonance-Monitored Coaxial Electrochemical Ablation--Preliminary Evaluation of Technical Feasibility

PURPOSE

To evaluate the technical feasibility of a coaxial electrode configuration to rapidly create a mechanically defined electrochemical ablation zone monitored by magnetic resonance (MR) imaging in real time.

MATERIALS AND METHODS

A direct current generator supplied the nitinol cathode cage and central platinum anode for coaxial electrochemical ablation. Safety and efficacy were evaluated by measuring local pH, temperature, and current scatter in saline solutions. Ablation zone diameters of 3-6 cm (n = 72) were created on ex vivo bovine liver and verified by gross pathology. Feasibility of MR monitoring was evaluated using 8 swine livers to create ablations of 3 cm (n = 12), 4 cm (n = 4), and 5 cm (n = 4) verified by histology.

RESULTS

Local pH was 3.2 at the anode and 13.8 at the cathode. Current scatter was negligible. Ablation progress increased relative to local ion concentration, and MR signal changes corresponded to histologic findings. In the ex vivo model, the times to achieve complete ablation were 15 minutes, 20 minutes, 35 minutes, and 40 minutes for diameters of 3 cm, 4 cm, 5 cm, and 6 cm, respectively. Ablation times for the in situ model were 15 minutes, 35 minutes, and 50 minutes for 3 cm, 4 cm, and 5 cm, respectively.

CONCLUSIONS

The coaxial configuration mechanically defined the electrochemical ablation zone with times similar to comparably sized thermal ablations. MR compatibility allowed for real-time monitoring of ablation progress.

Spatiotemporal filtering of MR-temperature artifacts arising from bowel motion during transurethral MR-HIFU

PURPOSE

Transurethral MR-HIFU is a minimally invasive image-guided treatment for localized prostate cancer that enables precise targeting of tissue within the gland. The treatment is performed within a clinical MRI to obtain real-time MR thermometry used as an active feedback to control the spatial heating pattern in the prostate and to monitor for potential damage to surrounding tissues. This requires that the MR thermometry measurements are an accurate representation of the true tissue temperature. The proton resonance frequency shift thermometry method used is sensitive to tissue motion and changes in the local magnetic susceptibility that can be caused by the motion of air bubbles in the rectum, which can impact the performance of transurethral MR-HIFU in these regions of the gland.

METHODS

A method is proposed for filtering of temperature artifacts based on the temporal variance of the temperature, using empirical and dynamic positional knowledge of the ultrasonic heating beam, and an estimation of the measurement noise. A two-step correction strategy is introduced which eliminates artifact-detected temperature variations while keeping the noise level low through spatial averaging.

RESULTS

The filter has been evaluated by postprocessing data from five human transurethral ultrasound treatments. The two-step correction process led to reduced final temperature standard deviation in the prostate and rectum areas where the artifact was located, without negatively affecting areas distal to the artifact. The performance of the filter was also found to be consistent across all six of the data sets evaluated. The evaluation of the detection criterion parameter M determined that a value of M = 3 achieves a conservative filter with minimal loss of spatial resolution during the process.

CONCLUSIONS

The filter was able to remove most artifacts due to the presence of moving air bubbles in the rectum during transurethral MR-HIFU. A quantitative estimation of the filter capabilities shows a systematic improvement in the standard deviation of the corrected temperature maps in the rectum zone as well as in the entire acquired slice.

International consensus on use of focused ultrasound for painful bone metastases: Current status and future directions

Focused ultrasound surgery (FUS), in particular magnetic resonance guided FUS (MRgFUS), is an emerging non-invasive thermal treatment modality in oncology that has recently proven to be effective for the palliation of metastatic bone pain. A consensus panel of internationally recognised experts in focused ultrasound critically reviewed all available data and developed consensus statements to increase awareness, accelerate the development, acceptance and adoption of FUS as a treatment for painful bone metastases and provide guidance towards broader application in oncology. In this review, evidence-based consensus statements are provided for (1) current treatment goals, (2) current indications, (3) technical considerations, (4) future directions including research priorities, and (5) economic and logistical considerations.

Influence of temperature-dependent thermal parameters on temperature elevation of tissue exposed to high-intensity focused ultrasound: numerical simulation

High-intensity focused ultrasound (HIFU) has been used successfully as a non-invasive modality in treating solid tumors. The temperature rise HIFU irradiation causes in a tissue depends on the thermal properties of the tissue. This study was motivated by our observation that the thermal properties of a tissue vary significantly with temperature (Guntur SR, Lee KI, Paeng DG, Coleman AJ, Choi MJ. Ultrasound Med Biol 2013;39:1771-1784). This research investigated how significantly the alteration of tissue thermal parameters, in the ranges of values measured at 25°C-90°C, affects prediction of the temperature elevation of tissue under the same HIFU exposure. The numerical simulation was performed by coupling a non-linear Khokhlov-Zabolotskaya-Kuznetsov equation with a bio-heat transfer function. In the conventional method of prediction, the thermal parameters were set as constants measured at room temperature (25°C). This study compared the conventional prediction with those predicted with different thermal parameters measured at the various temperatures up to 90°C. The results indicated that the conventional method significantly overestimated the rise in focal temperature in the liver tissue exposed to a clinical HIFU field, compared with the prediction made using thermal parameters measured at temperatures that cause thermal denaturation. This finding suggests that temperature-dependent thermal parameters should be considered in predicting the temperature rise in a tissue to avoid use of an insufficient thermal dose in treatment planning for HIFU surgery.

Thermometry and ablation monitoring with ultrasound

In this review we present the current status of ultrasound thermometry and ablation monitoring, with emphasis on the diverse approaches published in the literature and with an eye on which methods are closest to clinical reality. It is hoped that this review will serve as a guide to the expansion of sonographic methods for treatment monitoring and thermometry since the last brief review in 2007.

Frequency-dependent conductivity contrast for tissue characterization using a dual-frequency range conductivity mapping magnetic resonance method

Electrical conductivities of biological tissues show frequency-dependent behaviors, and these values at different frequencies may provide clinically useful diagnostic information. MR-based tissue property mapping techniques such as magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance electrical property tomography (MREPT) are widely used and provide unique conductivity contrast information over different frequency ranges. Recently, a new method for data acquisition and reconstruction for low- and high-frequency conductivity images from a single MR scan was proposed. In this study, we applied this simultaneous dual-frequency range conductivity mapping MR method to evaluate its utility in a designed phantom and two in vivo animal disease models. Magnetic flux density and B(1)(+) phase map for dual-frequency conductivity images were acquired using a modified spin-echo pulse sequence. Low-frequency conductivity was reconstructed from MREIT data by the projected current density method, while high-frequency conductivity was reconstructed from MREPT data by B(1)(+) mapping. Two different conductivity phantoms comprising varying ion concentrations separated by insulating films with or without holes were used to study the contrast mechanism of the frequency-dependent conductivities related to ion concentration and mobility. Canine brain abscess and ischemia were used as in vivo models to evaluate the capability of the proposed method to identify new electrical properties-based contrast at two different frequencies. The simultaneous dual-frequency range conductivity mapping MR method provides unique contrast information related to the concentration and mobility of ions inside tissues. This method has potential to monitor dynamic changes of the state of disease.

Gd-EOB-DTPA-enhanced MR guidance in thermal ablation of liver malignancies

OBJECTIVE

To evaluate the potency of Gd-EOB-DTPA to support hepatic catheter placement in laser ablation procedures by quantifying time-dependent delineation effects for instrumentation and target tumor within liver parenchyma. Monitoring potential influence on online MR thermometry during the ablation procedure is a secondary aim.

MATERIALS AND METHODS

30 cases of MR-guided laser ablation were performed after i.v. bolus injection of gadoxetic acid (0.025 mmol/Kg Gd-EOB-DTPA; Bayer Healthcare, Berlin, Germany). T1-weighted GRE sequences were used for applicator guidance (FLASH 3D) in the catheter placement phase and for therapy monitoring (FLASH 2D) in the therapy phase. SNR and consecutive CNR values were measured for elements of interest plotted over time both for catheter placement and therapy phase and compared with a non-contrast control group of 19 earlier cases. Statistical analysis was realized using the paired Wilcoxon test.

RESULTS

Sustainable signal elevation of liver parenchyma in the contrast-enhanced group was sufficient to silhouette both target tumor and applicator against the liver. Differences in time dependent CNR alteration were highly significant between contrast-enhanced and non-contrast interventions for parenchyma and target on the one hand (p = 0.020) and parenchyma and instrument on the other hand (p = 0.002). Effects lasted for the whole procedure (monitoring up to 60 min) and were specific for the contrast-enhanced group. Contrasting maxima were seen after median 30 (applicator) and 38 (tumor) minutes, in the potential core time of a multineedle procedure. Contrast influence on T1 thermometry for real-time monitoring of thermal impact was not significant (p = 0.068-0.715).

CONCLUSION

Results strongly support anticipated promotive effects of Gd-EOB-DTPA for MR-guided percutaneous liver interventions by proving and quantifying the delineating effects for therapy-relevant elements in the procedure. Time benefit, cost effectiveness and oncologic outcome of the described beneficiary effects will have to be part of further investigations.

Feasibility of volumetric MRI-guided high intensity focused ultrasound (MR-HIFU) for painful bone metastases

Background

Magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) has recently emerged as an effective treatment option for painful bone metastases. We describe here the first experience with volumetric MR-HIFU for palliative treatment of painful bone metastases and evaluate the technique on three levels: technical feasibility, safety, and initial effectiveness.

Methods

In this observational cohort study, 11 consecutive patients (7 male and 4 female; median age, 60 years; age range, 53–86 years) underwent 13 treatments for 12 bone metastases. All patients exhibited persistent metastatic bone pain refractory to the standard of care. Patients were asked to rate their worst pain on an 11-point pain scale before treatment, 3 days after treatment, and 1 month after treatment. Complications were monitored. All data were prospectively recorded in the context of routine clinical care. Response was defined as a ≥2-point decrease in pain at the treated site without increase in analgesic intake. Baseline pain scores were compared to pain scores at 3 days and 1 month using the Wilcoxon signed-rank test. For reporting, the STROBE guidelines were followed.

Results

No treatment-related major adverse events were observed. At 3 days after volumetric MR-HIFU ablation, pain scores decreased significantly (p = 0.045) and response was observed in a 6/11 (55%) patients. At 1-month follow-up, which was available for nine patients, pain scores decreased significantly compared to baseline (p = 0.028) and 6/9 patients obtained pain response (overall response rate 67% (95% confidence interval (CI) 35%–88%)).

Conclusions

This is the first study reporting on the volumetric MR-HIFU ablation for painful bone metastases. No major treatment-related adverse events were observed during follow-up. The results of our study showed that volumetric MR-HIFU ablation for painful bone metastases is technically feasible and can induce pain relief in patients with metastatic bone pain refractory to the standard of care. Future research should be aimed at standardization of the treatment procedures and treatment of larger numbers of patients to assess treatment effectiveness and comparison to the standard of care.

Thermal fixation of swine liver tissue after magnetic resonance-guided high-intensity focused ultrasound ablation

The aim of this study was to investigate experimental conditions for efficient and controlled in vivo liver tissue ablation by magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU) in a swine model, with the ultimate goal of improving clinical treatment outcome. Histological changes were examined both acutely (four animals) and 1 wk after treatment (five animals). Effects of acoustic power and multiple sonication cycles were investigated. There was good correlation between target size and observed ablation size by thermal dose calculation, post-procedural MR imaging and histopathology, when temperature at the focal point was kept below 90°C. Structural histopathology investigations revealed tissue thermal fixation in ablated regions. In the presence of cavitation, mechanical tissue destruction occurred, resulting in an ablation larger than the target. Complete extra-corporeal MR-guided HIFU ablation in the liver is feasible using high acoustic power. Nearby large vessels were preserved, which makes MR-guided HIFU promising for the ablation of liver tumors adjacent to large veins.

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

PURPOSE

MR thermometry (MRT) is a noninvasive method for measuring temperature that can potentially be used for radio frequency (RF) safety monitoring. This application requires measuring absolute temperature. In this study, a multigradient-echo (mGE) MRT sequence was used for that purpose. A drawback of this sequence, however, is that its accuracy is affected by background gradients. In this article, we present a method to minimize this effect and to improve absolute temperature measurements using MRI.

THEORY

By determining background gradients using a B0 map or by combining data acquired with two opposing readout directions, the error can be removed in a homogenous phantom, thus improving temperature maps.

METHODS

All scans were performed on a 3T system using ethylene glycol-filled phantoms. Background gradients were varied, and one phantom was uniformly heated to validate both compensation approaches. Independent temperature recordings were made with optical probes.

RESULTS

Errors correlated closely to the background gradients in all experiments. Temperature distributions showed a much smaller standard deviation when the corrections were applied (0.21°C vs. 0.45°C) and correlated well with thermo-optical probes.

CONCLUSION

The corrections offer the possibility to measure RF heating in phantoms more precisely. This allows mGE MRT to become a valuable tool in RF safety assessment.

MR-guided focused ultrasound surgery, present and future

MR-guided focused ultrasound surgery (MRgFUS) is a quickly developing technology with potential applications across a spectrum of indications traditionally within the domain of radiation oncology. Especially for applications where focal treatment is the preferred technique (for example, radiosurgery), MRgFUS has the potential to be a disruptive technology that could shift traditional patterns of care. While currently cleared in the United States for the noninvasive treatment of uterine fibroids and bone metastases, a wide range of clinical trials are currently underway, and the number of publications describing advances in MRgFUS is increasing. However, for MRgFUS to make the transition from a research curiosity to a clinical standard of care, a variety of challenges, technical, financial, clinical, and practical, must be overcome. This installment of the Vision 20∕20 series examines the current status of MRgFUS, focusing on the hurdles the technology faces before it can cross over from a research technique to a standard fixture in the clinic. It then reviews current and near-term technical developments which may overcome these hurdles and allow MRgFUS to break through into clinical practice.

Real-time monitoring of high-intensity focused ultrasound treatment using axial strain and axial-shear strain elastograms

Axial strain elastograms (ASEs) have been found to help visualize sonographically invisible thermal lesions. However, in most studies involving high-intensity focused ultrasound (HIFU)-induced thermal lesions, elastography imaging was performed separately later, after the lesion was formed. In this article, the feasibility of monitoring, in real time, tissue elasticity variation during HIFU treatment and immediately thereafter is explored using quasi-static elastography. Further, in addition to ASEs, we also explore the use of simultaneously acquired axial-shear strain elastograms (ASSEs) for HIFU lesion visualization. Experiments were performed on commercial porcine liver samples in vitro. The HIFU experiments were conducted at two applied acoustic power settings, 35 and 20 W. The experimental setup allowed us to interrupt the HIFU pulse momentarily several different times during treatment to perform elastographic compression and data acquisition. At the end of the experiments, the samples were cut along the imaging plane and photographed to compare size and location of the formed lesion with those visualized on ASEs and ASSEs. Single-lesion and multiple-lesion experiments were performed to assess the contribution of ASEs and ASSEs to lesion visualization and treatment monitoring tasks. At both power settings, ASEs and ASSEs provided accurate location information during HIFU treatment. At the low-power setting case, ASEs and ASSEs provide accurate lesion size in real-time monitoring. Lesion appearance in ASEs and ASSEs was affected by the cavitation bubbles produced at the high-power setting. The results further indicate that the cavitation bubbles influence lesion appearance more in ASEs than in ASSEs. Both ASEs and ASSEs provided accurate size information after a waiting period that allowed the cavitation bubbles to disappear. The results indicate that ASSEs not only improve lesion visualization and size measurement of a single lesion, but, under certain conditions, also help to identify untreated gaps between adjacent lesions with high contrast.

PRFS-based MR thermometry versus an alternative T1 magnitude method--comparative performance predicting thermally induced necrosis in hepatic tumor ablation

OBJECTIVE

To compare the accuracy of a semi-quantitative proton resonance frequency shift (PRFS) thermal mapping interface and an alternative qualitative T1 thermometry model in predicting tissue necrosis in an established routine setting of MRI-guided laser ablation in the human liver.

MATERIALS AND METHODS

34 cases of PRFS-guided (GRE) laser ablation were retrospectively matched with 34 cases from an earlier patient population of 73 individuals being monitored through T1 magnitude image evaluation (FLASH 2D). The model-specific real-time estimation of necrotizing thermal impact (above 54 °C zone and T1 signal loss, respectively) was correlated in size with the resulting necrosis as shown by lack of enhancement on the first-day contrast exam (T1). Matched groups were compared using the Mann-Whitney test.

RESULTS

Online PRFS guidance was available in 33 of 34 cases. Positive size correlation between calculated impact zone and contrast defect at first day was evident in both groups (p < 0.0004). The predictive error estimating necrosis was median 21% (range 1 %-52%) in the PRFS group and 61 % (range 22-84%) in the T1 magnitude group. Differences in estimating lethal impact were significant (p = 0.004), whereas the real extent of therapy-induced necrosis showed no significant difference (p > 0.28) between the two groups.

CONCLUSION

PRFS thermometry is feasible in a clinical setting of thermal hepatic tumor ablation. As an interference-free MR-tool for online therapy monitoring its accuracy to predict tissue necrosis is superior to a competing model of thermally induced alteration of the T1 magnitude signal.

Real-time temperature estimation and monitoring of HIFU ablation through a combined modeling and passive acoustic mapping approach

Passive acoustic mapping (PAM) has been recently demonstrated as a method of monitoring focused ultrasound therapy by reconstructing the emissions created by inertially cavitating bubbles (Jensen et al 2012 Radiology 262 252-61). The published method sums energy emitted by cavitation from the focal region within the tissue and uses a threshold to determine when sufficient energy has been delivered for ablation. The present work builds on this approach to provide a high-intensity focused ultrasound (HIFU) treatment monitoring software that displays both real-time temperature maps and a prediction of the ablated tissue region. This is achieved by determining heat deposition from two sources: (i) acoustic absorption of the primary HIFU beam which is calculated via a nonlinear model, and (ii) absorption of energy from bubble acoustic emissions which is estimated from measurements. The two sources of heat are used as inputs to the bioheat equation that gives an estimate of the temperature of the tissue as well as estimates of tissue ablation. The method has been applied to ex vivo ox liver samples and the estimated temperature is compared to the measured temperature and shows good agreement, capturing the effect of cavitation-enhanced heating on temperature evolution. In conclusion, it is demonstrated that by using PAM and predictions of heating it is possible to produce an evolving estimate of cell death during exposure in order to guide treatment for monitoring ablative HIFU therapy.

Magnetic resonance temperature imaging of laser-induced thermotherapy: assessment of fast sequences in ex vivo porcine liver

Aim: To evaluate magnetic resonance sequences for T1 and proton resonance frequency (PRF) thermometry during laser-induced thermotherapy (LITT) in liver tissue. Materials & methods: During LITT (1064 nm; 30 W; 3-cm diffuser; 2–3 min) in ex vivo porcine liver, temperature was measured (25–70°C) utilizing a fiberoptic thermometer and MRI was performed with a 1.5-T scanner through the following sequences: segmented echo planar imaging (seg-EPI) for the PRF method; fast low-angle shot (FLASH), inversion-recovery turbo FLASH (IRTF), saturation-recovery turbo FLASH (SRTF) and true-fast imaging (TRUFI) for the T1 method. Phase angle and signal amplitude (regarding PRF/T1) was recorded in regions of interest, on images under fiberoptic probe tips. Sequences’ thermal coefficients were determined by calibrating phase angle and signal amplitude against temperature and subsequently validated. Results: Coefficients of -0.0089 ± 0.0003 ppm °C-1 (seg-EPI) and -0.917 ± 0.046, -1.166 ± 0.058, -1.038 ± 0.054 and -1.443 ± 0.118°C-1 (FLASH, IRTF, SRTF and TRUFI, respectively) were obtained. Precisions of 0.71, 1.34, 2.07, 2.44 and 3.21°C and, through Bland–Altman analysis, accuracies of -0.67, 0.79, 1.65, 1.57 and 2.13°C (seg-EPI, FLASH, IRTF, SRTF and TRUFI, respectively) were determined. Conclusion: The PRF method with seg-EPI sequence is preferred for thermometry during LITT owing to higher precision and accuracy. Among T1-method sequences, FLASH showed higher accuracy and robustness.

Focal therapy for localized prostate cancer -choosing the middle ground

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Emerging technologies for image guidance and device navigation in interventional radiology

Recent developments in image-guidance and device navigation, along with emerging robotic technologies, are rapidly transforming the landscape of interventional radiology (IR). Future state-of-the-art IR procedures may include real-time three-dimensional imaging that is capable of visualizing the target organ, interventional tools, and surrounding anatomy with high spatial and temporal resolution. Remote device actuation is becoming a reality with the introduction of novel magnetic-field enabled instruments and remote robotic steering systems. Robots offer several degrees of freedom and unprecedented accuracy, stability, and dexterity during device navigation, propulsion, and actuation. Optimization of tracking and navigation of interventional tools inside the human body will be critical in converting IR suites into the minimally invasive operating theaters of the future with increased safety and unsurpassed therapeutic efficacy. In the not too distant future, individual image guidance modalities and device tracking methods could merge into autonomous, multimodality, multiparametric platforms that offer real-time data of anatomy, morphology, function, and metabolism along with on-the-fly computational modeling and remote robotic actuation. The authors provide a concise overview of the latest developments in image guidance and device navigation, while critically envisioning what the future might hold for 2020 IR procedures.

Evaluation of a robotic technique for transrectal MRI-guided prostate biopsies

OBJECTIVES

To evaluate the accuracy and speed of a novel robotic technique as an aid to perform magnetic resonance image (MRI)-guided prostate biopsies on patients with cancer suspicious regions.

METHODS

A pneumatic controlled MR-compatible manipulator with 5 degrees of freedom was developed in-house to guide biopsies under real-time imaging. From 13 consecutive biopsy procedures, the targeting error, biopsy error and target displacement were calculated to evaluate the accuracy. The time was recorded to evaluate manipulation and procedure time.

RESULTS

The robotic and manual techniques demonstrated comparable results regarding mean targeting error (5.7 vs 5.8 mm, respectively) and mean target displacement (6.6 vs 6.0 mm, respectively). The mean biopsy error was larger (6.5 vs 4.4 mm) when using the robotic technique, although not significant. Mean procedure and manipulation time were 76 min and 6 min, respectively using the robotic technique and 61 and 8 min with the manual technique.

CONCLUSIONS

Although comparable results regarding accuracy and speed were found, the extended technical effort of the robotic technique make the manual technique - currently - more suitable to perform MRI-guided biopsies. Furthermore, this study provided a better insight in displacement of the target during in vivo biopsy procedures.

Estimation of thermal dose from MR thermometry during application of nonablative pulsed high intensity focused ultrasound

PURPOSE

To evaluate whether MR thermometry is sufficiently fast, accurate, and spatially resolved for monitoring the thermal safety of nonablative pulsed high intensity ultrasound (pHIFU) treatments.

MATERIALS AND METHODS

A combination of real MR thermometry data and modeling was used to analyze the effects of temporal and spatial averaging as well as noise on the peak temperatures and thermal doses that would be measured by MR thermometry.

RESULTS

MR thermometry systematically underestimates the temperature and thermal doses during pHIFU treatment. Small underestimates of peak temperature can lead to large underestimates of thermal dose. Spatial averaging errors are small for ratios of pixel dimension to heating zone radius less than 0.25, which may be achieved by reducing the voxel size or steering the acoustic beam. Thermal dose might also be underestimated for very short, high power pulses due to temporal averaging. A simple correction factor based on the applied power and duty cycle may be applied to determine the upper bound of this effect.

CONCLUSION

The temperature and thermal dose measured using MR thermometry during pulsed HIFU treatment is probably sufficient in most instances. Simple corrections may be used to calculate an upper bound where this is a critical factor.

Threshold-based prediction of the coagulation zone in sequential temperature mapping in MR-guided radiofrequency ablation of liver tumours

OBJECTIVE

To evaluate different cut-off temperature levels for a threshold-based prediction of the coagulation zone in magnetic resonance (MR)-guided radiofrequency (RF) ablation of liver tumours.

METHODS

Temperature-sensitive measurements were acquired during RF ablation of 24 patients with primary (6) and secondary liver lesions (18) using a wide-bore 1.5 T MR sytem and compared with the post-interventional coagulation zone. Temperature measurements using the proton resonance frequency shift method were performed directly subsequent to energy application. The temperature maps were registered on the contrast-enhanced follow-up MR images acquired 4 weeks after treatment. Areas with temperatures above 50°, 55° and 60°C were segmented and compared with the coagulation zones. Sensitivity and positive predictive value were calculated.

RESULTS

No major complications occurred and all tumours were completely treated. No tumour recurrence was observed at the follow-up examination after 4 weeks. Two patients with secondary liver lesions showed local tumour recurrence after 4 and 7 months. The 60°C threshold level achieved the highest positive predictive value (87.7 ± 9.9) and the best prediction of the coagulation zone.

CONCLUSIONS

For a threshold-based prediction of the coagulation zone, the 60°C cut-off level achieved the best prediction of the coagulation zone among the tested levels.

KEY POINTS

• Temperature monitoring can be used to survey MR-guided radiofrequency ablation • The developing ablation zone can be estimated based on post-interventional temperature measurements • A 60°C threshold level can be used to predict the ablation zone • The 50°C and 55°C temperature zones tend to overestimate the ablation zone.

Spatiotemporal monitoring of high-intensity focused ultrasound therapy with passive acoustic mapping

PURPOSE

To demonstrate feasibility of monitoring high-intensity focused ultrasound (HIFU) treatment with passive acoustic mapping of broadband and harmonic emissions reconstructed from filtered-channel radiofrequency data in ex vivo bovine tissue.

MATERIALS AND METHODS

Both passive acoustic emissions and B-mode images were recorded with a diagnostic ultrasound machine during 180 HIFU exposures of five freshly excised, degassed bovine livers. Tissue was exposed to peak rarefactional pressures between 3.6 and 8.0 MPa for 2, 5, or 10 seconds. The B-mode images were analyzed for hyperechoic activity, and threshold levels were determined for the harmonic (1.17 mJ) and broadband (0.0137 mJ) components of the passively reconstructed source energy to predict tissue ablation. Both imaging methods were compared with tissue lesions after exposure to determine their spatial accuracy and their capability to help predict presence of ablated tissue. Performance of both methods as detectors was compared (matched-pair test design).

RESULTS

Passive mapping successfully aided prediction of the presence of tissue ablation more often than did conventional hyperechoic images (49 of 58 [84%] vs 31 of 58 [53%], P < .001). At 5.4-6.3-MPa exposures, sensitivity, specificity, negative predictive value, and positive predictive value of the two methods, respectively, were 15 of 20 versus five of 21 (P = .006), eight of nine versus eight of nine (P = .72), 15 of 16 versus five of six (P = .53), and eight of 13 versus eight of 24 (P = .011). Across HIFU exposure amplitude ranges, passive acoustic mapping also aided correct prediction of the visually detected location of ablation following tissue sectioning in 42 of 45 exposures for which the harmonic and broadband threshold levels for tissue ablation were exceeded. Early cavitation activity indicated the focal position within the tissue before irreversible tissue damage occurred.

CONCLUSION

Passive acoustic mapping significantly outperformed the conventional hyperecho technique as an ultrasound-based HIFU monitoring method, as both a detector of lesion occurrence and a method of mapping the position of ablated tissue.

Thermally-triggered 'off-on-off' response of gadolinium-hydrogel-lipid hybrid nanoparticles defines a customizable temperature window for non-invasive magnetic resonance imaging thermometry

For effective and safe thermotherapy, real-time, accurate, three-dimensional tissue thermometry is required. Magnetic resonance imaging (MRI)-based thermometry in combination with current temperature responsive contrast agents only provides an 'off-on' signal at a certain temperature, not indicating temperature increases beyond the desired therapeutic levels. To overcome this limitation, a novel Gd-chelated hydrogel-lipid hybrid nanoparticle (HLN) formulation was developed that provides an 'off-on-off' signal defining a thermometric window for MR thermometry. Novel thermally responsive poly(N-isopropylacrylamide-co-acrylamide) (NIPAM-co-AM) hydrogel nanoparticles (<15 nm) with bisallylamidodiethylenetriaminetriacetic acid, a novel crosslinker with Gd(3+) chelation functionality, were synthesized. The Gd-hydrogel nanoparticles were encapsulated in a solid lipid nanoparticle matrix that prevented T(1)-weighted contrast signal enhancement. Melting of the matrix lipid freed the Gd-hydrogel nanoparticles into the bulk water and an 'off-on' contrast signal enhancement occurred. As the temperature was further increased to temperatures greater than, the volume phase transition temperature of the hydrogel nanoparticles, they collapsed and provided an 'on-off' signal diminution. Both the 'off-on' and the 'on-off' transition temperature could be tailored by changing the lipid matrix and altering the NIPAM/AM ratio in the hydrogel, respectively. This allowed MRI thermometry of different temperature windows using the Gd-HLN system.

A liver-mimicking MRI phantom for thermal ablation experiments

PURPOSE

To develop a liver-mimicking MRI gel phantom for use in the development of temperature mapping and coagulation progress visualization tools needed for the thermal tumor ablation methods, including laser-induced interstitial thermotherapy (LITT) and radiofrequency ablation (RFA).

METHODS

A base solution with an acrylamide concentration of 30 vol. % was prepared. Different components were added to the solution; among them are bovine hemoglobin and MR signal-enhancing contrast agents (Magnevist as T1 and Lumirem as T2 contrast agent) for adjustment of the optical absorption and MR relaxation times, respectively. The absorption was measured in samples with various hemoglobin concentrations (0%-7.5%) at different temperatures (25-80 degrees C) using the near-infrared spectroscopy, measuring the transmitted radiation through the sample. The relaxation times were measured in samples with various concentrations of T1 (0.025%-0.325%) and T2 (0.4%-1.6%) contrast agents at different temperatures (25-75 degrees C), through the MRI technique, acquiring images with specific sequences. The concentrations of the hemoglobin and contrast agents of the gel were adjusted so that its absorption coefficient and relaxation times are equivalent to those of liver. To this end, the absorption and relaxation times of the gel samples were compared to reference values, measured in an ex vivo porcine liver at different temperatures through the same methods used for the gel. For validation of the constructed phantom, the absorption and relaxation times were measured in samples containing the determined amounts of the hemoglobin and contrast agents and compared with the corresponding liver values. To qualitatively test the heat resistance of the phantom, it was heated with the LITT method up to approximately 120 degrees C and then was cut to find out if it has been melted.

RESULTS

In contrast to liver, where the absorption change with temperature showed a sigmoidal form with a jump at T approximately equal 45 degrees C, the absorption of the gel varied slightly over the whole temperature range. However, the gel absorption presented a linear increase from approximately 1.8 to approximately 2.2 mm(-1) with the rising hemoglobin concentration. The gel relaxation times showed a linear decrease with the rising concentrations of the respective contrast agents. Conversely, with the rising temperature, both T1 and T2 increased linearly and showed almost the same trends as in liver. The concentrations of hemoglobin and T1 and T2 contrast agents were determined as 3.92 +/- 0.42 vol. %, 0.098 +/- 0.023 vol. %, and 2.980 +/- 0.067 vol. %, respectively. The measured ex vivo liver T1 value increased from approximately 300 to approximately 530 ms and T2 value from approximately 45 to approximately 52 ms over the temperature range. The phantom validation experiments resulted in absorption coefficients of 2.0-2.1 mm(-1) with variations of 1.5%-2.95% compared to liver below 50 degrees C, T1 of 246.6-597.2 ms and T2 of 40.8-67.1 ms over the temperature range of 25-75 degrees C. Using the Bland-Altman analysis, a difference mean of -6.1/1.9 ms was obtained for T1/T2 between the relaxation times of the phantom and liver. After heating the phantom with LITT, no evidence of melting was observed.

CONCLUSIONS

The constructed phantom is heat-resistant and MR-compatible and can be used as an alternative to liver tissue in the MR-guided thermal ablation experiments with laser to develop clinical tools for real-time monitoring and controlling the thermal ablation progress in liver.

Monitoring of thermal therapy based on shear modulus changes: II. Shear wave imaging of thermal lesions

The clinical applicability of high-intensity focused ultrasound (HIFU) for noninvasive therapy is currently hampered by the lack of robust and real-time monitoring of tissue damage during treatment. The goal of this study is to show that the estimation of local tissue elasticity from shear wave imaging (SWI) can lead to a precise mapping of the lesion. HIFU treatment and monitoring were respectively performed using a confocal setup consisting of a 2.5-MHz single element transducer focused at 34 mm on ex vivo samples and an 8-MHz ultrasound diagnostic probe. Ultrasound-based strain imaging was combined with shear wave imaging on the same device. The SWI sequences consisted of 2 successive shear waves induced at different lateral positions. Each wave was created with pushing beams of 100 μs at 3 depths. The shear wave propagation was acquired at 17,000 frames/s, from which the elasticity map was recovered. HIFU sonications were interleaved with fast imaging acquisitions, allowing a duty cycle of more than 90%. Thus, elasticity and strain mapping was achieved every 3 s, leading to real-time monitoring of the treatment. When thermal damage occurs, tissue stiffness was found to increase up to 4-fold and strain imaging showed strong shrinkages that blur the temperature information. We show that strain imaging elastograms are not easy to interpret for accurate lesion characterization, but SWI provides a quantitative mapping of the thermal lesion. Moreover, the concept of shear wave thermometry (SWT) developed in the companion paper allows mapping temperature with the same method. Combined SWT and shear wave imaging can map the lesion stiffening and temperature outside the lesion, which could be used to predict the eventual lesion growth by thermal dose calculation. Finally, SWI is shown to be robust to motion and reliable in vivo on sheep muscle.

Real-time passive acoustic monitoring of HIFU-induced tissue damage

Thermal ablation by high-intensity focused ultrasound (HIFU) shows great promise as a noninvasive cancer therapy. This work proposes a novel method of real-time HIFU treatment monitoring that uses the passively monitored acoustic signal emanating from the focus during HIFU exposure. We performed 212 exposures in seven freshly excised ox livers using 1.067-MHz HIFU at a 95% duty cycle for a range of insonation durations and acoustic intensities. Acoustic emissions were recorded using a 15-MHz passive detector aligned confocally and coaxially with the HIFU transducer. Lesion presence and size were ascertained by slicing the tissue in the transverse and axial focal planes post exposure. Our results demonstrate that successful formation of HIFU lesions in ex vivo ox liver is highly correlated with the presence of pronounced dips in the magnitude of the received signal at integer harmonics of the insonation frequency. A detector based on this observation predicted lesioning with >80% accuracy in regimes that were very likely to create lesions (≥60 J of energy) and had an error rate of <6% for exposures that were too short to cause lesioning (≤1 s long). The overall sensitivity and specificity of the detector were 75.6% and 74.2%, respectively. The proposed detector could therefore provide a low-cost means of effectively monitoring clinical HIFU treatments passively and in real time.

Evolution from active surveillance to focal therapy in the management of prostate cancer

Organ-preserving therapies are widely accepted in many facets of medicine and, more recently, in oncology. For example, partial nephrectomy is now accepted as a preferred alternative over radical nephrectomy for small (up to 4 cm or T1) tumors. Focal therapy (FT) is another organ-preserving strategy applying energy (cryotherapy, laser ablation and/or high-intensity focused ultrasound) to destroy tumors while leaving the majority of the organ, surrounding tissue and structures unscathed and functional. Owing to the perceived multifocality of prostate cancer (PCa) technology limitations, in the past PCa was not considered suitable for FT. However, with the rise of active surveillance for the management of low-risk PCa in carefully selected patients, FT is emerging as an alternative. This is owing to technology improvements in imaging and energy-delivery systems to ablate tissue, as well as the realization that many men and clinicians still desire tumor control. With the postulated ability to ablate tumors with minimal morbidity, FT may have found a role in the management of PCa; the aim of FT a being long-term cancer control without the morbidity associated with radical therapies. Data for FT in PCa have been derived from case series and small Phase I trials, with larger cohort studies with longer follow-up having only just commenced. More data from large trials on the safety and efficacy of FT are required before this approach can be recommended in men with PCa. Importantly, studies must confirm that no viable cancer cells remain in the region of ablation. FT might eventually prove to be a ‘middle ground’ between active surveillance and radical treatment, combining minimal morbidity with cancer control and the potential for retreatment.

Imaging in interventional oncology

Medical imaging in interventional oncology is used differently than in diagnostic radiology and prioritizes different imaging features. Whereas diagnostic imaging prioritizes the highest-quality imaging, interventional imaging prioritizes real-time imaging with lower radiation dose in addition to high-quality imaging. In general, medical imaging plays five key roles in image-guided therapy, and interventional oncology, in particular. These roles are (a) preprocedure planning, (b) intraprocedural targeting, (c) intraprocedural monitoring, (d) intraprocedural control, and (e) postprocedure assessment. Although many of these roles are still relatively basic in interventional oncology, as research and development in medical imaging focuses on interventional needs, it is likely that the role of medical imaging in intervention will become even more integral and more widely applied. In this review, the current status of medical imaging for intervention in oncology will be described and directions for future development will be examined.

A real-time controller for sustaining thermally relevant acoustic cavitation during ultrasound therapy

A novel method for sustaining inertial cavitation during high-intensity focused ultrasound (HIFU) exposure in an agar-based tissue-mimicking material is presented. Inertial cavitation occurs during HIFU therapy when the local rarefaction pressure exceeds the inertial cavitation threshold of the insonated medium, and is characterized by broadband acoustic emissions which can be easily detected non-invasively using a passive cavitation detector (PCD). Under the right conditions, inertial cavitation has been previously shown to greatly enhance the rate of heat deposition by redistributing part of the energy carried at the fundamental HIFU frequency to higher frequencies, which are more readily absorbed by visco-elastic media such as soft tissue. However, in the absence of any cavitation control, inertial cavitation activity at the focus decays rapidly over a few seconds of exposure because of the combined effects of cavitation nuclei depletion, bubble dissolution, bubble-bubble interactions, increased vapor pressure caused by heating, and focal shielding caused by pre-focal bubble activity. The present work describes the design, validation, and testing of a real-time adaptive controller, with integrated passive localization capabilities, for sustaining inertial cavitation within the focal region of a HIFU transducer by modulation of the HIFU amplitude. Use of the controller in agar gel, originally at room temperature, has enabled therapeutically relevant temperatures in excess of 55°C to be maintained continuously in the focal region for more than 20 s using significantly less acoustic energy than is required to achieve the same temperature rise in the absence of cavitation control.

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.

Focal therapy in prostate cancer: modalities, findings and future considerations

Focal therapy is emerging as an alternative to active surveillance for the management of low-risk prostate cancer in carefully selected patients. The aim of focal therapy is long-term cancer control without the associated morbidity that plagues all radical therapies. Different energy modalities have been used to focally ablate cancer tissue, and available techniques include cryotherapy, laser ablation, high-intensity focused ultrasound and photodynamic therapy. The majority of evidence for focal therapy has come from case series and small phase I trials, and larger cohort studies with longer follow-up are only now being commenced. More data from large trials on the safety and efficacy of focal therapy are therefore required before this approach can be recommended in men with prostate cancer; in particular, studies must confirm that no viable cells remain in the region of ablation. Focal therapy might eventually prove to be a 'middle ground' between active surveillance and radical treatment, combining minimal morbidity with cancer control and the potential for re-treatment.

MRI-guided transurethral ultrasound therapy of the prostate gland using real-time thermal mapping: initial studies

OBJECTIVE

To confirm the correlation between planning and thermal injury of the prostate as determined by magnetic resonance imaging (MRI) and histology in canine and humans treated with transurethral ultrasound.

MATERIAL AND METHODS

Canine studies: 2 sets of in vivo studies were performed under general anesthesia in 1.5 T clinical MRI. Nine dogs were treated using single transducer; 8 dogs were treated using urethral applicator with multiple transducers. Rectal cooling was maintained. After initial imaging, a target boundary was selected and high-intensity ultrasound energy delivered. The spatial temperature distribution was measured continuously every 5 seconds with MR thermometry using the proton-resonant frequency shift method. The goal was to achieve 55 °C at the target boundary. After treatment, the prostate was harvested and fixed with adjoining tissue, including rectum. Temperature maps, anatomical images, and histologic sections were registered to each other and compared. Human studies: To date, 5 patients with localized prostate cancer have been treated immediately before radical prostatectomy. Approximately 30% of the gland volume was targeted.

RESULTS

A continuous pattern of thermal coagulation was successfully achieved within the target region, with an average spatial precision of 1-2 mm. Radical prostatectomy was routine, with an uncomplicated postoperative course in all patients. The correlation between anatomical, thermal, and histologic images was ≤3 mm. Treatment time was <30 minutes. No thermal damage to rectal tissue was observed.

CONCLUSIONS

Thermal ablation within the prescribed target of the prostate has been successfully demonstrated in canine studies. The treatment is also feasible in humans.

Laser-induced thermotherapy (LITT)--evaluation of a miniaturised applicator and implementation in a 1.0-T high-field open MRI applying a porcine liver model

OBJECTIVE

To evaluate the feasibility and safety of a novel LITT applicator for thermal ablation of liver malignancies in 1.0-T high-field open MRI.

METHODS

A miniaturised 6-F double-tubed protective catheter with a closed cooling circuit was used with a flexible laser fibre, connected to a 1,064-nm Nd:YAG laser and evaluated in non-perfused porcine livers (18-30 W for 10-20 min, 2-W and 2-min increments; n = 210/applicator) in reference to an established 9-F system. As a proof of concept, MR-guided LITT was performed in two healthy domestic pigs in high-field open MRI.

RESULTS

Ex-vivo, the coagulation volumes induced by the 6-F system with maximum applicable power of 24 W for 20 min (33.0 ± 4.4 cm(3)) did not differ significantly from those set with the 9-F system at 30 W for 20 min (35.8 ± 4.9 cm(3)) (p = 0.73). A flow-rate of 15 ml/min of the cooling saline solution was sufficient. MR navigation and thermometry were feasible.

CONCLUSION

The miniaturised 6-F applicator can create comparable coagulation sizes to those of the 9-F system. Applicator guidance and online-thermometry in high-field open MRI are feasible.

Electrode displacement strain imaging of thermally-ablated liver tissue in an in vivo animal model

PURPOSE

Percutaneous thermal ablation is increasingly being used to destroy hepatic tumors in situ. The success of ablative techniques is highly dependent on adequate ablation zone monitoring, and ultrasound-based strain imaging could become a convenient and cost-effective means to delineate ablation zone boundaries. This study investigates in vivo electrode displacement-based strain imaging for monitoring hepatic ablation procedures that are difficult to perform with conventional elastography.

METHODS

a In our method, minute displacements (less than a millimeter) are applied to the unconstrained end of the ablation electrode, resulting in localized tissue deformation within the ablation zone that provides the mechanical stimuli required for strain imaging. This article presents electrode displacement strain images of radiofrequency ablation zones created in porcine liver in vivo (n = 13).

RESULTS

Cross-sectional area measurements from strain images of these ablation zones were obtained using manual and automated segmentation. Area measurements from strain images were highly correlated with areas measured on histopathology images, quantitated using linear regression (R = 0.894, P < 0.001 and R = 0.828, P < 0.001, respectively).

CONCLUSIONS

This study further demonstrates that electrode displacement elastography is capable of providing high-contrast images using widely available commercial ultrasound systems which may potentially be used to assess the extent of thermal ablation zones.

Periodic contrast-enhanced computed tomography for thermal ablation monitoring: a feasibility study

Image-guided tumor ablation is rapidly gaining acceptance for treating many tumors. While imaging diagnosis, treatment targeting and follow-up continue to improve, little progress has been made in developing practical imaging techniques for monitoring ablation treatments. In this study we demonstrate the feasibility of using contrast-enhanced computed tomography (CECT) to monitor ablation zone growth with 2 min temporal resolution. Highly constrained back-projection (HYPR) post-processing is applied to the time-series of CECT images, improving image quality by a factor of four after acquiring ten time frames. Such improvements limit the amount of radiation and iodinated contrast material required to visualize the ablation zone, especially at early time points. Additional study of periodic CECT with HYPR processing appears warranted.

Theoretical Modeling for Hepatic Microwave Ablation

Thermal tissue ablation is an interventional procedure increasingly being used for treatment of diverse medical conditions. Microwave ablation is emerging as an attractive modality for thermal therapy of large soft tissue targets in short periods of time, making it particularly suitable for ablation of hepatic and other tumors. Theoretical models of the ablation process are a powerful tool for predicting the temperature profile in tissue and resultant tissue damage created by ablation devices. These models play an important role in the design and optimization of devices for microwave tissue ablation. Furthermore, they are a useful tool for exploring and planning treatment delivery strategies. This review describes the status of theoretical models developed for microwave tissue ablation. It also reviews current challenges, research trends and progress towards development of accurate models for high temperature microwave tissue ablation.

Theoretical modeling for hepatic microwave ablation

Thermal tissue ablation is an interventional procedure increasingly being used for treatment of diverse medical conditions. Microwave ablation is emerging as an attractive modality for thermal therapy of large soft tissue targets in short periods of time, making it particularly suitable for ablation of hepatic and other tumors. Theoretical models of the ablation process are a powerful tool for predicting the temperature profile in tissue and resultant tissue damage created by ablation devices. These models play an important role in the design and optimization of devices for microwave tissue ablation. Furthermore, they are a useful tool for exploring and planning treatment delivery strategies. This review describes the status of theoretical models developed for microwave tissue ablation. It also reviews current challenges, research trends and progress towards development of accurate models for high temperature microwave tissue ablation.

Interactive real-time mapping and catheter ablation of the cavotricuspid isthmus guided by magnetic resonance imaging in a porcine model

Aims

We investigated the feasibility of real-time magnetic resonance imaging (RTMRI) guided ablation of the cavotricuspid isthmus (CTI) by using a MRI-compatible ablation catheter.

Methods and results

Cavotricuspid isthmus ablation was performed in an interventional RTMRI suite by using a novel 7 French, steerable, non-ferromagnetic ablation catheter in a porcine in vivo model (n = 20). The catheter was introduced and navigated by RTMRI visualization only. Catheter position and movement during manipulation were continuously visualized during the entire intervention. Two porcine prematurely died due to VT/VF. Anatomical completion of the CTI ablation line could be achieved after a mean of 6.3±3 RF pulses (RF energy: 1807±1016.4 Ws/RF pulse, temperature: 55.9±5.9°C) in n = 18 animals. In 15 of 18 procedures (83.3%) a complete CTI block was proven by conventional mapping in the electrophysiological (EP) lab.

Conclusion

Completely non-fluoroscopic ablation guided by RTMRI using a steerable and non-ferromagnetic catheter is a promising novel technology in interventional electrophysiology.

Radiofrequency ablation of small liver malignancies under magnetic resonance guidance: progress in targeting and preliminary observations with temperature monitoring

OBJECTIVES

To evaluate the feasibility and effectiveness of magnetic resonance (MR)-guided radiofrequency (RF) ablation for small liver tumours with poor conspicuity on both contrast-enhanced ultrasonography (US) and computed tomography (CT), using fast navigation and temperature monitoring.

METHODS

Sixteen malignant liver nodules (long-axis diameter, 0.6-2.4 cm) were treated with multipolar RF ablation on a 1.5-T wide-bore MR system in ten patients. Targeting was performed interactively, using a fast steady-state free precession sequence. Real-time MR-based temperature mapping was performed, using gradient echo-echo planar imaging (GRE-EPI) and hardware filtering. MR-specific treatment data were recorded. The mean follow-up time was 19 +/- 7 months.

RESULTS

Correct placement of RF electrodes was obtained in all procedures (image update, <500 ms; mean targeting time, 21 +/- 11 min). MR thermometry was available for 14 of 16 nodules (88%) with an accuracy of 1.6 degrees C in a non-heated region. No correlation was found between the size of the lethal thermal dose and the ablation zone at follow-up imaging. The primary and secondary effectiveness rates were 100% and 91%, respectively.

CONCLUSIONS

RF ablation of small liver tumours can be planned, targeted, monitored and controlled with MR imaging within acceptable procedure times. Temperature mapping is technically feasible, but the clinical benefit remains to be proven.

Evaluation of magnetic resonance imaging-compatible needles and interactive sequences for musculoskeletal interventions using an open high-field magnetic resonance imaging scanner

In this article, we study in vitro evaluation of needle artefacts and image quality for musculoskeletal laser-interventions in an open high-field magnetic resonance imaging (MRI) scanner at 1.0T with vertical field orientation. Five commercially available MRI-compatible puncture needles were assessed based on artefact characteristics in a CuSO4 phantom (0.1%) and in human cadaveric lumbar spines. First, six different interventional sequences were evaluated with varying needle orientation to the main magnetic field B0 (0 degrees to 90 degrees ) in a sequence test. Artefact width, needle-tip error, and contrast-to-noise ratio (CNR) were calculated. Second, a gradient-echo sequence used for thermometric monitoring was assessed and in varying echo times, artefact width, tip error, and signal-to-noise ratio (SNR) were measured. Artefact width and needle-tip error correlated with needle material, instrument orientation to B0, and sequence type. Fast spin-echo sequences produced the smallest needle artefacts for all needles, except for the carbon fibre needle (width <3.5 mm, tip error <2 mm) at 45 degrees to B0. Overall, the proton density-weighted spin-echo sequences had the best CNR (CNR(Muscle/Needle) >16.8). Concerning the thermometric gradient echo sequence, artefacts remained <5 mm, and the SNR reached its maximum at an echo time of 15 ms. If needle materials and sequences are accordingly combined, guidance and monitoring of musculoskeletal laser interventions may be feasible in a vertical magnetic field at 1.0T.