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, 0.6-2.4cm) 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 ± 7months. Results: Correct placement of RF electrodes was obtained in all procedures (image update, <500ms; mean targeting time, 21 ± 11min). MR thermometry was available for 14 of 16 nodules (88%) with an accuracy of 1.6°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 prove

Quantitative cardiac perfusion evaluation using first-pass contrast-enhanced magnetic resonance imaging

The measurement of the blood flow to the myocardium (or cardiac perfusion) is important for the clinical assessment of ischemia. Magnetic resonance imaging (MRI) may provide a technique for the quantitative assessment of cardiac perfusion that is potentially superior to other imaging modalities. However, to date, there is no standard method for the quantitative assessment of cardiac perfusion with MRI. This dissertation describes the development and implementation of a method for measuring the absolute blood flow in the myocardium in ml/min/g of tissue. This method includes: modifications to the fast gradient echo-echo planar imaging pulse sequence typically used for the acquisition of a time series of T1-weighted images in first-pass contrast-enhanced perfusion studies; a novel theory-based image signal calibration approach; a study of the spatial homogeneity of the preconditioning saturation RF pulses used to achieve T1-weighting; the development of an integrated GUI-based software for image processing and analysis; a tracer kinetics model used to generate blood flow estimates from observed tracer concentration-time curves. Our novel theory-based image signal calibration method relies on normalization of the T1-weighted images by an initial proton density-weighted image, the use of robust saturation RF pulses for consistent TI weighting, and dual-delay-time acquisitions for improving the dynamic range of the arterial input function determination. The signal calibration technique is used to map signal-time curves that correspond to the dynamic image enhancement during the passage of the tracer bolus through the heart to T1-time curves, and then to corresponding [Gd]-time curves. The [Gd]-time curves for the left ventricular cavity and the myocardium represent the inputs to the tracer kinetics model, which is used to fit the observed data and generate blood flow estimates. The described quantitative cardiac perfusion measurement method is used to measure blood flow in preliminary in vivo studies. We obtain reasonable absolute blood flow for rest and stress, and we see a clear increase in myocardial blood flow during adenosine stress. We conclude that this method can constitute the basis for a future clinically-oriented quantitative evaluation of cardiac perfusion

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

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

SNR enhancement of highly-accelerated real-time cardiac MRI acquisitions based on non-local means algorithm

Real-time cardiac MRI appears as a promising technique to evaluate the mechanical function of the heart. However, ultra-fast MRI acquisitions come with an important signal-to-noise ratio (SNR) penalty, which drastically reduces the image quality. Hence, a real-time denoising approach would be desirable for SNR amelioration. In the clinical context of cardiac dysfunction assessment, long acquisitions are required and for most patients the acquisition takes place with free breathing. Hence, it is necessary to compensate respiratory motion in real-time. In this article, a real-time and interactive method for sequential registration and denoising of real-time MR cardiac images is presented. The method has been experimented on 60 fast MRI acquisitions in five healthy volunteers and five patients. These experiments assessed the feasibility of the method in a real-time context

Rapid dynamic R1/R2*/temperature assessment: a method with potential for monitoring drug delivery

Local drug delivery by hyperthermia-induced drug release from thermosensitive liposomes (TSLs) may reduce the systemic toxicity of chemotherapy, whilst maintaining or increasing its efficacy. Relaxivity contrast agents can be co-encapsulated with the drug to allow the visualization of the presence of liposomes, by means of R 2 *, as well as the co-release of the contrast agent and the drug, by means of R 1, on heating. Here, the mathematical method used to extract both R 2 * and R 1 from a fast dynamic multi-echo spoiled gradient echo (ME-SPGR) is presented and analyzed. Finally, this method is used to monitor such release events. R 2 * was obtained from a fit to the ME-SPGR data. Absolute R 1 was calculated from the signal magnitude changes corrected for the apparent proton density changes and a baseline Look–Locker R 1 map. The method was used to monitor nearly homogeneous water bath heating and local focused ultrasound heating of muscle tissue, and to visualize the release of a gadolinium chelate from TSLs in vitro. R 2 *, R 1 and temperature maps were measured with a 5-s temporal resolution. Both R 2 *and R 1 measured were found to change with temperature. The dynamic R 1 measurements after heating agreed with the Look–Locker R 1 values if changes in equilibrium magnetization with temperature were considered. Release of gadolinium from TSLs was detected by an R 1 increase near the phase transition temperature, as well as a shallow R 2 * increase. Simultaneous temperature, R 2 * and R 1 mapping is feasible in real time and has the potential for use in image-guided drug delivery studies

Rapid Dynamic Temperature/R1/R2* Assessment : A Method With Potential For Monitoring Drug Delivery

International audienc

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

Radiomics-based detection of acute myocardial infarction on noncontrast enhanced midventricular short-axis cine CMR images

Cardiac magnetic resonance cine images are primarily used to evaluate functional consequences, whereas limited information is extracted from the noncontrast pixel-wise myocardial signal intensity pattern. In this study we want to assess whether characterizing this inherent contrast pattern of noncontrast-enhanced short axis (SAX) cine images via radiomics is sufficient to distinguish subjects with acute myocardial infarction (AMI) from controls. Cine balanced steady-state free-precession images acquired at 1.5 T from 99 AMI and 49 control patients were included. First, radiomic feature extraction of the left ventricular myocardium of end-diastolic (ED) and end-systolic (ES) frames was performed based on automated (AUTO) or manually corrected (MAN) segmentations. Next, top features were selected based on optimal classification results using a support vector machine (SVM) approach. The classification performances of the four radiomics models (using AUTO or MAN segmented ED or ES images), were measured by AUC, classification accuracy (CA), F1-score, sensitivity and specificity. The most accurate model was found when combining the features RunLengthNonUniformity, ClusterShade and Median obtained from the manually segmented ES images (CA = 0.846, F1 score = 0.847). ED analysis performed worse than ES, with lower CA and F1 scores (0.769 and 0.770, respectively). Manual correction of automated contours resulted in similar model features as the automated segmentations and did not improve classification results. A radiomics analysis can capture the inherent contrast in noncontrast mid-ventricular SAX cine images to distinguishing AMI from healthy subjects. The ES radiomics model was more accurate than the ED model. Manual correction of the autosegmentation did not provide significant classification improvements.</p

MRI contrast variation of thermosensitive magnetoliposomes triggered by focused ultrasound: a tool for image-guided local drug delivery

Improved drug delivery control during chemotherapy is a major concern to increase their therapeutic index. Drug accumulation in solid tumor can be visualized using MRI contrast agent such as iron oxide nanoparticles encapsulated in liposomes. Once accumulated in tumor, the combination of a thermosensitive composition with an external source of activation allows local release of drug. MRI guided-High intensity focused ultrasound (HIFU) represents a non invasive technique to generate local hyperthermia for drug release of thermosensitive magnetoliposomes (TSM). In this study we performed encapsulation of ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) in thermosensitive liposomes to obtain TSM. Magnetic behavior of this MRI contrast agent was observed during TSM membrane permeabilization. For this, measurement of transverse and longitudinal relaxivities on MRI, and real time experiments were performed on TSM samples loaded with USPIO during heating using a water bath or HIFU. Results showed significant differences for MRI signal enhancement and relaxivities ratios before and after heating, which were absent for non-thermosensitive liposomes and free nanoparticles used as controls. Thus, incorporation of USPIO as MRI-contrast agents into thermosensitive liposomes should, besides TSM tumor accumulation, allows the visualization of TSM membrane permeabilization upon temperature elevation. In conclusion, HIFU under MR image guidance in combination with USPIO loaded thermosensitive liposomes as drug delivery system has the potential for a better control of drug delivery and to increase the therapeutic index of chemotherapy