Jörg Roland

Accuracy of real-time MR temperature mapping in the brain: A comparison of fast sequences

PURPOSE To compare magnetic resonance (MR) thermometry based on the proton resonance frequency (PRF) method using a single shot echoplanar imaging (ss EPI) sequence to both of the standard sequences, gradient echo (GRE) and segmented echoplanar imaging (seg EPI) in the in vivo human brain, at 1.5T and 3T. MATERIAL AND METHODS Repetitive MR thermometry was performed on the brain of six volunteers using GRE, seg EPI, and ss EPI sequences on whole-body 1.5T and 3T clinical systems using comparable acquisition parameters. Phase stability and temperature data precision in the human head were determined over 12 min for the three sequences at both field strengths. An ex-vivo swine skeletal muscle model was used to evaluate temperature accuracy of the ss EPI sequence during heating by high intensity focused ultrasound (HIFU). RESULTS In-vivo examinations of brain revealed an average temperature precision of 0.37 °C/0.39 °C/0.16 °C at 3T for the GRE/seg EPI/ss EPI sequences. At 1.5T, a precision of 0.58 °C/0.63 °C/0.21 °C was achieved. In the ex-vivo swine model, a strong correlation of temperature data derived using ss EPI and GRE sequences was found with a temperature deviation <1 °C. CONCLUSION The ss EPI sequence was the fastest and the most precise sequence for MR thermometry, with significantly higher accuracy compared to GRE.

Prediction of cell necrosis with sequential temperature mapping after radiofrequency ablation.

To assess the feasibility of magnetic resonance (MR) thermometry after thermoablative therapy and to quantitatively evaluate the ability of two sequence types to predict cell necrosis.

A pilot study for clinical feasibility of the near-harmonic 2D referenceless PRFS thermometry in liver under free breathing using MR-guided LITT ablation data

Objectives: The conventional implementations of proton resonance frequency shift (PRFS) magnetic resonance thermometry (MRT) require the subtraction of single or multiple temporal references, a motion sensitive critical feature. A pilot study was conducted here to investigate the clinical feasibility of near-harmonic two-dimensional (2D) referenceless PRFS MRT, using patient data from MR-guided laser ablation of liver malignancies. Methods: PRFS MRT with respiratory-triggered multi-slice gradient-recalled (GRE) acquisition was performed under free breathing in six patients. The precision of the novel referenceless MRT was compared with the reference phase subtraction. Coupling the referenceless MRT with a model-based, real-time compatible regularisation algorithm was also investigated. Results: The precision of MRT was improved by a factor of 3.3 when using the referenceless method as compared to the reference phase subtraction. The approach combining referenceless PRFS MRT and model-based regularisation yielded an estimated precision of 0.7° to 2.1°C, resulting in millimetre-range agreement between the calculated thermal dose and the 24 h post-treatment unperfused regions in liver. Conclusions: The application of the near-harmonic 2D referenceless MRT method was feasible in a clinical scenario of MR-guided laser-induced thermal therapy (LITT) ablation in liver and permitted accurate prediction of the thermal lesion under free breathing in conscious patients, obviating the need for a controlled breathing under general anaesthesia.

Four-dimensional temperature distributions in red blood cells withdrawn from storage and exposed to ambient temperature: a magnetic resonance thermometry study

BACKGROUND: Recommended by current guidelines, red blood cell (RBC) temperature should not exceed 10°C during transport. Since warming is a generically three‐dimensional process that is not homogeneous, it is necessary to clarify the term “temperature during warming.” The purpose of this study was therefore to investigate laws and relations between surface, mean, and core temperature and the corresponding times when they exceed 10°C during warm‐up.

Thermometry of red blood cell concentrate: magnetic resonance decoding warm up process.

Purpose Temperature is a key measure in human red blood cell concentrate (RBC) quality control. A precise description of transient temperature distributions in RBC units removed from steady storage exposed to ambient temperature is at present unknown. Magnetic resonance thermometry was employed to visualize and analyse RBC warm up processes, to describe time courses of RBC mean, surface and core temperatures by an analytical model, and to determine and investigate corresponding model parameters. Methods Warm-up processes of 47 RBC units stored at 1–6°C and exposed to 21.25°C ambient temperature were investigated by proton resonance frequency thermometry. Temperature distributions were visualized and analysed with dedicated software allowing derivation of RBC mean, surface and core temperature-time courses during warm up. Time-dependence of mean temperature was assumed to fulfil a lumped capacitive model of heat transfer. Time courses of relative surface and core temperature changes to ambient temperature were similarly assumed to follow shifted exponential decays characterized by a time constant and a relative time shift, respectively. Results The lumped capacitive model of heat transfer and shifted exponential decays described time-dependence of mean, surface and core temperatures close to perfect (mean R2 were 0.999±0.001, 0.996±0.004 and 0.998±0.002, respectively). Mean time constants were τ mean = 55.3±3.7 min, τ surface = 41.4±2.9 min and τ core = 76.8±7.1 min, mean relative time shifts were Δsurface = 0.07±0.02 and Δcore = 0.04±0.01. None of the constants correlated significantly with temperature differences between ambient and storage temperature. Conclusion Lumped capacitive model of heat transfer and shifted exponential decays represent simple analytical formulas to describe transient mean, surface and core temperatures of RBC during warm up, which might be a helpful tool in RBC temperature monitoring and quality control. Independence of constants on differences between ambient and storage temperature suggests validity of models for arbitrary storage and ambient temperatures.

Interventional MR-imaging for thermal ablation therapy

Magnetic Resonance Imaging (MRI) has several unique advantages for guiding thermal ablation therapies. It not only provides excellent soft-tissue contrast and multiplanar capabilities, but also is sensitive to thermal effects. To make full use of these advantages for thermal ablation procedures, we present an integrated solution for a thermal therapy workflow that combines dedicated MRI pulse sequences and visualization/analysis tools for trajectory planning, automatic slice positioning for image-guided needle placement, and advanced MR thermal mapping. The paper highlights a novel approach to detect the needle in real-time MR images and to automatically realign the scan planes. In addition, a global approach to correct for B0 field shift during online MR thermometry is introduced. The application has been implemented using the open-source eXtensible Imaging Platform (XIP).

Real‐ Time Magnetic Resonance Temperature Mapping in the Brain

Repetitive MR thermometry was performed on the brain of nine volunteers using GRE, seg EPI, and ss EPI sequences on whole‐body 1.5 T and 3 T clinical systems using comparable acquisition parameters. Phase stability and temperature data precision in the human head were determined over 12 minutes for the three sequences at both field strengths. The ss EPI sequence was the fastest and the most precise sequence for MR thermometry, with significantly higher accuracy compared to GRE. An ex‐vivo swine skeletal muscle model was used to evaluate temperature accuracy of the ss EPI sequence during heating by high intensity focused ultrasound (HIFU).