Can we ablate liver lesions close to large portal and hepatic veins with MR-guided HIFU? An experimental study in a porcine model.

OBJECTIVES: Invasive treatment of tumors adjacent to large hepatic vessels is a continuous clinical challenge. The primary aim of this study was to examine the feasibility of ablating liver tissue adjacent to large hepatic and portal veins with magnetic resonance imaging-guided high-intensity focused ultrasound (MRgHIFU). The secondary aim was to compare sonication data for ablations performed adjacent to hepatic veins (HV) versus portal veins (PV). MATERIALS AND METHODS: MRgHIFU ablations were performed in six male land swine under general anesthesia. Ablation cells of either 4 or 8 mm diameter were planned in clusters (two/animal) adjacent either to HV (n = 6) or to PV (n = 6), with diameter ≥ 5 mm. Ablations were made using 200 W and 1.2 MHz. Post-procedure evaluation was made on contrast-enhanced MRI (T1w CE-MRI), histopathology, and ablation data from the HIFU system. RESULTS: A total of 153 ablations in 81 cells and 12 clusters were performed. There were visible lesions with non-perfused volumes in all animals on T1w CE-MRI images. Histopathology showed hemorrhage and necrosis in all 12 clusters, with a median shortest distance to vessel wall of 0.4 mm (range 0-2.7 mm). Edema and endothelial swelling were observed without vessel wall rupture. In 8-mm ablations (n = 125), heat sink was detected more often for HV (43%) than for PV (19%; p = 0.04). CONCLUSIONS: Ablations yielding coagulative necrosis of liver tissue can be performed adjacent to large hepatic vessels while keeping the vessel walls intact. This indicates that perivascular tumor ablation in the liver is feasible using MRgHIFU. KEY POINTS: • High-intensity focused ultrasound ablation is a non-invasive treatment modality that can be used for treatment of liver tumors. • This study shows that ablations of liver tissue can be performed adjacent to large hepatic vessels in an experimental setting. • Liver tumors close to large vessels can potentially be treated using this modality.

A theoretical framework for determining cerebral vascular function and heterogeneity from dynamic susceptibility contrast MRI.

Mapping the complex heterogeneity of vascular tissue in the brain is important for understanding cerebrovascular disease. In this translational study, we build on previous work using vessel architectural imaging (VAI) and present a theoretical framework for determining cerebral vascular function and heterogeneity from dynamic susceptibility contrast magnetic resonance imaging (MRI). Our tissue model covers realistic structural architectures for vessel branching and orientations, as well as a range of hemodynamic scenarios for blood flow, capillary transit times and oxygenation. In a typical image voxel, our findings show that the apparent MRI relaxation rates are independent of the mean vessel orientation and that the vortex area, a VAI-based parameter, is determined by the relative oxygen saturation level and the vessel branching of the tissue. Finally, in both simulated and patient data, we show that the relative distributions of the vortex area parameter as a function of capillary transit times show unique characteristics in normal-appearing white and gray matter tissue, whereas tumour-voxels in comparison display a heterogeneous distribution. Collectively, our study presents a comprehensive framework that may serve as a roadmap for in vivo and per-voxel determination of vascular status and heterogeneity in cerebral tissue.

Enhanced nutrient supply to very low birth weight infants is associated with improved white matter maturation and head growth.

BACKGROUND: Extrauterine growth restriction is common among very low birth weight infants (VLBW, BW <1,500 g). Optimal postnatal nutrient supply is essential to limit growth restriction and ensure adequate growth and neurodevelopment. OBJECTIVES: We compared an enhanced postnatal nutrient supply to a standard supply and evaluated the effects on growth velocity, head circumference growth and cerebral maturation - the latter by magnetic resonance diffusion tensor imaging (DTI). We hypothesized increased growth velocity, head circumference growth and decreased mean diffusivity (MD) in cerebral white matter (WM) areas, suggesting improved cerebral maturation among infants on the enhanced nutrient supply. METHODS: In this randomized controlled trial, infants on the enhanced nutrient supply received increased amounts of energy, protein, fat, essential fatty acids and vitamin A until discharge. DTI was performed close to term equivalent age. Outcomes were growth velocity, head circumference growth and WM mean diffusivity. RESULTS: Among the 50 included infants, 14 in the intervention group and 11 controls underwent a successful DTI. Infants on the enhanced diet achieved improved growth velocity (16.5 vs. 13.8 g/kg/day, p = 0.01) and increased head circumference (Δz score: 0.24 vs. -0.12, p = 0.15). A significantly lower MD was seen in a large WM area such as the superior longitudinal fasciculi (1.19 × 10(-3) vs. 1.24 × 10(-3) mm(2)/s, p = 0.04, adjusted for age when scanned). CONCLUSIONS: Enhanced nutrient supply to VLBW infants is associated with improved growth velocity, increased head circumference growth and decreased regional WM mean diffusivity, suggesting improved maturation of cerebral connective tracts.

MRI of axillary brachial plexus blocks: a randomised controlled study.

BACKGROUND: Axillary plexus blocks are usually guided by ultrasound, but alternative methods may be used when ultrasound equipment is lacking. For a nonultrasound-guided axillary block, the need for three injections has been questioned. OBJECTIVES: Could differences in block success between single, double and triple deposits methods be explained by differences in local anaesthetic distribution as observed by MRI? DESIGN: A blinded and randomised controlled study. SETTING: Conducted at Oslo University Hospital, Rikshospitalet, Norway from 2009 to 2011. PATIENTS: Forty-five ASA 1 to 2 patients scheduled for surgery were randomised to three equally sized groups. All patients completed the study. INTERVENTIONS: Patients in the single-deposit group had an injection through a catheter parallel to the median nerve. In the double-deposit group the patients received a transarterial block. In the triple-deposit group the injections of the two other groups were combined. Upon completion of local anaesthetic injection the patients were scanned by MRI, before clinical block assessment. The distribution of local anaesthetic was scored by its closeness to terminal nerves and cords of the brachial plexus, as seen by MRI. The clinical effect was scored by the degree of sensory block in terminal nerve innervation areas. MAIN OUTCOME MEASURES: Sensory block effect and MRI distribution pattern. RESULTS: The triple-deposit method had a higher success rate (100%) than the single-deposit method (67%) and the double-deposit method (67%) in blocking all cutaneous nerves distal to the elbow (P = 0.04). The patients in the triple-deposit group most often had the best MRI scores. For any nerve or cord, at least one of the single-deposit or double-deposit groups had a similarly high MRI score as the triple-deposit group. CONCLUSION: Distal to the elbow, the triple-deposit method had the highest sensory block success rate. This could be explained to some extent by analysis of the magnetic resonance images. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01033006.

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.

Catheter tip tracking for MR-guided interventions using discrete Kalman filter and mean shift localization.

PURPOSE: This paper presents and evaluates stochastic computer algorithms used to automatically detect and track marked catheter tip during MR-guided catheterization. The algorithms developed employ extraction and matching of regional features of the catheter tip to perform the localization. METHOD: To perform the tracking, a probability map that indicates the possible locations of the catheter tip in the MR images is first generated. This map is generated from the similarity to a given marker template. The method to assess the similarity between the marker template image and the different positions on each MR frame is based on speeded-up robust features extracted from the gradient image. The probability map is then used in two different stochastic localization frameworks mean shift (MS) localization and Kalman filter (KF) to track the position of the catheter using pairs of orthogonal projection of 2D MR images. The algorithm developed was tested on catheter tip marked with LC resonant circuit (of size 2 mm x 2 cm) tuned to the Larmor frequency of the MRI scanner and for different image resolutions (1, 3, 5 and 7 mm squared pixel). RESULTS: The tracking performance was very robust for the two algorithms MS and KF with image resolution as low as 3 mm where the localization error was about 1 mm for KF and 0.9 mm for MS. For the 5-mm resolution images, the error was 2.2 mm for both KF and MS, and for the 7-mm resolution images, the error was 3.6 and 3.7 mm for KF and MS, respectively. CONCLUSION: Both KF and MS gave comparable results when it comes to accuracy for the different image resolutions. The results showed that the two tracking algorithms tracked the catheter tip with high robustness for image resolution of 3 mm and with acceptable reliability for image resolution as poor as 5 mm with the resonant marker configuration used.

T2*-correction in dynamic contrast-enhanced MRI from double-echo acquisitions.

PURPOSE: To evaluate the importance of T2*-effects on the arterial input function (AIF) and on the resulting dynamic parameter estimation in dynamic contrast-enhanced (DCE) MRI of high-grade gliomas. MATERIALS AND METHODS: Seven patients with high-grade gliomas were imaged in total 50 times using a double-echo DCE sequence. Kinetic analysis using the extended Tofts model was performed using AIFs with and without correction for T2*-effects, and the resulting estimates of the transfer constant (K(trans) ), blood plasma volume (vp ), and the rate constant (kep ) were compared. Numerical simulations were done for comparison with clinical results as well as to further investigate the dependency of parameter values on the magnitude of T2*-induced errors. RESULTS: All kinetic parameters were found to be overestimated if T2*-effects in the AIF were not accounted for; with vp being most severely affected. The relative error in each parameter was dependent on the absolute parameter magnitude, resulting in incorrect parametric tumor distributions in the presence of uncorrected AIF T2*-effects. CONCLUSION: In DCE, a sufficiently short echo time should be used or corrections for T2*-effects based on double-echo acquisition should be made for correct quantification of kinetic parameters.

Volumetric ablation of uterine fibroids using Sonalleve high-intensity focused ultrasound in a 3 Tesla scanner--first clinical assessment.

INTRODUCTION: The purpose of this study was to evaluate the feasibility and safety of the Sonalleve high-intensity focused ultrasound (HIFU; Philips Healthcare, Vantaa, Finland) system in ablating uterine fibroids in a 3T magnet. MATERIAL AND METHODS: Seven women were included in this study. Treatment was performed according to the manufacturer's recommendation. Technical data describing the HIFU procedures were collected. On MR images at baseline, immediately and 30 days after ablation, we evaluated the volumes of the uterus, the dominant fibroid and the ablation zone. The Uterine Fibroid Symptom and Quality of Life (UFS-QOL) questionnaire was used to assess potential clinical response. RESULTS: The procedure was technically feasible in all patients. The median number of sonications performed during each procedure was 20 (range 2-27) per patient, the maximum temperature in all sonication cells was about 68°C. The median procedure time was 156 minutes (range 95-164). The non-perfused volume after treatment ranged from 1 to 27 ml and was unchanged or decreased in all but one patient at 30 days follow-up. There were no major adverse events. DISCUSSION: In our 3T magnet the system was able to heat tissue and induce areas of non-enhancement within uterine fibroids without major complications. Clinical benefit remains to be proven.

Anatomical standardization of small animal brain FDG-PET images using synthetic functional template: experimental comparison with anatomical template.

Anatomical standardization (also called spatial normalization) of positron emission tomography (PET) small animal brain images is required to make statistical comparisons across individuals. Frequently, PET images are co-registered to an individual MR or CT image of the same subject in order to transform the functional images to an anatomical space. In the present work, we evaluate the normalization of synthetic PET (synPET) images to a synthetic PET template. To provide absolute error in terms of pixel misregistration, we created a synthetic PET image from the individual MR image through segmentation of the brain into gray and white matter which produced functional and anatomical images in the same space. When comparing spatial normalization of synPET images to a synPET template with the gold standard (MR images to an MR template), a mean translation error of 0.24mm (±0.20) and a maximal mean rotational error of 0.85° (±0.91) were found. Significant decrease in misregistration error was measured when achieving spatial normalization of functional images to a functional template instead of an anatomical template. This accuracy strengthens the use of standardization methods where individual PET images are registered to a customized PET template in order to statistically assess physiological changes in rat brains.

Measurement of brain perfusion, blood volume, and blood-brain barrier permeability, using dynamic contrast-enhanced T(1)-weighted MRI at 3 tesla.

Assessment of vascular properties is essential to diagnosis and follow-up and basic understanding of pathogenesis in brain tumors. In this study, a procedure is presented that allows concurrent estimation of cerebral perfusion, blood volume, and blood-brain permeability from dynamic T(1)-weighted imaging of a bolus of a paramagnetic contrast agent passing through the brain. The methods are applied in patients with brain tumors and in healthy subjects. Perfusion was estimated by model-free deconvolution using Tikhonov's method (gray matter/white matter/tumor: 72 +/- 16/30 +/- 8/56 +/- 45 mL/100 g/min); blood volume (6 +/- 2/4 +/- 1/7 +/- 6 mL/100 g) and permeability (0.9 +/- 0.4/0.8 +/- 0.3/3 +/- 5 mL/100 g/min) were estimated by using Patlak's method and a two-compartment model. A corroboration of these results was achieved by using model simulation. In addition, it was possible to generate maps on a pixel-by-pixel basis of cerebral perfusion, cerebral blood volume, and blood-brain barrier permeability.

Automated assessment of whole-body adipose tissue depots from continuously moving bed MRI: a feasibility study.

PURPOSE: To present an automated algorithm for segmentation of visceral, subcutaneous, and total volumes of adipose tissue depots (VAT, SAT, TAT) from whole-body MRI data sets and to investigate the VAT segmentation accuracy and the reproducibility of all depot assessments. MATERIALS AND METHODS: Repeated measurements were performed on 24 volunteer subjects using a 1.5 Tesla clinical MRI scanner and a three-dimensional (3D) multi-gradient-echo sequence (resolution: 2.1 x 2.1 x 8 mm(3), acquisition time: 5 min 15 s). Fat and water images were reconstructed, and fully automated segmentation was performed. Manual segmentation of the VAT reference was performed by an experienced operator. RESULTS: Strong correlation (R = 0.999) was found between the automated and manual VAT assessments. The automated results underestimated VAT with 4.7 +/- 4.4%. The accuracy was 88 +/- 4.5% and 7.6 +/- 5.7% for true positive and false positive fractions, respectively. Coefficients of variation from the repeated measurements were: 2.32 % +/- 2.61%, 2.25% +/- 2.10%, and 1.01% +/- 0.74% for VAT, SAT, and TAT, respectively. CONCLUSION: Automated and manual VAT results correlated strongly. The assessments of all depots were highly reproducible. The acquisition and postprocessing techniques presented are likely useful in obesity related studies.

Lipid content in the musculature of the lower leg: evaluation with high-resolution spectroscopic imaging.

A novel spectroscopic imaging method with high spectral and spatial resolution was developed for the specific goal of assessing muscle fat. Sensitivity to the methylene and methyl protons of fatty acids was improved by the use of a binomial 1 1 excitation pulse instead of the standard radiofrequency (RF) pulse. Acceptable measurement time is achieved by using a narrow spectral bandwidth (6 ppm). The spectral resolution is sufficient to resolve extramyocellular (EMCL) and intramyocellular (IMCL) lipids. A post-detection data processing scheme that permits correction of spectral artifacts caused by chemical shifts, spectral line aliasing, and magnetic field inhomogeneities is suggested. The lipid content in different lower leg muscles was evaluated. Muscle fiber orientation was taken into account in assessing quantities of EMCL and IMCL. The proposed technique allows small amounts of inhomogeneously distributed muscle lipids to be quantified.

Pediatric excretory MR urography: comparative study of enhanced and non-enhanced techniques.

Our purpose was to compare the quality of ureteral imaging in pediatric patients using two different MR sequences: the non-enhanced heavily T2-weighted (W) turbo spin-echo sequence (TSE) and the gadolinium-enhanced T1W fast-field-echo sequence (T1 FFE). An experimental study on three pigs was first performed. The TSE, before and after furosemide injection, was followed by the T1 FFE sequence. The clinical study included 11 infants and 10 children. With some modifications the same MR parameters and techniques were used as in the animal study. The TSE with TR 8000 ms and TE 650 ms implied 6 radial stacks each of 40 mm thickness. The T1 FFE included TR 18 ms, TE 2.9 ms, flip angle 60, and 50 slices with thickness 0.7 mm. After post-processing, image reconstructions qualitative and quantitative analysis were performed. Complete visualization of the ureters was achieved in 35 of 42 (83%) cases. Seventy-four percent of the ureters were completely visualized with T1 FFE compared with only 19% with TSE. Sixty-nine percent of the ureters were better imaged with T1 FFE than TSE and 21% equally well imaged. Four ureters (10%), either obstructed or due to poor renal function, were better imaged with TSE. The two sequences are complementary. Visualization of non-obstructed ureters is excellent with T1 FFE and the sequence is superior to TSE. The TSE, however, may be equal to or even better than T1 FFE in visualizing obstructed ureters or ureters draining malfunctioning kidneys.