Feasibility of fast MR-thermometry during cardiac radiofrequency ablation.

Online MR temperature monitoring during radiofrequency (RF) ablation of cardiac arrhythmias may improve the efficacy and safety of the treatment. MR thermometry at 1.5 T using the proton resonance frequency (PRF) method was assessed in 10 healthy volunteers under normal breathing conditions, using a multi-slice, ECG-gated, echo planar imaging (EPI) sequence in combination with slice tracking. Temperature images were post-processed to remove residual motion-related artifacts. Using an MR-compatible steerable catheter and electromagnetic noise filter, RF ablation was performed in the ventricles of two sheep in vivo. The standard deviation of the temperature evolution in time (TSD) was computed. Temperature mapping of the left ventricle was achieved at an update rate of approximately 1 Hz with a mean TSD of 3.6 ± 0.9 °C. TSD measurements at the septum showed a higher precision (2.8 ± 0.9 °C) than at the myocardial regions at the heart-lung and heart-liver interfaces (4.1 ± 0.9 °C). Temperature rose maximally by 9 °C and 16 °C during 5 W and 10 W RF applications, respectively, for 60 s each. Tissue temperature can be monitored at an update rate of approximately 1 Hz in five slices. Typical temperature changes observed during clinical RF application can be monitored with an acceptable level of precision.

Dual-echo Dixon imaging with flexible choice of echo times.

In this work, a new two-point method for water-fat imaging is described and explored. It generalizes existing two-point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two-point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient-echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two-point methods. Notably, water images with higher spatial resolution and better signal-to-noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in- and opposed-phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two-point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three-point methods in this way, especially toward the edges of larger field of views.

Three-dimensional contrast-enhanced hepatic MR imaging: comparison between a centric technique and a linear approach with partial Fourier along both slice and phase directions.

PURPOSE: To compare the image quality of two variants of a three-dimensional (3D) gradient echo sequence (GRE) for hepatic MRI. MATERIALS AND METHODS: Thirty-nine patients underwent hepatic MRI on a 3.0 Tesla (T) magnet (Intera Achieva; Philips Medical Systems). The clinical protocol included two variants of a 3D GRE with fat suppression: (i) a "centric" approach, with elliptical centric k-space ordering and (ii) an "enhanced" approach using linear sampling and partial Fourier in both the slice and phase encoding direction. "Centric" and "Enhanced" 3D GRE images were obtained both precontrast (n = 32) and after gadoxetic acid injection (n = 39). Two reviewers jointly reviewed MR images for anatomic sharpness, overall contrast, homogeneity, and absence of artifacts. The liver-to-lesion signal difference ratio (SDR) was measured. Paired sample Wilcoxon test and paired t-tests were used. RESULTS: Enhanced 3D GRE images performed better than centric 3D GRE images with respect to anatomic sharpness (P = 0.0156), overall contrast (P = 0.0195), homogeneity (P < 0.0001), and absence of artifacts (P = 0.0003) on precontrast images. For postcontrast MRI, enhanced 3D GRE images showed better quality in terms of overall contrast (P = 0.0195), homogeneity (P < 0.0001), and absence of artifacts (P = 0.009). Liver-to-lesion SDR on enhanced 3D GRE images (0.48 ± 0.13) was significantly higher than that of conventional 3D GRE images (0.40 ± 0.19, P = 0.0004) on postcontrast images, but not on precontrast images. CONCLUSION: The enhanced 3D GRE sequence available on our scanner provided better hepatic image quality than the centric variant, without compromising lesion contrast.

Diffusion-weighted magnetic resonance imaging of the liver using tracking only navigator echo: feasibility study.

PURPOSE: To introduce and assess the TRacking Only Navigator echo (TRON) technique for diffusion-weighted magnetic resonance imaging (DWI) of the liver. SUBJECTS AND METHODS: A total of 10 volunteers underwent TRON, respiratory triggered (RT), and free breathing (FB) DWI of the liver. Scan times of TRON and RT DWI were measured, and image sharpness in TRON, RT, and FB DWI was assessed and compared using nonparametric tests. Furthermore, 14 patients with liver metastasis who had undergone TRON and RT DWI of the liver were retrospectively assessed. Relative contrast ratios (RCRs) and apparent diffusion coefficients (ADCs) of the largest hepatic metastasis in TRON and RT DWI were measured. RCRs were compared using a parametric test and agreement in ADCs was assessed using the Bland-Altman method. RESULTS: In the volunteers, mean scan times of TRON and RT relative to FB DWI were 110% to 112% and 261% to 290%, respectively. On axial images, there were no significant differences in images sharpness among TRON, RT, and FB DWI, but on coronal images image sharpness in TRON was nearly always significantly better (P < 0.05) than in RT and FB DWI. In the patients, mean RCRs between TRON and RT DWI were not significantly different (P = 0.9091). Mean difference in ADC +/- limits of agreement (in 10 mm/s) between TRON and RT DWI was -0.16 +/- 0.79. CONCLUSION: TRON offers sharp diffusion-weighted images of the liver using an efficient scan time, making it an excellent alternative to RT and FB DWI. The moderate to poor agreement in ADCs of liver metastases between TRON and RT DWI requires further investigation.

Influence of cardiac motion on diffusion-weighted magnetic resonance imaging of the liver.

PURPOSE: To assess cardiac motion-induced signal loss in diffusion-weighted magnetic resonance imaging (DWI) of the liver using dynamic DWI. MATERIALS AND METHODS: Three volunteers underwent dynamic coronal DWI of the liver under breathholding, in the diastolic (DWI(diast)) or systolic (DWI(syst)) cardiac phase, and with motion probing gradients (MPGs) in phase encoding (P, left-right), frequency encoding (M, superior-inferior), or slice select (S, anterior-posterior) direction. Liver-to-background contrasts (LBCs) of DWI(syst) were compared to those of DWI(diast), for both the left and right liver lobes, using nonparametric tests. Signal decrease ratios (SDRs) were calculated as (1-(LBCDWI(syst)/LBCDWI(diast))) x 100%. DWI(syst) was further analyzed to determine which direction of MPGs was most affected by cardiac motion. RESULTS: In the left liver lobe, LBCs of DWI(syst) (median 3.35) were significantly lower (P < 0.0001) than those of DWI(diast) (median 4.84). In the right liver lobe, LBCs of DWI(syst) (median 4.17) were also significantly lower (P < 0.0001) than those of DWI(diast) (median 5.35 ). SDRs of the left and right liver lobes were 25.5% and 17.3%, respectively. In DWI(syst), the significantly lowest (P < 0.05) LBCs were observed in the M direction (left liver lobe) and P direction (right liver lobe) of MPGs. CONCLUSION: Signal intensity of both liver lobes are affected by cardiac motion in DWI. In the left liver lobe, signal loss especially occurs in the superior-inferior direction of MPGs, whereas in the right lobe, signal loss especially occurs in the left-right direction of MPGs.

Modified electrocardiograph-triggered black-blood turbo spin-echo technique to improve T1-weighting in contrast-enhanced MRI of atherosclerotic carotid arteries.

PURPOSE: To assess the efficacy of a modified electrocardiograph (EKG)-triggered black-blood T1W (T1W) spin-echo sequence in improving contrast on post-gadolinium high-resolution carotid plaque imaging by implementing heart-rate-independent contrast preparation. MATERIALS AND METHODS: We used a standard EKG-triggered double inversion-recovery (DIR) turbo spin-echo (TSE) sequence modified with the addition of an extra saturation (90 degrees ) radio frequency (RF) pulse placed immediately after the DIR module, shortening the repetition time to a fixed value of 400 msec. A total of 10 patients with atherosclerotic disease were included in the study. Postinjection intraplaque contrast measurements were performed on each patient for the standard and the modified sequence. RESULTS: Post-gadolinium-injection intraplaque contrast was 31.7 +/- 12.8% with the standard T1W sequence (nT1-TSE), and 45.3 +/- 17.2% with the modified T1W sequence (mT1-TSE), showing a significant contrast enhancement of 13.6% (P < 0.001) without significant image quality modification. CONCLUSION: The addition of a RF pulse to the standard EKG-triggered T1W TSE sequence increased intraplaque contrast without increasing sequence acquisition time. Furthermore, it appeared to be a robust technique, easy to implement on clinical scanners.

Swallowing, arterial pulsation, and breathing induce motion artifacts in carotid artery MRI.

PURPOSE: To identify and quantify the potential sources of motion in carotid artery imaging. MATERIALS AND METHODS: Two healthy volunteers and 12 patients (20-75 years old) with atherosclerotic disease were scanned on a Philips Intera 1.5T system. A single-shot balanced-fast field echo (bFFE) sequence was used to acquire real-time axial views of the carotid artery wall (three images per second). A three-step acquisition protocol was performed to analyze the three types of motion (arterial pulsation, breathing, and swallowing) separately. The isocenter carotid artery motion amplitude in either the x or y direction was measured. Radial variation in the carotid lumen between the systolic and diastolic phases was analyzed. Motion frequency was reported for each patient. RESULTS: Significant motion related to arterial pulsation (amplitude = 0.27-0.93 mm, mean = 0.6, SD = 0.19), breathing (amplitude = 0.5-3.6 mm, mean = 1.56, SD = 0.99)), and swallowing (amplitude = 1.4-9.2 mm, mean = 4.7, SD = 2.4) were visualized. CONCLUSION: Pulsation, breathing, and swallowing are sources of significant motion in the carotid artery wall. Such motion should be considered in the future to improve carotid artery image quality.

Correlation between the occurrence of 1H-MRS lipid signal, necrosis and lipid droplets during C6 rat glioma development.

The aim of this study was to investigate the possible correlation between the 1H MRS mobile lipid signal, necrosis and lipid droplets in C6 rat glioma. First, the occurrence of necrosis and lipid droplets was determined during tumor development, by a histological analysis performed on 34 rats. Neither necrosis nor lipid droplets were observed before 18 days post-implantation. At later stages of development, both necrosis and lipid droplets were apparent, the lipid droplets being mainly located within the necrotic areas. Using a second group of eight rats, a temporal correlation was evidenced between mobile lipid signal detected by in vivo single-voxel one- (136 ms echo time) and two-dimensional J-resolved 1H MR spectroscopy, and the presence of necrosis and lipid droplets on the histological sections obtained from the brains of the same rats. Finally, spatial distribution of the mobile lipid signal was analyzed by chemical-shift imaging performed on a third group of eight animals, at the end of the tumor growth. The spectroscopic image corresponding to the resonance of mobile lipids had its maximum intensity in the center of the tumor where necrotic regions were observed on the histological sections. These necrotic areas contained large amounts of lipid droplets. All these results suggest that mobile lipids detected in vivo by 1H MRS (136 ms echo time) in C6 rat brain glioma arise mainly from lipid droplets located in necrosis.

Hemorrhagic shearing lesions in children and adolescents with posttraumatic diffuse axonal injury: improved detection and initial results.

PURPOSE: To compare the effectiveness of a high-spatial-resolution susceptibility-weighted (SW) magnetic resonance (MR) imaging technique with that of a conventional gradient-recalled-echo (GRE) MR imaging technique for detection of hemorrhage in children and adolescents with diffuse axonal injury (DAI). MATERIALS AND METHODS: Seven young patients with a mean Glasgow Coma Scale score of 7 +/- 4 (SD) at admission were imaged a mean of 5 days +/- 3 after injury. High-spatial-resolution three-dimensional GRE imaging performed with postprocessing by using a normalized phase mask was compared with conventional GRE MR imaging. The total and mean values of lesion number and apparent hemorrhage volume load determined with both examinations were compared. Mean values were compared by using paired t test analysis. Differences were considered to be significant at P < or =.05. RESULTS: Hemorrhagic lesions were much more visible on SW MR images than on conventional GRE MR images. SW MR imaging depicted 1,038 hemorrhagic DAI lesions with an apparent total hemorrhage volume of 57,946 mm3. GRE MR imaging depicted 162 lesions with an apparent total hemorrhage volume of 28,893 mm3. SW MR imaging depicted a significantly higher mean number of lesions in all patients than did GRE MR imaging, according to results of visual (P =.004) and computer (P =.004) counting analyses. The mean hemorrhage volume load for all patients also was significantly greater (P =.014) by using SW MR imaging according to computer analysis. SW MR imaging appeared to depict much smaller hemorrhagic lesions than GRE MR imaging. The majority (59%) of individual hemorrhagic DAI lesions seen on SW MR images were small in area (<10 mm(2)), whereas the majority (43%) of lesions seen on GRE images were larger in area (10-20 mm(2)). CONCLUSION: SW MR imaging depicts significantly more small hemorrhagic lesions than does conventional GRE MR imaging and therefore has the potential to improve diagnosis of DAI.