True real-time cardiac MRI in free breathing without ECG synchronization using a novel sequence with radial k-space sampling and balanced SSFP contrast mode.

We investigated a novel sequence with radial k-space sampling, gridding and sliding window reconstruction with bSSFP contrast that allows for true real-time functional cardiac evaluation independent from respiration and ECG triggering. 12 healthy volunteers underwent 1.5 T cardiac MRI. Single-shot short axis views were acquired with a) standard retrospectively ECG-gated segmented breath-hold (bh) bSSFP and with the real-time radial bSSFP sequence with a nominal temporal resolution of b) 16 fps (frames per second) and c) 40 fps. Radial bSSFP were acquired during free breathing without ECG synchronization. Left ventricular functional parameters (EDV, ESV, SV and EF) were compared and quality of wall motion depiction was assessed. Contrast-to-noise-ratio (CNR) of myocardium/blood pool in the left ventricle was calculated. EF showed excellent correlation (Bland-Altman r = 0.99; Lin rho = 0.91) between bh-bSSFP (65%) and 40 fps radial (64%) and moderate correlation (r = 0.84, rho = 0.20) with 16 fps radial bSSFP (56%). While EDV was in good agreement for all three sequences, ESV was significantly overestimated with 16 fps radial bSSFP. Despite lower CNR, image quality for wall motion assessment was rated significantly better for 40 fps compared to 16 fps radial bSSFP due to the faster temporal resolution. Left ventricular functional analysis with fast true real-time radial bSSFP is in good agreement with standard ECG-gated bh-bSSFP. The independency from ECG synchronization and breathing promises a robust method for patients with impaired cardiopulmonary status in whom breath-hold and good quality ECG cannot be achieved.

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.

Dual-energy CT for the assessment of chronic myocardial infarction in patients with chronic coronary artery disease: comparison with 3-T MRI.

OBJECTIVE: The purpose of this article is to compare the performance of dual-energy CT with that of 3-T MRI with late enhancement for the detection of chronic myocardial infarction during first-pass coronary CT angiography (CTA). SUBJECTS AND METHODS: Thirty-six patients underwent coronary CTA for the assessment of coronary bypass graft patency on a first-generation dual-source CT scanner in dual-energy mode. Gray-scale images (100 kV, 140 kV, and blended virtual 120 kV) were assessed for areas of hypodense myocardium during the arterial phase. In addition, a color-coded map of myocardial iodine distribution was calculated from the dual-energy data for perfusion analysis. Dual-energy CT data were compared with data from 3-T MRI with late enhancement, which served as the reference standard for scar detection using the American Heart Association's 17-segment model of the left ventricle. RESULTS: One hundred one (17%) of 612 myocardial segments in 22 (61%) of 36 patients showed late enhancement on MRI. Although myocardial iodine mapping was prone to artifacts, mostly arising from sternal wires (70% sensitivity), 100-kV gray-scale images showed the highest sensitivity (80%) for the detection of myocardial scar. Blended virtual 120-kV images with lower noise and higher resolution had the best diagnostic accuracy (77% sensitivity, 97% specificity, 85% positive predictive value, 96% negative predictive value, and 94% accuracy). CONCLUSION: Detection of chronic myocardial infarction on color-coded iodine distribution analysis with first-generation dual-energy CT is impeded by thoracic metallic devices. This group of patients benefits more from adequate blending of high- and low-kilovoltage gray-scale images. Further technical improvements are desirable to lower artifact burden and improve sensitivity on myocardial iodine distribution mapping.