A practical methodology to improve the dosimetric accuracy of MR-based radiotherapy simulation for brain tumors.

PURPOSE: To investigate the dosimetric accuracy of synthetic computed tomography (sCT) images generated by a clinically-ready voxel-based MRI simulation package, and to develop a simple and feasible method to improve the accuracy. METHODS: 20 patients with brain tumor were selected to undergo CT and MRI simulation. sCT images were generated by a clinical MRI simulation package. The discrepancy between planning CT and sCT in CT number and body contour were evaluated. To resolve the discrepancies, an sCT specific CT-relative electron density (RED) calibration curve was used, and a layer of pseudo-skin was created on the sCT. The dosimetric impact of these discrepancies, and the improvement brought about by the modifications, were evaluated by a planning study. Volumetric modulated arc therapy (VMAT) treatment plans for each patient were created and optimized on the planning CT, which were then transferred to the original sCT and the modified-sCT for dose re-calculation. Dosimetric comparisons and gamma analysis between the calculated doses in different images were performed. RESULTS: The average gamma passing rate with 1%/1 mm criteria was only 70.8% for the comparison of dose distribution between planning CT and original sCT. The mean dose difference between the planning CT and the original sCT were -1.2% for PTV D95 and -1.7% for PTV Dmax, while the mean dose difference was within 0.7 Gy for all relevant OARs. After applying the modifications on the sCT, the average gamma passing rate was increased to 92.2%. Mean dose difference in PTV D95 and Dmax were reduced to -0.1% and -0.3% respectively. The mean dose difference was within 0.2 Gy for all OAR structures and no statistically significant difference were found. CONCLUSIONS: The modified-sCT demonstrated improved dosimetric agreement with the planning CT. These results indicated the overall dosimetric accuracy and practicality of this improved MR-based treatment planning method.

Cellular uptake and imaging studies of gadolinium-loaded single-walled carbon nanotubes as MRI contrast agents.

We quantify here, for the first time, the intracellular uptake (J774A.1 murine macrophage cells) of gadolinium-loaded ultra-short single-walled carbon nanotubes (gadonanotubes or GNTs) in a 3 T MRI scanner using R(2) and R(2)* mapping in vitro. GNT-labeled cells exhibited high and linear changes in net transverse relaxations (ΔR(2) and ΔR 2*) with increasing cell concentration. The measured ΔR(2)* were about three to four times greater than the respective ΔR(2) for each cell concentration. The intracellular uptake of GNTs was validated with inductively coupled plasma optical emission spectrometry (ICP-OES), indicating an average cellular uptake of 0.44 ± 0.09 pg Gd per cell or 1.69 × 10(9) Gd(3+) ions per cell. Cell proliferation MTS assays demonstrated that the cells were effectively labeled, without cytotoxicity, for GNTs concentrations ≤28 µM Gd. In vivo relaxometry of a subcutaneously-injected GNT-labeled cell pellet in a mouse was also demonstrated at 3 T. Finally, the pronounced R(2)* effect of GNT-labeled cells enabled successful in vitro visualization of labeled cells at 9.4 T.

Simultaneous ultrasound and MRI system for breast biopsy: compatibility assessment and demonstration in a dual modality phantom.

Simultaneous capturing of ultrasound (US) and magnetic resonance (MR) images allows fusion of information obtained from both modalities. We propose an MR-compatible US system where MR images are acquired in a known orientation with respect to the US imaging plane and concurrent real-time imaging can be achieved. Compatibility of the two imaging devices is a major issue in the physical setup. Tests were performed to quantify the radio frequency (RF) noise introduced in MR and US images, with the US system used in conjunction with MRI scanner of different field strengths (0.5 T and 3 T). Furthermore, simultaneous imaging was performed on a dual modality breast phantom in the 0.5 T open bore and 3 T close bore MRI systems to aid needle-guided breast biopsy. Fiducial based passive tracking and electromagnetic based active tracking were used in 3 T and 0.5 T, respectively, to establish the location and orientation of the US probe inside the magnet bore. Our results indicate that simultaneous US and MR imaging are feasible with properly-designed shielding, resulting in negligible broadband noise and minimal periodic RF noise in both modalities. US can be used for real time display of the needle trajectory, while MRI can be used to confirm needle placement.

Multi-modal imaging: simultaneous MRI and ultrasound imaging for carotid arteries visualization.

The purpose of this study is to examine the feasibility of simultaneous Magnetic Resonance Imaging (MRI) and Ultrasound (US) imaging in visualizing anatomical structures and functions in human carotid arteries. US has high frame rate in visualizing dynamic changes while high resolution MRI is capable of capturing volumetric structures with the best tissue contrast. Concurrent multi-modal image acquisition allows fusion of US Doppler flow measurement with volumetric MRI. We present a method for acquiring MR images in a known orientation with respect to US image by passive fiducial tracking and demonstrate concurrent real-time imaging in the right Common Carotid Artery (CCA) in both modalities. Preliminary results suggest that US and MRI can operate concurrently with proper shielding. Dispensability measurements are feasible on both modalities at the co-incident plane.