MRI offers outstanding soft tissue contrast that may reduce uncertainties in target and organ-at-risk delineation and enable online adaptive image-guided treatment. Spatial distortions resulting from ...non-linearities in the gradient fields and non-uniformity in the main magnetic field must be accounted for across the imaging field-of-view to prevent systematic errors during treatment delivery. This work presents a modular phantom and software application to characterize geometric distortion (GD) within the large field-of-view MRI images required for radiation therapy simulation. The modular phantom is assembled from a series of rectangular foam blocks containing high-contrast fiducial markers in a known configuration. The modular phantom design facilitates transportation of the phantom between different MR scanners and MR-guided linear accelerators and allows the phantom to be adapted to fit different sized bores or coils. The phantom was evaluated using a 1.5T MR-guided linear accelerator (MR-Linac) and 1.5T and 3.0T diagnostic scanners. Performance was assessed by varying acquisition parameters to induce image distortions in a known manner. Imaging was performed using T1 and T2 weighted pulse sequences with 2D and 3D distortion correction algorithms and the receiver bandwidth (BW) varied as 250-815 Hz/pixel. Phantom set-up reproducibility was evaluated across independent set-ups. The software was validated by comparison with a non-modular phantom. Average geometric distortion was 0.94+/-0.58 mm for the MR-Linac, 0.90+/-0.53 mm for the 1.5 T scanner, and 1.15+/-0.62 mm for the 3.0T scanner, for a 400 mm diameter volume-of-interest. GD increased, as expected, with decreasing BW, and with the 2D versus 3D correction algorithm. Differences in GD attributed to phantom set-up were 0.13 mm or less. Differences in GD for the two software applications were less than 0.07 mm.
MR images have a magnitude and a phase, but in almost all clinical applications only the magnitude images are used, because the phase images have a smooth but strong background signal that masks ...useful information. The phase contains information such as the temperature during thermal ablation, and the iron content of brain tissue. We describe a novel method to suppress the background that is based on higher order edge detection and sparse image representation.
Interventional Magnetic Resonance Imaging (iMRI) is the application of MRI to guide and evaluate minimally invasive diagnostic and therapeutic procedures. For these interventions to be successful, ...MRI must provide fast acquisition times while maintaining image quality characteristics such as signal-to-noise ratio (SNR), tissue contrast, and resolution. Steady state sequences are often used in iMRI due to their high temporal resolution, but their signal and contrast properties are not suitable for some procedure types or stages. Also, negative factors outside of the sequence such as poor interface design or lack of understanding of contrast agent injection protocols may limit the effectiveness of these sequences. These limitations of current steady state techniques need to be overcome in order to utilize the vast potential of IMRI in minimally invasive procedures. This project covers a broad scope within iMRI, approaching several of these issues in various applications. Keyhole imaging is been shown to provide excellent temporal and spatial resolution for guidance when the phase encoding axis is aligned with the length of an interventional device. While the specific imaging axes orientation places additional demands on scan plane definition, a unique spatial input device with six degrees of freedom with realtime image feedback can perform this task with greater speed and convenience. Intraarterial catheter injections of contrast agent for MR angiography promise improved image quality at reduced dosages compared to remote intravenous injections. This study attempts to reduce contrast dosages even further by examining factors in the injection and image acquisition protocols that affect the quality of these angiographic acquisitions. Understanding these factors will allow an increased number of injections per intervention and enable more complex endovascular interventions. This study will also perform a thorough analysis of Missing Pulse Steady State Free Precession, a unique sequence that drops every third radiofrequency (RF) pulse to acquire the spin echo that occurs during this time. This echo will be evaluated by acquiring its components with a new method of gradient moment separation. Other echoes are also available in this sequence, and a thorough analysis of the signal and contrast properties of these previously unexplored echoes will be performed with mathematical simulations and with new sequences designed to acquire these echoes. Finally, a change in MPSSFP introduces a new class of sequences that can image spins excited by any two types of RF excitations while suppressing spins excited by just one type. Applications in intervolume and chemical shift imaging are presented. The success of this work will increase both the quality and range of steady state applications in iMRI, pushing the field of beyond the current state of the art.
