Lung Imaging with UTE MRI

Cross-sectional imaging of the lungs, or pulmonary imaging, has proven to be an incredibly valuable tool in a wide range of pulmonary diseases. The vast majority of lung imaging is done with CT, as it is fast enough to freeze respiratory motion and provides high spatial resolution to visualize fine structure of the lungs. MRI of the lungs is inherently challenging due to the presence of large local magnetic field gradients, relatively low proton density, and motion. The benefits of performing MRI for lung imaging include no ionizing radiation, opportunities for multiple contrasts, and integration with other MRI also offers the opportunity to obtain multiple tissue contrasts. The most common lung MRI techniques are structural T1-weighted scans, but also emerging are functional contrasts such as ventilation and perfusion, as well as other MRI contrast mechanisms including T2-weighting and diffusion-weighting. Finally, lung MRI can be combined with other MRI scanning techniques, including cardiac MRI, abdominal MRI, whole-body MRI, and PET/MRI, for increasing examination efficiency by only requiring a single scan session and providing more comprehensive assessment that includes evaluation of the pulmonary system. This article covers pulse sequences, motion management methods, image reconstruction, and contrast mechanisms of UTE MRI (e.g. T1-weighting, ventilation mapping) for imaging of the lung

T2 Relaxation during Radiofrequency (RF) pulses

Radiofrequency (RF) pulses are a critical part of every MRI pulse sequence, and must be specifically designed for ultrashort echo time (UTE) and zero echo time (ZTE) acquisitions. When considering the behavior of RF pulses, most often longitudinal T1 or transverse T2 relaxation is assumed to be negligible during the RF pulses themselves. This is usually valid with conventional sequences since most tissue T1s and T2s are much longer than typical RF pulse durations. However, when imaging tissues that have transverse relaxation times that are of the order of, or shorter than, the RF pulse duration, as is often the case with UTE and ZTE MRI, then relaxation during the pulse must be considered. This article covers the theory of T2/T2* relaxation during an RF pulse, and the implications and applications of this for imaging of ultrashort-T2* species

A Regional Bolus Tracking and Real-time B1_1 Calibration Method for Hyperpolarized 13^{13}C MRI

Purpose: Acquisition timing and B$_1$ calibration are two key factors that affect the quality and accuracy of hyperpolarized $^{13}$C MRI. The goal of this project was to develop a new approach using regional bolus tracking to trigger Bloch-Siegert B$_1$ mapping and real-time B$_1$ calibration based on regional B$_1$ measurements, followed by dynamic imaging of hyperpolarized $^{13}C$ metabolites in vivo. Methods: The proposed approach was implemented on a system which allows real-time data processing and real-time control on the sequence. Real-time center frequency calibration upon the bolus arrival was also added. The feasibility of applying the proposed framework for in vivo hyperpolarized $^{13}$C imaging was tested on healthy rats, tumor-bearing mice and a healthy volunteer on a clinical 3T scanner following hyperpolarized [1-$^{13}$C]pyruvate injection. Multichannel receive coils were used in the human study. Results: Automatic acquisition timing based on either regional bolus peak or bolus arrival was achieved with the proposed framework. Reduced blurring artifacts in real-time reconstructed images were observed with real-time center frequency calibration. Real-time computed B$_1$ scaling factors agreed with real-time acquired B$_1$ maps. Flip angle correction using B$_1$ maps results in a more consistent quantification of metabolic activity (i.e, pyruvate-to-lactate conversion, k$_{PL}$). Experiment recordings are provided to demonstrate the real-time actions during the experiment. Conclusion: The proposed method was successfully demonstrated on animals and a human volunteer, and is anticipated to improve the efficient use of the hyperpolarized signal as well as the accuracy and robustness of hyperpolarized $^{13}$C imaging

Bone Material Analogues for PET/MRI Phantoms

Purpose: To develop bone material analogues that can be used in construction of phantoms for simultaneous PET/MRI systems. Methods: Plaster was used as the basis for the bone material analogues tested in this study. It was mixed with varying concentrations of an iodinated CT contrast, a gadolinium-based MR contrast agent, and copper sulfate to modulate the attenuation properties and MRI properties (T1 and T2*). Attenuation was measured with CT and 68Ge transmission scans, and MRI properties were measured with quantitative ultrashort echo time pulse sequences. A proof-of-concept skull was created by plaster casting. Results: Undoped plaster has a 511 keV attenuation coefficient (~0.14 cm-1) similar to cortical bone (0.10-0.15 cm-1), but slightly longer T1 (~500 ms) and T2* (~1.2 ms) MR parameters compared to bone (T1 ~ 300 ms, T2* ~ 0.4 ms). Doping with the iodinated agent resulted in increased attenuation with minimal perturbation to the MR parameters. Doping with a gadolinium chelate greatly reduced T1 and T2*, resulting in extremely short T1 values when the target T2* values were reached, while the attenuation coefficient was unchanged. Doping with copper sulfate was more selective for T2* shortening and achieved comparable T1 and T2* values to bone (after 1 week of drying), while the attenuation coefficient was unchanged. Conclusions: Plaster doped with copper sulfate is a promising bone material analogue for a PET/MRI phantom, mimicking the MR properties (T1 and T2*) and 511 keV attenuation coefficient of human cortical bone