Validation of simple and robust protocols for high‐resolution lung proton MRI in mice

One fundamental limitation of spatial resolution for in vivo MR lung imaging is related to motion in the thoracic cavity. To overcome this limitation, several methods have been proposed, including scan‐synchronous ventilation and the cardiac gating approach. However, with cardiac and ventilation triggered techniques, the use of a predetermined and constant sequence repetition time is not possible, resulting in variable image contrast. In this study, the potential of two “constant repetition time” approaches based on retrospective self‐gating and signal averaging were investigated for lung imaging. Image acquisitions were performed at a very short echo time for visualization of the lung structures and the parenchyma. Highly spatially resolved images acquired using retrospective self‐gating, signal averaging technique and conventional cardiorespiratory gating are presented and compared. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

DC‐gated high resolution three‐dimensional lung imaging during free‐breathing

Purpose:

To use the acquisition of the k‐space center signal (DC signal) implemented into a Cartesian three‐dimensional (3D) FLASH sequence for retrospective respiratory self‐gating and, thus, for the examination of the whole human lung in high spatial resolution during free breathing.

Materials and Methods:

Volunteer as well as patient measurements were performed under free breathing conditions. The DC signal is acquired after the actual image data acquisition within each excitation of a 3D FLASH sequence. The DC signal is then used to track respiratory motion for retrospective respiratory gating.

Results:

It is shown that the acquisition of the DC signal after the imaging module can be used in a 3D FLASH sequence to extract respiratory motion information for retrospective respiratory self‐gating and allows for shorter echo times (TE) and therefore increased lung parenchyma SNR.

Conclusion:

The acquisition of the DC signal after image signal acquisition allows successful retrospective gating, enabling the reconstruction of high resolution images of the whole human lung under free breathing conditions. J. Magn. Reson. Imaging 2013;37:727–732. © 2012 Wiley Periodicals, Inc.     

Comparison of lung T2* during free‐breathing at 1.5 T and 3.0 T with ultrashort echo time imaging

Assessment of lung effective transverse relaxation time (T₂*) may play an important role in the detection of structural and functional changes caused by lung diseases such as emphysema and chronic bronchitis. While T₂* measurements have been conducted in both animals and humans at 1.5 T, studies on human lung at 3.0 T have not yet been reported. In this work, ultrashort echo time imaging technique was applied for the measurement and comparison of T₂* values in normal human lungs at 1.5 T and 3.0 T. A 2D ultrashort echo time pulse sequence was implemented and evaluated in phantom experiments, in which an eraser served as a homogeneous short T₂* sample. For the in vivo study, five normal human subjects were imaged at both field strengths and the results compared. The average T₂* values measured during free‐breathing were 2.11(±0.27) ms at 1.5 T and 0.74(±0.1) ms at 3.0 T, respectively, resulting in a 3.0 T/1.5 T ratio of 2.9. Furthermore, comparison of the relaxation values at end‐expiration and end‐inspiration, accomplished through self‐gating, showed that during normal breathing, differences in T₂* between the two phases may be negligible. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Lung imaging under free‐breathing conditions

Respiratory motion and pulsatile blood flow can generate artifacts in morphological and functional lung imaging. Total acquisition time, and thus the achievable signal to noise ratio, is limited when performing breath‐hold and/or electrocardiogram‐triggered imaging. To overcome these limitations, imaging during free respiration can be performed using respiratory gating/triggering devices or navigator echoes. However, these techniques provide only poor gating resolution and can induce saturation bands and signal fluctuations into the lung volume. In this work, acquisition schemes for nonphase encoded navigator echoes were implemented into different sequences for morphological and functional lung imaging at 1.5 Tesla (T) and 0.2T. The navigator echoes allow monitoring of respiratory motion and provide an ECG‐trigger signal for correction of the heart cycle without influencing the imaged slices. Artifact free images acquired during free respiration using a 3D GE, 2D multislice TSE or multi‐Gradient Echo sequence for oxygen‐enhanced T quantification are presented. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Respiratory and cardiac self‐gated free‐breathing cardiac CINE imaging with multiecho 3D hybrid radial SSFP acquisition

A respiratory and cardiac self‐gated free‐breathing three‐dimensional cine steady‐state free precession imaging method using multiecho hybrid radial sampling is presented. Cartesian mapping of the k‐space center along the slice encoding direction provides intensity‐weighted position information, from which both respiratory and cardiac motions are derived. With in plan radial sampling acquired at every pulse repetition time, no extra scan time is required for sampling the k‐space center. Temporal filtering based on density compensation is used for radial reconstruction to achieve high signal‐to‐noise ratio and contrast‐to‐noise ratio. High correlation between the self‐gating signals and external gating signals is demonstrated. This respiratory and cardiac self‐gated, free‐breathing, three‐dimensional, radial cardiac cine imaging technique provides image quality comparable to that acquired with the multiple breath‐hold two‐dimensional Cartesian steady‐state free precession technique in short‐axis, four‐chamber, and two‐chamber orientations. Functional measurements from the three‐dimensional cardiac short axis cine images are found to be comparable to those obtained using the standard two‐dimensional technique. Magn Reson Med 63:1230–1237, 2010. © 2010 Wiley‐Liss, Inc.