The 3D Cones k-space trajectory has many desirable properties for rapid and ultra-short echo time magnetic resonance imaging. An algorithm is presented that generates the 3D Cones gradient waveforms given a desired field of view and resolution. The algorithm enables a favorable trade-off between increases in readout time and decreases in the total number of required readouts. The resulting trajectory is very signal-to-noise ratio (SNR) efficient and has excellent aliasing properties. A rapid high-resolution ultra-short echo time imaging sequence is used to compare the 3D Cones trajectory to 3D projection reconstruction (3DPR) sampling schemes. Three-dimensional radial trajectories, such as 3D projection reconstruction (3DPR), allow very short echo times and repetition times, have good motion properties, and have diffuse aliasing patterns that allow scan-time reduction through undersampling. These properties have been exploited for applications such as rapid imaging of the coronary arteries (1), contrast-enhanced angiography (2), and ultra-short echo time (UTE) imaging (3). The 3D Cones trajectory (4) is a generalization of the 3DPR trajectory in which the spokes twist around one of the axes, resulting in shorter required scan times and increased signal-to-noise ratio (SNR) efficiency.As with all center-out radial trajectories, the 3D Cones trajectory does not require any phase encoding before the readout begins. Since sampling can start immediately, a higher readout time per repetition (readout duty cycle) is obtained, which leads to higher overall SNR. More twist can be added to the trajectory resulting in an increased readout time. As more twist is added, fewer total readouts are required (leading to shorter scan times) and the sampling density becomes more uniform (leading to a higher SNR efficiency). Thus, the 3D Cones trajectory provides the flexibility to reduce total scan time while increasing SNR efficiency in exchange for an increase in readout times.Several algorithms that generate the 3D Cones trajectory have been presented. Thedens (5) presents a method that uses a parametrically defined variable-density spiral in the x-y plane, which is specified by the number of rotations made by the spiral interleaf on the surface of each cone. This method forces the trajectory to start twisting immediately, which is unnecessary since the trajectory is already greatly oversampled at the center of k-space. For longer readouts, this is not a serious issue, but for very short readouts (as might be necessary in UTE or fast steady-state imaging), this method requires more interleaves than necessary and also reduces the achieved resolution of short-T 2 species.Boada et al. (6) use a method called twisted projection imaging (TPI) for sodium imaging. In this scheme, the trajectory consists of a radial line along each cone, which begins twisting according to the solution of the differential equation maintaining constant sampling density, starting at a specified fraction p of the maximum k-space value. This eliminates the i...

Ultrashort echo time (UTE) imaging has shown promise as a technique for imaging tissues with T 2 values of a few milliseconds or less. These tissues, such as tendons, menisci, and cortical bone, are normally invisible in conventional magnetic resonance imaging techniques but have signal in UTE imaging. They are difficult to visualize because they are often obscured by tissues with longer T 2 values. In this article, new long-T 2 suppression RF pulses that improve the contrast of short-T 2 species are introduced. These pulses are improvements over previous long-T 2 suppression pulses that suffered from poor off-resonance characteristics or T 1 sensitivity. Short-T 2 tissue contrast can also be improved by suppressing fat in some applications. Dual-band long-T 2 suppression pulses that additionally suppress fat are also introduced. Simulations, along with phantom and in vivo experiments using 2D and 3D UTE imaging, demonstrate the feasibility, improved contrast, and improved sensitivity of these new long-T 2 suppression pulses. The resulting images show predominantly short-T 2 species, while most long-T 2 species are suppressed. Magn Reson Med 56:94 -103, 2006.

Non-invasive visualization of the coronary arteries in vivo is one of the most important goals in cardiovascular imaging. Compared to other paradigms for coronary MR angiography (MRA), a free-breathing three-dimensional (3D) whole-heart iso-resolution approach simplifies prescription effort, requires less patient cooperation, reduces overall exam time, and supports retrospective reformats at arbitrary planes. However, this approach requires a long continuous acquisition and must account for respiratory and cardiac motion throughout the scan. In this work, a new free-breathing coronary MRA technique that reduces scan time and improves robustness to motion is developed. Data acquisition is accomplished using a 3D cones non-Cartesian trajectory, which can reduce the number of readouts three-fold or more compared to conventional 3D Cartesian encoding and provides greater robustness to motion/flow effects. To further enhance robustness to motion, 2D navigator images are acquired to directly track respiration-induced displacement of the heart and enable retrospective compensation of all acquired data (none discarded) for image reconstruction. In addition, multiple cardiac phases are imaged to support retrospective selection of the best phase(s) for visualizing each coronary segment. Experimental results demonstrate that whole-heart coronary angiograms can be obtained rapidly and robustly with this proposed technique.