In this work, 3D vastly undersampled isotropic projection (VIPR) acquisition is used simultaneously with continuous table motion to extend the superior/inferior (S/I) FOV for MR angiograms. The new technique is termed floating table isotropic PR (FLIPR). The use of 3D PR in conjunction with table motionobviates the need to locate and prescribe imaging volumes containing the major blood vessels over the large superiorinferior (S/I) ranges encountered in whole-body imaging. In addition, the FLIPR technique provides extended anterior-posterior (A/P) abdominal coverage, isotropic spatial resolution, and temporal resolution. In volunteer studies, FLIPR MR angiograms with 1.6-mm isotropic spatial resolution that approached whole body in extent were acquired in less than 2 min. Several methods have been developed to perform contrastenhanced MR angiography (CE-MRA) of the vasculature of the abdomen, thigh, lower leg, and foot using a single bolus of gadolinium contrast agent (1-7). These techniques are commonly referred to as "bolus chase" methods because they aim to image the bolus of injected contrast as it flows through the peripheral vascular system. Bolus chase methods initially were time-inefficient because of the delays encountered when the table was moved between stations. Researchers resolved this problem, in part, by manually moving the table with the body coil used for excitation and reception (1), and incorporating surface coil reception (2,4,6). The concept was subsequently extended to whole-body MRA (6) and then to non-CE applications, such as tumor screening (8).The concept of a continuous moving table (CMT) for more time-efficient bolus chase MRA was first introduced by Kruger et al. (7) as a means of eliminating time delays, overlap, and discarded acquisitions between stations. In this approach, one corrects the changing position of the imaged volume by translating each echo in hybrid space prior to performing a Fourier transform in the phase-encode direction. This technique is similar to the sliding interleaved ky (SLINKY) method, which was originally proposed for time-of-flight MRA (9). A similar, non-timeresolved method was introduced by Zhu and Dumoulin (10), whereby the center frequency of the RF excitation pulse is dynamically swept to track the table motion instead of the echoes being translated as a postprocessing step. The method of Kruger et al. (7) was recently extended to provide temporal resolution with the use of a variablerate k-space sampling approach employing a hybrid Cartesian and projection acquisition sampling scheme (11).More generally, Dietrich and Hajnal (12), Brittain et al. (13), and Scheffler (14) introduced the idea of CMT-MRI to improve the B 0 homogeneity of echo-planar, spiral, and PR methods for acquiring slices in the axial orientation. Axialoriented slices are less suitable for MRA due to reduced slice resolution and no temporal resolution. However, methods that make use of the entire imaging FOV (such as in the present work and Ref. 7) must correct for deviations from ...
The projection reconstruction (PR)-HyperTRICKS (time resolved imaging of contrast kinetics) acquisition integrates the benefits of through-plane Cartesian slice encoding and in-plane undersampled PR. It provides high spatial resolution both inplane (about 1 mm 2 ) and through-plane (1-2 mm), as well as relatively high temporal resolution (about 0.25 frames per second). However, undersampling artifacts that originate from anatomy superior or inferior to a coronal imaging FOV may severely degrade the image quality. In coronal MRA acquisitions, the slice coverage is limited in order to achieve high temporal resolution. In this report we describe an artifact reduction method that uses selective excitation in PR-Hyper-TRICKS. This technique significantly reduces undersampling streak artifacts while it increases the slice coverage. Both high spatial resolution and high temporal resolution are desired for peripheral magnetic resonance angiography (MRA), because the dynamic flow of the contrast material used in MRA can be quite complex in patients with peripheral vascular disease (1-5). Bolus-chase techniques can be used to image the abdomen, thigh, and calf stations consecutively with a single contrast bolus injection (6 -8).Although high-spatial-resolution angiograms can be achieved for the abdomen and thigh stations, images of the lower extremities often suffer from venous contamination because the data are acquired quite late after the arrival of contrast (2,5). Several methods have been proposed to improve the image quality of the lower-extremity vasculature. One such method is to start acquiring the central k-space data for the calf station as soon as possible by shortening the imaging time of the abdomen and thigh stations (9 -11). However, several problems remain, including the large variability of contrast arrival times (12) and the delayed filling or asymmetric filling of diseased vessels. Another approach is to image the lower extremities by means of a time-resolved acquisition with a separate injection of contrast agent. Wang et al. (5) recently proposed a 2D time-resolved acquisition for the lower extremities prior to the use of a bolus-chase acquisition for the abdomen and thigh stations only. A separate injection of 5-7 ml of gadolinium-based contrast material was used for the 2D time-resolved data acquisition. They found that the 2D time-resolved acquisition was quite robust in preventing venous contamination and depicting the contrast dynamics in the lower extremities.Given the advantages of 3D imaging over 2D imaging, it is desirable to depict the contrast dynamics for lowerextremity imaging with the use of a 3D data volume. The Cartesian time resolved imaging of contrast kinetics (TRICKS) technique was developed for 3D time-resolved imaging, in which spatial resolution is traded for temporal resolution (13,14). The projection reconstruction (PR)-TRICKS technique provides high in-plane spatial resolution but limited through-plane spatial resolution by means of in-plane PR and Cartesian slice encoding (...
The possibility of negative temperatures on the Kelvin scale is intriguing and confusing simultaneously. This is because students are used to thinking of temperature as a measure of the internal energy of a system. While this concept is good for many systems, it does not work for all systems. Nuclear and electron spin systems, along with lasers and other energy-bound systems, have negative absolute temperature. These systems have an upper limit to their energy, which arises out of the quantum nature of these systems. This is what gives rise to the negative absolute temperature. Temperature in the broadest sense is related to the change of the entropy of a system for a given change in the internal energy of the system. To help students understand this concept, we have developed a NetLogo model simulation of a binary spin lattice system using the 2D Ising model. For example, imagine a grid of tiny suspended bar magnets, with each magnet only allowed one of two orientations: north pole up or south pole up. NetLogo is a software tool that allows users to program agents that can interact with each other and the environment. Through using this simulation, students in a calculus-based physics or thermodynamics class will get a visual and hands-on experience of the full range of temperatures possible.
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