Tricks

Time Resolved Imaging of Contrast Kinetics (TRICKS) [7] uses an increased sampling rate for lower frequencies, temporal interpolation of k-space views, and zero-filling in the slice-encoding direction. When appropriately combined, these elements permit reconstruction of a series of 3D image sets, having an effective temporal frame rate of one volume every two to six seconds, with no serious compromise in spatial resolution. Conversely, given thelimited duration of the bolus of contrast agent in the arteries, and the potential for motion during MR angiography, there is a practical lower limit on the appropriate sampled voxel size. This lower limit on spatial resolution improves with field-of-view (FOV) minimization because less time is required to reach the desired sampling resolution. Furthermore, interference from venous signal intensity and respiratory motion can be minimized by using the elliptic centric view order, which makes longer acquisition times feasible. Consistent withthis approach, a high-spatial-resolution MR angiographic technique with reduced FOV and slightly extended acquisitiontime can be used: 26.0 cm (x axis [superior to inferior]) x 19.0 cm (y axis [right toleft]) x 6.4 cm (z axis [anterior to posterior]) andcovered the region of interest in a total acquisitiontime of 40 seconds [8].

Additional sequences may be added to the standard protocol in specific cases [9]. When the abdominal aorta is being evaluated for aneurysm, additional images should be acquired to depict the outer wall of the aorta and give an estimate of the true size of the aneurysm. The size of the aneurysm may be underestimated on CE MRA, as it is primarily a luminogram similar to the conventional angiogram. Since the background is sup

Fig. 2a-c. The MIP image (a) of a 3D CE MRA dataset (Gd-BOPTA, 0.1 mmol/kg) shows the luminal image of the aorta with extensive atherosclerotic disease of the iliac and femoral vessels and dilatation as well as wall irregularities of the distal abdominal aorta and right common iliac artery (arrows). Two post contrast T1w fat suppressed images at the level of the common iliac arteries just below the bifurcation (b) and at the level of the distal aorta (c), performed after the CE MRA study, demonstrate large aneurysm formation with thrombus formation (asterisk) of both common iliac arteries as well as of the distal abdominal aorta. Whereas the perfused lumen of the left iliac artery appears almost normal on the MIP image, the aneurysm is even larger compared with that of the right iliac artery [Images courtesy of Dr. G. Schneider]

pressed on CE MRA in order to depict the vessels accurately, it may become difficult to visualize the outer wall of the aneurysm even on native slices or MPR. This is often a major problem, especially when patients are being considered for endovascu-lar stent graft placement, which requires outer-to-outer wall measurements for placement of a stent graft. The solution is to use either an additional black-blood imaging sequence (Fig. 2) for evaluation of the vessel wall [10] or a true fast imaging with steady-state precession technique (true FISP) that uses a fully balanced gradient waveform to recycle transverse magnetization [11] (Fig. 3). Contrast is determined on the basisof the ratio of T2 to T1 rather than on the basis of inflow effects, as in spoiled gradient-echo methods. This difference eliminates sensitivity to saturation effects from absent or slow flow. PostcontrastT1-weighted images with fat suppression may also be helpful in some conditions of the aorta such as arteritis, mycotic aneurysm, or graft infection [12].

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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