Arterial pseudostenosis on first-pass gadolinium-enhanced three-dimensional MR angiography: new observation of a potential pitfall.

OBJECTIVE: The purpose of this study was to determine the frequency of apparent stenosis of normal aortic branches in patients on first-pass gadolinium-enhanced three-dimensional (3D) MR angiography and to reproduce the same phenomenon in a pulsatile flow phantom model. CONCLUSION: Apparent stenosis of normal vessels on gadolinium-enhanced 3D MR angiography seen on the first-pass acquisition was observed in only a small proportion (approximately 2%) of our patients. The pseudostenosis was reproducible in the phantom model using rapid injection. A stenosis on first-pass images should be interpreted with caution. Confirmation of the findings on other sequences, such as the second-pass gadolinium-enhanced 3D MR angiography or 3D phase-contrast MR angiography, prevented overdiagnosis of significant stenoses.

A simple method to improve image nonuniformity of brain MR images at the edges of a head coil.

One of the major sources of image nonuniformity in the high field MR scanners is the radiofrequency (RF) coil inhomogeneity. It degrades conspicuity of lesion(s) in the MR images of the brain and surrounding tissues and reduces accuracy of image postprocessing particularly at the edges of the coil. In this investigation, we have devised and tested a simple method to correct for nonuniformity of MR images of the brain at the edges of the RF head coil. Initially, a cylindrical oil phantom, which fit exactly in the head coil, was scanned on a 1.5 T imager. Then, a correction algorithm identified a reference pixel value in the phantom at the most homogeneous region of the RF coil. Next, every pixel inside the phantom was normalized relative to this reference value. The resulting set of coefficients or "correction matrices" was obtained for different types of MR contrast agent. Finally, brain MR images of normal subjects and multiple sclerosis patients were acquired and processed by the corresponding correction matrices obtained with different pulse sequences. Application of correction matrices to brain MR images showed a gain in pixel intensity particularly in the slices at the edge of the coil.

Computer simulation helps predict cerebral aneurysms.

Researchers have recently proven the ability of computer simulation to predict the behaviour of cerebral aneurysms. Accurately simulating the flow of blood within the aneurysm helps researchers to predict the growth pattern of the aneurysm and the danger of rupturing. As this tool is further developed into a practical diagnostic tool, it is expected to dramatically improve the ability of surgeons to weigh the results of alternate treatment methods. The simulation method used, computational fluid dynamics (CFD), provides much more information than current diagnostic tools, including particularly shear stress levels at various stages of the cardiac cycle, which help to pinpoint areas of aneurysm formation and growth.

Carotid artery bifurcation: multiple-flip-angle, three-dimensional, time-of-flight MR angiographic technique.

Multiple-flip-angle, three-dimensional, time-of-flight magnetic resonance (MR) angiography was performed in vitro and in vivo in the carotid artery bifurcation. Composite MR angiographic images were created from multiple data sets. Compared with images obtained with a low (20 degrees) flip angle, composite images obtained with flip angles of 20 degrees and 60 degrees demonstrated improved image quality and statistically significant improvement in signal-to-noise ratio (P < .05).

Carotid magnetic resonance angiography: improved image quality with dual 3-inch surface coils.

Magnetic resonance angiography (MRA) hs inherent artifacts due to variation in velocity and direction of flowing blood in the carotid bulb and regions of stenosis. We examined the efficiency of dual 3-inch surface coils to delineate carotid artery flow better. Carotid MRA was performed on ten healthy volunteers and six patients, on a 1.5 T system. A special adapter was constructed to use with 3-inch (receive-only) coils, which were placed over the carotid bifurcations. Routine anterior neck coils were also used. Contiguous axial two-dimensional (45/8.7, 1.5 mm, flip angle 60 degrees) time-of-flight sequences were used. Image matrix was 256 x 256 with two signals averaged and acquisition time 6-10 min. These images were postprocessed and reformatted into angiographic views using a maximum intensity projection algorithm. Computer simulation of carotid artery blood flow throughout the cardiac cycle based on vessel contours derived from digital subtraction angiography was carried out by finite element analysis. Improved definition of vessel margin, particularly at the carotid bifurcation, and substantially increased signal-to-background ratio of flowing blood were obtained with 3-in-chcoils. Apparent loss of signal in the carotid bulb was diminished. In one patient, contiguous flow throughout a high-grade stenosis was well defined, with the surface coil method, while drop-off of signal was observed with routine neck coil imaging.

Pulsation artifact in short TR MR imaging and angiography: exacerbation with signal averaging.

Averaging the signals from more than one excitation per phase-encoding view increases the signal-to-noise ratio and, in conventional spin-echo magnetic resonance imaging, reduces most motion artifacts. To determine the effects of signal averaging on two-dimensional gradient-echo images, acquisitions with different TRs and with no averaging versus multiple-signal averaging were compared in a pulsatile flow phantom and the human abdominal aorta. Intraview (each view repeated before changing the phase-encoding value) and interview (obtaining all views sequentially and then repeating the entire set) averaging methods were used. Pulsation artifacts were present on all images of the flow phantom and the aorta. Intraview signal averaging, the method most commonly used, exacerbated rather than ameliorated pulsation artifacts with short TR sequences. Pulsation artifacts on two-dimensional images obtained with a short TR can be minimized by completing the acquisition as rapidly as possible, avoiding signal averaging. If signal averaging is used for short TR images, it should be interview averaging.

Use of Ferrum in MRI of lung parenchyma and pulmonary embolism.

MRI of lung parenchyma and pulmonary embolism (PE) remains challenging. "Ferrum," a ferric hydroxide sucrose complex used clinically for iron deficiency anemia for more than 40 years, was investigated as a negative MRI contrast agent in five rabbits bearing experimental PE as well as in five normal volunteers. Clots were prepared by spontaneous coagulation of 0.1 ml In-111 labeled autologous red blood cells and introduced through the jugular vein. Scintigraphic imaging permitted anatomical localization of PE in vivo and thereby served as a control for MR imaging. MRI was performed on a 1.5 T GE Signa scanner before and after induction of PE, and before and after the injection of Ferrum. T1-weighted images were obtained continuously for up to 90 min using varying doses of Ferrum. In five normal human volunteers, a single dose of 100 mg each was administered. T1- and T2-weighted spin-echo and gradient-echo images of lung parenchyma were repeatedly obtained before and after agent administration. In rabbit, Ferrum remained in circulation for several hours where it shortened both T1 and T2 of blood, improving the contrast between PE and lung parenchyma (i.e., intravascular compartment). A dose of 3 mg/kg was enough to increase the contrast-to-noise ratio (CNR) between PE and lung parenchyma by almost three fold, substantially improving lesion detectability. CNR increased up to five-fold when the dose was increased up to 20 mg/kg at which point CNR reached a plateau. In humans, T2-weighted spin-echo sequence appeared to be most sensitive to changes in signal-to-noise ratio (SNR) of normal lung parenchyma. Within 60 min after injection of 100 mg of iron, SNR dropped by 34% (p < .025). However, 24 hr later, SNR returned to almost normal. Ferrum increased the contrast between PE and lung parenchyma in the rabbit and decreased the parenchymal SNR in humans in nontoxic doses. These results suggest that Ferrum is worthy of further investigation of PE imaging in humans.

Intracranial aneurysms: flow analysis of their origin and progression.

PURPOSE: To explain the origin and growth of intracranial aneurysms using the hemodynamic data obtained from a computer simulation. MATERIALS AND METHODS: Pulsatile flow in an intracranial aneurysm cavity was numerically simulated based on physiologic pulsatile flow observed in the aorta. A finite element method was applied to solve the equations of motion and the non-Newtonian viscosity of blood was taken into account in the analysis. An angiogram of a middle cerebral artery segment with aneurysm was used for the computer modeling of blood flow within the aneurysm cavity. Local shear stress and pressure on the wall at the neck of the aneurysm as well as blood flow motions inside the cavity were calculated as a function of time for various stages in the development of the aneurysm. FINDINGS: Blood moves into the aneurysm cavity along the proximal wall of the cavity and emerges along the distal wall during the acceleration period of systole; however, during the deceleration period of systole and diastole, blood changes its flow direction, entering along the distal wall of the cavity and leaving along the proximal cavity wall. Rapid changes of blood flow direction result in rapid changes in wall shear stress and pressure at the proximal and distal walls of the cavity, rendering continuous damage to the intima at the cavity neck. These hemodynamic stresses relate to the anatomy of a particular vessel may be responsible for the initiation of aneurysm formation and subsequent progression, thrombosis and/or rupture. CONCLUSION: Computer modeling can further our understanding of factors that determine the origin and progression of intracranial aneurysms.

Optimization of gradient-echo imaging parameters for intracaval filters and trapped thromboemboli.

Flow-phantom magnetic resonance (MR) gradient-echo (GRE) imaging at 1.5 T was performed on a titanium Greenfield filter containing trapped blood clots with a high concentration of either deoxyhemoglobin (DHb) or methemoglobin (MHb), simulating acute and older thromboemboli, respectively. Flip angle, repetition time (TR), and echo time (TE) were varied, and a contrast-to-noise ratio between trapped clots and flowing fluid (clot-flow contrast) was determined for each set of imaging parameters. Use of very low flip angles (less than or equal to 10 degrees) rendered MHb clots indistinguishable from flowing fluid. In general, DHb clots displayed greater clot-flow contrast than MHb clots regardless of flip angle. With increasing TE values, T2* effect was observed with MHb clots, and magnetic susceptibility artifacts increased. Overall, optimum clot-flow contrast for imaging of both DHb and MHb clots was achieved with a flip angle of 45 degrees-60 degrees, a TR of 50 msec, and the shortest TE possible. Using GRE parameters similar to the optimum parameters determined in vitro, the authors imaged four patients with nickel-titanium Simon filters and one dog with a titanium Greenfield filter. MR imaging was successful in demonstrating filter location, caval patency, and the presence and extent of intraluminal thrombus.

Low-artifact intravascular devices: MR imaging evaluation.

Flow-phantom magnetic resonance (MR) imaging, with use of both spin-echo (SE) and gradient-echo (GRE) techniques at 1.5 T, was performed on the percutaneous Greenfield (beta-III titanium alloy [TMA wire]), Amplatz (MP32-N alloy), and Simon nitinol filters and TMA wire facsimiles of the bird's nest, Gunther, new retrievable, and Amplatz vena caval filters. SE imaging allowed detection of thrombi as small as 5 X 5 mm trapped within the percutaneous Greenfield, Simon nitinol, and TMA-wire facsimile filters; with the MP32-N Amplatz filter, a larger volume of thrombus (10 X 20-mm clots) was necessary for clot detection. GRE imaging allowed detection of intraluminal tilting of the percutaneous Greenfield and facsimile Amplatz (TMA-wire) filters. GRE imaging was useful for demonstrating postfilter turbulence due to clots, which was greatest for the Amplatz filter. Imaging of facsimile vascular devices made of tantalum or TMA wire did not cause the severe "black-hole" MR artifacts typical of the stainless-steel devices. SE and GRE imaging were very useful for determining caval patency in two patients with previously placed Mobin-Uddin filters. Noninvasive MR evaluation of blood vessels in the presence of a variety of low-artifact intravascular devices appears feasible.