Velocity and flow quantitation in the superior sagittal sinus with ungated and cine (gated) phase-contrast MR imaging

Normal blood flow and velocity in the superior sagittal sinus were measured in 30 patients. A fast two-dimensional ungated phase-contrast (PC) pulse sequence was compared with a peripherally gated cine PC technique for velocity and flow quantitation. The same imaging parameters were used for both methods. Measured values for mean velocity and flow obtained with the two methods were compared by using regression analysis and t testing. For blood flow, the correlation coefficient was 0.976. For velocity measurements, r was 0.950. Mean flow was 285 mL/min ± 19 with the ungated PC method and 281 mL/min ± 19 with the cine PC method. The mean velocities measured with the two methods were 12.94 cm/sec ± 1.1 and 13.59 cm/sec ± 1.1, respectively. There was no significant difference (paired t test) between the methods for mean flow or velocity data. This was true even though flow in the superior sagittal sinus is moderately pulsatile, as shown with the cine PC technique. The ungated PC method provided these data in 13 seconds versus 3.5 minutes for the cine PC method.     

Measurement of skeletal muscle motion in vivo with phase-contrast MR imaging

The ability to measure skeletal muscle motion with phase-contrast magnetic resonance (MR) imaging was tested with a motion phantom that simulated muscle activity. Quantitative analytic data on unidimensional, bidirectional skeletal muscle motion measured in vivo was obtained in four healthy volunteers. MR images of the subjectss' forearms were obtained during flexion and extension of the fingers and of the anterior and posterior muscle compartments of the lower leg with various resistances to ankle dorsiflexion and plantar flexion. It was necessary to correct the data for the effects of eddy currents. In vitro evaluation of the technique was done by studying through-plane sinusoidal motion of solid objects. The largest error was underestimation of the peak excursion of 11.5 mm by 0.09 mm (the root mean square error for the cycle was 0.04 mm) In vivo experiments demonstrated the contraction of muscles in relation to each other. Data acquisition and analysis techniques must be refined, but measuring skeletal muscle motion with phase-contrast MR imaging should enhance the understanding of bioengineering fundamentals and muscular changes in disease and adaptation.     

Time-resolved MR imaging by automatic data segmentation

A method for time-resolved imaging that provides a flexible trade-off between imaging time and temporal resolution is presented. It is based on a view order selection technique that automatically segments the acquired raw data into appropriate temporal frames. When used with cardiac monitoring and phase-contrast imaging, data similar to that obtained with a conventional gated phase-contrast sequence are acquired rapidly. For many applications, the temporal resolution can be reduced enough to permit imaging within a breath-hold interval, while still allowing accurate time-averaged flow quantitation. This is a general technique that can be implemented within a variety of pulse sequences and can resolve other motion cycles, including the respiratory cycle.     

Gadoteridol dose dependence in MR imaging of a liver abscess model

A necrotic liver abscess model was studied with magnetic resonance (MR) imaging at 1.5 T before and after intravenous administration of gadoteridol at doses of 0.1, 0.25, and 0.5 mmol/kg in 24 rabbits. Enhancement characteristics and lesion delineation were assessed with both breath-hold and non-breath-hold imaging techniques. Lesion delineation, as assessed both by signal intensity measurements and evaluations by two image readers blinded to imaging technique, was greatest on high-dose (0.5 mmol/kg) breath-hold images. Lesion rim enhancement was seen consistently only on postcontrast images obtained at a dose of 0.5 mmol/kg and progressed with time after injection of contrast material.     

Efficacy evaluation of gadoteridol for MR angiography of intracranial vascular lesions

A phase III multicenter study was conducted in 89 patients with known intracranial vascular lesions to evaluate an extracellular gadolinium contrast agent, gadoteridol, for intracranial magnetic resonance (MR) angiography. The pre- and postcontrast MR angiograms of 82 patients were evaluated by the unblinded investigators and by two blinded readers (A and B) for visualization of lesions; arterial and venous anatomy; extent, size, and number of lesions; and disease classification. The unblinded readers indicated that lesions were visualized better on postcontrast images in the following categories: venous anatomy, 87 (81%) of 107 lesions; arterial anatomy, 43 lesions (40%); and extent or size of lesions, 38 lesions (36%). In 29 (35%) of 82 patients, the unblinded readers determined that enhanced MR angiography provided more diagnostic information than unenhanced MR angiography. The blinded readers determined that enhanced MR angiography provided more information for visualization of vascular anatomy in more than 60% of cases. The additional information provided with gadoteridol would have changed the diagnosis in nine (8%) of 107 lesions seen by the unblinded readers, 11 (12%) of 90 lesions seen by reader A, and three (3%) of 93 lesions seen by reader B. The results confirm that the use of gadoteridol improves the visualization of intracranial vascular lesions with MR angiography. The authors conclude that development of new postprocessing algorithms will improve the utility of contrast-enhanced MR angiography.     

Use of superparamagnetic contrast media to suppress signal from flowing spins: Preliminary experience

The authors describe their preliminary experience with the use of superparamagnetic magnetic resonance (MR) imaging contrast media for suppression of signal from flowing blood. The goal of this work was to determine if a superparamagnetic contrast agent could successfully eliminate blood signal during cardiac-gated MR imaging, thereby eliminating or reducing flow artifacts associated with the complex and variable hemodynamics within the heart chambers. Imaging and data analysis were performed in 17 dogs subjected to experimental myocardial infarction as part of a parallel project. Six doses (0.2, 1, 2, 3.5, 4, 5, and 10 mg/kg) of AMI-25, an experimental contrast agent, were used in the study. Spin-echo imaging was performed immediately before and every 5 minutes (for an average of 25 minutes) after bolus injection of the contrast agent. Variations in the image signal-to-noise ratio relative to a baseline (before injection of contrast agent) image were assessed as a function of dose and time. Preliminary results suggest that a considerable reduction in blood flow artifacts and, hence, increases in image signal-tonoise ratio can be achieved at doses greater than or equal to 3.5 mg/kg, for approximately 20 minutes after injection. Doses equal to or less than 2 mg/kg and images obtained more than 20 minutes after injection (regardless of dose) did not reliably show hemodynamic artifact suppression.     

Evaluation of the influence of the aqueous phase bioconstituent environment on the F-19 T1 of perfluorocarbon blood substitute emulsions

Oxygen-sensitive F-19 magnetic resonance imaging of perfluorocarbon compounds requires that fluorocarbon T1 changes correlate with the local Po₂ and not with the composition of the surrounding aqueous phase. The influence of various bioconstituents and paramagnetic ions within the aqueous phase on the F-19 fluorocarbon phase T1 for PFC emulsions was evaluated at 0.14 and 0.66 T. T1 was measured for FC-43, perflubron, and a fluorinated surfactant. Controlled variables introduced in the aqueous phase included annex solution constituents, blood, pH changes, and Gd-DTPA. For a constant Po₂, the F-19 T1s were independent of the emulsion constituents, blood concentration, and pH. For FC-43 and perflubron, F-19 T1 was independent of the Gd-DTPA concentration, while the aqueous phase T1 decreased by more than an order of magnitude. XMO-10 (smallest emulsion particle size) showed a slight decrease in F-19 T1 with increasing Gd-DTPA concentration at 0.66 T.     

In vivo comparison of two through-plane MR velocity mapping methods: Fast fourier encoding and phase mapping

Magnetic resonance imaging maps of velocity were acquired with a 1.5-T system in 10 subjects in a plane perpendicular to the main pulmonary artery. Velocity images were successively acquired with a method developed from Fourier-encoding velocity imaging (FEVI) principles with eight gradient steps and one excitation, and with two-point phase-subtraction mapping. Reconstruction in FEVI was implemented by zero-filling interpolation around the eight gradient steps and then around the four central steps. The methods were compared by using estimates of noise in velocity measurements based on the difference between the experimental map and a smooth fitted map. For the same acquisition time, FEVI with four encoding steps was more precise in velocity measurements than phase mapping. Precision was further increased by the use of eight encoding steps, but acquisition time was doubled.     

MR susceptometry: an external-phantom method for measuring bulk susceptibility from field-echo phase reconstruction maps

A novel method for estimating the susceptibility of an object by using the magnetic resonance (MR) imaging field distortions in an external-reference water bath next to the object is described. The field measurement was obtained with a phase reconstruction from a gradient-echo acquisition. A field model of an arbitrary object in a static magnetic field was discretely calculated from geometry determined from the magnitude reconstruction. Least-squares estimation yields the susceptibility of the object. Required (and proven) assumptions include validity of superposition, object homogeneity, negligible higher-order field terms, field-model accuracy, geometric-model accuracy, and correlation of gradient-echo phase to field distortion. MR susceptometry estimation of phantoms yielded susceptibility estimates that correlated well with known values (r >.9975). This MR susceptometry method is a first step toward quantitation of iron levels through MR imaging phase information in patients with iron overload.     

Assessment of hyperemia in acute appendicitis: Comparison between power Doppler and color Doppler sonography

The purpose of our study was to compare power Doppler sonography (US) and conventional color Doppler US in the diagnosis of acute appendicitis by revealing local signs of hyperemia. One hundred consecutive patients (46 males and 54 females, ages 7–61 years; mean, 24.7 years) with clinically suspected acute appendicitis were prospectively examined with power Doppler US and conventional color Doppler US. Of 38 patients who subsequently underwent surgery, appendicitis was proven in 34. The remaining 62 patients who were not submitted to surgery had no clinical evidence of appendicitis over a 2- to 6-month follow-up. At gray-scale US, the appendix was visualized in 30 (88.2%) of the 34 cases with pathologically proven acute appendicitis. Power Doppler US depicted a moderate to marked hypervascularity of the appendiceal wall and surrounding mesoappendix in 28 (93.3%) of these 30 patients. At conventional color Doppler US, flow signal within the appendiceal wall and surrounding mesoappendix was shown in only 21 (70%) of 30 cases (P<0.05). No false-positive diagnosis was made at either power or color Doppler US among the 62 patients without appendicitis. Power Doppler US is more sensitive than conventional color Doppler imaging for revealing signs of local hyperemia in acute appendicitis.