The influence of out-of-plane geometry on the flow within a distal end-to-side anastomosis.

This paper describes a computational and experimental investigation of flow in a proto-type model geometry of a fully occluded 45 deg distal end-to-side anastomosis. Previous investigations have considered a similar configuration where the centerlines of the bypass and host vessels lie within a plane, thereby producing a plane of symmetry within the flow. We have extended these investigations by deforming the bypass vessel out of the plane of symmetry, thereby breaking the symmetry of the flow and producing a nonplanar geometry. Experimental data were obtained using magnetic resonance imaging of flow within perspex models and computational data were obtained from simulations using a high-order spectral/hp element method. We found that the nonplanar three-dimensional flow notably alters the distribution of wall shear stress at the bed of the anastomosis, reducing the peak wall shear stress peak by approximately 10 percent when compared with the planar model. Furthermore, an increase in the absolute flux of velocity into the occluded region, proximal to the anastomosis, of 80 percent was observed in the nonplanar geometry when compared with the planar geometry.

Glenohumeral relationships during physiologic shoulder motion and stress testing: initial experience with open MR imaging and active imaging-plane registration.

PURPOSE: To test the hypotheses that open dynamic magnetic resonance (MR) imaging can (a) be used to evaluate and define normal shoulder motion in active joint motion and muscle contraction and (b) be used in conjunction with physical examination. MATERIALS AND METHODS: With an open-configuration, 0.5-T MR imaging system and active image-plane tracking, 10 shoulders were studied in five asymptomatic subjects to establish normal patterns of glenohumeral motion during abduction and adduction and internal and external rotation. Preliminary studies of physical examination during MR imaging, in which a physician examiner applied mechanical force to the humeral head, were also performed. RESULTS: During abduction and adduction and internal and external rotation maneuvers with active subjects muscle contraction, the humeral head remained precisely centered on the glenoid fossa in all asymptomatic subjects, which is in agreement with findings of previous radiographic studies. Application of force to the humeral head by an examiner was associated with as much as 6 mm of anterior translation and 13 mm of posterior translation. CONCLUSION: Dynamic MR imaging of the glenohumeral joint is possible over a wide range of physiologic motion in vertically open systems. Use of an MR tracking coil enabled accurate tracking of the anatomy of interest. These preliminary measurements of normal glenohumeral motion patterns begin to establish normal ranges of motion and constitute a necessary first step in characterizing pathologic motion in patients with common clinical problems such as instability and impingement.

Active MR tracking on a 0.2 Tesla MR imager.

An active MR tracking system was implemented on a 0.2 Tesla open MRI system. Interventional devices with receive-only microcoils at their tips were developed and investigated on the scanner. Microcoils having a diameter of about 1 mm and 20 turns were found to provide sufficient signal-to-noise ratios for stable tracking. Positional accuracy and precision were found to be acceptable under practical conditions. Simulation of MR-guided biopsy using biplane images with tracking was performed in a gelatin phantom and dog livers. Successful tracking of catheters with integrated microcoils was also demonstrated in the aorta and IVC of live dogs.

Joint motion in an open MR unit using MR tracking.

A system for active scan plane guidance during kinematic magnetic resonance (MR) examination of joint motion was developed utilizing an external tracking coil and MR tracking software. In a phantom study and during upright, weight-bearing, physiologic knee flexion, the external tracking coil maintained the scan plane through desired structures. Thus, MR tracking provides a robust method to guide the scan plane during MR imaging of active joint motion.

MR-guided percutaneous angioplasty: assessment of tracking safety, catheter handling and functionality.

PURPOSE: Magnetic resonance (MR)-guided percutaneous vascular interventions have evolved to a practical possibility with the advent of open-configuration MR systems and real-time tracking techniques. The purpose of this study was to assess an MR-tracking percutaneous transluminal angioplasty (PTA) catheter with regard to its safety profile and functionality. METHODS: Real-time, biplanar tracking of the PTA catheter was made possible by incorporating a small radiofrequency (RF) coil in the catheter tip and connecting it to a coaxial cable embedded in the catheter wall. To evaluate potentially hazardous thermal effects due to the incorporation of the coil, temperature measurements were performed within and around the coil under various scanning and tracking conditions at 1.5 Tesla (T). Catheter force transmission and balloon-burst pressure of the MR-tracking PTA catheter were compared with those of a standard PTA catheter. The dilatative capability of the angioplasty balloon was assessed in vitro as well as in vivo, in an isolated femoral artery segment in a swine. RESULTS: The degree of heating at the RF coil was directly proportional to the power of the RF pulses. Heating was negligible with MR tracking, conventional spin-echo and low-flip gradient-echo sequences. Sequences with higher duty cycles, such as fast spin echo, produced harmful heating effects. Force transmission of the MR-tracking PTA catheter was slightly inferior to that of the standard PTA catheter, while balloon-burst pressures were similar to those of conventional catheters. The MR-tracking PTA catheter functioned well both in vitro and in vivo. CONCLUSION: The in vivo use of an MR-tracking PTA catheter is safe under most scanning conditions.

Interactive coronary MRI.

The acquisition of complete three-dimensional (3D), segmented gradient-echo data sets to visualize the coronary arteries can be both time consuming and sensitive to motion, even with use of multiple breath-holding or respiratory gating. An alternate hybrid approach is demonstrated here, in which real-time interactive imaging is first used to locate an optimal oblique coronary scan plane. Then, a limited number of contiguous slices are acquired around that plane within a breath-hold with use of two-dimensional (2D) segmented gradient-echo imaging. Dual inversion nulling is used to suppress fat and myocardium. Finally, if needed, a limited reformat of the data is performed to produce images from relatively long sections of the coronaries. This approach yields relatively rapid visualization of portions of the coronary tree. Several different methods are compared for interactively moving the scan plane.

Visualization of vascular guidewires using MR tracking.

MR tracking of a vascular guidewire sized for a .035-inch (.89-mm) catheter lumen was performed. The guidewire was actively tracked by incorporation of a miniature radiofrequency (RF) receive coil built into its tip. After in vitro validation, simultaneous tracking of the guidewire and a catheter was performed in the aortic and abdominal vessels of a swine at 1.5 T. The ability to track such a small device and the ability to simultaneously track multiple devices are significant steps towards vascular interventions under MR guidance.

Tissue-independent MR tracking of invasive devices with an internal signal source.

An improved MR tracking coil design is described that provides more robust tracking performance. It is shown theoretically and experimentally that a coil equipped with an internal spin source increases the signal-to-noise ratio in comparison to a coil system without internal source. The tracking behavior is stable even in air, which is important for laparoscopic work, in which the device has to be placed inside the inflated abdomen. Two types of coil windings were tested: one with the windings perpendicular to the coil axis and the other one with the windings tilted relative to this axis. The experiments showed no significant effect on the signal-to-noise ratio between the two types. The improved MR-tracking coil design with internal source was successfully used in cholecystostomies and in laparoscopic interventions; both procedures were performed on swine.

Active biplanar MR tracking for biopsies in humans.

OBJECTIVE: The purpose of this study was to assess the potential of active biplanar MR needle tracking in a 0.5-T open-configuration MR system. A needle design characterized by an integrated internal signal source was used. To evaluate the performance of this new technique in vivo as well as in vitro, procedures were performed on patients. CONCLUSION: The results of this study prove that the biplanar tracking concept can be applied in a clinical environment. The applicability of biplanar tracking is further enhanced by integrating an internal signal source.

MR imaging-guided intravascular procedures: initial demonstration in a pig model.

With use of an open 1.5-T magnetic resonance (MR) imager and a tracking catheter, the authors successfully placed the catheter into the left or right sacral artery in pigs. The tracking catheter comprised a 5.3-F percutaneous transluminal angioplasty catheter with a small copper radio-frequency coil in its tip. With use of the coil as an antenna, the catheter tip position was projected in real time onto MR angiography road maps in two planes. Guidance of placement of the catheter with the MR angiography road maps allowed successful embolization, balloon occlusion, and transjugular intrahepatic puncture of the portal system. Specialized catheters can be tracked in vivo to allow MR guidance in intravascular interventional procedures.

Real-time biplanar needle tracking for interventional MR imaging procedures.

PURPOSE: To evaluate real-time biplanar tracking of a specially designed needle with magnetic resonance (MR) imaging guidance. MATERIALS AND METHODS: The needle is made of polyetheretherketone and has a miniature radio-frequency coil incorporated into the tip. Tracking software on two workstations is used to compute three-dimensional coordinates of the coil and to display the position as a moving symbol in two imaging planes. Validation of needle tracking was performed in a harvested human liver. T2-weighted fast spin-echo images were used to target a 1-cm cyst. Success of needle placement was confirmed with aspiration and with updated gradient-recalled-echo images. RESULTS: The cyst was successfully targeted from different approaches. Tracking procedures were monitored in real time simultaneously on two separate images. CONCLUSION: Real-time biplanar needle tracking may prove to be useful for both diagnostic and therapeutic interventional MR imaging procedures.

Phase contrast MR angiography techniques.

Phase contrast MR methods encode information from macroscopic motion into the phase of the MR signal. Phase contrast methods can be applied with small and large fields-of-view, can give quantitative measures of velocity, and provide excellent suppression of signals from stationary tissue. Unlike time-of-flight methods, phase contrast methods directly measure flow and thus are not hindered by the artifactual appearance of tissue having short T1. Phase contrast angiograms can be two-dimensional (thin slice or projectile), three-dimensional, and/or time resolved and have applications throughout the body.

Intravascular MR tracking catheter: preliminary experimental evaluation.

OBJECTIVE: This article reports on the preliminary evaluation of a new technique for guiding intravascular interventional procedures with MR imaging. Active real-time position monitoring of catheters with MR imaging is made possible by incorporating a small RF coil into the tip of the catheter. The purpose of this study was to evaluate the practicability and localizing precision of this MR catheter tracking technique in vitro and in vivo in comparison with fluoroscopy. MATERIALS AND METHODS: Feed cables employing a 0.9-mm-diameter coaxial cable, a 0.5-mm-diameter partially shielded coaxial cable, and a twisted pair cable attaching RF coils at the catheter tip to a coaxial plug at the catheter base were assessed. Further, miniature copper loop RF coils of two, three, and four turns were tested. In vitro validation of MR tracking was achieved by using a phantom consisting of a water-filled harvested segment of human aorta and iliac arteries embedded in gel. Accuracy of catheter placement was compared with MR and fluoroscopy. Subsequently, the MR tracking technique was evaluated in a swine model using a prototype 5-French MR tracking catheter. RESULTS: A fully shielded coaxial cable was found to be crucial for localizing the attached RF coil by means of the tracking technique. The number of coil turns had a lesser impact. Positions of the catheter tip measured with the MR technique and with fluoroscopy correlated well (r > .98), with a 6-mm 95% confidence interval of positional differences. Active real-time tracking of the coil-tipped catheters was achieved both in vitro and in vivo. The 5-French tracking catheter was successfully placed in the splenic and renal arteries of the swine. CONCLUSION: Robust in vivo tracking and accurate placement of catheters equipped with miniature RF coils are possible with MR imaging.

Noninvasive measurement of renal hemodynamic functions using gadolinium enhanced magnetic resonance imaging.

A technique for the assessment of single kidney hemodynamic functions utilizing a novel MR pulse sequence in conjunction with MR contrast material administration is described. Renal extraction fraction (EF) is derived by measuring the concentration of the incoming contrast agent in the renal artery and the outgoing concentration in the renal vein. The glomerular filtration rate (GFR) can then be determined by the product of EF and renal plasma flow. A modified inversion recovery MR pulse sequence is used to measure the T1 of moving blood. This pulse sequence uses a spatially nonselective inversion pulse. A series of small flip angle detection pulses are then used to monitor the recovery of longitudinal spin magnetization in an image plane intersecting the renal vessels. The recovery rate is measured in each vessel and the T1 of blood determined. These T1 measurements are then used to determine the ratio of contrast concentration in the renal arteries and veins. Blood flow measurements can be obtained simultaneously with T1 measurements by inserting flow-encoding magnetic field gradients into the pulse sequence. Preliminary results in human volunteers suggest the feasibility of noninvasively determining hemodynamic functions with magnetic resonance.

Simultaneous detection of multiple components of motion with MRI.

OBJECTIVE: Simultaneous detection of two or more components of motion using new magnetic resonance pulse sequences was investigated. MATERIALS AND METHODS: The technique employs Fourier phase encoding to encode the first component, and phase contrast detection to encode the second. Although the technique can be generalized to any number of spatial dimensions and motional orders, applications in which one or two spatial dimensions are obtained with a single Fourier velocity or acceleration dimension are most likely to be useful. For example, Fourier-encoded velocity and phase-contrasted acceleration information can be combined into the same image. RESULTS: Several variations of the pulse sequence were investigated in phantoms and human volunteers. The first variation acquired images having an appearance similar to that of Fourier velocity-encoded images in which signal displacement is proportional to velocity, but with pixel intensity determined by acceleration. In another variation two spatial dimensions were acquired with a third dimension that uses Fourier velocity encoding to measure axial velocity within a curved tube. Radial velocity components were determined simultaneously with a second velocity-encoding gradient pulse. CONCLUSION: The phantom and in vivo results presented here suggest that simultaneous detection of two or more components of motion is feasible.

Magnetic resonance angiography: today and tomorrow.

Magnetic resonance angiography (MRA) has enjoyed enthusiastic success at many research institutions where it is now routinely used in place of invasive x-ray angiography (XRA) for a variety of applications. While the physical principles of MRA are well understood, there is still plenty of opportunity for growth in the coming years. Recent improvements in instrumentation have permitted more rapid acquisition and manipulation of larger data sets. Instruments in the future are sure to continue this trend as computer hardware becomes more capable and less expensive. New clinical applications will also expand the utility of MRA beyond its current use. MRA is already being used in peripheral vessels and it appears to have great potential in the abdomen. Research into MRA methods for coronary vessel imaging is also beginning to show intriguing results. In addition, preliminary research results suggest that interventional MRA may one day become a reality.

Reduction of artifacts from breathing and peristalsis in phase-contrast MRA of the chest and abdomen.

Strategies for the acquisition of temporally resolved phase-contrast angiography are described and demonstrated. Projections are acquired through the entire thickness of the subject permitting excitation geometries to be independent of image orientation. Minimal delays are used between the acquisition of oppositely flow-encoded data resulting in fewer artifacts from movement of nominally stationary tissue. A protocol that suppresses artifacts from both peristalsis and breathing is presented for the aortic arch and iliac arteries.

Real-time position monitoring of invasive devices using magnetic resonance.

Techniques which can be used to follow the position of invasive devices in real-time using magnetic resonance (MR) are described. Tracking of an invasive device is made possible by incorporating one or more small RF coils into the device. These coils detect MR signals from only those spins near the coil. Pulse sequences which employ nonselective RF pulses to excite all nuclear spins within the field-of-view are used. Readout magnetic field gradient pulses, typically applied along one of the primary axes of the imaging system, are then used to frequency encode the position of the receive coil(s). Data are Fourier transformed and one or more peaks located to determine the position of each receiver coil in the direction of the applied field gradient. Subsequent data collected on orthogonal axes permits the localization of the receiver coil in three dimensions. The process can be repeated rapidly and the position of each coil can be displayed in real-time.

Quantitative measurement of velocity at multiple positions using comb excitation and Fourier velocity encoding.

An MR imaging technique that simultaneously acquires Fourier velocity encoded data from multiple stations is described. The technique employs a comb excitation rf pulse that excites an arbitrary number of slices. As the Fourier velocity phase encoding gradient pulse is advanced, the phase of each slice is the comb is advanced by a unique amount. This causes the signals from the spins in a particular slice to appear at a position in the phase encoding direction, which is the sum of the spin velocity and an offset arising from the phase increment given to that excitation slice. Acquisition of spin velocity information occurs simultaneously for all slices, permitting the calculation of wave velocities arising from pulsatile flow. These wave velocities can then be used to determine vessel distensibility.