Wide, short bore magnetic resonance at 1.5 t: reducing the failure rate in claustrophobic patients.

PURPOSE: Claustrophobic reactions in patients undergoing magnetic resonance imaging (MRI) have a significant impact on the workflow, patient acceptance and ultimately the costs involved in obtaining a diagnostic scan. The purpose of this study was to determine if the use of a wide, short bore MRI scanner could reduce the need for general anesthesia assistance in these cases. METHODS: Between September 2006 and March 2008, all patients for whom MRI examinations of the head and/or spine were canceled or prematurely terminated due to claustrophobia on a standard 60 cm bore, 1.5 T scanner were scheduled to be re-scanned on a 70 cm wide bore, 1.25 m long 1.5 T scanner. This re-scanning attempt was made 2 or more days prior to a scheduled anesthesia-assisted MRI appointment. If the patient successfully completed the wide bore MRI examination then the anesthesia-assisted MRI appointment was canceled. RESULTS: A total of 56 patients were included in this study. The examinations included individual body regions as well as combination examinations (head and cervical spine, entire spine etc.). A total of 72 body regions were examined in 56 patients. Of these regions, 65 (90%) were completed successfully, 50 patients (89%) successfully completed a diagnostic examination on the 70 cm scanner and 6 patients (11%), all of whom were scheduled for examinations which included the head, were unable to complete the examination on the wide bore scanner. CONCLUSIONS: A 1.5 T wide short bore scanner increases the examination success rate in patients with claustrophobia and substantially reduces the need for anesthesia-assisted MRI examinations even when claustrophobia is severe.

The utility of diffusion-weighted imaging for cholesteatoma evaluation.

DWI is a useful technique for the evaluation of cholesteatomas. It can be used to detect them when the physical examination is difficult and CT findings are equivocal, and it is especially useful in the evaluation of recurrent cholesteatoma. Initial DWI techniques only detected larger cholesteatomas, >5 mm, due to limitations of section thickness and prominent skull base artifacts. Newer techniques allow detection of smaller lesions and may be sufficient to replace second-look surgery in patients with prior cholesteatoma resection.

In vivo measurement of pulsewave velocity in small vessels using intravascular MR.

A one-dimensional intravascular MR (IVMR) technique for the measurement of pulsewave velocity in a single cardiac cycle is presented. The technique was used to measure pulsewave velocity in vivo in the intact rabbit model, where its sensitivity to different hemodynamic states was demonstrated using a pharmacological intervention with phenylephrine and nitroprusside. IVMR measurements of pulsewave velocity were found to increase with mean arterial pressure, as expected. Further, IVMR-based pulsewave velocity estimates were in agreement with those measured by pressure catheters and direct distensibility measurement. Because of their rapidity and highly localized nature, these measurements of vessel elasticity may complement the high-resolution vascular imaging information gained in an IVMR examination. This could allow assessment of atherosclerotic plaques and facilitate immediate treatment decisions. Magn Reson Med 45:53-60, 2001.

Minimizing dead-periods in flow-encoded or -compensated pulse sequences while imaging in oblique planes.

A technique is developed to minimize magnetic resonance pulse sequence dead-periods, with first and higher moment requirements, while imaging in oblique planes. Dead-period requirements of starting amplitude, ending amplitudes, area, and other moment requirements are transformed from the image coordinate system to the physical gradient coordinate system, where the waveforms are then designed. With this technique, the full capabilities of the gradient hardware are utilized. An online algorithm is presented to perform dead-period minimization and gradient waveform design when the first moment and area of the dead-period are specified. The algorithm is then used to implement three-axis flow-compensation in a fast spoiled gradient-echo sequence. This results in a minimal increase in TE and TR over an equivalent non-flow-compensated sequence, and little variation in the minimum TR over the entire range of oblique slice orientations. Applications of this algorithm extend to the optimization of any pulse sequence in which the first moment is important and oblique imaging is required. J. Magn. Reson. Imaging 1999;10:183-192.

Intravascular MR-monitored balloon angioplasty: an in vivo feasibility study.

PURPOSE: To develop a new method for monitoring balloon angioplasty by using an intravascular magnetic resonance (MR) imaging technique. MATERIALS AND METHODS: Nine New Zealand White rabbits were used: seven for technique refinement, including surgery, device insertion, stenosis creation, and MR protocol development; and two for the final MR imaging of the balloon angioplasty. The in vivo experimental method involved insertion of a catheter antenna and a balloon catheter, via femoral arteriotomies bilaterally, into the target site of the upper abdominal aorta, where a stenosis was artificially created by binding a plastic cable tie. Then, the entire process of the dilation of the stenosis with balloon inflation was monitored under MR fluoroscopy. RESULTS: Catheter insertions were successful, and a 5-mm-long stenosis of the aorta was produced in all nine rabbits. Eight complete balloon angioplasty procedures were satisfactorily monitored and recorded, showing clearly the stenosis of the aorta at the beginning of the procedure, the dilation of the stenosis during the balloon inflation, and the complete opening of the stenosis after balloon dilation. CONCLUSION: Preliminary results of in vivo balloon angioplasty monitored with intravascular MR imaging are presented. MR fluoroscopy, based on the intravascular MR imaging technique, may represent a potential alternative to x-ray fluoroscopy for guiding interventional treatment of cardiovascular diseases.

Accuracy of arterial pulse-wave velocity measurement using MR.

The performance of a one-dimensional MR technique for the estimation of pulse-wave velocity in the aorta was evaluated. An expression for the error in this estimate was formulated and verified both by simulation and by experiment. On the basis of this formulation, guidelines for increasing the efficiency of the acquisition were established. The technique was further validated by comparison with pulse-wave velocity measurements made with a pressure catheter. All data were acquired from a latex tube driven by a pulsatile flow system. MR measurements of pulse-wave velocity in the tube were found to be very reproducible in the presence of white noise. Measurements by other techniques were in good agreement, falling within 2 SD of the mean. Because of its sensitivity and spatial resolution, this technique shows promise for making spatially resolved estimates of vessel distensibility. This would allow assessment of diseases, such as atherosclerosis, that cause local changes in the material properties of the vasculature.

Improved sampling of myocardial motion with variable separation tagging.

A novel tagging method is introduced that increases the spatial resolution of estimates of myocardial radial thickening. The separation of adjacent parallel tag planes is customized to match the expected motion of specific regions of the heart wall. In regions in which the tags increase their separation over systole (radial thickening), the tag planes are placed close together at end diastole. In regions in which the tags decrease their separation over systole (circumferential shortening), the tags are placed farther apart so they remain detectable at end systole. With variable separation tagging (VTAG), parallel plane tagging can be used to obtain higher-resolution estimates of radial thickening and circumferential shortening simultaneously.

Quantification and reduction of ghosting artifacts in interleaved echo-planar imaging.

A mathematical analysis of ghosting artifacts often seen in interleaved echo-planar images (EPI) is presented. These artifacts result from phase and amplitude discontinuities between lines of k-space in the phase-encoding direction, and timing misregistrations from system filter delays. Phase offsets and time delays are often measured using "reference" scans, to reduce ghosting through postprocessing. From the expressions describing ghosting artifacts, criteria were established for reducing ghosting to acceptable levels. Subsequently, the signal-to-noise ratio (SNR) requirements for estimation of time delays and phase offsets, determined from reference scans, was evaluated to establish the effect of estimation error on artifact reduction for interleaved EPI. Artifacts resulting from these effects can be reduced to very low levels when appropriate reference scan estimation is used. This has important implications for functional MRI (fMRI) and applications involving small changes in signal intensity.

Pencil excitation with interleaved fourier velocity encoding: NMR measurement of aortic distensibility.

A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of pulse-wave velocity in the aorta. A cylinder of magnetization is excited along the aorta, with Fourier-velocity encoding and readout gradients applied along the cylinder axis. Cardiac gating and data interleaving improve the effective time resolution to as high as 3 ms. Wave velocities are determined from the position of the foot of the flow wave in the velocity profiles. Evidence of helical flow distal to the aortic arch can be seen in normal subjects, while disturbed flow patterns are visible in patients with aneurysms and dissections.

A one-dimensional velocity technique for NMR measurement of aortic distensibility.

A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of wave velocity in the aorta. A two-dimensional NMR selective-excitation pulse is used to repeatedly excite a cylinder of magnetization in the aorta, with magnetization read out along the cylinder axis each time. A toggled bipolar flow-encoding pulse is applied prior to readout, to produce a non-dimensional phase-contrast flow image. Cardiac gating and data interleaving are employed to improve the effective time resolution to 2 ms. Wave velocities are determined from the slope of the leading edge of flow measured on the resulting M-mode velocity image. The technique is sensitive over a range of distensibilities from 10(-6) to 10(-3) m s2/kg. The average value in the descending thoracic aorta in seven normal subjects was found to be 4.8 x 10(-5) m s2/kg, with a significant inverse correlation with age.

Myocardial tagging in polar coordinates with use of striped tags.

Regional deformation abnormalities in the heart wall provide a good indicator of ischemia. Myocardial tagging with magnetic resonance imaging is a new method of assessing heart wall motion during contraction. Current methods of myocardial tagging either do not provide two-dimensional information or lack a coordinate system well adapted to the morphology of the heart. In this article, the authors describe a new tagging method that provides a true polar coordinate system, with both radial and angular dimensions. This is accomplished with use of a section-selective version of spatially modulated magnetization resulting in striped tags (STAGs). These STAG planes are placed in the myocardium in a star pattern so that they intersect on the long axis of the heart and stripes appear through the width of the heart wall. In the short-axis view during contraction, rotation around the long axis yields angular information such as shear and twist, while separation of the stripes within the myocardium permits measurement of radial thickening. Therefore, this method provides a coordinate system for calculating two-dimensional strain that is adapted to the morphology of the left ventricle.