In vivo measurement of cerebral blood flow: a review of methods and applications.

This article reviews the concepts of cerebral blood flow for the clinician involved in the management of patients with carotid stenosis and/or ischaemic stroke. Methods of assessing cerebral blood flow in vivo using nuclear medicine, magnetic resonance and X-ray computed tomography are described. Applications of magnetic resonance and X-ray computed tomographic methods are reviewed and illustrated by examples from the authors' radiological practice.

Diffusion-weighted imaging of the spine using radial k-space trajectories.

INTRODUCTION: Diffusion-weighted MR imaging (DWI) of the spine requires robust imaging methods, that are insensitive to susceptibility effects caused by the transition from bone to soft tissue and motion artifacts due to breathing, swallowing, and cardiac motion. The purpose of this study was to develop a robust imaging method suitable for DWI of the spine. METHODS AND SUBJECTS: A radial k-space spin echo sequence has been implemented, which is self-navigating because each acquisition line passes through the origin of k-space. Influence of cardiac motion and associated flow of cerebrospinal fluid is minimized by cardiac gating with a finger photoplethysmograph. The sequence has been tested on a 1.5T system. Diffusion-weighted images of six normal volunteers were acquired in the sagittal plane with 4 b values between 50 and 500 s mm(-2). Because of the symmetries of the cord, diffusion measurements in the head-foot (HF) or left-right (LR) directions were sufficient to measure the dominant effects of anisotropy. RESULTS: The apparent diffusion coefficients (ADCs) measured, respectively, in the LR and HF directions were (0.699+/-0.050)x10(-3) and (1.805+/-0.086)x10(-3) mm(2) s(-1) in the spinal cord, (1.588+/-0.082)x10(-3) and (1.528+/-0.052)x10(-3) mm(2) s(-1) in the intervertebral disks, and (0.346+/-0.047)x10(-3) and (0.306+/-0.035)x10(-3) mm(2) s(-1) in the vertebrae of the cervicothoracic spine. CONCLUSION: Diffusion-weighted spin echo sequences with radial trajectories in k-space provide a means of achieving robust, high quality diffusion-weighted imaging and measuring ADCs in the spine. The application of the diffusion-weighting gradients in different directions allows diffusion anisotropy to be measured.

Echoplanar BOLD fMRI of brain activation induced by concurrent transcranial magnetic stimulation.

RATIONALE AND OBJECTIVES: The authors demonstrate the feasibility of combining transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) inside an MR scanner to noninvasively stimulate and image regional brain activity. METHODS: Echoplanar blood oxygen level dependent (BOLD)-based fMRI studies of TMS response were performed on three human volunteers inside a standard 1.5 T MR scanner using independent computer control to interleave echoplanar image acquisition and stimulation of right thumb primary motor cortex with a nonferromagnetic TMS coil. RESULTS: Significant (P< 0.001) response was observed in motor cortex under the TMS coil during stimulation compared to rest, as well in auditory cortex, the latter presumably due to the loud "snap" when the coil was pulsed. CONCLUSIONS: Concurrent TMS stimulation and echoplanar BOLD fMRI imaging is possible. This method has potential for tracing neural circuits with brain imaging, as well as investigating the effects of TMS.

Mapping transcranial magnetic stimulation (TMS) fields in vivo with MRI.

Transcranial magnetic stimulation (TMS) is a non-invasive technique for investigating brain function that uses pulsed magnetic fields created by special coils to induce localized neuronal depolarization. Despite the technique's expanding application, the exact magnetic field produced by TMS coils have never been directly measured in human subjects. Using a standard 1.5T MR scanner and TMS coils constructed from non magnetic materials, we have obtained 3D maps of the magnetic field created by TMS coils in human volunteers. Further, we mapped the combined field of two coils and demonstrated that combinations of coils might be used to focus the magnetic field to achieve improved stimulation patterns and, perhaps, reach areas out of reach of single coils.

Exact temporal eddy current compensation in magnetic resonance imaging systems.

A step-response method has been developed to extract the properties (amplitudes and decay time constants) of intrinsic-eddy-current-sourced magnetic fields generated in whole-body magnetic resonance imaging systems when pulsed field gradients are applied. Exact compensation for the eddy-current effect is achieved through a polynomial rooting procedure and matrix inversion once the 2 N properties of the N-term decay process are known. The output of the inversion procedure yields the required characteristics of the filter for spectrum magnitude and phase equalization. The method is described for the general case along with experimental results for one-, two-, and three-term inversions. The method's usefulness is demonstrated for the usually difficult case of long-term (200-1000-ms) eddy-current compensation. Field-gradient spectral flatness measurements over 30 mHz-100 Hz are given to validate the method.

Use of standard gradients with compound oblique angulation for optimal quantitative MR flow imaging in oblique vessels.

The earliest described phase-modulation techniques for flow quantification by MR imaging require a phase image obtained by modifying one of the imaging gradients and a reference phase image obtained without the modified gradient. However, by using the same gradients that are used for routine two-dimensional Fourier transform imaging, both anatomic and velocity-encoded images can be obtained in one scan. Although convenient, this technique is sensitive to flow both within and perpendicular to the imaging plane. Consequently, significant errors occur in the measurement of flow in vessels oblique to the image plane. To determine the relative accuracy and practicality of quantitatively measuring flow in oblique vessels, we used standard sequence gradients with routine orthogonal plane imaging and direct compound oblique plane imaging. Phantom studies of flow in a vessel aligned along the z axis showed a significant linear correlation (r = .999; p less than .05) between the spin phase and spin velocity. However, studies of flow at relatively low physiologic rates (12-17 cm/sec) in vessels angled 0-30 degrees off axis showed that obliquities of as little as 10 degrees result in significant quantification errors. This is due to a larger phase shift per unit velocity along the frequency-encoding direction vs along the slice-select direction and to a mixture of velocities within a voxel that is oblique to the flow direction. In most instances, resolution of these errors can be achieved satisfactorily only by electronic plane rotation with compound oblique angulation so that the image plane and vessel are perpendicular. When so used, this technique potentially might provide important adjunctive quantitative flow data in oblique vessels during routine clinical imaging.

Quantitative phase-flow MR imaging in dogs by using standard sequences: comparison with in vivo flow-meter measurements.

For evaluation of the feasibility and clinical potential of using the phase data from standard MR imaging sequences to measure blood flow, 11 vessels with diameters of 4 to 7 mm were imaged in seven dogs. The flow in either the superior mesenteric vein or the inferior vena cava was measured first at laparotomy (in ml/min) with electromagnetic flow meters. Immediately thereafter, these vessels were imaged by MR in 25-mm thick sections by using a standard spin echo (SE) 750/30 sequence with a Philips 0.5-T imager. Previous phase-flow calibration of the imager and sequence allowed calculation of the blood flow rates from the phase images that were used to measure the vessels' cross-sectional areas and blood phase values. Comparison of the measurements obtained with each technique showed a significant correlation (r = .977, p less than .05) between MR-imaging values and flow-meter measurements when the blood velocity was less than approximately 40 cm/sec, the known upper limit of the flow dynamic range for the MR hardware and sequence used. There was no correlation for blood velocities greater than 40 cm/sec. However, the range of blood flow velocities in dogs and man extends to more than 100 cm/sec. Thus, these results suggest that this technique might yield valuable adjunctive flow data in routine clinical imaging provided that improvements in hardware and software permit a larger dynamic range.

Fast-field-echo MR imaging with Gd-DTPA: physiologic evaluation of the kidney and liver.

To determine if magnetic resonance imaging with Gd-DTPA could be used to assess renal and hepatic perfusion and possibly function, a fast-field-echo technique was used to perform sequential imaging of the kidney and liver of five subjects. Sixteen 3-second images of the same section were obtained at 13-second intervals immediately after Gd-DTPA administration, and again at 30 or 50 minutes after injection. From these images, curves of renal and hepatic signal versus time were generated. In each case the renal signal intensity peaked within 2 minutes and decreased to 60% or less by 3.4 minutes, and 35% or less by 50 minutes. Hepatic curves peaked within 2 minutes and approached initial levels by 30 minutes. These results suggest that transit and clearance of the contrast agent can be imaged by this technique.

Jugular venous thrombosis: MR imaging.

Magnetic resonance (MR) imaging of jugular venous thrombosis was investigated in three patients who had symptoms suggestive of this condition; the diagnosis was later confirmed by computed tomography, by venography, and clinically. Bright intraluminal signal intensity was seen throughout the course of the affected jugular vein on MR images in all three patients, in sharp contrast to the lack of signal from the corresponding site in the uninvolved venous system. Temporal changes in signal intensity from the acute to subacute stage of thrombosis were evaluated for one patient. A relative increase in signal intensity for the subacute phase was believed to be related to a decrease in the T1 relaxation time. MR may be the imaging modality of choice in the investigation of venous thrombosis.