Rapid mapping of flow velocity using a new PARSE method.

A new method for flow velocity mapping is presented here. Instead of the conventional approach of employing two images (velocity sensitive and control) to generate velocity information, in the new method one determines the velocity directly from a single-shot acquisition by solving an inverse problem. This technique is a variant of single-shot parameter assessment by retrieval from signal encoding (SS-PARSE). The results of simulation and phantom studies show strong agreement with the actual velocities. The prototype method can measure velocities in the range of -50 to 50 cm/s, which is roughly appropriate for future applications in dynamic blood flow measurement in carotid arteries.     

Motion estimation of tagged cardiac magnetic resonance images using variational techniques

This work presents a new method for motion estimation of tagged cardiac magnetic resonance sequences based on variational techniques. The variational method has been improved by adding a new term in the optical flow equation that incorporates tracking points with high stability of phase. Results were obtained through simulated and real data, and were validated by manual tracking and with respect to a reference state-of-the-art method: harmonic phase imaging (HARP). The error, measured in pixels per frame, obtained with the proposed variational method is one order of magnitude smaller than the one achieved by the reference method, and it requires a lower computational cost.     

Visualization of moving fluid: quantitative analysis of blood flow velocity using MR imaging

A new method for the measurement of blood flow using magnetic resonance imaging has been developed. The flow velocities are calculated from the distances that the fluid has moved. The distances are directly visualized by a new pulse sequence. In a phantom study, the measured flow rates showed very good correlation with actual flow rates of up to 20 l/min (3 m/sec). In a volunteer study, pulsatile flow velocities of a large artery were measured with electrocardiographic gating. The flow pattern of a cardiac cycle at the abdominal aorta is similar to that revealed by other methods of measurement, such as Doppler ultrasound. This method allows reasonably accurate quantitative analysis of blood flow in the large arteries.     

Method of injection of contrast medium for brain perfusion CT

Perfusion computed tomography (CT) has great value for detecting stroke and evaluating blood flow in the brain. With perfusion CT, it is possible to obtain two absolute values, cerebral blood flow (CBF) (ml/min/100g) and cerebral blood volume (CBV) (%). In using this examination, the main problem is the method of iodine injection. The maximum slope of time-attenuation curve in organs must be reached before the peak enhancement time of the sagittal sinus. To solve this problem, we used a new method in which total injection volume is 30ml, and the rate of injection is 9ml/sec. The data acquisition time is one second for each scan, and the time interval is one second, for 20 scans in total. With this method, we can obtain reliable information on blood flow in the damaged brain. The most common examination used for the detection of brain blood flow is single-phased dynamic CT with Xe inhalation. However, the Xe inhalant examination is difficult to use in the routine clinical setting. Perfusion CT will be more useful for the detection of brain blood flow.     

Estimation of local aortic elastic properties with MRI.

Aortic compliance and pulse wave velocity (PWV) are important determiners of heart load, and are clinically useful indices of cardiovascular risk. Most direct methods to derive them require invasive pressure measurement. In this work a noninvasive technique to evaluate aortic compliance and PWV using MRI is proposed. MRI magnitude and phase images to measure area and flow in the ascending aorta were acquired in a group of 13 young healthy subjects. Assuming that the early systolic part of the wave was unidirectional and reflectionless, PWV was determined as the ratio between flow and area variations at early systole. Our results were compared to pulse wave velocities derived from a direct transit time, and to one using ascending aortic area and peripheral brachial pulse pressure. The new method proved to be accurate and in good agreement with the transit time method, as well as with previously published results.     

Nonsubtractive spiral phase contrast velocity imaging.

Phase contrast velocity imaging is a standard method for accurate in vivo flow measurement. One drawback, however, is that it lengthens the scan time (or reduces the achievable temporal resolution) because one has to acquire two or more images with different flow sensitivities and subtract their phases to produce the final velocity image. Without this step, non-flow-related phase variations will give rise to an erroneous, spatially varying background velocity. In this paper, we introduce a novel phase contrast velocity imaging technique that requires the acquisition of only a single image. The idea is to estimate the background phase variation from the flow-encoded image itself and then have it removed, leaving only the flow-related phase to generate a corrected flow image. This technique is sensitive to flow in one direction and requires 50% less scan time than conventional phase contrast velocity imaging. Phantom and in vivo results were obtained and compared with those of the conventional method, demonstrating the new method's effectiveness in measuring flow in various vessels of the body. Magn Reson Med 42:704-713, 1999.     

Rapid quantitation of high-speed flow jets.

Flow jets containing velocities up to 5-7 m/s are common in patients with congenital defects and patients with valvular disease (stenosis and regurgitation). The quantitation of peak velocity and flow volume in these jets is clinically significant but requires specialized imaging sequences. Conventional 2DFT phase contrast sequences require lengthy acquisitions on the order of several minutes. Conventional spiral phase contrast sequences are faster, but are highly corrupted by flow artifacts at these high velocities due to phase dispersion and motion during the excitation and readout. A new prospectively gated method based on spiral phase contrast is presented, which has a sufficiently short measurement interval (<4 ms) to minimize flow artifacts, while achieving high spatial resolution (2 x 2 x 4 mm(3)) to minimize partial volume effects, all within a single breathhold. A complete single-slice phase contrast movie loop with 22 ms true temporal resolution is acquired in one 10-heartbeat breathhold. Simulations indicate that this technique is capable of imaging through-plane jets with velocities up to 10 m/s, and initial studies in aortic stenosis patients show accurate in vivo measurement of peak velocities up to 4.2 m/s (using echocardiography as a reference).     

Differential flow imaging by NMR

A new method for spatially resolved NMR flow measurements, named differential flow imaging (DFI), is introduced and experimentally verified. The DFI technique is based on the fact that flow velocity in any direction may cause a pixel position shift in the phase-encoding direction of a 2DFT NMR image. In this method two flow-influenced magnitude images are obtained by properly encoding and/or compensating the flow velocity. A spatial map of the desired component of the flow velocity can consequently be calculated from these two images. Since the DFI technique uses only the magnitude information of the complex images, it is not sensitive to systematic phase errors in contrast to other methods which are based on the phase measurements. On the other hand, the DFI technique can be combined with the phase measurement methods to perform multidimensional flow measurements in a shorter data acquisition time when the phase errors are small or corrected.     

Cavernosal venocclusive insufficiency in male impotence: evaluation of degree and location

The author's experience with 72 patients with impotence, one patient with Peyronie disease (no impotence), and two healthy volunteers indicates that incompetence of the venocclusive mechanism of the corpora cavernosa is probably the most common cause of vasculogenic impotence. Manometric and angiographic studies are the key to diagnosis but only when performed while the venocclusive mechanism is activated. Activation is achieved by intracavernosal injection of a papaverine-phentolamine mixture. Pharmacomanometry, performed with the newly developed pharmacologic maintenance erectile flow (PMEF) method, enabled precise quantitation of the leak at a given cavernosal pressure. A new appreciation of the limited volume of normal flow in the cavernosal arteries indicates that cavernosal leak in the range of only 20 ml/min is probably capable of contributing to impotence. Pharmacocavernosography enabled identification of sites of leakage (key information in the planning of surgical correction) but was unreliable in the evaluation of the severity of leakage.     

Resolution-insensitive velocity and flow rate measurement in low-background phase-contrast MRA.

Magnetic resonance (MR) phase-contrast (PC) flow measurements are degraded by partial volume errors when the spatial resolution is low, in particular when a large difference in signal magnitude exists between the fluid and the surrounding material. The latter is often the case in phantom studies and may be encountered when flow is measured in prosthetic vessel segments (such as shunts, grafts, and bypasses) and in contrast-enhanced blood. This paper presents a new method that is designed to measure flow in vessels of circular cross-section with Poiseuille flow and negligible background signal arising from static material around the lumen. The method calculates the average flow velocity directly from the original complex image data by integrating the signal in oppositely velocity-sensitized PC images. The radius is calculated from the summed signal modulus. The method allows accurate and resolution-insensitive measurements of the average flow velocity to be obtained in both cross-sectional and in-plane acquisitions. It is not critical to any of the assumed conditions. The validity and capabilities of the proposed technique are demonstrated by in vitro experiments.     

Patient-specific computational modeling of cerebral aneurysms with multiple avenues of flow from 3D rotational angiography images

RATIONALE AND OBJECTIVES: Previous studies of aneurysm flow dynamics based on three-dimensional (3D) rotational angiography (RA) images were limited to aneurysms with a single route of blood inflow. However, aneurysms of the circle of Willis frequently involve locations with more than one source of inflow, such as aneurysms of the anterior communicating artery. The highest resolution images of cerebral vessels are from RA images, but this technique is limited to visualizing only one route of inflow at a time, leaving a significant limitation in the application of 3DRA image sets for clinical studies of patient-specific computational fluid dynamics (CFD) simulations. In this report, subject-specific models of cerebral aneurysms with multiple avenues of flow are constructed from RA images by using a novel combination of image co-registration and surface merging techniques. MATERIALS AND METHODS: RA images are obtained by means of contrast injection in each vessel that provides inflow to the aneurysm. Anatomic models are constructed independently of each of these vascular trees and fused together into a single model. The model is used to construct a finite element grid for CFD simulations of hemodynamics. RESULTS: Three examples of patient-specific models are presented: an anterior communicating artery aneurysm, a basilar tip aneurysm, and a model of an entire circle of Willis with five coincident aneurysms. The method is evaluated with a numeric phantom of an aneurysm in the anterior communicating artery. CONCLUSION: These examples show that this new technique can be used to create merged network numeric models for CFD modeling. Furthermore, intra-aneurysmal flow patterns are influenced strongly by merging of the two inflow streams. This effect decreases as distance from the merging streams increases.     

Computer-assisted evaluation of aortic stiffness using data acquired via magnetic resonance.

Aortic stiffness is frequently assessed through pulse wave velocity (PWV) measurements. Based on data acquired by magnetic resonance (MR) using a one-dimensional time-of-flight technique, a new computational tool has been developed to rapidly construct flow velocity images and automatically calculate PWV. Comparison between PWV results obtained from this and a manual analysis demonstrates good agreement (correlation coefficient of 0.9951), while the new method improves the time efficiency by more than 20 times. The new method can also significantly improve flow signal quality and yield more credible results when strong interfering background signals are present.     

A novel flow-suppression technique using tailored RF pulses

The pulsatile nature of blood flow makes zipper-like artifacts along the coding direction in the two-dimensional Fourier transform NMR image. So far, spatial presaturation, one of the correction methods, is known to be effective in eliminating flow artifacts when the Fourier spin echo acquisition is employed. However, this method requires an additional RF pulse and a spoiling gradient for presaturation. Described in this paper is a new flow suppression technique, based on spin dephasing, using a set of tailored RF pulses. The proposed method does not require additional saturation RF pulses or spoiling gradient pulses, making it advantageous over other methods. In addition, the method is relatively robust to flow velocity. The proposed technique is equivalent to the existing flow saturation technique except that the elimination of the flow component is achieved by a pair of tailored 90–180° RF pulses in the spin echo sequence. The principle of the proposed method is the creation of a linear phase gradient within the slice along the slice selection direction for the moving material by use of two opposing quadratic phase RF pulses, i.e., 90° and 180° RF pulses with opposing quadratic phase distributions. That is to say, all the spins of the moving materials along the slice selection direction become dephased. Therefore, no observable signal is generated. Computer simulations and experimental results obtained using a 2.0-T whole-body imaging system on both a phantom and a human volunteer are also presented.     

The REICA method for quantification of cerebral blood flow is less affected by lung washout of [123I] IMP than the graph-plot method

Annals of Nuclear Medicine - The $$\gamma$$ -Ray Evaluation with Iodoamphetamine for Cerebral blood flow Assessment (REICA) method is a new method for cerebral blood flow (CBF) quantification with...     

Rapid MR imaging of blood flow with a phase-sensitive, limited-flip-angle, gradient recalled pulse sequence: preliminary experience

To assess blood flow rapidly, a limited-flip-angle, gradient recalled pulse sequence was modified to acquire two views at the same phase-encoding step in successive repetitions. One view is obtained with first-moment flow compensation, while the second view is obtained with selectable flow encoding (non-zero first moment) along one direction. Blood flowing along the encoded direction acquires a phase difference between the two views, resulting in signal dependent on both direction and speed of flow. Stationary tissues undergo no phase change. Therefore, the phase shift between the two views produces an image that spatially renders flow direction and velocity. With a 24-msec repetition time, a 256 X 128 matrix, and two excitations, data acquisition is completed in 13 seconds per location (both a magnitude image and a flow image are produced at each location). Images generated with flow phantoms confirmed the accuracy of this method. Preliminary clinical evidence in 23 human subjects suggests that this method is useful in evaluating portal hypertension, distinguishing arterial from venous flow, distinguishing between slow flow and clot, and confirming the presence of clot. This method appears to be a fast, easy way to assess blood flow in large vessels.     

Echo-planar high-resolution flow velocity mapping

A technique for the very rapid measurement of blood flow with high spatial resolution is described. The method combines the previously validated technique of phase velocity mapping and echo-planar principles. The relatively small diameter of blood vessels enables a high-resolution echo-planar flow measurement to be made with as few as 16 echoes such that the method can be incorporated into a near standard NMR scanner. Two sequence variations are tested and validated in vitro and one is used to demonstrate in vivo blood flow measurement. The results are shown to compare well with a previously validated less rapid method. The technique should enhance the potential of NMR flow imaging by enabling sudden changes in flow to be studied. It should also simplify the measurement of blood flow in small mobile vessels such as the coronary arteries.     

Quantitative measurement of high flow velocities by a spin echo MR technique.

A new method of flow measurement using a spin echo (SE) technique has been developed on the basis of the flow effect that at high velocities signal intensity decreases linearly with increasing flow velocity. Flow velocity is calculated from the signal intensity ratio of the flowing material in two images with the same imaging parameters but different echo times. The linear relationship between the signal intensity and flow velocity was examined with a steady flow phantom. When assessed with steady flows in the phantom, flow velocities calculated by this method were in good agreement with velocities measured by a flow meter. This method was used with ECG gating to measure the blood flow of the right common carotid artery of a healthy volunteer. The measured peak flow velocity and the pattern of flow velocities during systole correlated well with the results obtained by Doppler ultrasound.     

Phase-contrast MR angiography with reduced acquisition time: New concepts in sequence design

A new acquisition scheme for three-dimensional (3D) phase-contrast MR angiography reduces by 33% the measurement time for a data set sensitive to flow in all three orthogonal directions. Background suppression is achieved by acquiring a flow-compensated data set and three data sets flow encoded in the three orthogonal directions, with subsequent complex subtraction. The data are acquired in an interleaved fashion, eliminating misregistration artifacts due to patient motion between measurements sensitive to different flow directions. A standard maximum-intensity-projection algorithm is applied to the combined 3D data set to obtain angiographic projections sensitive to all three orthogonal flow directions. The theory and implementation of the method are described and examples of its application to the intracranial and abdominal circulation are provided.     

Multiphasic MR imaging: a new method for direct imaging of pulsatile CSF flow

A new technique is described that allows for the creation of pure pulsatile flow magnetic resonance (MR) images in a single acquisition. Five to 16 electrocardiographically gated images spanning the entire cardiac cycle are obtained with use of a gradient-echo pulse sequence. The section can be varied from 4 mm thick to full thickness projection. Taken singly, each image provides direct assessment of flow direction and velocity. Subtraction of image pairs eliminates signal detected from stationary protons, producing images of pulsatile flow. In this study the technique was used to image the flow of cerebrospinal fluid (CSF) in healthy subjects and in one patient with syringohydromyelia. The data suggest that multiphasic MR imaging provides a powerful means for the noninvasive assessment of CSF pulsatile flow dynamics and may have potential clinical application for the investigation of a variety of abnormalities such as normal pressure hydrocephalus, syrinx, and spinal block.     

Tomographic multiphase flow measurement

Measurement of multiphase flow of gas, oil and water is not at all trivial and in spite of considerable achievements over the past two decades, important challenges remain (Corneliussen et al., 2005). These are related to reducing measurement uncertainties arising from variations in the flow regime, improving long term stability and developing new means for calibration, adjustment and verification of the multiphase flow meters. This work focuses on the first two issues using multi gamma beam (MGB) measurements for identification of the type of flow regime. Further gamma ray tomographic measurements are used for reference of the gas/liquid distribution. For the MGB method one Am-241 source with principal emission at 59.5 keV is used because this relatively low energy enables efficient collimation and thereby shaping of the beams, as well as compact detectors. One detector is placed diametrically opposite the source whereas the second is positioned to the side so that this beam is close to the pipe wall. The principle is then straight forward to compare the measured intensities of these detectors and through that identify the flow pattern, i.e. the instantaneous cross-sectional gas-liquid distribution. The measurement setup also includes Compton scattering measurements, which can provide information about the changes in the water salinity for flow segments with high water liquid ratio and low gas fractions. By measuring the transmitted intensity in short time slots (<100 ms), rapid regime variations are revealed. From this we can select the time sections suitable for salinity measurements. Since the salinity variations change at the time scale of hours, a running average can be performed to increase the accuracy of the measurements. Recent results of this work will be presented here.