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.     

横窦血液流量MR测量方法的比较研究

目的 通过与电影相位对比（PC）法和超声的比较，评价二维（2D）PC法测量横窦血液流速和流量的准确性。方法 （1）志愿者8例，共计测量12个横窦，在每个横窦相同层面上分别采用2DPC法和电影PC法进行血流信号面积，血液流速和流量测量，测量结果用配对t检验进行统计分析。（2）需要开颅手术患者5例。共计6个横窦，术前采用2DPC方法对横窦血液流速进行测量，术中暴露横窦以后，用TCD探测血液流速，测量结果采用相关回归分析。结果 统计结果表明：2DPC法和电影PC法测得的横窦血流信号面积（t=-1.106,P=0.293)。流速（t=0.262,P=0.798)和流量（t=0.439,P=0.669)均无显著性差异，2DPC测得的流速与TCD测得的流速相关性良好（y^=1.303x 0.62,r^2=0.88)。结论 2DPC法是测量横窦血液流量简便实用的方法。     

Calculation of flow velocities from the geometrical shape of contrast agent-induced time-density curves using DSA

A method for the calculation of flow velocities in vessels is introduced. The method is validated by measurements with an artificial model. Measurements with the carrier fluids sodium chloride solution and human blood using various scattering objects, show good correlation (r greater than +0.9) of the calculated and volumetrically measured velocities. With a menu-oriented computer program, measurement of flow velocities on DSA series can be made following the routine clinical examination.     

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.     

Blood flow mapping in the human liver by the xenon/CT method

In the noninvasive, nonradioactive xenon/CT method of blood flow measurement, xenon gas is inhaled, and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential CT; time-dependent xenon concentration within various tissue segments is then used to derive local blood flow maps. The usefulness of the method in the assessment of local cerebral blood flow has been documented. In this paper we explore its application to blood flow measurement in the human liver. In our preliminary clinical studies, hepatic blood flow ranged from 50 to 120 ml/100 cc/min in normal and adequately supplied tissue, and lower flow values were observed in tissue with abnormal function. The advantages and limitations of the method in such applications are discussed.     

Heart motion-adapted MR velocity mapping of blood velocity distribution downstream of aortic valve prostheses: initial experience

PURPOSE: To investigate blood flow velocities and shear rates at two distances downstream of an artificial aortic valve in patients. MATERIALS AND METHODS: Blood velocity was quantified downstream of the valve prosthesis (for replacement after aortic valve stenosis or combined stenosis and regurgitation) in 10 patients by using a magnetic resonance (MR) cine velocity mapping method in which the imaging section position is adapted according to the excursion of the valvular plane of the heart. Two acquisitions were performed to display the blood velocity distributions one-fourth valve diameter and one valve diameter downstream of the valve and to quantify blood volumes and shear rates. RESULTS: The velocity profiles measured during flow acceleration one-fourth valve diameter downstream were characterized by a distinct pattern of two lateral jets and one central jet of antegrade flow. High shear rates were found along the leaflet tips. The profiles obtained one valve diameter downstream were skewed, with varying velocity patterns among patients. Peak shear rates were found close to the vessel wall. With correction for through-plane motion of the valve, the mean apparent regurgitant fraction (+/- SD) was 14% +/- 6; the mean regurgitant fraction without correction was 9% +/- 5. CONCLUSION: The described noninvasive procedure for velocity mapping enables measurements close to the valve and thus evaluation of blood flow patterns with respect to valve design in humans.     

Quantifying MRI T1 relaxation in flowing blood: implications for arterial input function measurement in DCE-MRI

Objectives:To investigate the feasibility of accurately quantifying the concentration of MRI contrast agent in flowing blood by measuring its T1 in a large vessel. Such measures are often used to obtain patient-specific arterial input functions for the accurate fitting of pharmacokinetic models to dynamic contrast enhanced MRI data. Flow is known to produce errors with this technique, but these have so far been poorly quantified and characterised in the context of pulsatile flow with a rapidly changing T1 as would be expected in vivo.Methods:A phantom was developed which used a mechanical pump to pass fluid at physiologically relevant rates. Measurements of T₁ were made using high temporal resolution gradient recalled sequences suitable for DCE-MRI of both constant and pulsatile flow. These measures were used to validate a virtual phantom that was then used to simulate the expected errors in the measurement of an AIF in vivo.Results:The relationship between measured T1 values and flow velocity was found to be non-linear. The subsequent error in quantification of contrast agent concentration in a measured AIF was shown.Conclusions:The T1 measurement of flowing blood using standard DCE- MRI sequences are subject to large measurement errors which are non-linear in relation to flow velocity.Advances in knowledge:This work qualitatively and quantitatively demonstrates the difficulties of accurately measuring the T1 of flowing blood using DCE-MRI over a wide range of physiologically realistic flow velocities and pulsatilities. Sources of error are identified and proposals made to reduce these.     

Determination of instantaneous and average blood flow rates from digital angiograms of vessel phantoms using distance-density curves

We have developed a new method for quantitation of blood flow rates based on determination of the spatial shift of the distribution of contrast material in opacified vessels in digital angiograms that are acquired while the contrast material proceeds through the vessel. The distance that the contrast material travels during the time between image acquisitions is determined by comparison of density-vs.-distance curves, which represent the distribution of contrast material along the length of the vessel in the respective images. The flow rate between image acquisitions is calculated by multiplication of the distance traveled by the frame rate and the vessel cross-sectional area. Therefore, for high-frame-rate acquisitions, "instantaneous" flow rates can be determined. In these vessel-phantom studies, the root-mean-square (RMS) difference between instantaneous flow rates measured with our technique and those measured with an electromagnetic method was 2.9 ml/sec; the RMS difference for average flow rates was 0.8 ml/sec.     

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).     

High-resolution blood flow velocity measurements in the human finger.

MR phase contrast blood flow velocity measurements in the human index finger were performed with triggered, nontriggered, and cine acquisition schemes. A strong (G(max) = 200 mT/m), small bore (inner diameter 12 cm) gradient system inserted in a whole body 3 Tesla MR scanner allowed high-resolution imaging at short echo times, which decreases partial volume effects and flow artifacts. Arterial blood flow velocities ranging from 4.9-19 cm/sec were measured, while venous blood flow was significantly slower at 1.5-7.1 cm/sec. Taking into account the corresponding vessel diameters ranging from 800 microm to 1.8 mm, blood flow rates of 3.0-26 ml/min in arteries and 1.2-4.8 ml/min in veins are obtained. The results were compared to ultrasound measurements, resulting in comparable blood flow velocities in the same subjects. Magn Reson Med 45:716-719, 2001.     

Pulsed-injection method for blood flow velocity measurement in intraarterial digital subtraction angiography

The pulsed-injection method for measuring the velocity of blood flow in intraarterial digital subtraction angiography is described. With this technique, contrast material is injected at a pulsing frequency as high as 15 Hz, so that two or more boluses can be imaged simultaneously. The velocity of flow is determined by measuring the spacing between the boluses and multiplying it by the pulsing frequency. Results of tests with phantoms correlate well with flow measurements obtained with a graduated cylinder for velocities ranging from 8 to 60 cm/sec. The potential of the method for time-dependent velocity measurement has been demonstrated with simulated pulsatile flows.     

Effect of breath holding on blood flow measurement using fast velocity encoded cine MRI.

Breath-hold MR measurement of cardiac output was compared with results from respiratory triggered MR acquisitions, since flow measurement during breath-holding may be different from physiological blood flow. Cardiac output during large lung volume breath-holding (4.47 +/- 0.63 l/min in the aorta and 4.53 +/- 0.59 l/min in the pulmonary artery) was significantly lower than that measured during normal breathing (6.09 +/- 0.49 l/min and 6.48 +/- 0.67 l/min, P < 0.01). In contrast, no significant difference was found between measurements conducted with small lung volume breath-holding (5.87 +/- 0.53 l/min and 6.41 +/- 0.75 l/min) and normal breathing. In conclusion, breath-hold MR flow measurement using small lung volume by shallow inspiration can provide a blood flow quantification that is close to physiological blood flow. Magn Reson Med 45:346-348, 2001.     

Breath-hold MR measurements of blood flow velocity in internal mammary arteries and coronary artery bypass grafts

Breath-hold velocity-encoded cine MR (VENC-MR) imaging is a feasible method for measuring phasic blood flow velocity in small vessels that move during respiration. The purposes of the current study are to compare breathhold VENC-MR measurements of flow velocities in the internal mammary arteries (IMA) with nonbreath-hold measurements and to characterize the systolic and diastolic flow velocity curves in a cardiac cycle in native IMA and IMA grafts. Flow velocity in 30 native IMA and 8 IMA grafts were evaluated with a breath-hold VENC-MR sequence with K-space segmentation and view-sharing reconstruction(TR/TE=16/9 msec, VENC=100 cm/s). In 10 native IMA, nonbreathhold VENC-MR images were acquired as well for comparison. Breath-hold VENC-MR imaging showed significantly higher systolic and diastolic peak velocities in native IMA (43.1 cm/second ± 15.0 and 10.0 cm/second ± 4.8), in comparison to those of nonbreath-hold VENC-MR imaging (27.6 cm/second ± 10.2 and 7.3 cm/second ± 3.9, P<.05). The diastolic/systolic peak velocity ratio in the IMA grafts (.88 ± .41) was significantly higher than that in native IMA (.24 ± .08, P<.01). Interobserver variability in the flow velocity measurement was less than 4%. Breath-hold VENC-MR imaging demonstrated higher peak flow velocity in the IMA than nonbreath-hold VENC-MR imaging. This technique is a rapid and effective method for the noninvasive assessment of blood flow velocity in IMA grafts.     

Measurement of aortic blood flow with MR imaging: comparative study with Doppler US

An innovative magnetic resonance imaging technique was applied to the measurement of blood flow in the abdominal aorta. The technique combines selective excitation and visualization from an orthogonal view. The distance that fluid has moved is directly visualized. The blood flow velocity at every 50 msec throughout the cardiac cycle was measured in a short time (about 4 minutes) using electrocardiographic gating and repeated excitations in each cycle. Measurements were compared with those obtained by Doppler ultrasound (US) as a reference. The pulsatile change of flow velocity in the cycle correlated well with the Doppler US recording. Two flow velocity indexes, peak flow velocity and the velocity integral, also showed good correlation (r = .98 for both). This method is applicable for clinical use and is useful for measurement of high flow rates, as found in arteries.     

Semiquantitative fast flow velocity measurements using catheter coils with a limited sensitivity profile.

Flow measurements can be used to quantify blood flow during MR-guided intravascular interventional procedures. In this study, a fast flow measurement technique is proposed that quantifies flow velocities in the vicinity of a small RF coil attached to an intravascular catheter. Since the small RF coil receives signal from only a limited volume around the catheter, a spatially nonselective signal reception is employed. To enhance signal from flowing blood, and suppress unwanted signal contributions from static material, a slice-selective RF excitation is used. At a velocity sensitivity of 150 cm/s, a temporal resolution of 2 x TR = 10.2 ms can be achieved. The flow measurement is combined with an automatic slice positioning to facilitate measurements during interventional procedures. The influence of the catheter position in the blood vessel on the velocity measurement was analyzed in simulations. For blood vessels with laminar flow, the simulation showed a systematic deviation between catheter measurement and true flow between -15% and 80%. In four animal experiments, the catheter velocity measurement was compared with results from a conventional ECG-triggered 2D phase-contrast (PC) technique. The shapes of the velocity time curves in the abdominal aorta were nearly identical to the conventional measurements. A relative scaling factor of 0.69-1.19 was found between the catheter velocity measurement and the reference measurement, which could be partly explained by the simulation results.     

Parametric imaging of cerebral vascular reserve. 2. Reproducibility, response to CO2 and correlation with middle cerebral artery velocities

We report the reproducibility and response to change in end-tidal CO2 of a new method of quantifying regional mean cerebral transit time (MCTT) compared with the reproducibility and CO2 reactivity of middle cerebral artery (MCA) blood flow velocities measured using transcranial Doppler ultrasound. Within the range of end-tidal CO2 which could be achieved in conscious subjects breathing spontaneously, hemispheric MCTT, peak MCA velocity and mean MCA velocity showed a linear relationship with end-tidal CO2. After correction to a standardised end-tidal CO2, the coefficients of variation were 5.7% for hemispheric MCTT, 6.3% for peak MCA velocity and 6.8% for mean MCA velocity. Under the conditions of this study, MCA blood flow velocity was proportional to the reciprocal of MCTT, which in turn represents the ratio of blood flow to blood volume. Although the two methods appear to provide similar information, measurement of MCTT is quicker to perform, is less observer-dependent, provides regional information, uses conventional equipment present in most nuclear medicine departments and is less subject to problems associated with patient movement.     

Demonstration of pulsatile cerebrospinal-fluid flow using magnetic resonance phase imaging

The study of pulsatile cerebrospinal-fluid (CSF) flow may be useful in diagnosis of certain forms of intracranial disease. Previous techniques used to study CSF flow either are invasive or do not allow accurate measurement. Magnetic resonance imaging (MRI) offers a non-invasive method of studying the CSF pathways. Our technique uses MR phase images and allows quantitative measurement of flow velocities and volume-flow rates. Four volunteers were studied at the level of the second cervical vertebra (C2). The MRI pulse sequence was gated from the R-wave of the subject's electrocardiogram and 12 scans were taken corresponding to different times in the cardiac cycle. The variation in flow velocity throughout the cycle was plotted, and maximum caudad and cephalad flow velocities and flow rates were calculated. Good agreement was found between three of the four volunteers. The mean maximum caudad velocity was 2.91 cm s-1 occurring at a mean time of 190 ms after the R-wave. This corresponds to a mean maximum flow rate of 4.13 ml s-1. The total imaging time for each study was about 1 h. Technical developments, allowing simultaneous acquisition of several images throughout the cardiac cycle, will reduce this time significantly.     

Nuclear magnetic resonance velocity spectra of pulsatile flow in a rigid tube.

Velocity spectra can be derived from velocity-encoded nuclear magnetic resonance (NMR) images. Velocity spectra are histograms showing the amounts of fluid flowing at different velocities in the sensitive volume of the measurement. Velocity spectra may prove to be useful in characterizing the flow of blood in small vessels, for example, in detecting the presence of stenoses and in evaluating their severity. NMR velocity spectra acquired in vivo are sufficiently complicated that a model system was designed and tested to investigate the velocity spectra of pulsatile flow. This study measured the NMR velocity spectra of pulsatile flow in a rigid tube and compared them to velocity spectra derived from Doppler ultrasound measurements and to velocity spectra inferred from a theoretical model driven by the measured pressure difference function. The experimental results from each technique agree.     

MR measurement of blood flow in the cardiovascular system.

Methods for measurement of blood flow with MR were devised many years ago but have been used for diagnosis in only the past few years. The two methods of measurement that have been used most extensively are based on the principles of time of flight and phase shift. A number of factors can influence the accuracy of MR measurements of blood flow. In vitro studies using flow phantoms have verified the accuracy of the phase-shift technique for measuring flow velocities exceeding 5 m/sec, which for practical purposes, encompasses the peak flow encountered in cardiovascular disorders. The flow measurements have been used to quantify valvular heart disease, congenital heart disease, pulmonary arterial disease, thoracic aortic disease, and peripheral vascular disease.     

Can dynamic susceptibility contrast magnetic resonance imaging perfusion data be analyzed using a model based on directional flow?

PURPOSE: To examine the implications of a physiological model of cerebral blood that uses the contradictory assumption that blood flow in all voxels of DSCE-MRI data sets is directional in nature. Analysis of dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSCE-MRI) uses techniques based on indicator dilution theory. Underlying this approach is an assumption that blood flow through pixels of gray and white matter is entirely random in direction. MATERIALS AND METHODS: We have used a directional flow model to estimate theoretical blood flow velocities that would be observed through normal cerebral tissues. Estimates of flow velocities from individual pixels were made by measuring the mean transit time for net flow (nMTT). Measurements of nMTT were made for each voxel by estimating the mean difference in contrast arrival time between each of the adjacent six voxels. RESULTS: Examination of the spatial distribution of contrast arrival time from DSCE-MRI data sets in normal volunteers demonstrated clear evidence of directional flow both in large vessels and in gray and white matter. The mean velocities of blood flow in gray and white matter in 12 normal volunteers were 0.25 +/- 0.013 and 0.21 +/- 0.014 cm/second, respectively, compared to predicted values of 0.25 and 0.18 cm/second. These values give measured nMTT for a 1-mm isotropic voxel of gray and white matter of 0.45 +/- 0.12 and 0.52 +/- 0.11 seconds, respectively, compared to predicted values of 0.47 and 0.55 seconds. CONCLUSION: A directional model of blood flow provides an alternative approach to the calculation of cerebral blood flow from (CBF) DSCE-MRI data.