Magnetic resonance angiography of the carotid bifurcation

MRA methods may be categorized into TOF or PC techniques. TOF techniques utilize flow related enhancement to provide high signal intensity blood. Two common TOF methods to visualize the bifurcations are sequential 2D or 3D imaging. In 2D imaging, the carotid bifurcation is visualized by obtaining a series of thin 2D axial gradient echo images. This stack of images may then be subjected to postprocessing to show just the vessel geometry, with the background stationary tissue suppressed. Advantages of 3D techniques include a reduction of T2* effects and a theoretical increase in signal to noise. While the scan time is increased with 3D imaging with the additional direction of phase encoding, overall imaging times are comparable for 3D and sequential 2D techniques, with both being shorter than the 3D PC technique. Advantages of PC angiography include direct and effective suppression of background tissues and definition of slow flow states. This method also has the potential for quantitative flow measurements. Because the signal of blood from this technique depends on flow induced phase change, signal loss from more complex flow is more problematic than with TOF methods. It is apparent from the plethora of methods available that no single MRA technique can answer all clinical questions and situations. Specific techniques and parameters will have to be tailored to individual patient needs. While this makes the routine application of MRA more complex, it also will ensure that the maximum diagnostic yield is achieved.     

Intra-vascular blood velocity and volumetric flow rate calculated from dynamic 4D CT angiography using a time of flight technique

We examine a time of flight (TOF) approach for the analysis of contrast enhanced 4D volumetric CT angiography scans to derive and display blood velocity in arteries. Software was written to divide blood vessels into a series of cross sections and to track contrast bolus TOF along the central vessel axis, which was defined by a user, from 4D CT source data. Time density curves at each vessel cross section were fit with quadratic, Gaussian, and gamma variate functions to determine bolus time to peak (TTP). A straight line was used to plot TTP versus vessel path length for all three functions and the slope used to calculate intraluminal velocity. Software was validated in a simulated square channel and non-pulsatile flow phantom prior to the calculation of blood velocity in the major cerebral arteries of 8 normal patients. The TOF algorithm correctly calculates intra-luminal fluid velocity in eight flow conditions of the CT flow phantom where quadratic functions were used. Across all conditions, in phantoms and in vivo, the success of calculations depended strongly on having a sufficiently long path length to make measurements and avoiding venous contamination. Total blood flow into the brain was approximately 17 % of a normal 5 L cardiac output. The technique was explored in vivo in a patient with subclavian steal syndrome, in the pulmonary arteries and in the iliac artery from clinical 4D CT source data.  Intravascular blood velocity and flow may be calculated from 4D CT angiography using a TOF approach.     

Contrast-enhanced MR angiography of the intracranial circulation

NCE MRA can provide the authors with useful diagnostic information in patients suffering from intracranial vascular disease, often leading to improved or altered treatment decisions. Most centers have used 3D TOF for evaluation of stroke-the most common cerebral vascular disease. Because of slow and disturbed flow, conventional 3D TOF MRA tends to overestimate stenotic lesions and occluded arteries and this can confound neurovascular assessment in stroke patients. Post contrast 3D TOF techniques provide a more robust and more specific method for imaging the intracranial circulation that overcomes the drawbacks of conventional 3D TOF. In the setting of acute ischemic stroke, the authors have found that the combination of conventional and CE 3D TOF MRA improves their overall diagnostic ability. Dynamic and time-resolved CE MRA techniques have evolved rapidly. Time-resolved CE MRA, in particular, is emerging as a useful technique for imaging dynamic vascular pathologies such as AVMs. Unfortunately, time-resolved MRA of the intracranial circulation provides images with low spatial resolution and is currently limited to subsecond frame rate 2D acquisitions, and less than 2 seconds frame rates for 3D acquisitions. Nevertheless, like in other vascular regions, CE MRA represents a milestone for non-invasive intracranial vascular imaging. The continuing development of CE MRA techniques and of new contrast agents will lessen the need for intra-arterial angiography in the future.     

Novel and facile criterion to assess the accuracy of WSS estimation by 4D flow MRI

Four-dimensional flow magnetic resonance imaging (4D flow MRI) is a versatile tool to obtain hemodynamic information and anatomic information simultaneously. The wall shear stress (WSS), a force exerted on a vessel wall in parallel, is one of the hemodynamic parameters available with 4D flow MRI and is thought to play an important role in clinical applications such as assessing the development of atherosclerosis. Nevertheless, the accuracy of WSS obtained with 4D flow MRI is rarely evaluated or reported in literature, especially in the in vivo studies. We propose a novel and facile criterion called Reynolds resolution to assess the accuracy of WSS estimation in 4D flow MRI studies. Reynolds resolution consists of a spatial resolution, encoding velocity, kinematic viscosity of a working fluid, and signal-to-noise ratio, which are readily accessible information in 4D flow MRI measurements. We explored the relationship between Reynolds resolution and the WSS error. To include diverse and extensive cases, we measured three circular tubing flows with a diameter of 40, 8, and 2 mm. The 40 mm tubing flow was measured by 3 Tesla (T) human MR scanner with a knee coil and spatial resolution of 0.5 mm. The 8 and 2 mm tubing flows were both measured by 4.7 T MR scanner, but the scans were performed with a conventional birdcage coil (8 mm tubing) and a custom-made solenoid coil (2 mm tubing), respectively. The spatial resolution was varied from 0.2, 0.4 or 0.8 mm for the 8 mm tubing flow, but was fixed at 0.090 mm for 2 mm tubing flow. In addition, the near-wall velocity gradient, required to be determined prior to the WSS, was calculated using two methods; these included assuming a linear velocity profile or quadratic velocity profile near wall. The accuracy of WSS obtained using each method and tubing flow was evaluated against the theoretical WSS value. As a result, we found that Reynolds resolution is in logarithmic relation to the WSS error.     

MRI评价肺动静脉瘘

目的　评价MRI、MRA等对肺部血管畸形的临床应用价值。方法　分析了 6例手术或血管造影证实的AVF病例的多种表现 ,其中男 5例 ,女 1例 ,采用 1.0T超导MRI仪 ,体线圈技术。分别行SE、2D TOF、3D TOF及cine MRI多种序列成像 ,共行 3 5次MRI和MRA等检查。结果　 6例AVF中共发现 9个病灶。 2例共 3个病灶在SE序列中可见大小不等流空信号 ,其余为等信号之软组织块影、边缘不规则 ;在梯度回波序列 (2D、3D TOF及cine MRI)中病灶均呈高信号或极高信号影 ,在2D TOF中可见到 5例病灶的供血动脉及 3例引流静脉 ;在 3D TOF中可见到 4例病灶的供血动脉及 2例引流静脉 ;在cine MRI序列中可见到 3例病灶供血动脉及 2例引流静脉 ,且cine MRI可见到 5例病灶内信号强弱变化同心动周期一致。结论　目前 ,MRI多种序列的结合应用 ,可以较准确地诊断AVF ,作为一种无创性诊断方法有一定临床应用价值。     

Comprehensive 4D velocity mapping of the heart and great vessels by cardiovascular magnetic resonance

Background

Phase contrast cardiovascular magnetic resonance (CMR) is able to measure all three directional components of the velocities of blood flow relative to the three spatial dimensions and the time course of the heart cycle. In this article, methods used for the acquisition, visualization, and quantification of such datasets are reviewed and illustrated.

Methods

Currently, the acquisition of 3D cine (4D) phase contrast velocity data, synchronized relative to both cardiac and respiratory movements takes about ten minutes or more, even when using parallel imaging and optimized pulse sequence design. The large resulting datasets need appropriate post processing for the visualization of multidirectional flow, for example as vector fields, pathlines or streamlines, or for retrospective volumetric quantification.

Applications

Multidirectional velocity acquisitions have provided 3D visualization of large scale flow features of the healthy heart and great vessels, and have shown altered patterns of flow in abnormal chambers and vessels. Clinically relevant examples include retrograde streams in atheromatous descending aortas as potential thrombo-embolic pathways in patients with cryptogenic stroke and marked variations of flow visualized in common aortic pathologies. Compared to standard clinical tools, 4D velocity mapping offers the potential for retrospective quantification of flow and other hemodynamic parameters.

Conclusions

Multidirectional, 3D cine velocity acquisitions are contributing to the understanding of normal and pathologically altered blood flow features. Although more rapid and user-friendly strategies for acquisition and analysis may be needed before 4D velocity acquisitions come to be adopted in routine clinical CMR, their capacity to measure multidirectional flows throughout a study volume has contributed novel insights into cardiovascular fluid dynamics in health and disease.     

Can the degree of retrograde diastolic flow in abnormal umbilical artery flow velocity waveforms predict pregnancy outcome?

OBJECTIVE: Reverse end-diastolic flow is the most pathological type of the umbilical artery flow velocity waveform. We aimed to investigate whether additional prognostic information can be obtained from umbilical artery waveforms in cases with reverse end-diastolic flow. SUBJECTS AND METHODS: Umbilical artery Doppler velocity waveforms from 44 fetuses with reverse end-diastolic flow were analyzed and the following parameters measured: the highest amplitude and the area below the maximum velocity curve of forward and reverse flow (A, B and C, D, respectively) and the duration of forward and reverse flow (Tc and Td, respectively). Ratios A/B, C/D and Tc/Td were calculated. The cut-off values for A/B, C/D and Tc/Td with the best predictive values for perinatal death were established with the help of receiver operating characteristics curves. The three curves were compared with each other. RESULTS: Of the three ratios, A/B and C/D had the best capacity to predict perinatal death. Both ratios had acceptable sensitivities, specificities and positive predictive values. In this regard, A/B and C/D were comparable. The cut-off values for A/B and C/D were 4.3 and 4.52, respectively. Survivors had I significantly higher A/B and C/D ratios than non-survivors (P = 0.0001 and 0.0003, respectively). Significantly more fetuses with A/B or C/D below the established cut-off values had pulsations in the venous system (P < 0.05). In fetuses with a gestational age < =210 gestational days the survival rate was significantly higher in those with A/B or C/D above the cut-off values (P = 0.03 and 0.003, respectively). CONCLUSIONS: The A/B or C/D ratio can be used for quantification of the reverse end-diastolic flow waveforms in the umbilical artery and may offer additional information to the evaluation of fetal condition.     

Assessing cerebrospinal fluid flow connectivity using 3D gradient echo phase contrast velocity encoded MRI

The aim of this study is to investigate the feasibility of using three-directional velocity encoded 3D gradient echo (GE) phase contrast (PC) imaging to assess cerebrospinal fluid (CSF) flow connectivity in the human brain. Five healthy volunteers were scanned using low velocity sensitivity (V(enc) = 0.04-0.05 m s(-1)). Flow-time curves were compared to standard 2D PC scans. The 3D data were used to reconstruct in vivo CSF flow volumes based on time-averaged phase-difference information, and the patency of the CSF flow pathways was assessed using nearest-neighbour connectivity. A pulsatile flow phantom was used to gauge the measurement accuracy of the CSF flow volumes at low flow velocities. Flow connectivity from the lateral ventricles down to the cisterna magna was successfully demonstrated in all volunteers. The phantom tests showed a good distinction between the flow cavities and the background noise. 3D PC imaging results in CSF flow waveforms with similar pulsatility but underestimated peak velocities compared to 2D PC data. 3D time-resolved velocity encoded GE imaging has successfully been applied to assess CSF flow connectivity in normal subjects.     

二维平扫与三维增强法MRA在急性下肢动脉闭塞中的应用

目的 比较两种方法在急性下肢动脉闭塞中的应用价值。旨在临床急诊提供快速、准确的检查方法。方法 采用二维平扫ＴＯＦ法及三维增强法对１０例急性下肢动脉闭塞患进行检查。９例进行急诊手术,以手术结果为金标准进行效果评定。结果 二维平扫与三维增强诊断下肢动脉闭塞敏感性分别为７０％、１００％。三维增强对病变长度的估计准确性大于二维平扫。结论 三维增强法快速准确,是急性下肢动脉闭塞首选的检查方法《二维平扫法有价值,如定位明确,在大腿段病变可用二维平扫。     

Discrepancies in recommended criteria for grading of carotid stenosis with ultrasound

The accuracy of duplex ultrasound for grading of internal carotid artery stenosis has been widely tested and shown to be high. However, different methods for measurement of the degree of carotid stenosis with the golden standard conventional angiography have been used in the different studies. This, together with other factors, has led to some confusion regarding the relation between the ultrasonographically measured flow velocity and the angiographically measured degree of stenosis. The ultrasound criteria that are used in Sweden (and in Germany) differ in an important way from the criteria recommended in North America and the United Kingdom for the same degree of angiographic stenoses. Possible reasons for the discrepancies are discussed in this article. The authors recommend absolute agreement locally whether ECST or NASCET criteria shall be used in the communication between radiologists, clinical physiologists, vascular surgeons, neurologists and other physicians involved in patient management decisions. Angle‐dependent ultrasound criteria should be used and flow velocity measurements with ultrasound should be combined with assessment of plaque burden on 2D picture.     

Volumetric and quantitative imaging of retinal blood flow in rats with optical microangiography

In this paper, we present methods for 3D visualization and quantitative measurements of retinal blood flow in rats by the use of optical microangiography imaging technique (OMAG). We use ultrahigh sensitive OMAG to provide high-quality 3D RBF perfusion maps in the rat eye, from which the Doppler angle, as well as the diameters of blood vessels, are evaluated. Estimation of flow velocity (i.e. axial flow velocity) is achieved by the use of Doppler OMAG, which has its origins in phase-resolved Doppler optical coherence tomography. The measurements of the Doppler angle, vessel size, and the axial velocity lead to the quantitative assessment of the absolute flow velocity and the blood flow rate in selected retinal vessels. We demonstrate the feasibility of OMAG to provide 3D microangiograms and quantitative assessment of retinal blood flow in a rat model subjected to raised intra-ocular pressure (IOP). We show that OMAG is capable of monitoring the longitudinal response of absolute blood velocity and flow rate of retinal blood vessels to increased IOP in the rat, demonstrating its usefulness for ophthalmological research.     

三种TOF法MRA诊断实验性动脉瘤的对比分析

目的：比较分析三种ＴＯＦ法ＭＲＡ的图像质量，以及不同成像技术对动脉瘤的诊断价值。材料与方法：用静脉囊镶嵌法将１８只犬制成三种类型（单侧型、分叉型、末梢型）颈总动脉囊状动脉瘤模型，术后２周分别进行ＩＡＤＳＡ、２ＤＴＯＦ、单层块３ＤＴＯＦ、薄块多层重叠技术（ＭＯＴＳＡ）ＭＲＡ检查。结果：以ＩＡＤＳＡ图像为标准，２ＤＴＯＦ、单层块３ＤＴＯＦ、ＭＯＴＳＡＭＲＡ都能显示动脉瘤的位置、形态、类型、大小、瘤颈、瘤体伸展方向。其中以ＭＯＴＳＡＭＲＡ图像最具评价能力，显示动脉瘤的形态、类型、瘤颈优于２ＤＴＯＦ、单层块３ＤＴＯＦ，不足之处图像有层块梯阶样伪影。三种ＴＯＦ成像技术对动脉瘤内缓慢血流、湍流、血栓、大动脉瘤有不同程度的信号丢失。结论：三种ＴＯＦ法成像技术中，ＭＯＴＳＡ技术最有利于作为临床３ＤＴＯＦ法ＭＲＡ检查颅内动脉瘤的常规成像技术。     

A geometric flow for segmenting vasculature in proton-density weighted MRI

Modern neurosurgery takes advantage of magnetic resonance images (MRI) of a patient’s cerebral anatomy and vasculature for planning before surgery and guidance during the procedure. Dual echo acquisitions are often performed that yield proton-density (PD) and T2-weighted images to evaluate edema near a tumor or lesion. In this paper we develop a novel geometric flow for segmenting vasculature in PD images, which can also be applied to the easier cases of MR angiography data or Gadolinium enhanced MRI. Obtaining vasculature from PD data is of clinical interest since the acquisition of such images is widespread, the scanning process is non-invasive, and the availability of vessel segmentation methods could obviate the need for an additional angiographic or contrast-based sequence during preoperative imaging. The key idea is to first apply Frangi’s vesselness measure [Frangi, A., Niessen, W., Vincken, K.L., Viergever, M.A., 1998. Multiscale vessel enhancement filtering. In: International Conference on Medical Image Computing and Computer Assisted Intervention, vol. 1496 of Lecture Notes in Computer Science, pp. 130–137] to find putative centerlines of tubular structures along with their estimated radii. This measure is then distributed to create a vector field which allows the flux maximizing flow algorithm of Vasilevskiy and Siddiqi [Vasilevskiy, A., Siddiqi, K., 2002. Flux maximizing geometric flows. IEEE Transactions on Pattern Analysis and Machine Intelligence 24 (12), 1565–1578] to be applied to recover vessel boundaries. We carry out a qualitative validation of the approach on PD, MR angiography and Gadolinium enhanced MRI volumes and suggest a new way to visualize the segmentations in 2D with masked projections. We validate the approach quantitatively on a single-subject data set consisting of PD, phase contrast (PC) angiography and time of flight (TOF) angiography volumes, with an expert segmented version of the TOF volume viewed as the ground truth. We then validate the approach quantitatively on 19 PD data sets from a new digital brain phantom, with semi-automatically obtained labels from the corresponding angiography volumes viewed as ground truth. A significant finding is that both for the single-subject and multi-subject studies, 90% or more of the vasculature in the ground truth segmentation is recovered from the automatic segmentation of the other volumes.     

New developments in paediatric cardiac functional ultrasound imaging

Ultrasound imaging can be used to estimate the morphology as well as the motion and deformation of tissues. If the interrogated tissue is actively deforming, this deformation is directly related to its function and quantification of this deformation is normally referred as ‘strain imaging’. Tissue can also be deformed by applying an internal or external force and the resulting, induced deformation is a function of the mechanical tissue characteristics. In combination with the load applied, these strain maps can be used to estimate or reconstruct the mechanical properties of tissue. This technique was named ‘elastography’ by Ophir et al. in 1991. Elastography can be used for atherosclerotic plaque characterisation, while the contractility of the heart or skeletal muscles can be assessed with strain imaging. Rather than using the conventional video format (DICOM) image information, radio frequency (RF)-based ultrasound methods enable estimation of the deformation at higher resolution and with higher precision than commercial methods using Doppler (tissue Doppler imaging) or video image data (2D speckle tracking methods). However, the improvement in accuracy is mainly achieved when measuring strain along the ultrasound beam direction, so it has to be considered a 1D technique. Recently, this method has been extended to multiple directions and precision further improved by using spatial compounding of data acquired at multiple beam steered angles. Using similar techniques, the blood velocity and flow can be determined. RF-based techniques are also beneficial for automated segmentation of the ventricular cavities. In this paper, new developments in different techniques of quantifying cardiac function by strain imaging, automated segmentation, and methods of performing blood flow imaging are reviewed and their application in paediatric cardiology is discussed.     

Quantitative assessment of regurgitant flow with total digital three-dimensional reconstruction of color Doppler flow in the convergent region: in vitro validation.

BACKGROUND: This study was designed to develop and test a total digital 3-dimensional (3D) color flow map reconstruction for proximal isovelocity surface area (PISA) measurement in the convergent region. METHODS: Asymmetric flow convergent velocity field was created in an in vitro pulsatile model of mitral regurgitation. Image files stored in the echocardiographic scanner memory were digitally transferred to a computer workstation, and custom software decoded the file format, extracted velocity information, and generated 3D flow images automatically. PISA and volume flow rate were calculated without geometric assumption. For comparison, regurgitant volume was also calculated, using continuous wave Doppler, 2-dimensional (2D), and M-mode color flow Doppler with the hemispheric approach. RESULTS: Flows from 3D digital velocity profiles showed a closed, excellent relation with actual flow rates, especially for instantaneous flow rate. Regurgitant volume calculated with the 3D method underestimated the actual flow rate by 2.6%, whereas 2D and the M-mode method show greater underestimation (44.2% and 32.1%, respectively). CONCLUSION: Our 3D reconstruction of color flow Doppler images gives more exact information of the flow convergent zone, especially in complex geometric flow fields. Its total digital velocity process allows accurate measurement of convergent surface area and improves quantitation of valvular regurgitation.     

Fast block flow tracking of atrial septal defects in 4D echocardiography

We are working to develop beating-heart atrial septal defect (ASD) closure techniques using real-time 3D ultrasound guidance. The major image processing challenges are the low-image quality and the processing of information at high-frame rate. This paper presents comparative results for ASD tracking in time sequences of 3D volumes of cardiac ultrasound. We introduce a block flow technique, which combines the velocity computation from optical flow for an entire block with template matching. Enforcing adapted similarity constraints to both the previous and first frames ensures optimal and unique solutions. We compare the performance of the proposed algorithm with that of block matching and region-based optical flow on eight in vivo 4D datasets acquired from porcine beating-heart procedures. Results show that our technique is more stable and has higher sensitivity than both optical flow and block matching in tracking ASDs. Computing velocity at the block level, our technique tracks ASD motion at 2 frames/s, much faster than optical flow and comparable in computation cost to block matching, and shows promise for real-time (30 frames/s). We report consistent results on clinical intra-operative images and retrieve the cardiac cycle (in ungated images) from error analysis. Quantitative results are evaluated on synthetic data with maximum tracking errors of 1 voxel.     

Comparison of 4D flow and 2D velocity-encoded phase contrast MRI sequences for the evaluation of aortic hemodynamics

The purpose of this study was to compare aortic flow and velocity quantification using 4D flow MRI and 2D CINE phase-contrast (PC)-MRI with either one-directional (2D-1dir) or three-directional (2D-3dir) velocity encoding. 15 healthy volunteers (51?±?19 years) underwent MRI including (1) breath-holding 2D-1dir and (2) free breathing 2D-3dir PC-MRI in planes orthogonal to the ascending (AA) and descending (DA) aorta, as well as (3) free breathing 4D flow MRI with full thoracic aorta coverage. Flow quantification included the co-registration of the 2D PC acquisition planes with 4D flow MRI data, AA and DA segmentation, and calculation of AA and DA peak systolic velocity, peak flow and net flow volume for all sequences. Additionally, the 2D-3dir velocity taking into account the through-plane component only was used to obtain results analogous to a free breathing 2D-1dir acquisition. Good agreement was found between 4D flow and 2D-3dir peak velocity (differences?=??3 to 6?%), peak flow (?7?%) and net volume (?14 to ?9?%). In contrast, breath-holding 2D-1dir measurements exhibited indices significantly lower than free breathing 2D-3dir and 2D-1dir (differences?=??35 to ?7?%, p?<?0.05). Finally, high correlations (r?≥?0.97) were obtained for indices estimated with or without eddy current correction, with the lowest correlation observed for net volume. 4D flow and 2D-3dir aortic hemodynamic indices were in concordance. However, differences between respiration state and 2D-1dir and 2D-3dir measurements indicate that reference values should be established according to the PC-MRI sequence, especially for the widely used net flow (e.g. stroke volume in the AA).     

Vector Doppler imaging of a spinning disc ultrasound Doppler phantom

Vector Doppler methods are used to obtain angle independent in-plane velocity information. Velocity magnitude as well as direction are reconstructed from regular steered colour flow and from split-aperture Doppler acquisitions. Spatially resolved in-plane velocity was obtained through Doppler colour flow mode and subsequent data triangulation. A depth-invariant constant Doppler angle was achieved by using a depth expanding transmit-receive Doppler aperture. Velocities of up to 50 cm s(-1) and 360 degrees vector velocity directions were measured. This was achieved by creating a spinning solid disc phantom. Such a phantom was built to allow underwater mounting and spinning of a solid disc-shaped ultrasound phantom (maximum velocity of 50 cm s-1). Doppler triangulation was realised by steered Doppler and by a split-aperture approach. Results of both imaging methods are shown. Split-aperture results showed errors of less then 10% for velocity magnitude estimation and less then 2.5 degrees for directional information.     

Magnetic-resonance velocity mapping of the central circulation

Summary. In magnetic-resonance (MR) velocity mapping there exists a linear relationship between the velocity and signal in each element of a tomographic image. The technique can be used for quantitative measurements of linear velocities (m s^_1) and flow rates (1 min^-1). By using cinematographic images the flow profile during the cardiac cycle can be determined. This allows quantification of forward flow, regurgitant volume and regurgitant fraction in cases of heart-valve insufficiency. In valvular stenosis the transvalvular pressure gradient and valve area can be determined. Magnetic-resonance velocity mapping may also provide information about diastolic function of left ventricular function. Together with other MR imaging techniques, velocity mapping gives an accurate assessment of the severity of aortic dissection. Recent studies indicate that MR velocity mapping provides quantification of renal blood flow, and that MR imaging may be used even for coronary angiography and measurements of coronary blood flow. Therefore MR velocity mapping has the potential to become an important clinical tool for examination of the cardiovascular system providing high accuracy and quantitative measurements.     

Inflow effects on functional MRI

Blood inflow from the upstream has contribution or contamination to the blood oxygen level-dependent (BOLD) functional signal both in its magnitude and time courses. During neuronal activations, regional blood flow velocity increases which results in increased fMRI signals near the macrovasculatures. The inflow effects are dependent on RF pulse history, slice geometry, flow velocity, blood relaxation times and imaging parameters. In general, the effect is stronger with more T(1) weighting in the signal, e.g. by using a short repetition time and a large flip angle. This article reviews the basic principle of the inflow effects, its appearances in conventional GRE, fast spin-echo (FSE) and echo-planar imaging (EPI) acquisitions, methods for separating the inflow from the BOLD effect as well as the interplay between imaging parameters and other physiological factors with the inflow effects in fMRI. Based on theoretical derivation and human experiments, the inflow effects have been shown to contribute significantly in conventional GRE but negligible in FSE acquisitions. For gradient-echo EPI experiments, the blood inflow could modulate both amplitude and the temporal information of the fMRI signal, depending on the imaging parameters and settings.