Flow-related Signal Research Articles

Implementation of a rapid inversion-prepared dual-contrast gradient echo sequence for quantitative dynamic contrast-enhanced magnetic resonance imaging of the human prostate

The first step in quantitative pharmacokinetic modeling is to determine the arterial input function (AIF) by deriving the contrast medium (CM) concentration from an appropriate imaging sequence by monitoring changes in either the amplitude or the phase signal of an accommodative artery. The bolus passage is best detected on T2- or T2*-weighted images, while extravasation is best assessed on T1-weighted images. Here, an imaging sequence is used that employs a parallel acquisition technique for the interleaved acquisition of an inversion-prepared T1-weighted image and a T1/T2*-mixed-weighted image for determination of the AIF. The sequence was applied in six patients with prostate cancer. A method is presented for quantifying the AIF derived from the signal intensity-time courses of both the T1/T2*-mixed-weighted and the T1-weighted image. Furthermore, in some patients the signal intensity-time course of the T1-weighted image exhibits flow-induced signal modulations. To reduce the effect of this flow-related signal enhancement the corresponding phase information was used. The sequence presented here has the potential to improve the quantification of the AIF at all time points and pharmacokinetic modeling of the CM dynamics of the prostate.

On flow effects in balanced steady‐state free precession imaging: Pictorial description, parameter dependence, and clinical implications

To present a pictorial description of the origin of flow effects in balanced steady-state free precession (SSFP) imaging that can result in considerable frequency offset-dependent signal changes originating from outflow of spins that can still contribute to the total SSFP signal (out-of-slice contributions), to analyze the parameter dependence and slice broadening associated with outflow effects, and to illustrate clinical implications such as frequency offset-dependent flow artifacts and spatial misencoding. Computer simulations were used to create a pictorial description of flow effects that illustrates the origin and parameter dependence of the observed signal changes and links their frequency dependency to the phase distribution of spins flowing in and out of the imaging slice. Slice broadening associated with out-of-slice contributions and flow-related signal enhancement was characterized by an effective slice thickness, which depends on flow rate, the T2* decay of signal magnitude from spins that have left the imaging slice, and the ratio of the net out-of-slice magnetization relative to the thickness of the imaging slice. Both simulated SSFP signal intensities and effective slice broadening vary with flip angle and demonstrate a nonlinear dependence on inflow. Simulations with bloodlike T1 and T2 show that the effective slice thickness can be as large as 15 times the prescribed slice thickness. These effects can have significant clinical implications such as large frequency offset-dependent signal enhancement, pulsatile flow artifacts, and spatial misrepresentation of blood signal that has already left the imaging slice. Flow-related signal changes in SSFP imaging exhibit highly complex parameter dependence, which predominantly has to be associated with frequency offset-dependent outflow effects and a resulting broadening of the slice thickness.

Reduction of flow-related signal loss in flow-compensated 3D TOF MR angiography, using variable echo time (3D TOF-VTE).

High-resolution MRA with phase/frequency flow compensation may require very long echo times (TEs). Variable TE (VTE) was implemented into flow-compensated 3D TOF to minimize the effective TE and reduce the flow-related signal void. The k-space of the 3D TOF was divided into segment groups ranging from two to 32 segments with different TEs. The TEs were minimized and the flow-compensation gradient lobes were calculated to null the total first moment at the peak of the echo for each segment. Possible artifacts and off-resonance effects were evaluated, with respect to the number of TE segments, using the point spread function (PSF) and corresponding experiments. The optimal number of TE segments for the least artifact was determined to be one-half of the number of slices. Two types of artifacts caused by VTE were predicted and subsequently observed. The developed pulse sequence 3D TOF-VTE was tested on clinical MRI systems, by performing scans of the cervical carotid artery and intracranial carotid artery at the carotid siphon. The signal distribution near the bifurcation and the siphon was much more uniform with VTE, and the flow-related signal loss was greatly reduced. The resultant MR angiograms provided improved vessel detail. The results show that VTE improved the quality of flow-compensated 3D TOF MRA.

Inhibition and functional magnetic resonance imaging

This review summarizes our current knowledge on how inhibitory phenomena are reflected in the functional magnetic resonance imaging (fMRI) signal. It is well-established that activity-related changes of brain metabolism and blood flow are dominated by changes in synaptic activity. Both excitatory and inhibitory synaptic activities are associated with increased metabolic demands. The amount of energy consumption associated with inhibition vs. excitation is reflected in metabolism- and blood flow-related neuroimaging signals such as the blood oxygen level-dependent (BOLD) contrast; the relationship between the different “signals”, however, may not be linear. The influence on these signals depends on the number of active inhibiting synapses, the duration of inhibition and the degree of propagation within subsequent neuronal circuits. The relative influence of inhibition as compared to excitation on the metabolism/blood flow may also vary in different neuronal circuits. Inhibition leads to locally decreased discharge activity, which does not have a significant effect on the cerebral blood flow (CBF)/BOLD images. However, inhibition may also result in suppression of complex neuronal circuits, leading to a decrease in excitatory as well as inhibitory synaptic activity, and therewith, to a decreased metabolism and blood flow within those complex neuronal networks. The available fMRI data indicate that, depending on the abovementioned factors, inhibition may be reflected in positive, negative or no BOLD-signal at all. Thus, the BOLD-signal is ambiguous with respect to the underlying electrophysiological event. In the future, combining fMRI with electrophysiological methods will strengthen neuroimaging studies.

Endothelial Signaling in Kidney Morphogenesis: A Role for Hemodynamic Forces

The local presence of endothelial cells seems necessary for proper embryonic development of several organs. However, the signals involved are unknown. The glomerulus is generated by the coalescence of podocytes around an ingrowing capillary and is the site of blood ultrafiltration. In the absence of vessels, glomerular assembly does not occur. We describe mutations in the zebrafish that prevent glomerulogenesis. All mutants display cardiac dysfunction. Pharmacological interference with cardiac output and focal laser occlusion of the vessel similarly prevent glomerular formation. The unifying feature of all these perturbations is absence of blood flow. We find that expression of matrix metalloproteinase-2 (MMP-2), known in other systems to be regulated in a stretch-responsive manner, is in renal endothelial cells and is regulated by flow, suggesting that an MMP-2-sensitive event may be downstream of the flow-related signal. In support of this, blockade of MMP-2 activity by injection of TIMP-2 does not perturb circulation but does prevent glomerular assembly. Thus, vascular flow is required for glomerular assembly, most probably acting via a stretch-responsive signaling system in the vessel wall.

Endoscopic Third Ventriculocisternostomies In The Infant: Pre- & Post-Operative Magnetic Resonance Imaging EvaluationElective Project: Undergraduate Prize 2000

PURPOSE: To determine whether it is possible to select patients with obstructive hydrocephalus, in the under 1 age group for endoscopic third ventriculocisternostomy (ETV) using pre-operative T2 weighted turbo spin echo (T2W-TSE) sagittal sequence Magnetic Resonance (MR) imaging; and to assess ventriculocisternostomy patency using post-operative T2W-TSE MR.PATIENTS AND METHODS: A retrospective review of MR examinations and clinical notes of 11 patients under 1 year of age who had ETV, was performed. The post-operative flow MR images were divided into the presence or absence of flow-related signal changes.RESULTS: In 6 of the 11 patients, ETV was successful (54.5%) i.e. no VP shunt or revision of the ETV was required. 9 patients had post-operative T2W-TSE MR examinations — 8 of these 9 MR studies correlated to the clinical situation (89%). The remaining MR examination showed a CSF flow void but the ETV failed at 3 weeks.CONCLUSION: Pre-operative MR using T2W-TSE to select suitable candidates for ETV improves the success rate from < 40% to 54.5%. Post-operatively MRI is a good predictor of whether the ETV has been successful or not.Sharman, A. (2000). Clinical Radiology55, 989.

Intracranial black-blood MR angiography with high-resolution 3D fast spin echo.

Three-dimensional fast spin-echo (3DFSE) techniques are promising for black-blood imaging of cerebral vessels. In this study, flow-related signal dephasing was demonstrated as the primary mechanism for blood signal attenuation. Parameter optimization of TR (1500 to 3000 ms), receiver bandwidth (25 to 31.25 kHz), effective TE (25.7 to 30.1 ms), and ETL (7 to 8) was accomplished by making measurements of vessel-to-tissue contrast-to-noise ratios on vessels. A comparison of high-resolution 3DFSE and 3DTOF magnetic resonance angiography demonstrated that 3DFSE can generate images with equivalent or better small vessel detail than conventional techniques. 3DFSE black-blood techniques may provide improved sensitivity of small arteries and veins with slow or in-plane flow and immunity to flow-related distortions. Future studies with optimized parameters will determine the clinical efficacy of this technique.

Cerebral Venous Sinus Thrombosis: Prognostic and Therapeutic Significance of an Early Radiologic Diagnosis

We describe the CT and MRI patterns of cerebral venous sinus thrombosis (CVST) on the basis of the venous angioarchitecture and the underlying pathophysiological mechanism. We also investigated if any radiologic data exist to establish which patients can be followed conservatively and which warrant endovascular treatment. The clinical, CT-CTA and MRI-MRA findings of 11 patients (2 men; 9 women; 24 to 69 years-old) with CVST were reviewed. The morphological patterns of CVST were divided into two major groups: Vascular signs: spontaneous sinusal hyperdensity (9); spontaneous all-sequences hyperintensity (4); venous engorgement (9); empty delta sign (4); lack of sinusal contrast-enhancement (3); delayed sinusal transit-time (11); lack of flow-related signal (3). Parenchymal signs: mass effect and cortical sulcal effacement (8), white matter edema (7), venous ischemia (6), haemorrhagic infarct (3), breakdown of the blood-brain barrier (4), hydrocephalus (2). The clinical and radiologic pictures are related to cerebral venous angioarchitecture and underlying pathophysiologic mechanism of venous thrombosis. Reversibility of clinical symptoms and parenchymal lesions is far more frequent, because vessel damage slowly and progressively develops, whereas damage to brain tissue occurs later. Consequently, a prompt CT-MRI diagnosis may allow a good prognosis. Treatment using selective sinusal instillation of urokinase is considered only when the patient clinically and radiologically does not improve within the first two weeks after heparinization.

Sliding interleaved kY (SLINKY) acquisition: a novel 3D MRA technique with suppressed slab boundary artifact.

This work addresses the elimination of the slab boundary artifact (SBA) or venetian blind artifact in three-dimensional multiple overlapped thin slab acquisition (3D MOTSA) for magnetic resonance angiography (MRA). Our method uses a sliding-slab, interleaved kY (SLINKY) data acquisition strategy, equalizing flow-related signal intensity weighting across the entire slab dimension. This technique demodulates signal intensity changes along the slab direction and can essentially eliminate the SBA while retaining the same or better imaging time efficiency than that of conventional MOTSA, providing robustness to complicated flow patterns and thereby resulting in more accurate depiction of vascular morphology. In addition, this technique does not need specialized reconstruction and extra computation. The unique penalty of this technique is the sensitivity to phase inconsistency in the data. Both phantom and in vivo experiments verify the clinical significance of the technique. The new MRA images acquired with this imaging technique show highly reliable mapping of vascular morphology without the SBA and reduction of signal voids in complex/slow flow regions.

Endoscopic Third Ventriculocisternostomy: MR Assessment of Patency with 2-D Cine Phase-Contrast versus T2-Weighted Fast Spin Echo Technique

Purpose: To determine if fast spin-echo T2 (FSE) is of equal value to flow-sensitive 2D cine phase-contrast (CPC) to assess patency of endoscopic third ventriculocisternostomies (VC). Patients and Methods: We reviewed clinical charts and MR scans of 27 patients who underwent third VC for treatment of obstructive hydrocephalus. Thirty-nine postoperative scans included both sequences and were assessed for the presence or absence of flow-related signal changes. Results: In 28 cases, FSE, CPC, and clinical findings suggested patency. In 1 case, CPC and FSE suggested occlusion, which was confirmed clinically and operatively. In the remaining cases, FSE showed better clinical correlation than did CPC. Conclusion: The assessment of third VC patency with FSE, a sequence available on most clinical scanners without a requirement for special hardware and software, is at least as sensitive in the qualitative assessment of VC function as CPC.

Uterine arteriovenous malformations: gray-scale and Doppler US features with MR imaging correlation.

To describe the gray-scale and color and duplex Doppler ultrasound (US) and the magnetic resonance (MR) imaging features of uterine arteriovenous malformations (AVMs). Uterine AVMs in 10 patients were retrospectively evaluated. All patients underwent gray-scale US and color and duplex Doppler US. Nine underwent angiography with therapeutic embolization; four, MR imaging. The resistance index (RI), pulsatility index (PI), and peak systolic velocities (PSVs) were evaluated. At gray-scale US, uterine AVMs were nonspecific and manifested as subtle myometrial inhomogeneity, tubular spaces within the myometrium, intramural uterine mass, endometrial mass, or cervical mass or sometimes as prominent parametrial vessels. Color Doppler features were consistent and included intense juxtaposed signals with aliasing and apparent flow reversals. Spectral Doppler US revealed low-resistance flow (RI, 0.25-0.55; PI, 0.3-0.6) and PSVs greater than 96 cm/sec, which suggests arteriovenous shunting. MR imaging showed a bulky uterus, a focal uterine mass, disruption of the junctional zones, serpiginous flow-related signal voids, and prominent parametrial vessels. Gray-scale morphology and Doppler US features should allow noninvasive diagnosis of uterine AVMs. Doppler and MR imaging features of uterine AVMs may overlap with other causes of arteriovenous shunting, including abnormal placentation and gestational trophoblastic disease (GTD). These can be differentiated with serum beta human chorionic gonadotropin test results (negative with AVM, positive with GTD).

Artifacts and signal loss due to flow in the presence of B(o) inhomogeneity.

An in vitro study was performed to investigate the effects of B(o) inhomogeneity on magnetic resonance images of flow. Controlled inhomogeneity gradients (Gi) were applied and the magnitude of the artifacts produced was quantified for different echo delay times (TE). Both steady and pulsatile flows were examined. In the presence of an inhomogeneity gradient, signal loss is apparent if the flow is pulsatile and/or if the slice thickness is large. The signal loss increases with increasing TE and Gi. With pulsatile flow, ghosting artifacts are also generated. These increase in intensity with increasing TE and Gi. In vivo, field inhomogeneity due to susceptibility variations is large enough to produce these effects. Representative time-of-flight images obtained of a normal volunteer with two different TEs demonstrate the effect in vivo. Flow-related signal loss and artifacts, therefore, increase with increasing TE independent of the moments of the applied gradients.

Measurement of fluid-shear rate by Fourier-encoded velocity imaging.

A new technique for estimating the blood fluid shear rate at the vessel wall is presented. The technique uses Fourier-encoded velocity imaging to determine the velocity distribution within a spatial element (voxel) that straddles the blood-vessel wall interface. By appropriate processing, the velocity distribution (1) can determine the location of the wall-blood interface within the voxel and (2) estimate the velocity profile across the spatial extent of the voxel. From this information, accurate estimates of fluid shear rate may be obtained. Simulations are presented to illustrate this technique and to show the effects of various error sources, including differences in proton densities between blood and wall tissues and flow-related signal changes. Experimental evidence obtained for steady flow in straight tubes is also presented in support of the technique. The mean error in the experimental shear rate estimates found using the proposed technique was -15%. This represents a significant improvement over estimates obtained by extrapolation of the velocity profile over multiple voxels (mean error of -73%).

Cine magnetic resonance imaging of aqueductal stenosis

Cerebral aqueductal stenosis is one of the most common causes of congenital and acquired hydrocephalus, but the etiology, pathophysiology and cerebrospinal fluid (CSF) dynamics of aqueductal stenosis have yet to be clarified. Utilizing cardiac gated cine magnetic resonance (MR) imaging, we evaluated aqueductal configuration and pulsatile motion of brain and CSF flow stimulated by cardiac pulsation in five patients with non-tumoral aqueductal stenosis. Cine MR of four cases revealed obliteration of the aqueduct by thickening mesencephalic tectum, turbulent CSF flow in the III ventricle, and absence of flow-related signal void, which in all normal cases indicates CSF movement within the aqueduct. In the remaining fifth case, with proximal dilation of the aqueduct resulting from thinning of the tectum, distortion of caudal (distal) tectum related to pulsatile motion of the brain caused funnel-like narrowing of the aqueduct, leading to incomplete obstruction and the absence of upward CSF flow during diastole.

Real time blood flow imaging by spiral scan phase velocity mapping.

The work describes the development of a novel sequence that uses rapid spiral k-space sampling, combined with phase velocity mapping, for real time flow velocity imaging. The performance of the technique is assessed on phantoms for both through-plane and in-plane flows. The flow measurements compared well with those measured using a bucket and stopwatch. One advantage of the technique is that flow related signal loss is minimal due to the early acquisition of the center of k-space data. Flow artifacts were observed for in-plane flow and these were understood with the aid of computer simulations. In vivo studies involved cine velocity mapping in normal volunteers; aortic blood flow waveforms acquired by spiral scanning in two cardiac cycles compared well with data from a conventional gradient-echo sequence. Potential applications of the method are demonstrated by studying the response of aortic flow to physical exercise and the real time monitoring of aortic flow during a valsalver maneuver.

MR imaging with spatial modulation of magnetization in the evaluation of chronic central pulmonary thromboemboli.

To assess the diagnostic value of magnetic resonance (MR) imaging with SPAMM (spatial modulation of magnetization) in the identification of chronic central pulmonary thromboemboli. Twelve patients with pulmonary hypertension and five healthy volunteers were prospectively studied with a 1.5-T MR imaging system. The SPAMM technique was integrated into a conventional cardiac-synchronized spin-echo (SE) sequence. Six of the 12 patients had central thromboemboli. In the healthy subjects, intravascular stripes in the central pulmonary arteries disappeared as a result of flow within 100 msec after the R wave. Areas of persistent stripes were identified in seven of eight central pulmonary arteries with thromboemboli. Conversely, in the 16 central pulmonary arteries without clot, intraluminal stripes disappeared despite the presence of flow-related signal (sensitivity = 88%, specificity = 100%, accuracy = 96%). SPAMM appears to be a simple and effective technique for differentiating central pulmonary arterial thromboemboli from flow-related signal frequently observed with pulmonary hypertension.

Cine-MR Imaging of Aqueductal CSF Flow in Normal Pressure Hydrocephalus Syndrome before and after CSF Shunt

Reproducibility of the aqueductal CSF signal intensity on a gradient echo cine-MR sequence exploiting through plane inflow enhancement was tested in 11 patients with normal or dilated ventricles. Seven patients with normal pressure hydrocephalus (NPH) syndrome were investigated with the sequence before and after CSF shunting. Two patients exhibiting central flow void within a hyperintense aqueductal CSF improved after surgery and the flow void disappeared after shunting. One patient with increased maximum and minimum aqueductal CSF signal as compared to 18 healthy controls also improved and the aqueductal CSF signal was considerably decreased after shunting. Three patients with aqueductal CSF values similar to those in the controls did not improve, notwithstanding their maximum aqueductal CSF signals decreasing slightly after shunting. No appreciable aqueductal CSF flow related enhancement consistent with non-communicating hydrocephalus was found in the last NPH patient who improved after surgery. Cine-MR with inflow technique yields a reproducible evaluation of flow-related aqueductal CSF signal changes which might help in identifying shunt responsive NPH patients. These are likely to be those with hyperdynamic aqueductal CSF or aqueductal obstruction.

Membranous Obstruction of the Inferior Vena Cava with Budd-Chiari Syndrome: MR Imaging Findings

Membranous obstruction of the inferior vena cava (IVC) is a curable cause of a primary type of Budd-Chiari syndrome. Magnetic resonance (MR) imaging and vena cavography were performed on nine patients with membranous obstruction of the IVC. The MR findings were retrospectively analyzed and compared with computed tomographic findings in seven patients. The morphologic features of membranous obstruction of the IVC on spin-echo MR images were a curvilinear soft-tissue membrane (five cases) or an obliterated lumen of a hepatic segment of the IVC (four cases) in transverse or sagittal views. The lumen below the obstruction revealed flow-related signal (seven cases), intraluminal thrombus (one case), and thrombotic occlusion (one case). The hepatic veins were narrow and disoriented without connection to the hepatic segment of the IVC just below the diaphragm. On T2-weighted images, inhomogeneity with high signal intensity was shown more prominently in the hepatic parenchyma in Simson type II or III membranous obstruction. Other findings were hepatosplenomegaly, enlarged caudate lobe, cirrhotic liver, associated hepatoma, and presence of various collaterals.

Bright pleural effusion and ascites on gradient-echo MR images: a potential source of confusion in vascular MR studies.

Motion of fluids other than blood can cause flow-related signal enhancement on MR images, including MR angiograms. In order to study this problem, the appearance of ascites (20 patients) and pleural effusions (five patients) was assessed on MR images made during suspended respiration with flow-compensated gradient-echo sequences as well as T1- and T2-weighted sequences. Signal intensities of vessels, fluid collections, and muscle were measured and vessel/muscle and vessel/fluid contrast were calculated. Fluid motion was measured with a bolus tracking technique that tags a selected volume of fluid with an RF presaturation. Fluid collections had a bright signal in four of five patients with pleural effusion and in 15 of 20 patients with ascites. The average contrast ratio between bright components of the fluid collections and vessels was only 0.03 +/- 0.09. Bright fluid collections were seen on MR angiograms and could obscure blood vessels. Bolus tracking measurements of ascites revealed multidirectional flow, suggesting that its bright signal is related to motion that continues during suspended respiration. Fluid collections appeared dark on T1-weighted images in all patients, indicating that a short T1 relaxation time was not a cause of the high signal intensity. The results indicate that, despite breath-holding, ascites and pleural effusions can show bright signal intensity on gradient-echo images. Awareness of this phenomenon will avoid confusion between moving fluid collections and flowing blood and identify a source of image degradation on both gradient-echo and T2-weighted spin-echo MR acquisitions.

Mitral stenosis: evaluation with MR imaging after percutaneous balloon valvuloplasty.

To evaluate the pathoanatomic findings of mitral valve stenosis and changes after percutaneous balloon valvuloplasty (PBV), magnetic resonance (MR) imaging was performed in 23 patients. The patients were imaged with a 2.0-T system within 1 week before and 3-10 days after PBV. The angle of the interatrial septum was measured on the transverse image to facilitate a successful transseptal puncture. On MR images, the mean transverse and anteroposterior diameters of the left atrium at the level of the aortic root in the ventricular diastolic phase decreased significantly after PBV. Areas of flow-related intraluminal signal intensity detected in the left atrial cavity of 17 patients (74%) before the procedure disappeared in 15 patients after the procedure. Other MR imaging findings after PBV were the disappearance of intraluminal signal intensity in the pulmonary artery, normal curvature of the interatrial and interventricular septa, and pericardial effusion as a complication. MR imaging was thought to provide useful information before and after PBV in patients with mitral stenosis.