Magnetic resonance flow measurements in real time: comparison with a standard gradient-echo technique

Ultrafast gradient systems and hybrid imaging sequences offer the opportunity to acquire phase contrast flow data in real time. In a 1.5-Tesla magnetic resonance (MR)-tomograph, peak velocity and volume flow were assessed in 36 large vessels (aorta) and 33 medium-sized vessels (carotid and iliac artery) using a real-time (segmented k-space turbo gradient-echo planar imaging sequence) in comparison with a gradient-echo technique. With the real-time technique, the matrix was reduced from 116 to 64, and temporal resolution changed from 30 msec to 124 msec. Measurements of peak velocity correlated in large (r = 0.88) and medium-sized vessels (r = 0.81). Volume flow measurements correlated in large vessels (r = 0.87), however, a poor correlation (r = 0.64) was found in medium-sized vessels. Thus, scan time can be significantly reduced and images acquired without electrocardiogram (ECG)-triggering. Flow volume can only be determined in large vessels with sufficient accuracy, mainly due to reduced spatial resolution in smaller vessels.     

High-precision MR velocity mapping by 3D-fourier phase encoding with a small number of encoding steps

The final result of Fourier velocity mapping is a set of images, each representing the spatial distribution of spins at a given velocity. To acquire data in a short time, the number of encoding gradient steps must be as small as possible, but this can mean sacrificing velocity resolution. We used interpolation methods to obtain high velocity resolution with a small number of encoding steps involving linear interpolation from 16 encoding steps or more and zero-filling interpolation from two to eight encoding steps. Velocity measured by interpolated Fourier-flow encoding agreed well with values obtained using a calibrated phantom. A simulation of noise on the images of the phantom showed that, for a given acquisition time, increasing number of encoding steps in the Fourier flow encoding gave better precision for velocity measurement than did averaging identical signals in phase-mapping methods.     

DSC MR灌注成像在鉴别良恶性脑膜瘤中的价值

目的分析肿瘤实质和瘤周水肿区的血供特点,评价DSCMR灌注成像在鉴别良恶性脑膜瘤中的价值。方法对33例经病理证实的脑膜瘤患者(良性25例,恶性8例)行手术前DSCMR灌注成像扫描。测量肿瘤实质部分及瘤周水肿区最大rCBV值和相应部位rMTT数值,并将结果进行分组统计分析。结果良恶性脑膜瘤实质部分的最大rCBV值及相应部位rMTT数值之间无统计意义差异(t检验,Ρ>0.05)。恶性脑膜瘤瘤周水肿区的最大rCBV值和相应部位rMTT数值高于良性脑膜瘤,两者之间有统计意义差异(t检验,Ρ<0.05)。结论DSCMR灌注成像对良恶性脑膜瘤的鉴别有临床价值。     

磁共振脑血管血流分析的临床应用——脑血栓、脑缺血与正常组对比研究

目的：测定脑血栓、脑缺血患者和正常人的颅内动脉血流，为临床提供进一步的诊断资料。材料和方法：应用ＭＲ血流分析技术，对４２例大脑中动脉（ＭＣＡ）、颅内动脉（ＩＣＡ）及基底动脉（ＢＡ）血流进行了测量与分析。结果：（１）ＭＣＡ脑梗塞组血流速度峰值增高而平均血流量值降低；脑缺血组血流速度峰值增高或正常而平均血流量值正常或降低，二者必居其一；正常组之ＭＲ血流分析无异常。（２）ＩＣＡ和ＢＡ二者均未见明显异常。结论：推测ＭＲ脑血流分析技术在预测中风方面可能是有效的检测手段。     

High resolution measurement of cerebral blood flow using intravascular tracer bolus passages. Part II: Experimental comparison and preliminary results

This report evaluates several methods to map relative cerebral blood flow (rCBF) by applying both parametric and non-parametric techniques to deconvolve high resolution dynamic MRI measurements of paramagnetic bolus passages with noninvasively determined arterial inputs. We found a nonparametric (singular value decomposition (SVD)) deconvolution technique produced the most robust results, giving mean gray:white flow ratio of 2.7 ± 0.5 (SEM) in six normal volunteers, in excellent agreement with recent PET literature values for age-matched subjects. Similar results were obtained by using a model-dependent approach that assumes an exponential residue function, but not for a Gaussian-shaped residue function or for either Fourier or regularization-based model-independent approaches. Pilot studies of our CBF mapping techniques in patients with tumor, stroke, and migraine aura demonstrated that these techniques can be readily used on data routinely acquired by using current echo planar imaging technology. By using these techniques, the authors visualized important regional hemodynamic changes not detectable with rCBV mapping algorithms.     

Correction for vascular artifacts in cerebral blood flow values measured by using arterial spin tagging techniques

?Vascular”? artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of ?tagged”? arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to ?crush”? the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of ?tagged”? arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using ?dynamic”? spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be ～0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.     

Preoperative subtyping of meningiomas by perfusion MR imaging

Introduction  This paper aims to evaluate the value of perfusion magnetic resonance (MR) imaging in the preoperative subtyping of meningiomas by analyzing the relative cerebral blood volume (rCBV) of three benign subtypes and anaplastic meningiomas separately. Materials and methods  Thirty-seven meningiomas with peritumoral edema (15 meningothelial, ten fibrous, four angiomatous, and eight anaplastic) underwent perfusion MR imaging by using a gradient echo echo-planar sequence. The maximal rCBV (compared with contralateral normal white matter) in both tumoral parenchyma and peritumoral edema of each tumor was measured. The mean rCBVs of each two histological subtypes were compared using one-way analysis of variance and least significant difference tests. A p value less than 0.05 indicated a statistically significant difference. Results  The mean rCBV of meningothelial, fibrous, angiomatous, and anaplastic meningiomas in tumoral parenchyma were 6.93 ± 3.75, 5.61 ± 4.03, 11.86 ± 1.93, and 5.89 ± 3.85, respectively, and in the peritumoral edema 0.87 ± 0.62, 1.38 ± 1.44, 0.87 ± 0.30, and 3.28 ± 1.39, respectively. The mean rCBV in tumoral parenchyma of angiomatous meningiomas and in the peritumoral edema of anaplastic meningiomas were statistically different (p < 0.05) from the other types of meningiomas. Conclusion  Perfusion MR imaging can provide useful functional information on meningiomas and help in the preoperative diagnosis of some subtypes of meningiomas.     

High resolution measurement of cerebral blood flow using intravascular tracer bolus passages. Part I: Mathematical approach and statistical analysis

The authors review the theoretical basis of determination of cerebral blood flow (CBF) using dynamic measurements of nondiffusible contrast agents, and demonstrate how parametric and nonparametric deconvolution techniques can be modified for the special requirements of CBF determination using dynamic MRI. Using Monte Carlo modeling, the use of simple, analytical residue models is shown to introduce large errors in flow estimates when actual, underlying vascular characteristics are not sufficiently described by the chosen function. The determination of the shape of the residue function on a regional basis is shown to be possible only at high signal-to-noise ratio. Comparison of several nonparametric deconvolution techniques showed that a nonparametric deconvolution technique (singular value decomposition) allows estimation of flow relatively independent of underlying vascular structure and volume even at low signal-to-noise ratio associated with pixel-by-pixel deconvolution.     

Application of Spatial Modulation of Magnetization to Cervical Spinal Stenosis for Evaluation of the Hydrodynamic Changes Occurring in Cerebrospinal Fluid

Objective

To evaluate the hydrodynamic changes occurring in cerebrospinal fluid (CSF) flow in cervical spinal stenosis using the spatial modulation of magnetization (SPAMM) technique.

Materials and Methods

Using the SPAMM technique, 44 patients with cervical spinal stenosis and ten healthy volunteers were investigated. The degree of cervical spinal stenosis was rated as low-, intermediate-, or high-grade. Low-grade stenosis was defined as involving no effacement of the subarachnoid space, intermediate-grade as involving effacement of this space, and high-grade as involving effacement of this space, together with compressive myelopathy. The patterns of SPAMM stripes and CSF velocity were evaluated and compared between each type of spinal stenosis and normal spine.

Results

Low-grade stenosis (n = 23) revealed displacement or discontinuity of stripes, while intermediate- (n = 10) and high-grade (n = 11) showed a continuous straight band at the stenotic segment. Among low-grade cases, 12 showed wave separation during the systolic phase. Peak systolic CSF velocity at C4-5 level in these cases was lower than in volunteers (p < .05), but jet-like CSF propulsion was maintained. Among intermediate-grade cases, peak systolic velocity at C1-2 level was lower than in the volunteer group, but the difference was not significant (p > .05). In high-grade stenosis, both diastolic and systolic velocities were significantly lower (p < .05).

Conclusion

Various hydrodynamic changes occurring in CSF flow in cervical spinal stenosis were demonstrated by the SPAMM technique, and this may be a useful method for evaluating CSF hydrodynamic change in cervical spinal stenosis.     

Perfusion Imaging of the Brain Using Z-Score and Dynamic Images Obtained by Subtracting Images from before and after Contrast Injection

Objective

The aim of this study was to examine the feasibility of perfusion imaging of the brain using the Z-score and subtraction dynamic images obtained from susceptibility contrast MR images.

Materials and Methods

Five patients, each with a normal MRI, Moya-moya, a middle cerebral artery occlusion, post-trauma syndrome, and a metastatic brain tumor, were selected for a presentation. A susceptibility-contrast echo-planar image after a routine MRI was taken as the source image with a rapid manual injection of 0.1 mmol/kg of Gd-DTPA. The inflow and washout patterns were observed from the time-signal intensity curve of the serial scans using the standard program of an MRI machine. The repeated Z-score images of the peak and late phases were made using the threshold Z-score values between 1.4 and 2.0 in four to five studies of the pre-contrast, peak, and late phases. Dynamic subtraction images were produced by subtracting sequential post-contrast images from a pre-contrast image and coloring these images using a pseudocolor mapping method.

Results

In the diseases with perfusion abnormalities, the Z-score images revealed information about the degree of perfusion during the peak and late phases. However, the quality varied with the Z-score threshold and the studies selected in a group. The dynamic subtraction images were of sufficient quality with no background noise and more clearly illustrated the temporal changes in perfusion and delayed perfusion.

Conclusion

The Z-scores and dynamic subtraction images illustrated the degree of perfusion and sequential changes in the pattern of perfusion, respectively. These images can be used as a new complimentary method for observing the perfusion patterns in brain diseases.     

MR visualization of CSF flow through a ventriculo-cisternostomy

Summary The MRI findings in a 6-year-old boy with an astrocytoma of the mesancephalon are reported. A ventriculocisternostomy had been performed in order to reduce the hydrocephalus. At the site of the ventriculocisternostomy, the T2-weighted images showed a low signal in the anterior part of the third ventricle, the interpeduncular and the pontine cistern. This was attributed to CSF flow void. We conclude that MRI can provide information about the precise location and normal functioning of a ventriculocisternostomy.     

Cerebrospinal fluid flow

Cardiac-related motion of the cerebrospinal fluid (CSF) was investigated by analysis of the velocity-dependent phase of CSF protons and flow-dependent signal enhancement in magnitude images using ECG-gated FLASH sequences. In the cerebral aqueduct, CSF flow from the third to the fourth ventricle begins 200 ms after the R-wave of the ECG and simulates an arterial pulse wave pattern. It lasts about 60% of the cardiac cycle and is followed by backflow from the fourth to the third ventricle, which is slower and shorter. In the spinal canal, oscillating caudad motion precedes flow from the third to the fourth ventricle by about 50-100 ms and is superimposed on a bulk flow, which moves simultaneously in opposite directions in separate subarachnoid channels; it is directed mainly caudally in the anterior cervical subarachnoid space.     

MRA评估TIA患者脑血流量变化

目的 联合应用电影相位对比磁共振血管成像(cine PC MRA)、三维时间飞跃磁共振血管成像(3D TOF MRA)和三维对比增强磁共振血管成像 (3D CE -MRA)评估一过性脑缺血发作(TIA)患者全脑血流量和供血动脉病变对血流量的影响.资料与方法 采用MRA对41例TIA患者、30例正常人和30例脑梗死者的全脑血流量和脑供血动脉进行对照研究.结果 30例无脑动脉严重狭窄的TIA患者与30例正常人之间的全脑血流量差异无统计学意义(P=0.101).有颈内动脉(ICA)/椎-基底动脉(V-BA)严重狭窄的11例TIA患者与18例脑梗死患者的全脑血流量差异无统计学意义(P=0.150),但脑梗死患者合并大脑前动脉(ACA)、大脑中动脉(MCA)和大脑后动脉(PCA)狭窄或闭塞的概率较大(P=0.018).结论 无脑动脉严重狭窄的TIA患者与正常人之间的全脑血流量无明显差异,而有脑动脉严重狭窄的TIA脑梗死患者的全脑血流量无明显差异.     

Zero filled partial fourier phase contrast MR imaging: in vitro and in vivo assessment

PURPOSE: To validate partial Fourier phase contrast magnetic resonance (PC MR) with full number of excitation (NEX) PC MR measurements in vitro and in vivo. MATERIALS AND METHODS: MR flow measurements were performed using a partial Fourier and a full NEX PC MR sequence in a flow phantom and in 10 popliteal and renal arteries of 10 different healthy volunteers. Average velocity, peak velocity, and flow results were calculated and compared with regression analysis. RESULTS: Excellent correlations in average velocities (r = 0.99, P < 0.001), peak velocities (r = 0.99, P < 0.001), and flow rates (r = 0.98, P < 0.001) were demonstrated in vitro between the two different acquisitions. For the popliteal arteries there was excellent correlation between peak velocities for both acquisitions (r = 0.98, P < 0.0001); the correlation of average velocity measurements when using all data points in the cardiac cycle for all volunteers was 0.96 (P < 0.001). For the renal arteries the same comparison resulted in a good correlation for average velocity (0.93, P < 0.001) and peak velocity measurements (r = 0.91, P = 0.002), although the correlation coefficient for flow rates was 0.88 (P = 0.004). Blurring of the vessel margins was consistently observed on magnitude images acquired with the partial Fourier method, causing overestimation of the vessel area and some error in the flow measurements. CONCLUSION: Partial Fourier PC MR is able to provide comparable average and peak velocity values when using 1 NEX PC MRI as a reference.     

An asymmetric slice profile: Spatial alteration of flow signal response in 3d time-of-flight NMR angiography

We introduce an asymmetric slice profile technique, which alters the spatial response of the flow signal in 3D time-of-flight NMR angiography. By gradually increasing the flip angle from the inflow to the outflow portions of the slab, the inflow refreshment effect is distributed over a wide slab thickness. The asymmetric slice profile is simply produced by using a Gaussian RF excitation with an overlapping presaturation. The spatial distribution of steady flow signal in a phantom study demonstrated an essential agreement with a numerical simulation. 3D time-of-flight NMR angiography of volunteers' heads using this technique provided a smooth vascular depiction over a wide slab thickness.     

Motion Induced Phase Shifts in MR: Acceleration Effects in Quantitative Flow Measurements—A Reconsideration

Magnetic resonance phase difference techniques are commonly used to study flow velocities in the human body. Acceleration is often present, either in the form of pulsatile flow, or in the form of convective acceleration. Questions have arisen about the exact time point at which the velocity is encoded, and also about the sensitivity to (convective) acceleration and higher order motion derivatives. It has become common practice to interpret the net phase shifts measured with a phase difference velocity technique as being the velocity at a certain (Taylor) expansion time point, chosen somewhere between the RF excitation and the echo readout. However, phase shifts are developed over the duration of the encoding magnetic field gradient wave form, and should therefore be interpreted as a more or less time-averaged velocity. It will be shown that the phase shift as measured with a phase difference velocity technique represents the velocity at the “gravity” center of the encoding bipolar gradient (difference) function, without acceleration contribution. Any attempt to interpret the measured phase shift in terms of velocity on any other time point than the gradient gravity point will automatically introduce acceleration sensitivity.     

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.