High efficiency, low distortion 3D diffusion tensor imaging with variable density spiral fast spin echoes (3D DW VDS RARE)

We present an acquisition and reconstruction method designed to acquire high resolution 3D fast spin echo diffusion tensor images while mitigating the major sources of artifacts in DTI-field distortions, eddy currents and motion. The resulting images, being 3D, are of high SNR, and being fast spin echoes, exhibit greatly reduced field distortions. This sequence utilizes variable density spiral acquisition gradients, which allow for the implementation of a self-navigation scheme by which both eddy current and motion artifacts are removed. The result is that high resolution 3D DTI images are produced without the need for eddy current compensating gradients or B(0) field correction. In addition, a novel method for fast and accurate reconstruction of the non-Cartesian data is employed. Results are demonstrated in the brains of normal human volunteers.

Collecting patient race/ethnicity and primary language data in ambulatory care settings: a case study in methodology

OBJECTIVE

To collect patient race/ethnicity and language (r/e/l) in an ambulatory care setting.

DATA SOURCES/STUDY SETTING

The Palo Alto Medical Foundation (PAMF), December 2006-May 2008.

STUDY DESIGN

Three pilot studies: (1) Comparing mail versus telephone versus clinic visit questionnaire distribution; (2) comparing the front desk method (FDM) versus exam room method (ERM) in the clinic visit; and (3) determining resource allocation necessary for data entry.

DATA COLLECTION/EXTRACTION METHODS

Studies were planned and executed by PAMF's Quality and Planning division.

PRINCIPAL FINDINGS

Collecting r/e/l data during clinic visits elicited the highest response rate. The FDM yielded higher response rate than the ERM. One full-time equivalent is initially necessary for data entry.

CONCLUSIONS

Conducting sequential studies can help guide r/e/l collection in a short time frame.

Perfusion tensor imaging

Arterial spin labeling (ASL) provides a method by which to noninvasively measure the spatial and temporal characteristics of local tissue perfusion. Standard methods employ spatial tagging schemes, but recently methods based on velocity dependent tags, called Velocity Selective ASL (VSASL), have been introduced wherein the tagging depends upon the vascular velocity profile. In this article, we point out an interesting feature of VSASL: the velocity can be encoded in any direction, thereby allowing for the measurement of perfusion with a specified angular resolution. This then facilitates the reconstruction of the local perfusion field, characterized by a perfusion tensor P, from which can be derived quantities related to the structure of the local perfusion field, such as the mean perfusion, the perfusion anisotropy, and the principal directions of flow feeding each voxel. We demonstrate this new method, Perfusion Tensor Imaging (PTI), in both the brain and skeletal muscle of normal human volunteers and discuss possible applications.

Collateral circulation imaging: MR perfusion territory arterial spin-labeling at 3T

BACKGROUND AND PURPOSE

Current knowledge of the collateral circulation remains sparse, and a noninvasive method to better characterize the role of collaterals is desirable. The aim of our study was to investigate the presence and distal flow of collaterals by using a new MR perfusion territory imaging, vessel-encoded arterial spin-labeling (VE-ASL).

MATERIALS AND METHODS

Fifty-six patients with internal carotid artery (ICA) or middle cerebral artery (MCA) stenosis were identified by sonography. VE-ASL was performed to assess the presence and function of collateral flow. The perfusion information was combined with VE maps into high signal-intensity-to-noise-ratio 3-colored maps of the left carotid, right carotid, and posterior circulation territories. The presence of the anterior and posterior collateral flow was demonstrated by the color of the standard anterior cerebral artery/MCA flow territory. The distal function of collateral flow was categorized as adequate (cerebral blood flow [CBF] >/=10 mL/min/100 g) or deficient (CBF <10 mL/min/100 g). The results were compared with those of MR angiography (MRA) and intra-arterial digital subtraction angiography (DSA) in cross table, and kappa coefficients were calculated to determine the agreement among different methods.

RESULTS

The kappa coefficients of the presence of anterior and posterior collaterals by using VE-ASL and MRA were 0.785 and 0.700, respectively. The kappa coefficient of the function of collaterals by using VE-ASL and DSA was 0.726. Apart from collaterals through the circle of Willis, VE-ASL showed collateral flow via leptomeningeal anastomoses.

CONCLUSIONS

In patients with ICA or MCA stenosis, VE-ASL could show the presence, the origin, and distal function of collateral flow noninvasively.

Quantitative assessment of mixed cerebral vascular territory supply with vessel encoded arterial spin labeling MRI

BACKGROUND AND PURPOSE

Recent advances in arterial spin labeling MRI have permitted noninvasive evaluation of vascular territories. In the present study, we quantitatively assess mixing of internal carotid and basilar artery blood through cerebrovascular anastomoses using vessel-encoded arterial spin labeling and a new postprocessing method.

METHODS

Vessel-encoded arterial spin labeling was used to determine the territories of the internal carotid and basilar arteries in 14 healthy subjects and one patient with asymptomatic high-grade carotid artery stenosis before and after endarterectomy. Contributions to individual vascular territories were quantified using a voxelwise supply fraction algorithm and the results were correlated with MR angiography.

RESULTS

Vascular territories were consistent with cerebrovascular anatomy and the presence of pathology. The supply fraction method allowed quantification of mixed territorial supply arising from collateral flow and showed vascular supply changes in a patient with carotid artery stenosis after endarterectomy.

CONCLUSIONS

Vascular territories obtained with vessel-encoded arterial spin labeling correlate with cerebrovascular anatomy and allow quantitative assessment of mixed territorial supply in subjects with and without pathology.

Longitudinal MRI studies in the isoflurane-anesthetized rat: long-term effects of a short hypoxic episode on regulation of cerebral blood flow as assessed by pulsed arterial spin labelling

MRI is a powerful tool for measuring cerebral blood flow (CBF) longitudinally. However, most animal studies require anesthesia, potentially interfering with normal physiology. Isoflurane anesthesia was used here to study CBF regulation during repetitive scanning in rats. MR perfusion images were acquired using FAIR (flow-sensitive alternating inversion recovery) arterial spin labeling, and absolute CBF was calculated. CBF changes in response to a hypoxic (12% O2) and hypercapnic (5% CO2) gas stimulus were monitored. Hypercapnia led to a robust increase in CBF compared with baseline (195.5+/-21.5 vs 123.6+/-17.9 ml/100 g/min), and hypoxia caused a smaller non-significant increase in mean CBF values (145.4+/-13.4 ml/100 g/min). Strikingly, when measurements were repeated 5 days later, CBF was dramatically reduced in hypoxia (93.2+/-8.1 ml/100 g/min) compared with the first imaging session. Without application of the hypoxic and hypercapnic gases during the first MRI, baseline CBF and CBF changes in response to hypoxia at the second MRI were similar to naive rats. Blood gas analyses revealed a slight reduction in arterial oxygenation during the second period of anesthesia compared with the first. These findings indicate that, in isoflurane-anesthetized rats, even a short hypoxic episode can have long-lasting effects on cerebrovascular regulation.

Imaging periodic currents using alternating balanced steady-state free precession

Existing functional brain MR imaging methods detect neuronal activity only indirectly via a surrogate signal such as deoxyhemoglobin concentration in the vascular bed of cerebral parenchyma. It has been recently proposed that neuronal currents may be measurable directly using MRI (ncMRI). However, limited success has been reported in neuronal current detection studies that used standard gradient or spin echo pulse sequences. The balanced steady-state free precession (bSSFP) pulse sequence is unique in that it can afford the highest known SNR efficiency and is exquisitely sensitive to perturbations in free precession phase. It is reported herein that when a spin phase-perturbing periodic current is locked to an RF pulse train, phase perturbations are accumulated across multiple RF excitations and the spin magnetization reaches an alternating balanced steady state (ABSS) that effectively amplifies the phase perturbations due to the current. The alternation of the ABSS signal therefore is highly sensitive to weak periodic currents. Current phantom experiments employing ABSS imaging resulted in detection of magnetic field variations as small as 0.15nT in scans lasting for 36 sec, which is more sensitive than using gradient-recalled echo imaging.

Vessel-encoded arterial spin-labeling using pseudocontinuous tagging

A new signal-to-noise ratio (SNR) efficient method is introduced for the mapping of vascular territories based on pseudocontinuous arterial spin labeling (ASL). A pseudocontinuous tagging pulse train is modified using additional transverse gradient pulses and phase cycling to place some arteries in a tag condition, while others passing through the same tagging plane are in a control condition. This is combined with a Hadamard or similar encoding scheme such that all vessels of interest are fully inverted or relaxed for nearly all of the encoding cycles, providing optimal SNR. The relative tagging efficiency for each vessel is measured directly from the ASL data and is used in the decoding process to improve the separation of vascular territories. High SNR maps of left carotid, right carotid, and basilar territories are generated in 6 min of scan time.

Vascular space occupancy weighted imaging with control of residual blood signal and higher contrast-to-noise ratio

It has been recently proposed that the local cerebral blood volume change during brain activation can be measured by a series of images whose contrast is dependent on vascular space occupancy (VASO). VASO takes advantage of the inversion recovery sequence to acquire images when the longitudinal magnetization (Mz) of blood is relaxing through zero. The degree of blood suppression, however, is not always well controlled as a consequence of spatial variations in inversion efficiency and blood T1. Furthermore, while blood is eliminated, the Mz of other tissues is also small, which makes the contrast-to-noise ratio inherently low in VASO. In this paper, diffusion gradients were applied to demonstrate residual intravascular signal in the original VASO. An alternative VASO-weighted imaging was then proposed using a longer inversion time at which the Mz difference between blood and gray matter was optimized. A global saturation immediately after image acquisition was employed to eliminate the Mz disparity between inflowing blood and the residual in-plane blood from previous acquisition. Feasibility was evaluated by numerical simulation and functional experiments. In human visual cortex, the fractional VASO signal and cerebral blood volume changes were found to be -0.6% and 44%, respectively (voxel size = 3.4 x 3.4 x 5.0 mm3). As compared to the original VASO, the presented method provided a largely comparable activation map and hemodynamic curve but was not confounded by the existence of blood. Results also demonstrated its advantages of 1.6-fold higher CNR and insensitivity to variant tissue/blood T1 as well as inversion efficiency.

Feasibility of velocity selective arterial spin labeling in functional MRI

Arterial spin labeling (ASL) magnetically inverts or saturates the spins in arterial blood and uses them as endogenous tracers. Conventionally, the tagging band is upstream or nonselective to the target slices. In the brain, ASL-based functional magnetic resonance imaging (fMRI) has been shown to detect activation better localized in gray matter than blood oxygenation level dependent contrast. More recently, velocity selective-ASL (VS-ASL) was proposed to tag spins according to their flow velocity. One desirable characteristic of VS-ASL is its capability to generate tags sufficiently close to the target slices and thereby circumvent the complication of non-zero transit delay. In this study, we investigate the feasibility of VS-ASL in fMRI by comparing it with a conventional ASL method (PICORE). The results from the visual cortex of healthy volunteers show that VS-ASL and PICORE have comparable spatial specificity in detecting the flow change induced by neuronal activity. Velocity selective-arterial spin labeling can further distinguish the contribution from different flow directions but spurious elevation of fractional signal change may occur when the VS tagging is applied off the direction of blood supply. The flow reaches the vicinity of perfusion at a cutoff velocity (Vc) of 2 cm/sec whereas the activation exclusively detected by Vc=4 cm/sec implies the arteriolar response to the neuronal activity and a respondent vessel diameter up to 240 microm. Velocity selective imaging can remove intravascular signal from the vessels where the flow velocity is above Vc.

Quantification of rodent cerebral blood flow (CBF) in normal and high flow states using pulsed arterial spin labeling magnetic resonance imaging

PURPOSE

To implement a pulsed arterial spin labeling (ASL) technique in rats that accounts for cerebral blood flow (CBF) quantification errors due to arterial transit times (dt)-the time that tagged blood takes to reach the imaging slice-and outflow of the tag.

MATERIALS AND METHODS

Wistar rats were subjected to air or 5% CO(2), and flow-sensitive alternating inversion-recovery (FAIR) perfusion images were acquired. For CBF calculation, we applied the double-subtraction strategy (Buxton et al., Magn Reson Med 1998;40:383-396), in which data collected at two inversion times (TIs) are combined.

RESULTS

The ASL signal fell off more rapidly than expected from TI = one second onward, due to outflow effects. Inversion times for CBF calculation were therefore chosen to be larger than the longest transit times, but short enough to avoid systematic errors caused by outflow of tagged blood. Using our method, we observed a marked regional variability in CBF and dt, and a region dependent response to hypercapnia.

CONCLUSION

Even when flow is accelerated, CBF can be accurately determined using pulsed ASL, as long as dt and outflow of the tag are accounted for.

The effects of flow dispersion and cardiac pulsation in arterial spin labeling

The blood in the carotid arteries exhibits time-varying flow velocity as a function of cardiac phases. Despite this flow velocity variation, most current methods set forth for the analysis of arterial spin labeling (ASL) data have assumed that the tagged blood is delivered from the tagging region to the imaging region via simple plug flow, i.e., a single transit delay (deltat). In this study, we used a pulse oximeter to synchronize image acquisition at systole and diastole separately. The deltat dispersion was modeled with a Gaussian distribution and the effect of cardiac pulsation upon the ASL signal was evaluated on five healthy volunteers. ASL signals were collected at a series of inflow times (TI) using PICORE QUIPSS II: TR/TE/TI1 = 2400/3.2/700 ms, TI = {300, 500, 700, 900, 1100, 1300, 1500} ms, matrix size = 64 x 64, repetition = 100. Transit delay was found significantly shorter in systolic tag than diastolic tag (paired student's t-test, p < 0.001; mean difference across subjects = 54 ms). When the tag was applied in late systole, the ASL signal arrived in the target brain slice earlier, and was higher by 16% with TI = 700 ms. Intervoxel dispersion (-350 ms) dominated over intravoxel dispersion (< 200 ms). The disparity of ASL signals found between systolic and diastolic tags indicated that ASL imaging was sensitive to cardiac pulsations. We conclude that both flow dispersion and fluctuations in the ASL signal due to cardiac pulsations are significant.

Quantification of regional pulmonary blood flow using ASL-FAIRER

Pulsed arterial spin labeling (ASL) techniques have been theoretically and experimentally validated for cerebral blood flow (CBF) quantification. In this study ASL-FAIRER was used to measure regional pulmonary blood flow (rPBF) in seven healthy subjects. Two general ASL strategies were investigated: 1) a single-subtraction approach using one tag-control pair acquisition at an inversion time (TI) matched to the RR-interval, and 2) a multiple-subtraction approach using tag-control pairs acquired at various TIs. The mean rPBF averaged 1.70 +/- 0.38 ml/min/ml when measured with the multiple-subtraction approach, and was approximately 2% less when measured with the single-subtraction method (1.66 +/- 0.24 ml/min/ml). Assuming an average lung density of 0.33 g/ml, this translates into a regional perfusion of approximately 5.5 ml/g/min, which is comparable to other measures of pulmonary perfusion. As with other ASL applications, a key problem with quantitative interpretation of the results is the physical gap between the tagging region and imaged slice. Because of the high pulsatility of PBF, ASL acquisition and data analysis differ significantly between the lung and the brain. The advantages and drawbacks of the single- vs. multiple-subtraction approaches are considered within a theoretical framework tailored to PBF.

Velocity-selective arterial spin labeling

In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues.

Intravascular effect in velocity-selective arterial spin labeling: the choice of inflow time and cutoff velocity

Velocity-selective arterial spin labeling (VS-ASL) tags spins on a basis of flow velocity, instead of spatial distribution that has been commonly adopted in conventional ASL techniques. VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the error source of transit delay (deltat) variation independent of inflow time (TI). In practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (V(c)). The presented study takes advantage of multiple TI and V(c) to systematically investigate the intravascular effect. Results demonstrate the presence of significant signal from large vessels in VS-ASL images for V(c) down to 4 cm/s. For perfusion measurement in human brain, low V(c) (<4 cm/s) is recommended. With V(c) = 2 cm/s, quantitative cerebral blood flow is 72.8 ml/100 ml/min, which is in agreement with the reported range using conventional ASL methods. In field strength of 3 T, numerical simulation shows that optimal signal-to-noise ratio efficiency can be achieved with TR/TI = 2092 ms/1664 ms for single slice and 4493 ms/1404 ms for slab imaging.

Quantifying CBF with pulsed ASL: technical and pulse sequence factors

We summarize here current methods for the quantification of CBF using pulsed arterial spin labeling (ASL) methods. Several technical issues related to CBF quantitation are described briefly, including transit delay, signal from larger arteries, radio frequency (RF) slice profiles, magnetization transfer, tagging efficiency, and tagging geometry. Many pulsed tagging schemes have been devised, which differ in the type of tag or control pulses, and which have various advantages and disadvantages for quantitation. Several other modifications are also available that can be implemented as modules in an ASL pulse sequence, such as varying the wash-in time to estimate the transit delay. Velocity-selective ASL (VS-ASL) uses a new type of pulse labeling in which inflowing arterial spins are tagged based on their velocity rather than their spatial location. In principle, this technique may allow ASL measurement of cerebral blood flow (CBF) that is insensitive to transit delays.

A signal processing model for arterial spin labeling functional MRI

A model of the signal path in arterial spin labeling (ASL)-based functional magnetic resonance imaging (fMRI) is presented. Three subtraction-based methods for forming a perfusion estimate are considered and shown to be specific cases of a generalized estimate consisting of a modulator followed by a low pass filter. The performance of the methods is evaluated using the signal model. Contamination of the perfusion estimate by blood oxygenation level dependent contrast (BOLD) is minimized by using either sinc subtraction or surround subtraction for block design experiments and by using pair-wise subtraction for randomized event-related experiments. The subtraction methods all tend to decorrelate the 1/f type low frequency noise often observed in fMRI experiments. Sinc subtraction provides the flattest noise power spectrum at low frequencies, while pair-wise subtraction yields the narrowest autocorrelation function. The formation of BOLD estimates from the ASL data is also considered and perfusion weighting of the estimates is examined using the signal model.