A Short Introduction to Arterial Spin Labeling and its Application to Flow Territory Mapping

Evaluation of 3-dimensional stereotactic surface projection rendering of arterial spin labeling data in a clinical cohort

BACKGROUND AND PURPOSE

To assess the feasibility of 3-dimensional stereotactic surface projection (3D-SSP) as applied to arterial spin labeling (ASL) in a clinical pilot study.

METHODS

A retrospective sample of 10 consecutive patients who underwent ASL as part of a clinically indicated MR examination was collected during this pilot study. Five additional subjects with normal cerebral perfusion served as a control group. Following voxel-wise M0-correction, cerebral blood flow (CBF) quantification, and stereotactic anatomic standardization, voxel-wise CBF from an individual's ASL dataset was extracted to a set of predefined surface pixels (3D-SSP). A normal database was created from averaging the extracted CBF datasets of the control group. Patients' datasets were compared individually with the normal database by calculating a Z-score on a pixel-by-pixel basis and were displayed in 3D-SSP views for visual inspection. Independent, two-expert reader assessment, using a 3-point scale, compared standard quantitative CBF images to the 3D-SSP maps.

RESULTS

Patterns and severities of regionally reduced CBF were identified, by both independent readers, in the 3D-SSP maps. Reader assessment demonstrated preference for 3D-SSP over traditionally displayed standard quantitative CBF images in three of four evaluated imaging metrics (p = .026, .031, and .013, respectively); 3D-SSP maps were never found to be inferior to the standard quantitative CBF images.

CONCLUSIONS

Three-dimensional SSP maps are feasible in a clinical population and enable quantitative data extraction and localization of perfusion abnormalities by means of stereotactic coordinates in a condensed display. The proposed method is a promising approach for interpreting cerebrovascular pathophysiology.

Perioperative cerebral blood flow measured by arterial spin labeling with different postlabeling delay in patients undergoing carotid endarterectomy: a comparison study with CT perfusion

Background

Arterial spin labeling (ASL) is a non-invasive technique for measuring cerebral perfusion. Its accuracy is affected by the arterial transit time. This study aimed to (1) evaluate the accuracy of ASL in measuring the cerebral perfusion of patients who underwent carotid endarterectomy (CEA) and (2) determine a better postlabeling delay (PLD) for pre- and postoperative perfusion imaging between 1.5 and 2.0 s.

Methods

A total of 24 patients scheduled for CEA due to severe carotid stenosis were included in this study. All patients underwent ASL with two PLDs (1.5 and 2.0 s) and computed tomography perfusion (CTP) before and after surgery. Cerebral blood flow (CBF) values were measured on the registered CBF images of ASL and CTP. The correlation in measuring perioperative relative CBF (rCBF) and difference ratio of CBF (DRCBF) between ASL with PLD of 1.5 s (ASL1.5) or 2.0 s (ASL2.0) and CTP were also determined.

Results

There were no significant statistical differences in preoperative rCBF measurements between ASL1.5 and CTP (p = 0.17) and between ASL2.0 and CTP (p = 0.42). Similarly, no significant differences were found in rCBF between ASL1.5 and CTP (p = 0.59) and between ASL2.0 and CTP (p = 0.93) after CEA. The DRCBF measured by CTP was found to be marginally lower than that measured by ASL2.0_1.5 (p = 0.06) and significantly lower than that measured by ASL1.5_1.5 (p = 0.01), ASL2.0_2.0 (p = 0.03), and ASL1.5_2.0 (p = 0.007). There was a strong correlation in measuring perioperative rCBF and DRCBF between ASL and CTP (r = 0.67-0.85, p < 0.001). Using CTP as the reference standard, smaller bias can be achieved in measuring rCBF by ASL2.0 (-0.02) than ASL1.5 (-0.07) before CEA. In addition, the same bias (0.03) was obtained by ASL2.0 and ASL1.5 after CEA. The bias of ASL2.0_2.0 (0.31) and ASL2.0_1.5 (0.32) on DRCBF measurement was similar, and both were smaller than that of ASL1.5_1.5 (0.60) and ASL1.5_2.0 (0.60).

Conclusion

Strong correlation can be found in assessing perioperative cerebral perfusion between ASL and CTP. During perioperative ASL imaging, the PLD of 2.0 s is better than 1.5 s for preoperative scan, and both 1.5 and 2.0 s are suitable for postoperative scan.

Superselective pseudocontinuous arterial spin labeling in patients with meningioma: utility in prediction of feeding arteries and preoperative embolization feasibility

OBJECTIVE

Superselective pseudocontinuous arterial spin labeling (ss-pCASL) is an MRI technique in which individual vessels are labeled to trace their perfusion territories. In this study, the authors assessed its merit in defining feeding vessels and gauging preoperative embolization feasibility for patients with meningioma, using digital subtraction angiography (DSA) as the reference method.

METHODS

Thirty-one consecutive patients with meningiomas were prospectively recruited, each undergoing DSA (and embolization, if feasible) before resection. All ss-pCASL imaging studies were performed 1 day prior to DSA. Two neuroradiologists independently reviewed ss-pCASL images, rating the contribution of each labeled vessel to tumor blood supply as none, minor, or major. Two neuroradiologists also gauged the feasibility of embolization in each patient, based on ss-pCASL images. Interobserver and intermodality agreement were determined using Cohen's kappa statistic. The diagnostic performance of ss-pCASL was assessed in terms of discerning tumor blood supply and the potential for embolization.

RESULTS

Interobserver agreement in the rating of blood supply by ss-pCASL was very good (κ = 0.817, 95% CI 0.771-0.863), and intermodality agreement (consensus ss-pCASL readings vs DSA findings) was good (κ = 0.688, 95% CI 0.632-0.744). In delineating tumor blood supply, ss-pCASL showed high sensitivity (87.1%) and specificity (87.2%). The positive and negative predictive values for embolization feasibility were 85.2% and 100%, respectively.

CONCLUSIONS

In patients with meningiomas, feeding vessels are reliably predicted by ss-pCASL. This noninvasive approach, involving no iodinated contrast or radiation exposure, is particularly beneficial if there are no prospects of embolization.

In vivo 13C-MRI using SAMBADENA

Magnetic Resonance Imaging (MRI) is a powerful imaging tool but suffers from a low sensitivity that severely limits its use for detecting metabolism in vivo. Hyperpolarization (HP) methods have demonstrated MRI signal enhancement by several orders of magnitude, enabling the detection of metabolism with a sensitivity that was hitherto inaccessible. While it holds great promise, HP is typically relatively slow (hours), expensive (million $, €) and requires a dedicated device (“polarizer”). Recently, we introduced a new method that creates HP tracers without an external polarizer but within the MR-system itself based on parahydrogen induced polarization (PHIP): Synthesis Amid the Magnet Bore Allows Dramatically Enhanced Nuclear Alignment (SAMBADENA). To date, this method is the simplest and least cost-intensive method for hyperpolarized ¹³C-MRI. HP of P_13C > 20% was demonstrated for 5mM tracer solutions previously. Here, we present a setup and procedure that enabled the first in vivo application of SAMBADENA: Within seconds, a hyperpolarized angiography tracer was produced and injected into an adult mouse. Subsequently, fast ¹³C-MRI was acquired which exhibited the vena cava, aorta and femoral arteries of the rodent. This first SAMBADENA in vivo¹³C-angiography demonstrates the potential of the method as a fast, simple, low-cost alternative to produce HP-tracers to unlock the vast but hidden powers of MRI.

Optimized super-selective Arterial Spin Labeling for quantitative flow territory mapping

Flow territory mapping using Arterial Spin Labeling (ASL) methods allows for deeper insights into the physiology of the brain. However, in most approaches assumptions of the labeling efficiency are used for calculation of brain perfusion which might result in inaccuracies. This becomes more important in super-selective ASL as the labeling focus could be positioned not exactly at the artery of interest. Therefore, measuring the labeling efficiency of the current scan seems important to obtain reliable results and to correct for potential errors. In this study, an optimized super-selective ASL tagging scheme is presented and the labeling efficiency is measured using quantitative phase-contrast angiography of the tagged artery and considering the volume of the supplied flow territory. The aim then is to evaluate the labeling efficiency of super-selective ASL and considering these values when shifting the labeling spot away from the artery of interest. The measured efficiency is compared to simulations performed for the optimized super-selective ASL approach. Considering the values of labeling efficiency after shifting the labeling focus, the values of cerebral blood flow still show accurate results despite the expected lower SNR up to an offset of 3 mm. Following this, to obtain accurate results for quantifying super-selective ASL perfusion images, measuring the labeling efficiency seems necessary to prevent false results.

Self-controlled super-selective arterial spin labelling

OBJECTIVES

Arterial spin labelling (ASL) is a method of non-contrast-enhanced perfusion imaging that is generally based on the acquisition of two images which must be subtracted in order to obtain perfusion-weighted images. This is also the case for some flow territory mapping approaches that require the acquisition of two images for each artery of interest, thereby prolonging scan time and yielding largely redundant information. The aim of this study is to accelerate flow territory mapping using ASL by eliminating the acquisition of a control condition.

METHODS

Using super-selective ASL, only one artery of interest is tagged, while the contralateral arteries are in a state similar to the control condition. By using an arithmetic combination of the label images of all territories, selective images of flow territories can be obtained without the need to acquire an additional control condition. This approach for obtaining artery-selective perfusion-weighted images without acquiring a control condition is presented in this study and is referred to as "self-controlled super-selective ASL".

RESULTS

Quantitative perfusion measurements were similar to conventional super-selective and non-selective perfusion imaging across all subjects.

CONCLUSION

Super-selective arterial spin labelling can be performed without acquiring a control image.

KEY POINTS

• An accelerated method of flow territory mapping is presented. • Super-selective arterial spin labelling is performed without a control condition. • A new approach for calculating individual flow territories is presented. • The presented technique is compared to established approaches. • The outcome is similar to that using conventional techniques.

Aberrant patterns of brain cerebral blood flow in Chinese han first-episode drug-naïve depressive patients with and without a family history of depression

A positive family history plays a key role in the brain pathology of depression patients and previous research has confirmed that disturbed mood maintenance may be related to abnormal regional cerebral blood flow (rCBF). However, little is known about whether the rCBF is different between depression patients with and without family histories. To address this question, we examined the rCBF in drug-naïve, first-episode depression patients with and without family histories of depression using a 3D pseudo-continuous arterial spin-labelling technique. We found that decreased rCBF was predominantly observed in the patients without family histories, while decreased and increased rCBF co-existed in patients with family histories. The observed brain regions with altered rCBF were associated with affection processing, such as the prefrontal, occipital and insular areas. However the patterns of rCBF alteration observed in the present study were different from those found in previous studies where patients were compared with healthy controls. Our present findings, together with the findings from previous studies have prompted the need of a long-term follow-up study to characterize the brain features of the developmental trajectory of depression and investigate the targets for precise, personalized treatments.