Arterial spin-labeled imaging

Arterial Spin Labeling (ASL) is a non-invasive magnetic resonance imaging (MRI) technique used to measure cerebral blood flow (CBF). ASL uses magnetically labeled arterial blood water as an endogenous tracer, eliminating the need for exogenous contrast agents. Here’s an overview of the main ASL imaging modalities and their characteristics:

- Principle: Labels a large bolus of arterial blood with a short magnetic pulse. - Advantages:

1. High labeling efficiency.
2. Fast acquisition.
3. Widely available and straightforward implementation.

- Disadvantages:

1. Limited spatial coverage.
2. Sensitive to transit time variability.

—

### 2. Continuous ASL (CASL): - Principle: Uses a continuous radiofrequency (RF) pulse and a magnetic gradient to label inflowing arterial blood continuously. - Advantages:

1. High signal-to-noise ratio (SNR).
2. Accurate quantification of CBF.

- Disadvantages:

1. Requires high RF power, which may cause patient heating (specific absorption rate concerns).
2. More technically demanding than PASL.

—

### 3. Pseudo-Continuous ASL (pCASL): - Principle: A hybrid method that applies a train of short RF pulses to approximate continuous labeling. - Advantages:

1. Combines the efficiency of CASL with reduced RF power.
2. High labeling efficiency.
3. Widely adopted in clinical and research settings.

- Disadvantages:

1. Slightly more complex than PASL in terms of implementation.

—

### 4. Velocity-Selective ASL (VS-ASL): - Principle: Labels blood based on its velocity rather than its location. - Advantages:

1. Insensitive to arterial transit time.
2. Provides more consistent measurements in patients with impaired perfusion.

- Disadvantages:

1. Lower SNR compared to pCASL.

—

### 5. Multi-Delay ASL: - Principle: Acquires images at multiple post-labeling delay times. - Advantages:

1. Accounts for variations in arterial transit time.
2. Improves accuracy in regions with slow or delayed blood flow.

- Disadvantages:

1. Increased scan time.
2. Higher computational demand for post-processing.

—

### Applications of ASL: - Clinical: Stroke, dementia, tumors, epilepsy, and cerebrovascular disorders. - Research: Functional brain mapping, neurodevelopment, and aging studies.

—

### Future Directions: Emerging ASL techniques focus on improving spatial resolution, SNR, and robustness to physiological variability. Advanced post-processing methods, such as machine learning, are being integrated to enhance data interpretation and clinical applicability.

Although perfusion imaging plays a key role in the management of steno-occlusive diseases, the clinical usefulness of arterial spin labeling (ASL) is limited by technical issues.

The ASL perfusion technique offers similar information as conventional Dynamic susceptibility weighted contrast enhanced perfusion imaging; however, it does not require intravenous contrast and can be quantified. The appearance of pathology is significantly impacted by the ASL techniques used.

Arterial spin labeling perfusion-weighted imaging (ASL-PWI) enables quantification of tissue perfusion.

Arterial spin-labeled imaging of glioma

see Arterial spin labelled imaging for cerebral blood flow

Arterial spin labelled imaging for Moyamoya disease.

ASL data obtained in 129 children between 2011 and 2015 were retrospectively analyzed. CBF and relative CBF in the most perfused area of each neoplasm and contrast enhancement were quantified with a semiquantitative ratio. The correlation between CBF and microvascular density was analyzed in specimens stained with anti-CD34. Results were controlled in two validation cohorts with 1.5- and 3.0-T magnetic resonance (MR) imaging.

Mean CBF was significantly higher for high-grade than for low-grade hemispheric (116 mL/min/100 g [interquartile range {IQR}, 73-131 mL/min/100 g] vs 29 mL/min/100 g [IQR, 23-35 29 mL/min/100 g], P < .001), thalamic (87 mL/min/100 g [IQR, 73-100 mL/min/100 g] vs 36 mL/min/100 g [IQR, 30-40 mL/min/100 g], P = .016), and posterior fossa (59 mL/min/100 g [IQR, 45-91 mL/min/100 g] vs 33 mL/min/100 g [IQR, 25-40 mL/min/100 g], P < .001) tumors. With a cutoff of 50 mL/min/100 g, sensitivity and specificity were 90% (95% confidence interval [CI]: 68, 100) and 93% (95% CI: 66, 100), respectively, for hemispheric tumors; 100% (95% CI: 48, 100) and 80% (95% CI: 28, 100), respectively, for thalamic tumors; and 65% (95% CI: 51, 78) and 94% (95% CI: 80, 99), respectively, for posterior fossa tumors. In posterior fossa tumors, additional use of the CBF-to-contrast enhancement ratio yielded sensitivity and specificity of 96% (95% CI: 87, 100) and 97% (95% CI: 84, 100), respectively. Use of a simple algorithm based on these values yielded an accuracy of 93% (95% CI: 87, 97). Validation sets yielded similar results, with grading accuracy of 88% (95% CI: 62, 98) with 1.5-T MR imaging and 77% (95% CI: 46, 95) with 3.0-T MR imaging. CBF was strongly correlated with microvascular density (R = 0.66, P < .001).

High-grade pediatric brain tumors display higher CBF than do low-grade tumors, and they may be accurately graded by using these values. CBF is correlated with tumor microvascular density ¹⁾.

Arterial spin-labeling perfusion of 54 children (mean age, 7.5 years; 33 boys and 21 girls) with treatment-naive brain tumors was retrospectively evaluated. The 3D pseudocontinuous spin-echo arterial spin-labeling technique was acquired at 3T MR imaging. Maximal relative tumor blood flow was obtained by use of the ROI method and was compared with tumor histologic features and grade.

Tumors consisted of astrocytic (20), embryonal (11), ependymal (3), mixed neuronal-glial (8), choroid plexus (5), craniopharyngioma (4), and other pathologic types (3). The maximal relative tumor blood flow of high-grade tumors (grades III and IV) was significantly higher than that of low-grade tumors (grades I and II) (P < .001). There was a wider relative tumor blood flow range among high-grade tumors (2.14 ± 1.78) compared with low-grade tumors (0.60 ± 0.29) (P < .001). Across the cohort, relative tumor blood flow did not distinguish individual histology; however, among posterior fossa tumors, relative tumor blood flow was significantly higher for medulloblastoma compared with pilocytic astrocytoma (P = .014).

Characteristic arterial spin-labeling perfusion patterns were seen among diverse pathologic types of brain tumors in children. Arterial spin-labeling perfusion can be used to distinguish high-grade and low-grade tumors ²⁾.