Nonenhanced hybridized arterial spin-labeled magnetic resonance angiography of the extracranial carotid arteries at 3 Tesla using a fast low-angle shot readout
Ioannis Koktzoglou1,2, Matthew T Walker1,2, Joel R Meyer1,2, Ian G Murphy1,3, and Robert R Edelman1,3

1Radiology, NorthShore University HealthSystem, Evanston, IL, United States, 2University of Chicago Pritzker School of Medicine, Chicago, IL, United States, 3Northwestern University Feinberg School of Medicine, Chicago, IL, United States


Nonenhanced hybridized arterial spin labeling (hASL) magnetic resonance angiography (MRA) using a fast low-angle shot (FLASH) readout was used to image the extracranial carotid arteries at 3 Tesla. Comparisons were made with 2D time-of-flight (TOF) MRA and contrast-enhanced MRA. Image quality obtained hASL FLASH MRA was found to be superior to that 2D TOF, with the method also providing improved inter-rater agreement, quantification of arterial cross-sectional area, and vessel sharpness.


To evaluate nonenhanced hybridized arterial spin labeling (hASL) magnetic resonance angiography (MRA) of the carotid arteries using a fast low-angle shot (FLASH) readout at 3 Tesla.


Time-of-flight (TOF) magnetic resonance angiography (MRA) is a well-established method for depicting the extracranial carotid arteries without the use of contrast agents1,2. TOF MRA, however, has drawbacks including artifacts from saturation and dephasing of flowing spins, as well as limited vascular coverage with respect to contrast-enhanced MRA (CEMRA). Hybridized arterial spin-labeling (hASL) using a balanced steady-state free precession (bSSFP) readout has recently been suggested as a potential alternative for nonenhanced MRA of the carotid arteries at 1.5 Tesla3-5, but experience with the technique at 3 Tesla, the preferred field strength for neurovascular imaging, has been limited because of artifacts from B0 inhomogeneity. The purpose of this study was to evaluate nonenhanced hASL MRA using a FLASH readout for imaging the extracranial carotid arteries at 3 Tesla in patients undergoing standard-of-care 2D TOF and CEMRA protocols.


In this IRB-approved study, 37 patients (13 men, 24 women; mean age 68 years) presenting with neurologic symptoms were imaged with hASL, 2D TOF and CEMRA (0.1mmol/kg gadobutrol, Bayer HealthCare, Whippany, NJ) on a 3 Tesla system (MAGNETOM Skyra, Siemens, Erlangen, Germany) in the same scan session. hASL MRA consisted of an ungated 3D coronal FLASH readout that was preceded by pseudo-continuous and pulsed RF labeling (Figure 1). Using a 4-point scale (1: poor, 4: excellent) image quality was scored independently by two neuroradiologists in 10 locations of the left and right carotid arteries. Inter-observer agreement of image quality was computed using Gwet’s AC16. Arterial cross-sectional area and arterial sharpness were measured using objective computer-assisted algorithms. The presence of pathology was noted and discrepancies were settled by consensus review.


Across all locations within the carotid arteries, CEMRA provided the best image quality, with median scores of 4|4 (reader 1|2) (P<0.001 versus hASL and TOF), followed by hASL MRA with scores of 4|3 (P<0.001 versus TOF) and TOF with scores of 3|2. Image quality in one patient with a carotid stenosis is shown in Figure 2; vis-à-vis CEMRA, better correlation of arterial contours was observed with hASL MRA than with TOF MRA. For both readers, hASL MRA provided significantly better image quality than TOF at the following 8 locations: bilateral proximal internal carotid arteries (ICAs), bilateral mid-cervical ICAs, bilateral petrous ICAs, and bilateral external carotid arteries (P<0.05) (Figure 3). Inter-observer agreement was substantial for hASL MRA (AC1=0.61, 95% confidence interval (CI): 0.54-0.67), moderate for TOF MRA (AC1=0.43, 95% CI: 0.36-0.50) and almost perfect for CEMRA (AC1=0.87, 95% CI: 0.83-0.91) (P<0.001; all comparisons between techniques). Compared to CEMRA, better agreement of cross-sectional lumen area was obtained with hASL MRA than with TOF at the common carotid artery (intraclass correlation coefficient (ICC)=0.90 for hASL versus 0.66 for TOF, P<0.05), carotid bifurcation (ICC=0.87 versus 0.53, P<0.05), and internal carotid artery (ICC=0.65 versus 0.57, P=NS). Arterial sharpness was best with hASL MRA (0.74±0.12mm-1) (P<0.001 versus TOF and CEMRA), followed by TOF (0.63±0.13mm-1) and CEMRA (0.57±0.10mm-1). Using CEMRA as the reference standard, hASL MRA detected all 5 instances of carotid pathology (4 stenoses, 1 aneurysm) with no false positives.

Discussion and Conclusion

Image quality obtained with nonenhanced hASL MRA using a FLASH readout at 3 Tesla was improved with respect to 2D TOF MRA for displaying the extracranial carotid arteries, with improved inter-observer agreement. Furthermore, cross-sectional arterial area measurements obtained with hASL were in better agreement with CEMRA values, and arterial sharpness was improved at the carotid bifurcation. In conclusion, hASL FLASH offers an appealing alternative to 2D TOF for nonenhanced MRA of the extracranial carotid arteries at 3 Tesla. The protocol may have utility as a pre-contrast scout, in the assessment of the carotid arteries in patients with renal insufficiency, and when it is desirable to save contrast agents for cerebral perfusion imaging.


No acknowledgement found.


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4. Gooneratne et al. Proc ISMRM 2015 #3588
5. Xu et al. Proc ISMRM 2015 #3603
6. Gwet KL. Br J Math Stat Psychol. 2008; 61(Pt 1): 29-48.


Figure 1. Diagram of the hASL FLASH MRA pulse sequence.

Figure 2. Images obtained with nonenhanced hASL FLASH MRA, nonenhanced 2D TOF MRA, and CEMRA in a patient with a carotid stenosis. As compared to 2D TOF, arterial contours depicted with hASL FLASH better correlated with CEMRA.

Figure 3. Mean image quality versus location within the carotid arteries. Although CEMRA provided the best overall image quality, hASL FLASH MRA provided significantly better image quality than 2D TOF in eight of ten locations within the extracranial carotid arteries.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)