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Table of Contents    
NI FEATURE: THE EDITORIAL DEBATE IV-- PROS AND CONS
Year : 2017  |  Volume : 65  |  Issue : 5  |  Page : 977-978

Can arterial spin labelling really replace dynamic susceptibility contrast perfusion techniques for assessing brain tumours in clinical practice?


Department of Neuroradiology, Queen Elizabeth Hospital Birmingham, Birmingham, United Kingdom

Date of Web Publication6-Sep-2017

Correspondence Address:
Vijay Sawlani
Department of Neuroradiology, Queen Elizabeth Hospital Birmingham, University Hospitals Birmingham NHS Foundation Trust, Mindelsohn Way, Edgbaston, Birmingham, B15 2TH
United Kingdom
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/neuroindia.NI_727_17

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How to cite this article:
Patel MD, Sawlani V. Can arterial spin labelling really replace dynamic susceptibility contrast perfusion techniques for assessing brain tumours in clinical practice?. Neurol India 2017;65:977-8

How to cite this URL:
Patel MD, Sawlani V. Can arterial spin labelling really replace dynamic susceptibility contrast perfusion techniques for assessing brain tumours in clinical practice?. Neurol India [serial online] 2017 [cited 2019 Sep 22];65:977-8. Available from: http://www.neurologyindia.com/text.asp?2017/65/5/977/214069


The article by Soni et al., titled ‘Perfusion MR imaging of enhancing brain tumors: Comparison of arterial spin labelling technique with dynamic susceptibility contrast technique'is very thought-provoking and raises the important question about whether or not arterial spin-labelled (ASL) magnetic resonance imaging (MRI) can be used in clinical practice in the place of dynamic susceptibility contrast (DSC) imaging. Their results in this small cohort of patients showed a very strong correlation between the blood flow assessment in various tumours analysed with both the techniques, suggesting that non-invasive ASL imaging could potentially be an alternative to DSC imaging.[1] Both of these perfusion imaging techniques visualise blood flow at a microscopic level and, therefore, can demonstrate angiogenesis, something that the conventional MRI cannot.

The DSC technique is the most widely used, fast and robust method in the assessment of brain tumours,[2] which uses measurements of relative cerebral blood volume (CBV). This is shown to relate directly to the grade of brain tumour.[2] The technique involves analysis of a first pass gadolinium bolus with T2/T2*-weighted sequences, leading to susceptibility artefact and a loss in signal intensity proportional to the amount of contrast within the voxel. DSC imaging allows for the detection of sites where the blood-brain barrier has been impaired; however, leakage of contrast into the extravascular space can cause permanent signal loss on T2-weighted sequences, and thereby, results in errors in DSC perfusion mapping, also making repeated measurements difficult. Where there is signal loss from calcification, blood or air interfaces, evaluation will also be limited. DSC imaging is fast with less than one minute required for assessing the whole brain, over which time, a signal-time intensity curve is generated and various parameters can be calculated. The quantification of absolute cerebral blood flow (CBF), however, requires an accurate estimation of the mean transit time (MTT) which can be difficult to assess due to the presence of a number of intrinsic factors.

ASL imaging, on the other hand, is a low-cost and non-invasive method of perfusion mapping as it does not require gadolinium-based contrast agents; instead, the technique utilizes magnetically labelled arterial blood as an endogenous tracer. The high signal intensity seen on images acquired through ASL is from tissue hyperperfusion and intravessel signals, rather than from disruption of the blood-brain barrier and vascular structures, as is seen in DSC imaging. The ASL perfusion maps are, therefore, less influenced by an impaired blood brain barrier. Consequently, patterns of high signal can be different and can sometimes be seen outside the contrast enhancing area.

Not requiring gadolinium contrast is particularly of benefit in patients known to be having contrast-reactions, in subjects during during the pregnancy and the lactation period, in patients with renal insufficiency, in the paediatric population in whom obtaining an intravenous access becomes difficult, and also in patients where repeated measurements are required. There is no risk of contrast extravasation, nephrogenic systemic fibrosis or gadolinium deposition disease from repeated studies, which are problems often encountered with the linear gadolinium-based contrast agents.

ASL currently only provides absolute quantification of the cerebral blood flow; the maps are automatically created without the need for manual post-processing. Its major advantage over the DSC imaging is that it allows the use of threshold or reference values for a more objective assessment, allowing for comparisons of any given patient during treatment. CBF is, however, affected by the transit time, which differs based upon the patient's age and the grey and white matter present; therefore, corrections must be made while assessing for absolute CBF. The assessment of relative CBF is, however, sufficient for brain tumour evaluation. ASL cannot directly provide the CBV or MTT measurements, although these parameters can theoretically be derived, and there is less robust data on the ASL imaging usage compared to that of the DSC techniques.[2],[3]

ASL imaging requires rapid echo-planar imaging (EPI) sequences to be performed before labelled blood relaxes to its equilibrium state. This is at the expense of a lower signal-to-noise ratio, lower temporal and spatial resolution and greater susceptibility artefact from the metallic hardware, blood, large vessels and air interfaces. This also poses a problem if the patient moves during the imaging or in patients unable to cooperate. An artefact can significantly limit its interpretation, especially if there are neurosurgical components near a resection cavity or where there are calcified brain lesions, causing blooming artefact and signal void. Hypoperfusion and a lengthened transit time will reduce the signal magnitude with a subsequent increase in the vascular artefact. Furthermore, it may not be possible to evaluate the entire posterior fossa due to the inversion labelling slab,[4] which should be considered in posterior fossa tumours, although DSC imaging can often produce artefacts in the same region. Knowledge of artefacts and their causes is particularly important for the correct interpretation of ASL imaging.

The acquisition time is longer in ASL than in DSC imaging; typically, it is between 4-10 minutes depending on the magnetic field strength of the scanner, being faster at higher strengths. Image quality will improve (without an increase in the artefacts) with the use of higher field (3T) strength scanners,[4] a phased-array coil receiver, and ultra-fast 3D sequences by combining gradient and spin echo sequences instead of the traditional echo planar imaging (EPI) and post-processing to reduce the artefacts. Ultra-high field (7T) strength imaging opens up the possibility of a more accurate CBF measurement in areas of relatively low perfusion and an improvement in quantification by an increase in signal-to-noise ratio of approximately four times,[5] although there is higher energy deposition.

Although ASL has been around for more than two decades, it has only recently begun to make the transition from a research tool to clinical use.[6] Whilst it may not be appropriate to directly compare perfusion values from the ASL and DSC techniques, multiple studies suggest that they both correlate well in the field of neuro-oncology with similar sensitivities and specificities,[2],[3],[4] and ASL has been shown to be able to guide cerebral biopsy. It demonstrates evidence in multiple clinical applications in terms of an improved and earlier diagnosis;[5] however, more methodological developments are required to increase its robustness and decrease its variability in CBF estimation.[6] Furthermore, standardisation of acquisition parameters, education about image post-processing and better guidelines for interpretation would help to implement the wider use of ASL.[5] For the time being, ASL would be ideally suited for patients in whom gadolinium-based contrast is contraindicated or in patients who require repeated contrast-enhanced perfusion imaging.

 
  References Top

1.
Soni N, Dhanota DPS, Kumar S, Jaiswal AK, Srivastava AK. Perfusion MR imaging of enhancing brain tumors: Comparison of arterial spin labelling technique with dynamic susceptibility contrast technique. Neurol India 2017;65:1046-52.  Back to cited text no. 1
  [Full text]  
2.
Mohindra N, Neyaz Z. Magnetic resonance sequences: Practical neurological applications. Neurol India 2015;63:241-9.  Back to cited text no. 2
[PUBMED]  [Full text]  
3.
Ata ES, Turgut M, Eraslan C, Dayanır YÖ. Comparison between dynamic susceptibility contrast magnetic resonance imaging and arterial spin labeling techniques in distinguishing malignant from benign brain tumors. Eur J Radiol 2016;85:1545-53.  Back to cited text no. 3
    
4.
Lehmann P, Monet P, de Marco G, Saliou G, Perrin M, Stoquart-Elsankari S, et al. A comparative study of perfusion measurement in brain tumours at 3 Tesla MR: Arterial spin labeling versus dynamic susceptibility contrast-enhanced MRI. Eur Neurol 2010;64:21-6.  Back to cited text no. 4
    
5.
Grade M, Hernandez tamames JA, Pizzini FB, Achten E, Golay X, Smits M. A neuroradiologist's guide to arterial spin labeling MRI in clinical practice. Neuroradiology 2015;57:1181-202.  Back to cited text no. 5
    
6.
Haller S, Zaharchuk G, Thomas DL, Lovblad KO, Barkhof F, Golay X. Arterial spin labeling perfusion of the brain: Emerging clinical applications. Radiology 2016;281:337-56.  Back to cited text no. 6
    




 

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