Essential guide to arterial spin labelling: Review of key artifacts and applications

Abstract

Arterial spin labelling (ASL) is a magnetic resonance imaging perfusion technique used to study cerebral blood flow. It differs from magnetic susceptibility-based perfusion sequences in that it does not require contrast media, but rather uses the signals emitted by pre-labelled protons. It is useful in the exploration of several different kinds of neurological pathologies including ischaemic stroke, brain tumours, dementia and epilepsy. However, it presents numerous artifacts that radiologists must be familiar with to ensure a correct interpretation of findings.

Resumen

Arterial spin labelling (ASL) es una técnica de perfusión cerebral por de resonancia magnética utilizada para el estudio del flujo sanguíneo cerebral. Se diferencia de las secuencias de perfusión basadas en susceptibilidad magnética por no requerir medios de contraste, sino que aprovecha la señal emitida por los protones previamente marcados. Tiene utilidad en el estudio de diversas patologías neurológicas de diferente naturaleza como el ictus isquémico, los tumores cerebrales, las demencias o la epilepsia entre otros. No obstante, presenta numerosos artefactos que deben ser conocidos por el radiólogo para una correcta interpretación de los hallazgos.

Introduction

Arterial spin labelling (ASL) is a magnetic resonance (MR) perfusion technique used in neuroradiology. Images obtained with ASL are reconstructed and represented as cerebral blood flow (CBF) maps. Unlike other magnetic susceptibility techniques,¹ ASL does not use paramagnetic contrast agents, but rather takes advantage of the radiofrequency signal emitted by protons previously labelled at the level of the neck arteries, which supply blood flow to the brain.

The absence of exogenous contrast makes ASL a less invasive and safer sequence, especially in individuals allergic to contrast media used in MRI, patients with kidney disease or paediatric patients. It also avoids the potential deleterious effects of gadolinium deposition, especially in patients undergoing successive MRI scans.² Another advantage is that it provides a quantitative measure of CBF, expressed in units of perfusion in millilitres per 100 g of brain tissue per minute.³

However, one limitation of this technique is that ASL offers lower spatial resolution compared to other perfusion imaging sequences. This is largely due to the fact that the signal provided by blood is lower than that provided by paramagnetic contrast. The lower resolution in ASL may be a drawback in studies where greater anatomical precision is required, such as in tumour imaging studies.

The aim of this paper is to review the usefulness of this technique, and the different types of artifacts that can lead to diagnostic errors in daily clinical practice.

Technique

Radiofrequency pulses and gradients are used to magnetically label hydrogen protons within arterial blood. This occurs in a plane situated at 1 cm below the inferior border of the cerebellum.⁴ This is referred to as the labelling plane and is ideally positioned perpendicular to the course of the arteries that supply the brain. Within seconds, the labelled spins reach the cerebral parenchyma to be studied, constituting the acquisition plane. Through the relaxation process, protons emit the radiofrequency signals required to generate an image.

So the signal attributable to blood flow overlaps with the signal originating from the brain tissue itself, which is assumed to be static. The result is a small signal difference between control image taken of the brain tissue just before the labelled spins arrive (control image) and the post-labelling image acquired after. The post-labelling acquisition is performed after a specific time interval called the post-labelling delay (PLD). Perfusion maps are obtained by calculating the difference between the post-labelling signal and the control.

Optimal PLD varies according to the age of the patient under study. While in a healthy adult it is usually 1800 ms, in patients over 70 years of age it is 2000 ms. In the case of paediatric patients, PLD of 2000 ms in neonates and 1500 ms in children are recommended.⁴ Multiphase acquisition, with multiple PLDs, also known as dynamic ASL, is an alternative technique that not only enables better tailoring of the delay time to individual patients, but also improves the quality of parenchymal imaging.⁵ Dynamic ASL is valuable in the study of neurovascular disease, as it provides a visual representation of regions of the brain experiencing delayed flow.

Various methods exist for spin labelling in the neck: continuous (c-ASL), pulsed (p-ASL) and pseudocontinuous (pc-ASL).⁵

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  c-ASL uses long radiofrequency pulses, with long labelling durations. It has a higher signal-to-noise ratio and requires a shorter transit time compared to p-ASL.

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  p-ASL uses short duration pulses over a short labelling duration. In comparison with c-ASL, it offers more efficient labelling and a lower specific absorption rate.

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  pc-ASL attempts to combine the virtues of the above techniques in such a way that it uses short pulses over long labelling durations. It is the most recommended and most widely used labelling method today. In pc-ASL the labelling plane is usually located 2 cm from the acquisition area.

EDUCATIONAL POINT 1

Remember: the ASL sequence is a perfusion technique that does not use paramagnetic contrast agents. This makes it a useful alternative in paediatric patients as it is less invasive. In addition, ASL avoids potential adverse reactions to gadolinium, which is especially relevant in patients with contrast media allergies and in people with chronic kidney disease.

Normal cerebral blood flow patterns

Physiologically, it is common to find brain areas with higher and lower blood flow (Fig. 1). The relevant areas are listed below.⁶

Figure 1.

Left image shows physiological hyperperfusion pattern with greater flow in frontal and occipital cortices, as well as in posterior third of cingulate gyrus. Right image shows physiological hypoperfusion of globi pallidi (circles with solid line) and periventricular white matter (areas marked with a dashed line).

Hyperperfused areas

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  Posterior cingulate gyrus and the precuneus, probably because they are integrated within the default mode network.

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  Occipital cortices due to activation of the visual area.

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  Frontal lobes, especially frontobasal and parasagittal areas.

Hypoperfused areas

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  Basal ganglia, given the longer delay in CBF relative to the cortex.

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  Globi pallidi, probably due to the T1 shortening artifact resulting from the presence of iron deposits.⁶

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  White matter around the frontal and occipital horns of the lateral ventricles. This is because these are border zones and therefore more prone to hypoperfusion in cases of atherosclerotic disease.

In addition, CBF distribution is dynamic and varies with age. In neonates, there is a progressive increase in flow to the frontal and occipital cortices. In infants, increased blood supply to areas corresponding to the default mode network is a sign of brain maturation.⁷ In elderly patients, there is a decrease in flow to the periventricular white matter and border zones.⁶

ASL artifacts

We can classify the different types of ASL sequence artifacts according to when they occur: during labelling, during the transit of protons from the neck to the brain parenchyma or during acquisition.⁸

During labelling

Ineffective labelling artifact

Ineffective labelling occurs when the labelling plane cannot be positioned perpendicular to the arterial flow (Fig. 2), such as in elderly patients with tortuous carotid arteries. It may also result from the presence of metallic material in the neck, as magnetic susceptibility effects can interfere with the labelling process. In either of these situations, artifactual hypoperfusion will be observed due to the absence of enhancement (Fig. 3). MR angiography can be useful in determining the correct placement of the labelling plane.⁸ A low-resolution vascular sequence such as a time-of-flight (TOF) vessel scout can be performed. This has the advantage of being fast and including the neck arteries. Otherwise, a higher resolution TOF sequence can be acquired: the latter provides more morphological information on intracranial arteries (Table 1). In the specific case of carotid stents, the labelling plane should be placed above the stents.⁹

EDUCATIONAL POINT 2

Remember: the use of rapid vascular localisers or MR angiography sequences prior to ASL helps to inform the placement of the labelling plane, thus avoiding mislabelling.

Figure 2.

Six-year-old boy under investigation for generalised convulsive seizures. Solid lines show lower limits of acquisition planes. Dashed lines show labelling planes. A) The planes are initially placed too high in the study planning. Moreover, the patient’s head tilts to the right in coronal plane. This positioning error causes labelling failure, producing artifactual hypoperfusion in the right hemisphere in arterial spin labelling maps. B) After adjusting positioning of the planes, correct maps are obtained.

Figure 3.

A 67-year-old female with right hemispheric clinical deficit. Computed tomography (CT) and CT angiography did not reveal any findings to explain symptoms. The patient had a left carotid artery stent (A). Magnetic resonance imaging with arterial spin labelling sequence was performed, which demonstrated artifactual hypoperfusion in the left hemisphere (B). Finding due to labelling failure, in this case caused by presence of stent.

Table 1.

Summary of main ASL artifacts and how to resolve them to achieve technically optimal CBF maps.

Artifact	Cause	Solution
Ineffective labelling	Presence of metallic object in the labelling plane	Placement of the labelling plane above the metallic object
	Arterial tortuosity	MR angiography performed prior to the ASL to inform plane placement
Arterial transit	Delay in arterial flow	Dynamic ASL
Label loss	Gadolinium administration	Perform ASL prior to gadolinium administration
Motion	Patient motion	Ask the patient to stay still during the study
Occipital hyperperfusion	Visual stimulation	Ask the patient to close eyes during the acquisition

During transit

Arterial transit artifacts

In certain circumstances, such as low cardiac output, arterial stenosis (Fig. 4) or ischaemic stroke, CBF is slowed, and thus there is a delay in the passage of blood from the arterial compartment to the capillary and to the encephalic parenchyma. This means that the labelled protons emit their relaxation signal while they are still in the intra-arterial compartment. This results in a linear, often serpiginous, hyperintense signal, consistent with a delayed or impaired arterial flow. Arterial aneurysms are a special case, in which turbulent or slow flows effectively ‘trap’ the signal, resulting in foci of hyperintense signals.

Figure 4.

A) A 58-year-old male patient with significant stenosis of right internal carotid artery. B) Arterial spin labelling (ASL) maps in both grey-scale and colour. Asymmetric distribution of cerebral flow between both hemispheres is observed. In right hemisphere, cortical arterial branch pathway can be predicted due to the persistence of intra-arterial signal at this level (arrows). This phenomenon occurs due to prolonged transit time in right carotid territory caused by stenosis. ASL maps in grey scales make it easier to distinguish intra-arterial signal persistence artifacts. C) Sequence of images relating to previous patient shows dynamic ASL study, with successive post-labelling delay times. Dynamic ASL helps reveal asymmetry in blood flow, which is reduced in the left hemisphere. Persistence of intra-arterial signal (arrows) is observed, accompanied by delayed parenchymal filling in the right carotid territory (asterisks).

Although the arterial transit artifact is diagnostically useful,¹⁰ it can be mitigated by lengthening the PLD or by performing a study with multiple PLDs (Fig. 4).

Loss of labelled spin

Under no circumstances should gadolinium be administered prior to acquiring the ASL sequence. The paramagnetic contrast shortens the relaxation time of the labelled spins, meaning that by the time the labelled protons reach the acquisition plane, they no longer emit a signal. This can result in artifactual generalised hypoperfusion¹¹ (Fig. 5A and Table 1).

EDUCATIONAL POINT 3

Remember: ASL sequences must be acquired prior to the administration of paramagnetic contrast, otherwise a generalised signal drop occurs.

Remember that arterial transit artifacts can be useful in the diagnosis of some neurovascular diseases, e.g. ischaemic stroke. Intra-arterial signal persistence can be decreased by prolonging the PLD or by performing a study with multiple PLDs (dynamic ASL).

Figure 5.

Various artifacts are presented: A) Artifactual reduction to overall flow signal due to inadvertent administration of paramagnetic contrast agent prior to image acquisition. B) Image degraded by patient motion artifacts, rendering it completely uninterpretable. C) Two-year-old child under examination due to epileptic seizures: an artifactual pattern of hyperperfusion in both occipital lobes is observed, due to eye opening during acquisition.

During acquisition

Patient motion artifacts

Changes to the position of anatomical structures between the control and post-labelling images disrupts k-space filling. The characteristic result is a completely uninterpretable image composed of spirals or semicircles¹² (Fig. 5B).

Occipital hyperperfusion

If the patient opens their eyes during the examination, physiologically this triggers increased blood flow to the occipital lobes. It is therefore important to instruct the patient to not open their eyes during the examination¹² (Fig. 5C and Table 1).

EDUCATIONAL POINT 4

Remember: patient cooperation is critical during ASL acquisition as they must remain motionless to prevent artifacts. They should also keep their eyes closed during the study to avoid physiological occipital hyperperfusion.

Clinical applications of ASL

The ASL sequence is useful in numerous neurological pathologies, not only in neurovascular disorders such as stroke, but also in other conditions in which CBF may be altered, such as tumours, seizures and neurodegenerative pathology (Table 2).

Table 2.

Summary of main clinical applications of ASL.

Neurovascular pathology

Arterial stenosis

Ischaemic stroke

Posterior reversible encephalopathy syndrome (PRES)

Hyperperfusion syndrome

Tumour

Dementia

Migraine

Epileptic seizures

Encephalitis

Psychiatric pathology

Neurovascular pathology

Arterial stenosis

Proximally, arterial stenoses tend to lead to the persistence of intra-arterial signal, known as a transit artifact; distally, they result in an area of hypoperfusion with reduced signal, corresponding to the oligohaemic vascular territory. This phenomenon is applicable to stenosis affecting either the internal carotid arteries or the cerebral arteries¹³ (Fig. 6).

Figure 6.

A 68-year-old male with history of significant stenosis of internal carotid artery after cervical radiotherapy for head and neck cancer. Patient presents with episodes of left hemisphere neurological deficits. A) CT angiography of supra-aortic trunks reveals occlusion of left internal carotid artery from its origin at the carotid bulb, with associated calcified atheromatosis. B) ASL map reveals diffuse hypoperfusion in left hemisphere that explains patient’s symptoms. C) Hyperintense foci observed on FLAIR imaging in left parieto-occipital cortex and centrum semiovale, following a linear distribution along deep border zone, suggestive of ischaemic infarcts due to carotid hypoperfusion. D) Foci of diffusion restriction at parieto-occipital level, with smaller foci in left centrum semiovale and precentral sulcus, all consistent with acute ischaemic lesions.

Ischaemic stroke

ASL can be a useful tool in the assessment of ischaemic stroke. Intra-arterial signal persistence artifacts can be observed in segments proximal to the filling defect causing the neurological symptoms.¹⁴ The sensitivity of this artifact for detecting filling defects is 76%, while its specificity is 52.7%.¹⁵

By comparing diffusion-restricted areas—which represent the infarct core—with areas with low CBF, it is possible to determine the presence of a mismatch (an indicator of the volume of brain tissue potentially recoverable by reperfusion therapy)¹⁶ (Fig. 7).

Figure 7.

A) An 80-year-old male with two-hour history of right hemiparesis and aphasia. Computed tomography (CT) and magnetic resonance imaging (MRI) performed. Diffusion-weighted imaging (DWI) reveals single focus of restricted diffusion in posterior portion of lentiform nucleus (circle), consistent with acute infarction. ASL map shows drop in flow throughout middle cerebral artery territory (arrow). Extensive ischaemic penumbra, with mismatch between diffusion and ASL. B) A 74-year-old man with activated stroke code due to left hemispheric neurological deficit. Non-contrast scan shows loss of grey-white matter differentiation in left parietal convexity, consistent with acute ischaemic infarction (circle). No discrepancy is observed between area of acute infarction on DWI and area of hypoperfusion on arterial spin labelling (circles). No filling defects identified in time-to-flight sequence. Furthermore, patient presented with right temporoparietal lesion (partially included in these images and indicated with arrow). On CT, appears as subcortical hypodensity with cortical rim. On MRI, appears as cortical hyperintensity on FLAIR, with no diffusion restriction, normal apparent diffusion coefficient and reduced cerebral blood flow (arrow). Image consistent with subacute ischaemic infarction.

A strong correlation has been demonstrated between the areas of ischaemia shown by ASL and other sequences, although ASL tends to overestimate the area of ischaemic penumbra,¹⁶ especially in patients with small areas of infarction.¹⁷

Ischaemic injury to the blood-brain barrier may result in failure of CBF autoregulation and hyperperfusion in infarcted areas. In these cases, increased CBF is observed, which is also associated with a higher risk of haemorrhagic transformation (Fig. 8).

Figure 8.

A 23-year-old male in first month following bilateral lung transplant due to cystic fibrosis. Patient presents with tonic-clonic seizures and language abnormalities. A) FLAIR imaging shows cortico-juxtacortical hyperintense area in right temporal lobe; (B) with gyriform enhancement following contrast administration; (C, D) no diffusion restriction observed. Together, findings are consistent with subacute ischaemic infarction. E) Arterial spin labelling map reveals area of increased flow, presumed to represent luxury perfusion within infarcted region.

On the other hand, increased flow in ASL soon after thrombectomy is associated with a smaller volume of infarcted tissue, as well as better functional outcomes between the first 24 h and the first 3 months. However, it is also associated with an increased risk of bleeding.¹⁸

Posterior reversible encephalopathy syndrome (PRES)

The pathophysiology of PRES is not fully understood. The latest evidence seems to suggest cerebral endothelial dysfunction as the origin of this disorder. This leads to increased vascular permeability, disruption of the blood-brain barrier and extravasation of water and proteins. Certain risk factors or triggers for PRES have been described, which include arterial hypertension, renal failure, eclampsia, and certain drugs such as tacrolimus or cyclosporine.

The fundamental imaging sign is the presence of vasogenic oedema, which most frequently displays a bilateral occipitoparietal distribution, although other patterns may be observed, such as holohemispheric border zones or predominance in the superior frontal gyrus.

Both hyperperfusion (Fig. 9A) and hypoperfusion phenomena have been described in perfusion studies of patients with PRES. There are different theories about the pathophysiological mechanism behind these flow abnormalities. Hyperperfusion has been attributed to vasodilatation secondary to failure of the cerebral vascular autoregulation system, whereas hypoperfusion appears to be due to direct endothelial damage and reactive vasoconstriction¹⁹ (Fig. 10).

Figure 9.

A) A 61-year-old woman, in third week following bilateral lung transplant. During admission, laboratory findings revealed thrombotic microangiopathy secondary to tacrolimus treatment, as well as reduced visual acuity. Both T2 and FLAIR images show hypersignal in subcortical white matter in both occipital lobes, consistent with vasogenic oedema (arrows). Arterial spin labelling (ASL) map shows bilateral increase in flow throughout occipital region, associated with PRES. B) A 62-year-old male who underwent angioplasty with stent in right internal carotid artery and restored flow. A week after the intervention, patient presents with generalised tonic-clonic seizures. On T2-weighted imaging, only finding observed is effacement of right frontal sulci. ASL map shows increase in cerebral blood flow values in right hemisphere. Hyperperfusion syndrome suggested as cause of seizures.

Figure 10.

A 53-year-old male was examined due to a cerebral mass. MRI shows a heterogeneous intra-axial lesion with mass effect, surrounding oedema, diffusion restriction and enhancement on post-contrast T1-weighted imaging. The arterial spin labelling map reveals increased flow in the described lesion. All these findings are consistent with high-grade glioma.

Hyperperfusion syndrome

Cerebral hyperperfusion syndrome may occur following revascularisation of longstanding carotid artery stenosis, either by endarterectomy or stenting. It has an incidence of approximately 7.5%, although only a minority of these patients are symptomatic. Clinical presentation may take the form of headaches, seizures or neurological deficits, with haemorrhage being the most serious complication.²⁰

Unilateral hemispheric vasogenic oedema is the main finding on both CT and MRI, which may or may not be associated with a mass effect. As would be expected, ASL maps show asymmetry between the two hemispheres, with hyperperfusion on the repermeabilised side (Fig. 9B).

Tumours

Tumour perfusion studies are useful in both in the characterisation of the lesion under study and in the determination of aggressiveness, in surgical planning and also in follow-up.

A strong correlation has been found between ASL and other perfusion sequences in the study of tumour blood flow, especially for gliomas with contrast uptake.²¹

It is important to note that the presence of haemorrhage, calcifications, cystic formations or necrosis can produce areas of low signal intensity, which does not necessarily indicate genuinely reduced tumour perfusion.²¹

Assessing the response to treatment of high-grade gliomas is challenging, especially when trying to discern whether areas of contrast uptake correspond to radionecrosis lesions or to tumour recurrence. ASL can help in these cases, as unlike recurrence, radionecrosis does not show increased flow^4,22 (Fig. 11).

EDUCATIONAL POINT 5

Remember: ASL can help in differentiating between tumour recurrence and areas of radionecrosis based on the flow pattern. In areas of tumour viability the ASL will demonstrate hyperperfusion, whereas radionecrosis lesions are hypoperfused.

Figure 11.

A 63-year-old male in follow-up for cerebral metastases from melanoma treated with radiotherapy. In the superior temporal gyrus, adjacent to frontal operculum, a small, well-defined lesion is seen. It is markedly hypointense on T2-weighted and FLAIR sequences (A and B), shows intense enhancement following contrast administration (C), hypoperfused on arterial spin labelling (ASL) (D) and associated with vasogenic oedema in the adjacent white matter (yellow arrows). ASL is key to determining that the lesion is secondary to radionecrosis and not tumour recurrence. Subsequent follow-ups show no changes to lesion in flow or size. Additionally, an extra axial lesion is observed in posterior portion of falx cerebri with intense and homogeneous enhancement consistent with a small left parafalcine meningioma (white arrow).

Primary CNS lymphomas and metastases of certain tumours, such as melanoma or renal cell carcinoma, among others, may show elevated CBF values, thus mimicking high-grade gliomas²³ (Fig. 12).

Figure 12.

A) A 76-year-old female with no relevant history presenting with disorientation and speech abnormalities for two weeks. Magnetic resonance imaging (MRI) reveals lesion with marked enhancement, diffusion restriction and hyperperfusion on arterial spin labelling. It was a primary cerebral lymphoma. B) A 57-year-old male with history of melanoma was admitted due to epileptic seizures. MRI shows small, well-defined lesion in right superior frontal gyrus (arrow), with surrounding oedema, enhancement and focal increase in perfusion. Given the clinical context, it was attributed to metastasis.

Figure 14.

A 12-year-old boy attends hospital with acute onset headache and dysarthria. Upon admission, magnetic resonance imaging (MRI) is performed. A) T2-weighted images show no pathological signs. Susceptibility weighted imaging identifies venous thickening of the veins, particularly in the left hemisphere (arrows). Arterial spin labelling maps show clear asymmetry, with left hemispheric hypoperfusion. During admission, patient improves spontaneously. The patient’s father mentions having suffered similar episodes as a child. B) Follow-up MRI a month later confirms normalisation of perfusion map. Subsequent genetic study confirmed suspicion of familial hemiplegic migraine.

Figure 15.

A) Eleven-year-old boy with episodes of focal impaired awareness seizures and with EEG revealing left frontal focal discharges. Cortical thickening is identified in the left frontal parasagittal region with subtle hyperintensity on FLAIR and small millimetric cystic-appearing foci (not shown in image). The described lesion does not exert mass effect, with no enhancement or diffusion restriction. The arterial spin labelling (ASL) map reveals increased flow within the described lesion, indicating activity in the epileptogenic focus (arrow). The lesion has all the radiological features of dysembryoplastic neuroepithelial tumour (DNET). B) A 76-year-old female under examination for right focal seizures which go on to be generalised. In addition to cortical thickening and FLAIR hyperintensity in the right parietal and temporo-occipital regions (arrows), there is subtle diffusion restriction in the right pulvinar nucleus (circle) and in the right temporo-occipital cortex (arrow). ASL map identifies areas of increased flow in the right parietal and temporo-occipital regions. Findings characteristic of patients in status epilepticus. C) A 65-year-old male patient with a left temporoinsular low-grade glioma in status epilepticus. FLAIR hyperintensity and focal increase in flow in the left pulvinar nucleus region (circle), as well as hypoperfusion of the contralateral cerebellar hemisphere (diaschisis phenomenon).

Other pathologies

Dementia

In neurodegenerative diseases, hypoperfusion (studied in ASL maps) and hypometabolism (observable by nuclear medicine techniques) precede anatomical abnormalities. In Alzheimer's disease, areas of hypoperfusion are typically seen in the temporal and parietal lobes, the posterior part of the cingulate gyrus and the precuneus²⁴ (Fig. 13).

Figure 13.

A 59-year-old female presenting with cognitive decline and memory failure. Although no significant parenchymal atrophy on morphological sequences, arterial spin labelling demonstrated asymmetrical pattern of hypoperfusion in anterior and medial regions of both temporal lobes, more pronounced on the right. Finding suggestive of Alzheimer’s disease.

Migraine

Migraine is one of the most common stroke simulators that a radiologist encounters in daily practice. Some patients present with auras preceding the headache, which may consist of visual and sensory symptoms, aphasia or motor deficits, as in the case of hemiplegic migraine.

CT, MRI or angiographic studies do not present characteristic findings; however, perfusion studies can provide information. Vasoconstriction and subsequent hypoperfusion phenomena usually occur during auras, whereas in the subsequent headache phase, a reactive vasodilation occurs, leading to hyperperfusion of the previously affected territories²⁵ (Fig. 14).

Epileptic seizures

Seizures lead to increased cellular metabolism and energy demands in abnormally activated areas. This is why during the ictal phase, epileptogenic foci display a local increase in perfusion²⁶ (Fig. 15), whereas in the post-ictal phase, and especially in chronic phases, a pattern of hypoperfusion is observed.

Chronic or recurrent seizures bring with them a certain degree of cortical atrophy, proportional to the hypoperfusion visualised on ASL.

Encephalitis

The inclusion of the ASL sequence in imaging studies of patients with autoimmune or herpetic encephalitis enhances diagnostic value by offering information not captured by conventional MRI sequences. In this regard, the measurement of CBF in the affected areas is related to the evolutionary phase of the process and the response to treatment. In acute or subacute phases there is an increase in flow due to inflammation phenomena, while in chronic phases there is a decrease in flow secondary to tissue destruction and loss of neuronal function.^27,28

Psychiatric illnesses

In recent years, ASL has been used in the study of psychiatric disorders. In the case of schizophrenia or major depression, decreased blood flow can be observed in certain areas such as the prefrontal cortex, while in anxiety disorder there is increased blood flow in areas related to the stress response, such as the tonsils.²⁹

Conclusion

ASL is a brain perfusion technique that does not use paramagnetic contrast, relying instead on the signal emitted by protons previously labelled or tagged in the region of the arteries supplying the brain.

It is associated with multiple artifacts that radiologists should be familiar with as otherwise they can lead to misinterpretations, but also can be indicative of certain pathological processes.

ASL has applications in several areas, including vascular pathology, tumours, dementia and epilepsy. In the case of stroke, ASL can be useful for delineating the ischaemic region and, together with diffusion-weighted imaging, for defining the ischaemic penumbra. In the radiological assessment of treated brain tumours, ASL can help to distinguish tumour recurrence, which causes increased CBF, from radionecrosis, which is characterised by decreased flow.

CRediT authorship contribution statement

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  Study conception and design: not applicable.

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  Data collection: not applicable.

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  Data analysis and interpretation: not applicable.

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  Critical content review: RSB, DCV, DHG, MDP, EML.

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  Approval of the final manuscript: RSB, DCV, DHG, MDP, EML.

Funding

This research has not received funding support from public sector agencies, the business sector or any non-profit organisations.