Imaging in stroke

Cerebrovascular diseases are a major cause of global mortality and morbidity. From initial diagnosis to determining and guiding further treatment, imaging examinations play a critical role in the management of stroke patients.

Comparison of CT and MRI in acute ischaemic stroke

In recent times, imaging techniques have made immense progress in that imaging times have shortened, signal-to-noise ratios and resolution has improved, and new imaging modalities have been developed.1 Even though both computed tomography (CT) and magnetic resonance imaging (MRI) are used to detect strokes, both these techniques have advantages and disadvantages. While MRI is more complicated and time-consuming, CT scans expose the patients to high radiation doses.2 Some studies stress that MRI is the gold standard in imaging acute stroke, however most studies still rely on CT scans alone.3

MRI scan and CT scan

Figure 1. Left image - MRI scan. Right image – CT scan. Note the soft tissue detail in the MRI is not visualised in the CT.

 

According to a guideline publication, diffusion MRI is more useful than a CT scan for diagnosing acute ischaemic stroke within 12 hours of a person’s first stroke symptom. In a large study review, it was found that stroke was accurately detected 83% of the time by MRI versus 26% of the time by CT.4

MRI has major benefits like including whole brain coverage with a standard scanner, absence of radiation, and a relatively safe contrast applied in lower doses. Moreover, MRI technique like arterial spin labelling (ASL) allow for basic perfusion imaging without administering contrast agents.2

However, CT scanners are much more widely available and accessible in emergency rooms than are MRI scanners, and imaging patients with acute stroke as rapidly as possible is of paramount importance.5

The mismatch between infarct core and ischaemic penumbra can be defined using diffusion-weighted MRI (DW-MRI) or CT cerebral blood volume imaging (CT-CBV), and penumbra with either MR or CT perfusion-weighted imaging (MRP or CTP).5,6 Advanced MRP and CTP imaging measurements of core/penumbra mismatch correlate up 97.8% when CT coverage is adequate.5,7

 

CT

MRI

Advantages

Disadvantages

Advantages

Disadvantages

General

Rapid, cheap, widely available, selection of lytic eligible patients8

Radiation exposure, contrast exposure limitations

No radiation exposure, no iodinated media injection

Less available, more expensive, specialist training. Contraindications (e.g. metal implants, claustrophobia)

Early infarction Imaging

Cost effective, validated in clinical trials

Moderate accuracy and interobserver reliability. Uncertainty beyond 4.5hrs

Highly accurate and sensitive

Requires use of apparent diffusion coefficient to confirm vasogenic vs. cytotoxic oedema

Vasculature imaging

Nearly 100% accurate. Less motion artefacts

Radiation exposure, iodinated contrast enhancement required

Indicated in renally impaired patients

Limited accuracy

Penumbra imaging

Use for intra-arterial thrombolysis (IAT) supported by clinical trials

Lack of standardization of methods and post- processing software. Limited in posterior circulation strokes

Use with intravenous recombinant tissue Plasminogen Activator (i.v. rtPA) within 6 hrs supported by clinical trials

Overestimation of penumbra; mismatch of 20% is arbitrary. Lack of studies with endovascular therapies

Table 1. Advantages and disadvantages of multimodal CT and MRI modalities. Adapted from García-Bermejo et al, 2012.

 

Despite the benefits of MRI, some patients refuse this method of imaging due to factors like claustrophobia, or the presence of pacemakers, aneurysm clips, or other ferromagnetic material in their bodies. In addition, patients with depressed consciousness, disorientations, aphasia or confusion cannot give a reliable history on potential contraindications to MR imaging.1 In such cases, CT scans are the preferred option.

In comparison to MRI scans, both CT imaging and scanners cost approximately half that of MR imaging. Hence, in cases where access to MRI is unavailable or restricted, perfusion CT combined with CT angiography continues to be an inexpensive and promising alternative.3

In terms of result interpretation, CT images require extensive training and are not highly reproducible; trained experts are difficult to find round-the-clock. MRIs are however easier since there is no intra- or interobserver variability.1

Thus, both imaging techniques present various pros and cons; however, MRI is preferred over CT scans for improved diagnosis and assessment of stroke.

Rapid imaging is essential

Every minute of an untreated large vessel stroke leads in the loss of 1.9 million neurons, 13.8 billion synapses and 12 km (7 miles) of axonal fibres.9 As per national guidelines, patients with acute stroke must undergo brain imaging within 25 minutes of emergency department arrival. Delays in brain imaging may reduce effectiveness of thrombolysis or render patients ineligible.10

Examining MRI scan

Figure 2. Examining MRI scan performed rapidly following stroke.

 

Most patients do not receive brain imaging within the stipulated time frame. Certain groups that have a lesser likelihood of imaging being completed within 25 minutes include:10

  • Age >70 years
  • Women
  • Non-white race (racial minorities)
  • History of diabetes, peripheral vascular disease, or prosthetic heart valve
  • Transportation to emergency department other than ambulance

It is therefore crucial that these populations that are at a greater risk of delays in imaging are prioritised.10

Fast brain imaging leads to quick differentiation between ischaemia or haemorrhage that directs further therapy. It also identifies brain injury extent (irreversible infarction) and potential complications (midline shift or herniation). Quick reaction also helps to rule out stroke mimics, and identify at-risk, under-perfused tissue that might undergo infarction if perfusion is not restored on time.10 For further information on acute stroke mimics, please click here.

Screening with MRI for Accurate and Rapid Stroke Treatment (SMART) study has shown that it is both feasible and practical to consistently and rapidly deliver intravenous tPA (IV tPA) to acute ischaemic stroke patients within national benchmark times.11

As time is brain, non-contrast computed tomography (NCCT) is the most widely suitable imaging method for fibrinolysis patient selection due to its’ speed.12,13 However, the task of identifying the signs of acute ischaemia and quantifying areas of brain involvement on NCCT scan is not easy due to its subtle findings.

The reliability of early ischaemic sign detection can be improved with experience, clinical history and the use of stroke window width and level on viewing the images. The Alberta Stroke Program Early CT Score (ASPECTS) was developed to overcome the difficulty of volume estimation in patients eligible for thrombolysis. It is a systematic, robust and practical method that can standardize the detection and reporting of the extent of acute ischaemic stroke.13

ASPECTS provides a quantitate score that measures the extent of early ischaemic changes. It score was originally designed to help identify patients who were likely to demonstrate the most clinical benefit from intravenous thrombolysis. Subsequently, this score was applied in imaging selection for endovascular therapies to isolate patients with greatest extent of ischaemic damage, in whom recanalization would be futile or harmful. More recently, ASPECTS has been recognized as a key selection criterion in the updated American Heart Association guidelines on the management of acute stroke, where endovascular therapy in patients with baseline ASPECTS≥6 is recommended.14

Stroke imaging guidelines

Guidelines for the early management of patients with acute ischaemic stroke were amended by the AHA/ASA in 2018. The guidelines describe prehospital care, urgent and emergency evaluation and treatment with intravenous and intra-arterial therapies, as well as in-hospital management and secondary prevention measures. Below are the latest guidelines for stroke management in brain imaging:15

Brain imaging AHA/ASA guidelines:15

  1. All patients admitted to hospital with suspected acute stroke should receive brain imaging evaluation on arrival to hospital. In most cases, non-contrast CT (NCCT) will provide the necessary information to make decisions about acute management.
  2. Systems should be established so that brain imaging studies can be performed within 20 minutes of arrival in the ED in at least 50% of patients who may be candidates for IV alteplase and/or mechanical thrombectomy.
  3. There remains insufficient evidence to identify a threshold of acute CT hypoattenuation severity or extent that affects treatment response to IV alteplase. The extent and severity of acute hypoattenuation or early ischaemic changes should not be used as a criterion to withhold therapy for such patients who otherwise qualify.
  4. The CT hyperdense MCA sign should not be used as a criterion to withhold IV alteplase from patients who otherwise qualify.
  5. Routine use of magnetic resonance imaging (MRI) to exclude cerebral microbleeds (CMBs) before administration of IV alteplase is not recommended.
  6. Use of imaging criteria to select ischaemic stroke patients who awoke with stroke or have unclear time of symptom onset for treatment with IV alteplase is not recommended outside a clinical trial.
  7. Multimodal CT and MRI, including perfusion imaging, should not delay administration of IV alteplase.
  8. For patients who otherwise meet criteria for EVT, a non-invasive intracranial vascular study is recommended during the initial imaging evaluation of the acute stroke patient but should not delay IV alteplase if indicated. For patients who qualify for IV alteplase according to guidelines from professional medical societies, initiating IV alteplase before non-invasive vascular imaging is recommended for patients who have not had non-invasive vascular imaging as part of their initial imaging assessment for stroke. Non-invasive intracranial vascular imaging should then be obtained as quickly as possible.
  9. For patients who otherwise meet criteria for EVT, it is reasonable to proceed with CTA if indicated in patients with suspected intracranial LVO before obtaining a serum creatinine concentration in patients without a history of renal impairment.
  10. In patients who are potential candidates for mechanical thrombectomy, imaging of the extracranial carotid and vertebral arteries, in addition to the intracranial circulation, is reasonable to provide useful information on patient eligibility and endovascular procedural planning.
  11. In selected patients with AIS within 6 to 24 hours of last known normal who have LVO in the anterior circulation, obtaining CTP, DW-MRI, or MRI perfusion is recommended to aid in patient selection for mechanical thrombectomy, but only when imaging and other eligibility criteria from RCTs showing benefit are being strictly applied in selecting patients for mechanical thrombectomy.
  12. It may be reasonable to incorporate collateral flow status into clinical decision making in some candidates to determine eligibility for mechanical thrombectomy.
References 
  1. Tatlisumak T. Is CT or MRI the method of choice for imaging patients with acute stroke? Why should men divide if fate has united? Stroke. 2002;33(9):2144-2145.
  2. Vymazal J, Rulseh AM, Keller J, Janouskova L. Comparison of CT and MR imaging in ischemic stroke. Insights Imaging. 2012;3(6):619-627. doi:10.1007/s13244-012-0185-9
  3. Meissner W, Sibon I, Rouanet F, Ménégon P, Orgogozo J-M. MRI versus CT in acute stroke. Lancet Lond Engl. 2007;369(9570):1342. doi:10.1016/S0140-6736(07)60623-0
  4. Schellinger PD, Bryan RN, Caplan LR, et al. Evidence-based guideline: The role of diffusion and perfusion MRI for the diagnosis of acute ischemic stroke: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Neurology. 2010;75(2):177-185. doi:10.1212/WNL.0b013e3181e7c9dd
  5. Schaefer PW, Barak ER, Kamalian S, et al. Quantitative Assessment of Core/Penumbra Mismatch in Acute Stroke: CT and MR Perfusion Imaging Are Strongly Correlated When Sufficient Brain Volume Is Imaged. Stroke. 2008;39(11):2986-2992. doi:10.1161/STROKEAHA.107.513358
  6. García-Bermejo P, Castaño C, Dávalos A. Multimodal CT versus MRI in Selecting Acute Stroke Patients for Endovascular Treatment. Interv Neurol. 2012;1(2):65-76. doi:10.1159/000346653
  7. Wintermark M, Meuli R, Browaeys P, et al. Comparison of CT perfusion and angiography and MRI in selecting stroke patients for acute treatment. Neurology. 2007;68(9):694-697. doi:10.1212/01.wnl.0000255959.30107.08
  8. Campbell BCV, Ma H, Ringleb PA, et al. Extending thrombolysis to 4·5–9 h and wake-up stroke using perfusion imaging: A systematic review and meta-analysis of individual patient data. The Lancet. 2019;394(10193):139-147. doi:10.1016/S0140-6736(19)31053-0
  9. Saver JL. Time is brain—Quantified. Stroke. 2006;37(1):263-266. doi:10.1161/01.STR.0000196957.55928.ab
  10. Kelly AG, Hellkamp AS, Olson D, Smith EE, Schwamm LH. Predictors of rapid brain imaging in acute stroke: Analysis of the Get With the Guidelines-Stroke program. Stroke. 2012;43(5):1279-1284. doi:10.1161/STROKEAHA.111.626374
  11. Shah S, Luby M, Poole K, et al. Screening with MRI for Accurate and Rapid Stroke Treatment: SMART. Neurology. 2015;84(24):2438-2444. doi:10.1212/WNL.0000000000001678
  12. Kobkitsuksakul C, Tritanon O, Suraratdecha V. Interobserver agreement between senior radiology resident, neuroradiology fellow, and experienced neuroradiologist in the rating of Alberta Stroke Program Early Computed Tomography Score (ASPECTS). Diagn Interv Radiol. 2018;24(2):104-107. doi:10.5152/dir.2018.17336
  13. Radhiana H. Non-contrast Computed Tomography in Acute Ischaemic Stroke: A Pictorial Review. 2013;68(1):8.
  14. Mokin M, Primiani CT, Siddiqui AH, Turk AS. ASPECTS (Alberta Stroke Program Early CT Score) Measurement Using Hounsfield Unit Values When Selecting Patients for Stroke Thrombectomy. Stroke. 2017;48(6):1574-1579. doi:10.1161/STROKEAHA.117.016745
  15. Powers WJ, Rabinstein AA, Ackerson T, et al. 2018 Guidelines for the Early Management of Patients With Acute Ischemic Stroke: A Guideline for Healthcare Professionals From the American Heart Association/American Stroke Association. Stroke. 2018;49(3):e46-e110. doi:10.1161/STR.0000000000000158