Silent Cerebral Events/Lesions Related to AF Ablation
Silent Cerebral Events/Lesions Related to AF Ablation
An SCE is defined as an acute new MRI-detected brain lesion typical to cerebral ischemia in a patient without clinically apparent neurological deficit.
Diffusion-weighted MRI (DWI) is the cornerstone of diagnosing cerebral stroke as it is able to detect cerebral ischemia within minutes after its onset. Therefore, all definitions of silent cerebral ischemic events are based on the identification of a hyperintense lesion on DWI indicating cellular edema in acute ischemia. The apparent diffusion coefficient map (ADC-map) is a display of calculated MRI sequences on a pixel-by-pixel mode. The ADC of a diffusion-positive lesion is reduced, which prevents the "overdetection" of shine-through effects on DWI. A hyperintense DWI lesion (diffusion-positive) and a corresponding reduced ADC map (hypointense) build the basis for detecting cerebral ischemia, even though potential reversibility of diffusion changes may occur in reversible ischemic areas.
In contrast, the T2-weighted fluid attenuated inverse recovery sequence (FLAIR) has been shown to turn positive only with delay (in larger infarcts up to 20 hours and even longer for smaller infarct sizes), and FLAIR positivity is related to the volume of DWI-hyperintensity. Therefore, MRI to detect AF-ablation-related events performed on day 1 after the procedure may underestimate the true incidence of lesions if FLAIR positivity is a prerequisite (Fig. 1). On the other hand, most SCE are of small volume and may not relate to FLAIR positivity due to small lesion size. Therefore, FLAIR should rather be used to detect chronic cerebral ischemic lesions and as a cornerstone of identifying "older" cerebral damage. There is still debate about when an ischemic lesion may become positive on FLAIR in the course of formation of these lesions. Due to this FLAIR-to-DWI mismatch, FLAIR sequences may not adequately identify acute to hyperacute cerebral ischemic events related to also as "tissue-clock" of FLAIR imaging.
In this context, differences in published definitions for SCE have to be kept in mind (Table 1). Whereas some groups have defined SCE based on a hyperintense DWI lesion plus reduced ADC plus FLAIR positivity (hyperintensity), newer reports have used a more sensitive definition without FLAIR positivity (Fig. 1). Applying these 2 different definitions onto the same set of MRI data, Wieczorek et al. identified FLAIR positivity in only 1 of 3 of all diffusion-positive lesions if performing MRI 1 day after AF ablation.
(Enlarge Image)
Figure 1.
MRI 1 day after AF ablation using—A: T2-weighted FLAIR. B: DWI. C: ADC-map. Three acute lesions are identified on DWI (circle) with corresponding reduced ADC (circle) but no FLAIR positivity (circle). These lesions are called silent cerebral events (SCE) and detection is more sensitive to acute cerebral ischemia than adding FLAIR-positivity.
In a recent animal experimental setting producing massive cerebral strokes due to particulate or gaseous emboli injection into the carotid arteries in a canine model, FLAIR-positive MRI-detected lesions were found to be equivalent to cerebral stroke as documented on histopathology, but this was not explicitly tested for FLAIR-negative DWI-positive lesions. Therefore, so far FLAIR-positive diffusion-positive MRI lesions may be considered lesions leading to brain scar formation and should be termed SCL, whereas FLAIR-negative diffusion-positive MRI lesions are considered SCE, as future progression of these lesions (they may become FLAIR-positive and may lead to brain scars) is unclear.
If FLAIR imaging is added, attention to congruence of slices of DWI and FLAIR sequences should be drawn. If different slice thicknesses or different slicing orientations are used, correlating FLAIR signals to DWI may be impossible. Differences between FLAIR and DWI imaging modalities may modulate the detection rate of SCE/SCL.
Whereas most reports on SCE/SCL are performed with a 1.5-Tesla MRI, 1 report on using a 3-Tesla high-resolution MRI exists. In this report, incidence of SCL related to cryoballoon ablation is approximately doubled to tripled, compared with the incidences detected with 1.5-Tesla MRIs (Table 1). Overall, it can be estimated that stronger magnets in MRI may detect incidences of cerebral ischemic events more sensitively due to higher spatial resolution. So far, no consensus on the exact sequences and definitions for SCE/SCL is available, but should be considered for future evaluation to make studies more comparable.
The optimum time point for postablation brain MRI evaluation in asymptomatic patients remains unclear. Whereas DWI may turn positive usually within the first 15 minutes after onset of cerebral ischemia, SCE may disappear within days 2 to 4 on DWI. Due to the delay of positivity, the FLAIR sequence alone may not adequately reflect the overall incidence of embolic brain events within the first hours after occurrence, especially in small size lesions. One day after ablation, only 1 of 3 of SCE are FLAIR-positive. Events not positive on FLAIR initially may become positive during the first days and, therefore, FLAIR imaging may be postponed to later time intervals. As a pragmatic recommendation, DWI should be performed within 24 hours after AF ablation but if attempting to detect SCL days 2 to 7 may be more adequate for postablation MRI. Further investigations are needed to optimize timing within the everyday schedule of ablation centers.
Definitions
An SCE is defined as an acute new MRI-detected brain lesion typical to cerebral ischemia in a patient without clinically apparent neurological deficit.
Diffusion-weighted MRI (DWI) is the cornerstone of diagnosing cerebral stroke as it is able to detect cerebral ischemia within minutes after its onset. Therefore, all definitions of silent cerebral ischemic events are based on the identification of a hyperintense lesion on DWI indicating cellular edema in acute ischemia. The apparent diffusion coefficient map (ADC-map) is a display of calculated MRI sequences on a pixel-by-pixel mode. The ADC of a diffusion-positive lesion is reduced, which prevents the "overdetection" of shine-through effects on DWI. A hyperintense DWI lesion (diffusion-positive) and a corresponding reduced ADC map (hypointense) build the basis for detecting cerebral ischemia, even though potential reversibility of diffusion changes may occur in reversible ischemic areas.
In contrast, the T2-weighted fluid attenuated inverse recovery sequence (FLAIR) has been shown to turn positive only with delay (in larger infarcts up to 20 hours and even longer for smaller infarct sizes), and FLAIR positivity is related to the volume of DWI-hyperintensity. Therefore, MRI to detect AF-ablation-related events performed on day 1 after the procedure may underestimate the true incidence of lesions if FLAIR positivity is a prerequisite (Fig. 1). On the other hand, most SCE are of small volume and may not relate to FLAIR positivity due to small lesion size. Therefore, FLAIR should rather be used to detect chronic cerebral ischemic lesions and as a cornerstone of identifying "older" cerebral damage. There is still debate about when an ischemic lesion may become positive on FLAIR in the course of formation of these lesions. Due to this FLAIR-to-DWI mismatch, FLAIR sequences may not adequately identify acute to hyperacute cerebral ischemic events related to also as "tissue-clock" of FLAIR imaging.
In this context, differences in published definitions for SCE have to be kept in mind (Table 1). Whereas some groups have defined SCE based on a hyperintense DWI lesion plus reduced ADC plus FLAIR positivity (hyperintensity), newer reports have used a more sensitive definition without FLAIR positivity (Fig. 1). Applying these 2 different definitions onto the same set of MRI data, Wieczorek et al. identified FLAIR positivity in only 1 of 3 of all diffusion-positive lesions if performing MRI 1 day after AF ablation.
(Enlarge Image)
Figure 1.
MRI 1 day after AF ablation using—A: T2-weighted FLAIR. B: DWI. C: ADC-map. Three acute lesions are identified on DWI (circle) with corresponding reduced ADC (circle) but no FLAIR positivity (circle). These lesions are called silent cerebral events (SCE) and detection is more sensitive to acute cerebral ischemia than adding FLAIR-positivity.
In a recent animal experimental setting producing massive cerebral strokes due to particulate or gaseous emboli injection into the carotid arteries in a canine model, FLAIR-positive MRI-detected lesions were found to be equivalent to cerebral stroke as documented on histopathology, but this was not explicitly tested for FLAIR-negative DWI-positive lesions. Therefore, so far FLAIR-positive diffusion-positive MRI lesions may be considered lesions leading to brain scar formation and should be termed SCL, whereas FLAIR-negative diffusion-positive MRI lesions are considered SCE, as future progression of these lesions (they may become FLAIR-positive and may lead to brain scars) is unclear.
If FLAIR imaging is added, attention to congruence of slices of DWI and FLAIR sequences should be drawn. If different slice thicknesses or different slicing orientations are used, correlating FLAIR signals to DWI may be impossible. Differences between FLAIR and DWI imaging modalities may modulate the detection rate of SCE/SCL.
Whereas most reports on SCE/SCL are performed with a 1.5-Tesla MRI, 1 report on using a 3-Tesla high-resolution MRI exists. In this report, incidence of SCL related to cryoballoon ablation is approximately doubled to tripled, compared with the incidences detected with 1.5-Tesla MRIs (Table 1). Overall, it can be estimated that stronger magnets in MRI may detect incidences of cerebral ischemic events more sensitively due to higher spatial resolution. So far, no consensus on the exact sequences and definitions for SCE/SCL is available, but should be considered for future evaluation to make studies more comparable.
The optimum time point for postablation brain MRI evaluation in asymptomatic patients remains unclear. Whereas DWI may turn positive usually within the first 15 minutes after onset of cerebral ischemia, SCE may disappear within days 2 to 4 on DWI. Due to the delay of positivity, the FLAIR sequence alone may not adequately reflect the overall incidence of embolic brain events within the first hours after occurrence, especially in small size lesions. One day after ablation, only 1 of 3 of SCE are FLAIR-positive. Events not positive on FLAIR initially may become positive during the first days and, therefore, FLAIR imaging may be postponed to later time intervals. As a pragmatic recommendation, DWI should be performed within 24 hours after AF ablation but if attempting to detect SCL days 2 to 7 may be more adequate for postablation MRI. Further investigations are needed to optimize timing within the everyday schedule of ablation centers.
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