Comparison of MRI perfusion imaging and single photon emission computed tomography in chronic stroke

The ischemic penumbra: From concept to reality

The discovery that brain tissue could potentially be salvaged from ischaemia due to stroke, has led to major advances in the development of therapies for ischemic stroke. In this review, we detail the advances in the understanding of this area termed the ischaemic penumbra, from its discovery to the evolution of imaging techniques, and finally some of the treatments developed. Evolving from animal studies from the 70s and 80s and translated to clinical practice, the field of ischemic reperfusion therapy has largely been guided by an array of imaging techniques developed to positively identify the ischemic penumbra, including positron emission tomography, computed tomography and magnetic resonance imaging. More recently, numerous penumbral identification imaging studies have allowed for a better understanding of the progression of the ischaemic core at the expense of the penumbra, and identification of patients than can benefit from reperfusion therapies in the acute phase. Importantly, 40 years of critical imaging research on the ischaemic penumbra have allowed for considerable extension of the treatment time window and better patient selection for reperfusion therapy. The translation of the penumbra concept into routine clinical practice has shown that "tissue is at least as important as time."

Can Arterial Spin-Labeling with Multiple Postlabeling Delays Predict Cerebrovascular Reserve?

BACKGROUND AND PURPOSE

The effect of delayed transit time is the main source of error in the quantitative measurement of CBF in arterial spin-labeling. In the present study, we evaluated the usefulness of the transit time-corrected CBF and arterial transit time delay from multiple postlabeling delays arterial spin-labeling compared with basal/acetazolamide stress technetium Tc99m-hexamethylpropylene amineoxime (Tc99m-HMPAO) SPECT in predicting impairment in the cerebrovascular reserve.

MATERIALS AND METHODS

Transit time-corrected CBF maps and arterial transit time maps were acquired in 30 consecutive patients with unilateral ICA or MCA steno-occlusive disease (severe stenosis or occlusion). Internal carotid artery territory-based ROIs were applied to both perfusion maps. Additionally, impairment in the cerebrovascular reserve was evaluated according to both qualitative and quantitative analyses of the ROIs on basal/acetazolamide stress Tc99m-HMPAO SPECT using a previously described method. The area under the receiver operating characteristic curve was used to evaluate the diagnostic accuracy of arterial spin-labeling in depicting impairment of the cerebrovascular reserve. The correlation between arterial spin-labeling and cerebrovascular reserve was evaluated.

RESULTS

The affected hemisphere had a decreased transit time-corrected CBF and increased arterial transit time compared with the corresponding values of the contralateral normal hemisphere, which were statistically significant (P < .001). The percentage change of transit time-corrected CBF and the percentage change of arterial transit time were independently differentiating variables (P < .001) for predicting cerebrovascular reserve impairment. The correlation coefficient between the arterial transit time and cerebrovascular reserve index ratio was -0.511.

CONCLUSIONS

Our results demonstrate that the transit time-corrected CBF and arterial transit time based on arterial spin-labeling perfusion MR imaging can predict cerebrovascular reserve impairment.

The impact of carotid or intracranial atherosclerosis on perioperative stroke in patients undergoing open aortic arch surgery

OBJECTIVES

This study aimed to clarify the impact of carotid or intracranial atherosclerosis on perioperative stroke in patients undergoing open aortic arch surgery.

METHODS

Between 2008 and 2015, 200 consecutive patients underwent elective aortic arch surgery with selective antegrade cerebral perfusion and moderate hypothermic circulatory arrest. Nonselective screening for carotid or intracranial atherosclerosis was performed using carotid ultrasonography or magnetic resonance angiography. Carotid or intracranial atherosclerosis was classified as below moderate (0%-49% stenosis), moderate (50%-69%), or severe (70%-100%). In patients with moderate or severe stenosis, cerebral hemodynamics were evaluated using single-photon emission computed tomography with acetazolamide.

RESULTS

None of the 37 patients undergoing preoperative single-photon emission computed tomography with acetazolamide had impaired cerebral hemodynamics. In-hospital mortality rate was 3.5% (7/200). Postoperative neurologic morbidity included permanent stroke in 8 patients (4.0%) and transient neurologic deficits in 27 patients (14%). Permanent stroke occurred in 3 of 159 patients (1.9%) with below moderate stenosis and 5 of 41 patients (12.2%) with moderate or severe stenosis (P = .008). Seven of 8 patients (87.5%) with stroke experienced multiple atherothrombotic embolizations, and 1 patient experienced a stroke of unknown cause. In multivariate analysis, previous cerebrovascular accident (odds ratio, 5.0; 95% confidence interval, 2.07-12.42; P = .0004) and shaggy aorta (odds ratio, 4.2; 95% confidence interval, 1.58-10.98; P = .0045) were significant determinants of neurologic morbidity.

CONCLUSIONS

Embolism was the major cause of permanent stroke in our patient population. Preoperative craniocervical and aortic screening may aid in modifying the operative strategy to reduce the occurrence of stroke.

Arterial spin labelling shows functional depression of non-lesion tissue in chronic Wernicke's aphasia

Behavioural impairment post-stroke is a consequence of structural damage and altered functional network dynamics. Hypoperfusion of intact neural tissue is frequently observed in acute stroke, indicating reduced functional capacity of regions outside the lesion. However, cerebral blood flow (CBF) is rarely investigated in chronic stroke. This study investigated CBF in individuals with chronic Wernicke's aphasia (WA) and examined the relationship between lesion, CBF and neuropsychological impairment.

Arterial spin labelling CBF imaging and structural MRIs were collected in 12 individuals with chronic WA and 13 age-matched control participants. Joint independent component analysis (jICA) investigated the relationship between structural lesion and hypoperfusion. Partial correlations explored the relationship between lesion, hypoperfusion and language measures.

Joint ICA revealed significant differences between the control and WA groups reflecting a large area of structural lesion in the left posterior hemisphere and an associated area of hypoperfusion extending into grey matter surrounding the lesion. Small regions of remote cortical hypoperfusion were observed, ipsilateral and contralateral to the lesion. Significant correlations were observed between the neuropsychological measures (naming, repetition, reading and semantic association) and the jICA component of interest in the WA group. Additional ROI analyses found a relationship between perfusion surrounding the core lesion and the same neuropsychological measures.

This study found that core language impairments in chronic WA are associated with a combination of structural lesion and abnormal perfusion in non-lesioned tissue. This indicates that post-stroke impairments are due to a wider disruption of neural function than observable on structural T1w MRI.

Parametric Cerebrovascular Reserve Images Using Acetazolamide (99m)Tc-HMPAO SPECT: A Feasibility Study of Quantitative Assessment

PURPOSE

Basal/acetazolamide stress (99m)Tc-HMPAO single-photon emission computed tomography (SPECT) has been widely used for evaluation of hemodynamics; however, qualitative and subjective visual assessment of cerebrovascular reserve (CVR) has been performed in clinical settings. The aim of this study was to generate parametric CVR images and evaluate its feasibility of quantification.

METHODS

Basal/acetazolamide stress (99m)Tc-HMPAO SPECT data from 17 patients who underwent bypass surgery or percutaneous transluminal angioplasty were used. Spatial normalization was performed and parametric CVR images were generated using relative CVR (rCVR) of each voxel proportional to CVR of the whole brain. Binary parametric maps to show area of relatively reduced CVR were generated also using threshold of rCVR < 90 %. We calculated rCVR of internal carotid artery (ICA) using the parametric CVR images and probabilistic maps for ICA territory. Pre- and postprocedural parametric CVR images were obtained and quantitative rCVRs were compared. The rCVRs were evaluated according to visual grades for regional decreased CVR.

RESULTS

Postprocedural rCVR obtained from parametric CVR images increased significantly from preprocedural rCVR. The rCVR was significantly correlated with visual grades of reduced CVR for each side of ICA territories.

CONCLUSIONS

We generated parametric CVR images for basal/acetazolamide stress (99m)Tc-HMPAO SPECT. As a quantitative measurement, rCVR obtained from the parametric image was feasibly assessed hemodynamic abnormalities with preserved anatomical information.

Automated macrovessel artifact correction in dynamic susceptibility contrast magnetic resonance imaging using independent component analysis

Dynamic susceptibility contrast-MRI is the most commonly used functional MRI-based method for studying changes in cerebral perfusion. However, several studies indicated a systematic overestimation of perfusion parameters compared with other imaging modalities related to the high sensitivity of dynamic susceptibility contrast-MRI for blood flow in large vessels. In this study, we therefore suggest an improved, automated, robust, and efficient method allowing for generating hemodynamic parameter maps where signal influence from large vessels is minimized. Based on independent component analysis, this fully automated approach corrects dynamic susceptibility contrast-MRI data without any user interaction, thus making a clinical applicability possible. The accuracy of the proposed method was tested in 10 patients with cerebrovascular disease. Application of our correction algorithm resulted in a significant reduction of the effect of macrovessel signal on hemodynamic parameters like the cerebral blood flow and the cerebral blood volume compared with uncorrected data. As desired, our method specifically corrected for macrovessel artifacts in cortical grey matter tissue, leaving white matter tissue parameters largely unaffected. This may increase sensitivity and reliability of detecting perfusion abnormalities in patient groups, in particular with regard to stroke and other cerebrovascular disorders.

Imaging the penumbra - strategies to detect tissue at risk after ischemic stroke

The aim of thrombolytic therapy after acute ischemic stroke is salvage of the ischemic penumbra. Several imaging techniques have been used to identify the penumbra in patients who may benefit from reperfusion beyond the currently narrow 3-hour time-window for thrombolysis. We discuss the advantages and disadvantages of positron emission tomography (PET), single photon emission computed tomography (SPECT), MRI and CT scans. We comment on concepts of clinical-imaging mismatch models and we explore the implications for clinical trials.

Statistical parametric mapping and statistical probabilistic anatomical mapping analyses of basal/acetazolamide Tc-99m ECD brain SPECT for efficacy assessment of endovascular stent placement for middle cerebral artery stenosis

INTRODUCTION

Statistical parametric mapping (SPM) and statistical probabilistic anatomical mapping (SPAM) were applied to basal/acetazolamide Tc-99m ECD brain perfusion SPECT images in patients with middle cerebral artery (MCA) stenosis to assess the efficacy of endovascular stenting of the MCA.

METHODS

Enrolled in the study were 11 patients (8 men and 3 women, mean age 54.2 +/- 6.2 years) who had undergone endovascular stent placement for MCA stenosis. Using SPM and SPAM analyses, we compared the number of significant voxels and cerebral counts in basal and acetazolamide SPECT images before and after stenting, and assessed the perfusion changes and cerebral vascular reserve index (CVRI).

RESULTS

The numbers of hypoperfusion voxels in SPECT images were decreased from 10,083 +/- 8,326 to 4,531 +/- 5,091 in basal images (P = 0.0317) and from 13,398 +/- 14,222 to 7,699 +/- 10,199 in acetazolamide images (P = 0.0142) after MCA stenting. On SPAM analysis, the increases in cerebral counts were significant in acetazolamide images (90.9 +/- 2.2 to 93.5 +/- 2.3, P = 0.0098) but not in basal images (91 +/- 2.7 to 92 +/- 2.6, P = 0.1602). The CVRI also showed a statistically significant increase from before stenting (median 0.32; 95% CI -2.19-2.37) to after stenting (median 1.59; 95% CI -0.85-4.16; P = 0.0068).

CONCLUSION

This study revealed the usefulness of voxel-based analysis of basal/acetazolamide brain perfusion SPECT after MCA stent placement. This study showed that SPM and SPAM analyses of basal/acetazolamide Tc-99m brain SPECT could be used to evaluate the short-term hemodynamic efficacy of successful MCA stent placement.

Correlation of magnetic resonance imaging findings with hexamethylpropyleneamine oxime brain single photon emission computed tomography in ischemic stroke patients in the subacute stage

PURPOSE

To evaluate the correlation between magnetic resonance imaging (MRI) findings and 99mTc-hexamethylpropyleneamine oxime (HMPAO) brain single photon emission computed tomography (SPECT) during the subacute stage in ischemic stroke patients.

MATERIAL AND METHODS

The T1 and T2-weighted images and brain SPECT findings of 84 patients (mean age 60.69 +/- 12.47 years) with subacute cerebral ischemia during the period 1998-2004 were reviewed. All HMPAO SPECT and MRI studies were performed between 3 and 7 days (mean time delay 4.76 +/- 1.29 days) after the onset of stroke symptoms.

RESULTS

An ischemic lesion was seen both in T1 and T2-weighted images with perfusion defects above 60% (severe defect) according to count/pixel data of the lesion in HMPAO SPECT studies in 30 (90.9%) of 33 patients. Otherwise, the ischemic lesion was seen only on T2-weighted images with perfusion defects between 30% and 60% (moderate defect) in HMPAO SPECT studies in 25 (89.3%) of 28 patients. In 20 (87%) of 23 patients who had perfusion defects below 30% (mild defect) on HMPAO SPECT, only non-specific findings such as cerebral atrophy and/or periventricular ischemic-gliotic lesions could be seen in MRI. The difference between these ratios was statistically significant (P < 0.01).

CONCLUSION

Brain 99mTc-HMPAO SPECT findings indicate good correlation with MRI findings. When the ischemic lesions could be seen in both T1 and T2-weighted images, the patients frequently had severe perfusion defects. When only seen in T2-weighted images, the perfusion defect was moderate. When only non-specific findings were revealed by MRI, only mild perfusion defects were found by SPECT.

Cervical Arterial Dissection

Background and Purpose— Cervical arterial dissection is a major cause of stroke in young adults, yet despite standard treatment with anticoagulants or antiplatelet drugs, its management remains uncertain. The goal of this study was to assess the natural history of the disorder and to decide on the feasibility of a therapeutic trial.

Methods— Collaborating members of the Canadian Stroke Consortium prospectively enrolled consecutively referred patients with angiographically proven acute vertebral or carotid arterial dissection. Data recorded included clinical and radiological details, recurrence of ischemic cerebral events, and medical or surgical treatment.

Results— Of 116 patients, 67 had vertebral and 49 had carotid dissections, with no difference in age or sex. In 68 (59%), trauma occurred at the time of dissection. During the course of a 1-year follow-up, at least 17 patients (15%) had recurrent transient ischemic attacks, stroke, or death, mainly in the weeks immediately after the dissection. In 105 patients with complete follow-up, the event rate in those treated with anticoagulants was 8.3% and in those treated with aspirin was 12.4%, a nonsignificant difference of 4.1%. Using these data, we calculate that for a 2-arm trial (aspirin versus anticoagulants) with 80% power and 5% significance, 913 patients are needed in each group.

Conclusions— From our data indicating an initial relatively high recurrence rate, a multicenter trial of anticoagulants versus aspirin involving a total of 2000 patients is feasible.

Cerebral hemodynamic evaluation using perfusion-weighted magnetic resonance imaging: comparison with positron emission tomography values in chronic occlusive carotid disease

BACKGROUND AND PURPOSE

Perfusion-weighted magnetic resonance imaging (PWI) is a reliable and semiquantitative method for estimating cerebral hemodynamics. We sought to evaluate the potential of PWI for assessing cerebral blood flow (CBF) and metabolism compared with positron emission tomography (PET) in patients with chronic occlusive carotid disease.

METHODS

In 24 patients with chronic unilateral occlusive carotid disease, time-to-peak (TTP) delay (TTP-D) measured by PWI was compared with CBF, cerebral blood volume (CBV), and oxygen extraction fraction (OEF) obtained by PET. TTP indicates the time from the start of PWI to the bolus peak. TTP-D indicates the difference in TTP values between the occlusive and contralateral hemispheres. TTP-D was compared between patients with normal and reduced CBF/CBV and also between patients with normal and elevated OEF.

RESULTS

TTP-D in patients with reduced CBF/CBV was significantly longer than that in patients with normal CBF/CBV (3.4+/-1.8 versus 1.4+/-0.7 seconds; P<0.001). In the patients with reduced CBF/CBV, TTP-D correlated with OEF significantly (r=0.710, P<0.0001). TTP-D in patients with elevated OEF was significantly longer than that in patients with normal OEF (4.8+/-2.2 versus 2.0+/-0.9 seconds; P<0.01). In all 5 patients with TTP-D >or=4 seconds, OEF was elevated markedly.

CONCLUSIONS

TTP-D >or=4 seconds is considered to indicate a high risk of hemodynamic failure. The measurement of TTP-D by PWI appears to provide important clinical information for evaluating cerebral hemodynamics in chronic occlusive carotid disease.

Semi-quantitative CBF and CBF ratios obtained using perfusion-weighted MR imaging

To clarify the utility of semi-quantitative cerebral blood flow (CBF) measurements using perfusion-weighted MR imaging (PWI), a comparison of this method with quantitative CBF obtained using PET was conducted in 10 patients with chronic occlusive cerebrovascular disease and unilateral occlusion of the cerebral artery. Semi-quantitative CBF obtained using PWI and quantitative CBF obtained using PET showed no statistically significant correlation. The CBF ratios of the affected side relative to the contralateral unaffected side obtained using PWI and PET were 0.94 +/- 0.22 and 0.88 +/- 0.19, respectively. A statistically significant positive correlation was obtained between these ratios (p < 0.01). The CBF ratio, but not the semi-quantitative CBF, obtained using PWI has a potential to detect changes in the CBF.

The haemodynamic effect of carotid endarterectomy

OBJECTIVES

to assess the haemodynamic effect of carotid artery surgery, and to relate postoperative changes to the state of cerebral circulation before revascularisation.

MATERIALS AND METHODS

using transcranial Doppler we studied bilateral middle cerebral artery (MCA) flow velocities before and on 1st day, 2nd or 3rd day and 4th or 5th day and 3 months after carotid surgery in 61 patients. In addition, ipsilateral MCA flow velocity was monitored continuously during surgery. Data were related to the internal carotid artery (ICA) perfusion pressure (cerebral perfusion pressure index, CPPI), measured directly before ICA clamping.

RESULTS

postoperatively, MCA flow velocities increased significantly overall (p<0.01), mainly due to pronounced and longer lasting flow velocities in the group of 18 patients with CPPI<0.7 (p<0.05). Flow velocities peaked - absolute as well as relative - on the first postoperative day and then gradually levelled off to reach preoperative values after 4-5 days in patients with high CPPI, whereas MCA flow velocities remained increased in the group of patients with low CPPI. At 3 months flow velocities in both groups were normalised. New neurological symptoms occurred in four patients, who all had low CPPI preoperatively (22% (4/18) vs 0%; Fisher's exact test: p=0.006).

CONCLUSION

some degree of hyperperfusion was seen in most patients, but the changes were significantly more pronounced in patients with preoperative hypoperfusion, who also suffered significantly more neurological complications.

Perfusion magnetic resonance imaging maps in hyperacute stroke: relative cerebral blood flow most accurately identifies tissue destined to infarct

BACKGROUND AND PURPOSE

In ischemic stroke, perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) provide important pathophysiological information. A PWI>DWI mismatch pattern suggests the presence of salvageable tissue. However, improved methods for distinguishing PWI>DWI mismatch tissue that is critically hypoperfused from benign oligemia are required.

METHODS

We investigated the usefulness of maps of relative cerebral blood flow (rCBF), volume (rCBV), and mean transit time (rMTT) to predict transition to infarction in hyperacute (<6 hours) stroke patients with PWI>DWI mismatch patterns. Semiquantitative color-thresholded analysis was used to measure hypoperfusion volumes, including increasing color signal intensity thresholds of rMTT delay, which were compared with infarct expansion, outcome infarct size, and clinical status.

RESULTS

Acute rCBF lesion volume had the strongest correlation with final infarct size (r=0.91, P<0.001) and clinical outcome (r=0.67, P<0.01). There was a trend for acute rCBF>DWI mismatch volume to overestimate infarct expansion between the acute and outcome study (P=0.06). Infarct expansion was underestimated by acute rCBV>DWI mismatch (P<0.001). When rMTT lesions included tissue with moderately prolonged transit times (mean delay 4.3 seconds, signal intensity values 50% to 70%), infarct expansion was overestimated. In contrast, when rMTT lesions were restricted to more severely prolonged transit times (mean delay 6.1 seconds, signal intensity >70%), these regions progressed to infarction in all except 1 patient, but infarct expansion was underestimated (P<0.001).

CONCLUSIONS

The acute rCBF lesion most accurately identified tissue in the PWI>DWI mismatch region at risk of infarction. Color-thresholded PWI maps show potential for use in an acute clinical setting to prospectively predict tissue outcome.