Correlation of regional cerebral blood flow measured by stable xenon CT and perfusion MRI

Post-traumatic hydrocephalus: An overview of classification, diagnosis, treatment, and post-treatment imaging evaluation

The syndrome of post-traumatic hydrocephalus (PTH) has been recognized since Dandy's report in 1914. The pathogenesis of PTH has not been fully clarified. At present, it is believed that the obstacles of cerebrospinal fluid (CSF) secretion, absorption and circulation pathways are the reasons for the development of PTH. However, recent studies have also suggested that the osmotic pressure load of CSF and the pathological changes of CSF dynamics are caused by the development of hydrocephalus. Therefore, a better understanding of the definition, classification, diagnostic criteria, treatment, and evaluation of post-treatment effects of PTH is critical for the effective prevention and treatment of PTH. In this paper, we reviewed the classification and diagnosis of PTH and focused on the treatment and the imaging evaluation of post-treatment effects of PTH. This review might provide a judgment criterion for diagnosis of PTH and a basis for the effective prevention and treatment of PTH in the future.

Brain perfusion: computed tomography and magnetic resonance techniques

Cerebral perfusion imaging provides assessment of regional microvascular hemodynamics in the living brain, enabling in vivo measurement of a variety of different hemodynamic parameters. Perfusion imaging techniques that are used in the clinical setting usually rely upon X-ray computed tomography (CT) or magnetic resonance imaging (MRI). This chapter reviews CT- and MRI-based perfusion imaging techniques, with attention to image acquisition, clinically relevant aspects of image postprocessing, and fundamental differences between CT- and MRI-based techniques. Correlations with cerebrovascular physiology and potential clinical applications of perfusion imaging are reviewed, focusing upon the two major classes of neurologic disease in which perfusion imaging is most often performed: primary perfusion disorders (including ischemic stroke, transient ischemic attack, and reperfusion syndrome), and brain tumors.

Imaging of cerebral blood flow in patients with severe traumatic brain injury in the neurointensive care

Ischemia is a common and deleterious secondary injury following traumatic brain injury (TBI). A great challenge for the treatment of TBI patients in the neurointensive care unit (NICU) is to detect early signs of ischemia in order to prevent further advancement and deterioration of the brain tissue. Today, several imaging techniques are available to monitor cerebral blood flow (CBF) in the injured brain such as positron emission tomography (PET), single-photon emission computed tomography, xenon computed tomography (Xenon-CT), perfusion-weighted magnetic resonance imaging (MRI), and CT perfusion scan. An ideal imaging technique would enable continuous non-invasive measurement of blood flow and metabolism across the whole brain. Unfortunately, no current imaging method meets all these criteria. These techniques offer snapshots of the CBF. MRI may also provide some information about the metabolic state of the brain. PET provides images with high resolution and quantitative measurements of CBF and metabolism; however, it is a complex and costly method limited to few TBI centers. All of these methods except mobile Xenon-CT require transfer of TBI patients to the radiological department. Mobile Xenon-CT emerges as a feasible technique to monitor CBF in the NICU, with lower risk of adverse effects. Promising results have been demonstrated with Xenon-CT in predicting outcome in TBI patients. This review covers available imaging methods used to monitor CBF in patients with severe TBI.

Progressive changes in cerebral perfusion after carotid stenting: a dynamic susceptibility contrast perfusion weighted imaging study

BACKGROUND

We aimed to assess baseline inter-hemispheric perfusion differences, before carotid artery stenting (CAS), of severe internal carotid artery (ICA) stenosis, and to evaluate perfusion changes over time after CAS by means of perfusion weighted imaging (PWI).

METHODS

Dynamic susceptibility contrast PWI was performed in 33 patients with severe ICA stenosis 1 day before and 1 day after CAS, and repeated in 23 patients 30 days after CAS. Cerebral blood volume, cerebral blood flow, arrival time (T0), mean transit time, and time to peak (TTP) relative values comparing symptomatic and asymptomatic hemispheres before CAS were obtained. Pre- and post-CAS values were also compared in the treated hemisphere and middle cerebral artery (MCA) territories. The influence of collateral circulation and contralateral ICA stenosis were evaluated.

RESULTS

Before CAS, TTP was significantly increased in the affected hemisphere and MCA territory in all patients (p=0.007 and p=0.021, respectively). After treatment, normalization was observed for TTP and T0 30 days after CAS in all patients, mainly in patients with any grade of contralateral stenosis and with one or both functioning communicating arteries (42.4% of patients had both, and 42.4% showed only one-anterior communicating artery=11 patients (78.6%) and posterior communicating artery=3 patients (21.4%)).

CONCLUSIONS

Inter-hemispheric perfusion differences in patients with severe ICA stenosis normalize after CAS in the long term, up to 30 days after the procedure.

MR perfusion imaging in acute ischemic stroke

Magnetic resonance (MR) perfusion imaging offers the potential for measuring brain perfusion in acute stroke patients, at a time when treatment decisions based on these measurements may affect outcomes dramatically. Rapid advancements in both acute stroke therapy and perfusion imaging techniques have resulted in continuing redefinition of the role that perfusion imaging should play in patient management. This review discusses the basic pathophysiology of acute stroke, the utility of different kinds of perfusion images, and research on the continually evolving role of MR perfusion imaging in acute stroke care.

A systematic review of the literature on the effects of dexamethasone on the brain from in vivo human-based studies: implications for physiological brain imaging of patients with intracranial tumors

BACKGROUND

Among glucocorticoids, dexamethasone is most widely used for treatment of cerebral edema because of its long biological half-life and its low mineralocorticoid activity (sodium retaining).

OBJECTIVE

A systematic review of the literature on the effects of dexamethasone on the brain from in vivo studies in humans.

METHODS

A MEDLINE database search (via the PubMed interface) and an EMBASE database search (via the Dialog interface) of the past 35 years was performed. Every article relating to human use reported in English was included. In addition, references of all eligible articles were searched to identify other possible sources.

RESULTS

Twenty-four articles matched the eligibility criteria. There were disparate methodologies and conflicting results, although they tended to indicate a decrease in blood-tumor barrier permeability, decreased tumoral perfusion, decreased tumoral diffusivity, and the possibility of decreased perfusion in contralateral normal-appearing brain tissue.

CONCLUSION

Treatment with dexamethasone may alter imaging parameters from cerebral perfusion studies used in the management of brain tumors. In adequately powered studies, it may be possible to assess the longer term effects of dexamethasone on normal brain tissue to help optimize use with longer term survivors that are emerging as improvements in glioma treatment are made.

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.

Improving dynamic susceptibility contrast MRI measurement of quantitative cerebral blood flow using corrections for partial volume and nonlinear contrast relaxivity: A xenon computed tomographic comparative study

PURPOSE

To test whether dynamic susceptibility contrast MRI-based CBF measurements are improved with arterial input function (AIF) partial volume (PV) and nonlinear contrast relaxivity correction, using a gold-standard CBF method, xenon computed tomography (xeCT).

MATERIALS AND METHODS

Eighteen patients with cerebrovascular disease underwent xeCT and MRI within 36 h. PV was measured as the ratio of the area under the AIF and the venous output function (VOF) concentration curves. A correction was applied to account for the nonlinear relaxivity of bulk blood (BB). Mean CBF was measured with both techniques and regression analyses both within and between patients were performed.

RESULTS

Mean xeCT CBF was 43.3 +/- 13.7 mL/100g/min (mean +/- SD). BB correction decreased CBF by a factor of 4.7 +/- 0.4, but did not affect precision. The least-biased CBF measurement was with BB but without PV correction (45.8 +/- 17.2 mL/100 g/min, coefficient of variation [COV] = 32%). Precision improved with PV correction, although absolute CBF was mildly underestimated (34.3 +/- 10.8 mL/100 g/min, COV = 27%). Between patients correlation was moderate even with both corrections (R = 0.53).

CONCLUSION

Corrections for AIF PV and nonlinear BB relaxivity improve bolus MRI-based CBF maps. However, there remain challenges given the moderate between-patient correlation, which limit diagnostic confidence of such measurements in individual patients.

The performance of MRI-based cerebral blood flow measurements in acute and subacute stroke compared with 15O-water positron emission tomography: identification of penumbral flow

BACKGROUND AND PURPOSE

Perfusion-weighted MRI-based maps of cerebral blood flow (CBF(MRI)) are considered a good MRI measure of penumbral flow in acute ischemic stroke but are seldom used in clinical routine due to methodical issues. We validated CBF(MRI) on quantitative CBF measurement by 15O-water positron emission tomography (CBF(PET)).

MATERIAL AND METHODS

Comparative CBF(MRI) and CBF(PET) were performed in patients with acute and subacute stroke. In a voxel-based seed-growing technique, predefined CBF(MRI) thresholds (<40, <30, <20, <10 mL/100 g/min) were applied and the resulting volumes were compared with the hypoperfusion volume detected by the penumbral threshold (<20 mL/100 g/min) on CBF(PET). The volumetric comparison was expressed as the C-ratio (volume CBF(MRI)/volume CBF(PET)) to identify the best MRI threshold. The influence of vessel pathology, hypoperfusion size, and time point of imaging was described. The proportion of voxels correctly classified as hypoperfused and the proportion of voxel correctly classified as nonhypoperfused of the best CBF(MRI) threshold was calculated and a Bland-Altman plot illustrated the method-specific differences.

RESULTS

In 24 patients (median time MRI to PET: 68 minutes; 16 patients imaged within 24 hours after stroke), the median volume of hypoperfusion <20 mL/100 g/min (CBF(PET)) was 78.5 cm(3). Median hypoperfusion volume on CBF(MRI) ranged from 245.9 cm(3) (<40 mL/100 g/min) to 35.5 cm(3) (<10 mL/10 g/min). On visual inspection, an excellent qualitative congruence was found. The quantitative congruence was best for the MRI-CBF threshold <20 mL/100 g/min (median C-ratio: 1.0), reaching a proportion of voxels correctly classified as hypoperfused of 76% and a proportion of voxel correctly classified as nonhypoperfused of 96%, but a wide interindividual range (C-ratio 0.3 to 3.5) was found. Ipsilateral vessel pathology, time point of imaging, and size of hypoperfusion did not significantly influence the C-ratio. The Bland-Altman analysis for the volumetric difference of CBF(MRI) and CBF(PET) found a good overall agreement but a large SD.

CONCLUSIONS

Hypoperfusion areas below the CBF(PET) penumbral threshold can be well identified by the CBF(MRI) threshold <20 mL/10 g/min at a group level, but a large individual variance (exceeding 20% of volume in nearly half of the patients) could not be explained. Our results support a prudent use of MRI-based quantitative CBF measurement in clinical routine.

Perfusion MRI (Tmax and MTT) correlation with xenon CT cerebral blood flow in stroke patients

BACKGROUND

While stable xenon CT (Xe-CT) cerebral blood flow (CBF) is an accepted standard for quantitative assessment of cerebral hemodynamics, the accuracy of magnetic resonance perfusion-weighted imaging (PWI-MRI) is unclear. The Improved PWI Methodology in Acute Clinical Stroke Study compares PWI findings with Xe-CT CBF values in patients experiencing symptomatic severe cerebral hypoperfusion.

METHODS

We compared mean transit time (MTT) and Tmax PWI-MRI with the corresponding Xe-CT CBF values in 25 coregistered regions of interest (ROIs) of multiple sizes and locations in nine subacute stroke patients. Comparisons were performed with Pearson correlation coefficients (R). We performed receiver operating characteristic (ROC) curve analyses to define the threshold of Tmax and absolute MTT that could best predict a Xe-CT CBF <20 mL/100 g/minute.

RESULTS

The subjects' mean (SD) age was 50 (15) years, the median (interquartile range [IQR]) NIH Stroke Scale score was 2 (2-6), and the median (IQR) time between MRI and Xe-CT was 12 (-7-19) hours. The total number of ROIs was 225, and the median (IQR) ROI size was 550 (360-960) pixels. Tmax correlation with Xe-CT CBF (R = 0.63, p < 0.001) was stronger than absolute MTT (R = 0.55, p < 0.001), p = 0.049. ROC curve analysis found that Tmax >4 seconds had 68% sensitivity, 80% specificity, and 77% accuracy and MTT >10 seconds had 68% sensitivity, 77% specificity, and 75% accuracy for predicting ROIs with Xe-CT CBF <20 mL/100 g/minute.

CONCLUSION

Our results suggest that in subacute ischemic stroke patients, Tmax correlates better than absolute mean transit time (MTT) with xenon CT cerebral blood flow (Xe-CT CBF) and that both Tmax >4 seconds and MTT >10 seconds are strongly associated with Xe-CT CBF <20 mL/100 g/minute. CBF = cerebral blood flow; DBP = diastolic blood pressure; DEFUSE = Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution; DWI = diffusion-weighted imaging; EPITHET = Echoplanar Imaging Thrombolytic Evaluation Trial; FOV = field of view; ICA = internal carotid artery; IQR = interquartile range; MCA = middle cerebral artery; MTT = mean transit time; NIHSS = NIH Stroke Scale; PWI = perfusion-weighted imaging; PWI-MRI = magnetic resonance perfusion-weighted imaging; ROC = receiver operating characteristic; ROI = region of interest; SBP = systolic blood pressure; SVD = singular value decomposition; Xe-CT = xenon CT.

Cerebrovascular insufficiency as the criterion for revascularization procedures in selected patients: a correlation study of xenon contrast-enhanced CT and PWI

In order to identify patients who suffer from hemodynamic cerebral insufficiency and can benefit from cerebral revascularization procedures, xenon-CT scanning has been established to reliably measure the critical cerebrovascular reserve capacity. As a need for alternative quantification methods arises, this study aims to characterize the significance of both time-to-peak (TTP) and mean transit time (MTT) in perfusion-weighted imaging (PWI) in this particular subset of patients. Ten patients in routine preoperative work-up for cerebral revascularization were prospectively enrolled and underwent both XeCT scanning and PWI. Cerebrovascular reserve capacity (CVRC) was calculated for each region of interest (ROI, n = 504) after administration of a vasoactive stimulus. ROIs were anatomically matched with those of PWI after TTP and MTT were calculated. Highly significant negative correlation was found for TTP and CVRC for all ROIs (r = -0.3954, p < 0.0001; symptomatic ROIs: r = -0.4867, p < 0.0001). Correlation was weak for MTT and CVCR (r = -0.1287; p < 0.01). The optimum threshold for TTP to detect impaired cerebrovascular reactivity in our patient group was 4 s (specificity 90.8%, sensitivity 44.4%) for all ROIs (TTP > 4.4 s for symptomatic ROIs, specificity 88.4%, sensitivity 62.7%). An approximative equation to calculate the probability of pathological findings could be derived from the data. The positive predictive value (PPV) was 0.76 (symptomatic 0.78) with a negative predictive value (NPV) of 0.71 (symptomatic 0.78). While PWI currently is not able to replace XeCT in the direct quantification of CVRC, it may serve as a readily available follow-up tool. A TTP threshold of greater than 4 s allows to confirm a cerebrovascular compromise in a selected high-risk subgroup of patients.

Absolute quantification of cerebral blood flow in normal volunteers: correlation between Xe-133 SPECT and dynamic susceptibility contrast MRI

PURPOSE

To compare absolute cerebral blood flow (CBF) estimates obtained by dynamic susceptibility contrast MRI (DSC-MRI) and Xe-133 SPECT.

MATERIALS AND METHODS

CBF was measured in 20 healthy volunteers using DSC-MRI at 3T and Xe-133 SPECT. DSC-MRI was accomplished by gradient-echo EPI and CBF was calculated using a time-shift-insensitive deconvolution algorithm and regional arterial input functions (AIFs). To improve the reproducibility of AIF registration the time integral was rescaled by use of a venous output function. In the Xe-133 SPECT experiment, Xe-133 gas was inhaled over 8 minutes and CBF was calculated using a biexponential analysis.

RESULTS

The average whole-brain CBF estimates obtained by DSC-MRI and Xe-133 SPECT were 85 +/- 23 mL/(min 100 g) and 40 +/- 8 mL/(min 100 g), respectively (mean +/- SD, n = 20). The linear CBF relationship between the two modalities showed a correlation coefficient of r = 0.76 and was described by the equation CBF(MRI) = 2.4 . CBF(Xe)-7.9 (CBF in units of mL/(min 100 g)).

CONCLUSION

A reasonable positive linear correlation between MRI-based and SPECT-based CBF estimates was observed after AIF time-integral correction. The use of DSC-MRI typically results in overestimated absolute perfusion estimates and the present study indicates that this trend is further enhanced by the use of high magnetic field strength (3T).

Patients with chronic fatigue syndrome have reduced absolute cortical blood flow

Prior studies on brain blood flow in chronic fatigue syndrome (CFS) did not find consistent results. This may be because they used single-photon emission computed tomography to measure brain blood flow, which could not measure absolute blood flow. Therefore, the aim of this study was to test the hypothesis that patients with CFS have reduced absolute cerebral blood flow. Xenon-computed tomography blood flow studies were done on 25 CFS patients and seven healthy controls. Analyses were done after stratifying the CFS patients based on the presence or absence of a current psychiatric disorder. Flow was diminished in both groups as follows: patients with no current psychiatric disorders had reduced cortical blood flow in the distribution of both right and left middle cerebral arteries (P<0.05 for both) while those with current psychiatric disorders had reduced blood flow only in the left middle cerebral artery territory (P<0.05). These data indicate that patients with CFS have reduced absolute cortical blood flow in rather broad areas when compared with data from healthy controls and that those devoid of psychopathology had the most reductions in cortical flow. These data support, in part, our earlier findings that patients devoid of psychopathology are the group most at risk of having some of the symptoms of CFS due to brain dysfunction.

Comparative overview of brain perfusion imaging techniques

Numerous imaging techniques have been developed and applied to evaluate brain hemodynamics. Among these are: Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), Xenon-enhanced Computed Tomography (XeCT), Dynamic Perfusion-computed Tomography (PCT), Magnetic Resonance Imaging Dynamic Susceptibility Contrast (DSC), Arterial Spin-Labeling (ASL), and Doppler Ultrasound. These techniques give similar information about brain hemodynamics in the form of parameters such as cerebral blood flow (CBF) or volume (CBV). All of them are used to characterize the same types of pathological conditions. However, each technique has its own advantages and drawbacks. This article addresses the main imaging techniques dedicated to brain hemodynamics. It represents a comparative overview, established by consensus among specialists of the various techniques. For clinicians, this paper should offers a clearer picture of the pros and cons of currently available brain perfusion imaging techniques, and assist them in choosing the proper method in every specific clinical setting.

Magnetic resonance brain perfusion imaging with voxel-specific arterial input functions

PURPOSE

To propose an automatic method for estimating voxel-specific arterial input functions (AIFs) in dynamic contrast brain perfusion imaging.

MATERIALS AND METHODS

Voxel-specific AIFs were estimated blindly using the theory of homomorphic transformations and complex cepstrum analysis. Wiener filtering was used in the subsequent deconvolution. The method was verified using simulated data and evaluated in 10 healthy adults.

RESULTS

Computer simulations accurately estimated differently shaped, normalized AIFs. Simple Wiener filtering resulted in underestimation of flow values. Preliminary in vivo results showed comparable cerebral flow value ratios between gray matter (GM) and white matter (WM) when using blindly estimated voxel-specific AIFs or a single manually selected AIF. Significant differences (P < or = 0.0125) in mean transit time (MTT) and time-to-peak (TTP) in GM compared to WM was seen with the new method.

CONCLUSION

Initial results suggest that the proposed method can replace the tedious and difficult task of manually selecting an AIF, while simultaneously providing better differentiation between time-dependent hemodynamic parameters.

The application of diffusion- and perfusion-weighted magnetic resonance imaging in the diagnosis and therapy of acute cerebral infarction

Diffusion- and perfusion-weighted magnetic resonance imaging (DWI and PWI) was applied for stroke diagnose in 120 acute (< 48 h) ischemic stroke patients. At hyperacute (< 6 h) stage, it is difficult to find out the infarction zone in conventional T1 or T2 image, but it is easy in DWI, apparent diffusion coefficient (ADC) map; when at 3–6-hour stage it is also easy in PWI, cerebral blood flow (CBF) map, cerebral blood volume (CBV) map, and mean transit time (MTT) map; at acute (6–48 h) stage, DWI or PWI is more sensitive than conventional T1 or T2 image too. Combining DWI with ADC, acute and chronic infarction can be distinguished. Besides, penumbra which should be developed in meaning was used as an indication or to evaluate the therapeutic efficacy. There were two cases (< 1.5 h) that broke the model of penumbra because abnormity was found in DWI but not that in PWI, finally they recovered without any sequela.

Comparative overview of brain perfusion imaging techniques

BACKGROUND AND PURPOSE

Numerous imaging techniques have been developed and applied to evaluate brain hemodynamics. Among these are positron emission tomography, single photon emission computed tomography, Xenon-enhanced computed tomography, dynamic perfusion computed tomography, MRI dynamic susceptibility contrast, arterial spin labeling, and Doppler ultrasound. These techniques give similar information about brain hemodynamics in the form of parameters such as cerebral blood flow or cerebral blood volume. All of them are used to characterize the same types of pathological conditions. However, each technique has its own advantages and drawbacks.

SUMMARY OF REVIEW

This article addresses the main imaging techniques dedicated to brain hemodynamics. It represents a comparative overview established by consensus among specialists of the various techniques.

CONCLUSIONS

For clinicians, this article should offer a clearer picture of the pros and cons of currently available brain perfusion imaging techniques and assist them in choosing the proper method for every specific clinical setting.

Perfusion imaging of the brain using Z-score and dynamic images obtained by subtracting images from before and after contrast injection

Objective

The aim of this study was to examine the feasibility of perfusion imaging of the brain using the Z-score and subtraction dynamic images obtained from susceptibility contrast MR images.

Materials and Methods

Five patients, each with a normal MRI, Moya-moya, a middle cerebral artery occlusion, post-trauma syndrome, and a metastatic brain tumor, were selected for a presentation. A susceptibility-contrast echo-planar image after a routine MRI was taken as the source image with a rapid manual injection of 0.1 mmol/kg of Gd-DTPA. The inflow and washout patterns were observed from the time-signal intensity curve of the serial scans using the standard program of an MRI machine. The repeated Z-score images of the peak and late phases were made using the threshold Z-score values between 1.4 and 2.0 in four to five studies of the pre-contrast, peak, and late phases. Dynamic subtraction images were produced by subtracting sequential post-contrast images from a pre-contrast image and coloring these images using a pseudocolor mapping method.

Results

In the diseases with perfusion abnormalities, the Z-score images revealed information about the degree of perfusion during the peak and late phases. However, the quality varied with the Z-score threshold and the studies selected in a group. The dynamic subtraction images were of sufficient quality with no background noise and more clearly illustrated the temporal changes in perfusion and delayed perfusion.

Conclusion

The Z-scores and dynamic subtraction images illustrated the degree of perfusion and sequential changes in the pattern of perfusion, respectively. These images can be used as a new complimentary method for observing the perfusion patterns in brain diseases.

Cerebral perfusion imaging in acute stroke

Herein, the author discusses four perfusion technologies, including the diffusible tracer methods-xenon-enhanced computed tomography (CT) and single photon emission CT with various radioisotopes-and the nondiffusible tracer techniques-CT perfusion and magnetic resonance (MR) perfusion and diffusion. The methods for and important issues in the performance of each technique are presented, along with the accuracy of the data acquired with each technique, as demonstrated with experimental studies. In addition, the use of each technique in the evaluation of patients with acute stroke and their relative advantages and disadvantages are presented.

[Diffusion, perfusion and activation functional MRI studies of brain arteriovenous malformations]

The management of Brain Arteriovenous Malformations continues to be challenged by a lack of understanding and control of pathophysiological processes implied in the clinical symptoms. New data from functional MRI with diffusion-weighted, perfusion-weighted and neuronal activation highlight abnormal brain areas near or remote to the AVM nidus. Moreover, these techniques are able to show hemodynamic and neuronal adaptative phenomena involved in brain plasticity. They reflect the instantaneous hemodynamic brain conditions that may help to correlate the clinical symptoms with the anatomical and functional substratum and to influence any invasive therapy.

Validation of the CBF, CBV, and MTT values by perfusion MRI in chronic occlusive cerebrovascular disease

RATIONALE AND OBJECTIVES

To evaluate the reliability of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) values obtained by deconvolution algorithm perfusion-weighted MR imaging (D-PWI), we compared these values with those obtained by first-moment algorithm perfusion-weighted MR imaging (F-PWI) and 15O-PET.

SUBJECTS AND METHODS

Six healthy volunteers and eleven patients with chronic occlusive cerebrovascular disease were studied with both perfusion-weighted MR imaging and 15O-PET, and region-of-interest analyses were performed. Normalization factors for CBF and CBV values obtained by D-PWI were determined as the mean values of 15O-PET divided by those of D-PWI in healthy volunteers. Then these values were used in analyzing the data of the patients.

RESULTS

The MTT value obtained by D-PWI was 6.1 +/- 0.5 seconds on the non-occluded side, 6.4 +/- 0.7 seconds on the minimally to moderately stenosed side, and 6.7 +/- 1.2 seconds on the severely stenosed to occluded side. These values were significantly correlated with those obtained by F-PWI (r = 0.83; P < .001), and with those obtained by 15O-PET (r = 0.78; P < .05). However, the CBF and CBV values obtained by D-PWI did not correlate with those obtained by 15O-PET.

CONCLUSION

MTT values obtained by D-PWI were reliable parameters of cerebral hemodynamics, but the CBF and CBV values obtained by D-PWI were not always reliable.

MR perfusion imaging in proliferative angiopathy

Seizures, which may be the main expression of cerebral arteriovenous malformations (CAVM) can be difficult to control medically. Our goal was to use perfusion-weighted imaging (PWI) in correlation with clinical data to detect abnormal areas of the cerebrum related to a particular type of CAVM (proliferative angiopathy) and to study the pathophysiology. We use PWI, with a bolus injection of contrast medium, to investigate seven patients with proliferative angiopathy and fits producing language disturbance. Perfusion parameters were calculated using the first-pass moment theory. Five patients had perimalformative and/or contralateral abnormal areas with relative hyperperfusion (cerebral blood volume +20.7+/-16.2%, blood flow 92.5+/-68.8 ml/min/100 g). Areas of hypoperfusion and venous congestion were detected in two patients. One patient who underwent MRI after a severe focal deficit had no significant haemodynamic abnormality.

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.

New techniques in cerebral imaging

This article presents a review of the current techniques in Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) that enhance the usefulness of these tests for the diagnosis of cerebral disease. We were able to analyze concepts and definitions and give a comparative description for each test studied including Multidetector/Perfusion CT, Xenon CT, CT-Angiogram, MRI-Perfusion, MRI-Diffusion, MRI-Spectroscopy, MRI CSF Flow, Functional MRI studies and Cerebral Angiography.

Assessment of perfusion of facial microvascular transplants and early detection of ischemia by perfusion-CT scan

OBJECTIVE

Perfusion-computed tomography (CT) is a promising new technique to assess ischemic lesions caused by ischemic brain stroke. In this study, the use of perfusion-CT scans to predict ischemia in microvascular transplants of the face was examined.

STUDY DESIGN

Thirty-eight patients with microvascular latissimus dorsi transplants after tumor surgery were assessed by perfusion-CT scan 34 to 72 hours after surgery. In these cases, clinical examination of the transplant and examination by means of O(2)-probes were either unsuccessful or impossible. An electron beam tomography of the region of interest was performed by using an intravenous nonionic iodine-containing contrast medium (Ultravist 300, Nycomed, Germany) that was applied with an injector at a flow rate of 5 mL/min. Twenty scans with a scanning time of 300 ms and an interscanning time of 3 seconds were carried out. Changes in the Houndsfield units within the transplant as well as the region of the contralateral erector spinae muscle were measured.

RESULTS

Central malperfusion resulting in later complete transplant loss was detected in 2 cases. Peripheral malperfusion was found in 6 cases, resulting in localized resection and secondary wound closure. When no malperfusion was registered, the straightforward healing process took place.

CONCLUSION

Perfusion-CT scans are of great aid in the assessment of microvascular transplant perfusion in the face, when adequate perfusion is not verifiable clinically or by O(2)-probe because of removal or malfunction.

Pathophysiological changes after traumatic brain injury: comparison of two experimental animal models by means of MRI

In an experimental study MRI was used to compare the pathophysiological changes of brain tissue after lateral fluid percussion injury (FPI) versus cold injury (CI) as models of traumatic brain injury (TBI). Two groups of Sprague-Dawley rats (n=23) were subjected to mild FPI, respectively, CI localized over the right parietal cortex. MRI was performed at different time points including T1w, T2w and T1w-CE (Gd-DTPA 0.2 mmol/kg BW) sequences as well as perfusion-weighted imaging with calculation of regional cerebral blood volume (rCBV) and regional cerebral blood flow (rCBF). T2w and T1w-CE images showed hyperintense areas in the traumatised cortex demonstrating brain edema and blood-brain barrier (BBB)-breakdown increasing up to 12 h. Perfusion-weighted imaging demonstrated a significant decrease of rCBV and rCBF in the ipsilateral cortex of CI animals compared with the contralateral hemisphere. In contrast, rats of the FPI group showed only slight differences in rCBF and rCBV comparing the left and right cortex. The results of our study confirm that both mild FPI and CI produced focal brain edema with concomitant breakdown of the BBB as a model of TBI. Since differences regarding perfusion are much more pronounced in CI our results suggest that, this model more likely seems to reflect pathophysiological changes of brain ischemia, whereas FPI seems to be better suited to model the pathophysiological characteristics of TBI.

CT perfusion scanning with deconvolution analysis: pilot study in patients with acute middle cerebral artery stroke

PURPOSE

To measure mean cerebral blood flow (CBF) in ischemic and nonischemic territories and in low-attenuation regions in patients with acute stroke by using deconvolution-derived hemodynamic imaging.

MATERIALS AND METHODS

Twelve patients with acute middle cerebral artery stroke and 12 control patients were examined by using single-section computed tomography (CT) perfusion scanning. Analysis was performed with a deconvolution-based algorithm. Comparisons of mean CBF, cerebral blood volume (CBV), and mean transit time (MTT) were determined between hemispheres in all patients and between low- and normal-attenuation regions in patients with acute stroke. Two independent readers examined the images for extent of visually apparent regional perfusion abnormalities. The data were compared with extent of final infarct in seven patients with acute stroke who underwent follow-up CT or magnetic resonance imaging.

RESULTS

Significant decreases in CBF (-50%, P =.001) were found in the affected hemispheres of patients with acute stroke. Significant changes in CBV (-26%, P =.03) and MTT (+111%, P =.004) were also seen. Significant alterations in perfusion were also seen in low- compared with normal-attenuation areas. Pearson correlation between readers for extent of CBF abnormality was 0.94 (P =.001). Intraobserver variation was 8.9% for CBF abnormalities.

CONCLUSION

Deconvolution analysis of CT perfusion data is a promising method for evaluation of cerebral hemodynamics in patients with acute stroke.

Extracting parametric images from dynamic contrast-enhanced MRI studies of the brain using factor analysis

Factor analysis of dynamic studies (FADS) is a technique that allows structures with different temporal characteristics to be extracted from dynamic contrast enhanced studies without making any a priori assumptions about physiology. These dynamic structures may correspond to different tissue types or different organs or they may simply be a useful way of characterising the data. This paper describes a method of automatically extracting factor images and curves from contrast enhanced MRI studies of the brain. This method has been applied to 107 studies carried out on patients with acute stroke. The results show that FADS is able to extract factor curves correlated to arterial and venous signal intensity curves and that the corresponding factor images allow a distinction to be made between areas of the brain with normal and abnormal perfusion. The method is robust and can be applied routinely to dynamic studies of the brain. The constraints described are sufficiently general to be applicable to other dynamic MRI contrast enhanced studies where an increase in contrast concentration produces an increase in signal intensity.

Absolute cerebral blood flow measured by dynamic susceptibility contrast MRI: a direct comparison with Xe-133 SPECT

Absolute regional cerebral blood flow (CBF) was measured in ten healthy volunteers, using both dynamic susceptibility-contrast (DSC) magnetic resonance imaging (MRI) and Xe-133 SPECT within 4 h. After i.v. injection of Gd-DTPA-BMA (0.3 mmol/kg b.w.), the bolus was monitored with a Simultaneous Dual FLASH pulse sequence (1.5 s/image), providing one slice through brain tissue and a second slice through the carotid artery. Concentration C(t) is proportional to -(1/TE) ln[S(t)/S(0)] was related to CBF as C(t) = CBF [AIF(t) x R(t)], where AIF is the arterial input function and R(t) is the residue function. A singular-value-decomposition-based deconvolution technique was used for retrieval of R(t). Absolute CBF was given by Zierler's area-to-height relation and the central volume principle. For elimination of large vessels (ELV), all MRI-based CBF values exceeding 2.5 times the mean CBF value of the slice were excluded. A correction for partial-volume effects (CPVE) in the artery used for AIF monitoring was based on registration of signal in a phantom with tubes of various diameters (1.5-6.5 mm), providing an individual concentration correction factor applied to AIF data registered in vivo. In the Xe-133 SPECT investigation, 3,000-4,000 MBq of Xe-133 was administered intravenously, and CBF was calculated using the Kanno Lassen algorithm. When ELV and CPVE were applied, DSC-MRI showed average CBF values from the entire slice of 43 +/- 10 ml/(min 100 g) (small-artery AIF) and 48 +/- 17 ml/(min 100 g) (carotid-artery AIF) (mean +/- S.D., n = 10). The corresponding Xe-133-SPECT-based CBF was 33 +/- 6 ml/(min 100 g) (n = 10). The relationships of CBF(MRI) versus CBF(SPECT) showed good linear correlation (r = 0.74-0.83).

Systematic review of diffusion and perfusion imaging in acute ischemic stroke

BACKGROUND AND PURPOSE

Recent advances in neuroimaging have raised hopes of early and accurate identification of ischemic brain and the discrimination of dead from salvageable tissue. We sought to determine whether the data published so far are enough to establish the roles of these techniques in everyday clinical practice.

METHODS

A systematic review of studies of MR diffusion-weighted imaging (DWI), perfusion imaging (PI), or a combination of the two, in human stroke, excluding abstracts and case reports. One reviewer extracted information on the size of each study, its main purpose, methodological details, and results.

RESULTS

We identified 47 studies of DWI, 18 studies of MR PI alone or in combination with another advanced imaging modality, and 19 studies of DWI and PI together. Although high proportions of the studies were prospective and gave good details of the imaging sequences used, the majority gave very limited details on patient selection and clinical characteristics or blinded imaging assessment. Pathophysiological changes were inferred from DWI/PI patterns that were not supported by other data.

CONCLUSIONS

Despite considerable enthusiasm for and promise of these techniques, there is not sufficient information available in these studies to enable us to draw firm conclusions about the sensitivity and specificity of these techniques for identification of either ischemic lesions not visible by other means or salvageable tissue. Future studies should include larger numbers of carefully described patients, assess the contribution of DWI over and above other imaging, obtain follow-up at an appropriate time interval to determine accurate clinical and neuroradiological outcomes, and assess DWI/PI abnormality with reperfusion in randomized treatment trials. Investigators should also be encouraged to combine their individual patient data in meta-analyses to obtain a more robust assessment of the value of DWI and PI from larger sample sizes.

Whole brain quantitative CBF, CBV, and MTT measurements using MRI bolus tracking: implementation and application to data acquired from hyperacute stroke patients

A robust whole brain magnetic resonance (MR) bolus tracking technique based on indicator dilution theory, which could quantitatively calculate cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) on a regional basis, was developed and tested. T2*-weighted gradient-echo echoplanar imaging (EPI) volumes were acquired on 40 hyperacute stroke patients after gadolinium diethylene triamine pentaacetic acid (Gd-DTPA) bolus injection. The thalamus, white matter (WM), infarcted area, penumbra, and mirror infarcted and penumbra regions were analyzed. The calculation of the arterial input function (AIF) needed for absolute quantification of CBF, CBV, and MTT was shown to be user independent. The CBF values (ml/min/100 g units) and CBV values (% units, in parentheses) for the thalamus, WM, infarct, mirror infarct, penumbra, and mirror penumbra (averaged over all patients) were 69.8 +/- 22.2 (9.0 +/- 3.0 SD); 28.1 +/- 6.9 (3.9 +/- 1.2); 34.4 +/- 22.4 (7.1 +/- 2.7); 60.3 +/- 20.7 (8.2 +/- 2.3); 50.2 +/- 17.5 (10.4 +/- 2.4); and 64.2 +/- 17.0 (9.5 +/- 2.3), respectively, and the corresponding MTT values (in seconds) were 8.0 +/- 2.1; 8.6 +/- 3.0; 16.1 +/- 8.9; 8.6 +/- 2.9; 13.3 +/- 3.5; and 9.4 +/- 3.2. The infarct and penumbra CBV values were not significantly different from their corresponding mirror values, whereas the CBF and MTT values were (P < 0.01). Quantitative measurements of CBF, CBV, and MTT were calculated on a regional basis on data acquired from hyperacute stroke patients, and the CBF and MTT values showed greater sensitivity to areas with perfusion defects than the CBV values. J. Magn. Reson. Imaging 2000;12:400-410.

Contrast-reduced imaging of tissue concentration and arterial level (CRITICAL) for assessment of cerebral hemodynamics in acute stroke by magnetic resonance

RATIONALE AND OBJECTIVES

To compare cerebral perfusion data obtained by using a low-dose, T1-weighted MRI technique with radionuclide (single positron emission computed tomography [SPECT]) brain imaging and to assess the reproducibility of parametric MRI data (cerebral blood flow [CBF], cerebral blood volume [CBV], and time to peak [TTP]) by applying a previously described nuclear medicine technique to derive quantitative perfusion data.

METHODS

Single-slice brain and neck images were rapidly acquired during the passage of a small (1/10th of normal dose) bolus of contrast. Parametric images were constructed from the MR data by extracting the bolus transit curve for the brain and the peak arterial input curve from the carotid vessels in the neck. These were compared with SPECT perfusion imaging. Twenty-four patients with acute stroke were studied with both techniques; 13 underwent repeated scanning to assess data reproducibility.

RESULTS

Relative CBF data were comparable to SPECT data (r = 0.584, P = 0.01). Results were reproducible for relative CBF, CBV, and TTP. The arterial input function was significantly different on the second injection with an average difference of 73.5, suggesting that the signal-concentration relationship had lost linearity with increased contrast load. Absolute quantitative MRI data produced values in the expected range (CBF = 42.6 mL x 100 g(-1) x min(-1)).

CONCLUSIONS

This technique allows estimation of CBF in the setting of acute stroke with quantitative values in the expected range. Repeated measurements in the same patients showed that this technique provides a reproducible measure of relative CBF, CBV, and TTP.

Quantification of cerebral blood flow by bolus tracking and artery spin tagging methods

This study deals with perfusion quantification in healthy volunteers using two types of dynamic magnetic resonance imaging (MRI) methods. Absolute cerebral blood flow (CBF) measurements were performed in 11 subjects by applying both bolus tracking of exogenous contrast agent and non-invasive arterial spin labeling MRI techniques. Both methods produced CBF images with good tissue contrast and CBF values are in good agreement with literature data. The correlation between cerebral blood volume (CBV) and CBF is also discussed.

Whole brain quantitative CBF and CBV measurements using MRI bolus tracking: comparison of methodologies

Three different deconvolution techniques for quantifying cerebral blood flow (CBF) from whole brain T*(2)-weighted bolus tracking images were implemented (parametric Fourier transform P-FT, parametric single value decomposition P-SVD and nonparametric single value decomposition NP-SVD). The techniques were tested on 206 regions from 38 hyperacute stroke patients. In the P-FT and P-SVD techniques, the tissue and arterial concentration time curves were fit to a gamma variate function and the resulting CBF values correlated very well (CBF(P-FT) = 1.02 x CBF(P-SVD), r(2) = 0.96). The NP-SVD CBF values (i.e., original unfitted curves were used) correlated well with the P-FT CBF values only when a sufficient number of time series volumes were acquired to minimize tracer time curve truncation (CBF(P-FT) x 0.92 x CBF(NP-SVD), r(2) = 0.88). The correlation between the fitted CBV and the unfitted CBV values was also maximized in regions with minimal tracer time curve truncation (CBV(fit) = 1.00 x CBV(unfit), r(2) = 0.89). When a sufficient number of time series volumes could not be acquired (due to scanner limitations) to avoid tracer time curve truncation, the P-FT and P-SVD techniques gave more reliable estimates of CBF than the NP-SVD technique.