Perfusion-weighted imaging of interictal hypoperfusion in temporal lobe epilepsy using FAIR-HASTE: comparison with H(2)(15)O PET measurements

Lateralization of interictal temporal lobe hypoperfusion in lesional and non-lesional temporal lobe epilepsy using arterial spin labeling MRI

PURPOSE

Non-invasive imaging studies play a critical role in the presurgical evaluation of patients with drug-resistant temporal lobe epilepsy (TLE), particularly in helping to lateralize the seizure focus. Arterial Spin Labeling (ASL) MRI has been widely used to non-invasively study cerebral blood flow (CBF), with somewhat variable interictal alterations reported in TLE. Here, we compare temporal lobe subregional interictal perfusion and symmetry in lesional (MRI+) and non-lesional (MRI-) TLE compared to healthy volunteers (HVs).

METHODS

Twenty TLE patients (9 MRI+, 11 MRI-) and 14 HVs under went 3 T Pseudo-Continuous ASL MRI through an epilepsy imaging research protocol at the NIH Clinical Center. We compared normalized CBF and absolute asymmetry indices in multiple temporal lobe subregions.

RESULTS

Compared to HVs, both MRI+ and MRI- TLE groups demonstrated significant ipsilateral mesial and lateral temporal hypoperfusion, specifically in the hippocampal and anterior temporal neocortical subregions, with additional hypoperfusion in the ipsilateral parahippocampal gyrus in the MRI+ and contralateral hippocampus in the MRI- TLE groups. Contralateral to the seizure focus, there was significant relative hypoperfusion in multiple subregions in the MRI- compared to the MRI+ TLE groups. The MRI+ group therefore had significantly greater asymmetry across multiple temporal subregions compared to the MRI- TLE and HV groups. No significant differences in asymmetry were found between the MRI- TLE and HV groups.

CONCLUSION

We found a similar extent of interictal ipsilateral temporal hypoperfusion in MRI+ and MRI- TLE. However, significantly increased asymmetries were found only in the MRI+ group due to differences in perfusion contralateral to the seizure focus between the patient groups. The lack of asymmetry in the MRI- group may negatively impact the utility of interictal ASL for seizure focus lateralization in this patient population.

The utility of arterial spin labelled perfusion-weighted magnetic resonance imaging in measuring the vascularity of high grade gliomas - A prospective study

Background

Dynamic susceptibility contrast (DSC) perfusion weighted imaging (PWI) currently remains the gold standard technique for measuring cerebral perfusion in glioma diagnosis and surveillance. Arterial spin labelling (ASL) PWI is a non-invasive alternative that does not require gadolinium contrast administration, although it is yet to be applied in widespread clinical practice. This study aims to assess the utility of measuring signal intensity in ASL PWI in predicting glioma vascularity by measuring maximal tumour signal intensity in patients based on pre-operative imaging and comparing this to maximal vessel density on histopathology.

Methods

Pseudocontinuous ASL (pCASL) and DSC images were acquired pre-operatively in 21 patients with high grade gliomas. The maximal signal intensity within the gliomas over a region of interest of 100 mm² was measured and also normalised to the contralateral cerebral cortex (nTBF-C), and cerebellum (nTBF-Cb). Maximal vessel density per 1 mm² was determined on histopathology using CD31 and CD34 immunostaining on all participants.

Results

Using ASL, statistically significant correlation was observed between maximal signal intensity (p < 0.05) and nTBF-C (p < 0.05) to maximal vessel density based on histopathology. Although a positive trend was also observed nTBF-Cb, this did not reach statistical significance. Using DSC, no statistically significant correlation was found between signal intensity, nTBF-C and nTBF-Cb. There was no correlation between maximal signal intensity between ASL and DSC. Average vessel density did not correlate with age, sex, previous treatment, or IDH status.

Conclusions

ASL PWI imaging is a reliable marker of evaluating the vascularity of high grade gliomas and may be used as an adjunct to DSC PWI.

Radiological Differentiation Between Intracranial Meningioma and Solitary Fibrous Tumor/Hemangiopericytoma: A Systematic Literature Review

BACKGROUND

Intracranial solitary fibrous tumor (SFT) is characterized by aggressive local behavior and high post-resection recurrence rates. It is difficult to distinguish between SFT and meningiomas, which are typically benign. The goal of this study was to systematically review radiological features that differentiate meningioma and SFT.

METHODS

We performed a systematic review in accordance with PRISMA guidelines to identify studies that used imaging techniques to identify radiological differentiators of SFT and meningioma.

RESULTS

Eighteen studies with 1565 patients (SFT: 662; meningiomas: 903) were included. The most commonly used imaging modality was diffusion weighted imaging, which was reported in 11 studies. Eight studies used a combination of diffusion weighted imaging and T1- and T2-weighted sequences to distinguish between SFT and meningioma. Compared to all grades/subtypes of meningioma, SFT is associated with higher apparent diffusion coefficient, presence of narrow-based dural attachments, lack of dural tail, less peritumoral brain edema, extensive serpentine flow voids, and younger age at initial diagnosis. Tumor volume was a poor differentiator of SFT and meningioma, and overall, there were less consensus findings in studies exclusively comparing angiomatous meningiomas and SFT.

CONCLUSIONS

Clinicians can differentiate SFT from meningiomas on preoperative imaging by looking for higher apparent diffusion coefficient, lack of dural tail/narrow-based dural attachment, less peritumoral brain edema, and vascular flow voids on neuroimaging, in addition to younger age at diagnosis. Distinguishing between angiomatous meningioma and SFT is much more challenging, as both are highly vascular pathologies. Tumor volume has limited utility in differentiating between SFT and various grades/subtypes of meningioma.

An interictal measurement of cerebral oxygen extraction fraction in MRI-negative refractory epilepsy using quantitative susceptibility mapping

PURPOSE

Oxygen extraction fraction (OEF) can be a factor to identify brain tissue's disability in epileptic patients. This study aimed to assess the OEF's level measurement in refractory epileptic patients (REPs) using a quantitative susceptibility mapping (QSM) method and to determine whether the OEF parameters change.

METHODS

QSM-OEF maps of 26 REPs and 16 healthy subjects were acquired using 3T MRI with a 64-channel coil. Eighteen regions-of-interest (ROIs) were chosen around the cortex in one appropriate slice of the brain and the mean QSM-OEF for each ROI was obtained. The correlations of QSM-OEF among different clinical characteristics of the disease, as well as between the patients and normal subjects, were also investigated.

RESULTS

QSM-OEF was shown to be significantly higher in REPs (44.9 ± 5.8) than that in HS (41.9 ± 6.2) (p < 0.05). Mean QSM-OEF was statistically lower in the ipsilateral side (44.5 ± 6.6) compared to the contralateral side (46.4 ± 6.8) (P < 0.01). QSM-OEF was illustrated to have a strong positive correlation with the attack duration (r = 0.6), and a moderate negative correlation with the attack frequency (r = -0.3). Using an optimized support vector machine algorithm, we could predict the disease in subjects having abnormal OEF values in the brain-selected-ROIs with sensitivity, specificity, AUC, and the precision of 0.96, 1, 0.98, and 1, respectively.

CONCLUSIONS

The results of this study revealed that QSM-OEF of the REPs' brain is higher than that of HS, which indicates that QSM-OEF is associated with disease activity.

The utility of arterial spin labeling in the presurgical evaluation of poorly defined focal epilepsy in children

OBJECTIVE

The authors sought to assess the utility of arterial spin labeling (ASL) perfusion 3T-MRI for the presurgical evaluation of poorly defined focal epilepsy in pediatric patients.

METHODS

Pseudocontinuous ASL perfusion 3T-MRI was performed in 25 consecutive children with poorly defined focal epilepsy. ASL perfusion abnormalities were detected qualitatively by visual inspection and quantitatively by calculating asymmetry index (AI) maps and significant z-score cluster maps based on successfully operated cases. ASL results were prospectively compared to scalp EEG, structural 3T-MRI, FDG-PET, ictal/interictal SPECT, magnetoencephalography (MEG), and intracranial recording results, as well as the final surgically proven epileptogenic zone (EZ) in operated patients who had at least 1 year of good (Engel class I/II) seizure outcome and positive histopathology results.

RESULTS

Qualitative ASL perfusion abnormalities were found in 17/25 cases (68%), specifically in 17/20 MRI-positive cases (85.0%) and in none of the 5 MRI-negative cases. ASL was concordant with localizing scalp EEG findings in 66.7%, structural 3T-MRI in 90%, FDG-PET in 75%, ictal/interictal SPECT in 62.5%, and MEG in 75% of cases, and with intracranial recording results in 40% of cases. Eleven patients underwent surgery; in all 11 cases the EZ was surgically proven by positive histopathology results and the patient having at least 1 year of good seizure outcome. ASL results were concordant with this final surgically proven EZ in 10/11 cases (sensitivity 91%, specificity 50%). All 10 ASL-positive patients who underwent surgery had positive surgical pathology results and good long-term postsurgical seizure outcome at a mean follow-up of 39 months. Retrospective quantitative analysis based on significant z-score clusters found 1 true-positive result that was missed by qualitative analysis and 3 additional false-positive results (sensitivity 100%, specificity 23%).

CONCLUSIONS

ASL supports the hypothesis regarding the EZ in poorly defined focal epilepsy cases in children. Due to its convenience and noninvasive nature, the authors recommend that ASL be added routinely to the presurgical MRI evaluation of epilepsy. Future optimized quantitative methods may improve the diagnostic yield of this technique.

ExploreASL: An image processing pipeline for multi-center ASL perfusion MRI studies

Arterial spin labeling (ASL) has undergone significant development since its inception, with a focus on improving standardization and reproducibility of its acquisition and quantification. In a community-wide effort towards robust and reproducible clinical ASL image processing, we developed the software package ExploreASL, allowing standardized analyses across centers and scanners. The procedures used in ExploreASL capitalize on published image processing advancements and address the challenges of multi-center datasets with scanner-specific processing and artifact reduction to limit patient exclusion. ExploreASL is self-contained, written in MATLAB and based on Statistical Parameter Mapping (SPM) and runs on multiple operating systems. To facilitate collaboration and data-exchange, the toolbox follows several standards and recommendations for data structure, provenance, and best analysis practice. ExploreASL was iteratively refined and tested in the analysis of >10,000 ASL scans using different pulse-sequences in a variety of clinical populations, resulting in four processing modules: Import, Structural, ASL, and Population that perform tasks, respectively, for data curation, structural and ASL image processing and quality control, and finally preparing the results for statistical analyses on both single-subject and group level. We illustrate ExploreASL processing results from three cohorts: perinatally HIV-infected children, healthy adults, and elderly at risk for neurodegenerative disease. We show the reproducibility for each cohort when processed at different centers with different operating systems and MATLAB versions, and its effects on the quantification of gray matter cerebral blood flow. ExploreASL facilitates the standardization of image processing and quality control, allowing the pooling of cohorts which may increase statistical power and discover between-group perfusion differences. Ultimately, this workflow may advance ASL for wider adoption in clinical studies, trials, and practice.

MRI perfusion analysis using freeware, standard imaging software

Background

Perfusion-weighted imaging is only scarcely used in veterinary medicine. The exact reasons are unclear. One reason might be the typically high costs of the software packages for image analysis. In addition, a great variability concerning available programs makes it hard to compare results between different studies. Moreover, these algorithms are tuned for their usage in human medicine and often difficult to adapt to veterinary studies.

In order to address these issues, our aim is to deliver a free open source package for calculating quantitative perfusion parameters. We develop an “R package” calculating mean transit time, cerebral blood flow and cerebral blood volume from data obtained with freely imaging software (OsiriX Light®). We hope that the free availability, in combination with the fact that the underlying algorithm is open and adaptable, makes it easier for scientists in veterinary medicine to use, compare and adapt perfusion-weighted imaging analysis.

In order to demonstrate the usage of our software package, we reviewed previously acquired perfusion-weighted images from a group of eight purpose-breed healthy beagle dogs and twelve client-owned dogs with idiopathic epilepsy. In order to obtain the data needed for our algorithm, the following steps were performed: First, regions of interest (ROI) were drawn around different, previously reported, brain regions and the middle cerebral artery. Second, a ROI enhancement curve was generated for each ROI using a freely available PlugIn. Third, the signal intensity curves were exported as a comma-separated-value file. These files constitute the input to our software package, which then calculates the PWI parameters.

Results

We used our software package to re-assess perfusion weighted images from two previous studies. The clinical results were similar, showing a significant increase in the mean transit time and a significant decrease in cerebral blood flow for diseased dogs.

Conclusion

We provide an “R package” for computing the main perfusion parameters from measurements taken with standard imaging software and describe in detail how to obtain these measurements. We hope that our contribution enables users in veterinary medicine to easily obtain perfusion parameters using standard Open Source software in a standard, adaptable and comparable way.

A neuroradiologist's guide to arterial spin labeling MRI in clinical practice

Arterial spin labeling (ASL) is a non-invasive MRI technique to measure cerebral blood flow (CBF). This review provides a practical guide and overview of the clinical applications of ASL of the brain, as well its potential pitfalls. The technical and physiological background is also addressed. At present, main areas of interest are cerebrovascular disease, dementia and neuro-oncology. In cerebrovascular disease, ASL is of particular interest owing to its quantitative nature and its capability to determine cerebral arterial territories. In acute stroke, the source of the collateral blood supply in the penumbra may be visualised. In chronic cerebrovascular disease, the extent and severity of compromised cerebral perfusion can be visualised, which may be used to guide therapeutic or preventative intervention. ASL has potential for the detection and follow-up of arteriovenous malformations. In the workup of dementia patients, ASL is proposed as a diagnostic alternative to PET. It can easily be added to the routinely performed structural MRI examination. In patients with established Alzheimer’s disease and frontotemporal dementia, hypoperfusion patterns are seen that are similar to hypometabolism patterns seen with PET. Studies on ASL in brain tumour imaging indicate a high correlation between areas of increased CBF as measured with ASL and increased cerebral blood volume as measured with dynamic susceptibility contrast-enhanced perfusion imaging. Major advantages of ASL for brain tumour imaging are the fact that CBF measurements are not influenced by breakdown of the blood–brain barrier, as well as its quantitative nature, facilitating multicentre and longitudinal studies.

pASL versus DSC perfusion MRI in lateralizing temporal lobe epilepsy

BACKGROUND

Accurate lateralization of the epileptogenic focus in temporal lobe epilepsy (TLE) is crucial. Pulsed arterial spin labeling (pASL) has the capability of quantifying local relative cerebral blood flow (rCBF) by measuring the inflow of electromagnetically labeled arterial blood into the target area, and can be used in the presurgical workup of refractory TLE.

PURPOSE

To evaluate pASL in detecting mesial temporal lobe (mTL) perfusion asymmetry for the lateralization of the epileptogenic focus in patients with refractory TLE and to compare it with dynamic susceptibility contrast enhanced (DSC) magnetic resonance imaging (MRI) technique.

MATERIAL AND METHODS

This study was approved by the local ethical committee, and written informed consent was obtained in each patient. Thirty-six patients with medically refractory TLE and 11 healthy volunteer was enrolled in this study. Following brain MRI, pASL and DSC perfusion were performed in all subjects at 3T. rCBF measurements with two different perfusion MRI technique were compared between the patient and healthy volunteers. Lateralization based on perfusion asymmetry index (AI) were also evaluated and compared with clinical lateralization.

RESULTS

rCBF ratios measured in healthy volunteers by two different perfusion technique did not show any statistically significant difference. In TLE patients rCBF ratio of the ipsilateral (affected) side was found to be significantly lower than the contralateral (unaffected) side with both technique. The AI in the patient group was 8.86 ± 3.88 with pASL and 8.39 ± 4.06 with DSC. Correlation coefficient between clinical laterality and perfusion AI were 0.86 for pASL and 0.83 for DSC.

CONCLUSION

pASL can successfully detect interictal asymmetry in patients with TLE and can readily be combined with routine structural assessment for lateralization, providing an alternative to DSC perfusion.

Application of dynamic susceptibility contrast-enhanced perfusion in temporal lobe epilepsy

BACKGROUND

Accurately locating the epileptogenic focus in temporal lobe epilepsy (TLE) is important in clinical practice. Single-photon emission computed tomography (SPECT) and positron-emission tomography (PET) have been widely used in the lateralization of TLE, but both have limitations. Magnetic resonance perfusion imaging can accurately and reliably reflect differences in cerebral blood flow and volume.

PURPOSE

To investigate the diagnostic value of dynamic susceptibility contrast-enhanced (DSC) perfusion magnetic resonance imaging (MRI) in the lateralization of the epileptogenic focus in TLE.

MATERIAL AND METHODS

Conventional MRI and DSC-MRI scanning was performed in 20 interictal cases of TLE and 20 healthy volunteers. The relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) of the bilateral mesial temporal lobes of the TLE cases and healthy control groups were calculated. The differences in the perfusion asymmetry indices (AIs), derived from the rCBV and rCBF of the bilateral mesial temporal lobes, were compared between the two groups.

RESULTS

In the control group, there were no statistically significant differences between the left and right sides in terms of rCBV (left 1.55 ± 0.32, right 1.57 ± 0.28) or rCBF (left 99.00 ± 24.61, right 100.38 ± 23.46) of the bilateral mesial temporal lobes. However, in the case group the ipsilateral rCBV and rCBF values (1.75 ± 0.64 and 96.35 ± 22.63, respectively) were markedly lower than those of the contralateral side (2.01 ± 0.79 and 108.56 ± 26.92; P < 0.05). Both the AI of the rCBV (AI(rCBV); 13.03 ± 10.33) and the AI of the rCBF (AI(rCBF); 11.24 ± 8.70) of the case group were significantly higher than that of the control group (AI(rCBV) 5.55 ± 3.74, AI(rCBF) 5.12 ± 3.48; P < 0.05). The epileptogenic foci of nine patients were correctly lateralized using the 95th percentile of the AI(rCBV) and AI(rCBF) of the control group as the normal upper limits.

CONCLUSION

In patients with TLE interictal, both rCBV and rCBF of the ipsilateral mesial temporal lobe were markedly lower than that of healthy control subjects. DSC-MRI can provide lateralization for TLE.

Cerebral blood flow quantification in the rat: a direct comparison of arterial spin labeling MRI with radioactive microsphere PET

Background

Arterial spin labeling magnetic resonance imaging (ASL-MRI) has been recognised as a valuable method for non-invasive assessment of cerebral blood flow but validation studies regarding quantification accuracy by comparison against an accepted gold standard are scarce, especially in small animals. We have conducted the present study with the aim of comparing ASL flow-sensitive alternating inversion recovery (FAIR)-derived unidirectional water uptake (K₁) and ⁶⁸Ga/⁶⁴Cu microsphere (MS)-derived blood flow (f) in the rat brain.

Methods

In 15 animals, K₁and f were determined successively in dedicated small animal positron emission tomography and MR scanners. The Renkin-Crone model modified by a scaling factor was used for the quantification of f and K₁.

Results

Below about 1 mL/min/mL, we obtain an approximately linear relationship between f and K₁. At higher flow values, the limited permeability of water at the blood brain barrier becomes apparent. Within the accessed dynamic flow range (0.2 to 1.9 mL/min/mL), the data are adequately described by the Renkin-Crone model yielding a permeability surface area product of (1.53±0.46) mL/min/mL.

Conclusion

The ASL-FAIR technique is suitable for absolute blood flow quantification in the rat brain when using a one-compartment model including a suitable extraction correction for data evaluation.

Trial registration

24-9168.21-4/2004-1 (registered in Freistadt Sachsen, Landesdirektion Dresden)

Novel MRI approaches for assessing cerebral hemodynamics in ischemic cerebrovascular disease

Changes in cerebral hemodynamics underlie a broad spectrum of ischemic cerebrovascular disorders. An ability to accurately and quantitatively measure hemodynamic (cerebral blood flow and cerebral blood volume) and related metabolic (cerebral metabolic rate of oxygen) parameters is important for understanding healthy brain function and comparative dysfunction in ischemia. Although positron emission tomography, single-photon emission tomography, and gadolinium-MRI approaches are common, more recently MRI approaches that do not require exogenous contrast have been introduced with variable sensitivity for hemodynamic parameters. The ability to obtain hemodynamic measurements with these new approaches is particularly appealing in clinical and research scenarios in which follow-up and longitudinal studies are necessary. The purpose of this review is to outline current state-of-the-art MRI methods for measuring cerebral blood flow, cerebral blood volume, and cerebral metabolic rate of oxygen and provide practical tips to avoid imaging pitfalls. MRI studies of cerebrovascular disease performed without exogenous contrast are synopsized in the context of clinical relevance and methodological strengths and limitations.

Magnetic resonance imaging of the retina: A brief historical and future perspective

This invited review starts with a brief introduction of retinal anatomy and magnetic resonance imaging techniques with contrast to optics, followed by a history and future perspective on MRI applications to investigate the retinas of rodents, non-human primates and humans.

Hemodynamic studies of intracranial dural arteriovenous fistulas using arterial spin-labeling MR imaging

Arterial spin-labeling (ASL) magnetic resonance imaging (MRI) enables non-invasive acquisition of the brain perfusion information in cerebrovascular disease. We investigated hemodynamic changes in intracranial dural arteriovenous fistulas (DAVFs) using ASL-MRI. ASL-MRI by a Q2TIPS sequence on a 3.0-Tesla MRI was performed for three patients with Cognard's IIa+b type of DAVFs before and after treatment. Perfusion images obtained by ASL-MRI (ASL images) before treatment were visually compared with those by single-photon emission computed tomography images (SPECT images). Increasing rates of temporal changes of regional perfusion values in ASL images (ASL values) before and after treatment were also calculated. In all three patients, ASL images before treatment demonstrated high perfusion in regions around the shunting areas, where normal or low perfusion were detected on SPECT images; thus, ASL images might have demonstrated the abundant arterial shunting flow via the fistulas. On days eight to 20 after treatment, ASL values around the shunt areas remained the same or decreased, and those in the regions other than the shunt areas increased in all three patients. This might have been due to a combination of the following: a decrease in shunt flow volume, an amelioration of venous congestion, and a sustained an upward shift in the autoregulation of the brain perfusion pressure. All regional ASL values decreased on days 112 and 120 after treatment in two patients, which possibly reflects a reduction in the upward shift in autoregulation. ASL-MRI might be useful for identifying the hemodynamic behavior of DAVFs before and after treatment.

Arterial spin labeling in neuroimaging

Arterial spin labeling (ASL) is a magnetic resonance imaging technique for measuring tissue perfusion using a freely diffusible intrinsic tracer. As compared with other perfusion techniques, ASL offers several advantages and is now available for routine clinical practice in many institutions. Its noninvasive nature and ability to quantitatively measure tissue perfusion make ASL ideal for research and clinical studies. Recent technical advances have increased its sensitivity and also extended its potential applications. This review focuses on some basic knowledge of ASL perfusion, emerging techniques and clinical applications in neuroimaging.

Cerebral blood flow measurement using fMRI and PET: a cross-validation study

An important aspect of functional magnetic resonance imaging (fMRI) is the study of brain hemodynamics, and MR arterial spin labeling (ASL) perfusion imaging has gained wide acceptance as a robust and noninvasive technique. However, the cerebral blood flow (CBF) measurements obtained with ASL fMRI have not been fully validated, particularly during global CBF modulations. We present a comparison of cerebral blood flow changes (ΔCBF) measured using a flow-sensitive alternating inversion recovery (FAIR) ASL perfusion method to those obtained using H₂¹⁵O PET, which is the current gold standard for in vivo imaging of CBF. To study regional and global CBF changes, a group of 10 healthy volunteers were imaged under identical experimental conditions during presentation of 5 levels of visual stimulation and one level of hypercapnia. The CBF changes were compared using 3 types of region-of-interest (ROI) masks. FAIR measurements of CBF changes were found to be slightly lower than those measured with PET (average ΔCBF of 21.5 ± 8.2% for FAIR versus 28.2 ± 12.8% for PET at maximum stimulation intensity). Nonetheless, there was a strong correlation between measurements of the two modalities. Finally, a t-test comparison of the slopes of the linear fits of PET versus ASL ΔCBF for all 3 ROI types indicated no significant difference from unity (P > .05).

Magnetic resonance imaging of the retina

This paper reviews recent developments in high-resolution magnetic resonance imaging (MRI) and its application to image anatomy, physiology, and function in the retina of animals. It describes technical issues and solutions in performing retinal MRI, anatomical MRI, blood oxygenation level-dependent functional MRI (fMRI), and blood-flow MRI both of normal retinas and of retinal degeneration. MRI offers unique advantages over existing retinal imaging techniques, including the ability to image multiple layers without depth limitation and to provide multiple clinically relevant data in a single setting. Retinal MRI has the potential to complement existing retinal imaging techniques.

Current trends in intraoperative optical imaging for functional brain mapping and delineation of lesions of language cortex

Resection of a cerebral arteriovenous malformation (AVM), epileptic focus, or glioma, ideally has a prerequisite of microscopic delineation of the lesion borders in relation to the normal gray and white matter that mediate critical functions. Currently, Wada testing and functional magnetic resonance imaging (fMRI) are used for preoperative mapping of critical function, whereas electrical stimulation mapping (ESM) is used for intraoperative mapping. For lesion delineation, MRI and positron emission tomography (PET) are used preoperatively, whereas microscopy and histological sectioning are used intraoperatively. However, for lesions near eloquent cortex, these imaging techniques may lack sufficient resolution to define the relationship between the lesion and language function, and thus not accurately determine which patients will benefit from neurosurgical resection of the lesion without iatrogenic aphasia. Optical techniques such as intraoperative optical imaging of intrinsic signals (iOIS) show great promise for the precise functional mapping of cortices, as well as delineation of the borders of AVMs, epileptic foci, and gliomas. Here we first review the physiology of neuroimaging, and then progress towards the validation and justification of using intraoperative optical techniques, especially in relation to neurosurgical planning of resection AVMs, epileptic foci, and gliomas near or in eloquent cortex. We conclude with a short description of potential novel intraoperative optical techniques.

Measurement of deep gray matter perfusion using a segmented true-fast imaging with steady-state precession (True-FISP) arterial spin-labeling (ASL) method at 3T

PURPOSE

To study the feasibility of using the MRI technique of segmented true-fast imaging with steady-state precession arterial spin-labeling (True-FISP ASL) for the noninvasive measurement and quantification of local perfusion in cerebral deep gray matter at 3T.

MATERIALS AND METHODS

A flow-sensitive alternating inversion-recovery (FAIR) ASL perfusion preparation was used in which the echo-planar imaging (EPI) readout was replaced with a segmented True-FISP data acquisition strategy. The absolute perfusion for six selected regions of deep gray matter (left and right thalamus, putamen, and caudate) were calculated in 11 healthy human subjects (six male, five female; mean age = 35.5 years +/- 9.9).

RESULTS

Preliminary measurements of the average absolute perfusion values at the six selected regions of deep gray matter are in agreement with published values for mean absolute cerebral blood flow (CBF) baselines acquired from healthy volunteers using positron emission tomography (PET).

CONCLUSION

Segmented True-FISP ASL is a practical and quantitative technique suitable to measure local tissue perfusion in cerebral deep gray matter at a high spatial resolution without the susceptibility artifacts commonly associated with EPI-based methods of ASL.

Usefulness of pulsed arterial spin labeling MR imaging in mesial temporal lobe epilepsy

PURPOSE

Arterial spin labeling (ASL) is a developing magnetic resonance imaging (MRI) method for noninvasive measurement of cerebral blood flow (CBF). The purpose of this study was to evaluate the usefulness of ASL for detecting interictal temporal hypoperfusion in temporal lobe epilepsy (TLE). ASL-derived CBF measurements were compared with those derived from H(2)(15)O positron emission tomography (PET).

METHODS

11 normal controls and 10 patients with medically intractable TLE were studied. Pulsed ASL (PASL) with quantitative imaging of perfusion using a single subtraction, second version (QUIPSS II) was performed in all subjects and H(2)(15)O PET was performed in patients. Regional CBF values in the mesial and lateral temporal lobes were measured utilizing quantitative analysis of perfusion images. A perfusion asymmetry index (AI) was calculated for each region.

RESULTS

In patients, mean CBF in the mesial temporal lobe was not significantly different between PASL and H(2)(15)O PET, and ipsilateral mesial temporal CBF was lower than contralateral CBF with both techniques. PASL detected significant mesial temporal perfusion asymmetry agreeing with EEG laterality in four patients. H(2)(15)O PET found ipsilateral interictal hypoperfusion in three. Both scans found unilateral hypoperfusion in one patient with bilateral EEG discharges.

CONCLUSIONS

Pulsed ASL may be a promising approach to detecting interictal hypoperfusion in TLE. This method has potential as a clinical alternative to H(2)(15)O PET due to noninvasiveness and easy accessibility.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

Pulsed-arterial-spin-labeling perfusion 3T MRI following single seizure: a first case report study

A 41-year-old woman with focal epilepsy was analyzed immediately and 1 week post-ictally using pulsed-arterial-spin-labeling perfusion 3T MRI, a non-invasive method. Seizure-associated blood flow alterations were characterized by increased ictal perfusion in the occipito-parietal right cortex, and hypoperfusion in the same location 1 week later. These results indicate that non-invasive perfusion MRI can be used routinely to detect vascular alterations in epilepsy, and possibly to identify the location of the seizure focus.

Perfusion imaging of brain tumors using arterial spin-labeling: correlation with histopathologic vascular density

BACKGROUND AND PURPOSE

We investigated the relationship between tumor blood-flow measurement based on perfusion imaging by arterial spin-labeling (ASL-PI) and histopathologic findings in brain tumors.

MATERIALS AND METHODS

We used ASL-PI to examine 35 patients with brain tumors, including 11 gliomas, 9 meningiomas, 9 schwannomas, 1 diffuse large B-cell lymphoma, 4 hemangioblastomas, and 1 metastatic brain tumor. As an index of tumor perfusion, the relative signal intensity (SI) of each tumor (%Signal intensity) was determined as a percentage of the maximal SI within the tumor per averaged SI within normal cerebral gray matter on ASL-PI. Relative vascular attenuation (%Vessel) was determined as the total microvessel area per the entire tissue area on CD-34-immunostained histopathologic specimens. MIB1 indices of gliomas were also calculated. The differences in %Signal intensity among different histopathologic types and between high- and low-grade gliomas were compared. In addition, the correlations between %Signal intensity and %Vessel or MIB1 index were evaluated in gliomas.

RESULTS

Statistically significant differences in %Signal intensity were observed between hemangioblastomas versus gliomas (P < .005), meningiomas (P < .05), and schwannomas (P < .005). Among gliomas, %Signal intensity was significantly higher for high-grade than for low-grade tumors (P < .05). Correlation analyses revealed significant positive correlations between %Signal intensity and %Vessel in 35 patients, including all 6 histopathologic types (rs = 0.782, P < .00005) and in gliomas (rs = 0.773, P < .05). In addition, in gliomas, %Signal intensity and MIB1 index were significantly positively correlated (rs = 0.700, P < .05).

CONCLUSION

ASL-PI may predict histopathologic vascular densities of brain tumors and may be useful in distinguishing between high- and low-grade gliomas and in differentiating hemangioblastomas from other brain tumors.

Functional imaging in temporal lobe epilepsy

This article highlights the importance of functional imaging methods in the diagnosis and preoperative planning in temporal lobe epilepsy TLE). It starts with a discussion of the role of ictal and interictal single-photon emission computed tomography (SPECT) and interictal positron emission tomography (PET) in lateralizing TLE. The next section discusses the role of blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) and perfusion MRI, particularly for lateralization of language and memory in TLE patients. The final section explores the relatively new role that the emerging technique of diffusion tensor imaging (DTI) is playing in the evaluation of TLE.

Effect of cerebrovascular risk factors on regional cerebral blood flow

PURPOSE

To prospectively investigate which cerebrovascular risk factors are related to regional cerebral blood flow (rCBF), as measured noninvasively with arterial spin-labeling (ASL) magnetic resonance (MR) imaging, in a large group of patients with symptomatic atherosclerotic disease.

MATERIALS AND METHODS

Ethics committee approval and informed consent were obtained. One hundred thirty consecutive patients (107 men, 23 women; mean age, 58 years +/- 10 [standard deviation]) with symptomatic atherosclerotic disease were included in the study. Cerebrovascular risk factors (body mass index, carotid artery stenosis, diabetes mellitus, hyperhomocysteinemia, hyperlipidemia, hypertension, and smoking) were assessed by means of a questionnaire and physical, ultrasonographic, and laboratory examinations. The control group consisted of 10 subjects (eight men, two women; mean age, 58 years +/- 15) without symptomatic atherosclerotic disease. rCBF measurements were performed with ASL MR imaging. The effects of the individual cerebrovascular risk factors on the rCBF were assessed by using linear regression analysis.

RESULTS

Hypertension was significantly associated with higher rCBF (adjusted beta = 6.5 mL/min/100 g; 95% confidence interval: 1.4 mL/min/100 g, 11.7 mL/min/100 g). Hyperhomocysteinemia was significantly related to lower rCBF (adjusted beta = -7.4 mL/min/100 g; 95% confidence interval: -12.7 mL/min/100 g, -2.1 mL/min/100 g). No significant associations between rCBF and the other cerebrovascular risk factors were found.

CONCLUSION

In patients with symptomatic atherosclerotic disease, hypertension is related to higher rCBF and hyperhomocysteinemia is related to lower rCBF.

Clinical neuroimaging using arterial spin-labeled perfusion magnetic resonance imaging

The two most common methods for measuring perfusion with MRI are based on dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL). Although clinical experience to date is much more extensive with DSC perfusion MRI, ASL methods offer several advantages. The primary advantages are that completely noninvasive absolute cerebral blood flow (CBF) measurements are possible with relative insensitivity to permeability, and that multiple repeated measurements can be obtained to evaluate one or more interventions or to perform perfusion-based functional MRI. ASL perfusion and perfusion-based functional MRI methods have been applied in many clinical settings, including acute and chronic cerebrovascular disease, CNS neoplasms, epilepsy, aging and development, neurodegenerative disorders, and neuropsychiatric diseases. Recent technical advances have improved the sensitivity of ASL perfusion MRI, and increasing use is expected in the coming years. The present review focuses on ASL perfusion MRI and applications in clinical neuroimaging.

Contrast-enhanced perfusion and diffusion MRI accurately lateralize temporal lobe epilepsy: a pilot study

AIMS

To undertake a pilot study to assess whether magnetic resonance (MR) contrast-enhanced perfusion imaging (CEPI) and diffusion-weighted imaging (DWI) provide lateralizing information in medically refractory temporal lobe epilepsy (TLE),and to compare this to standard quantitative hippocampal assessments (volumetric measurements and T2 relaxometry).

METHODS

Ten patients with 'non-lesional' TLE and 10 control subjects were studied. Quantification of the relative cerebral blood flow (rCBF) and apparent diffusion coefficient (ADC) was performed for the hippocampal regions. The ratios of the ipsilateral-to-contralateral side (to the EEG lateralization) were compared with the side-to-side ratios in the controls.

RESULTS

Six patients (60%) had an ADC ratio outside the control range (the larger ADC ipsilateral to the EEG lateralization in all cases). The CBF ratios were outside the control range in all eight patients (100%) in whom CEPI was performed (the lower value ipsilateral to the EEG lateralization in all cases). The magnitude of the hippocampal volume (HV) ratios showed no significant correlation with the magnitude of the ADC ratios (R=-0.03, p=0.93) or CBF ratios (R=0.36, p=0.39). There was a closer relationship with the T2 relaxometry ratios, but this was also not significant (R=-0.40, p=0.32; R=0.58, p=0.08).

CONCLUSIONS

DWI and CEPI show potential as reliable tools for the lateralization of non-lesional TLE. Further studies with larger numbers are necessary to determine whether these techniques provide independent data to established MR quantitative measures.

Measurement of cerebral perfusion with arterial spin labeling: Part 2. Applications

Arterial spin labeling (ASL) uses magnetic resonance imaging methods to measure cerebral blood flow (CBF) non-invasively. ASL CBF validly localizes brain function and may be especially useful for studies where the time frame of behavioral change is more than a few minutes, such as in longitudinal and treatment studies. ASL measures of cerebral perfusion are highly accurate in detecting lesion laterality in temporal lobe epilepsy, stenotic-occlusive disease, and brain tumors. Among lesioned patients, ASL CBF has excellent concurrent validity when correlated with CBF measured by Positron Emission Tomography or with dynamic susceptibility-weighted magnetic resonance. ASL CBF can predict tumor grading in vivo and can predict six-month response to the surgical treatment of brain tumors. ASL's capability to selectively and non-invasively tag flow in major vessels may refine the monitoring of treatment of cerebrovascular disease and brain tumors. Conclusions about the utility of ASL are limited by the small sample sizes of the studies currently in the literature and by the uncertainty caused by the effect of brain disease on transit times of the magnetic tag. As the method evolves, ASL techniques will likely become more widely used in clinical research and practice.

FAIR-TrueFISP imaging of cerebral perfusion in areas of high magnetic susceptibility differences at 1.5 and 3 Tesla

PURPOSE

To estimate cerebral blood perfusion in areas of strong magnetic susceptibility changes with high spatial and temporal resolution using a flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) method.

MATERIALS AND METHODS

We implemented an ASL method that is capable of imaging perfusion in areas of high magnetic susceptibility changes by combining a FAIR spin preparation with a true fast imaging in steady precession (TrueFISP) data acquisition strategy. A TrueFISP readout sequence was applied especially in regions with magnetic field inhomogeneities and compared with corresponding FAIR measurements obtained with a standard echo-planar imaging (EPI) readout. Quantitative perfusion images were obtained at 1.5 Tesla (1.5T) from eight healthy volunteers (24-42 years old) and one patient (23 years old). FAIR-TrueFISP perfusion images were compared with FAIR echo-planar images. T1 maps, which are necessary for quantitative perfusion estimation, were obtained with inversion recovery (IR) TrueFISP and IR EPI. Additionally, high-resolution perfusion measurements were performed on four volunteers at 3T.

RESULTS

The two ASL perfusion imaging modalities yielded comparable diagnostic image quality in brain areas with low susceptibility differences at 1.5T. Cerebral perfusion of gray matter (GM) areas was 105.7 +/- 5.2 mL/100 g/minute for FAIR-TrueFISP and 88.8 +/- 14.6 mL/100 g/minute for FAIR-EPI at 1.5T, and 70.4 +/- 7.1 mL/100 g/minute for FAIR-TrueFISP and 63.5 +/- 6.9 mL/100 g/minute for FAIR-EPI at 3.0T. Higher perfusion values at 1.5T were due to more pronounced partial-volume effects from fast moving spins at lower spatial resolution. The FAIR-TrueFISP sequence showed no significant distortions and markedly reduced signal void artifacts in areas of high susceptibility changes (e.g., near brain-bone transitions and close to metallic clips) compared to FAIR-EPI. At 3T, highly resolved FAIR-TrueFISP perfusion images were acquired with an in-plane resolution of up to 1.3 mm.

CONCLUSION

FAIR-TrueFISP allows for assessment of cerebral perfusion in areas of critically high susceptibility changes with conventional ASL methods.

Temporal lobe perfusion in the deaf: MR measurement with pulsed arterial spin labeling (FAIR)

RATIONALE AND OBJECTIVES

Experimental studies in animals have shown that loss of a primary sensory modality early in life may result in substantial alterations in cortical organization. This study was performed to measure cerebral perfusion in auditory cortex in congenitally deaf adults using the FAIR (Flow-sensitive Alternating Inversion Recovery) magnetic resonance imaging technique. Our hypothesis was that there would be relatively intact perfusion in auditory cortex.

MATERIALS AND METHODS

Twenty-six profoundly congenitally deaf subjects were compared with 15 control subjects. A FAIR perfusion slice was scanned through the superior temporal gyrus parallel to the Sylvian fissure while subjects were at rest. Perfusion maps were calculated and regions of interest were drawn over the superior temporal gyrus including auditory cortex and the medial occipital lobe.

RESULTS

The relative perfusion of the superior temporal gyrus (STG) was slightly less in the deaf (right STG = 0 .79 +/- 0.16, left = 0.93 +/- 0.29) compared with the hearing (right STG = 0.90 +/- 0.14, left = 0.98 +/- 0.31) when normalized to the occipital cortex, but the differences were not statistically significant. Both showed moderate left lateralization; however, only in the deaf did this reach statistical significance (P < .01).

CONCLUSIONS

In the resting state, the deaf demonstrate a relatively normal perfusion in the region of cortex usually associated with auditory function. Although the presumed underlying electrical activity may represent some degree of residual auditory function, it is likely that the normal level of perfusion reflects cortical reorganization and the early migration of nonauditory processing into this area.

Pulsed arterial spin labeling parameter optimization for an elderly population

PURPOSE

To optimize pulsed arterial spin labeling (PASL) parameters for the elderly to take into account possible perfusion changes that occur in the brain with age.

MATERIALS AND METHODS

Healthy young (N = 14, age range = 21-27 years) and elderly (N = 12, age range = 61-67 years) subjects were scanned using Q2TIPS (QUIPSS II with thin-slice TI, periodic saturation) with varying inversion times (TI(2)) at 1.5T. The difference signal (DeltaM), transit time (deltat), and cerebral blood flow (CBF) were calculated in segmented gray matter (GM).

RESULTS

The young displayed more perfusion-weighted signal difference than the elderly at all TI(2)'s. The peak DeltaM occurred at TI(2) approximately 1300 msec and 1500 msec in the young and elderly groups, respectively. Qualitatively, intravascular signal was minimal in the younger group by TI(2) = 1500 msec, whereas a longer TI(2) of 1800 msec was needed to minimize this signal in the elderly. The transit time was approximately 100 msec longer in the elderly, and CBF was in the range of literature values.

CONCLUSION

For acquiring perfusion-weighted images with minimal intravascular signal and adequate tissue signal for PASL studies of cerebral perfusion in the elderly, a longer inversion time is advantageous.

Brain Perfusion Following Single Seizures

PURPOSE

The aim of this study was to assess the regional relative interictal and postictal perfusion changes in temporal and parietal lobe epilepsy.

METHODS

We investigated interictal and postictal magnet resonance perfusion changes in five patients with temporal lobe epilepsy either with hippocampal sclerosis (n = 3) or without (n = 2), and in one patient with extratemporal (parietal lobe) epilepsy. T(2)*-weighted single-shot echo-planar images were acquired after bolus application of 0.2 mmol/kg gadolinium-diethylene triamine pentaacetic acid (GD-DTPA) at baseline and after intervals of 2-12 min, 15-23 min, 28-50 min, 63-72 min, and 180-240 min. The bolus-peak ratio was calculated in regions of interest in the hippocampus (HIP), parahippocampal gyrus (PHG), thalamus (THA), cortex (COR), and white matter (WM), yielding relative perfusion changes.

RESULTS

Interictally, we found relative hyperperfusion of the ictogenic side in five of six patients in the HIP. Postictally, the perfusion decreased in the HIP by 25-39% as compared to baseline, whereas the PHG showed a reverse pattern. In the late postictal phase, perfusion increased in the HIP again and decreased in the PHG. In the THA, the inter- and postictal changes were small (5-19%). COR and WM showed equivocal results.

CONCLUSION

Postictal relative hypoperfusion in the HIP appears to be associated with the cessation of neuronal ictal discharge, whereas postictal hyperperfusion in the PHG lags behind and may reflect increased metabolism to restore the interictal state of neuronal excitability.

Functional magnetic resonance imaging in the treatment of epilepsy

Although structural magnetic resonance imaging (MRI) is now in routine use in the evaluation and management of epilepsy, functional MRI (fMRI) has recently begun to provide a noninvasive and widely available modality for assessing regional brain function. fMRI studies of language and memory are able to show discrete areas of activation in cerebral cortex, are useful in lateralizing language and memory during presurgical evaluation, and are providing further insight into the processes underlying cerebral plasticity in the brains of epilepsy patients. The use of fMRI for localization of ictal phenomena may also contribute to the localization of seizure foci and to a better understanding of the pathophysiology of electrographic spikes. The combination of fMRI with electroencephalogram and other advanced structural imaging techniques may not only improve seizure localization, but may also contribute valuable information towards a better understanding of the pathophysiology of epilepsy and its consequences on brain development.

In vivo functional NMR imaging of resistance artery control

Arterial spin labeling (ASL) in combination with NMR imaging is an in vivo technique that quantifies tissue perfusion in absolute values (ml blood x min(-1) x g tissue(-1)) with high temporal (1-10 s) and spatial (0.1-3 mm) resolution. It uses the arterial water spins as endogenous freely diffusible markers of perfusion and, hence, is a totally noninvasive method. The technique has been successfully applied to quantify baseline perfusion in many organs, including the heart, in humans and animals, and results were validated by comparison with gold standards, PET and microspheres, respectively. Because of the high sampling rate of perfusion with ASL and the possibility that measurements could be obtained without harm over indefinite periods of time, the technique has the potential for use in functional investigations of microcirculation regulation and resistance artery control in vivo. We describe examples of the use of ASL to this end. With use of specific technological developments, ASL determination of perfusion can be coupled with simultaneous acquisitions of (1)H and (31)P NMR spectroscopy data. These protocols offer new possibilities whereby the microcirculatory control of cell oxygenation and high-energy phosphate metabolism can be explored.

Internal Carotid Artery Occlusion Assessed at Pulsed Arterial Spin-labeling Perfusion MR Imaging at Multiple Delay Times

Magnetic resonance (MR) imaging with pulsed arterial spin labeling (ASL) was performed at six different inversion times in nine patients with internal carotid artery (ICA) occlusion and in 11 control subjects. The hospital's commission on scientific research on human subjects approved the study protocol, and all study subjects gave informed consent. Cerebral blood flow (CBF) in the middle cerebral artery territories was calculated from the combined signal intensities measured with ASL at the multiple inversion times. In the patients with ICA occlusion, mean CBF values were decreased in the gray matter of the hemisphere ipsilateral to the occlusion, as compared with values in the gray matter of the contralateral hemisphere (P < .05) and with values in the gray matter of the control subjects (P < .05). Quantification of CBF with ASL at multiple inversion times can compensate for the blood transit delays in patients with ICA occlusion.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

CBF changes during brain activation: fMRI vs. PET

The changes in regional cerebral blood flow (rCBF) associated with the changes in neuronal activity are routinely measured both by positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) techniques. However, direct comparison has not been performed to determine similarities and differences of PET and fMRI techniques in determining the rCBF response to brain activation. In the present study, a quantitative comparison of the functional rCBF maps obtained by PET and fMRI are made by performing an activation study in a single group of subjects under precisely controlled conditions and using identical visual stimuli. Twelve healthy volunteers participated in the activation study using the visual checkerboard stimulation with flip frequency at 8 Hz. By selecting the conjunctive pixels which activated on both PET and fMRI maps, the change in rCBF measured by fMRI was 36.95 +/- 2.54%, whereas the value measured by PET was 38.79 +/- 2.63%. Our results have demonstrated that there is no statistically significant difference (P = 0.22) in the measurements of rCBF change between MRI and PET methods.

Assessment of Irradiated Brain Metastases by Means of Arterial Spin-Labeling and Dynamic Susceptibility-Weighted Contrast-Enhanced Perfusion MRI

RATIONALE AND OBJECTIVES

To assess if preradiation and early follow-up measurements of relative regional cerebral blood flow (rrCBF) can predict treatment outcome in patients with cerebral metastases and to evaluate rrCBF changes in tumor and normal tissue after stereotactic radiosurgery using arterial spin-labeling (ASL) and first-pass dynamic susceptibility-weighted contrast-enhanced (DSC) perfusion MRI.

METHODS

In 25 patients with a total of 28 brain metastases, DSC MRI and ASL perfusion MRI using the Q2TIPS sequence were performed with a 1.5-T unit. Measurements were performed prior to and at 6 weeks, 12 weeks, and 24 weeks after stereotactic radiosurgery. Follow-up examinations were completely available in 25 patients for Q2TIPS and 17 patients with 18 metastases for DSC MRI. The rrCBF of the metastases and the normal brain tissue was determined by a region-of-interest analysis. rrCBF values were correlated with the treatment outcome that was classified according to tumor volume changes at 6 months.

RESULTS

The alteration of the rrCBF at the 6-week follow-up was highly predictive for treatment outcome. A decrease of the rrCBF value predicted tumor response correctly in all metastases for Q2TIPS and in 13 of 16 metastases for DSC MRI. The pretherapeutic rrCBF was not able to predict treatment outcome. The rrCBF values in normal brain tissue affected by radiation doses less than 0.5 Gy remained unchanged after therapy.

CONCLUSION

These preliminary results suggest that ASL and DSC MRI techniques determining rrCBF changes in brain metastases after stereotactic radiosurgery allow the prediction of treatment outcome.

Magnetization transfer, HASTE, and FLAIR imaging

Continuous technologic developments and research have increased the clinical applications of MT, HASTE, and FLAIR imaging in neuroradiology. HASTE has become the MR imaging sequence of choice for fetal neuroimaging. Other promising uses, such as for diffusion-weighted imaging, have not been fully exploited. FLAIR has been firmly established as one of the cornerstones of brain imaging; however, post-contrast FLAIR images have not offered a clear advantage over standard T1-weighted images as suggested by early studies. FLAIR imaging with echoplanar acquisition is not considered advantageous, because the decreased imaging times are obtained at the expense of lower sensitivity. For a number of applications, diffusion-weighted imaging has surpassed FLAIR. Nevertheless, FLAIR images may be more sensitive for the detection of acute brain infarction. Recently described methods for the elimination of CSF flow artifacts may lead to improved quality and reliability of FLAIR images for subarachnoid space disease. MT preparation is now routinely incorporated in time-of-flight MR angiography and gradient-echo T2*-weighted spine imaging sequences and provides increased sensitivity for postcontrast MR imaging. These applications may not be advantageous in all clinical settings. MTR analysis offers valuable information for an increasing number of pathologic processes but has not yet gained wide clinical acceptance owing to sophisticated postprocessing and significant intercenter variations. Different modifications of these techniques are being evaluated, and further developments are expected.

Reduced susceptibility effects in perfusion fMRI with single-shot spin-echo EPI acquisitions at 1.5 tesla

Arterial spin labeling (ASL) perfusion contrast is not based on susceptibility effects and can therefore be used to study brain function in regions of high static inhomogeneity. As a proof of concept, single-shot spin-echo echo-planar imaging (EPI) acquisition was carried out with a multislice continuous ASL (CASL) method at 1.5T. A bilateral finger tapping paradigm was used in the presence of an exogenously induced susceptibility artifact over left motor cortex. The spin-echo CASL technique was compared with a regular gradient-echo EPI sequence with the same slice thickness, as well as other imaging methods using thin slices and spin-echo acquisitions. The results demonstrate improved functional sensitivity and efficiency of the spin-echo CASL approach as compared with gradient-echo EPI techniques, and a trend of improved sensitivity as compared with spin-echo EPI approach in the brain regions affected by the susceptibility artifact. ASL images, either with or without subtraction of the control, provide a robust alternative to blood oxygenation level dependant (BOLD) methods for activation imaging in regions of high static field inhomogeneity.

Independent cerebral vasoconstrictive effects of hyperoxia and accompanying arterial hypocapnia at 1 ATA

Breathing 100% O2 at 1 atmosphere absolute (ATA) is known to be associated with a decrease in cerebral blood flow (CBF). It is also accompanied by a fall in arterial Pco2 leading to uncertainty as to whether the cerebral vasoconstriction is totally or only in part caused by arterial hypocapnia. We tested the hypothesis that the increase in arterial Po2 while O2 was breathed at 1.0 ATA decreases CBF independently of a concurrent fall in arterial Pco2. CBF was measured in seven healthy men aged 21-62 yr by using noninvasive continuous arterial spin-labeled-perfusion MRI. The tracer in this technique, magnetically labeled protons in blood, has a half-life of seconds, allowing repetitive measurements over short time frames without contamination. CBF and arterial blood gases were measured while breathing air, 100% O2, and 4 and 6% CO2 in air and O2 backgrounds. Arterial Po2 increased from 91.7 +/- 6.8 Torr in air to 576.7 +/- 18.9 Torr in O2. Arterial Pco2 fell from 43.3 +/- 1.8 Torr in air to 40.2 +/- 3.3 Torr in O2. CBF-arterial Pco2 response curves for the air and hyperoxic runs were nearly parallel and separated by a distance representing a 28.7-32.6% decrement in CBF. Regression analysis confirmed the independent cerebral vasoconstrictive effect of increased arterial Po2. The present results also demonstrate that the magnitude of this effect at 1.0 ATA is greater than previously measured.

Comparison of Arterial Spin-Labeling Techniques and Dynamic Susceptibility-Weighted Contrast-Enhanced MRI in Perfusion Imaging of Normal Brain Tissue

OBJECTIVES

To evaluate relative cerebral blood flow (rCBF) in normal brain tissue using arterial spin-labeling (ASL) methods and first-pass dynamic susceptibility-weighted contrast-enhanced (DSC) magnetic resonance imaging (MRI).

METHODS

Sixty-two patients with brain metastases were examined on a 1.5 T-system up to 6 times during routine follow-up after stereotactic radiosurgery. Perfusion values in normal gray and white matter were measured using the ASL techniques ITS-FAIR in 38 patients, Q2TIPS in 62 patients, and the first-pass DSC echo-planar (EPI) MRI after bolus administration of gadopentetate dimeglumine in 42 patients. Precision of the ASL sequences was tested in follow-up examinations in 10 healthy volunteers.

RESULTS

Perfusion values in normal brain tissue obtained by all sequences correlated well by calculating Pearson's correlation coefficients (P < 0.0001) and remained unchanged after stereotactic radiosurgery as shown by analysis of variance (P > 0.05).

CONCLUSION

Both ASL and DSC EPI MRI yield highly comparable perfusion values in normal brain tissue.

Dynamic susceptibility contrast (DSC) MRI and interictal epileptiform activity in cryptogenic partial epilepsy

PURPOSE

To study the possible correlation between interictal EEG patterns and neuroradiologic data obtained by dynamic susceptibility contrast (DSC) magnetic resonance imaging (MRI) in patients with partial epilepsy.

METHODS

Seventeen subjects with cryptogenic partial epilepsy underwent long-term video-EEG monitoring and DSC-MRI in the same session. Ten patients had temporal lobe epilepsy (TLE) and seven, epilepsy of extratemporal origin (ExTE). MRI data were compared with EEG findings, and the accuracy of DSC-MRI was analyzed considering spiking rate (number of interictal epileptiform abnormalities, IEA/min) and type of epilepsy.

RESULTS

DSC-MRI showed a relevant asymmetry in the frontal, temporal, and occipital regions in eight (47%) of 17 patients, consisting of a relative regional cerebral blood volume (rCBV) increase in these areas. Because this region corresponded to the interictal EEG focus (IEF) or to the hemisphere involved in the genesis of epileptic discharges in most patients showing a higher spiking rate, patients were classified in two groups: patients with high spiking rate (HSR, n = 9) and with low spiking rate (LSR, n = 8); the cutoff corresponded to the median value of IEA/min. The rCBV increase corresponded to the IEF or to the hemisphere involved in the genesis of epileptic discharges in seven (77.7%) of nine HSR patients. No patients with LSR showed significant asymmetries in rCBV pattern. In five of six patients with TLE-HSR (83.3%), DSC-MRI showed a relative rCBV increase concordant with IEF or hemisphere involved in the genesis of epileptic discharges; in patients with ExTE-HSR, the concordance was 66%.

CONCLUSIONS

DSC-MRI is a noninvasive procedure that may provide useful additional information to lateralize and/or localize the IEF when interictal epileptiform activity is sufficiently elevated.

Comparison of quantitative perfusion imaging using arterial spin labeling at 1.5 and 4.0 Tesla

High-field arterial spin labeling (ASL) perfusion MRI is appealing because it provides not only increased signal-to-noise ratio (SNR), but also advantages in terms of labeling due to the increased relaxation time T(1) of labeled blood. In the present study, we provide a theoretical framework for the dependence of the ASL signal on the static field strength, followed by experimental validation in which a multislice pulsed ASL (PASL) technique was carried out at 4T and compared with PASL and continuous ASL (CASL) techniques at 1.5T, both in the resting state and during motor activation. The resting-state data showed an SNR ratio of 2.3:1.4:1 in the gray matter and a contrast-to-noise ratio (CNR) of 2.7:1.1:1 between the gray and white matter for the difference perfusion images acquired using 4T PASL, 1.5T CASL, and 1.5T PASL, respectively. However, the functional data acquired using 4T PASL did not show significantly improved sensitivity to motor cortex activation compared with the 1.5T functional data, with reduced fractional perfusion signal change and increased intersubject variability. Possible reasons for these experimental results, including susceptibility effects and physiological noise, are discussed.

Technical aspects and utility of fMRI using BOLD and ASL

Functional magnetic resonance imaging (fMRI) is an emerging methodology which provides various approaches to visualizing regional brain activity non-invasively. Although the exact mechanisms underlying the coupling between neural function and fMRI signal changes remain unclear, fMRI studies have been successful in confirming task-specific activation in a variety of brain regions, providing converging evidence for functional localization. In particular, fMRI methods based on blood oxygenation level dependent (BOLD) contrast and arterial spin labeling (ASL) perfusion contrast have enabled imaging of changes in blood oxygenation and cerebral blood flow (CBF). While BOLD contrast has been widely used as the surrogate marker for neural activation and can provide reliable information on the neuroanatomy underlying transient sensorimotor and cognitive functions, recent evidence suggests perfusion contrast is suitable for studying relatively long term effects on CBF both at rest or during activation. New developments in combining or simultaneously measuring the electrophysiological and fMRI signals allow a new class of studies that capitalize on dynamic imaging with high spatiotemporal resolution. This article reviews the biophysical bases and methodologies of fMRI and its applications to the clinical neurosciences, with emphasis on the spatiotemporal resolution of fMRI and its coupling with neurophysiology under both normal and pathophysiological conditions.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.