Quantitative measurements of cerebral blood flow in patients with unilateral carotid artery occlusion: a PET and MR study

Augmentation of perfusion with simultaneous vasodilator and inotropic agents in experimental acute middle cerebral artery occlusion: a pilot study

BACKGROUND

This study tests the hypothesis that simultaneous cerebral blood pressure elevation and potent vasodilation augments perfusion to ischemic tissue in acute ischemic stroke and it varies by degree of pial collateral recruitment.

METHODS

Fifteen mongrel canines were included. Subjects underwent permanent middle cerebral artery occlusion; pial collateral recruitment was scored before treatment. Seven treatment subjects received a continuous infusion of norepinephrine (0.1-1.52 µg/kg/min; titrated 25-45 mmHg above baseline mean arterial pressure while keeping systolic blood pressure below 180 mmHg) and hydralazine (20 mg) starting 30 min post-occlusion. Perfusion (cerebral blood flow-CBF) was evaluated with quantitative dynamic susceptibility contrast MRI 2.5 hours post-occlusion to produce images in mL/100 g/min, and relative CBF measured as ratios. Mean region of interest (ROI) values were reported, and compared and subject to regression analysis to elucidate trends.

RESULTS

Differences in quantitative CBF (qCBF) between treatment and control group varied by degree of pial collateral recruitment, based on Wilcoxon rank sum scores and regression model fit. For poorly collateralized subjects, ipsilateral anatomic, core infarct, and penumbra regions showed treatment with higher qCBF, raised above the ischemic threshold, compared with the control, while well collateralized subjects showed a paradoxical decrease maintained above the ischemic threshold for neuronal death. qCBF on the contralateral side increased regardless of collateralization.

CONCLUSION

Results suggest that perfusion can be augmented in ischemic stroke with norepinephrine and hydralazine. Perfusion augmentation depends on degree of collateralization and territory in question, with some evidence of vascular steal.

Whole-brain perfusion mapping in mice by dynamic BOLD MRI with transient hypoxia

Non-invasive mapping of cerebral perfusion is critical for understanding neurovascular and neurodegenerative diseases. However, perfusion MRI methods cannot be easily implemented for whole-brain studies in mice because of their small size. To overcome this issue, a transient hypoxia stimulus was applied to induce a bolus of deoxyhemoglobins as an endogenous paramagnetic contrast in blood oxygenation level-dependent (BOLD) MRI. Based on stimulus-duration-dependent studies, 5 s anoxic stimulus was chosen, which induced a decrease in arterial oxygenation to 59%. Dynamic susceptibility changes were acquired with whole-brain BOLD MRI using both all-vessel-sensitive gradient-echo and microvascular-sensitive spin-echo readouts. Cerebral blood flow (CBF) and cerebral blood volume (CBV) were quantified by modeling BOLD dynamics using a partial-volume-corrected arterial input function. In the mouse under ketamine/xylazine anesthesia, total CBF and CBV were 112.0 ± 15.0 ml/100 g/min and 3.39 ± 0.59 ml/100 g (n = 15 mice), respectively, whereas microvascular CBF and CBV were 85.8 ± 6.9 ml/100 g/min and 2.23 ± 0.27 ml/100 g (n = 7 mice), respectively. Regional total vs. microvascular perfusion metrics were highly correlated but a slight mismatch was observed in the large-vessel areas and cortical depth profiles. Overall, this non-invasive, repeatable, simple hypoxia BOLD-MRI approach is viable for perfusion mapping of rodents.

Quantitative perfusion mapping with induced transient hypoxia using BOLD MRI

PURPOSE

Gadolinium-based dynamic susceptibility contrast (DSC) is commonly used to characterize blood flow in patients with stroke and brain tumors. Unfortunately, gadolinium contrast administration has been associated with adverse reactions and long-term accumulation in tissues. In this work, we propose an alternative deoxygenation-based DSC (dDSC) method that uses a transient hypoxia gas paradigm to deliver a bolus of paramagnetic deoxygenated hemoglobin to the cerebral vasculature for perfusion imaging.

METHODS

Through traditional DSC tracer kinetic modeling, the MR signal change induced by this hypoxic bolus can be used to generate regional perfusion maps of cerebral blood flow, cerebral blood volume, and mean transit time. This gas paradigm and blood-oxygen-level-dependent (BOLD)-MRI were performed concurrently on a cohort of 66 healthy and chronically anemic subjects (age 23.5 ± 9.7, female 64%).

RESULTS

Our results showed reasonable global and regional agreement between dDSC and other flow techniques, such as phase contrast and arterial spin labeling.

CONCLUSION

In this proof-of-concept study, we demonstrated the feasibility of using transient hypoxia to generate a contrast bolus that mimics the effect of gadolinium and yields reasonable perfusion estimates. Looking forward, optimization of the hypoxia boluses and measurement of the arterial-input function is necessary to improve the accuracy of dDSC. Additionally, a cross-validation study of dDSC and DSC in brain tumor and ischemic stroke subjects is warranted to evaluate the clinical diagnostic utility of this approach.

Bayesian Estimation of CBF Measured by DSC-MRI in Patients with Moyamoya Disease: Comparison with 15O-Gas PET and Singular Value Decomposition

BACKGROUND AND PURPOSE

CBF analysis of DSC perfusion using the singular value decomposition algorithm is not accurate in patients with Moyamoya disease. This study compared the Bayesian estimation of CBF against the criterion standard PET and singular value decomposition methods in patients with Moyamoya disease.

MATERIALS AND METHODS

Nineteen patients with Moyamoya disease (10 women; 22-52 years of age) were evaluated with both DSC and ¹⁵O-gas PET within 60 days. DSC-CBF maps were created using Bayesian analysis and 3 singular value decomposition analyses (standard singular value decomposition, a block-circulant deconvolution method with a fixed noise cutoff, and a block-circulant deconvolution method that adopts an occillating noise cutoff for each voxel according to the strength of noise). Qualitative and quantitative analyses of the Bayesian-CBF and singular value decomposition-CBF methods were performed against ¹⁵O-gas PET and compared with each other.

RESULTS

In qualitative assessments of DSC-CBF maps, Bayesian-CBF maps showed better visualization of decreased CBF on PET (sensitivity = 62.5%, specificity = 100%, positive predictive value = 100%, negative predictive value = 78.6%) than a block-circulant deconvolution method with a fixed noise cutoff and a block-circulant deconvolution method that adopts an oscillating noise cutoff for each voxel according to the strength of noise (P < .03 for all except for specificity). Quantitative analysis of CBF showed that the correlation between Bayesian-CBF and PET-CBF values (ρ = 0.46, P < .001) was similar among the 3 singular value decomposition methods, and Bayesian analysis overestimated true CBF (mean difference, 47.28 mL/min/100 g). However, the correlation between CBF values normalized to the cerebellum was better in Bayesian analysis (ρ = 0.56, P < .001) than in the 3 singular value decomposition methods (P < .02).

CONCLUSIONS

Compared with previously reported singular value decomposition algorithms, Bayesian analysis of DSC perfusion enabled better qualitative and quantitative assessments of CBF in patients with Moyamoya disease.

Absolute quantitative MR perfusion and comparison against stable-isotope microspheres

PURPOSE

This work sought to compare a quantitative T₁ bookend dynamic susceptibility contrast MRI based perfusion protocol for absolute cerebral blood flow (qCBF) against CBF measured by the stable-isotope neutron capture microsphere method, a recognized reference standard for measuring tissue blood flow, at normocapnia, hypercapnia, and in acute stroke.

METHODS

CBF was measured in anesthetized female canines by MRI and microspheres over 2 consecutive days for each case. On day 1, 5 canines were measured before and during a physiological challenge induced by carbogen inhalation; on day 2, 4 canines were measured following permanent occlusion of the middle cerebral artery. CBF and cerebrovascular reactivity measured by MRI and microsphere deposition were compared.

RESULTS

MRI correlated strongly with microspheres at the hemispheric level for CBF during normo- and hypercapnic states (r² = 0.96), for individual cerebrovascular reactivity (r² = 0.84), and for postocclusion CBF (r² = 0.82). Correction for the delay and dispersion of the contrast bolus resulted in a significant improvement in the correlation between MRI and microsphere deposition in the ischemic state (r² = 0.96). In all comparisons, moderate correlations were found at the regional level.

CONCLUSION

In an experimental canine model with and without permanent occlusion of the middle cerebral artery, MRI-based qCBF yielded moderate to strong correlations for absolute quantitative CBF and cerebrovascular reactivity measurements during normocapnia and hypercapnia. Correction for delay and dispersion greatly improved the quantitation during occlusion of the middle cerebral artery, underscoring the importance for this correction under focal ischemic condition.

Arterial input function in a dedicated slice for cerebral perfusion measurements in humans

OBJECT

We aimed to modify our previously published method for arterial input function measurements for evaluation of cerebral perfusion (dynamic susceptibility contrast MRI) such that it can be applied in humans in a clinical setting.

MATERIALS AND METHODS

Similarly to our previous work, a conventional measurement sequence for dynamic susceptibility contrast MRI is extended with an additional measurement slice at the neck. Measurement parameters at this slice were optimized for the blood signal (short echo time, background suppression, magnitude and phase images). Phase-based evaluation of the signal in the carotid arteries is used to obtain quantitative arterial input functions.

RESULTS

In all pilot measurements, quantitative arterial input functions were obtained. The resulting absolute perfusion parameters agree well with literature values (gray and white matter mean values of 46 and 24 mL/100 g/min, respectively, for cerebral blood flow and 3.0% and 1.6%, respectively, for cerebral blood volume).

CONCLUSIONS

The proposed method has the potential to quantify arterial input functions in the carotid arteries from a direct measurement without any additional normalization.

Dynamic Susceptibility Contrast Perfusion Magnetic Resonance Imaging Demonstrates Reduced Periventricular Cerebral Blood Flow in Dogs with Ventriculomegaly

The nature of ventriculomegaly in dogs is still a matter of debate. Signs of increased intraventricular pressure and atrophy of the cerebral white matter have been found in dogs with ventriculomegaly, which would imply increased intraventricular pressure and, therefore, a pathological condition, i.e., to some extent. Reduced periventricular blood flow was found in people with high elevated intraventricular pressure. The aim of this study was to compare periventricular brain perfusion in dogs with and without ventriculomegaly using perfusion weighted-magnetic-resonance-imaging to clarify as to whether ventriculomegaly might be associated with an increase in intraventricular pressure. Perfusion was measured in 32 Cavalier King Charles spaniels (CKCS) with ventriculomegaly, 10 CKCSs were examined as a control group. Cerebral blood flow (CBF) was measured using free-hand regions of interest (ROI) in five brain regions: periventricular white matter, caudate nucleus, parietal cortex, hippocampus, and thalamus. CBF was significantly lower in the periventricular white matter of the dogs with ventriculomegaly (p = 0.0029) but not in the other ROIs. Reduction of periventricular CBF might imply increase of intraventricular pressure in ventriculomegaly.

Robustness of spatio-temporal regularization in perfusion MRI deconvolution: An application to acute ischemic stroke

PURPOSE

The robustness of a recently introduced globally convergent deconvolution algorithm with temporal and edge-preserving spatial regularization for the deconvolution of dynamic susceptibility contrast perfusion magnetic resonance imaging is assessed in the context of ischemic stroke.

THEORY AND METHODS

Ischemic tissues are not randomly distributed in the brain but form a spatially organized entity. The addition of a spatial regularization term allows to take into account this spatial organization contrarily to the sole temporal regularization approach which processes each voxel independently. The robustness of the spatial regularization in relation to shape variability, hemodynamic variability in tissues, noise in the magnetic resonance imaging apparatus, and uncertainty on the arterial input function selected for the deconvolution is addressed via an original in silico validation approach.

RESULTS

The deconvolution algorithm proved robust to the different sources of variability, outperforming temporal Tikhonov regularization in most realistic conditions considered. The limiting factor is the proper estimation of the arterial input function.

CONCLUSION

This study quantified the robustness of a spatio-temporal approach for dynamic susceptibility contrast-magnetic resonance imaging deconvolution via a new simulator. This simulator, now accessible online, is of wide applicability for the validation of any deconvolution algorithm. Magn Reson Med 78:1981-1990, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Comparison of cerebral blood flow measurement with [15O]-water positron emission tomography and arterial spin labeling magnetic resonance imaging: A systematic review

Noninvasive imaging of cerebral blood flow provides critical information to understand normal brain physiology as well as to identify and manage patients with neurological disorders. To date, the reference standard for cerebral blood flow measurements is considered to be positron emission tomography using injection of the [(15)O]-water radiotracer. Although [(15)O]-water has been used to study brain perfusion under normal and pathological conditions, it is not widely used in clinical settings due to the need for an on-site cyclotron, the invasive nature of arterial blood sampling, and experimental complexity. As an alternative, arterial spin labeling is a promising magnetic resonance imaging technique that magnetically labels arterial blood as it flows into the brain to map cerebral blood flow. As arterial spin labeling becomes more widely adopted in research and clinical settings, efforts have sought to standardize the method and validate its cerebral blood flow values against positron emission tomography-based cerebral blood flow measurements. The purpose of this work is to critically review studies that performed both [(15)O]-water positron emission tomography and arterial spin labeling to measure brain perfusion, with the aim of better understanding the accuracy and reproducibility of arterial spin labeling relative to the positron emission tomography reference standard.

Quantification of cerebral perfusion using dynamic quantitative susceptibility mapping

PURPOSE

The purpose of this study is to develop a dynamic quantitative susceptibility mapping (QSM) technique with sufficient temporal resolution to map contrast agent concentration in cerebral perfusion imaging.

METHODS

The dynamic QSM used a multiecho three-dimensional (3D) spoiled gradient echo golden angle interleaved spiral sequence during contrast bolus injection. Four-dimensional (4D) space-time resolved magnetic field reconstruction was performed using the temporal resolution acceleration with constrained evolution reconstruction method. Deconvolution of the gadolinium-induced field was performed at each time point with the morphology enabled dipole inversion method to generate a 4D gadolinium concentration map, from which three-dimensional spatial distributions of cerebral blood volume and cerebral blood flow were computed.

RESULTS

Initial in vivo brain imaging demonstrated the feasibility of using dynamic QSM for generating quantitative 4D contrast agent maps and imaging three-dimensional perfusion. The cerebral blood flow obtained with dynamic QSM agreed with that obtained using arterial spin labeling.

CONCLUSION

Dynamic QSM can be used to perform 4D mapping of contrast agent concentration in contrast-enhanced magnetic resonance imaging. The perfusion parameters derived from this 4D contrast agent concentration map were in good agreement with those obtained using arterial spin labeling.

Calibration of diffuse correlation spectroscopy blood flow index with venous-occlusion diffuse optical spectroscopy in skeletal muscle

We investigate and assess the utility of a simple scheme for continuous absolute blood flow monitoring based on diffuse correlation spectroscopy (DCS). The scheme calibrates DCS using venous-occlusion diffuse optical spectroscopy (VO-DOS) measurements of arm muscle tissue at a single time-point. A calibration coefficient (γ) for the arm is determined, permitting conversion of DCS blood flow indices to absolute blood flow units, and a study of healthy adults (N=10) is carried out to ascertain the variability of γ. The average DCS calibration coefficient for the right (i.e., dominant) arm was γ=(1.24±0.15)×10(8) (mL·100  mL(−1)·min(−1))/(cm(2)/s). However, variability can be significant and is apparent in our site-to-site and day-to-day repeated measurements. The peak hyperemic blood flow overshoot relative to baseline resting flow was also studied following arm-cuff ischemia; excellent agreement between VO-DOS and DCS was found (R(2)=0.95, slope=0.94±0.07, mean difference=−0.10±0.45). Finally, we show that incorporation of subject-specific absolute optical properties significantly improves blood flow calibration accuracy.

Differences in dynamic susceptibility contrast MR perfusion maps generated by different methods implemented in commercial software

PURPOSE

There are several potential sources of difference that can influence the reproducibility of magnetic resonance (MR) perfusion values. We aimed to investigate the reproducibility and variability of dynamic susceptibility contrast (DSC) MR imaging (MRI) parameters obtained from identical source data by using 2 commercially available software applications with different postprocessing algorithms.

METHODS AND MATERIALS

We retrospectively evaluated DSC-MRI data sets of 24 consecutive patients with glioblastoma multiforme. Perfusion data were postprocessed with 2 commercial software packages, NordicICE (NordicNeuroLab, Bergen, Norway) and GE Brainstat (GE Healthcare, Milwaukee, Wis), each of which offers the possibility of different algorithms. We focused the comparison on their main analysis issues, that is, the gamma-variate fitting function (GVF) and the arterial input function (AIF). Two regions of interest were placed on maps of perfusion parameters (cerebral blood volume [CBV], cerebral blood flow [CBF], mean transit time [MTT]): one around tumor hot spot and one in the contralateral normal brain. A one-way repeated-measures analysis of variance was conducted to determine whether there was a significant difference in the calculated MTT, CBV, and CBF values.

RESULTS

As regards NordicICE software application, the use of AIF is significant (P = 0.048) but not the use of GVF (P = 0.803) for CBV values. Additionally, in GE, the calculation method discloses a statistical effect on data. Comparing similar GE-NordicICE algorithms, both method (P = 0.005) and software (P < 0.0001) have a statistical effect in the difference. Leakage-corrected and uncorrected normalized CBV (nCBV) values are statistically equal. No statistical differences have been found in nMTT values when directly calculated. Values of nCBF are affected by the use of GVF.

CONCLUSION

The use of a different software application determines different results, even if the algorithms seem to be the same. The introduction of AIF in the data postprocessing determines a higher estimates variability that can make interhospital and intrahospital examinations not completely comparable. A simpler approach based on raw curve analysis produces more stable results.

Middle cerebral artery blood flows by combining TCD velocities and MRA diameters: in vitro and in vivo validations

Non-invasive transcranial Doppler (TCD) is widely used for blood velocity (BV, cm/sec) measurements in the human middle cerebral artery (MCA). MCABV measurements are accepted as linear with MCA blood flow (MCABF). Magnetic resonance angiography (MRA) provides measurements of MCA lumen diameters that can be combined with TCD MCABV to calculate MCABF (mL/min). We tested the precision and accuracy of this method against a flow phantom and in vivo proximal internal carotid artery blood flow (ICABF). In vitro precision (repeated measures) and accuracy (vs. time collection) gave correlations coefficients of 0.97 and 0.98, respectively (both p < 0.05). In vivo precision (repeated measures) and accuracy (vs. ICABF) gave correlation coefficients of 0.90 (left and right), 0.94 (left) and 0.93 (right) (all p < 0.05). Bilateral MCABF in 35 adults were similar (left, 168 ± 72 mL/min; right, 180 ± 69 mL/min; p > 0.05). Results suggest that blood velocity by TCD and lumen diameter by MRA can be combined to estimate absolute values of MCABF.

Improvements in the quantitative assessment of cerebral blood volume and flow with the removal of vessel voxels from MR perfusion images

Objective. To improve the quantitative assessment of cerebral blood volume (CBV) and flow (CBF) in the brain voxels from MR perfusion images. Materials and Methods. Normal brain parenchyma was automatically segmented with the time-to-peak criteria after cerebrospinal fluid removal and preliminary vessel voxel removal. Two scaling factors were calculated by comparing the relative CBV and CBF of the segmented normal brain parenchyma with the absolute values in the literature. Using the scaling factors, the relative values were converted to the absolute CBV and CBF. Voxels with either CBV > 8 mL/100 g or CBF > 100 mL/100 g/min were characterized as vessel voxels and were excluded from the quantitative measurements. Results. The segmented brain parenchyma with normal perfusion was consistent with the angiographic findings for each patient. We confirmed the necessity of dual thresholds including CBF and CBV for proper removal of vessel voxels. The scaling factors were 0.208 ± 0.041 for CBV, and 0.168 ± 0.037, 0.172 ± 0.037 for CBF calculated using standard and circulant singular value decomposition techniques, respectively. Conclusion. The automatic scaling and vessel removal techniques provide an alternative method for obtaining improved quantitative assessment of CBV and CBF in patients with thromboembolic cerebral arterial disease.

Cerebrovascular occlusive disease: quantitative cerebral blood flow using dynamic susceptibility contrast mr imaging correlates with quantitative H2[15O] PET

PURPOSE

To compare quantitative values of cerebral blood flow (CBF) derived from dynamic susceptibility contrast (DSC) magnetic resonance (MR) imaging with reference standard positron emission tomography (PET) in patients with confirmed cerebrovascular occlusive disease.

MATERIALS AND METHODS

Local institutional review board approval and informed consent were obtained for a prospective study of 18 patients (six men, 12 women; age range, 28-71 years; mean age, 45 years ± 10.4 [standard deviation]) with angiographically confirmed Moyamoya (n = 8) or internal carotid artery occlusions (n = 10). DSC MR images and oxygen 15-labeled water (H(2)[(15)O]) PET images were acquired on the same day. DSC images were postprocessed to yield parametric images of CBF (in mL/100 g/min), coregistered, and analyzed using grid-based regions of interest. Mean values of CBF in each region of interest from MR imaging and PET data sets were compared. Correlations for each patient were determined and overall agreement between pooled MR imaging and PET CBF was reported using linear regression analysis and Bland-Altman plots.

RESULTS

Strong correlations (r(2) ≥ 0.55) were found between MR imaging and PET CBF values in all patients. Use of the bookend approach was found to underestimate CBF predictably across the patient cohort (mean slope, 0.82; standard deviation, 0.18; slope of aggregated data, 0.75). This allowed for a simple rescaling of MR imaging values producing strong agreement with PET values in the aggregated data (r(2) = 0.66; slope = 1.00; intercept = 0.00).

CONCLUSION

The data show that the bookend MR imaging technique produces similar results for quantitative CBF between DSC MR imaging and H(2)[(15)O] PET. Although MR-derived CBF underestimated PET-derived CBF, the patient-to-patient variability in the slopes of the linear MR and PET relationships was significantly smaller than a competing quantitation technique. As a result, the bookend technique appears to more predictably measure quantitative CBF in a clinical setting.

Evaluating quantitative approaches to dynamic susceptibility contrast MRI among carotid endarterectomy patients

PURPOSE

To evaluate two dynamic susceptibility contrast (DSC) quantification methods in symptomatic carotid artery disease patients undergoing carotid endarterectomy (CEA) surgery by comparing methods directly and assessing the reliability of each method in the hemisphere contralateral to surgery.

MATERIALS AND METHODS

Absolute cerebral blood flow (CBF) and volume (CBV) was calculated in putamen and sensorimotor gray matter of 17 patients using two methods: 1) The Bookend method that scales relative DSC images to CBV values calculated from the ratio of pre- and postcontrast T1-weighted images, and 2) the Tail-scaling method that uses the ratio of area under the tails of the venous and arterial concentration time-courses to scale the DSC images.

RESULTS

There was a positive correlation between the methods with significant correlation post-CEA (P < 0.035). Intersession correlation was greater when using the Tail-scaling method contralateral to surgery (P < 0.004).

CONCLUSION

We have demonstrated correlation between methods that is significant after surgery and have found that the Tail-scaling method produces better test-retest reliability than our implementation of the Bookend method. Results from this study suggest that DSC has the potential to measure hemodynamic changes after endarterectomy and future work is required to establish clinical value.

The Kety-Schmidt technique for quantitative perfusion and oxygen metabolism measurements in the MR imaging environment

SUMMARY

The Kety-Schmidt technique provides quantitative measurement of whole-brain CBF. CBF is measured as the area between the arterial and venous washout curves of a diffusible tracer. Oxygen extraction and metabolism may be calculated from arterial and venous samples. In this report, we present a method for performing these measurements in an MR imaging environment. This technique could be useful for validation of MR imaging methods of hemodynamic and metabolic measurements in humans.

Validation and absolute quantification of MR perfusion compared with CT perfusion in patients with unilateral cerebral arterial stenosis

OBJECTIVE

The aim of the study was to assess absolute quantification of dynamic susceptibility contrast-enhanced magnetic resonance perfusion (MRP) comparing with computed tomography perfusion (CTP) in patients with unilateral stenosis.

MATERIALS AND METHODS

We retrospectively post-processed MRP in 20 patients with unilateral occlusion or stenosis of >79% at the internal carotid artery or the middle cerebral artery (MCA). Absolute quantification of MRP was performed after applying the following techniques: cerebrospinal fluid removal, vessel removal, and automatic segmentation of brain to calculate the scaling factors to convert relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) values to absolute values. For comparison between MRP and CTP, we manually deposited regions of interest in bilateral MCA territories at the level containing the body of the lateral ventricle.

RESULTS

The correlation between MRP and CTP was best for mean transit time (MTT) (r=0.83), followed by cerebral blood flow (CBF) (r=0.52) and cerebral blood volume (CBV) (r=0.43). There was no significant difference between CTP and MRP for CBV, CBF, and MTT on the lesion side, the contralateral side, the lesion-contralateral differences, or the lesion-to-contralateral ratios (P>0.05). The mean differences between MRP and CTP were as follows: CBV -0.57 mL/100g, CBF 2.50 mL/100g/min, and MTT -0.90 s.

CONCLUSION

Absolute quantification of MRP is possible. Using the proposed method, measured values of MRP and CTP had acceptable linear correlation and quantitative agreement.

An improved model for describing the contrast bolus in perfusion MRI

PURPOSE

Quantification of perfusion measurements using dynamic, susceptibility-weighted contrast-enhanced (DSC) MRI depends on estimating the size and shape of the tracer bolus. Typically, the bolus is described as a gamma variate function (GV) fitted to the bolus portion of tracer concentration time curve (CTC). However, the last point to fit is arbitrary which can lead to considerable variation in the fitted curve in the presence of noise. In this technical note, we present a model which takes into account recirculation explicitly and fits robustly to the entire CTC in the presence of noise.

METHODS

Signal data measurements from ten DSC MRI patients were fitted with our new model and a GV function using four different methods of estimating the end of the bolus. Estimates of the area under the curves (AUC) and first moments (FMs) of the bolus were compared at different noise levels.

RESULTS

The new model gave errors similar to or smaller than those of the most effective methods for fitting a GV.

CONCLUSIONS

The single compartment recirculation (SCR) model is the most robust fitting technique with respect to noise both for bias and variability.

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.

Feasibility of semiquantitative liver perfusion assessment by ferucarbotran bolus injection in double-contrast hepatic MRI

PURPOSE

To evaluate the feasibility of semiquantitative measurement of liver perfusion from analysis of ferucarbotran induced signal-dynamics in double-contrast liver MR-imaging (DC-MRI).

MATERIALS AND METHODS

In total 31 patients (21 men; 58 ± 10 years) including 18 patients with biopsy proven liver cirrhosis prospectively underwent clinically indicated DC-MRI at 1.5 Tesla (T) with dynamic T2-weighted gradient-echo imaging after ferucarbotran bolus injection. Breathing artefacts in tissue and input time curves were reduced by Savitzky-Golay-filtering and semiquantitative perfusion maps were calculated using a model free approach. Hepatic blood flow index (HBFI) and splenic blood flow index (SBFI) were determined by normalization of arbitrary perfusion values to the perfusion of the erector spinae muscle resulting in a semiquantitative perfusion measure.

RESULTS

In 30 of 31 patients the evaluated protocol could successfully be applied. Mean HBF was 7.7 ± 2.46 (range, 4.6-12.8) and mean SBF was 13.20 ± 2.57 (range, 8.5-17.8). A significantly lower total HBF was seen in patients with cirrhotic livers as compared to patients with noncirrhotic livers (P < 0.05). In contrast, similar SBF was observed in cirrhotic and noncirrhotic patients (P = 0.11).

CONCLUSION

Capturing the signal dynamics during bolus injection of ferucarbotran in DC-MRI of the liver allows for semiquantitative assessment of hepatic perfusion that may be helpful for a more precise characterisation of liver cirrhosis and focal liver lesions.

Feasibility of pseudocontinuous arterial spin labeling at 7 T with whole-brain coverage

Object

We studied the feasibility of pseudocontinuous arterial spin labeling (pCASL) at 7 T.

Materials and methods

Simulations were performed to find the optimal labeling parameters for pCASL, with particular attention to the maximum-allowed specific absorption rate (SAR). Subsequently, pCASL experiments (four volunteers) were performed to find the B1 efficiency at the labeling position with and without high-permittivity pads placed around the head, and to study the optimal labeling duration (four separate volunteers). Finally, feasibility of whole-brain pCASL imaging was tested.

Results

Simulations showed that a lower B1 efficiency should be compensated by a lower effective flip angle of the labeling, a moderately shorter labeling duration, and a longer repetition time. B1 efficiency in the internal carotid arteries just below the carotid siphon was approximately 55% and 35% with and without high-permittivity pads, respectively. In vivo experiments showed an optimal labeling duration of 1,500 ms, although longer labeling durations up to 2,500 ms resulted in similar signal-to-noise efficiency. Whole-brain pCASL imaging was demonstrated in a single volunteer.

Conclusion

Despite decreased B1 efficiency, sufficient labeling efficiency can be achieved for whole-brain pCASL at 7 T with high-permittivity pads. However, image quality is still limited compared with 3 T, probably due to imaging instabilities, and further research is needed to elucidate this.

Automatic selection of arterial input function on dynamic contrast-enhanced MR images

Dynamic susceptibility contrast-magnetic resonance imaging (DSC-MRI) data analysis requires the knowledge of the arterial input function (AIF) to quantify the cerebral blood flow (CBF), volume (CBV) and the mean transit time (MTT). AIF can be obtained either manually or using automatic algorithms. We present a method to derive the AIF on the middle cerebral artery (MCA). The algorithm draws a region of interest (ROI) where the MCA is located. Then, it uses a recursive cluster analysis on the ROI to select the arterial voxels. The algorithm had been compared on simulated data to literature state of art automatic algorithms and on clinical data to the manual procedure. On in silico data, our method allows to reconstruct the true AIF and it is less affected by partial volume effect bias than the other methods. In clinical data, automatic AIF provides CBF and MTT maps with a greater contrast level compared to manual AIF ones. Therefore, AIF obtained with the proposed method improves the estimate reliability and provides a quantitatively reliable physiological picture.

Deconvolution-Based CT and MR Brain Perfusion Measurement: Theoretical Model Revisited and Practical Implementation Details

Deconvolution-based analysis of CT and MR brain perfusion data is widely used in clinical practice and it is still a topic of ongoing research activities. In this paper, we present a comprehensive derivation and explanation of the underlying physiological model for intravascular tracer systems. We also discuss practical details that are needed to properly implement algorithms for perfusion analysis. Our description of the practical computer implementation is focused on the most frequently employed algebraic deconvolution methods based on the singular value decomposition. In particular, we further discuss the need for regularization in order to obtain physiologically reasonable results. We include an overview of relevant preprocessing steps and provide numerous references to the literature. We cover both CT and MR brain perfusion imaging in this paper because they share many common aspects. The combination of both the theoretical as well as the practical aspects of perfusion analysis explicitly emphasizes the simplifications to the underlying physiological model that are necessary in order to apply it to measured data acquired with current CT and MR scanners.

SCALE-PWI: A pulse sequence for absolute quantitative cerebral perfusion imaging

The Bookend technique is a magnetic resonance imaging (MRI) dynamic susceptibility contrast method that provides reliable quantitative measurement of cerebral blood flow (CBF) and cerebral blood volume (CBV). The quantification is patient specific, is derived from a steady-state measurement of CBV, and is obtained from T(1) changes in the white matter and the blood pool after contrast agent injection. In the current implementation, the Bookend technique consists of three scanning steps requiring a cumulative scan time of 3 minutes 47 seconds, a well-trained technologist, and extra time for offline image reconstruction. We present an automation and acceleration of the multiscan Bookend protocol through a self-calibrating pulse sequence, namely Self-Calibrated Epi Perfusion-Weighted Imaging (SCALE-PWI). The SCALE-PWI is a single-shot echo-planar imaging pulse sequence with three modules and a total scan time of under 2 minutes. It provides the possibility of performing online, quantitative perfusion image reconstruction, which reduces the latency to obtain quantitative maps. A validation study in healthy volunteers (N=19) showed excellent agreement between SCALE-PWI and the conventional Bookend protocol (P>0.05 with Student's t-test, r=0.95/slope=0.98 for quantitative CBF, and r=0.91/slope=0.94 for quantitative CBV). A single MRI pulse sequence for absolute quantification of cerebral perfusion has been developed.

Quantitative cerebral MR perfusion imaging: preliminary results in stroke

PURPOSE

To evaluate quantitative cerebral blood flow (qCBF) with traditional time-based measurements or metrics of cerebral perfusion: time to peak (Tmax) and mean transit time (MTT) in stroke patients.

MATERIALS AND METHODS

Nine ischemic stroke patients (four male, five female, 63 ± 16 years old) were included in the study which was Health Insurance Portability and Accountability Act compliant and institutional review board approved. Cerebral perfusion was quantified using the Bookend method. Mean values of qCBF, Tmax, and MTT were determined in regions of interest (ROIs). ROIs were drawn on diffusion weighted images in diffusion positive, critically ischemic (CI), in ipsilateral normal region immediately surrounding the critically ischemic region, the presumed penumbra (PP), and in contralateral diffusion negative control, presumed normal region (PN) of gray and white matter separately (GM and WM).

RESULTS

In both GM and WM, qCBF measures distinguished the studied brain regions with the most markedly reduced values in regions corresponding to extent of likely ischemic injury. In planned comparisons, only qCBF measurements differed significantly between CI and PP tissues. ROC analysis supported the utility of qCBF for discriminating brain regions differing in the likely extent of ischemic injury (CI and PN regions - qCBF: area under the curve [AUC] = 0.96, Tmax: AUC = 0.96, MTT: AUC = 0.72). Importantly, qCBF afforded the best discrimination of CI and PP regions (qCBF: AUC = 0.82, Tmax: AUC = 0.65, MTT: AUC = 0.52).

CONCLUSION

This initial evaluation indicates that quantitative MRI perfusion is feasible in ischemic stroke patients. qCBF derived with this strategy provide enhanced discrimination of CI and PP compared to time-based imaging metrics. This approach merits investigation in larger clinical studies.

Quantitative cerebral blood flow in dynamic susceptibility contrast MRI using total cerebral flow from phase contrast magnetic resonance angiography

Dynamic susceptibility contrast magnetic resonance imaging during bolus injection of gadolinium contrast agent is commonly used to investigate cerebral hemodynamics. The large majority of clinical applications of dynamic susceptibility contrast magnetic resonance imaging to date have reported relative cerebral blood flow values because of dependence of the result on the accuracy of determining the arterial input function, the robustness of the singular value decomposition algorithm, and others. We propose a calibration approach that directly measures the total (i.e., whole brain) cerebral blood flow in individual subjects using phase contrast magnetic resonance angiography. The method was applied to data from 11 patients with intracranial pathology. The sum of squares variance about the mean (uncorrected: white matter = 105.6, gray matter = 472.2; corrected: white matter = 34.1, gray matter = 99.8) after correction was significantly lower for white matter (P = 0.045) and for gray matter (P = 0.011). However, the mean gray and white matter cerebral blood flow in the contralateral hemisphere were not significantly altered by the correction. The proposed phase contrast magnetic resonance angiography calibration technique appears to be one of the most direct correction schemes available for dynamic susceptibility contrast magnetic resonance imaging cerebral blood flow values and can be performed rapidly, requiring only a few minutes of additional scan time.

Combined arterial spin label and dynamic susceptibility contrast measurement of cerebral blood flow

Dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL) are both used to measure cerebral blood flow (CBF), but neither technique is ideal. Absolute DSC-CBF quantitation is challenging due to many uncertainties, including partial- volume errors and nonlinear contrast relaxivity. ASL can measure quantitative CBF in regions with rapidly arriving flow, but CBF is underestimated in regions with delayed arrival. To address both problems, we have derived a patient-specific correction factor, the ratio of ASL- and DSC-CBF, calculated only in short-arrival-time regions (as determined by the DSC-based normalized bolus arrival time [Tmax]). We have compared the combined CBF method to gold-standard xenon CT in 20 patients with cerebrovascular disease, using a range of Tmax threshold levels. Combined ASL and DSC CBF demonstrated quantitative accuracy as good as the ASL technique but with improved correlation in voxels with long Tmax. The ratio of MRI-based CBF to xenon CT CBF (coefficient of variation) was 90 +/- 30% (33%) for combined ASL and DSC CBF, 43 +/- 21% (47%) for DSC, and 91 +/- 31% (34%) for ASL (Tmax threshold 3 sec). These findings suggest that combining ASL and DSC perfusion measurements improves quantitative CBF measurements in patients with cerebrovascular disease.

Dynamic susceptibility contrast MRI with localized arterial input functions

Compared to gold-standard measurements of cerebral perfusion with positron emission tomography using H(2)[(15)O] tracers, measurements with dynamic susceptibility contrast MR are more accessible, less expensive, and less invasive. However, existing methods for analyzing and interpreting data from dynamic susceptibility contrast MR have characteristic disadvantages that include sensitivity to incorrectly modeled delay and dispersion in a single, global arterial input function. We describe a model of tissue microcirculation derived from tracer kinetics that estimates for each voxel a unique, localized arterial input function. Parameters of the model were estimated using Bayesian probability theory and Markov-chain Monte Carlo, circumventing difficulties arising from numerical deconvolution. Applying the new method to imaging studies from a cohort of 14 patients with chronic, atherosclerotic, occlusive disease showed strong correlations between perfusion measured by dynamic susceptibility contrast MR with localized arterial input function and perfusion measured by quantitative positron emission tomography with H(2)[(15)O]. Regression to positron emission tomography measurements enabled conversion of dynamic susceptibility contrast MR to a physiologic scale. Regression analysis for localized arterial input function gave estimates of a scaling factor for quantitation that described perfusion accurately in patients with substantial variability in hemodynamic impairment.

A simple positron emission tomography-based calibration for perfusion-weighted magnetic resonance maps to optimize penumbral flow detection in acute stroke

BACKGROUND AND PURPOSE

Perfusion-weighted (PW) MRI is increasingly used to identify the tissue at risk. The adequate PW-MRI map and threshold remain controversial due to a considerable individual variation of values. By comparative positron emission tomography, we evaluated a simple MR-based and positron emission tomography-validated calibration of PW maps.

METHODS

PW-MRI and quantitative positron emission tomography (15O-water) of patients with acute stroke were used to calculate averaged as well as individual thresholds of penumbral flow (positron emission tomography cerebral blood flow (<20 mL/100 g/min) for maps of time to peak, mean transit time, cerebral blood flow, and cerebral blood volume. A linear regression analysis studied the variability of the individual thresholds using 3 different PW reference regions (hemispheric, white matter, gray matter). The best model was used for volumetric analysis to compare averaged and scaled individual thresholds and to calculate look-up tables for PW maps.

RESULTS

In 26 patients, the averaged thresholds were (median/interquartile range): cerebral blood flow 21.7 mL/100 g/min (19.9 to 32); cerebral blood volume 1.5 mL/100 g (0.9 to 1.8); mean transit time seconds 5.2 (3.9 to 6.9); and relative time to peak 4.2 seconds (2.8 to 5.8). The large individual variability was best explained by the mean value of the hemispheric reference derived from a region of interest on a level with the basal ganglia of the unaffected hemisphere (R(2): cerebral blood flow 0.76, cerebral blood volume 0.55, mean transit time 0.83, time to peak 0.95). Hemispheric reference-corrected thresholds clearly improved the detection of penumbral flow. Look-up tables were calculated to identify the individual thresholds according to the hemispheric reference value.

CONCLUSIONS

The individual variation of PW values, even if calculated by deconvolution, remains a major obstacle in quantitative PW imaging and can be significantly improved by a simple MR-based calibration. Easily applicable look-up tables identify the individual best threshold for each PW map to optimize mismatch detection.

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).

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.

Quantification of cerebral blood flow, cerebral blood volume, and blood-brain-barrier leakage with DCE-MRI

Dynamic susceptibility contrast MRI (DSC-MRI) is the current standard for the measurement of Cerebral Blood Flow (CBF) and Cerebral Blood Volume (CBV), but it is not suitable for the measurement of Extraction Flow (EF) and may not allow for absolute quantification. The objective of this study was to develop and evaluate a methodology to measure CBF, CBV, and EF from T1-weighted dynamic contrast-enhanced MRI (DCE-MRI). A two-compartment modeling approach was developed, which applies both to tissues with an intact and with a broken Blood-Brain-Barrier (BBB). The approach was evaluated using measurements in normal grey matter (GM) and white matter (WM) and in tumors of 15 patients. Accuracy and precision were estimated with simulations of normal brain tissue. All tumor and normal tissue curves were accurately fitted by the model. CBF (mL/100 mL/min) was 82 +/- 21 in GM and 23 +/- 14 in WM, CBV (mL/100 mL) was 2.6 +/- 0.8 in GM and 1.3 +/- 0.4 in WM. EF (mL/100 mL/min) was close to zero in GM (-0.009 +/- 0.05) and WM (-0.03 +/- 0.08). Simulations show an overlap between CBF values of WM and GM, which is eliminated when Contrast-to-Noise (CNR) is improved. The model provides a consistent description of tracer kinetics in all brain tissues, and an accurate assessment of perfusion and permeability in reference tissues. The measurement sequence requires optimization to improve CNR and the precision in the perfusion parameters. With this approach, DCE-MRI presents a promising alternative to DSC-MRI for quantitative bolus-tracking in the brain.

Absolute quantification of cerebral blood flow: correlation between dynamic susceptibility contrast MRI and model-free arterial spin labeling

PURPOSE

To compare absolute cerebral blood flow (CBF) estimates obtained by model-free arterial spin labeling (ASL) and dynamic susceptibility contrast MRI (DSC-MRI), corrected for partial volume effects (PVEs).

METHODS

CBF was measured using DSC-MRI and model-free ASL (quantitative signal targeting with alternating radiofrequency labeling of arterial regions) at 3 T in 15 subjects with brain tumor, and the two modalities were compared with regard to CBF estimates in normal gray matter (GM) and DSC-to-ASL CBF ratios in selected tumor regions. The DSC-MRI CBF maps were calculated using a global arterial input function (AIF) from the sylvian-fissure region, but, in order to minimize PVEs, the AIF time integral was rescaled by a venous output function time integral obtained from the sagittal sinus.

RESULTS

In GM, the average DSC-MRI CBF estimate was 150+/-45 ml/(min 100 g) (mean+/-SD) while the corresponding ASL CBF was 44+/-10 ml/(min 100 g). The linear correlation between GM CBF estimates obtained by DSC-MRI and ASL was r=.89, and observed DSC-to-ASL CBF ratios differed by less than 3% between GM and tumor regions.

CONCLUSIONS

A satisfactory positive linear correlation between the CBF estimates obtained by model-free ASL and DSC-MRI was observed, and DSC-to-ASL CBF ratios showed no obvious tissue dependence.

Arterial spin-label imaging in patients with normal bolus perfusion-weighted MR imaging findings: pilot identification of the borderzone sign

PURPOSE

To determine whether perfusion abnormalities are depicted on arterial spin-labeling (ASL) images obtained in patients with normal bolus perfusion-weighted (PW) magnetic resonance (MR) imaging findings.

MATERIALS AND METHODS

Institutional review board approval and written informed patient consent were obtained. This study was HIPAA compliant. Consecutive patients suspected or known to have cerebrovascular disease underwent 1.5-T brain MR imaging, including MR angiography, gradient-echo PW imaging, and pseudocontinuous ASL imaging, between October 2007 and January 2008. Patients with normal bolus PW imaging findings were retrospectively identified, and two neuroradiologists subsequently evaluated the ASL images for focal abnormalities. The severity of the borderzone sign-that is, bilateral ASL signal dropout with surrounding cortical areas of hyperintensity in the middle cerebral artery borderzone regions-was classified by using a four-point scale. For each group, the ASL-measured mean mixed cortical cerebral blood flow (CBF) at the level of the centrum semiovale was evaluated by using the Jonckheere-Terpstra test.

RESULTS

One hundred thirty-nine patients met the study inclusion criteria, and 41 (30%) of them had normal bolus PW imaging findings. Twenty-three (56%) of these 41 patients also had normal ASL imaging findings. The remaining 18 (44%) patients had the ASL borderzone sign; these patients were older (mean age, 71 years +/- 11 [standard deviation] vs 57 years +/- 16; P < .005) and had lower mean CBF (30 mL/100 g/min +/- 12 vs 46 mL/100 g/min +/- 12, P < .003) compared with the patients who had normal ASL imaging findings. Five patients had additional focal ASL findings that were related to either slow blood flow in a vascular structure or postsurgical perfusion defects and were not visible on the PW images.

CONCLUSION

Approximately half of the patients with normal bolus PW imaging findings had abnormal ASL findings-most commonly the borderzone sign. Results of this pilot study suggest that ASL imaging in patients who have this sign and are suspected of having cerebrovascular disease yields additional and complementary hemodynamic information.

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.

Theoretic basis and technical implementations of CT perfusion in acute ischemic stroke, part 2: technical implementations

CT perfusion (CTP) is a functional imaging technique that provides important information about capillary-level hemodynamics of the brain parenchyma and is a natural complement to the strengths of unenhanced CT and CT angiography in the evaluation of acute stroke, vasospasm, and other neurovascular disorders. CTP is critical in determining the extent of irreversibly infarcted brain tissue (infarct "core") and the severely ischemic but potentially salvageable tissue ("penumbra"). This is achieved by generating parametric maps of cerebral blood flow, cerebral blood volume, and mean transit time.

The acetazolamide challenge: techniques and applications in the evaluation of chronic cerebral ischemia

The acetazolamide (ACZ) challenge test is a useful clinical tool and a reliable predictor of critically reduced perfusion. In patients with chronic steno-occlusive disease, the ability to maintain normal cerebral blood flow by reducing vascular resistance secondary to autoregulatory vasodilation is compromised. Identification of the presence and degree of autoregulatory vasodilation (reflecting the cerebrovascular reserve) is a significant prognostic factor in patients with chronic cerebrovascular disease. The pharmacologic challenge of a vasodilatory stimulus such as ACZ can also be used to optimize the treatment strategies for these patients. The pathophysiology, methods, and clinical applications of the ACZ challenge test are discussed in this article.

Contrast agent dose effects in cerebral dynamic susceptibility contrast magnetic resonance perfusion imaging

PURPOSE

To study the contrast agent dose sensitivity of hemodynamic parameters derived from brain dynamic susceptibility contrast MRI (DSC-MRI).

MATERIALS AND METHODS

Sequential DSC-MRI (1.5T gradient-echo echo-planar imaging using an echo time of 61-64 msec) was performed using contrast agent doses of 0.1 and 0.2 mmol/kg delivered at a fixed rate of 5.0 mL/second in 12 normal subjects and 12 stroke patients.

RESULTS

1) Arterial signal showed the expected doubling in relaxation response (DeltaR2*) to dose doubling. 2) The brain signal showed a less than doubled DeltaR2* response to dose doubling. 3) The 0.2 mmol/kg dose studies subtly underestimated cerebral blood volume (CBV) and cerebral blood flow (CBF) relative to the 0.1 mmol/kg studies. 4) In the range of low CBV and CBF, the 0.2 mmol/kg studies overestimated the CBV and CBF compared with the 0.1 mmol/kg studies. 5) The 0.1 mmol/kg studies reported larger ischemic volumes in stroke.

CONCLUSION

Subtle but statistically significant dose sensitivities were found. Therefore, it is advisable to carefully control the contrast agent dose when DSC-MRI is used in clinical trials. The study also suggests that a 0.1 mmol/kg dose is adequate for hemodynamic measurements.

Method for rapid calculation of quantitative cerebral perfusion

PURPOSE

To evaluate an algorithm based on algebraic estimation of T1 values (three-point estimation) in comparison with computational curve-fitting for the postprocessing of quantitative cerebral perfusion scans.

MATERIALS AND METHODS

Computer simulations were performed to quantify the magnitude of the expected error on T1 and consequently cerebral perfusion using the three-point estimation technique on a Look-Locker (LL) EPI scan. In 50 patients, quantitative cerebral perfusion was calculated using the bookend method with three-point estimation and curve-fitting. The bookend method, a novel approach for calculating quantitative cerebral perfusion based on changes in T1 values after a contrast injection, is currently being validated. The number of computations was used as a measure of computation speed for each method. Student's paired t-test, Bland-Altman, and correlation analyses were performed to evaluate the accuracy of estimation.

RESULTS

There was a 99.65% reduction in the number of computations with three-point estimation. Student's t-test showed no significant difference in cerebral perfusion (P=0.80, 0.49, paired t-test N=50, quantitative cerebral blood flow-white matter [qCBF-WM], qCBF-gray matter [qCBF-GM]) when compared to curve-fitting. The results of the two techniques were strongly correlated in patients (slope=0.99, intercept=1.58 mL/(100 g/minute), r=0.86) with a small systemic bias of -0.97 mL/(100 g/minute) in Bland-Altman analysis.

CONCLUSION

The three-point estimation technique is adequate for rapid calculation of qCBF. The estimation scheme drastically reduces processing time, thus making the method feasible for clinical use.