The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

Synthesis and Characterization of a Novel Photocleavable Fluorescent Dye Dyad for Diffusion Imaging

We report the synthesis and characterization of a photocleavable fluorescent dye dyad. The two constituting dyes show a large spectral overlap and are in close proximity to each other, leading to efficient Förster Resonance Energy Transfer (FRET). Photocleavage of the dyad and the subsequent independent diffusion of both fluorophores qualifies the system to be used for high accuracy diffusion measurements. In contrast to previous work, the dyad reported here can be applied in polar solvents and cleaved by UV-A light. Beneficially, the photolabile linker provides two orthogonal labeling sites for various commercially available fluorescent labels. In this work, we chose the cationic organic dyes ATTO565 and ATTO647N. We outline the synthesis and spectral characterization of the system with UV-Vis and fluorescence spectroscopy as well as fluorescence lifetime and fluorescence quantum yield measurements. Furthermore, we performed proof-of-principle microscopy experiments to demonstrate its capability in polyvinyl acetate films.

Accelerated dynamic magnetic resonance imaging from Spatial-Subspace Reconstructions (SPARS)

Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) ideally requires a high spatial and a high temporal resolution, but hardware limitations prevent acquisitions from achieving both simultaneously-either high temporal resolution is exchanged for spatial resolution, or vice versa. Even state-of-the-art image reconstruction techniques that infer missing data in a sparse acquisition space cannot recover the loss of spatial detail, especially at high temporal acceleration rates. The purpose of this paper is to introduce the concept of spatial subspace reconstructions (SPARS) and demonstrate its ability to reconstruct high spatial resolution dynamic images from as few as one acquired k-space spoke per time frame in a dynamic series. Briefly, a low-temporal-high-spatial resolution organization of the acquired raw data is used to estimate the basis vectors of the spatial subspace in which the high-temporal-high-spatial ground truth data resides. This subspace is then used to estimate entire images from single k-space spokes. In both simulated and human in-vivo data, the proposed SPARS reconstruction method outperformed standard GRASP and GRASP-Pro reconstruction, providing a shorter reconstruction time and yielding higher accuracy from both a spatial and temporal perspective.

Macro- and microvascular contributions to cerebral structural alterations in patients with asymptomatic carotid artery stenosis

Atherosclerosis can underly internal carotid artery stenosis (ICAS), a major risk factor for ischemic stroke, as well as small vessel disease (SVD). This study aimed to investigate hemodynamics and structural alterations associated with SVD in ICAS patients. 28 patients with unilateral asymptomatic ICAS and 30 age-matched controls underwent structural (T1-/T2-weighted and diffusion tensor imaging [DTI]) and hemodynamic (pseudo-continuous arterial spin labeling and dynamic susceptibility contrast) magnetic resonance imaging. SVD-related alterations were assessed using free water (FW), FW-corrected DTI, and peak-width of skeletonized mean diffusivity (PSMD). Furthermore, cortical thickness, cerebral blood flow (CBF), and capillary transit time heterogeneity (CTH) were analyzed. Ipsilateral to the stenosis, cortical thickness was significantly decreased in the posterior dorsal cingulate cortex (p = 0.024) and temporal pole (p = 0.028). ICAS patients exhibited elevated PSMD (p = 0.005), FW (p < 0.001), and contralateral alterations in FW-corrected DTI metrics. We found significantly lateralized CBF (p = 0.011) and a tendency for lateralized CTH (p = 0.067) in the white matter (WM) related to ICAS. Elevated PSMD and FW may indicate a link between SVD and WM changes. Contralateral alterations were seen in FW-corrected DTI, whereas hemodynamic and cortical changes were mainly ipsilateral, suggesting SVD might influence global brain changes concurrent with ICAS-related hemodynamic alterations.

ASL lexicon and reporting recommendations: A consensus report from the ISMRM Open Science Initiative for Perfusion Imaging (OSIPI)

The 2015 consensus statement published by the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group and the European Cooperation in Science and Technology ( COST) Action ASL in Dementia aimed to encourage the implementation of robust arterial spin labeling (ASL) perfusion MRI for clinical applications and promote consistency across scanner types, sites, and studies. Subsequently, the recommended 3D pseudo-continuous ASL sequence has been implemented by most major MRI manufacturers. However, ASL remains a rapidly and widely developing field, leading inevitably to further divergence of the technique and its associated terminology, which could cause confusion and hamper research reproducibility. On behalf of the ISMRM Perfusion Study Group, and as part of the ISMRM Open Science Initiative for Perfusion Imaging (OSIPI), the ASL Lexicon Task Force has been working on the development of an ASL Lexicon and Reporting Recommendations for perfusion imaging and analysis, aiming to (1) develop standardized, consensus nomenclature and terminology for the broad range of ASL imaging techniques and parameters, as well as for the physiological constants required for quantitative analysis; and (2) provide a community-endorsed recommendation of the imaging parameters that we encourage authors to include when describing ASL methods in scientific reports/papers. In this paper, the sequences and parameters in (pseudo-)continuous ASL, pulsed ASL, velocity-selective ASL, and multi-timepoint ASL for brain perfusion imaging are included. However, the content of the lexicon is not intended to be limited to these techniques, and this paper provides the foundation for a growing online inventory that will be extended by the community as further methods and improvements are developed and established.

Exploring arterial tissue microstructural organization using non-Gaussian diffusion magnetic resonance schemes

The purpose of this study was to characterize the alterations in microstructural organization of arterial tissue using higher-order diffusion magnetic resonance schemes. Three porcine carotid artery models namely; native, collagenase treated and decellularized, were used to estimate the contribution of collagen and smooth muscle cells (SMC) on diffusion signal attenuation using gaussian and non-gaussian schemes. The samples were imaged in a 7 T preclinical scanner. High spatial and angular resolution diffusion weighted images (DWIs) were acquired using two multi-shell (max b-value = 3000 s/mm²) acquisition protocols. The processed DWIs were fitted using monoexponential, stretched-exponential, kurtosis and bi-exponential schemes. Directionally variant and invariant microstructural parametric maps of the three artery models were obtained from the diffusion schemes. The parametric maps were used to assess the sensitivity of each diffusion scheme to collagen and SMC composition in arterial microstructural environment. The inter-model comparison showed significant differences across the considered models. The bi-exponential scheme based slow diffusion compartment (Ds) was highest in the absence of collagen, compared to native and decellularized microenvironments. In intra-model comparison, kurtosis along the radial direction was the highest. Overall, the results of this study demonstrate the efficacy of higher order dMRI schemes in mapping constituent specific alterations in arterial microstructure.

Validation and parameter optimization of a hybrid embedded/homogenized solid tumor perfusion model

The goal of this paper is to investigate the validity of a hybrid embedded/homogenized in-silico approach for modeling perfusion through solid tumors. The rationale behind this novel idea is that only the larger blood vessels have to be explicitly resolved while the smaller scales of the vasculature are homogenized. As opposed to typical discrete or fully resolved 1D-3D models, the required data can be obtained with in-vivo imaging techniques since the morphology of the smaller vessels is not necessary. By contrast, the larger vessels, whose topology and structure is attainable noninvasively, are resolved and embedded as one-dimensional inclusions into the three-dimensional tissue domain which is modeled as a porous medium. A sound mortar-type formulation is employed to couple the two representations of the vasculature. We validate the hybrid model and optimize its parameters by comparing its results to a corresponding fully resolved model based on several well-defined metrics. These tests are performed on a complex data set of three different tumor types with heterogeneous vascular architectures. The correspondence of the hybrid model in terms of mean representative elementary volume blood and interstitial fluid pressures is excellent with relative errors of less than 4%. Larger, but less important and explicable errors are present in terms of blood flow in the smaller, homogenized vessels. We finally discuss and demonstrate how the hybrid model can be further improved to apply it for studies on tumor perfusion and the efficacy of drug delivery.

Optical incoherent technique for noninvasive assessment of blood flow in tissues: Theoretical model and experimental study

Laser noninvasive methods for assessment of a tissue blood flow (BF), for example, the Laser Doppler Flowmetry (LDF), are well-known today. However, in such methods, low-frequency fluctuations (LFFs) in the registered optical signal caused by blood volume changes inside a tissue have not been studied in details until now. The aim of this study is to investigate the LFFs formation and to justify the LFFs-based diagnostic technique for cutaneous BF assessment. LFFs are theoretically described and experimentally shown in the input LDF signal inside the frequency range 0 to 10 Hz. They are substantiated as the basis of the new diagnostic method, in which BF is defined as the magnitude of blood volume changes in a tissue per unit time. The hand-made prototype of the promising diagnostic tool with light emitted diodes is used to validate the technique in experiments in vivo on 16 healthy volunteers in comparison with the LDF method. Experimental results show a good similarity of the recorded BF for both coherent and incoherent method. The proposed technique makes it possible the creation of inexpensive diagnostic equipment for assessment of cutaneous BF without using lasers and coherent light, completely and functionally comparable to LDF devices.

Laparoscopic Ablation System for Complete Circumferential Renal Sympathetic Denervation

GOAL

The catheter-based renal denervation (RDN) showed promising results for patients in lowering BP, but there were also many non-responders. One of the possible reasons was the incomplete neural ablation due to the ablation of renal nerves at random sites resulting in asymmetric innervation patterns along the renal artery.

METHODS

We developed a laparoscopic ablation system that is optimized for complete RDN regardless of renal arterial innervation and size. To demonstrate its effectiveness, we evaluated the system using computational simulation and 28-day survival model using pigs.

RESULTS

The ablations were focused around the tunica externa, and the ablation patterns could be predicted numerically during RDN treatment. In the animal study, the mean reduction of systolic BP and diastolic BP in the bilateral main renal arteries was 22.8 mmHg and 14.4 mmHg (P<0.001), respectively. The respond to immunostaining targeting tyrosine hydroxylase was significantly reduced at treatment site (108.2 ± 7.5 (control) vs. 63.4 ± 8.7 (treatment), P<0.001), and an increased degree of sympathetic signals interruption to kidneys was associated with the efficacy of RDN.

CONCLUSION

The laparoscopic ablation system achieved complete circumferential RDN at the treatment site and could numerically predict the ablation patterns.

SIGNIFICANCE

These findings clearly suggest that the proposed system can significantly improve the RDN effectiveness by reducing the variation to the percentage of injured nerves and open up a new opportunity to treat uncontrolled hypertension.

Dynamic contrast-enhanced MRI in oncology: how we do it

Magnetic resonance imaging (MRI) is particularly attractive for clinical application in perfusion imaging thanks to the absence of ionizing radiation and limited volumes of contrast agent (CA) necessary. Dynamic contrast-enhanced MRI (DCE-MRI) involves sequentially acquiring T1-weighted images through an organ of interest during the passage of a bolus administration of CA. It is a particularly flexible approach to perfusion imaging as the signal intensity time course allows not only rapid qualitative assessment, but also quantitative measures of intrinsic perfusion and permeability parameters. We examine aspects of the T1-weighted image series acquisition, CA administration, post-processing that constitute a DCE-MRI study in clinical practice, before considering some heuristics that may aid in interpreting the resulting contrast enhancement time series. While qualitative DCE-MRI has a well-established role in the diagnostic assessment of a range of tumours, and a central role in MR mammography, clinical use of quantitative DCE-MRI remains limited outside of clinical trials. The recent publication of proposals for standardized acquisition and analysis protocols for DCE-MRI by the Quantitative Imaging Biomarker Alliance may be an opportunity to consolidate and advance clinical practice.

Laparoscopic Renal Denervation System for Treating Resistant Hypertension: Overcoming Limitations of Catheter-Based Approaches

GOAL

In a pivotal clinical trial, the percutaneous catheter-based renal denervation system developed to treat resistant hypertension did not show effectiveness in reducing blood pressure because of its fundamental limitation to ablate deeper nerves present around the renal artery.

METHODS

We propose a new renal denervation strategy called laparoscopicdenervation system (LDS) based-on laparoscopy procedure to ablate the renal nerves completely but inhibit the thermal arterial damage.The system has flexible electrodes to bend around the arterial wall to ablate nervesThe simulation study using validated in-silico models evaluated the heat distributionon the outer arterial wall,and an acute animal study (swine model) was conducted to demonstrate the feasibility of LDS in vivo.

RESULTS

The simulation studyconfirmedthat LDS could localize the heat distributionbetween the electrode and the outer arterial wall. In the animal study, we could maximize nerve denervation by the localizing ablation energy within the renal nerves and achieve nerve denaturationand decrease in neural density by 20.78% (P < 0.001), while maintaining a constant tip temperature of 65 °C for the duration of 70 s treatment. The study confirmed intact lumen artery through histological analysis and acute reduction in systolic blood pressure by 9.55 mmHg (p < 0.001) Conclusion: The LDS presented here has potential to effectively and safely ablate the renal nerves, independent of anatomical variation and nerve distribution, to control hypertension in real clinical conditions.

SIGNIFICANCE

LDS approach is innovative, inventive, and presents a novel technique totreat hypertension.

Axial variation of deoxyhemoglobin density as a source of the low-frequency time lag structure in blood oxygenation level-dependent signals

Perfusion-related information is reportedly embedded in the low-frequency component of a blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal. The blood-propagation pattern through the cerebral vascular tree is detected as an interregional lag variation of spontaneous low-frequency oscillations (sLFOs). Mapping of this lag, or phase, has been implicitly treated as a projection of the vascular tree structure onto real space. While accumulating evidence supports the biological significance of this signal component, the physiological basis of the “perfusion lag structure,” a requirement for an integrative resting-state fMRI-signal model, is lacking. In this study, we conducted analyses furthering the hypothesis that the sLFO is not only largely of systemic origin, but also essentially intrinsic to blood, and hence behaves as a virtual tracer. By summing the small fluctuations of instantaneous phase differences between adjacent vascular regions, a velocity response to respiratory challenges was detected. Regarding the relationship to neurovascular coupling, the removal of the whole lag structure, which can be considered as an optimized global-signal regression, resulted in a reduction of inter-individual variance while preserving the fMRI response. Examination of the T2* and S₀, or non-BOLD, components of the fMRI signal revealed that the lag structure is deoxyhemoglobin dependent, while paradoxically presenting a signal-magnitude reduction in the venous side of the cerebral vasculature. These findings provide insight into the origin of BOLD sLFOs, suggesting that they are highly intrinsic to the circulating blood.

Age-Related Alterations in Retinal Tissue Perfusion and Volumetric Vessel Density

Purpose

To determine age-related alterations in the retinal tissue perfusion (RTP) and volumetric vessel density (VVD) in healthy subjects.

Methods

Total 148 healthy subjects (age 18 to 83 years) were enrolled and divided into four groups (G1, <35 years; G2, 35 ∼ 49 years; G3, 50 ∼ 64 years; and G4, ≥65 years). The RTP and VVD were measured at the macula. The RTP was calculated as the blood flow supplying the macular area (ϕ 2.5 mm) divided by the perfused tissue volume of the inner retina from the inner limiting membrane to the outer plexiform layer. The VVD of the macula (ϕ 2.5 mm) was calculated as the vessel density divided by the corresponding tissue volume.

Results

The RTP and VVD of the retinal vascular network and deep vascular plexus (DVP) reached a peak in G2. Compared to G2, G4 had significantly lower RTP and VVD of DVP (P < 0.05). After 35 years old, age was negatively related to the RTP (r = −0.26, P = 0.02) and VVD of the DVP (r = −0.47, P < 0.001). However, age was positively related to VVD of the superficial vascular plexus (SVP; r = 0.24, P = 0.04) in subjects aged more than 35 years. The RTP was correlated to VVD measurements (r = 0.23–0.37, P < 0.01).

Conclusions

This is the first study to reveal the age-related alterations in the RTP and VVD during normal aging in a healthy population. Decreased RTP and VVD in the DVP along with increased VVD in the SVP may represent a characteristic pattern of normal aging in the healthy population.

A Multilayer Monte Carlo Model for the Investigation of Optical Path and Penetration Depth at Different Perfusion States of the Colon

There is a great interest in monitoring the oxygen supply delivered to the colon. Insufficient oxygen delivery may lead to hypoxia, sepsis, multiorgan dysfunction and death. For assessing colonic perfusion, more information and understanding is required relating to the light-interaction within the colonic tissue. A multilayer Monte Carlo model of a healthy human colon has been developed to investigate the light-tissue behavior during different perfusion states within the mucosal layer of the colon. Results from a static multilayer model of optical path and reflectance at two wavelengths, 660 nm and 880 nm, through colon tissue, containing different volume fractions of blood with a fixed oxygen saturation are presented. The effect on the optical path and penetration depth with varying blood volumes within the mucosa for each wavelength has been demonstrated. The simulation indicated both wavelengths of photons penetrated similar depths, entering the muscularis layer.

Magnetic Resonance Imaging Parameters and Their Impact on Survival of Patients with Glioblastoma: Tumor Perfusion Predicts Survival

BACKGROUND

Many prognostic factors influence overall survival (OS) of patients with glioblastoma. Despite gross total resection and Stupp protocol adherence, many patients have poor survival. Perfusion magnetic resonance imaging may assist in diagnosis, treatment monitoring, and prognostication.

METHODS

This retrospective study of 36 patients with glioblastoma assessed influence of preoperative magnetic resonance imaging parameters reflecting tumor cell density and vascularity and patient age on OS.

RESULTS

The area under curve based on optimal receiver operating characteristic curves for the perfusion parameters normalized relative tumor blood volume (n_rTBV) and normalized relative tumor blood flow (n_rTBF) were 0.92 and 0.89, respectively, and the highest among all imaging parameters and age. OS showed strongly negative correlations with corrected n_rTBV (R = -0.70; P < 0.001) and n_rTBF (R = -0.67; P < 0.001). The Cox model, which included age and imaging parameters, demonstrated that n_rTBV and n_rTBF were most predictive of OS, with hazard ratios of 5.97 (P = 0.0001) and 8.76 (P = 0.0001), respectively, compared with 1.63 (P = 0.19) for age. Eighteen patients with corrected n_rTBV ≤2.5 (best cutoff value) had a median OS of 15.1 months (95% confidence interval (CI), 11.34-21.25) compared with 2.8 months (95% CI, 1.48-4.03; P < 0.001) for 18 patients with corrected n_rTBV >2.5. Twenty-four patients with n_rTBF ≤2.79 had a median OS of 12 months (95% CI, 10.46-17.9) compared with 2.8 months for 12 patients with n_rTBF >2.79 (95% CI, 1.31-4.2; P < 0.001).

CONCLUSIONS

The dominant predictors of OS are normalized perfusion parameters n_rTBV and n_rTBF. Preoperative perfusion imaging may be used as a surrogate to predict glioblastoma aggressiveness and survival independent of treatment.

Computational fluid dynamics with imaging of cleared tissue and of in vivo perfusion predicts drug uptake and treatment responses in tumours

Understanding the uptake of a drug by diseased tissue, and the drug's subsequent spatiotemporal distribution, are central factors in the development of effective targeted therapies. However, the interaction between the pathophysiology of diseased tissue and individual therapeutic agents can be complex, and can vary across tissue types and across subjects. Here, we show that the combination of mathematical modelling, high-resolution optical imaging of intact and optically cleared tumour tissue from animal models, and in vivo imaging of vascular perfusion predicts the heterogeneous uptake, by large tissue samples, of specific therapeutic agents, as well as their spatiotemporal distribution. In particular, by using murine models of colorectal cancer and glioma, we report and validate predictions of steady-state blood flow and intravascular and interstitial fluid pressure in tumours, of the spatially heterogeneous uptake of chelated gadolinium by tumours, and of the effect of a vascular disrupting agent on tumour vasculature.

Simultaneous magnetic resonance diffusion and pseudo-diffusion tensor imaging

PURPOSE

An emerging topic in diffusion magnetic resonance is imaging blood microcirculation alongside water diffusion using the intravoxel incoherent motion (IVIM) model. Recently, a combined IVIM diffusion tensor imaging (IVIM-DTI) model was proposed, which accounts for both anisotropic pseudo-diffusion due to blood microcirculation and anisotropic diffusion due to tissue microstructures. In this article, we propose a robust IVIM-DTI approach for simultaneous diffusion and pseudo-diffusion tensor imaging.

METHODS

Conventional IVIM estimation methods can be broadly divided into two-step (diffusion and pseudo-diffusion estimated separately) and one-step (diffusion and pseudo-diffusion estimated simultaneously) methods. Here, both methods were applied on the IVIM-DTI model. An improved one-step method based on damped Gauss-Newton algorithm and a Gaussian prior for the model parameters was also introduced. The sensitivities of these methods to different parameter initializations were tested with realistic in silico simulations and experimental in vivo data.

RESULTS

The one-step damped Gauss-Newton method with a Gaussian prior was less sensitive to noise and the choice of initial parameters and delivered more accurate estimates of IVIM-DTI parameters compared to the other methods.

CONCLUSION

One-step estimation using damped Gauss-Newton and a Gaussian prior is a robust method for simultaneous diffusion and pseudo-diffusion tensor imaging using IVIM-DTI model. Magn Reson Med 79:2367-2378, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

MRI of cerebral micro-vascular flow patterns: A multi-direction diffusion-weighted ASL approach

The study and clinical assessment of brain disease is currently hindered by a lack of non-invasive methods for the detailed and accurate evaluation of cerebral vascular pathology. Angiography can detect aberrant flow in larger feeding arteries/arterioles but cannot resolve the micro-vascular network. Small vessels are a key site of vascular pathology that can lead to haemorrhage and infarction, which may in turn trigger or exacerbate neurodegenerative processes. In this study, we describe a method to investigate microvascular flow anisotropy using a hybrid arterial spin labelling and multi-direction diffusion-weighted MRI sequence. We present evidence that the technique is sensitive to the mean/predominant direction of microvascular flow in localised regions of the rat cortex. The data provide proof of principle for a novel and non-invasive imaging tool to investigate cerebral micro-vascular flow patterns in healthy and disease states.

Enhanced Laws textures: A potential MRI surrogate marker of hepatic fibrosis in a murine model

PURPOSE

To compare enhanced Laws textures derived from parametric proton density (PD) maps to other MRI surrogate markers (T₂, PD, apparent diffusion coefficient (ADC)) in assessing degrees of liver fibrosis in an ex vivo murine model of hepatic fibrosis imaged using 11.7T MRI.

METHODS

This animal study was IACUC approved. Fourteen male, C57BL/6 mice were divided into control and experimental groups. The latter were fed a 3,5-dicarbethoxy-1,4-dihydrocollidine (DDC) supplemented diet to induce hepatic fibrosis. Ex vivo liver specimens were imaged using an 11.7T scanner, from which the parametric PD, T₂, and ADC maps were generated from spin-echo pulsed field gradient and multi-echo spin-echo acquisitions. A sequential enhanced Laws texture analysis was applied to the PD maps: automated dual-clustering algorithm, optimal thresholding algorithm, global grayscale correction, and Laws texture features extraction. Degrees of fibrosis were independently assessed by digital image analysis (a.k.a. %Area Fibrosis). Scatterplot graphs comparing enhanced Laws texture features, T₂, PD, and ADC values to degrees of fibrosis were generated and correlation coefficients were calculated.

RESULTS

Hepatic fibrosis and the enhanced Laws texture features were strongly correlated with higher %Area Fibrosis associated with higher Laws textures (r=0.89). Without the proposed enhancements, only a moderate correlation was detected between %Area Fibrosis and unenhanced Laws texture features (r=0.70). Correlation also existed between %Area Fibrosis and ADC (r=0.86), PD (r=0.65), and T₂ (r=0.66).

CONCLUSIONS

Higher degrees of hepatic fibrosis are associated with increased Laws textures. The proposed enhancements could improve the accuracy of Laws texture features significantly.

Diffusion weighted MR and apparent diffusion coefficient measurement in classification and characterization of noncystic focal liver lesions: does a clinical role exist?

The objective of this study was to assess the clinical role of apparent diffusion coefficient (ADC) analysis in noncystic focal liver lesion (FLL) classification/characterization.Six hundred liver magnetic resonances with multi-b (b = 50, 400, 800 s/mm) diffusion-weighted imaging (DwI) were retrospectively reviewed. Mean ADC was measured in 388 lesions (195 benign and 193 malignant) excluding internal necrotic areas. Cystic benign lesions were excluded from analysis. Sensitivity and specificity in distinguishing benign from malignant lesions were calculated. Analysis of variance was performed to detect differences among subgroups of solid lesions.Mean ADC of malignant lesions was 0.980 × 10 mm/s, significantly (P < 0.05) lower than mean ADC of benign lesions (1.433 × 10 mm/s). Applying an ADC cutoff of 1.066 × 10 mm/s, specificity and sensitivity for malignancy were respectively 86.6% and 73.6%. Of all lesions, >1/3 (39.5%) presented values lower than 1 × 10 mm/s, with 90.0% chance of malignancy. Above 1.5 × 10 mm/s (about 20% of all lesions) chance of malignancy was 9.5%.DwI cannot assist in noncystic FLL characterization, but can help in FLL classification in about half the cases.

Kinetic modeling in the context of cerebral blood flow quantification by H2(15)O positron emission tomography: the meaning of the permeability coefficient in Renkin-Crone׳s model revisited at capillary scale

One the one hand, capillary permeability to water is a well-defined concept in microvascular physiology, and linearly relates the net convective or diffusive mass fluxes (by unit area) to the differences in pressure or concentration, respectively, that drive them through the vessel wall. On the other hand, the permeability coefficient is a central parameter introduced when modeling diffusible tracers transfer from blood vessels to tissue in the framework of compartmental models, in such a way that it is implicitly considered as being identical to the capillary permeability. Despite their simplifying assumptions, such models are at the basis of blood flow quantification by H2(15)O Positron Emission Tomgraphy. In the present paper, we use fluid dynamic modeling to compute the transfers of H2(15)O between the blood and brain parenchyma at capillary scale. The analysis of the so-obtained kinetic data by the Renkin-Crone model, the archetypal compartmental model, demonstrates that, in this framework, the permeability coefficient is highly dependent on both flow rate and capillary radius, contrarily to the central hypothesis of the model which states that it is a physiological constant. Thus, the permeability coefficient in Renkin-Crone׳s model is not conceptually identical to the physiologic permeability as implicitly stated in the model. If a permeability coefficient is nevertheless arbitrarily chosen in the computed range, the flow rate determined by the Renkin-Crone model can take highly inaccurate quantitative values. The reasons for this failure of compartmental approaches in the framework of brain blood flow quantification are discussed, highlighting the need for a novel approach enabling to fully exploit the wealth of information available from PET data.

Arterial input function in perfusion MRI: a comprehensive review

Cerebral perfusion, also referred to as cerebral blood flow (CBF), is one of the most important parameters related to brain physiology and function. The technique of dynamic-susceptibility contrast (DSC) MRI is currently the most commonly used MRI method to measure perfusion. It relies on the intravenous injection of a contrast agent and the rapid measurement of the transient signal changes during the passage of the bolus through the brain. Central to quantification of CBF using this technique is the so-called arterial input function (AIF), which describes the contrast agent input to the tissue of interest. Due to its fundamental role, there has been a lot of progress in recent years regarding how and where to measure the AIF, how it influences DSC-MRI quantification, what artefacts one should avoid, and the design of automatic methods to measure the AIF. The AIF is also directly linked to most of the major sources of artefacts in CBF quantification, including partial volume effect, bolus delay and dispersion, peak truncation effects, contrast agent non-linearity, etc. While there have been a number of good review articles on DSC-MRI over the years, these are often comprehensive but, by necessity, with limited in-depth discussion of the various topics covered. This review article covers in greater depth the issues associated with the AIF and their implications for perfusion quantification using DSC-MRI.

Serial diffusion MRI to monitor and model treatment response of the targeted nanotherapy CRLX101

PURPOSE

Targeted nanotherapies are being developed to improve tumor drug delivery and enhance therapeutic response. Techniques that can predict response will facilitate clinical translation and may help define optimal treatment strategies. We evaluated the efficacy of diffusion-weighted magnetic resonance imaging to monitor early response to CRLX101 (a cyclodextrin-based polymer particle containing the DNA topoisomerase I inhibitor camptothecin) nanotherapy (formerly IT-101), and explored its potential as a therapeutic response predictor using a mechanistic model of tumor cell proliferation.

EXPERIMENTAL DESIGN

Diffusion MRI was serially conducted following CRLX101 administration in a mouse lymphoma model. Apparent diffusion coefficients (ADCs) extracted from the data were used as treatment response biomarkers. Animals treated with irinotecan (CPT-11) and saline were imaged for comparison. ADC data were also input into a mathematical model of tumor growth. Histological analysis using cleaved-caspase 3, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, Ki-67, and hematoxylin and eosin (H&E) were conducted on tumor samples for correlation with imaging results.

RESULTS

CRLX101-treated tumors at day 2, 4, and 7 posttreatment exhibited changes in mean ADC = 16 ± 9%, 24 ± 10%, 49 ± 17%, and size (TV) = -5 ± 3%, -30 ± 4%, and -45 ± 13%, respectively. Both parameters were statistically greater than controls [p(ADC) ≤ 0.02, and p(TV) ≤ 0.01 at day 4 and 7], and noticeably greater than CPT-11-treated tumors (ADC = 5 ± 5%, 14 ± 7%, and 18 ± 6%; TV = -15 ± 5%, -22 ± 13%, and -26 ± 8%). Model-derived parameters for cell proliferation obtained using ADC data distinguished CRLX101-treated tumors from controls (P = 0.02).

CONCLUSIONS

Temporal changes in ADC specified early CRLX101 treatment response and could be used to model image-derived cell proliferation rates following treatment. Comparisons of targeted and nontargeted treatments highlight the utility of noninvasive imaging and modeling to evaluate, monitor, and predict responses to targeted nanotherapeutics.

The importance of RF bandwidth for effective tagging in pulsed arterial spin labeling MRI at 9.4T

The movement towards MRI at higher field strengths (>7T) has enhanced the appeal of arterial spin labeling (ASL) for many applications due to improved SNR of the measurements. Greater field strength also introduces increased magnetic susceptibility effects resulting in marked B(0) field inhomogeneity. Although B(0) field perturbations can be minimised by shimming over the imaging volume, marked field inhomogeneity is likely to remain within the labeling region for pulsed ASL (PASL). This study highlights a potential source of error in cerebral blood flow quantification using PASL at high field. We show that labeling efficiency in flow-sensitive alternating inversion recovery (FAIR) displayed marked sensitivity to the RF bandwidth of the inversion pulse in a rat model at 9.4T. The majority of preclinical PASL studies have not reported the bandwidth of the inversion pulse. We show that a high bandwidth pulse of > = 15 kHz was required to robustly overcome the field inhomogeneity in the labeling region at high field strength, which is significantly greater than the inversion bandwidth ~2-3 kHz used in previous studies. Unless SAR levels are at their limit, we suggest the use of a high bandwidth labeling pulse for most PASL studies.

Applications of imaging technology in radiation research

Imaging research and advances in systems engineering have enabled the transition of medical imaging from a means for accomplishing traditional anatomic visualization (i.e., orthopedic planar film X ray) to a means for noninvasively assessing a variety of functional measures. Perfusion imaging is one of the major highlights in functional imaging. In this work, various methods for measuring perfusion using widely-available commercial imaging modalities and contrast agents, specifically X ray and MR (magnetic resonance), will be described. The first section reviews general methods used for perfusion imaging, and the second section provides modality-specific information, focusing on the contrast mechanisms used to calculate perfusion-related parameters. The goal of these descriptions is to illustrate how perfusion imaging can be applied to radiation biology research.

Chronic brain tissue remodeling after stroke in rat: a 1-year multiparametric magnetic resonance imaging study

Rats subjected to 2h of transient middle cerebral artery occlusion were studied temporally over 1 year by magnetic resonance imaging (MRI) and behavioral testing. Multiparameter MRI measures of T(2), T(1), T(1) in the presence of off-resonance saturation of the bound proton signal (T(1sat)), apparent diffusion coefficient (ADC) and susceptibility-weighted imaging (SWI) were obtained at 1 day, 1, 2, 3 and 4 weeks, and 3, 6, 9 and 12 months post-ischemia. Regions of interest included: ischemic core (damaged both at 1 day and later); new lesion (normal at 1 day, but damaged later); and recovery (damaged at 1 day, but normal later) areas. Hematoxylin and eosin, Prussian blue and ED-1, a monoclonal antibody murine macrophage marker, stainings were performed for histological assessment. Core area T(2) and ADC values increased until ~6 months, and T(1) and T(1sat) until ~12 months. New lesion area MRI parameter values increased until ~6 months (T(2), T(1) and ADC), or ~1 year (T(1sat)). Lesion area was largest at 1day (mean±SD: 37.0±13.7mm(2)) and smallest at 1 year (18.1±10.5mm(2)). Recovery area was largest at 3 weeks (8.9±3.8mm(2)) and smallest at 1year (6.4±3.3mm(2)). The ipsilateral/contralateral ventricle area ratio was 0.7±0.2 at 1 day and increased significantly at 1 year (2.4±0.7). Iron-laden macrophages, histologically confirmed at 1 year, were detected in the lesion borders by SWI at 3, 6, 9 and 12 months. Our data indicate that MRI detectable changes of ischemia-damaged brain tissue continue for at least 1 year post-ischemia.

Track-density imaging (TDI): super-resolution white matter imaging using whole-brain track-density mapping

Neuroimaging advances have given rise to major progress in neurosciences and neurology, as ever more subtle and specific imaging methods reveal new aspects of the brain. One major limitation of current methods is the spatial scale of the information available. We present an approach to gain spatial resolution using post-processing methods based on diffusion MRI fiber-tracking, to reveal structures beyond the resolution of the acquired imaging voxel; we term such a method as super-resolution track-density imaging (TDI). A major unmet challenge in imaging is the identification of abnormalities in white matter as a cause of illness; super-resolution TDI is shown to produce high-quality white matter images, with high spatial resolution and outstanding anatomical contrast. A unique property of these maps is demonstrated: their spatial resolution and signal-to-noise ratio can be tailored depending on the chosen image resolution and total number of fiber-tracks generated. Super-resolution TDI should greatly enhance the study of white matter in disorders of the brain and mind.

Characterizing the origin of the arterial spin labelling signal in MRI using a multiecho acquisition approach

Arterial spin labelling (ASL) can noninvasively isolate the MR signal from arterial blood water that has flowed into the brain. In gray matter, the labelled bolus is dispersed within three main compartments during image acquisition: the intravascular compartment; intracellular tissue space; and the extracellular tissue space. Changes in the relative volumes of the extracellular and intracellular tissue space are thought to occur in many pathologic conditions such as stroke and brain tumors. Accurate measurement of the distribution of the ASL signal within these three compartments will yield better understanding of the time course of blood delivery and exchange, and may have particular application in animal models of disease to investigate the relationship between the source of the ASL signal and pathology. In this study, we sample the transverse relaxation of the ASL perfusion weighted and control images acquired with and without vascular crusher gradients at a range of postlabelling delays and tagging durations, to estimate the tricompartmental distribution of labelled water in the rat cortex. Our results provide evidence for rapid exchange of labelled blood water into the intracellular space relative to the transit time through the vascular bed, and provide a more solid foundation for cerebral blood flow quantification using ASL techniques.

Feasibility study of particle-assisted laser ablation of brain tumors in orthotopic canine model

We report on a pilot study showing a proof of concept for the passive delivery of nanoshells to an orthotopic tumor where they induce a local, confined therapeutic response distinct from that of normal brain resulting in the photothermal ablation of canine transmissible venereal tumor (cTVT) in a canine brain model. cTVT fragments grown in severe combined immunodeficient mice were successfully inoculated in the parietal lobe of immunosuppressed, mixed-breed hound dogs. A single dose of near-IR (NIR)-absorbing, 150-nm nanoshells was infused i.v. and allowed time to passively accumulate in the intracranial tumors, which served as a proxy for an orthotopic brain metastasis. The nanoshells accumulated within the intracranial cTVT, suggesting that its neovasculature represented an interruption of the normal blood-brain barrier. Tumors were thermally ablated by percutaneous, optical fiber-delivered, NIR radiation using a 3.5-W average, 3-minute laser dose at 808 nm that selectively elevated the temperature of tumor tissue to 65.8 +/- 4.1 degrees C. Identical laser doses applied to normal white and gray matter on the contralateral side of the brain yielded sublethal temperatures of 48.6 +/- 1.1 degrees C. The laser dose was designed to minimize thermal damage to normal brain tissue in the absence of nanoshells and compensate for variability in the accumulation of nanoshells in tumor. Postmortem histopathology of treated brain sections showed the effectiveness and selectivity of the nanoshell-assisted thermal ablation.

Protective effect of post-ischaemic viral delivery of heat shock proteins in vivo

Heat shock proteins (HSPs) function as molecular chaperones involved in protein folding, transport and degradation and, in addition, they can promote cell survival both in vitro and in vivo after a range of stresses. Although some in vivo studies have suggested that HSP27 and HSP70 can be neuroprotective, current evidence is limited, particularly when HSPs have been delivered after an insult. The effect of overexpressing HSPs after transient occlusion of the middle cerebral artery in rats was investigated by delivering an attenuated herpes simplex viral vector (HSV-1) engineered to express HSP27 or HSP70 30 mins after tissue reperfusion. Magnetic resonance imaging scans were used to determine lesion size and cerebral blood flow at six different time points up to 1 month after stroke. Animals underwent two sensorimotor tests at the same time points to assess the relationship between lesion size and function. Results indicate that post-ischaemic viral delivery of HSP27, but not of HSP70, caused a statistically significant reduction in lesion size and induced a significant behavioural improvement compared with controls. This is the first evidence of effective post-ischaemic gene therapy with a viral vector expressing HSP27 in an experimental model of stroke.

Experimental comparison of four FAIR arterial spin labeling techniques for quantification of mouse cerebral blood flow at 4.7 T

Pulsed arterial spin labeling (ASL) is an attractive and robust method for quantification of rodent cerebral blood flow (CBF) in particular, although there is a need for sensitivity optimization. Look-Locker flow-sensitive alternating inversion recovery (FAIR) echo planar imaging (EPI) (LLFAIREPI) was expected to be a likely candidate for assessing sensitivity, although it has not yet been applied to rodents. In this study, the performance of two FAIR techniques and two Look-Locker FAIR techniques were compared in mouse brain at 4.7 T. FAIR-EPI (single inversion time, FAIREPI-1TI), FAIR-EPI (eight inversion times, FAIREPI-8TI), LLFAIREPI and Look-Locker FAIR gradient echo (LLFAIRGE) sequences were implemented with equal spatial resolution and equal FAIR preparation modules. Measurements were carried out sequentially on the brain in 10 healthy mice, and quantitative CBF maps were obtained after different acquisition times up to 23 min. All methods gave similar group variability in CBF. Especially at shorter acquisition times, LLFAIREPI gave lower relative variations in CBF within selected brain regions than the other techniques at the same acquisition time. The Look-Locker techniques, however, overestimated CBF compared with classical FAIR-EPI, which was attributed to bulk flow in arterioles and T(2) effects. The image quality with LLFAIREPI was less reproducible within the group. Both FAIREPI-1TI and LLFAIREPI appear to be good candidates for serial rapid measurement of CBF, but LLFAIREPI has the additional advantage that apparent T(1) can be measured simultaneously with CBF.

Mapping spatiotemporal diffusion inside the human brain using a numerical solution of the diffusion equation

Diffusion is an important mechanism for molecular transport in living biological tissues. Diffusion magnetic resonance imaging (dMRI) provides a unique probe to examine microscopic structures of the tissues in vivo, but current dMRI techniques usually ignore the spatiotemporal evolution process of the diffusive medium. In the present study, we demonstrate the feasibility to reveal the spatiotemporal diffusion process inside the human brain based on a numerical solution of the diffusion equation. Normal human subjects were scanned with a diffusion tensor imaging (DTI) technique on a 3-T MRI scanner, and the diffusion tensor in each voxel was calculated from the DTI data. The diffusion equation, a partial-derivative description of Fick's law for the diffusion process, was discretized into equivalent algebraic equations. A finite-difference method was employed to obtain the numerical solution of the diffusion equation with a Crank-Nicholson iteration scheme to enhance the numerical stability. By specifying boundary and initial conditions, the spatiotemporal evolution of the diffusion process inside the brain can be virtually reconstructed. Our results exhibit similar medium profiles and diffusion coefficients as those of light fluorescence dextrans measured in integrative optical imaging experiments. The proposed method highlights the feasibility to noninvasively estimate the macroscopic diffusive transport time for a molecule in a given region of the brain.

The influence of diffusion- and relaxation-related factors on signal intensity: an introductive guide to magnetic resonance diffusion-weighted imaging studies

In magnetic resonance diffusion-weighted imaging, signal intensity is influenced simultaneously by temperature, diffusivity, b value, pseudodiffusion, macroscopic motion, and T2-weighted intensity value. The purpose of this pictorial essay is to discuss and exemplify the influence that such factors and the related modifications have on signal intensity. Apparent diffusion coefficient, shine-through and pseudodiffusion will also be examined to show how T2-weighted signal intensity value and nondiffusional intravoxel incoherent motion can affect the diffusion-weighted imaging.

Multi-modal magnetic resonance imaging alterations in two rat models of mild neurotrauma

Magnetic resonance imaging (MRI) is increasingly used in the assessment of the severity and progression of neurotrauma. We evaluated temporal and regional changes after mild fluid percussion (FPI) and controlled cortical impact (CCI) injury using T2-weighted-imaging (T2WI) and diffusion-weighted imaging (DWI) MRI over 7 days. Region of interest analysis of brain areas distant to the injury site (such as the hippocampus, retrosplenial and piriform cortices, and the thalamus) was undertaken. In the hippocampus of CCI animals, we found a slow increase (51%) in apparent diffusion coefficients (ADC) over 72 h, which returned to control values. The hippocampal T2 values in the CCI animals were elevated by 18% over the 7-day time course compared to control, indicative of edema formation. Histological analysis supported the lack of overt cellular loss in most brain regions after mild CCI injury. FPI animals showed a generalized decrease in hippocampal ADC values over the first 72 h, which then returned to sham levels, with decreased T2 values over the same period, which remained depressed at 7 days. Histological assessment of FPI animals revealed numerous shrunken cells in the hippocampus and thalamus, but other regions showed little damage. Increased immunohistochemical staining for microglia and astroglia at 7 days post-injury was greater in FPI animals within the affected brain regions. In summary, traumatic brain injury is less severe in mild CCI than FPI, based on the temporal events assessed with MRI.

In vivo MRI reveals the dynamics of pathological changes in the brains of cathepsin D-deficient mice and correlates changes in manganese-enhanced MRI with microglial activation

Cathepsin D (CTSD; EC 3.4.23.5) is essential for normal development and/or maintenance of neurons in the central nervous system: its deficiency causes a devastating neurological disorder with severely shortened life span in man, sheep and mouse. Neuropathologically, the CTSD deficiencies are characterized by selective neuronal degeneration, gliosis and accumulation of autofluorescent proteinaceous storage material in neurons. Our aim was to study the dynamics behind the pathological alterations occurring in the brains of CTSD-deficient (CTSD-/-) mice by using in vivo magnetic resonance imaging (MRI) and histology. In order to do this, we measured T(2) signal intensity (SI), apparent diffusion coefficient, area and volume of multiple brain structures from MR images acquired using T(2)-, T(1)- and diffusion-weighted sequences at three time points during disease progression. MRI revealed no differences in the brains between CTSD-/- and control mice at postnatal day 15+/-1 (P15+/-1), representing an initial stage of the disease. In the intermediate stage of the disease, P19(+/-1), SI alterations in the thalami of the affected mice became evident in both T(1)- and T(2)-weighted images. The terminal stage of the disease, P25, was characterized by marked alterations in the T(2) SI, apparent diffusion coefficient and volume of multiple brain structures in CTSD-/- mice. In addition, manganese enhanced high-resolution T(1)-weighted 3D sequences (MEMRI) and histological stainings revealed that the hyperintense signal areas in MEMRI matched perfectly with areas of microglial activation in the brains of CTSD-/- mice at the terminal disease stage. In conclusion, the SI alterations in the thalami of CTSD-/- mice preceded other changes, and the degenerative process was greatly enhanced at the age P19(+/-1), leading to severely reduced brain volume in just 6 days.

In vivo measurement of the longitudinal relaxation time of arterial blood (T1a) in the mouse using a pulsed arterial spin labeling approach

A novel method for measuring the longitudinal relaxation time of arterial blood (T1a) is presented. Knowledge of T1a is essential for accurately quantifying cerebral perfusion using arterial spin labeling (ASL) techniques. The method is based on the flow-sensitive alternating inversion recovery (FAIR) pulsed ASL (PASL) approach. We modified the standard FAIR acquisition scheme by incorporating a global saturation pulse at the beginning of the recovery period. With this approach the FAIR tissue signal difference has a simple monoexponential dependence on the recovery time, with T1a as the time constant. Therefore, FAIR measurements performed over a range of recovery times can be fitted to a monoexponential recovery curve and T1a can be calculated directly. This eliminates many of the difficulties associated with the measurement of T1a. Experiments performed in vivo in the mouse at 2.35T produced a mean value of 1.51 s for T1a, consistent with previously published values.

The chronic vascular and haemodynamic response after permanent bilateral common carotid occlusion in newborn and adult rats

Vascular growth and redistribution of flow can compensate for arterial occlusion and possibly reduce the effects of hypoperfusion. As yet there is limited information on the age-dependent nature of vasculature remodelling. In this study, we have monitored the vascular and morphologic changes using magnetic resonance imaging and histology in a chronic bilateral common carotid artery occlusion (BCCAO) model in both newborn and adult rats. Acutely, cerebral blood flow (CBF) decreased immediately after BCCAO, producing a state of oligemic hypoperfusion. At 6 months after BCCAO in both adult and neonatal rats, the CBF had normalised at control values. To investigate the underlying mechanism for the return of CBF to control values, intra- and extracerebral magnetic resonance angiograms (MRAs) were acquired. As expected, signal from the common carotid arteries was present in the sham-operated rats, but was absent in the BCCAO animals. India ink angiograms demonstrated more tortuous basilar arteries in the adult rats post-BCCAO and MRAs demonstrated more extracerebral midline collaterals in the neonatal rats post-BCCAO, indicating different modes of vascular adaptation dependent on the age at onset of the insult. Both groups had collateral vessels arising from the vertebral arteries, and BCCAO was also associated with increased diameter of basilar, posterior cerebral, posterior communicating, internal carotid, middle cerebral and anterior cerebral arteries. Our study suggests that the developing and mature animals exhibit different patterns of vascular remodelling and that the BCCAO hypoperfusion model will be useful for investigating age-dependent vascular events in response to vaso-occlusive disease.

Stroke Imaging at 3.0 T

Stroke is a devastating disease with a complex pathophysiology. It is a major cause of death and disability in North America. To fully characterize its extent and effects, one requires numerous specialized anatomical and functional MR techniques, specifically diffusion-weighted imaging, MR angiography, and perfusion-weighted imaging. The advent of 3.0 T clinical scanners has the potential to provide higher quality information in potentially less time compared with 1.5 T stroke-specific MR imaging protocols. This article gives a brief overview of stroke, presents the principles and clinical applications of the relevant MR techniques required for diagnostic stroke imaging at high field, and discusses the advantages, challenges, and limitations of 3.0 T imaging as they relate to stroke.

Structural MRI of carotid artery atherosclerotic lesion burden and characterization of hemispheric cerebral blood flow before and after carotid endarterectomy

Collateral circulation plays a major role in maintaining cerebral blood flow (CBF) in patients with internal carotid artery (ICA) stenosis. CBF can remain normal despite severe ICA stenosis, making the benefit of carotid endarterectomy (CEA) or stenting difficult to assess. Before and after surgery, we assessed CBF supplied through the ipsilateral (stenotic) or contralateral ICA individually with a novel hemisphere-selective arterial spin-labeling (ASL) perfusion MR technique. We further explored the relationship between CBF and ICA obstruction ratio (OR) acquired with a multislice black-blood imaging sequence. For patients with unilateral ICA stenosis (n = 19), conventional bilateral labeling did not reveal interhemispheric differences. With unilateral labeling, CBF in the middle cerebral artery (MCA) territory on the surgical side from the ipsilateral supply (53.7 +/- 3.3 ml/100 g/min) was lower than CBF in the contralateral MCA territory from the contralateral supply (58.5 +/- 2.7 ml/100 g/min), although not statistically significant (p = 0.09). The ipsilateral MCA territory received significant (p = 0.02) contralateral supply (7.0 +/- 2.7 ml/100 g/min), while ipsilateral supply to the contralateral side was not reciprocated. After surgery (n = 11), ipsilateral supply to the MCA territory increased from 57.3 +/- 5.7 to 67.3 +/- 5.4 ml/100 g/min (p = 0.03), and contralateral supply to the ipsilateral MCA territory decreased. The best predictor of increased CBF on the side of surgery was normalized presurgical ipsilateral supply (r(2) = 0.62, p = 0.004). OR was less predictive of change, although the change in normalized contralateral supply was negatively correlated with OR(excess) (=OR(ipsilateral) - OR(contralateral)) (r(2) = 0.58, p = 0.006). The results demonstrate the effect of carotid artery stenosis on blood supply to the cerebral hemispheres, as well as the relative role of collateral pathways before surgery and redistribution of blood flow through these pathways after surgery. Unilateral ASL may better predict hemodynamic surgical outcome (measured by improved perfusion) than ICA OR.

Regional variation of cerebral blood flow and arterial transit time in the normal and hypoperfused rat brain measured using continuous arterial spin labeling MRI

Continuous arterial spin labeling (CASL) is a noninvasive magnetic resonance (MR) method for measuring cerebral perfusion. In its most widely used form, CASL incorporates a postlabeling delay to minimize the sensitivity of the technique to transit time effects, which otherwise corrupt cerebral blood flow (CBF) quantification. For this delay to work effectively, it must be longer than the longest transit time present in the system. In this work, CASL measurements were made in four coronal slices in the rat brain using a range of postlabeling delays. By doing this, direct estimation of both CBF and arterial transit time (delta(a)) was possible. These measurements were performed in the normal brain and during hypoperfusion induced by occlusion of the common carotid arteries. It was found that, in the normal rat brain, significant regional variation exists for both CBF and delta(a). Mean values of CBF and delta(a) in the selected gray matter regions of interest were 233 mL/100 g min and 266 ms, respectively, with the latter ranging from 100 to 500 ms. Therefore, use of a 500-ms postlabeling delay is suitable for any location in the normal rat brain. After common carotid artery occlusion, CBF decreased and delta(a) increased by regionally dependent amounts. In the sensory cortex, delta(a) increased to a mean value of 740 ms, significantly greater than 500 ms. These results highlight the importance of either (a) determining delta(a) as part of the CASL measurement or (b) knowing the approximate range of values delta(a) is likely to take for a given application, so that the parameters of the CASL sequence can be chosen appropriately.