Perfusion and diffusion MR imaging

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.

Early Changes in Glutamate Metabolism and Perfusion in Basal Ganglia following Hypoxia-Ischemia in Neonatal Piglets: A Multi-Sequence 3.0T MR Study

The excitotoxicity of glutamate metabolism as well as hemodynamic disorders of the brain are both risk factors for neonatal hypoxic–ischemic brain damage (HIBD). In the present study, changes in glutamate metabolism in the basal ganglia were detected by proton magnetic resonance spectroscopy (¹H-MRS) at 0–6, 8–12, 24–30, and 48–60 h after the induction of hypoxia-ischemia (HI) in newborn piglets. Meanwhile, correlation analysis was performed by combining the microcirculatory perfusion informations acquired by intravoxel incoherent motion (IVIM) scan to explore their possible interaction mechanism. The results suggested that Glu level in the basal ganglia underwent a “two-phase” change after HI; perfusion fraction f, an IVIM-derived perfusion parameter, was clearly decreased in the early stage after HI, then demonstrated a transient and slight recovery process, and thereafter continued to decrease. The changes in f and Glu level were in a significant negative correlation (r = −0.643, P = 0.001). Our study results revealed that Glu level is closely associated with the microcirculatory perfusion changes in the acute stage of HIBD.

Evaluation of Hepatic Tumors Using Intravoxel Incoherent Motion Diffusion-Weighted MRI

Background

This study aimed to evaluate the diagnostic value of the D value, D* value, and f magnitude for identifying benign and malignant hepatic tumors using intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI).

Material/Methods

Data of 89 cases (123 lesions) with hepatic tumor confirmed by surgical pathology and postoperative follow-up were retrospectively collected. Among these cases, 40 cases were benign hepatic tumors (57 lesions) and 49 cases were malignant hepatic tumors (66 lesions). All subjects underwent conventional MRI with T₁WI, T₂WI, multi-b-value DWI, and dynamic enhanced LAVA scan. Diffusion-weighted images with 11 b values (0, 10, 20, 30, 50, 80, 100, 200, 400, 800, and 1000 s/mm²) were obtained to calculate true molecular diffusion (D), perfusion-related diffusion coefficient (D*), and perfusion fraction (f). The diagnostic performance in differentiating between malignant and benign hepatic lesions was analyzed.

Results

Malignant lesions had a significantly lower D value ([1.04±0.34]×10⁻³ mm²/s) and D* value ([16.5±7.7]×10⁻³ mm²/s) compared to benign lesions (D value: [1.70±0.55]×10⁻³ mm²/s, P<0.01; D* value: [21.7±9.9]×10⁻³ mm²/s, P<0.01). There was no statistically significant difference in f values between malignant (23.3±9.5) and benign lesions (33.5±14.9, P=0.13). In addition, D exhibited a better diagnostic performance than D* in terms of the area under the curve, sensitivity, and specificity when identifying malignancies from benign lesions.

Conclusions

D and D* are significant parameters for diagnosing hepatic tumors. Moreover, the D value is a more reliable parameter in distinguishing benign and malignant hepatic tumors.

Impact of the calculation algorithm on biexponential fitting of diffusion-weighted MRI in upper abdominal organs

PURPOSE

To compare the variability, precision, and accuracy of six different algorithms (Levenberg-Marquardt, Trust-Region, Fixed-Dp , Segmented-Unconstrained, Segmented-Constrained, and Bayesian-Probability) for computing intravoxel-incoherent-motion-related parameters in upper abdominal organs.

METHODS

Following the acquisition of abdominal diffusion-weighted magnetic resonance images of 10 healthy men, six distinct algorithms were employed to compute intravoxel-incoherent-motion-related parameters in the left and right liver lobe, pancreas, spleen, renal cortex, and renal medulla. Algorithms were evaluated regarding inter-reader and intersubject variability. Comparability of results was assessed by analyses of variance. The algorithms' precision and accuracy were investigated on simulated data.

RESULTS

A Bayesian-Probability based approach was associated with very low inter-reader variability (average Intraclass Correlation Coefficients: 96.5-99.6%), the lowest inter-subject variability (Coefficients of Variation [CV] for the pure diffusion coefficient Dt : 3.8% in the renal medulla, 6.6% in the renal cortex, 10.4-12.1% in the left and right liver lobe, 15.3% in the spleen, 15.8% in the pancreas; for the perfusion fraction Fp : 15.5% on average; for the pseudodiffusion coefficient Dp : 25.8% on average), and the highest precision and accuracy. Results differed significantly (P < 0.05) across algorithms in all anatomical regions.

CONCLUSION

The Bayesian-Probability algorithm should be preferred when computing intravoxel-incoherent-motion-related parameters in upper abdominal organs.

Intravoxel Incoherent Motion Diffusion Weighted MR Imaging for Monitoring the Instantly Therapeutic Efficacy of Radiofrequency Ablation in Rabbit VX2 Tumors without Evident Links between Conventional Perfusion Weighted Images

OBJECTIVE

To investigate the intravoxel incoherent motion diffusion weighted imaging (IVIM-DWI) as a potential valuable marker to monitor the therapy responses of VX2 to radiofrequency ablation (RF Ablation).

METHODS

The institutional animal care and use committee approved this study. In 10 VX2 tumor-bearing rabbits, IVIM-DWI examinations were performed with a 3.0T imaging unit by using 16 b values from 0 to 800 sec/mm2. The true diffusion coefficient (D), pseudodiffusion coefficient (D*) and perfusion fraction (f) of tumors were compared between before and instantly after RF Ablation treatment. The differences of D, D* and f and conventional perfusion parameters (from perfusion CT and dynamic enhanced magnetic resonance imaging, DCE-MRI) in the coagulation necrosis area, residual unablated area, untreated area, and normal control had been calculated by compared t-test. The correlation between f or D* with perfusion weighted CT including blood flow, BF (milliliter per 100 mL/min), blood volume, BV (milliliter per 100 mL/min), and capillary permeability-surface area, PMB (as a fraction) or from DCE-MRI: transfer constant (Ktrans), extra-vascular extra-cellular volume fraction (Ve) and reflux constant (Kep) values had been analyzed by region-of-interest (ROI) methods to calculate Pearson's correlation coefficients.

RESULTS

In the ablated necrosis areas, f and D* significantly decreased and D significantly increased, compared with residual unblazed areas or untreated control groups and normal control groups (P < 0.001). The IVIM-DWI derived f parameters showed significant increases in the residual unablated tumor area. There was no significant correlations between f or D* and conventional perfusion parameters.

CONCLUSIONS

The IVIM-DW derived f, D and D* parameters have the potential to indicate therapy response immediately after RF Ablation treatment, while no significant correlations with classical tumor perfusion metrics were derived from DCE-MRI and perfusion-CT measurements.

Correlative assessment of tumor microcirculation using contrast-enhanced perfusion MRI and intravoxel incoherent motion diffusion-weighted MRI: is there a link between them?

The purpose of this study was to correlate intravoxel incoherent motion (IVIM) imaging with classical perfusion-weighted MRI metrics in human gliomas. Parametric images for slow diffusion coefficient (D), fast diffusion coefficient (D*), and fractional perfusion-related volume (f) in patients with high-grade gliomas were generated. Maps of Fp (plasma flow), vp (vascular plasma volume), PS (permeability surface-area product), ve (extravascular, extracellular volume), E (extraction ratio), ke (influx ratio into the interstitium), and tc (vascular transit time) from dynamic contrast-enhanced (DCE) and dynamic susceptibility contrast-enhanced (DSC) MRI were also generated. A region-of-interest analysis on the contralateral healthy white matter and on the tumor areas was performed and the extracted parameter values were tested for any significant differences among tumor grades or any correlations. Only f could be significantly correlated to DSC-derived vp and tc in healthy brain tissue. Concerning the tumor regions, Fp was significantly positively correlated with D* and inversely correlated with f in DSC measurements. The D*, f, and f × D* values in the WHO grade III gliomas were non-significantly different from those in the grade IV gliomas. There was a trend to significant negative correlations between f and PS as well as between f × D* and ke in DCE experiments. Presumably due to different theoretical background, tracer properties and modeling of the tumor vasculature in the IVIM theory, there is no clearly evident link between D*, f and DSC- and DCE-derived metrics.

Diffusion-weighted magnetic resonance imaging of the prostate: improved robustness with stretched exponential modeling

PURPOSE

This study aimed to compare the intraclass correlation coefficients of parameters estimated with stretched exponential and biexponential diffusion models of in vivo diffusion-weighted magnetic resonance imaging (MRI) of the prostate.

METHODS

After the institutional review board issued a waiver of informed consent for this Health Insurance Portability and Accountability Act-compliant study, 25 patients with biopsy-proven prostate cancer underwent 3T endorectal MRI and diffusion-weighted MRI of the prostate at 10 b values (0, 45, 75, 105, 150, 225, 300, 600, 900, and 1200 s/mm). The full set of b values was collected twice within a single acquisition. Intraclass correlation coefficients were calculated for intra-acquisition variability. From the biexponential model, the quantitative parameters diffusion coefficient (D), perfusion coefficient (D*), and perfusion fraction (f) were estimated. From the stretched exponential model, the quantitative parameters Kohlrausch decay constant (DK) and alpha (α) were estimated.

RESULTS

For the 25 patient data sets, the average intraclass correlation coefficients for DK and α were 95.8%, and 64.1%, respectively, whereas those for D, D*, and f were 84.4%, 25.3%, and 41.3%, respectively.

CONCLUSIONS

The stretched exponential diffusion model captures the nonlinear effects of intravoxel incoherent motion in the prostate. The parameters derived from this model are more reliable and reproducible than the parameters derived from the standard, widely used biexponential diffusion/perfusion model.

Measuring biexponential transverse relaxation of the ASL signal at 9.4 T to estimate arterial oxygen saturation and the time of exchange of labeled blood water into cortical brain tissue

The transverse decay of the arterial spin labeling (ASL) signal was measured at four inflow times in the rat brain cortex at 9.4 T. Biexponential T2 decay was observed that appears to derive from different T2 values associated with labeled water in the intravasculature (IV) and extravascular (EV) compartments. A two compartment biexponential model was used to assess the relative contribution of the IV and EV compartments to the ASL signal, without assuming a value for T2 of labeled blood water in the vessels. This novel methodology was applied to estimate the exchange time of blood water into EV tissue space and the oxygen saturation of blood on the arterial side of the vasculature. The mean exchange time of labeled blood water was estimated to be 370±40 ms. The oxygen saturation of the arterial side of the vasculature was significantly less than 100% (∼85%), which may have implications for quantitative functional magnetic resonance imaging studies where the arterial oxygen saturation is frequently assumed to be 100%.

Fat deposition decreases diffusion parameters at MRI: a study in phantoms and patients with liver steatosis

PURPOSE

Assess the effect of fat deposition on the MRI diffusion coefficients in lipid emulsion-based phantoms and patients with proven isolated liver steatosis.

MATERIALS AND METHODS

Diffusion-weighted MRI with 11 b values from 0-500 s/mm(2) was performed in phantoms (fat fractions 0-18 %) with and without fat suppression and in 19 patients with normal liver (n = 14) or isolated liver steatosis (n = 5) proven by histopathology. The apparent, pure and perfusion-related diffusion coefficients and the perfusion fraction were measured. Spearman correlation coefficient and Mann-Whitney U test were used for comparisons.

RESULTS

A strong correlation between the apparent and pure diffusion coefficients and fat fractions was seen in phantoms. The pure diffusion coefficient decreased significantly in patients with liver steatosis (0.96 ± 0.16 × 10(-3) mm(2)/s versus 1.18 ± 0.09 × 10(-3) mm(2)/s in normal liver, P = 0.005), whereas the decrease in apparent diffusion coefficient did not reach statistical significance (1.26 ± 0.25 × 10(-3) mm(2)/s versus 1.41 ± 0.14 × 10(-3) mm(2)/s in normal liver, P = 0.298).

CONCLUSIONS

Fat deposition decreases the apparent and pure diffusion coefficients in lipid emulsion-based phantoms and patients with isolated liver steatosis proven by histopathology.

Intravoxel incoherent motion and diffusion-tensor imaging in renal tissue under hydration and furosemide flow challenges

PURPOSE

To assess the reproducibility and the distribution of intravoxel incoherent motion (IVIM) and diffusion-tensor (DT) imaging parameters in healthy renal cortex and medulla at baseline and after hydration or furosemide challenges.

MATERIALS AND METHODS

Using an institutional review board-approved HIPAA-compliant protocol with written informed consent, IVIM and DT imaging were performed at 3 T in 10 volunteers before and after water loading or furosemide administration. IVIM (apparent diffusion coefficient [ADC], tissue diffusivity [D(t)], perfusion fraction [f(p)], pseudodiffusivity [D(p)]) and DT (mean diffusivity [MD], fractional anisotropy [FA], eigenvalues [λ(i)]) imaging parameters and urine output from serial bladder volumes were calculated. (a)Reproducibility was quantified with coefficient of variation, intraclass correlation coefficient, and Bland-Altman limits of agreement; (b) contrast and challenge response were quantified with analysis of variance; and (c) Pearson correlations were quantified with urine output.

RESULTS

Good reproducibility was found for ADC, D(t), MD, FA, and λ(i) (average coefficient of variation, 3.7% [cortex] and 5.0% [medulla]), and moderate reproducibility was found for D(p), f(p), and f(p) · D(p) (average coefficient of variation, 18.7% [cortex] and 25.9% [medulla]). Baseline cortical diffusivities significantly exceeded medullary values except D(p), for which medullary values significantly exceeded cortical values, and λ(1,) which showed no contrast. ADC, D(t), MD, and λ(i) increased significantly for both challenges. Medullary diffusivity increases were dominated by transverse diffusion (1.72 ± 0.09 [baseline] to 1.79 ± 0.10 [hydration] μm(2)/msec, P = .0059; or 1.86 ± 0.07 [furosemide] μm(2)/msec, P = .0094). Urine output correlated with cortical ADC with furosemide (r = 0.7, P = .034) and with medullary λ(1) (r = 0.83, P = .0418), λ(2) (r = 0.85, P = .0301), and MD (r = 0.82, P = .045) with hydration.

CONCLUSION

Diffusion MR metrics are sensitive to flow changes in kidney induced by diuretic challenges. The results of this study suggest that vascular flow, tubular dilation, water reabsorption, and intratubular flow all play important roles in diffusion-weighted imaging contrast.

Reducing the influence of b-value selection on diffusion-weighted imaging of the prostate: evaluation of a revised monoexponential model within a clinical setting

PURPOSE

To compare the accuracy of standard and revised monoexponential models of diffusion-weighted magnetic resonance imaging (DW-MRI) data for differentiating malignant from benign prostate tissue, using surgical pathology as the reference standard.

MATERIALS AND METHODS

The Institutional Review Board waived informed consent for this Health Insurance Portability and Accountability Act (HIPAA)-compliant, retrospective study of 46 patients (median age = 61 years; range: 42-85 years) who underwent DW-MRI between May and December 2008 before radical prostatectomy for biopsy-proven prostate cancer, had no prior treatment, and had whole-mount step-section pathology maps available showing at least one peripheral zone (PZ) lesion >0.1 cm(3) . DW-MRI data were obtained for b-values of 0, 400, and 700 s/mm(2) . Apparent diffusion coefficients (ADCs) were estimated from PZ regions of interest (ROIs) on b = 0, 700 and b = 0, 400 s/mm(2) images, using a standard monoexponential model. The true diffusion coefficient (D) and perfusion fraction (f) were measured using a revised monoexponential model incorporating all three b-values. Areas under receiver operating characteristic curves (AUCs) were calculated to assess the accuracy of individual parameters and a logistic regression model combining D and f (D+f) in distinguishing malignant ROIs; P < 0.05 denoted significance.

RESULTS

ADC(400) (AUC = 0.81, P < 0.0001), ADC(700) (AUC = 0.79, P < 0.0001), D (AUC = 0.71, P = 0.0001) and D + f distinguished malignant from benign ROIs (AUC = 0.82, P < 0.0001), but f did not (AUC = 0.56, P = 0.28); D + f was significantly more accurate than D (P = 0.016) but not more accurate than ADC(400) (P = 0.26) or ADC(700) (P = 0.12).

CONCLUSION

The true diffusion coefficient provides an additional DW-MRI parameter for distinguishing prostate cancer that is less influenced than the ADC by b-value selection.

Investigation of prostate cancer using diffusion-weighted intravoxel incoherent motion imaging

PURPOSE

The objective of this work was to evaluate the diagnostic performance of the intravoxel incoherent motion (IVIM) model to differentiate between healthy and malignant prostate tissue.

MATERIALS AND METHODS

Regions of interest were drawn in healthy and cancerous tissue of 13 patients with histologically proven prostate carcinoma and fitted to a monoexponential model [yielding the apparent diffusion coefficient (ADC)] and the IVIM signal equation (yielding the perfusion fraction f, the diffusion constant D and the pseudodiffusion coefficient of perfusion D⁎). Parameter maps were calculated for all parameters.

RESULTS

The ADC, D and f were significantly (P<.005) lowered in cancerous tissue (1.01±0.22 μm(2)/ms, 0.84±0.19 μm(2)/ms and 14.27±7.10%, respectively) compared to benign tissue (1.49±0.17 μm(2)/ms, 1.21±0.22 μm(2)/ms and 21.25±8.32%, respectively). Parameter maps of D and f allowed for a delineation of the tumor, but showed higher variations compared to the ADC map.

CONCLUSION

Apparent diffusion coefficient maps provide better diagnostic performance than IVIM maps for tumor detection. However, the results suggest that the reduction of the ADC in prostate cancer stems not only from changes in cellularity but also from perfusion effects. IVIM imaging might hold promise for the diagnosis of other prostatic lesions.

Optimization of b-value sampling for diffusion-weighted imaging of the kidney

Diffusion-weighted imaging (DWI) involves data acquisitions at multiple b values. In this paper, we presented a method of selecting the b values that maximize estimation precision of the biexponential analysis of renal DWI data. We developed an error propagation factor for the biexponential model, and proposed to optimize the b-value samplings by minimizing the error propagation factor. A prospective study of four healthy human subjects (eight kidneys) was done to verify the feasibility of the proposed protocol and to assess the validity of predicted precision for DWI measures, followed by Monte Carlo simulations of DWI signals based on acquired data from renal lesions of 16 subjects. In healthy subjects, the proposed methods improved precision (P = 0.003) and accuracy (P < 0.001) significantly in region-of-interest based biexponential analysis. In Monte Carlo simulation of renal lesions, the b-sampling optimization lowered estimation error by at least 20-30% compared with uniformly distributed b values, and improved the differentiation between malignant and benign lesions significantly. In conclusion, the proposed method has the potential of maximizing the precision and accuracy of the biexponential analysis of renal DWI.

An in vivo verification of the intravoxel incoherent motion effect in diffusion-weighted imaging of the abdomen

To investigate the vascular contribution to the measured apparent diffusion coefficient and to validate the Intra Voxel Incoherent Motion theory, the signal as a function of the b-value was measured in the healthy pancreas with and without suppression of the vascular component and under varying echo times (TE = 50, 70, and 100 msec). The perfusion fraction f and the diffusion coefficient D were extracted from the measured DW-data using the original Intra Voxel Incoherent Motion-equation and a modified version of this equation incorporating relaxation effects. First, the perfusion fraction f in the blood suppressed pancreatic tissue decreased significantly (P = 0.03), whereas the diffusion coefficient D did not change with suppression (P = 0.43). Second, the perfusion fraction f increased significantly with increasing echo time (P = 0.0025), whereas the relaxation time compensated perfusion fraction f' showed no significant dependence on TE (P = 0.31). These results verify a vascular contribution to the diffusion weighted imaging measurement at low b values and support the Intra Voxel Incoherent Motion-theory.

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.

In vivo measurement of the apparent diffusion coefficient in normal and malignant prostatic tissues using echo-planar imaging

PURPOSE

To measure for the first time the apparent diffusion coefficient (ADC) values in anatomical regions of the prostate for normal and patient groups, and to investigate its use as a differentiating parameter between healthy and malignant tissue within the patient group.

MATERIALS AND METHODS

Single-shot diffusion-weighted echo-planar imaging (DW-EPI) was used to measure the ADC in the prostate in normal (N = 7) and patient (N = 19) groups. The spin-echo images comprised 96 x 96 pixels (field of view of 16 cm, TR/TE = 4000/120 msec) with six b-factor values ranging from 64 to 786 seconds/mm(2).

RESULTS

The ADC values averaged over all patients in non-cancerous and malignant peripheral zone (PZ) tissues were 1.82 +/- 0.53 x 10(-3) (mean +/- SD) and 1.38 +/- 0.52 x 10(-3) mm(2)/second, respectively (P = 0.00045, N = 17, paired t-test). The ADC values were found to be higher in the non-cancerous PZ (1.88 +/- 0.48 x 10(-3)) than in healthy or benign prostatic hyperplasia central gland (BPH-CG) region (1.62 +/- 0.41 x 10(-3)). For the normal group, the mean values were 1.91 +/- 0.46 x 10(-3) and 1.63 +/- 0.30 x 10(-3) mm(2)/second for the PZ and CG, respectively (P = 0.011, N = 7). Significant overlap exists between individual values among all tissue types. Furthermore, ADC values for the same tissue type showed no statistically significant difference between the two subject groups.

CONCLUSION

ADC is quantified in the prostate using DW-EPI. Values are lower in cancerous than in healthy PZ in patients, and in BPH-CG than PZ in volunteers.

Effect of vasodilator hydralazine on tumor microvascular random flow and blood volume as measured by intravoxel incoherent motion (IVIM) weighted MRI in conjunction with Gd-DTPA-Albumin enhanced MRI

We studied the effect of hydralazine on tumor blood volume fraction and microvascular random flow velocity magnitude by IVIM weighted MRI in conjunction with dynamic Gd-DTPA-Albumin enhanced MRI. Blood volume fraction maps were obtained from the dynamic Gd-DTPA-Albumin enhanced MRI measurements. The average blood volume fraction of R3230 AC adenocarcinoma decreased from 0.125 +/- 0.022 (s.d.) ml/g to 0.105 +/- 0.018 (s.d.) ml/g (p < 0.001) after the administration of hydralazine at a dose of 5 mg/kg. The microvascular random flow velocity magnitude maps were obtained from the IVIM weighted MRI signals by utilizing the Gd-DTPA-Albumin measured blood volume fractions as an input in the compartmental modeling analysis of the IVIM weighted MRI signal. The random-directional microvascular flow induced MRI signal attenuation was separated from the molecular diffusion induced signal attenuation. Flow induced attenuation was more significant after the administration of hydralazine. The mean microvascular random flow velocity magnitude increased from 0.52 +/- 0.15 (s.d.) mm/sec to 0.73 +/- 0.23 (s.d.) mm/sec (p < 0.05) in the presence of the above blood volume fraction change.

RF excitation profiles with FAIR: impact of truncation of the arterial input function on quantitative perfusion

This study investigates the impact of imaging coil length and consequent truncation of the arterial input function on the perfusion signal contrast obtained in the flow-sensitive alternating inversion recovery (FAIR) perfusion imaging measurement. We examined the difference in perfusion contrast achieved with head, head and neck, and body imaging coils based on the hypothesis that the standard head coil provides a truncated input function compared with that provided by the body coil and that this effect will be accentuated at long inversion times. The TI-dependent cerebral response of the FAIR sequence was examined at 1.5 T by varying the TI from 200 to 3500 msec with both the head and whole body coils (n = 5) as well as using a head and neck coil (n = 3). Difference signal intensity DeltaM and quantitative cerebral blood flow (CBF) were plotted against TI for each coil configuration. Despite a lower signal-to-noise ratio, relative CBF was significantly greater when measured with the body or head and neck coil compared with the standard head coil for longer inversion times (two-way ANOVA, P < or = 0.002). This effect is attributed to truncation of the arterial input function of labeled water by the standard head coil and the resultant inflow of unlabeled spins to the image slice during control image acquisition, resulting in overestimation of CBF. The results support the conclusion that the arterial input function depends on the anatomic extent of the inversion pulse in FAIR, particularly at longer mixing times (TI > 1200 msec at 1.5 T). Use of a head and neck coil ensures adequate inversion while preserving SNR that is lost in the body coil.

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

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Parametric mapping of scaled fitting error in dynamic susceptibility contrast enhanced MR perfusion imaging

The purpose of this study was to examine the benefits of routine generation of a parametric image of scaled curve fitting errors in the analysis of dynamic susceptibility contrast enhanced MR perfusion imaging. We describe the scaled fitting error (SFE), which reflects the magnitude of potential errors in the estimation of perfusion parameters from dynamic susceptibility contrast enhanced studies. The SFE is the root-mean-square error between the observed values in the time course of change of effective transverse relaxation rate (delta R2* (t)) in tissue and the theoretical values derived by gamma variate curve fitting, scaled with a simple function related to the area under the fitted gamma variate curve. The SFE was tested using Monte Carlo simulation and by observations in normal volunteers and patients. This demonstrated that the SFE was linearly related to uncertainties in calculation of the values of relative cerebral blood volume (rCBV) and relative mean transit time (rMTT). High spatial resolution SFE maps were obtained in all volunteers and patients. In normal brain, SFE was consistently higher in white matter than in grey matter. In 54/85 patients with neurodegenerative or vascular brain disease, SFE maps showed focal areas with high values owing to poor signal to noise ratio in delta R2*(t). Increased SFE was also found in 11/54 brain tumours owing to loss of conformance of delta R2*(t) to the gamma variate function. SFE mapping is simple to implement and the computational overhead is negligible. It is concluded that parametric maps of SFE allow visual and quantitative comparison of fitting errors with the theoretical gamma variate model between anatomical regions and provide a quality control device to rapidly assess the reliability of the associated rCBV and rMTT estimations.

Measurement of tumor vascular volume and mean microvascular random flow velocity magnitude by dynamic Gd-DTPA-albumin enhanced and diffusion-weighted MRI

Tumor vascular volume fraction and the magnitude of the mean microvascular random flow velocity were measured in an animal tumor model by combining dynamic Gd-DTPA-albumin enhanced MRI and diffusion-weighted MRI in conjunction with a compartmental modeling analysis. The vascular volume fraction maps were obtained from the dynamic Gd-DTPA-albumin enhanced MRI measurement. It was found that the vascular volume fraction for Walker 256 tumor was higher within the outgrowing rim and decreased towards the central region. The average value obtained from five animals was 0.062 +/- 0.009 ml/g. By using the vascular volume fraction from the Gd-DTPA-albumin enhanced MRI measurement, maps of the magnitude of the mean microvascular random flow velocity were obtained from the diffusion-weighted MRI measurements with the compartmental modeling analysis. The relative extravascular and intravascular contributions to the diffusion-weighted MRI signal were determined for three tissue groups with different Gd-DTPA-albumin enhancement characteristics, and the flow and molecular diffusion-induced attenuation factors for the intravascular compartment were also compared. The mean microvascular random flow velocity magnitude maps were obtained with an average value of 0.67 +/- 0.06 mm/s.

Can the IVIM model be used for renal perfusion imaging?

OBJECTIVE

Renal perfusion imaging may provide information about the hemodynamic significance of a renal artery stenosis and could improve noninvasive characterization when combined with angiography. It was proposed previously that diffusion sequences could provide useful perfusion indices based on the intravoxel incoherent motion (IVIM) model. Owing to motion artifacts, diffusion imaging has been restricted to relatively immobile organs like the brain. With the availability of single-shot echo-planar imaging (EPI) our purpose was to evaluate the IVIM model in renal perfusion.

METHODS AND MATERIAL

Eight volunteers underwent diffusion-sensitive magnetic resonance (MR) imaging of the kidneys using a spin echo (SE) EPI sequence. The diffusion coefficients determined by a linear regression analysis and fits to the IVIM function were calculated.

RESULTS AND CONCLUSION

Our preliminary experience does not support the possibility of obtaining perfusion information using the IVIM model in the kidneys.

Unraveling diffusion constants in biological tissue by combining Carr-Purcell-Meiboom-Gill imaging and pulsed field gradient NMR

A diffusion-weighted multi-spin-echo pulse sequence is presented, which allows for simultaneous measurement of T2, the fractional amplitude, and the diffusion constant of different fractions. Monte Carlo simulations demonstrate an improvement of this sequence with respect to the accuracy of diffusion constant and fractional amplitude for slow exchange. Examples are shown for a simple phantom containing two fractions. In addition, experiments on cat brain in healthy condition and following occlusion of the middle cerebral artery show that the fractional amplitude and the diffusion constant of cerebral spinal fluid and normal brain tissue can be analyzed within each pixel with acceptable accuracy.

Diffusion MRI: precision, accuracy and flow effects

After a decade of evolution and application of diffusion imaging, a large body of literature has been accumulated. It is in this context that the accuracy and precision of diffusion-weighted and quantitative diffusion MRI are reviewed. The emphasis of the review is on practical methods for clinical human imaging, particularly in the brain. The requirements for accuracy and precision are reviewed for various clinical and basic science applications. The methods of measuring and calculating diffusion effects with MRI are reviewed. The pulse gradient spin echo (PGSE) methods are emphasized as these methods are used most commonly in the clinical setting. Processing of PGSE data is reviewed. Various PGSE encoding schemes are also reviewed in terms of the accuracy and precision of isotropic and anisotropic diffusion measurements. The broad range of factors impacting the accuracy of the PGSE methods and other encoding schemes is then considered. Firstly, system inaccuracies such as background imaging gradients, gradient linearity, refocusing RF pulses, eddy currents, image misregistration, noise and dynamic range are considered. A second class of inaccuracies is contributed by the bulk effects of the imaged object, and include sample background gradients, subject motion of cerebrospinal fluid and organs, and aperiodic organ motion. A final category of potential inaccuracies is classified as being contributed by microscopic, biophysical tissue properties and include partial volume effects, anisotropy, restriction, diffusion distance, compartmentation, exchange, multiexponential diffusion decay, T2 weighting and microvascular perfusion. Finally, the application of diffusion methods to studies of blood flow in the microvasculature (i.e. the arterioles, capillaries and venules) are reviewed in detail, particularly in terms of feasibility and the stringent accuracy and precision requirements. Recent provocative studies examining the use of PGSE approaches to suppress microvascular signals in brain functional MRI (fMRI) are also reviewed.

In vivo measurement of diffusion and pseudo-diffusion in skeletal muscle at rest and after exercise

To investigate whether diffusion-related compartmentalization could be observed in skeletal muscle and whether this compartmentalization was affected by exercising, attenuation curves of signal against diffusion weighting were obtained in skeletal muscle of nine healthy volunteers at rest and after an exercise. Fifteen points were obtained for each diffusion curve with diffusion weightings ranging between approximately 0 and 560 x 10(6) s/m2. Data were fitted a biexponential model using three parameters to yield two apparent diffusion coefficients, a long one, ADCL, and a short one, ADCS, together with the fractional volume, fL, associated with the long one. At rest, values of parameters ADCL, ADCS, and fL were 46 x 10(-9) +/- 37 x 10(-9) m2/s, 1.74 x 10(-9) +/- 0.11 x 10(-9) m2/s, and 3.6 +/- 1.3%, respectively. After exercise, these values were 89 x 10(-9) +/- 37 x 10(-9) m2/s (p < .001 vs. rest), 1.94 x 10(-9) +/- 0.13 x 10(-9) m2/s (p < .001), and 5.2 +/- 1.3% (p < .05), respectively. These variations demonstrate significant changes in attenuation curves between rest and postexercise in skeletal muscle and may support an interpretation of the long and the short components in terms of a microvascular and an extra-microvascular compartments.

Self-diffusion in CNS tissue by volume-selective proton NMR

We have used a volume-selective NMR pulse sequence to measure the diffusion coefficient (D) inside a selected cuboid volume. The sequence is a variant of the stimulated echo method for volume-selective spectroscopy, and allows sequential measurement of D in any of three orthogonal directions and for a range of diffusion times. The echo attenuation from which D is calculated is free from the cross terms that can arise because of interactions between volume selection and diffusion-sensitizing gradients. Anisotropic and diffusion-time-dependent behavior is observed in brain structures known to have axon fibers aligned primarily in one direction.

Monitoring the initial expansion of focal ischaemic changes by diffusion-weighted MRI using a remote controlled method of occlusion

The purpose of the present study was to quantify the early changes which occur on diffusion-weighted imaging following focal cerebral ischaemia. We have developed a method for the remote occlusion of the middle cerebral artery in the rat, allowing early changes to be monitored and images to be acquired before and after ischaemia under identical conditions. Sequential diffusion-weighted images were acquired before and for up to 6 h following the occlusion. The data show that a hyperintense area appears soon after occlusion which expands during the first 2 h. It has previously been shown that the hyperintensity observed after ischaemia is linked to events associated with energy failure. Our findings are in good agreement with previous pathophysiological studies of focal ischaemia, which suggest a growing of the energy depleted area. In conclusion, our data extend the usefulness of diffusion-weighted imaging to the study of the early spatial evolution of the most compromised area after focal ischaemia. Diffusion-weighted imaging may constitute a very powerful tool to investigate pharmacological therapies which may interact with this evolution, both in experimental and clinical studies.

Quantitation of non-Einstein diffusion behavior of water in biological tissues by proton MR diffusion imaging: synthetic image calculations

The non-Einstein diffusion behavior of water in a model biological tissue system, intact duck embryos, has been investigated by the use of an in vivo proton pulsed-gradient spin-echo (PGSE) MR imaging technique. Multiple-frame MR images of the intact duck embryos and control solution (0.5 mM CuSO4 doped water) were acquired systematically at different diffusion times and strengths of the diffusion-sensitizing magnetic field gradients of the PGSE sequence. These raw images were then used to generate various dynamic (self-diffusion coefficient) and structural (fractal, residual attenuation, and compartment fraction) diffusion parameter maps of water in the imaging objects on the basis of different Einstein and higher order (non-Brownian, Residual, and 2-compartment) diffusion models. The self-diffusion coefficients of the body tissues of the embryos obtained from all diffusion models were significantly lower than those of the surrounding embryonic fluid. The structural diffusion parameter maps obtained from the higher order diffusion models revealed that water molecules exhibited either non-Brownian, restricted, or compartmentalized diffusion behavior in the embryonic tissues, but Einstein or Brownian diffusion behavior in the embryonic fluid and control solution. The diffusion parameter maps, both dynamic and structural, were found to provide much better contrasts than the conventional relaxation time (T1, T2, and biexponential T2) maps in separating the tissues from the surrounding embryonic fluid in the duck embryos. The mathematical models and procedures for generating the dynamic and structural diffusion parameter maps are also presented in this paper.

Diffusion-weighted imaging studies of cerebral ischemia in gerbils. Potential relevance to energy failure

BACKGROUND AND PURPOSE

Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients.

METHODS

Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia.

RESULTS

During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15-20 ml.100 g-1.min-1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia.

CONCLUSIONS

The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.