In vivo brain edema classification: New insight offered by large b-value diffusion-weighted MR imaging

Apparent diffusion coefficient (ADC) does not correlate with different serological parameters in myositis and myopathy

Background Magnetic resonance imaging (MRI) is widely used in several muscle disorders. Diffusion-weighted imaging (DWI) is an imaging modality, which can reflect microstructural tissue composition. The apparent diffusion coefficient (ADC) is used to quantify the random motion of water molecules in tissue. Purpose To investigate ADC values in patients with myositis and non-inflammatory myopathy and to analyze possible associations between ADC and laboratory parameters in these patients. Material and Methods Overall, 17 patients with several myositis entities, eight patients with non-inflammatory myopathies, and nine patients without muscle disorder as a control group were included in the study (mean age = 55.3 ± 14.3 years). The diagnosis was confirmed by histopathology in every case. DWI was obtained in a 1.5-T scanner using two b-values: 0 and 1000 s/mm². In all patients, the blood sample was acquired within three days to the MRI. The following serological parameters were estimated: C-reactive protein, lactate dehydrogenase, alanine aminotransferase, aspartate aminotransferase, creatine kinase, and myoglobine. Results The estimated mean ADC value for the myositis group was 1.89 ± 0.37 × 10^-3mm²/s and for the non-inflammatory myopathy group was 1.79 ± 0.33 × 10^-3 mm²/s, respectively. The mean ADC values (1.15 ± 0.37 × 10^-3 mm²/s) were significantly higher to unaffected muscles (vs. myositis P = 0.0002 and vs. myopathy P = 0.0021). There were no significant correlations between serological parameters and ADC values. Conclusion Affected muscles showed statistically significantly higher ADC values than normal muscles. No linear correlations between ADC and serological parameters were identified.

The diagnostic value of biexponential apparent diffusion coefficients in myopathy

To investigate the performance of a biexponential signal decay model using DWI in myopathies and to differentiate Polymyositis (PM)/Dermatomyositis (DM), Glycogen Storage Diseases (GSDs) and Muscular Dystrophies (MDs) utilizing diffusion-weighted imaging. 11 healthy volunteers (control group) and 46 patients with myopathy were enrolled in the retrospective study. 27 of 46 patients had PM/DM, 7 patients GSDs and 12 patients MDs. After conventional MR sequences, diffusion weighted imaging with a b-factor ranging from 0 to 1200 s/mm(2) was performed on both thighs. The intra-muscular signal-to-noise ratios (SNRs) on multiple-b DWI images were measured for 7 different muscles and compared among the different groups. The median T2 signal intensity and biexponential apparent diffusion coefficients (ADC), including standard ADC, fast ADC, and slow ADC values, were compared among the different groups. The intra-muscular SNRs were statistically significantly different depending on the b value, and also found among the 4 groups (p < 0.05). The median T2 signal intensity of the normal muscles in control group was statistically significantly lower than that of edematous muscles in the PM/DM, GSDs and MDs groups (p = 0.000), while there were no statistically significant differences among the PM/DM, GSDs, and MDs groups (p > 0.05). The median standard ADC value of the edematous muscles in GSDs was statistically significantly lower than that of normal muscles in the control group (p = 0.000) and the median ADC value of the edematous muscles in PM/DM patients was statistically significantly greater than that of the GSDs (p = 0.000) and MDs groups (p = 0.005). The median slow ADC value of the edematous muscles in MDs patients and PM/DM patients was statistically significantly greater than that of GSDs patients (p < 0.05). Intra-muscular SNR decay curves and biexponential ADC parameters are useful in distinguishing among PM/DM, GSDs, and MDs.

Biexponential diffusion alterations in the normal-appearing white matter of glioma patients might indicate the presence of global vasogenic edema

PURPOSE

To investigate normal-appearing white matter (NAWM) microstructure of glioma patients with biexponential diffusion analysis in order to reveal the nature of diffusion abnormalities and to assess whether they are region-specific or global.

MATERIALS AND METHODS

Twenty-four newly diagnosed glioma patients (grade II-IV) and 24 matched control subjects underwent diffusion-weighted imaging at 3T. Diffusion parameters were calculated using monoexponential and biexponential models. Apparent diffusion coefficient (ADC) values were measured in the entire NAWM of the hemisphere contralateral and ipsilateral to the tumor. In the contralateral NAWM, regional ADC values were assessed in the frontal, parietal, occipital, and temporal NAWM.

RESULTS

ADCmono and ADCfast were significantly higher than control values in all investigated regions except the temporal NAWM (P < 0.04). ADCslow was significantly increased in the total contralateral, frontal, and parietal NAWM (P < 0.03), while pslow was decreased in both total hemispheric NAWM and the parietal NAWM of glioma patients compared to controls (P < 0.04). ADCmono , ADCfast , ADCslow , and pslow were significantly different among the NAWM of the four lobes of the contralateral hemisphere in both groups (P < 0.0001), and these regional differences were similar in patients and controls (P > 0.05). Hemispheric ADCmono and pslow differences were different between groups (P < 0.05).

CONCLUSION

Globally altered diffusion parameters suggest the presence of global vasogenic edema in the NAWM of glioma patients, which is further supported by the finding that regional differences in patients follow those found in controls. Alternatively, some tumor infiltration might contribute to diffusion abnormalities in the NAWM, especially in the tumor-affected hemisphere. J. Magn. Reson. Imaging 2016;44:633-641.

Brain magnetic resonance in hepatic encephalopathy

The term hepatic encephalopathy (HE) covers a wide spectrum of neuropsychiatric abnormalities caused by portal-systemic shunting. The diagnosis requires demonstration of liver dysfunction or portal-systemic shunts and exclusion of other neurologic disorders. Most patients with this condition have liver dysfunction caused by cirrhosis, but it also occurs in patients with acute liver failure and less commonly, in patients with portal-systemic shunts that are not associated with hepatocellular disease. Various magnetic resonance (MR) techniques have improved our knowledge about the pathophysiology of HE. Proton MR spectroscopy and T1-weighted imaging can detect and quantify accumulations of brain products that are normally metabolized or eliminated such as glutamine and manganese. Other MR techniques such as T2-weighted and diffusion-weighted imaging can identify white matter abnormalities resulting from disturbances in cell volume homeostasis secondary to brain hyperammonemia. Partial or complete recovery of these abnormalities has been observed with normalization of liver function or after successful liver transplantation. MR studies have undoubtedly improved our understanding of the mechanisms involved in the pathogenesis of HE, and some findings can be considered biomarkers for monitoring the effects of therapeutic measures focused on correcting this condition.

Differentiating white matter lesions in multiple sclerosis and migraine using monoexponential and biexponential diffusion measurements

PURPOSE

To compare the white matter lesions seen in multiple sclerosis and migraine using monoexponential and high b-value biexponential diffusion measurements.

MATERIALS AND METHODS

Diffusion-weighted images were acquired on a 3.0-Tesla magnetic resonance imaging system. Diffusion parameters were estimated using monoexponential (0-1000 s/mm(2) ) and biexponential (0-5000 s/mm(2) ) approaches from 15 multiple sclerosis patients, 15 patients with migraine and 15 healthy control subjects. The study was performed in accordance with the approval of the Regional Research Ethics Committee. The apparent diffusion coefficient (ADC) values were measured in the lesions and the normal-appearing white matter of patients and in the white matter of controls.

RESULTS

High lesional ADCmono values were detected in both patient groups without significant differences between the groups (10.72 and 9.86 × 10(-4) mm(2) /s for MS and migraine respectively, P = 0.2134). The biexponential measurements showed significantly higher ADCfast , ADCslow , and Pslow values in the migraine lesions than in the multiple sclerosis lesions (16.47 versus 14.29, 1.41 versus 0.76, and 20.34 versus 12.01 all values in 10(-4) mm(2) /s; P = 0.0344, P = 0.0019, P = 0.0021, respectively).

CONCLUSION

Biexponential diffusion analysis may help to differentiate multiple sclerosis-related white matter lesions from migraine-related ones.

The role of tissue microstructure and water exchange in biophysical modelling of diffusion in white matter

Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. Model development has been paralleled by microscopy studies of the microstructure of excised and fixed nerves, confirming that axon diameter estimates from diffusion measurements agree with those from microscopy. However, results obtained in vivo are less conclusive. For example, the amount of slowly diffusing water is lower than expected, and the diffusion-encoded signal is apparently insensitive to diffusion time variations, contrary to what may be expected. Recent understandings of the resolution limit in diffusion MRI, the rate of water exchange, and the presence of microscopic axonal undulation and axonal orientation dispersions may, however, explain such apparent contradictions. Knowledge of the effects of biophysical mechanisms on water diffusion in tissue can be used to predict the outcome of diffusion tensor imaging (DTI) and of diffusion kurtosis imaging (DKI) studies. Alterations of DTI or DKI parameters found in studies of pathologies such as ischemic stroke can thus be compared with those predicted by modelling. Observations in agreement with the predictions strengthen the credibility of biophysical models; those in disagreement could provide clues of how to improve them. DKI is particularly suited for this purpose; it is performed using higher b-values than DTI, and thus carries more information about the tissue microstructure. The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.

Brain magnetic resonance spectroscopy in episodic hepatic encephalopathy

Brain magnetic resonance (MR) study has shown metabolic abnormalities and changes in water distribution of the brain tissue that may relate to the pathogenesis of hepatic encephalopathy (HE). We designed a study to investigate the disturbances in brain water and metabolites during episodic HE using a 3-T MR scanner. Cirrhotic patients with different grades of HE underwent MR during hospitalization (n=18). The MR was repeated at 6 weeks' follow-up (n=14). The results were compared with those of a group of healthy volunteers (n=8). During episodic HE, brain diffusion-weighted imaging showed a high apparent diffusion coefficient (ADC) (12% to 14%) that decreased during follow-up (-1% to -4%). These disturbances were accompanied by high glutamine (581%), low choline (-31%), and low myo-inositol (-86%) peaks on MR spectroscopy. In overt HE, patients showed high glutamine that decreased during follow-up (-22%). In addition, these patients exhibited a rise in plasma S100 beta and enlargement of brain white-matter lesions. In conclusion, several disturbances detected by MR support the presence of impaired brain water homeostasis during episodic HE. Although astrocytes have a major role in this condition, brain edema during episodic HE may be extracellular and does not appear to be directly responsible for the development of neurologic manifestations.

Brain magnetic resonance in experimental acute-on-chronic liver failure

BACKGROUND & AIM

Acute-on-chronic liver failure is the term that refers to sustained liver injury with acute decompensation, usually induced by a precipitating factor. A common link between ensuing failures of various organs is impairment of the vascular supply, which may also induce vasogenic oedema in the brain. The aim of this study was to perform magnetic resonance (MR) study of the brain in a rat model combining bile duct ligation (BDL) and lipopolysaccharide (LPS) administration to investigate brain oedema in liver failure.

METHODS

Bile duct-ligated rats underwent in vivo brain MR imaging at 4, 5 and 6 weeks, and after superimposed administration of LPS. The MR techniques applied enabled assessment of brain metabolites, and intra- or extracellular water distribution. Brain water content was assessed by gravimetry.

RESULTS

MR spectroscopy showed an increase in brain glutamine and a decrease in myo-inositol and choline in relation to progression of liver disease. BDL rats showed a slight, progressive increase in the amount of cortical brain water that was significant after LPS injection. These changes did not modify the apparent diffusion coefficient, supporting a mixed origin of brain oedema (vasogenic and cytotoxic).

CONCLUSIONS

The mechanisms leading to the development of brain oedema in an experimental liver disease model were related to the time course of liver failure and to pro-inflammatory stimuli. MR findings support the presence of cytotoxic and vasogenic mechanisms in induced brain oedema in BDL rats exposed to LPS.

Quantitative MRI analysis of the brain after twenty-two years of neuromyelitis optica indicates focal tissue damage

BACKGROUND

The long-term effect of neuromyelitis optica (NMO) on the brain is not well established.

METHODS

After 22 years of NMO, a patient's brain was examined by quantitative T1- and T2-weighted mono- and biexponential diffusion and proton spectroscopy. It was compared to 3 cases with short-term NMO and 20 healthy subjects.

RESULTS

Although routine T1- and T2-weighted images appeared to be normal, quantitative T1-, T2- and diffusion-weighted magnetic resonance imaging identified areas with high diffusivity and altered relaxation time in 'normal appearing white matter' (NAWM). In such abnormal NAWM regions, biexponential diffusion analysis and quantitative spectroscopy indicated extracellular edema and axonal loss, respectively. Repeated analysis 6 months later identified the same alterations. Such patchy alterations were not detectable in the NAWM of the 3 cases with short-term NMO, and they were also not quantitatively different from the controls.

CONCLUSION

After NMO of 22-year duration, metabolic changes, altered diffusivity and magnetic resonance relaxation features of patchy brain areas may suggest tissue damage in NAWM that persist for at least 6 months.

Preliminary study of whole-body diffusion-weighted imaging in detecting pulmonary metastatic lesions from clear cell renal cell carcinoma: comparison with CT

BACKGROUND

Whole-body diffusion-weighted imaging (DWI) has been widely used in detecting malignant metastases, including pulmonary metastases.

PURPOSE

To evaluate the possible utility of whole-body DWI in detecting pulmonary metastases of patients with clear cell renal cell carcinoma (ccRCC) and compare the exact differences between MR and CT in detecting pulmonary lesions.

MATERIAL AND METHODS

Whole-body DWI and chest CT examinations were performed on nine consecutive patients (8 men and 1 woman) with histologically confirmed ccRCC and possible metastatic lesions before chemotherapy.

RESULTS

CT and MR demonstrated pulmonary metastases in seven patients and no metastatic lesions in two patients. The numbers of pulmonary metastases detected on CT, DWI-only, T1WI-only and DWI in combination with T1WI were 83, 35, 34 and 39, respectively. Metastases with a diameter above 1.0 cm could all be detected by DWI and a diameter above 0.7 cm could all be detected by DWI in combination with T1WI. Significant differences were obtained both for correlationship between diameter and detection rates of DWI and T1WI by using Spearman rank correlation analysis.

CONCLUSION

Although MR cannot be considered a replacement for CT in pulmonary metastases from ccRCC, whole-body DWI, with the combination of T1 dual echo, might be helpful for the evaluation of tumor response to chemotherapy in the follow-up of patients when the diameter of the pulmonary metastases is over 1.0 cm.

Neuroimaging in acute liver failure

Acute liver failure (ALF) is frequently complicated by the development of brain edema that can lead to intracranial hypertension and severe brain injury. Neuroimaging techniques allow a none-invasive assessment of brain tissue and cerebral hemodynamics by means of transcranial Doppler ultrasonography, magnetic resonance and nuclear imaging with radioligands. These methods have been very helpful to unravel the pathogenesis of this process and have been applied to patients and experimental models. They allow monitoring the outcome of patients with ALF and neurological manifestations. The increase in brain water can be detected by observing changes in brain volume and disturbances in diffusion weighted imaging. Neurometabolic changes are detected by magnetic resonance spectroscopy, which provides a pattern of abnormalities characterized by an increase in glutamine and a decrease in myo-inositol. Disturbances in cerebral blood flow are depicted by SPECT or PET and can be monitored and the bedside by assessing the characteristics of the waveform provided by transcranial Doppler ultrasonography. Neuroimaging methods, which are rapidly evolving, will undoubtedly lead to future diagnostic and therapeutic progress that could be very helpful for patients with ALF.

Biexponential analysis of diffusion-tensor imaging of the brain in patients with cirrhosis before and after liver transplantation

BACKGROUND AND PURPOSE

DTI has shown increased MD of water molecules in the brain of patients with cirrhosis, consistent with low-grade edema. This study further characterizes this edema by using biexponential analysis of DTI data, a technique that may differentiate cytotoxic and vasogenic edema.

MATERIALS AND METHODS

A total of 41 patients with cirrhosis awaiting liver transplantation and 16 healthy controls were studied by DTI by using a single-shot echo-planar technique with 11 b-values (range, 0-7500 s/mm(2)) and 6 noncollinear directions. Measurements were fitted to biexponential function to determine MD and FA for the fast and slow diffusion components. Regions of interest were selected in the parietal white matter and corticospinal tract. The assessment was repeated 1 year after liver transplantation in 24 of these patients.

RESULTS

In parietal white matter, patients with cirrhosis showed an increase in fast MD and a decrease in fast FA that normalized after liver transplantation. In the corticospinal tract, there was an increase in fast and slow MD that normalized after transplantation, and a decrease in FA that persisted posttransplantation. There was no association of DTI parameters with minimal HE (n =12).

CONCLUSIONS

Biexponential analysis of DTI supports the presence of edema in the brain of patients with cirrhosis that reverts after transplantation. In parietal white matter, the increase in brain water was mainly located in the interstitial compartment, while the corticospinal tract showed a mixed pattern (intra- and extracellular). In addition, the findings on posttransplantation were consistent with microstructural damage along the corticospinal tract.

A biexponential DWI study in rat brain intracellular oedema

PURPOSE

To examine the changes in MR parameters derived from diffusion weighted imaging (DWI) biexponential analysis in an in vivo intracellular brain oedema model, and to apply electron microscopy (EM) to shed more light on the morphological background of MR-related observations.

MATERIALS AND METHODS

Intracellular oedema was induced in ten male Wistar rats (380-450g) by way of water load, using a 20% body weight intraperitoneal injection of 140mmol/L dextrose solution. A 3T MRI instrument was used to perform serial DWI, and MR specroscopy (water signal) measurements. Following the MR examination the brains of the animals were analyzed for EM.

RESULTS

Following the water load induction, apparent diffusion coefficient (ADC) values started declining from 724±43μm(2)/s to 682±26μm(2)/s (p<0.0001). ADC-fast values dropped from 948±122 to 840±66μm(2)/s (p<0.001). ADC-slow showed a decrease from 226±66 to 191±74μm(2)/s (p<0.05). There was a shift from the slow to the fast component at 110min time point. The percentage of the fast component demonstrated moderate, yet significant increase from 76.56±7.79% to 81.2±7.47% (p<0.05). The water signal was increasing by 4.98±3.52% compared to the base line (p<0.01). The results of the E.M. revealed that water was detected intracellularly, within astrocytic preivascular end-feet and cell bodies.

CONCLUSION

The unexpected volume fraction changes (i.e. increase in fast component) detected in hypotonic oedema appear to be substantially different from those observed in stroke. It may suggest that ADC decrease in stroke, in contrast to general presumptions, cannot be explained only by water shift from extra to intracellular space (i.e. intracellular oedema).

Diffusion imaging of brain tumors

MRI offers a tremendous armamentarium of different methods that can be employed in brain tumor characterization. MR diffusion imaging has become a widely accepted method to probe for the presence of fluid pools and molecular tissue water mobility. For most clinical applications of diffusion imaging, it is assumed that the diffusion signal vs diffusion weighting factor b decays monoexponentially. Within this framework, the measurement of a single diffusion coefficient in brain tumors permits an approximate categorization of tumor type and, for some tumors, definitive diagnosis. In most brain tumors, when compared with normal brain tissue, the diffusion coefficient is elevated. The presence of peritumoral edema, which also exhibits an elevated diffusion coefficient, often precludes the delineation of the tumor on the basis of diffusion information alone. Serially obtained diffusion data are useful to document and even predict the cellular response to drug or radiation therapy. Diffusion measurements in tissues over an extended range of b factors have clearly shown that the monoparametric description of the MR diffusion signal decay is incomplete. Very high diffusion weighting on clinical systems requires substantial compromise in spatial resolution. However, after suitable analysis, superior separation of malignant brain tumors, peritumoral edema and normal brain tissue can be achieved. These findings are also discussed in the light of tissue-specific differences in membrane structure and the restrictions exerted by membranes on diffusion. Finally, measurement of the directional dependence of diffusion permits the assessment of white matter integrity and dislocation. Such information, particularly in conjunction with advanced post-processing, is considered to be immensely useful for therapy planning. Diffusion imaging, which permits monoexponential analysis and provides directional diffusion information, is performed routinely in brain tumor patients. More advanced methods require improvement in acquisition speed and spatial resolution to gain clinical acceptance.

Regional differences in osmotic behavior in brain during acute hyponatremia: an in vivo MRI-study of brain and skeletal muscle in pigs

Brain edema is suggested to be the principal mechanism underlying the symptoms in acute hyponatremia. Identification of the mechanisms responsible for global and regional cerebral water homeostasis during hyponatremia is, therefore, of utmost importance. To examine the osmotic behavior of different brain regions and muscles, in vivo-determined water content (WC) was related to plasma sodium concentration ([Na(+)]) and brain/muscle electrolyte content. Acute hyponatremia was induced with desmopressin acetate and infusion of a 2.5% glucose solution in anesthetized pigs. WC in different brain regions and skeletal muscle was estimated in vivo from T(1) maps determined by magnetic resonance imaging (MRI). WC, expressed in gram water per 100 g dry weight, increased significantly in slices of the whole brain [342(SD = 14) to 363(SD = 21)] (6%), thalamus [277(SD = 13) to 311(SD = 24)] (12%) and white matter [219(SD = 7) to 225(SD = 5)] (3%). However, the WC increase in the whole brain and white mater WC was less than expected from perfect osmotic behavior, whereas in the thalamus, the water increase was as expected. Brain sodium content was significantly reduced. Muscle WC changed passively with plasma [Na(+)]. WC determined with deuterium dilution and tissue lyophilzation correlated well with MRI-determined WC. In conclusion, acute hyponatremia induces brain and muscle edema. In the brain as a whole and in the thalamus, regulatory volume decrease (RVD) is unlikely to occur. However, RVD may, in part, explain the observed lower WC in white matter. This may play a potential role in osmotic demyelination.

Value of quantitative MRI biomarkers (Evans' index, aqueductal flow rate, and apparent diffusion coefficient) in idiopathic normal pressure hydrocephalus

PURPOSE

To define the value of Evans' index (EI), aqueductal flow rate (FR), and apparent diffusion coefficient (ADC) in the diagnosis of normal pressure hydrocephalus (NPH) and to assess the ability of these markers preoperatively to predict shunt response. To shed some light as to the mechanisms responsible for the symptoms of NPH.

MATERIALS AND METHODS

Preoperative EI, FR, and ADC readings in nine cases of clinically diagnosed NPH were compared with those of age- and gender-matched controls. Similar pre- and postoperative readings of responders and nonresponders were subsequently compared.

RESULTS

Compared with the controls, all measurements were statistically significant except for peak systolic flow rate (pSfr), which was near statistical significance. Comparison of pre- and postoperative readings of responders and nonresponders revealed a decrease in ADC in all responders (P = 0.032). Subdural hemorrhage was found in all nonresponders (P = 0.012).

CONCLUSION

For patients presenting with signs and symptoms of NPH, readings on MRI greater than 0.3, 10 mL/min, -9.0 mL/min, and 10.65 x 10(-4) mm(2)/s for EI, peak diastolic flow rate (pDfr), pSfr, and ADC, respectively, add further weight to the diagnosis. The strong correlation between shunt response and ADC decline support our hypothesis that water accumulation in the cerebrum is the major cause for the symptoms of NPH. The presence of subdural hemorrhage in all nonresponders raises suspicion of decreased compliance as the other major cause.

High b-value apparent diffusion-weighted images from CURVE-ball DTI

PURPOSE

To investigate the utility of a proposed clinical diffusion imaging scheme for rapidly generating multiple b-value diffusion contrast in brain magnetic resonance imaging (MRI) with high signal-to-noise ratio (SNR).

MATERIALS AND METHODS

Our strategy for efficient image acquisition relies on the invariance property of the diffusion tensor eigenvectors to b-value. A simple addition to the conventional diffusion tensor MR imaging (DTI) data acquisition scheme used for tractography yields diffusion-weighted images at twice and three times the conventional b-value. An example from a neurosurgical brain tumor is shown. Apparent diffusion-weighted (ADW) images were calculated for b-values 800, 1600, and 2400 s/mm(2), and a map of excess diffusive kurtosis was computed from the three ADWs.

RESULTS

High b-value ADW images demonstrated decreased contrast between normal gray and white matter, while the heterogeneity and contrast of the lesion was emphasized relative to conventional b-value data. Kurtosis maps indicated the deviation from Gaussian diffusive behavior.

CONCLUSION

DTI data with multiple b-values and good SNR can be acquired in clinically reasonable times. High b-value ADW images show increased contrast and add information to conventional DWI. Ambiguity in conventional b-value images over whether hyperintense signal results from abnormally low diffusion, or abnormally long T(2), is better resolved in high b-value images.

Diffusion tensor imaging and fiber tractography of skeletal muscle: optimization of B value for imaging at 1.5 T

OBJECTIVE

The aim of our study was to determine the optimal b value for 1.5-T diffusion tensor (DT) MRI and fiber tractography of in vivo human skeletal muscle.

SUBJECTS AND METHODS

Five healthy volunteers were included in this prospective study. DT MRI of the proximal calf was performed with 15 directions of diffusion sensitization and parallel imaging with an acceleration factor of 2. Each single-shot spin-echo echo-planar imaging sequence was performed in each volunteer with eight b values ranging from 125 s/mm(2) to 1,000 s/mm(2). Fiber tractography of two calf muscles (anterior tibialis and lateral gastrocnemius) was performed, and image quality was ranked 1-8. Fractional anisotropy, signal-to-noise ratio, and fiber density index were assessed for both muscles at each b value. Statistical analysis was performed with repeated measurement analysis of variance.

RESULTS

Among all fiber tractographic images assessed, those obtained at a b value of 625 s/mm(2) were qualitatively ranked first (best image quality) or second (second-best image quality). Signal-to-noise ratio was highest for both muscles at a b value of 125 s/mm(2) (anterior tibialis, 66.2; lateral gastrocnemius, 60.1). The highest fiber density indexes were found for both muscles at a b value of 625 s/mm(2) (anterior tibialis index, 7.82; lateral gastrocnemius index, 5.75). In the analysis of variance no significant differences were found between fractional anisotropy and b values for either muscle (p = 0.359 and p = 0.078).

CONCLUSION

The optimal b value for in vivo DT MRI and tractographic assessment of human skeletal muscle in the calf at 1.5 T MRI was found to be 625 s/mm(2).