In vivo study of microcirculation in canine myocardium using the IVIM method

Optimal diffusion weighting for in vivo cardiac diffusion tensor imaging

PURPOSE

To investigate the influence of the diffusion weighting on in vivo cardiac diffusion tensor imaging (cDTI) and obtain optimal parameters.

METHODS

Ten subjects were scanned using stimulated echo acquisition mode echo planar imaging with six b-values, from 50 to 950 s·mm(-2) , plus b = 15 s·mm(-2) reference. The relationship between b-value and both signal loss and signal-to-noise ratio measures was investigated. Mean diffusivity, fractional anisotropy, and helical angle maps were calculated using all possible b-value pairs to investigate the effects of diffusion weighting on the main and reference data.

RESULTS

Signal decay at low b-values was dominated by processes with high apparent diffusion coefficients, most likely microvascular perfusion. This effect could be avoided by diffusion weighting of the reference images. Parameter maps were improved with increased b-value until the diffusion-weighted signal approached the noise floor. For the protocol used in this study, b = 750 s·mm(-2) combined with 150 s·mm(-2) diffusion weighting of the reference images proved optimal.

CONCLUSION

Mean diffusivity, fractional anisotropy, and helical angle from cDTI are influenced by the b-value of the main and reference data. Using optimal values improves parameter maps and avoids microvascular perfusion effects. This optimized protocol should provide greater sensitivity to pathological changes in parameter maps.

Intravoxel incoherent motion (IVIM) imaging at different magnetic field strengths: what is feasible?

BACKGROUND

Due to limited SNR the cerebral applications of the intravoxel incoherent motion (IVIM) concept have been sparse. MRI hardware developments have resulted in improved SNR and this may justify a reassessment of IVIM imaging for non-invasive quantification of the cerebral blood volume (CBV) as a first step toward determining the optimal field strength.

PURPOSE

To investigate intravoxel incoherent motion imaging for its potential to assess cerebral blood volume (CBV) at three different MRI field strengths.

MATERIALS AND METHODS

Four volunteers were scanned twice at 1.5 T, 3 T as well as 7 T. By correcting for field-strength-dependent effects of relaxation, estimates of corrected CBV (cCBV) were obtained in deep gray matter (DGM), frontal gray matter (FGM) and frontal white matter (FWM), using Bayesian analysis. In addition, simulations were performed to facilitate the interpretation of experimental data.

RESULTS

In DGM, FGM and FWM we obtained cCBV estimates of 2.2 ml/100 ml, 2.7 ml/100 ml, 1.4 ml/100 ml at 1.5 T; 3.7 ml/100 ml, 5.0 ml/100 ml, 3.2 ml/100 ml at 3 T and 15.5 ml/100 ml, 20.3 ml/100 ml, 7.0 ml/100 ml at 7 T.

CONCLUSION

Quantitative cCBV values obtained at 1.5 T and 3 T corresponded better to physiological reference values, while 7 T showed the largest deviation from expected values. Simulations of synthetic tissue voxels indicated that the discrepancy at 7 T can partly be explained by SNR issues. Results were generally more repeatable at 7 T (intraclass correlation coefficient, ICC=0.84) than at 1.5 T (ICC=0.68) and 3 T (ICC=0.46).

Interrelations of muscle functional MRI, diffusion-weighted MRI and (31) P-MRS in exercised lower back muscles

Exercise-induced changes of transverse proton relaxation time (T2 ), tissue perfusion and metabolic turnover were investigated in the lower back muscles of volunteers by applying muscle functional MRI (mfMRI) and diffusion-weighted imaging (DWI) before and after as well as dynamic (31) P-MRS during the exercise. Inner (M. multifidus, MF) and outer lower back muscles (M. erector spinae, ES) were examined in 14 healthy young men performing a sustained isometric trunk-extension. Significant phosphocreatine (PCr) depletions ranging from 30% (ES) to 34% (MF) and Pi accumulations between 95% (left ES) and 120%-140% (MF muscles and right ES) were observed during the exercise, which were accompanied by significantly decreased pH values in all muscles (∆pH ≈ -0.05). Baseline T2 values were similar across all investigated muscles (approximately 27 ms at 3 T), but revealed right-left asymmetric increases (T2 ,inc ) after the exercise (right ES/MF: T2 ,inc  = 11.8/9.7%; left ES/MF: T2 ,inc  = 4.6/8.9%). Analyzed muscles also showed load-induced increases in molecular diffusion D (p = .007) and perfusion fraction f (p = .002). The latter parameter was significantly higher in the MF than in the ES muscles both at rest and post exercise. Changes in PCr (p = .03), diffusion (p < .01) and perfusion (p = .03) were strongly associated with T2,inc , and linear mixed model analysis revealed that changes in PCr and perfusion both affect T2,inc (p < .001). These findings support previous assumptions that T2 changes are not only an intra-cellular phenomenon resulting from metabolic stress but are also affected by increased perfusion in loaded muscles.

Combined intravoxel incoherent motion and diffusion tensor imaging of renal diffusion and flow anisotropy

PURPOSE

We used a combined intravoxel incoherent motion-diffusion tensor imaging (IVIM-DTI) methodology to distinguish structural from flow effects on renal diffusion anisotropy.

METHODS

Eight volunteers were examined with IVIM-DTI at 3T with 20 diffusion directions and 10 b-values. Mean diffusivity (MD) and fractional anisotropy (FA) from DTI analysis were calculated for low (b ≤ 200 s/mm(2) ), high (b > 200 s/mm(2) ), and full b-value ranges. IVIM-parameters perfusion-fraction fP , pseudo-diffusivity Dp , and tissue-diffusivity Dt were first calculated independently on a voxelwise basis for all directions. After estimating a fixed isotropic fp from these data, global anisotropies of Dt and Dp in the cortex and medulla were determined in a constrained cylindrical description and visualized using polar plots and cosine scatterplots.

RESULTS

For all b-value ranges, medullary FA was significantly higher than that of the cortex. The corticomedullary difference was smaller for the high b-value range. Significantly higher fp and Dt were determined for the cortex and showed a significantly higher directional variance in the medulla. Polar plot analysis displayed nearly isotropic Dp and Dt in the cortex and anisotropy in the medulla.

CONCLUSION

Both flow and microstructure apparently contribute to the medullary diffusion anisotropy. The described novel method may be useful in separating decreased tubular flow from irreversible structural tubular damage, for example, in diabetic nephropathy or during allograft rejection.

Diffusion-weighted intravoxel incoherent motion imaging of renal tumors with histopathologic correlation

PURPOSE

The aim of this study was to use intravoxel incoherent motion diffusion-weighted imaging to discriminate subtypes of renal neoplasms and to assess agreement between intravoxel incoherent motion (perfusion fraction, fp) and dynamic contrast-enhanced magnetic resonance imaging (MRI) metrics of tumor vascularity.

SUBJECTS AND METHODS

In this Health Insurance Portability and Accountability Act-compliant, institutional review board-approved prospective study, 26 patients were imaged at 1.5-T MRI using dynamic contrast-enhanced MRI with high temporal resolution and diffusion-weighted imaging using 8 b values (range, 0-800 s/mm). Perfusion fraction (fp), tissue diffusivity (Dt), and pseudodiffusivity (Dp) were calculated using biexponential fitting of the diffusion data. Apparent diffusion coefficient (ADC) was calculated with monoexponential fit using 3 b values of 0, 400, and 800 s/mm. Dynamic contrast-enhanced data were processed with a semiquantitative method to generate model-free parameter cumulative initial area under the curve of gadolinium concentration at 60 seconds (CIAUC60). Perfusion fraction, Dt, Dp, ADC, and CIAUC60 were compared between different subtypes of renal lesions. Perfusion fraction was correlated with CIAUC60.

RESULTS

We examined 14 clear cell, 4 papillary, 5 chromophobe, and 3 cystic renal cell carcinomas (RCCs). Although fp had higher accuracy (area under the curve, 0.74) for a diagnosis of clear cell RCC compared with Dt or ADC, the combination of fp and Dt had the highest accuracy (area under the curve, 0.78). The combination of fp and Dt diagnosed papillary RCC and cystic RCC with 100% accuracy, and clear cell RCC and chromophobe RCC, with 86.5% accuracy. There was significant strong correlation between fp and CIAUC60 (r = 0.82; P < 0.001).

CONCLUSION

Intravoxel incoherent motion parameters fp and Dt can discriminate renal tumor subtypes. Perfusion fraction demonstrates good correlation with CIAUC60 and can assess degree of tumor vascularity without the use of exogenous contrast agent.

Intra-voxel incoherent motion MRI in rodent model of diethylnitrosamine-induced liver fibrosis

RATIONALE AND OBJECTIVES

To compare the apparent diffusion coefficient (ADC) and the perfusion fraction measured by intra-voxel incoherent motion (IVIM) Magnetic Resonance Imaging (MRI) with liver fibrosis degrees in a rodent model.

MATERIALS AND METHODS

All experiments received approval from our institutional animal care and use committee. Liver fibrosis was induced in 13 rats by oral gavage with diethylnitrosamine; 4 untreated rats with normal livers were used as controls. Diffusion Weighted MRI was performed and 8 gradient factors (0, 50, 100, 150, 200, 300, 400 and 500s/mm(2)) were acquired. The values of ADC, true diffusion coefficient D and perfusion fraction f were measured based on Li Bihan's method. The percentage of liver fibrosis was assessed via quantitative analysis of Masson trichrome staining using an average of 30 fields per section. The MRI measurements were compared to the histological fibrotic grade to evaluate the correlation between them.

RESULTS

ADC contained the contribution of diffusion and perfusion. The ADC and f values decreased significantly with the increasing fibrosis level (correlation coefficient: ADC: ρ=-0.781, p<0.001; f: ρ=-0.720, p=0.001); but D was poorly correlated with fibrosis level (ρ=-0.502, p=0.040).

CONCLUSION

The hepatic ADC and the perfusion fraction f were significantly correlated with the liver fibrosis level; however, D was not. This might suggest that hepatic perfusion is altered during the progression of hepatic fibrosis.

Cardiac diffusion-weighted MR imaging in recent, subacute, and chronic myocardial infarction: a pilot study

PURPOSE

To investigate the clinical feasibility of diffusion-weighted imaging (DWI) to detect recent myocardial infarction (MI) and to differentiate it from subacute and chronic MI, with late-gadolinium enhancement (LGE) sequence as reference. Furthermore, to measure variation of the myocardial apparent diffusion coefficient (ADC) according to the age of MI.

MATERIALS AND METHODS

Seventy-four MI patients were separated in 3 groups. Group A included 34 recent (< 8 days) MI patients; group B, 22 subacute (9-90 days) MI patients; group C, 18 chronic (> 90 days) MI patients; a fourth group (group D) included 24 controls. DWI and LGE images were acquired on a 1.5T system. DWI and LGE matched images were assessed visually by two blinded observers for hyperintense areas in corresponding segments.

RESULTS

Qualitative assessment of DWI compared with LGE images yielded a sensitivity of 97% and a specificity of 61%/14% to differentiate recent from chronic/subacute MI, respectively. The absolute ADCs (recent 0.00632 ± 0.00037 mm(2) /s, subacute 0.00639 ± 0.00035 mm(2) /s, chronic 0.00743 ± 0.00056 mm(2) /s, remote or normal 0.00895 ± 0.00019 mm(2) /s) and relative ADCs were significantly different between groups (P < 0.001) except between recent and subacute MIs.

CONCLUSION

DWI is a sensitive technique to diagnose recent MI. DWI MR sequences could help differentiate recent from chronic MI. From these preliminary results, one should expect DWI to be used in the triage of emergency patients with atypical chest pain, to clarify if an MI is present or not in just a few minutes.

In vivo cardiac diffusion-weighted magnetic resonance imaging: quantification of normal perfusion and diffusion coefficients with intravoxel incoherent motion imaging

OBJECTIVES

Diffusion-weighted imaging (DWI) and the introduction of the intravoxel incoherent motion (IVIM) model have provided a unique method for evaluating perfusion and diffusion within a tissue without the need for a contrast agent. Despite its relevance, cardiac DWI has thus far been limited by low b values because of signal loss induced by physiological motion. The goal of this study was to develop a methodology for estimating IVIM parameters of in vivo cardiac magnetic resonance imaging using an efficient DWI acquisition framework. This was achieved by investigating various acquisition strategies (principal component analysis [PCA] filtering and temporal maximum intensity projection [PCATMIP] and single trigger delay [TD]) and fitting methods.

MATERIAL AND METHODS

Simulations were performed on a synthetic dataset of diffusion-weighted signal intensity (SI) to determine the fitting method that would yield IVIM parameters with the greatest accuracy. The required number of b values to correctly estimate IVIM parameters was also investigated. Breath-hold DWI scans were performed for 12 volunteers to collect several TD values during diastole. Thirteen b values ranging from 0 to 550 s/mm were used. The IVIM parameters derived using the data from all the acquired TDs (PCATMIP technique) were compared with those derived using a single acquisition performed at an optimized diastolic time point (1TD).

RESULTS

The main result of this study was that PCATMIP, when combined with a fitting model that accounted for T1 and T2 relaxation, provided IVIM parameters with less variability. However, an acquisition performed with 1 optimized diastolic TD provided results that were as good as those provided using PCATMIP if the R-R variability during the acquisition was sufficiently low (± 5%). Furthermore, the use of only 9 b values (that could be acquired in 2 breath-holds), instead of 13 b values (requiring 3 breath-holds), was sufficient to determine the IVIM parameters.

CONCLUSIONS

This study demonstrates that IVIM is technically feasible in vivo and reports for the first time the perfusion fraction, f, and the diffusion coefficients, D and D*, for the cardiac DWI of healthy volunteers. Motion-induced signal loss, which is the main problem associated with cardiac DWI, could be avoided with the combined use of sliding acquisition during the cardiac cycle and image postprocessing with the PCATMIP algorithm. This study provides new perspectives for perfusion imaging without a contrast agent and demonstrates that IVIM parameters can act as promising tools to further characterize microvascular abnormalities or dysfunction.

Effects of gadoxetic acid on quantitative diffusion-weighted imaging of the liver

PURPOSE

To prospectively evaluate the effect of gadoxetic acid (Gd-EOB-DTPA; Primovist, Bayer-Schering, Berlin, Germany) on quantitative diffusion-weighted imaging (DWI) using the Le Bihan IntraVoxel Incoherent Motion model and considering separately the following parameters: slow diffusion coefficient (D), fast diffusion coefficient (D*), perfusion fraction (PF), and apparent diffusion coefficient (ADC).

MATERIALS AND METHODS

Twenty-four consecutive patients were submitted to the same magnetic resonance (MR)-DWI acquisition before and after gadoxetic acid administration. Patients were divided into four groups according to the time at which the DW sequence was repeated, then 5, 10, 15, 20 minutes after contrast agent administration. A total of 48 manually drawn regions of interest (ROIs) of about 1200 pixels were placed in the middle right liver lobe. The mean and standard deviation (SD) were calculated in each group/patient for every DWI-related parameter. Analysis of variance was performed (threshold P = 0.05). Bonferroni and Games-Howell post-hoc tests were applied if significant differences were found among groups; otherwise, data were averaged together.

RESULTS

D, D*, PF, and ADC did not show any significant difference before and after contrast agent administration, at any time.

CONCLUSION

It is possible to perform DW acquisitions after gadoxetic acid administration without any significant variation of the values of DW-related parameters under consideration in this study.

Grading of uterine cervical cancer by using the ADC difference value and its correlation with microvascular density and vascular endothelial growth factor

OBJECTIVE

To investigate the application value of the ADC(difference) value in evaluating the pathological grade of uterine cervical cancer and to analyse the correlations among microvascular density (MVD), vascular endothelial growth factor (VEGF) expression and maximum ADC(difference) value.

METHODS

Fifty-six patients with uterine cervical cancer were included in this prospective study. All underwent conventional MRI and DWI. MVD and VEGF were evaluated by immunohistochemical staining with anti-CD34 and anti-VEGF, respectively.

RESULTS

Maximum ADC(difference) value and MVD count showed statistical differences among different pathological grades (P < 0.001, P < 0.001). There was a significant positive linear correlation between the maximum ADC(difference) value and pathological tumour grade (P < 0.001), and also between MVD count and pathological tumour grade (P < 0.001). No significant differences were found between the level of VEGF expression and pathological tumour grade (P = 0.222). The maximum ADC(difference) value correlated positively with both the MVD count and the level of VEGF expression (P < 0.001, P < 0.001).

CONCLUSIONS

Quantitative analysis of maximum ADC(difference) value of uterine cervical cancer may represent the grade of tumour differentiation and provide valuable information on tumour microcirculation and perfusion, thus allowing a promising new method of non-invasively assessing the pathological grade, which could serve as a substitution for assessing tumour angiogenesis.

Extension of the intravoxel incoherent motion model to non-gaussian diffusion in head and neck cancer

PURPOSE

To extend the intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) model to restricted diffusion and to simultaneously quantify the perfusion and restricted diffusion parameters in neck nodal metastases.

MATERIALS AND METHODS

The non-gaussian (NG)-IVIM model was developed and tested on diffusion-weighted MRI data collected on a 1.5-Tesla MRI scanner from eight patients with head and neck cancer. Voxel-wise parameter quantification was performed by using a noise-rectified least-square fitting method. The NG-IVIM, IVIM, Kurtosis, and ADC (apparent diffusion coefficient) models were used for comparison. For each voxel, within the metastatic node, the optimal model was determined using the Bayesian Information Criterion. The voxel percentage preferred by each model was calculated and the optimal model map was generated. Monte Carlo simulations were performed to evaluate the accuracy and precision dependency of the new model.

RESULTS

For the eight neck nodes, the range of voxel percentage preferred by the NG-IVIM model was 2.3-79.3%. The optimal modal maps showed heterogeneities within the tumors. The Monte Carlo simulations demonstrated that the accuracy and precision of the NG-IVIM model improved by increasing signal-to-noise ratio and b value.

CONCLUSION

The NG-IVIM model characterizes perfusion and restricted diffusion simultaneously in neck nodal metastases.

Interstitial fluid pressure correlates with intravoxel incoherent motion imaging metrics in a mouse mammary carcinoma model

The effective delivery of a therapeutic drug to the core of a tumor is often impeded by physiological barriers, such as the interstitial fluid pressure (IFP). There are a number of therapies that can decrease IFP and induce tumor vascular normalization. However, a lack of a noninvasive means to measure IFP hinders the utilization of such a window of opportunity for the maximization of the treatment response. Thus, the purpose of this study was to investigate the feasibility of using intravoxel incoherent motion (IVIM) diffusion parameters as noninvasive imaging biomarkers for IFP. Mice bearing the 4T1 mammary carcinoma model were studied using diffusion-weighted imaging (DWI), immediately followed by wick-in-needle IFP measurement. Voxelwise analysis was conducted with a conventional monoexponential diffusion model, as well as a biexponential model taking IVIM into account. There was no significant correlation of IFP with either the median apparent diffusion coefficient from the monoexponential model (r = 0.11, p = 0.78) or the median tissue diffusivity from the biexponential model (r = 0.30, p = 0.44). However, IFP was correlated with the median pseudo-diffusivity (D(p)) of apparent vascular voxels (r = 0.76, p = 0.02) and with the median product of the perfusion fraction and pseudo-diffusivity (f(p)D(p)) of apparent vascular voxels (r = 0.77, p = 0.02). Although the effect of IVIM in tumors has been reported previously, to our knowledge, this study represents the first direct comparison of IVIM metrics with IFP, with the results supporting the feasibility of the use of IVIM DWI metrics as noninvasive biomarkers for tumor IFP.

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.

Low b-value diffusion-weighted cardiac magnetic resonance imaging: initial results in humans using an optimal time-window imaging approach

OBJECTIVES

Diffusion-weighted imaging (DWI) using low b-values permits imaging of intravoxel incoherent motion in tissues. However, low b-value DWI of the human heart has been considered too challenging because of additional signal loss due to physiological motion, which reduces both signal intensity and the signal-to-noise ratio (SNR). We address these signal loss concerns by analyzing cardiac motion during a heartbeat to determine the time-window during which cardiac bulk motion is minimal. Using this information to optimize the acquisition of DWI data and combining it with a dedicated image processing approach has enabled us to develop a novel low b-value diffusion-weighted cardiac magnetic resonance imaging approach, which significantly reduces intravoxel incoherent motion measurement bias introduced by motion.

MATERIALS AND METHODS

Simulations from displacement encoded motion data sets permitted the delineation of an optimal time-window with minimal cardiac motion. A number of single-shot repetitions of low b-value DWI cardiac magnetic resonance imaging data were acquired during this time-window under free-breathing conditions with bulk physiological motion corrected for by using nonrigid registration. Principal component analysis (PCA) was performed on the registered images to improve the SNR, and temporal maximum intensity projection (TMIP) was applied to recover signal intensity from time-fluctuant motion-induced signal loss. This PCATMIP method was validated with experimental data, and its benefits were evaluated in volunteers before being applied to patients.

RESULTS

Optimal time-window cardiac DWI in combination with PCATMIP postprocessing yielded significant benefits for signal recovery, contrast-to-noise ratio, and SNR in the presence of bulk motion for both numerical simulations and human volunteer studies. Analysis of mean apparent diffusion coefficient (ADC) maps showed homogeneous values among volunteers and good reproducibility between free-breathing and breath-hold acquisitions. The PCATMIP DWI approach also indicated its potential utility by detecting ADC variations in acute myocardial infarction patients.

CONCLUSIONS

Studying cardiac motion may provide an appropriate strategy for minimizing the impact of bulk motion on cardiac DWI. Applying PCATMIP image processing improves low b-value DWI and enables reliable analysis of ADC in the myocardium. The use of a limited number of repetitions in a free-breathing mode also enables easier application in clinical conditions.

Liver magnetic resonance diffusion weighted imaging: 2011 update

Diffusion-Weighted-Imaging (DWI) assesses proton motion on a cellular scale. Owing to recent instrumentation developments, diffusion sequences are now routinely used for liver imaging. This review will go through the physical principles that underlie this technique, and then highlight up-to-date liver applications including quantification of liver fibrosis, focal lesions detection and characterization, and therapy response monitoring.

Comparison of biexponential and monoexponential model of diffusion weighted imaging in evaluation of renal lesions: preliminary experience

OBJECTIVES

To obtain intravoxel incoherent motion (IVIM) parameters with biexponential analysis of multiple b-value diffusion-weighted imaging (DWI) and compare these parameters to apparent diffusion coefficient (ADC) obtained with monoexponential modeling in their ability to discriminate enhancing from nonenhancing renal lesions.

MATERIALS AND METHODS

Twenty-eight patients were imaged at 1.5 T utilizing contrast-enhanced (CE) magnetic resonance imaging (MRI) and breath-hold DWI using 8 b values (range: 0-800 s/mm(2)). Perfusion fraction (f(p)), tissue diffusivity (D(t)), and pseudo-diffusion coefficient (D(p)) were calculated using segmented biexponential analysis. ADC(total) and ADC(0-400-800) were calculated with monoexponential fitting of the DWI data. f(p), D(t), D(p), ADC(total), and ADC(0-400-800) were compared between enhancing and nonenhancing renal lesions. Receiver operating characteristic analysis was performed for all DWI parameters. f(p) was correlated with percent enhancement.

RESULTS

There were a total of 31 renal lesions (15 enhancing and 16 nonenhancing) in 28 patients on CE-MRI. f(p) of enhancing masses was significantly higher (27.9 vs. 6.1) and D(t) was significantly lower (1.47 vs. 2.40 ×10(-3) mm(2)/s). IVIM parameters f(p) and D(t) demonstrated higher accuracy in differentiating enhancing from nonenhancing renal lesions compared with monoexponential parameters ADC(0-400-800) and ADC(total), with area under the curve of 0.946, 0.896, 0.854, and 0.675, respectively. There was a good correlation between f(p) and percent enhancement (r = 0.7; P < 0.001).

CONCLUSION

IVIM parameters f(p) and D(t) obtained with biexponential fitting of multi-b value DWI have higher accuracy compared with ADC (obtained with monoexponential fit) in discriminating enhancing from nonenhancing renal lesions. Furthermore, f(p) demonstrates good correlation with percent enhancement and can provide information regarding lesion vascularity without the use of exogenous contrast agent.

Magnetic resonance diffusion-weighted imaging: quantitative evaluation of age-related changes in healthy liver parenchyma

The purpose of this study was to verify in healthy liver parenchyma the possible influence of age on DwI-related parameters: apparent diffusion coefficient (ADC), perfusion fraction (PF), diffusion and pseudodiffusion coefficient (D and D*). Forty healthy adult volunteers (age range 26-86 years), divided into four age groups, were prospectively submitted to a breath-hold magnetic resonance diffusion imaging (MR-DwI) (two b values, 0-300 and 0-1000 s/mm²). A smaller cohort of 16 subjects underwent a free-breath multi-b acquisition (16 b values, 0-750 s/mm²). Quantitative analysis was performed by two observers with manually defined regions of interest, on the most homogeneous portion of the right liver lobe. Individual and group statistical analysis of data was performed: ANOVA to establish differences between groups and Pearson correlation coefficient to investigate the association between DwI parameters and age. The mean, S.D. and 95% limits of agreement of ADC values for each age-defined group are reported. ANOVA showed no significant differences between group means (P always >.05). No significant correlation between subjects' age and DwI parameters was established, both in breath-hold and free-breath acquisitions, on the whole range of adopted b values. Our study conducted on healthy liver parenchyma shows that there are no significant differences in ADC, PF, D and D* of younger or older subjects.

Intravoxel incoherent motion imaging of tumor microenvironment in locally advanced breast cancer

Diffusion-weighted imaging plays important roles in cancer diagnosis, monitoring, and treatment. Although most applications measure restricted diffusion by tumor cellularity, diffusion-weighted imaging is also sensitive to vascularity through the intravoxel incoherent motion effect. Hypervascularity can confound apparent diffusion coefficient measurements in breast cancer. We acquired multiple b-value diffusion-weighted imaging at 3 T in a cohort of breast cancer patients and performed biexponential intravoxel incoherent motion analysis to extract tissue diffusivity (D(t)), perfusion fraction (f(p)), and pseudodiffusivity (D(p)). Results indicated significant differences between normal fibroglandular tissue and malignant lesions in apparent diffusion coefficient mean (±standard deviation) values (2.44 ± 0.30 vs. 1.34 ± 0.39 μm(2)/msec, P < 0.01) and D(t) (2.36 ± 0.38 vs. 1.15 ± 0.35 μm(2)/msec, P < 0.01). Lesion diffusion-weighted imaging signals demonstrated biexponential character in comparison to monoexponential normal tissue. There is some differentiation of lesion subtypes (invasive ductal carcinoma vs. other malignant lesions) with f(p) (10.5 ± 5.0% vs. 6.9 ± 2.9%, P = 0.06), but less so with D(t) (1.14 ± 0.32 μm(2)/msec vs. 1.18 ± 0.52 μm(2)/msec, P = 0.88) and D(p) (14.9 ± 11.4 μm(2)/msec vs. 16.1 ± 5.7 μm(2)/msec, P = 0.75). Comparison of intravoxel incoherent motion biomarkers with contrast enhancement suggests moderate correlations. These results suggest the potential of intravoxel incoherent motion vascular and cellular biomarkers for initial grading, progression monitoring, or treatment assessment of breast tumors.

Robust optimal design of diffusion-weighted magnetic resonance experiments for skin microcirculation

Skin microcirculation plays an important role in several diseases including chronic venous insufficiency and diabetes. Magnetic resonance (MR) has the potential to provide quantitative information and a better penetration depth compared with other non-invasive methods such as laser Doppler flowmetry or optical coherence tomography. The continuous progress in hardware resulting in higher sensitivity must be coupled with advances in data acquisition schemes. In this article, we first introduce a physical model for quantifying skin microcirculation using diffusion-weighted MR (DWMR) based on an effective dispersion model for skin leading to a q-space model of the DWMR complex signal, and then design the corresponding robust optimal experiments. The resulting robust optimal DWMR protocols improve the worst-case quality of parameter estimates using nonlinear least squares optimization by exploiting available a priori knowledge of model parameters. Hence, our approach optimizes the gradient strengths and directions used in DWMR experiments to robustly minimize the size of the parameter estimation error with respect to model parameter uncertainty. Numerical evaluations are presented to demonstrate the effectiveness of our approach as compared to conventional DWMR protocols.

Diagnosis of cirrhosis with intravoxel incoherent motion diffusion MRI and dynamic contrast-enhanced MRI alone and in combination: preliminary experience

PURPOSE

To report our preliminary experience with the use of intravoxel incoherent motion (IVIM) diffusion-weighted magnetic resonance imaging (DW-MRI) and dynamic contrast-enhanced (DCE)-MRI alone and in combination for the diagnosis of liver cirrhosis.

MATERIALS AND METHODS

Thirty subjects (16 with noncirrhotic liver, 14 with cirrhosis) were prospectively assessed with IVIM DW-MRI (n = 27) and DCE-MRI (n = 20). IVIM parameters included perfusion fraction (PF), pseudodiffusion coefficient (D*), true diffusion coefficient (D), and apparent diffusion coefficient (ADC). Model-free DCE-MR parameters included time to peak (TTP), upslope, and initial area under the curve at 60 seconds (IAUC60). A dual input single compartmental perfusion model yielded arterial flow (Fa), portal venous flow (Fp), arterial fraction (ART), mean transit time (MTT), and distribution volume (DV). The diagnostic performances for diagnosis of cirrhosis were evaluated for each modality alone and in combination using logistic regression and receiver operating characteristic analyses. IVIM and DCE-MR parameters were compared using a generalized estimating equations model.

RESULTS

PF, D*, D, and ADC values were significantly lower in cirrhosis (P = 0.0056-0.0377), whereas TTP, DV, and MTT were significantly increased in cirrhosis (P = 0.0006-0.0154). There was no correlation between IVIM- and DCE-MRI parameters. The highest Az (areas under the curves) values were observed for ADC (0.808) and TTP-DV (0.952 for each). The combination of ADC with DV and TTP provided 84.6% sensitivity and 100% specificity for diagnosis of cirrhosis.

CONCLUSION

The combination of DW-MRI and DCE-MRI provides an accurate diagnosis of cirrhosis.

Balanced multipoint displacement encoding for DENSE MRI

Displacement encoding with stimulated echoes (DENSE) is a quantitative imaging technique that encodes tissue displacement in the phase of the acquired signal. Various DENSE sequences have encoded displacement using methods analogous to the simple multipoint methods of phase contrast (PC) MRI. We developed general n-dimension balanced multipoint encoding for DENSE. Using these methods, phase noise variance decreased experimentally by 73.7%, 65.6%, and 61.9% compared with simple methods, which closely matched the theoretical decreases of 75%, 66.7%, and 62.5% for one-dimensional (1D), 2D, and 3D encoding, respectively. Phase noise covariances decreased by 99.2% and 99.3% for balanced 2D and 3D encoding, consistent with the zero-covariance prediction. The direction bias inherent to the simple methods was decreased to almost zero using balanced methods. Reduced phase noise and improved displacement and strain maps using balanced methods were visually observed in phantom and volunteer images. Balanced multipoint encoding can also be applied to PC MRI.

Circumferential strain in the wall of the common carotid artery: comparing displacement-encoded and cine MRI in volunteers

The walls of conduit arteries undergo cyclic stretching from the periodic fluctuation of arterial pressure. Atherosclerotic lesions have been shown to localize to regions of excessive stretching of the arterial wall. We employed a displacement encoding with stimulated echoes (DENSE) sequence to image the motion of the common carotid artery wall and map the two-dimensional (2D) circumferential strain. The sequence utilizes a fully-balanced steady-state free-precession (SSFP) readout with 0.60 mm in-plane resolution. Preliminary results in volunteers at 1.5T (N = 4) and 3.0T (N = 17) are compared to measurements of the lumen circumference from cine images. The agreement between the two independent measurements at both field strengths (P < or = 0.001) supports the use of DENSE as a means to map the pulsatile strain in the carotid artery wall.

Selective suppression of artifact-generating echoes in cine DENSE using through-plane dephasing

In displacement-encoded imaging with stimulated echoes (DENSE), tissue displacement is encoded in the phase of the stimulated echo. However, three echoes generally contribute to the acquired signal (the stimulated echo, the complex conjugate of the stimulated echo, and an echo due to T(1) relaxation). It is usually desirable to suppress all except the stimulated echo, since otherwise the additional echoes will cause displacement measurement errors. Ideally, suppression of the artifact-generating echoes would be independent of time, T(1), and displacement-encoding frequency, and would not require additional acquisitions. In this study through-plane gradients were used to selectively dephase artifact-generating echoes without causing significant signal loss of the stimulated echo. A cine DENSE sequence was modified to include dephasing gradients and perform complementary spatial modulation of magnetization (CSPAMM). For single-acquisition cine DENSE using dephasing alone, artifact suppression was similar to CSPAMM with two acquisitions. The use of dephasing with CSPAMM required two acquisitions, but demonstrated greater artifact suppression than CSPAMM alone or dephasing alone.

Noninvasive measurement of myocardial tissue volume change during systolic contraction and diastolic relaxation in the canine left ventricle

In coronary circulation the flow in epicardial arteries and veins is observed to be pulsatile and out of phase with each other. Theoretical considerations predict that this phenomenon extends to all levels of the vascular tree and leads to a cyclic fluctuation of regional tissue volume. Intramyocardial tissue volume change between end-systole and end-diastole was measured noninvasively with MRI in 10 closed-chest beagles. The displacement encoding with stimulated-echo technique was used to obtain pixel-by-pixel tissue displacement field between end-diastole and end-systole and vice versa in the midlevel left ventricle, from which the 3D strain matrix and volume changes were calculated. The volume change was between 0.8+/-0.5% (mean+/-STD) in the epicardial layer and 1.5+/-0.6% in the subendocardial layer of the left ventricle. Tissue volume fluctuation reflects the amount of arterial inflow in a heartbeat under the assumption that regional arterial inflow and venous outflow have little time overlap. The corresponding perfusion level was estimated to be from (1.0+/-0.6) ml/min/g in the epicardial layer to (1.7+/-0.6) ml/min/g in the subendocardial layer, in good agreement with microsphere measurements in the same dog model. The result supports the notion of high arterial resistance at the microvascular level from intramyocardial pressure during systole.

Magnetic resonance imaging assessment of myocardial elastic modulus and viscosity using displacement imaging and phase-contrast velocity mapping

Approximately half of patients experiencing congestive heart failure present with a normal left ventricular ejection fraction. Perturbations in material properties affecting ventricular pressure/volume relationships likely play an important role in the "stiff heart syndrome" yet noninvasive tools permitting the accurate assessment of myocardial elasticity are extremely limited. We developed an MRI-based technique to examine regional left ventricular stress/strain relationships by incorporating displacement-encoding with stimulated-echoes (DENSE) and phase-contrast (PC) velocity mapping and compared regional elastic moduli (EM) and viscous delay time constants (VDTCs) (N=10) with immediate postmortem direct strain gauge measurements (N=8) and global chamber compliance (literature) in normal dogs. EMs by MRI were significantly greater in papillary muscle columns when compared with lateral wall and septal locations by MRI (7.59+/-1.65 versus 3.40+/-0.87 versus 2.55+/-0.93 kPa, P<0.0001) and were in agreement with direct strain gauge measurements (3.78+/-0.93 and 2.96+/-0.88 kPa for the lateral wall and the septum, P=ns for both versus MRI). MRI-determined VDTCs were similar in the three regions (VDTC=-1.15+/-12.37 versus 3.04+/-7.25 versus 4.17+/-5.76 ms, P=ns) and did not differ from lateral and septal wall strain gauge assessment (VDTC=3.09+/-0.40 and 4.57+/-1.86 ms, P=ns for both versus MRI). Viscoelastic measurements obtained in six normal volunteers demonstrated the feasibility of this technique in humans. Noninvasive, regional assessment of myocardial stiffness using DENSE and PC velocity mapping techniques is accurate in a canine model and feasible in humans.