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An improved model for prostate diffusion incorporating the results of Monte Carlo simulations of diffusion in the cellular compartment

Gilani, Nima; Malcolm, Paul; Johnson, Glyn
The purpose of this work was to refine a previously published model of prostate diffusion by incorporating improved estimates of cellular diffusivity obtained by Monte Carlo simulation. Stromal and epithelial cell size and intracellular volume fraction in different grades of cancer were determined from histological images. Diffusion in different mixtures of cells, corresponding to different tumor grades, was simulated and cellular apparent diffusion coefficient and kurtosis values determined. These values were incorporated into the previously published model of prostate diffusion and model predictions compared with values found in the literature. Stromal cell radius and intracellular volume fraction were 3.74 +/- 0.96 mum and 13 +/- 3% respectively in normal peripheral zone (PZ), and were similar in all grades of cancer. Epithelial cell radius and intracellular volume fraction were 3.40 +/- 0.15 mum and 45 +/- 5% respectively in normal PZ, rising to 4.75 +/- 0.20 mum and 70 +/- 8% in high grade cancer. Cellular apparent diffusion coefficient and kurtosis were 1.02 mum2 ms-1 and 0.58 respectively in normal PZ, and 0.61 mum2 ms-1 and 1.15 in high grade cancer (variation in simulation values are less than 0.1%). Agreement between model predictions and measurements were good, with a mean square error of 0.22 mum2 ms-1 . Incorporation of cellular diffusion coefficient and kurtosis values obtained by Monte Carlo simulation into a model of prostate diffusion gives good agreement with published results.
PMID: 28915319
ISSN: 1099-1492
CID: 2705952

Minimization of errors in biexponential T measurements of the prostate

Gilani, Nima; Rosenkrantz, Andrew B; Malcolm, Paul; Johnson, Glyn
PURPOSE: To determine the echo times that provide the greatest precision in measurements of prostate T2 s. T2 relaxation time measurements in the prostate are complicated by the structure of prostate tissue, which consists of fluid-filled glands surrounded by epithelial and stromal cells. Since the glands are large relative to diffusion distances, there is little water exchange between the two compartments and T2 s are biexponential. Because the relative size and characteristics of the two compartments change in prostate tumors, accurate measurement of the characteristics of each may provide useful information on tumor grade. MATERIALS AND METHODS: T2 s were measured in a group of 25 men with biopsy-proven prostate cancer. Subjects were scanned at 3T with a 16-echo turbo-spin echo T2 -mapping sequence. Normal prostate T2 s were measured in areas showing no disease. Optimum echo times for measurement of normal prostate T2 s were found by calculating the covariance matrix, which provides estimates of parameter variance. Echo times that minimize T2 variance were then found by searching over grids of different echo times. Optima for four to eight echo acquisitions were found. Optima were tested by Monte Carlo simulation. RESULTS: Fast and slow T2 s were 60 msec and 360 msec, respectively. The fast signal fraction was 0.6. Optimum echo times were between 0 and 780 msec, depending on the number of echoes acquired. CONCLUSION: Use of optimum echo times can substantially improve the precision of biexponential T2 measurements. This optimization is anticipated to improve prostate cancer characterization using T2 measurements. J. Magn. Reson. Imaging 2015.
PMID: 25704897
ISSN: 1053-1807
CID: 1473442

Intravascular contrast agent T(2) (*) relaxivity in brain tissue

Patil, Vishal; Jensen, Jens H; Johnson, Glyn
Dynamic susceptibility-weighted contrast-enhanced (DSC) MRI perfusion measurements depend on estimating intravascular contrast agent (CA) concentrations (C) from signal intensity changes in T2*-weighted images after bolus injection. Generally, linearity is assumed between relaxation and C, but previous studies have shown that compartmentalization of CA and secondary magnetic field perturbations generate deviations from linearity. Physical phantoms using bulk blood have been used to empirically determine the relationship between relaxation rate and C in large vessels. However, the relaxivity of CA in the microvasculature is not easily measured since constructing appropriate phantoms is difficult. Instead, theoretical relaxivity models have been developed. In this study, we empirically tested a non-linear expression based on static dephasing regime (SDR) and linear approximation. Signal-time curves in white (WM) and grey matter (GM) were converted to concentration time curves (CTCs) using both expressions. Parameters for both linear and non-linear formulations were adjusted to give a best agreement between cerebral blood volumes (CBV) calculated from WM and arterial CTCs in a group of normal subjects scanned at 3T. Optimized parameters were used to calculate blood volume in WM and GM in healthy subjects scanned at 3T and in meningioma patients scanned at 1.5T. Results from this study showed that a non-linear SDR formulation gave an acceptable functional form for tissue relaxivity, giving reliable CBV estimates at different field strengths and echo times
PMCID:3672249
PMID: 23225224
ISSN: 0952-3480
CID: 378682

DeltaR2 (*) gadolinium-diethylenetriaminepentacetic acid relaxivity in venous blood

Patil, Vishal; Johnson, Glyn
The accuracy of perfusion measurements using dynamic, susceptibility-weighted, contrast-enhanced MRI depends on estimating contrast agent concentration in an artery, i.e., the arterial input function. One of the difficulties associated with obtaining an arterial input function are partial volume effects when both blood and brain parenchyma occupy the same pixel. Previous studies have attempted to correct arterial input functions which suffer from partial volume effects using contrast concentration in venous blood. However, the relationship between relaxation and concentration (C) in venous blood has not been determined in vivo. In this note, a previously employed fitting approach is used to determine venous relaxivity in vivo. In vivo relaxivity is compared with venous relaxivity measured in vitro in bulk blood. The results show that the fitting approach produces relaxivity calibration curves which give excellent agreement with arterial measurements. Magn Reson Med 69:1104-1108, 2013. (c) 2012 Wiley Periodicals, Inc.
PMCID:3437243
PMID: 22576560
ISSN: 0740-3194
CID: 255172

Analytical derivation of the b matrix of a time-efficient isotropic diffusion weighting gradient waveform

Trampel, Robert; Johnson, Glyn
PURPOSE: Diffusion-weighted magnetic resonance imaging of (3)He provides information about lung structure. If rotationally invariant measures of diffusion are desired, an equal diffusion weighting in all three spatial directions is necessary to obtain. In order to achieve such isotropic diffusion weighting, gradients have to be applied in these three spatial directions, which can be time consuming. Therefore, the purpose of this study was the analytic derivation of a time-efficient isotropic diffusion weighting scheme. METHODS: The complete b matrix of a preselected gradient waveform was derived analytically. The effect of ramp times and the contribution of the imaging gradients were included in the calculation. The time-efficient waveform was compared to a standard isotropic diffusion weighting scheme by determining the mean diffusivity of hyperpolarized (3)He in human lungs. RESULTS: An analytically derived expression of the b matrix for a time-efficient gradient scheme allowing isotropic diffusion weighting was derived. Additionally, the b matrix of a common set of imaging gradients was calculated. Diffusion measurements of hyperpolarized (3)He in human lungs using the derived optimized gradient scheme and a standard gradient waveform used for isotropic diffusion weighting, respectively, gave results for the mean diffusivity which did not show any statistical difference. However, the echo time using the optimized scheme was reduced by 2.5 ms in comparison with the standard scheme which leads to a theoretical signal increase of 30%. CONCLUSIONS: The analytically derived b matrix allows for the straightforward determination of time-efficient isotropic diffusion weighting schemes. By using those schemes, a substantial gain in signal can be achieved whereas the resulting values for the mean diffusivity did not show any statistical difference to the values obtained when using standard waveforms for isotropic diffusion weighting.
PMID: 23464333
ISSN: 0094-2405
CID: 823262

USE OF MULTIVOXEL DSC-MRI PERFUSION DATA IN STEREOTACTIC-GUIDED GLIOMA SURGERY AND CORRELATION WITH TUMOR PATHOLOGY [Meeting Abstract]

Parker, Erik; Fatterpekar, Girish; Raz, Eytan; Narayana, Ashwatha; Johnson, Glyn; Placantonakis, Dimitris; Zagzag, David
ISI:000310971300496
ISSN: 1522-8517
CID: 204992

Prostate Cancer: Feasibility and Preliminary Experience of a Diffusional Kurtosis Model for Detection and Assessment of Aggressiveness of Peripheral Zone Cancer

Rosenkrantz, AB; Sigmund, EE; Johnson, G; Babb, JS; Mussi, TC; Melamed, J; Taneja, SS; Lee, VS; Jensen, JH
Purpose: To assess the feasibility of diffusional kurtosis (DK) imaging for distinguishing benign from malignant regions, as well as low- from high-grade malignant regions, within the peripheral zone (PZ) of the prostate in comparison with standard diffusion-weighted (DW) imaging. Materials and Methods: The institutional review board approved this retrospective HIPAA-compliant study and waived informed consent. Forty-seven patients with prostate cancer underwent 3-T magnetic resonance imaging by using a pelvic phased-array coil and DW imaging (maximum b value, 2000 sec/mm(2)). Parametric maps were obtained for apparent diffusion coefficient (ADC); the metric DK (K), which represents non-Gaussian diffusion behavior; and corrected diffusion (D) that accounts for this non-Gaussianity. Two radiologists reviewed these maps and measured ADC, D, and K in sextants positive for cancer at biopsy. Data were analyzed by using mixed-model analysis of variance and receiver operating characteristic curves. Results: Seventy sextants exhibited a Gleason score of 6; 51 exhibited a Gleason score of 7 or 8. K was significantly greater in cancerous sextants than in benign PZ (0.96 ± 0.24 vs 0.57 ± 0.07, P < .001), as well as in cancerous sextants with higher rather than lower Gleason score (1.05 ± 0.26 vs 0.89 ± 0.20, P < .001). K showed significantly greater sensitivity for differentiating cancerous sextants from benign PZ than ADC or D (93.3% vs 78.5% and 83.5%, respectively; P < .001), with equal specificity (95.7%, P > .99). K exhibited significantly greater sensitivity for differentiating sextants with low- and high-grade cancer than ADC or D (68.6% vs 51.0% and 49.0%, respectively; P ≤ .004) but with decreased specificity (70.0% vs 81.4% and 82.9%, respectively; P ≤ .023). K had significantly greater area under the curve for differentiating sextants with low- and high-grade cancer than ADC (0.70 vs 0.62, P = .010). Relative contrast between cancerous sextants and benign PZ was significantly greater for D or K than ADC (0.25 ± 0.14 and 0.24 ± 0.13, respectively, vs 0.18 ± 0.10; P < .001). Conclusion: Preliminary findings suggest increased value for DK imaging compared with standard DW imaging in prostate cancer assessment. Supplemental material: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12112290/-/DC1
PMID: 22550312
ISSN: 0033-8419
CID: 167146

Treatment-Related Change Versus Tumor Recurrence in High-Grade Gliomas: A Diagnostic Conundrum--Use of Dynamic Susceptibility Contrast-Enhanced (DSC) Perfusion MRI

Fatterpekar, Girish M; Galheigo, Diogo; Narayana, Ashwatha; Johnson, Glyn; Knopp, Edmond
OBJECTIVE: The purpose of this article is to address radiation necrosis, pseudoprogression, and pseudoresponse relative to high-grade gliomas and evaluate the role of conventional MRI and, in particular, dynamic susceptibility contrast-enhanced perfusion MRI in assessing such treatment-related changes from tumor recurrence. CONCLUSION: Posttreatment imaging assessment of high-grade gliomas remains challenging. Familiarity with the expected MR imaging appearances of treatment-related change and tumor recurrence will help distinguish these entities allowing appropriate management
PMCID:3105250
PMID: 22194475
ISSN: 1546-3141
CID: 147218

An improved model for describing the contrast bolus in perfusion MRI

Patil, Vishal; Johnson, Glyn
Purpose: Quantification of perfusion measurements using dynamic, susceptibility-weighted contrast-enhanced (DSC) MRI depends on estimating the size and shape of the tracer bolus. Typically, the bolus is described as a gamma variate function (GV) fitted to the bolus portion of tracer concentration time curve (CTC). However, the last point to fit is arbitrary which can lead to considerable variation in the fitted curve in the presence of noise. In this technical note, we present a model which takes into account recirculation explicitly and fits robustly to the entire CTC in the presence of noise.Methods: Signal data measurements from ten DSC MRI patients were fitted with our new model and a GV function using four different methods of estimating the end of the bolus. Estimates of the area under the curves (AUC) and first moments (FMs) of the bolus were compared at different noise levels.Results: The new model gave errors similar to or smaller than those of the most effective methods for fitting a GV.Conclusions: The single compartment recirculation (SCR) model is the most robust fitting technique with respect to noise both for bias and variability
PMCID:3230640
PMID: 22149821
ISSN: 0094-2405
CID: 146258

Systematic differences between lean and obese adolescents in brain spin-lattice relaxation time: a quantitative study

Cazettes, F; Tsui, W H; Johnson, G; Steen, R G; Convit, A
BACKGROUND AND PURPOSE: Emerging evidence suggests that obese adolescents show changes in brain structure compared with lean adolescents. In addition, obesity impacts body development during adolescence. We tested a hypothesis that T1, a marker of brain maturation, can show brain differences associated with obesity. MATERIALS AND METHODS: Adolescents similar in sex, family income, and school grade were recruited by using strict entry criteria. We measured brain T1 in 48 obese and 31 lean adolescents by quantitative MR imaging at 1.5T. We combined MPRAGE and inversion-recovery sequences with normalization to standard space and automated skull stripping to obtain T1 maps with a symmetric voxel volume of 1 mm(3). RESULTS: Sex, income, triglycerides, total cholesterol, and fasting glucose did not differ between groups, but obese adolescents had significantly lower HDL, higher LDL, and higher fasting insulin levels than lean adolescents. Intracranial vault volume did not differ between groups, but obese adolescents had smaller intracranial vault-adjusted brain parenchymal volumes. Obese adolescents had 4 clusters (>100 contiguous voxels) of T1 relaxation that were significantly different (P < .005) from those in lean adolescents. Three of these clusters had longer T1s in obese adolescents (in the orbitofrontal and parietal regions), and 1 cluster had shorter T1s, compared with lean adolescents. CONCLUSIONS: Our results suggest that obesity may have a significant impact on brain development, especially in the frontal and parietal lobes. It is unclear if these changes persist into adulthood or whether they indicate that obese subjects follow a different developmental trajectory during adolescence
PMCID:3237848
PMID: 21960489
ISSN: 1936-959x
CID: 150559