The purpose of this study was to determine the accuracy and sources of error in estimating single-kidney glomerular filtration rate (GFR) derived from low-dose gadolinium-enhanced T1-weighted MR renography. To analyze imaging data, MR signal intensity curves were converted to concentration vs. time curves, and a three-compartment, six-parameter model of the vascular-nephron system was used to analyze measured aortic, cortical, and medullary enhancement curves. Reliability of the parameter estimates was evaluated by sensitivity analysis and by Monte Carlo analyses of model solutions to which random noise had been added. The dominant sensitivity of the medullary enhancement curve to GFR 1-4 min after tracer injection was supported by a low coefficient of variation in model-fit GFR values (4%) when measured data were subjected to 5% noise. These analyses also showed the minimal effects of bolus dispersion in the aorta on parameter reliability. Single-kidney GFR from MR renography analyzed by the three-compartment model (4.0-71.4 ml/min) agreed well with reference measurements from (99m)Tc-DTPA clearance and scintigraphy (r = 0.84, P < 0.001). Bland-Altman analysis showed an average difference of 11.9 ml/min (95% confidence interval = 5.8-17.9 ml/min) between model and reference values. We conclude that a nephron-based multicompartmental model can be used to derive clinically useful estimates of single-kidney GFR from low-dose MR renography.

Purpose To study the differentiation of malignant breast lesions from benign lesions and fibroglandular tissue (FGT) using apparent diffusion coefficient (ADC) and intravoxel incoherent motion (IVIM) parameters. Materials and Methods This retrospective study included 26 malignant and 14 benign breast lesions in 35 patients who underwent diffusion-weighted MRI at 3.0T and nine b-values (0–1000 s/mm2). ADC and IVIM parameters (perfusion fraction fp, pseudodiffusion coefficient Dp, and true diffusion coefficient Dd) were determined in lesions and FGT. For comparison, IVIM was also measured in 16 high-risk normal patients. A predictive model was constructed using linear discriminant analysis. Lesion discrimination based on ADC and IVIM parameters was assessed using receiver operating characteristic (ROC) and area under the ROC curve (AUC). Results In FGT of normal subjects, fp was 1.1 ± 1.1%. In malignant lesions, fp (6.4 ± 3.1%) was significantly higher than in benign lesions (3.1 ± 3.3%, P = 0.0025) or FGT (1.5 ± 1.2%, P < 0.001), and Dd ((1.29 ± 0.28) × 10−3 mm2/s) was lower than in benign lesions ((1.56 ± 0.28) × 10−3 mm2/s, P = 0.011) or FGT ((1.86 ± 0.34) × 10−3 mm2/s, P < 0.001). A combination of Dd and fp provided higher AUC for discrimination between malignant and benign lesions (0.84) or FGT (0.97) than ADC (0.72 and 0.86, respectively). Conclusion The IVIM parameters provide accurate identification of malignant lesions.

For time-resolved acquisitions with k-space undersampling, a simulation method was developed for selecting imaging parameters based on minimization of errors in signal intensity versus time and physiologic parameters derived from tracer kinetic analysis. Optimization was performed for time-resolved angiography with stochastic trajectories (TWIST) algorithm applied to contrast-enhanced MR renography. A realistic 4D phantom comprised of aorta and two kidneys, one healthy and one diseased, was created with ideal tissue time-enhancement pattern generated using a three-compartment model with fixed parameters, including glomerular filtration rate (GFR) and renal plasma flow (RPF). TWIST acquisitions with different combinations of sampled central and peripheral k-space portions were applied to this phantom. Acquisition performance was assessed by the difference between simulated signal intensity ( Key words: time-resolved MRI; dynamic contrast-enhanced MRI; MR renography; optimal sampling; TWIST Dynamic contrast-enhanced MR imaging (DCE MRI) plays an important role in many applications, such as perfusion imaging in oncology (1), MR angiography (2), and MR renography (MRR) (3,4). Among the key requirements of DCE MRI is achieving sufficiently high temporal resolution without sacrificing spatial resolution and anatomic coverage. Strategies for achieving both high temporal and spatial resolution often employ k-space undersampling, such as keyhole imaging (5), blocked regional interpolation scheme for k-space (BRISK) (6), continuous update with random encoding (CURE) (7), time-resolved imaging of contrast kinetics (TRICKS) (8,9), and k-t Broad-use Linear Acquisition Speed-up Technique (k-t BLAST) (10). The resulting image artifacts and spatial resolution depend on the size of the frequently updated portion of k-space (the "center") and on the nature and extent of undersampling of the periphery. A large central portion of k-space is likely to produce high-quality images but lower temporal resolution. On the other hand, undersampling of the peripheral k-space regions can result in ringing artifacts, which not only impair postprocessing steps, such as image segmentation, but may also obscure visualization and characterization of smaller structures. Furthermore, undersampling may distort enhancement curves, especially when the signal is changing rapidly, for example, during firstpass perfusion, and can affect the accuracy of kinetic modeling parameters.Despite increasing use of fast acquisition techniques and DCE MRI in diagnostic radiology, few studies have explored the problem of balancing the temporal and spatial properties of the acquisition protocol. A number of studies have evaluated the minimum temporal resolution required for accurate derivation of parameters using tracer kinetic modeling from dynamic data (11); however, there is no general methodology to guide the selection of optimal imaging parameters necessary to achieve proper temporal resolution as well as good-quality images. In humans, the main obstacle to opti...

The crossover between thermally assisted and pure quantum tunneling has been studied in single crystals of high spin (S = 10) uniaxial molecular magnet Mn12 using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow ( approximately 0. 1 K) or broad ( approximately 1 K) temperature interval depending on the magnitude of the field transverse to the anisotropy axis.

A three-compartment model is proposed for analyzing magnetic resonance renography (MRR) and computed tomography renography (CTR) data to derive clinically useful parameters such as glomerular filtration rate (GFR) and renal plasma flow (RPF). The model fits the convolution of the measured input and the predefined impulse retention functions to the measured tissue curves. A MRR study of 10 patients showed that relative root mean square errors by the model were significantly lower than errors for a previously reported three-compartmental model (11.6% ؎ 4.9 vs 15.5% ؎ 4.1; P < 0.001). GFR estimates correlated well with reference values by 99m Tc-DTPA scintigraphy (correlation coefficient r ‫؍‬ 0.82), and for RPF, r ‫؍‬ 0.80. Parameter-sensitivity analysis and Monte Carlo simulation indicated that model parameters could be reliably identified. Key words: computed tomography; glomerular filtration rate; impulse retention function; magnetic resonance renography; renal plasma flow MR renography (MRR) and computed tomography renography (CTR) are increasingly used for noninvasive measurement of single-kidney function (1-7). These dynamic imaging techniques record the transit of a tracer, such as Gd-DTPA or iodinated contrast agents, from the aorta through the renal system. Tracer activity versus time curves can then be derived for intrarenal regions such as renal cortex, medulla, and collecting system. Design of an appropriate physiologic model is an essential part of accurate quantification of renal function (1,2).Several models have been proposed to estimate glomerular filtration rate (GFR) from MRR (3-6) and CTR (7). Baumann and Rudin (3) computed the GFR from the medullary uptake of the tracer using the cortical concentration as the input function. Another method (4) used a PatlakRutland plot to estimate GFR from the clearance of the tracer from the vascular compartment. This approach used whole-kidney concentration, obviating the need for regional segmentation of the kidneys. Both of these methods ignored the outflow of the tracer, and the results can be biased by improper selection of the "upslope" interval. Annet et al. (5) extended these techniques to account for tracer leaving the nephron space, thus enabling fitting of the model to measured data over a longer time period. All of these models assume instantaneous mixing of tracer within every compartment.More recently, models have been proposed with the aim of extending physiologic measures beyond GFR. Krier et al. (7) represented the cortex and medulla curves as extended gamma-variate functions with parameters shown to yield renal plasma flow (RPF) and tubular transit times in addition to GFR. GFR and RPF measures were validated against the reference values in pig model using CT renography. Hermoye et al. (8) determined RPF and GFR in rabbits from the cortical impulse response function by numerical deconvolution of renal cortical enhancement curves. The impulse response function exhibited three sequential peaks presumed to reflect the contrast in glomeruli, pro...

Purpose: To investigate whether the loss of corticomedullary differentiation (CMD) on T1-weighted MR images due to renal insufficiency can be attributed to changes in T1 values of the cortex, medulla, or both. Materials and Methods:Study subjects included 10 patients (serum creatinine range 0.6 -3.0 mg/dL) referred for suspected renovascular disease who underwent 99m Tc-diethylene triamine pentaacetic acid (DTPA) renography to determine single kidney glomerular filtration rate (SKGFR) and same-day MRI, which included T1 measurements and unenhanced T1-weighted gradient echo imaging. Corticomedullary differentiation on T1-weighted images was assessed qualitatively and quantitatively.Results: SKGFR values ranged from 3.5 to 89.4 mL/minute based on radionuclide studies. T1 relaxation times of the medulla exceeded those of renal cortex by 147.9 Ϯ 176.0 msec (mean Ϯ standard deviation [SD]). Regression analysis showed a negative correlation between cortex T1 and SKGFR (r ϭ -0.5; P ϭ 0.03), whereas there was no significant correlation between medullary T1 and SKGFR. The difference between medullary and cortical T1s correlated significantly with SKGFR (r ϭ 0.58; P Ͻ 0.01). In all five kidneys with a corticomedullary contrast-to-noise ratio (CNR) Ͻ5.0 on T1-weighted images, SKGFR was less than 20 mL/minute. Conclusion:In our subject population, loss of CMD with decreasing SKGFR can be attributed primarily to an increased T1 relaxation time of the cortex. Medullary T1 values vary but do not appear to correlate with degree of renal insufficiency. ON T1-WEIGHTED MR IMAGES of the normal healthy kidney, the cortex can be clearly differentiated from the medulla, a characteristic referred to as corticomedullary differentiation (CMD). CMD reflects the T1 differences between the cortex and medulla, where the cortex, due to its shorter T1 relaxation time, appears hyperintense with respect to the medulla. Loss of CMD has been observed in renal insufficiency, secondary to a variety of etiologies, including glomerulonephritis, acute tubular necrosis, end-stage chronic renal failure, obstructive hydronephrosis, Fabry's disease, and acute allograft rejection (1-8). While average T1 values in normal adult kidneys at 1.5 T have been reported to be 882 Ϯ 59 msec (mean Ϯ standard deviation [SD]) for cortex and 1163 Ϯ 118 msec for medulla (9), to our knowledge, the underlying changes in T1 at 1.5 T that result in loss of CMD in renal insufficiency have not been determined. Our purpose was to investigate whether the loss of CMD is attributable to changes in T1 values of the cortex, medulla, or both. MATERIALS AND METHODS PatientsThis study included a total of 10 patients (five female, five male) ranging from 38 to 90 years of age (69.8 Ϯ 16.9 years) who were referred for suspected renovascular disease. Patients had underlying diagnoses of chronic renal failure and hypertension (N ϭ 1) and hypertension alone (N ϭ 9) and were being evaluated for renal artery stenosis. Serum creatinine levels in five patients were less than 1.0 mg/dL (range ϭ 0...

The accuracy and precision of an automated graph-cuts (GC) segmentation technique for dynamic contrast-enhanced (DCE) 3D MR renography (MRR) was analyzed using 18 simulated and 22 clinical datasets. For clinical data, the error was 7.2 ؎ 6.1 cm 3 for the cortex and 6.5 ؎ 4.6 cm 3 for the medulla. The precision of segmentation was 7.1 ؎ 4.2 cm 3 for the cortex and 7.2 ؎ 2.4 cm 3 for the medulla. Compartmental modeling of kidney function in 22 kidneys yielded a renal plasma flow (RPF) error of 7.5% ؎ 4.5% and single-kidney GFR error of 13.5% ؎ 8.8%. The precision was 9.7% ؎ 6.4% for RPF and 14.8% ؎ 11.9% for GFR. It took 21 min to segment one kidney using GC, compared to 2.5 hr for manual segmentation. One technique to determine renal function consists of the intravenous injection of radioactive tracer followed by assessment of its plasma clearance 2-4 hr later. This technique is time-consuming, requires multiple blood samples, and measures only the global glomerular filtration rate (GFR)-a disadvantage when asymmetric or unilateral renal disease is present. The gold standard technique for assessing single-kidney GFR is inulin clearance, but this method is too invasive and complex for routine clinical application. As an alternative, dynamic gamma camera imaging with 99m Tc-DTPA has been shown to provide single-kidney GFR by analysis of the renal radioactivity. By combining measures of renal physiology with depiction of anatomical detail, dynamic contrast-enhanced (DCE) 3D MR renography (MRR) has the potential to improve upon nuclear medicine techniques and also provide useful functional information to supplement anatomic renal MRI examinations (1). Good spatial, temporal, and contrast resolution is achievable with current contrast-enhanced dynamic protocols, whereby serial 3D MR images of the kidneys are generated following an injection of contrast material. Gadolinium (Gd) chelates, such as gadopentetate dimeglumine (Gd-DTPA), are suitable MR contrast agents because they are freely filtered at the glomerulus without tubular secretion or resorption.Several approaches have been proposed to analyze renography data, including the upslope method (2), deconvolution (3,4), the Rutland-Patlak method (5,6), and renal kinetic modeling (7-10). The key prerequisite is the ability to segment dynamic MR images into functional regions (i.e., the cortical and medullary compartments). Fitting concentration-time activity (CTA) curves to kinetic models yields perfusion and filtration rates per unit volume of tissue. These kinetic rates multiplied by the cortical and medullary volumes (determined from segmented images) give the renal plasma flow (RPF) and GFR for the entire kidney.Accurate segmentation of contrast-enhanced MRR data remains a difficult task. Dynamic 3D MR images of the abdomen suffer from partial-volume and respiratory-motion artifacts and have a relatively low signal-to-noise ratio (SNR). Other sources of error include signal nonuniformity and wraparound artifacts. The presence of cysts and renal masses, and reduced...

PACS. 75.45+j -Macroscopic quantum phenomena in magnetic systems. PACS. 75.60 Ej -Magnetisation curves, hysteresis, Barkhausen and related effects. PACS. 75.50 Tt -Fine-particle systems.Abstract. -Precise magnetic hysteresis measurements of small single crystals of Mn12 acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (> 1.6 K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.Introduction. -The high spin (S = 10) molecular magnets Mn 12 acetate and Fe 8 have become prototypes for the study of the transition from classical superparamagnetism to quantum tunnelling of mesoscopic spins. Much of the recent interest in these materials has been stimulated by the observation of a remarkably regular series of steps and plateaus in the magnetic hysteresis loops of Mn 12 at low temperature (below a blocking temperature of 3 K), first in oriented powders [1] and shortly thereafter in single crystals [2]. These results indicate that the relaxation rate of the magnetization toward equilibrium is greatly enhanced at well-defined intervals of magnetic field. These observations have been interpreted within a simple effective spin Hamiltonian for these molecules and a model of thermally assisted tunnelling of the magnetization, first suggested in reference [3]. This model describes a regime intermediate between thermal activation over the anisotropy barrier (superparamagnetism)