Comparison of Gd-DTPA and Gd-BOPTA for studying renal perfusion and filtration

Purpose:

To measure the systematic error in perfusion and filtration parameters derived from magnetic resonance (MR) renography caused by protein binding of MR contrast agents.

Materials and Methods:

Eight healthy Danish Landrace pigs were examined with dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI). In four pigs a bolus of gadopentetate‐dimeglumine (Gd‐DTPA; no protein binding) was injected, followed by gadobenate‐dimeglumine (Gd‐BOPTA; 10% protein binding). The order was reversed in the other four pigs. A two‐compartment filtration model was generalized to allow for protein binding and fitted to whole‐cortex region of interest (ROI) curves. Single‐kidney plasma flow and volume, tubular flow (or GFR), and tubular transit time of both agents were compared.

Results:

The data show a strong systematic underestimation (P < 0.001) in GFR by Gd‐BOPTA (33 ± 7.2%), and no significant differences (P > 0.05) in plasma flow (2.2 ± 18%), plasma volume (?1.7 ± 7.8%) and tubular transit time (3.1 ± 7.2%). The order of injection had no significant effect.

Conclusion:

Theory and experiments agree that perfusion parameters of both agents are comparable, whereas GFR is underestimated with Gd‐BOPTA due to the dependence of relaxivity on protein content. Hence, GFR cannot be measured with protein‐bound contrast agents, but the proposed dual‐agent protocol may produce new functional indices measuring protein filtration. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.

     

In vivo MRI and histopathological assessment of tumor microenvironment in luminal-like and basal-like breast cancer xenografts

Purpose:

To explore tumor pathophysiology with special attention to the microenvironment in two molecular subtypes of human breast cancer using in vivo magnetic resonance imaging (MRI) and histopathology. The impact of tumor growth, size, and the influence of estradiol were also investigated.

Materials and Methods:

Two orthotopic and directly transplanted human breast cancer models representing luminal‐like and basal‐like molecular subtypes were characterized by dynamic contrast‐enhanced MRI and diffusion‐weighted MRI. Ex vivo measurements of vascularization, hypoxia, mitoses, and the level of VEGF activations were associated with the calculated in vivo MRI parameters of the tumors.

Results:

The vascular permeability and perfusion (K^trans) was significantly higher in basal‐like compared to luminal‐like tumors. These findings were confirmed by a 4‐fold higher proliferating microvessel density (pMVD) in basal‐like tumors, reflecting the difference in aggressiveness between the subtypes. No effect of tumor growth was observed during 6 days of growth in any of the models; however, large tumors had lower K^trans, higher extracellular extravascular volume fraction (v_e), and more hypoxia than medium‐sized tumors. Estradiol withdrawal induced increased K^trans, v_e, and tumor water diffusion (ADC) in luminal‐like tumors, corresponding to increased VEGFR2 activation, which is likely to cause increased tumor vessel permeability.

Conclusion:

These novel data confirm the potential of functional MRI methods to map histopathologically proven changes in breast tumor vasculature and microenvironment in vivo. J. Magn. Reson. Imaging 2012;35:1098‐1107. © 2011 Wiley Periodicals, Inc.     

Concurrent MR blood volume and vessel size estimation in tumors by robust and simultaneous ΔR2 and ΔR2* quantification

This work presents a novel method for concurrent estimation of the fractional blood volume and the mean vessel size of tumors based on a multi‐gradient‐echo‐multi‐spin‐echo sequence and the injection of a super‐paramagnetic blood‐pool agent. The approach further comprises a post‐processing technique for simultaneous estimation of changes in the transverse relaxation rates R₂ and R, which is robust against global B₀ and B₁ field inhomogeneities and slice imperfections. The accuracy of the simultaneous ΔR₂ and ΔR quantification approach is evaluated in a phantom. The simultaneous blood volume and vessel size estimates, obtained with MR, compare well to the immunohistological findings in a preclinical experiment (HT1080 cells, implanted in nude mice). Clinical translation is achieved in a patient with a pleomorphic sarcoma in the left pubic bone. The latter demonstrates the robustness of the technique against changes in the contrast agent concentration in blood during washout. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Exercise experience influences affective and motivational outcomes of prescribed and self‐selected intensity exercise

The purpose of the study was to compare affective and motivational responses to exercise performed at self‐selected and prescribed intensity [close to ventilatory threshold (VT)] between physically active and sedentary women. Following a graded exercise test, the women completed two 30 min bouts of treadmill exercise (on separate days, order counterbalanced). Intensity was prescribed in one session and self‐selected in the other. Exercise intensity, exercise‐efficacy, perceived competence, autonomy and affective responses were assessed. Results showed that the active women self‐selected to exercise at a significantly higher %HR_peak than their sedentary counterparts but, importantly, both groups exercised close to their VT. The order of conditions influenced affective and motivational responses. The active women experienced more positive affect during exercise and greater competence than sedentary women when the self‐selected condition was completed first. Autonomy was higher for the self‐selected condition. Self‐efficacy and competence were higher in the active women. Differences in self‐efficacy perceptions before the exercise depended on which condition was completed first. In conclusion, sedentary women felt relatively positive in the self‐selected condition but would benefit from familiarization and experience with exercise to enhance their self‐efficacy and competence.     

MRI at 7 tesla and above: Demonstrated and potential capabilities

With more than 40 installed MR systems worldwide operating at 7 Tesla or higher, ultra‐high‐field (UHF) imaging has been established as a platform for clinically oriented research in recent years. Along with technical developments that, in part, have also been successfully transferred to lower field strengths, MR imaging and spectroscopy at UHF have demonstrated capabilities and potentials for clinical diagnostics in a variety of studies. In terms of applications, this overview article focuses on already achieved advantages for in vivo imaging, i.e., in imaging the brain and joints of the musculoskeletal system, but also considers developments in body imaging, which is particularly challenging. Furthermore, new applications for clinical diagnostics such as X‐nuclei imaging and spectroscopy, which only really become feasible at ultra‐high magnetic fields, will be presented. J. Magn. Reson. Imaging 2015;41:13–33. © 2014 Wiley Periodicals, Inc .     

Loss of fine structure and edge sharpness in fast-spin-echo carotid wall imaging: measurements and comparison with multiple-spin-echo in normal and atherosclerotic subjects

Purpose:

To test whether the k‐space acquisition strategy used by fast‐spin‐echo (FSE) is a major source of blurring in carotid wall and plaque imaging, and investigate an alternative acquisition approach.

Materials and Methods:

The effect of echo train length (ETL) and T₂ on the amount of blurring was studied in FSE simulations of vessel images. Edge sharpness was measured in black‐blood T₁ and proton‐density weighted (T₁W and PDW) carotid images acquired from 5 normal volunteers and 19 asymptomatic patients using both FSE and multiple‐spin‐echo (Multi‐SE) sequences at 3 Tesla. Plaque images were classified and divided in group α (tissues' average T₂ ～40–70 ms) and group β (plaque components with shorter T₂).

Results:

Simulations predicted 26.9% reduction of vessel edge sharpness from Multi‐SE to FSE images (ETL = 9, T₂ = 60 ms). This agreed with in vivo measurements in normal volunteers (27.4%) and in patient group α (26.2%), while in group β the loss was higher (31.6%).

Conclusion:

FSE significantly reduced vessel edge sharpness along the phase‐encoding direction in T₁W and PDW images. Blurring was stronger in the presence of plaque components with short T₂ times. This study shows a limitation of FSE and the potential of Multi‐SE to improve the quality of carotid imaging. J. Magn. Reson. Imaging 2011;33:1136–1143. © 2011 Wiley‐Liss, Inc.     

Free-breathing renal magnetic resonance angiography with steady-state free-precession and slab-selective spin inversion combined with radial k-space sampling and water-selective excitation.

The impact of radial k-space sampling and water-selective excitation on a novel navigator-gated cardiac-triggered slab-selective inversion prepared 3D steady-state free-precession (SSFP) renal MR angiography (MRA) sequence was investigated. Renal MRA was performed on a 1.5-T MR system using three inversion prepared SSFP approaches: Cartesian (TR/TE: 5.7/2.8 ms, FA: 85 degrees), radial (TR/TE: 5.5/2.7 ms, FA: 85 degrees) SSFP, and radial SSFP combined with water-selective excitation (TR/TE: 9.9/4.9 ms, FA: 85 degrees). Radial data acquisition lead to significantly reduced motion artifacts (P < 0.05). SNR and CNR were best using Cartesian SSFP (P < 0.05). Vessel sharpness and vessel length were comparable in all sequences. The addition of a water-selective excitation could not improve image quality. In conclusion, radial k-space sampling reduces motion artifacts significantly in slab-selective inversion prepared renal MRA, while SNR and CNR are decreased. The addition of water-selective excitation could not improve the lower CNR in radial scanning.     

Simultaneous imaging and R2* mapping using a radial multi-gradient-echo (rMGE) sequence

PURPOSE: To demonstrate a rapid MR technique that combines imaging and R2* mapping based on a single radial multi-gradient-echo (rMGE) data set. The technique provides a fast method for online monitoring of the administration of (super-)paramagnetic contrast agents as well as image-guided drug delivery. MATERIALS AND METHODS: Data are acquired using an rMGE sequence, resulting in interleaved undersampled radial k-spaces representing different echo times (TEs). These data sets are reconstructed separately, yielding a series of images with different TEs used for pixelwise R2* mapping. A fast numerical algorithm implemented on a real-time reconstruction platform provides online estimation of the relaxation rate R2*. Simultaneously the images are summed for the computation of a high-resolution image. RESULTS: Convenient high-resolution R2* maps of phantoms and the liver of a healthy volunteer were obtained. In addition to stable intrinsic baseline maps, the proposed technique provides particularly accurate results for the high relaxation rates observed during the presence of (super-)paramagnetic contrast agents. Assuming that the change in R2* is proportional to the concentration of the agent, the technique offers a rough estimate for dynamic dosage. CONCLUSION: The simultaneous online display of morphological and parametric information permits convenient, quantitative surveillance of contrast-agent administration.     

Combined spin- and gradient-echo perfusion-weighted imaging

In this study, a spin- and gradient-echo echo-planar imaging (SAGE EPI) MRI pulse sequence is presented that allows simultaneous measurements of gradient-echo and spin-echo dynamic susceptibility-contrast perfusion-weighted imaging data. Following signal excitation, five readout trains were acquired using spin- and gradient-echo echo-planar imaging, all of them with echo times of less than 100 ms. Contrast agent concentrations in brain tissue were determined based on absolute R2* and R(2) estimates rather than relative changes in the signals of individual echo trains, producing T(1)-independent dynamic susceptibility-contrast perfusion-weighted imaging data. Moreover, this acquisition technique enabled vessel size imaging through the simultaneous quantification of R2* and R(2), without an increase in acquisition time. In this work, the concepts of SAGE EPI pulse sequence and results in stroke and tumor imaging are presented. Overall, SAGE EPI combined the advantages of higher sensitivity to contrast agent passage of gradient-echo perfusion-weighted imaging with better microvascular selectivity of spin-echo perfusion-weighted imaging.     

Quantitative measurement of N‐acetyl‐aspartyl‐glutamate at 3 T using TE‐averaged PRESS spectroscopy and regularized lineshape deconvolution

This article introduces regularized lineshape deconvolution in conjunction with TE‐averaged PRESS spectroscopy to measure N‐acetyl‐aspartyl‐glutamate (NAAG). Averaging different echo times suppressed the signals of multiplets from strongly coupled spin systems near 2 ppm; thus, minimizing the interfering signals to detect the acetyl proton signal of NAAG. Signal distortion was corrected by lineshape deconvolution, and Tikhonov regularization was introduced to reduce noise amplification arising from deconvolution; as a result, spectral resolution was enhanced without significantly sacrificing signal‐to‐noise ratio (SNR). This new approach was used to measure NAAG in the two regions of interest of healthy volunteers, dominated by gray matter and white matter, respectively. The acetyl proton signal of NAAG was directly quantified by fitting the deconvoluted spectra to a Voigt‐lineshape spectral model function, yielding the NAAG–N‐acetyl‐aspartate (NAA) ratios of 0.11 ± 0.02 for the gray matter voxels (n = 8) and 0.18 ± 0.02 for the white matter voxels (n = 12). Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Diagnosis of colorectal hepatic metastases: comparison of contrast-enhanced CT, contrast-enhanced US, superparamagnetic iron oxide-enhanced MRI, and gadoxetic acid-enhanced MRI

Purpose:

To compare the diagnostic accuracy of contrast‐enhanced computed tomography (CE‐CT), contrast‐enhanced ultrasonography (CE‐US), superparamagnetic iron oxide‐enhanced magnetic resonance imaging (SPIO‐MRI), and gadoxetic acid‐enhanced MRI (Gd‐EOB‐MRI) in the evaluation of colorectal hepatic metastases.

Materials and Methods:

In all, 111 patients with colorectal cancers were enrolled in this study. Of the 112 metastases identified in 46 patients, 31 in 18 patients were confirmed histologically and the remaining 81 in 28 patients were confirmed by follow‐up imaging. CE‐CT, CE‐US, SPIO‐MRI, and Gd‐EOB‐MRI were evaluated. Mean (of three readers, except for CE‐US) area under the receiver operating characteristic curve (A_z), sensitivities, and positive predictive values (PPV) were calculated. Each value was compared to the others by variance z‐test or chi‐square test with Bonferroni correction.

Results:

For all lesions, mean A_z and sensitivity of Gd‐EOB‐MRI (0.992, 95% [56/59]) were significantly greater than those of CE‐CT (0.847, 63% [71/112]) and CE‐US (0.844, 73% [77/106]). For lesions ≤1 cm, mean A_z and sensitivity of Gd‐EOB‐MRI (0.999, 92% [22/24]) were significantly greater than those of CE‐CT (0.685, 26% [13/50]) and CE‐US (0.7, 41% [18/44]). Mean A_z (95% CI) of SPIO‐MRI for all lesions (0.966 [0.929–0.987]) and lesions ≤ 1 cm (0.961 [0.911–0.988]) were significantly greater than those of CE‐CT and CE‐US. Mean sensitivity of SPIO‐MRI for lesions ≤1 cm (63%, 26/41) was significantly greater than that of CE‐CT.

Conclusion:

Gd‐EOB‐MRI and SPIO‐MRI were more accurate than CE‐CT and CE‐US for evaluation of liver metastasis in patients with colorectal carcinoma. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.