Magnetic Resonance Imaging - Articles in Press

Automatic white matter lesion segmentation using an adaptive outlier detection method - Corrected Proof

Kok Haur Ong, Dhanesh Ramachandram, Rajeswari Mandava, Ibrahim Lutfi Shuaib — 2012-05-14

Abstract: White matter (WM) lesions are diffuse WM abnormalities that appear as hyperintense (bright) regions in cranial magnetic resonance imaging (MRI). WM lesions are often observed in older populations and are important indicators of stroke, multiple sclerosis, dementia and other brain-related disorders. In this paper, a new automated method for WM lesions segmentation is presented. In the proposed method, the presence of WM lesions is detected as outliers in the intensity distribution of the fluid-attenuated inversion recovery (FLAIR) MR images using an adaptive outlier detection approach. Outliers are detected using a novel adaptive trimmed mean algorithm and box–whisker plot. In addition, pre- and postprocessing steps are implemented to reduce false positives attributed to MRI artifacts commonly observed in FLAIR sequences. The approach is validated using the cranial MRI sequences of 38 subjects. A significant correlation (R=0.9641, P value=3.12×10−3) is observed between the automated approach and manual segmentation by radiologist. The accuracy of the proposed approach was further validated by comparing the lesion volumes computed using the automated approach and lesions manually segmented by an expert radiologist. Finally, the proposed approach is compared against leading lesion segmentation algorithms using a benchmark dataset.

Compressed sensing MR image reconstruction using a motion-compensated reference - Corrected Proof

Huiqian Du, Fan Lam — 2012-05-14

Abstract: Compressed sensing (CS)-based methods have been proposed for image reconstruction from undersampled magnetic resonance data. Recently, CS-based schemes using reference images have also been proposed to further reduce the sampling requirement. In this study, we propose a new reference-constrained CS reconstruction method that accounts for the misalignment between the reference and the target image to be reconstructed. The proposed method uses a new image model that represents the target image as a linear combination of a motion-dependent reference image and a sparse difference image. We then use an efficient iterative algorithm to jointly estimate the motion parameters and the difference image from sparsely sampled data. Simulation results from a numerical phantom data set and an in vivo data set show that the proposed method can accurately compensate the motion effects between the reference and the target images and improve reconstruction quality. The proposed method should prove useful for several applications such as interventional imaging, longitudinal imaging studies and dynamic contrast-enhanced imaging.

Detecting cortical lesions in multiple sclerosis at 7 T using white matter signal attenuation - Corrected Proof

2012-05-14

Abstract: Cortical lesions have recently been a focus of multiple sclerosis (MS) MR research. In this study, we present a white matter signal attenuating sequence optimized for cortical lesion detection at 7 T. The feasibility of white matter attenuation (WHAT) for cortical lesion detection was determined by scanning eight patients (four relapsing/remitting MS, four secondary progressive MS) at 7 T. WHAT showed excellent gray matter–white matter contrast, and cortical lesions were hyperintense to the surrounding cortical gray matter, The sequence was then optimized for cortical lesion detection by determining the set of sequence parameters that produced the best gray matter–cortical lesion contrast in a 10-min scan. Despite the B1 inhomogeneities common at ultra-high field strengths, WHAT with an adiabatic inversion pulse showed good cortical lesion detection and would be a valuable component of clinical MS imaging protocols.

Paradoxical uptake of Gd-EOB-DTPA on the hepatobiliary phase in the evaluation of hepatic metastasis from breast cancer: is the “target sign” a common finding? - Corrected Proof

2012-05-14

Abstract: Purpose: The purpose was to describe magnetic resonance imaging (MRI) findings of breast cancer liver metastasis using gadoxetic acid (Gd-EOB-DTPA) with an emphasis on the added value of the hepatobiliary phase (HBP).Material and methods: Nine patients with 13 liver metastases were included in the study after the medical records of 29 breast cancer patients who underwent Gd-EOB-DTPA-enhanced MRI between February 2008 and June 2010 were reviewed. The diagnoses of liver metastasis were established by percutaneous liver biopsy or surgery and on the basis of image findings. Two radiologists retrospectively evaluated signal intensity (SI) and sizes of metastases and patterns of enhancement in an HBP. The SI ratio was calculated as the SI of the central hyperintense portion in “target” lesions divided by the SI of nearby normal liver parenchyma on the HBP. We also measured apparent diffusion coefficient (ADC) values from Diffusion Weighted Image (DWI).Results: Liver metastases were all hypointense [n=13/13 (100%)] on T1-weighted imaging (WI), and many lesions had a “target” appearance with a central high SI and a peripheral low SI rim (47%) on T2WI. Dynamic study showed rim enhancement on the arterial phase (85%) and a “target” appearance, consisting of a central enhancing portion with peripheral washout or hypointense rim, on the HBP (62%). The mean SI ratio was 0.7. The mean ADC value of “target” appearing metastases was 1.25 (×10−3 mm2/s; range 1.3–1.6) compared with a mean value of 0.8 (×10−3 mm2/s; range 0.8–1.4) in homogeneous defect on the HBP. There was statistically significant difference (P<.05).Conclusion: Breast cancer liver metastases commonly demonstrated as a peripheral ring enhancement on arterial dominant phase and a target sign with a central round enhancing portion and a peripheral hypointense rim on the HBP.

Ultrasmall superparamagnetic iron oxides enhanced MR imaging in rats with experimentally induced endometriosis - Corrected Proof

Hyun Jung Lee, Hui Joong Lee, Jong Min Lee, Yongmin Chang, Seung Tae Woo — 2012-05-04

Abstract: Purpose: The purpose of our study was to evaluate the feasibility of magnetic resonance imaging (MRI) using ultrasmall superparamagnetic iron oxides (USPIO) in the detection of experimentally induced endometriosis.Materials and methods: Endometriosis was surgically induced in rats by transplanting an autologous fragment of uterine tissue onto the inner surface of the abdominal wall, the posterior surface of the uterine body and the arterial cascades of the small intestines adjacent to mesenteric blood vessels. Six weeks later, MRI using Gd-DTPA and USPIO was performed for the evaluation of the ectopic uterine tissue (EUT). A scoring system was developed for image interpretation (0=absence, 1=probably absence, 2=probably presence and 3=presence). We defined MR index (MRIx) as the sum of T1-weighted and enhanced T1-weighted and T2-weighted image scores, and USPIO MRIx (MRIx+USPIO) as the MRIx score plus the score of USPIO-enhanced T2-weighted image.Results: The MRIx+USPIO was also higher in the successfully autotransplanted group than in the failed group (6.19±1.72 versus 3.94±1.20, P<.001). There was also a significant linear relationship between MRIx+USPIO and pathologic status (R2=0.494, P<.001). Thirty-one (64.6%) of the 48 implanted uterine tissues were histologically confirmed on pathologic review. The area of MRIx and MRIx+USPIO in the detection of EUT more than 3 mm in size was 0.739 and 0.913, respectively.Conclusion: Our results suggest that USPIO-enhanced MRI could be a novel diagnostic tool for diagnosis in experimentally induced peritoneal endometriosis.

Imaging biomarkers to monitor response to the hypoxia-activated prodrug TH-302 in the MiaPaCa2 flank xenograft model - Corrected Proof

2012-05-03

Abstract: TH-302, a hypoxia-activated anticancer prodrug, was evaluated for antitumor activity and changes in dynamic contrast-enhanced (DCE) and diffusion-weighted (DW) magnetic resonance imaging (MRI) in a mouse model of pancreatic cancer. TH-302 monotherapy resulted in a significant delay in tumor growth compared to vehicle-treated controls. TH-302 treatment was also associated with a significant decrease in the volume transfer constant (Ktrans) compared to vehicle-treated controls 1 day following the first dose measured using DCE-MRI. This early decrease in Ktrans following the first dose as measured is consistent with selective killing of the hypoxic fraction of cells which are associated with enhanced expression of hypoxia inducible transcription factor-1 alpha that regulates expression of permeability and perfusion factors including vascular endothelial growth factor-A. No changes were observed in DW-MRI following treatment with TH-302, which may indicate that this technique is not sensitive enough to detect changes in small hypoxic fractions of the tumor targeted by TH-302. These results suggest that changes in tumor permeability and/or perfusion may be an early imaging biomarker for response to TH-302 therapy.

Combined renal MRA and perfusion with a single dose of contrast - Corrected Proof

Parmede Vakil, James C. Carr, Timothy J. Carroll — 2012-04-23

Abstract: Both anatomical and functional scans are often performed when diagnosing renovascular diseases, which in many cases require two separate contrast injections. With nephrogenic systemic fibrosis being associated with gadolinium, minimizing contrast injection dosage is desirable. In this study, a technique which performs time-resolved renal magnetic resonance angiography (MRA) and perfusion with a single scan and single dose of contrast has been evaluated in six healthy volunteers. A previously developed three-dimensional MRA technique called Contrast-enhanced Angiography with Multi-Echo and Radial k-space (CAMERA) has been used to acquire images, and perfusion analysis was performed using deconvolution methods. Time-resolved MRA, as well as renal blood flow, renal volume of distribution and mean transit time maps, were acquired.

Gadofosveset-enhanced magnetic resonance angiography as a means of evaluating pulmonary arteriovenous malformation: a case report - Corrected Proof

Josephine Pressacco, Konstantin Papas — 2012-04-23

Abstract: This case report is a unique presentation of a new potential indication for Gadofosvest (Ablavar), a blood pool contrast agent for magnetic resonance angiography (MRA). Ablavar is an excellent MRA contrast agent because it provides optimal contrast opacification of both the arterial and venous system, unlike the conventional extracellular agents that are used for arterial imaging only. The present case report demonstrates the ability of Ablavar to demonstrate pulmonary arteriovenous malformation (AVM), showing both its arterial feeders as well as its venous drainage tract.

An approach for computer-aided detection of brain metastases in post-Gd T1-W MRI - Corrected Proof

Reza Farjam, Hemant A. Parmar, Douglas C. Noll, Christina I. Tsien, Yue Cao — 2012-04-23

Abstract: Purpose: To develop an approach for computer-aided detection (CAD) of small brain metastases in post-Gd T1-weighted magnetic resonance imaging (MRI).Method: A set of unevenly spaced 3D spherical shell templates was optimized to localize brain metastatic lesions by cross-correlation analysis with MRI. Theoretical and simulation analyses of effects of lesion size and shape heterogeneity were performed to optimize the number and size of the templates and the cross-correlation thresholds. Also, effects of image factors of noise and intensity variation on the performance of the CAD system were investigated. A nodule enhancement strategy to improve sensitivity of the system and a set of criteria based upon the size, shape and brightness of lesions were used to reduce false positives. An optimal set of parameters from the FROC curves was selected from a training dataset, and then the system was evaluated on a testing dataset including 186 lesions from 2753 MRI slices. Reading results from two radiologists are also included.Results: Overall, a 93.5% sensitivity with 0.024 of intra-cranial false positive rate (IC-FPR) was achieved in the testing dataset. Our investigation indicated that nodule enhancement was very effective in improving both sensitivity and specificity. The size and shape criteria reduced the IC-FPR from 0.075 to 0.021, and the brightness criterion decreases the extra-cranial FPR from 0.477 to 0.083 in the training dataset. Readings from the two radiologists had sensitivities of 60% and 67% in the training dataset and 70% and 80% in the testing dataset for the metastatic lesions <5 mm in diameter.Conclusion: Our proposed CAD system has high sensitivity and fairly low FPR for detection of the small brain metastatic lesions in MRI compared to the previous work and readings of neuroradiologists. The potential of this method for assisting clinical decision- making warrants further evaluation and improvements.

Noncontrast dynamic MRA in intracranial arteriovenous malformation (AVM): comparison with time of flight (TOF) and digital subtraction angiography (DSA) - Corrected Proof

2012-04-23

Abstract: Digital subtraction angiography (DSA) remains the gold standard to diagnose intracranial arteriovenous malformations (AVMs) but is invasive. Existing magnetic resonance angiography (MRA) is suboptimal for assessing the hemodynamics of AVMs. The objective of this study was to evaluate the clinical utility of a novel noncontrast four-dimensional (4D) dynamic MRA (dMRA) in the evaluation of intracranial AVMs through comparison with DSA and time-of-flight (TOF) MRA. Nineteen patients (12 women, mean age 26.2±10.7 years) with intracranial AVMs were examined with 4D dMRA, TOF and DSA. Spetzler–Martin grading scale was evaluated using each of the above three methods independently by two raters. Diagnostic confidence scores for three components of AVMs (feeding artery, nidus and draining vein) were also rated. Kendall's coefficient of concordance was calculated to evaluate the reliability between two raters within each modality (dMRA, TOF, TOF plus dMRA). The Wilcoxon signed-rank test was applied to compare the diagnostic confidence scores between each pair of the three modalities. dMRA was able to detect 16 out of 19 AVMs, and the ratings of AVM size and location matched those of DSA. The diagnostic confidence scores by dMRA were adequate for nidus (3.5/5), moderate for feeding arteries (2.5/5) and poor for draining veins (1.5/5). The hemodynamic information provided by dMRA improved diagnostic confidence scores by TOF MRA. As a completely noninvasive method, 4D dMRA offers hemodynamic information with a temporal resolution of 50–100 ms for the evaluation of AVMs and can complement existing methods such as DSA and TOF MRA.

The effect of temporal sampling on quantitative pharmacokinetic and three-time-point analysis of breast DCE-MRI - Corrected Proof

Jacob U. Fluckiger, Matthias C. Schabel, Edward V.R. DiBella — 2012-04-18

Abstract: The effects of temporal sampling on the previously published three-time-point (3TP) method are compared with those of a Tofts–Kety model using an arterial input function from the alternating minimization with model (AMM) method. Computer simulations are done to estimate the expected error in both the 3TP and Tofts–Kety models as a function of the temporal sampling rate of the data. The error in the 3TP model parameters remained essentially constant with respect to temporal sampling. The Tofts–Kety model showed a linear increase in parameter error with respect to temporal sampling. Both analysis methods were also applied to 87 clinically acquired breast scans. These scans were downsampled in time by a factor of 2 and 4, and the methods were reapplied. The spatial resolution was held constant throughout this study. At temporal resolutions less than 19.4 s, the Tofts–Kety model outperformed the 3TP model using receiver operating characteristic curve analysis (area under the ROC curve [AUC] of 0.94 compared to 0.91). As the temporal sampling rate decreased, the 3TP model outperformed the Tofts–Kety model (AUC of 0.89 versus 0.85). When the temporal sampling rate of the data was less than 20 s, the Tofts–Kety model with the AMM method had lower parameter error than the 3TP method.

Enhanced choline metabolism in a rodent rhabdomyosarcoma model: correlation between RT-PCR and translational 3 T H-MRS - Corrected Proof

2012-04-18

Abstract: Purpose: To investigate which transmembrane choline transporters and intracellular choline kinases play a prominent role at gene expression level in the rise of the total choline (tCho) peak at proton MR spectra in a rodent rhabdomyosarcoma model.Materials and Methods: Twenty-two rats bearing grafted bilateral syngenic rhabdomyosarcoma were examined on a clinical 3 T MR system. Total choline concentration was measured from proton MR spectra using cubic centimeter volumes of interest (VOIs) located contiguously along the greater axis of the tumour. After euthanasia, cubic centimetre tissue specimens corresponding to Proton magnetic resonance spectroscopy (H-MRS) VOIs were frozen in liquid nitrogen. Out of 89 H-MRS voxels, only 39 with a corresponding tissue specimen suitable for biochemical processing were included in the analysis. RNA was extracted from all the 39 samples and reverse-transcribed into cDNA. Choline kinase α and β gene expression as well as genes of the transmembrane transporters OCT1, OCT2, OCT3, CTL1, CTL3, CTL4 and CHT1 were studied using reverse transcriptase polymerase chain reaction. The expression level of each gene (ΔCt), was normalized referred to that of the RPL19 gene. The Spearman rank correlation coefficient was used to analyse variables.Results: There was no overexpression of genes coding for kinases; however, significant correlation was observed between kinase α sub-type and the tCho peak (P=.002; r=0.51). OCT1 was the most overexpressed transporter gene. Less overexpressed CTL1 gene was significantly correlated with the tCho peak (P=.02; r=0.38).Conclusion: Choline transporters seem to play a predominant role in the increase in total choline concentration at the gene expression level in our model.

Detection of rectal cancer and response to concurrent chemoradiotherapy by proton magnetic resonance spectroscopy - Corrected Proof

2012-04-16

Abstract: Introduction: To diagnose rectal cancer and monitor treatment response after preoperative concurrent chemoradiotherapy (CCRT) in rectal cancer patients using proton-1 magnetic resonance spectroscopy (1H-MRS).Materials and Methods: We enrolled 134 rectal cancer patients before treatment, of whom 34 underwent preoperative CCRT and follow-up MR spectroscopy before surgery. 1H-MRS was performed using a six-channel phased-array coil at 3.0 T. We evaluated the presence of a choline peak at 3.2 ppm, and lipid peaks at 0.9 and 1.3 ppm, and glutamine and glutamate peaks at 2.1-2.3 and 2.7 ppm seen at two TEs (40 and 135 ms). We divided MR spectra patterns into two groups (A and B).Results: A choline peak at 3.2 ppm seen in both TEs was characteristic for rectal cancer before treatment. Of 103 patients, 55 (53%) showed an elevated choline peak before treatment (type A). Type A spectra were seen in 68% of patients (23/34) before preoperative CCRT. After CCRT, the choline peak disappeared, resulting in only the lipid peak at 1.3 ppm (type B) in 97% of patients (33/34).Discussion: We optimized a localized in vivo 1H-MRS method for detection of rectal adenocarcinoma and monitoring treatment response after preoperative CCRT. The method appears to be a promising and feasible noninvasive modality.

Signal displacement in spiral-in acquisitions: simulations and implications for imaging in SFG regions - Corrected Proof

2012-04-16

Abstract: Susceptibility field gradients (SFGs) cause problems for functional magnetic resonance imaging (fMRI) in regions like the orbital frontal lobes, leading to signal loss and image artifacts (signal displacement and “pile-up”). Pulse sequences with spiral-in k-space trajectories are often used when acquiring fMRI in SFG regions such as inferior/medial temporal cortex because it is believed that they have improved signal recovery and decreased signal displacement properties.Previously postulated theories explain differing reasons why spiral-in appears to perform better than spiral-out; however it is clear that multiple mechanisms are occurring in parallel. This study explores differences in spiral-in and spiral-out images using human and phantom empirical data, as well as simulations consistent with the phantom model.Using image simulations, the displacement of signal was characterized using point spread functions (PSFs) and target maps, the latter of which are conceptually inverse PSFs describing which spatial locations contribute signal to a particular voxel. The magnitude of both PSFs and target maps was found to be identical for spiral-out and spiral-in acquisitions, with signal in target maps being displaced from distant regions in both cases. However, differences in the phase of the signal displacement patterns that consequently lead to changes in the intervoxel phase coherence were found to be a significant mechanism explaining differences between the spiral sequences.The results demonstrate that spiral-in trajectories do preserve more total signal in SFG regions than spiral-out; however, spiral-in does not in fact exhibit decreased signal displacement. Given that this signal can be displaced by significant distances, its recovery may not be preferable for all fMRI applications.

Ex vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues - Corrected Proof

2012-04-16

Abstract: Purpose: Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T1, T2 and T2⁎ relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents.Methods: Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T1, T2 and T2⁎ relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents.Results: T2 and T2⁎ relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T2: lipid/necrotic core was lower 47±13.7 ms than connective tissue (67±22.5 ms) and fibrous cap (60±13.2 ms); T2⁎: fibrous cap was higher (48±15.5ms) than connective tissue (19±10.6 ms) and lipid/necrotic core (24±8.2 ms)]. T1 relaxation times were not significantly different (P=.287) [T1: Fibrous cap: 933±271.9 ms; connective tissue (1002±272.9 ms) and lipid/necrotic core (1044±304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing.Conclusions: This study demonstrates that there is a significant difference between qT2 and qT2⁎ in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents.

Undersampled MRI reconstruction with patch-based directional wavelets - Corrected Proof

Xiaobo Qu, Di Guo, Bende Ning, Yingkun Hou, Yulan Lin, Shuhui Cai, Zhong Chen — 2012-04-16

Abstract: Compressed sensing has shown great potential in reducing data acquisition time in magnetic resonance imaging (MRI). In traditional compressed sensing MRI methods, an image is reconstructed by enforcing its sparse representation with respect to a preconstructed basis or dictionary. In this paper, patch-based directional wavelets are proposed to reconstruct images from undersampled k-space data. A parameter of patch-based directional wavelets, indicating the geometric direction of each patch, is trained from the reconstructed image using conventional compressed sensing MRI methods and incorporated into the sparsifying transform to provide the sparse representation for the image to be reconstructed. A reconstruction formulation is proposed and solved via an efficient alternating direction algorithm. Simulation results on phantom and in vivo data indicate that the proposed method outperforms conventional compressed sensing MRI methods in preserving the edges and suppressing the noise. Besides, the proposed method is not sensitive to the initial image when training directions.

Calibration-Less Multi-coil MR image reconstruction - Corrected Proof

Angshul Majumdar, Rabab K. Ward — 2012-04-16

Abstract: In parallel magnetic resonance imaging (MRI), the problem is to reconstruct an image given the partial K-space scans from all the receiver coils. Depending on its position within the scanner, each coil has a different sensitivity profile. All existing parallel MRI techniques require estimation of certain parameters pertaining to the sensitivity profile, e.g., the sensitivity map needs to be estimated for the SENSE and SMASH and the interpolation weights need to be calibrated for GRAPPA and SPIRiT. The assumption is that the estimated parameters are applicable at the operational stage. This assumption does not always hold, consequently the reconstruction accuracies of existing parallel MRI methods may suffer.We propose a reconstruction method called Calibration-Less Multi-coil (CaLM) MRI. As the name suggests, our method does not require estimation of any parameters related to the sensitivity maps and hence does not require a calibration stage. CaLM MRI is an image domain method that produces a sensitivity encoded image for each coil. These images are finally combined by the sum-of-squares method to yield the final image. It is based on the theory of Compressed Sensing (CS). During reconstruction, the constraint that “all the coil images should appear similar” is introduced within the CS framework. This leads to a CS optimization problem that promotes group-sparsity. The results from our proposed method are comparable (at least for the data used in this work) with the best results that can be obtained from state-of-the-art methods.

Cardiovascular magnetic resonance for the assessment of pulmonary arterial hypertension: toward a comprehensive CMR exam - Corrected Proof

El-Sayed H. Ibrahim, Richard D. White — 2012-04-16

Abstract: Pulmonary arterial hypertension (PAH) is a progressive disorder characterized by abnormally increased blood pressure of the pulmonary circulation. The clinical course of the untreated PAH involves rapid progression to right ventricular (RV) failure and death. Right heart catheterization is the gold-standard method for confirming PAH. However, the technique's invasiveness and associated risks preclude its use on a regular basis. Different imaging techniques have been implemented for evaluating PAH, including echocardiography, computed tomography and nuclear medicine. However, these techniques have their own limitations. During the past decade, cardiovascular magnetic resonance (CMR) has been increasingly used for the evaluation of different cardiovascular diseases, including PAH, due to its high resolution, high tissue contrast, and the plethora of anatomical and physiological parameters that can be measured with this modality. This article presents an up-to-date review of the implementation of CMR for evaluating PAH. This is achieved by describing a comprehensive CMR protocol that includes several imaging sequences for assessing different cardiovascular parameters pertaining to PAH. In contrast to the previously published articles, the presented CMR protocol evaluates both RV function and pulmonary artery hemodynamics, which are both affected in PAH. Each imaging sequence is explained along with the image analysis steps required for deriving the cardiovascular parameters of interest. Then, based on an extensive literature review, the article illustrates the significance of the derived cardiovascular parameters and their association with PAH. The article concludes with a discussion of the advantages of the proposed CMR exam for better understanding of the disease pathophysiology and treatment planning.

Pediatric primary and metastatic neuroblastoma: MRI findings: Pictorial review - Corrected Proof

2012-04-16

Abstract: Magnetic resonance imaging (MRI) has become one of the most valuable modalities for initial and follow-up imaging of suspected or known neuroblastoma (NBL) owing to its excellent inherent contrast, lack of ionizing radiation and multiplanar imaging capability.Importantly, NBL has a variable appearance on different imaging modalities, and this is particularly pertinent to MRI. MRI is a cornerstone for management of NBL, providing essential information at initial presentation regarding diagnosis, staging, resectability and relation to vital structures. It can also define the extent of residual disease after surgical resection or assess the efficacy of treatment. Follow-up MRI is frequently performed to ensure sustained complete remission or to monitor known residual disease.This pictorial review article aims to provide the reader with a concise, yet comprehensive, collection of MR images of primary and metastatic NBL lesions with relevant correlation with other imaging modalities.

Transient and sustained components of the sensorimotor BOLD response in fMRI - Corrected Proof

Michael Marxen, Ryan J. Cassidy, Tara L. Dawson, Bernhard Ross, Simon J. Graham — 2012-04-11

Abstract: Blood oxygenation level-dependent (BOLD) signal time courses in functional magnetic resonance imaging are estimated within the framework of general linear modeling by convolving an input function, that represents neural activity, with a canonical hemodynamic response function (HRF). Here we investigate the performance of different neural input functions and latency-optimized HRFs for modeling BOLD signals in response to vibrotactile somatosensory stimuli of variable durations (0.5, 1, 4, 7 s) in 14 young, healthy adults who were required to make button press responses at each stimulus cessation. Informed by electrophysiology and the behavioral task, three nested models with an increasing number of parameters were considered: a boxcar; boxcar and offset transient; and onset transient, boxcar and offset transient (TBT). The TBT model provided the best fit of the group-averaged BOLD time courses based on χ2 and F statistics. Only the TBT model was capable of fitting the bimodal shape of the BOLD response to the 7-s stimulus and the relative peak amplitudes for all stimulus lengths in key somatosensory and motor areas. This suggests that the TBT model provides a more comprehensive description of brain sensorimotor responses in this experiment than provided by the simple boxcar model. Work comparing the activation maps obtained with the TBT model with magnetoencephalography data is under way.

Effect of cerebrovascular changes on brain DTI quantitation: a hypercapnia study - Corrected Proof

Abby Y. Ding, Kevin C. Chan, Ed X. Wu — 2012-04-11

Abstract: Quantitative diffusion tensor imaging (DTI) offers a valuable tool to probe the microstructural changes in neural tissues in vivo, where absolute quantitation accuracy and reproducibility are essential. It has been long recognized that measurement of apparent diffusion coefficient (ADC) using DTI could be influenced by the presence of water molecules in cerebrovasculature. However, little is known about to what extent such blood signal affects DTI quantitation. In this study, we quantitatively examined the effect of cerebral hemodynamic change on DTI indices by using a standard multislice echo planar imaging (EPI) spin echo (SE) DTI acquisition protocol and a rat model of hypercapnia. In response to 5% CO2 challenge, mean, radial and axial diffusivities measured with diffusion factor (b-value) of b=1.0 ms/μm2 were found to increase in whole brain (1.52%±0.22%, 1.66%±0.16% and 1.35%±0.37%, respectively), gray matter (1.56%±0.23%, 1.63%±0.14% and 1.47%±0.45%, respectively) and white matter regions (1.45%±0.28%, 1.88%±0.33% and 1.10%±0.26%, respectively). Fractional anisotropy (FA) was found to decrease by 1.67%±0.38%, 1.91%±0.59% and 1.46%±0.30% in whole brain, gray matter and white matter regions, respectively. In addition, these diffusivity increases and FA decreases became more pronounced at a lower b-value (b=0.3 ms/μm2). The results indicated that in vivo DTI quantitation in brain can be contaminated by vascular factors on the order of few percentages. Consequently, alterations in cerebrovasculature and hemodynamics can affect the DTI quantitation and its efficacy in characterizing the neural tissue microstructures in normal and diseased states. Caution should be taken in designing and interpreting quantitative DTI studies as all DTI indices can be potentially confounded by physiologic conditions and by cerebrovascular and hemodynamic characteristics.

Magnetic resonance differentiation between T2 and T1 gallbladder carcinoma: significance of subserosal enhancement on the delayed phase dynamic study - Corrected Proof

2012-04-11

Abstract: Purpose: The aim of this study is to investigate whether subserosal enhancement on the delayed-phase dynamic magnetic resonance (MR) study (SED) can differentiate T2 from T1 gallbladder carcinoma (GBC).Methods: The institutional research board approved this retrospective study. Between 1997 and 2006, there were surgically proven 11 T1 and 21 T2 GBC in 30 patients, all of whom had undergone preoperative contrast enhanced dynamic MR study, either with a 2D sequence (n=17) or 3D sequences (n=15). All images were reviewed by two radiologists for the presence of SED, and receiver operating characteristic (ROC) curve analysis was performed. Sensitivity, specificity, positive and negative predictive values were calculated by consensus.Results: The areas under the ROC curves of the two readers were 0.91 and 0.86, and the kappa value was 0.78. Of the 21 T2 GBC, 18 and 3 showed positive and negative SED, respectively. Of the 11 T1 GBC, 1 and 10 showed positive and negative SED, respectively. The sensitivity, specificity, positive and negative predictive values of SED for diagnosing T2 lesions were 86%, 88%, 91% and 77%, respectively.Conclusions: In conclusion, SED may be a useful sign to differentiate T2 from T1 GBC, which would affect the preoperative surgical planning of the patients.

Imaging features of small (≤3 cm) pancreatic solid tumors on gadoxetic-acid-enhanced MR imaging and diffusion-weighted imaging: an initial experience - Corrected Proof

2012-04-11

Abstract: Objective: The objective was to determine imaging features that distinguish small (≤3cm) solid pancreatic adenocarcinoma, neuroendocrine tumor (NET) and solid pseudopapillary tumor (SPT) on gadoxetic-acid-enhanced magnetic resonance imaging (MRI) and diffusion-weighed imaging (DWI).Materials and methods: Twenty-four adenocarcinomas, 10 NETs and 8 SPTs were retrospectively included. Two radiologists analyzed morphologic features, signal intensity of the tumors on MR images including DWI (b=800) and dynamic enhancement pattern with consensus. Tumor-to-parenchyma ratio and tumor apparent diffusion coefficients (ADCs) were quantitatively assessed.Results: All adenocarcinomas had an ill-defined margin and irregular shape, and more frequently had pancreatic duct dilatation compared with other tumors (P<.05). All SPTs and all but one of the adenocarcinomas (95.8%) had no arterial enhancement with progressively increased enhancement, whereas seven NETs (70%) had arterial enhancement with progressively decreased enhancement (P<.01). The mean value of tumor-to-parenchyma ratio on arterial and portal phases was significantly higher for NETs, and the mean value of tumor ADCs was significantly lower for SPTs than for other tumors (P<.05).Conclusions: Gadoxetic-acid-enhanced MRI may aid in differentiation between small adenocarcinomas, NETs and SPTs based on morphologic features with dynamic enhancement pattern in adenocarcinomas, dynamic enhancement pattern with tumor-to-parenchyma ration on arterial and portal phases in NETs, and dynamic enhancement pattern with lower ADC value in SPTs.

Manifestation and post hoc correction of gradient cross-term artifacts in DTI - Corrected Proof

Govind Nair, Xiaoping P. Hu — 2012-04-11

Abstract: Cross-terms between imaging and diffusion gradients, unaccounted for during tensor calculations, can lead to erroneous estimation of diffusivity and fractional anisotropy (FA) in regions of isotropic and anisotropic diffusion. Cross-term of magnitude 136.8±1.6 s/mm2, artificially introduced in the slice-encode direction, caused an increase in FA in isotropic phantom from 0.0546±0.0001 to 0.0996±0.0001, while the change in chimpanzee brain depended on the orientation of the white matter (WM). Mean diffusivity (MD) remained unchanged in isotropic phantom, but increased by ∼20% in the WM due to cross-terms. A bias was observed in the principal eigenvectors in both phantom and chimpanzee brain, resulting in significant increase in midline crossing fibers along the bias than perpendicular to it in tractography in chimpanzee brain. Post hoc correction of these artifacts was achieved by estimating the cross-term factors using calibration scans on an isotropic phantom and modifying the b-matrix before tensor calculation. Upon correction, the FA and MD values closely resembled the values obtained from sequence without cross-terms, and the bias in principal eigenvectors was eliminated. Customized sequences involving large b-values, high-resolution imaging, or long diffusion or echo times should therefore be evaluated and any residual cross-terms corrected before implementation.

Coronary artery stent mimicking intracardiac thrombus on cardiac magnetic resonance imaging due to signal loss: case report - Corrected Proof

2012-04-11

Abstract: Since the introduction of percutaneous coronary intervention for coronary artery disease, thousands of patients have been treated with the implantation of coronary stents. Moreover, several of the patients with coronary stent undergo cardiac magnetic resonance (CMR) imaging every year.This case report is of a 77-year-old man who was previously treated with the implantation of a coronary stent in the left circumflex artery. He underwent CMR imaging, which revealed a process 14×21 mm in the left atrium. Cardiac contrast computed tomography did not demonstrate any cardiac pathology.While the signal loss on MRI associated with implanted metallic devices is known, we report a case where an implanted coronary stent in the left circumflex artery led to an intracardiac signal loss mimicking intracardiac thrombus/tumor.

Dynamic properties of water in silicalite-1 powder - Corrected Proof

2012-04-11

Abstract: Self-diffusion of D2O in partially filled silicalite-1 crystals was studied at 25°C by 2H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10−7 to ∼10−10 m2/s, which include values much higher and lower than that of bulk water (∼10−9 m2/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.

Use of fat suppression in R2 relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients - Corrected Proof

2012-04-11

Abstract: Purpose: To assess the performance and results of R2 relaxometry using a fat-suppressed (FS) multiecho sequence and compare these to conventional R2 relaxometry in estimating tissue iron overload.Materials and Methods: Relaxation rate values (R2=1/T2) of the liver, spleen, pancreas and vertebral bone marrow (VBM) were estimated in 21 patients with β-thalassemia major, using a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence before (R2) and after (R2 FS) the application of chemically selective fat suppression.Results: Hepatic and splenic R2 FS values correlated with respective R2 values (r=0.98 and r=0.96, P<.001), whereas correlations between R2 FS and R2 values for pancreas and VBM were not statistically significant. Bland–Altman plots show disagreement between R2 and R2 FS values, particularly for pancreas and VBM. Hepatic, pancreatic and VBM R2 FS values correlated with serum ferritin (r=0.88, P<.001; r=0.51, P<.003; and r=0.75, P<.002, respectively). Hepatic R2 FS values correlated with splenic R2 FS (r=0.77, P<.03), pancreatic R2 FS (r=0.61, P<.006) and VBM R2 FS values (r=0.70, P<.001), whereas pancreatic R2 FS values correlated also with VMB R2 FS values. On the contrary, among the R2 values of the above tissues, obtained without fat suppression, only hepatic R2 values correlated with serum ferritin, whereas no correlation was documented between hepatic and pancreatic or VBM R2 values. The application of fat suppression did not improve breathing or flow artifacts.Conclusion: Application of fat suppression in the standard CPMG sequence improved the capability of MRI in noninvasive quantification of iron, particularly in lipid-rich tissues, such as vertebral bone marrow (VBM) and pancreas.

Temporal/spatial resolution improvement of in vivo DCE-MRI with compressed sensing-optimized FLASH - Corrected Proof

2012-04-05

Abstract: Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) provides critical information regarding tumor perfusion and permeability by injecting a T1 contrast agent, such as Gd-DTPA, and making a time-resolved measurement of signal increase. Both temporal and spatial resolutions are required to be high to achieve an accurate and reproducible estimation of tumor perfusion. However, the dynamic nature of the DCE experiment limits simultaneous improvement of temporal and spatial resolution by conventional methods. Compressed sensing (CS) has become an important tool for the acceleration of imaging times in MRI, which is achieved by enabling the reconstruction of subsampled data. Similarly, CS algorithms can be utilized to improve the temporal/spatial resolution of DCE-MRI, and several works describing retrospective simulations have demonstrated the feasibility of such improvements. In this study, the fast low angle shot sequence was modified to implement a Cartesian, CS-optimized, sub-Nyquist phase encoding acquisition/reconstruction with multiple two-dimensional slice selections and was tested on water phantoms and animal tumor models. The mean voxel-level concordance correlation coefficient for Akep values obtained from ×4 and ×8 accelerated and the fully sampled data was 0.87±0.11 and 0.83±0.11, respectively (n=6), with optimized CS parameters. In this case, the reduction of phase encoding steps made possible by CS reconstruction improved effectively the temporal/spatial resolution of DCE-MRI data using an in vivo animal tumor model (n=6) and may be useful for the investigation of accelerated acquisitions in preclinical and clinical DCE-MRI trials.

A semiautomatic postprocessing of liver R2* measurement for assessment of liver iron overload - Corrected Proof

Jie Deng, Cynthia K. Rigsby, Samantha Schoeneman, Emma Boylan — 2012-04-05

Abstract: Purpose: The purpose was to propose and evaluate a semiautomatic postprocessing method to measure liver R2⁎ values in patients with a broad range of liver iron content.Materials and Methods: Multiecho gradient echo magnetic resonance images were acquired in patients diagnosed with thalassemia or other types of congenital anemias. Liver R2⁎ values were measured using a routine manually defined region-of-interest (mROI) method and a semiautomatic (SA) method. In the semiautomatic method, pixelwise (pSA) and averaged (aSA) signal fitting was performed on the segmented liver tissues after hepatic vessel extraction. The pixelwise fitting approach resulted in a liver R2⁎ map with an overlay of nonfitted pixels associated with noise performance. The following aSA approach derived overall R2⁎ by fitting the averaged signal intensities of all pixels within the liver ROI excluding vessels and nonfitted pixels. The measurement accuracy and interobserver agreement using mROI and the two semiautomatic approaches (pSA and aSA) were evaluated.Results: In a total of 45 exams with R2⁎ ranging from 30 to 1500 s−1, the R2⁎ measurements using all three methods were overall highly correlated and concordant with each other. R2⁎ values measured by aSA were consistently higher than those measured by mROI. At lower R2⁎ (<1000 s−1), R2⁎ values measured by pSA were consistent with aSA but higher than mROI; with increasing R2⁎, the pSA method became less stable and underestimated R2⁎ due to increased noise level. The interobserver agreement was higher for the aSA method compared to pSA and mROI.Conclusion: The semiautomatic postprocessing method provides a promising tool for reliable liver R2⁎ measurement with additional information for overall evaluation of iron distribution and measurement confidence. This method may offer the potential of reducing interoperator variability and improving diagnostic confidence in patients with liver iron overload.

Atlas-based automatic mouse brain image segmentation revisited: model complexity vs. image registration - Corrected Proof

Jordan Bai, Thi Lan Huong Trinh, Kai-Hsiang Chuang, Anqi Qiu — 2012-04-05

Abstract: Although many atlas-based segmentation methods have been developed and validated for the human brain, limited work has been done for the mouse brain. This paper investigated roles of image registration and segmentation model complexity in the mouse brain segmentation. We employed four segmentation models [single atlas, multiatlas, simultaneous truth and performance level estimation (STAPLE) and Markov random field (MRF) via four different image registration algorithms (affine, B-spline free-form deformation (FFD), Demons and large deformation diffeomorphic metric mapping (LDDMM)] for delineating 19 structures from in vivo magnetic resonance microscopy images. We validated their accuracies against manual segmentation. Our results revealed that LDDMM outperformed Demons, FFD and affine in any of the segmentation models. Under the same registration, increasing segmentation model complexity from single atlas to multiatlas, STAPLE or MRF significantly improved the segmentation accuracy. Interestingly, the multiatlas-based segmentation using nonlinear registrations (FFD, Demons and LDDMM) had similar performance to their STAPLE counterparts, while they both outperformed their MRF counterparts. Furthermore, when the single-atlas affine segmentation was used as reference, the improvement due to nonlinear registrations (FFD, Demons and LDDMM) in the single-atlas segmentation model was greater than that due to increasing model complexity (multiatlas, STAPLE and MRF affine segmentation). Hence, we concluded that image registration plays a more crucial role in the atlas-based automatic mouse brain segmentation as compared to model complexity. Multiple atlases with LDDMM can best improve the segmentation accuracy in the mouse brain among all segmentation models tested in this study.

Effect of nonrigid registration algorithms on deformation-based morphometry: a comparative study with control and Williams syndrome subjects - Corrected Proof

2012-03-29

Abstract: Deformation-based morphometry (DBM) is a widely used method for characterizing anatomical differences across groups. DBM is based on the analysis of the deformation fields generated by nonrigid registration algorithms, which warp the individual volumes to a DBM atlas. Although several studies have compared nonrigid registration algorithms for segmentation tasks, few studies have compared the effect of the registration algorithms on group differences that may be uncovered through DBM. In this study, we compared group atlas creation and DBM results obtained with five well-established nonrigid registration algorithms using 13 subjects with Williams syndrome and 13 normal control subjects. The five nonrigid registration algorithms include the following: (1) the adaptive bases algorithm, (2) the image registration toolkit, (3) The FSL nonlinear image registration tool, (4) the automatic registration tool, and (5) the normalization algorithm available in Statistical Parametric Mapping (SPM8). Results indicate that the choice of algorithm has little effect on the creation of group atlases. However, regions of differences between groups detected with DBM vary from algorithm to algorithm both qualitatively and quantitatively. Some regions are detected by several algorithms, but their extent varies. Others are detected only by a subset of the algorithms. Based on these results, we recommend using more than one algorithm when performing DBM studies.