Bayesian estimation of relaxation times T(1) in MR images of irradiated Fricke-agarose gels

The authors present a novel method for processing T(1)-weighted images acquired with Inversion-Recovery (IR) sequence. The method, developed within the Bayesian framework, takes into account a priori knowledge about the spatial regularity of the parameters to be estimated. Inference is drawn by means of Markov Chains Monte Carlo algorithms. The method has been applied to the processing of IR images from irradiated Fricke-agarose gels, proposed in the past as relative dosimeter to verify radiotherapeutic treatment planning systems. Comparison with results obtained from a standard approach shows that signal-to noise ratio (SNR) is strongly enhanced when the estimation of the longitudinal relaxation rate (R1) is performed with the newly proposed statistical approach. Furthermore, the method allows the use of more complex models of the signal. Finally, an appreciable reduction of total acquisition time can be obtained due to the possibility of using a reduced number of images. The method can also be applied to T(1) mapping of other systems.     

MR imaging of radiation osteitis in the sacroiliac joints

The purpose of this study was to analyze magnetic resonance (MR) images of radiation osteitis of sacroiliac joints, retrospectively. Seven patients with radiation osteitis, which was diagnosed by pelvic plain radiographs and CT images, underwent MRI. T(1)-weighted spin echo images and T(2)-weighted fast spin echo images were obtained in all patients. Four patients were examined after gadolinium injection. Major signal changes of radiation osteitis were distributed on the iliac side. T(1)-weighted images showed diffuse low intensity both in sacral and iliac sides. T(2)-weighted images showed very low intensity adjacent to sacroiliac joints, but mixed intensity was illustrated apart from joints, and high intensity in the peripheral areas. Radiation osteitis showed slight to mild, but irregular enhancement in four patients after gadolinium administration. MRI can illustrate abnormal bone change distribution and is useful for diagnosing this entity by characteristic intensity patterns on T(1)-weighted images with and without gadolinium and T(2)-weighted image. However, the diagnosis of accompanied insufficiency fractures in the area of radiation osteitis is occasionally difficult with conventional MRI.     

Non-uniformity correction of human brain imaging at high field by RF field mapping of B1+ and B1-

A new method of non-uniform image correction is proposed. Image non-uniformity is originated from the spatial distribution of RF transmission and reception fields, represented as B(1)(+) and B(1)(-), respectively. In our method, B(1)(+) mapping was performed invivo by a phase method. In B(1)(-) mapping, images with multiple TEs were acquired with a multi-echo adiabatic spin echo (MASE) sequence which enables homogeneous excitation. By T(2) fitting of these images an M(0) map (M(0)(MASE)) was obtained, in which signal intensity was expressed as the product of B(1)(-) and M?(1-e?(TR/T1)) . The ratio of this M(0)(MASE) map to the B(1)(+) map showed a similar spatial pattern in different human brains. These ratios of M(0)(MASE) to B(1)(+) in 24 subjects were averaged and then fitted with a spatially polynomial function to obtain a ratio map of B(1)(-)/B(1)(+)(α). Uniform image was achieved in spin echo (SE), MASE and inversion recovery turboFLASH (IRTF) images using measured B(1)(+) and calculated B(1)(-) by αB(1)(+). Water fractions in gray and white matters obtained from the M(0) images corrected by this method were in good agreement with previously reported values. From these experimental results, the proposed method of non-uniformity correction is validated at 4.7 T imaging.     

Feasibility of fat-saturated T2-weighted magnetic resonance imaging with slice encoding for metal artifact correction (SEMAC) at 3T

Background and Purpose

Fluid-sensitive MR imaging in postoperative evaluation is important, however, metallic artifacts is inevitable. The purpose is to investigate the feasibility of fat-saturated slice encoding for metal artifact correction (SEMAC)-corrected T2-weighted magnetic resonance (MR) at 3T in patients with spinal prostheses.

Methods

Following institutional review board approval, 27 SEMAC-encoded spinal MRs between September 2012 and October 2013 in patients with spinal metallic prostheses were analyzed. The MR images were scanned on a 3T MR system including SEMAC-corrected and uncorrected fast spin echo (FSE) T2-weighted MR images with fat-saturation. Two musculoskeletal radiologists compared the image sets and qualitatively analyzed the images using a five-point scale in terms of artifact reduction around the prosthesis, visualization of the prosthesis and pedicle, and intervertebral neural foramina. Quantitative assessments were performed by calculating the ratio of signal intensity from the fixated vertebra and that from upper level vertebra. For statistical analyses, paired t-test was used.

Results

Fat-saturated SEMAC-corrected T2-weighted MR images enabled significantly improved metallic artifact reduction (P < 0.05). Quantitative evaluation of the signal intensity ratio of screw-fixated vertebra and upper level vertebra showed a significantly lower ratio on fat-saturated SEMAC images (P < 0.05), however, the high signal intensity of signal pile-up could be not completely corrected.

Conclusion

SEMAC correction in fat-suppressed T2-weighted MR images can overcome the signal loss of metallic artifacts and provide improved delineation of the pedicle screw and peri-prosthetic region. Signal pile-up, however, could not be corrected completely, therefore readers should be cautious in the evaluation of marrow around the prosthesis.     

A novel content-based active contour model for brain tumor segmentation

Brain tumor segmentation is a crucial step in surgical and treatment planning. Intensity-based active contour models such as gradient vector flow (GVF), magneto static active contour (MAC) and fluid vector flow (FVF) have been proposed to segment homogeneous objects/tumors in medical images. In this study, extensive experiments are done to analyze the performance of intensity-based techniques for homogeneous tumors on brain magnetic resonance (MR) images. The analysis shows that the state-of-art methods fail to segment homogeneous tumors against similar background or when these tumors show partial diversity toward the background. They also have preconvergence problem in case of false edges/saddle points. However, the presence of weak edges and diffused edges (due to edema around the tumor) leads to oversegmentation by intensity-based techniques. Therefore, the proposed method content-based active contour (CBAC) uses both intensity and texture information present within the active contour to overcome above-stated problems capturing large range in an image. It also proposes a novel use of Gray-Level Co-occurrence Matrix to define texture space for tumor segmentation. The effectiveness of this method is tested on two different real data sets (55 patients - more than 600 images) containing five different types of homogeneous, heterogeneous, diffused tumors and synthetic images (non-MR benchmark images). Remarkable results are obtained in segmenting homogeneous tumors of uniform intensity, complex content heterogeneous, diffused tumors on MR images (T1-weighted, postcontrast T1-weighted and T2-weighted) and synthetic images (non-MR benchmark images of varying intensity, texture, noise content and false edges). Further, tumor volume is efficiently extracted from 2-dimensional slices and is named as 2.5-dimensional segmentation.     

MRI with superparamagnetic iron oxide: efficacy in the detection and characterization of focal hepatic lesions

The purpose of this study was to evaluate the potential of superparamagnetic iron oxide particles (SPIO) as tissue specific contrast agent in magnetic resonance (MR) imaging in detection and characterization of focal hepatic lesions. We investigated 45 patients with focal hepatic lesions. T1-weighted SE (TR 650/TE 15 ms) and T2-weighted SE (TR 2015-2030/TE 45 and 90 ms) unenhanced images were obtained. After SPIO application we performed T1-weighted images with and T2-weighted images with and without fat suppression using the same image parameters. Liver signal intensity decreased by 74% (min 47%, max 83%) on T2-weighted images after application of the contrast agent. Benign lesions (FNH, adenoma) showed an average signal drop of 40% (min 20%, max 47%) whereas malignant lesions showed no significant change of signal intensity on post-contrast images. The mean tumor-to-liver contrast-to-noise ratio (C/N) was improved in all post-contrast sequences irrespective of the lesion type. An additional increase of tumor-to-liver contrast by use of fat suppression technique could be established in the slightly T2-weighted sequence (TE 45 ms). In metastases, divided in different size groups, we could determine a significant size relation of tumor-to-liver C/N. After SPIO application the number of detected lesions increased distinctly, especially small foci are more easily demonstrated. SPIO particles are a efficacious contrast agent for MR examinations of the liver. For tumor characterization T1- and T2-weighted pre- and post-contrast images are necessary. The T1-weighted sequences are helpful to differentiate benign lesions such as cysts and hemangiomas from malignant lesions. Detection and differential diagnoses of hepatic lesions are improved by use of the SPIO-particles.     

Ovarian Brenner tumors: MR imaging characteristics

This study describes the appearance of Brenner tumors on MR imaging and compares quantitative signal intensity measurements of Brenner tumors with that of other ovarian tumors. A search of pathologic and MR records disclosed patients who had MRIs showing Brenner tumors prior to surgical excision. Patients (21) with other surgically proven ovarian masses were randomly selected for comparison. MR imaging was performed at 1.5 T with phased array multicoils and fast spin echo T2-weighted images. Region-of-interest measurements of signal intensity (SI) were made to calculate signal intensity ratios (SIR = mass SI/muscle SI). Brenner tumors showed significantly lower SIR than other tumors on T2-weighted images (p = 0 .004) and similar SIR on T1-weighted images. Brenner tumors show lower signal intensity on T2-weighted images than other non-fibrous ovarian tumors. This lower signal intensity may result from the extensive fibrous content of these tumors.     

Correction of intensity nonuniformity in spin-echo T(1)-weighted images

This paper presents a method to correct intensity nonuniformity in spin-echo T(1)-weighted images and particularly the inhomogeneities due to RF transmission imperfections which have tissue-dependent effects through the T(1) relaxation times. This method is based on the use of a uniform phantom, first for classic normalization by division by the phantom images, and second for T(1)-correction using the RF transmitted cartography. We present experimental results from a bi-phasic (oil/water) phantom and from a salmon with a 0.2 T imager. The results demonstrate the efficiency of the method in the two cases and its ability to cope with partial volume effect.     

Implementation of a rapid inversion-prepared dual-contrast gradient echo sequence for quantitative dynamic contrast-enhanced magnetic resonance imaging of the human prostate

The first step in quantitative pharmacokinetic modeling is to determine the arterial input function (AIF) by deriving the contrast medium (CM) concentration from an appropriate imaging sequence by monitoring changes in either the amplitude or the phase signal of an accommodative artery. The bolus passage is best detected on T2- or T2*-weighted images, while extravasation is best assessed on T1-weighted images. Here, an imaging sequence is used that employs a parallel acquisition technique for the interleaved acquisition of an inversion-prepared T1-weighted image and a T1/T2*-mixed-weighted image for determination of the AIF.

The sequence was applied in six patients with prostate cancer. A method is presented for quantifying the AIF derived from the signal intensity-time courses of both the T1/T2*-mixed-weighted and the T1-weighted image. Furthermore, in some patients the signal intensity-time course of the T1-weighted image exhibits flow-induced signal modulations. To reduce the effect of this flow-related signal enhancement the corresponding phase information was used.

The sequence presented here has the potential to improve the quantification of the AIF at all time points and pharmacokinetic modeling of the CM dynamics of the prostate.     

Small hyperintense hepatic lesions on T1-weighted images in patients with cirrhosis: evaluation with serial MRI and imaging features for clinical benignity

PURPOSE: The aim of this study was to evaluate the frequency and magnetic resonance imaging (MRI) features of clinically benign, small (<2 cm) hyperintense hepatic lesions in the cirrhotic liver on T1-weighted MR images seen at serial MRI. MATERIALS AND METHODS: This study included 189 patients with cirrhosis, who underwent hepatic MRI more than twice with an interval of at least 12 months. The initial MR images were reviewed for the presence of small hyperintense lesions on T1-weighted images. The size, location and signal intensity on T2-weighted images as well as enhancement patterns of the corresponding lesions were recorded. RESULTS: On the initial T1-weighted MR images, 43 small hyperintense hepatic lesions were detected in 23 (12%) of 189 patients. Twelve (28%) of 43 lesions showed early enhancement and were pathologically diagnosed as hepatocellular carcinoma (HCC) during the follow-up period. Thirty-one (72%) of 43 lesions showed no early enhancement with various signal intensity on T2-weighted images (hyperintensity=4, isointensity=20, hypointensity=7). Among these 31 lesions, 12 showed no interval change, while 11 disappeared (n=10) or decreased in size (n=1). In the remaining eight lesions, seven were diagnosed as HCC on the basis of pathologic confirmation or the interval growth. CONCLUSION: Small hyperintense hepatic lesions on T1-weighted magnetic resonance (MR) images without early enhancement on the arterial-phase contrast-enhanced dynamic studies in patients with cirrhosis usually showed no interval growth or disappeared during the serial MRI. These lesions with additional findings of iso- or hypointensity on the T2-weighted MR images without "washout effect" on the contrast-enhanced equilibrium-phase images may more frequently be clinically benign or hyperplastic nodules than HCCs.     

A novel, fast entropy-minimization algorithm for bias field correction in MR images

A novel, fast entropy-minimization algorithm for bias field correction in magnetic resonance (MR) images is suggested to correct the intensity inhomogeneity degradation of MR images that has become an increasing problem with the use of phased-array coils. Four important modifications were made to the conventional algorithm: (a) implementation of a modified two-step sampling strategy for stacked 2D image data sets, which included reducing the size of the measured image on each slice with a simple averaging method without changing the number of slices and then using a binary mask generated by a histogram threshold method to define the sampled voxels in the reduced image; (b) improvement of the efficiency of the correction function by using a Legendre polynomial as an orthogonal base function polynomial; (c) use of a nonparametric Parzen window estimator with a Gaussian kernel to calculate the probability density function and Shannon entropy directly from the image data; and (d) performing entropy minimization with a conjugate gradient method. Results showed that this algorithm could correct different types of MR images from different types of coils acquired at different field strengths very efficiently and with decreased computational load.     

Comparison of T2 and LASER T2dagger image contrast in a rat model of acute cerebral ischemia

Previous studies have shown that T2(dagger)-weighted magnetic resonance images acquired using localization by adiabatic selective refocusing (LASER) can provide early tissue contrast following ischemia, possibly due to alterations in microscopic susceptibility within the tissue. The purpose of this study was to make a direct in vivo comparison of T2-, T2(dagger)- and diffusion-weighted image contrast during acute ischemia. Acute middle cerebral artery (MCA) occlusion was attempted in 14 rats using a modified Tamura approach incorporating electrocoagulation of the left MCA. T2(dagger)-weighted LASER images (Echo Time [TE]=108 ms), T2-weighted Carr-Purcell-Meiboom-Gill (CPMG) images (TE=110 ms) and diffusion-weighted images (b value=105 s/mm(2)) were acquired at 4 T within 1.5 h of ischemia onset. Tissue contrast in the MCA territory was quantified for histologically verified ischemic tissue (n=6) and in sham controls (n=4). T2(dagger)-weighted LASER images demonstrated greater contrast compared to the T2-weighted CPMG images, and more focal contrast compared to the diffusion-weighted images, suggesting different contrast mechanisms were involved.     

A dual image approach for bias field correction in magnetic resonance imaging

In this paper, we propose a dual image approach to correcting intensity inhomogeneities for MR images acquired using surface coils. Previous methods are usually not satisfactory due to restricted application domains, considerable human interactions, or some undesirable artifacts. The proposed algorithm provides nice correction results for a variety of surface-coil MR images. It is accomplished by using an additional body-coil MR image of a smaller size captured at the same position as that of the surface-coil image to facilitate the estimation of the bias field function. The correction algorithm consists of aligning the surface-coil image with the body-coil image and fitting a spline surface from a sparse set of data points for the associated bias field function. Experiments on some real images show satisfactory correction results by using the proposed algorithm.     

Spatial normalization of multiple sclerosis brain MRI data depends on analysis method and software package

BackgroundSpatially normalizing brain MRI data to a template is commonly performed to facilitate comparisons between individuals or groups. However, the presence of multiple sclerosis (MS) lesions and other MS-related brain pathologies may compromise the performance of automated spatial normalization procedures. We therefore aimed to systematically compare five commonly used spatial normalization methods for brain MRI – including linear (affine), and nonlinear MRIStudio (LDDMM), FSL (FNIRT), ANTs (SyN), and SPM (CAT12) algorithms – to evaluate their performance in the presence of MS-related pathologies.Methods3 Tesla MRI images (T1-weighted and T2-FLAIR) were obtained for 20 participants with MS from an ongoing cohort study (used to assess a real dataset) and 1 healthy control participant (used to create a simulated lesion dataset). Both raw and lesion-filled versions of each participant's T1-weighted brain images were warped to the Montreal Neurological Institute (MNI) template using all five normalization approaches for the real dataset, and the same procedure was then repeated using the simulated lesion dataset (i.e., total of 400 spatial normalizations). As an additional quality-assurance check, the resulting deformations were also applied to the corresponding lesion masks to evaluate how each processing pipeline handled focal white matter lesions. For each normalization approach, inter-subject variability (across normalized T1-weighted images) was quantified using both mutual information (MI) and coefficient of variation (COV), and the corresponding normalized lesion volumes were evaluated using paired-sample t-tests.ResultsAll four nonlinear warping methods outperformed conventional linear normalization, with SPM (CAT12) yielding the highest MI values, lowest COV values, and proportionately-scaled lesion volumes. Although lesion-filling improved spatial normalization accuracy for each of the methods tested, these effects were small compared to differences between normalization algorithms.ConclusionsSPM (CAT12) warping, ideally combined with lesion-filling, is recommended for use in future MS brain imaging studies requiring spatial normalization.     

Magnetic resonance imaging features in melanoma

T1 and T2 relaxation time shortening secondary to paramagnetic compounds has been described in melanoma. The purpose of this paper is to evaluate the signal behavior of melanoma involved in various body areas using short TR, TE and long TR, TE sequences. Twenty-seven sites of melanoma were evaluated with MR using T1 weighted and T2-weighted techniques. Using fat and muscle signal intensities as references tissues, lesions were graded into high, low or intermediate intensity categories for each of the sequences. Four signal patterns emerged. The typical pattern characterized by high signal on T1-weighted images and low signal on T2-weighted images reflected T1 and T2 shortening. The other pattern categories comprised of lesions demonstrating low signal T1-weighted images and high signal on T2-weighted images, high signal on both T1- and T2-weighted images and lesions showing intermediate signal on either T1- or T2-weighted images. We observed a tendency away from the typical signal pattern in extraocular melanoma cases with only one of 14 demonstrating this pattern. Moreover, only seven of thirteen ocular melanomas exhibited such behavior. Possible explanations for this findings as well as the existence of a variety of MR appearances to melanoma are offered. We conclude that while signal patterns showing T1 and T2 shortening are typical of melanoma, the absence of these does not exclude the diagnosis.     

Giant hemangioma of the liver: MR imaging characteristics in 24 patients

We retrospectively reviewed the magnetic resonance imaging (MRI) of giant hemangiomas in 24 patients. MRI studies comprised T1-weighted, T2-weighted and serial gadolinium-enhanced spoiled gradient echo (SGE) images. Morphologic features, signal characteristics and enhancement patterns were assessed. Histopathologic evaluation was obtained in nine patients. On T2-weighted images all lesions (size 5.7-24 cm) were hyperintense relative to the spleen and two dominant patterns of heterogeneity were demonstrated: a central heterogeneous area of either bright, dark, or mixed signal intensity, and a network of multiple fibrous septa of low signal intensity. Histopathologic evaluation of two lesions with a central bright area demonstrated the presence of hypocellular myxoid tissue. Central enhancement (9 lesions) and an irregular flame-shaped peripheral pattern of enhancement (12 lesions) were present in lesions with a mean diameter greater than 10 cm. Although giant hemangiomas show greater variability in their MR imaging appearance, an accurate diagnosis can be made through still characteristic features of high signal intensity on T2-weighted images and discontinuous peripheral enhancement.     

Synovial sarcoma: MR imaging

Ten patients with biopsy-proved synovial sarcoma were evaluated by magnetic resonance (MR) imaging on a 1.5-T unit. The lesions showed intermediate signal intensity on T1-weighted images and heterogeneous high signal intensity on T2-weighted images. Tumors were well-demarcated from normal tissues. Additional information included adjacent bone involvement (one case), femoral vein invasion by tumor (one case), and hemorrhage within the tumors (one case). Four patients underwent a repeat MR examination following chemotherapy. This showed a decrease in size and increase in the signal intensity of three tumors on T2-weighted images, proven to be due to necrosis in one. These changes correlated with clinical regression of disease. While MR in synovial sarcoma does not have any specific signal intensity, it proved to be useful in defining the extent of disease and in determining the response to chemotherapy.     

Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times

In order to predict the most sensitive MR imaging sequence for detecting liver metastases at 1.5 T, in vivo measurements of T1 and T2 relaxation times and proton density were obtained using multipoint techniques. Based on these measurements, two-dimensional contrast contour plots were constructed demonstrating signal intensity contrast between hepatic lesions and surrounding liver parenchyma for different pulse sequences and pulse timing parameters. The data predict that inversion recovery spin echo (IRSE) imaging should yield the greatest contrast between liver metastases and liver parenchyma at 1.5 T, followed by short tau inversion recovery (STIR) and spin-echo (SE) pulse sequences. T2-weighted SE images provided greater liver/lesion contrast than T1-weighted SE pulse sequences. Calculated T1, T2, and proton density values of the spleen were similar to those of hepatic metastatic lesions, indicating that the signal intensity of the spleen may be used as an internal standard to predict the signal intensity of hepatic metastases on T1- and T2-weighted images at 1.5 T.     

Recurrent hepatocellular carcinoma versus radiation-induced hepatic injury: differential diagnosis with MR imaging

The objective of this study was to assess the value of MR imaging in the differentiation between a recurrent hepatocellular carcinoma (HCC) and a radiation-induced hepatic injury. Nine male patients with suspected recurrence after radiotherapy for HCC underwent T(2)-, T(1)-weighted imaging and Gd-DTPA enhanced dynamic studies. T(2) relaxation times, signal intensity ratios in T(1)-weighted images (WI) and the relative enhancement of the dynamic study were calculated. Recurrent tumors and the irradiated area showed similar image characteristics: hypointense in T(1)-WI and hyperintense in T(2)-WI. T(2) values and signal intensity ratios in the T(1)-WI were not significantly different. In the gadolinium-enhanced dynamic study, a recurrent HCC showed early enhancement, followed by a rapid washout. However, the irradiated liver parenchyma showed hyperintensity from an early phase, and contrast enhancement tended to be more prominent and prolonged at the end of the dynamic studies. The characteristic findings of the dynamic MR study enable us to distinguish between a recurrent HCC and a radiation-induced hepatic injury.     

HASTE sequence with parallel acquisition and T2 decay compensation: application to carotid artery imaging

T2-weighted carotid artery images acquired using the turbo spin-echo (TSE) sequence frequently suffer from motion artifacts due to respiration and blood pulsation. The possibility of using HASTE sequence to achieve motion-free carotid images was investigated. The HASTE sequence suffers from severe blurring artifacts due to signal loss in later echoes due to T2 decay. Combining HASTE with parallel acquisition (PHASTE) decreases the number of echoes acquired and thus effectively reduces the blurring artifact caused by T2 relaxation. Further improvement in image sharpness can be achieved by performing T2 decay compensation before reconstructing the PHASTE data. Preliminary results have shown successful suppression of motion artifacts with PHASTE imaging. The image quality was enhanced relative to the original HASTE image, but was still less sharp than a non-motion-corrupted TSE image.