The voxel sensitivity function in Fourier transform imaging: applications to magnetic resonance angiography

Super-resolution Deep Learning Reconstruction Cervical Spine 1.5T MRI: Improved Interobserver Agreement in Evaluations of Neuroforaminal Stenosis Compared to Conventional Deep Learning Reconstruction

The aim of this study was to investigate whether super-resolution deep learning reconstruction (SR-DLR) is superior to conventional deep learning reconstruction (DLR) with respect to interobserver agreement in the evaluation of neuroforaminal stenosis using 1.5T cervical spine MRI. This retrospective study included 39 patients who underwent 1.5T cervical spine MRI. T2-weighted sagittal images were reconstructed with SR-DLR and DLR. Three blinded radiologists independently evaluated the images in terms of the degree of neuroforaminal stenosis, depictions of the vertebrae, spinal cord and neural foramina, sharpness, noise, artefacts and diagnostic acceptability. In quantitative image analyses, a fourth radiologist evaluated the signal-to-noise ratio (SNR) by placing a circular or ovoid region of interest on the spinal cord, and the edge slope based on a linear region of interest placed across the surface of the spinal cord. Interobserver agreement in the evaluations of neuroforaminal stenosis using SR-DLR and DLR was 0.422-0.571 and 0.410-0.542, respectively. The kappa values between reader 1 vs. reader 2 and reader 2 vs. reader 3 significantly differed. Two of the three readers rated depictions of the spinal cord, sharpness, and diagnostic acceptability as significantly better with SR-DLR than with DLR. Both SNR and edge slope (/mm) were also significantly better with SR-DLR (12.9 and 6031, respectively) than with DLR (11.5 and 3741, respectively) (p < 0.001 for both). In conclusion, compared to DLR, SR-DLR improved interobserver agreement in the evaluations of neuroforaminal stenosis using 1.5T cervical spine MRI.

A generalized order mixture model for tracing connectivity of white matter fascicles complexity in brain from diffusion MRI

This paper focuses on tracing the connectivity of white matter fascicles in the brain. In particular, a generalized order algorithm based on mixture of non-central Wishart distribution model is proposed for this purpose. The proposed algorithm utilizes the generalization of integer order based approach with the mixture of non-central Wishart distribution model. Pseudo super anomalous behavior of water diffusion inside human brain is the prime motivation of the the present study. We have shown results on multiple synthetic simulations with fibers orientations in two and three directions in each voxel as well as experiments on real data. Synthetic simulations were performed with varying noise levels and diffusion weighting gradient i.e. $b-$values. The proposed model performed outstanding especially for distinguishing closely oriented fibers.

Influence of cerebrospinal fluid on power absorption during transcranial magnetic resonance-guided focused ultrasound treatment

BACKGROUND

Ultrasound beam aberration correction is vital when focusing ultrasound through the skull bone in transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) applications. Current methods make transducer element phase adjustments to compensate for the variation in skull properties (shape, thickness, and acoustic properties), but do not account for variations in the internal brain anatomy.

PURPOSE

Our objective is to investigate the effect of cerebrospinal fluid (CSF) and brain anatomy on beam focusing in tcMRgFUS treatments.

METHODS

Simulations were conducted with imaging data from 20 patients previously treated with focused ultrasound for disabling tremor. The Hybrid Angular Spectrum (HAS) method was used to test the effect of including cerebral spinal fluid (CSF) and brain anatomy in determining the element phases used for aberration correction and beam focusing. Computer tomography (CT) and magnetic resonance imaging (MRI) images from patient treatments were used to construct a segmented model of each patient's head. The segmented model for treatment simulation consisted of water, skin, fat, brain, CSF, diploë, and cortical bone. Transducer element phases used for treatment simulation were determined using time reversal from the desired focus, generating a set of phases assuming a homogeneous brain in the intracranial volume, and a second set of phases assigning CSF acoustic properties to regions of CSF. In addition, for three patients, the relative effect of separately including CSF speed of sound values compared to CSF attenuation values was found.

RESULTS

We found that including CSF acoustic properties (speed of sound and attenuation) during phase planning compared to phase correction without considering CSF increased the absorbed ultrasound power density ratios at the focus over a range of 1.06 to 1.29 (mean of 17% ± 6%) for 20 patients. Separately considering the CSF speed of sound and CSF attenuation showed that the increase was due almost entirely to including the CSF speed of sound; considering only the CSF attenuation had a negligible effect.

CONCLUSIONS

Based on HAS simulations, treatment planning phase determination using morphologically realistic CSF and brain anatomy yielded an increase of up to 29% in the ultrasound focal absorbed power density. Future work will be required to validate the CSF simulations.

Simultaneous proton resonance frequency T1 - MR shear wave elastography for MR-guided focused ultrasound multiparametric treatment monitoring

PURPOSE

To develop an efficient MRI pulse sequence to simultaneously measure multiple parameters that have been shown to correlate with tissue nonviability following thermal therapies.

METHODS

A 3D segmented EPI pulse sequence was used to simultaneously measure proton resonance frequency shift (PRFS) MR thermometry (MRT), T1 relaxation time, and shear wave velocity induced by focused ultrasound (FUS) push pulses. Experiments were performed in tissue mimicking gelatin phantoms and ex vivo bovine liver. Using a carefully designed FUS triggering scheme, a heating duty cycle of approximately 65% was achieved by interleaving FUS ablation pulses with FUS push pulses to induce shear waves in the tissue.

RESULTS

In phantom studies, temperature increases measured with PRFS MRT and increases in T1 correlated with decreased shear wave velocity, consistent with material softening with increasing temperature. During ablation in ex vivo liver, temperature increase measured with PRFS MRT initially correlated with increasing T1 and decreasing shear wave velocity, and after tissue coagulation with decreasing T1 and increasing shear wave velocity. This is consistent with a previously described hysteresis in T1 versus PRFS curves and increased tissue stiffness with tissue coagulation.

CONCLUSION

An efficient approach for simultaneous and dynamic measurements of PRSF, T1 , and shear wave velocity during treatment is presented. This approach holds promise for providing co-registered dynamic measures of multiple parameters, which correlates to tissue nonviability during and following thermal therapies, such as FUS.

Essentials for Interpreting Intracranial Vessel Wall MRI Results: State of the Art

Intracranial vessel wall (VW) MRI has become widely available in clinical practice, providing multiple uses for evaluation of neurovascular diseases. The Vessel Wall Imaging Study Group of the American Society of Neuroradiology has recently reported expert consensus recommendations for the clinical implementation of this technique. However, the complexity of the neurovascular system and caveats to the technique may challenge its application in clinical practice. The purpose of this article is to review concepts essential for accurate interpretation of intracranial VW MRI results. This knowledge is intended to improve diagnostic confidence and performance in the interpretation of VW MRI scans. © RSNA, 2021 Online supplemental material is available for this article.

Improved MR thermometry for laser interstitial thermotherapy

OBJECTIVES

To develop, test and evaluate improved 2D and 3D protocols for proton resonance frequency shift magnetic resonance temperature imaging (MRTI) of laser interstitial thermal therapy (LITT). The objective was to develop improved MRTI protocols in terms of temperature measurement precision and volume coverage compared to the 2D MRTI protocol currently used with a commercially available LITT system.

METHODS

Four different 2D protocols and four different 3D protocols were investigated. The 2D protocols used multi-echo readouts to prolong the total MR sampling time and hence the MRTI precision, without prolonging the total acquisition time. The 3D protocols provided volumetric thermometry by acquiring a slab of 12 contiguous slices in the same acquisition time as the 2D protocols. The study only considered readily available pulse sequences (Cartesian 2D and 3D gradient recalled echo and echo planar imaging [EPI]) and methods (partial Fourier and parallel imaging) to ensure wide availability and rapid clinical implementation across vendors and field strengths. In vivo volunteer studies were performed to investigate and compare MRTI precision and image quality. Phantom experiments with LITT heating were performed to investigate and compare MRTI precision and accuracy. Different coil setups were used in the in vivo studies to assess precision differences between using local (such as flex and head coils) and non-local (i.e., body coil) receive coils. Studies were performed at both 1.5 T and 3 T.

RESULTS

The improved 2D protocols provide up to a factor of two improvement in the MRTI precision in the same acquisition time, compared to the currently used clinical protocol. The 3D echo planar imaging protocols provide comparable precision as the currently used 2D clinical protocol, but over a substantially larger field of view, without increasing the acquisition time. As expected, local receive coils perform substantially better than the body coil, and 3 T provides better MRTI accuracy and precision than 1.5 T. 3D data can be zero-filled interpolated in all three dimensions (as opposed to just two dimensions for 2D data), reducing partial volume effects and measuring higher maximum temperature rises.

CONCLUSIONS

With the presented protocols substantially improved MRTI precision (for 2D imaging) or greatly improved field of view coverage (for 3D imaging) can be achieved in the same acquisition time as the currently used protocol. Only widely available pulse sequences and acquisition methods were investigated, which should ensure quick translation to the clinic. Lasers Surg. Med. 51:286-300, 2019. © 2019 Wiley Periodicals, Inc.

Patents on Quantitative Susceptibility Mapping (QSM) of Tissue Magnetism

BACKGROUND

Quantitative susceptibility mapping (QSM) depicts biodistributions of tissue magnetic susceptibility sources, including endogenous iron and calcifications, as well as exogenous paramagnetic contrast agents and probes. When comparing QSM with simple susceptibility weighted MRI, QSM eliminates blooming artifacts and shows reproducible tissue susceptibility maps independent of field strength and scanner manufacturer over a broad range of image acquisition parameters. For patient care, QSM promises to inform diagnosis, guide surgery, gauge medication, and monitor drug delivery. The Bayesian framework using MRI phase data and structural prior knowledge has made QSM sufficiently robust and accurate for routine clinical practice.

OBJECTIVE

To address the lack of a summary of US patents that is valuable for QSM product development and dissemination into the MRI community.

METHOD

We searched the USPTO Full-Text and Image Database for patents relevant to QSM technology innovation. We analyzed the claims of each patent to characterize the main invented method and we investigated data on clinical utility.

RESULTS

We identified 17 QSM patents; 13 were implemented clinically, covering various aspects of QSM technology, including the Bayesian framework, background field removal, numerical optimization solver, zero filling, and zero-TE phase.

CONCLUSION

Our patent search identified patents that enable QSM technology for imaging the brain and other tissues. QSM can be applied to study a wide range of diseases including neurological diseases, liver iron disorders, tissue ischemia, and osteoporosis. MRI manufacturers can develop QSM products for more seamless integration into existing MRI scanners to improve medical care.

Simultaneous MR thermometry and acoustic radiation force imaging using interleaved acquisition

PURPOSE

A novel and practical method for simultaneously performing MR acoustic radiation force imaging (ARFI) and proton resonance frequency (PRF)-shift thermometry has been developed and tested. This could be an important tool for evaluating the success of MR-guided focused ultrasound procedures for which MR-thermometry measures temperature and thermal dose and MR-ARFI detects changes in tissue mechanical properties.

METHODS

MR imaging was performed using a gradient recalled echo segmented echo-planar imaging pulse sequence with bipolar motion encoding gradients (MEG). Images with ultrasound pulses (ON) and without ultrasound pulses (OFF) during the MEG were interleaved at the repetition time (TR) level. ARFI displacements were calculated by complex subtraction of ON-OFF images, and PRF temperature maps were calculated by baseline subtraction. Evaluations in tissue-mimicking phantoms and ex vivo porcine brain tissue were performed. Constrained reconstruction improved the temporal resolution of dynamic measurements.

RESULTS

Simultaneous maps of displacement and temperature were acquired in 2D and 3D while keeping tissue heating < 1°C. Accuracy of the temperature maps was comparable to the standard PRF sequence. Using constrained reconstruction and subsampled k-space (R = 4.33), 3D simultaneous temperature and displacement maps can be acquired every 4.7 s.

CONCLUSION

This new sequence acquires simultaneous temperature and displacement maps with minimal tissue heating, and can be applied dynamically for monitoring tissue mechanical properties during ablation procedures. Magn Reson Med 79:1515-1524, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Quantitative MR imaging of intra-orbital structures: Tissue-specific measurements and age dependency compared to extra-orbital structures using multispectral quantitative MR imaging

The orbit can be affected by unique pathologic conditions and often requires MRI evaluation. The purpose of this study was to investigate the age-related changes in multiple intra-orbital structures using quantitative MRI (qMRI). Thirty-eight subjects (20 males, 18 females; ages 0.5-87 years) underwent MRI with a mixed turbo spin echo sequence. T1 and T2 measurements were obtained within ROI in 6 intra-orbital structures (medial and lateral rectus muscles, medial and lateral retrobulbar fat, lacrimal gland, and optic nerve), and compared with those of corresponding extra-orbital structures (masseter muscle, subcutaneous cheek fat, buccal fat, parotid gland, and frontal white matter). Statistical analyses were performed using Pearson's correlation coefficients. T1 and T2 values of the extra-ocular muscles increased with age, with higher T1 and T2 values compared to the masseter muscles. Retrobulbar fat showed significant age-associated increases in T1 values in the lateral side and in T2 values in both sides. T1 and T2 values in the lacrimal gland increased with age, while the parotid gland showed an age-associated increase in T2 values and decrease in T1 values. Optic nerves demonstrated age-related changes, similar to that of frontal white matter; rapid decreases with age in T1 and T2 times in early stages of life, and slight increases in T1 and T2 times later in life. Intra-orbital structures demonstrated specific qMRI measurements and aging patterns, which were different from extra-orbital structures. Location-specific age-related changes of intra-orbital structures should be considered in the qMRI assessment of the orbital pathology.

Susceptibility underestimation in a high-susceptibility phantom: Dependence on imaging resolution, magnitude contrast, and other parameters

PURPOSE

We assessed the accuracy of quantitative susceptibility mapping in a gadolinium balloon phantom with a large range of susceptibility values and imaging resolutions at 1.5 and 3 Tesla (T).

THEORY AND METHODS

The phantom contained sources with susceptibility values of 0.4, 0.8, 1.6, and 3.2 ppm and was imaged at isotropic resolutions of 0.7, 0.8, 1.2, and 1.8 mm. Numerical simulations were performed to match the experimental findings. Voxel sensitivity effects were used to explain the susceptibility underestimations.

RESULTS

Both phantom data and simulation demonstrated that systematic underestimation of the susceptibility values increased with voxel size, field strength, and object susceptibility.

CONCLUSION

The underestimation originates from the signal formation in a voxel, which can be described by the voxel sensitivity function. The amount of underestimation is thus affected by imaging resolution, magnitude contrast, image filtering, and details of the susceptibility inclusions such as the susceptibility value and geometry. High-resolution imaging is therefore needed for accurate reconstruction of QSM values, especially at higher susceptibilities. Magn Reson Med 78:1080-1086, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

On the influence of zero-padding on the nonlinear operations in Quantitative Susceptibility Mapping

PURPOSE

Zero padding is a well-studied interpolation technique that improves image visualization without increasing image resolution. This interpolation is often performed as a last step before images are displayed on clinical workstations. Here, we seek to demonstrate the importance of zero padding before rather than after performing non-linear post-processing algorithms, such as Quantitative Susceptibility Mapping (QSM). To do so, we evaluate apparent spatial resolution, relative error and depiction of multiple sclerosis (MS) lesions on images that were zero padded prior to, in the middle of, and after the application of the QSM algorithm.

MATERIALS AND METHODS

High resolution gradient echo (GRE) data were acquired on twenty MS patients, from which low resolution data were derived using k-space cropping. Pre-, mid-, and post-zero padded QSM images were reconstructed from these low resolution data by zero padding prior to field mapping, after field mapping, and after susceptibility mapping, respectively. Using high resolution QSM as the gold standard, apparent spatial resolution, relative error, and image quality of the pre-, mid-, and post-zero padded QSM images were measured and compared.

RESULTS

Both the accuracy and apparent spatial resolution of the pre-zero padded QSM was higher than that of mid-zero padded QSM (p<0.001; p<0.001), which was higher than that of post-zero padded QSM (p<0.001; p<0.001). The image quality of pre-zero padded reconstructions was higher than that of mid- and post-zero padded reconstructions (p=0.004; p<0.001).

CONCLUSION

Zero padding of the complex GRE data prior to nonlinear susceptibility mapping improves image accuracy and apparent resolution compared to zero padding afterwards. It also provides better delineation of MS lesion geometry, which may improve lesion subclassification and disease monitoring in MS patients.

T2* Measurement bias due to concomitant gradient fields

PURPOSE

To demonstrate that concomitant magnetic fields can cause significant spatially dependent biases in T2* relaxometry measurements with implications for clinical applications such as BOLD and dynamic susceptibility contrast-enhanced MRI.

THEORY AND METHODS

After developing a theoretical framework for intravoxel dephasing and signal loss from concomitant magnetic fields, this framework and the effect of concomitant fields on T2* are validated with phantom experiments and numerical simulation. In lower leg and renal T2* mapping, we quantify measurement bias for imaging protocols with high gradient amplitude multiecho readouts, comparable to those used in clinical applications.

RESULTS

Concordance between phantom experiment and numerical simulation validate the theoretical framework. Changes in T2* measured in the lower leg and kidney varied by up to 15% and 35%, respectively, as a result of concomitant gradient effects when compared with the control measurements.

CONCLUSION

Concomitant magnetic fields produced by imaging gradient coils can cause clinically significant T2* mapping errors when high amplitude, long duration gradient waveforms are used. While we have shown that measurement biases can be quite large, modification of imaging parameters can potentially reduce concomitant field-induced measurement errors to acceptable levels. Magn Reson Med 77:1562-1572, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Sampling strategies for subsampled segmented EPI PRF thermometry in MR guided high intensity focused ultrasound

PURPOSE

To investigate k-space subsampling strategies to achieve fast, large field-of-view (FOV) temperature monitoring using segmented echo planar imaging (EPI) proton resonance frequency shift thermometry for MR guided high intensity focused ultrasound (MRgHIFU) applications.

METHODS

Five different k-space sampling approaches were investigated, varying sample spacing (equally vs nonequally spaced within the echo train), sampling density (variable sampling density in zero, one, and two dimensions), and utilizing sequential or centric sampling. Three of the schemes utilized sequential sampling with the sampling density varied in zero, one, and two dimensions, to investigate sampling the k-space center more frequently. Two of the schemes utilized centric sampling to acquire the k-space center with a longer echo time for improved phase measurements, and vary the sampling density in zero and two dimensions, respectively. Phantom experiments and a theoretical point spread function analysis were performed to investigate their performance. Variable density sampling in zero and two dimensions was also implemented in a non-EPI GRE pulse sequence for comparison. All subsampled data were reconstructed with a previously described temporally constrained reconstruction (TCR) algorithm.

RESULTS

The accuracy of each sampling strategy in measuring the temperature rise in the HIFU focal spot was measured in terms of the root-mean-square-error (RMSE) compared to fully sampled "truth." For the schemes utilizing sequential sampling, the accuracy was found to improve with the dimensionality of the variable density sampling, giving values of 0.65 °C, 0.49 °C, and 0.35 °C for density variation in zero, one, and two dimensions, respectively. The schemes utilizing centric sampling were found to underestimate the temperature rise, with RMSE values of 1.05 °C and 1.31 °C, for variable density sampling in zero and two dimensions, respectively. Similar subsampling schemes with variable density sampling implemented in zero and two dimensions in a non-EPI GRE pulse sequence both resulted in accurate temperature measurements (RMSE of 0.70 °C and 0.63 °C, respectively). With sequential sampling in the described EPI implementation, temperature monitoring over a 192×144×135 mm3 FOV with a temporal resolution of 3.6 s was achieved, while keeping the RMSE compared to fully sampled "truth" below 0.35 °C.

CONCLUSIONS

When segmented EPI readouts are used in conjunction with k-space subsampling for MR thermometry applications, sampling schemes with sequential sampling, with or without variable density sampling, obtain accurate phase and temperature measurements when using a TCR reconstruction algorithm. Improved temperature measurement accuracy can be achieved with variable density sampling. Centric sampling leads to phase bias, resulting in temperature underestimations.

MR relaxometry for the facial ageing assessment: the preliminary study of the age dependency in the MR relaxometry parameters within the facial soft tissue

OBJECTIVES

To investigate the location-specific tissue properties and age-related changes of the facial fat and facial muscles using quantitative MRI (qMRI) analysis of longitudinal magnetization (T1) and transverse magnetization (T2) values.

METHODS

38 subjects (20 males and 18 females, 0.5-87 years old) were imaged with a mixed turbo-spin echo sequence at 1.5 T. T1 and T2 measurements were obtained within regions of interest in six facial fat regions including the buccal fat and subcutaneous cheek fat, four eyelid fat regions (lateral upper, medial upper, lateral lower and medial lower) and five facial muscles including the orbicularis oculi, orbicularis oris, buccinator, zygomaticus major and masseter muscles bilaterally.

RESULTS

Within the zygomaticus major muscle, age-associated T1 decreases in females and T1 increases in males were observed in later life with an increase in T2 values with age. The orbicularis oculi muscles showed lower T1 and higher T2 values compared to the masseter, orbicularis oris and buccinator muscles, which demonstrated small age-related changes. The dramatic age-related changes were also observed in the eyelid fat regions, particularly within the lower eyelid fat; negative correlations with age in T1 values (p<0.0001 for age) and prominent positive correlation in T2 values in male subjects (p<0.0001 for male×age). Age-related changes were not observed in T2 values within the subcutaneous cheek fat.

CONCLUSIONS

This study demonstrates proof of concept using T1 and T2 values to assess age-related changes of the facial soft tissues, demonstrating tissue-specific qMRI measurements and non-uniform ageing patterns within different regions of facial soft tissues.

Quantitative susceptibility mapping (QSM): Decoding MRI data for a tissue magnetic biomarker

In MRI, the main magnetic field polarizes the electron cloud of a molecule, generating a chemical shift for observer protons within the molecule and a magnetic susceptibility inhomogeneity field for observer protons outside the molecule. The number of water protons surrounding a molecule for detecting its magnetic susceptibility is vastly greater than the number of protons within the molecule for detecting its chemical shift. However, the study of tissue magnetic susceptibility has been hindered by poor molecular specificities of hitherto used methods based on MRI signal phase and T2* contrast, which depend convolutedly on surrounding susceptibility sources. Deconvolution of the MRI signal phase can determine tissue susceptibility but is challenged by the lack of MRI signal in the background and by the zeroes in the dipole kernel. Recently, physically meaningful regularizations, including the Bayesian approach, have been developed to enable accurate quantitative susceptibility mapping (QSM) for studying iron distribution, metabolic oxygen consumption, blood degradation, calcification, demyelination, and other pathophysiological susceptibility changes, as well as contrast agent biodistribution in MRI. This paper attempts to summarize the basic physical concepts and essential algorithmic steps in QSM, to describe clinical and technical issues under active development, and to provide references, codes, and testing data for readers interested in QSM.

Enhancing the detection of BOLD signal in fMRI by reducing the partial volume effect

Purpose. To investigate the advantages of reducing the partial volume effect (PVE) to enhance the detection of the BOLD signal in fMRI. Methods. A linear phase term was added in k-space to obtain half-voxel shifting of 64 × 64 T₂*-weighted echo-planar images. Three sets of image data shifted in the x, y, and diagonal direction, respectively, are combined with the original 64 × 64 data to form the 128 × 128 voxel-shifted interpolated data. Results. A simulation of a synthetic fMRI dataset shows that the voxel-shifted interpolation (VSI) can increase the t-score up to 50% in single-voxel activations. An fMRI study (n = 7) demonstrates that 20.4% of the interpolated voxels have higher t-scores than their nearest neighboring voxels in the original maps. The average increase of the t-score in these interpolated voxels is 13.3%. Conclusion. VSI yields increased sensitivity in detecting voxel-size BOLD activations, improved spatial accuracy of activated regions, and improved detection of the peak BOLD signal of an activated region. VSI can potentially be used as an alternative to the high-resolution fMRI studies in which reduction in SNR and increase in imaging time become prohibitive.

Voxel spread function method for correction of magnetic field inhomogeneity effects in quantitative gradient-echo-based MRI

PURPOSE

Macroscopic magnetic field inhomogeneities adversely affect different aspects of MRI images. In quantitative MRI when the goal is to quantify biological tissue parameters, they bias and often corrupt such measurements. The goal of this article is to develop a method for correction of macroscopic field inhomogeneities that can be applied to a variety of quantitative gradient-echo-based MRI techniques.

METHODS

We have reanalyzed a basic theory of gradient echo MRI signal formation in the presence of background field inhomogeneities and derived equations that allow for correction of magnetic field inhomogeneity effects based on the phase and magnitude of gradient echo data. We verified our theory by mapping effective transverse relaxation rate in computer simulated, phantom, and in vivo human data collected with multi-gradient echo sequences.

RESULTS

The proposed technique takes into account voxel spread function effects and allowed obtaining virtually free from artifacts effective transverse relaxation rate maps for all simulated, phantom and in vivo data except of the edge areas with very steep field gradients.

CONCLUSION

The voxel spread function method, allowing quantification of tissue specific effective transverse relaxation rate-related tissue properties, has a potential to breed new MRI biomarkers serving as surrogates for tissue biological properties similar to longitudinal and transverse relaxation rate constants widely used in clinical and research MRI.

Improved susceptibility weighted imaging method using multi-echo acquisition

PURPOSE

To introduce novel acquisition and postprocessing approaches for susceptibility weighted imaging (SWI) to remove background field inhomogeneity artifacts in both magnitude and phase data.

METHODS

The proposed method acquires three echoes in a three-dimensional gradient echo (GRE) sequence, with a field compensation gradient (z-shim gradient) applied to the third echo. The artifacts in the magnitude data are compensated by signal estimation from all three echoes. The artifacts in phase signals are removed by modeling the background phase distortions using Gaussians. The method was applied in vivo and compared with conventional SWI.

RESULTS

The method successfully compensates for background field inhomogeneity artifacts in magnitude and phase images, and demonstrated improved SWI images. In particular, vessels in frontal lobe, which were not observed in conventional SWI, were identified in the proposed method.

CONCLUSION

The new method improves image quality in SWI by restoring signal in the frontal and temporal regions.

Accuracy limits for the thickness measurement of the hip joint cartilage in 3-D MR images: simulation and validation

This paper describes a theoretical simulation method for ascertaining the inherent limits on the accuracy of thickness measurement of hip joint cartilage in 3-D MR images. This method can specify where and how thickness can be measured with sufficient accuracy under the certain MR imaging conditions. In the numerical simulation, we present a mathematical model for two adjacent sheet structures separated by a small distance, which simulated the femoral and acetabular cartilage and the joint space width in the hip joint; moreover, we perform the numerical simulation of MR imaging and postprocessing for thickness measurement. We especially focused on the effects of voxel anisotropy in MR imaging with variable orientation of cartilage surface and different joint space width. Also, thickness measurement is performed in MR imaging with isotropic voxel. The results from MR data with isotropic voxels show that accurate measurement of cartilage thickness at location of measured values of the hip joint space width and the cartilage thickness being two times as large as the voxel size or above should be possible. The simulation method is validated by comparison with the actual results obtained from the experiments using three phantoms, five normal cadaver hip specimens, and nine patients with osteoarthritis.

Improved T2 mapping accuracy with dual-echo turbo spin echo: effect of phase encoding profile orders

Turbo spin echo (TSE) pulse sequences have been applied to estimate T(2) relaxation times in clinically feasible scan times. However, T(2) estimations using TSE pulse sequences has been shown to differ considerable from reference standard sequences due to several sources of error. The purpose of this work was to apply voxel-sensitivity formalism to correct for one such source of error introduced by differing phase encoding profile orders with dual-echo TSE pulse sequences. The American College of Radiology phantom and the brains of two healthy volunteers were imaged using dual-echo TSE as well as 32-echo spin-echo acquisitions and T(2) estimations from uncorrected and voxel-sensitivity formalism-corrected dual-echo TSE and 32-echo acquisitions were compared. In all regions of the brain and the majority of the analyses of the American College of Radiology phantom, voxel-sensitivity formalism correction resulted in considerable improvements in dual-echo TSE T(2) estimation compared with the 32-echo acquisition, with improvements in T(2) value accuracy ranging from 5.2% to 18.6%.

R*(2) mapping in the presence of macroscopic B₀ field variations

R₂ mapping has important applications in MRI, including functional imaging, tracking of super-paramagnetic particles, and measurement of tissue iron levels. However, R₂ measurements can be confounded by several effects, particularly the presence of fat and macroscopic B₀ field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting R₂ maps and make the estimated R₂ values dependent on the acquisition parameters. In this study, we develop a complex-reconstruction, confounder-corrected R₂ mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift-encoded methods for R₂, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets.

The effects of spatial sampling choices on MR temperature measurements

The purpose of this article is to quantify the effects that spatial sampling parameters have on the accuracy of magnetic resonance temperature measurements during high intensity focused ultrasound treatments. Spatial resolution and position of the sampling grid were considered using experimental and simulated data for two different types of high intensity focused ultrasound heating trajectories (a single point and a 4-mm circle) with maximum measured temperature and thermal dose volume as the metrics. It is demonstrated that measurement accuracy is related to the curvature of the temperature distribution, where regions with larger spatial second derivatives require higher resolution. The location of the sampling grid relative temperature distribution has a significant effect on the measured values. When imaging at 1.0 × 1.0 × 3.0 mm(3) resolution, the measured values for maximum temperature and volume dosed to 240 cumulative equivalent minutes (CEM) or greater varied by 17% and 33%, respectively, for the single-point heating case, and by 5% and 18%, respectively, for the 4-mm circle heating case. Accurate measurement of the maximum temperature required imaging at 1.0 × 1.0 × 3.0 mm(3) resolution for the single-point heating case and 2.0 × 2.0 × 5.0 mm(3) resolution for the 4-mm circle heating case.

The kynurenine pathway in adolescent depression: preliminary findings from a proton MR spectroscopy study

BACKGROUND

Cytokine induction of the enzyme indoleamine 2,3-dioxygenase (IDO) has been implicated in the development of major depressive disorder (MDD). IDO metabolizes tryptophan (TRP) into kynurenine (KYN), thereby decreasing TRP availability to the brain. KYN is further metabolized into several neurotoxins. The aims of this pilot were to examine possible relationships between plasma TRP, KYN, and 3-hydroxyanthranilic acid (3-HAA, neurotoxic metabolite) and striatal total choline (tCho, cell membrane turnover biomarker) in adolescents with MDD. We hypothesized that MDD adolescents would exhibit: i) positive correlations between KYN and 3-HAA and striatal tCho and a negative correlation between TRP and striatal tCho; and, ii) the anticipated correlations would be more pronounced in the melancholic subtype group.

METHODS

Fourteen adolescents with MDD (seven with melancholic features) and six healthy controls were enrolled. Minimums of 6 weeks MDD duration and a severity score of 40 on the Children's Depression Rating Scale-Revised were required. All were scanned at 3T with MRI, multi-voxel 3-dimensional, high, 0.75 cm(3), spatial resolution proton magnetic resonance spectroscopic imaging. Striatal tCho concentrations were assessed using phantom replacement. Spearman correlation coefficients were Bonferroni-corrected.

RESULTS

Positive correlations were found only in the melancholic group, between KYN and 3-HAA and tCho in the right caudate (r=0.93, p=0.03) and the left putamen (r=0.96, p=.006), respectively.

CONCLUSIONS

These preliminary findings suggest a possible role of the KYN pathway in adolescent melancholic MDD. Larger studies should follow.

Isolated kidney phantom for development of biothermal vascular models with application to high intensity focused ultrasound therapy

A methodology using magnetic resonance angiography (MRA) is presented for identifying thermally significant blood vessels in isolated kidneys, specifically for use in biothermal model development with application to high intensity focused ultrasound (HIFU). A combination of a proven preservation technique, newly developed MR-compatible experimental procedures and the refinement of MR pulse sequence parameters was used to determine vascular characteristics using high-resolution three-dimensional time-of-flight MRA image of flow through isolated kidneys. Results presented are twofold. First, improved vessel visibility was attained through decreasing the magnetic resonance imaging bandwidth from 150 to 30 Hz/pixel while simultaneously increasing the echo time, repetition time, and flip angle; vascular center line extraction showed an 18% improvement in the number of vessel segments detected and a 23% increase in length of the terminal segments over a base line technique without improvements. Second, the overall system was shown to be practical to determine vascular flow effects during HIFU heating; testing results from heating the kidney with HIFU are presented, showing a decrease of average kidney temperature with an increase of flow rate through the kidney with localized cooling demonstrated surrounding known vessel locations.

Hip cartilage thickness measurement accuracy improvement

Accurate measurement of the distance separating two adjacent sheet structures, such as femoral cartilage and acetabular cartilage in the hip joint is important in evaluation of osteoarthritis. A new method, insensitive to the influence of adjacent sheet structures, was developed to improve the accuracy of hip cartilage thickness measurement. A theoretical simulation for investigating the influence of adjacent sheet structures on the accuracy of cartilage thickness measurement in MR images was performed. The thickness is defined as the distance between zero-crossings of the second directional derivatives along the sheet surface normal direction. The simulation measurement showed considerable underestimation in thickness measurement occurred due to the influence of the adjacent sheet. A new method based on a model of the MR imaging process to eliminate the influence of adjacent sheet structure was developed and tested using phantoms and two cadaveric human hip joint MR scans. The new method reduced the influence of the adjacent sheet structure was more accurate than the conventional method for measuring hip cartilage thickness.

Rapid cardiac-output measurement with ungated spiral phase contrast

An ungated spiral phase-contrast (USPC) method was used to measure cardiac output (CO) rapidly and conveniently. The USPC method, which was originally designed for small peripheral vessels, was modified to assess CO by measuring flow in the ascending aorta (AA). The modified USPC used a 12-interleaf spiral trajectory to acquire full-image data every 283 ms with 2-mm spatial resolution. The total scan time was 5 s. For comparison, a triggered real-time (TRT) method was used to indirectly calculate CO by measuring left-ventricular (LV) volume. The USPC and TRT measurements from all normal volunteers agreed. In a patient with patent ductus arteriosus (PDA), high CO was measured with USPC, which agreed well with the invasive cardiac-catheterized measurement. In normal volunteers, CO dropped about 20-30% with Valsalva maneuvering, and increased about 100% after exercise. Continuous 28-s cycling between Valsalva maneuvering and free-breathing showed that USPC can temporally resolve physiological CO changes.

Reproducibility of 3D free-breathing magnetic resonance coronary vessel wall imaging

AIMS

Although the coronary artery vessel wall can be imaged non-invasively using magnetic resonance imaging (MRI), the in vivo reproducibility of wall thickness measures has not been previously investigated. Using a refined magnetization preparation scheme, we sought to assess the reproducibility of three-dimensional (3D) free-breathing black-blood coronary MRI in vivo.

METHODS AND RESULTS

MRI vessel wall scans parallel to the right coronary artery (RCA) were obtained in 18 healthy individuals (age range 25-43, six women), with no known history of coronary artery disease, using a 3D dual-inversion navigator-gated black-blood spiral imaging sequence. Vessel wall scans were repeated 1 month later in eight subjects. The visible vessel wall segment and the wall thickness were quantitatively assessed using a semi-automatic tool and the intra-observer, inter-observer, and inter-scan reproducibilities were determined. The average imaged length of the RCA vessel wall was 44.5+/-7 mm. The average wall thickness was 1.6+/-0.2 mm. There was a highly significant intra-observer (r=0.97), inter-observer (r=0.94), and inter-scan (r=0.90) correlation for wall thickness (all P<0.001). There was also a significant agreement for intra-observer, inter-observer, and inter-scan measurements on Bland-Altman analysis. The intra-class correlation coefficients for intra-observer (r=0.97), inter-observer (r=0.92), and inter-scan (r=0.86) analyses were also excellent.

CONCLUSION

The use of black-blood free-breathing 3D MRI in conjunction with semi-automated analysis software allows for reproducible measurements of right coronary arterial vessel-wall thickness. This technique may be well-suited for non-invasive longitudinal studies of coronary atherosclerosis.

A comparison of T2*-weighted magnitude and phase imaging for measuring the arterial input function in the rat aorta following intravenous injection of gadolinium contrast agent

The arterial input function (AIF) is important for quantitative MR imaging perfusion experiments employing Gd contrast agents. This study compared the accuracy of T(2)*-weighted magnitude and phase imaging for noninvasive measurement of the AIF in the rat aorta. Twenty-eight in vivo experiments were performed involving simultaneous arterial blood sampling and MR imaging following Gd injection. In vitro experiments were also performed to confirm the in vivo results. At 1.89 T and TE=3 ms, the relationship between changes in 1/T(2)* in blood (estimated from MR signal magnitude) and Gd concentration ([Gd]) was measured to be approximately 19 s(-1) mM(-1), while that between phase and [Gd] was approximately 0.19 rad mM(-1). Both of these values are consistent with previously published results. The in vivo phase data had approximately half as much scatter with respect to [Gd] than the in vivo magnitude data (r(2)=.34 vs. r(2)=.17, respectively). This is likely due to the fact that the estimated change in 1/T(2)* is more sensitive than the phase to a variety of factors such as partial volume effects and T(1) weighting. Therefore, this study indicates that phase imaging may be a preferred method for measuring the AIF in the rat aorta compared to T(2)*-weighted magnitude imaging.

Zero-Filling Interpolation Processing Technique Can Efficiently Suppress Segmental Stenosis Artifact on Small Arteries of Anatomical Phantoms

The purpose of this study was to evaluate the zero-filling interpolation processing (ZIP) technique for contrast-enhanced MR angiography (CE-MRA). Phantoms of arteries were made with different lumen diameters. Gadolinium-enhanced three-dimensional MR angiography was performed on a GE 1.5T scanner. The parameters of FSPGR pulse sequence were: flip angle 30°, TR 6ms, TE 1.4ms, bandwidth 31.25 kHz, slice thickness 1.2 mm, matrix 256×256. The sequence parameters were kept constant for the studies, whereas four selections were chosen: 1) with ZIP1024 and ZIP×4 techniques; 2) only with ZIP1024 technique; 3) only with ZIP×4 technique; 4) without ZIP technique. For image quality evaluation, MR maximum intensity projection (MIP) images were created. Signal-to-noise ratio (SNR) was measured on MIP images. Vessel edge was determined using full width at half maximum (FWHM) for lumen diameter calculation and results were compared with the actual lumen diameter. The distinctness of the vessel edge and the artifacts on the phantoms were compared for all sequences. Three experienced radiologists made consensus evaluation. The FWHM results of lumen measurements for all the sequences with ZIP techniques were more accurate than that of the sequence without ZIP technique in all phantoms, no matter what the size of the artery. The vessel edge with ZIP1024 technique was more distinct. However, the highest average SNR was obtained with the sequence without ZIP technique. The segmental stenosis artifacts on small artery of phantoms were only efficiently suppressed with the ZIP×4 technique. ZIP technique is excellent for CE-MRA to obtain high quality MR angiography. It not only can improve the spatial resolution and the distinctness of the vessel edge on CE-MRA, but can also efficiently suppress segmental stenosis artifact on small arteries of phantoms.

Resolution-insensitive velocity and flow rate measurement in low-background phase-contrast MRA

Magnetic resonance (MR) phase-contrast (PC) flow measurements are degraded by partial volume errors when the spatial resolution is low, in particular when a large difference in signal magnitude exists between the fluid and the surrounding material. The latter is often the case in phantom studies and may be encountered when flow is measured in prosthetic vessel segments (such as shunts, grafts, and bypasses) and in contrast-enhanced blood. This paper presents a new method that is designed to measure flow in vessels of circular cross-section with Poiseuille flow and negligible background signal arising from static material around the lumen. The method calculates the average flow velocity directly from the original complex image data by integrating the signal in oppositely velocity-sensitized PC images. The radius is calculated from the summed signal modulus. The method allows accurate and resolution-insensitive measurements of the average flow velocity to be obtained in both cross-sectional and in-plane acquisitions. It is not critical to any of the assumed conditions. The validity and capabilities of the proposed technique are demonstrated by in vitro experiments.

Limits on the accuracy of 3-D thickness measurement in magnetic resonance images--effects of voxel anisotropy

Measuring the thickness of sheet-like thin anatomical structures, such as articular cartilage and brain cortex, in three-dimensional (3-D) magnetic resonance (MR) images is an important diagnostic procedure. This paper investigates the fundamental limits on the accuracy of thickness determination in MR images. We defined thickness here as the distance between the two sides of boundaries measured at the subvoxel resolution, which are the zero-crossings of the second directional derivatives combined with Gaussian blurring along the normal directions of the sheet surface. Based on MR imaging and computer postprocessing parameters, characteristics for the accuracy of thickness determination were derived by a theoretical simulation. We especially focused on the effects of voxel anisotropy in MR imaging with variable orientation of sheet-like structure. Improved and stable accuracy features were observed when the standard deviation of Gaussian blurring combined with thickness determination processes was around square root of 2/2 times as large as the pixel size. The relation between voxel anisotropy in MR imaging and the range of sheet normal orientation within which acceptable accuracy is attainable was also clarified, based on the dependences of voxel anisotropy and the sheet normal orientation obtained by numerical simulations. Finally, in vitro experiments were conducted using an acrylic plate phantom and a resected femoral head to validate the results of theoretical simulation. The simulated thickness was demonstrated to be well-correlated with the actual in vitro thickness.

The need for phase-encoding flow compensation in high-resolution intracranial magnetic resonance angiography

PURPOSE

To demonstrate that the time delay between phase and frequency encoding and the presence of pulsatile blood flow in high-resolution time-of-flight (TOF) imaging of the intracranial arteries (especially near the circle of Willis) can distort the appearance of blood vessels and result in a cross-hatch-appearing artifact in surrounding tissue.

MATERIALS AND METHODS

Two techniques to reduce the artifact, tri-directional flow compensation (3DFC) and elliptical-centric (EC) phase-encoding order, are investigated in five volunteer studies.

RESULTS

3DFC eliminates the pulsation-related artifacts and the vessel distortion. A residual amplitude variation artifact is observed. EC phase encoding nearly eliminates the pulsatile motion-related artifact, but it does not eliminate vessel distortion.

CONCLUSION

The combination of 3DFC and EC phase encoding appears to provide the greatest artifact reduction in the five volunteer studies performed.

High-resolution 3D T2-weighted fast spin echo: new applications in the orbit

Recent developments have made available for ophthalmologic MR imaging a very high-resolution 3D fast spin echo T2 (3D FSE T2) sequence, which runs in a standard head coil. A modification of this technique, 3D FSEz T2, uses a zero-filled slice interpolation method during post-processing to further improve spatial resolution. We describe the technique and share our early clinical observations in patients with ocular masses. Briefly, the additional information from the 3D FSEz T2 resulted in a change in diagnosis from the conventional imaging series in 11 of (41%) 27 studies, usually through the identification of previously treated retinoblastoma lesions. The new sequence significantly increased diagnostic confidence in six (38%) of the remaining 16 cases, usually through better anatomical detail and lesion conspicuity, and did not change interpretation in 10 cases. Such an approach improves diagnostic confidence and may eliminate the need for a dedicated surface coil examination.

Improved depiction of small anatomic structures in MR images using Gaussian-weighted spirals and zero-filled interpolation

Partial-volume artifacts reduce the contrast and continuity of small structures in magnetic resonance images. Zero-filled interpolation (ZFI) has been known for some time as a useful technique to reduce partial-volume artifacts and improve the appearance of small structures and edges. However, its use is limited by the fact that ZFI can exacerbate image artifacts. For example, it can exacerbate Gibbs ringing, also known as the truncation artifact, which manifests itself as spurious ringing around sharp edges. Currently, the most common technique to address this problem is post-acquisition filtering, which causes blurring in the image. Using ZFI in conjunction with a variable-density sampling method designed to reduce ringing is proposed as a possible solution to this problem. This approach is demonstrated with a Gaussian-weighted spiral and is compared to conventional spiral sampling both with and without the application of a filter used to reduce ringing. The two spiral sampling techniques are compared using simulations, phantom images, and in vivo brain images. The Gaussian-weighted spiral demonstrates reduced ringing without the loss of spatial resolution commonly associated with post-acquisition filtering. Additionally, this sampling technique is shown to work well in conjunction with ZFI to reduce partial-volume artifacts without the apparent increase in Gibbs ringing usually associated with zero-filled reconstruction. This approach will be most useful for imaging techniques such as MR angiography which are known to be sensitive to partial-volume effects, as well as when imaging anatomic regions associated with more severe Gibbs ringing.

Real-time MR image acquisition during high-dose dobutamine hydrochloride stress for detecting left ventricular wall-motion abnormalities in patients with coronary arterial disease

PURPOSE

To compare the accuracy of real-time magnetic resonance (MR) imaging with that of standard echo-planar MR imaging for detecting myocardial wall-motion abnormalities at rest and during dobutamine hydrochloride-induced stress in patients with coronary arterial disease.

MATERIALS AND METHODS

In 22 patients with coronary arterial disease, left ventricular wall motion was examined at rest and during dobutamine hydrochloride stress, by using echo-planar MR imaging and a new technique with real-time segmented k-space turbo gradient-echo echo-planar MR imaging (repetition time, 16.5 msec; echo time, 6.8 msec). Wall-motion abnormalities were determined visually for each perfusion territory, and Cohen kappa coefficients were calculated for real-time imaging in comparison with echo-planar imaging. Coronary angiography was performed in all patients. Sensitivity and specificity for real-time and echo-planar imaging were calculated for detecting significant coronary arterial stenosis.

RESULTS

kappa values for detecting wall-motion abnormalities at real-time imaging, in comparison with echo-planar MR imaging, were 0.97 at rest and 0.94 at maximum dobutamine hydrochloride stress. At comparison with those of angiography, the sensitivity and specificity for detecting significant coronary arterial stenosis were 88% (14 of 16 patients) and 83% (five of six patients), respectively, for echo-planar imaging and 81% (13 of 16 patients) and 83% (five of six patients), respectively, for real-time imaging.

CONCLUSION

Real-time MR imaging is possible under stress conditions and allows accurate detection of wall-motion abnormalities.

Correction of concomitant magnetic field-induced image artifacts in nonaxial echo-planar imaging

Echo-planar images acquired in nonaxial planes are often distorted. Such image distortion has limited the applications of the echo-planar imaging (EPI) technique. In this article, it is demonstrated that a considerable amount of the distortion is caused by the higher-order magnetic field concomitant with the linear magnetic field gradient, or the concomitant magnetic field. The image distortion caused by the concomitant magnetic field is more prominent when a higher gradient amplitude is used for readout. It is also shown that the concomitant magnetic field can cause ghosting and blurring. A theoretical analysis is performed for the concomitant field effect in nonaxial EPI images. A point-by-point (or line-by-line) phase correction algorithm is developed to correct the image distortion, ghosting, and blurring. A postreconstruction processing algorithm is also developed to correct image distortion with much higher computational efficiency. Experimental results show that both correction methods effectively reduce the image distortion in coronal or sagittal images.

Three-dimensional black-blood cardiac magnetic resonance coronary vessel wall imaging detects positive arterial remodeling in patients with nonsignificant coronary artery disease

BACKGROUND

Direct noninvasive visualization of the coronary vessel wall may enhance risk stratification by quantifying subclinical coronary atherosclerotic plaque burden. We sought to evaluate high-resolution black-blood 3D cardiovascular magnetic resonance (CMR) imaging for in vivo visualization of the proximal coronary artery vessel wall.

METHODS AND RESULTS

Twelve adult subjects, including 6 clinically healthy subjects and 6 patients with nonsignificant coronary artery disease (10% to 50% x-ray angiographic diameter reduction) were studied with the use of a commercial 1.5 Tesla CMR scanner. Free-breathing 3D coronary vessel wall imaging was performed along the major axis of the right coronary artery with isotropic spatial resolution (1.0x1.0x1.0 mm(3)) with the use of a black-blood spiral image acquisition. The proximal vessel wall thickness and luminal diameter were objectively determined with an automated edge detection tool. The 3D CMR vessel wall scans allowed for visualization of the contiguous proximal right coronary artery in all subjects. Both mean vessel wall thickness (1.7+/-0.3 versus 1.0+/-0.2 mm) and wall area (25.4+/-6.9 versus 11.5+/-5.2 mm(2)) were significantly increased in the patients compared with the healthy subjects (both P<0.01). The lumen diameter (3.6+/-0.7 versus 3.4+/-0.5 mm, P=0.47) and lumen area (8.9+/-3.4 versus 7.9+/-3.5 mm(2), P=0.47) were similar in both groups.

CONCLUSIONS

Free-breathing 3D black-blood coronary CMR with isotropic resolution identified an increased coronary vessel wall thickness with preservation of lumen size in patients with nonsignificant coronary artery disease, consistent with a "Glagov-type" outward arterial remodeling. This novel approach has the potential to quantify subclinical disease.

T1 measurements in cell cultures: a new tool for characterizing contrast agents at 1.5T

The objective of this work was to assess the feasibility and accuracy of T1 and relaxivity measurements in cell cultures using 1.5T magnetic resonance imaging (MRI) with the long-term goal to develop a tool for evaluation of novel paramagnetic agents in a realistic macromolecular environment. This initial study was carried out using MCF-7 cells treated with independently determined concentrations of Gd-DTPA. Two cell culture systems were evaluated: cell pellets and single layers of cells grown on microporous inserts. High-resolution T1 measurements of cell cultures were acquired with two dimensional Inversion Recovery Fast Spin Echo (2D-IR-FSE), three dimensional Inversion Recovery Fast Spin Echo (3D-IR-FSE), and 3D-SPGR sequences. The T1 and relaxivity accuracy of these sequences was confirmed with aqueous Gd-DTPA samples of known concentration. Relaxivities of 1.71 +/- 0.15 [mM(-1)second(-1)] and 1.55 +/- 0.50 [mM(-1)second(-1)] were measured in the cell pellets and cell monolayers, respectively, and were different from the value of 4.3 [mM(-1)second(-1)] for Gd-DTPA in water. Both cell pellets and monolayers are suitable for initial assessment of novel MR contrast agents.

Effect of windowing and zero-filled reconstruction of MRI data on spatial resolution and acquisition strategy

Standard, MR spin-warp sampling strategies acquire data on a rectangular k-space grid. That method samples data from the "corners" of k-space, i.e., data that lie in a region of k-space outside of an ellipse just inscribed in the rectangular boundary. Illustrative calculations demonstrate that the data in the corners of k-space contribute to the useful resolution only if an interpolation method such as a zero-filled reconstruction is used. The consequences of this finding on data acquisition and data windowing strategies are discussed. A further implication of this result is that the spatial resolution of images reconstructed with zero-filling (but without radial windowing) is expected to display angular dependence, even when the phase- and frequency-encoded resolutions are identical. This hypothesis is experimentally verified with a slit geometry phantom. It is also observed that images reconstructed without zero-filling do not display the angular dependence of spatial resolution predicted solely by the maximal k-space extent of the raw data. The implications of these results for 3D contrast-enhanced angiographic acquisitions with elliptical centric view ordering are explored with simulations.

Magnetic resonance flow measurements in real time: comparison with a standard gradient-echo technique

Ultrafast gradient systems and hybrid imaging sequences offer the opportunity to acquire phase contrast flow data in real time. In a 1.5-Tesla magnetic resonance (MR)-tomograph, peak velocity and volume flow were assessed in 36 large vessels (aorta) and 33 medium-sized vessels (carotid and iliac artery) using a real-time (segmented k-space turbo gradient-echo planar imaging sequence) in comparison with a gradient-echo technique. With the real-time technique, the matrix was reduced from 116 to 64, and temporal resolution changed from 30 msec to 124 msec. Measurements of peak velocity correlated in large (r = 0.88) and medium-sized vessels (r = 0.81). Volume flow measurements correlated in large vessels (r = 0.87), however, a poor correlation (r = 0.64) was found in medium-sized vessels. Thus, scan time can be significantly reduced and images acquired without electrocardiogram (ECG)-triggering. Flow volume can only be determined in large vessels with sufficient accuracy, mainly due to reduced spatial resolution in smaller vessels.

Correcting partial volume artifacts of the arterial input function in quantitative cerebral perfusion MRI

To quantify cerebral perfusion with dynamic susceptibility contrast MRI (DSC-MRI), one needs to measure the arterial input function (AIF). Conventionally, one derives the contrast concentration from the DSC sequence by monitoring changes in either the amplitude or the phase signal on the assumption that the signal arises completely from blood. In practice, partial volume artifacts are inevitable because a compromise has to be reached between the temporal and spatial resolution of the DSC acquisition. As the concentration of the contrast agent increases, the vector of the complex blood signal follows a spiral-like trajectory. In the case of a partial-volume voxel, the spiral is located around the static contribution of the surrounding tissue. If the static contribution of the background tissue is disregarded, estimations of the contrast concentration will be incorrect. By optimizing the correspondence between phase information and amplitude information one can estimate the origin of the spiral, and thereupon correct for partial volume artifacts. This correction is shown to be accurate at low spatial resolutions for phantom data and to improve the AIF determination in a clinical example. Magn Reson Med 45:477-485, 2001.

Comparison of magnetic resonance real-time imaging of left ventricular function with conventional magnetic resonance imaging and echocardiography

This study analyzes the accuracy of a new real-time magnetic resonance imaging (MRI) technique (acquisition duration, 62 ms/image) and echocardiography for the determination of left ventricular (LV) end-diastolic volume, end-systolic volume, ejection fraction, and muscle mass when compared with turbo gradient echo imaging as the reference standard. Thirty-four patients were examined with digital echocardiography, standard, and real-time MRI. A close correlation was found between the results of real-time imaging and the reference standard for end-diastolic volume, end-systolic volume, and ejection fraction (r >0.95), with a lower correlation for LV muscle mass (r = 0.81). Correlations between echocardiography and the reference standard were lower for all parameters. Real-time MRI enables the acquisition of high-quality cine loops of the entire heart in minimal time without electrocardiographic triggering or breath holding. Thus, patient setup and scan time can be reduced considerably. Results are similar to the reference standard and superior to echocardiography for determining LV volumes and ejection fraction. This technique is a valid alternative to current approaches and can form the basis of every cardiac MRI examination.

Comparison of three-dimensional fast spin echo and gradient echo sequences for high-resolution temporal bone imaging

T2-weighted high-resolution gradient and fast spin echo sequences are widely used as an alternative or adjunct to contrast-enhanced T1-weighted temporal bone imaging. However, to date no systematic comparison has been presented. The purpose of this work is to identify optimal acquisition parameters and to compare volume gradient and fast spin echo techniques. Signal intensities and scan efficiency were computed for gradient echo segment-interleaved motion-compensated acquisition into steady state (SIMCAST), standard fast spin echo (FSE), and fast recovery fast spin echo (FR-FSE). Computations were compared with inner ear images acquired with cubic voxel sizes of 0.35-0.40 mm(3)in 5-8 minutes. Given otherwise identical conditions, the FR-FSE sequence produces images with improved SNR in shorter scan times than standard FSE. For FR-FSE, the scan efficiency is optimal for specific pairs of TR and echo train length, eg, 400 ms/8, 735 ms/16, and 2,050 ms/48. FR-FSE images with large TR and echo trains, while achieving better SNR, are severely compromised by blurring. Imaging with echo train lengths of 16-24 and TR of 800-1,200 ms is a good compromise, and FR-FSE signal-to-noise ratio (SNR) and scan efficiency become comparable to SIMCAST. In vivo image quality is excellent with both FR-FSE and SIMCAST, but SIMCAST images have slightly higher SNR and are significantly more crisp. J. Magn. Reson. Imaging 2000;12:814-825.

Enhanced image detail using continuity in the MIP Z-buffer: applications to magnetic resonance angiography

In this paper a new algorithm is presented for the segmentation and display of blood vessels from images obtained with magnetic resonance angiography (MRA) and other three-dimensional (3D) angiographic imaging techniques. The algorithm developed is based on the observation that vessels are strongly evident in the maximum intensity projection (MIP) Z-buffer as regions of high continuity and low local roughness. Roughness is measured here by the minimum chi2 value of a low-order local least-squares fit in the principal directions through each point in the MIP Z-buffer. Although some background pixels in the Z-buffer exhibit low local roughness, the size of the connected region is nearly always much smaller than even the very smallest vessels that appear in the MIP image. It is shown that by applying connectivity to the regions of low roughness, there is nearly complete separation between vascular detail and background. When connectivity is further applied in the original 3D image, vascular bed segmentation becomes nearly complete. The algorithm consists of three basic steps: a) determination of the minimum local roughness at each point in the MIP Z-buffer; b) connection of all neighboring points of low local roughness; and c) connection of all points in the original 3D image matrix that are connected to the points determined in the MIP Z-buffer and that are above an intensity threshold. The algorithm as presented is not optimized but demonstrates a very strong potential for improved portrayal of vascular detail.

A median filter for 3D FAST spin echo black blood images of cerebral vessels

High-resolution black-blood MRA images of intracranial vascular anatomy can be obtained using 3D fast spin-echo techniques. Although these images demonstrate excellent contrast between vessels and surrounding soft tissues, the dark signal from air and bone can obscure the desired vascular information when a minimum intensity projection image is created. In this paper, we describe an image processing technique based upon a median filter that is effective for detecting narrow vessel-like structures. Minimum intensity projection images of the filtered MRA volume can be obtained in any orientation without prior segmentation of the skull or surrounding air spaces. The filter is very effective for detecting and visualizing small vessels, but is much less effective for detecting vessels and vascular pathology larger than the filter detection width. The filtering technique is demonstrated on black-blood MRA images from a volunteer study.