Phase-derivative analysis in MR angiography: reduced Venc dependency and improved vessel wall detection in laminar and disturbed flow

Quantification of susceptibility change at high-concentrated SPIO-labeled target by characteristic phase gradient recognition

Phase map cross-correlation detection and quantification may produce highlighted signal at superparamagnetic iron oxide nanoparticles, and distinguish them from other hypointensities. The method may quantify susceptibility change by performing least squares analysis between a theoretically generated magnetic field template and an experimentally scanned phase image. Because characteristic phase recognition requires the removal of phase wrap and phase background, additional steps of phase unwrapping and filtering may increase the chance of computing error and enlarge the inconsistence among algorithms. To solve problem, phase gradient cross-correlation and quantification method is developed by recognizing characteristic phase gradient pattern instead of phase image because phase gradient operation inherently includes unwrapping and filtering functions. However, few studies have mentioned the detectable limit of currently used phase gradient calculation algorithms. The limit may lead to an underestimation of large magnetic susceptibility change caused by high-concentrated iron accumulation. In this study, mathematical derivation points out the value of maximum detectable phase gradient calculated by differential chain algorithm in both spatial and Fourier domain. To break through the limit, a modified quantification method is proposed by using unwrapped forward differentiation for phase gradient generation. The method enlarges the detectable range of phase gradient measurement and avoids the underestimation of magnetic susceptibility. Simulation and phantom experiments were used to quantitatively compare different methods. In vivo application performs MRI scanning on nude mice implanted by iron-labeled human cancer cells. Results validate the limit of detectable phase gradient and the consequent susceptibility underestimation. Results also demonstrate the advantage of unwrapped forward differentiation compared with differential chain algorithms for susceptibility quantification at high-concentrated iron accumulation.

Total atherosclerotic burden by whole body magnetic resonance angiography predicts major adverse cardiovascular events

OBJECTIVE

The purpose of the present study was to investigate the relationship between the Total Atherosclerotic Score (TAS), a measurement of the overall atherosclerotic burden of the arterial tree by whole body magnetic resonance angiography (WBMRA), and the risk of major adverse cardiovascular events (MACE), defined as cardiac death, myocardial infarction, stroke and/or coronary revascularization, assuming that TAS predicts MACE.

METHODS AND RESULTS

305 randomly selected 70 year-old subjects (47% women) underwent WBMRA. Their atherosclerotic burden was evaluated and TAS > 0, that is atherosclerotic changes, were found in 68% of subjects. During follow-up (mean 4.8 years), MACE occurred in 25 subjects (8.2%). Adjusting for multiple risk factors, TAS was associated with MACE (OR 8.86 for any degree of vessel lumen abnormality, 95%CI 1.14-69.11, p = 0.037). In addition, TAS improved discrimination and reclassification when added to the Framingham risk score (FRS), and ROC (Receiver Operator Curve) increased from 0.681 to 0.750 (p = 0.0421).

CONCLUSION

In a population-based sample of 70 year old men and women WBMRA, with TAS, predicted MACE independently of major cardiovascular risk factors.

Positive contrast technique for the detection and quantification of superparamagnetic iron oxide nanoparticles in MRI

In vivo detection and quantification of cells labeled with superparamagnetic iron oxide (SPIO) nanoparticles has been attracting increasing attention. In particular, positive contrast methods, such as susceptibility gradient mapping (SGM) and phase gradient mapping (PGM), have been proposed for the improved detection of SPIO nanoparticles. In this study, a different implementation of the PGM method is introduced; it calculates the phase gradient in the image space using a fast Fourier transform without the need for phase unwrapping. We first compared positive contrast generation between the PGM and SGM methods, which estimates the susceptibility gradient in k space through echo shift measurements. Next, PGM was applied to quantify SPIO concentrations by fitting the resulting phase gradient maps to those of a theoretical model. MR experiments were conducted using a 3-T magnet scanner to acquire two datasets: the first was acquired from a gelatin phantom with three SPIO-doped vials of different concentrations, and the second was obtained in vivo from a nude rat with SPIO-labeled C6 glioma cells implanted in the flanks. The sensitivity of the PGM and SGM methods was compared using various factors, including different SPIO concentrations, TEs and signal-to-noise ratios. Based on the theoretical model of an infinite cylinder, the results demonstrated that, without loss of spatial resolution, the PGM method presents positive contrast maps with a higher sensitivity than SGM at medium and low SPIO concentrations, whereas SGM is more sensitive than PGM at longer TEs. The quantification of SPIO concentrations using the phantom dataset was also reported. On the basis of the same infinite cylinder model, it was shown that the PGM method provides an accurate estimation of SPIO concentration.

Quantification of SPIO nanoparticles in vivo using the finite perturber method

The susceptibility gradients generated by super-paramagnetic iron oxide (SPIO) nanoparticles make them an ideal contrast agent in magnetic resonance imaging. Traditional quantification methods for SPIO nanoparticle-based contrast agents rely on either mapping T₂* values within a region or by modeling the magnetic field inhomogeneities generated by the contrast agent. In this study, a new model-based SPIO quantification method is introduced. The proposed method models magnetic field inhomogeneities by approximating regions containing SPIOs as ensembles of magnetic dipoles, referred to as the finite perturber method. The proposed method was verified using data acquired from a phantom and in vivo mouse models. The phantom consisted of an agar solution with four embedded vials, each vial containing known but different concentrations of SPIO nanoparticles. Gaussian noise was also added to the phantom data to test performance of the proposed method. The in vivo dataset was acquired using five mice, each of which was subcutaneously implanted in the flanks with 1 × 10(5) labeled and 1 × 10(6) unlabeled C6 glioma cells. For the phantom data set, the proposed algorithm was generate accurate estimations of the concentration of SPIOs. For the in vivo dataset, the method was able to give estimations of the concentration within SPIO-labeled tumors that are reasonably close to the known concentration.

Recursive approach to the moment-based phase unwrapping method

The moment-based phase unwrapping algorithm approximates the phase map as a product of Gegenbauer polynomials, but the weight function for the Gegenbauer polynomials generates artificial singularities along the edge of the phase map. A method is presented to remove the singularities inherent to the moment-based phase unwrapping algorithm by approximating the phase map as a product of two one-dimensional Legendre polynomials and applying a recursive property of derivatives of Legendre polynomials. The proposed phase unwrapping algorithm is tested on simulated and experimental data sets. The results are then compared to those of PRELUDE 2D, a widely used phase unwrapping algorithm, and a Chebyshev-polynomial-based phase unwrapping algorithm. It was found that the proposed phase unwrapping algorithm provides results that are comparable to those obtained by using PRELUDE 2D and the Chebyshev phase unwrapping algorithm.

Cyclic motion encoding for enhanced MR visualization of slip interfaces

PURPOSE

To develop and test a magnetic resonance imaging-based method for assessing the mechanical shear connectivity across tissue interfaces with phantom experiments and in vivo feasibility studies.

MATERIALS AND METHODS

External vibrations were applied to phantoms and tissue and the differential motion on either side of interfaces within the media was mapped onto the phase of the MR images using cyclic motion encoding gradients. The phase variations within the voxels of functional slip interfaces reduced the net magnitude signal in those regions, thus enhancing their visualization. A simple two-compartment model was developed to relate this signal loss to the intravoxel phase variations. In vivo studies of the abdomen and forearm were performed to visualize slip interfaces in healthy volunteers.

RESULTS

The phantom experiments demonstrated that the proposed technique can assess the functionality of shear slip interfaces and they provided experimental validation for the theoretical model developed. Studies of the abdomen showed that the slip interface between the small bowel and the peritoneal wall can be visualized. In the forearm, this technique was able to depict the slip interfaces between the functional compartments of the extrinsic forearm muscles.

CONCLUSION

Functional shear slip interfaces can be visualized sensitively using cyclic motion encoding of externally applied tissue vibrations.

Magnetic resonance imaging in neonatal stroke

Neonatal stroke occurs in 1 in 2300-5000 live births, the incidence of which is lower than that in adults, but still higher than that in childhood. The higher incidence of perinatal stroke in preterm and term infants compared to stroke in childhood may be partly explained by higher detection rates using routine fetal ultrasound and postnatal cranial sonography. In addition, there is greater availability of magnetic resonance imaging (MRI) for neuroimaging in preterm and full-term infants, which is due in part to the availability of MR-compatible incubators and MR systems at or near the neonatal intensive care unit. In addition, the wide range of MR techniques, such as T2-, diffusion- and susceptibility-weighted imaging allows improved visualization and quantification of neonatal stroke or hypoxic-ischemic injury. This chapter reviews the MR neuroimaging modalities that actually assist the clinician in the detection of neonatal stroke.

Quantification of SPIO nanoparticles using phase gradient mapping

A new method is developed to quantify the concentration of super-paramagnetic iron oxide (SPIO) contrast agent using magnetic resonance imaging (MRI). The proposed method utilizes a positive contrast method, known as phase gradient mapping (PGM), to find the gradient of the field map. Then the concentration is calculated by fitting the gradient of the field map to the gradient of an ideal geometric model. The proposed method was compared to relaxivity-based SPIO quantification method and was applied to calculate the concentration of SPIO contrast agent for MRI experiments performed on a phantom with known concentrations. The results obtained from the proposed method accord well with the true concentrations.

Phase gradient mapping as an aid in the analysis of object-induced and system-related phase perturbations in MRI

In this note we wish to demonstrate the utility of phase gradient mapping (PGM) as an aid in the analysis and characterization of object-induced and system-related macroscopic phase perturbations in MRI. To achieve this goal, phase gradient maps and, if applicable, field gradient maps were derived from standard phase images via a forward difference operator taking into account phase wraps. By way of phantom experiments, PGM was shown to provide reliable phase and field gradient information, even in regions with multiple phase wraps. Phase gradient mapping was further shown to allow positive identification of local phase and field perturbations and global discrimination between positive and negative local susceptibility deviations. The suitability of PGM for in vivo studies was demonstrated by a 3D brain examination of a healthy volunteer.

Magnetic resonance evaluation of the cerebral circulation in obstructive arterial disease

BACKGROUND

The aim of the current overview is to highlight the possibilities of magnetic resonance imaging (MRI) in the assessment of patients with obstructive arterial disease. The anatomic and hemodynamic aspects of the extra- and intracranial cerebral circulation were analyzed and show the importance of combining both aspects in studying cerebral hemodynamic changes.

RESULTS

Three levels of cerebral circulation are distinguished: blood flow to the brain (level 1); the distribution of blood flow in the brain (level 2), and finally perfusion of the brain (level 3). To investigate the anatomy of the arteries in the neck and the circle of Willis, contrast-enhanced, time-of-flight and phase contrast MR angiography (MRA) are available. To evaluate the hemodynamics at the 1st and 2nd level of the cerebral circulation two-dimensional phase contrast (volume flow and flow direction) MRA can be used. In addition, the distribution of blood via the circle of Willis can be visualized with dynamic MRA. At the 3rd level, measurements of regional brain perfusion can be obtained by injecting gadolinium, dynamic susceptibility contrast MRI, or noninvasively with arterial spin labeling (ASL) MRI. In addition, selective ASL MRI is able to evaluate the perfused territories of individual brain-feeding arteries.

CONCLUSION

The currently available MR techniques allow evaluation of the cerebral circulation from the aortic arch upwards towards the microvasculature and brain tissue perfusion in a comprehensive 20-min protocol. The combined use of the described MR methods in patients with steno-occlusive disease will further clarify the pathophysiological relations between the vasculature, perfusion and brain function.

Gadolinium-enhanced Magnetic Resonance Angiography, Colour Duplex and Digital Subtraction Angiography of the Lower Limb Arteries from the Aorta to the Tibio-peroneal Trunk in Patients with Intermittent Claudication

OBJECTIVES

To evaluate the sensitivity, specificity, positive and negative predictive value of contrast-enhanced (gadolinium) magnetic resonance imaging (CE-MRA) and colour duplex ultrasound (CDU) of lower limb arteries.

DESIGN

Prospective, single centre study.

MATERIAL AND METHODS

A consecutive series of 58 patients with intermittent claudication (IC) were examined with CE-MRA and CDU from the infrarenal aorta to the tibio-peroneal trunk with digital subtraction angiography (DSA) as reference. The arterial tree was divided into 15 segments, pooled into three regions; suprainguinal, thigh and knee. Sensitivity, specificity, positive and negative predictive values for significant obstructions were calculated. Cohen Kappa statistics was used to establish agreement between the three methods.

RESULTS

The sensitivity (specificity in parentheses) for significant obstructions in the suprainguinal region were 96% (94%) for CE-MRA and 91% (96%) for CDU, in the thigh region 92% (95%) for CE-MRA and 76% (99%) for CDU, and in the knee region 93% (96%) for CE-MRA and 33% (98%) for CDU. CDU failed to visualize 10% of suprainguinal, 2% of thigh and 13% of knee-region arterial segments.

CONCLUSIONS

Both CE-MRA and CDU are good alternatives to DSA in the suprainguinal- and thigh-region. In the knee region only CE-MRA can be relied upon as an alternative to DSA. Imaging by CDU is not suited to situations were evaluation of runoff vessels is important.

Shear stiffness estimation using intravoxel phase dispersion in magnetic resonance elastography

Dynamic MR elastography (MRE) is a phase-contrast technique in which the periodic shear motion of an object is encoded as variations in the phase of the reconstructed images. An alternative MRE method is presented whereby waves are depicted as intensity variations in the magnitude images due to intravoxel phase dispersion (IVPD). A theoretical framework is developed to model how the IVPD magnitude data are related to the underlying shear wave motion, and how they can be used to estimate shear stiffness. The results are shown in a series of phantom experiments to demonstrate that IVPD MRE complements phase-contrast MRE.

An adaptive segmentation algorithm for time-of-flight MRA data

A three-dimensional (3-D) representation of cerebral vessel morphology is essential for neuroradiologists treating cerebral aneurysms. However, current imaging techniques cannot provide such a representation. Slices of MR angiography (MRA) data can only give two-dimensional (2-D) descriptions and ambiguities of aneurysm position and size arising in X-ray projection images can often be intractable. To overcome these problems, we have established a new automatic statistically based algorithm for extracting the 3-D vessel information from time-of-flight (TOF) MRA data. We introduce distributions for the data, motivated by a physical model of blood flow, that are used in a modified version of the expectation maximization (EM) algorithm. The estimated model parameters are then used to classify statistically the voxels into vessel or other brain tissue classes. The algorithm is adaptive because the model fitting is performed recursively so that classifications are made on local subvolumes of data. We present results from applying our algorithm to several real data sets that contain both artery and aneurysm structures of various sizes.

MR phase-contrast flow measurement with limited spatial resolution in small vessels: value of model-based image analysis

Magnetic resonance phase-contrast volume flow rate (VFR) measurement with limited resolution in small vessels is subject to two major sources of error: a) partial volume artifacts, causing systematic overestimation of the VFR, and b) errors related to the selection of vessel pixels [region of interest (ROI)], causing large inter-observer and intra-observer variability. Additionally, limited resolution results in Gibbs-ringing around vessels, which adversely affects VFR determination. In this paper, a semi-automatic model-based method is presented that effectively eliminates errors due to both partial volume effect and Gibbs-ringing and also minimizes errors from variability in the ROI selection. The model assumes a parabolic flow profile and cylindrical vessel geometry, incorporates inflow effects, and takes into account the point-spread function of the acquisition. The method automatically estimates maximum velocity, vessel radius, and VFR. The method is validated in phantoms under various conditions and evaluated in vivo. For small vessels with moderately pulsatile flow, it is demonstrated that accurate VFRs and diameter estimates are obtained, virtually independent of the ROI selection, even in vessels covered by just a few pixels. Compared with conventional VFR analysis, both accuracy and reproducibility improve significantly.

Limits to the accuracy of vessel diameter measurement in MR angiography

This work addresses the fundamental limits imposed by the MRI process on the accuracy with which vessel diameters and cross-sectional areas can be derived from time-of-flight (TOF) and phase-contrast (PC) MR source images. By means of simulations and in vitro experiments, it is demonstrated that, even in the absence of flow-related artifacts, severe inaccuracies in the determination of diameters or cross-sectional areas may occur solely because of the physical process of the MR image acquisition. Resolution and intraluminal saturation have strong effects on the vessel appearance and thus on the diameter estimation error. It is shown that low resolution leads to diameter overestimation or even underestimation and that intraluminal saturation causes severe underestimation, even for relatively low flip angles. Velocity and velocity encoding do not have a major influence on lumen appearance in PC images. Accurate diameter estimations can be attained only if lumen diameters constitute at least three pixels for both TOF and PC acquisitions, provided that intraluminal saturation is suppressed or avoided. Additionally, since the constitution of TOF and PC images is dissimilar, lumina should be analyzed differently to obtain accurate diameters and cross-sectional areas.