Mapping complex tissue architecture with diffusion spectrum magnetic resonance imaging.

Methods are presented to map complex fiber architectures in tissues by imaging the 3D spectra of tissue water diffusion with MR. First, theoretical considerations show why and under what conditions diffusion contrast is positive. Using this result, spin displacement spectra that are conventionally phase-encoded can be accurately reconstructed by a Fourier transform of the measured signal's modulus. Second, studies of in vitro and in vivo samples demonstrate correspondence between the orientational maxima of the diffusion spectrum and those of the fiber orientation density at each location. In specimens with complex muscular tissue, such as the tongue, diffusion spectrum images show characteristic local heterogeneities of fiber architectures, including angular dispersion and intersection. Cerebral diffusion spectra acquired in normal human subjects resolve known white matter tracts and tract intersections. Finally, the relation between the presented model-free imaging technique and other available diffusion MRI schemes is discussed.

Coronary magnetic resonance angiography.

Coronary magnetic resonance angiography (MRA) is a powerful noninvasive technique with high soft-tissue contrast for the visualization of the coronary anatomy without X-ray exposure. Due to the small dimensions and tortuous nature of the coronary arteries, a high spatial resolution and sufficient volumetric coverage have to be obtained. However, this necessitates scanning times that are typically much longer than one cardiac cycle. By collecting image data during multiple RR intervals, one can successfully acquire coronary MR angiograms. However, constant cardiac contraction and relaxation, as well as respiratory motion, adversely affect image quality. Therefore, sophisticated motion-compensation strategies are needed. Furthermore, a high contrast between the coronary arteries and the surrounding tissue is mandatory. In the present article, challenges and solutions of coronary imaging are discussed, and results obtained in both healthy and diseased states are reviewed. This includes preliminary data obtained with state-of-the-art techniques such as steady-state free precession (SSFP), whole-heart imaging, intravascular contrast agents, coronary vessel wall imaging, and high-field imaging. Simultaneously, the utility of electron beam computed tomography (EBCT) and multidetector computed tomography (MDCT) for the visualization of the coronary arteries is discussed.

Noninvasive coronary vessel wall and plaque imaging with magnetic resonance imaging.

BACKGROUND: Conventional x-ray angiography frequently underestimates the true burden of atherosclerosis. Although intravascular ultrasound allows for imaging of coronary plaque, this invasive technique is inappropriate for screening or serial examinations. We therefore sought to develop a noninvasive free-breathing MR technique for coronary vessel wall imaging. We hypothesized that such an approach would allow for in vivo imaging of coronary atherosclerosis. METHODS AND RESULTS: Ten subjects, including 5 healthy adult volunteers (aged 35+/-17 years, range 19 to 56 years) and 5 patients (aged 60+/-4 years, range 56 to 66 years) with x-ray-confirmed coronary artery disease (CAD), were studied with a T2-weighted, dual-inversion, fast spin-echo MR sequence. Multiple adjacent 5-mm cross-sectional images of the proximal right coronary artery were obtained with an in-plane resolution of 0.5x1.0 mm. A right hemidiaphragmatic navigator was used to facilitate free-breathing MR acquisition. Coronary vessel wall images were readily acquired in all subjects. Both coronary vessel wall thickness (1.5+/-0.2 versus 1.0+/-0.2 mm) and wall area (21.2+/-3.1 versus 13.7+/-4.2 mm(2)) were greater in patients with CAD (both P:<0.02 versus healthy adults). CONCLUSIONS: In vivo free-breathing coronary vessel wall and plaque imaging with MR has been successfully implemented in humans. Coronary wall thickness and wall area were significantly greater in patients with angiographic CAD. The presented technique may have potential applications in patients with known or suspected atherosclerotic CAD or for serial evaluation after pharmacological intervention.

Continuous arterial spin labeling of mouse cerebral blood flow using an actively-detuned two-coil system at 9.4T.

Among numerous magnetic resonance imaging (MRI) techniques, perfusion MRI provides insight into the passage of blood through the brain's vascular network non-invasively. Studying disease models and transgenic mice would intrinsically help understanding the underlying brain functions, cerebrovascular disease and brain disorders. This study evaluates the feasibility of performing continuous arterial spin labeling (CASL) on all cranial arteries for mapping murine cerebral blood flow at 9.4 T. We showed that with an active-detuned two-coil system, a labeling efficiency of 0.82 ± 0.03 was achieved with minimal magnetization transfer residuals in brain. The resulting cerebral blood flow of healthy mouse was 99 ± 26 mL/100g/min, in excellent agreement with other techniques. In conclusion, high magnetic fields deliver high sensitivity and allowing not only CASL but also other MR techniques, i.e. (1)H MRS and diffusion MRI etc, in studying murine brains.

Artifact-free coronary magnetic resonance angiography and coronary vessel wall imaging in the presence of a new, metallic, coronary magnetic resonance imaging stent.

BACKGROUND: Coronary in-stent restenosis cannot be directly assessed by magnetic resonance angiography (MRA) because of the local signal void of currently used stainless steel stents. The aim of this study was to investigate the potential of a new, dedicated, coronary MR imaging (MRI) stent for artifact-free, coronary MRA and in-stent lumen and vessel wall visualization. METHODS AND RESULTS: Fifteen prototype stents were deployed in coronary arteries of 15 healthy swine and investigated with a double-oblique, navigator-gated, free-breathing, T2-prepared, 3D cartesian gradient-echo sequence; a T2-prepared, 3D spiral gradient-echo sequence; and a T2-prepared, 3D steady-state, free-precession coronary MRA sequence. Furthermore, black-blood vessel wall imaging by a dual-inversion-recovery, turbo spin-echo sequence was performed. Artifacts of the stented vessel segment and signal intensities of the coronary vessel lumen inside and outside the stent were assessed. With all investigated sequences, the vessel lumen and wall could be visualized without artifacts, including the stented vessel segment. No signal intensity alterations inside the stent when compared with the vessel lumen outside the stent were found. CONCLUSIONS: The new, coronary MRI stent allows for completely artifact-free coronary MRA and vessel wall imaging.

MP2RAGE Multiple Sclerosis Magnetic Resonance Imaging at 3 T.

OBJECTIVES: Lesion detection and characterization in multiple sclerosis (MS) are an essential part of its clinical diagnosis and an important research field. In this pilot study, we applied the recently introduced two inversion-contrast magnetization-prepared rapid gradient echo sequence (MP2RAGE) to patients with early-stage MS.¦MATERIALS AND METHODS: The MP2RAGE is a 3-dimensional (3D) magnetization-prepared rapid gradient echo derivative providing homogeneous T1 weighting and simultaneous T1 mapping. The MP2RAGE performance was compared with that of 2 clinical routine sequences (2D fluid-attenuated inversion recovery [FLAIR] and 3D magnetization-prepared rapid gradient echo [MP-RAGE]) and 2 state-of-the art clinical research sequences (the 3D FLAIR-SPACE [sampling perfection with application-optimized contrasts by using different flip-angle evolutions], a fluid-attenuated variable flip-angle fast spin echo technique, and the 3D double-inversion recovery SPACE). A cohort of 10 early-stage female MS patients (age, 31.6 ± 4.7 years; disease duration, 3.8 ± 1.9 years; median expanded disability status scale score, 1.75) and 10 age- and gender-matched controls were enrolled after approval of the local institutional review board was obtained. Multiple sclerosis lesions were identified and assigned to brain locations and tissue types by two experienced physicians in all 5 contrasts. Subsequently, lesions were manually delineated for comparison and statistical analysis of lesion count, volume and quantitative measures.¦RESULTS AND CONCLUSIONS: The results show that the 3D T1-weighted high-resolution MP2RAGE contrast provides a sensitive means for MS lesion assessment. The additional quantitative T1 relaxation time maps obtained with the MP2RAGE provide further potential diagnostic and prognostic information that could help (a) to better discriminate lesion subtypes and (b) to stage and predict the activity and the evolution of MS. Results also indicate that the T2-weighted double-inversion recovery and FLAIR-SPACE contrasts are attractive complements to the MP2RAGE for lesion detection.

Renal arteries: navigator-gated balanced fast field-echo projection MR angiography with aortic spin labeling: initial experience.

A cardiac-triggered free-breathing three-dimensional balanced fast field-echo projection magnetic resonance (MR) angiographic sequence with a two-dimensional pencil-beam aortic labeling pulse was developed for the renal arteries. For data acquisition during free breathing in eight healthy adults and seven consecutive patients with renal artery disease, real-time navigator technology was implemented. This technique allows high-spatial-resolution and high-contrast renal MR angiography and visualization of renal artery stenosis without exogenous contrast agent or breath hold. Initial promising results warrant larger clinical studies.

Free-breathing renal MR angiography with steady-state free-precession (SSFP) and slab-selective spin inversion: initial results.

BACKGROUND: The aim of our study was the investigation of a novel navigator-gated three-dimensional (3D) steady-state free-precession (SSFP) sequence for free-breathing renal magnetic resonance angiography (MRA) without contrast medium, and to examine the advantage of an additional inversion prepulse for improved contrast. METHODS: Eight healthy volunteers (mean age 29 years) and eight patients (mean age 53 years) were investigated on a 1.5 Tesla MR system (ACS-NT, Philips, Best, The Netherlands). Renal MRA was performed using three navigator-gated free-breathing cardiac-triggered 3D SSFP sequences [repetition time (TR) = 4.4 ms, echo time (TE) = 2.2 ms, flip angle 85 degrees, spatial resolution 1.25 x 1.25 x 4.0 mm(3), scanning time approximately 1 minute 30 seconds]. The same sequence was performed without magnetization preparation, with a non-slab selective and a slab-selective inversion prepulse. Signal-to-noise ratio (SNR), contrast-to-noise (CNR) vessel length, and subjective image quality were compared. RESULTS: Three-dimensional SSFP imaging combined with a slab-selective inversion prepulse enabled selective and high contrast visualization of the renal arteries, including the more distal branches. Standard SSFP imaging without magnetization preparation demonstrated overlay by veins and renal parenchyma. A non-slab-selective prepulse abolished vessel visualization. CNR in SSFP with slab-selective inversion was 43.6 versus 10.6 (SSFP without magnetization preparation) and 0.4 (SSFP with non-slab-selective inversion), P < 0.008. CONCLUSION: Navigator-gated free-breathing cardiac-triggered 3D SSFP imaging combined with a slab-selective inversion prepulse is a novel, fast renal MRA technique without the need for contrast media.

High-resolution three-dimensional aortic magnetic resonance angiography and quantitative vessel wall characterization of different atherosclerotic stages in a rabbit model

PURPOSE: Atherosclerosis results in a considerable medical and socioeconomic impact on society. We sought to evaluate novel magnetic resonance imaging (MRI) angiography and vessel wall sequences to visualize and quantify different morphologic stages of atherosclerosis in a Watanabe hereditary hyperlipidemic (WHHL) rabbit model. MATERIAL AND METHODS: Aortic 3D steady-state free precession angiography and subrenal aortic 3D black-blood fast spin-echo vessel wall imaging pre- and post-Gadolinium (Gd) was performed in 14 WHHL rabbits (3 normal, 6 high-cholesterol diet, and 5 high-cholesterol diet plus endothelial denudation) on a commercial 1.5 T MR system. Angiographic lumen diameter, vessel wall thickness, signal-/contrast-to-noise analysis, total vessel area, lumen area, and vessel wall area were analyzed semiautomatically. RESULTS: Pre-Gd, both lumen and wall dimensions (total vessel area, lumen area, vessel wall area) of group 2 + 3 were significantly increased when compared with those of group 1 (all P < 0.01). Group 3 animals had significantly thicker vessel walls than groups 1 and 2 (P < 0.01), whereas angiographic lumen diameter was comparable among all groups. Post-Gd, only diseased animals of groups 2 + 3 showed a significant (>100%) signal-to-noise ratio and contrast-to-noise increase. CONCLUSIONS: A combination of novel 3D magnetic resonance angiography and high-resolution 3D vessel wall MRI enabled quantitative characterization of various atherosclerotic stages including positive arterial remodeling and Gd uptake in a WHHL rabbit model using a commercially available 1.5 T MRI system.

Local re-inversion coronary MR angiography: arterial spin-labeling without the need for subtraction.

PURPOSE: To implement a double-inversion bright-blood coronary MR angiography sequence using a cylindrical re-inversion prepulse for selective visualization of the coronary arteries. MATERIALS AND METHODS: Local re-inversion bright-blood magnetization preparation was implemented using a nonselective inversion followed by a cylindrical aortic re-inversion prepulse. After an inversion delay that allows for in-flow of the labeled blood-pool into the coronary arteries, three-dimensional radial steady-state free-precession (SSFP) imaging (repetition/echo time, 7.2/3.6 ms; flip angle, 120 degrees, 16 profiles per RR interval; field of view, 360 mm; matrix, 512, twelve 3-mm slices) is performed. Coronary MR angiography was performed in three healthy volunteers and in one patient on a commercial 1.5 Tesla whole-body MR System. RESULTS: In all subjects, coronary arteries were selectively visualized with positive contrast. In addition, a middle-grade stenosis of the proximal right coronary artery was seen in one patient. CONCLUSION: A novel T1 contrast-enhancement strategy is presented for selective visualization of the coronary arteries without extrinsic contrast medium application. In comparison to former arterial spin-labeling schemes, the proposed magnetization preparation obviates the need for a second data set and subtraction.

Is magnetic resonance imaging of hepatic hemangioma any different in liver fibrosis and cirrhosis compared to normal liver?

PURPOSE: To compare qualitative and quantitative magnetic resonance (MR) imaging characteristics of hepatic hemangiomas in patients with normal, fibrotic and cirrhotic livers. MATERIALS AND METHODS: Retrospective, institutional review board approved study (waiver of informed consent). Eighty-nine consecutive patients with 231 hepatic hemangiomas who underwent liver MR imaging for lesion characterization were included. Lesions were classified into three groups according to the patients' liver condition: no underlying liver disease (group 1), fibrosis (group 2) and cirrhosis (group 3). Qualitative and quantitative characteristics (number, size, signal intensities on T1-, T2-, and DW MR images, T2 shine-through effect, enhancement patterns (classical, rapidly filling, delayed filling), and ADC values) were compared. RESULTS: There were 160 (69%), 45 (20%), and 26 (11%) hemangiomas in groups 1, 2 and 3, respectively. Lesions were larger in patients with normal liver (group 1 vs. groups 2 and 3; P=.009). No difference was found between the groups on T2-weighted images (fat-suppressed fast spin-echo (P=.82) and single-shot (P=.25)) and in enhancement patterns (P=.56). Mean ADC values of hemangiomas were similar between groups 1, 2 and 3 (2.11±.52×10(-3)mm(2)/s, 2.1±.53×10(-3)mm(2)/s and 2.14±.44×10(-3)mm(2)/s, P=87, respectively). T2 shine-through effect was less frequently observed in cirrhosis (P=.02). CONCLUSION: MR imaging characteristics of hepatic hemangioma were similar in patients with normal compared to fibrotic and cirrhotic livers. Smaller lesion size was observed with liver disease and less T2 shine-through effect was seen in hemangiomas developed on cirrhosis, the latter being an important finding to highlight in these patients at risk of developing hepatocellular carcinoma.

Nonenhanced arterial spin labeled carotid MR angiography using three-dimensional radial balanced steady-state free precession imaging.

PURPOSE: To optimize and preliminarily evaluate a three-dimensional (3D) radial balanced steady-state free precession (bSSFP) arterial spin labeled (ASL) sequence for nonenhanced MR angiography (MRA) of the extracranial carotid arteries. MATERIALS AND METHODS: The carotid arteries of 13 healthy subjects and 2 patients were imaged on a 1.5 Tesla MRI system using an undersampled 3D radial bSSFP sequence providing a scan time of ∼4 min and 1 mm(3) isotropic resolution. A hybridized scheme that combined pseudocontinuous and pulsed ASL was used to maximize arterial coverage. The impact of a post label delay period, the sequence repetition time, and radiofrequency (RF) energy configuration of pseudocontinuous labeling on the display of the carotid arteries was assessed with contrast-to-noise ratio (CNR) measurements. Faster, higher undersampled 2 and 1 min scans were tested. RESULTS: Using hybridized ASL MRA and a 3D radial bSSFP trajectory, arterial CNR was maximized with a post label delay of 0.2 s, repetition times ≥ 2.5 s (P < 0.05), and by eliminating RF energy during the pseudocontinuous control phase (P < 0.001). With higher levels of undersampling, the carotid arteries were displayed in ≤ 2 min. CONCLUSION: Nonenhanced MRA using hybridized ASL with a 3D radial bSSFP trajectory can display long lengths of the carotid arteries with 1 mm(3) isotropic resolution. J. Magn. Reson. Imaging 2015;41:1150-1156. © 2014 Wiley Periodicals, Inc.

Fast spin echo imaging of carotid artery dynamics.

PURPOSE: We propose the use of a retrospectively gated cine fast spin echo (FSE) sequence for characterization of carotid artery dynamics. The aim of this study was to compare cine FSE measures of carotid dynamics with measures obtained on prospectively gated FSE images. METHODS: The common carotid arteries in 10 volunteers were imaged using two temporally resolved sequences: (i) cine FSE and (ii) prospectively gated FSE. Three raters manually traced a common carotid artery area for all cardiac phases on both sequences. Measured areas and systolic-diastolic area changes were calculated and compared. Inter- and intra-rater reliability were assessed for both sequences. RESULTS: No significant difference between cine FSE and prospectively gated FSE areas were observed (P = 0.36). Both sequences produced repeatable cross-sectional area measurements: inter-rater intraclass correlation coefficient (ICC) = 0.88 on cine FSE images and 0.87 on prospectively gated FSE images. Minimum detectable difference (MDD) in systolic-diastolic area was 4.9 mm(2) with cine FSE and 6.4 mm(2) with prospectively gated FSE. CONCLUSION: This cine FSE method produced repeatable dynamic carotid artery measurements with less artifact and greater temporal efficiency compared with prospectively gated FSE. Magn Reson Med 74:1103-1109, 2015. © 2014 Wiley Periodicals, Inc.

Fat Suppression with Dixon Techniques in Musculoskeletal Magnetic Resonance Imaging: A Pictorial Review.

Dixon techniques are part of the methods used to suppress the signal of fat in MRI. They present many advantages compared with other fat suppression techniques including (1) the robustness of fat signal suppression, (2) the possibility to combine these techniques with all types of sequences (gradient echo, spin echo) and different weightings (T1-, T2-, proton density-, intermediate-weighted sequences), and (3) the availability of images both with and without fat suppression from one single acquisition. These advantages have opened many applications in musculoskeletal imaging. We first review the technical aspects of Dixon techniques including their advantages and disadvantages. We then illustrate their applications for the imaging of different body parts, as well as for tumors, neuromuscular disorders, and the imaging of metallic hardware.