Refocusing Flip Angle Research Articles

An efficient 3D stack-of-stars turbo spin echo pulse sequence for simultaneous T2-weighted imaging and T2 mapping.

To design a pulse sequence for efficient 3D T2-weighted imaging and T2 mapping. A stack-of-stars turbo spin echo pulse sequence with variable refocusing flip angles and a flexible pseudorandom view ordering is proposed for simultaneous T2-weighted imaging and T2 mapping. An analytical framework is introduced for the selection of refocusing flip angles to maximize relative tissue contrast while minimizing T2 estimation errors and maintaining low specific absorption rate. Images at different echo times are generated using a subspace constrained iterative reconstruction algorithm. T2 maps are obtained by modeling the signal evolution using the extended phase graph model. The technique is evaluated using phantoms and demonstrated in vivo for brain, knee, and carotid imaging. Numerical simulations demonstrate an improved point spread function with the proposed pseudorandom view ordering compared to golden angle view ordering. Phantom experiments show that T2 values estimated from the stack-of-stars turbo spin echo pulse sequence with variable refocusing flip angles have good concordance with spin echo reference values. In vivo results show the proposed pulse sequence can generate qualitatively comparable T2-weighted images as conventional Cartesian 3D SPACE in addition to simultaneously generating 3D T2 maps. The proposed stack-of-stars turbo spin echo pulse sequence with pseudorandom view ordering and variable refocusing flip angles allows high resolution isotropic T2 mapping in clinically acceptable scan times. The optimization framework for the selection of refocusing flip angles improves T2 estimation accuracy while generating T2-weighted contrast comparable to conventional Cartesian imaging.

Optimization of Fat Suppression Technique and Imaging Parameters for MR Neurography Using 3D Turbo Spin Echo with Variable Refocusing Flip Angle at 3.0 T: Visualization of Brachial Plexus

Magnetic resonance neurography (MRN) has been used to evaluate abnormal conditions of entire nerves and nerve bundles. A fat-suppressed 3D turbo spin echo (TSE) sequence is one of the imaging techniques for MRN, which has been widely adopted at 1.5 T. However, MRN of the brachial plexus using a 3D TSE sequence with short-term inversion recovery (STIR) reduces the effect of fat suppression at 3.0 T. Moreover, the use of spectral pre-saturation with inversion recovery (SPIR) does not result in uniform fat suppression due to the inhomogeneity of the static magnetic field. On the other hand, it is well known that the visibility of the brachial plexus using a 3D TSE sequence greatly changes with the equivalent echo time (TEequiv). Therefore, we optimized the fat suppression technique and TEequiv so that the 3D TSE sequence, using a combination of STIR with SPIR and an optimal TEequiv (from 73 to 110 ms), achieved better visualization of the brachial plexus without residual fat.

Rapid and accurate dictionary-based T2 mapping from multi-echo turbo spin echo data at 7 Tesla.

Using lower refocusing flip angles in multi-echo turbo spin echo (ME-TSE) sequences at ultra-high magnetic field leads to non-monoexponential signal decay and overestimation of T2 values due to stimulated and secondary echoes. To investigate the feasibility of a fast and accurate reconstruction of quantitative T2 values using an ME-TSE sequence with reduced refocusing flip angles at 7 Tesla, a dictionary-based reconstruction method was developed and is presented in this work. Prospective. Phantom measurements with relaxation phantom, four healthy volunteers. 7 Tesla MRI, multi-echo turbo spin echo (ME-TSE), spin echo (SE), and B1 mapping. Based on Bloch simulations and the extended phase graph model, signal decay curves were calculated to account for nonrectangular slice profile, B1 inhomogeneity, and reduced refocusing flip angles and stored in a dictionary. Data obtained with an ME-TSE sequence at 7 Tesla were matched to this dictionary to obtain T2 values. To compare the proposed method to reference T2 values, a spin echo sequence with different echo times was used. Welch's t-test was used to compare T2 values in phantom measurements. T2 values obtained with the proposed ME-TSE method coincided with the T2 values from the spin echo experiment in phantom measurements (P = 0.89 for 120° flip angle, P = 0.75 for 180° flip angle). Only for very low B1 transmit fields, a slight overestimation of T2 values was observed. In vivo measurements showed lower T2 values in gray matter (55 ± 2 millisecond) and white matter (39 ± 5 millisecond) compared with literature values of 3 Tesla data. The proposed dictionary-based ME-TSE approach provided accurate T2 values in short measurement time at 7 Tesla with low specific absorption rate burden due to the reduction of refocusing flip angles. Therefore, it can provide new opportunities in clinical high-field MRI to further improve radiographic diagnosis by using quantitative imaging. 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1253-1262.

Clinical Utility of a Novel Ultrafast T2-Weighted Sequence for Spine Imaging.

TSE-based T2-weighted imaging of the spine has long scan times. This work proposes a fast imaging protocol using variable refocusing flip angles, optimized for blurring and specific absorption rate. A variable refocusing flip angle echo-train was optimized for the spine to improve the point spread function and minimize the specific absorption rate, yielding images with improved spatial resolution and SNR compared with the constant flip angle sequence. Data were acquired from 51 patients (35 lumbar, 16 whole-spine) using conventional TSE and the proposed sequence, with a single-shot variant for whole-spine. Noninferiority analysis was performed to evaluate the efficiency of the proposed technique. The proposed multishot sequence resulted in a 2× shorter scan time with a >1.5× lower specific absorption rate. The variable flip angle sequence was noninferior to the conventional TSE (P < .025) for all image-quality and clinical criteria except signal-to-noise ratio for the lumbar spine protocol. However, mean image scores for the TSE-variable refocusing flip angle were ≥4.3 for all criteria, and concordance analysis showed high agreement (>90%) with the TSE, indicating clinical equivalence. The single-shot sequence resulted in 4× shorter whole-spine scans, and image scores were ≥4.4 for all criteria, attesting to its clinical utility. We present a fast T2-weighted spine protocol using variable refocusing flip angles, including a single-shot variant. The sequences have better point spread function behavior than their constant flip angle counterparts and, being faster, should be less sensitive to patient motion, often seen in the longer TSE scans.

Limitations of skipping echoes for exponential T2 fitting.

Exponential fitting of multiecho spin echo sequences with skipped echoes is still commonly used for quantification of transverse relaxation (T2 ). To examine the efficacy of skipped echo methods for T2 quantification against computational modeling of the exact signal decay. Prospective comparison of methods. Eight volunteers were imaged at 4.7T, six volunteers at 1.5T, and phantoms ([MnCl2 ] = 68-270 mM). 1.5T and 4.7T; multiple-echo spin echo. Exponential fitting for T2 using all echoes, skipping the first echo or skipping all odd echoes, compared with Bloch simulations. Resulting T2 values were examined over a range of T2 (10-150 msec), refocusing flip angles (90-270°), and echo train lengths (ETL = 6-32). Shapiro-Wilk tests and Q-Q plots were used to check for normality of data. Paired sample t-tests and Wilcoxon rank tests were used to compare fitting models using α = 0.05. Multiple comparisons were accounted for with Bonferroni correction. In examined regions of interest, typical incorrect estimation of T2 ranged from 23-39% for exponential fitting of all echoes, or 15-32% for skipped echo methods. In vivo, T2 estimation error was reduced to as little as 10% with skipped echo methods using 180° refocusing and ETL = 8, although error varied due to refocusing angle, T2 , and ETL. In vivo, skipped echo T2 values were significantly different than all echo exponential fitting (P < 0.004), but also were significantly different from reference values (P < 0.002, except frontal white matter). Simulations showed skipping the first echo was the most effective form of exponential fitting, in particular for T2 <50 msec and ETL = 8, with potential to reduce T2 errors to 10%, depending on refocusing angle and T2 . Skipping echoes is insufficient for avoiding stimulated echo contamination. Resulting T2 errors depend on a complicated interplay of T2 , refocusing angle, and ETL. Modeling of the multiecho sequence is recommended. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:1432-1440.

Retrospective multi-phase non-contrast-enhanced magnetic resonance angiography (ROMANCE MRA) for robust angiogram separation in the presence of cardiac arrhythmia.

PurposeTo develop a novel, retrospective multi‐phase non‐contrast‐enhanced MRA (ROMANCE MRA) in a single acquisition for robust angiogram separation even in the presence of cardiac arrhythmia.MethodsIn the proposed ROMANCE MRA, data were continuously acquired over all cardiac phases using retrospective, multi‐phase flow‐sensitive single‐slab 3D fast spin echo (FSE) with variable refocusing flip angles, while an external pulse oximeter was in sync with pulse repetitions in FSE to record real‐time information on cardiac cycles. Data were then sorted into k‐bin space using the real‐time cardiac information. Angiograms were reconstructed directly from k‐bin space by solving a constrained optimization problem with both subtraction‐induced sparsity and low rank priors. Peripheral MRA was performed in normal volunteers with/without caffeine consumption and a volunteer with cardiac arrhythmia using conventional fresh blood imaging (FBI) and the proposed ROMANCE MRA for comparison.ResultsThe proposed ROMANCE MRA shows superior performance in accurately delineating both major and small vessel branches with robust background suppression if compared with conventional FBI. Even in the presence of irregular heartbeats, the proposed method exhibits clear depiction of angiograms over conventional methods within clinically reasonable imaging time.ConclusionWe successfully demonstrated the feasibility of the proposed ROMANCE MRA in generating robust angiograms with background suppression.

Controlling T2 blurring in 3D RARE arterial spin labeling acquisition through optimal combination of variable flip angles and k-space filtering.

To improve the SNR efficiency and reduce the T2 blurring of 3D rapid acquisition with relaxation enhancement stack-of-spiral arterial spin labeling imaging by using variable refocusing flip angles and k-space filtering. An algorithm for determining the optimal combination of variable flip angles and filtering correction is proposed. The flip angles are designed using extended phase graph physical simulations in an analytical and global optimization framework, with an optional constraint on deposited power. Optimal designs for correcting to Hann and Fermi window functions were compared with conventional constant amplitude or variable flip angle only designs on 6 volunteers. With the Fermi window correction, the proposed optimal designs provided 39.8 and 27.3% higher SNR (P < .05) than conventional constant amplitude and variable flip angle designs. Even when power deposition was limited to 50% of the constant amplitude design, the proposed method outperformed the SNR (P < .05) of these 2 conventional approaches by 32.5 and 20.7%. The sharpness and the contrast between gray and white matter were improved with the k-space filtering correction for all of the flip angle designs. The improvements were moderate for the Hann window correction. This work demonstrates that variable flip angles can be derived as the output of an optimization problem. The combined design of variable flip angle and k-space filtering provided superior SNR to designs primarily emphasizing either approach singly.

Contrast-Enhanced High-Resolution MRI for Evaluating Time Course Changes in Middle Cerebral Artery Plaques.

It is clinically important to evaluate time course changes in symptomatic middle cerebral artery (MCA) stenotic plaques because of likely recurrence. The objective of this study is to determine whether contrast-enhanced high-resolution magnetic resonance imaging (MRI) is a feasible method for this purpose. Contrast-enhanced, high-resolution, 3D turbo spin-echo images with low refocusing flip angle control (3D LOWRAT) applied to 7 patients with symptomatic MCA stenosis were evaluated at the initial (1 month after stroke onset) and follow-up (7 months after stroke onset) stages, and statistical variables, including plaque-to-thalamus signal intensity ratio, degree of stenosis, and stroke recurrence obtained at the 2 stages, were compared. Stenotic change at the initial stage was compared to that at the follow-up stage using MR angiography. In 4 of the 7 patients, the signal intensity ratio measured at the follow-up stage was lower than that measured at the initial stage and in 1 patient, the stenosis subsequently improved. We used a Chi-Square Test. In the other 3 patients, the signal intensity ratios did not differ between the 2 stages, and ischemic stroke occurred in 2 of these 3 patients. Gadolinium contrast enhancement was found to be useful for effective evaluation of time course changes in the stability of symptomatic MCA stenotic plaques.

Variable refocusing flip angle single-shot fast spin echo imaging of liver lesions: increased speed and lesion contrast.

To evaluate acquisition time and clinical image quality of a variable refocusing flip angle (vrf) single-shot fast spin echo (SSFSE) sequence in comparison with a conventional SSFSE sequence for imaging of liver lesions in patients undergoing whole-body PET/MRI for oncologic staging. A vrfSSFSE sequence was acquired in 43 patients with known pancreatic neuroendocrine tumors undergoing 68Ga-DOTA-TOC PET on a simultaneous time-of-flight 3.0T PET/MRI. Liver lesions≥1.5cm with radionucleotide uptake were analyzed. Contrast-to-noise ratios (CNRs) were measured, and four blinded radiologists assessed overall image quality. Differences in repetition time and CNR were assessed using a paired Student's t test with p<0.05 considered statistically significant. Inter-reader variability was assessed with Fleiss' kappa statistic. 53 eligible lesions in 27 patients were included for analysis. vrfSSFSE demonstrated higher mean lesion CNR compared to SSFSE (9.9±4.1 vs. 6.7±4.1, p<0.001). Mean repetition time (TR) was 679±97ms for the vrfSSFSE sequence compared to 1139±106ms for SSFSE (p<0.0001), corresponding to a 1.7-fold decrease in acquisition time. Overall quality of liver lesion and common bile duct images with the vrfSSFSE sequence was graded as superior than or equivalent to the SSFSE sequence for 59% and 67% of patients, respectively. Compared to conventional SSFSE, vrfSSFSE resulted in improved lesion contrast on simultaneous PET/MRI in patients with liver metastases. Due to decreased SAR demands, vrfSSFSE significantly decreased TR, allowing coverage of the entire liver in a single twenty-second breath hold. This may have important clinical implications in the setting of PET/MRI, where scan time is limited by the necessity of whole-body image acquisition in addition to bed specific imaging.

Slice profile effects on nCPMG SS-FSE.

To determine the effects of the RF refocusing pulse profile on the magnitude of the transverse signal smoothness throughout the echo train in non-Carr-Purcell-Meiboom-Gill (nCPMG) single-shot fast spin echo (SS-FSE) imaging and to design an RF refocusing pulse that provides improved signal stability. THEORY AND METHODS: nCPMG SS-FSE quadratic phase modulation requires sufficiently high and uniform refocusing flip angle to achieve a stable signal. Typically, refocusing pulses used in SS-FSE sequences are designed for minimum duration to minimize echo spacing and as a consequence have poor selectivity. However, delay-insensitive variable rate excitation Shinnar-Le Roux (DV-SLR) refocusing pulses can achieve both improved selectivity as well as a short duration. This class of RF pulse is compared against a traditional low time-bandwidth refocusing pulse in a nCPMG SS-FSE in simulation, phantom, and in vivo. DV-SLR pulses achieve a more stable signal in simulation, phantom, and in vivo cases while maintaining an appropriately short duration as well as not dramatically increasing specific absorption rate (SAR) accumulation. The nCPMG SS-FSE method demonstrates improved robustness when a more selective refocusing pulse is used. Refocusing pulses that use a time-varying excitation gradient can achieve this selectivity while maintaining short echo spacing. Magn Reson Med 79:430-438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Fundamental Study of Three-dimensional Fast Spin-echo Imaging with Spoiled Equilibrium Pulse.

Three-dimensional fast spin-echo (3D FSE) imaging with variable refocusing flip angle has been recently applied to pre- or post-enhanced T1-weighted imaging. To reduce the acquisition time, this sequence requires higher echo train length (ETL), which potentially causes decreased T1 contrast. Spoiled equilibrium (SpE) pulse consists of a resonant +90° radiofrequency (RF) pulse and is applied at the end of the echo train. This +90° RF pulse brings residual transverse magnetization to the negative longitudinal axis, which makes it possible to increase T1 contrast. The purpose of our present study was to examine factors that influence the effect of spoiled equilibrium pulse and the relationship between T1 contrast improvement and imaging parameters and to understand the characteristics of spoiled equilibrium pulse. Phantom studies were conducted using an magnetic resonance imaging (MRI) phantom made of polyvinyl alcohol gel. To evaluate the effect of spoiled equilibrium pulse with changes in repetition time (TR), ETL, and refocusing flip angle, we measured the signal-to-noise ratio and contrast-to-noise ratio (CNR). The effect of spoiled equilibrium pulse was evaluated by calculating the enhancement rate of CNR. The factors that influence the effect of spoiled equilibrium pulse are TR, ETL, and relaxation time of tissues. Spoiled equilibrium pulse is effective with increasing TR and decreasing ETL. The shorter the T1 value, the better the spoiled equilibrium pulse functions. However, for tissues in which the T1 value is long (>600 ms), at a TR of 600 ms, improvement in T1 contrast by applying spoiled equilibrium pulse cannot be expected.

Increased Speed and Image Quality for Pelvic Single-Shot Fast Spin-Echo Imaging with Variable Refocusing Flip Angles and Full-Fourier Acquisition.

Purpose To assess image quality and speed improvements for single-shot fast spin-echo (SSFSE) with variable refocusing flip angles and full-Fourier acquisition (vrfSSFSE) pelvic imaging via a prospective trial performed in the context of uterine leiomyoma evaluation. Materials and Methods Institutional review board approval and informed consent were obtained. vrfSSFSE and conventional SSFSE sagittal and coronal oblique acquisitions were performed in 54 consecutive female patients referred for 3-T magnetic resonance (MR) evaluation of known or suspected uterine leiomyomas. Two radiologists who were blinded to the image acquisition technique semiquantitatively scored images on a scale from -2 to 2 for noise, image contrast, sharpness, artifacts, and perceived ability to evaluate uterine, ovarian, and musculoskeletal structures. The null hypothesis of no significant difference between pulse sequences was assessed with a Wilcoxon signed rank test by using a Holm-Bonferroni correction for multiple comparisons. Results Because of reductions in specific absorption rate, vrfSSFSE imaging demonstrated significantly increased speed (more than twofold, P < .0001), with mean repetition times compared with conventional SSFSE imaging decreasing from 1358 to 613 msec for sagittal acquisitions and from 1494 to 621 msec for coronal oblique acquisitions. Almost all assessed image quality and perceived diagnostic capability parameters were significantly improved with vrfSSFSE imaging. These improvements included noise, sharpness, and ability to evaluate the junctional zone, myometrium, and musculoskeletal structures for both sagittal acquisitions (mean values of 0.56, 0.63, 0.42, 0.56, and 0.80, respectively; all P values < .0001) and coronal oblique acquisitions (mean values of 0.81, 1.09, 0.65, 0.93, and 1.12, respectively; all P values < .0001). For evaluation of artifacts, there was an insufficient number of cases with differences to allow statistical testing. Conclusion Compared with conventional SSFSE acquisition, vrfSSFSE acquisition increases 3-T imaging speed via reduced specific absorption rate and leads to significant improvements in perceived image quality and perceived diagnostic capability when evaluating pelvic structures. © RSNA, 2016 Online supplemental material is available for this article.

Abstract TP219: Eight-minute Whole-brain Intracranial Vessel Wall MRI at 3 Tesla: Toward a Practical Imaging Protocol for Diagnosis of Stroke Etiology

Introduction: T1-weighted variable-flip-angle 3D turbo spin-echo (TSE), has emerged as a vessel wall MRI technique promising for elucidating underlying intracranial vessel wall pathologies associated with stroke. Whole-brain 0.5-mm isotropic-resolution 3D TSE has recently been proposed at 3 Tesla. However, its scan time of 12 min renders it impractical for a clinical setting. This work aimed to design an expedited protocol and conduct a pilot study to demonstrate its potential for diagnosis of wall abnormalities. Methods: To expedite the acquisition, one may exploit an elliptical data sampling strategy and prolonged echo train length (ETL). However, this would reduce image SNR and compromise vessel wall delineation. On the other hand, SNR is intimately related to refocusing flip angles that are calculated for a prescribed tissue with specific T1 and T2 values (denoted as simulation T1 and T2). In this work, the effects of simulation T2 and ETL on the SNR/CNR performance were first explored on 9 healthy subjects. An optimized imaging protocol was then determined from a narrowed range of choices on 7 healthy subjects and finally applied to a pilot study of 10 patients with known wall disease. Results: Wall SNR, wall-CSF CNR, and white-gray matter CNR generally increased with simulation T2 and decreased with ETL. Optimization was focused on a narrowed range of protocol choices (ETL = 52; T2 = 140, 170, 200 ms) which enable a scan time of 8 min. ETL=52/T2=170ms was shown (Figure a.) to provide significantly higher (t-test) SNR/CNR performance than the original 12-min protocol (ETL=36/T2=100ms). With the 8-min protocol pre/post contrast, wall abnormalities (Figure b.) were correctly detected in all patients. Conclusion: An 8-min whole-brain intracranial vessel wall imaging protocol is developed which has demonstrated the potential of accurately identifying various wall abnormalities. A large-scale trial on using the technique for diagnosis of stroke etiology is underway.

Fresh Blood Imaging using variable TR and variable refocusing flip angle for non-contrast peripheral MR angiography at 3T: a 7-Minute iliac to calf station run-offs scan

Fresh Blood Imaging using variable TR and variable refocusing flip angle for non-contrast peripheral MR angiography at 3T: a 7-Minute iliac to calf station run-offs scan

Mono-Exponential Fitting in T2-Relaxometry: Relevance of Offset and First Echo.

IntroductionT2 relaxometry has become an important tool in quantitative MRI. Little focus has been put on the effect of the refocusing flip angle upon the offset parameter, which was introduced to account for a signal floor due to noise or to long T2 components. The aim of this study was to show that B1 imperfections contribute significantly to the offset. We further introduce a simple method to reduce the systematic error in T2 by discarding the first echo and using the offset fitting approach.Materials and MethodsSignal curves of T2 relaxometry were simulated based on extended phase graph theory and evaluated for 4 different methods (inclusion and exclusion of the first echo, while fitting with and without the offset). We further performed T2 relaxometry in a phantom at 9.4T magnetic resonance imaging scanner and used the same methods for post-processing as in the extended phase graph simulated data. Single spin echo sequences were used to determine the correct T2 time.ResultsThe simulation data showed that the systematic error in T2 and the offset depends on the refocusing pulse, the echo spacing and the echo train length. The systematic error could be reduced by discarding the first echo. Further reduction of the systematic T2 error was reached by using the offset as fitting parameter. The phantom experiments confirmed these findings.ConclusionThe fitted offset parameter in T2 relaxometry is influenced by imperfect refocusing pulses. Using the offset as a fitting parameter and discarding the first echo is a fast and easy method to minimize the error in T2, particularly for low to intermediate echo train length.

Imaging of the lumbar plexus: Optimized refocusing flip angle train design for 3D TSE.

To study the effects of refocusing angle modulation with 3D turbo spin echo (TSE) on signal and sharpness of small oblique nerves embedded in muscle and suppressed fat in the lumbar plexus. Flip angle trains were generated with extended phase graphs (EPG) for a sequence parameter subspace. Signal loss and width broadening were simulated for a single-pixel nerve embedded in muscle and suppressed fat to prescribe a flip angle modulation that gives the best compromise between signal and sharpness of small nerves. Two flip angle trains were defined based on the simulations of small embedded nerves: design denoted A, predicting maximum global signal, and design denoted B, predicting maximum signal for minimum width broadening. In vivo data of the lumbar plexus in 10 healthy volunteers was acquired at 3.0T with 3D TSE employing flip angle trains A and B. Quantitative and qualitative analyses of the acquired data were made to assess changes in width and signal intensity. Changing flip angle modulation from A to B resulted in: 1) average signal losses of 23% in (larger) L5 nerves and 9% in (smaller) L3 nerves; 2) average width reductions of 4% in L5 nerves and of 16% in L3 nerves; and 3) statistically significant sharpness improvement (P = 0.005) in L3 nerves. An optimized flip angle train in 3D TSE imaging of the lumbar plexus considering geometry-specific blurring effects from both the nerve and the surrounding tissue can improve the delineation of small nerves.

Magnetic resonance imaging of focal cortical dysplasia: Comparison of 3D and 2D fluid attenuated inversion recovery sequences at 3 T

PurposeFocal cortical dysplasia (FCD) is a frequent finding in drug resistant epilepsy. The aim of our study was to evaluate an isotropic high-resolution 3-dimensional Fluid-attenuated inversion recovery sequence (3D FLAIR) at 3T in comparison to standard 2D FLAIR in the diagnosis of FCD. Materials and methodsIn a prospective study, 19 epilepsy patients with the MR diagnosis of FCD were examined with a sagittal 3D FLAIR sequence with modulated refocusing flip angle (slice thickness 1.10mm) and a 2D FLAIR in the coronal (thk. 3mm) and axial planes (thk. 2mm). Manually placed regions of interest were used for quantitative analysis. Qualitative image analysis was performed by two neuroradiologists in consensus. ResultsContrast between gray and white matter (p≤0.02), the lesion (p≤0.031) or hyperintense extension to the ventricle (p≤0.021) and white matter was significantly higher in 2D than in 3D FLAIR sequences. In the visual analysis there was no difference between 2D and 3D sequences. ConclusionConventional 2D FLAIR sequences yield a higher image contrast compared to the employed 3D FLAIR sequence in patients with FCDs. Potential advantages of 3D imaging using surface rendering or automated techniques for lesion detection have to be further elucidated.

Usefulness of R2* maps generated by iterative decomposition of water and fat with echo asymmetry and least-squares estimation quantitation sequence for cerebral artery dissection

Acute intramural hematoma resulting from cerebral artery dissection is usually visualized as a region of intermediate signal intensity on T1-weighted images (WI). This often causes problems with distinguishing acute atheromatous lesions from surrounding parenchyma and dissection. The present study aimed to determine whether or not R2* maps generated by the iterative decomposition of water and fat with echo asymmetry and least-squares estimation quantitation sequence (IDEAL IQ) can distinguish cerebral artery dissection more effectively than three-dimensional variable refocusing flip angle TSE T1WI (T1-CUBE) and T2*WI. We reviewed data from nine patients with arterial dissection who were assessed by MR images including R2* maps, T2*WI, T1-CUBE, and 3D time-of-flight (TOF)-MRA. We visually assessed intramural hematomas in each patient as positive (clearly visible susceptibility effect reflecting intramural hematoma as hyperintensity on R2* map and hypointensity on T2*WI), negative (absent intramural hematoma), equivocal (difficult to distinguish between intramural hematoma and other paramagnetic substances such as veins, vessel wall calcification, or hemorrhage) and not evaluable (difficult to determine intramural hematoma due to susceptibility artifacts arising from skull base). Eight of nine patients were assessed during the acute phase. Lesions in all eight patients were positive for intramural hematoma corresponding to dissection sites on R2* maps, while two lesions were positive on T2*WI and three lesions showed high-intensity on T1-CUBE reflected intramural hematoma during the acute phase. R2* maps generated using IDEAL IQ can detect acute intramural hematoma associated with cerebral artery dissection more effectively than T2*WI and earlier than T1-CUBE.

T2 Relaxation time quantitation differs between pulse sequences in articular cartilage.

To compare T2 relaxation time measurements between MR pulse sequences at 3 Tesla in agar phantoms and in vivo patellar, femoral, and tibial articular cartilage. T2 relaxation times were quantified in phantoms and knee articular cartilage of eight healthy individuals using a single echo spin echo (SE) as a reference standard and five other pulse sequences: multi-echo SE (MESE), fast SE (2D-FSE), magnetization-prepared spoiled gradient echo (3D-MAPSS), three-dimensional (3D) 3D-FSE with variable refocusing flip angle schedules (3D vfl-FSE), and quantitative double echo steady state (qDESS). Cartilage was manually segmented and average regional T2 relaxation times were obtained for each sequence. A regression analysis was carried out between each sequence and the reference standard, and root-mean-square error (RMSE) was calculated. Phantom measurements from all sequences demonstrated strong fits (R(2) > 0.8; P < 0.05). For in vivo cartilage measurements, R(2) values, slope, and RMSE were: MESE: 0.25/0.42/5.0 ms, 2D-FSE: 0.64/1.31/9.3 ms, 3D-MAPSS: 0.51/0.66/3.8 ms, 3D vfl-FSE: 0.30/0.414.2 ms, qDESS: 0.60/0.90/4.6 ms. 2D-FSE, qDESS, and 3D-MAPSS demonstrated the best fits with SE measurements as well as the greatest dynamic ranges. The 3D-MAPSS, 3D vfl-FSE, and qDESS demonstrated the closest average measurements to SE. Discrepancies in T2 relaxation time quantitation between sequences suggest that care should be taken when comparing results between studies.

Optimization of Magnetization-Prepared 3-Dimensional Fluid Attenuated Inversion Recovery Imaging for Lesion Detection at 7 T

The aim of this study was to optimize the 3-dimensional (3D) fluid attenuated inversion recovery (FLAIR) pulse sequence for isotropic high-spatial-resolution imaging of white matter (WM) and cortical lesions at 7 T. We added a magnetization-prepared (MP) FLAIR module to a Cube 3D fast spin echo sequence and optimized the refocusing flip angle train using extended phase graph simulations, taking into account image contrast, specific absorption rate (SAR), and signal-to-noise ratio (SNR) as well as T1/T2 values of the different species of interest (WM, grey matter, lesions) at 7 T. We also effected improved preparation homogeneity at 7 T by redesigning the refocusing pulse used in the MP segments. Two sets of refocusing flip angle trains-(a) an SNR-optimal and (b) a contrast-optimal set-were derived and used to scan 7 patients with Alzheimer disease/cognitive impairment and 7 patients with multiple sclerosis. Conventional constant refocusing flip MP-FLAIR images were also acquired for comparison. Lesion SNR, contrast, and lesion count were compared between the 2 optimized and the standard FLAIR sequences. Whole brain coverage with 0.8 mm isotropic spatial resolution in ∼5-minute scan times was achieved using the optimized 3D FLAIR sequences at clinically acceptable SAR levels. The SNR efficiency of the SNR-optimal sequence was significantly better than that of conventional constant refocusing flip MP-FLAIR sequence, whereas the scan time was reduced more than 2-fold (∼5 vs >10 minutes). The contrast efficiency of the contrast-optimal sequence was comparable with that of the constant refocusing flip sequence. Lesion load ascertained by lesion counting was not significantly different among the sequences. Magnetization-prepared FLAIR-Cube with refocusing flip angle trains optimized for SNR and contrast can be used to characterize WM and cortical lesions at 7 T with 0.8 mm isotropic resolution in short scan times and without SAR penalty.