Quantitative mapping of T2 using partial spoiling

Fast quantitative MRI has become an important tool for biochemical characterization of tissue beyond conventional T₁, T₂, and T₂*‐weighted imaging. As a result, steady‐state free precession (SSFP) techniques have attracted increased interest, and several methods have been developed for rapid quantification of relaxation times using steady‐state free precession. In this work, a new and fast approach for T₂ mapping is introduced based on partial RF spoiling of nonbalanced steady‐state free precession. The new T₂ mapping technique is evaluated and optimized from simulations, and in vivo results are presented for human brain at 1.5 T and for human articular cartilage at 3.0 T. The range of T₂ for gray and white matter was from 60 msec (for the corpus callosum) to 100 msec (for cortical gray matter). For cartilage, spatial variation in T₂ was observed between deep (34 msec) and superficial (48 msec) layers, as well as between tibial (33 msec), femoral, (54 msec) and patellar (43 msec) cartilage. Excellent correspondence between T₂ values derived from partially spoiled SSFP scans and the ones found with a reference multicontrast spin‐echo technique is observed, corroborating the accuracy of the new method for proper T₂ mapping. Finally, the feasibility of a fast high‐resolution quantitative partially spoiled SSFP T₂ scan is demonstrated at 7.0 T for human patellar cartilage. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Fast three‐dimensional proton spectroscopic imaging of the human brain at 3 T by combining spectroscopic missing pulse steady‐state free precession and echo planar spectroscopic imaging

The combination of the principles of two fast spectroscopic imaging (SI) methods, spectroscopic missing pulse steady‐state free precession and echo planar SI (EPSI) is described as an approach toward fast 3D SI. This method, termed missing pulse steady‐state free precession echo planar SI, exhibits a considerably reduced minimum total measurement time T_min, allowing a higher temporal resolution, a larger spatial matrix size, and the use of k‐space weighted averaging and phase cycling, while maintaining all advantages of the original spectroscopic missing pulse steady‐state free precession sequence. The minor signal‐to‐noise ratio loss caused by using oscillating read gradients can be compensated by applying k‐space weighted averaging. The missing pulse steady‐state free precession echo planar SI sequence was implemented on a 3 T head scanner, tested on phantoms and applied to healthy volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Characterization and reduction of saturation banding in multiplanar coherent and incoherent steady‐state imaging

Hypointense band artifacts occur at intersections of nonparallel imaging planes in rapidly acquired MR images; quantitative or numerical analysis of these bands and strategies to mitigate their appearance have largely gone unexplored. The magnetization evolution in the different regions of multiplanar images was simulated for three common rapid steady‐state techniques (spoiled gradient echo, steady state free precession, balanced steady state free precession). Saturation banding was found to be highly dependent on the pulse sequence, acquisition time, and phase‐encoding order. Encoding the center of k‐space at the end of the acquisition of each slice (i.e., reverse centric phase encoding) is demonstrated to be a simple and robust method for significantly reducing the relative saturation in all imaging planes. View ordering and resolution dependence were confirmed in multiplanar abdominal images. The added importance of reducing the artifact in accelerated acquisition techniques (e.g., parallel imaging) is particularly notable in multiplanar balanced steady state free precession images in the brain. Magn Reson Med 63:1415–1421, 2010. © 2010 Wiley‐Liss, Inc.     

Outer volume suppression in steady state sequences (OVSuSS) for percutaneous interventions

Magnetic resonance‐guided percutaneous interventions with needles require fast pulse sequences with acquisition times less than 1 s to image the needle trajectory within moving organs. To guide the movement of a rigid instrument with high sampling rate, an magnetic resonance imaging method was developed that reduces the acquisition time down to a few hundred milliseconds by restricting the field of view to a small stripe around the instrument trajectory. To maintain the dynamic steady state, saturation pulses for outer volume suppression were inserted into additional repetition time‐intervals. These saturation intervals were combined with three sequence variants: a spoiled gradient echo sequence, an echo‐shifted steady state free precession and a balanced steady state free precession sequence. The magnetization dynamics were analyzed by means of numerical optimized simulations. Results were compared with phantom experiments and an average signal‐to‐suppression‐ratio of 15.5 could be achieved. With a field of view reduction of up to 12.5% an update rate of six images per second could be achieved. Finally, animal experiments demonstrated the fast and reliable needle tip visualization during percutaneous magnetic resonance‐guided interventions with the help of a robotic assistance system. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

CEST‐FISP: A novel technique for rapid chemical exchange saturation transfer MRI at 7 T

Chemical exchange saturation transfer (CEST) and magnetization transfer techniques provide unique and potentially quantitative contrast mechanisms in multiple MRI applications. However, the in vivo implementation of these techniques has been limited by the relatively slow MRI acquisition techniques, especially on high‐field MRI scanners. A new, rapid CEST‐fast imaging with steady‐state free precession technique was developed to provide sensitive CEST contrast in ～20 sec. In this study at 7 T with in vitro bovine glycogen samples and initial in vivo results in a rat liver, the CEST‐fast imaging with steady‐state free precession technique was shown to provide equivalent CEST sensitivity in comparison to a conventional CEST‐spin echo acquisition with a 50‐fold reduction in acquisition time. The sensitivity of the CEST‐fast imaging with steady‐state free precession technique was also shown to be dependent on k‐space encoding with centric k‐space encoding providing a 30–40% increase in CEST sensitivity relative to linear encoding for 256 or more k‐space lines. Overall, the CEST‐fast imaging with steady‐state free precession acquisition technique provides a rapid and sensitive imaging platform with the potential to provide quantitative CEST and magnetization transfer imaging data. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Design and use of variable flip angle schedules in transient balanced SSFP subtractive imaging

In subtractive imaging modalities, the differential longitudinal magnetization decays with time, necessitating signal‐efficient scanning methods. Balanced steady‐state free precession pulse sequences offer greater signal strength than conventional spoiled gradient echo sequences, even during the transient approach to steady state. Although traditional balanced steady‐state free precession requires that each excitation pulse use the same flip angle, operating in the transient regimen permits the application of variable flip angle schedules that can be tailored to optimize certain signal characteristics. A computationally efficient technique is presented to generate variable flip angle schedules efficiently for any optimization metric. The validity of the technique is shown using two phantoms, and its potential is demonstrated in vivo with a variable angle schedule to increase the signal‐to‐noise ratio (SNR) in myocardial tissue. Using variable flip angles, the mean SNR improvement in subtractive imaging of myocardial tissue was 18.2% compared to conventional, constant flip angle, balanced steady‐state free precession (P = 0.0078). Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

磁共振不同成像序列对膝关节软骨损伤诊断价值的Meta分析

目的评价磁共振不同成像序列诊断膝关节软骨损伤的临床价值。资料与方法计算机检索关于磁共振诊断膝关节软骨损伤的研究,并追溯纳入文献的参考文献。按照QUADAS质量评价条目评价纳入研究的质量,采用RevMan 5.0和Meta-Disc 1.4对资料进行统计分析,计算不同序列的合并敏感度、特异度,并描绘出受试者工作特征曲线(SROC)。结果最终纳入20个研究。结果显示,与关节镜相比,快速自旋回波序列、质子加权序列、稳态扰相梯度回波序列、相干梯度回波序列、稳态自由进动序列诊断膝关节软骨损伤的合并敏感度、特异度和SROC曲线下面积分别为0.753、0.967和0.9835;0.745、0.945和0.9588;0.851、0.958和0.9881;0.869、0.961和0.9153;0.740和0.901。结论当前证据表明,磁共振对关节软骨损伤具有较高诊断效能,结合不同序列的敏感度、特异度和SROC曲线下面积,推荐临床使用快速回波序列和稳态扰相梯度回波序列。     

Comparison of wideband steady‐state free precession and T2‐weighted fast spin echo in spine disorder assessment at 1.5 and 3 T

Wideband steady‐state free precession (WB‐SSFP) is a modification of balanced steady‐state free precession utilizing alternating repetition times to reduce susceptibility‐induced balanced steady‐state free precession limitations, allowing its use for high‐resolution myelographic‐contrast spinal imaging. Intertissue contrast and spatial resolution of complete‐spine‐coverage 3D WB‐SSFP were compared with those of 2D T₂‐weighted fast spin echo, currently the standard for spine T₂‐imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal‐to‐noise ratio efficiency (SNR per unit‐time and unit‐volume) of several tissues was measured, along with four intertissue contrast‐to‐noise ratios; nerve‐ganglia:fat, intradural‐nerves:cerebrospinal fluid, nerve‐ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB‐SSFP clinical advantages and disadvantages. At 3 T, WB‐SSFP provided spinal contrast‐to‐noise ratios 3.7–5.2 times that of fast spin echo. At 1.5 T, WB‐SSFP contrast‐to‐noise ratio was 3–3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural‐nerves:cerebrospinal fluid. WB‐SSFP signal‐to‐noise ratio efficiency was also higher. Three‐dimensional WB‐SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB‐SSFP's high resolution and contrast‐to‐noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal‐stenosis. WB‐SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Fundamental study of the fat-suppressed three-dimensional coherent oscillatory state acquisition for the manipulation of image contrast (3D-COSMIC) sequence in the knee joint cartilage

Fat-suppressed three-dimensional coherent oscillatory state acquisition for the manipulation of image contrast (3D-COSMIC) is a sequence that is based on fast imaging employing steady state acquisition (FIESTA) of balanced steady-state free precession (balanced SSFP). Since the data acquisition of steady-state transition is filled up with the center of k-space, improvement in the contrast of the cartilage, which is a low T?/T? value domain, is expected. This time we report on the usability in applying the above sequence to cartilage imaging of the knee joint and comparing and examining this sequence with the sequence in the past from the viewpoints of the contrast and scan time. As a result, compared with fat-suppressed three-dimensional spoiled gradient echo (3D-SPGR), the contrast of marrow and synovial fluid was equivalent to that of the cartilage, and imaging time was shorter than half of that with the cartilage. Compared with a fat-suppressed two-dimensional proton density weighted image (2D-PDWI), the contrast of the cartilage and synovial fluid was significantly improved, and spatial resolution was also excellent. As a short imaging time and a high resolution image pick-up are possible for fat-suppressed 3D-COSMIC, and it can describe minute damage of the cartilage since it depicts synovial fluid as high-level signals, I think this technique is useful.     

Imaging iron‐loaded mouse glioma tumors with bSSFP at 3 T

The poor prognosis for patients with high‐grade glioma is partly due to the invasion of tumor cells into surrounding brain tissue. The goal of the present work was to develop a mouse model of glioma that included the potential to track cell invasion using MRI by labeling GL261 cells with iron oxide contrast agents prior to intracranial injection. Two types of agents were compared with several labeling schemes to balance between labeling with sufficient iron to curb the dilution effect of cell division while avoiding overwhelming signal loss that could prevent adequate visualization of tumor boundaries. The balanced steady‐state free precession (bSSFP) pulse sequence was evaluated for its suitability for imaging glioma tumors and compared to T₂‐weighted two‐dimensional fast spin echo (FSE) and T₁‐weighted spoiled gradient recalled echo (SPGR) at 3 T in terms of signal‐to‐noise ratio and contrast‐to‐noise ratio efficiencies. Ultimately, a three‐dimensional bSSFP protocol consisting of a set of two images with complementary contrasts was developed, allowing excellent tumor visualization with minimal iron contrast when using pulse repetition time = 6 ms and α = 40°, and extremely high sensitivity to iron when using pulse repetition time = 22 ms and α = 20°. Quantitative histologic analysis validated that the strong signal loss seen in balanced steady state free precession pulse sequence images of iron‐loaded tumors correlated well with the presence of iron. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Cartilage imaging: motivation,techniques, current and future significance

Cartilage repair techniques and pharmacological therapies are currently areas of major clinical interest and research, in particular to prevent and treat osteoarthritis. MR imaging-based techniques to visualize cartilage are prerequisites to guide and monitor these therapies. In this review article, standard MR imaging sequences are described, including proton density-weighted fast spin echo, spoiled gradient echo and dual echo steady state sequences. In addition, new sequences that have been developed and are currently being investigated are presented, including driven equilibrium Fourier transform and steady-state free precession-based imaging. Using high-field MR imaging at 3.0-T, visualization of cartilage and the related pathology has been improved. Volumetric quantitative cartilage MR imaging was developed as a tool to monitor the progression of osteoarthritis and to evaluate new pharmacological cartilage protective therapies. The most exciting developments, however, are in the field of cartilage matrix assessment with quantitative dGEMRIC, T2 and T1rho mapping techniques. These techniques aim at detecting cartilage damage at a stage when changes are potentially still reversible, before cartilage tissue is lost. There is currently substantial interest in these techniques from rheumatologists and orthopedists; radiologists therefore need to keep up with these developments.     

Juxtapapillary diverticulum: findings on MRI

The purpose of our study was to describe the imaging findings of juxtapapillary diverticulum on magnetic resonance imaging (MRI). The MRI and magnetic resonance cholangiopancreatography (MRCP) examinations of 14 patients with juxtapapillary diverticula that were diagnosed on endoscopic retrograde cholangiopancreatography (ERCP) (N = 8) or endoscopy (N = 6) were retrospectively evaluated. T1-weighted spoiled gradient-echo, T2-weighted half Fourier single shot fast spin-echo (HASTE), and T2-weighted True FISP (fast imaging with steady state precession) images and thin-slice MRCP images were obtained on all patients. In five patients, diluted gadolinium DPTA (1/100) was used as an oral contrast. T2-weighted True FISP and HASTE images demonstrated air-fluid levels within all diverticula. Hyperintense oral contrast on T1-weighted spoiled gradient-echo images aided detection of the smaller diverticula. MRCP images obtained in the coronal plane best demonstrated the relationship of the diverticula to the papilla. MRI with the use of HASTE, True FISP, and oral contrast-enhanced T1-weighted sequences was able to depict juxtapapillary diverticula in our series.     

Intrascanner and interscanner variability of magnetization transfer‐sensitized balanced steady‐state free precession imaging

Recently, a new and fast three‐dimensional imaging technique for magnetization transfer ratio (MTR) imaging has been proposed based on a balanced steady‐state free precession protocol with modified radiofrequency pulses. In this study, optimal balanced steady‐state free precession MTR protocol parameters were derived for maximum stability and reproducibility. Variability between scans was assessed within white and gray matter for nine healthy volunteers using two different 1.5 T clinical systems at six different sites. Intrascanner and interscanner MTR measurements were well reproducible (coefficient of variation: c_v < 0.012 and c_v < 0.015, respectively) and results indicate a high stability across sites (c_v < 0.017) for optimal flip angle settings. This study demonstrates that balanced steady‐state free precession MTR not only benefits from short acquisition time and high signal‐to‐noise ratio but also offers excellent reproducibility and low variability, and it is thus proposed for clinical MTR scans at individual sites as well as for multicenter studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Determination of optimal angles for variable nutation proton magnetic spin-lattice, T1, and spin-spin, T2, relaxation times measurement.

T1 and T2 can be rapidly determined with a combination of multiangle spoiled gradient recalled echo (SPGR) and steady-state free precession (SSFP) imaging. Previously, we demonstrated a simple method for determining the set of SPGR and SSFP angles that provided greater T1 and T2 precision than a set of uniformly spaced angles. In this article a more rigorous approach for determining angles is described. Weighted least-squares is also introduced for T1 and T2 estimation and a novel weighting function described. This new approach, suited for imaging applications where large T1 and T2 ranges are anticipated, provides high and uniform precision over a wide range of T1 and T2 values.     

Flip angle profile correction for T1 and T2 quantification with look‐locker inversion recovery 2D steady‐state free precession imaging

Fast methods using balanced steady‐state free precession have been developed to reduce the scan time of T₁ and T₂ mapping. However, flip angle (FA) profiles created by the short radiofrequency pulses used in steady‐state free precession deviate substantially from the ideal rectangular profile, causing T₁ and T₂ mapping errors. The purpose of this study was to develop a FA profile correction for T₁ and T₂ mapping with Look‐Locker 2D inversion recovery steady‐state free precession and to validate this method using 2D spin echo as a reference standard. Phantom studies showed consistent improvement in T₁ and T₂ accuracy using profile correction at multiple FAs. Over six human calves, profile correction provided muscle T₁ estimates with mean error ranging from excellent (?0.6%) at repetition time/FA = 18 ms/60° to acceptable (6.8%) at repetition time/FA = 4.9 ms/30°, while muscle T₂ estimates were less accurate with mean errors of 31.2% and 47.9%, respectively. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Fast 31P chemical shift imaging using SSFP methods.

Steady-state free precession (SSFP) methods have been very successful due to their high signal and short imaging times. These properties make them good candidates for applications that intrinsically suffer from low signal such as low gamma nuclei imaging. A new chemical shift imaging (CSI) technique based on the SSFP signal formation has been implemented and applied to (31)P. The signal properties of the SSFP CSI method have been evaluated and the steady-state signal of (31)P has been measured in human muscles. Due to the T(2) and T(1) signal dependence of SSFP, the steady-state signal mainly consists of phosphocreatine (PCr). The technique allows fast CSI acquisitions with high SNR of the PCr signal. The SNR gain for PCr over a FLASH-based CSI method is approx. 4-5. Fast in vivo CSI of human muscle with subcentimeter resolution and high SNR is demonstrated at 2 T.     

Rapid combined T1 and T2 mapping using gradient recalled acquisition in the steady state.

A novel, fully 3D, high-resolution T(1) and T(2) relaxation time mapping method is presented. The method is based on steady-state imaging with T(1) and T(2) information derived from either spoiling or fully refocusing the transverse magnetization following each excitation pulse. T(1) is extracted from a pair of spoiled gradient recalled echo (SPGR) images acquired at optimized flip angles. This T(1) information is combined with two refocused steady-state free precession (SSFP) images to determine T(2). T(1) and T(2) accuracy was evaluated against inversion recovery (IR) and spin-echo (SE) results, respectively. Error within the T(1) and T(2) maps, determined from both phantom and in vivo measurements, is approximately 7% for T(1) between 300 and 2000 ms and 7% for T(2) between 30 and 150 ms. The efficiency of the method, defined as the signal-to-noise ratio (SNR) of the final map per voxel volume per square root scan time, was evaluated against alternative mapping methods. With an efficiency of three times that of multipoint IR and three times that of multiecho SE, our combined approach represents the most efficient of those examined. Acquisition time for a whole brain T(1) map (25 x 25 x 10 cm) is less than 8 min with 1 mm(3) isotropic voxels. An additional 7 min is required for an identically sized T(2) map and postprocessing time is less than 1 min on a 1 GHz PIII PC. The method therefore permits real-time clinical acquisition and display of whole brain T(1) and T(2) maps for the first time.     

Imaging single mammalian cells with a 1.5 T clinical MRI scanner.

In the present work, we demonstrate that the steady-state free precession (SSFP) imaging pulse sequence FIESTA (fast imaging employing steady state acquisition) used in conjunction with a custom-built insertable gradient coil and customized RF coils can be used to detect individual SPIO-labeled cells using a commonly available 1.5 T clinical MRI scanner. This work provides the first evidence that single-cell tracking will be possible using clinical MRI scanners, opening up new possibilities for cell tracking and monitoring of cellular therapeutics in vivo in humans.     

Optimization of alternating TR‐SSFP for fat‐suppression in abdominal images at 3T

Magnetic resonance imaging is widely used in the work‐up and monitoring of patients with Crohn's disease. Balanced steady‐state free precession sequences are an important part of the imaging protocol and until now primarily 1.5T scanners have been used in daily clinical practice. This is largely because running balanced steady‐state free precession sequences in 3T magnets has technical problems related to increased B₀ inhomogeneity and specific absorption rate (SAR) deposition. A modified form of alternating repetition time steady‐state free precession sequence is presented to acquire 3D‐isotropic abdominal images with fat‐suppression at 3T within a breath‐hold. The modifications include an adjusted radiofrequency pulse shape, suitable phase‐cycling scheme and TR₁/TR₂ ratio. Results show that the proposed sequence is successful in obtaining high contrast 3D‐isotropic abdominal images within a breath‐hold. Furthermore, the proposed methodology is easy to implement in a clinical setting and does not require any postprocessing steps. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.