Single‐breath‐hold multiple‐slice DENSE MRI

A method to acquire multiple displacement encoded slices within a single breath hold is presented. Efficiency is improved over conventional DENSE without compromising image quality by readout of multiple slices in the same cardiac cycle, thus utilizing the position‐encoded stimulated echo available in the whole heart. The method was evaluated by comparing strain values obtained using the proposed method to strain values obtained by conventional separate breath‐hold single‐slice DENSE acquisitions. Good agreement (Lagrangian E₂ strain bias = 0.000, 95% limits of agreement ± 0.04, root‐mean‐square‐difference 0.02 [9.4% of mean end‐systolic E₂]) was found between the methods, indicating that the proposed method can replace a multiple breath‐hold acquisition. Eliminating the need for multiple breath holds reduces the risk of changes in breath‐hold positions or heart rate, results in higher patient comfort, and facilitates inclusion of DENSE in a clinical routine protocol. Magn Reson Med 63:1411–1414, 2010. © 2010 Wiley‐Liss, Inc.     

Optimized three‐dimensional fast‐spin‐echo MRI

Spin‐echo‐based acquisitions are the workhorse of clinical MRI because they provide a variety of useful image contrasts and are resistant to image artifacts from radio‐frequency or static field inhomogeneity. Three‐dimensional (3D) acquisitions provide datasets that can be retrospectively reformatted for viewing in freely selectable orientations, and are thus advantageous for evaluating the complex anatomy associated with many clinical applications of MRI. Historically, however, 3D spin‐echo‐based acquisitions have not played a significant role in clinical MRI due to unacceptably long acquisition times or image artifacts associated with details of the acquisition method. Recently, optimized forms of 3D fast/turbo spin‐echo imaging have become available from several MR‐equipment manufacturers (for example, CUBE [GE], SPACE [Siemens], and VISTA [Philips]). Through specific design strategies and optimization, including short non–spatially selective radio‐frequency pulses to significantly shorten the echo spacing and variable flip angles for the refocusing radio‐frequency pulses to suppress blurring or considerably lengthen the useable duration of the spin‐echo train, these techniques permit single‐slab 3D imaging of sizeable volumes in clinically acceptable acquisition times. These optimized fast/turbo spin‐echo pulse sequences provide a robust and flexible approach for 3D spin‐echo‐based imaging with a broad range of clinical applications. J. Magn. Reson. Imaging 2014;39:745–767. © 2014 Wiley Periodicals, Inc .     

Fast‐spin‐echo imaging of inner fields‐of‐view with 2D‐selective RF excitations

Purpose:

To demonstrate the feasibility of two‐dimensional selective radio frequency (2DRF) excitations for fast‐spin‐echo imaging of inner fields‐of‐view (FOVs) in order to shorten acquisitions times, decrease RF energy deposition, and reduce image blurring.

Materials and Methods:

Fast‐spin‐echo images (in‐plane resolution 1.0 × 1.0 mm² or 0.5 × 1.0 mm²) of inner FOVs (40 mm, 16 mm oversampling) were obtained in phantoms and healthy volunteers on a 3 T whole‐body MR system using blipped‐planar 2DRF excitations.

Results:

Positioning the unwanted side excitations in the blind spot between the image section and the slice stack to measure yields minimum 2DRF pulse durations (about 6 msec) that are compatible with typical echo spacings of fast‐spin‐echo acquisitions. For the inner FOVs, the number of echoes and refocusing RF pulses is considerably reduced which compared to a full FOV (182 mm) reduces the RF energy deposition by about a factor of three and shortens the acquisition time, e.g., from 39 seconds to 12 seconds for a turbo factor of 15 or from 900 msec to 280 msec for a single‐shot acquisition, respectively. Furthermore, image blurring occurring for high turbo factors as in single‐shot acquisitions is considerably reduced yielding effectively higher in‐plane resolutions.

Conclusion:

Inner‐FOV acquisitions using 2DRF excitations may help to shorten acquisitions times, ameliorate image blurring, and reduce specific absorption rate (SAR) limitations of fast‐spin‐echo (FSE) imaging, in particular at higher static magnetic fields. J. Magn. Reson. Imaging 2010;31:1530–1537. © 2010 Wiley‐Liss, Inc.     

Comparison of ferucarbotran‐enhanced fluid‐attenuated inversion‐recovery echo‐planar,T2‐weighted turbo spin‐echo,T2*‐weighted gradient‐echo,and diffusion‐weighted echo‐planar imaging for detection of malignant liver lesions

Purpose:

To compare the diagnostic accuracy of superparamagnetic iron oxide (SPIO)‐enhanced fluid‐attenuated inversion‐recovery echo‐planar imaging (FLAIR EPI) for malignant liver tumors with that of T2‐weighted turbo spin‐echo (TSE), T2*‐weighted gradient‐echo (GRE), and diffusion‐weighted echo‐planar imaging (DW EPI).

Materials and Methods:

SPIO‐enhanced magnetic resonance imaging (MRI) that included FLAIR EPI, T2‐weighted TSE, T2*‐weighted GRE, and DW EPI sequences was performed using a 3 T system in 54 consecutive patients who underwent surgical exploration with intraoperative ultrasonography. A total of 88 malignant liver tumors were evaluated. Images were reviewed independently by two blinded observers who used a 5‐point confidence scale to identify lesions. Results were correlated with results of histopathologic findings and surgical exploration with intraoperative ultrasonography. The accuracy of each MRI sequence was measured with jackknife alternative free‐response receiver operating characteristic analysis. The sensitivity of each observer with each MRI sequence was compared with McNemar's test.

Results:

Accuracy values were significantly higher with FLAIR EPI sequence (0.93) than with T2*‐weighted GRE (0.80) or DW EPI sequences (0.80) (P < 0.05). Sensitivity was significantly higher with the FLAIR EPI sequence than with any of the other sequences.

Conclusion:

SPIO‐enhanced FLAIR EPI sequence was more accurate in the diagnosis of malignant liver tumors than T2*‐weighted GRE and DW EPI sequences. SPIO‐enhanced FLAIR EPI sequence is helpful for the detection of malignant liver tumors. J. Magn. Reson. Imaging 2010;31:607–616. ©2010 Wiley‐Liss, Inc.     

Phase‐sensitive,dual‐acquisition,single‐slab, 3D,turbo‐spin‐echo pulse sequence for simultaneous T2‐weighted and fluid‐attenuated whole‐brain imaging

Conventional T₂‐weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid‐attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. The purpose of this work is to develop a phase‐sensitive, dual‐acquisition, single‐slab, three‐dimensional, turbo/fast spin echo imaging, simultaneously achieving both conventional T₂‐weighted and fluid‐attenuated inversion recovery–like high‐resolution whole‐brain images in a single pulse sequence, without an apparent increase of imaging time. Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip‐down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF‐suppressed image is then reconstructed by weighted averaging the in‐ and opposed‐phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T₂‐weighted imaging and fluid‐attenuated inversion recovery. Magn Reson Med 63:1422–1430, 2010. © 2010 Wiley‐Liss, Inc.     

Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

Small‐tip fast recovery (STFR) imaging is a new steady‐state imaging sequence that is a potential alternative to balanced steady‐state free precession. Under ideal imaging conditions, STFR may provide comparable signal‐to‐noise ratio and image contrast as balanced steady‐state free precession, but without signal variations due to resonance offset. STFR relies on a tailored “tip‐up,” or “fast recovery,” radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip‐up pulse is based on the acquisition of a separate off‐resonance (B0) map. Unfortunately, the design of fast (a few ms) slice‐ or slab‐selective radiofrequency pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on “non‐slice‐selective” tip‐up pulses, which simplifies the radiofrequency pulse design problem significantly. Out‐of‐slice magnetization pathways are suppressed using radiofrequency‐spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady‐state T₂/T₁‐weighted imaging with intrinsic suppression of cerebrospinal fluid, through‐plane vessel signal, and off‐resonance artifacts. In the future, we expect STFR imaging to benefit significantly from parallel excitation hardware and high‐order gradient shim systems. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Highly k‐t‐space–accelerated phase‐contrast MRI

The purpose of this study was to combine a recently introduced spatiotemporal parallel imaging technique, PEAK‐GRAPPA (parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisition), with two‐dimensional (2D) cine phase‐contrast velocity mapping. Phase‐contrast MRI was applied to measure the blood flow in the thoracic aorta and the myocardial motion of the left ventricle. To evaluate the performance of different reconstruction methods, fully acquired k‐space data sets were used to compare conventional parallel imaging using GRAPPA with reduction factors of R = 2–6 and PEAK‐GRAPPA as well as sliding window reconstruction with reduction factors R = 2–12 (net acceleration factors up to 5.2). PEAK‐GRAPPA reconstruction resulted in improved image quality with considerably reduced artifacts, which was also supported by error analysis. To analyze potential blurring or low‐pass filtering effects of spatiotemporal PEAK‐GRAPPA, the velocity time courses of aortic flow and myocardial tissue motion were evaluated and compared with conventional image reconstructions. Quantitative comparisons of blood flow velocities and pixel‐wise correlation analysis of velocities highlight the potential of PEAK‐GRAPPA for highly accelerated dynamic phase‐contrast velocity mapping. Magn Reson Med 60:1169–1177, 2008. © 2008 Wiley‐Liss, Inc.     

Hybrid of opposite‐contrast MR angiography (HOP‐MRA) combining time‐of‐flight and flow‐sensitive black‐blood contrasts

For the purpose of visualizing low‐flow as well as high‐flow blood vessels without using contrast agents, we propose a new technique called a hybrid of opposite‐contrast MR angiography (HOP‐MRA). HOP‐MRA is a combination of standard time‐of‐flight (TOF) using a full first‐order velocity‐compensation for white‐blood (WB) and flow‐sensitive black‐blood (FSBB) techniques, which use motion‐probing gradients to introduce intravoxel flow dephasing. A dual‐echo three‐dimensional gradient echo sequence was used to reduce both imaging time and misregistration. HOP‐MRA images were obtained using a simple‐weighted subtraction (SWS) or a frequency‐weighted subtraction (FWS) applying different spatial filtering for WB and BB images. We then assessed the relationships among the contrast‐to‐noise ratios (CNR) of the blood‐to‐background signals for those three images. In both volunteer and clinical brain studies, low‐flow vessels were well visualized and the background signal was well suppressed by HOP‐MRA compared with standard TOF‐ or BB‐MRA. The FWS was better than the SWS when whole‐maximum intensity projection was performed on a larger volume including with different types of tissue. The proposed HOP‐MRA was proven to visualize low‐flow to high‐flow vessels and, therefore, demonstrates excellent potential to become a clinically useful technique, especially for visualizing collateral vessels which is difficult with standard TOF‐MRA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.