Time‐resolved three‐dimensional (3D) phase‐contrast (PC) balanced steady‐state free precession (bSSFP)

In this study the feasibility of a time‐resolved, three‐dimensional (3D), three‐directional flow‐sensitive balanced steady‐state free precession (bSSFP) sequence is demonstrated. Due to its high signal‐to‐noise ratio (SNR) in blood and cerebrospinal fluid (CSF) this type of sequence is particularly effective for acquisition of blood and CSF flow velocities. Flow sensitivity was achieved with the phase‐contrast (PC) technique, implementing a custom algorithm for calculation of optimal gradient parameters. Techniques to avoid the most important sources of bSSFP‐related artifacts (including distortion due to eddy currents and signal voids due to flow‐related steady‐state disruption) are also presented. The technique was validated by means of a custom flow phantom, and in vivo experiments on blood and CSF were performed to demonstrate the suitability of this sequence for human studies. Accurate depiction of blood flow in the cerebral veins and of CSF flow in the cervical portion of the neck was obtained. Possible applications of this technique might include the study of CSF flow patterns, direct in vivo study of pathologies such as hydrocephalus and Chiari malformation, and validation for the existing CSF circulation model. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Inflow‐based vascular‐space‐occupancy (iVASO) MRI

Vascular‐space‐occupancy (VASO) MRI, a blood nulling approach for assessing changes in cerebral blood volume (CBV), is hampered by low signal‐to‐noise ratio (SNR) because only 10–20% of tissue signal is recovered when using nonselective inversion for blood nulling. A new approach, called inflow‐VASO (iVASO), is introduced in which only blood flowing into the slice has experienced inversion, thereby keeping tissue and cerebrospinal fluid (CSF) signal in the slice maximal and reducing CSF partial volume effects. SNR increases of 198% ± 12% and 334% ± 9% (mean ± SD, n = 7) with respect to VASO were found at TR values of 5s and 2s, respectively. When using inflow approaches, data interpretation is complicated by the fact that signal changes are affected by vascular transit times. An optimal TR‐range (1.5–2.5s) was derived in which the iVASO response during activation predominantly reflects arterial/arteriolar CBV (CBV_a) changes. In this TR‐range, perfusion contributions to the signal change are negligible because arterial label has not yet undergone capillary exchange, and arterial and precapillary blood signals are nulled. For TR = 2s, the iVASO signal change upon visual stimulation corresponded to a CBV_a increase of 58% ± 7%, in agreement with arteriolar CBV changes previously reported. The onset of the hemodynamic response for iVASO occurred 1.2 ± 0.5s (n = 7) faster than for conventional VASO. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin‐locked balanced steady‐state free‐precession (slSSFP) pulse sequence is described that combines a balanced gradient‐echo acquisition with an off‐resonance spin‐lock pulse for fast MRI. The transient and steady‐state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady‐state free‐precession (bSSFP) for a range of T₂/T₁ and flip angles, although the slSSFP steady‐state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal‐to‐noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Robust GRAPPA‐accelerated diffusion‐weighted readout‐segmented (RS)‐EPI

Readout segmentation (RS‐EPI) has been suggested as a promising variant to echo‐planar imaging (EPI) for high‐resolution imaging, particularly when combined with parallel imaging. This work details some of the technical aspects of diffusion‐weighted (DW)‐RS‐EPI, outlining a set of reconstruction methods and imaging parameters that can both minimize the scan time and afford high‐resolution diffusion imaging with reduced distortions. These methods include an efficient generalized autocalibrating partially parallel acquisition (GRAPPA) calibration for DW‐RS‐EPI data without scan time penalty, together with a variant for the phase correction of partial Fourier RS‐EPI data. In addition, the role of pulsatile and rigid‐body brain motion in DW‐RS‐EPI was assessed. Corrupt DW‐RS‐EPI data arising from pulsatile nonlinear brain motion had a prevalence of ～7% and were robustly identified via k‐space entropy metrics. For DW‐RS‐EPI data corrupted by rigid‐body motion, we showed that no blind overlap was required. The robustness of RS‐EPI toward phase errors and motion, together with its minimized distortions compared with EPI, enables the acquisition of exquisite 3 T DW images with matrix sizes close to 512². Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Dilution method of gadolinium ethoxybenzyl diethylenetriaminepentaacetic acid (Gd‐EOB‐DTPA)‐enhanced magnetic resonance imaging (MRI)

Purpose

To elucidate whether a contrast agent dilution method (dilution method), in which gadoxetate disodium (Gd‐EOB‐DTPA) is diluted with saline, is useful for good‐quality arterial‐phase images.

Materials and Methods

In this study we observed 494 hypervascular hepatocellular carcinomas (HCCs) in 327 patients with chronic liver disease. Three Gd‐EOB‐DTPA injection methods were adopted for comparison: 1) test injection method (undiluted Gd‐EOB‐DTPA and modified scan delay), in which a test dose of 0.5 mL of Gd‐EOB‐DTPA was injected to determine scan delay; 2) conventional method (undiluted Gd‐EOB‐DTPA and fixed scan delay); and ( 3 ) dilution method (diluted Gd‐EOB‐DTPA and fixed scan delay), in which Gd‐EOB‐DTPA was diluted to 20 mL with saline. Lesion‐liver contrast was calculated. Image quality and lesion detectability were evaluated by two radiologists blinded to the injection methods.

Results

The lesion‐liver contrast of the dilution method was significantly higher than that of the other two methods. Lesion detectability of the conventional method (64%) was significantly lower than that of the other two methods (contrast agent dilution method, 95%; test injection method, 93%). The image quality of the contrast agent dilution method was significantly better than that of the other two methods.

Conclusion

The dilution method contributed to improved image quality, high lesion‐liver contrast, and high lesion detectability in the arterial‐phase images of GD‐EOB‐DTPA‐enhanced MRI. J. Magn. Reson. Imaging 2009;30:849–854. © 2009 Wiley‐Liss, Inc.     

Single‐shot multiecho parallel echo‐planar imaging (EPI) for diffusion tensor imaging (DTI) with improved signal‐to‐noise ratio (SNR) and reduced distortion

In this work, a multiecho parallel echo‐planar imaging (EPI) acquisition strategy is introduced as a way to improve the acquisition efficiency in parallel diffusion tensor imaging (DTI). With the use of an appropriate echo combination strategy, the sequence can provide signal‐to‐noise ratio (SNR) enhancement while maintaining the advantages of parallel EPI. Simulations and in vivo experiments demonstrate that a weighted summation of multiecho images provides a significant gain in SNR over the first echo image. It is experimentally demonstrated that this SNR gain can be utilized to reduce the number of measurements often required to ensure adequate SNR for accurate DTI measures. Furthermore, the multiple echoes can be used to derive a T₂ map, providing additional information that might be useful in some applications. Magn Reson Med 60:1512–1517, 2008. © 2008 Wiley‐Liss, Inc.     

Improved arterial visibility using short‐tau inversion‐recovery (STIR) fat suppression in non‐contrast‐enhanced time–spatial labeling inversion pulse (Time‐SLIP) renal MR angiography (MRA)

Purpose

To evaluate whether short‐tau inversion‐recovery (STIR) fat suppression is worthwhile in non‐contrast‐enhanced respiration‐triggered free‐breathing time–spatial labeling inversion pulse (Time‐SLIP) renal magnetic resonance angiography (MRA) compared with chemical shift selective (CHESS) fat suppression.

Materials and Methods

Simulation‐based analyses of inversion time (TI) for spatial‐selective inversion‐recovery (ssIR) pulse and breathing rate were performed, and confirmed on a phantom and in human subjects using a three‐dimensional (3D) coherent steady‐state free precession (SSFP) sequence on a 1.5T Toshiba scanner.

Results

The STIR fat suppression successfully suppressed signals from the intestines and parenchymous organs and provided better contrast between the arteries and the background, although an extension of TI was required for the ssIR pulse when a patient's respiration was extremely slow.

Conclusion

STIR fat suppression provides better renal artery contrast than CHESS fat suppression in non‐contrast free‐breathing Time‐SLIP MRA; it is also an effective screening tool for renal artery stenosis because of the lack of interference from intestinal signals. However, close attention is needed if the patient has slow respiration. As the TI for the ssIR pulse decreases, the STIR method requires faster‐paced respiration. J. Magn. Reson. Imaging 2009;29:1471–1477. © 2009 Wiley‐Liss, Inc.     

Magnetization transfer enhanced vascular‐space‐occupancy (MT‐VASO) functional MRI

Vascular‐space‐occupancy (VASO) MRI is a novel technique that uses blood signal nulling to detect blood volume alterations through changes in tissue signal. VASO has relatively low signal to noise ratio (SNR) because only 10–20% of tissue signal remain at the time of blood nulling. Here, it is shown that by adding a magnetization transfer (MT) prepulse it is possible to increase SNR either by attenuating the initial tissue magnetization when the MT pulse is placed before inversion, or, accelerating the recovery process when the pulse is applied after the inversion. To test whether the MT pulse would affect the blood nulling time in VASO, MT effects in blood were measured both ex vivo in a bovine blood phantom and in vivo in human brain. Such effects were found to be sufficiently small (< 2.5%) under a saturation power ≤ 3 μT, length = 500 ms, and frequency offset ≥40 ppm to allow use of the same nulling time. Subsequently, functional MRI experiments using MT‐VASO were performed in human visual cortex at 3 Tesla. The relative signal changes in MT‐VASO were of the same magnitude as in VASO, while the contrast to noise ratio (CNR) was enhanced by 44 ± 12% and 36 ± 11% respectively. Therefore, MT‐VASO should provide a means for increasing inherently low CNR in VASO experiments while preserving the CBV sensitivity. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Separate MRI quantification of dispersed (ferritin‐like) and aggregated (hemosiderin‐like) storage iron

A new MRI method is proposed for separately quantifying the two principal forms of tissue storage (nonheme) iron: ferritin iron, a dispersed, soluble fraction that can be rapidly mobilized, and hemosiderin iron, an aggregated, insoluble fraction that serves as a long‐term reserve. The method utilizes multiple spin echo sequences, exploiting the fact that aggregated iron can induce nonmonoexponential signal decay for multiple spin echo sequences. The method is validated in vitro for agarose phantoms, simulating dispersed iron with manganese chloride, and aggregated iron with iron oxide microspheres. To demonstrate feasibility for human studies, preliminary in vivo data from two healthy controls and six patients with transfusional iron overload are presented. For both phantoms and human subjects, conventional R₂ and R₂* relaxation rates are also measured in order to contrast the proposed method with established MRI iron quantification techniques. Quantification of dispersed (ferritin‐like) iron may provide a new means of monitoring the risk of iron‐induced toxicity in patients with iron overload and, together with quantification of aggregated (hemosiderin‐like) iron, improve the accuracy of estimates for total storage iron. Magn Reson Med 63:1201–1209, 2010. © 2010 Wiley‐Liss, Inc.     

Evaluation of a positron emission tomography (PET)‐compatible field‐cycled MRI (FCMRI) scanner

Field‐cycled MRI (FCMRI) uses two independent, actively controlled resistive magnets to polarize a sample and to provide the magnetic field environment during data acquisition. This separation of tasks allows for novel forms of contrast, reduction of susceptibility artifacts, and a versatility in design that facilitates the integration of a second imaging modality. A 0.3T/4‐MHz FCMRI scanner was constructed with a 9‐cm‐wide opening through the side for the inclusion of a photomultiplier‐tube–based positron emission tomography (PET) system. The performance of the FCMRI scanner was evaluated prior to integrating PET detectors. Quantitative measurements of the system's signal, phase, and temperature were recorded. The polarizing and readout magnets could be operated continuously at 100 A without risk of damage to the system. Transient instabilities in the readout magnet, caused by the pulsing of the polarizing magnet, dissipated in 50 ms; this resulted in a steady‐state homogeneity of 32 Hz over a 7‐cm‐diameter volume. The short‐ and long‐term phase behaviors of the readout field were sufficiently stable to prevent visible readout or phase‐encode artifacts during imaging. Preliminary MR images demonstrated the potential of the FCMRI scanner and the efficacy of integrating a PET system. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Radial sliding‐window magnetic resonance angiography (MRA) with highly‐constrained projection reconstruction (HYPR)

Sufficient temporal resolution is required to image the dynamics of blood flow, which may be critical for accurate diagnosis and treatment of various intracranial vascular diseases, such as arteriovenous malformations (AVMs) and aneurysms. Highly‐constrained projection reconstruction (HYPR) has recently become a technique of interest for high‐speed contrast‐enhanced magnetic resonance angiography (CE‐MRA). HYPR provides high frame rates by preferential weighting of radial projections while maintaining signal‐to‐noise ratio (SNR) by using a high SNR composite. An analysis was done to quantify the effects of HYPR on SNR, contrast‐to‐noise ratio (CNR), and temporal blur compared to the previously developed radial sliding‐window technique using standard filtered backprojection or regridding methods. Computer simulations were performed to study the effects of HYPR processing on image error and the temporal information. Additionally, in vivo imaging was done on patients with angiographically confirmed AVMs to measure the effects of alteration of various HYPR parameters on SNR as well as the fidelity of the temporal information. The images were scored by an interventional radiologist in a blinded read and were compared with X‐ray digital subtraction angiography (DSA). It was found that with the right choice of parameters, modest improvements in both SNR and dynamic information can be achieved as compared to radial sliding‐window MRA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Simultaneous single‐quantum and triple‐quantum‐filtered MRI of 23Na (SISTINA)

The low MR sensitivity of the sodium nucleus and its low concentration in the human body constrain acquisition time. The use of both single‐quantum and triple‐quantum sodium imaging is, therefore, restricted. In this work, we present a novel MRI sequence that interleaves an ultra‐short echo time radial projection readout into the three‐pulse triple‐quantum preparation. This allows for simultaneous acquisition of tissue sodium concentration weighted as well as triple‐quantum filtered images. Performance of the sequence is shown on phantoms. The method is demonstrated on six healthy informed volunteers and is applied to three cases of brain tumors. A comparison with images from tumor specific O‐(2‐[18F]fluoroethyl)‐L ‐tyrosine positron emission tomography and standard MR images is presented. The combined information of the triple‐quantum‐filtered images with single‐quantum images may enable a better understanding of tissue viability. Future studies can benefit from the evaluation of both contrasts with shortened acquisition times. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.