Reduction of the specific absorption ratio (SAR) is essential in MR systems with high magnetic fields. Therefore, a variety of SAR-reduced sequences are available on modern 3T-MR systems, and some sequences can be applied under the same conditions as conventional sequences. The purpose of this study was to compare the imaging properties of a phantom acquired by SAR-reduced sequences with those of conventional sequences. Our data showed that effective relaxation time differed in each of the sequences, because the echo space and RF pulse of the SAR-reduced sequences varied from those of the conventional sequences. Further, the image contrast of the SAR-reduced sequences differed from the image contrast of the conventional ones.
Magnetic field inhomogeneity causes artifacts in MRI. For example, in single-shot echo-planar imaging (EPI), they often appear as severe geometric distortions along the phase-encoding direction. Sensitivity encoding (SENSE) is useful in reducing the distortion in EPI since it only acquires partial k-space data using multiple receiver channels. In SENSE, a reference scan usually needs to be performed to create a sensitivity profile of each receiver channel. Gradient echo (GRE) sequences are often used in the reference scan. In diffusion-weighted imaging (DWI) using single-shot EPI with SENSE, non-negligible aliasing artifacts often remain in the reconstructed images. We suppose that these artifacts result from misregistration between the reference images acquired using GRE sequences and the DWI acquired using EPI. In this study, we used two types of acquisition methods to create sensitivity profiles, GRE sequences and EPI, and have compared the residual artifacts in the reconstructed images. The sensitivity profiles were created from the data acquired using the GRE and EPI sequences. Artifacts were reduced when EPI sensitivity profiles were used. This resulted from the fact that off-resonance effects, e.g., magnetic field inhomogeneity, susceptibility, and chemical shift, often cause severe image distortion in EPI, and, therefore, there is misregistration between images reconstructed from the data acquired using GRE and EPI sequences. Our study suggests that EPI sensitivity profiles be used when imaging data are acquired using a single-shot EPI with SENSE, although GRE sensitivity profiles have often been used in practice.
The purpose of this study is to evaluate the general stability and image properties of a 3T MRI system newly installed at the ATR-Brain Activity Imaging Center (ATR-BAIC), in addition to a conventional 1.5T system. In this study, we focused on the echo planar imaging (EPI) sequence since continuous EPI with a relatively long duration of up to 30 min is routinely used, and the stabilization of EPI is always a concern. The following five results were obtained: (1) Significant image shifts along the phase direction were observed in the 1.5T data but not in the 3T data, although B0 shifts in both the 1.5T and 3T systems were the same level (1.3 Hz/min); (2) The signal fluctuations were 1/2-1/3 smaller in the 3T system compared with the 1.5T system; (3) The temporal signal-to-noise ratio (TSNR) of the 3T system was 1.7-2.0 (CP-coil) and 2.5-4.0 (12ch-coil) greater than the 1.5T system; (4) We found a low frequency periodic fluctuation (cycles of approximately 30-40 sec), and an increase in noise in the latter half of the long term series, which might originate from the 3T MRI scanner; and (5) Spatial non-uniformity of TSNR and voxels with a linear-trend were observed in the 3T data.
The purpose of this study was to suppress CSF flow artifacts in the fast FLAIR sequence at 3.0T MRI. We investigated the influence of thickness of the inversion pulse in the sequence on the high-intensity CSF flow artifacts based on the flow phantom and in-vivo studies at 1.5T and 3.0T. Results demonstrated that CSF flow artifacts at 3.0T were clearly stronger than at as 1.5T. Moreover, 3.0T was influenced by the crosstalk between each inversion pulse compared with 1.5T. The optimal setting of inversion pulse for two interleaving acquisitions for fast FLAIR imaging at 3.0T was approximately 1.5 fold on the basis of sum of slice thickness and slice gap. The appropriate setting of thickness of inversion pulse in fast FLAIR imaging reduces the incidence of CSF flow artifacts at 3.0T.
SWI and MRA employ the FLASH sequence of gradient echo, and there are many similarities in acquisition parameters between the 2 sequences. We designed an acquisition sequence for the simultaneous collection of SWI and MRA. MRA images were simultaneously collected by changing the TE of the SWI sequence to double echo. The first TE was set to 6.15 msec, 8.61 msec, 11.10 msec of the opposed phase to acquire MRA information. The second TE was set to 20.00 msec to acquire magnetic susceptibility-weighted image information. Double echo, MRA, and SWI were performed in 6 volunteers. The MRI system used was the MAGNETOM Trio A Tim System of SIEMENS Co. Magnetic susceptibility-weighted and MRA images could be simultaneously collected. Evaluation of the main trunk of the cerebral artery in the MRA image was possible, but blood vessels became poorly imaged with transition toward the periphery. Arteries and veins were imaged due to the change to double echo on magnetic susceptibility-weighted imaging. The TE to acquire the best MRA was 6.15 msec of the opposed phase. The imaging of blood vessels by simultaneously collected MRA was impaired because of the inability to use TONE and a short TE. Arteries and veins were imaged by susceptibility-weighted imaging because flow compensation did not function at the second TE.
Time-resolved MRA has recently been reported, and high-resolution MRA that does not miss timing is possible. In this examination, CEMRA that used 4D time-resolved angiography using keyhole (4D-TRAK) at 3T for the pelvic region was studied. 4D-TRAK is a method of using keyhole imaging together with sensitivity encoding (SENSE) and contrast-enhanced timing robust angio (CENTRA). The method changed flip angle (FA) and examined relative signal intensity, TR, TE, dynamic keyhole scan duration (DKSD) of the dilution contrast media, and keyhole% (KP). Image quality was examined with a blood vessel phantom. The presence of the partial echo method (PE) was examined in all cases. Relative signal intensity rose when FA increased. It has decreased in the PE method. TR did not show a difference by the PE method in FA15°or more. DKSD was extended by the PE method. In the blood vessel Phantom, the PE method made the ringing artifacts remarkable. The artifacts that originated in keyhole imaging were observed. Shortening TR is difficult because of peculiar SAR to 3T. The PE method is not effective and becomes useless. It is necessary to note a point different from the parameter setting by 1.5T.
Whole heart coronary MRA (WHCA) is a noninvasive method used to image all coronary arteries with cardiac and real-time respiratory gating. We compared the coronary depiction ability of 3T WHCA with that of 1.5T using healthy volunteers. In addition, we compared the study performance rate, which might differ at 3T and 1.5T due to the difference in specific absorption rate (SAR) limits. The coronary artery was classified into nine segments, based on the classification of the American Heart Association (AHA). Each observer was asked to evaluate WHCA with the three-point scale rating for each segment, and to measure the visible length of each coronary artery utilizing reconstructed CPR and VR images. Depiction at 3T was superior to that at 1.5T. The completion rate of study was 100% at 1.5T, but just 63% at 3T owing to SAR limits. Thus it was suggested that 3T WHCA might be feasible with the advantage of high depiction ability, if adequate SAR reduction techniques were developed.
T2* value measurement of the liver parenchyma with a 3.0T MR scanner may be useful for evaluating focal liver function. Currently, there are 2 sequences for measurement of T2* value of the liver: multi-echo fast field echo (mFFE), and multi-echo planar imaging (EPI). We can correct inhomogeneity of the local magnetic field with the EPI sequence; however, the spatial resolution is poor. On the other hand, mFFE has a relatively high spatial resolution but cannot correct inhomogeneity of the local magnetic field. We investigated the two measurement methods of a T2* map that measured the T2* images obtained with mFFE and EPI sequences by using a 3.0T MR scanner in the phantom and patient studies. In the phantom studies, T2* values measured on images with the mFFE sequence were affected by inhomogeneity of the local magnetic field, but T2* values measured on images with the EPI sequence were showed no difference by corrected inhomogeneity of local magnetic field. However, in the clinical study, we found good agreement in T2* values of the liver measured on images with mFFE and EPI sequences. Therefore, the mFFE sequence can be an alternative to the EPI sequence in the clinical setting. It is occasionally difficult to identify normal or pathological structures on images obtained with the EPI sequence because of its low spatial resolution. The spatial resolution of images obtained with the mFFE sequence is much better than that with the EPI sequence. Based on these discussions, we believe that the mFFE sequence may be appropriate for the measurement of T2* values in the liver in the clinical setting.
Cardiac late Gadolinium enhancement MR imaging has been shown to allow assessment of myocardial viability in patients with ischemic heart disease. The current standard approach is a 3D inversion recovery sequence at 1.5 Tesla. The aims of this study were to evaluate the technique feasibility and clinical utility of MR viability imaging at 3.0 Tesla in patients with myocardial infarction and cardiomyopathy. In phantom and volunteer studies, the inversion time required to suppress the signal of interests and tissues was prolonged at 3.0 Tesla. In the clinical study, the average inversion time to suppress the signal of myocardium at 3.0 Tesla with respect to MR viability imaging at 1.5 Tesla was at 15 min after the administration of contrast agent (304.0±29.2 at 3.0 Tesla vs. 283.9±20.9 at 1.5 Tesla). The contrast between infarction and viable myocardium was equal at both field strengths (4.06±1.30 at 3.0 Tesla vs. 4.42±1.85 at 1.5 Tesla). Even at this early stage, MR viability imaging at 3.0 Tesla provides high quality images in patients with myocardial infarction. The inversion time is significantly prolonged at 3.0 Tesla. The contrast between infarction and viable myocardium at 3.0 Tesla are equal to 1.5 Tesla. Further investigation is needed for this technical improvement, for clinical evaluation, and for limitations.
We studied the feasibility of high spatial and temporally resolved imaging by a combination of keyhole imaging, which is a fast imaging method, and a 3.0T MRI system which inherently has a high signal-to-noise ratio (SNR). In keyhole imaging, full k-space reference data are collected, followed by repeated dynamic acquisition of the central low-frequency data alone, and this makes dynamic acquisition faster. The purpose of our article is to describe the effects of the reference data on the reconstructed dynamic images with the assumption of vessel visualization. For efficient use of keyhole imaging, it is important to optimize the keyhole percentage and timing of reference data acquisition for the object of interest.
A high signal-to-noise ratio (SNR) can be obtained in three-Tesla (3T) MRI, and it is possible to use it to shorten imaging time and improve spatial resolution. However, reports of its disadvantages have been increasing. We attempted to describe a high-resolution evaluation image that made the best use of a decrease in SAR and high SNR by using the LAVA (liver acquisition with volume acceleration) method, a kind of three-dimensional GRE (3D gradient echo) method that did not show the above-mentioned disadvantage in obtaining a shadow inspection of the female pelvic area with 3T MRI. A 0.8 mm isovoxel image of excellent SNR could be obtained within about one and one-half minutes by using the LAVA method as a result of the examination. Moreover, a SAR that was problematic with the 3T MR device was able to be decreased, and was useful.