[Effect of Contrast on FLAIR by Changing the Number of Packages].

We have found that the number of packages influences contrast for brain tissue signals on fluid-attenuated inversion recovery (FLAIR). The purpose of this study was to evaluate the contrast of white and gray matters by changing the number of packages. In a volunteer study (n=8), FLAIR images were obtained with the various number of packages (number of package=2, 3, 4, 5). We investigated the same imaging condition at both 1.5 and 3.0T. The signal intensity of white and gray matters in all volunteers was increased as increasing the number of packages. Moreover, the contrast ratio between white and gray matters was slightly decreased. In our conclusion, the contrast between the gray and white matters on FLAIR was influenced by the number of packages.

Optimized 4D time-of-flight MR angiography using saturation pulse.

PURPOSE: To assess arterial visibility on 4D time-of-flight (4D-TOF) by temporal magnetization transfer contrast pulse (t-MTC) and temporal tilted optimized nonsaturating excitation (t-TONE). 3D-TOF magnetic resonance angiography (MRA) is used for the noninvasive assessment of the intracranial arteries. However, it does not provide temporal information for diagnosing hemodynamics. To noninvasively obtain more detailed hemodynamics-related information, we developed a novel time-resolved MRA without the arterial spin labeling technique, termed 4D-TOF MRA using saturation pulse. MATERIALS AND METHODS: On a 3.0T MRI, three techniques were compared to optimize the visibility of the arteries above the circle of Willis; 1) simple 4D-TOF, 2) 4D-TOF with t-MTC, and 3) 4D-TOF with t-MTC and t-TONE. Eight healthy volunteers were scanned with these three sequences. The contrast changes between the background tissue and the arteries in temporal phases were assessed and compared quantitatively and qualitatively. RESULTS: The contrast between the background and the arteries for 4D-TOF with t-MTC and t-TONE was significantly higher than those for the other methods in delayed phases (P < 0.001).The qualitative assessment showed that 4D-TOF with t-MTC and with t-MTC and t-TONE provided better visualization of the intracranial artery than simple 4D-TOF (P < 0.001). CONCLUSION: 4D-TOF with t-TONE and t-MTC enabled observation of the intracranial hemodynamics. Optimized 4D-TOF provides high-quality images without image subtraction, and good visibility of the intracranial arteries even in the prolonged observation time. J. Magn. Reson. Imaging 2016;43:1320-1326.

[Problem of spectral attenuated with inversion recovery fat suppression method with respiratory-gated].

The purpose of this study was to investigate the effect of fat suppression when we use respiratory-gated spectral attenuated with inversion recovery (SPAIR) method with respiratory-gated. We experimented on phantom and in-vivo study using simulated wave of respiratory-gated SPAIR at 1.5 tesla and 3.0 tesla. As a result, the effect of fat suppression becomes wrong with longer intervals of inspiration and expiration by wave of respiratory-gated. The signal intensity also varies with each slice. This result had the same trend on phantom and in-vivo study. The longitudinal magnetization of fat becomes a stable state when SPAIR pulse is shot more than once. However, the SPAIR method with respiratory-gated collect signal before the longitudinal magnetization of fat to be stable state, and fat suppression effect becomes bad, because the inversion time does not match the null point of the fat. Therefore, when we use SPAIR method with respiratory-gated it always causes bad fat suppression.

Evaluation of motion artefact reduction depending on the artefacts' directions in head MRI using conditional generative adversarial networks.

Motion artefacts caused by the patient's body movements affect magnetic resonance imaging (MRI) accuracy. This study aimed to compare and evaluate the accuracy of motion artefacts correction using a conditional generative adversarial network (CGAN) with an autoencoder and U-net models. The training dataset consisted of motion artefacts generated through simulations. Motion artefacts occur in the phase encoding direction, which is set to either the horizontal or vertical direction of the image. To create T2-weighted axial images with simulated motion artefacts, 5500 head images were used in each direction. Of these data, 90% were used for training, while the remainder were used for the evaluation of image quality. Moreover, the validation data used in the model training consisted of 10% of the training dataset. The training data were divided into horizontal and vertical directions of motion artefact appearance, and the effect of combining this data with the training dataset was verified. The resulting corrected images were evaluated using structural image similarity (SSIM) and peak signal-to-noise ratio (PSNR), and the metrics were compared with the images without motion artefacts. The best improvements in the SSIM and PSNR were observed in the consistent condition in the direction of the occurrence of motion artefacts in the training and evaluation datasets. However, SSIM > 0.9 and PSNR > 29 dB were accomplished for the learning model with both image directions. The latter model exhibited the highest robustness for actual patient motion in head MRI images. Moreover, the image quality of the corrected image with the CGAN was the closest to that of the original image, while the improvement rates for SSIM and PSNR were approximately 26% and 7.7%, respectively. The CGAN model demonstrated a high image reproducibility, and the most significant model was the consistent condition of the learning model and the direction of the appearance of motion artefacts.

[Improvement of Motion Artifacts in Brain MRI Using Deep Learning by Simulation Training Data].

PURPOSE: To test whether deep learning can be used to effectively reduce artifacts in MR images of the brain. METHODS: In this study, a large set of images with and without motion artifacts is needed for training. It is difficult to collect training data from clinical images because it requires a lot of effort and time. We have created motion artifact images of the brain by computer simulation. As an experimental study, we obtained original images for deep learning from 20 volunteers. These original images were used to create various images of different artifacts by computer simulation and these were used the input images for deep learning. The same method was used to create test images and these images were used to compare the structural similarity (SSIM) index and peak signal-to-noise ratio (PSNR) between the input images and output images using the three denoising methods. The network models used were U-shaped fully convolutional network (U-Net), denoising convolutional neural network (DnCNN) and wide inference network and 5 layers Residual learning and batch normalization (Win5RB). RESULTS: U-Net was the most effective model for reducing motion artifacts. The SSIM and PSNR were 0.978 and 32.5 dB. CONCLUSION: This is an effective method to reduce artifacts without degrading the image quality of brain MRI images.

[Development of Motion Artifact Generator for Deep Learning in Brain MRI].

PURPOSE: We focused on deep learning for a reduction of motion artifacts in MRI. It is difficult to collect a large number of images with and without motion artifacts from clinical images. The purpose of this study was to create motion artifact images in MRI by simulation. METHODS: We created motion artifact images by computer simulation. First, 20 different types of vertical pixel-shifted images were created with different shifts, and the amount of pixel shift was set from -10 to 10 pixels. The same method was used to create pixel-shifted images for horizontal shift, diagonal shift, and rotational shift, and a total of 80 types of pixel-shifted images were prepared. These images were Fourier transformed to create 80 types of k-space data. Then, phase encodings in these k-space data were randomly sampled and Fourier transformed to create artifact images. The reproducibility of the simulation images was verified using the deep learning network model of U-net. In this study, the evaluation indices used were the structural similarity index measure (SSIM) and peak signal-to-noise ratio (PSNR). RESULTS: The average SSIM and PSNR for the simulation images were 0.95 and 31.5, respectively; those for the clinical images were 0.96 and 31.1, respectively. CONCLUSION: Our simulation method enables us to create a large number of artifact images in a short time, equivalent to clinical artifact images.

[Influence of imaging parameters on the measurement of apparent diffusion coefficient].

The apparent diffusion coefficient (ADC) values are calculated by using signal intensity in diffusion-weighted images (DWIs) with two or more different b-value. Therefore, the signal to noise ratio (SNR) of DWI with higher b-value may have a big influence on the measured ADC value. We examined the influence of the imaging parameters on the calculated ADC values. The SNR of DWI increased by using a larger voxel size, by means of a decreased number of matrix, an increased slice thickness, and an increased field of view (FOV). However, when the number of excitations was increased to improve the SNR of DWI, the signal intensity of background noise was observed to be slightly increased. It was suggested that the consistency of measured ADC was not preserved when the signal of the DWIs with higher b-value dropped close to the noise level.

Optimal strategy for measuring intraventricular temperature using acceleration motion compensation diffusion-weighted imaging.

DWI thermometry is affected by CSF pulsation. To achieve more accurate determination of intraventricular temperature, we compared conventional DWI (c-DWI), acceleration motion compensation DWI (aMC-DWI), and motion compensation DWI (MC-DWI) when using two different b values (commonly used b value [1000 s/mm2] and theoretically optimized b value according to the diffusion coefficient of the CSF [400 s/mm2]). Eight healthy volunteers were scanned using a 3.0-T magnetic resonance (MR) system. The temperature map was created using the diffusion coefficient from DWI with b = 1000 and 400 s/mm2, respectively. The intraventricular temperatures in the lateral ventricles (LV) with less CSF pulsation, and the third ventricle (TV), which has more CSF pulsation, were compared between three techniques using the Friedman test. We measured the body temperature in the axilla to compare it with the intraventricular temperature. With b = 1000 s/mm2, the intraventricular temperatures in TV for c-DWI were significantly higher (43.12 ± 2.86 °C) than those for the aMC-DWI (37.68 ± 1.66 °C; P < 0.05), whereas those in LV were not significantly different (P = 0.093). With b = 400 s/mm2, the intraventricular temperatures in TV for c-DWI (75.07 ± 5.48 °C) were significantly higher than those for the aMC-DWI (38.63 ± 0.92 °C; P < 0.05), whereas those in LV were not significantly different (P = 0.093). aMC-DWI provided an intraventricular temperature that was close to or slightly higher than the body temperature in either condition. However, c-DWI- and MC-DWI-measured temperatures were higher than the body temperature, particularly in the TV. Thus, aMC-DWI can accurately determine the intraventricular temperature.

[Distortion in diffusion weighted imaging].

The definitional equation of the distortion of echo planar imaging(EPI)was examined. We compared measured values with calculated values by using the definitional equation of the chemical shift of the EPI method that first composed the diffusion-weighted image, and examined the possibility of applying it to the distortion. The results showed that the chemical shift with the definitional equation and the measurement corresponded, and the correlation between the chemical shift and the distortion was acquired. Next, the distortion of each images that composed DWI with the increase and the sign acceptable method difference of b value. The difference in the distortion between each image has increased with the increase in b value. It was assumed that the influence of the eddy currents was due to the high motion probing gradient.

[Study of aliasing ERROR with SENSE in body diffusion image using single-shot EPI at 3T].

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.

[Basic examination of CEMRA with 4D time-resolved angiography using keyhole (4D-TRAK) in 3T pelvic region].

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 degrees 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.

[Trial of artifact reduction in body diffusion weighted imaging development and basic examination of "TRacking Only Navigator"(TRON method)].

In recent years, the utility of body diffusion weighted imaging as represented by diffusion weighted whole body imaging with background body signal suppression (DWIBS), the DWIBS method, is very high. However, there was a problem in the DWIBS method involving the artifact corresponding to the distance of the diaphragm. To provide a solution, the respiratory trigger (RT) method and the navigator echo method were used together. A problem was that scan time extended to the compensation and did not predict the extension rate, although both artifacts were reduced. If we used only navigator real time slice tracking (NRST) from the findings obtained by the DWIBS method, we presumed the artifacts would be ameliorable without the extension of scan time. Thus, the TRacking Only Navigator (TRON) method was developed, and a basic examination was carried out for the liver. An important feature of the TRON method is the lack of the navigator gating window (NGW) and addition of the method of linear interpolation prior to NRST. The method required the passing speed and the distance from the volunteer's diaphragm. The estimated error from the 2D-selective RF pulse (2DSRP) of the TRON method to slice excitation was calculated. The condition of 2D SRP, which did not influence the accuracy of NRST, was required by the movement phantom. The volunteer was scanned, and the evaluation and actual scan time of the image quality were compared with the RT and DWIBS methods. Diaphragm displacement speed and the quantity of displacement were determined in the head and foot directions, and the result was 9 mm/sec, and 15 mm. The estimated error was within 2.5 mm in b-factor 1000 sec/mm(2). The FA of 2DSRP was 15 degrees, and the navigator echo length was 120 mm, which was excellent. In the TRON method, the accuracy of NRST was steady because of line interpolation. The TRON method obtained image quality equal to that of the RT method with the b-factor in the volunteer scanning at short actual scan time. The TRON method can obtain image quality equal to that of the RT method in body diffusion weighted imaging within a short time. Moreover, because scan time during planning becomes actual scan time, inspection can be efficiently executed.

Time-spatial Labeling Inversion Pulse (Time-SLIP) with Pencil Beam Pulse: A Selective Labeling Technique for Observing Cerebrospinal Fluid Flow Dynamics.

We assessed labeling region selectivity on time-spatial labeling inversion pulse (Time-SLIP) with pencil beam pulse (PB Time-SLIP) for the use of visualizing cerebrospinal fluid (CSF) flow dynamics. We compared the selectivity of labeling to the third and fourth ventricles between PB Time-SLIP and conventional Time-SLIP (cTime-SLIP) in eight volunteers and one patient using a 1.5T MRI. PB Time-SLIP provided more selective labeling in CSF than cTime-SLIP, particularly in complex anatomical regions.

[Examination of imaging parameter influence on image distortion in EPI].

Echo planar imaging (EPI) is highly sensitive to static magnetic field inhomogeneities. The degree of local image compression and stretching is a function of the static field gradient in the phase-encoding direction. This is caused by the accumulation of a phase shift. Any static field shift will lead to a position shift in the image, and it is the regions with large static fields that are the most difficult to correct. We reduce image distortion by SENSE with an array coil. However, we often use a surface coil because we cannot use an array coil in clinical studies. In this case, image distortion becomes greater, and reduction of distortion is very important. For the purpose of this study, we examined the relation between imaging parameters and image distortion. Image distortion of EPI is unrelated to the following parameters: number of phase encodings, half scan, echo time, and diffusion b-value. However, the following parameters influenced image distortion: FOV, number of frequency encodings, rectangle FOV, and multi-shot imaging. Image distortion of EPI is decided by the area of the phase-encoding gradient and the interval of readout gradients. We hope that many institutions will find these data useful.

[Basis examination of image contrast in fluid attenuated inversion recovery balanced turbo field echo (FLAIR-B-TFE)].

PURPOSE: We have made clinical use of FLAIR-B-TFE, where an image is taken at the null point (NP) of water with the addition of inversion pulse to B-TFE, and obtained highly effective results in many areas. Changes in NP and image contrast were reviewed to optimize this sequence. MATERIALS AND METHODS: Oil, water, and venous blood before and after Gd-DTPA dispensation as well as diluted (by 500/4000 times) Gd-DTPA solution were designated as the standard phantoms wherein shot intervals (SI), scan modes, k-space ordering, TFE factor, dummy pulse, and presence or absence of IR pulses were changed. RESULTS: What affects the NP of water most is the SI, and unless SI is long enough so that the longitudinal magnetization of water can recover to the full, NP will change. There is little difference in image contrast between the NP of water and that of blood, and a sluggish blood signal ascendance will necessitate intentional blood signal ascendance by contrast-enhancement. The signal intensity of blood after the angiographies will almost reach a plateau at an SI level of more than 2000 ms. Therefore, it is appropriate to apply SI 2000 ms, in view of the time necessary for contrast-enhancement.

[Influence of respiratory motion in body diffusion weighted imaging under free breathing (examination of a moving phantom)].

Diffusion weighted imaging (DWI) theoretically aims to detect random motion over small distances, such as Brownian motion. Therefore, breath-hold scanning has been considered the only way to acquire DWI in the body without artifacts from bulk motion. Recent reports suggest that non-breath-hold scanning is feasible. The purpose of this study was to evaluate the influence of respiratory motion on DWI using a moving phantom model. Our results showed that the difference in apparent diffusion coefficient (ADC) was less than 10% between a static phantom and a moving phantom. There was no relation between the speed and stroke of the moving phantom and the calculated ADC. The results indicate that stable motion such as calm respiration does not cause signal loss on DWI, in contrast to intra-voxel incoherent motion (IVIM). The images obtained using this method showed high resolution and signal-to-noise ratio (SNR), suitable for three-dimensional display of the lesion.

Activation of pre-supplementary motor area (SMA) and SMA proper during unimanual and bimanual complex sequences: an analysis using functional magnetic resonance imaging.

Several functional imaging studies have shown that the extent of activation and percentage change in cerebral blood flow in the supplementary motor area (SMA) during a bimanual mirror performance of a simple repetitive movement are almost identical to those during a unimanual movement. The aim of this study was to investigate whether this finding was also applicable to a more complex movement. Eight right-handed, healthy volunteers performed unimanually (with their right and left hands) and bimanually (in a mirror fashion) thumb-finger opposition in a nonconsecutive order (index-middle-index-ring-index-little-index-middle ... fingers). The SMA proper was more activated during the bimanual movement than the unimanual movement with either hand. This is in accordance with the hypothesis that bimanual movement, even in a mirror fashion, is more difficult than unimanual movement when the task is complex but not when the task is simple. Pre-SMA was inconsistently activated. The results suggest that the SMA proper plays an active role in executive processing during bimanual mirror performance of complex movements.

[Studies of artifacts with TFE factor increase in heart delayed enhancement MRI sequence IR-T1TFE].

The characteristic of delayed enhancement MRI is high spatial resolution, which makes it possible to evaluate the degree of damage to the myocardium from the inner to the outer membrane of patients with ischemic heart disease. Therefore, this MRI technique is unique in its ability to detect myocardial condition and is necessary to obtain high-quality images. We experienced artifacts induced by TFE factor increase with delayed enhancement of IR-T1TFE. The purpose of this study was to determine the cause of such artifacts. IR-T1TFE changed signal intensity with phase direction as the TFE factor increased. Streak artifacts occurred because signal intensity caused changes with phase direction. Increases in TFE factor prolonged data collection time, such that marked artifacts were created because of changes in signal intensity. Ghost artifacts occurred because signal intensity changed between shots. When the TFE factor was increased, the difference in signal intensity was diminished between shots. The interval of acquired noise decreased in the raw data. Therefore, the interval of ghost artifacts became wider on images.