[Optimization of Echo Train Length in Non-contrast Enhanced MR Angiography for Clinical Examination of the Calf Arteries].

PURPOSE: Non-contrast magnetic resonanse angiography (MRA) using the three-dimensional electrocardiogram-synchronized fast spin echo method uses systolic and diastolic arterial signal differences. The method relies on the flow void signal of the arterial flow because of dephasing during systole. However, depiction of slow flow such as that in a calf artery was degraded because of insufficient dephasing during systole. In this study, we optimized echo train length (ETL) using a flow phantom and normal volunteers for clinical examination of the calf arteries. METHODS: Flow phantom and normal volunteer images were obtained with various ETLs (40, 50, 60, and 70). An averaged profile across the tube in the phantom was used for detailed investigation of flow dephasing. Visual evaluation was performed and signal intensity change along vessels was measured using normal volunteer images. Comparison with peak systolic velocity (PSV) measured using ultrasound equipment was also conducted. RESULTS: Results of the flow phantom and normal volunteer study indicated that the overall depictability was improved with ETL 60 and 70, which was higher than the standard value. Additionally, the visualization of the peroneal artery with low PSV of ETL 70 had better depictability than ETL 60. CONCLUSION: This study suggested that ETL 70 might be better for clinical examination of the calf arteries.

[Distortion Reduction Effect of View Angle Tilting (VAT) in Large Field of View Magnetic Resonance Imaging].

With shortening of the gantry of magnetic resonance imaging (MRI) systems, large field-of-view (FOV) imaging has become difficult because static magnetic field nonuniformity and gradient magnetic field nonlinearity exacerbate geometric distortion of MR images. However, results of earlier studies have demonstrated that view angle tilting (VAT) can reduce severe image distortion attributable to local susceptibility effects of metals. Although VAT is usually applied to local magnetic field nonuniformity, in principle VAT is expected to correct distortion also for peripheral images in large-FOV MRI. Results from this phantom experiment using VAT with large-FOV verified the effectiveness of distortion correction. The experiment using VAT showed reduction of maximum distortion from 23.6 to -1.9 mm. Furthermore, results of a volunteer study confirmed the distortion correction capability of VAT: it reduced distortion and improved visibility of the anatomical structure. In conclusion, experimentally obtained results underscore VAT effectiveness for improving distortion in large-FOV MRI.

[Evaluation of Resolution Characteristics in Three-dimensional MR Imaging Using Single Plate Thin-Ramp Method].

Recent remarkable progress of fast imaging techniques in 3D MRI has emphasized the importance of evaluation of its resolution characteristics. A trial point spread function (PSF) measurement was conducted using the ramp method, a conventional measurement method for the slice profile of 2D imaging, as an approach to evaluate the resolution characteristics of 3D imaging. However, problems peculiar to 3D imaging have arisen, such as artifacts and offsets in the slice selective direction. Therefore, we attempted PSF measurement using a phantom having a single-plate construction with only a simple ramp part (Single Plate Thin-Ramp Method) to respond to these shortcomings. By employing appropriate positioning and simple post processing, we obtained a PSF easily in both phase encoding and slice selective directions without the artifacts and offsets described above. Furthermore, it was possible to evaluate the change of resolution characteristics depending on the scan condition in three-dimensional MR image.

[Evaluation of Vessel Depictability in Compressed Sensing MR Angiography Using Numerical Phantom Model].

As an acceleration technique for use with magnetic resonance imaging (MRI), compressed sensing MRI (CSMRI) was introduced recently to obtain MR images from under sampled k-space data. Images generated using a nonlinear iterative procedure based on sophisticated theory in informatics using data sparsity have complicated characteristics. Therefore, the factors affecting image quality (IQ) in CS-MRI must be elucidated. This article specifically describes the examination of the IQ of clinically important MR angiography (MRA). For MRA, the depictability of thin blood vessels is extremely important, but quantitative evaluation of thin blood vessel depictability is difficult. Therefore, we conducted numerical experiments using a simple numerical phantom model mimicking the cerebral arteries so that the experimental conditions, including the thin vessel positions, can be given. Results show that vessel depictability changed depending on the noise intensity when the wavelet transform was used as the sparsifying transform. Decreased vessel depictability might present difficulties at the clinical signal-to-noise ratio (SNR) level. Therefore, selecting data acquisition and reconstruction conditions carefully in terms of the SNR is crucially important for CS-MRI study.

Scan parameters and the diffusion emphasis effect in diffusion-weighted imaging using a motion-probing gradient preparation pulse.

Diffusion-sensitized driven equilibrium preparation (DSDE) is a gradient echo (GRE) diffusion-weighted imaging (DWI) sequence that employs a motion-probing gradient (MPG) preparation pulse and phase cycling. In DSDE, several scan parameters of the MPG preparation pulse and the GRE sequence affect diffusion sensitivity. Our investigation of the relationship between these scan parameters and the diffusion emphasis effect revealed the importance of "prep.TE" in the MPG preparation pulse and "TFE shot interval" in the gradient echo sequence. Appropriate choice of these parameters allows DSDE to provide a similar DWI to that of conventional single-shot SEEPI DWI. We therefore concluded DSDE to be a useful DWI method.

Denoising parameter dependence of coronary artery depictability in compressed sensing magnetic resonance angiography.

Using numerical indices and visual evaluation, we evaluated the dependence of coronary-artery depictability on the denoising parameter in compressed sensing magnetic resonance angiography (CS-MRA). This study was conducted to clarify the acceleration factor (AF) and denoising factor (DF) dependence of CS-MRA image quality. Vascular phantoms and clinical images were acquired using three-dimensional CS-MRA on a clinical 1.5 T system. For the phantom measurements, we compared the full width at half maximum (FWHM), sharpness, and contrast ratio of the vascular profile curves for various AFs and DFs. In the clinical cases, the FWHM, sharpness, contrast ratio, signal-to-noise ratio, noise level values, and visual evaluation results were compared for various DFs. Phantom image analyses demonstrated that the respective measurements of the FWHM, sharpness, and contrast ratios did not significantly change with an increase in AF. The FWHM and sharpness measurements slightly changed with the DF level. However, the contrast ratio tended to increase with an increase in the DF level. In the clinical cases, the FWHM and sharpness showed no significant differences, even when the DF level was changed. However, the contrast ratio tended to decrease as the DF level increased. When the DF levels of the clinical cases increased, the background signals of the myocardium, fat, and noise levels decreased. We investigated the dependence of the coronary-artery depictability on AF and DF using CS-MRA. Analysis of the coronary-artery profile curves indicated that a better image quality was achieved with a stronger DF on coronary CS-MRA.

Modulation transfer function measurement of three-dimensional T1-weighted turbo spin echo sequence with low refocusing flip angles using single-plate method.

This study aimed to demonstrate the usefulness of modulation transfer function (MTF) measurements using the single-plate method to evaluate changes in resolution properties that are dependent on three parameters: echo train length (ETL), low refocusing flip angle (RFA), and start-up echo in three-dimensional T1-weighted turbo spin echoes (TSE) with a low RFA and to optimize these parameters. Although the MTFs were slightly degraded with an RFA of 120°, they were considerably degraded with an RFA of ≤ 90°. On the other hand, the MTF of low RFA was greatly improved by setting the start-up echo, allowing setting a long ETL. The single-plate method provided a clear and easy evaluation of the resolution properties of low RFA TSE. Furthermore, this method allows us to visualize changes in the signal intensity of each echo in k-space, depending on the sequence variation. These results suggest that the MTF measurement using the single-plate method is useful for evaluating the resolution properties of TSE sequences and optimizing the measured parameters.

Using Spin-labeled MR Imaging to Depict Slow Pancreatic Juice Flow.

This study evaluated the dependence of the signal characteristics of time-spatial labeling inversion pulse (time-SLIP) on flow velocity and tag thickness to depict the pancreatic juice flow by analyzing signal profile using a tube phantom study. The tag edge property was evaluated by edge rise distance (ERD). For various slow flow velocities and tag thicknesses, the signal profile characteristics were evaluated using two indices: the tag center value (RTCV) reduction rate and the total signal value along the tube (TSVT). ERD, which was about 10% of the tag thickness, was higher for thicker tags, making slow flow detection difficult. TSVT was proportional to the thickness of the tag and was preserved irrespective of the flow velocity. RTCV became lower with higher flow velocity and decreased significantly with thinner tags. These results suggest that the visualization of pancreatic juice flow might improve stability by considering the appropriate tag thickness.

Comparison of Slab and Pencil Beam Labeling in Spin-labeled MR Imaging for Pancreatic Juice Flow Visualization.

The usefulness of a highly targeted pencil beam (PB) label was compared with the commonly used slab label for direct visualization of pancreaticobiliary reflux using the time-spatial labeling inversion pulse (time-SLIP) technique. Signal profiles of flow phantom images obtained with a 1.5T MRI were analyzed. Both labels had similar labeling capabilities, but the edge characteristics of the PB label were blunt. Next, sixty-eight patients were classified into two groups according to the angle of the pancreaticobiliary ducts, and the displacement of the pancreatic ducts in respiratory fluctuation was measured. The results were approximately 7 mm in both groups. The blunt edge characteristics of the PB label suggest that it is robust to respiratory fluctuations. The overall labeling ability of the PB is comparable to that of the slab. In the larger angle of pancreaticobiliary ducts, the PB label may be able to label the pancreatic duct more selectively.

[Effects of region of interest settings in signal-to-noise ratio measurement of clinical images using arrays of multiple receiver coils].

In many clinical imaging procedures using arrays of multiple receiver coils, a uniform sensitivity process is performed using the sensitivity distribution from the body coil. This causes the noise to be uneven, and background noise cannot be used when measuring the signal-to-noise ratio (SNR). The SNR of clinical images with sensitivity correction using arrays of multiple receiver coils sets the region of interest (ROI) in the region where the signal is uniform, and is limited to the identical ROI method where measurements are taken with noise from the identical region. When SNR is measured with the identical ROI method, uneven noise caused by sensitivity correction as well as the signal strength distribution within the ROI of the object is reflected in the noise. Therefore, evaluation must be performed in as localized a position as possible. Measurement error becomes small on images with higher resolution, and if ROI larger than 10×10 pixels can be set in a region of even signal, SNR measurement of clinical images with less underestimation may be possible.

Noise power spectrum in compressed sensing magnetic resonance imaging.

Compressed sensing magnetic resonance imaging (CS-MRI) uses random undersampling and nonlinear iterative reconstruction. This study was conducted to clarify the noise power spectrum (NPS) characteristics of CS-MRI. We measured two-dimensional (2D) NPS of CS-MRI with various acceleration factors (AF) and denoising factors (DF) and compared their appearance to those of conventional parallel MR images. Results showed that the 2D NPS of CS-MRI exhibited the following characteristics: (1) local decrease in the low-frequency region, (2) gradual decrease in the high-frequency region, and (3) a stripe pattern aligned at unequal intervals in the phase-encoding direction. Specifically, the 2D NPS of CS-MRI reflects the random undersampling pattern of k-space data. Additionally, 2D NPS allowed visualization of AF-dependent noise characteristics of CS-MRI. Furthermore, 1D NPS graph shapes clarified the CS-MRI noise characteristic dependence on AF and DF.

Single-plate method for practical modulation transfer function measurement in magnetic resonance imaging.

A new "single-plate method" is presented for measuring the modulation transfer function (MTF) in magnetic resonance imaging (MRI). This method sets a slice plane perpendicular to a single-plate phantom to eliminate contamination effects from the direction perpendicular to the measurement direction in the image plane, which occur with a conventional ramp method. As no practical method for measuring the MTF has been established for MRI, we examined whether the MTF can be measured practically using the single-plate method for various fast imaging sequences. Furthermore, the MTFs of T1-weighted (T1W) fast spin echo (FSE) and conventional spin echo (CSE) images obtained using the single-plate method and ramp method were compared. The measured MTFs of T1W CSE images revealed rectangular shapes with a sharp decrease near the Nyquist cutoff frequency in both phase-encoding (PE) and frequency-encoding (readout, RO) directions. The measured MTFs of T1W FSE images obtained with centric-order acquisition showed symmetric step-function shapes reflecting k-space segmentation determined by the echo train length (ETL). The measured MTFs of T2-weighted (T2W) FSE images showed asymmetric step-function shapes reflecting differences in T2 decay of signals from samples. The MTFs obtained using the single-plate method significantly reduced the collapse caused by the contamination effect, which is observed in all the MTF measurements of the ramp method. The proposed "single-plate method" simplified the complicated MTF measurement procedure and eliminated the contamination effect. This method is expected to be useful for evaluating the resolution properties of MR fast imaging techniques with a complicated k-space trajectory.

[Evaluation of an adaptive filter for CT under low-CNR condition: comparison with linear filter].

The use of an adaptive filter for CT images is becoming a common procedure and is said to reduce image noise while preserving sharpness and helping to reduce the required X-ray dose. Although many reports support this view, the validity of such evaluations is arguable. When the linearity of a system is in question, physical performance indexes should be measured under conditions similar to those of clinical use. Evaluations of diagnosis using clinical images may be fallible because the non-filtered image used as the reference might not have been optimally reconstructed. We have chosen simple, but commonly used, adaptive filters for our evaluation. As a reference for comparing performance, we designed linear filters that best approximate the noise characteristics of the adaptive filters. MTF is measured through observation of the edge-spread function. Clinical abdominal images are used to compare the performance of adaptive filters and linear filters. We conclude that the performance of the type of adaptive filter we have chosen is virtually the same as that of the linear filter, as long as the image quality of soft tissues is our interest. Both the noise SD and MTF are virtually the same if the contrast of the object is not substantially higher than 150 HU. Images of soft tissues obtained with the use of adaptive filters are also virtually the same as those obtained by linear filters. The edge-preservation characteristic of this adaptive filter is not observable for soft tissues.

[Signal-to-noise ratio measurement in parallel MRI with subtraction mapping and consecutive methods].

When measuring the signal-to-noise ratio (SNR) of an image the used parallel magnetic resonance imaging, it was confirmed that there was a problem in the application of past SNR measurement. With the method of measuring the noise from the background signal, SNR with parallel imaging was higher than that without parallel imaging. In the subtraction method (NEMA standard), which sets a wide region of interest, the white noise was not evaluated correctly although SNR was close to the theoretical value. We proposed two techniques because SNR in parallel imaging was not uniform according to inhomogeneity of the coil sensitivity distribution and geometry factor. Using the first method (subtraction mapping), two images were scanned with identical parameters. The SNR in each pixel divided the running mean (7 by 7 pixels in neighborhood) by standard deviation/radical2 in the same region of interest. Using the second (consecutive) method, more than fifty consecutive scans of the uniform phantom were obtained with identical scan parameters. Then the SNR was calculated from the ratio of mean signal intensity to the standard deviation in each pixel on a series of images. Moreover, geometry factors were calculated from SNRs with and without parallel imaging. The SNR and geometry factor using parallel imaging in the subtraction mapping method agreed with those of the consecutive method. Both methods make it possible to obtain a more detailed determination of SNR in parallel imaging and to calculate the geometry factor.

[Method of SNR determination using clinical images].

Parallel magnetic resonance imaging (MRI) with arrays of receiver coils such as those of sensitivity encoding (SENSE) are being widely used. However, conventional methods of image signal-to-noise ratio (SNR) determination cannot be used for parallel MRI, and a novel method has been reported in JSRT. However, this method of SNR determination is for phantom images not for clinical images. Therefore, we researched accurate measurement of the image noise of parallel MRI reconstruction including the unfolding process and uniformity filters. The possibility of a subtraction method using clinical images and the accuracy of standard deviation (SD) of clinical images for optimum ROI were studied because it was not possible to use the air-signal method. The results indicated that the position of the ROI was selected for uniformity of signal intensity area and that the size of the ROI was about 50 pixels. However, under these conditions, the noise value of SNR was higher than that using the phantom-subtraction method. In addition, the tissue-subtraction method was useful when the two scanning images were in agreement.

Relation between one- and two-dimensional noise power spectra of magnetic resonance images.

Our purpose in this study was to elucidate the relation between the one-dimensional (1D) and two-dimensional (2D) noise power spectra (NPSs) in magnetic resonance imaging (MRI). We measured the 1D NPSs using the slit method and the radial frequency method. In the slit method, numerical slits 1 pixel wide and L pixels long were placed on a noise image (128 × 128 pixels) and scanned in the MR image domain. We obtained the 1D NPS using the slit method (1D NPS_Slit) and the 2D NPS of the noise region scanned by the slit (2D NPS_Slit). We also obtained 1D NPS using the radial frequency method (1D NPS_Radial) by averaging the NPS values on the circumference of a circle centered at the origin of the original 2D NPS. The properties of the 1D NPS_Slits varied with L and the scanning direction in PROPELLER MRI. The 2D NPS_Slit shapes matched that of the original 2D NPS, but were compressed by L/128. The central line profiles of the 2D NPS_Slits and the 1D NPS_Slits matched exactly. Therefore, the 1D NPS_Slits reflected not only the NPS values on the central axis of the original 2D NPS, but also the NPS values around the central axis. Moreover, the measurement precisions of the 1D NPS_Slits were lower than those of the 1D NPS_Radial. Consequently, it is necessary to select the approach applied for 1D NPS measurements according to the data acquisition method and the purpose of the noise evaluation.

Effectiveness of Disaster-prevention Technologies against Quake-induced Damage of MR Scanners during the Great East Japan Earthquake.

In the present study, we have performed a statistical analysis to investigate damages in magnetic resonance (MR) scanners caused by the Great East Japan Earthquake (GEJE, magnitude 9.0) and evaluated whether these disaster-prevention technologies contributed to the reduction of damages in the GEJE or not. It was confirmed that the extent of damage was significantly different between seismic scale (SS) 5 and SS over 6. Our survey study demonstrated that anchoring of MR facilities reduced damages due to quakes and demonstrated that anchoring is an efficient method for quake-induced damage prevention. The odds ratio revealed that base isolation was very useful to prevent damages in MR scanners.

Enantiomeric purity determination of acetyl-L-carnitine by NMR with chiral lanthanide shift reagents.

Enantiomer signal separation of acetyl-carnitine chloride was obtained on a 500 MHz Nuclear Magnetic Resonance (1H NMR) analysis by fast diastereomeric interaction with chiral shift reagents such as chiral lanthanide-camphorato or chiral samarium-pdta shift reagents. Effects of the kinds of chiral shift reagents and the molar ratio of chiral shift reagent to acetyl-carnitine chloride on enantiomer signal separation were investigated and evaluated. Optimization of the experimental conditions provided two significant split signals for the enantiomers, leading to the successful quantitative analysis. Distinguishment of 0.5% of the minor enantiomer (D-form) in acetyl-L-carnitine chloride was found to be possible by 1H NMR with tris[3-(heptafluoropropylhydroxymethylene)-D-camphorato] and praseodymium derivative, (Pr[hfc]3), as chiral shift reagents.