[Evaluation of the Effectiveness of a Neck Fixation Device for Improving Image Quality in Diffusion-weighted Whole-body Imaging with Background Suppression].

The effectiveness of a neck fixation device to improve the image quality of DWIBS was investigated. Healthy volunteers were examined while chewing with and without a neck fixation device using a 3-T MRI system. Distance of mandibular movement was measured using true-fast imaging of steady-state precession (true FISP). Signal-to-noise ratio (SNR) and apparent diffusion coefficient (ADC) of DWIBS were measured. Image quality of DWIBS was scored by visual evaluation. These values were compared with and without a neck fixation device. Regarding results, the mandibular displacement and ADC were decreased, and the SNR and visual score were increased by the use of the fixation device. There is a significant difference between with and without a neck fixation device in each measurement. The technique using a neck fixation device helps improve image quality of DWIBS in the head and neck region.

Fast B1 Mapping Based on Double-Angle Method with T1 Correction Using Standard Pulse Sequence.

Radiofrequency (RF) field (B1) mapping by combining the double-angle method (DAM) and T1 correction was investigated. The signal intensities S1 and S2 acquired by flip angle (FA) α and double FA 2α at short repetition time (TR) were converted to a signal intensity at TR=∞ by T1 correction. Then, these were used for DAM calculation. The T1 values are measured from two different images acquired with different TRs based on the saturation recovery (SR) method preliminarily. The effects of imaging parameters for T1 estimation and measured FA were investigated using CuSO4-doped water phantoms. A two-dimensional gradient echo type echo planar imaging pulse sequence was used. T1 values obtained by the 2-SR method were underestimated compared to the multipoint inversion recovery method. FA error was less than 5% when the appropriate imaging parameters were used. The acquisition time could be shortened to under 25 s by the use of T1-corrected DAM.

[Imaging Parameter Optimization of Non-contrast Three-dimensional Time-of-flight Magnetic Resonance Angiography for Patients with Intracranial Stents Using a 1.5 T Magnetic Resonance Imaging System].

The imaging parameters of non-contrast three-dimensional time-of-flight magnetic resonance angiography (3D TOF-MRA) were optimized to improve the image quality for patients treated using stent-assisted coiling. A simulated blood flow phantom with three types of stents (Enterprise 2, Neuroform Atlas, and LVIS) was imaged by changing echo time (TE), band width (BW), flip angle (FA), and matrix (phase, frequency). The difference between the signal intensity in the simulated vessel and the background was measured at each imaging condition. The ratio of this difference with and without the stent was evaluated as the relative in-stent signal (RIS). In addition, the error ratio of the stent lumen diameter was assessed by comparing the full width at half maximum (FWHM) to that measured by 3D X-ray angiography. The RIS was higher in order of LVIS, Neuroform Atlas, and Enterprise 2 in all conditions. The RIS was higher in imaging conditions with short TE, narrow BW, high FA, and large phase matrix. The highest RIS was seen with a frequency matrix of 320 in the Enterprise 2 and 256 in the others. FWHM error ratio was smaller in the same order as the RIS. FWHM error ratio was smaller in imaging conditions with short TE, large frequency matrix (>384), large phase matrix (>224), and high FA (>20°). Imaging conditions of 3D TOF-MRA that were effective to improve the image quality for stent lumen evaluation were short TE and high spatial resolution.

Wide slab is useful for routine quality control of MRI slice thickness.

Although slice thickness accuracy is important for the performance of magnetic resonance imaging systems, long scan times are required to perform reliable measurements. Inclined slabs and wedges are conventionally used as test devices to obtain slice profiles. In this study, a novel dedicated device with a widened slab was created, and its efficacy was compared with that of a conventional wedge. The signal-to-noise ratio (SNR) of the profile and the coefficient of variation (CV) of the measured slice thickness were measured. Wide slab usage showed sufficient SNR by averaging multiple profile lines, even with single acquisition. Therefore, it is possible to substantially shorten the measurement time. When ≥ 20 lines were averaged, CV was < 1%. Furthermore, a 200-mm slab width enabled evaluation of the positional dependence of slice thickness in a single imaging. Thus, quality control of MRI slice thickness can be easily implemented with this device.

Proposal of a New Index Based on Signal-to-Noise Ratio for Low-contrast Detectability in Computed Tomographic Imaging.

The low-contrast detectability of computed tomography (CT) images is commonly evaluated by the contrast-to-noise ratio (CNR) because of its convenience to measure. However, the correlation between CNR and visual detectability is poor because the CNR is a simple index determined by both the contrast of the object and the standard deviation of the image noise. On the other hand, the signal-to-noise ratio (SNR), especially SNR based on the statistical decision theory model (SNRS, D) and SNR based on the matched-filter model (SNRM) are considered superior to CNR. In this study, we investigated a new physical image quality index for evaluating low-contrast detectability (SNRA), which is approximately derived from SNRS, D and SNRM. The new index, which was calculated using the object size, contrast of the object and the noise power spectrum, provided good approximations when the diameter of the rod object was equal and >5 mm. The diameter dependency of the SNRA was also found to provide better sensitivity than the sensitivities of CNR and object-specific CNR, similar to SNRS, D and SNRM. The results suggested that the proposed convenient index should be useful for evaluating the low-contrast detectability of CT images.

Simultaneous acquisition of high-contrast and quantitative liver T1 images using 3D phase-sensitive inversion recovery: a feasibility study.

Background Tumor-to-liver contrast is low in images of chronically diseased livers because gadolinium-based hepatocyte-specific contrast agents (Gd-EOB-DTPA) accumulate less to hepatocytes. Purpose To determine whether phase-sensitive inversion recovery (PSIR) could improve the T1 contrasts of Gd-based contrast agents and liver parenchyma and simultaneously provide accurate T1 values for abdominal organs. Material and Methods The image contrasts of phantoms with different Gd concentrations that were obtained using PSIR were compared to conventional turbo field echo (TFE) results. T1 value was estimated using PSIR by performing iterations to investigate the two IR magnetization evolutions. The estimated T1 values were validated using IR-spin echo (IR-SE) and Look-Locker (L-L) sequences. In an in vivo study, the liver-to-spleen and liver-to-muscle contrasts of the PSIR and TFE images of seven volunteers were compared, as were the T1 values of liver parenchyma, spleen, and muscle obtained using PSIR and L-L sequences. Results The PSIR images showed T1 contrasts higher than those in the TFE results. The PSIR and IR-SE T1 values were linearly correlated. Additionally, the R1 estimated using PSIR were correlated with those measured using IR-SE and L-L. In the in vivo study, the liver-to-spleen and liver-to-muscle contrasts of PSIR were significantly higher than those obtained using TFE. T1 values of abdominal organs obtained using PSIR and L-L were clearly correlated. Conclusion PSIR may be capable of improving liver image T1 contrasts when Gd-based contrast agents are employed and simultaneously yielding accurate T1 values of abdominal organs.

[A Low-temperature Manganese Chloride Tetrahydrate Improves the Image Quality of Magnetic Resonance Cholangiopancreatography].

Manganese chloride tetrahydrate (MCT) is one of the oral negative contrast agents which is indispensable for imaging of magnetic resonance cholangiopancreatography (MRCP). In this study, improvement of the image quality of MRCP by using low-temperature MCT is verified. All MR imagings were performed using 1.5 T scanner. The T(1) and T(2) values of the different temperature MCTs were measured in the phantom study. Different concentrations of MCT-doped water (30%, 50%, 70%, and 90%) were measured at several temperature conditions (10°C, 15°C, 23°C, 35°C, and 40°C). As a result, the T(1) and T(2) values became larger with a temperature rise. It was more remarkable in low-concentration MCT. Then, 17 healthy subjects were scanned two times with different temperatures of MCT. The MCT of the normal temperature (23°C) and low temperature (10°C) were taken at consecutive 2 days. The contrast between the stomach and the spleen were significantly higher in 2D half Fourier acquisition single shot turbo spin echo (HASTE) images by use of the low-temperature MCT. The contrast between the common bile duct and the adjacent background were significantly higher in the source images of 3D MRCP by use of the low temperature MCT. In addition, 76% of subjects answered in the questionnaire that the low temperature MCT is easier to drink. The low temperature MCT improves the image quality of MRCP and contributes to performing noninvasive examination.

Fat-subtracted three-dimensional time-of-flight MR angiography of the neck by use of fat-only images with the two-point Dixon technique.

For improvement of three-dimensional time-of-flight magnetic resonance angiography (3D-TOF-MRA) image quality in the neck, fat-subtracted MRA by use of the two-point Dixon technique was compared with conventional fat-suppressed MRA techniques. Three different types of neck 3D-TOF-MRA were obtained [minimum echo time (TE) (1.9 ms), opposed-phase TE (3.4 ms), and chemical shift selective fat suppression (CHESS) (TE = 1.9 ms)] on five volunteers at 3.0 T. MRA was obtained with subtraction of fat-only images (produced by a two-point Dixon sequence) from minimum-TE MRA images, and compared with other fat-suppressed MRA images. Fat-subtracted MRA demonstrated uniform fat suppression compared with other techniques. The mean vessel-to-fat contrast in fat-subtracted MRA was significantly higher (p < 0.01) than in other MRA images (minimum-TE: 0.137 ± 0.086, opposed-phase TE: 0.268 ± 0.102, CHESS: 0.307 ± 0.052, fat-subtracted: 0.965 ± 0.101). The mean vessel-to-muscle contrast in opposed-phase TE MRA was significantly lower (p < 0.01) than in other MRA images (minimum-TE: 0.526 ± 0.036, opposed-phase TE: 0.419 ± 0.188, CHESS: 0.511 ± 0.023, fat-subtracted: 0.573 ± 0.016). Fat-subtracted MRA by use of the two-point Dixon technique improves the image quality of neck MRA. This technique would be a useful method for MRA, especially in areas with inhomogeneous magnetic fields, such as the neck.

Optimization of inversion time for postmortem short-tau inversion recovery (STIR) MR imaging.

PURPOSE: Signal intensity and image contrast differ between postmortem magnetic resonance (PMMR) images and images acquired from the living body. We sought to achieve sufficient fat suppression with short-tau inversion recovery (STIR) PMMR imaging by optimizing inversion time (TI). MATERIAL AND METHODS: We subjected 37 deceased adult patients to PMMR imaging at 1.5 tesla 8 to 60 hours after confirmation of death and measured T1 values of areas of subcutaneous fat with relaxation time maps. Rectal temperature (RT) measured immediately after PMMR ranged from 6 to 31°C. We used Pearson's correlation coefficient to analyze the relationship between T1 and relaxation time (RT). We compared STIR images from 4 cadavers acquired with a TI commonly used in the living body and another TI calculated from the linear regression of T1 and RT. RESULTS: T1 values of subcutaneous fat ranged from 89.4 to 182.2 ms. There was a strong, positive, and significant correlation between T1 and RT (r = 0.91, P < 0.0001). The regression expression for the relationship was T1 = 2.6*RT + 90 at a field strength of 1.5T. The subcutaneous fat signal was suppressed more effectively with the optimized TI. CONCLUSION: The T1 value of subcutaneous fat in PMMR correlates linearly with body temperature. Using this correlation to determine TI, fat suppression with PMMR STIR imaging can be easily improved.

Feasibility of MR perfusion-weighted imaging by use of a time-spatial labeling inversion pulse.

Perfusion-weighted imaging (PWI) by use of arterial spin labeling (ASL) has been introduced to the clinical setting. However, it is not widely available because it requires specialized pulse sequences. Imaging using a time-spatial labeling inversion pulse (time-SLIP), which is a magnetic resonance angiography (MRA) technique that is based on ASL, can be used in various situations. In this study, we examined the feasibility of time-SLIP PWI. Two types of time-SLIP sequences were evaluated: (1) a single inversion recovery (IR) pulse sequence, which is the same as that used in conventional time-SLIP MRA except for the timing of data acquisition, and (2) a dual IR pulse sequence, where a second, non-selective, IR pulse was added during the inflow time to suppress background signals. Subtraction processing is performed between the "on" and "off" settings of the first IR pulse (time-SLIP tag) to obtain PWI. The average signal intensity was measured in a uniform phantom as the residual of the background, and in five healthy subjects as the perfusion signal. The average signal-to-noise ratio (SNR) was also measured in the five subjects. All imaging was performed with a 1.5-T MR scanner. Images using the dual IR method showed lower background signals and higher perfusion signals compared with images using the single IR method. However, the SNR was lower in images with the dual IR method. These results demonstrate that a time-SLIP, which is an MRA method, can be used for obtaining cerebral PWI simply by adjusting the imaging parameters.

[Measurement of slice thickness in magnetic resonance image by the impulse response].

The purpose of this study was to verify the applicability of measurement of slice thickness of magnetic resonance imaging (MRI) by the delta method, and to discuss the measurement precision by the disk diameter and baseline setup of the slice profile of the delta method. The delta method used the phantom which put in the disk made of acrylic plastic. The delta method measured the full width at half maximum (FWHM) and the full width at tenth maximum (FWTM) from the slice profile of the disk signal. Evaluation of the measurement precision of the delta method by the disk diameter and baseline setup were verified by comparison of the FWHM and FWTM. In addition, evaluation of the applicability of the delta method was verified by comparison of the FWHM and FWTM using the wedge method. The baseline setup had the proper signal intensity of an average of 10 slices in the disk images. There were statistically significant difference in the FWHM between disk diameter of 10 mm and disk diameter of 30 mm and 5 mm. The FWHM of the disk diameter of 10 mm was smaller than the disk diameter of 30 mm and 5 mm. There was no statistically significant difference in the FWHM between the delta method and the wedge method. There is no difference in the effective slice thickness of the delta method and the wedge method. The delta method has an advantage in measurement of thin slice thickness.

[Comparison of diffusion tensor imaging-derived fractional anisotropy in multiple centers for identical human subjects].

The fractional anisotropy (FA) is calculated by using diffusion tensor imaging (DTI) with multiple motion probing gradients (MPG). While FA has become a widely used tool to detect moderate changes in water diffusion in brain tissue, the measured value is sensitive to scan parameters (e.g. MPG-direction, signal to noise ratio, etc.). Therefore, it is paramount to address the reproducibility of DTI measurements among multiple centers. The purpose of this study was to assess the inter-center variability of FA. We studied five healthy volunteers who underwent DTI brain scanning three times at three different centers (I-III), each with a 1.5 T scanner having a different MPG-schema. Then, we compared the FA and eigenvalue from the three centers measured in seven brain regions: splenium of corpus callosum (CCs), genu of corpus callosum (CCg), putamen, posterior limb of internal capsule, cerebral peduncle, optic radiation, and middle cerebellar peduncle. At the CCs and CCg, there was a statistical difference (p<0.05) between center Iand center IIfor the same MPG-directions. Furthermore, at CCs and CCg, there was a statistical difference (p<0.05) between center II and center III for different MPG-directions. Conversely, no statistical differences were found between center I and center III for the different MPG-directions for all regions. These results indicate that the FA value was affected by the MPG-schema as well as by the MPG-directions.

MR perfusion imaging by alternate slab width inversion recovery arterial spin labeling (AIRASL): a technique with higher signal-to-noise ratio at 3.0 T.

OBJECT: To propose a new arterial spin labeling (ASL) perfusion-imaging method (alternate slab width inversion recovery ASL: AIRASL) that takes advantage of the qualities of 3.0 T. MATERIALS AND METHODS: AIRASL utilizes alternate slab width IR pulses for labeling blood to obtain a higher signal-to-noise ratio (SNR). Numerical simulations were used to evaluate perfusion signals. In vivo studies were performed to show the feasibility of AIRASL on five healthy subjects. We performed a statistical analysis of the differences in perfusion SNR measurements between flow-sensitive alternating inversion recovery (FAIR) and AIRASL. RESULTS: In signal simulation, the signal obtained by AIRASL at 3.0 and 1.5 T was 1.14 and 0.85%, respectively, whereas the signal obtained by FAIR at 3.0 and 1.5 T was 0.57 and 0.47%, respectively. In an in vivo study, the SNR of FAIR (3.0 T) and FAIR (1.5 T) were 1.73 ± 0.49 and 1.02 ± 0.20, respectively, whereas the SNRs of AIRASL (3.0 T) and AIRASL (1.5 T) were 3.93 ± 1.65 and 1.34 ± 0.31, respectively. SNR in AIRASL at 3.0 T was significantly greater than that in FAIR at 3.0 T. CONCLUSION: The most significant potential advantage of AIRASL is its high SNR, which takes advantage of the qualities of 3.0 T. This sequence can be easily applied in the clinical setting and will enable ASL to become more relevant for clinical application.

Time spatial labeling inversion pulse cerebral MR angiography without subtraction by use of dual inversion recovery background suppression.

Time-spatial labeling inversion pulse (time-SLIP), which is a technique of nonenhanced magnetic resonance angiography (MRA) based on arterial spin labeling (ASL), is used in various situations. Although subtraction between the images obtained with and without ASL is usually employed in cerebral time-SLIP MRA, to reduce the imaging time, dual inversion recovery (IR) has been applied for suppression of the background signals in this study. Appropriate timings for the 1st IR, 2nd IR, and the interval of data acquisition were investigated using computer simulation and a phantom experiment. With a short interval of data acquisition, the visibility of the simulated vessel was inadequate because replacement of the suppressed flow was insufficient. With a long interval of data acquisition, the contrast between the vessel and background was reduced. The reasons for this appeared to be the following: the longitudinal magnetization of the replaced flow is reduced because of the prolonged 2nd inversion time, causing a mismatch of the null point between the calculated and the actual values to become prominent. As a result, 3-4 s seemed to be an appropriate interval for data acquisition. Sufficient angiographic information could be obtained by use of dual IR background suppression in a volunteer study. With this technique, cerebral time-SLIP MRA can be performed in half of the imaging time required with the conventional subtraction technique.

[Examination of safety improvement by failure record analysis that uses reliability engineering].

How the maintenance checks of the medical treatment system, including start of work check and the ending check, was effective for preventive maintenance and the safety improvement was verified. In this research, date on the failure of devices in multiple facilities was collected, and the data of the trouble repair record was analyzed by the technique of reliability engineering. An analysis of data on the system (8 general systems, 6 Angio systems, 11 CT systems, 8 MRI systems, 8 RI systems, and the radiation therapy system 9) used in eight hospitals was performed. The data collection period assumed nine months from April to December 2008. Seven items were analyzed. (1) Mean time between failures (MTBF) (2) Mean time to repair (MTTR) (3) Mean down time (MDT) (4) Number found by check in morning (5) Failure generation time according to modality. The classification of the breakdowns per device, the incidence, and the tendency could be understood by introducing reliability engineering. Analysis, evaluation, and feedback on the failure generation history are useful to keep downtime to a minimum and to ensure safety.

[Development of a linear stage compatible for magnetic resonance imaging systems].

A magnetic resonance imaging (MRI) system compatible linear stage was developed. The stage was made of acrylic plastic and moving power was applied by an ultrasonic motor. Moving distance of the stage was detected by counting the number of motor rotations using a optic fiber sensor. Accuracy and precision of the stage control were measured inside and outside the magnet using a micrometer and a laser distance meter. As a result, a value of more than 95% was achieved in both of them in the 1.5 T magnetic field when it was applied for more than a 0.3 mm movement. Measurement of the slice sensitivity profile (SSP) by the delta method was performed. Slice selection by this linear stage and by radio frequency (RF) offset were compared. The result by linear stage was in good agreement with the result by RF offset. With this linear stage, a performance evaluation test of MRI equipments that need micromotion can be performed.

[Suppression of CSF artifact in fast FLAIR sequence at 3.0 Tesla].

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.

Fat quantification by use of phase change in dual-echo magnetic resonance imaging.

The demand for measurement of visceral fat in medical checkups is increasing because of attention to central obesity. Computed tomography is widely used for this purpose, but carries risks associated with radiation exposure. Fast imaging with a dual-echo technique acquiring water and fat signals both in-phase and out-of-phase using magnetic resonance imaging is desirable, but not all scanners are capable of this technique. We tried to quantify fat content by using a dual-echo gradient echo pulse sequence with an arbitrary echo time (TE) set. We calculated the phase change during the TE interval and extracted the region of fat by threshold processing. Variations in the precision of counting fat pixels caused by differences between TE1 and TE2 were measured in phantom experiments. We then evaluated the validity of this method by examination of a volunteer. The phantom study showed a large error when the TE interval was a multiple of the in-phase time for fat and water, and the error increased according to the prolongation of the TE. In the human study, phase wrapping readily occurred in the phase-change image. However, the region of fat was easily extracted with high-pass filtering. The fat signal can also be extracted by use of scanners that cannot be set simultaneously to in-phase and out-of-phase TE.

Correction of inhomogeneous RF field using multiple SPGR signals for high-field spin-echo MRI.

PURPOSE: To propose a simple and useful method for correcting nonuniformity of high-field (3 Tesla) T(1)-weighted spin-echo (SE) images based on a B1 field map estimated from gradient recalled echo (GRE) signals. METHODS: To estimate B1 inhomogeneity, spoiled gradient recalled echo (SPGR) images were collected using a fixed repetition time of 70 ms, flip angles of 45 and 90 degrees, and echo times of 4.8 and 10.4 ms. Selection of flip angles was based on the observation that the relative intensity changes in SPGR signals were very similar among different tissues at larger flip angles than the Ernst angle. Accordingly, spatial irregularity that was observed on a signal ratio map of the SPGR images acquired with these 2 flip angles was ascribed to inhomogeneity of the B1 field. Dual echo time was used to eliminate T(2)(*) effects. The ratio map that was acquired was scaled to provide an intensity correction map for SE images. Both phantom and volunteer studies were performed using a 3T magnetic resonance scanner to validate the method. RESULTS: In the phantom study, the uniformity of the T(1)-weighted SE image improved by 23%. Images of human heads also showed practically sufficient improvement in the image uniformity. CONCLUSION: The present method improves the image uniformity of high-field T(1)-weighted SE images.

Assessment of the vascularity of uterine leiomyomas using double-echo dynamic perfusion-weighted MRI with the first-pass pharmacokinetic model: correlation with histopathology.

OBJECTIVES: To retrospectively evaluate the feasibility of perfusion-weighted MRI (PWI) in uterine leiomyomas. MATERIALS AND METHODS: : Eighteen uterine leiomyomas in 15 patients were evaluated. PWI was performed using a double-echo T2*-weighted spoiled gradient-recalled acquisition sequence, and the first-pass pharmacokinetic model was applied to calculate relative blood volume (rBV). Histopathologic analysis was performed to measure vascular area (VA). RESULTS: PWI was successful in 13 of 15 patients. On quantitative analysis, mean (+/-SD) rBV calculated from PWI was 0.17 +/- 0.13 (range, 0.06-0.55), whereas mean VA was 3.3% +/- 1.6% (range, 1.7-8.5%). A significant correlation was identified between rBV and VA (r = 0.87, P < 0.001). CONCLUSIONS: The rBV determined at PWI correlates with histologic vascular area in uterine leiomyomas.