Diffusion MR Imaging with T2-based Water Suppression (T2wsup-dMRI).

PURPOSE: This study proposes and assesses a new diffusion MRI (dMRI) technique to solve problems related to the quantification of parameter maps (apparent diffusion coefficient [ADC] or mean diffusivity [MD], fractional anisotropy [FA]) and misdrawing of fiber tractography (FT) due to cerebrospinal fluid (CSF)-partial volume effects (PVEs) for brain tissues by combining with the T2-based water suppression (T2wsup) technique. METHODS: T2wsup-diffusion-weighted imaging (DWI) images were obtained by subtracting those images from the acquired multi-b value (b) DWI images after correcting the signal intensities of multiecho time (TE) images using long TE water signal-dominant images. Quantitative parameter maps and FT were obtained from minimum data points and were compared with those using the standard (without wsup) DWI method, and partly compared with those obtained using other alternative DWI methods of applying fluid attenuation inversion recovery (FLAIR), non-b-zero (NBZ) by theoretical or noise-added simulation and MR images. RESULTS: In the T2wsup-dMRI method, the hyperintense artifacts due to CSF-PVEs in MRI data were dramatically suppressed even at lower b (≲ 500 s/mm2) while keeping the tissue SNR. The quantitative parameter map values became precisely close to the pure tissue values precisely even in water (CSF) PVE voxels in healthy brain tissues (T2 ≲ 100 ms). Furthermore, the fiber tracts were correctly connected, particularly at the fornix in closest contact to the CSF. CONCLUSION: Solving the problem of CSF-PVE in the current dMRI technique using our proposed T2wsup-dMRI technique is easy, with higher SNR than those obtained with FLAIR or NBZ methods when applying to healthy brain tissues. The proposed T2wsup-dMRI could be useful in clinical settings, although further optimization of the pulse sequence and processing techniques and clinical assessments are required, particularly for long T2 lesions.

Synthetic MRI with T2-based Water Suppression to Reduce Hyperintense Artifacts due to CSF-Partial Volume Effects in the Brain.

PURPOSE: Our purpose was to assess our proposed new synthetic MRI (synMRI) technique, combined with T2-based water suppression (T2wsup), to reduce cerebral spinal fluid (CSF)-partial volume effects (PVEs). These PVEs are problematic in the T2-weighted fluid-attenuation inversion recovery (FLAIR) images obtained by conventional synMRI techniques. METHODS: Our T2wsup was achieved by subtracting additionally acquired long TE spin echo (SE) images of water signals dominant from the originally acquired images after T2 decay correction and a masking on the long TE image using the water volume (Vw) map to preserve tissue SNR, followed by quantitative mapping and then calculation of the synthetic images. A simulation study based on a two-compartment model including tissue and water in a voxel and a volunteer MR study were performed to assess our proposed method. Parameters of long TE and a threshold value in the masking were assessed and optimized experimentally. Quantitative parameter maps of standard and with T2wsup were generated, then wsup-synthetic FLAIR and SE images were calculated using those suitable combinations and compared. RESULTS: Our simulation clarified that the CSF-PVE artifacts in the standard synthetic FLAIR increase T2 as the water volume increases in a voxel, and the volunteer MR brain study demonstrated that the hyperintense artifacts on synthetic images were reduced to <10% of Vw in those with the standard synMRI while keeping the tissue SNR by selecting optimal masking parameters on additional long TE images of TE = 300 ms. In addition, the wsup-synthetic SE provided better gray-white matter contrasts compared with the wsup-synthetic FLAIR while keeping CSF suppression. CONCLUSION: Our proposed T2wsup-synMRI technique makes it easy to reduce the CSF-PVE artifacts problematic in the synthetic FLAIR images using the current synMRI technique by adding long TE images and simple processing. Although further optimizations in data acquisition and processing techniques are required before actual clinical use, we expect our technique to become clinically useful.

Real-based Polarity-preserving Asymmetric Fourier Imaging (RepAFI).

We proposed and assessed a modified asymmetric Fourier imaging (AFI) technique named real-based polarity-preserving AFI (RepAFI), in which the low-pass filter kernel for background phase estimation in AFI is optimized to preserve the magnetization polarity information for blood vessels and cerebrospinal fluid (CSF) even for data obtained using phase-sensitive inversion-recovery spin-echo-based (PSIR-SE) sequences with asymmetrical sampling in the k-space. Our proposed RepAFI technique achieves a practical balance of image quality and simplicity to provide better performance than conventional AFI methods.

Evaluation of Appropriate Readout Sequence for Renal MRI Perfusion Using ASTAR (ASL) Technique.

The aim of this study was to compare true-steady state free precession (True-SSFP) with fast field echo (FFE) as readout imaging sequences for renal arterial spin labeling (ASL), and to optimize the imaging condition. Renal ASL perfusion images were acquired using signal targeting with alternated radio frequency using asymmetric inversion slab (ASTAR) technique with respiratory triggering at 3T MRI system, using either 3D True-SSFP or FFE as the readout sequence. Inversion time (TI) varied from 800 to 2400 ms. Appropriate flip angles were estimated for each sequence by simulating signal intensity (SI). The SI of the renal cortex, vertebral body, and intestinal tract were measured, and the contrast ratio of the cortex (CRcortex) or intestine (CRintestine) related to vertebra was calculated. The image quality of the kidneys, background signal suppression, and misregistration were evaluated by four-point scales. As a result, in quantitative evaluation, the average of CRcortex of each TI (800, 1200, 1600, 2000, and 2400 msec) were 0.49, 0.57, 0.63, 0.63, and 0.56 in FFE, and 0.59, 0.71, 0.73, 0.73, and 0.68 in True-SSFP, respectively. IN qualitative evaluation, ASL images with True-SSFP readout were significantly better than those with FFE readout. In conclusion, True-SSFP sequences will be recommended as read out imaging sequence for obtaining ASL image compared with FFE image.

Magnitude-based Asymmetric Fourier Imaging (MagAFI).

PURPOSE: We propose and assess 2 novel asymmetric Fourier imaging (AFI) techniques, magnitude-based AFI (MagAFI) and MagAFI combined with projection on to convex sets (POCS) (MagAFI+POCS). MagAFI does not require phase information because it uses only the magnitude image with zero-filling. MagAFI+POCS requires phase information but further reduces image errors. MATERIALS AND METHODS: We initially compared phase maps obtained using asymmetrically sampled data for the whole of the k-space and symmetrically sampled data for the low frequency part of the k-space. We used one-dimensional simulation data and 3-dimensional gradient echo data for 2 different echo times (TEs) of the brains of volunteers and assessed the differences between the image reconstructed from the full k-space data and AFI images reconstructed from truncated k-space data. We generated AFI images in this study using the zero-filling, Margosian (homodyne), Margosian+POCS (standard POCS), MagAFI, and MagAFI+POCS techniques. RESULTS: We confirmed the assumption of smaller phase errors for the full k-space data than for the symmetric low frequency k-space data. Our proposed MagAFI technique provides images with smaller phase-induced errors than those obtained using conventional methods, including standard POCS methods, which have been regarded as the best methods. MagAFI+POCS improves image quality as well as robustness. CONCLUSION: Our proposed MagAFI technique achieves a practical balance of image quality and simplicity to perform better than conventional methods using only the 0-filled magnitude image. Combined with POCS, this technique can produce images of even better quality.

Computed diffusion-weighted imaging using 3-T magnetic resonance imaging for prostate cancer diagnosis.

OBJECTIVE: To assess the efficacy of computed diffusion-weighted images (cDWIs) of b = 2000 s/mm(2) (cDWI2000) generated from DWIs of b = 0 and 1000 for prostate cancer (PCa) diagnosis in comparison with that of measured original DWIs of b = 1000 (mDWI1000) and b = 2000(mDWI2000) using 3-T MRI. METHODS: Eighty patients who underwent a preoperative MRI examination, including T2WI and DWI (b = 0, 1000, 2000 s/mm(2)), were enrolled in this study. Four combinations of images, protocol A (T2WI alone), B (T2WI + mDWI1000), C (T2WI + mDWI2000) and D (T2WI + cDWI2000), were assessed for their diagnostic capability. Areas under the receiver operating characteristic curve (Az) and diagnostic performance were evaluated, as well as contrast ratios (CR) between cancerous and non-cancerous lesions for each DWI. RESULTS: The highest CR was obtained with cDWI2000 (0.29 ± 0.16). Sensitivity, specificity, accuracy, and Az of the protocols were: A: 66.3 %, 59.4 %, 63.0 %, 0.67; B: 82.6 %, 62.0 %, 72.5 %, 0.80; C: 84.1 %, 66.5 %, 75.5 %, 0.86; D: 83.2 %, 70.0 %, 76.6 %, and 0.84, respectively The specificities and accuracies of protocol C and D were significantly higher than those of protocol B (P < 0.05). CONCLUSION: cDWI2000 appears to be more effective than mDWI1000, and at least as effective as mDWI2000 for PCa diagnosis.

Inhomogeneous noise correction combined with uniform filter and sensitivity map (INCUS) for multi-coil imaging including parallel imaging.

PURPOSE: We have developed and assess INCUS (inhomogeneous noise correction combined with uniform filter and sensitivity map), a novel technique to correct spatially inhomogeneous noise in surface-coil-based standard or parallel imaging in magnetic resonance (MR) imaging. MATERIALS AND METHODS: We employed a weighted summation of 2 images uniformly filtered with both a strong filter and a weak or no filter to achieve spatially nonuniform filtering by utilizing a coil-sensitivity and/or noise map to give greater weight to the strongly denoised components of the region with lower signal-to-noise ratio (SNR). We compared the image quality and difference between INCUS and standard uniform filter techniques employing several types of linear or nonlinear filter in abdominal or diffusion-weighted brain images each for 1 volunteer acquired on a 1.5-tesla magnetic resonance (MR) imager with parallel imaging after adding simulated noise. RESULTS: The INCUS technique inherently reduced inhomogeneous noise on multiple surface-coil imaging and minimized blur obtained with every filter type. Overall errors were smaller for the INCUS filter than the uniform filter when either of these was used in combination with any of the other filters. CONCLUSION: The INCUS technique provides a straightforward, higher degree of freedom, and computationally feasible implementation of a general purpose de-noising filter for surface coil-based imaging including parallel imaging in commercial MR imaging.

Postoperative evaluation of superficial temporal artery-middle cerebral artery bypass using an MR angiography technique with combined white-blood and black-blood sequences.

PURPOSE: To assess the performance of the hybrid of opposite-contrast MR angiography (HOP MRA) technique, which combines flow dephasing and compensating sequences, in the postoperative evaluation of superficial temporal artery (STA)-middle cerebral artery (MCA) bypass. MATERIALS AND METHODS: Both 3D-HOP MRA and 3D-time-of-flight (TOF) MRA at 1.5 Tesla were performed in 19 patients after STA-MCA bypass. The two techniques were visually evaluated to compare the visualization of distal MCA branches and the length and number of depicted recipient MCA branches. Additionally, for the depicted recipient MCA branches, the contrast-to-noise ratio (CNR) and length were compared between the two techniques. RESULTS: The 3D-HOP MRA provided better visualization of the recipient MCA branches than 3D-TOF MRA in 10 of the 19 patients, while the depicted recipient MCA branches were longer on 3D-HOP MRA than on 3D-TOF MRA in 9 patients. Although not statistically significant, the average number of depicted recipient branches by 3D-HOP MRA (2.16) was greater than that by 3D-TOF MRA (1.79). 3D-HOP MRA was significantly superior to 3D-TOF MRA in both CNR (119.0 versus 74.3) and length of the recipient MCA branches (72.0 versus 49.9 pixels). CONCLUSION: 3D-HOP MRA is superior to 3D-TOF MRA for the detailed evaluation of STA-MCA bypass.

Phase enhancement for time-of-flight and flow-sensitive black-blood MR angiography.

A magnitude-based MR angiography method of standard time-of-flight (TOF) employing a three-dimensional gradient-echo sequence with flow rephasing is widely used. A recently proposed flow-sensitive black-blood (FSBB) method combining three-dimensional gradient-echo sequence with a flow-dephasing gradient and a hybrid technique, called hybrid of opposite-contrast, allow depiction of smaller blood vessels than does standard TOF. To further enhance imaging of smaller vessels, a new enhancement technique combining phase with magnitude is proposed. Both TOF and FSBB pulse sequences were used with only 0th-order gradient moment nulling, and suitable dephasing gradients were added to increase the phase shift introduced mainly by flow. Magnitude-based vessel-to-background contrast-to-noise ratios in TOF and FSBB were further enhanced to increase the dynamic range between positive and negative signals through the use of cosine-function-based filters for white- and black-blood imaging. The proposed phase-enhancement processing both improved visualization of slow-flow vessels in the brains of volunteer subjects with shorter echo time in TOF, FSBB, and hybrid of opposite-contrast and reduced wraparound artifacts with smaller b values without sacrificing vessel-to-background contrast in FSBB. This method of enhancement processing has excellent potential to become clinically useful.

Improved visualization of intracranial vessels by gradient moment nulling in hybrid of opposite-contrast magnetic resonance angiography (HOP MRA).

Hybrid of opposite-contrast (HOP) magnetic resonance angiography (MRA) is a new method that combines the advantages of 3-dimensional (3D) time-of-flight (TOF) MRA and black-blood (BB) MRA without prolonging acquisition time. In phantom and clinical studies, we focused on image differences when we applied gradient moment nulling (GMN) to 2 or 3 axes in the first echo. We made an original phantom with a semicircular tube of 3- and 5-mm internal diameter, with flow rate in the tube of 0, 20, 60, 80, or 120 cm/s. In original images of the phantom obtained with TOF MRA and flow-sensitive BB MRA and in filter frequency-weighted subtraction (FWS) processed images acquired with HOP MRA, we measured the contrast-to-noise ratio (CNR) of both the inside and outside of the tubes. In FWS processed images with GMN applied to 2 axes, the CNR was high at various flow rates in both straight and bending portions of the tubes in comparison with TOF images. In a clinical study in 15 patients, we evaluated vessel visualization in images obtained using conventional TOF MRA with magnetization transfer contrast (MTC) and HOP MRA. In clinical studies, visualization scores of HOP MRA were equivalent to those of conventional TOF MRA in the bilateral internal carotid arteries (ICA) and inferior in the basilar arteries. However, visualization of the peripheral portion of the middle cerebral artery (MCA) improved significantly in HOP MRA with GMN applied to 2 and 3 axes. Visualization of the main trunk of the ICA and MCA was superior in HOP MRA with GMN applied to 2 axes. HOP MRA with 2-axis GMN may be useful for excellent visualization of both major arteries and peripheral vessels in the head.

Hybrid of opposite-contrast magnetic resonance angiography of the brain by combining time-of-flight and black blood sequences: its value in moyamoya disease.

OBJECTIVE: We assessed the value of a new magnetic resonance angiography (MRA) technique named hybrid of opposite-contrast (HOP) MRA in the diagnosis of moyamoya disease. METHODS: Using a dual-echo sequence, we obtained the first echo for time-of-flight (TOF) MRA followed by the second echo for black blood MRA. We then subtracted the black blood MRA data set from that of the TOF MRA followed by maximum-intensity projection. In 14 patients, we performed HOP MRA and compared the findings with those on 3-dimensional TOF MRA and MR images. The HOP MRA was also compared with a radionuclide perfusion study (7 patients). RESULTS: The HOP MRA technique improved the demonstration of distal arteries in 13 patients. The findings in HOP MRA correlated with MR images in 9 patients as well as with the perfusion study in 6 patients. CONCLUSIONS: The HOP MRA technique demonstrated the distal arterial branches in moyamoya disease well and facilitated the perfusion assessment with MR imaging.

Hybrid of opposite-contrast MRA of the brain by combining time-of-flight and black-blood sequences: initial experience in major trunk stenoocclusive diseases.

PURPOSE: To assess the feasibility of a new MR angiography (MRA) technique named hybrid of opposite-contrast MRA (HOP MRA) that combined the time-of-flight (TOF) MRA with a flow-sensitive black-blood (FSBB) sequence in the diagnosis of major trunk stenoocclusive diseases. MATERIALS AND METHODS: On a 1.5 Tesla imager using a dual-echo three-dimensional (3D)-gradient-echo sequence, we obtained the first echo for TOF MRA followed by the second echo for FSBB. We then subtracted the FSBB data set from that of TOF MRA followed by maximum intensity projection. In four normal volunteers and 19 patients with chronic stenoocclusive disease of the major trunk, we performed HOP MRA along with 3D-TOF MRA and compared the findings. RESULTS: In the volunteer group, the HOP MRA technique improved the demonstration of distal arterial branches. In 12 of the 19 patients, the HOP MRA better visualized branches distal to the lesion as well as distal branches of normal trunks than 3D-TOF MRA, while both techniques provided equivalent depiction of branches distal to the lesion but better depiction of normal distal branches in three patients. CONCLUSION: The HOP-MRA technique is promising in major trunk stenoocclusive diseases as it better demonstrates distal branches probably representing collaterals than 3D-TOF MRA.

Reference-based maximum upslope: a CBF quantification method without using arterial input function in dynamic susceptibility contrast MRI.

PURPOSE: We assessed errors in cerebral blood flow (CBF) obtained from our proposed reference-based method without using arterial input function (AIF) indices in dynamic susceptibility contrast (DSC) magnetic resonance imaging (MRI). MATERIALS AND METHODS: We calculated CBF and the referential tissue-related ratio (CBFratio) using numerical simulation and 3 nondeconvolution methods and a deconvolution method of block-circulant singular value decomposition (cSVD). We compared errors with and without simulated noise as parameters of mean transit time (MTT), AIF delay and temporal resolution, and clinical DSC-MRI maps. RESULTS: The errors in CBF obtained using maximum upslope (US) were smallest among the nondeconvolution methods and almost equivalent to errors in the cSVD method under practical imaging conditions. In addition, errors in the CBFratio obtained using reference-based US (Ref-US) referring to white matter were smallest, even compared to all errors in CBF and CBFratio. The Ref-US method introduced less error than the cSVD method, especially at low flow rates, was further robust against AIF noise and coarse temporal resolution, and was comparably robust against transit delay. In pixel-by-pixel correlations between absolute value maps for US and for cSVD-CBF in clinical DSC-MRI, those correlation coefficients (r) between the 2 maps were stable, r > 0.9, despite variation in the slopes of the linear regression line, so the 2 CBFratio maps were visually well correlated in any case. CONCLUSION: The Ref-US technique without AIF measurement can become a practical perfusion methodology for DSC-MRI in patients even with acute stroke because it balances robustness with systematic and random errors and it is simply performed.

Hybrid of opposite-contrast MR angiography (HOP-MRA) combining time-of-flight and flow-sensitive black-blood contrasts.

For the purpose of visualizing low-flow as well as high-flow blood vessels without using contrast agents, we propose a new technique called a hybrid of opposite-contrast MR angiography (HOP-MRA). HOP-MRA is a combination of standard time-of-flight (TOF) using a full first-order velocity-compensation for white-blood (WB) and flow-sensitive black-blood (FSBB) techniques, which use motion-probing gradients to introduce intravoxel flow dephasing. A dual-echo three-dimensional gradient echo sequence was used to reduce both imaging time and misregistration. HOP-MRA images were obtained using a simple-weighted subtraction (SWS) or a frequency-weighted subtraction (FWS) applying different spatial filtering for WB and BB images. We then assessed the relationships among the contrast-to-noise ratios (CNR) of the blood-to-background signals for those three images. In both volunteer and clinical brain studies, low-flow vessels were well visualized and the background signal was well suppressed by HOP-MRA compared with standard TOF- or BB-MRA. The FWS was better than the SWS when whole-maximum intensity projection was performed on a larger volume including with different types of tissue. The proposed HOP-MRA was proven to visualize low-flow to high-flow vessels and, therefore, demonstrates excellent potential to become a clinically useful technique, especially for visualizing collateral vessels which is difficult with standard TOF-MRA.