Assessment of cerebral gliomas by a new dark fluid sequence, high intensity REduction (HIRE): a preliminary study

Technical Note: Novel imaging method to obtain gray matter-attenuated inversion recovery image using low-field magnetic resonance imaging systems

INTRODUCTION

The double inversion recovery (DIR) technique suppresses two types of tissue signals with different T1 values by applying two inversion recovery (IR) pulses with different inversion times (TI). In contrast, the double tissue suppression with multi-echo acquisition and single TI combining HIRE (DOMUST-HIRE) method, is a technique enabling the white-matter-attenuated inversion recovery (WAIR) images by setting one inversion time (TI) in a sequence based on the multi-echo method and subtracting the second echo image from the first echo image. Here, we propose a new sequence that can provide the gray-matter-attenuated inversion recovery image based on the DOMUST-HIRE method.

METHODS

In this small clinical study, we performed determination of optimal TI and physical evaluation by imaging a subject's head with T1WI and our proposed method for GAIR images.

RESULTS

Our proposed method could increase the contrast ratio and the contrast-to-noise ratio between white matter (WM) and gray matter (GM), whereas the signal-to-noise ratio WM and GM decreased than with T1WI method.

CONCLUSIONS

Our proposed method can be used to suppress GM and CSF signals.

IMPLICATIONS FOR PRACTICE

The use of our proposed method in low-field MRI systems could provide GAIR image.

A New Magnetic Resonance Imaging Method for 2 Tissue Suppression: Double Tissue Suppression With Multiecho Acquisition and Single Inversion Time Combining High-Intensity Reduction (DOMUST-HIRE)

INTRODUCTION

The fluid-attenuated inversion recovery (FLAIR) method is one of the most important magnetic resonance imaging techniques for the brain, and the high-intensity reduction (HIRE) method is an imaging technique to obtain cerebrospinal fluid suppression images by subtracting long echo time images from short echo time images. In contrast, the double inversion recovery technique suppresses 2 types of tissue signals with different T1 values by applying 2 inversion recovery pulses with different inversion times. However, the double inversion recovery method requires the setting of 2 inversion times in a sequence; thus, its use is limited to relatively high-specification equipment. Here, we propose a new sequence called double tissue suppression with multiecho acquisition and single inversion time combining high-intensity reduction (DOMUST-HIRE) that suppresses the 2 tissues by adding single inversion recovery pulses to a sequence based on the HIRE method.

METHODS

In this small clinical study, we performed physical evaluation by imaging a subject's head with FLAIR and DOMUST-HIRE method.

RESULTS

The DOMUST-HIRE method can increase the contrast ratio and the contrast-to-noise ratio between white matter (WM) and gray matter, whereas the signal-to-noise ratio between WM and gray matter decreased than with FLAIR method.

CONCLUSIONS

The DOMUST-HIRE method can be used to suppress WM and cerebrospinal fluid signals.

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 (V_w) 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 V_w 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.

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.

Magnetic resonance neurography of the lumbosacral plexus at 3 Tesla - CSF-suppressed imaging with submillimeter resolution by a three-dimensional turbo spin echo sequence

PURPOSE

To investigate magnetic resonance neurography (MRN) of the lumbosacral plexus (LSP) with cerebrospinal fluid (CSF) suppression by using submillimeter resolution for three-dimensional (3D) turbo spin echo (TSE) imaging.

MATERIALS AND METHODS

Using extended phase graph (EPG) analysis, the signal response of CSF was simulated considering dephasing from coherent motion for frequency-encoding voxel sizes ranging from 0.3 to 1.3 mm and for CSF velocities ranging from 0 to 4 cm/s. In-vivo MRN included 3D TSE data with frequency encoding parallel to the feet/head axis from 15 healthy adults (mean age: 28.5 ± 3.8 years, 5 females; acquisition voxel size: 2 × 2 × 2 mm³) and 16 pediatric patients (mean age: 6.7 ± 4.1 years, 7 females; acquisition voxel size: 0.7 × 0.7 × 1.4 mm³) acquired at 3 Tesla. Five of the adults were scanned repetitively with changing acquisition voxel sizes (1 × 2 × 2 mm³, 0.7 × 2× 2 mm³, and 0.5 × 2 × 2 mm³). Measurements of the bilateral ganglion of the L5 nerve root, averaged between sides, as well as the CSF in the thecal sac were obtained for all included subjects and compared between adults and pediatric patients and between voxel sizes, using a CSF-to-nerve signal ratio (CSFNR).

RESULTS

According to simulations, the CSF signal is reduced along the echo train for moving spins. Specifically, it can be reduced by over 90% compared to the maximum simulated signal for flow velocities above 2 cm/s, and could be most effectively suppressed by considering a frequency-encoding voxel size of 0.8 mm or less. For in-vivo measurements, mean CSFNR was 1.52 ± 0.22 for adults and 0.10 ± 0.03 for pediatric patients (p < .0001). Differences in CSFNR were significant between measurements using a voxel size of 2 × 2 × 2 mm³ and measurements in data with reduced voxel sizes (p ≤ .0012), with submillimeter resolution (particularly 0.5 × 2 × 2 mm³) providing highest CSF suppression.

CONCLUSIONS

Applying frequency-encoding voxel sizes in submillimeter range for 3D TSE imaging with frequency encoding parallel to the feet/head axis may considerably improve MRN of LSP pathology in adults in the future because of favorable CSF suppression.

Use of Multiplied, Added, Subtracted and/or FiTted Inversion Recovery (MASTIR) pulse sequences

The group of Multiplied, Added, Subtracted and/or fiTted Inversion Recovery (MASTIR) pulse sequences in which usually two or more inversion recovery (IR) images of different types are combined is described, and uses for this type of sequence are outlined. IR sequences of different types can be multiplied, added, subtracted, and/or fitted together to produce variants of the MASTIR sequence. The sequences provide a range of options for increasing image contrast, demonstrating specific tissues and fluids of interest, and suppressing unwanted signals. A formalism using the concept of pulse sequences as tissue property filters is used to explain the signal, contrast and weighting of the pulse sequences with both univariate and multivariate filter models. Subtraction of one magnitude reconstructed IR image from another with a shorter TI can produce very high T₁ dependent positive contrast from small increases in T₁. The reverse subtracted IR sequence can provide high positive contrast enhancement with gadolinium chelates and iron deposition which decrease T₁. Additional contrast to that arising from increases in T₁ can be produced by supplementing this with contrast arising from concurrent increases in ρ_m and T₂, as well as increases or decreases in diffusion using subtraction IR with echo subtraction and/or diffusion subtraction. Phase images may show 180º differences as a result of rotating into the transverse plane both positive and negative longitudinal magnetization. Phase images with contrast arising in this way, or other ways, can be multiplied by magnitude IR images to increase the contrast of the latter. Magnetization Transfer (MT) and susceptibility can be used with IR sequences to improve contrast. Selective images of white and brown adipose tissue lipid and water components can be produced using different TIs and in and out-of-phase TEs. Selective images of ultrashort and short T₂ tissue components can be produced by nulling long T₂ tissue components with an inversion pulse and subtraction of images with longer TEs from images with ultrashort TEs. The Double Echo Sliding IR (DESIRE) sequence provides images with a wide range of TIs from which it is possible to choose values of TI to achieve particular types of tissue and/or fluid contrast (e.g., for subtraction with different TIs, as described above, and for long T₂ tissue signal nulling with UTE sequences). Unwanted tissue and fluid signals can be suppressed by addition and subtraction of phase-sensitive (ps) and magnitude reconstructed images. The sequence also offers options for synergistic use of the changes in blood and tissue ρ_m, T₁, T₂/T₂*, D* and perfusion that can be seen with fMRI of the brain. In-vivo and ex-vivo illustrative examples of normal brain, cartilage, multiple sclerosis, Alzheimer's disease, and peripheral nerve imaged with different forms of the MASTIR sequence are included.

Whole-body MRI for metastatic cancer detection using T2 -weighted imaging with fat and fluid suppression

PURPOSE

To develop a whole-body MRI technique at 3T with improved lesion conspicuity for metastatic cancer detection using fast, high-resolution and high SNR T₂ -weighted (T₂ W) imaging with simultaneous fat and fluid suppression.

THEORY AND METHODS

The proposed dual-echo T₂ -weighted acquisition for enhanced conspicuity of tumors (DETECT) acquires 4 images, in-phase (IP) and out-of-phase (OP) at a short and a long TE using single-shot turbo spin echo. The IP/OP images at the short and long TEs are reconstructed using the standard Dixon and shared-field-map Dixon reconstruction respectively, for robust fat-water separation. An adaptive complex subtraction between the 2 TE water-only images achieves fluid attenuation. DETECT imaging was optimized and evaluated in whole-body imaging of 5 healthy volunteers, and compared against diffusion-weighted imaging with background suppression (DWIBS) in 5 patients with known metastatic renal cell carcinoma.

RESULTS

Robust fat-water separation and fluid attenuation were achieved using the shared-field-map Dixon reconstruction and adaptive complex subtraction, respectively. DETECT imaging technique generated co-registered T₂ W images with and without fat suppression, heavily T₂ W, and fat and fluid suppressed T₂ W whole-body images in <7 min. Compared to DWIBS acquired in 17 min, the DETECT imaging achieved better detection and localization of lesions in patients with metastatic cancer.

CONCLUSION

DETECT imaging technique generates T₂ W images with high resolution, high SNR, minimal geometric distortions, and provides good lesion conspicuity with robust fat and fluid suppression in <7 min for whole-body imaging, demonstrating efficient and reliable metastatic cancer detection at 3T.

Improved short tau inversion recovery (iSTIR) for increased tumor conspicuity in the abdomen

OBJECT

To develop an improved short tau inversion recovery (iSTIR) technique with simultaneous suppression of fat, blood vessels and fluid to increase tumor conspicuity in the abdomen for cancer screening.

MATERIALS AND METHODS

An adiabatic spectrally selective inversion pulse was used for fat suppression to overcome the reduced signal to noise ratio associated with chemically non-selective inversion pulse of STIR. A motion-sensitizing driven equilibrium was used for blood vessel suppression and a dual-echo single-shot fast spin echo acquisition was used for fluid suppression. The technique was optimized on four normal subjects and later tested on five patients referred for metastatic tumor evaluation.

RESULTS

A velocity encoding of 2 cm/s achieved effective blood suppression even in small vessels. Subtraction of two images (one with 60 ms and the other with 280 ms echo time) acquired in the same echo train achieved excellent fluid suppression (>70% reduction). Simultaneous suppression of fat, blood vessels and fluid improved the tumor conspicuity compared to corresponding fat-suppressed (STIR) image.

CONCLUSION

This technique generated two complementary images from a single scan: one that is equivalent to a STIR image and the other that qualitatively resembles a diffusion-weighted image and may have potential for magnetic resonance imaging cancer screening.

Positive contrast MR imaging of tendons, ligaments, and menisci by subtraction of signals from a double echo steady state sequence (Sub-DESS)

PURPOSE

To improve the visualization of fibrous tissues as tendons, ligaments and fibrocartilage structures as menisci by positive contrast using a new 3D Double Echo Steady State (DESS) sequence.

METHODS

The proposed 3D DESS sequence works with separate acquisition of a first echo with an echo time (TE1 ) of 1.2 ms followed by a more heavily T2 -weighted second echo recorded at time TE2 . Subtraction of images from both echoes leads to positive signal from fibrous tissues, whereas in other tissues as musculature and fat the subtraction signal nearly vanishes due to almost similar signal strength in both echoes. Systematic measurements in healthy volunteers with different sets of pulse repetition time (TR), TE1 , readout bandwidth and flip angle were performed to determine optimal sequence parameters.

RESULTS

The presented 3D sequence with Cartesian readout requires relatively short measuring time, provides reasonable signal-to-noise ratio and can be easily implemented in protocols for clinical musculoskeletal MR imaging. Degenerative changes or tears of tendons, ligaments and fibrocartilage are known to cause increased water content and therefore prolongation of transverse relaxation times, which leads to reduced signal intensities in the "subtraction images."

CONCLUSION

Positive contrast of fibrous tissue as demonstrated by the proposed sub-DESS approach provides improved conspicuity and allows for three-dimensional reconstruction especially of structures with curved geometry.

Contrast-enhanced T1-weighted fluid-attenuated inversion-recovery BLADE magnetic resonance imaging of the brain: an alternative to spin-echo technique for detection of brain lesions in the unsedated pediatric patient?

RATIONALE AND OBJECTIVES

We compared contrast-enhanced T1-weighted magnetic resonance (MR) imaging of the brain using different types of data acquisition techniques: periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER, BLADE) imaging versus standard k-space sampling (conventional spin-echo pulse sequence) in the unsedated pediatric patient with focus on artifact reduction, overall image quality, and lesion detectability.

MATERIALS AND METHODS

Forty-eight pediatric patients (aged 3 months to 18 years) were scanned with a clinical 1.5-T whole body MR scanner. Cross-sectional contrast-enhanced T1-weighted spin-echo sequence was compared to a T1-weighted dark-fluid fluid-attenuated inversion-recovery (FLAIR) BLADE sequence for qualitative and quantitative criteria (image artifacts, image quality, lesion detectability) by two experienced radiologists. Imaging protocols were matched for imaging parameters. Reader agreement was assessed using the exact Bowker test.

RESULTS

BLADE images showed significantly less pulsation and motion artifacts than the standard T1-weighted spin-echo sequence scan. BLADE images showed statistically significant lower signal-to-noise ratio but higher contrast-to-noise ratios with superior gray-white matter contrast. All lesions were demonstrated on FLAIR BLADE imaging, and one false-positive lesion was visible in spin-echo sequence images.

CONCLUSION

BLADE MR imaging at 1.5 T is applicable for central nervous system imaging of the unsedated pediatric patient, reduces motion and pulsation artifacts, and minimizes the need for sedation or general anesthesia without loss of relevant diagnostic information.

Quantitative analysis of focal masses at MR imaging: a plea for standardization

PURPOSE

To assess the effects of changing analytic method variables on the signal intensity (SI) difference-to-noise ratios (SDNRs) for the contrast between lesions and background organs depicted on magnetic resonance (MR) images and to propose a standardized analytic method for the quantitative analysis of focal masses seen at MR imaging.

MATERIALS AND METHODS

The SIs of 48 liver metastases (originating from colorectal cancer) in 20 patients, the surrounding liver parenchyma, and the background noise were measured on T2-weighted MR images. All 2000 and 2001 issues of the American Journal of Roentgenology, the Journal of Magnetic Resonance Imaging, Magnetic Resonance Imaging, and Radiology were searched for articles describing quantitative analyses. SDNRs were calculated by using formulas from these articles and various region-of-interest (ROI) locations to measure metastasis and background noise SIs. The Wilcoxon signed rank test was used to compare the various SDNR calculations.

RESULTS

In 34 articles in which quantitative analyses of focal masses are described, the reported SDNRs were calculated with four different formulas. The SDNRs for our study material calculated with the four formulas reported in the literature differed grossly in both number and unit. The SDNRs for ROIs encompassing the entire metastasis differed significantly (P =.034) from the SDNRs for ROIs in a homogeneous area of the metastasis margin. Differences in SDNRs between various noise ROI locations were significant (P <.022).

CONCLUSION

Slight changes in the variables of quantitative analysis of focal masses had marked effects on reported SDNRs. To overcome these effects, the use of a standardized method involving one formula, a lesion ROI in a homogeneous area at the metastasis margin, and a background noise ROI along the phase-encoding axis in the air (including systematic noise) is proposed for the quantitative analysis of findings on magnitude MR images.

Magnetization transfer, HASTE, and FLAIR imaging

Continuous technologic developments and research have increased the clinical applications of MT, HASTE, and FLAIR imaging in neuroradiology. HASTE has become the MR imaging sequence of choice for fetal neuroimaging. Other promising uses, such as for diffusion-weighted imaging, have not been fully exploited. FLAIR has been firmly established as one of the cornerstones of brain imaging; however, post-contrast FLAIR images have not offered a clear advantage over standard T1-weighted images as suggested by early studies. FLAIR imaging with echoplanar acquisition is not considered advantageous, because the decreased imaging times are obtained at the expense of lower sensitivity. For a number of applications, diffusion-weighted imaging has surpassed FLAIR. Nevertheless, FLAIR images may be more sensitive for the detection of acute brain infarction. Recently described methods for the elimination of CSF flow artifacts may lead to improved quality and reliability of FLAIR images for subarachnoid space disease. MT preparation is now routinely incorporated in time-of-flight MR angiography and gradient-echo T2*-weighted spine imaging sequences and provides increased sensitivity for postcontrast MR imaging. These applications may not be advantageous in all clinical settings. MTR analysis offers valuable information for an increasing number of pathologic processes but has not yet gained wide clinical acceptance owing to sophisticated postprocessing and significant intercenter variations. Different modifications of these techniques are being evaluated, and further developments are expected.

[Detection of acute cerebral infarction by dual echo subtraction technique in MR imaging]

The purpose of this study was to develop an image enhancement technique to detect acute cerebral infarct regions in brain MR images. Transverse relaxation times for abnormal changes tend to be longer than those for normal tissues. In order to obtain MR images with two different echo times, we employed the fast spin echo sequence. We then employed the image subtraction technique using two T(2)-weighted images to enhance acute cerebral infarct regions. As a result, the areas of acute cerebral infarct regions were enhanced as regions of higher signal than normal regions of brain tissue. Further, high signal areas in dual echo subtraction images corresponded to cerebral infarct regions of high signal areas in diffusion weighted images (DWI). We found that the image subtraction technique is useful to enhance very subtle regions of acute cerebral infarction in MR images. Because we employ the difference between transverse relaxation times for normal and abnormal tissues, which does not depend on the strength of the magnetic field, the dual echo subtraction method can be used in many hospitals.

Suppression of cerebrospinal fluid and blood flow artifacts in FLAIR MR imaging with a single-slab three-dimensional pulse sequence: initial experience

The authors compared high-signal-intensity flow-related artifacts present with a conventional two-dimensional (2D) fluid-attenuated inversion recovery (FLAIR) sequence with those seen with a single-slab, three-dimensional (3D) FLAIR sequence. Four readers graded the subarachnoid space and intraventricular artifacts, the pulsation artifacts, and the conspicuity of cranial nerves in the posterior fossa. For all comparisons, differences between 2D and 3D images were highly statistically significant, with 3D imaging being superior in all cases.