Initial Experience of Applying TWIST-Dixon With Flexible View Sharing in Breast DCE-MRI.

INTRODUCTION: We developed a new fast imaging technique with flexible time-resolved angiography with stochastic trajectories (TWIST) view sharing to achieve variable temporal resolution and with flexible echo time Dixon to achieve robust fat suppression and to evaluate its application in breast dynamic contrast enhanced-magnetic resonance imaging (DCE-MRI). MATERIALS AND METHODS: The TWIST-Dixon technique was improved with more flexible view sharing and echo times (TWIST-Dixon-Flex). In a dynamic series, each measurement can be separately prescribed as "full," "partial," or "center-only." The spatial and temporal resolution can then be adjusted throughout the measurements to match the dynamic characteristics of contrast enhancement at different phases. The potential advantages of TWIST-Dixon-Flex were evaluated with 18 clinical breast DCE MRI cases. A mixed-effects analysis of variance (ANOVA) was performed to compare the image quality with that of the conventional images. RESULTS: The ANOVA showed that the quality of postcontrast TWIST-Dixon-Flex images was significantly higher than that of the conventional images. The TWIST-Dixon-Flex technique also provided the capability to detect differences in rapid contrast uptake from different regions of the breast tumor, which is not possible with conventional breast DCE-MRI. CONCLUSION: The new TWIST-Dixon-Flex technique provides potentially valuable information about early tumor enhancement, and maintains excellent image quality at peak and postcontrast enhancement. This technique could help overcome the compromise on spatial over temporal resolution in clinical breast imaging.

Free-breathing variable flip angle balanced SSFP cardiac cine imaging with reduced SAR at 3T.

PURPOSE: To develop a free-breathing variable flip angle (VFA) balanced steady-state free precession (bSSFP) cardiac cine imaging technique with reduced specific absorption rate (SAR) at 3 Tesla. METHODS: Free-breathing VFA (FB-VFA) images in the short-axis and four-chamber views were acquired using an optimal VFA scheme, then compared with conventional breath-hold constant flip angle (BH-CFA) acquisitions. Two cardiac MRI experts used a 5-point scale to score images from healthy subjects (N = 10). The left ventricular ejection fraction, end diastolic volume (LVEDV), end systolic volume, stroke volume (LVSV), and end diastolic myocardial mass (LVEDM) were determined by manual contour analysis for BH-CFA and FB-VFA. A pilot evaluation of FB-VFA was performed in one patient with Duchenne muscular dystrophy. RESULTS: FB-VFA SAR was 25% lower than BH-CFA with similar blood-myocardium contrast. The qualitative FB-VFA score was lower than the BH-CFA for the short-axis (3.1 ± 0.5 versus 4.3 ± 0.8; P < 0.05) and the four-chamber view (3.4 ± 0.4 versus 4.6 ± 0.6; P < 0.05). The LVEDV and the LVSV were 5% and 12% larger (P < 0.05) for FB-VFA compared with BH-CFA. There was no difference in LVEDM. CONCLUSION: FB-VFA bSSFP cardiac cine imaging decreased the SAR at 3T with image quality sufficient to perform cardiac functional analysis. Magn Reson Med 76:1210-1216, 2016. © 2015 Wiley Periodicals, Inc.

High spatial and temporal resolution dynamic contrast-enhanced magnetic resonance angiography using compressed sensing with magnitude image subtraction.

PURPOSE: We propose a compressed-sensing (CS) technique based on magnitude image subtraction for high spatial and temporal resolution dynamic contrast-enhanced MR angiography (CE-MRA). METHODS: Our technique integrates the magnitude difference image into the CS reconstruction to promote subtraction sparsity. Fully sampled Cartesian 3D CE-MRA datasets from 6 volunteers were retrospectively under-sampled and three reconstruction strategies were evaluated: k-space subtraction CS, independent CS, and magnitude subtraction CS. The techniques were compared in image quality (vessel delineation, image artifacts, and noise) and image reconstruction error. Our CS technique was further tested on seven volunteers using a prospectively under-sampled CE-MRA sequence. RESULTS: Compared with k-space subtraction and independent CS, our magnitude subtraction CS provides significantly better vessel delineation and less noise at 4× acceleration, and significantly less reconstruction error at 4× and 8× (P < 0.05 for all). On a 1-4 point image quality scale in vessel delineation, our technique scored 3.8 ± 0.4 at 4×, 2.8 ± 0.4 at 8×, and 2.3 ± 0.6 at 12× acceleration. Using our CS sequence at 12× acceleration, we were able to acquire dynamic CE-MRA with higher spatial and temporal resolution than current clinical TWIST protocol while maintaining comparable image quality (2.8 ± 0.5 vs. 3.0 ± 0.4, P = NS). CONCLUSION: Our technique is promising for dynamic CE-MRA.

Steady-state first-pass perfusion (SSFPP): a new approach to 3D first-pass myocardial perfusion imaging.

PURPOSE: To describe and characterize a new approach to first-pass myocardial perfusion utilizing balanced steady-state free precession acquisition without the use of saturation recovery or other magnetization preparation. THEORY: The balanced steady-state free precession sequence is inherently sensitive to contrast agent enhancement of the myocardium. This sensitivity can be used to advantage in first-pass myocardial perfusion imaging by eliminating the need for magnetization preparation. METHODS: Bloch equation simulations, phantom experiments, and in vivo 2D imaging studies were run comparing the proposed technique with three other methods: saturation recovery spoiled gradient echo, saturation recovery steady-state free precession, and steady-state spoiled gradient echo without magnetization preparation. Additionally, an acquisition-reconstruction strategy for 3D perfusion imaging is proposed and initial experience with this approach is demonstrated in healthy subjects and one patient. RESULTS: Phantom experiments verified simulation results showing the sensitivity of the balanced steady-state free precession sequence to contrast agent enhancement in solid tissue is similar to that of magnetization-prepared acquisitions. Images acquired in normal volunteers showed the proposed technique provided superior signal and signal-to-noise ratio compared with all other sequences at baseline as well as postcontrast. CONCLUSIONS: A new approach to first-pass myocardial perfusion is presented that obviates the need for magnetization preparation and provides high signal-to-noise ratio.

Clinical evaluation of CAIPIRINHA: comparison against a GRAPPA standard.

PURPOSE: To evaluate image quality when using a CAIPIRINHA sampling pattern in comparison to a standard GRAPPA sampling pattern in patients undergoing a routine three-dimensional (3D) breathheld liver exam. CAIPIRINHA uses an optimized phase encoding sampling strategy to alter aliasing artifacts in 3D acquisitions to improve parallel imaging reconstruction. MATERIALS AND METHODS: Twenty patient volunteers were scanned using a 3D VIBE acquisition with an acceleration factor of four using a CAIPIRINHA and standard GRAPPA sampling pattern. CAIPIRINHA and GRAPPA images were evaluated by three radiologists in a two alternative forced choice test, and the Wilcoxon signed rank test was performed. RESULTS: The CAIPIRINHA sampling pattern was preferred in an average of 68% of the comparisons, and the Wilcoxon signed rank test showed a significant improvement in CAIPIRINHA images (P = 0.014). This analysis indicates that in the given sample set, CAIPIRINHA preference over the GRAPPA standard was statistically significant. CONCLUSION: This work shows that for an acceleration factor of four, a CAIPIRINHA accelerated VIBE acquisition provides significantly improved image quality in comparison to the current GRAPPA standard. This allows a further reduction in imaging time for similar spatial resolutions, which can reduce long breathhold requirements in abdominal imaging, and may be particularly helpful in patients who cannot provide requisite breathholds with current protocols.

Application of time-resolved angiography with stochastic trajectories (TWIST)-Dixon in dynamic contrast-enhanced (DCE) breast MRI.

PURPOSE: To evaluate a magnetic resonance imaging (MRI) technique that integrates time-resolved angiography with stochastic trajectories (TWIST) view sharing and Dixon for a breast dynamic contrast-enhanced (DCE)-MRI application. MATERIALS AND METHODS: Simulation study: K-space data at six timepoints (1 pre-, 5 postcontrast) were generated by performing Fourier transform on a digital "phantom" with 3-9 mm enhancing lesions and three types of enhancement curves (persistent, plateau, washout). Images were reconstructed with and without TWIST. Clinical study: Six TWIST-Dixon image sets (one pre-, five postcontrast) were acquired in 18 patients on a 3T scanner, followed by one conventional image set. The last TWIST-Dixon and the conventional images were scored for seven criteria: perceived signal-to-noise ratio (P.SNR), visualization of anatomy, fat suppression (FS) accuracy, FS uniformity, ghosting artifact, edge ringing artifact, and overall image quality (IQ). RESULTS: Simulation study: With proper TWIST parameters (pA ≥33%, pB ≥50%), the enhancement underestimation was 5% or less for tumor size ≥5 mm. Clinical study: TWIST-Dixon images have significantly better scores in all criteria except for ghosting artifacts, where the difference was not significant. CONCLUSION: With proper parameters, TWIST-Dixon provides higher perceived SNR, more accurate fat suppression, and better overall image quality for breast DCE-MRI without sacrificing accuracy in the enhancement estimation.

CAIPIRINHA-Dixon-TWIST (CDT)-volume-interpolated breath-hold examination (VIBE): a new technique for fast time-resolved dynamic 3-dimensional imaging of the abdomen with high spatial resolution.

PURPOSE: The purpose of this study was to assess the feasibility and image quality of a novel, highly accelerated T1-weighted sequence for time-resolved imaging of the abdomen during the first pass of contrast media transit using controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) under sampling, view-sharing techniques, and Dixon water-fat separation (CAIPRINHA-Dixon-time-resolved imaging with interleaved stochastic trajectories-volumetric interpolated breath-hold examination [CDT-VIBE]). MATERIALS AND METHODS: In this retrospective, institutional review board-approved study, 47 patients (median age, 62 years; 25 men, 22 women) scanned on a 3.0-T magnetic resonance system (Skyra; Siemens) were included. The CDT-VIBE (repetition time/echo time1/echo time2, 4.1/1.33/2.56 milliseconds; acquisition time, 29 seconds) was used in place of the standard arterial phase acquisition and started 15 seconds after the injection of 0.1 mmol/kg Gd-DOTA (Dotarem, Guerbet). Within 29 seconds, 14 high spatial resolution (1.2 × 1.2 × 3 mm) 3-dimensional data sets were acquired and reconstructed using view sharing (temporal resolution, 2.1 seconds). The CDT-VIBE images were evaluated independently by 2 blinded, experienced radiologists with regard to image quality and the number of hepatic arterial-dominant phases present on an ordinal 5-point scale (5, excellent; 1, nondiagnostic). Added diagnostic information with CDT-VIBE relative to portal venous phase VIBE was assessed. RESULTS: In all patients, CDT-VIBE measurements were successfully acquired. The image quality was diagnostic in 46 of the 47 patients. Both readers assessed the highest image quality present in the data sets with a median score of 4 (range, 3-5 for both readers; κ, 0.789) and the worst image quality with a median score of 3 (range, 1-4 for both readers; κ, 0.689). With a range between 1 and 8 (median, 5), hepatic arterial-dominant data sets (of the 14 acquired) were obtained in each case. There was an added diagnostic value with CDT-VIBE in 10 of the 47 patients (21%). CONCLUSIONS: The CDT-VIBE is a robust approach allowing, for the first time, dynamic imaging of the upper abdomen with high temporal resolution and preservation of high spatial resolution.

Development and evaluation of TWIST Dixon for dynamic contrast-enhanced (DCE) MRI with improved acquisition efficiency and fat suppression.

PURPOSE: To develop a new pulse sequence called time-resolved angiography with stochastic trajectories (TWIST) Dixon for dynamic contrast enhanced magnetic resonance imaging (DCE-MRI). MATERIALS AND METHODS: The method combines dual-echo Dixon to generate separated water and fat images with a k-space view-sharing scheme developed for 3D TWIST. The performance of TWIST Dixon was compared with a volume interpolated breathhold examination (VIBE) sequence paired with spectrally selective adiabatic inversion Recovery (SPAIR) and quick fat-sat (QFS) fat-suppression techniques at 3.0T using quantitative measurements of fat-suppression accuracy and signal-to-noise ratio (SNR) efficiency, as well as qualitative breast image evaluations. RESULTS: The water fraction of a uniform phantom was calculated from the following images: 0.66 ± 0.03 for TWIST Dixon; 0.56 ± 0.23 for VIBE-SPAIR, and 0.53 ± 0.14 for VIBE-QFS, while the reference value is 0.70 measured by spectroscopy. For phantoms with contrast (Gd-BOPTA) concentration ranging from 0-6 mM, TWIST Dixon also provides consistently higher SNR efficiency (3.2-18.9) compared with VIBE-SPAIR (2.8-16.8) and VIBE-QFS (2.4-12.5). Breast images acquired with TWIST Dixon at 3.0T show more robust and uniform fat suppression and superior overall image quality compared with VIBE-SPAIR. CONCLUSION: The results from phantom and volunteer evaluation suggest that TWIST Dixon outperforms conventional methods in almost every aspect and it is a promising method for DCE-MRI and contrast-enhanced perfusion MRI, especially at higher field strength where fat suppression is challenging.

Automatic view planning for cardiac MRI acquisition.

Conventional cardiac MRI acquisition involves a multi-step approach, requiring a few double-oblique localizers in order to locate the heart and prescribe long- and short-axis views of the heart. This approach is operator-dependent and time-consuming. We propose a new approach to automating and accelerating the acquisition process to improve the clinical workflow. We capture a highly accelerated static 3D full-chest volume through parallel imaging within one breath-hold. The left ventricle is localized and segmented, including left ventricle outflow tract. A number of cardiac landmarks are then detected to anchor the cardiac chambers and calculate standard 2-, 3-, and 4-chamber long-axis views along with a short-axis stack. Learning-based algorithms are applied to anatomy segmentation and anchor detection. The proposed algorithm is evaluated on 173 localizer acquisitions. The entire view planning is fully automatic and takes less than 10 seconds in our experiments.

Optimal k-space sampling for dynamic contrast-enhanced MRI with an application to MR renography.

For time-resolved acquisitions with k-space undersampling, a simulation method was developed for selecting imaging parameters based on minimization of errors in signal intensity versus time and physiologic parameters derived from tracer kinetic analysis. Optimization was performed for time-resolved angiography with stochastic trajectories (TWIST) algorithm applied to contrast-enhanced MR renography. A realistic 4D phantom comprised of aorta and two kidneys, one healthy and one diseased, was created with ideal tissue time-enhancement pattern generated using a three-compartment model with fixed parameters, including glomerular filtration rate (GFR) and renal plasma flow (RPF). TWIST acquisitions with different combinations of sampled central and peripheral k-space portions were applied to this phantom. Acquisition performance was assessed by the difference between simulated signal intensity (SI) and calculated GFR and RPF and their ideal values. Sampling of the 20% of the center and 1/5 of the periphery of k-space in phase-encoding plane and data-sharing of the remaining 4/5 minimized the errors in SI (<5%), RPF, and GFR (both <10% for both healthy and diseased kidneys). High-quality dynamic human images were acquired with optimal TWIST parameters and 2.4 sec temporal resolution. The proposed method can be generalized to other dynamic contrast-enhanced MRI applications, e.g., MR angiography or cancer imaging.

Time-resolved MR angiography with generalized autocalibrating partially parallel acquisition and time-resolved echo-sharing angiographic technique for hemodialysis arteriovenous fistulas and grafts.

PURPOSE: To evaluate the imaging of hemodialysis arteriovenous (AV) fistulas and grafts with use of magnetic resonance (MR) angiography with generalized autocalibrating partially parallel acquisition (GRAPPA) and time-resolved echo-sharing angiographic technique (TREAT) and compare the findings with those of digital subtraction angiography (DSA). MATERIALS AND METHODS: The vascular tree directly related to AV fistulas and grafts was divided into nine segments. Images of each segment obtained on GRAPPA MR angiography were evaluated for the presence of stenosis, occlusion, and any other disease (eg, pseudoaneurysm) by two independent observers and compared with a consensus reading of the same segments on DSA imaging. Sensitivity and specificity were calculated with use of DSA as the gold standard modality, and each image on MR angiography and DSA was rated for quality. Linear-weighted kappa scores were calculated as a measure of interobserver variability in the detection of pathologic processes. RESULTS: A total of 80 segments were evaluated by each observer. For both observers, sensitivity rates for the detection of stenosis, occlusion, and any disease were 100% (95% CI, 52%-100%), 100% (95% CI, 20%-100%), and 100% (95% CI, 60%-100%), respectively. For observer 1, specificity rates for the detection of stenosis, occlusion, and any disease were 96% (95% CI, 88%-99%), 100% (95% CI, 94%-100%), and 96% (95% CI, 88%-99%), respectively. For observer 2, the specificity rates for the detection of stenosis, occlusion, and any disease were 93% (95% CI, 84%-98%), 100% (95% CI, 94%-100%), and 93% (95% CI, 84%-97%), respectively. Linear-weighted kappa values for MR angiography and DSA were 0.78+/-0.084 and 0.62+/-0.152, respectively. CONCLUSION: Time-resolved MR angiography with GRAPPA and TREAT offers excellent image quality and provides an accurate and reliable modality for the detection of pathologic processes in hemodialysis AV fistulas and grafts.

Time-resolved contrast-enhanced three-dimensional magnetic resonance angiography of the chest: combination of parallel imaging with view sharing (TREAT).

RATIONALE AND OBJECTIVES: In view sharing, some parts of k-space are updated more often than others, leading to an effective shortening of the total acquisition time. Undersampling of high-frequency k-space data, however, can result in artifacts at the edges of blood vessels, especially during the rapid signal intensity changes. The objective of this study was to evaluate a new time-resolved echo-shared angiographic technique (TREAT) combining parallel imaging with view sharing. First, the presence of artifacts arising from different temporal interpolation schemes was evaluated in simulations of the point spread function. Second, the image quality and presence of artifacts of time-resolved parallel three-dimensional magnetic resonance angiography (3D MRA) of the chest, acquired with and without view sharing, was assessed in a clinical study of patients with cardiovascular or pulmonary disease. MATERIALS AND METHODS: Using parameters from a time-resolved parallel 3D MRA sequence without view sharing (parallel MRA), giving a 33% increase in spatial resolution, our simulations have revealed that k-space segmentation in 3 regions provides acceptable artifacts. Thirty-six consecutive patients (mean age, 50 +/- 16 years; 15 females, 22 males) were examined in a clinical study with TREAT. The image data were compared with that of a group of 31 consecutive patients (mean age, 46 +/- 19 years; 12 females, 19 males) examined with a conventional time-resolved parallel MRA sequence without view sharing (parallel MRA). The image quality and presence of artifacts was assessed in a blind comparison by 2 radiologists in consensus using MPR and MIP reconstructions. Furthermore, the peak SNR of the pulmonary artery and aorta was compared between both MRA sequences. RESULTS: The image quality of TREAT was rated significantly higher than that of the parallel MRA sequence without view sharing: depending on the orientation of MPR and MIP reconstructions, an excellent image quality was found in 69-89% with TREAT and in 45-71% with the parallel MRA protocol without view sharing, respectively. The presence of artifacts was equal with both sequences. CONCLUSION: View sharing can be successfully combined with other acceleration techniques, such as parallel imaging. TREAT allows the assessment of the thoracic vasculature with a high temporal and spatial resolution.

Thorax: low-dose contrast-enhanced three-dimensional MR angiography with subsecond temporal resolution--initial results.

The purpose of the study was to implement a three-dimensional (3D) magnetic resonance (MR) angiographic technique with acquisition times on the order of 800 msec with use of a spoiled gradient-echo pulse sequence (repetition time, 1.60 msec; echo time, 0.65 msec) and bolus intravenous injection of contrast material doses as small as 6 mL. High-spatial-resolution conventional MR angiography performed with 30 mL of gadopentetate dimeglumine was the reference standard. As implemented, subsecond 3D MR angiography allowed temporal sampling that was rapid enough to depict short-lived processes, as illustrated in patients with shunts and dissections. With small contrast material doses and subsecond frame rates, it is also possible to measure pulmonary arteriovenous circulation times with this 3D MR angiographic technique.