Magnetic resonance angiography with compressed sensing: An evaluation of moyamoya disease

High-resolution magnetic resonance vessel wall imaging provides new insights into Moyamoya disease

Moyamoya disease (MMD) is a rare condition that affects the blood vessels of the central nervous system. This cerebrovascular disease is characterized by progressive narrowing and blockage of the internal carotid, middle cerebral, and anterior cerebral arteries, which results in the formation of a compensatory fragile vascular network. Currently, digital subtraction angiography (DSA) is considered the gold standard in diagnosing MMD. However, this diagnostic technique is invasive and may not be suitable for all patients. Hence, non-invasive imaging methods such as computed tomography angiography (CTA) and magnetic resonance angiography (MRA) are often used. However, these methods may have less reliable diagnostic results. Therefore, High-Resolution Magnetic Resonance Vessel Wall Imaging (HR-VWI) has emerged as the most accurate method for observing and analyzing arterial wall structure. It enhances the resolution of arterial walls and enables quantitative and qualitative analysis of plaque, facilitating the identification of atherosclerotic lesions, vascular entrapment, myofibrillar dysplasia, moyamoya vasculopathy, and other related conditions. Consequently, HR-VWI provides a new and more reliable evaluation criterion for diagnosing vascular lesions in patients with Moyamoya disease.

Use of Compressed Sensing Accelerated, Low-Velocity Encoded, Isotropic Resolution, Phase Contrast Magnetic Resonance Angiography for SEEG Electrode Implantation

OBJECTIVE

We assessed the feasibility of using compressed sensing accelerated, low-velocity encoded, isotropic resolution phase contrast (CLIP) magnetic resonance angiography (MRA) for avascular trajectory planning of stereoelectroencephalography.

METHODS

Ten healthy subjects (1 woman and 9 men; age, 33.6 ± 9.0 years) and 20 consecutive patients (12 female patients; age, 22 ± 13.6 years) were enrolled in the present study. The healthy subjects underwent CLIP-MRA, and 3 other phase contrast MRA protocols with conventional parallel imaging (PI) acceleration, including low resolution with twofold PI (PI2), high resolution (HR) with fivefold PI (PI5), and HR-PI2. The patients underwent CLIP-MRA and computed tomography angiography (CTA). The image qualities were evaluated. The numbers and locations of trajectory-vessel conflict detected using CLIP-MRA were noted.

RESULTS

With similar scan durations, CLIP-MRA achieved higher spatial resolution compared with low resolution with PI2 and detected significantly more branches compared with HR-PI5. With the same spatial resolution, the signal/noise and contrast/noise ratios of CLIP-MRA were higher than those with HR-PI2 with a shorter scan duration. For the 12 adult patients (10 female patients; 28.8 ± 12.7 years), CLIP-MRA had better signal/noise and contrast/noise ratios than CTA. The trajectory had required modification for 14 of the 20 patients (70%), with a proportion of trajectory modification of 10.7% (23 of 215 electrodes). The middle meningeal artery, cortical vessel, and skull vessel were the main vessels with conflict (n = 11, n = 7, and n = 5, respectively).

CONCLUSIONS

In the present study, CLIP-MRA provided a clear cortical vascular display noninvasively without intravascular contrast and radiation. The middle meningeal artery and diploic and emissary veins were the main conflict vessels and could be clearly displayed using CLIP-MRA but not CTA.

Ultra-High-Resolution Time-of-Flight MR-Angiography for the Noninvasive Assessment of Intracranial Aneurysms, Alternative to Preinterventional DSA?

PURPOSE

The 3D time-of-flight (TOF) magnetic resonance angiography (MRA) at 3T shows high sensitivity for intracranial aneurysms but is inferior to three-dimensional digital subtraction angiography (3D-DSA) regarding aneurysm characteristics. We applied an ultra-high-resolution (UHR) TOF-MRA using compressed sensing reconstruction to investigate the diagnostic performance in preinterventional evaluation of intracranial aneurysms compared to conventional TOF-MRA and 3D-DSA.

METHODS

In this study 17 patients with unruptured intracranial aneurysms were included. Aneurysm dimensions, configuration, image quality and sizing of endovascular devices were compared between conventional TOF-MRA at 3T and UHR-TOF with 3D-DSA as gold standard. Quantitatively, contrast-to-noise ratios (CNR) were compared between TOF-MRAs.

RESULTS

On 3D-DSA, 25 aneurysms in 17 patients were detected. On conventional TOF, 23 aneurysms were detected (sensitivity: 92.6%). On UHR-TOF, 25 aneurysms were detected (sensitivity: 100%). Image quality was not significantly different between TOF and UHR-TOF (p = 0.17). Aneurysm dimension measurements were significantly different between conventional TOF (3.89 mm) and 3D-DSA (4.2 mm, p = 0.08) but not between UHR-TOF (4.12 mm) and 3D-DSA (p = 0.19). Irregularities and small vessels at the aneurysm neck were more frequently correctly depicted on UHR-TOF compared to conventional TOF. Comparison of the planned framing coil diameter and flow-diverter (FD) diameter revealed neither a statistically significant difference between TOF and 3D-DSA (coil p = 0.19, FD p = 0.45) nor between UHR-TOF and 3D-DSA (coil: p = 0.53, FD 0.33). The CNR was significantly higher in conventional TOF (p = 0.009).

CONCLUSION

In this pilot study, ultra-high-resolution TOF-MRA visualized all aneurysms and accurately depicted aneurysm irregularities and vessels at the base of the aneurysm comparably to DSA, outperforming conventional TOF. UHR-TOF with compressed sensing reconstruction seems to represent a non-invasive alternative to pre-interventional DSA for intracranial aneurysms.

Accelerating TOF-MRA: The impact of the combined use of compressed sensitivity encoding and spiral imaging

PURPOSE

To evaluate the image quality of the combined technique of compressed sensitivity encoding (CS) and spiral imaging in time-of-flight magnetic resonance angiography (TOF-MRA), which is approximately 2.5 times faster than conventional methods.

METHODS

Twenty volunteers underwent four TOF-MRA sequences: sensitivity encoding (SENSE) with acceleration factor of 4 (acquisition time: 4:55 min), CS with acceleration factor of 10.9, and spiral and CS-spiral (both 1:55 min). A quantitative image analysis (signal-to-noise ratio [SNR], contrast, and full width at half maximum [FWHM] edge criterion measurements) was performed on four TOF sequences. For qualitative image analysis, two board-certified radiologists evaluated the overall depiction of the proximal, intermediate, and distal branches in CS, spiral, and CS-spiral images using SENSE as a reference.

RESULTS

The SNR of BA in spiral and CS-spiral imaging was significantly lower than that in SENSE (p = 0.009). The contrasts of ACA and BA in CS-spiral were significantly higher and those in spiral were significantly lower than those in SENSE (p < 0.001). The FWHM in the CS image was significantly higher than that of SENSE; however, no significant differences were observed between the spiral or CS-spiral and SENSE. In qualitative analysis, the depiction of proximal vascular branches was significantly impaired in spiral than in others and that of distal vascular branches was significantly impaired in CS than in others (p < 0.001).

CONCLUSIONS

In TOF-MRA, which is approximately 2.5 times faster than conventional methods, the combined use of CS and spiral imaging demonstrated an improvement in image quality compared to either CS or spiral imaging alone.

SUMMARY STATEMENT

The image quality of Compressed SENSE and spiral imaging is particularly poor in the proximal and distal vascular branches, respectively at an extremely high acceleration factor; however, CS-spiral provided stable image quality in all regions as compared with the SENSE technique.

Highly accelerated intracranial time-of-flight magnetic resonance angiography using wave-encoding

PURPOSE

To develop an accelerated 3D intracranial time-of-flight (TOF) magnetic resonance angiography (MRA) sequence with wave-encoding (referred to as 3D wave-TOF) and to evaluate two variants: wave-controlled aliasing in parallel imaging (CAIPI) and compressed-sensing wave (CS-wave).

METHODS

A wave-TOF sequence was implemented on a 3 T clinical scanner. Wave-encoded and Cartesian k-space datasets from six healthy volunteers were retrospectively and prospectively undersampled with 2D-CAIPI sampling and variable-density Poisson disk sampling. 2D-CAIPI, wave-CAIPI, standard CS, and CS-wave schemes were compared at various acceleration factors. Flow-related artifacts in wave-TOF were investigated, and a set of practicable wave parameters was developed. Quantitative analysis of wave-TOF and traditional Cartesian TOF MRA was performed by comparing the contrast-to-background ratio between the vessel and background tissue in source images, and the structural similarity index measure (SSIM) between the maximum intensity projection images from accelerated acquisitions and their respective fully sampled references.

RESULTS

Flow-related artifacts caused by the wave-encoding gradients in wave-TOF were eliminated by properly chosen parameters. Images from wave-CAIPI and CS-wave acquisitions had a higher SNR and better-preserved contrast than traditional parallel imaging (PI) and CS methods. Maximum intensity projection images from wave-CAIPI and CS-wave acquisitions had a cleaner background, with vessels that were better depicted. Quantitative analyses indicated that wave-CAIPI had the highest contrast-to-background ratio, SSIM, and vessel-masked SSIM among the sampling schemes studied, followed by the CS-wave acquisition.

CONCLUSION

3D wave-TOF improves the capability of accelerated MRA and provides better image quality at higher acceleration factors compared to traditional PI- or CS-accelerated TOF, suggesting the potential use of wave-TOF in cerebrovascular disease.

MEDL-Net: A model-based neural network for MRI reconstruction with enhanced deep learned regularizers

PURPOSE

To improve the MRI reconstruction performance of model-based networks and to alleviate their large demand for GPU memory.

METHODS

A model-based neural network with enhanced deep learned regularizers (MEDL-Net) was proposed. The MEDL-Net is separated into several submodules, each of which consists of several cascades to mimic the optimization steps in conventional MRI reconstruction algorithms. Information from shallow cascades is densely connected to latter ones to enrich their inputs in each submodule, and additional revising blocks (RB) are stacked at the end of the submodules to bring more flexibility. Moreover, a composition loss function was designed to explicitly supervise RBs.

RESULTS

Network performance was evaluated on a publicly available dataset. The MEDL-Net quantitatively outperforms the state-of-the-art methods on different MR image sequences with different acceleration rates (four-fold and six-fold). Moreover, the reconstructed images showed that the detailed textures are better preserved. In addition, fewer cascades are required when achieving the same reconstruction results compared with other model-based networks.

CONCLUSION

In this study, a more efficient model-based deep network was proposed to reconstruct MR images. The experimental results indicate that the proposed method improves reconstruction performance with fewer cascades, which alleviates the large demand for GPU memory.

Ultrasonic hemodynamic changes of superficial temporal artery graft in different angiogenesis outcomes of Moyamoya disease patients treated with combined revascularization surgery

Objective

Combined bypass is commonly used in adult Moyamoya disease (MMD) for revascularization purposes. The blood flow from the external carotid artery system supplied by the superficial temporal artery (STA), middle meningeal artery (MMA), and deep temporal artery (DTA) can restore the impaired hemodynamics of the ischemic brain. In this study we attempted to evaluate the hemodynamic changes of the STA graft and predict the angiogenesis outcomes in MMD patients after combined bypass surgery by using quantitative ultrasonography.

Methods

We retrospectively studied Moyamoya patients who were treated by combined bypass between September 2017 and June 2021 in our hospital. We quantitatively measured the STA with ultrasound and recorded the blood flow, diameter, pulsatility index (PI) and resistance index (RI) to assess graft development preoperatively and at 1 day, 7 days, 3 months, and 6 months after surgery. All patients received both pre- and post- operative angiography evaluation. Patients were divided into either well- or poorly-angiogenesis groups according to the transdural collateral formation status on angiography at 6 months after surgery (W group or P group). Patients with matshushima grade A or B were divided into W group. Patients with matshushima grade C were divided into P group, indicating a poor angiogenesis development.

Results

A total of 52 patients with 54 operated hemispheres were enrolled, including 25 men and 27 women with an average age of 39 ± 14.3 years. Compared to preoperative values, the average blood flow of an STA graft at day 1 postoperation increased from 16.06 ± 12.47 to 117.47± 73.77 (mL/min), diameter increased from 1.14 ± 0.33 to 1.81 ± 0.30 (mm), PI dropped from 1.77 ± 0.42 to 0.76 ± 0.37, and RI dropped from 1.77 ± 0.42 to 0.50 ± 0.12. According to the Matsushima grade at 6 months after surgery, 30 hemispheres qualified as W group and 24 hemispheres as P group. Statistically significant differences were found between the two groups in diameter (p = 0.010) as well as flow (p = 0.017) at 3 months post-surgery. Flow also remained significantly different at 6 months after surgery (p = 0.014). Based on GEE logistic regression evaluation, the patients with higher levels of flow post-operation were more likely to have poorly-compensated collateral. ROC analysis showed that increased flow of ≥69.5 ml/min (p = 0.003; AUC = 0.74) or a 604% (p = 0.012; AUC = 0.70) increase at 3 months post-surgery compared with the pre-operative value is the cut-off point which had the highest Youden's index for predicting P group. Furthermore, a diameter at 3 months post-surgery that is ≥0.75 mm (p = 0.008; AUC = 0.71) or 52% (p =0.021; AUC = 0.68) wider than pre-operation also indicates a high risk of poor indirect collateral formation.

Conclusions

The hemodynamic of the STA graft changed significantly after combined bypass surgery. An increased flow of more than 69.5 ml/min at 3 months was a good predictive factor for poor neoangiogenesis in MMD patients treated with combined bypass surgery.

Vessel wall MR imaging in neuroradiology

Vessel wall MR imaging (VW-MRI) has been introduced into clinical practice and applied to a variety of diseases, and its usefulness has been reported. High-resolution VW-MRI is essential in the diagnostic workup and provides more information than other routine MR imaging protocols. VW-MRI is useful in assessing lesion location, morphology, and severity. Additional information, such as vessel wall enhancement, which is useful in the differential diagnosis of atherosclerotic disease and vasculitis could be assessed by this special imaging technique. This review describes the VW-MRI technique and its clinical applications in arterial disease, venous disease, vasculitis, and leptomeningeal disease.

Practical Aspects of novel MRI Techniques in Neuroradiology: Part 2 - Acceleration Methods and Implications for Individual Regions

BACKGROUND

 Recently introduced MRI techniques facilitate accelerated examinations or increased resolution with the same duration. Further techniques offer homogeneous image quality in regions with anatomical transitions. The question arises whether and how these techniques can be adopted for routine diagnostic imaging.

METHODS

 Narrative review with an educational focus based on current literature research and practical experiences of different professions involved (physicians, MRI technologists/radiographers, physics/biomedical engineering). Different hardware manufacturers are considered.

RESULTS AND CONCLUSIONS

 Compressed sensing and simultaneous multi-slice imaging are novel acceleration techniques with different yet complimentary applications. They do not suffer from classical signal-to-noise-ratio penalties. Combining 3 D and acceleration techniques facilitates new broader examination protocols, particularly for clinical brain imaging. In further regions of the nervous systems mainly specific applications appear to benefit from recent technological improvements.

KEY POINTS

  · New acceleration techniques allow for faster or higher resolution examinations.. · New brain imaging approaches have evolved, including more universal examination protocols.. · Other regions of the nervous system are dominated by targeted applications of recently introduced MRI techniques..

CITATION FORMAT

· Sundermann B, Billebaut B, Bauer J et al. Practical Aspects of novel MRI Techniques in Neuroradiology: Part 2 - Acceleration Methods and Implications for Individual Regions. Fortschr Röntgenstr 2022; DOI: 10.1055/a-1800-8789.

Follow-Up Assessment of Intracranial Aneurysms Treated with Endovascular Coiling: Comparison of Compressed Sensing and Parallel Imaging Time-of-Flight Magnetic Resonance Angiography

The aim of our study was to compare compressed sensing (CS) time-of-flight (TOF) magnetic resonance angiography (MRA) with parallel imaging (PI) TOF MRA in the evaluation of patients with intracranial aneurysms treated with coil embolization or stent-assisted coiling. We enrolled 22 patients who underwent follow-up imaging after intracranial aneurysm coil embolization. All patients underwent both PI TOF and CS TOF MRA during the same examination. Image evaluation aimed to compare the performance of CS to PI TOF MRA in determining the degree of aneurysm occlusion, as well as the depiction of parent vessel and vessels adjacent to the aneurysm dome. The reference standard for the evaluation of aneurysm occlusion was PI TOF MRA. The inter-modality agreement between CS and PI TOF MRA in the evaluation of aneurysm occlusion was almost perfect (κ  =  0.98, p  <  0.001) and the overall inter-rater agreement was substantial (κ  =  0.70, p  <  0.001). The visualization of aneurysm parent vessel in CS TOF images compared with PI TOF images was evaluated to be better in 11.4%, equal in 86.4%, and worse in 2.3%. CS TOF MRA, with almost 70% scan time reduction with respect to PI TOF MRA, yields comparable results for assessing the occlusion status of coiled intracranial aneurysms. Short scan times increase patient comfort, reduce the risk of motion artifacts, and increase patient throughput, with a resulting reduction in costs. CS TOF MRA may therefore be a potential replacement for PI TOF MRA as a first-line follow-up examination in patients with intracranial aneurysms treated with coil embolization.

High-resolution compressed sensing time-of-flight MR angiography outperforms CT angiography for evaluating patients with Moyamoya disease after surgical revascularization

Background

To evaluate the utility of high-resolution compressed sensing time-of-fight MR angiography (CS TOF-MRA) for assessing patients with moyamoya disease (MMD) after surgical revascularization, by comparison with computer tomography angiography (CTA).

Methods

Twenty patients with MMD after surgical revascularizations who underwent CS TOF-MRA and CTA were collected. The scan time of CS TOF-MRA was 5 min and 4 s, with a reconstructed resolution of 0.4 × 0.4 × 0.4 mm³. Visualization of superficial temporal artery and middle cerebral artery (STA–MCA) bypass, neovascularization into the brain pial surface and Moyamoya vessels (MMVs) were independently ranked by two neuroradiologists on CS TOF-MRA and CTA, respectively. The patency of anastomosis was assessed as patent or occluded, using digital subtraction angiography and expert’s consensus as ground truth. Interobserver agreement was calculated using the weighted kappa statistic. Wilcoxon signed-rank or Chi-square test was performed to investigate diagnostic difference between CS TOF-MRA and CTA.

Results

Twenty-two hemispheres from 20 patients were analyzed. The inter-reader agreement for evaluating STA–MCA bypass, neovascularization and anastomosis patency was good to excellent (κ_{CS TOF-MRA}, 0.738–1.000; κ_CTA, 0.743–0.909). The STA–MCA bypass and MMVs were better visualized on CS TOF-MRA than CTA (both P < 0.05). CS TOF-MRA had a higher sensitivity than CTA (94.7% vs. 73.7%) for visualizing anastomoses. Neovascularization was better observed in 13 (59.1%) sides on CS TOF-MRA, in comparison to 7 (31.8%) sides on CTA images (P = 0.005).

Conclusion

High-resolution CS TOF-MRA outperforms CTA for visualization of STA–MCA bypass, neovascularization and MMVs within a clinically reasonable time in MMD patients after revascularization.

Optimization of undersampling parameters for 3D intracranial compressed sensing MR angiography at 7 T

Purpose

3D time‐of‐flight MRA can accurately visualize the intracranial vasculature but is limited by long acquisition times. Compressed sensing reconstruction can be used to substantially accelerate acquisitions. The quality of those reconstructions depends on the undersampling patterns used. In this work, we optimize sets of undersampling parameters for various acceleration factors of Cartesian 3D time‐of‐flight MRA.

Methods

Fully sampled datasets, acquired at 7 Tesla, were retrospectively undersampled using variable‐density Poisson disk sampling with various autocalibration region sizes, polynomial orders, and acceleration factors. The accuracy of reconstructions from the different undersampled datasets was assessed using the vessel‐masked structural similarity index. Identified optimal undersampling parameters were then evaluated in additional prospectively undersampled datasets. Compressed sensing reconstruction parameters were chosen based on a preliminary reconstruction parameter optimization.

Results

For all acceleration factors, using a fully sampled calibration area of 12 × 12 k‐space lines and a polynomial order of 2 resulted in the highest image quality. The importance of parameter optimization of the sampling was found to increase for higher acceleration factors. The results were consistent across resolutions and regions of interest with vessels of varying sizes and tortuosity. The number of visible small vessels increased by 7.0% and 14.2% when compared to standard parameters for acceleration factors of 7.2 and 15, respectively.

Conclusion

The image quality of compressed sensing time‐of‐flight MRA can be improved by appropriate choice of undersampling parameters. The optimized sets of parameters are independent of the acceleration factor and enable a larger number of vessels to be visualized.

Non-contrast hemodynamic imaging of Moyamoya disease with MR fingerprinting ASL: A feasibility study

PURPOSE

MR Fingerprinting (MRF) Arterial Spin Labeling (ASL) is a non-contrast technique to estimate multiple brain hemodynamic and structural parameters in a single scan. The purpose of this study is to examine the feasibility and initial utility of MRF-ASL in Moyamoya disease.

METHODS

MRF-ASL, conventional single-delay ASL, Time-of-flight (TOF) MR angiography, and contrast-based dynamic-susceptibility-contrast (DSC) MRI were prospectively collected from a group of Moyamoya patients in North America (N = 21, 4 men and 17 women). Sixteen healthy subjects (7 men and 9 women) also underwent an MRF-ASL scan. Cerebral blood flow (CBF), bolus arrival time (BAT), and tissue T1 were compared between Moyamoya patients and healthy controls. Perfusion parameters from MRF-ASL were compared to those from other MRI sequences. Multi-linear regression was used for comparisons of parameter values between Moyamoya and control groups. Linear mixed-effects models was used when comparing MRF-ASL to PCASL and DSC parameters. Spearman's Rank Correlation Coefficient was calculated when comparing MRF-ASL to and MRA grades. A P value of 0.05 or less was considered significant.

RESULTS

BAT in stenotic internal carotid artery (ICA) territories was prolonged (P < 0.001) in Moyamoya patients, when compared with healthy controls. CBF in stenotic ICA territories of Moyamoya patients was not different from CBF in healthy controls; but in the PCA territories, CBF in Moyamoya patients was higher (P < 0.01) than controls. Quantitative T1 values in the stenotic ICA territories was longer (P < 0.05) than that in controls. Hemodynamic parameters estimated from MRF-ASL were significantly correlated with single-delay ASL and DSC. Longer BAT was associated with more severe intracranial artery stenosis in ICA.

CONCLUSIONS

MRF-ASL is a promising technique to assess perfusion and structural abnormalities in Moyamoya patients.

Acceleration of Brain TOF-MRA with Compressed Sensitivity Encoding: A Multicenter Clinical Study

BACKGROUND AND PURPOSE

The clinical practice of three-dimensional TOF-MRA, despite its capability in brain artery assessment, has been hampered by the relatively long scan time, while recent developments in fast imaging techniques with random undersampling has shed light on an improved balance between image quality and imaging speed. Our aim was to evaluate the effectiveness of TOF-MRA accelerated by compressed sensitivity encoding and to identify the optimal acceleration factors for routine clinical use.

MATERIALS AND METHODS

One hundred subjects, enrolled at 5 centers, underwent 8 brain TOF-MRA sequences: 5 sequences using compressed sensitivity encoding with acceleration factors of 2, 4, 6, 8, and 10 (CS2, CS4, CS6, CS8, and CS10), 2 using sensitivity encoding with factors of 2 and 4 (SF2 and SF4), and 1 without acceleration as a reference sequence (RS). Five large arteries, 6 medium arteries, and 6 small arteries were evaluated quantitatively (reconstructed signal intensity, structural similarity, contrast ratio) and qualitatively (scores on arteries, artifacts, overall image quality, and diagnostic confidence for aneurysm and stenosis). Comparisons were performed among the 8 sequences.

RESULTS

The quantitative measurements showed that the reconstructed signal intensities of the assessed arteries and the structural similarity consistently decreased as the compressed sensitivity encoding acceleration factor increased, and no significant difference was found for the contrast ratios in pair-wise comparisons among SF2, CS2, and CS4. Qualitative evaluations showed no significant difference in pair-wise comparisons among RS, SF2, and CS2 (P > .05). The visualization of all the assessed arteries was acceptable for CS2 and CS4, while 2 small arteries in images of CS6 were not reliably displayed, and the visualization of large arteries was acceptable in images of CS8 and CS10.

CONCLUSIONS

CS4 is recommended for routine brain TOF-MRA with balanced image quality and acquisition time; CS6, for examinations when small arteries are not evaluated; and CS10, for fast visualization of large arteries.

Noncontrast Magnetic Resonance Angiography in the Era of Nephrogenic Systemic Fibrosis and Gadolinium Deposition

ABSTRACT

Gadolinium-based contrast agents for clinical magnetic resonance imaging are overall safe. However, the discovery of nephrogenic systemic fibrosis in patients with severe renal impairment and gadolinium deposition in patients receiving contrast have generated developments in contrast-free imaging of the vasculature, that is, noncontrast magnetic resonance angiography. This article presents an update on noncontrast magnetic resonance angiography techniques, with comparison to other imaging alternatives. Potential benefits and challenges to implementation, and evidence to date for various clinical applications are discussed.

Compressed sensing time-of-flight magnetic resonance angiography with high spatial resolution for evaluating intracranial aneurysms: comparison with digital subtraction angiography

BACKGROUND AND PURPOSE

Compressed sensing is used for accelerated acquisitions with incoherently under-sampled k-space data, and intracranial time-of-flight magnetic resonance angiography is suitable for compressed sensing. Compressed sensing time-of-flight is beneficial in decreasing acquisition time and increasing spatial resolution while maintaining acquisition time. In this retrospective study, we aimed to evaluate the image quality and diagnostic performance of compressed sensing time-of-flight with high spatial resolution and compare with parallel imaging time-of-flight using digital subtraction angiography as a reference.

MATERIAL AND METHODS

In total, 39 patients with 46 intracranial aneurysms underwent parallel imaging and compressed sensing time-of-flight in the same imaging session and digital subtraction angiography before or after magnetic resonance angiography. The overall image quality, artefacts and diagnostic confidence were assessed by two observers. The contrast ratio, maximal aneurysm diameters and diagnostic performance were evaluated.

RESULTS

Compressed sensing time-of-flight showed significantly better overall image quality, degree of artefacts and diagnostic confidence in both observers, with better inter-observer agreement. The contrast ratio was significantly higher for compressed sensing time-of-flight than for parallel imaging time-of-flight in both observers (source images, P < 0.001; maximum intensity projection images, P < 0.05 for both observers); however, the measured maximal diameters of aneurysms were not significantly different. Compressed sensing time-of-flight showed higher sensitivity, specificity, accuracy and positive and negative predictive values for detecting aneurysms than parallel imaging time-of-flight in both observers, with better inter-observer agreement. Compressed sensing time-of-flight was preferred over parallel imaging time-of-flight by both observers; however, parallel imaging time-of-flight was preferred in cases of giant and large aneurysms.

CONCLUSIONS

Compressed sensing-time-of-flight provides better image quality and diagnostic performance than parallel imaging time-of-flight. However, neuroradiologists should be aware of under-sampling artefacts caused by compressed sensing.

Highly accelerated submillimeter resolution 3D GRASE with controlled T 2 blurring in T 2 -weighted functional MRI at 7 Tesla: A feasibility study

PURPOSE

To achieve highly accelerated submillimeter resolution T 2 -weighted functional MRI at 7T by developing a three-dimensional gradient and spin echo imaging (GRASE) with inner-volume selection and variable flip angles (VFA).

METHODS

GRASE imaging has disadvantages in that (a) k-space modulation causes T 2 blurring by limiting the number of slices and (b) a VFA scheme results in partial success with substantial SNR loss. In this work, accelerated GRASE with controlled T 2 blurring is developed to improve a point spread function (PSF) and temporal signal-to-noise ratio (tSNR) with a large number of slices. To this end, the VFA scheme is designed by minimizing a trade-off between SNR and blurring for functional sensitivity, and a new GRASE-optimized random encoding, which takes into account the complex signal decays of T 2 and T 2 ∗ weightings, is proposed by achieving incoherent aliasing for constrained reconstruction. Numerical and experimental studies were performed to validate the effectiveness of the proposed method over regular and VFA GRASE (R- and V-GRASE).

RESULTS

The proposed method, while achieving 0.8 mm isotropic resolution, functional MRI compared to R- and V-GRASE improves the spatial extent of the excited volume up to 36 slices with 52%-68% full width at half maximum (FWHM) reduction in PSF but approximately 2- to 3-fold mean tSNR improvement, thus resulting in higher BOLD activations.

CONCLUSIONS

We successfully demonstrated the feasibility of the proposed method in T 2 -weighted functional MRI. The proposed method is especially promising for cortical layer-specific functional MRI.

Novel Ultrafast Spiral Head MR Angiography Compared to Standard MR and CT Angiography

BACKGROUND AND PURPOSE

Intracranial vessel imaging by time-of-flight MR angiography (TOF-MRA) is one of the most frequently performed investigations in clinical neuroradiology. Particularly in the acute setting, fast imaging is needed for diagnostics, with a sequence ideally depicting even small vessels. The purpose of this study was to compare image and diagnostic quality of a novel ultrashort TOF-MRA sequence accelerated by spiral imaging (TOF-Spiral-short) to a standard TOF-MRA sequence accelerated by compressed sensing (TOF-CS) and to CT angiography (CTA).

METHODS

Forty-one patients (36.6% showing vessel pathologies) who had undergone TOF-CS (acquisition duration: 4 minutes 8 seconds), TOF-Spiral-short (acquisition duration: 51 seconds; spiral imaging [accelerating factor 1.3], decreased field of view [accelerating factor 1.2], and increased voxel size [accelerating factor 3.3]), and CTA were retrospectively evaluated. Assessment of image quality, diagnostic confidence, and quantification of stenosis or aneurysm diameter were performed by two readers.

RESULTS

Image quality at the skull base was slightly reduced with TOF-Spiral-short compared to CTA and TOF-CS (P < .05). Delineation of small intracranial vessels was improved by TOF-Spiral-short compared to CTA (P < .0001). In TOF-Spiral-short, diagnostic confidence was not reduced compared to TOF-CS in patients with vessel pathologies. We observed no significant difference in quantitative pathology assessment between TOF-Spiral-short and the other two modalities. TOF-Spiral-short enabled the correct identification of all vessel pathologies.

CONCLUSIONS

Accelerating TOF-MRA of brain-feeding arteries by a novel ultrashort spiral imaging sequence shows adequate image quality and sufficient diagnostic performance. Thus, TOF-Spiral-short holds potential for fast and reliable diagnostics of vessel pathologies, particularly in the acute setting.

Evaluation of cerebral arteriovenous shunts: a comparison of parallel imaging time-of-flight magnetic resonance angiography (TOF-MRA) and compressed sensing TOF-MRA to digital subtraction angiography

PURPOSE

Time-of-flight (TOF)-MR angiography (MRA) is an important imaging sequence for the surveillance and analysis of cerebral arteriovenous shunt (AVS), including arteriovenous malformation (AVM) and arteriovenous fistula (AVF). However, this technique has the disadvantage of a relatively long scan time. The aim of this study was to compare diagnostic accuracy between compressed sensing (CS)-TOF and conventional parallel imaging (PI)-TOF-MRA for detecting and characterizing AVS.

METHODS

This study was approved by the institutional review board for human studies. Participants comprised 56 patients who underwent both CS-TOF-MRA and PI-TOF-MRA on a 3-T MR unit with or without cerebral AVS between June 2016 and September 2018. Imaging parameters for both sequences were almost identical, except the acceleration factor of 3× for PI-TOF-MRA and 6.5× for CS-TOF-MRA, and the scan time of 5 min 19 s for PI-TOF-MRA and 2 min 26 s for CS-TOF-MRA. Two neuroradiologists assessed the accuracy of AVS detection on each sequence and analyzed AVS angioarchitecture. Concordance between CS-TOF, PI-TOF, and digital subtraction angiography was calculated using unweighted and weighted kappa statistics.

RESULTS

Both CS-TOF-MRA and PI-TOF-MRA yielded excellent sensitivity and specificity for detecting intracranial AVS (reviewer 1, 97.3%, 94.7%; reviewer 2, 100%, 100%, respectively). Interrater agreement on the angioarchitectural features of intracranial AVS on CS-MRA and PI-MRA was moderate to good.

CONCLUSION

The diagnostic performance of CS-TOF-MRA is comparable to that of PI-TOF-MRA in detecting and classifying AVS with a reduced scan time under 2.5 min.

Contemporary and emerging magnetic resonance imaging methods for evaluation of moyamoya disease

Numerous recent technological advances offer the potential to substantially enhance the MRI evaluation of moyamoya disease (MMD). These include high-resolution volumetric imaging, high-resolution vessel wall characterization, improved cerebral angiographic and perfusion techniques, high-field imaging, fast scanning methods, and artificial intelligence. This review discusses the current state-of-the-art MRI applications in these realms, emphasizing key imaging findings, clinical utility, and areas that will benefit from further investigation. Although these techniques may apply to imaging of a wide array of neurovascular or other neurological conditions, consideration of their application to MMD is useful given the comprehensive multidimensional MRI assessment used to evaluate MMD. These MRI techniques span from basic cross-sectional to advanced functional sequences, both qualitative and quantitative.The aim of this review was to provide a comprehensive summary and analysis of current key relevant literature of advanced MRI techniques for the evaluation of MMD with image-rich case examples. These imaging methods can aid clinical characterization, help direct treatment, assist in the evaluation of treatment response, and potentially improve the understanding of the pathophysiology of MMD.

Clinical feasibility of ultrafast intracranial vessel imaging with non-Cartesian spiral 3D time-of-flight MR angiography at 1.5T: An intra-individual comparison study

Objectives

Non-Cartesian Spiral readout can be implemented in 3D Time-of-flight (TOF) MR angiography (MRA) with short acquisition times. In this intra-individual comparison study we evaluated the clinical feasibility of Spiral TOF MRA in comparison with compressed sensing accelerated TOF MRA at 1.5T for intracranial vessel imaging as it has yet to be determined.

Materials and methods

Forty-four consecutive patients with suspected intracranial vascular disease were imaged with two Spiral 3D TOFs (Spiral, 0.82x0.82x1.2 mm³, 01:32 min; Spiral 0.8, 0.8x0.8x0.8 mm^3, 02:12 min) and a Compressed SENSE accelerated 3D TOF (CS 3.5, 0.82x0.82x1.2 mm³, 03:06 min) at 1.5T. Two neuroradiologists assessed qualitative (visualization of central and peripheral vessels) and quantitative image quality (Contrast Ratio, CR) and performed lesion and variation assessment for all three TOFs in each patient. After the rating process, the readers were questioned and representative cases were reinspected in a non-blinded fashion. For statistical analysis, the Friedman and Nemenyi post-hoc test, Kendall W tests, repeated measure ANOVA and weighted Cohen's Kappa tests were used.

Results

The Spiral and Spiral 0.8 outperformed the CS 3.5 in terms of peripheral image quality (p<0.001) and performed equally well in terms of central image quality (p>0.05). The readers noted slight differences in the appearance of maximum intensity projection images. A good to high degree of interstudy agreement between the three TOFs was observed for lesion and variation assessment (W = 0.638, p<0.001 –W = 1, p<0.001). CR values did not differ significantly between the three TOFs (p = 0.534). Interreader agreement ranged from good (K = 0.638) to excellent (K = 1).

Conclusions

Compared to the CS 3.5, both the Spiral and Spiral 0.8 exhibited comparable or better image quality and comparable diagnostic performance at much shorter acquisition times.

High-resolution compressed-sensing time-of-flight MRA in a case of acute ICA/MCA dissection

PURPOSE

Acute, isolated intracranial dissection (ICD) represents a rare and challenging cause of acute stroke. DSA is considered to be the gold standard imaging modality in patients with ICD. The role of novel, high-resolution (HR) compressed-sensing (CS) time-of-flight (TOF) MRA techniques in ICDs is unclear.

METHODS

A 22-year-old male patient with an isolated right ICA/MCA intracranial dissection underwent "conventional" 3-T TOF MRA, HR CS TOF MRA and also DSA including digital rotational angiography.

RESULTS

Unlike the "conventional" TOF MRA, HR CS TOF MRA provided comparable image quality to rotational angiography and a dissection membrane was clearly visible in both techniques.

CONCLUSION

In this single case study, we demonstrated the feasibility of a novel HR CS TOF in a case of an acute isolated intracranial ICA/MCA dissection, which needs to be validated in a larger case series.

Accelerated Internal Auditory Canal Screening Magnetic Resonance Imaging Protocol With Compressed Sensing 3-Dimensional T2-Weighted Sequence

BACKGROUND AND PURPOSE

High-resolution T2-weighted sequences are frequently used in magnetic resonance imaging (MRI) studies to assess the cerebellopontine angle and internal auditory canal (IAC) in sensorineural hearing loss patients but have low yield and lengthened examinations. Because image content in the Wavelet domain is sparse, compressed sensing (CS) that uses incoherent undersampling of k-space and iterative reconstruction can accelerate MRI acquisitions. We hypothesized that an accelerated CS T2 Sampling Perfection with Application optimized Contrasts using different flip angle Evolution (SPACE) sequence would produce acceptable diagnostic quality for IAC screening protocols.

MATERIAL AND METHODS

Seventy-six patients underwent 3 T MRI using conventional SPACE and a CS T2 SPACE prototype sequence for screening the IACs were identified retrospectively. Unilateral reconstructions for each sequence were separated, then placed into mixed folders for independent, blinded review by 3 neuroradiologists during 2 sessions 4 weeks apart. Radiologists reported if a lesion was present. Motion and visualization of specific structures were rated using ordinal scales. McNemar, Wilcoxon, Cohen κ, and Mann-Whitney U tests were performed for accuracy, equivalence, and interrater and intrarater reliability.

RESULTS

T2 SPACE using CS reconstruction reduced scan time by 80% to 50 seconds and provided 98.7% accuracy for IAC mass detection by 3 raters. Radiologists preferred conventional images (0.7-1.0 reduction on 5-point scale, P < 0.001), but rated CS SPACE acceptable. The 95% confidence for reduction in any cerebellopontine angle, IAC, or fluid-filled inner ear structure assessment with CS SPACE did not exceed 0.5.

CONCLUSIONS

Internal auditory canal screening MRI protocols can be performed using a 5-fold accelerated T2 SPACE sequence with compressed sensing while preserving diagnostic image quality and acceptable lesion detection rate.

Quantitative Assessment of Age-Associated Alterations in Brain Vasculature in Wild-Type Mice and in Bigenic Mice that Model Alzheimer's Disease

PURPOSE

Vascular dysfunction is a major hallmark of Alzheimer's disease (AD). However, studies that investigated vascular dysfunction in mice modeling AD using magnetic resonance angiography (MRA) are typically limited to qualitative and/or scoring-based paradigms, which are labor-intensive and observer-dependent.

PROCEDURES

We developed and validated a semi-automatic MRA processing pipeline and applied this to high-resolution in vivo MRA images acquired on a 9.4T small animal MRI scanner. We assessed vascular morphology at 3, 6, and 12 months in wild-type (WT) and bigenic (APP.V717IxTau.P301L: biAT) mice.

RESULTS

Vessel radius or length can increase with age regardless of genotype depending on the respective vessel. We also observed significantly lower internal carotid artery length in biAT mice compared to WT.

CONCLUSIONS

The results demonstrate that even subtle changes in vessel morphology can be noninvasively quantified. This is of great interest for AD, but also to other models of neurodegenerative diseases involving macrovascular dysfunction.

Clinical feasibility study of 3D intracranial magnetic resonance angiography using compressed sensing

BACKGROUND

Compressed sensing (CS) has been widely used to improve the speed of MRI, but the feasibility of application in 3D intracranial MR angiography (MRA) needs to be evaluated in clinical practice.

PURPOSE

To evaluate the clinical feasibility of CS-MRA in comparison with conventional 3D-MRA (Con-MRA).

STUDY TYPE

Retrospective.

SUBJECTS

Forty-nine consecutive patients with suspected intracranial arterial disease.

FIELD STRENGTH/SEQUENCE

3T MRI. 3D time-of-flight (TOF) MRA using a CS algorithm and conventional 3D TOF MRA scan.

ASSESSMENT

Three radiologists (4, 11, and 12 years of experience in neuroradiology) independently assessed the image quality, vascular lesions, and variations of intracranial arteries of both CS-MRA and Con-MRA, respectively.

STATISTICAL TESTS

The Kendall W test was performed to assess the interobserver agreement of image quality and intracranial arterial stenosis. A nonparametric test (Wilcoxon test) was used for comparison of the image quality and definition of the external carotid artery (ECA). Weighted kappa analysis was performed for the interstudy agreement of intracranial arterial stenosis. The aneurysm, decreased branches, congenital hypoplasia, absence, and variant branching of intracranial arteries were observed and evaluated for interobserver agreement and interstudy agreement by kappa analysis. Paired-t-tests for signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were conducted.

RESULTS

Image quality is better for CS-MRA compared with Con-MRA with significance (Z = -3.710 to -2.673, with P < 0.01). The interstudy agreement of lesion and variation of intracranial arteries assessment for each observer was excellent. The SNR and CNR were significantly higher in CS-MRA compared with Con-MRA (P < 0.001). The definition of ECA of CS-MRA was significantly better (Z = -4.9, P < 0.001).

DATA CONCLUSION

CS-MRA showed significantly higher image quality with less blur, comparable image diagnostic performance of intracranial arteries, and better display of ECA than Con-MRA.

LEVEL OF EVIDENCE

3 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;50:1843-1851.

Hybrid of Compressed Sensing and Parallel Imaging Applied to Three-dimensional Isotropic T2-weighted Turbo Spin-echo MR Imaging of the Lumbar Spine

Purpose:

The hybrid compressed sensing (hybrid-CS) technique can shorten the acquisition time compared with the sensitivity encoding (SENSE) technique in lumbar MRI. To evaluate the feasibility of a hybrid-CS technique in comparison with 3D isotropic T₂-weighted turbo spin-echo (3D volume isotropic turbo spin-echo acquisition [VISTA]) MRI of the lumbar spine.

Materials and Methods:

The Institutional Review Board approved this study and informed consent was obtained from participants prior to study entry. Sixteen healthy volunteers underwent lumbar spine 3D VISTA with conventional parallel imaging for SENSE and hybrid-CS at 3T. We recorded the image acquisition times of SENSE and hybrid-CS. We compared the signal-to-noise ratio (SNR) in spine, cerebrospinal fluid (CSF), lumbar disc, epidural fat, and erector spinae muscle, and the contrast of spine, CSF, and disc, and performed qualitative image analysis assessment, between the two image sequences.

Results:

The image acquisition time for hybrid-CS was 39.2% shorter than that of SENSE (218.4/358.8 s). The contrast of CSF and SNR of the spine was significantly higher with hybrid-CS than with SENSE (P < 0.05). The SNR of the disc and muscle was significantly higher with SENSE than with hybrid-CS (P < 0.05). There were no significant differences in the contrast of spine, disc, and fat, and SNR of CSF and fat between hybrid-CS and SENSE. There were no significant differences in the qualitative evaluation between hybrid-CS and SENSE.

Conclusion:

Compared with SENSE, hybrid-CS for 3D VISTA can shorten image acquisition time without sacrificing image quality.

Performance of sparse-view CT reconstruction with multi-directional gradient operators

To further reduce the noise and artifacts in the reconstructed image of sparse-view CT, we have modified the traditional total variation (TV) methods, which only calculate the gradient variations in x and y directions, and have proposed 8- and 26-directional (the multi-directional) gradient operators for TV calculation to improve the quality of reconstructed images. Different from traditional TV methods, the proposed 8- and 26-directional gradient operators additionally consider the diagonal directions in TV calculation. The proposed method preserves more information from original tomographic data in the step of gradient transform to obtain better reconstruction image qualities. Our algorithms were tested using two-dimensional Shepp–Logan phantom and three-dimensional clinical CT images. Results were evaluated using the root-mean-square error (RMSE), peak signal-to-noise ratio (PSNR), and universal quality index (UQI). All the experiment results show that the sparse-view CT images reconstructed using the proposed 8- and 26-directional gradient operators are superior to those reconstructed by traditional TV methods. Qualitative and quantitative analyses indicate that the more number of directions that the gradient operator has, the better images can be reconstructed. The 8- and 26-directional gradient operators we proposed have better capability to reduce noise and artifacts than traditional TV methods, and they are applicable to be applied to and combined with existing CT reconstruction algorithms derived from CS theory to produce better image quality in sparse-view reconstruction.

Clinical Evaluation of Highly Accelerated Compressed Sensing Time-of-Flight MR Angiography for Intracranial Arterial Stenosis

BACKGROUND AND PURPOSE

Time-of-flight MR angiography is the preferred imaging technique to assess intracranial arterial stenosis but is limited by a relatively long acquisition time. Compressed sensing provides an innovative approach in undersampling k-space to minimize the data-acquisition time. We aimed to evaluate the diagnostic accuracy of compressed sensing TOF for detecting intracranial arterial stenosis by comparison with conventional parallel imaging TOF-MRA.

MATERIALS AND METHODS

Compressed sensing TOF and parallel imaging TOF were performed in 22 patients with intracranial arterial stenosis. The MRA scan times were 2 minutes and 31 seconds and 4 minutes and 48 seconds for compressed sensing TOF and parallel imaging TOF, respectively. The reconstructed resolutions were 0.4 × 0.4 × 0.4 and 0.4 × 0.4 × 0.6 mm³ for compressed sensing TOF and parallel imaging TOF, respectively. The diagnostic quality of the images and visibility of the stenoses were independently ranked by 2 neuroradiologists blinded to the type of method and were compared using the Wilcoxon signed rank test. Concordance was calculated with the Cohen κ. Edge sharpness of the arteries and the luminal stenosis ratio were analyzed and compared using a paired-sample t test.

RESULTS

The interrater agreement was good to excellent. Compressed sensing TOF resulted in image quality comparable with that of parallel imaging TOF but boosted confidence in diagnosing arterial stenoses (P = .025). The edge sharpness of the intracranial arteries for compressed sensing TOF was significantly higher than that for parallel imaging TOF (P < .001). The luminal stenosis ratio on compressed sensing TOF showed no significant difference compared with that on parallel imaging TOF.

CONCLUSIONS

Compressed sensing TOF both remarkably reduced the scan time and provided adequate image quality for the diagnosis of intracranial arterial stenosis.