Four-dimensional phase contrast MRI with accelerated dual velocity encoding

Purpose:

To validate a novel approach for accelerated four‐dimensional phase contrast MR imaging (4D PC‐MRI) with an extended range of velocity sensitivity.

Materials and Methods:

4D PC‐MRI data were acquired with a radially undersampled trajectory (PC‐VIPR). A dual V_enc (dV_enc) processing algorithm was implemented to investigate the potential for scan time savings while providing an improved velocity‐to‐noise ratio. Flow and velocity measurements were compared with a flow pump, conventional 2D PC MR, and single V_enc 4D PC‐MRI in the chest of 10 volunteers.

Results:

Phantom measurements showed excellent agreement between accelerated dV_enc 4D PC‐MRI and the pump flow rate (R² ≥ 0.97) with a three‐fold increase in measured velocity‐to‐noise ratio (VNR) and a 5% increase in scan time. In volunteers, reasonable agreement was found when combining 100% of data acquired with V_enc = 80 cm/s and 25% of the high V_enc data, providing the VNR of a 80 cm/s acquisition with a wider velocity range of 160 cm/s at the expense of a 25% longer scan.

Conclusion:

Accelerated dual V_enc 4D PC‐MRI was demonstrated in vitro and in vivo. This acquisition scheme is well suited for vascular territories with wide ranges of flow velocities such as congenital heart disease, the hepatic vasculature, and others. J. Magn. Reson. Imaging 2012;35:1462–1471. © 2012 Wiley Periodicals, Inc.     

A case of paraspinal arteriovenous fistula in the lumbar spinal body assessed with time resolved three‐dimensional phase contrast MRI

A 93‐year‐old female with a paraspinal arteriovenous fistula (AVF) occurred within the lumbar spinal vertebral body was assessed with time resolved three‐dimensional (3D) phase‐contrast MRI (4D‐Flow) on 1.5 Tesla MR scanner (GE Healthcare). The 3D vector field, streamlines, and pathlines analyses demonstrated uni‐directional flow from the aorta to the large vascular cavity in the lumbar vertebral body by means of the lumbar artery as well as dilated paravertebral veins as drainers, which confirmed AVF, not aortic pseudoaneurysm. The 4D‐Flow also showed an added value in planned endovascular surgery concerning localization of the precise shunting point and the shunting volume quantification. J. Magn. Reson. Imaging 2012;36:1231–1233. © 2012 Wiley Periodicals, Inc.     

Three‐dimensional velocity mapping of thoracic aorta and supra‐aortic arteries in takayasu arteritis

Takayasu arteritis is an inflammatory disease of unknown etiology that involves the aorta, its major branches, and the pulmonary artery. We describe three patients with Takayasu arteritis who showed abnormal velocity profile of the thoracic aorta and supra‐aortic arteries on time‐resolved three‐dimensional (3D) phase‐contrast MR imaging and velocity mapping techniques. Compared with two comparative subjects, velocity reduction was observed in these arteries. The velocity reduction was prominent along the thickened arterial wall, even with normal luminal caliber, and the highest velocity was observed on the contralateral side. In one patient, the arterial flow velocity and its profile at systole were partly improved after the treatment. The time‐resolved 3D velocity mapping visualized the changes in the blood velocity profile at systole in Takayasu arteritis. J. Magn. Reson. Imaging 2010;31:1481–1485. ©2010 Wiley‐Liss, Inc.     

Reducing artifacts in one‐dimensional Fourier velocity encoding for fast and pulsatile flow

When evaluating the severity of valvular stenosis, the peak velocity of the blood flow is routinely used to estimate the transvalvular pressure gradient. One‐dimensional Fourier velocity encoding effectively detects the peak velocity with an ungated time series of spatially resolved velocity spectra in real time. However, measurement accuracy can be degraded by the pulsatile and turbulent nature of stenotic flow and the existence of spatially varying off‐resonance. In this work, we investigate the feasibility of improving the peak velocity detection capability of one‐dimensional Fourier velocity encoding for stenotic flow using a novel echo‐shifted interleaved readout combined with a variable‐density circular k‐space trajectory. The shorter echo and readout times of the echo‐shifted interleaved acquisitions are designed to reduce sensitivity to off‐resonance. Preliminary results from limited phantom and in vivo results also indicate that some artifacts from pulsatile flow appear to be suppressed when using this trajectory compared to conventional single‐shot readouts, suggesting that peak velocity detection may be improved. The efficiency of the new trajectory improves the temporal and spatial resolutions. To realize the proposed readout, a novel multipoint‐traversing algorithm is introduced for flexible and automated gradient‐waveform design. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Shared velocity encoding: a method to improve the temporal resolution of phase-contrast velocity measurements

Phase-contrast magnetic resonance imaging (PC-MRI) is used routinely to measure fluid and tissue velocity with a variety of clinical applications. Phase-contrast magnetic resonance imaging methods require acquisition of additional data to enable phase difference reconstruction, making real-time imaging problematic. Shared Velocity Encoding (SVE), a method devised to improve the effective temporal resolution of phase-contrast magnetic resonance imaging, was implemented in a real-time pulse sequence with segmented echo planar readout. The effect of SVE on peak velocity measurement was investigated in computer simulation, and peak velocities and total flow were measured in a flow phantom and in volunteers and compared with a conventional ECG-triggered, segmented k-space phase-contrast sequence as a reference standard. Computer simulation showed a 36% reduction in peak velocity error from 8.8 to 5.6% with SVE. A similar reduction of 40% in peak velocity error was shown in a pulsatile flow phantom. In the phantom and volunteers, volume flow did not differ significantly when measured with or without SVE. Peak velocity measurements made in the volunteers using SVE showed a higher concordance correlation (0.96) with the reference standard than non-SVE (0.87). The improvement in effective temporal resolution with SVE reconstruction has a positive impact on the precision and accuracy of real-time phase-contrast magnetic resonance imaging peak velocity measurements.     

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.     

Accuracy and repeatability of fourier velocity encoded M‐mode and two‐dimensional cine phase contrast for pulse wave velocity measurement in the descending aorta

Purpose:

To assess the accuracy and repeatability of Fourier velocity encoded (FVE) M‐mode and two‐dimensional (2D) phase contrast with through‐plane velocity encoding (2D‐PC) for pulse wave velocity (PWV) evaluation in the descending aorta using five different analysis techniques.

Materials and Methods:

Accuracy experiments were conducted on a tubular human‐tissue‐mimicking phantom integrated into a flow simulator. The theoretical PWV value was derived from the Moens‐Korteweg equation after measurement of the tube elastic modulus by uniaxial tensile testing (PWV = 6.6 ± 0.7 m/s). Repeatability was assessed on 20 healthy volunteers undergoing three consecutive MR examinations.

Results:

FVE M‐mode PWV was more repeatable than 2D‐PC PWV independently of the analysis technique used. The early systolic fit (ESF) method, followed by the maximum of the first derivative (1st der.) method, was the most accurate (PWV = 6.8 ± 0.4 m/s and PWV = 7.0 ± 0.6 m/s, respectively) and repeatable (inter‐scan within‐subject variation δ = 0.096 and δ = 0.107, respectively) for FVE M‐mode. For 2D‐PC, the 1st der. method performed best in terms of accuracy (PWV = 6.8 ± 1.1 m/s), whereas the ESF algorithm was the most repeatable (δ = 0.386).

Conclusion:

FVE M‐mode allows rapid, accurate and repeatable central PWV evaluation when the ESF algorithm is used. 2D‐PC requires long scan times and can provide accurate although much less repeatable PWV measurements when the 1st der. method is used. J. Magn. Reson. Imaging 2010;31:1185–1194. © 2010 Wiley‐Liss, Inc.     

Contrast‐enhanced,three‐dimensional,whole‐brain,black‐blood imaging: Application to small brain metastases

Contrast‐enhanced three‐dimensional T₁‐weighted imaging based on magnetization‐prepared rapid‐gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T₁‐weighted, black‐blood version of single‐slab three‐dimensional turbo/fast spin echo whole‐brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. For blood suppression, variable refocusing flip angles with flow‐sensitizing gradients are employed. To avoid a signal loss resulting from the flow‐sensitizing scheme, the first refocusing flip angle is forced to 180°. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T₁‐weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases. Magn Reson Med 63:553–561, 2010. © 2010 Wiley‐Liss, Inc.     

Improved SNR in phase contrast velocimetry with five‐point balanced flow encoding

Phase contrast velocimetry can be utilized to measure complex flow for both quantitative and qualitative assessment of vascular hemodynamics. However, phase contrast requires that a maximum measurable velocity be set that balances noise and phase aliasing. To efficiently reduce noise in phase contrast images, several investigators have proposed extended velocity encoding schemes that use extra encodings to unwrap phase aliasing; however, existing techniques can lead to significant increases in echo and scan time, limiting their clinical benefits. In this work, we have developed a novel five‐point velocity encoding scheme that efficiently reduces noise with minimal increases in scan and echo time. Investigations were performed in phantoms, demonstrating a 63% increase in velocity‐to‐noise ratio compared to standard four‐point encoding schemes. Aortic velocity measurements were performed in healthy volunteers, showing similar velocity‐to‐noise ratio improvements. In those volunteers, it was also demonstrated that, without sacrificing accuracy, low‐resolution images can be used for the fifth encoding point, reducing the scan time penalty from 25% down to less than 1%. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Clinical blood flow quantification with segmented k-space magnetic resonance phase velocity mapping

PURPOSE: To evaluate the accuracy of segmented k-space magnetic resonance phase velocity mapping (PVM) in quantifying aortic blood flow from through-plane velocity measurements. MATERIALS AND METHODS: Two segmented PVM schemes were evaluated, one with seven lines per segment (seg-7) and one with nine lines per segment (seg-9), in twenty patients with cardiovascular disease. A non-segmented (non-seg) PVM acquisition was also performed to provide the reference data. RESULTS: There was agreement between the aortic flow curves acquired with segmented and non-segmented PVM. The calculated systolic and total flow volume per cycle from the seg-7 and the seg-9 scans correlated and agreed with the flow volumes from the non-seg scans (differences < 5%). Sign tests showed that there were no statistically significant differences (P-values > 0.05) between the segmented and the non-segmented PVM measurements [corrected]. Seg-9, which was the fastest among the three sequences, provided adequate spatial and temporal resolution (> 10 phases per cycle). CONCLUSION: Segmented k-space PVM shows great clinical potential in blood flow quantification.     

Analysis of complex cardiovascular flow with three‐component acceleration‐encoded MRI

Functional information regarding cardiac performance, pressure gradients, and local flow derangement are available from blood acceleration fields. Thus, this study examines a 2D and 3D phase contrast sequence optimized to efficiently encode three‐directional, time‐resolved acceleration in vitro and in vivo. Stenosis phantom acceleration measurements were compared to acceleration derived from standard velocity encoded phase contrast‐magnetic resonance imaging (i.e., “velocity‐derived acceleration”). For in vivo analysis, three‐directional 2D acceleration maps were compared to velocity‐derived acceleration using regions proximal and distal to the aortic valve in six healthy volunteers at 1.5 and 3.0 T (voxel size = 1.4 × 2.1 × 8 mm, temporal resolution = 16–20 ms). In addition, a 4D acceleration sequence was evaluated for feasibility in a healthy volunteer and postrepair biscuspid aortic valve patient with an ascending aortic aneurysm. The phantom magnetic resonance acceleration measurements were more accurate (nonturbulent root mean square error = 2.2 vs. 5.1 m/s² for phase contrast‐magnetic resonance imaging) and 10 times less noisy (nonturbulent σ = 0.9 vs. 13.6 m/s² for phase contrast‐magnetic resonance imaging) than velocity‐derived acceleration. Acceleration mapping of the left ventricular outflow tract and aortic arch exhibited signal voids colocated with complex flow events such as vortex formation and high order motion. 4D acceleration data, visualized in combination with the velocity data, may provide new insight into complex flow phenomena. Magn Reson Med 67:50–61, 2012. © 2011 Wiley Periodicals, Inc.     

Variable-density one-shot Fourier velocity encoding.

In areas of highly pulsatile and turbulent flow, real-time imaging with high temporal, spatial, and velocity resolution is essential. The use of 1D Fourier velocity encoding (FVE) was previously demonstrated for velocity measurement in real time, with fewer effects resulting from off-resonance. The application of variable-density sampling is proposed to improve velocity measurement without a significant increase in readout time or the addition of aliasing artifacts. Two sequence comparisons are presented to improve velocity resolution or increase the velocity field of view (FOV) to unambiguously measure velocities up to 5 m/s without aliasing. The results from a tube flow phantom, a stenosis phantom, and healthy volunteers are presented, along with a comparison of measurements using Doppler ultrasound (US). The studies confirm that variable-density acquisition of kz-kv space improves the velocity resolution and FOV of such data, with the greatest impact on the improvement of FOV to include velocities in stenotic ranges.