Flow-independent angiography for peripheral vascular disease: initial in-vivo results

Coronary Artery Plaque Imaging

PURPOSE OF REVIEW

This short review summarizes the recent development in clinical and experimental imaging techniques for coronary atherosclerosis.

RECENT FINDINGS

Coronary atherosclerosis is the underlying disease of myocardial infarction, the leading cause of death in the industrialized world. Conventional ways of risk assessment, including evaluation of traditional risk factors and interrogation of luminal stenosis, have proven imprecise for the prediction of major events. Rapid advances in noninvasive imaging techniques including MRI, CT, and PET, as well as catheter-based methods, have opened the doors to more in-depth interrogation of plaque burden, composition, and many crucial pathological processes such as inflammation and hemorrhage. These emerging imaging modalities and methodologies, combined with conventional imaging evidences of anatomy and ischemia, offer the promises to provide comprehensive information of the disease status. There is tremendous clinical potential for imaging to improve the current management of coronary atherosclerosis, including the identification of high-risk patients for aggressive therapies and guiding personalized treatment. In this review, we provide an overview of the state-of-the-art coronary plaque imaging techniques focusing on their respective strengths and weaknesses, as well as their clinical outlook.

In silico modeling of magnetic resonance flow imaging in complex vascular networks

The paper presents a computational model of magnetic resonance (MR) flow imaging. The model consists of three components. The first component is used to generate complex vascular structures, while the second one provides blood flow characteristics in the generated vascular structures by the lattice Boltzmann method. The third component makes use of the generated vascular structures and flow characteristics to simulate MR flow imaging. To meet computational demands, parallel algorithms are applied in all the components. The proposed approach is verified in three stages. In the first stage, experimental validation is performed by an in vitro phantom. Then, the simulation possibilities of the model are shown. Flow and MR flow imaging in complex vascular structures are presented and evaluated. Finally, the computational performance is tested. Results show that the model is able to reproduce flow behavior in large vascular networks in a relatively short time. Moreover, simulated MR flow images are in accordance with the theoretical considerations and experimental images. The proposed approach is the first such an integrative solution in literature. Moreover, compared to previous works on flow and MR flow imaging, this approach distinguishes itself by its computational efficiency. Such a connection of anatomy, physiology and image formation in a single computer tool could provide an in silico solution to improving our understanding of the processes involved, either considered together or separately.

Non-contrast-enhanced peripheral angiography using a sliding interleaved cylinder acquisition

PURPOSE

To develop a new sequence for non-contrast-enhanced peripheral angiography using a sliding interleaved cylinder (SLINCYL) acquisition.

METHODS

A venous saturation pulse was incorporated into a three-dimensional magnetization-prepared balanced steady-state free precession sequence for non-contrast-enhanced peripheral angiography to improve artery-vein contrast. The SLINCYL acquisition, which consists of a series of overlapped thin slabs for volumetric coverage similar to the original sliding interleaved ky (SLINKY) acquisition, was used to evenly distribute the venous-suppression effects over the field of view. In addition, the thin-slab-scan nature of SLINCYL and the centric-ordered sampling geometry of its readout trajectory were exploited to implement efficient fluid-suppression and parallel imaging schemes. The sequence was tested in healthy subjects and a patient.

RESULTS

Compared to a multiple overlapped thin slab acquisition, both SLINKY and SLINCYL suppressed the venetian blind artifacts and provided similar artery-vein contrast. However, SLINCYL achieved this with shorter scan times and less noticeable artifacts from k-space amplitude modulation than SLINKY. The fluid-suppression and parallel imaging schemes were also validated. A patient study using the SLINCYL-based sequence well identified stenoses at the superficial femoral arteries, which were also confirmed with digital subtraction angiography.

CONCLUSION

Non-contrast-enhanced angiography using SLINCYL can provide angiograms with improved artery-vein contrast in the lower extremities.

Simultaneous fat suppression and band reduction with large-angle multiple-acquisition balanced steady-state free precession

Balanced steady-state free precession (bSSFP) MRI is a rapid and signal-to-noise ratio-efficient imaging method, but suffers from characteristic bands of signal loss in regions of large field inhomogeneity. Several methods have been developed to reduce the severity of these banding artifacts, typically involving the acquisition of multiple bSSFP datasets (and the accompanying increase in scan time). Fat suppression with bSSFP is also challenging; most existing methods require an additional increase in scan time, and some are incompatible with bSSFP band-reduction techniques. This work was motivated by the need for both robust fat suppression and band reduction in the presence of field inhomogeneity when using bSSFP for flow-independent peripheral angiography. The large flip angles used in this application to improve vessel conspicuity and contrast lead to specific absorption rate considerations, longer repetition times, and increased severity of banding artifacts. In this work, a novel method that simultaneously suppresses fat and reduces bSSFP banding artifact with the acquisition of only two phase-cycled bSSFP datasets is presented. A weighted sum of the two bSSFP acquisitions is taken on a voxel-by-voxel basis, effectively synthesizing an off-resonance profile at each voxel that puts fat in the stop band while keeping water in the pass band. The technique exploits the near-sinusoidal shape of the bSSFP off-resonance spectrum for many tissues at large (>50°) flip angles.

Three-dimensional fluid-suppressed T2-prep flow-independent peripheral angiography using balanced SSFP

Accurate depiction of the vessels of the lower leg, foot or hand benefits from suppression of bright MR signal from lipid (such as bone marrow) and long-T1 fluid (such as synovial fluid and edema). Signal independence of blood flow velocities, good arterial/muscle contrast and arterial/venous separation are also desirable. The high SNR, short scan times and flow properties of balanced steady-state free precession (SSFP) make it an excellent candidate for flow-independent angiography. In this work, a new magnetization-prepared 3D SSFP sequence for flow-independent peripheral angiography is presented. The technique combines a number of component techniques (phase-sensitive fat detection, inversion recovery, T2-preparation and square-spiral phase-encode ordering) to achieve high-contrast peripheral angiograms at only a modest scan time penalty over simple 3D SSFP. The technique is described in detail, a parameter optimization performed and preliminary results presented achieving high contrast and 1-mm isotropic resolution in a normal foot.

3D noncontrast MR angiography of the distal lower extremities using flow-sensitive dephasing (FSD)-prepared balanced SSFP

While three-dimensional contrast-enhanced MR angiography (MRA) is becoming the method of choice for clinical peripheral arterial disease (PAD) examinations, safety concerns with contrast administration in patients with renal insufficiency have triggered a renaissance of noncontrast MRA. In this work, a noncontrast-MRA technique using electrocardiography-triggered three-dimensional segmented balanced steady-state free precession with flow-sensitive dephasing (FSD) magnetization preparation was developed and tested in the distal lower extremities. FSD preparation was used to induce arterial flow voids at systolic cardiac phase while having little effect on venous blood and static tissues. High-spatial-resolution MRA was obtained by means of magnitude subtraction between a dark-artery scan with FSD preparation at systole and a bright-artery scan without FSD preparation at mid-diastole. In nine healthy volunteers, FSD parameters, including the gradient waveform and the first-order gradient moment, were optimized for excellent MRA image quality. Furthermore, arterial stenosis and occlusion in two peripheral arterial disease patients were identified using the noncontrast-MRA technique, as confirmed by contrast-enhanced MRA. In conclusion, FSD-prepared balanced steady-state free precession in conjunction with electrocardiography gating and image subtraction provides a promising noncontrast-MRA strategy for the distal lower extremities.

Non-contrast-enhanced flow-independent peripheral MR angiography with balanced SSFP

Flow-independent angiography is a non-contrast-enhanced technique that can generate vessel contrast even with reduced blood flow in the lower extremities. A method is presented for producing these angiograms with magnetization-prepared balanced steady-state free precession (bSSFP). Because bSSFP yields bright fat signal, robust fat suppression is essential for detailed depiction of the vasculature. Therefore, several strategies have been investigated to improve the reliability of fat suppression within short scan times. Phase-sensitive SSFP can efficiently suppress fat; however, partial volume effects due to fat and water occupying the same voxel can lead to the loss of blood signal. In contrast, alternating repetition time (ATR) SSFP minimizes this loss; however, the level of suppression is compromised by field inhomogeneity. Finally, a new double-acquisition ATR-SSFP technique reduces this sensitivity to off-resonance. In vivo results indicate that the two ATR-based techniques provide more reliable contrast when partial volume effects are significant.

Improving non-contrast-enhanced steady-state free precession angiography with compressed sensing

Flow-independent angiography offers the ability to produce vessel images without contrast agents. Angiograms are acquired with magnetization-prepared three-dimensional balanced steady-state free precession sequences, where the phase encodes are interleaved and the preparation is repeated before each interleaf. The frequent repetition of the preparation significantly decreases the scan efficiency. The number of excitations can instead be reduced with compressed sensing by exploiting the compressibility of the angiograms. Hence, the phase encodes can be undersampled to save scan time without significantly degrading image quality. These savings can be allotted for preparing the magnetization more often, or alternatively, improving resolution. The enhanced resolution and contrast achieved with the proposed method are demonstrated with lower leg angiograms. Depiction of the vasculature is significantly improved with the increased resolution in the phase-encode plane and higher blood-to-background contrast.

Nonenhanced MR angiography

While nonenhanced magnetic resonance (MR) angiographic methods have been available since the earliest days of MR imaging, prolonged acquisition times and image artifacts have generally limited their use in favor of gadolinium-enhanced MR angiographic techniques. However, the combination of recent technical advances and new concerns about the safety of gadolinium-based contrast agents has spurred a resurgence of interest in methods that do not require exogenous contrast material. After a review of basic considerations in vascular imaging, the established methods for nonenhanced MR angiographic techniques, such as time of flight and phase contrast, are considered and their advantages and disadvantages are discussed. This article then focuses on new techniques that are becoming commercially available, such as electrocardiographically gated partial-Fourier fast spin-echo methods and balanced steady-state free precession imaging both with and without arterial spin labeling. Challenges facing these methods and possible solutions are considered. Since different imaging techniques rely on different mechanisms of image contrast, recommendations are offered for which strategies may work best for specific angiographic applications. Developments on the horizon include techniques that provide time-resolved imaging for assessment of flow dynamics by using nonenhanced approaches.

Flow-independent magnetic resonance venography of the calf

PURPOSE

To show that flow-independent venography that exploit the intrinsic MR properties of blood to isolate vessels from surrounding structures can be used for depiction of peripheral veins and for detection of deep venous thrombosis (DVT).

MATERIALS AND METHODS

Sequence and parameters were first determined on a theoretical basis. The sequence was then optimized in volunteers (N = 4) for the depiction of the peripheral venous vessels. Qualitative evaluation of the normal venous anatomy was performed in five volunteers. The feasibility of diagnosing DVT of the calf with this method was evaluated with preliminary clinical studies.

RESULTS

Excellent depiction of the venous anatomy was achieved in all volunteers with the optimized technique. Very small venous structures, such as superficial, muscular, and perforator veins, were clearly depicted because of the high spatial and contrast resolution capacities of the sequences. In all six patients, DVT findings diagnosed by duplex sonography were also seen on MR venography.

CONCLUSION

Venous anatomy mapping and detection of lower-limb DVT appear feasible using flow-independent MR venograms.

Pulse sequences for steady-state saturation of flowing spins

It is useful to be able to suppress the NMR signal from spins in a flowing fluid, for example for "black-blood" visualization of blood vessels in vivo, for the suppression of flow artifacts, and for the estimation of tissue perfusion by continuous labeling of inflowing arterial spins. This work considers the flow of fluid through a region in which it is subjected to a train of saturation pulses. Computer simulations and in vitro measurements show that a train of equal-duration spoiler pulses produces less effective suppression than does a train of pulses of geometrically increasing duration. It is shown analytically that a long train of ideal equal-duration spoiler pulses converts initial magnetization (0, 0, M0) into a combination of longitudinal and transverse magnetization equal to 0. 29 (-M0, 0, M0) and is therefore unsatisfactory for continuous saturation.

Three-dimensional flow-independent peripheral angiography

A magnetization-prepared sequence, T2-Prep-IR, exploits T1, T2, and chemical shift differences to suppress background tissues relative to arterial blood. The resulting flow-independent angiograms depict vessels with any orientation and flow velocity. No extrinsic contrast agent is required. Muscle is the dominant source of background signal in normal volunteers. However, long-T2 deep venous blood and nonvascular fluids such as edema also contribute background signal in some patients. Three sets of imaging parameters are described to address patient-specific contrast requirements. A rapid, spiral-based, three-dimensional readout is utilized to generate high-resolution angiograms of the lower extremities. Comparisons with x-ray angiography and two-dimensional time-of-flight angiography indicate that this flow-independent technique has unique capabilities to accurately depict stenoses and to visualize slow flow and in-plane vessels.