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- MATERIALS AND METHODS
Inversion-prepared pulse sequences can be used for noncontrast MR angiography (MRA) but suffer from long scan times when acquired using conventional nonaccelerated techniques. This work proposes a subtraction-based spin-labeling, three-dimensional fast inversion recovery MRA (FIR-MRA) method for imaging the intracranial arteries. FIR-MRA uses alternating cycles of nonselective and slab-selective inversions, leading to dark-blood and bright-blood images, respectively. The signal difference between these images eliminates static background tissue and generates the angiogram. To reduce scan time, segmented fast gradient recalled echo readout and parallel imaging are applied. The inversion recovery with embedded self-calibration method used allows for parallel acceleration at factors of 2 and above. An off-resonance selective inversion provides effective venous suppression, with no detriment to the depiction of arteries. FIR-MRA was compared against conventional three-dimensional time-of-flight angiography at 3 T in eight normal subjects. Results showed that FIR-MRA had superior vessel conspicuity in the distal vessels (P < 0.05), and equal or better vessel continuity and venous suppression. However, FIR-MRA had inferior vessel sharpness (P < 0.05) in four of nine vessel groups. The clinical utility of FIR-MRA was demonstrated in three MRA patients. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc.
As has been recently reviewed (1), non-contrast-enhanced MR angiography (MRA) methods have had a long history of development and have been applied in virtually every vascular system in the body. The discovery in the last several years of an association between gadolinium-based contrast agents and nephrogenic systemic fibrosis in patients with renal impairment (2, 3) has prompted renewed interest in non-contrast-enhanced MRA (CE-MRA) methods. One class of non-CE-MRA utilizes flow-related enhancement of blood into a region previously saturated by selective excitation. This class includes spin-labeling or tagging methods (4, 5) and time-of-flight (TOF) methods (6, 7). These methods are suited for imaging the relatively fast flow of the intracranial arteries, specifically for detection and diagnostic evaluation of aneurysms, arteriovenous malformations (AVM), and vascular stenosis. The pulse sequence most frequently utilized for intracranial MRA at our institution is three-dimensional (3D) TOF (7) acquired with flow compensation (8) and the multiple overlapping thin-slab acquisition technique (9). Because the TOF sequence is subject to signal saturation, multiple overlapping thin-slab acquisition can provide improved vessel conspicuity over a single-slab acquisition of equal thickness. However, the spatial saturation pulses that are applied for venous suppression do not necessarily discriminate between veins and arteries, which can result in the appearance of discontinuous vessels at slab interfaces.
The goal of this work was to develop a 3D non-CE-MRA method for imaging the intracranial arteries, with high spatial resolution and image quality. Techniques for generating vascular contrast using inversion recovery have been described in the literature (4, 5, 10, 11). One permutation of such techniques (4) uses nonselective and selective inversion cycles to generate respectively control (dark-blood) and labeled (bright-blood) images, similar to that described in perfusion imaging with spin labeling (12–15). Rather than acquiring two separate scans (4), both datasets can be acquired in an alternating fashion as a single scan (13–15) to reduce misregistration between both datasets. The complex difference between both datasets results in the elimination of static tissue and the generation of vascular contrast from unsaturated blood flowing into the selective inversion slab. Such spin-labeling MRA can provide high vessel-to-background contrast and high vessel conspicuity, as seen in 2DFT carotid and intracranial imaging with cine (16, 17). Sampling at high spatial resolution and 3DFT imaging would also provide superior depiction of vessel morphology (18), similar to that seen in 3D TOF. However, 3D spin-labeling MRA can potentially have very long scan times (>15 min) due to the long inversion intervals and the need for two inversion cycles for data collection.
In this work, we propose the fast inversion recovery (FIR-MRA) technique for high-resolution, 3DFT, non-CE-MRA. FIR-MRA uses nonselective and selective inversion pulses, along with an appropriate inversion time (TI) for interleaved acquisition of dark-blood and bright-blood T1-weighted images in a single scan. Subtraction of the two images yields an MR angiogram. Use of multiple repetitions of a fast gradient recalled echo (GRE) sequence for readout (19) provides an initial speedup. Parallel imaging (20–22) at factors of 2 and above is applied. To exploit the intrinsic delay intervals within the inversion-prepared sequence for calibration acquisition, the inversion recovery with embedded self-calibration (IRES) method (23) is applied, which provides effectively increased acceleration over that of standard self-calibration. Additionally, an off-resonance selective inversion provides effective venous suppression. The principal parameters of FIR-MRA were investigated using simulations and in vivo imaging. The image quality of 3D FIR-MRA was compared to 3D TOF in eight normal subjects, using qualitative evaluation criteria. FIR-MRA was further assessed in three patient studies.
- Top of page
- MATERIALS AND METHODS
We have developed a high-resolution, 3D, non-contrast-enhanced, FIR-MRA technique utilizing spin-labeling for intracranial angiography. The principal parameters of the sequence were identified and optimized. The technique was both tested in normal human subjects and used in three patients referred for clinical MRA. The choice of employing a single-scan spin-labeling technique provided superior conspicuity of distal vessels over 3D TOF. Simultaneously, dark-blood and bright-blood T1-weighted images are produced, which may have added diagnostic benefits in discriminating arteries from veins and brain tissue. The off-center selective inversion provides an effective venous suppression scheme that discriminates venous blood from arterial blood. The combination of fast GRE and parallel imaging allowed for a high-resolution, 3D, intracranial, MRA examination of 7.7 min. In particular, the IRES method for parallel imaging resulted in neither loss of net acceleration nor further signal modulation. The principal limitation of FIR-MRA versus TOF was some loss of vessel sharpness. This is due to the signal modulation during the readout. FIR-MRA also did have a somewhat longer scan time than TOF (6.4 min with calibration scan). Conceivably, improved receiver coils could allow higher acceleration, thereby reducing the degree of signal modulation and reducing the loss of sharpness.
To evaluate vessel continuity apart from vessel conspicuity, vessels that were conspicuous in FIR-MRA but not in TOF were given a zero score for vessel continuity. This was noted most prominently in the anterior frontal branches (vessel group 5). The aliasing artifacts in TOF were typically seen in the pontine region of the brain and were attributed to the image-space-based reconstruction used, rather than to the TOF technique. Ghosting artifacts were observed in TOF but did not factor in the evaluations as these were attributed mostly to the linear phase-encoding order of TOF. While the presence of veins in TOF did not result in nondiagnostic images, the application of saturation bands for venous suppression also suppressed arteries that reenter the imaging slab, contributing to the appearance of vessel discontinuity. In comparison, the venous suppression method in FIR-MRA was more effective and was not detrimental to the depiction of arteries.
There are some other limitations of FIR-MRA. In spite of FIR-MRA being acquired in a single-scan manner, motion that occurs at time scales smaller than TC may result in image artifacts. Motion between the acquisitions of consecutive slabs may also occur in both FIR-MRA and TOF. However, no motion artifacts were observed (Table 3b). A limitation of spin-labeling-based techniques mentioned in the literature (26) is the signal dropout in the proximal vessels at large TI (∼1500 msec). This arises from the scanner's maximum field of view that limits the extent of the labeling region proximal to the imaging slab. In this work, these signal dropout effects were not observed, given the smaller TI (≤900 msec) used, and the maximum field of view was 48 cm. Finally, because the FIR-MRA technique relies on the long T1 of blood, FIR-MRA must be performed prior to any intravenous injection of gadolinium-based contrast agents.
There are several parameters that were not demonstrated in this work. TC could be increased to increase vascular contrast but would also result in scan time increase. The number of slabs could be reduced by increasing the slab thickness of each slab, but this could potentially reduce vessel conspicuity. The readout bandwidth could be reduced to increase SNR, but doing so would increase signal dephasing due to a longer echo time. The use of balanced steady-state free precession as seen in other non-CE-MRA methods (29–31) could provide increased efficiency due to shorter echo time and increased received signal. In this work, however, the choice of spoiled GRE avoided the issues of banding artifacts and specific absorption rate intrinsic to balanced steady-state free precession. All acquisitions in this study were obtained at 3 T, which from theory is advantageous versus 1.5 T because a longer T1 of blood at 3 T increases vessel signal.
There are several potential benefits of FIR-MRA. The improved conspicuity of distal vessels, aneurysm remnants (Fig. 7), and the nidus of the AVM (Fig. 8) all suggest that FIR-MRA may be better than TOF in depicting vascular territories with slow flow. The ability to distinguish vessel components from tissue with draining veins appearing dark in bright-blood FIR-MRA may provide a new avenue for evaluating AVMs. The complete background suppression in FIR-MRA allowed visualizations with volume rendering and MIPs to be performed without the need for image segmentation, facilitating, for example, automatic aneurysm detection algorithms (32, 33). Branches of the external carotid arteries were unevaluated but were observed to also be better depicted in FIR-MRA due to obscuration by bright, subcutaneous fat in TOF. Hence, FIR-MRA may be useful for evaluation of giant cell arteritis, which characteristically involves the superficial temporal artery. Finally, for multicontrast carotid plaque imaging (34), FIR-MRA can simultaneously provide high luminal signal and blood-nulled T1 contrast, hence avoiding the need for image registration.