Sensitivity encoding (SENSE) and partial Fourier techniques have been shown to reduce the acquisition time and provide high diagnostic quality images. However, for time-resolved acquisitions there is a need for both high temporal and spatial resolution. View sharing can be used to provide an increased frame rate but at the cost of acquiring spatial frequencies over a duration longer than a frame time. In this work we hypothesize that a CArtesian Projection Reconstruction-like (CAPR) technique in combination with 2D SENSE, partial Fourier, and view sharing can provide 1-2 mm isotropic resolution with sufficient temporal resolution to distinguish intracranial arterial and venous phases of contrast passage in whole-brain angiography. In doing so, the parameter of "temporal footprint" is introduced as a descriptor for characterizing and comparing time-resolved view-shared pulse sequences. It is further hypothesized that short temporal footprint sequences have higher temporal fidelity than similar sequences with longer temporal footprints. The tradeoff of temporal footprint and temporal acceleration is presented and characterized in numerical simulations. Since its initial description over a decade ago (1), 3D contrast-enhanced MR angiography (CE-MRA) has become a widely used technique (2,3). Over the interim the method has undergone a number of technical advances allowing improved spatial and temporal resolution, including reduction of TR times, altered view orders to allow extended acquisition times (4), means for timing the acquisition to the arterial phase (5-7), development of stack of stars and 3D projection reconstruction (PR) techniques with application to MRA (8,9), and use of partial Fourier acquisition (10). More recently, parallel imaging methods (11-14) have been applied to 3D CE-MRA (15-23). A number of these methods can be applied synergistically.Along with improvements in spatial resolution, there has been progress in the generation of time-resolved 3D CE-MRA datasets. This can be done by simply recycling an unaccelerated 3D pulse sequence (24,25) or by using view sharing to provide an image update rate shorter than the intrinsic acquisition time (26 -28). Other versions of these methods have been developed (29 -36), including the use of such techniques as projection reconstruction (37) and spiral acquisition (38). Also, a method based on viewshared PR and slice encoding combined with nonlinear processing has been developed for time-resolved imaging (39). A number of these methods for time-resolved MRA have been integrated with the above-mentioned parallel imaging for either improved temporal or spatial resolution. Applying parallel imaging along one dimension has been used to provide acceleration factors as high as 3 to 4 for time-resolved sequences (11,16,18,32,40 -42). However, it has been shown for sensitivity encoding (SENSE) that for a given acceleration factor, 2D acceleration has markedly less signal-to-noise ratio (SNR) penalty than 1D (13). To our knowledge the first applications of 2D paral...

Purpose:To prospectively evaluate the feasibility of performing high-spatial-resolution (1-mm isotropic) time-resolved three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiography of the peripheral vasculature with Cartesian acquisition with projectionreconstruction-like sampling (CAPR) and eightfold accelerated two-dimensional (2D) sensitivity encoding (SENSE). Materials and Methods:All studies were approved by the institutional review board and were HIPAA compliant; written informed consent was obtained from all participants. There were 13 volunteers (mean age, 41.9; range, 27-53 years). The CAPR sequence was adapted to provide 1-mm isotropic spatial resolution and a 5-second frame time. Use of different receiver coil element sizes for those placed on the anterior-to-posterior versus left-to-right sides of the field of view reduced signal-to-noise ratio loss due to acceleration. Results from eight volunteers were rated independently by two radiologists according to prominence of artifact, arterial to venous separation, vessel sharpness, continuity of arterial signal intensity in major arteries (anterior and posterior tibial, peroneal), demarcation of origin of major arteries, and overall diagnostic image quality. MR angiographic results in two patients with peripheral vascular disease were compared with their results at computed tomographic angiography. Results:The sequence exhibited no image artifact adversely affecting diagnostic image quality. Temporal resolution was evaluated to be sufficient in all cases, even with known rapid arterial to venous transit. The vessels were graded to have excellent sharpness, continuity, and demarcation of the origins of the major arteries. Distal muscular branches and the communicating and perforating arteries were routinely seen. Excellent diagnostic quality rating was given for 15 (94%) of 16 evaluations. Conclusion:The feasibility of performing high-diagnostic-quality timeresolved 3D contrast-enhanced MR angiography of the peripheral vasculature by using CAPR and eightfold accelerated 2D SENSE has been demonstrated.

CAPR is a SENSE-type parallel 3DFT acquisition paradigm for 4D contrast-enhanced magnetic resonance angiography (CE-MRA) that has been demonstrated capable of providing high spatial and temporal resolution, diagnostic-quality images at very high acceleration rates. However, CAPR images are typically reconstructed online using Tikhonov regularization and partial Fourier methods, which are prone to exhibit noise amplification and undersampling artifacts when operating at very high acceleration rates. In this work, a sparsity-driven offline reconstruction framework for CAPR is developed and demonstrated to consistently provide improvements over the currently-employed reconstruction strategy against these ill-effects. Moreover, the proposed reconstruction strategy requires no changes to the existing CAPR acquisition protocol, and an efficient numerical optimization and hardware system are described that allow for a 256×160×80 volume CE-MRA volume to be reconstructed from an 8-channel data set in less than two minutes.

Various methods have been used for time-resolved contrastenhanced magnetic resonance angiography (CE-MRA), many involving view sharing. However, the extent to which the resultant image time series represents the actual dynamic behavior of the contrast bolus is not always clear. Although numerical simulations can be used to estimate performance, an experimental study can allow more realistic characterization. The purpose of this work was to use a computer-controlled motion phantom for study of the temporal fidelity of three-dimensional (3D) time-resolved sequences in depicting a contrast bolus. It is hypothesized that the view order of the acquisition and the selection of views in the reconstruction can affect the positional accuracy and sharpness of the leading edge of the bolus and artifactual signal preceding the edge. Phantom studies were performed using dilute gadolinium-filled vials that were moved along tabletop tracks by a computer-controlled motor. Several view orders were tested using view-sharing and Cartesian sam- Contrast-enhanced magnetic resonance angiography (CE-MRA) is a noninvasive technique that is widely used for the diagnosis of vascular disease (1-3). Technical challenges in CE-MRA have included imaging the arterial phase of the contrast bolus without venous contamination (4) and obtaining images with adequately high spatial resolution. Also, CE-MRA methods can possibly be prone to artifacts due to the motion of flowing blood, the varying concentration of contrast material, and the manner in which k-space is acquired (4,5). The issue of timing the data acquisition to the arterial phase has been addressed with a variety of techniques such as a test bolus (6) or automated triggering (7,8). Also, centric view orders (9,10), if initiated effectively, can allow extended acquisition times, and thus high-spatial-resolution arterial-phase images without appreciable venous signal. Alternatively, time-resolved MRA allows matching the central k-space sampling to peak arterial enhancement by repeatedly acquiring images as contrast material moves through the area of interest, either with two-dimensional (2D) (11,12) or three-dimensional (3D) (13) acquisition. However, due to the extended (typically tens of seconds) dwell time of intravenously-injected contrast material in the vasculature, this generally involves a tradeoff: time spent in resampling low spatial frequencies for improved temporal resolution could have been spent in sampling high spatial frequencies for improved spatial resolution (14). This in turn has been addressed by attempting to speed up the acquisition with short repetition times or by using view sharing (13,15,16), a method by which images are reconstructed more frequently than the intrinsic image acquisition time.Recently, the tradeoff of time with spatial resolution in time-resolved sequences has been changed with the advent of parallel acquisition techniques (17)(18)(19). This is particularly true when 2D techniques (20) are applied to 3D imaging, as is typically the case for CE-M...

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