An MR angiographic technique, referred to as 3D TRICKS (3D time-resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k-space views, and zero-filling in the slice-encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2-6 s. Acquiring a temporal series of images offers advantages over the current contrast-enhanced 3D MRA techniques in that it I) increases the likelihood that an arterial-only 3D image set will be obtained. II) permits the passage of the contrast agent to be observed, and III) allows temporal-processing techniques to be applied to yield additional information, or improve image quality.

Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase‐encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X‐ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the kx–ky plane and phase encodings in kz. PR resolution and artifact characteristics are demonstrated in a phantom and in contrast‐enhanced volunteer studies. Magn Reson Med 43:91–101, 2000. © 2000 Wiley‐Liss, Inc.

Time-resolved contrast-enhanced 3D MR angiography (MRA) methods have gained in popularity but are still limited by the tradeoff between spatial and temporal resolution. A method is presented that greatly reduces this tradeoff by employing undersampled 3D projection reconstruction trajectories. The variable density k-space sampling intrinsic to this sequence is combined with temporal k-space interpolation to provide time frames as short as 4 s. This time resolution reduces the need for exact contrast timing while also providing dynamic information. Spatial resolution is determined primarily by the projection readout resolution and is thus isotropic across the FOV, which is also isotropic. Although undersampling the outer regions of k-space introduces aliased energy into the image, which may compromise resolution, this is not a limiting factor in highcontrast applications such as MRA. Results from phantom and volunteer studies are presented demonstrating isotropic resolution, broad coverage with an isotropic field of view (FOV), minimal projection reconstruction artifacts, and temporal information. In one application, a single breath-hold exam covering the entire pulmonary vasculature generates high-resolution, isotropic imaging volumes depicting the bolus passage. Contrast-enhanced magnetic resonance angiography (MRA) of the chest or abdomen is typically accomplished by completing the scan in a single breath-hold to limit respiratory artifacts, and during the first pass of a contrast agent for maximum arterial enhancement (1,2). As the arterial-to-venous delay decreases, timing the bolus arrival and rapid scanning become more important. Time-resolved methods have become of increasing interest as they can mitigate the need for precise bolus timing, in addition to providing important dynamic information (3,4). Repetitive 3D Cartesian acquisitions of k-space acquired at high speed have recently generated dynamic MRA exams of the pulmonary vasculature (5,6), and when combined with correlation postprocessing, they can be used to calculate arterial and venous image volumes (7). Time-resolved methods generally require further compromises between temporal resolution, spatial resolution, and field of view (FOV) (8). Speed is generated by acquiring less throughplane resolution or by a partial acquisition of k-space in all three dimensions.Sparse sampling of k-space, analyzed generally in Ref. 9, has increased resolution per unit time in cases wherein the artifacts from aliased energy are acceptable due to high signal contrast. Peters et al. (10) increased in-plane resolution by a factor of 4 relative to Cartesian techniques by sampling the in-plane k-space dimensions with projections, and the slice dimension with Cartesian encoding. As is well known in computed tomography (CT), the readout resolution of the projections determines the resolution of the image, and the total number of acquired projections determines the level of artifact (11). Imaging time with these hybrid project reconstruction (PR) methods, termed PRojecti...

Recent work in k-t BLAST and undersampled projection angiography has emphasized the value of using training data sets obtained during the acquisition of a series of images. These techniques have used iterative algorithms guided by the training set information to reconstruct time frames sampled at well below the Nyquist limit. We present here a simple non-iterative unfiltered backprojection algorithm that incorporates the idea of a composite image consisting of portions or all of the acquired data to constrain the backprojection process. This significantly reduces streak artifacts and increases the overall SNR, permitting decreased numbers of projections to be used when acquiring each image in the image time series. For undersampled 2D projection imaging applications, such as cine phase contrast (PC) angiography, our results suggest that the angular undersampling factor, relative to Nyquist requirements, can be increased from the present factor of 4 to about 100 while increasing SNR per individual time frame. Results are presented for a contrast-enhanced PR HYPR TRICKS acquisition in a volunteer using an angular undersampling factor of 75 and a TRICKS temporal undersampling factor of 3 for an overall undersampling factor of 225. There are many applications for which it is desirable to have high spatial and high temporal resolution. K-space sampling that obeys the Nyquist theorem usually precludes simultaneous achievement of these aims in MR imaging. Among other approaches, radial acquisitions have been proposed for accelerated sampling schemes. Peters (1) and Vigen (2) reported on the use of 3D MR angiography acquisitions in which 2 dimensions were encoded using undersampled projection reconstruction and the third was encoded using phase encoding. In these applications, the projections are rotated around a single axis and, even if the planes containing the projections are completely sampled in the Fourier encoded direction, the undersampling factor, relative to that required by the Nyquist theorem, is limited to about 6 due to the streaks in the axial reformatted images.When radial sampling is extended by distributing the projections in all directions in 3D as in VIPR (3), significantly higher acceleration factors relative to fully sampled acquisition can be achieved. We recently reported on a relatively artifact free PC VIPR (phase contrast Vastly undersampled Isotropic PRojection imaging) acquisition in which an acceleration factor of 61 relative to conventional Cartesian 3D PC was achieved (4). This acceleration factor was defined as the ratio of an imaging speed index for PC VIPR and Cartesian 3D PC acquisitions. This index was determined as the volume covered divided by the product of scan duration times voxel size.Despite such large increases in acquisition speed, some applications would benefit from further accelerations. For example, in recent cine PC VIPR measurements with 3D flow encoding for pressure mapping in 1-2 mm thick vessels using an acquisition matrix of 256 ϫ 256 ϫ 256 voxels and 10 cardiac ph...

Phase contrast (PC) magnetic resonance imaging with a three-dimensional, radially undersampled acquisition allows for the acquisition of high resolution angiograms and velocimetry in dramatically reduced scan times. However, such an acquisition is sensitive to blurring and artifacts from offresonance and trajectory errors. A dual-echo trajectory is proposed with a novel trajectory calibration from prescan data coupled with a multi-frequency reconstruction to correct for these errors. Comparisons of phantom data and in vivo results from volunteer, and patients with arteriovenous malformations patients are presented with and without these corrections and show significant improvement of image quality when both corrections are applied. The results demonstrate significantly improved visualization of vessels, allowing for highly accelerated PC acquisitions without sacrifice in image quality. Volumetric phase contrast (PC) MR imaging with velocity encoding in three spatial dimensions holds the potential to be a comprehensive vascular imaging method; providing both anatomical and quantitative velocity measurements, all without the use of a contrast agent. As a lumenographic imaging tool, it has been effectively used for the identification of aneurysms, arteriovenous malformations (1), and vascular stenoses (2) in the cerebrovascular system, great vessels, and renal arteries. Additional hemodynamic information can be obtained through postprocessing of the acquired anatomical and velocity data, providing either velocity visualization and/or quantitative hemodynamic analysis. Visualization of complex velocity fields can be performed by flow vectors, streamlines, and particle traces to visually identify pathologic flow patterns (3). Quantitative flow measurements can be accomplished retrospectively with oblique reformats, avoiding difficulties of prospectively targeted two-dimensional (2D) PC measurements. Hemodynamic measures such as wall sheer stress and relative pressure can be determined directly from the velocity data (4,5). However, despite the plethora of diagnostic measures available from 3D PC, its clinical use has been hindered by relatively lengthy imaging times and the occurrence of flow related artifacts.For 3D PC to become a viable clinical solution, the scan time for images of diagnostic resolution must be reduced. This has been achieved through protocol optimization for vascular territories with larger vessels (6), which usually still results in extended imaging times. Parallel imaging techniques (7,8) can be used in conjunction with optimized protocols, but generally only allow accelerations on the order of 2-4 and can lead to additional signal-to-noise ratio (SNR) degradation. In addition to these accelerated imaging approaches, non-Cartesian trajectories may be used for more efficient sampling schemes, accelerated imaging by undersampling, and the reduction of flow related artifacts.We have previously introduced Vastly undersampled Isotropic PRojection (VIPR) imaging (9), a 3D radial trajectory with angul...

MS-325 provides excellent vascular and selective arterial enhancement during dynamic MR angiography. The long blood residence time also allows acquisition of steady-state images of the arteries and veins with excellent spatial resolution.

HighlY constrained backPRojection (HYPR) is a promising image-processing strategy with widespread application in time-resolved MRI that is also well suited for PET applications requiring time series data. The HYPR technique involves the creation of a composite image from the entire time series. The individual time frames then provide the basis for weighting matrices of the composite. The signal-to-noise ratio (SNR) of the individual time frames can be dramatically improved using the high SNR of the composite image. In this study, we introduced the modified HYPR algorithm (the HYPR method constraining the backprojections to local regions of interest [HYPR-LR]) for the processing of dynamic PET studies. We demonstrated the performance of HYPR-LR in phantom, small-animal, and human studies using qualitative, semiquantitative, and quantitative comparisons. The results demonstrate that significant improvements in SNR can be realized in the PET time series, particularly for voxel-based analysis, without sacrificing spatial resolution. HYPR-LR processing holds great potential in nuclear medicine imaging for all applications with low SNR in dynamic scans, including for the generation of voxel-based parametric images and visualization of rapid radiotracer uptake and distribution.

Dynamic MR imaging applications often require compromises in spatial and/or temporal resolution when standard reconstruction schemes are used. Acquisition windows are limited by the passage of contrast agents, as with hyperpolarized nuclei and contrast enhanced angiography, and/or clinical feasibility, as in 3D cine flow imaging. Recently, several alternative sampling and reconstruction methods have been introduced that explore data redundancies in such applications. These methods include model-based reconstructions (1-3) that rely on a priori information and compressed sensing methods (4,5), which aim to reduce the number of k-space points to represent a given object.Recently, HighlY constrained backPRojection (HYPR) (3) reconstruction has been used in conjunction with undersampled radial acquisitions to permit radial undersampling factors of up to 80 in 2D and 1000 in 3D (6 -8) in selected time-resolved applications in which the images are sparse and have a high degree of spatiotemporal correlation. Unlike other acceleration methods, where signalto-noise ratio (SNR) tends to decrease in proportion to the square root of the acceleration factor, HYPR maintains SNR from the composite image used to constrain the unfiltered backprojection process. While originally formulated for angiography, HYPR has been applied to a wide range of imaging methods including hyperpolarized gas imaging, cerebral diffusion, and cine phase contrast, all of which have temporal information that is spatially correlated.In the original HYPR method, a series of radial acquisitions with interleaved k-space projection sets is acquired. Using 1D discrete Fourier transform, we obtain image space profiles P t i , i ϭ 1…N p , where N p is the number of projections acquired at each timeframe. Each of these Radon projections is then normalized by the corresponding Radon projections P c i , i ϭ 1…N p , of the composite image I c that is reconstructed by conventional methods from the projections in several or all of the acquired timeframes. An unfiltered backprojection operator B is applied to each normalized projection. The average of all the backprojected information for each timeframe may be regarded as a weighting image I w . The individual timeframe weighting images provide dynamic information. The final HYPR images I H are obtained by multiplication of the individual timeframe weighting images with the composite image, and can be described as:In the limit of extremely sparse images or images with complete spatiotemporal correlation the HYPR algorithm provides near exact reconstruction. However, as the sparsity and spatiotemporal correlation deteriorate, there can be crosstalk of signals from different portions of the imaging volume. This crosstalk has generally forced the use of narrow sliding window composites to improve waveform fidelity. Since the sliding window composite has fewer projections, it has more artifact than a full-length composite would. A HYPR-based method presented here uses the concept of local reconstruction (HYPR LR) by con...