HYPR TOF: time-resolved contrast-enhanced intracranial MR angiography using time-of-flight as the spatial constraint.

FOR SEVERAL DECADES, time-resolved serial imaging using x-ray digital subtraction angiography (DSA) has played a major role in the diagnosis of vascular disease (1). Non–contrast-enhanced MR angiography and computed tomography (CT) angiography are widely used methods that permit evaluation of the cerebrovascular system at reduced cost and low risk compared with DSA but do not provide temporal information (2,3). Time-resolved contrast-enhanced MR angiography is essential for the characterization of high-flow lesions such as arteriovenous malformations and dural fistulas (1,3). Recently, there has been extensive work directed at simultaneously obtaining high spatial and temporal resolution in contrast-enhanced MRA. Clinical studies have been reported using parallel imaging approaches such as sensitivity encoding (SENSE) (4,5), simultaneous acquisition of spatial harmonics (SMASH) (6) and refinements of these techniques (7,8), which use information from multiple receiver coils to reduce the number of k-space samples required to reconstruct images with significantly fewer artifacts than would be obtained using conventional undersampled Cartesian acquisition. These techniques usually require inversion of a matrix that leads to noise amplification at high accelerations. Some are computationally intensive. Nevertheless, acceleration factors of 2–4 are commercially available and higher accelerations are becoming available with the increasing number of elements in receiver coils. Undersampled radial acquisitions like two-dimensional (2D) PR (9, 10), VIPR (11), and PC VIPR (12) have been proved to be effective to accelerate MR angiography. However, as the undersampling factor increases, images are degraded by increased under-sampling artifacts and decreased signal to noise ratio (SNR). A recent innovative reconstruction algorithm called HYPR (HighlY Constrained backPRojection) is able to reduce undersampling artifacts and maintain good SNR even at high levels of acceleration (13, 14). HYPR LR (Local Reconstruction) (15) permits the use of a longer constraint window, resulting in increased SNR and quantitative reconstruction accuracy. Phase Contrast (PC) HYPR FLOW (16, 17), which uses post-contrast PC VIPR images as the spatial constraint and reconstructs the first pass time-resolved contrast-enhanced VIPR acquisition using HYPR LR technique, for the first time, splits the tasks of providing spatial resolution/SNR and temporal resolution between two separate scans with the same acquisition geometry and uses HYPR constrained reconstruction to obtain a time series of images with all of these features. Time-of-flight (TOF) angiography is a more accepted clinical standard for imaging of the intracranial arterial system due to the improved vessel lumen depiction, shorter echo times (TEs), and improved acquisition efficiency (18, 19). The high SNR and large spatial coverage of 3D multi-slab TOF have made this technique one of the clinical routines for intracranial MRA. One drawback is that it does not provide temporal information, which, may be essential for accurate diagnosis and treatment planning for certain cerebral vascular diseases. In this study, we use TOF images as a constraint in the reconstruction of a time series of undersampled first pass contrast-enhanced angiograms using HYPR processing. The result, HYPR TOF, is a set of time-resolved angiograms with spatial resolution and SNR comparable to TOF and sub-second temporal resolution.