Time‐resolved MR angiography with limited projections

Abstract

A method for reconstruction of time-resolved MRI called highlyconstrained backprojection (HYPR) has been developed. To evaluate the HYPR reconstruction in relation to data sparsity and temporal dynamics, computer simulations were performed, investigating signal modulations under different situations that reflect dynamic contrast-enhanced MR angiography (MRA). In vivo studies were also performed with gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) for abdominal MRA in a canine model to demonstrate the application of HYPR for threedimensional (3D) time-resolved MRA. When contrast dynamics vary over space, large vessels (e.g., veins) tend to introduce signal interference to small vessels (e.g., arteries) in HYPR, particularly when the vessels are in close proximity. The enhancement of background tissue signals may also alter the arterial and venous temporal profiles in HYPR. However, the artifacts are manifest as intensity modulation rather than structural interference, and therefore have little impact on structural diagnosis. Increasing the number of projections per time point increases temporal blur while reducing corruption of temporal behavior from adjacent tissues. Uniformly interleaved acquisition order, such as the bit-reversed order, is important to reduce artifacts. With high signal-to-noise ratio ( Spatial resolution, temporal resolution, signal-to-noise ratio (SNR), field of view (FOV), and the extent of artifacts are common tradeoffs in magnetic resonance imaging (MRI). This is particularly an issue in contrast-enhanced studies such as peripheral MR angiography (MRA), in which contrast dynamics are often of interest for revealing pathological information and for timing the acquisition to the peak arterial enhancement. Submillimeter spatial resolution is necessary to visualize small arteries, large threedimensional (3D) imaging volumes are needed to provide coverage of the lower extremities, sufficient SNR is mandatory, and artifacts cannot interfere with clinical evaluations. In current clinical practice, a tradeoff is made sacrificing temporal resolution in order to gain sufficient spatial resolution. That is, static 3D volumes are acquired, in which the arterial signals are a weighted average of the contrast enhancement throughout the data acquisition. Although elliptical-centric ordered acquisition (1,2) and the application of parallel imaging techniques (3) have greatly improved the robustness of the current protocols, mistiming of the contrast enhancement and venous contamination are still potential issues due to the often irregular blood-flow patterns in the patient population (4).Alternatively, contrast-enhanced MRA can be performed in a time-resolved manner by taking advantage of the data sparsity to shorten the data acquisition and to optimize reconstruction. Projection reconstruction (PR) techniques achieve high undersampling factors with acceptable artifacts due to the limited artifactual interference caused by high-frequency undersampling of sparse data sets (5,6). The acceptable un...

Max CAPR: High‐resolution 3D contrast‐enhanced MR angiography with acquisition times under 5 seconds

“…Sharing of data from frame to frame introduces temporal correlation, as has been described for both Cartesian (9) and non-Cartesian (15,16,46) sequences. This can cause phenomena occurring in one frame to be artifactually manifest in other frames and, in the case of CE-MRA, lead to anticipation or persistence artifact emanating from the contrast-enhanced vasculature (40).…”

HYPR-spectral photoacoustic CT for preclinical imaging

“…16,17 In x-ray CT, the new approach can reduce patient radiation exposure by 5-10-fold for perfusion and other time-resolved exams. 18,19 HYPR was conceived shortly after the 2005 ISMRM meeting and was stimulated by previous ideas about exploiting the redundancy of information in imaging sequences 20,21,22 and by a plenary talk in which Juergen Hennig showed a hypothetical example of how images could be updated using a single k-space point if every point in an image volume had the same time dependence, a condition we have subsequently referred to as the "Hennig Limit". The question arose as to how many k-space measurements would be required to update an image series if a priori information regarding the image signal distribution were available and the degree of spatial-temporal correlation were substantial, although not perfect.…”

Undersampled radial MR acquisition and highly constrained back projection (HYPR) reconstruction: Potential medical imaging applications in the post‐Nyquist era

“…These techniques generally involve the use of a training set of data (10,36) that is used to constrain the reconstruction of individual time frames. The exact relationship between these techniques and the HYPR technique introduced below is not completely clear at this point in time, especially with regard to the ability to greatly increase the SNR of images in a series while maintaining accurate response to changes in the image parameter characterizing the series.…”

“…Several ideas came together at this time. These included the ideas of undersampled radial acquisition and the formation of a training image, as had been discussed in the context of kt-blast (10) and a radial acquisition technique of Huang and Wright (36). It appeared that this training image, if formed from interleaved versions of undersampled radial acquisitions, could generate a reconstruction constraint with high SNR and something much closer to Nyquist sampling than the individual frames in the image series.…”