Time-resolved angiography: Past, present, and future.

The introduction of digital subtraction angiography (DSA) in 1980 provided a method for real time 2D subtraction imaging. Later, 4D magnetic resonance (MR) angiography emerged beginning with techniques like Keyhole and time-resolved imaging of contrast kinetics (TRICKS) that provided frame rates of one every 5 seconds with limited spatial resolution. Undersampled radial acquisition was subsequently developed. The 3D vastly undersampled isotropic projection (VIPR) technique allowed undersampling factors of 30-40. Its combination with phase contrast displays time-resolved flow dynamics within the cardiac cycle and has enabled the measurement of pressure gradients in small vessels. Meanwhile similar accelerations were achieved using Cartesian acquisition with projection reconstruction (CAPR), a Cartesian acquisition with 2D parallel imaging. Further acceleration is provided by constrained reconstruction techniques such as highly constrained back-projection reconstruction (HYPR) and its derivatives, which permit acceleration factors approaching 1000. Hybrid MRA combines a separate phase contrast, time-of flight, or contrast-enhanced acquisition to constrain the reconstruction of contrast-enhanced time frames providing exceptional spatial and temporal resolution and signal-to-noise ratio (SNR). This can be extended to x-ray imaging where a 3D DSA examination can be used to constrain the reconstruction of time-resolved 3D volumes. Each 4D DSA (time-resolved 3D DSA) frame provides spatial resolution and SNR comparable to 3D DSA, thus removing a major limitation of intravenous DSA. Similar techniques have provided the ability to do 4D fluoroscopy.