Controlled experimental study depicting moving objects in view‐shared time‐resolved 3D MRA

Abstract

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...

Time-of-Arrival Mapping at Three-dimensional Time-resolved Contrast-enhanced MR Angiography

“…In other words, if the k-space center is measured early within the acquisition time for a given first image, then it should be measured at the same relative time point for all subsequent images. This ensures that the leading edge of the contrast material bolus moving at a fixed velocity will be accurately depicted at equally spaced intervals in the resultant time series of MR images, as shown experimentally with phantoms (23). Measurement of this edge forms the basis of the TOA estimation.…”

“…The following additional properties are desirable: (a) The MR image acquisition strategy should be consistent, (b) the k-space center should be compactly measured, (c) the duration of the acquisition per image, referred to as the temporal footprint, should be minimized, and (d) other artifactual SIs should be minimized or avoided. These properties have been studied in related studies (23) and are explained as follows:…”

“…This reduces blurring of the leading edge of the contrast material bolus (23). To meet the third criterion, reduction in the overall acquisition time per image, use of an acceleration technique such as two-dimensional sensitivity encoding (14) is useful.…”

“…Even if all of the preceding criteria are met, the image of an advancing contrast material bolus reconstructed at some time point will invariably deviate from the ideal and unattainable image on which all data are instantaneously acquired. For example, for any one time frame, measurement of highspatial-frequency k-space values after the central k-space is sampled results in a nonzero SI value in advance of the leading edge of the contrast material bolus (23). Such "anticipation" artifact can be reduced by measuring the central k-space near the end of the acquisition time for each time frame.…”

Peripheral Vasculature: High-Temporal- and High-Spatial-Resolution Three-dimensional Contrast-enhanced MR Angiography

“…For the same approximate 5-second frame time, the 20- (43,44). CAPR avoids k-space interpolation of time-resolved imaging of contrast kinetics (or TRICKS) (10) and extensive sampling of peripheral k-space after central k-space of time-resolved angiography with interleaved stochastic trajectories (or TWIST) (45), characteristics that can degrade the sharpness in portraying the leading edge of the contrast material bolus (46). The radial-like kspace pattern of the CAPR sampling provides some intrinsic resistance to artifact, similar to that demonstrated in conventional projection reconstruction acquisition (47).…”